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Zhu X, Zhang H, Mendell JT. Ribosome Recycling by ABCE1 Links Lysosomal Function and Iron Homeostasis to 3' UTR-Directed Regulation and Nonsense-Mediated Decay. Cell Rep 2021; 32:107895. [PMID: 32668236 PMCID: PMC7433747 DOI: 10.1016/j.celrep.2020.107895] [Citation(s) in RCA: 32] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2020] [Revised: 06/09/2020] [Accepted: 06/22/2020] [Indexed: 12/26/2022] Open
Abstract
Nonsense-mediated decay (NMD) is a pathway that degrades mRNAs containing premature termination codons. Here we describe a genome-wide screen for NMD factors that uncovers an unexpected mechanism that broadly governs 3' untranslated region (UTR)-directed regulation. The screen reveals that NMD requires lysosomal acidification, which allows transferrin-mediated iron uptake, which, in turn, is necessary for iron-sulfur (Fe-S) cluster biogenesis. This pathway maintains the activity of the Fe-S cluster-containing ribosome recycling factor ABCE1, whose impaired function results in movement of ribosomes into 3' UTRs, where they displace exon junction complexes, abrogating NMD. Importantly, these effects extend beyond NMD substrates, with ABCE1 activity required to maintain the accessibility of 3' UTRs to diverse regulators, including microRNAs and RNA binding proteins. Because of the sensitivity of the Fe-S cluster of ABCE1 to iron availability and reactive oxygen species, these findings reveal an unanticipated vulnerability of 3' UTR-directed regulation to lysosomal dysfunction, iron deficiency, and oxidative stress.
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Affiliation(s)
- Xiaoqiang Zhu
- Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390-9148, USA
| | - He Zhang
- Quantitative Biomedical Research Center, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA; Department of Population and Data Sciences, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Joshua T Mendell
- Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390-9148, USA; Harold C. Simmons Comprehensive Cancer Center, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA; Hamon Center for Regenerative Science and Medicine, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA; Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
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Schäfer GG, Pedrini-Martha V, Jackson DJ, Dallinger R, Lieb B. The evolution of hemocyanin genes in Tectipleura: a multitude of conserved introns in highly diverse gastropods. BMC Ecol Evol 2021; 21:36. [PMID: 33663373 PMCID: PMC7931591 DOI: 10.1186/s12862-021-01763-3] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2020] [Accepted: 02/08/2021] [Indexed: 02/07/2023] Open
Abstract
Background Hemocyanin is the oxygen transporter of most molluscs. Since the oxygen affinity of hemocyanin is strongly temperature-dependent, this essential protein needs to be well-adapted to the environment. In Tectipleura, a very diverse group of gastropods with > 27,000 species living in all kinds of habitats, several hemocyanin genes have already been analyzed. Multiple independent duplications of this gene have been identified and may represent potential adaptations to different environments and lifestyles. The aim of this study is to further explore the evolution of these genes by analyzing their exon–intron architectures. Results We have reconstructed the gene architectures of ten hemocyanin genes from four Tectipleura species: Aplysia californica, Lymnaea stagnalis, Cornu aspersum and Helix pomatia. Their hemocyanin genes each contain 53 introns, significantly more than in the hemocyanin genes of Cephalopoda (9–11), Vetigastropoda (15) and Caenogastropoda (28–33). The gene structures of Tectipleura hemocyanins are identical in terms of intron number and location, with the exception of one out of two hemocyanin genes of L. stagnalis that comprises one additional intron. We found that gene structures that differ between molluscan lineages most probably evolved more recently through independent intron gains. Conclusions The strict conservation of the large number of introns in Tectipleura hemocyanin genes over 200 million years suggests the influence of a selective pressure on this gene structure. While we could not identify conserved sequence motifs within these introns, it may be simply the great number of introns that offers increased possibilities of gene regulation relative to hemocyanin genes with less introns and thus may have facilitated habitat shifts and speciation events. This hypothesis is supported by the relatively high number of introns within the hemocyanin genes of Pomacea canaliculata that has evolved independently of the Tectipleura. Pomacea canaliculata belongs to the Caenogastropoda, the sister group of Heterobranchia (that encompass Tectipleura) which is also very diverse and comprises species living in different habitats. Our findings provide a hint to some of the molecular mechanisms that may have supported the spectacular radiation of one of Metazoa’s most species rich groups.
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Affiliation(s)
- Gabriela Giannina Schäfer
- Institute of Molecular Physiology, Johannes Gutenberg-University of Mainz, Johann-Joachim-Becher-Weg 7, 55128, Mainz, Germany.
| | - Veronika Pedrini-Martha
- Institute of Zoology and Center of Molecular Biosciences, University of Innsbruck, Technikerstraße 25, 6020, Innsbruck, Austria
| | - Daniel John Jackson
- Department of Geobiology, Georg-August-University of Göttingen, Goldschmidtstr. 3, 37077, Göttingen, Germany
| | - Reinhard Dallinger
- Institute of Zoology and Center of Molecular Biosciences, University of Innsbruck, Technikerstraße 25, 6020, Innsbruck, Austria
| | - Bernhard Lieb
- Institute of Molecular Physiology, Johannes Gutenberg-University of Mainz, Johann-Joachim-Becher-Weg 7, 55128, Mainz, Germany
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Single-Molecule Imaging Uncovers Rules Governing Nonsense-Mediated mRNA Decay. Mol Cell 2019; 75:324-339.e11. [PMID: 31155380 PMCID: PMC6675935 DOI: 10.1016/j.molcel.2019.05.008] [Citation(s) in RCA: 96] [Impact Index Per Article: 19.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2018] [Revised: 03/15/2019] [Accepted: 05/07/2019] [Indexed: 01/09/2023]
Abstract
Nonsense-mediated decay (NMD) is a surveillance system that degrades mRNAs containing a premature termination codon (PTC) and plays important roles in protein homeostasis and disease. The efficiency of NMD is variable, impacting the clinical outcome of genetic mutations. However, limited resolution of bulk analyses has hampered the study of NMD efficiency. Here, we develop an assay to visualize NMD of individual mRNA molecules in real time. We find that NMD occurs with equal probability during each round of translation of an mRNA molecule. However, this probability is variable and depends on the exon sequence downstream of the PTC, the PTC-to-intron distance, and the number of introns both upstream and downstream of the PTC. Additionally, a subpopulation of mRNAs can escape NMD, further contributing to variation in NMD efficiency. Our study uncovers real-time dynamics of NMD, reveals key mechanisms that influence NMD efficiency, and provides a powerful method to study NMD. Nonsense-mediated decay (NMD) of single mRNA molecules is visualized in live cells Each terminating ribosome has an equal probability of inducing NMD NMD efficiency is affected by the number and position of introns Kinetics of XRN1-dependent decay of the 3′ mRNA cleavage fragment are uncovered
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Pouresmaeili F, Kamalidehghan B, Kamarehei M, Goh YM. A comprehensive overview on osteoporosis and its risk factors. Ther Clin Risk Manag 2018; 14:2029-2049. [PMID: 30464484 PMCID: PMC6225907 DOI: 10.2147/tcrm.s138000] [Citation(s) in RCA: 203] [Impact Index Per Article: 33.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
Osteoporosis is a bone disorder with remarkable changes in bone biologic material and consequent bone structural distraction, affecting millions of people around the world from different ethnic groups. Bone fragility is the worse outcome of the disease, which needs long term therapy and medical management, especially in the elderly. Many involved genes including environmental factors have been introduced as the disease risk factors so far, of which genes should be considered as effective early diagnosis biomarkers, especially for the individuals from high-risk families. In this review, a number of important criteria involved in osteoporosis are addressed and discussed.
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Affiliation(s)
- Farkhondeh Pouresmaeili
- Infertility and Reproductive Health Research Center (IRHRC), Shahid Beheshti University of Medical Sciences, Tehran, Iran
- Medical Genetics Department, Faculty of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran,
| | - Behnam Kamalidehghan
- Medical Genetics Department, Faculty of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran,
- Medical Genetics Center, National Institute of Genetics Engineering and Biotechnology (NIGEB), Tehran, Iran,
| | - Maryam Kamarehei
- Department of Biochemistry and Biophysics (IBB), University of Tehran, Tehran, Iran,
| | - Yong Meng Goh
- Department of Veterinary Preclinical Sciences, Faculty of Veterinary Medicine, Universiti Putra Malaysia (UPM), Serdang, Malaysia
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5
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Variability in genes regulating vitamin D metabolism is associated with vitamin D levels in type 2 diabetes. Oncotarget 2018; 9:34911-34918. [PMID: 30405883 PMCID: PMC6201852 DOI: 10.18632/oncotarget.26178] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2018] [Accepted: 09/17/2018] [Indexed: 12/19/2022] Open
Abstract
Mortality rate is increased in type 2 diabetes (T2D). Low vitamin D levels are associated with increased mortality risk in T2D. In the general population, genetic variants affecting vitamin D metabolism (DHCR7 rs12785878, CYP2R1 rs10741657, GC rs4588) have been associated with serum vitamin D. We studied the association of these variants with serum vitamin D in 2163 patients with T2D from the “Sapienza University Mortality and Morbidity Event Rate (SUMMER) study in diabetes”. Measurements of serum vitamin D were centralised. Genotypes were obtained by Eco™ Real-Time PCR. Data were adjusted for gender, age, BMI, HbA1c, T2D therapy and sampling season. DHCR7 rs12785878 (p = 1 x 10–4) and GC rs4588 (p = 1 x 10–6) but not CYP2R1 rs10741657 (p = 0.31) were significantly associated with vitamin D levels. One unit of a weighted genotype risk score (GRS) was strongly associated with vitamin D levels (p = 1.1 x 10–11) and insufficiency (<30 ng/ml) (OR, 95%CI = 1.28, 1.16–1.41, p = 1.1 x 10–7). In conclusion, DHCR7 rs12785878 and GC rs4588, but not CYP2R1 rs10741657, are significantly associated with vitamin D levels. When the 3 variants were considered together as GRS, a strong association with vitamin D levels and vitamin D insufficiency was observed, thus providing robust evidence that genes involved in vitamin D metabolism modulate serum vitamin D in T2D.
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Baird TD, Cheng KCC, Chen YC, Buehler E, Martin SE, Inglese J, Hogg JR. ICE1 promotes the link between splicing and nonsense-mediated mRNA decay. eLife 2018. [PMID: 29528287 PMCID: PMC5896957 DOI: 10.7554/elife.33178] [Citation(s) in RCA: 35] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
The nonsense-mediated mRNA decay (NMD) pathway detects aberrant transcripts containing premature termination codons (PTCs) and regulates expression of 5–10% of non-aberrant human mRNAs. To date, most proteins involved in NMD have been identified by genetic screens in model organisms; however, the increased complexity of gene expression regulation in human cells suggests that additional proteins may participate in the human NMD pathway. To identify proteins required for NMD, we performed a genome-wide RNAi screen against >21,000 genes. Canonical members of the NMD pathway were highly enriched as top hits in the siRNA screen, along with numerous candidate NMD factors, including the conserved ICE1/KIAA0947 protein. RNAseq studies reveal that depletion of ICE1 globally enhances accumulation and stability of NMD-target mRNAs. Further, our data suggest that ICE1 uses a putative MIF4G domain to interact with exon junction complex (EJC) proteins and promotes the association of the NMD protein UPF3B with the EJC. The DNA in our cells contains the hereditary information that makes each of us unique. Molecules called mRNAs are copies of this information and are used as templates for making proteins. When a strand of incorrectly copied mRNA, or one including errors from the original DNA template, is recognized, our cells destroy the mRNA to prevent it from producing a damaged protein. Organisms from yeast to humans have evolved a mechanism for finding and destroying faulty mRNAs, called mRNA surveillance. Animals are particularly reliant on mRNA surveillance, as their proteins are often made from cutting and pasting together mRNA from different portions of DNA, in a process known as splicing. Despite being a vital process, we still lack a good understanding of how mRNA surveillance works. Now, Baird et al. used human kidney cells that produced an error-containing mRNA that could be tracked. To investigate how efficient RNA surveillance is under different conditions, the levels of individual proteins were reduced one at a time. By tracking the amount of faulty mRNA, it was possible to find out if a single protein plays a role in human mRNA surveillance. If the number of faulty mRNAs is high when a protein is reduced, it suggests that this protein may be involved in mRNA surveillance. Baird et al. screened more than 21,000 proteins, the majority of proteins made in human cells. Many of the proteins that stood out as important in mRNA surveillance were the ones already known to be relevant in yeast and worm cells. But the experiments also identified new proteins that appear to play a role specifically in human RNA surveillance. One of the proteins, ICE1, is essential for the relationship between mRNA splicing and mRNA surveillance. Without ICE1, the mRNA surveillance machinery can no longer find and destroy faulty mRNAs. Nearly one-third of genetic diseases are caused by mutations that result in faulty mRNAs, which can be detected by mRNA surveillance pathways. Depending on the disease, destroying these error-containing mRNAs can either improve or worsen disease symptoms. A better understanding of the factors that control human RNA surveillance could one day help to develop treatments that affect mRNA surveillance to improve disease outcomes.
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Affiliation(s)
- Thomas D Baird
- Biochemistry and Biophysics Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, United States
| | - Ken Chih-Chien Cheng
- National Center for Advancing Translational Sciences, National Institutes of Health, Rockville, United States
| | - Yu-Chi Chen
- National Center for Advancing Translational Sciences, National Institutes of Health, Rockville, United States
| | - Eugen Buehler
- National Center for Advancing Translational Sciences, National Institutes of Health, Rockville, United States
| | - Scott E Martin
- National Center for Advancing Translational Sciences, National Institutes of Health, Rockville, United States
| | - James Inglese
- National Center for Advancing Translational Sciences, National Institutes of Health, Rockville, United States
| | - J Robert Hogg
- Biochemistry and Biophysics Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, United States
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Lejeune F. Nonsense-mediated mRNA decay at the crossroads of many cellular pathways. BMB Rep 2018; 50:175-185. [PMID: 28115040 PMCID: PMC5437961 DOI: 10.5483/bmbrep.2017.50.4.015] [Citation(s) in RCA: 49] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2017] [Indexed: 12/22/2022] Open
Abstract
Nonsense-mediated mRNA decay (NMD) is a surveillance mechanism ensuring the fast decay of mRNAs harboring a premature termination codon (PTC). As a quality control mechanism, NMD distinguishes PTCs from normal termination codons in order to degrade PTC-carrying mRNAs only. For this, NMD is connected to various other cell processes which regulate or activate it under specific cell conditions or in response to mutations, mis-regulations, stresses, or particular cell programs. These cell processes and their connections with NMD are the focus of this review, which aims both to illustrate the complexity of the NMD mechanism and its regulation and to highlight the cellular consequences of NMD inhibition.
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Affiliation(s)
- Fabrice Lejeune
- University Lille, UMR8161 - M3T - Mechanisms of Tumorigenesis and Target Therapies; CNRS, UMR 8161, 3Institut Pasteur de Lille, F-59000 Lille, France
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8
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Optimized approach for the identification of highly efficient correctors of nonsense mutations in human diseases. PLoS One 2017; 12:e0187930. [PMID: 29131862 PMCID: PMC5683606 DOI: 10.1371/journal.pone.0187930] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/14/2017] [Accepted: 10/27/2017] [Indexed: 11/23/2022] Open
Abstract
About 10% of patients with a genetic disease carry a nonsense mutation causing their pathology. A strategy for correcting nonsense mutations is premature termination codon (PTC) readthrough, i.e. incorporation of an amino acid at the PTC position during translation. PTC-readthrough-activating molecules appear as promising therapeutic tools for these patients. Unfortunately, the molecules shown to induce PTC readthrough show low efficacy, probably because the mRNAs carrying a nonsense mutation are scarce, as they are also substrates of the quality control mechanism called nonsense-mediated mRNA decay (NMD). The screening systems previously developed to identify readthrough-promoting molecules used cDNA constructs encoding mRNAs immune to NMD. As the molecules identified were not selected for the ability to correct nonsense mutations on NMD-prone PTC-mRNAs, they could be unsuitable for the context of nonsense-mutation-linked human pathologies. Here, a screening system based on an NMD-prone mRNA is described. It should be suitable for identifying molecules capable of efficiently rescuing the expression of human genes harboring a nonsense mutation. This system should favor the discovery of candidate drugs for treating genetic diseases caused by nonsense mutations. One hit selected with this screening system is presented and validated on cells from three cystic fibrosis patients.
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Chorev M, Joseph Bekker A, Goldberger J, Carmel L. Identification of introns harboring functional sequence elements through positional conservation. Sci Rep 2017. [PMID: 28646210 PMCID: PMC5482813 DOI: 10.1038/s41598-017-04476-0] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
Many human introns carry out a function, in the sense that they are critical to maintain normal cellular activity. Their identification is fundamental to understanding cellular processes and disease. However, being noncoding elements, such functional introns are poorly predicted based on traditional approaches of sequence and structure conservation. Here, we generated a dataset of human functional introns that carry out different types of functions. We showed that functional introns share common characteristics, such as higher positional conservation along the coding sequence and reduced loss rates, regardless of their specific function. A unique property of the data is that if an intron is unknown to be functional, it still does not mean that it is indeed non-functional. We developed a probabilistic framework that explicitly accounts for this unique property, and predicts which specific human introns are functional. We show that we successfully predict function even when the algorithm is trained on introns with a different type of function. This ability has many implications in studying regulatory networks, gene regulation, the effect of mutations outside exons on human disease, and on our general understanding of intron evolution and their functional exaptation in mammals.
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Affiliation(s)
- Michal Chorev
- Department of Genetics, The Alexander Silberman Institute of Life Sciences, Faculty of Science, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, 91904, Israel.,The Rachel and Selim Benin School of Computer Science and Engineering, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Jerusalem, 91904, Israel
| | | | - Jacob Goldberger
- Faculty of Engineering, Bar-Ilan University, Ramat Gan, 52900, Israel
| | - Liran Carmel
- Department of Genetics, The Alexander Silberman Institute of Life Sciences, Faculty of Science, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, 91904, Israel.
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10
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Nickless A, Bailis JM, You Z. Control of gene expression through the nonsense-mediated RNA decay pathway. Cell Biosci 2017; 7:26. [PMID: 28533900 PMCID: PMC5437625 DOI: 10.1186/s13578-017-0153-7] [Citation(s) in RCA: 118] [Impact Index Per Article: 16.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2017] [Accepted: 05/12/2017] [Indexed: 11/25/2022] Open
Abstract
Nonsense-mediated RNA decay (NMD) was originally discovered as a cellular surveillance pathway that safeguards the quality of mRNA transcripts in eukaryotic cells. In its canonical function, NMD prevents translation of mutant mRNAs harboring premature termination codons (PTCs) by targeting them for degradation. However, recent studies have shown that NMD has a much broader role in gene expression by regulating the stability of many normal transcripts. In this review, we discuss the function of NMD in normal physiological processes, its dynamic regulation by developmental and environmental cues, and its association with human disease.
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Affiliation(s)
- Andrew Nickless
- Department of Cell Biology & Physiology, Washington University School of Medicine, Campus Box 8228, 660 S. Euclid Ave., St. Louis, MO 63110 USA
| | - Julie M Bailis
- Department of Oncology Research, Amgen, South San Francisco, CA 94080 USA
| | - Zhongsheng You
- Department of Cell Biology & Physiology, Washington University School of Medicine, Campus Box 8228, 660 S. Euclid Ave., St. Louis, MO 63110 USA
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11
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Zahdeh F, Carmel L. The role of nucleotide composition in premature termination codon recognition. BMC Bioinformatics 2016; 17:519. [PMID: 27927164 PMCID: PMC5142417 DOI: 10.1186/s12859-016-1384-z] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2016] [Accepted: 11/26/2016] [Indexed: 11/25/2022] Open
Abstract
Background It is not fully understood how a termination codon is recognized as premature (PTC) by the nonsense-mediated decay (NMD) machinery. This is particularly true for transcripts lacking an exon junction complex (EJC) along their 3’ untranslated region (3’UTR), and thus degrade through the EJC-independent NMD pathway. Results Here, we analyzed data of transcript stability change following NMD repression and identified over 200 EJC-independent NMD-targets. We examined many features characterizing these transcripts, and compared them to NMD-insensitive transcripts, as well as to a group of transcripts that are destabilized following NMD repression (destabilized transcripts). Conclusions We found that none of the known NMD-triggering features, such as the presence of upstream open reading frames, significantly characterizes EJC-independent NMD-targets. Instead, we saw that NMD-targets are strongly enriched with G nucleotides upstream of the termination codon, and even more so along their 3’UTR. We suggest that high G content around the termination codon impedes translation termination as a result of mRNA folding, thus triggering NMD. We also suggest that high G content in the 3’UTR helps to activate NMD by allowing for the accumulation of UPF1, or other NMD-promoting proteins, along the 3’UTR. Electronic supplementary material The online version of this article (doi:10.1186/s12859-016-1384-z) contains supplementary material, which is available to authorized users.
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Affiliation(s)
- Fouad Zahdeh
- Department of Genetics, The Alexander Silberman Institute of Life Sciences, Faculty of Science, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, 91904, Israel.,Hereditary Research Lab, Life Sciences Department, Bethlehem University, Bethlehem, Palestine
| | - Liran Carmel
- Department of Genetics, The Alexander Silberman Institute of Life Sciences, Faculty of Science, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, 91904, Israel.
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12
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Xu G, Jiang X, Jaffrey SR. A mental retardation-linked nonsense mutation in cereblon is rescued by proteasome inhibition. J Biol Chem 2013; 288:29573-85. [PMID: 23983124 DOI: 10.1074/jbc.m113.472092] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
A nonsense mutation in cereblon (CRBN) causes autosomal recessive nonsyndromic mental retardation. Cereblon is a substrate receptor for the Cullin-RING E3 ligase complex and couples the ubiquitin ligase to specific ubiquitination targets. The CRBN nonsense mutation (R419X) results in a protein lacking 24 amino acids at its C terminus. Although this mutation has been linked to mild mental retardation, the mechanism by which the mutation affects CRBN function is unknown. Here, we used biochemical and mass spectrometric approaches to explore the function of this mutant. We show that the protein retains its ability to assemble into a Cullin-RING E3 ligase complex and catalyzes the ubiquitination of CRBN-target proteins. However, we find that this mutant exhibits markedly increased levels of autoubiquitination and is more readily degraded by the proteasome than the wild type protein. We also show that the level of the mutant protein can be restored by a treatment of cells with a clinically utilized proteasome inhibitor, suggesting that this agent may be useful for the treatment of mental retardation associated with the CRBN R419X mutation. These data demonstrate that enhanced autoubiquitination and degradation account for the defect in CRBN activity that leads to mental retardation.
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Affiliation(s)
- Guoqiang Xu
- From the Department of Pharmacology, College of Pharmaceutical Sciences, Jiangsu Key Laboratory of Translational Research for Neuro-Psycho-Diseases, Soochow University, Suzhou, Jiangsu 215123, China and
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Abstract
In mammalian cells, aberrant transcripts harboring a premature termination codon (PTC) can be generated by abnormal or inefficient biogenesis of mRNAs or by somatic mutation. Truncated polypeptides synthesized from these aberrant transcripts could be toxic to normal cellular functions. However, mammalian cells have evolved sophisticated mechanisms for monitoring the quality of mRNAs. The faulty transcripts harboring PTC are subject to nonsense-mediated mRNA decay (NMD), nonsense-mediated translational repression (NMTR), nonsense-associated alternative splicing (NAS), or nonsense-mediated transcriptional gene silencing (NMTGS). In this review, we briefly outline the molecular characteristics of each pathway and suggest mRNA quality control mechanisms as a means to regulate normal gene expression. [BMB Reports 2013; 46(1): 9-16]
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Affiliation(s)
- Jungwook Hwang
- Graduate School of Biomedical Science and Engineering, Hanyang University, Seoul, Korea
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Abstract
The intron–exon architecture of many eukaryotic genes raises the intriguing question of whether this unique organization serves any function, or is it simply a result of the spread of functionless introns in eukaryotic genomes. In this review, we show that introns in contemporary species fulfill a broad spectrum of functions, and are involved in virtually every step of mRNA processing. We propose that this great diversity of intronic functions supports the notion that introns were indeed selfish elements in early eukaryotes, but then independently gained numerous functions in different eukaryotic lineages. We suggest a novel criterion of evolutionary conservation, dubbed intron positional conservation, which can identify functional introns.
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Affiliation(s)
- Michal Chorev
- Department of Genetics, The Alexander Silberman Institute of Life Sciences, Faculty of Science, The Hebrew University of Jerusalem Jerusalem, Israel
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Kudryashova E, Struyk A, Mokhonova E, Cannon SC, Spencer MJ. The common missense mutation D489N in TRIM32 causing limb girdle muscular dystrophy 2H leads to loss of the mutated protein in knock-in mice resulting in a Trim32-null phenotype. Hum Mol Genet 2011; 20:3925-32. [PMID: 21775502 DOI: 10.1093/hmg/ddr311] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
Mutations in tripartite motif protein 32 (TRIM32) are responsible for several hereditary disorders that include limb girdle muscular dystrophy type 2H (LGMD2H), sarcotubular myopathy (STM) and Bardet Biedl syndrome. Most LGMD2H mutations in TRIM32 are clustered in the NHL β-propeller domain at the C-terminus and are predicted to interfere with homodimerization. To get insight into TRIM32's role in the pathogenesis of LGMD2H and to create an accurate model of disease, we have generated a knock-in mouse (T32KI) carrying the c.1465G > A (p.D489N) mutation in murine Trim32 corresponding to the human LGMD2H/STM pathogenic mutation c.1459G > A (p.D487N). Our data indicate that T32KI mice have both a myopathic and a neurogenic phenotype, very similar to the one described in the Trim32-null mice that we created previously. Analysis of Trim32 gene expression in T32KI mice revealed normal mRNA levels, but a severe reduction in mutant TRIM32 (D489N) at the protein level. Our results suggest that the D489N pathogenic mutation destabilizes the protein, leading to its degradation, and results in the same mild myopathic and neurogenic phenotype as that found in Trim32-null mice. Thus, one potential mechanism of LGMD2H might be destabilization of mutated TRIM32 protein leading to a null phenotype.
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Affiliation(s)
- Elena Kudryashova
- Department of Neurology, Center for Duchenne Muscular Dystrophy at UCLA, David Geffen School of Medicine at UCLA, Los Angeles, CA 90095, USA
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Mahowald GK, Mahowald MA, Moon C, Khor B, Sleckman BP. Out-of-frame T cell receptor beta transcripts are eliminated by multiple pathways in vivo. PLoS One 2011; 6:e21627. [PMID: 21765899 PMCID: PMC3135592 DOI: 10.1371/journal.pone.0021627] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2011] [Accepted: 06/03/2011] [Indexed: 11/21/2022] Open
Abstract
Non-productive antigen receptor genes with frame shifts generated during the assembly of these genes are found in many mature lymphocytes. Transcripts from these genes have premature termination codons (PTCs) and could encode truncated proteins if they are not either inactivated or destroyed by nonsense-mediated decay (NMD). In mammalian cells, NMD can be activated by pathways that rely on the presence of an intron downstream of the PTC; however, NMD can also be activated by pathways that do not rely on these downstream introns, and pathways independent of NMD can inactivate PTC-containing transcripts. Here, through the generation and analysis of mice with gene-targeted modifications of the endogenous T cell receptor beta (Tcrb) locus, we demonstrate that in T cells in vivo, optimal clearance of PTC-containing Tcrb transcripts depends on the presence of an intron downstream of the PTC.
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Affiliation(s)
- Grace K. Mahowald
- Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Michael A. Mahowald
- Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Clara Moon
- Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Bernard Khor
- Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri, United States of America
| | - Barry P. Sleckman
- Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri, United States of America
- * E-mail:
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17
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Exon junction complex subunits are required to splice Drosophila MAP kinase, a large heterochromatic gene. Cell 2010; 143:238-50. [PMID: 20946982 DOI: 10.1016/j.cell.2010.09.036] [Citation(s) in RCA: 92] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/29/2009] [Revised: 08/04/2010] [Accepted: 09/02/2010] [Indexed: 02/04/2023]
Abstract
The exon junction complex (EJC) is assembled on spliced mRNAs upstream of exon-exon junctions and can regulate their subsequent translation, localization, or degradation. We isolated mutations in Drosophila mago nashi (mago), which encodes a core EJC subunit, based on their unexpectedly specific effects on photoreceptor differentiation. Loss of Mago prevents epidermal growth factor receptor signaling, due to a large reduction in MAPK mRNA levels. MAPK expression also requires the EJC subunits Y14 and eIF4AIII and EJC-associated splicing factors. Mago depletion does not affect the transcription or stability of MAPK mRNA but alters its splicing pattern. MAPK expression from an exogenous promoter requires Mago only when the template includes introns. MAPK is the primary functional target of mago in eye development; in cultured cells, Mago knockdown disproportionately affects other large genes located in heterochromatin. These data support a nuclear role for EJC components in splicing a specific subset of introns.
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De Nicolo A, Tancredi M, Lombardi G, Flemma CC, Barbuti S, Di Cristofano C, Sobhian B, Bevilacqua G, Drapkin R, Caligo MA. A novel breast cancer-associated BRIP1 (FANCJ/BACH1) germ-line mutation impairs protein stability and function. Clin Cancer Res 2008; 14:4672-80. [PMID: 18628483 DOI: 10.1158/1078-0432.ccr-08-0087] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
PURPOSE BRCA1-interacting protein 1 (BRIP1; FANCJ/BACH1), which encodes a DNA helicase that interacts with BRCA1, has been suggested to be a low-penetrance breast cancer predisposing gene. We aimed to assess whether BRIP1 mutations contribute to breast cancer susceptibility in our population and, if so, to investigate the effect of such mutation(s) on BRIP1 function. EXPERIMENTAL DESIGN A series of 49 breast/ovarian cancer families, devoid of a BRCA1/BRCA2 mutation, were screened for BRIP1 mutations. Functional analyses, including coimmunoprecipitation and stability assays, were employed to further characterize a previously unreported variant. RESULTS Five sequence alterations were identified, of which four had been already described. Herein, we report a novel BRIP1 germ-line mutation identified in a woman with early-onset breast cancer. The mutation consists of a 4-nucleotide deletion (c.2992-2995delAAGA) in BRIP1 exon 20 that causes a shift in the reading frame, disrupts the BRCA1-binding domain of BRIP1, and creates a premature stop codon. Functional analysis of the recombinant mutant protein in transfected cells showed that the truncation interferes with the stability of the protein and with its ability to interact with BRCA1. Loss of the wild-type BRIP1 allele with retention of the mutated one was observed in the patient's breast tumor tissue. CONCLUSIONS These results, by showing that the newly identified BRIP1 c.2992-2995delAAGA mutation is associated with instability and functional impairment of the encoded protein, provide further evidence of a breast cancer-related role for BRIP1.
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Affiliation(s)
- Arcangela De Nicolo
- Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, MA 02115, USA
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19
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You KT, Li LS, Kim NG, Kang HJ, Koh KH, Chwae YJ, Kim KM, Kim YK, Park SM, Jang SK, Kim H. Selective translational repression of truncated proteins from frameshift mutation-derived mRNAs in tumors. PLoS Biol 2007; 5:e109. [PMID: 17456004 PMCID: PMC1854916 DOI: 10.1371/journal.pbio.0050109] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/08/2006] [Accepted: 02/16/2007] [Indexed: 01/28/2023] Open
Abstract
Frameshift and nonsense mutations are common in tumors with microsatellite instability, and mRNAs from these mutated genes have premature termination codons (PTCs). Abnormal mRNAs containing PTCs are normally degraded by the nonsense-mediated mRNA decay (NMD) system. However, PTCs located within 50-55 nucleotides of the last exon-exon junction are not recognized by NMD (NMD-irrelevant), and some PTC-containing mRNAs can escape from the NMD system (NMD-escape). We investigated protein expression from NMD-irrelevant and NMD-escape PTC-containing mRNAs by Western blotting and transfection assays. We demonstrated that transfection of NMD-irrelevant PTC-containing genomic DNA of MARCKS generates truncated protein. In contrast, NMD-escape PTC-containing versions of hMSH3 and TGFBR2 generate normal levels of mRNA, but do not generate detectable levels of protein. Transfection of NMD-escape mutant TGFBR2 genomic DNA failed to generate expression of truncated proteins, whereas transfection of wild-type TGFBR2 genomic DNA or mutant PTC-containing TGFBR2 cDNA generated expression of wild-type protein and truncated protein, respectively. Our findings suggest a novel mechanism of gene expression regulation for PTC-containing mRNAs in which the deleterious transcripts are regulated either by NMD or translational repression.
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Affiliation(s)
- Kwon Tae You
- Department of Pathology, Yonsei University College of Medicine, Seoul, Korea
- Brain Korea 21 Projects for Medical Sciences, Yonsei University College of Medicine, Seoul, Korea
| | - Long Shan Li
- Department of Pathology, Yonsei University College of Medicine, Seoul, Korea
- Brain Korea 21 Projects for Medical Sciences, Yonsei University College of Medicine, Seoul, Korea
| | - Nam-Gyun Kim
- Department of Pathology, Yonsei University College of Medicine, Seoul, Korea
| | - Hyun Ju Kang
- Department of Pathology, Yonsei University College of Medicine, Seoul, Korea
- Brain Korea 21 Projects for Medical Sciences, Yonsei University College of Medicine, Seoul, Korea
| | - Kwi Hye Koh
- Department of Pathology, Yonsei University College of Medicine, Seoul, Korea
- Brain Korea 21 Projects for Medical Sciences, Yonsei University College of Medicine, Seoul, Korea
| | - Yong-Joon Chwae
- Brain Korea 21 Projects for Medical Sciences, Yonsei University College of Medicine, Seoul, Korea
- Department of Microbiology, Pochon Cha University College of Medicine, Pocheon, Gyeonggi, Korea
| | - Kyoung Mi Kim
- School of Life Sciences and Biotechnology, Korea University, Seoul, Korea
| | - Yoon Ki Kim
- School of Life Sciences and Biotechnology, Korea University, Seoul, Korea
| | - Sung Mi Park
- Department of Life Science, Pohang University of Science and Technology, Pohang, Kyungbuk, Korea
| | - Sung Key Jang
- Department of Life Science, Pohang University of Science and Technology, Pohang, Kyungbuk, Korea
| | - Hoguen Kim
- Department of Pathology, Yonsei University College of Medicine, Seoul, Korea
- Brain Korea 21 Projects for Medical Sciences, Yonsei University College of Medicine, Seoul, Korea
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20
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Harries LW, Bingham C, Bellanne-Chantelot C, Hattersley AT, Ellard S. The position of premature termination codons in the hepatocyte nuclear factor -1 beta gene determines susceptibility to nonsense-mediated decay. Hum Genet 2005; 118:214-24. [PMID: 16133182 DOI: 10.1007/s00439-005-0023-y] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2005] [Accepted: 06/15/2005] [Indexed: 11/24/2022]
Abstract
The nonsense-mediated decay (NMD) pathway is an mRNA surveillance mechanism that detects and degrades transcripts containing premature termination codons. The position of a truncating mutation can govern the resulting phenotype as mutations in the last exon evade NMD. In this study we investigated the susceptibility to NMD of six truncating HNF-1beta mutations by allele-specific quantitative real-time PCR using transformed lymphoblastoid cell lines. Four of six mutations (R181X, Q243fsdelC, P328L329fsdelCCTCT and A373fsdel29) showed evidence of NMD with levels of mutant transcript at 71% (p=0.009), 24% (p=0.008), 22% (p=0.008) and 3% (p=0.016) of the wild-type allele respectively. Comparable results were derived from lymphoblastoid cells and renal tubule cells isolated from a patient's overnight urine confirming that cell lines provide a good model for mRNA analysis. Two mutations (H69fsdelAC and P159fsdelT) produced transcripts unexpectedly immune to NMD. We conclude that truncating mutant transcripts of the HNF-1beta gene do not conform to the known rules governing NMD susceptibility, but instead demonstrate a previously unreported 5' to 3' polarity. We hypothesise that this may be due to reinitiation of translation downstream of the premature termination codon. Our study suggests that reinitiation of translation may be an important mechanism in the evasion of NMD, but that other factors such as the distance from the native initiation codon may also play a part.
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Affiliation(s)
- L W Harries
- Institute of Biomedical and Clinical Sciences, Peninsula Medical School, Barrack Road, Exeter, UK
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21
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Baek D, Green P. Sequence conservation, relative isoform frequencies, and nonsense-mediated decay in evolutionarily conserved alternative splicing. Proc Natl Acad Sci U S A 2005; 102:12813-8. [PMID: 16123126 PMCID: PMC1192826 DOI: 10.1073/pnas.0506139102] [Citation(s) in RCA: 112] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Studies of expressed sequence tag data sets have revealed large numbers of splicing variants for human genes, but it remains challenging to distinguish functionally important variants from aberrant splicing, clarify the nature of the alternative functions, and understand the signals that regulate splicing choices. To help address these issues, we have constructed and analyzed a large data set of 1,478 exon-skipping alternative splicing (AS) variants evolutionarily conserved in human and mouse. In about one-fifth of cases, one isoform appears subject to nonsense-mediated mRNA decay (NMD), supporting the idea that a major role of AS is to regulate gene expression; one-quarter of these NMD-inducing cases involve a conserved exon whose apparent sole purpose is to mediate destruction of the message when included. We explore sequence conservation likely related to splicing regulation, using in part a measure of the overall amount of conserved information in a sequence, and find that the increased conservation that has been observed within AS exons primarily affects synonymous sites, suggesting that regulatory signals significantly constrain synonymous substitution rates. We show that a lower frequency of the inclusion isoform relative to the exclusion isoform tends to be associated with weaker splice site signals, smaller exon size, and higher intronic sequence conservation, and provide evidence that all of these factors are under selection to control relative isoform frequencies. Some conserved instances of AS appear to represent aberrant splicing events that by chance have occurred in both species, and we develop a nonparametric likelihood approach to identify these.
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Affiliation(s)
- Daehyun Baek
- Department of Bioengineering, University of Washington, Box 357730, Seattle, WA 98195, USA.
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22
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McMinn JE, Liu SM, Dragatsis I, Dietrich P, Ludwig T, Eiden S, Chua SC. An allelic series for the leptin receptor gene generated by CRE and FLP recombinase. Mamm Genome 2005; 15:677-85. [PMID: 15389315 DOI: 10.1007/s00335-004-2340-1] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2003] [Accepted: 05/18/2004] [Indexed: 01/23/2023]
Abstract
Body weight regulation is mediated through several major signaling pathways, some of which have been delineated by positional cloning of spontaneous genetic mutations in mice. Lepr(db/db) mice are obese due to a defect in the signaling portion of the leptin receptor, which has led to extensive study of this highly conserved system over the past several years. We have created an allelic series at Lepr for the further examination of LEPR signaling phenotypes using both the FLP /frt and CRE /loxP systems. By inserting a frt-PGK-neo-frt sequence in Lepr intron 16, we have generated a conditional gene repair Lepr allele ( Lepr-neo) that elicits morbid obesity, diabetes, and infertility in homozygous mice, recapitulating the obesity syndrome of Lepr(db/db) mice. Thus, in vivo excision of the PGK-neo cassette with a FLP recombinase transgene restores the lean and fertile phenotype to Lepr(flox/flox) mice. In the same construct, we have also inserted loxP sites that flank Lepr coding exon 17, a region that encodes a JAK docking site required for STAT3 signaling. CRE-mediated excision of Lepr coding exon 17 from Lepr with a frameshift in subsequent exons results in a syndrome of obesity, diabetes, and infertility in LeprDelta17/Delta17 mice, which is indistinguishable from Lepr(neo/neo) and Lepr(db/db) mice. We conclude that suppression of Lepr gene expression by PGK-neo is phenotypically equivalent to deletion of the Lepr signaling motifs, and therefore the Lepr(neo/neo) mouse may be used to investigate conditional gene repair of Lepr signaling deficiency.
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Affiliation(s)
- Julie E McMinn
- Department of Pediatrics, Division of Molecular Genetics, Columbia University College of Physicians and Surgeons, New York, New York 10032, USA
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23
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Bühler M, Paillusson A, Mühlemann O. Efficient downregulation of immunoglobulin mu mRNA with premature translation-termination codons requires the 5'-half of the VDJ exon. Nucleic Acids Res 2004; 32:3304-15. [PMID: 15210863 PMCID: PMC443527 DOI: 10.1093/nar/gkh651] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Premature translation-termination codons (PTCs) elicit rapid degradation of the mRNA by a process called nonsense-mediated mRNA decay (NMD). NMD appears to be significantly more efficient for mRNAs of genes belonging to the immunoglobulin superfamily, which frequently acquire PTCs during VDJ rearrangment, than for mRNAs of other genes. To identify determinants for efficient NMD, we developed a minigene system derived from a mouse immunoglobulin micro gene (Ig-micro) and measured the effect of PTCs at different positions on the mRNA level. This revealed that PTCs located downstream of the V-D junction in the VDJ exon of Ig-micro minigenes and of endogenous Ig-micro genes elicit very strong mRNA downregulation, whereas NMD efficiency decreases gradually further upstream in the V segment where a PTC was inserted. Interestingly, two PTCs are in positions where they usually do not trigger NMD (<50 nt from the 3'-most 5' splice site) still resulted in reduced mRNA levels. Using a set of hybrid constructs comprised of Ig-micro and an inefficient substrate for NMD, we identified a 177 nt long element in the V segment that is necessary for efficient downregulation of PTC-containing hybrid transcripts. Moreover, deletion of this NMD-promoting element from the Ig-micro minigene results in loss of strong NMD.
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Affiliation(s)
- Marc Bühler
- Institute of Cell Biology, University of Bern, Baltzerstrasse 4, CH-3012 Bern, Switzerland
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24
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Chiu SY, Lejeune F, Ranganathan AC, Maquat LE. The pioneer translation initiation complex is functionally distinct from but structurally overlaps with the steady-state translation initiation complex. Genes Dev 2004; 18:745-54. [PMID: 15059963 PMCID: PMC387415 DOI: 10.1101/gad.1170204] [Citation(s) in RCA: 110] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
The bulk of cellular proteins derive from the translation of eukaryotic translation initiation factor (eIF)4E-bound mRNA. However, recent studies of nonsense-mediated mRNA decay (NMD) indicate that cap-binding protein (CBP)80-bound mRNA, which is a precursor to eIF4E-bound mRNA, can also be translated during a pioneer round of translation. Here, we report that the pioneer round, which can be assessed by measuring NMD, is not inhibited by 4E-BP1, which is known to inhibit steady-state translation by competing with eIF4G for binding to eIF4E. Therefore, at least in this way, the pioneer round of translation is distinct from steady-state translation. eIF4GI, poly(A)-binding protein (PABP)1, eIF3, eIF4AI, and eIF2alpha coimmunopurify with both CBP80 and eIF4E, which suggests that each factor functions in both modes of translation. Consistent with roles for PABP1 and eIF2alpha in the pioneer round of translation, PABP-interacting protein 2, which is known to destabilize PABP1 binding to poly(A) and inhibit steady-state translation, as well as inactive eIF2alpha, which is also known to inhibit steady-state translation, also inhibit NMD. Polysome profiles indicate that CBP80-bound mRNAs are translated less efficiently than their eIF4E-bound counterparts.
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Affiliation(s)
- Shang-Yi Chiu
- Department of Biochemistry and Biophysics, School of Medicine and Dentistry, University of Rochester, Rochester, New York 14642, USA
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25
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Schell T, Köcher T, Wilm M, Seraphin B, Kulozik AE, Hentze MW. Complexes between the nonsense-mediated mRNA decay pathway factor human upf1 (up-frameshift protein 1) and essential nonsense-mediated mRNA decay factors in HeLa cells. Biochem J 2003; 373:775-83. [PMID: 12723973 PMCID: PMC1223536 DOI: 10.1042/bj20021920] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2002] [Revised: 04/24/2003] [Accepted: 04/30/2003] [Indexed: 11/17/2022]
Abstract
mRNAs harbouring premature translation-termination codons are usually degraded by the nonsense-mediated mRNA decay (NMD) pathway. Human up-frameshift protein 1 (Hupf1) is an NMD factor that is conserved between yeast and mammals. To isolate cellular complexes that are formed with Hupf1 and to explore the role of cellular proteins in NMD, we generated a HeLa cell line that stably expresses Hupf1 bearing a double-affinity tag (termed Hupf1-2tag). Hupf1-2tag is localized in the cytoplasm similar to the endogenous Hupf1 protein, and the Hupf1-2tag cell line is fully NMD-competent. Using affinity chromatography, Hupf1-2tag-associated proteins were isolated. MS and immunoblotting identified the NMD factors Hupf2 and Hupf3a/b as interaction partners of Hupf1. Size-exclusion chromatography indicates that the NMD factors Hupf1, Hupf2 and the large isoform of Hupf3a might exist in a stable, high-molecular-mass complex of approx. 1.3 MDa. Interestingly, the poly(A)-binding protein was also identified by MS to be associated specifically with Hupf1-2tag. In contrast with the interaction with Hupf2 and Hupf3a/b, the association of poly(A)-binding protein with Hupf1 is highly sensitive to treatment of the isolated complexes with RNase. Components of the exon-exon junction complex or the translational eukaryotic release factor (eRF) 3 were not identified in complexes associated with Hupf1-2tag. We discuss these findings in the context of current models of NMD.
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Affiliation(s)
- Thomas Schell
- European Molecular Biology Laboratory Heidelberg, Gene Expression Programme, Meyerhofstrasse 1, 69117 Heidelberg, Germany
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26
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Alonso CR, Akam M. A Hox gene mutation that triggers nonsense-mediated RNA decay and affects alternative splicing during Drosophila development. Nucleic Acids Res 2003; 31:3873-80. [PMID: 12853602 PMCID: PMC167643 DOI: 10.1093/nar/gkg482] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2003] [Accepted: 05/07/2003] [Indexed: 11/14/2022] Open
Abstract
Nonsense mutations are usually assumed to affect protein function by generating truncated protein products. Nonetheless, it is now clear that these mutations affect not just protein synthesis but also messenger RNA stability. The surveillance mechanism responsible for the detection and degradation of 'nonsense' RNA messages is termed nonsense-mediated RNA decay (NMD). Essential biochemical components of the NMD machinery have been defined in several species. Here we identify the Drosophila orthologue of one of these factors, Upf1, and document its expression during embryogenesis. To test whether NMD acts during Drosophila development, we make use of a mutation that introduces a stop codon into a variably spliced exon of the Hox gene Ultrabithorax (Ubx). Using real-time quantitative RT-PCR we demonstrate that Ubx transcripts containing the premature stop codon are expressed at lower levels than their wild type counterpart. Unexpectedly, we also find that the same mutation significantly increases the levels of a Ubx splicing isoform that lacks the exon containing the premature termination codon. These findings indicate that NMD is operational during Drosophila development and suggest that nonsense mutations may affect development by altering the spectrum of splicing products formed, as well as by reducing or eliminating protein synthesis.
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Affiliation(s)
- Claudio R Alonso
- Laboratory for Development and Evolution, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK.
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27
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Abstract
Nonsense-mediated decay (NMD) is an RNA surveillance mechanism that degrades mRNAs containing premature termination (nonsense) codons. The second signal for this pathway in mammalian cells is an intron that must be at least approximately 55 nucleotides downstream of the nonsense codon. Although the functional significance of this '-55 boundary rule' is not known, it is widely thought to reflect the important role of an exon junction protein complex deposited just upstream of exon-exon junctions after RNA splicing. Here we report that a T-cell receptor (TCR)-beta gene did not conform to this rule. Rather than a definitive boundary position, nonsense codons had a polar effect, such that nonsense codons distant from the terminal downstream intron triggered robust NMD and proximal nonsense codons caused modest NMD. We identified a region of the TCR-beta gene that conferred this boundary-independent polar expression pattern on a heterologous gene. Collectively, our results suggest that TCR-beta transcripts contain one or more sequence elements that elicit an unusual NMD response triggered by a novel second signal that ultimately causes boundary-independent polar regulation. TCR genes may have evolved this unique NMD response because they frequently acquire nonsense codons during normal development.
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Affiliation(s)
- Jun Wang
- Department of Immunology, The University of Texas M.D. Anderson Cancer Center, Houston, TX 77030, USA
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28
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Byers PH. Killing the messenger: new insights into nonsense-mediated mRNA decay. J Clin Invest 2002; 109:3-6. [PMID: 11781342 PMCID: PMC150830 DOI: 10.1172/jci14841] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Affiliation(s)
- Peter H Byers
- Department of Pathology, University of Washington, Seattle, Washington 98195-7470, USA.
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29
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Le Hir H, Gatfield D, Izaurralde E, Moore MJ. The exon-exon junction complex provides a binding platform for factors involved in mRNA export and nonsense-mediated mRNA decay. EMBO J 2001; 20:4987-97. [PMID: 11532962 PMCID: PMC125616 DOI: 10.1093/emboj/20.17.4987] [Citation(s) in RCA: 611] [Impact Index Per Article: 26.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We recently reported that spliceosomes alter messenger ribonucleoprotein particle (mRNP) composition by depositing several proteins 20-24 nucleotides upstream of mRNA exon-exon junctions. When assembled in vitro, this so-called 'exon-exon junction complex' (EJC) contains at least five proteins: SRm160, DEK, RNPS1, Y14 and REF. To better investigate its functional attributes, we now describe a method for generating spliced mRNAs both in vitro and in vivo that either do or do not carry the EJC. Analysis of these mRNAs in Xenopus laevis oocytes revealed that this complex is the species responsible for enhancing nucleocytoplasmic export of spliced mRNAs. It does so by providing a strong binding site for the mRNA export factors REF and TAP/p15. Moreover, by serving as an anchoring point for the factors Upf2 and Upf3, the EJC provides a direct link between splicing and nonsense-mediated mRNA decay. Finally, we show that the composition of the EJC is dynamic in vivo and is subject to significant evolution upon mRNA export to the cytoplasm.
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Affiliation(s)
| | - David Gatfield
- Howard Hughes Medical Institute, Department of Biochemistry, Brandeis University, 415 South Street, Waltham, MA 02454, USA and
European Molecular Biology Laboratory (EMBL), Meyerhofstrasse 1, D-69117 Heidelberg, Germany Corresponding author e-mail:
| | - Elisa Izaurralde
- Howard Hughes Medical Institute, Department of Biochemistry, Brandeis University, 415 South Street, Waltham, MA 02454, USA and
European Molecular Biology Laboratory (EMBL), Meyerhofstrasse 1, D-69117 Heidelberg, Germany Corresponding author e-mail:
| | - Melissa J. Moore
- Howard Hughes Medical Institute, Department of Biochemistry, Brandeis University, 415 South Street, Waltham, MA 02454, USA and
European Molecular Biology Laboratory (EMBL), Meyerhofstrasse 1, D-69117 Heidelberg, Germany Corresponding author e-mail:
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30
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Abstract
Nonsense-mediated mRNA decay (NMD), the loss of mRNAs carrying premature stop codons, is a process by which cells recognize and degrade nonsense mRNAs to prevent possibly toxic effects of truncated peptides. Most mammalian nonsense mRNAs are degraded while associated with the nucleus, but a few are degraded in the cytoplasm; at either site, there is a requirement for translation and for an intron downstream of the early stop codon. We have examined the NMD of a mutant HEXA message in lymphoblasts derived from a Tay-Sachs disease patient homozygous for the common frameshift mutation 1278ins4. The mutant mRNA was nearly undetectable in these cells and increased to approximately 40% of normal in the presence of the translation inhibitor cycloheximide. The stabilized transcript was found in the cytoplasm in association with polysomes. Within 5 h of cycloheximide removal, the polysome-associated nonsense message was completely degraded, while the normal message was stable. The increased lability of the polysome-associated mutant HEXA mRNA shows that NMD of this endogenous mRNA occurred in the cytoplasm. Transfection of Chinese hamster ovary cells showed that expression of an intronless HEXA minigene harboring the frameshift mutation or a closely located nonsense codon resulted in half the normal mRNA level. Inclusion of multiple downstream introns decreased the abundance further, to about 20% of normal. Thus, in contrast to other systems, introns are not absolutely required for NMD of HEXA mRNA, although they enhance the low-HEXA-mRNA phenotype.
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Affiliation(s)
- K S Rajavel
- Department of Biological Chemistry, University of California Los Angeles, Los Angeles, California 90095-1737, USA
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31
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Kim V, Yong J, Kataoka N, Abel L, Diem MD, Dreyfuss G. The Y14 protein communicates to the cytoplasm the position of exon-exon junctions. EMBO J 2001; 20:2062-8. [PMID: 11296238 PMCID: PMC125236 DOI: 10.1093/emboj/20.8.2062] [Citation(s) in RCA: 158] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
We recently described an RNA-binding protein, Y14, that binds preferentially to spliced mRNAs and persists in the cytoplasm. Y14 is part of a multi-protein complex that also contains the mRNA export factor TAP. This suggests that splicing imprints the mRNA with a unique set of proteins that communicate the history of the transcript to the cytoplasm. Here, using microinjection of pre-mRNAs into Xenopus oocyte nuclei followed by immunoprecipitation of RNase-fragmented mRNAs from the cytoplasm, we show that Y14 is stably bound to sequences immediately upstream of exon-exon junctions. This feature appears to be unique to Y14. Using monoclonal antibodies that we produced against Aly/REF, another component recently reported to be an mRNA export factor, we show that Aly/REF is associated with spliced mRNAs in the nucleus but is not detectable on mRNAs in the cytoplasm. Thus, we propose that the splicing- dependent binding of Y14 provides a position-specific molecular memory that communicates to the cytoplasm the location of exon and intron boundaries. This novel mechanism may play an important role in post-splicing events.
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Affiliation(s)
| | | | | | | | | | - Gideon Dreyfuss
- Howard Hughes Medical Institute and Department of Biochemistry & Biophysics, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6148, USA
Corresponding author e-mail: V.N.Kim and J.Yong contributed equally to this work
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32
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Neu-Yilik G, Gehring NH, Thermann R, Frede U, Hentze MW, Kulozik AE. Splicing and 3' end formation in the definition of nonsense-mediated decay-competent human beta-globin mRNPs. EMBO J 2001; 20:532-40. [PMID: 11157759 PMCID: PMC133467 DOI: 10.1093/emboj/20.3.532] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Premature translation termination codons are common causes of genetic disorders. mRNAs with such mutations are degraded by a surveillance mechanism termed nonsense-mediated decay (NMD), which represents a phylogenetically widely conserved post-transcriptional mechanism for the quality control of gene expression. How NMD-competent mRNPs are formed and specified remains a central question. Here, we have used human beta-globin mRNA as a model system to address the role of splicing and polyadenylation for human NMD. We show that (i) splicing is an indispensable component of the human beta-globin NMD pathway, which cannot be compensated for by exonic beta-globin 'failsafe' sequences; (ii) the spatial requirements of human beta-globin NMD, as signified by the maximal distance of the nonsense mutation to the final exon-exon junction, are less constrained than in yeast; and (iii) non-polyadenylated mRNAs with a histone 3' end are NMD competent. Thus, the formation of NMD-competent mRNP particles critically depends on splicing but does not require the presence of a poly(A) tail.
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Affiliation(s)
| | | | | | | | - Matthias W. Hentze
- Children’s Hospital, Charité, Humboldt University, Augustenburger Platz 1, D-13353 Berlin and
European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany Corresponding authors e-mail: or G.Neu-Yilik, N.H.Gehring and R.Thermann contributed equally to this work
| | - Andreas E. Kulozik
- Children’s Hospital, Charité, Humboldt University, Augustenburger Platz 1, D-13353 Berlin and
European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany Corresponding authors e-mail: or G.Neu-Yilik, N.H.Gehring and R.Thermann contributed equally to this work
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33
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Serin G, Gersappe A, Black JD, Aronoff R, Maquat LE. Identification and characterization of human orthologues to Saccharomyces cerevisiae Upf2 protein and Upf3 protein (Caenorhabditis elegans SMG-4). Mol Cell Biol 2001; 21:209-23. [PMID: 11113196 PMCID: PMC88795 DOI: 10.1128/mcb.21.1.209-223.2001] [Citation(s) in RCA: 196] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
Nonsense-mediated mRNA decay (NMD), also called mRNA surveillance, is an important pathway used by all organisms that have been tested to degrade mRNAs that prematurely terminate translation and, as a consequence, eliminate the production of aberrant proteins that could be potentially harmful. In mammalian cells, NMD appears to involve splicing-dependent alterations to mRNA as well as ribosome-associated components of the translational apparatus. To date, human (h) Upf1 protein (p) (hUpf1p), a group 1 RNA helicase named after its Saccharomyces cerevisiae orthologue that functions in both translation termination and NMD, has been the only factor shown to be required for NMD in mammalian cells. Here, we describe human orthologues to S. cerevisiae Upf2p and S. cerevisiae Upf3p (Caenorhabditis elegans SMG-4) based on limited amino acid similarities. The existence of these orthologues provides evidence for a higher degree of evolutionary conservation of NMD than previously appreciated. Interestingly, human orthologues to S. cerevisiae Upf3p (C. elegans SMG-4) derive from two genes, one of which is X-linked and both of which generate multiple isoforms due to alternative pre-mRNA splicing. We demonstrate using immunoprecipitations of epitope-tagged proteins transiently produced in HeLa cells that hUpf2p interacts with hUpf1p, hUpf3p-X, and hUpf3p, and we define the domains required for the interactions. Furthermore, we find by using indirect immunofluorescence that hUpf1p is detected only in the cytoplasm, hUpf2p is detected primarily in the cytoplasm, and hUpf3p-X localizes primarily to nuclei. The finding that hUpf3p-X is a shuttling protein provides additional indication that NMD has both nuclear and cytoplasmic components.
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Affiliation(s)
- G Serin
- Department of Cancer Genetics, Roswell Park Cancer Institute, Buffalo, New York 14263, USA
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34
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Mendell JT, Medghalchi SM, Lake RG, Noensie EN, Dietz HC. Novel Upf2p orthologues suggest a functional link between translation initiation and nonsense surveillance complexes. Mol Cell Biol 2000; 20:8944-57. [PMID: 11073994 PMCID: PMC86549 DOI: 10.1128/mcb.20.23.8944-8957.2000] [Citation(s) in RCA: 126] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Transcripts harboring premature signals for translation termination are recognized and rapidly degraded by eukaryotic cells through a pathway known as nonsense-mediated mRNA decay (NMD). In addition to protecting cells by preventing the translation of potentially deleterious truncated peptides, studies have suggested that NMD plays a broader role in the regulation of the steady-state levels of physiologic transcripts. In Saccharomyces cerevisiae, three trans-acting factors (Upf1p to Upf3p) are required for NMD. Orthologues of Upf1p have been identified in numerous species, showing that the NMD machinery, at least in part, is conserved through evolution. In this study, we demonstrate additional functional conservation of the NMD pathway through the identification of Upf2p homologues in Schizosaccharomyces pombe and humans (rent2). Disruption of S. pombe UPF2 established that this gene is required for NMD in fission yeast. rent2 was demonstrated to interact directly with rent1, a known trans-effector of NMD in mammalian cells. Additionally, fragments of rent2 were shown to possess nuclear targeting activity, although the native protein localizes to the cytoplasmic compartment. Finally, novel functional domains of Upf2p and rent2 with homology to eukaryotic initiation factor 4G (eIF4G) and other translational regulatory proteins were identified. Directed mutations within these so-called eIF4G homology (4GH) domains were sufficient to abolish the function of S. pombe Upf2p. Furthermore, using the two-hybrid system, we obtained evidence for direct interaction between rent2 and human eIF4AI and Sui1, both components of the translation initiation complex. Based on these findings, a novel model in which Upf2p and rent2 effects decreased translation and accelerated decay of nonsense transcripts through competitive interactions with eIF4G-binding partners is proposed.
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Affiliation(s)
- J T Mendell
- Institute of Genetic Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
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35
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Sun X, Moriarty PM, Maquat LE. Nonsense-mediated decay of glutathione peroxidase 1 mRNA in the cytoplasm depends on intron position. EMBO J 2000; 19:4734-44. [PMID: 10970865 PMCID: PMC302051 DOI: 10.1093/emboj/19.17.4734] [Citation(s) in RCA: 65] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
mRNA for glutathione peroxidase 1 (GPx1) is subject to cytoplasmic nonsense-mediated decay (NMD) when the UGA selenocysteine (Sec) codon is recognized as nonsense. Here, we demonstrate by moving the sole intron of the GPx1 gene that either the Sec codon or a TAA codon in its place elicits NMD when located >/=59 bp but not </=43 bp upstream of the intron. Therefore, the exon-exon junction of GPx1 mRNA positions the boundary between nonsense codons that do and do not elicit NMD, as has been shown for the 3'-most junctions of mRNAs subject to nucleus-associated NMD. We also demonstrate by using a regulatable promoter to drive GPx1 gene expression that cytoplasmic NMD is characteristic of steady-state mRNA, in contrast to nucleus-associated NMD. These findings clarify the mechanistic relationship between cytoplasmic and nucleus-associated NMD and offer the first demonstration that nuclear introns can influence cytoplasmic NMD. Finally, by analyzing hybrid GPx1 genes, we disprove the idea that the cellular site of NMD is determined by the efficiency of translation initiation.
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Affiliation(s)
- X Sun
- Department of Cancer Genetics, Roswell Park Cancer Institute, Buffalo, NY 14263, USA
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36
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Buzina A, Shulman MJ. Infrequent translation of a nonsense codon is sufficient to decrease mRNA level. Mol Biol Cell 1999; 10:515-24. [PMID: 10069800 PMCID: PMC25184 DOI: 10.1091/mbc.10.3.515] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
In many organisms nonsense mutations decrease the level of mRNA. In the case of mammalian cells, it is still controversial whether translation is required for this nonsense-mediated RNA decrease (NMD). Although previous analyzes have shown that conditions that impede translation termination at nonsense codons also prevent NMD, the residual level of termination was unknown in these experiments. Moreover, the conditions used to impede termination might also have interfered with NMD in other ways. Because of these uncertainties, we have tested the effects of limiting translation of a nonsense codon in a different way, using two mutations in the immunoglobulin mu heavy chain gene. For this purpose we exploited an exceptional nonsense mutation at codon 3, which efficiently terminates translation but nonetheless maintains a high level of mu mRNA. We have shown 1) that translation of Ter462 in the double mutant occurs at only approximately 4% the normal frequency, and 2) that Ter462 in cis with Ter3 can induce NMD. That is, translation of Ter462 at this low (4%) frequency is sufficient to induce NMD.
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Affiliation(s)
- A Buzina
- Departments of Immunology and Molecular and Medical Genetics, University of Toronto, Toronto, Ontario, Canada M5S 1A8
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37
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Zhang J, Sun X, Qian Y, LaDuca JP, Maquat LE. At least one intron is required for the nonsense-mediated decay of triosephosphate isomerase mRNA: a possible link between nuclear splicing and cytoplasmic translation. Mol Cell Biol 1998; 18:5272-83. [PMID: 9710612 PMCID: PMC109113 DOI: 10.1128/mcb.18.9.5272] [Citation(s) in RCA: 220] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/1997] [Accepted: 06/01/1998] [Indexed: 11/20/2022] Open
Abstract
Mammalian cells have established mechanisms to reduce the abundance of mRNAs that harbor a nonsense codon and prematurely terminate translation. In the case of the human triosephosphate isomerase (TPI gene), nonsense codons located less than 50 to 55 bp upstream of intron 6, the 3'-most intron, fail to mediate mRNA decay. With the aim of understanding the feature(s) of TPI intron 6 that confer function in positioning the boundary between nonsense codons that do and do not mediate decay, the effects of deleting or duplicating introns have been assessed. The results demonstrate that TPI intron 6 functions to position the boundary because it is the 3'-most intron. Since decay takes place after pre-mRNA splicing, it is conceivable that removal of the 3'-most intron from pre-mRNA "marks" the 3'-most exon-exon junction of product mRNA so that only nonsense codons located more than 50 to 55 nucleotides upstream of the "mark" mediate mRNA decay. Decay may be elicited by the failure of translating ribosomes to translate sufficiently close to the mark or, more likely, the scanning or looping out of some component(s) of the translation termination complex to the mark. In support of scanning, a nonsense codon does not elicit decay if some of the introns that normally reside downstream of the nonsense codon are deleted so the nonsense codon is located (i) too far away from a downstream intron, suggesting that all exon-exon junctions may be marked, and (ii) too far away from a downstream failsafe sequence that appears to function on behalf of intron 6, i.e., when intron 6 fails to leave a mark. Notably, the proposed scanning complex may have a greater unwinding capability than the complex that scans for a translation initiation codon since a hairpin structure strong enough to block translation initiation when inserted into the 5' untranslated region does not block nonsense-mediated decay when inserted into exon 6 between a nonsense codon residing in exon 6 and intron 6.
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Affiliation(s)
- J Zhang
- Department of Cancer Genetics, Roswell Park Cancer Institute, New York State Department of Health, Buffalo, New York 14263, USA
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38
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Sun X, Perlick HA, Dietz HC, Maquat LE. A mutated human homologue to yeast Upf1 protein has a dominant-negative effect on the decay of nonsense-containing mRNAs in mammalian cells. Proc Natl Acad Sci U S A 1998; 95:10009-14. [PMID: 9707591 PMCID: PMC21452 DOI: 10.1073/pnas.95.17.10009] [Citation(s) in RCA: 152] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
All eukaryotic cells analyzed have developed mechanisms to eliminate the production of mRNAs that prematurely terminate translation. The mechanisms are thought to exist to protect cells from the deleterious effects of in-frame nonsense codons that are generated by routine inefficiencies and inaccuracies in RNA metabolism such as pre-mRNA splicing. Depending on the particular mRNA and how it is produced, nonsense codons can mediate a reduction in mRNA abundance either (i) before its release from an association with nuclei into the cytoplasm, presumably but not certainly while the mRNA is being exported to the cytoplasm and translated by cytoplasmic ribosomes, or (ii) in the cytoplasm. Here, we provide evidence for a factor that functions to eliminate the production of nonsense-containing RNAs in mammalian cells. The factor, variously referred to as Rent1 (regulator of nonsense transcripts) or HUPF1 (human Upf1 protein), was identified by isolating cDNA for a human homologue to Saccharomyces cerevisiae Upf1p, which is a group I RNA helicase that functions in the nonsense-mediated decay of mRNA in yeast. Using monkey COS cells and human HeLa cells, we demonstrate that expression of human Upf1 protein harboring an arginine-to-cysteine mutation at residue 844 within the RNA helicase domain acts in a dominant-negative fashion to abrogate the decay of nonsense-containing mRNA that takes place (i) in association with nuclei or (ii) in the cytoplasm. These findings provide evidence that nonsense-mediated mRNA decay is related mechanistically in yeast and in mammalian cells, regardless of the cellular site of decay.
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Affiliation(s)
- X Sun
- Roswell Park Cancer Institute, Department of Genetics, Elm and Carlton Streets, Buffalo, NY 14263, USA
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39
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Thermann R, Neu-Yilik G, Deters A, Frede U, Wehr K, Hagemeier C, Hentze MW, Kulozik AE. Binary specification of nonsense codons by splicing and cytoplasmic translation. EMBO J 1998; 17:3484-94. [PMID: 9628884 PMCID: PMC1170685 DOI: 10.1093/emboj/17.12.3484] [Citation(s) in RCA: 341] [Impact Index Per Article: 13.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Premature translation termination codons resulting from nonsense or frameshift mutations are common causes of genetic disorders. Complications arising from the synthesis of C-terminally truncated polypeptides can be avoided by 'nonsense-mediated decay' of the mutant mRNAs. Premature termination codons in the beta-globin mRNA cause the common recessive form of beta-thalassemia when the affected mRNA is degraded, but the more severe dominant form when the mRNA escapes nonsense-mediated decay. We demonstrate that cells distinguish a premature termination codon within the beta-globin mRNA from the physiological translation termination codon by a two-step specification mechanism. According to the binary specification model proposed here, the positions of splice junctions are first tagged during splicing in the nucleus, defining a stop codon operationally as a premature termination codon by the presence of a 3' splicing tag. In the second step, cytoplasmic translation is required to validate the 3' splicing tag for decay of the mRNA. This model explains nonsense-mediated decay on the basis of conventional molecular mechanisms and allows us to propose a common principle for nonsense-mediated decay from yeast to man.
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Affiliation(s)
- R Thermann
- Department of Pediatrics, Charité-Virchow Medical Center, Humboldt University, Berlin, Germany
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40
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Moriarty PM, Reddy CC, Maquat LE. Selenium deficiency reduces the abundance of mRNA for Se-dependent glutathione peroxidase 1 by a UGA-dependent mechanism likely to be nonsense codon-mediated decay of cytoplasmic mRNA. Mol Cell Biol 1998; 18:2932-9. [PMID: 9566912 PMCID: PMC110672 DOI: 10.1128/mcb.18.5.2932] [Citation(s) in RCA: 184] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/1997] [Accepted: 02/17/1998] [Indexed: 02/07/2023] Open
Abstract
The mammalian mRNA for selenium-dependent glutathione peroxidase 1 (Se-GPx1) contains a UGA codon that is recognized as a codon for the nonstandard amino acid selenocysteine (Sec). Inadequate concentrations of selenium (Se) result in a decrease in Se-GPx1 mRNA abundance by an uncharacterized mechanism that may be dependent on translation, independent of translation, or both. In this study, we have begun to elucidate this mechanism. We demonstrate using hepatocytes from rats fed either a Se-supplemented or Se-deficient diet for 9 to 13 weeks that Se deprivation results in an approximately 50-fold reduction in Se-GPx1 activity and an approximately 20-fold reduction in Se-GPx1 mRNA abundance. Reverse transcription-PCR analyses of nuclear and cytoplasmic fractions revealed that Se deprivation has no effect on the levels of either nuclear pre-mRNA or nuclear mRNA but reduces the level of cytoplasmic mRNA. The regulation of Se-GPx1 gene expression by Se was recapitulated in transient transfections of NIH 3T3 cells, and experiments were extended to examine the consequences of converting the Sec codon (TGA) to either a termination codon (TAA) or a cysteine codon (TGC). Regardless of the type of codon, an alteration in the Se concentration was of no consequence to the ratio of nuclear Se-GPx1 mRNA to nuclear Se-GPx1 pre-mRNA. The ratio of cytoplasmic Se-GPx1 mRNA to nuclear Se-GPx1 mRNA from the wild-type (TGA-containing) allele was reduced twofold when cells were deprived of Se for 48 h after transfection, which has been shown to be the extent of the reduction for the endogenous Se-GPx1 mRNA of cultured cells incubated as long as 20 days in Se-deficient medium. In contrast to the TGA allele, Se had no effect on expression of either the TAA allele or the TGC allele. Under Se-deficient conditions, the TAA and TGC alleles generated, respectively, 1.7-fold-less and 3-fold-more cytoplasmic Se-GPx1 mRNA relative to the amount of nuclear Se-GPx1 mRNA than the TGA allele. These results indicate that (i) under conditions of Se deprivation, the Sec codon reduces the abundance of cytoplasmic Se-GPx1 mRNA by a translation-dependent mechanism and (ii) there is no additional mechanism by which Se regulates Se-GPx1 mRNA production. These data suggest that the inefficient incorporation of Sec at the UGA codon during mRNA translation augments the nonsense-codon-mediated decay of cytoplasmic Se-GPx1 mRNA.
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Affiliation(s)
- P M Moriarty
- Department of Human Genetics, Roswell Park Cancer Institute, Buffalo, New York 14263, USA
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41
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Zhang J, Maquat LE. Evidence that translation reinitiation abrogates nonsense-mediated mRNA decay in mammalian cells. EMBO J 1997; 16:826-33. [PMID: 9049311 PMCID: PMC1169683 DOI: 10.1093/emboj/16.4.826] [Citation(s) in RCA: 144] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Nonsense codons upstream of and including position 192 of the human gene for triosephosphate isomerase (TPI) have been found to reduce the abundance of TPI mRNA to approximately 25% of normal. The reduction is due to the decay of newly synthesized TPI mRNA that co-purifies with nuclei. TPI mRNA that co-purifies with cytoplasm is immune to nonsense-mediated decay. Until now, a nonsense codon at position 23 has been the 5'-most nonsense codon that has been analyzed. Here, we provide evidence that a nonsense codon at position 1, 2 or 10 reduces the abundance of nucleus-associated TPI mRNA to an average of only 84% of normal because translation reinitiates at the methionine codon at position 14. First, converting codon 14 to one for valine increased the effectiveness with which an upstream nonsense codon reduces mRNA abundance. Second, when TPI gene sequences, including codon 14, were fused upstream of and in-frame to the translational reading frame of an Escherichia coli chloramphenicol acetyl transferase (CAT) gene that lacked an initiation codon, a nonsense codon at TPI position 1 or 2 allowed for the production of TPI-CAT that was an estimated 14 amino acids smaller than TPI-CAT produced by a nonsense-free gene, whereas a nonsense codon at TPI position 23 precluded the production of TPI-CAT. These and related findings lend credence to the concept that the nonsense-mediated reduction in the half-life of nucleus-associated TPI mRNA involves cytoplasmic ribosomes.
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Affiliation(s)
- J Zhang
- Department of Human Genetics, Roswell Park Cancer Institute, Buffalo, NY 14263, USA
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42
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Kessler O, Chasin LA. Effects of nonsense mutations on nuclear and cytoplasmic adenine phosphoribosyltransferase RNA. Mol Cell Biol 1996; 16:4426-35. [PMID: 8754843 PMCID: PMC231441 DOI: 10.1128/mcb.16.8.4426] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
We have analyzed Chinese hamster ovary (CHO) cell mutants bearing nonsense codons in four of the five exons of the adenine phosphoribosyltransferase (aprt) gene and have found a pattern of mRNA reduction similar to that seen in systems studied previously: a decrease in steady-state mRNA levels of 5- to 10-fold for mutations in exons 1, 2, and 4 but little effect for mutations in the 3'-most exon (exon 5). Nuclear aprt mRNA levels showed a similar decrease. Nonsense-containing aprt mRNA decayed at the same rate as wild-type mRNA in these cell lines after inhibition of transcription with actinomycin D. Nonsense-containing aprt mRNA is associated with polysomes, ruling out a model in which stable residual mRNA escapes degradation by avoiding translation initiation. A tetracycline-responsive form of the aprt gene was used to compare the stability of nonsense-containing and wild-type aprt mRNAs without globally inhibiting transcription. In contrast to measurements made in the presence of actinomycin D, after inhibition of aprt transcription with tetracycline, a nonsense-mediated destabilization of aprt mRNA was indeed demonstrable. The increased rate of decay of cytoplasmic aprt mRNA seen here could account for the nonsense-mediated reduction in steady-state levels of aprt mRNA. However, the low levels of nonsense-bearing aprt mRNA in the nucleus suggest a sensibility of mRNA to translation or translatability before it exits that compartment. Quantitation of the steady-state levels of transcripts containing introns revealed no accumulation of partially spliced aprt RNA and hence no indication of nonsense-mediated aberrancies in splicing. Our results are consistent with a model in which translation facilitates the export of mRNA through a nuclear pore. However, the mechanism of this intriguing nucleocytoplasmic communication remains to be determined.
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Affiliation(s)
- O Kessler
- Department of Biological Sciences, Columbia University, New York, New York 10027, USA
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Lavenu A, Pistoi S, Pournin S, Babinet C, Morello D. Both coding exons of the c-myc gene contribute to its posttranscriptional regulation in the quiescent liver and regenerating liver and after protein synthesis inhibition. Mol Cell Biol 1995; 15:4410-9. [PMID: 7623834 PMCID: PMC230681 DOI: 10.1128/mcb.15.8.4410] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
In vivo, the steady-state level of c-myc mRNA is mainly controlled by posttranscriptional mechanisms. Using a panel of transgenic mice in which various versions of the human c-myc proto-oncogene were under the control of major histocompatibility complex H-2Kb class I regulatory sequences, we have shown that the 5' and the 3' noncoding sequences are dispensable for obtaining a regulated expression of the transgene in adult quiescent tissues, at the start of liver regeneration, and after inhibition of protein synthesis. These results indicated that the coding sequences were sufficient to ensure a regulated c-myc expression. In the present study, we have pursued this analysis with transgenes containing one or the other of the two c-myc coding exons either alone or in association with the c-myc 3' untranslated region. We demonstrate that each of the exons contains determinants which control c-myc mRNA expression. Moreover, we show that in the liver, c-myc exon 2 sequences are able to down-regulate an otherwise stable H-2K mRNA when embedded within it and to induce its transient accumulation after cycloheximide treatment and soon after liver ablation. Finally, the use of transgenes with different coding capacities has allowed us to postulate that the primary mRNA sequence itself and not c-Myc peptides is an important component of c-myc posttranscriptional regulation.
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Affiliation(s)
- A Lavenu
- Department of Immunology, Institut Pasteur, Paris, France
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44
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Kugler W, Enssle J, Hentze MW, Kulozik AE. Nuclear degradation of nonsense mutated beta-globin mRNA: a post-transcriptional mechanism to protect heterozygotes from severe clinical manifestations of beta-thalassemia? Nucleic Acids Res 1995; 23:413-8. [PMID: 7885837 PMCID: PMC306691 DOI: 10.1093/nar/23.3.413] [Citation(s) in RCA: 53] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
Nonsense mutations of the beta-globin gene are a common cause of beta-thalassemia. It is a hallmark of these mutations not only to cause a lack of protein synthesis but also a reduction of mRNA expression. Both the pathophysiologic significance and the underlying mechanisms for this surprising phenomenon have so far remained enigmatic. We report that the reduction of the fully spliced mutant beta-globin mRNA already manifests itself within the nucleus. In contrast, the levels of mutant pre-mRNA are normal. The promoter and the 5'-untranslated region (5'-UTR) of the herpes simplex virus type 1 thymidine kinase (HSV1 Tk) gene can independently circumvent this recognition/response mechanism in cis and restore nonsense mutated beta-globin mRNA expression to normal levels. These two genetic elements can thus exert a dominant influence on the post-transcriptional control of nonsense mutated beta-globin gene expression. While wild-type mRNA levels are restored by fusion of the HSV1 Tk 5'-UTR to the nonsense mutated beta-globin reading frame, translation of a wildtype reading frame in such a hybrid is precluded. In contrast, the HSV1 Tk promoter appears to efficiently deliver the mRNA to the translational apparatus. The 5'-UTR and the promoter sequences therefore control the nuclear fate of nonsense mutated beta-globin mRNA by separable pathways. The nuclear mRNA degradation mechanisms examined here may prevent the synthesis of C-terminally truncated beta-globin chain fragments and may protect heterozygotes from clinically relevant symptoms of beta-thalassemia.
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Affiliation(s)
- W Kugler
- Department Pediatrics II, University of Ulm, Germany
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45
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Abstract
Frameshift and nonsense mutations within the gene for human triosephosphate isomerase (TPI) that generate a nonsense codon within the first three-fourths of the protein coding region have been found to reduce the abundance of the product mRNA that copurifies with nuclei. The cellular process and location of the nonsense codon-mediated reduction have proven difficult to elucidate for technical reasons. We show here, using electron microscopy to judge the purity of isolated nuclei, that the previously established reduction to 25% of the normal mRNA level is evident for nuclei that are free of detectable cytoplasmic contamination. Therefore, the reduction is likely to be characteristic of bona fide nuclear RNA. Fully spliced nuclear mRNA is identified by Northern (RNA) blot hybridization and a reverse transcription-PCR assay as the species that undergoes decay in experiments that used the human c-fos promoter to elicit a burst and subsequent shutoff of TPI gene transcription upon the addition of serum to serum-deprived cells. Finally, the finding that deletion of a 5' splice site of the TPI gene results predominantly but not exclusively in the removal by splicing (i.e., skipping) of the upstream exon as a part of the flanking introns has been used to demonstrate that decay is specific to those mRNA products that maintain the nonsense codon. This result, together with our previous results that implicate translation by ribosomes and charged tRNAs in the decay mechanism, indicate that nonsense codon recognition takes place after splicing and triggers decay solely in cis. The possibility that decay takes place during the process of mRNA export from the nucleus to the cytoplasm is discussed.
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Belgrader P, Cheng J, Zhou X, Stephenson LS, Maquat LE. Mammalian nonsense codons can be cis effectors of nuclear mRNA half-life. Mol Cell Biol 1994; 14:8219-28. [PMID: 7969159 PMCID: PMC359361 DOI: 10.1128/mcb.14.12.8219-8228.1994] [Citation(s) in RCA: 50] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Frameshift and nonsense mutations within the gene for human triosephosphate isomerase (TPI) that generate a nonsense codon within the first three-fourths of the protein coding region have been found to reduce the abundance of the product mRNA that copurifies with nuclei. The cellular process and location of the nonsense codon-mediated reduction have proven difficult to elucidate for technical reasons. We show here, using electron microscopy to judge the purity of isolated nuclei, that the previously established reduction to 25% of the normal mRNA level is evident for nuclei that are free of detectable cytoplasmic contamination. Therefore, the reduction is likely to be characteristic of bona fide nuclear RNA. Fully spliced nuclear mRNA is identified by Northern (RNA) blot hybridization and a reverse transcription-PCR assay as the species that undergoes decay in experiments that used the human c-fos promoter to elicit a burst and subsequent shutoff of TPI gene transcription upon the addition of serum to serum-deprived cells. Finally, the finding that deletion of a 5' splice site of the TPI gene results predominantly but not exclusively in the removal by splicing (i.e., skipping) of the upstream exon as a part of the flanking introns has been used to demonstrate that decay is specific to those mRNA products that maintain the nonsense codon. This result, together with our previous results that implicate translation by ribosomes and charged tRNAs in the decay mechanism, indicate that nonsense codon recognition takes place after splicing and triggers decay solely in cis. The possibility that decay takes place during the process of mRNA export from the nucleus to the cytoplasm is discussed.
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Affiliation(s)
- P Belgrader
- Department of Human Genetics, Roswell Park Cancer Institute, Buffalo, New York 14263
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47
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Nonsense but not missense mutations can decrease the abundance of nuclear mRNA for the mouse major urinary protein, while both types of mutations can facilitate exon skipping. Mol Cell Biol 1994. [PMID: 8065364 DOI: 10.1128/mcb.14.9.6326] [Citation(s) in RCA: 73] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
In an effort to understand the mechanisms by which nonsense codons affect RNA metabolism in mammalian cells, nonsense mutations were generated within the gene for the secretory major urinary protein (MUP) of mice. The translation of MUP mRNA normally begins within exon 1 and terminates within exon 6, the penultimate exon. Through the use of Northern (RNA) blot hybridization and assays that couple reverse transcription and PCR, a nonsense mutation within codon 50 of exon 2 or codon 143 of exon 5 was found to reduce the abundance of fully spliced, nuclear MUP mRNA to 10 to 20% of normal without an additional reduction in the abundance of cytoplasmic mRNA. In contrast, a nonsense mutation within codon 172 of exon 5 was found to have no effects on the abundance of MUP mRNA. These findings suggest that a boundary between nonsense mutations that do and do not reduce the abundance of nuclear mRNA exists within the exon preceding the exon that harbors the normal site of translation termination. In this way, the boundary is analogous to the boundary that exists within the penultimate exon of the human gene for the cytosolic enzyme triosephosphate isomerase. Assays for exon skipping, i.e., the removal of an exon as a part of the flanking introns during the process of splicing, reveal that 0.1, 2.0, and 0.1% of MUP mRNA normally lack exon 5, exon 6, and exons 5 plus 6, respectively. Relative to normal, the two nonsense mutations within exon 5 increase the abundance of RNA lacking exon 5 on average 20-fold and increase the abundance of RNA lacking exons 5 plus 6 on average 5-fold. Since only one of these nonsense mutations also reduces the abundance of fully spliced nuclear mRNA to 10 to 20% of normal, the two mechanisms by which a nonsense mutation can alter nuclear RNA metabolism must be distinct. The analysis of missense mutations within codons 143 and 172, some of which retain the nonsense mutation, indicates that the reduction in the abundance of fully spliced nuclear mRNA is dependent upon the premature termination of MUP mRNA translation, whereas skipping is attributable to nonsense mutation-mediated changes in exon 5 structure rather than to the premature termination of translation. The increase in exon 5 skipping by either the nonsense or missense mutations within codon 172 correlates with a decrease in the complementarity of exon 5 to U1 snRNA. This suggests that a 5' splice site may extend as far as 12 nucleotides into the upstream exon, which is, to our knowledge, the largest extension.
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48
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Belgrader P, Maquat LE. Nonsense but not missense mutations can decrease the abundance of nuclear mRNA for the mouse major urinary protein, while both types of mutations can facilitate exon skipping. Mol Cell Biol 1994; 14:6326-36. [PMID: 8065364 PMCID: PMC359159 DOI: 10.1128/mcb.14.9.6326-6336.1994] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
In an effort to understand the mechanisms by which nonsense codons affect RNA metabolism in mammalian cells, nonsense mutations were generated within the gene for the secretory major urinary protein (MUP) of mice. The translation of MUP mRNA normally begins within exon 1 and terminates within exon 6, the penultimate exon. Through the use of Northern (RNA) blot hybridization and assays that couple reverse transcription and PCR, a nonsense mutation within codon 50 of exon 2 or codon 143 of exon 5 was found to reduce the abundance of fully spliced, nuclear MUP mRNA to 10 to 20% of normal without an additional reduction in the abundance of cytoplasmic mRNA. In contrast, a nonsense mutation within codon 172 of exon 5 was found to have no effects on the abundance of MUP mRNA. These findings suggest that a boundary between nonsense mutations that do and do not reduce the abundance of nuclear mRNA exists within the exon preceding the exon that harbors the normal site of translation termination. In this way, the boundary is analogous to the boundary that exists within the penultimate exon of the human gene for the cytosolic enzyme triosephosphate isomerase. Assays for exon skipping, i.e., the removal of an exon as a part of the flanking introns during the process of splicing, reveal that 0.1, 2.0, and 0.1% of MUP mRNA normally lack exon 5, exon 6, and exons 5 plus 6, respectively. Relative to normal, the two nonsense mutations within exon 5 increase the abundance of RNA lacking exon 5 on average 20-fold and increase the abundance of RNA lacking exons 5 plus 6 on average 5-fold. Since only one of these nonsense mutations also reduces the abundance of fully spliced nuclear mRNA to 10 to 20% of normal, the two mechanisms by which a nonsense mutation can alter nuclear RNA metabolism must be distinct. The analysis of missense mutations within codons 143 and 172, some of which retain the nonsense mutation, indicates that the reduction in the abundance of fully spliced nuclear mRNA is dependent upon the premature termination of MUP mRNA translation, whereas skipping is attributable to nonsense mutation-mediated changes in exon 5 structure rather than to the premature termination of translation. The increase in exon 5 skipping by either the nonsense or missense mutations within codon 172 correlates with a decrease in the complementarity of exon 5 to U1 snRNA. This suggests that a 5' splice site may extend as far as 12 nucleotides into the upstream exon, which is, to our knowledge, the largest extension.
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Affiliation(s)
- P Belgrader
- Department of Human Genetics, Roswell Park Cancer Institute, Buffalo, New York 14263
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