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Bétermier M, Klobutcher LA, Orias E. Programmed chromosome fragmentation in ciliated protozoa: multiple means to chromosome ends. Microbiol Mol Biol Rev 2023; 87:e0018422. [PMID: 38009915 PMCID: PMC10732028 DOI: 10.1128/mmbr.00184-22] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2023] Open
Abstract
SUMMARYCiliated protozoa undergo large-scale developmental rearrangement of their somatic genomes when forming a new transcriptionally active macronucleus during conjugation. This process includes the fragmentation of chromosomes derived from the germline, coupled with the efficient healing of the broken ends by de novo telomere addition. Here, we review what is known of developmental chromosome fragmentation in ciliates that have been well-studied at the molecular level (Tetrahymena, Paramecium, Euplotes, Stylonychia, and Oxytricha). These organisms differ substantially in the fidelity and precision of their fragmentation systems, as well as in the presence or absence of well-defined sequence elements that direct excision, suggesting that chromosome fragmentation systems have evolved multiple times and/or have been significantly altered during ciliate evolution. We propose a two-stage model for the evolution of the current ciliate systems, with both stages involving repetitive or transposable elements in the genome. The ancestral form of chromosome fragmentation is proposed to have been derived from the ciliate small RNA/chromatin modification process that removes transposons and other repetitive elements from the macronuclear genome during development. The evolution of this ancestral system is suggested to have potentiated its replacement in some ciliate lineages by subsequent fragmentation systems derived from mobile genetic elements.
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Affiliation(s)
- Mireille Bétermier
- Department of Genome Biology, Université Paris-Saclay, CEA, CNRS, Institute for Integrative Biology of the Cell, Gif-sur-Yvette, France
| | - Lawrence A. Klobutcher
- Department of Molecular Biology and Biophysics, UCONN Health (University of Connecticut), Farmington, Connecticut, USA
| | - Eduardo Orias
- Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara, California, USA
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Maurer-Alcalá XX, Yan Y, Pilling OA, Knight R, Katz LA. Twisted Tales: Insights into Genome Diversity of Ciliates Using Single-Cell 'Omics. Genome Biol Evol 2018; 10:1927-1939. [PMID: 29945193 PMCID: PMC6101598 DOI: 10.1093/gbe/evy133] [Citation(s) in RCA: 27] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 06/25/2018] [Indexed: 12/30/2022] Open
Abstract
The emergence of robust single-cell 'omics techniques enables studies of uncultivable species, allowing for the (re)discovery of diverse genomic features. In this study, we combine single-cell genomics and transcriptomics to explore genome evolution in ciliates (a > 1 Gy old clade). Analysis of the data resulting from these single-cell 'omics approaches show: 1) the description of the ciliates in the class Karyorelictea as "primitive" is inaccurate because their somatic macronuclei contain loci of varying copy number (i.e., they have been processed by genome rearrangements from the zygotic nucleus); 2) gene-sized somatic chromosomes exist in the class Litostomatea, consistent with Balbiani's (1890) observation of giant chromosomes in this lineage; and 3) gene scrambling exists in the underexplored Postciliodesmatophora (the classes Heterotrichea and Karyorelictea, abbreviated here as the Po-clade), one of two major clades of ciliates. Together these data highlight the complex evolutionary patterns underlying germline genome architectures in ciliates and provide a basis for further exploration of principles of genome evolution in diverse microbial lineages.
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Affiliation(s)
- Xyrus X Maurer-Alcalá
- Program in Organismic and Evolutionary Biology, University of Massachusetts Amherst.,Department of Biological Sciences, Smith College, Northampton, Massachusetts.,Institute of Cell Biology, University of Bern, Bern, Switzerland
| | - Ying Yan
- Department of Biological Sciences, Smith College, Northampton, Massachusetts
| | - Olivia A Pilling
- Department of Biological Sciences, Smith College, Northampton, Massachusetts
| | - Rob Knight
- Department of Pediatrics, University of California San Diego, San Diego.,Department of Computer Science and Engineering, University of California San Diego, San Diego.,Center for Microbiome Innovation, University of California San Diego, San Diego
| | - Laura A Katz
- Program in Organismic and Evolutionary Biology, University of Massachusetts Amherst.,Department of Biological Sciences, Smith College, Northampton, Massachusetts
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3
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Abou Assi H, El-Khoury R, González C, Damha MJ. 2'-Fluoroarabinonucleic acid modification traps G-quadruplex and i-motif structures in human telomeric DNA. Nucleic Acids Res 2017; 45:11535-11546. [PMID: 29036537 PMCID: PMC5714228 DOI: 10.1093/nar/gkx838] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2017] [Accepted: 09/15/2017] [Indexed: 12/30/2022] Open
Abstract
Human telomeres and promoter regions of genes fulfill a significant role in cellular aging and cancer. These regions comprise of guanine and cytosine-rich repeats, which under certain conditions can fold into G-quadruplex (G4) and i-motif structures, respectively. Herein, we use UV, circular dichroism and NMR spectroscopy to study several human telomeric sequences and demonstrate that G4/i-motif-duplex interconversion kinetics are slowed down dramatically by 2'-β-fluorination and the presence of G4/i-motif-duplex junctions. NMR-monitored kinetic experiments on 1:1 mixtures of native and modified C- and G-rich human telomeric sequences reveal that strand hybridization kinetics are controlled by G4 or i-motif unfolding. Furthermore, we provide NMR evidence for the formation of a hybrid complex containing G4 and i-motif structures proximal to a duplex DNA segment at neutral pH. While the presence of i-motif and G4 folds may be mutually exclusive in promoter genome sequences, our results suggest that they may co-exist transiently as intermediates in telomeric sequences.
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Affiliation(s)
- Hala Abou Assi
- Department of Chemistry, McGill University, Montreal, QC H3A 0B8, Canada
| | - Roberto El-Khoury
- Department of Chemistry, McGill University, Montreal, QC H3A 0B8, Canada
| | - Carlos González
- Instituto de Química Física 'Rocasolano', CSIC, Serrano 119, 28006 Madrid, Spain
| | - Masad J Damha
- Department of Chemistry, McGill University, Montreal, QC H3A 0B8, Canada
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4
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Wu RA, Tam J, Collins K. DNA-binding determinants and cellular thresholds for human telomerase repeat addition processivity. EMBO J 2017; 36:1908-1927. [PMID: 28495680 PMCID: PMC5494469 DOI: 10.15252/embj.201796887] [Citation(s) in RCA: 30] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2017] [Revised: 04/10/2017] [Accepted: 04/11/2017] [Indexed: 12/30/2022] Open
Abstract
The reverse transcriptase telomerase adds telomeric repeats to chromosome ends. Purified human telomerase catalyzes processive repeat synthesis, which could restore the full ~100 nucleotides of (T2AG3)n lost from replicated chromosome ends as a single elongation event. Processivity inhibition is proposed to be a basis of human disease, but the impacts of different levels of processivity on telomere maintenance have not been examined. Here, we delineate side chains in the telomerase active-site cavity important for repeat addition processivity, determine how they contribute to duplex and single-stranded DNA handling, and test the cellular consequences of partial or complete loss of repeat addition processivity for telomere maintenance. Biochemical findings oblige a new model for DNA and RNA handling dynamics in processive repeat synthesis. Biological analyses implicate repeat addition processivity as essential for telomerase function. However, telomeres can be maintained by telomerases with lower than wild-type processivity. Furthermore, telomerases with low processivity dramatically elongate telomeres when overexpressed. These studies reveal distinct consequences of changes in telomerase repeat addition processivity and expression level on telomere elongation and length maintenance.
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Affiliation(s)
- Robert Alexander Wu
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA, USA
| | - Jane Tam
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA, USA
| | - Kathleen Collins
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA, USA
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5
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Multiple DNA Interactions Contribute to the Initiation of Telomerase Elongation. J Mol Biol 2017; 429:2109-2123. [PMID: 28506636 DOI: 10.1016/j.jmb.2017.04.023] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2017] [Revised: 04/12/2017] [Accepted: 04/12/2017] [Indexed: 02/05/2023]
Abstract
Telomerase maintains telomere length and chromosome integrity by adding short tandem repeats of single-stranded DNA to the 3' ends, via reverse transcription of a defined template region of its RNA subunit. To further understand the telomerase elongation mechanism, we studied the primer utilization and extension activity of the telomerase from the budding yeast Naumovozyma castellii (Saccharomyces castellii), which displays a processive nucleotide and repeat addition polymerization. For the efficient initiation of canonical elongation, telomerase required 4-nt primer 3' end complementarity to the template RNA. This DNA-RNA hybrid formation was highly important for the stabilization of an initiation-competent telomerase-DNA complex. Anchor site interactions with the DNA provided additional stabilization to the complex. Our studies indicate three additional separate interactions along the length of the DNA primer, each providing different and distinct contributions to the initiation event. A sequence-independent anchor site interaction acts immediately adjacent to the base-pairing 3' end, indicating a protein anchor site positioned very close to the catalytic site. Two additional anchor regions further 5' on the DNA provide sequence-specific contributions to the initiation of elongation. Remarkably, a non-telomeric sequence in the distal 25- to 32-nt region negatively influences the initiation of telomerase elongation, suggesting an anchor site with a regulatory role in the telomerase elongation decision.
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Abstract
Telomerase is the essential reverse transcriptase required for linear chromosome maintenance in most eukaryotes. Telomerase supplements the tandem array of simple-sequence repeats at chromosome ends to compensate for the DNA erosion inherent in genome replication. The template for telomerase reverse transcriptase is within the RNA subunit of the ribonucleoprotein complex, which in cells contains additional telomerase holoenzyme proteins that assemble the active ribonucleoprotein and promote its function at telomeres. Telomerase is distinct among polymerases in its reiterative reuse of an internal template. The template is precisely defined, processively copied, and regenerated by release of single-stranded product DNA. New specificities of nucleic acid handling that underlie the catalytic cycle of repeat synthesis derive from both active site specialization and new motif elaborations in protein and RNA subunits. Studies of telomerase provide unique insights into cellular requirements for genome stability, tissue renewal, and tumorigenesis as well as new perspectives on dynamic ribonucleoprotein machines.
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Affiliation(s)
- R Alex Wu
- Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3202; , , ,
| | - Heather E Upton
- Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3202; , , ,
| | - Jacob M Vogan
- Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3202; , , ,
| | - Kathleen Collins
- Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3202; , , ,
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Telomeric G-quadruplexes are a substrate and site of localization for human telomerase. Nat Commun 2015; 6:7643. [PMID: 26158869 PMCID: PMC4510649 DOI: 10.1038/ncomms8643] [Citation(s) in RCA: 185] [Impact Index Per Article: 20.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2014] [Accepted: 05/27/2015] [Indexed: 12/14/2022] Open
Abstract
It has been hypothesized that G-quadruplexes can sequester the 3' end of the telomere and prevent it from being extended by telomerase. Here we purify and characterize stable, conformationally homogenous human telomeric G-quadruplexes, and demonstrate that human telomerase is able to extend parallel, intermolecular conformations in vitro. These G-quadruplexes align correctly with the RNA template of telomerase, demonstrating that at least partial G-quadruplex resolution is required. A highly purified preparation of human telomerase retains this extension ability, establishing that the core telomerase enzyme complex is sufficient for partial G-quadruplex resolution and extension. The parallel-specific G-quadruplex ligand N-methyl mesoporphyrin IX (NMM) causes an increase in telomeric G-quadruplexes, and we show that telomerase colocalizes with a subset of telomeric G-quadruplexes in vivo. The ability of telomerase to partially unwind, extend and localize to these structures implies that parallel telomeric G-quadruplexes may play an important biological role.
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Sandin S, Rhodes D. Telomerase structure. Curr Opin Struct Biol 2014; 25:104-10. [PMID: 24704747 PMCID: PMC4045397 DOI: 10.1016/j.sbi.2014.02.003] [Citation(s) in RCA: 49] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2014] [Revised: 02/17/2014] [Accepted: 02/21/2014] [Indexed: 12/18/2022]
Abstract
First of telomerase architecture. Human telomerase functions as a dimer. Conserved RNA/reverse transcriptase core.
The telomerase reverse transcriptase has an essential role in telomere maintenance and in cancer biology. Progress during the last year has revealed the three-dimensional architecture of both human and ciliate telomerase at about 25 Å resolution, obtained using single particle electron microscopy (EM). The structural analysis of the two holoenzyme complexes isolated from cells shows that whilst the ciliate telomerase is monomeric, the human telomerase is dimeric and only functional as a dimer. We critically discuss the approaches taken to assign the location of protein and RNA subunits, as well as fitting the crystal structure of the catalytic protein subunit in the medium resolution EM density maps. Comparison of the two structural interpretations reveals not only a common RNA/reverse transcriptase core, but also significant differences due to different RNA subunit size and protein composition. These differences suggest that the oligomeric state and subunit composition of telomerase in evolutionary distant organism have evolved.
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Affiliation(s)
- Sara Sandin
- School of Biological Sciences, Nanyang Technological University, 60 Nanyang Drive, Singapore 637551, Singapore
| | - Daniela Rhodes
- School of Biological Sciences and LKC Medicine, Proteos, 61 Biopolis Drive, Singapore 138673, Singapore.
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9
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Sugimoto M. A cascade leading to premature aging phenotypes including abnormal tumor profiles in Werner syndrome (review). Int J Mol Med 2013; 33:247-53. [PMID: 24356923 DOI: 10.3892/ijmm.2013.1592] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2013] [Accepted: 12/11/2013] [Indexed: 11/06/2022] Open
Abstract
This perspective review focused on the Werner syndrome (WS) by addressing the issue of how a single mutation in a WRN gene encoding WRN DNA helicase induces a wide range of premature aging phenotypes accompanied by an abnormal pattern of tumors. The key event caused by WRN gene mutation is the dysfunction of telomeres. Studies on normal aging have identified a molecular circuit in which the dysfunction of telomeres caused by cellular aging activates the TP53 gene. The resultant p53 suppresses cell growth and induces a shorter cellular lifespan, and also compromises mitochondrial biogenesis leading to the overproduction of reactive oxygen species (ROS) causing multiple aging phenotypes. As an analogy of the mechanism in natural aging, we described a hypothetical mechanism of premature aging in WS: telomere dysfunction induced by WRN mutation causes multiple premature aging phenotypes of WS, including shortened cellular lifespan and inflammation induced by ROS, such as diabetes mellitus. This model also explains the relatively late onset of the disorder, at approximately age 20. Telomere dysfunction in WS is closely correlated with abnormality in tumorigenesis. Thus, the majority of wide and complex pathological phenotypes of WS may be explained in a unified manner by the cascade beginning with telomere dysfunction initiated by WRN gene mutation.
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Affiliation(s)
- Masanobu Sugimoto
- GeneCare Research Institute, Co. Ltd., Kamakura, Kanagawa 247-0063, Japan
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10
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Abstract
The International Space Station provides an extraordinary facility to study the accelerated aging process in microgravity, which could be triggered by significant reductions in magnesium (Mg) ion levels with, in turn, elevations of catecholamines and vicious cycles between the two. With space flight there are significant reductions of serum Mg (P < 0.0001) that have been shown in large studies of astronauts and cosmonauts. The loss of the functional capacity of the cardiovascular system with space flight is over ten times faster than the course of aging on Earth. Mg is an antioxidant and calcium blocker and in space there is oxidative stress, insulin resistance, and inflammatory conditions with evidence in experimental animals of significant endothelial injuries and damage to mitochondria. The aging process is associated with progressive shortening of telomeres, repetitive DNA sequences, and proteins that cap and protect the ends of chromosomes. Telomerase can elongate pre-existing telomeres to maintain length and chromosome stability. Low telomerase triggers increased catecholamines while the sensitivity of telomere synthesis to Mg ions is primarily seen for the longer elongation products. Mg stabilizes DNA and promotes DNA replication and transcription, whereas low Mg might accelerate cellular senescence by reducing DNA stability, protein synthesis, and function of mitochondria. Telomerase, in binding to short DNAs, is Mg dependent. On Earth, in humans, a year might be required to detect changes in telomeres, but in space there is a predictably much shorter duration required for detection, which is therefore more reasonable in time and cost. Before and after a space mission, telomere lengths and telomerase enzyme activity can be determined and compared with age-matched control rats on Earth. The effect of Mg supplementation, both on maintaining telomere length and extending the life span, can be evaluated. Similar studies in astronauts would be fruitful.
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Single-stranded DNA repeat synthesis by telomerase. Curr Opin Chem Biol 2011; 15:643-8. [PMID: 21816660 DOI: 10.1016/j.cbpa.2011.07.011] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2011] [Accepted: 07/11/2011] [Indexed: 12/13/2022]
Abstract
The eukaryotic ribonucleoprotein reverse transcriptase (RT) telomerase uses a template within its integral RNA subunit to extend chromosome ends by synthesis of single-stranded telomeric repeats. Telomerase is adapted to its unique cellular role by an ability to release product DNA in single-stranded form, regenerating free template from the product-template hybrid. Furthermore, by retaining a template-independent grip on the single-stranded product, telomerase can catalyze processive repeat synthesis. These specialized nucleic acid handling properties are dependent on the protein and RNA domain network within an active RNP. RNP domain architecture and mechanisms for single-stranded DNA handling have been a focus of recent studies highlighted here.
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Sugimoto M, Furuichi Y, Ide T, Goto M. Involvement of WRN helicase in immortalization and tumorigenesis by the telomeric crisis pathway (Review). Oncol Lett 2011; 2:609-611. [PMID: 22848235 DOI: 10.3892/ol.2011.298] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2011] [Accepted: 04/21/2011] [Indexed: 11/06/2022] Open
Abstract
The repeated replication of cells shortens telomeres, culminating in their instability, after which most cells cease to replicate and die. However, a small fraction of the cells become immortalized by maintaining telomeres with activated telomerase activity. It has been proposed that WRN helicase encoded by the WRN gene, the causative gene of Werner syndrome (WS), is required for immortalization by the telomeric crisis pathway (TCP) in a system that uses lymphoblastoid cell lines transformed by the Epstein-Barr virus. Taken together, these characteristics indicate that WRN helicase is also required for the immortalization of epithelial cells by TCP and consequent carcinogenesis, suggesting that the tumorigenesis of epithelial cells by TCP is suppressed in WS lacking the WRN helicase function. Notably, in WS the pathway of alternative lengthening of telomeres without activation of telomerase activity has been suggested to be involved in immortalization and tumorigenesis. This factor is consistent with the abundance of non-epithelial cancers in WS in that the ratio of epithelial to non-epithelial cancers is approximately 1:1 in WS patients compared to 10:1 in the general population. A hypothetical scheme showing the role of WRN helicase in immortalization by means of the supposed 'breakage-fusion-bridge cycle' of chromosomes at telomeric crisis is described.
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Blackburn EH, Collins K. Telomerase: an RNP enzyme synthesizes DNA. Cold Spring Harb Perspect Biol 2011; 3:cshperspect.a003558. [PMID: 20660025 DOI: 10.1101/cshperspect.a003558] [Citation(s) in RCA: 229] [Impact Index Per Article: 17.6] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Telomerase is a eukaryotic ribonucleoprotein (RNP) whose specialized reverse transcriptase action performs de novo synthesis of one strand of telomeric DNA. The resulting telomerase-mediated elongation of telomeres, which are the protective end-caps for eukaryotic chromosomes, counterbalances the inevitable attrition from incomplete DNA replication and nuclease action. The telomerase strategy to maintain telomeres is deeply conserved among eukaryotes, yet the RNA component of telomerase, which carries the built-in template for telomeric DNA repeat synthesis, has evolutionarily diverse size and sequence. Telomerase shows a distribution of labor between RNA and protein in aspects of the polymerization reaction. This article first describes the underlying conservation of a core set of structural features of telomerase RNAs important for the fundamental polymerase activity of telomerase. These include a pseudoknot-plus-template domain and at least one other RNA structural motif separate from the template-containing domain. The principles driving the diversity of telomerase RNAs are then explored. Much of the diversification of telomerase RNAs has come from apparent gain-of-function elaborations, through inferred evolutionary acquisitions of various RNA motifs used for telomerase RNP biogenesis, cellular trafficking of enzyme components, and regulation of telomerase action at telomeres. Telomerase offers broadly applicable insights into the interplay of protein and RNA functions in the context of an RNP enzyme.
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Wyatt HDM, West SC, Beattie TL. InTERTpreting telomerase structure and function. Nucleic Acids Res 2010; 38:5609-22. [PMID: 20460453 PMCID: PMC2943602 DOI: 10.1093/nar/gkq370] [Citation(s) in RCA: 117] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/13/2010] [Revised: 04/20/2010] [Accepted: 04/26/2010] [Indexed: 12/15/2022] Open
Abstract
The Nobel Prize in Physiology or Medicine was recently awarded to Elizabeth Blackburn, Carol Greider and Jack Szostak for their pioneering studies on chromosome termini (telomeres) and their discovery of telomerase, the enzyme that synthesizes telomeres. Telomerase is a unique cellular reverse transcriptase that contains an integral RNA subunit, the telomerase RNA and a catalytic protein subunit, the telomerase reverse transcriptase (TERT), as well as several species-specific accessory proteins. Telomerase is essential for genome stability and is associated with a broad spectrum of human diseases including various forms of cancer, bone marrow failure and pulmonary fibrosis. A better understanding of telomerase structure and function will shed important insights into how this enzyme contributes to human disease. To this end, a series of high-resolution structural studies have provided critical information on TERT architecture and may ultimately elucidate novel targets for therapeutic intervention. In this review, we discuss the current knowledge of TERT structure and function, revealed through the detailed analysis of TERT from model organisms. To emphasize the physiological importance of telomeres and telomerase, we also present a general discussion of the human diseases associated with telomerase dysfunction.
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Affiliation(s)
- Haley D. M. Wyatt
- London Research Institute, Cancer Research UK, Clare Hall Laboratories, South Mimms, EN6 3LD, UK and Southern Alberta Cancer Research Institute and Departments of Biochemistry and Molecular Biology and Oncology, Calgary, Alberta, T2N 4N1, Canada
| | - Stephen C. West
- London Research Institute, Cancer Research UK, Clare Hall Laboratories, South Mimms, EN6 3LD, UK and Southern Alberta Cancer Research Institute and Departments of Biochemistry and Molecular Biology and Oncology, Calgary, Alberta, T2N 4N1, Canada
| | - Tara L. Beattie
- London Research Institute, Cancer Research UK, Clare Hall Laboratories, South Mimms, EN6 3LD, UK and Southern Alberta Cancer Research Institute and Departments of Biochemistry and Molecular Biology and Oncology, Calgary, Alberta, T2N 4N1, Canada
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Mestres M, Caballin MR, Schmid E, Stephan G, Sachs R, Barrios L, Barquinero JF. Analysis ofα‐particle induced chromosome aberrations in human lymphocytes, using pan‐centromeric and pan‐telomeric probes. Int J Radiat Biol 2009; 80:737-44. [PMID: 15799619 DOI: 10.1080/09553000400017416] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
PURPOSE The aim of the present study has been the evaluation of the incomplete chromosome aberrations induced after alpha-particle irradiation by the simultaneous detection of all centromeres and telomeres present in human lymphocytes. Moreover, a study on the lengths of the different acentric fragments is presented. MATERIALS AND METHODS Attached lymphocytes were irradiated at doses of 0.2, 0.5, 0.7 and 1 Gy using a 241Am source. Flourescent in-situ hybridization (FISH) techniques were applied using pan-centromeric and pan-telomeric probes. All abnormal cells were digitalised and analysed using a Cytovision FISH workstation. The description of all abnormalities observed, and the length of the acentric fragments was recorded. RESULTS A total of 378 incomplete chromosomes plus incomplete acentrics was found. Cases with more than 92 telomeres were not detected. The ratio between total incomplete elements and multicentrics was 1.00. The total number of acentric (ace) fragments was 822; 57% of them were complete fragments ace (+,+), 26% incomplete fragments ace (+,-), and 17% interstitial fragments ace(-,-); the mean relative lengths were 2.91 +/- 0.06, 1.91 +/- 0.07 and 1.63 +/- 0.07, respectively. In all three cases a secondary peak in the length distribution was found, corresponding to a relative length between 3.5 and 4. CONCLUSION The percentage of incomplete rejoinings is higher after alpha-particle exposure than that described previously for low-linear energy transfer (LET) radiation exposures. The results seem to indicate that compared to low-LET radiation, after alpha-particle exposure centromere-containing elements are more likely to be repaired.Many interstitial fragments are large linear forms that cannot be considered as non-distinguishable acentric rings.
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Affiliation(s)
- M Mestres
- Unitat d'Antropologia, Dpt. Biologia Animal, Biologia Vegetal i Ecologia, Facultat de Ciències, E-08193, Universitat Autònoma de Barcelona, Bellaterra, Spain
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17
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Finger SN, Bryan TM. Multiple DNA-binding sites in Tetrahymena telomerase. Nucleic Acids Res 2008; 36:1260-72. [PMID: 18174223 PMCID: PMC2275084 DOI: 10.1093/nar/gkm866] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2007] [Revised: 09/13/2007] [Accepted: 09/24/2007] [Indexed: 12/20/2022] Open
Abstract
Telomerase is a ribonucleoprotein enzyme that maintains chromosome ends through de novo addition of telomeric DNA. The ability of telomerase to interact with its DNA substrate at sites outside its catalytic centre ('anchor sites') is important for its unique ability to undergo repeat addition processivity. We have developed a direct and quantitative equilibrium primer-binding assay to measure DNA-binding affinities of regions of the catalytic protein subunit of recombinant Tetrahymena telomerase (TERT). There are specific telomeric DNA-binding sites in at least four regions of TERT (the TEN, RBD, RT and C-terminal domains). Together, these sites contribute to specific and high-affinity DNA binding, with a K(d) of approximately 8 nM. Both the K(m) and K(d) increased in a stepwise manner as the primer length was reduced; thus recombinant Tetrahymena telomerase, like the endogenous enzyme, contains multiple anchor sites. The N-terminal TEN domain, which has previously been implicated in DNA binding, shows only low affinity binding. However, there appears to be cooperativity between the TEN and RNA-binding domains. Our data suggest that different DNA-binding sites are used by the enzyme during different stages of the addition cycle.
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Affiliation(s)
| | - Tracy M. Bryan
- Children's Medical Research Institute, 214 Hawkesbury Road, Westmead NSW 2145 and University of Sydney, NSW 2006, Australia
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18
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Bryan TM, Jarstfer MB. Interrogation of G-quadruplex–protein interactions. Methods 2007; 43:332-9. [DOI: 10.1016/j.ymeth.2007.05.001] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2007] [Accepted: 05/01/2007] [Indexed: 10/22/2022] Open
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Endings in the middle: current knowledge of interstitial telomeric sequences. Mutat Res 2007; 658:95-110. [PMID: 17921045 DOI: 10.1016/j.mrrev.2007.08.006] [Citation(s) in RCA: 98] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2007] [Revised: 08/28/2007] [Accepted: 08/30/2007] [Indexed: 01/24/2023]
Abstract
Interstitial telomeric sequences (ITSs) consist of tandem repeats of the canonical telomeric repeat and are common in mammals. They are localized at intrachromosomal sites, including those repeats located close to the centromeres and those found at interstitial sites, i.e., between the centromeres and the telomeres. ITSs might originate from ancestral intrachromosomal rearrangements (inversions and fusions), from differential crossing-over or from the repair of double-strand break during evolution. Three classes of ITSs have been described in the human genome, namely, short ITSs, long subtelomeric ITSs and fusion ITSs. The fourth class of ITSs, pericentromeric ITSs, has been found in other species. The function of ITSs can be inferred from the association of heritable diseases with ITS polymorphic variants, both in copy number and sequence. This is one of the most attractive aspects of ITS studies because it leads to new and useful markers for genetic linkage studies, forensic applications, and detection of genetic instability in tumors. Some ITSs also might be hotspots of chromosome breakage, rearrangement and amplification sites, based on the type of clastogens and the nature of ITSs. This study will contribute new knowledge with respect to ITSs' biology and mechanism, prevalence of diseases, risk evaluation and prevention of related diseases, thus facilitates the design of early detection markers for diseases caused by genomic instability.
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Abstract
The structure and integrity of telomeres are essential for genome stability. Telomere dysregulation can lead to cell death, cell senescence, or abnormal cell proliferation. The maintenance of telomere repeats in most eukaryotic organisms requires telomerase, which consists of a reverse transcriptase (RT) and an RNA template that dictates the synthesis of the G-rich strand of telomere terminal repeats. Structurally, telomerase reverse transcriptase (TERT) contains unique and variable N- and C-terminal extensions that flank a central RT-like domain. The enzymology of telomerase includes features that are both similar to and distinct from those characteristic of other RTs. Two distinguishing features of TERT are its stable association with the telomerase RNA and its ability to repetitively reverse transcribe the template segment of RNA. Here we discuss TERT structure and function; its regulation by RNA-DNA, TERT-DNA, TERT-RNA, TERT-TERT interactions, and TERT-associated proteins; and the relationship between telomerase enzymology and telomere maintenance.
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Affiliation(s)
- Chantal Autexier
- Bloomfield Center for Research in Aging, Lady Davis Institute for Medical Research, Sir Mortimer B. Davis Jewish General Hospital, Quebec, Canada.
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21
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Juranek SA, Lipps HJ. New Insights into the Macronuclear Development in Ciliates. INTERNATIONAL REVIEW OF CYTOLOGY 2007; 262:219-51. [PMID: 17631190 DOI: 10.1016/s0074-7696(07)62005-1] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
During macronuclear differentiation in ciliated protozoa, most amazing "DNA gymnastics" takes place, which includes DNA excision, DNA elimination, DNA reorganization, and DNA-specific amplification. Although the morphological events occurring during macronuclear development are well described, a detailed knowledge of the molecular mechanisms and the regulation of this differentiation process is still missing. However, recently several models have been proposed for the molecular regulation of macronuclear differentiation, but these models have yet to be verified experimentally. The scope of this review is to summarize recent discoveries in different ciliate species and to compare and discuss the different models proposed. Results obtained in these studies are not only relevant for our understanding of nuclear differentiation in ciliates, but also for cellular differentiation in eukaryotic organisms in general as well as for other disciplines such as bioinformatics and computational biology.
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Affiliation(s)
- Stefan A Juranek
- Howard Hughes Medical Institute, Laboratory of RNA Molecular Biology, Rockefeller University, New York, New York 10021, USA
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22
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Bolzán AD, Bianchi MS. Telomeres, interstitial telomeric repeat sequences, and chromosomal aberrations. Mutat Res 2006; 612:189-214. [PMID: 16490380 DOI: 10.1016/j.mrrev.2005.12.003] [Citation(s) in RCA: 106] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/17/2005] [Revised: 12/29/2005] [Accepted: 12/30/2005] [Indexed: 11/18/2022]
Abstract
Telomeres are specialized nucleoproteic complexes localized at the physical ends of linear eukaryotic chromosomes that maintain their stability and integrity. The DNA component of telomeres is characterized by being a G-rich double stranded DNA composed by short fragments tandemly repeated with different sequences depending on the species considered. At the chromosome level, telomeres or, more properly, telomeric repeats--the DNA component of telomeres--can be detected either by using the fluorescence in situ hybridization (FISH) technique with a DNA or a peptide nucleic acid (PNA) (pan)telomeric probe, i.e., which identifies simultaneously all of the telomeres in a metaphase cell, or by the primed in situ labeling (PRINS) reaction using an oligonucleotide primer complementary to the telomeric DNA repeated sequence. Using these techniques, incomplete chromosome elements, acentric fragments, amplification and translocation of telomeric repeat sequences, telomeric associations and telomeric fusions can be identified. In addition, chromosome orientation (CO)-FISH allows to discriminate between the different types of telomeric fusions, namely telomere-telomere and telomere-DNA double strand break fusions and to detect recombination events at the telomere, i.e., telomeric sister-chromatid exchanges (T-SCE). In this review, we summarize our current knowledge of chromosomal aberrations involving telomeres and interstitial telomeric repeat sequences and their induction by physical and chemical mutagens. Since all of the studies on the induction of these types of aberrations were conducted in mammalian cells, the review will be focused on the chromosomal aberrations involving the TTAGGG sequence, i.e., the telomeric repeat sequence that "caps" the chromosomes of all vertebrate species.
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Affiliation(s)
- Alejandro D Bolzán
- Laboratorio de Citogenética y Mutagénesis, Instituto Multidisciplinario de Biología Celular (IMBICE), C.C. 403, 1900 La Plata, Argentina.
| | - Martha S Bianchi
- Laboratorio de Citogenética y Mutagénesis, Instituto Multidisciplinario de Biología Celular (IMBICE), C.C. 403, 1900 La Plata, Argentina
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23
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Dome JS, Bockhold CA, Li SM, Baker SD, Green DM, Perlman EJ, Hill DA, Breslow NE. High Telomerase RNA Expression Level Is an Adverse Prognostic Factor for Favorable-Histology Wilms' Tumor. J Clin Oncol 2005; 23:9138-45. [PMID: 16172460 DOI: 10.1200/jco.2005.00.562] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Purpose A primary objective of the fifth National Wilms Tumor Study (NWTS-5) was to identify prognostic indicators for patients with favorable-histology Wilms' tumor. The prognostic significance of telomerase expression level in primary tumor samples was assessed. Patients and Methods A case-cohort study was conducted involving 291 NWTS-5 registrants. Telomerase activity was measured using the telomeric repeat amplification protocol (TRAP). Expression levels of TERT mRNA (encoding the telomerase catalytic component) and TERC/hTR (the telomerase RNA template) were measured using quantitative real-time polymerase chain reaction. Results After excluding samples because of lack of viable tumor, RNA degradation, or insufficient clinical information, 244 patients remained for the final analysis (96 with relapse and 148 without relapse). Univariate analysis revealed a positive correlation between relative risk (RR) of relapse and levels of TERT mRNA and TERC expression. For each doubling in TERT mRNA and TERC level, the RR increased by a factor of 1.16 (95% CI, 1.04 to 1.29; P = .01) and 1.35 (95% CI, 1.11 to 1.64; P = .003), respectively. The one third of patients whose tumors had the highest TERC expression level had an RR of 2.06 (95% CI, 1.14 to 3.70; P = .02) compared with patients with the lowest level. TERC expression level remained a significant prognostic indicator in a multivariate analysis adjusting for TERT mRNA, tumor stage, and patient age. TRAP level did not correlate with RR of relapse. Telomerase expression levels were not predictive of overall survival. Conclusion Telomerase RNA expression level may provide a clinically useful adjunct to the current risk classification schema for favorable-histology Wilms' tumor.
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Affiliation(s)
- Jeffrey S Dome
- Department of Hematology/Oncology, St Jude Children's Research Hospital, Memphis, TN 38105, USA.
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24
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Liao XH, Zhang ML, Yang CP, Xu LX, Zhou JQ. Characterization of recombinant Saccharomyces cerevisiae telomerase core enzyme purified from yeast. Biochem J 2005; 390:169-76. [PMID: 15813705 PMCID: PMC1184572 DOI: 10.1042/bj20050208] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Telomerase is a cellular reverse transcriptase that elongates the single-stranded chromosome ends and oligonucleotides in vivo and in vitro. In Saccharomyces cerevisiae, Est2p (telomerase catalytic subunit) and Tlc1 (telomerase RNA template subunit) constitute the telomerase core complex. We co-overexpressed GST (glutathione S-transferase)-Est2p and Tlc1 in S. cerevisiae, and reconstituted the telomerase activity. The GST-Est2p-Tlc1 complex was partially purified by ammonium sulphate fractionation and affinity chromatography on glutathione beads, and the partially purified telomerase did not contain the other two subunits of the telomerase holoenzyme, Est1p and Est3p. The purified recombinant GST-Est2p-Tlc1 telomerase core complex could specifically add nucleotides on to the single-stranded TG(1-3) primer in a processive manner, but could not translocate to synthesize more than one telomeric repeat. The purified telomerase core complex exhibited different activities when primers were paired with the Tlc1 template at different positions. The procedure of reconstitution and purification of telomerase core enzyme that we have developed now allows for further mechanistic studies of the functions of other subunits of the telomerase holoenzyme as well as other telomerase regulation proteins.
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Affiliation(s)
- Xin-Hua Liao
- Max-Planck Junior Research Group in the State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences; Graduate School of the Chinese Academy of Sciences, Shanghai, 200031, China
| | - Ming-Liang Zhang
- Max-Planck Junior Research Group in the State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences; Graduate School of the Chinese Academy of Sciences, Shanghai, 200031, China
| | - Cui-Ping Yang
- Max-Planck Junior Research Group in the State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences; Graduate School of the Chinese Academy of Sciences, Shanghai, 200031, China
| | - Lu-Xia Xu
- Max-Planck Junior Research Group in the State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences; Graduate School of the Chinese Academy of Sciences, Shanghai, 200031, China
| | - Jin-Qiu Zhou
- Max-Planck Junior Research Group in the State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences; Graduate School of the Chinese Academy of Sciences, Shanghai, 200031, China
- To whom correspondence should be addressed (email )
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25
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Cunningham DD, Collins K. Biological and biochemical functions of RNA in the tetrahymena telomerase holoenzyme. Mol Cell Biol 2005; 25:4442-54. [PMID: 15899850 PMCID: PMC1140614 DOI: 10.1128/mcb.25.11.4442-4454.2005] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023] Open
Abstract
Telomerase extends chromosome ends by the synthesis of tandem simple-sequence repeats. Studies of minimal recombinant telomerase ribonucleoprotein (RNP) reconstituted in vitro have revealed sequences within the telomerase RNA subunit (TER) that are required to establish its internal template and other unique features of enzyme activity. Here we test the significance of these motifs following TER assembly into telomerase holoenzyme in vivo. We established a method for stable expression of epitope-tagged TER and TER variants in place of wild-type Tetrahymena TER. We found that sequence substitutions in nontemplate regions of TER altered telomere length maintenance in vivo, with an increase or decrease in the set point for telomere length homeostasis. We also characterized the in vitro activity of the telomerase holoenzymes reconstituted with TER variants, following RNA-based RNP affinity purification from cell extracts. We found that nontemplate sequence substitutions imposed specific defects in the fidelity and processivity of template use. These findings demonstrate nontemplate functions of TER that are critical for the telomerase holoenzyme catalytic cycle and for proper telomere length maintenance in vivo.
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Affiliation(s)
- Doreen D Cunningham
- Department of Molecular and Cell Biology, 16 Barker Hall, University of California, Berkeley, CA 94720-3204, USA
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26
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Simpson EB, Ross SL, Marchetti SE, Kennell JC. Relaxed primer specificity associated with reverse transcriptases encoded by the pFOXC retroplasmids of Fusarium oxysporum. EUKARYOTIC CELL 2005; 3:1589-600. [PMID: 15590832 PMCID: PMC539014 DOI: 10.1128/ec.3.6.1589-1600.2004] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The pFOXC mitochondrial retroplasmids are small, autonomously replicating linear DNAs that have a telomere-like repeat of a 5-bp sequence at their termini. The plasmids are possible evolutionary precursors of the ribonucleoprotein complex telomerase, as they encode an active reverse transcriptase (RT) that is involved in plasmid replication. Using an in vitro system to study reverse transcription, we show that the pFOXC RT is capable of copying in vitro-synthesized RNAs by use of cDNA primers or extension of snapped-back RNA templates. The ability of the pFOXC RT to use base-paired primers distinguishes it from the closely related RTs encoded by the Mauriceville and Varkud mitochondrial retroplasmids of Neurospora spp. Reaction products are similar, but not identical, to those obtained with conventional RTs, and differences reflect the ability of the pFOXC RT to initiate cDNA synthesis with loosely associated primers. The pFOXC RT can also copy DNA templates and extend 3' mismatched DNA oligonucleotide primers. Analysis of pFOXC in vivo replication intermediates suggests that telomeric repeats are added during reverse transcription, and the ability to extend loosely associated primers could play a role in repeat formation by mechanisms similar to those associated with telomerase and certain non-long-terminal-repeat retrotransposons.
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MESH Headings
- 3' Untranslated Regions
- Base Pair Mismatch
- Base Sequence
- Blotting, Southern
- DNA/metabolism
- DNA Primers/chemistry
- DNA Transposable Elements
- DNA, Complementary/metabolism
- DNA, Fungal
- DNA, Mitochondrial/metabolism
- DNA-Directed DNA Polymerase/metabolism
- Dose-Response Relationship, Drug
- Fusarium/metabolism
- Mitochondria/metabolism
- Models, Genetic
- Molecular Sequence Data
- Plasmids/metabolism
- RNA/metabolism
- RNA, Fungal
- RNA, Messenger/metabolism
- RNA, Mitochondrial
- RNA-Directed DNA Polymerase/metabolism
- Retroelements/genetics
- Reverse Transcription
- Sequence Homology, Nucleic Acid
- Time Factors
- Transcription, Genetic
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Affiliation(s)
- E Barry Simpson
- Department of Biology, Saint Louis University, St. Louis, MO 63103, USA
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27
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McHugh MM, Gawron LS, Matsui SI, Beerman TA. The Antitumor Enediyne C-1027 Alters Cell Cycle Progression and Induces Chromosomal Aberrations and Telomere Dysfunction. Cancer Res 2005; 65:5344-51. [PMID: 15958582 DOI: 10.1158/0008-5472.can-05-0015] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
This study examined the extent of chromosome instability induced in cultured human colon carcinoma HCT116 cells by the antitumor radiomimetic enediyne antibiotic C-1027. Spectral karyotype analysis showed frequent intrachromosomal fusions and fragmentations 26 hours after addition of as little as 0.035 nmol/L C-1027. When the concentration was increased to 0.14 nmol/L C-1027, 92% of cells showed chromosomal aberrations compared with only 2.9% after treatment with an equivalent growth inhibitory dose of ionizing radiation (20 Gy). Thus, chromosome misrejoining was associated to a much greater extent with C-1027-induced than with ionizing radiation-induced cell growth inhibition. Despite these aberrations, a large fraction of C-1027-treated cells progressed into G1. Comet analysis showed that these extensive chromosomal anomalies were not due to increased induction or reduced repair of C-1027-induced compared with ionizing radiation-induced strand breaks. Fluorescence in situ hybridization analysis showed that misrejoining of telomere repeats (i.e., chromosomes joined end to end at their telomeres or fused together after complete loss of telomere sequences) was observed within 26 hours of C-1027 addition. The extreme cytotoxicity of C-1027 may reflect both induction and erroneous repair of DNA double-strand break in the whole genome and/or in subgenomic targets such as telomere sequences.
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Affiliation(s)
- Mary M McHugh
- Department of Pharmacology and Therapeutics, Roswell Park Cancer Institute, Buffalo, New York 14263, USA
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28
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Moriarty TJ, Ward RJ, Taboski MAS, Autexier C. An anchor site-type defect in human telomerase that disrupts telomere length maintenance and cellular immortalization. Mol Biol Cell 2005; 16:3152-61. [PMID: 15857955 PMCID: PMC1165400 DOI: 10.1091/mbc.e05-02-0148] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
Abstract
Telomerase-mediated telomeric DNA synthesis is important for eukaryotic cell immortality. Telomerase adds tracts of short telomeric repeats to DNA substrates using a unique repeat addition form of processivity. It has been proposed that repeat addition processivity is partly regulated by a telomerase reverse transcriptase (TERT)-dependent anchor site; however, anchor site-mediating residues have not been identified in any TERT. We report the characterization of an N-terminal human TERT (hTERT) RNA interaction domain 1 (RID1) mutation that caused telomerase activity defects consistent with disruption of a template-proximal anchor site, including reduced processivity on short telomeric primers and reduced activity on substrates with nontelomeric 5' sequences, but not on primers with nontelomeric G-rich 5' sequences. This mutation was located within a subregion of RID1 previously implicated in biological telomerase functions unrelated to catalytic activity (N-DAT domain). Other N-DAT and C-terminal DAT (C-DAT) mutants and a C-terminally tagged hTERT-HA variant were defective in elongating short telomeric primers, and catalytic phenotypes of DAT variants were partially or completely rescued by increasing concentrations of DNA primers. These observations imply that RID1 and the hTERT C terminus contribute to telomerase's affinity for its substrate, and that RID1 may form part of the human telomerase anchor site.
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Affiliation(s)
- Tara J Moriarty
- Department of Anatomy and Cell Biology, Experimental Medicine Division, McGill University, Montréal, Québec H3A 2B2, Canada
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29
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La Torre D, de Divitiis O, Conti A, Angileri FF, Cardali S, Aguennouz M, Aragona M, Panetta S, d'Avella D, Vita G, La Torre F, Tomasello F. Expression of Telomeric Repeat Binding Factor-1 in Astroglial Brain Tumors. Neurosurgery 2005; 56:802-10. [PMID: 15792519 DOI: 10.1227/01.neu.0000156468.41461.6f] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2004] [Accepted: 11/01/2004] [Indexed: 11/19/2022] Open
Abstract
OBJECTIVE In human somatic cells, telomeres shorten with successive cell divisions, resulting in progressive genomic instability, altered gene expression, and cell death. Recently, telomere-specific deoxyribonucleic acid-binding proteins, such as telomeric repeat binding factor-1 (TRF1), have been proposed as candidates for the role of molecules regulating telomerase activity, and they have been suggested to play key roles in the maintenance of telomere function. The present study was designed to assess TRF1 expression in human astroglial brain tumors and to speculate on the clinical implications of its expression. METHODS Twenty flash-frozen surgical specimens obtained from adult patients who underwent craniotomy for microsurgical tumor resection, histologically verified as World Health Organization Grade II to IV astrocytomas, were used. Expression of TRF1 in astrocytomas of different grades was studied by means of both immunohistochemical and Western blotting analysis. The correlation between the extent of TRF1 expression and histological grading, performance status, and length of survival of patients underwent statistical analyses. RESULTS TRF1 was expressed in all tumor samples. The level of its expression was variable, decreasing from low-grade through high-grade astrocytomas (P = 0.0032). TRF1 expression correlated with the patient's length of survival (P < 0.001) and performance status (P < 0.001) and proved to be an independent indicator of length of survival. CONCLUSION Our findings suggest that the loss of TRF1 expression capability, as a result of down-regulation of TRF1 expression in malignant gliomas cells, may play a role in the malignant progression of astroglial brain tumors.
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Affiliation(s)
- Domenico La Torre
- Neurosurgical Clinic, Department of Neurosciences, Psychiatric and Anesthesiological Sciences, University of Messina School of Medicine, Messina, Italy
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30
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Abstract
Trypanosomatid parasite infections have a devastating impact on human health. Little is known about the requirements for parasite growth during any stage of their complex, multi-host life cycle. In most eukaryotic organisms, sustained cell proliferation requires telomerase-dependent telomere length maintenance. Here we investigate the regulation and biochemical features of telomerase from Trypanosoma cruzi, the causative agent of Chagas disease. We found that T.cruzi telomerase is active in extracts from multiple developmental stages of the parasite life cycle. Detailed characterization of the enzymatic properties of telomerase using epimatigote-stage extract revealed a unique combination of substrate specificities, consistent with the evolutionary divergence of trypanosomes from previously established model systems for telomerase biochemical characterization. Results from partial purification of T.cruzi telomerase suggest that the catalytically active enzyme is a large ribonucleoprotein complex and that the internal RNA template has an atypical, cytosine-rich permutation. These results expand our understanding of telomerase enzymology and should encourage the development of parasite-specific telomerase inhibitors as a method for disease therapy.
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Affiliation(s)
- Denise P Muñoz
- Department of Molecular and Cell Biology, 16 Barker Hall, University of California at Berkeley, Berkeley, CA 94720-3204, USA.
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31
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Rivera MA, Blackburn EH. Processive utilization of the human telomerase template: lack of a requirement for template switching. J Biol Chem 2004; 279:53770-81. [PMID: 15456773 DOI: 10.1074/jbc.m407768200] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The ribonucleoprotein telomerase is a specialized reverse transcriptase minimally composed of an RNA, TER, and a protein catalytic subunit, TERT. The TER and TERT subunits of telomerase associate to form a dimeric enzyme in several organisms, including human. A small portion of TER, the template domain, is used by telomerase for the synthesis of tandem repeats of telomeric DNA. We studied some of the requirements for processive template usage by human telomerase. A blunt-ended duplex DNA primer was not utilized by telomerase. With a duplex telomeric DNA primer, a single-stranded 3' overhang with a minimum length of approximately 6 bases was required for efficient priming activity. Large substitutions in the human TER templating domain did not abolish enzymatic activity, although insertion of two residues into this sequence reduced processivity, as did a template mutation that results in a mismatch between the template region used for copying DNA and the region used for alignment of the substrate primer. Finally, by using a complementary pair of catalytically inactive telomerase RNA pseudoknot mutants in combination with a marked template, we demonstrated that processive synthesis by an obligatory dimer of human telomerase does not require template switching. These results indicate that processive template usage by human telomerase, like that of Tetrahymena telomerase, is strongly dependent on the base identities in the template domain and that a dimeric human telomerase can processively utilize a single template.
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Affiliation(s)
- Melissa A Rivera
- Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94143-2200, USA
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32
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Maser RS, DePinho RA. Telomeres and the DNA damage response: why the fox is guarding the henhouse. DNA Repair (Amst) 2004; 3:979-88. [PMID: 15279784 DOI: 10.1016/j.dnarep.2004.05.009] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
DNA double strand breaks (DSBs) are repaired by an extensive network of proteins that recognize damaged DNA and catalyze its repair. By virtue of their similarity, the normal ends of linear chromosomes and internal DNA DSBs are both potential substrates for DSB repair enzymes. Thus, telomeres, specialized nucleo-protein complexes that cap chromosomal ends, serve a critical function to differentiate themselves from internal DNA strand breaks, and as a result prevent genomic instability that can result from their inappropriate involvement in repair reactions. Telomeres that become critically short due to failure of telomere maintenance mechanisms, or which become dysfunctional by loss of telomere binding proteins, elicit extensive checkpoint responses that in normal cells blocks proliferation. In this situation, the DNA DSB repair machinery plays a major role in responding to these "damaged" telomeres - creating chromosome fusions or capturing telomeres from other chromosomes in an effort to rid the cell of the perceived damage. However, a surprising aspect of telomere maintenance is that many of the same proteins that facilitate this repair of damaged telomeres are also necessary for their proper integrity. Here, we review recent work defining the roles for DSB repair machinery in telomere maintenance and in response to telomere dysfunction.
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Affiliation(s)
- Richard S Maser
- Department of Medical Oncology, Dana-Farber Cancer Institute, Departments of Medicine and Genetics, Harvard Medical School, Boston, MA 02115, USA.
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33
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Witkin KL, Collins K. Holoenzyme proteins required for the physiological assembly and activity of telomerase. Genes Dev 2004; 18:1107-18. [PMID: 15131081 PMCID: PMC415636 DOI: 10.1101/gad.1201704] [Citation(s) in RCA: 109] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2004] [Accepted: 04/05/2004] [Indexed: 11/25/2022]
Abstract
Many proteins have been implicated in the physiological function of telomerase, but specific roles of telomerase-associated proteins other than telomerase reverse transcriptase (TERT) remain ambiguous. To gain a more comprehensive understanding of catalytically active enzyme composition, we performed affinity purification of epitope-tagged, endogenously assembled Tetrahymena telomerase. We identified and cloned genes encoding four telomerase proteins in addition to TERT. We demonstrate that both of the two new proteins characterized in detail, p65 and p45, have essential roles in the maintenance of telomere length as part of a ciliate telomerase holoenzyme. The p65 subunit contains an La motif characteristic of a family of direct RNA-binding proteins. We find that p65 in cell extract is associated specifically with telomerase RNA, and that genetic depletion of p65 reduces telomerase RNA accumulation in vivo. These findings demonstrate that telomerase holoenzyme proteins other than TERT play critical roles in RNP biogenesis and function.
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Affiliation(s)
- Keren L Witkin
- Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, California 94720-3204, USA
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34
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Abstract
Ciliate and yeast telomerase possess a nucleolytic activity capable of removing DNA from the 3' end of a single-stranded oligonucleotide substrate. The nuclease activity is thought to assist in enzyme proofreading and/or processivity. Herein, we report a previously uncharacterized human telomerase-associated nuclease activity that shares several properties with ciliate and yeast telomerases. Partially purified human telomerase, either from cell extracts or recombinantly produced, demonstrated an ability to remove 3' nontelomeric nucleotides from a substrate containing 5' telomeric DNA, followed by extension of the newly exposed telomeric sequence. This cleavage/extension activity was apparent at more than one position within the telomeric DNA and was influenced by sequences 5' to the telomeric/nontelomeric boundary and by substitution with a methylphosphonate moiety at the telomeric/nontelomeric DNA boundary. Our data suggest that human telomerase is associated with an evolutionarily conserved nucleolytic activity and support a model in which telomerase-substrate interactions can occur distal from the 3' primer end.
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Affiliation(s)
- Rena Oulton
- Department of Medical Biophysics, Ontario Cancer Institute/Advanced Medical Discovery Institute, University of Toronto, Toronto, Ontario M5G 2C1, Canada
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35
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Abstract
Telomerase is a reverse transcriptase that uses an integral RNA molecule to add de novo G-rich repeats onto telomeric DNA, or onto nontelomeric DNA generated during chromosome fragmentation and breakage events. A telomerase-mediated DNA substrate cleavage activity has been reported in ciliates and yeasts. Nucleolytic cleavage may serve a proofreading function, enhance processivity or ensure that nontemplate telomerase RNA sequences are not copied into DNA. We identified and characterized a human telomerase-mediated nucleolytic cleavage activity using enzyme reconstituted in a rabbit reticulocyte lysate in vitro transcription/translation system and native enzyme extracted from cells. We found that telomerase catalyzed the removal of nucleotides from DNA substrates including those that can form a mismatch with the RNA template or that contain nontelomeric sequences located 3' to a telomeric sequence. Unlike Tetrahymena telomerase, human telomerase catalyzed the removal of more than one nucleotide (up to 13) from telomeric primers. DNA substrates predicted to align at the 3'-end of the RNA template were not cleaved, consistent with cleavage being dictated by the template 5'-end. We also found some differences in the nuclease activity between RRL-reconstituted human telomerase and native enzyme.
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Affiliation(s)
- Sylvain Huard
- Department of Anatomy and Cell Biology, McGill University, Montréal, Québec H3A 2B4, Canada
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36
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Bosoy D, Lue NF. Yeast telomerase is capable of limited repeat addition processivity. Nucleic Acids Res 2004; 32:93-101. [PMID: 14704347 PMCID: PMC373262 DOI: 10.1093/nar/gkg943] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
Telomerase is a ribonucleoprotein reverse transcriptase responsible for the maintenance of one strand of telomere terminal repeats. Telomerase-mediated sequence addition is dictated by a short 'template' region of the RNA component. Despite the short template segment, telomerases from many organisms have been shown to mediate the synthesis of long extension products. This synthesis presumably depends on two types of translocation events: simultaneous translocation of the RNA-DNA duplex relative to the active site after each nucleotide incorporation (type I or nucleotide addition processivity), and translocation of the RNA relative to the DNA product after each round of repeat synthesis (type II or repeat addition processivity). In contrast, telomerases from yeasts have been shown to synthesize mostly short products, implying a defect in one or both types of translocation. In this report, we analyzed the processivity of yeast telomerase in vitro, and identified two position-specific elongation barriers within the 5' region of the RNA template that can account for the synthesis of incomplete first round products. These barriers respond differently to variations in nucleotide concentration, primer sequence and mutations in the catalytic protein subunit, consistent with their having distinct mechanistic bases. In addition, by using optimal primers and high concentrations of dGTP, we were able to detect significant type II translocation by the yeast enzyme. Thus, the difference between the elongation property of yeast and other telomerases appears to be quantitative rather than qualitative. Our results suggest that yeast may be a useful system for investigating the physiologic significance of repeat addition processivity.
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Affiliation(s)
- Dimitry Bosoy
- Department of Microbiology and Immunology, W. R. Hearst Microbiology Research Center, Weill Medical College of Cornell University, 1300 York Avenue, New York, NY 10021, USA
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37
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Sýkorová E, Lim KY, Kunická Z, Chase MW, Bennett MD, Fajkus J, Leitch AR. Telomere variability in the monocotyledonous plant order Asparagales. Proc Biol Sci 2003; 270:1893-904. [PMID: 14561302 PMCID: PMC1691456 DOI: 10.1098/rspb.2003.2446] [Citation(s) in RCA: 87] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
A group of monocotyledonous plants within the order Asparagales, forming a distinct clade in phylogenetic analyses, was reported previously to lack the 'typical' Arabidopsis-type telomere (TTTAGGG)(n). This stimulated us to determine what has replaced these sequences. Using slot-blot and fluorescent in situ hybridization (FISH) to species within this clade, our results indicate the following. 1. The typical Arabidopsis-type telomeric sequence has been partly or fully replaced by the human-type telomeric sequence (TTAGGG)(n). Species in Allium lack the human-type variant. 2. In most cases the human variant occurs along with a lower abundance of two or more variants of the minisatellite sequences (of seven types evaluated), usually these being the consensus telomeric sequence of Arabidopsis, Bombyx (TTAGG)(n) and Tetrahymena (TTGGGG)(n). FISH shows that the variants can occur mixed together at the telomere. 3. Telomerases generate products with a 6 base pair periodicity and when sequenced they reveal predominantly a reiterated human-type motif. These motifs probably form the 'true telomere' but the error rate of motif synthesis is higher compared with 'typical' plant telomerases. The data indicate that the Asparagales clade is unified by a mutation resulting in a switch from synthesis of Arabidopsis-like telomeres to a low-fidelity synthesis of human-like telomeres.
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Affiliation(s)
- E Sýkorová
- School of Biological Sciences, Queen Mary University of London, London E1 4NS, UK
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38
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Klapper W, Qian W, Schulte C, Parwaresch R. DNA damage transiently increases TRF2 mRNA expression and telomerase activity. Leukemia 2003; 17:2007-15. [PMID: 14513051 DOI: 10.1038/sj.leu.2403086] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Telomerase activity transiently increases when HL60 cells are treated with the topoisomerase II inhibitor etoposide. A quantitative assessment revealed that telomerase is activated by etoposide treatment in a number of cell lines and that the increase is reversible after withdrawal of etoposide from the cell culture. Telomerase activation correlated with the occurrence of DNA damage but not with cell cycle arrest. We did not detect any transcriptional upregulation of hTERT mRNA, suggesting a post-transcriptional mechanism of telomerase activation. Furthermore, the mRNA expression of the telomere binding protein TRF2 was upregulated early and reversibly after etoposide treatment. TRF1 mRNA expression levels were unchanged after DNA damage, but increased when the cells accumulated in the G2/M phase. The data show that the telosome reacts after DNA damage by upregulating telomerase activity and TRF2 expression in malignant cells. It has previously been shown that overexpression of TRF2 can repress senescence signals arising from critically shortened telomeres. We show here that TRF2 is upregulated by undirected DNA damage that also affects the telomeric DNA. These data suggest that upregulation of telomerase activity and TRF2 expression might act as antiapoptotic mechanisms in the DNA-damage response of malignant cells.
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Affiliation(s)
- W Klapper
- Department of Hematopathology and Lymph Node Registry Kiel, Niemannsweg Kiel, Germany
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39
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McGuire JM, Gana JA, Petcherskaia M, Kirk KE. Protein binding to expanded telomere repeats in Tetrahymena thermophila. J Eukaryot Microbiol 2003; 50:341-8. [PMID: 14563172 DOI: 10.1111/j.1550-7408.2003.tb00146.x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
The ends of eukaryotic chromosomes are protected by DNA-protein structures called telomeres. Telomeric DNA is highly conserved, usually consisting of long tracts of a repeating G-rich sequence. Tetrahymena thermophila telomeric DNA consists of alternating blocks of GGGG and TT sequences (i.e. a G4T2 repeat sequence). We examined the relative importance of the guanine and thymine elements of the repeat sequence in promoting in vitro binding by T. thermophila proteins. We identified single- and, for the first time, double-stranded telomere binding activities from a crude T. thermophila protein extract and tested the binding of these activities to altered telomere repeat sequences. All deletions or substitutions made to the guanine element virtually abolished binding, indicating that four G's are essential for recognition by the binding activity. However, G's alone are not sufficient for efficient binding, as elimination of the thymine element dramatically reduced binding. By contrast, substantial expansion of the thymine element was well tolerated, even though one such change, G4T4, is lethal in vivo. We tested up to a four-fold expansion of the thymine element and found that highly efficient binding was still achieved. These results suggest a minimal recognition sequence for T. thermophila proteins, with the T element providing an important spacer between essential G elements.
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Affiliation(s)
- Jennifer M McGuire
- Department of Biology, Lake Forest College, 555 N. Sheridan Road, Lake Forest, Illinois 60045, USA
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40
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Lee SR, Wong JMY, Collins K. Human telomerase reverse transcriptase motifs required for elongation of a telomeric substrate. J Biol Chem 2003; 278:52531-6. [PMID: 14565961 DOI: 10.1074/jbc.m311359200] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The reverse transcriptase telomerase copies an internal RNA template to synthesize telomeric simple-sequence repeats. In the cellular context, telomerase must elongate its few intended substrates (authentic chromosome ends) without spurious activity on other potential substrates (chromosome ends created by damage, repair, or recombination). Many mechanisms have been proposed to account for the biological substrate specificity of telomerase, with most models focusing on protein-protein interactions between telomerase and telomeric chromatin. Telomerase activity assays testing the elongation of model oligonucleotide substrates have revealed that in addition to hybridization with the RNA template, optimal DNA substrates also engage telomerase protein-based interaction sites. The physiological significance of these non-template interaction sites has not been established. We used in vivo reconstitution to assemble telomerase enzymes with variant telomerase reverse transcriptase proteins. Several telomerase enzyme variants retained a wild-type level of catalytic function in vitro when assayed using an artificial sequence substrate but exhibited reduced activity on a more physiological telomeric-sequence substrate. Telomerases that demonstrated this defect in telomeric substrate usage in vitro also failed to support telomere length maintenance in vivo. Our findings suggest that non-template interactions of the telomerase ribonucleoprotein with telomeric DNA play a critical role in supporting telomerase function on its appropriate cellular substrates.
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Affiliation(s)
- Suzanne R Lee
- Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, California 94720-3204, USA
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41
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Lydall D. Hiding at the ends of yeast chromosomes: telomeres, nucleases and checkpoint pathways. J Cell Sci 2003; 116:4057-65. [PMID: 12972499 DOI: 10.1242/jcs.00765] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
Telomeres stabilise DNA at the ends of chromosomes, preventing chromosome fusion and genetic instability. Telomeres differ from double strand breaks in that they activate neither DNA repair nor DNA damage checkpoint pathways. Paradoxically DNA repair and checkpoint genes play critical roles in telomere stability. Recent work has provided insights into the roles of DNA repair and DNA damage checkpoint pathways in the physiological maintenance of telomeres and in cellular responses when telomeres become uncapped. In budding yeast the Mre11p nuclease, along with other unidentified nucleases, plays critical roles in physiological telomere maintenance. However, when telomeres are uncapped, the 5'-to-3' exonuclease, Exo1p, plays a critical role in generating single-stranded DNA and activating checkpoint pathways. Intriguingly Exo1p does not play an important role in normal telomere maintenance. Although checkpoint pathways are not normally activated by telomeres, at least four different types of telomere defect activate checkpoint pathways. Interestingly, each of these telomere defects depends on a different subset of checkpoint proteins to induce cell cycle arrest. A model for how a spectrum of telomeric states might interact with telomerase and checkpoint pathways is proposed.
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Affiliation(s)
- David Lydall
- School of Biological Sciences, University of Manchester, G38 Stopford Building, Oxford Road, Manchester M13 9PT, UK.
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42
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Abstract
Telomerase is an enzyme that utilizes an internal RNA molecule as a template for the extension of chromosomal DNA ends. The catalytic core of telomerase consists of the RNA subunit and a protein reverse transcriptase subunit, known as telomerase reverse transcriptase (TERT). It has previously been shown that both yeast and human telomerase can form dimers or multimers in which one RNA in the complex can influence the activity of another. To test the proposal that dimerization might be essential for telomerase activity, we sought to determine whether Tetrahymena thermophila telomerase is active as a dimer or a monomer. Recombinant Tetrahymena telomerase eluted from a gel filtration column at the size of a monomeric complex (one RNA plus one TERT), and those fractions showed processive telomerase activity. We were unable to detect dimerization of Tetrahymena telomerase by coprecipitation experiments, by using tags on either the TERT protein or telomerase RNA. Therefore, a majority, if not all, of the recombinant Tetrahymena telomerase in our reconstitution system is present as a monomeric complex. We were also unable to detect dimerization of native telomerase from mating and vegetative Tetrahymena cell extracts. These results demonstrate that Tetrahymena telomerase does not need to dimerize to be active and processive.
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Affiliation(s)
- Tracy M Bryan
- Children's Medical Research Institute, Westmead, New South Wales 2145, Australia.
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43
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Abstract
Arthur Kornberg "never met a dull enzyme" (For the Love of Enzymes: The Odyssey of a Biochemist, Harvard University Press, 1989) and telomerase is no exception. Telomerase is a remarkable polymerase that uses an internal RNA template to reverse-transcribe telomere DNA, one nucleotide at a time, onto telomeric, G-rich single-stranded DNA. In the 17 years since its discovery, the characterization of telomerase enzyme components has uncovered a highly conserved family of telomerase reverse transcriptases that, together with the telomerase RNA, appear to comprise the enzymatic core of telomerase. While not as comprehensively understood as yet, some telomerase-associated proteins also serve crucial roles in telomerase function in vivo, such as telomerase ribonudeoprotein (RNP) assembly, recruitment to the telomere, and the coordination of DNA replication at the telomere. A selected overview of the biochemical properties of this unique enzyme, in vitro and in vivo, will be presented.
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44
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Abstract
The germline genomes of ciliated protozoa are dynamic structures, undergoing massive DNA rearrangement during the formation of a functional macronucleus. Macronuclear development involves chromosome fragmentation coupled with de novo telomere synthesis, numerous DNA splicing events that remove internal segments of DNA, and, in some ciliates, the reordering of scrambled gene segments. Despite the fact that all ciliates share similar forms of DNA rearrangement, there appears to be great diversity in both the nature of the rearranged DNA and the molecular mechanisms involved. Epigenetic effects on rearrangement have also been observed, and recent work suggests that chromatin differentiation plays a role in specifying DNA segments either for rearrangement or for elimination.
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Affiliation(s)
- Carolyn L Jahn
- Department of Cell and Molecular Biology, Northwestern University Medical School, Chicago, Illinois 60611, USA.
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45
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Baran N, Haviv Y, Paul B, Manor H. Studies on the minimal lengths required for DNA primers to be extended by the Tetrahymena telomerase: implications for primer positioning by the enzyme. Nucleic Acids Res 2002; 30:5570-8. [PMID: 12490725 PMCID: PMC140050 DOI: 10.1093/nar/gkf676] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Telomerase is a specialized reverse transcriptase that contains an integral RNA subunit including a short template sequence. It extends telomeric 3' overhangs and chromosome breakpoints by catalyzing reiterative copying of this internal template into single-stranded telomeric DNA repeats. Here we report for the first time that in vitro the ciliate Tetrahymena telomerase can efficiently extend very short single-stranded DNA primers (<6 nt). These data indicate that interactions with nucleotides further upstream are not essential for elongation of longer primers. We also report that the minimal lengths required for primers to be extended by the telomerase depend on the positions along the template at which the primers are initially aligned. At a primer concentration of 2.5 micro M, primers aligned in the beginning, middle and next to the end of the template, respectively, must consist of at least 4, 5 and 6 nt to be extended by the telomerase. At a primer concentration of 50 micro M, the corresponding minimal lengths are 3, 4 and 5 nt. The systematic variation of the minimal required primer lengths supports the presence of a site within the telomerase ribonucleoprotein complex that mediates specific positioning of 3' termini of telomeric and non-telomeric DNA in the beginning of the template during telomere synthesis.
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Affiliation(s)
- Nava Baran
- Department of Biology, Technion-Israel Institute of Technology, Haifa 32,000, Israel
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46
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Kelleher C, Teixeira MT, Förstemann K, Lingner J. Telomerase: biochemical considerations for enzyme and substrate. Trends Biochem Sci 2002; 27:572-9. [PMID: 12417133 DOI: 10.1016/s0968-0004(02)02206-5] [Citation(s) in RCA: 118] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
Telomerase extends chromosome ends by iterative reverse transcription of its RNA template. Following the addition of each telomeric repeat, the RNA template and the telomeric substrate reset their relative position in the active site provided by the telomerase reverse transcriptase (TERT). This step might require the formation of guanine-rich secondary structures in the nascent product. Results from numerous studies begin to delineate TERT sub-domains that orchestrate these events and support the model of cooperative action between distinct active sites within telomerase multimers. Natural telomere substrates are protein-DNA complexes that show an asymmetry between the two ends of a chromosome, possibly reflecting their differential mode of replication.
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Affiliation(s)
- Colleen Kelleher
- Swiss Institute for Experimental Cancer Research (ISREC), CH-1066 Epalinges, Switzerland
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47
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Abstract
The telomere is a special functional complex at the end of linear eukaryotic chromosomes, consisting of tandem repeat DNA sequences and associated proteins. It is essential for maintaining the integrity and stability of linear eukaryotic genomes. Telomere length regulation and maintenance contribute to normal human cellular aging and human diseases. The synthesis of telomeres is mainly achieved by the cellular reverse transcriptase telomerase, an RNA-dependent DNA polymerase that adds telomeric DNA to telomeres. Expression of telomerase is usually required for cell immortalization and long-term tumor growth. In humans, telomerase activity is tightly regulated during development and oncogenesis. The modulation of telomerase activity may therefore have important implications in antiaging and anticancer therapy. This review describes the currently known components of the telomerase complex and attempts to provide an update on the molecular mechanisms of human telomerase regulation.
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Affiliation(s)
- Yu-Sheng Cong
- Department of Cell Biology, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9039, USA.
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48
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Williams KR, Doak TG, Herrick G. Telomere formation on macronuclear chromosomes of Oxytricha trifallax and O. fallax: alternatively processed regions have multiple telomere addition sites. BMC Genet 2002; 3:16. [PMID: 12199911 PMCID: PMC128808 DOI: 10.1186/1471-2156-3-16] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/28/2002] [Accepted: 08/28/2002] [Indexed: 11/13/2022] Open
Abstract
BACKGROUND Ciliates employ massive chromatid breakage and de novo telomere formation during generation of the somatic macronucleus. Positions flanking the 81-MAC locus are reproducibly cut. But those flanking the Common Region are proposed to often escape cutting, generating three nested macronuclear chromosomes, two retaining "arms" still appended to the Common Region. Arm-distal positions must differ (in cis) from the Common Region flanks. RESULTS The Common-Region-flanking positions also differ from the arm-distal positions in that they are "multi-TAS" regions: anchored PCR shows heterogeneous patterns of telomere addition sites, but arm-distal sites do not. The multi-TAS patterns are reproducible, but are sensitive to the sequence of the allele being processed. Thus, random degradation following chromatid cutting does not create this heterogeneity; these telomere addition sites also must be dictated by cis-acting sequences. CONCLUSIONS Most ciliates show such micro-heterogeneity in the precise positions of telomere addition sites. Telomerase is believed to be tightly associated with, and act in concert with, the chromatid-cutting nuclease: heterogeneity must be the result of intervening erosion activity. Our "weak-sites" hypothesis explains the correlation between alternative chromatid cutting at the Common Region boundaries and their multi-TAS character: when the chromatid-breakage machine encounters either a weak binding site or a weak cut site at these regions, then telomerase dissociates prematurely, leaving the new end subject to erosion by an exonuclease, which pauses at cis-acting sequences; telomerase eventually heals these resected termini. Finally, we observe TAS positioning influenced by trans-allelic interactions, reminiscent of transvection.
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Affiliation(s)
- Kevin R Williams
- Department of Pathology, University of Utah School of Medicine, Salt Lake City UT 84132-2501, USA
| | - Thomas G Doak
- Department of Pathology, University of Utah School of Medicine, Salt Lake City UT 84132-2501, USA
| | - Glenn Herrick
- Department of Pathology, University of Utah School of Medicine, Salt Lake City UT 84132-2501, USA
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49
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Moriarty TJ, Dupuis S, Autexier C. Rapid upregulation of telomerase activity in human leukemia HL-60 cells treated with clinical doses of the DNA-damaging drug etoposide. Leukemia 2002; 16:1112-20. [PMID: 12040441 PMCID: PMC5235897 DOI: 10.1038/sj.leu.2402522] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2001] [Accepted: 02/14/2002] [Indexed: 11/09/2022]
Abstract
The enzyme telomerase is implicated in cellular resistance to apoptosis, but the mechanism for this resistance remains to be elucidated. The ability of telomerase to synthesize new DNA at telomeres suggests that this enzyme might function in the repair of double-stranded DNA breaks. To distinguish the effects of double-stranded DNA break damage and apoptosis on human telomerase activity, we treated the HL-60 human hematopoietic cancer cell line with clinical doses of the chemotherapeutic drug etoposide (0.5 to 5 microM), which allowed us to distinguish between events associated with DNA damage-induced cell cycle arrest, and events associated with apoptosis. Large (three- to seven-fold) upregulation of telomerase activity occurred soon after etoposide treatment (3 h) in S/G2/M-arresting populations; this upregulation was abolished at onset of apoptotic cell death. No upregulation of telomerase activity was observed in cells treated with a larger dose of etoposide (5 microM) that caused cells to undergo rapid apoptosis without intervening cell cycle arrests. These observations are consistent with a possible role for telomerase upregulation during the DNA damage response.
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MESH Headings
- Antineoplastic Agents, Phytogenic/administration & dosage
- Antineoplastic Agents, Phytogenic/pharmacology
- Antineoplastic Agents, Phytogenic/toxicity
- Apoptosis
- Cell Cycle
- Cell Nucleus/ultrastructure
- DNA Damage
- DNA Fragmentation
- DNA, Neoplasm/genetics
- DNA, Neoplasm/ultrastructure
- Diploidy
- Dose-Response Relationship, Drug
- Enzyme Activation
- Etoposide/administration & dosage
- Etoposide/pharmacology
- Etoposide/toxicity
- HL-60 Cells
- Humans
- Kinetics
- Leukemia, Promyelocytic, Acute/enzymology
- Leukemia, Promyelocytic, Acute/genetics
- Leukemia, Promyelocytic, Acute/pathology
- Telomerase/metabolism
- Up-Regulation
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Affiliation(s)
- T J Moriarty
- Department of Anatomy and Cell Biology, McGill University, Montréal, Québec, Canada
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50
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Miller MC, Collins K. Telomerase recognizes its template by using an adjacent RNA motif. Proc Natl Acad Sci U S A 2002; 99:6585-90. [PMID: 11997465 PMCID: PMC124446 DOI: 10.1073/pnas.102024699] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Telomerase adds telomeric repeats to chromosome 3' ends, forestalling the cellular senescence, apoptosis, and genomic instability that result from telomere loss caused by incomplete DNA replication. The telomerase ribonucleoprotein is dedicated to synthesis of tandem, simple-sequence repeats by virtue of its specialization for copying only a specific template region within the integral RNA. Here, using circularly permuted variants of Tetrahymena thermophila telomerase RNA, we identify the features that allow recognition of the template region within the RNA. We engineered a template-less telomerase ribonucleoprotein that can position and reverse transcribe an exchangeable RNA oligonucleotide template accurately. Only a short "template-recognition" element sequence tag is required to direct efficient use of adjacent 5' residues as a template for telomeric repeat synthesis. Our findings reveal molecular requirements for template selection by telomerase and physically resolve templating from other RNA functions in catalysis.
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Affiliation(s)
- Michael C Miller
- Department of Molecular and Cell Biology, University of California, 401 Barker Hall, Berkeley, CA 94720-3204, USA
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