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Spegg V, Altmeyer M. Genome maintenance meets mechanobiology. Chromosoma 2024; 133:15-36. [PMID: 37581649 PMCID: PMC10904543 DOI: 10.1007/s00412-023-00807-5] [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] [Received: 04/04/2023] [Revised: 06/20/2023] [Accepted: 07/26/2023] [Indexed: 08/16/2023]
Abstract
Genome stability is key for healthy cells in healthy organisms, and deregulated maintenance of genome integrity is a hallmark of aging and of age-associated diseases including cancer and neurodegeneration. To maintain a stable genome, genome surveillance and repair pathways are closely intertwined with cell cycle regulation and with DNA transactions that occur during transcription and DNA replication. Coordination of these processes across different time and length scales involves dynamic changes of chromatin topology, clustering of fragile genomic regions and repair factors into nuclear repair centers, mobilization of the nuclear cytoskeleton, and activation of cell cycle checkpoints. Here, we provide a general overview of cell cycle regulation and of the processes involved in genome duplication in human cells, followed by an introduction to replication stress and to the cellular responses elicited by perturbed DNA synthesis. We discuss fragile genomic regions that experience high levels of replication stress, with a particular focus on telomere fragility caused by replication stress at the ends of linear chromosomes. Using alternative lengthening of telomeres (ALT) in cancer cells and ALT-associated PML bodies (APBs) as examples of replication stress-associated clustered DNA damage, we discuss compartmentalization of DNA repair reactions and the role of protein properties implicated in phase separation. Finally, we highlight emerging connections between DNA repair and mechanobiology and discuss how biomolecular condensates, components of the nuclear cytoskeleton, and interfaces between membrane-bound organelles and membraneless macromolecular condensates may cooperate to coordinate genome maintenance in space and time.
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Affiliation(s)
- Vincent Spegg
- Department of Molecular Mechanisms of Disease, University of Zurich, Zurich, Switzerland
| | - Matthias Altmeyer
- Department of Molecular Mechanisms of Disease, University of Zurich, Zurich, Switzerland.
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2
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Silonov SA, Mokin YI, Nedelyaev EM, Smirnov EY, Kuznetsova IM, Turoverov KK, Uversky VN, Fonin AV. On the Prevalence and Roles of Proteins Undergoing Liquid-Liquid Phase Separation in the Biogenesis of PML-Bodies. Biomolecules 2023; 13:1805. [PMID: 38136675 PMCID: PMC10741438 DOI: 10.3390/biom13121805] [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] [Received: 11/14/2023] [Revised: 12/08/2023] [Accepted: 12/12/2023] [Indexed: 12/24/2023] Open
Abstract
The formation and function of membrane-less organelles (MLOs) is one of the main driving forces in the molecular life of the cell. These processes are based on the separation of biopolymers into phases regulated by multiple specific and nonspecific inter- and intramolecular interactions. Among the realm of MLOs, a special place is taken by the promyelocytic leukemia nuclear bodies (PML-NBs or PML bodies), which are the intranuclear compartments involved in the regulation of cellular metabolism, transcription, the maintenance of genome stability, responses to viral infection, apoptosis, and tumor suppression. According to the accepted models, specific interactions, such as SUMO/SIM, the formation of disulfide bonds, etc., play a decisive role in the biogenesis of PML bodies. In this work, a number of bioinformatics approaches were used to study proteins found in the proteome of PML bodies for their tendency for spontaneous liquid-liquid phase separation (LLPS), which is usually caused by weak nonspecific interactions. A total of 205 proteins found in PML bodies have been identified. It has been suggested that UBC9, P53, HIPK2, and SUMO1 can be considered as the scaffold proteins of PML bodies. It was shown that more than half of the proteins in the analyzed proteome are capable of spontaneous LLPS, with 85% of the analyzed proteins being intrinsically disordered proteins (IDPs) and the remaining 15% being proteins with intrinsically disordered protein regions (IDPRs). About 44% of all proteins analyzed in this study contain SUMO binding sites and can potentially be SUMOylated. These data suggest that weak nonspecific interactions play a significantly larger role in the formation and biogenesis of PML bodies than previously expected.
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Affiliation(s)
- Sergey A. Silonov
- Laboratory of Structural Dynamics, Stability and Folding of Proteins, Institute of Cytology, Russian Academy of Sciences, St. Petersburg 194064, Russia; (S.A.S.); (Y.I.M.); (E.M.N.); (E.Y.S.); (I.M.K.); (K.K.T.)
| | - Yakov I. Mokin
- Laboratory of Structural Dynamics, Stability and Folding of Proteins, Institute of Cytology, Russian Academy of Sciences, St. Petersburg 194064, Russia; (S.A.S.); (Y.I.M.); (E.M.N.); (E.Y.S.); (I.M.K.); (K.K.T.)
| | - Eugene M. Nedelyaev
- Laboratory of Structural Dynamics, Stability and Folding of Proteins, Institute of Cytology, Russian Academy of Sciences, St. Petersburg 194064, Russia; (S.A.S.); (Y.I.M.); (E.M.N.); (E.Y.S.); (I.M.K.); (K.K.T.)
| | - Eugene Y. Smirnov
- Laboratory of Structural Dynamics, Stability and Folding of Proteins, Institute of Cytology, Russian Academy of Sciences, St. Petersburg 194064, Russia; (S.A.S.); (Y.I.M.); (E.M.N.); (E.Y.S.); (I.M.K.); (K.K.T.)
| | - Irina M. Kuznetsova
- Laboratory of Structural Dynamics, Stability and Folding of Proteins, Institute of Cytology, Russian Academy of Sciences, St. Petersburg 194064, Russia; (S.A.S.); (Y.I.M.); (E.M.N.); (E.Y.S.); (I.M.K.); (K.K.T.)
| | - Konstantin K. Turoverov
- Laboratory of Structural Dynamics, Stability and Folding of Proteins, Institute of Cytology, Russian Academy of Sciences, St. Petersburg 194064, Russia; (S.A.S.); (Y.I.M.); (E.M.N.); (E.Y.S.); (I.M.K.); (K.K.T.)
| | - Vladimir N. Uversky
- Department of Molecular Medicine and USF Health Byrd Alzheimer’s Research Institute, Morsani College of Medicine, University of South Florida, Tampa, FL 33612, USA;
| | - Alexander V. Fonin
- Laboratory of Structural Dynamics, Stability and Folding of Proteins, Institute of Cytology, Russian Academy of Sciences, St. Petersburg 194064, Russia; (S.A.S.); (Y.I.M.); (E.M.N.); (E.Y.S.); (I.M.K.); (K.K.T.)
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3
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ATRX proximal protein associations boast roles beyond histone deposition. PLoS Genet 2021; 17:e1009909. [PMID: 34780483 PMCID: PMC8629390 DOI: 10.1371/journal.pgen.1009909] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2021] [Revised: 11/29/2021] [Accepted: 10/23/2021] [Indexed: 12/31/2022] Open
Abstract
The ATRX ATP-dependent chromatin remodelling/helicase protein associates with the DAXX histone chaperone to deposit histone H3.3 over repetitive DNA regions. Because ATRX-protein interactions impart functions, such as histone deposition, we used proximity-dependent biotinylation (BioID) to identify proximal associations for ATRX. The proteomic screen captured known interactors, such as DAXX, NBS1, and PML, but also identified a range of new associating proteins. To gauge the scope of their roles, we examined three novel ATRX-associating proteins that likely differed in function, and for which little data were available. We found CCDC71 to associate with ATRX, but also HP1 and NAP1, suggesting a role in chromatin maintenance. Contrastingly, FAM207A associated with proteins involved in ribosome biosynthesis and localized to the nucleolus. ATRX proximal associations with the SLF2 DNA damage response factor help inhibit telomere exchanges. We further screened for the proteomic changes at telomeres when ATRX, SLF2, or both proteins were deleted. The loss caused important changes in the abundance of chromatin remodelling, DNA replication, and DNA repair factors at telomeres. Interestingly, several of these have previously been implicated in alternative lengthening of telomeres. Altogether, this study expands the repertoire of ATRX-associating proteins and functions. ATRX is a protein that is needed to keep repetitive DNA regions organized. It does so in part by binding the DAXX histone chaperone to deposit histone proteins on DNA and assemble structures known as nucleosomes. While important, ATRX has additional functions that remain understudied. To better understand its various biological roles, we first identified the other proteins that are found in its proximity. ATRX-associating proteins were implicated in a range of functions, in addition to histone deposition. Our results suggest that ATRX-associating proteins likely help compact DNA after it is assembled into nucleosomes, and also promote its stability. We then examined the effect of ATRX on telomeres (repetitive DNA regions at the end of chromosomes). ATRX and at least one of its associating proteins suppressed spurious DNA exchanges at telomeres. To understand why, we then identified proteomic changes that occur at telomeres when ATRX was deleted. Loss of ATRX altered the enrichment of a surprising number of proteins at telomeres, including several DNA damage response and chromatin remodelling proteins.
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Yang CW, Hsieh MH, Sun HJ, Teng SC. Nuclear envelope tethering inhibits the formation of ALT-associated PML bodies in ALT cells. Aging (Albany NY) 2021; 13:10490-10516. [PMID: 33820871 PMCID: PMC8064153 DOI: 10.18632/aging.202810] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2020] [Accepted: 02/16/2021] [Indexed: 12/12/2022]
Abstract
Telomere length homeostasis is essential for maintaining genomic stability and cancer proliferation. Telomerase-negative cancer cells undergo recombination-mediated alternative lengthening of telomeres. Telomeres associate with the nuclear envelope through the shelterin RAP1 and nuclear envelope SUN1 proteins. However, how the associations between telomeres and the nuclear envelope affect the progression of telomere recombination is not understood. Here, we show that telomere anchorage might inhibit telomere-telomere recombination. SUN1 depletion stimulates the formation of alternative lengthening of telomeres-associated promyelocytic leukemia bodies in ALT cells. In contrast, overexpression of a telomere-nuclear envelope-tethering chimera protein, RAP1-SUN1, suppresses APB formation. Moreover, inhibition of this nuclear envelope attachment alleviates the requirement of TOP3α for resolving the supercoiling pressure during telomere recombination. A coimmunoprecipitation assay revealed that the SUN1 N-terminal nucleoplasmic domain interacts with the RAP1 middle coil domain, and phosphorylation-mimetic mutations in RAP1 inhibit this interaction. However, abolishing the RAP1-SUN1 interaction does not hinder APB formation, which hints at the existence of another SUN1-dependent telomere anchorage pathway. In summary, our results reveal an inhibitory role of telomere-nuclear envelope association in telomere-telomere recombination and imply the presence of redundant pathways for the telomere-nuclear envelope association in ALT cells.
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Affiliation(s)
- Chia-Wei Yang
- Department of Microbiology, College of Medicine, National Taiwan University, Taipei 10051, Taiwan
| | - Meng-Hsun Hsieh
- Department of Microbiology, College of Medicine, National Taiwan University, Taipei 10051, Taiwan
| | - Hao-Jhe Sun
- Department of Microbiology, College of Medicine, National Taiwan University, Taipei 10051, Taiwan
| | - Shu-Chun Teng
- Department of Microbiology, College of Medicine, National Taiwan University, Taipei 10051, Taiwan.,Center of Precision Medicine, National Taiwan University, Taipei 10051, Taiwan
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Differential Regulation of Telomeric Complex by BCR-ABL1 Kinase in Human Cellular Models of Chronic Myeloid Leukemia-From Single Cell Analysis to Next-Generation Sequencing. Genes (Basel) 2020; 11:genes11101145. [PMID: 33003326 PMCID: PMC7601685 DOI: 10.3390/genes11101145] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/08/2020] [Revised: 09/15/2020] [Accepted: 09/25/2020] [Indexed: 11/17/2022] Open
Abstract
Telomeres are specialized nucleoprotein complexes, localized at the physical ends of chromosomes, that contribute to the maintenance of genome stability. One of the features of chronic myeloid leukemia (CML) cells is a reduction in telomere length which may result in increased genomic instability and progression of the disease. Aberrant telomere maintenance in CML is not fully understood and other mechanisms such as the alternative lengthening of telomeres (ALT) are involved. In this work, we employed five BCR-ABL1-positive cell lines, namely K562, KU-812, LAMA-84, MEG-A2, and MOLM-1, commonly used in the laboratories to study the link between mutation, copy number, and expression of telomere maintenance genes with the expression, copy number, and activity of BCR-ABL1. Our results demonstrated that the copy number and expression of BCR-ABL1 are crucial for telomere lengthening. We observed a correlation between BCR-ABL1 expression and telomere length as well as shelterins upregulation. Next-generation sequencing revealed pathogenic variants and copy number alterations in major tumor suppressors, such as TP53 and CDKN2A, but not in telomere-associated genes. Taken together, we showed that BCR-ABL1 kinase expression and activity play a crucial role in the maintenance of telomeres in CML cell lines. Our results may help to validate and properly interpret results obtained by many laboratories employing these in vitro models of CML.
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6
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Hsu RYC, Lin YC, Redon C, Sun Q, Singh DK, Wang Y, Aggarwal V, Mitra J, Matur A, Moriarity B, Ha T, Aladjem MI, Prasanth KV, Prasanth SG. ORCA/LRWD1 Regulates Homologous Recombination at ALT-Telomeres by Modulating Heterochromatin Organization. iScience 2020; 23:101038. [PMID: 32344376 PMCID: PMC7186530 DOI: 10.1016/j.isci.2020.101038] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2019] [Revised: 03/26/2020] [Accepted: 04/01/2020] [Indexed: 12/23/2022] Open
Abstract
Telomeres are maintained by telomerase or in a subset of cancer cells by a homologous recombination (HR)-based mechanism, Alternative Lengthening of Telomeres (ALT). The mechanisms regulating telomere-homeostasis in ALT cells remain unclear. We report that a replication initiator protein, Origin Recognition Complex-Associated (ORCA/LRWD1), by localizing at the ALT-telomeres, modulates HR activity. ORCA's localization to the ALT-telomeres is facilitated by its interaction to SUMOylated shelterin components. The loss of ORCA in ALT-positive cells elevates the levels of two mediators of HR, RPA and RAD51, and consistent with this, we observe increased ALT-associated promyelocytic leukemia body formation and telomere sister chromatid exchange. ORCA binds to RPA and modulates the association of RPA to telomeres. Finally, the loss of ORCA causes global chromatin decondensation, including at the telomeres. Our results demonstrate that ORCA acts as an inhibitor of HR by modulating RPA binding to ssDNA and inducing chromatin compaction.
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Affiliation(s)
- Rosaline Y C Hsu
- Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, 601S Goodwin Avenue, Urbana, IL 61801, USA
| | - Yo-Chuen Lin
- Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, 601S Goodwin Avenue, Urbana, IL 61801, USA
| | - Christophe Redon
- Developmental Therapeutics Branch, Center for Cancer Research, NCI, NIH, Bethesda MD 20892, USA
| | - Qinyu Sun
- Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, 601S Goodwin Avenue, Urbana, IL 61801, USA
| | - Deepak K Singh
- Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, 601S Goodwin Avenue, Urbana, IL 61801, USA
| | - Yating Wang
- Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, 601S Goodwin Avenue, Urbana, IL 61801, USA
| | - Vasudha Aggarwal
- Biophysics and Biophysical Chemistry, Johns Hopkins University, Baltimore, MD 21205, USA
| | - Jaba Mitra
- Materials Engineering Department, UIUC, Urbana, IL 61801, USA
| | - Abhijith Matur
- Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, 601S Goodwin Avenue, Urbana, IL 61801, USA
| | | | - Taekjip Ha
- Biophysics and Biophysical Chemistry, Johns Hopkins University, Baltimore, MD 21205, USA; Howard Hughes Medical Institute, Johns Hopkins University, Baltimore, MD 21205, USA
| | - Mirit I Aladjem
- Developmental Therapeutics Branch, Center for Cancer Research, NCI, NIH, Bethesda MD 20892, USA
| | - Kannanganattu V Prasanth
- Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, 601S Goodwin Avenue, Urbana, IL 61801, USA; Cancer Center at Illinois, UIUC, Urbana, IL 61801, USA
| | - Supriya G Prasanth
- Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, 601S Goodwin Avenue, Urbana, IL 61801, USA; Cancer Center at Illinois, UIUC, Urbana, IL 61801, USA.
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7
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Zhang JM, Zou L. Alternative lengthening of telomeres: from molecular mechanisms to therapeutic outlooks. Cell Biosci 2020; 10:30. [PMID: 32175073 PMCID: PMC7063710 DOI: 10.1186/s13578-020-00391-6] [Citation(s) in RCA: 74] [Impact Index Per Article: 18.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2019] [Accepted: 02/23/2020] [Indexed: 02/06/2023] Open
Abstract
To escape replicative senescence, cancer cells have to overcome telomere attrition during DNA replication. Most of cancers rely on telomerase to extend and maintain telomeres, but 4-11% of cancers use a homologous recombination-based pathway called alternative lengthening of telomeres (ALT). ALT is prevalent in cancers from the mesenchymal origin and usually associates with poor clinical outcome. Given its critical role in protecting telomeres and genomic integrity in tumor cells, ALT is an Achilles heel of tumors and an attractive target for cancer therapy. Here, we review the recent progress in the mechanistic studies of ALT, and discuss the emerging therapeutic strategies to target ALT-positive cancers.
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Affiliation(s)
- Jia-Min Zhang
- Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129 USA
| | - Lee Zou
- Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129 USA.,2Department of Pathology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114 USA
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8
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Zhang JM, Yadav T, Ouyang J, Lan L, Zou L. Alternative Lengthening of Telomeres through Two Distinct Break-Induced Replication Pathways. Cell Rep 2020; 26:955-968.e3. [PMID: 30673617 PMCID: PMC6366628 DOI: 10.1016/j.celrep.2018.12.102] [Citation(s) in RCA: 167] [Impact Index Per Article: 41.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2018] [Revised: 11/09/2018] [Accepted: 12/26/2018] [Indexed: 12/11/2022] Open
Abstract
Alternative lengthening of telomeres (ALT) is a telomerase-independent but recombination-dependent pathway that maintains telomeres. Here, we describe an assay to visualize ALT-mediated telomeric DNA synthesis in ALT-associated PML bodies (APBs) without DNA-damaging agents or replication inhibitors. Using this assay, we find that ALT occurs through two distinct mechanisms. One of the ALT mechanisms requires RAD52, a protein implicated in break-induced DNA replication (BIR). We demonstrate that RAD52 directly promotes telomeric D-loop formation in vitro and is required for maintaining telomeres in ALT-positive cells. Unexpectedly, however, RAD52 is dispensable for C-circle formation, a hallmark of ALT. In RAD52-knockout ALT cells, C-circle formation and RAD52-independent ALT DNA synthesis gradually increase as telomeres are shortened, and these activities are dependent on BLM and BIR proteins POLD3 and POLD4. These results suggest that ALT occurs through a RAD52-dependent and a RAD52-independent BIR pathway, revealing the bifurcated framework and dynamic nature of this process.
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Affiliation(s)
- Jia-Min Zhang
- Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129, USA
| | - Tribhuwan Yadav
- Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129, USA
| | - Jian Ouyang
- Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129, USA
| | - Li Lan
- Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129, USA; Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129, USA
| | - Lee Zou
- Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129, USA; Department of Pathology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA.
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9
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Feng E, Batenburg NL, Walker JR, Ho A, Mitchell TRH, Qin J, Zhu XD. CSB cooperates with SMARCAL1 to maintain telomere stability in ALT cells. J Cell Sci 2020; 133:jcs234914. [PMID: 31974116 DOI: 10.1242/jcs.234914] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2019] [Accepted: 01/13/2020] [Indexed: 01/01/2023] Open
Abstract
Elevated replication stress is evident at telomeres of about 10-15% of cancer cells, which maintain their telomeres via a homologous recombination (HR)-based mechanism, referred to as alternative lengthening of telomeres (ALT). How ALT cells resolve replication stress to support their growth remains incompletely characterized. Here, we report that CSB (also known as ERCC6) promotes recruitment of HR repair proteins (MRN, BRCA1, BLM and RPA32) and POLD3 to ALT telomeres, a process that requires the ATPase activity of CSB and is controlled by ATM- and CDK2-dependent phosphorylation. Loss of CSB stimulates telomeric recruitment of MUS81 and SLX4, components of the structure-specific MUS81-EME1-SLX1-SLX4 (MUS-SLX) endonuclease complex, suggesting that CSB restricts MUS-SLX-mediated processing of stalled forks at ALT telomeres. Loss of CSB coupled with depletion of SMARCAL1, a chromatin remodeler implicated in catalyzing regression of stalled forks, synergistically promotes not only telomeric recruitment of MUS81 but also the formation of fragile telomeres, the latter of which is reported to arise from fork stalling. These results altogether suggest that CSB-mediated HR repair and SMARCAL1-mediated fork regression cooperate to prevent stalled forks from being processed into fragile telomeres in ALT cells.
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Affiliation(s)
- Emily Feng
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
| | - Nicole L Batenburg
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
| | - John R Walker
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
| | - Angus Ho
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
| | - Taylor R H Mitchell
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
| | - Jian Qin
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
| | - Xu-Dong Zhu
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
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10
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Porreca RM, Herrera-Moyano E, Skourti E, Law PP, Gonzalez Franco R, Montoya A, Faull P, Kramer H, Vannier JB. TRF1 averts chromatin remodelling, recombination and replication dependent-break induced replication at mouse telomeres. eLife 2020; 9:49817. [PMID: 31934863 PMCID: PMC6986873 DOI: 10.7554/elife.49817] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2019] [Accepted: 01/11/2020] [Indexed: 12/29/2022] Open
Abstract
Telomeres are a significant challenge to DNA replication and are prone to replication stress and telomere fragility. The shelterin component TRF1 facilitates telomere replication but the molecular mechanism remains uncertain. By interrogating the proteomic composition of telomeres, we show that mouse telomeres lacking TRF1 undergo protein composition reorganisation associated with the recruitment of DNA damage response and chromatin remodellers. Surprisingly, mTRF1 suppresses the accumulation of promyelocytic leukemia (PML) protein, BRCA1 and the SMC5/6 complex at telomeres, which is associated with increased Homologous Recombination (HR) and TERRA transcription. We uncovered a previously unappreciated role for mTRF1 in the suppression of telomere recombination, dependent on SMC5 and also POLD3 dependent Break Induced Replication at telomeres. We propose that TRF1 facilitates S-phase telomeric DNA synthesis to prevent illegitimate mitotic DNA recombination and chromatin rearrangement.
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Affiliation(s)
- Rosa Maria Porreca
- Telomere Replication and Stability group, Medical Research Council - London Institute of Medical Sciences, London, United Kingdom.,Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London, United Kingdom
| | - Emilia Herrera-Moyano
- Telomere Replication and Stability group, Medical Research Council - London Institute of Medical Sciences, London, United Kingdom.,Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London, United Kingdom
| | - Eleni Skourti
- Telomere Replication and Stability group, Medical Research Council - London Institute of Medical Sciences, London, United Kingdom.,Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London, United Kingdom
| | - Pui Pik Law
- Telomere Replication and Stability group, Medical Research Council - London Institute of Medical Sciences, London, United Kingdom.,Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London, United Kingdom
| | - Roser Gonzalez Franco
- Telomere Replication and Stability group, Medical Research Council - London Institute of Medical Sciences, London, United Kingdom.,Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London, United Kingdom
| | - Alex Montoya
- Biological Mass Spectrometry and Proteomics, Medical Research Council - London Institute of Medical Sciences, London, United Kingdom
| | - Peter Faull
- Biological Mass Spectrometry and Proteomics, Medical Research Council - London Institute of Medical Sciences, London, United Kingdom.,The Francis Crick Institute, Proteomics Mass Spectrometry Science and Technology Platform, London, United Kingdom
| | - Holger Kramer
- Biological Mass Spectrometry and Proteomics, Medical Research Council - London Institute of Medical Sciences, London, United Kingdom
| | - Jean-Baptiste Vannier
- Telomere Replication and Stability group, Medical Research Council - London Institute of Medical Sciences, London, United Kingdom.,Institute of Clinical Sciences, Faculty of Medicine, Imperial College London, London, United Kingdom
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11
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FANCM limits ALT activity by restricting telomeric replication stress induced by deregulated BLM and R-loops. Nat Commun 2019; 10:2253. [PMID: 31138795 PMCID: PMC6538666 DOI: 10.1038/s41467-019-10179-z] [Citation(s) in RCA: 116] [Impact Index Per Article: 23.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2018] [Accepted: 04/23/2019] [Indexed: 12/13/2022] Open
Abstract
Telomerase negative immortal cancer cells elongate telomeres through the Alternative Lengthening of Telomeres (ALT) pathway. While sustained telomeric replicative stress is required to maintain ALT, it might also lead to cell death when excessive. Here, we show that the ATPase/translocase activity of FANCM keeps telomeric replicative stress in check specifically in ALT cells. When FANCM is depleted in ALT cells, telomeres become dysfunctional, and cells stop proliferating and die. FANCM depletion also increases ALT-associated marks and de novo synthesis of telomeric DNA. Depletion of the BLM helicase reduces the telomeric replication stress and cell proliferation defects induced by FANCM inactivation. Finally, FANCM unwinds telomeric R-loops in vitro and suppresses their accumulation in cells. Overexpression of RNaseH1 completely abolishes the replication stress remaining in cells codepleted for FANCM and BLM. Thus, FANCM allows controlled ALT activity and ALT cell proliferation by limiting the toxicity of uncontrolled BLM and telomeric R-loops. In cancer cells, telomeres can be elongated through homology directed-repair pathways in a process known as Alternative Lengthening of Telomeres (ALT). Here, the authors reveal that FANCM regulates ALT activity and ALT cell proliferation by limiting the activity of uncontrolled BLM and telomeric R-loops.
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12
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PML nuclear bodies, membrane-less domains acting as ROS sensors? Semin Cell Dev Biol 2017; 80:29-34. [PMID: 29157919 DOI: 10.1016/j.semcdb.2017.11.007] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2017] [Revised: 11/04/2017] [Accepted: 11/06/2017] [Indexed: 12/23/2022]
Abstract
PML Nuclear bodies (PML NBs) are spherical domains associated with a broad range of activities upon stress responses such as apoptosis, senescence DNA repair, epigenetic control, as well as control of oncogenesis. These bodies are considered as privileged sites for post-translational modifications, where sumoylation plays a key role. Here we summarize recent in vitro and in vivo findings on the link between PML NBs and ROS, in particular PML contributions to oxidative stress response. We discuss how it may regulate switch from cell protection against stress to cell arrest/cell death.
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13
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Stabilization of Telomere G-Quadruplexes Interferes with Human Herpesvirus 6A Chromosomal Integration. J Virol 2017; 91:JVI.00402-17. [PMID: 28468887 DOI: 10.1128/jvi.00402-17] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2017] [Accepted: 04/29/2017] [Indexed: 11/20/2022] Open
Abstract
Human herpesviruses 6A and 6B (HHV-6A/B) can integrate their genomes into the telomeres of human chromosomes using a mechanism that remains poorly understood. To achieve a better understanding of the HHV-6A/B integration mechanism, we made use of BRACO-19, a compound that stabilizes G-quadruplex secondary structures and prevents telomere elongation by the telomerase complex. First, we analyzed the folding of telomeric sequences into G-quadruplex structures and their binding to BRACO-19 using G-quadruplex-specific antibodies and surface plasmon resonance. Circular dichroism studies indicate that BRACO-19 modifies the conformation and greatly stabilizes the G-quadruplexes formed in G-rich telomeric DNA. Subsequently we assessed the effects of BRACO-19 on the HHV-6A initial phase of infection. Our results indicate that BRACO-19 does not affect entry of HHV-6A DNA into cells. We next investigated if stabilization of G-quadruplexes by BRACO-19 affected HHV-6A's ability to integrate its genome into host chromosomes. Incubation of telomerase-expressing cells with BRACO-19, such as HeLa and MCF-7, caused a significant reduction in the HHV-6A integration frequency (P < 0.002); in contrast, BRACO-19 had no effect on HHV-6 integration frequency in U2OS cells that lack telomerase activity and elongate their telomeres through alternative lengthening mechanisms. Our data suggest that the fluidity of telomeres is important for efficient chromosomal integration of HHV-6A and that interference with telomerase activity negatively affects the generation of cellular clones containing integrated HHV-6A.IMPORTANCE HHV-6A/B can integrate their genomes into the telomeres of infected cells. Telomeres consist of repeated hexanucleotides (TTAGGG) of various lengths (up to several kilobases) and end with a single-stranded 3' extension. To avoid recognition and induce a DNA damage response, the single-stranded overhang folds back on itself and forms a telomeric loop (T-loop) or adopts a tertiary structure, referred to as a G-quadruplex. In the current study, we have examined the effects of a G-quadruplex binding and stabilizing agent, BRACO-19, on HHV-6A chromosomal integration. By stabilizing G-quadruplex structures, BRACO-19 affects the ability of the telomerase complex to elongate telomeres. Our results indicate that BRACO-19 reduces the number of clones harboring integrated HHV-6A. This study is the first of its kind and suggests that telomerase activity is essential to restore a functional telomere of adequate length following HHV-6A integration.
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Chang HR, Munkhjargal A, Kim MJ, Park SY, Jung E, Ryu JH, Yang Y, Lim JS, Kim Y. The functional roles of PML nuclear bodies in genome maintenance. Mutat Res 2017; 809:99-107. [PMID: 28521962 DOI: 10.1016/j.mrfmmm.2017.05.002] [Citation(s) in RCA: 58] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2017] [Revised: 04/28/2017] [Accepted: 05/04/2017] [Indexed: 02/07/2023]
Abstract
In the nucleus, there are several membraneless structures called nuclear bodies. Among them, promyelocytic leukemia nuclear bodies (PML-NBs) are involved in multiple genome maintenance pathways including the DNA damage response, DNA repair, telomere homeostasis, and p53-associated apoptosis. In response to DNA damage, PML-NBs are coalesced and divided by a fission mechanism, thus increasing their number. PML-NBs also play a role in repairing DNA double-strand breaks (DSBs) by homologous recombination (HR). Clinically, the dominant negative PML-RARα fusion protein expressed in acute promyelocytic leukemia (APL) inhibits the transactivation of downstream factors and disrupts PML function, revealing the tumor suppressor role of PML-NBs. All-trans retinoic acid and arsenic trioxide treatment has been implemented for promyelocytic leukemia to target the PML-RARα fusion protein. PML-NBs are associated with various factors implicated in genome maintenance, and are found at the sites of DNA damage. Their interaction with proteins such as p53 indicates that PML-NBs may play a significant role in apoptosis and cancer. Decades of research have revealed the importance of PML-NBs in diverse cellular pathways, yet the underlying molecular mechanisms and exact functions of PML-NBs remain elusive. In this review, PML protein modifications and the functional relevance of PML-NB and its associated factors in genome maintenance will be discussed.
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Affiliation(s)
- Hae Ryung Chang
- Department of Biological Sciences, Sookmyung Women's University, Seoul 04310, Republic of Korea
| | - Anudari Munkhjargal
- Department of Biological Sciences, Sookmyung Women's University, Seoul 04310, Republic of Korea
| | - Myung-Jin Kim
- Department of Biological Sciences, Sookmyung Women's University, Seoul 04310, Republic of Korea
| | - Seon Young Park
- Department of Biological Sciences, Sookmyung Women's University, Seoul 04310, Republic of Korea
| | - Eunyoung Jung
- Department of Biological Sciences, Sookmyung Women's University, Seoul 04310, Republic of Korea
| | - Jae-Ha Ryu
- Research Center for Cell Fate Control, Sookmyung Women's University, Seoul 04310, Republic of Korea
| | - Young Yang
- Department of Biological Sciences, Sookmyung Women's University, Seoul 04310, Republic of Korea
| | - Jong-Seok Lim
- Department of Biological Sciences, Sookmyung Women's University, Seoul 04310, Republic of Korea
| | - Yonghwan Kim
- Department of Biological Sciences, Sookmyung Women's University, Seoul 04310, Republic of Korea.
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15
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Berardinelli F, Coluzzi E, Sgura A, Antoccia A. Targeting telomerase and telomeres to enhance ionizing radiation effects in in vitro and in vivo cancer models. MUTATION RESEARCH-REVIEWS IN MUTATION RESEARCH 2017; 773:204-219. [PMID: 28927529 DOI: 10.1016/j.mrrev.2017.02.004] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [Subscribe] [Scholar Register] [Received: 10/18/2016] [Revised: 02/13/2017] [Accepted: 02/14/2017] [Indexed: 01/05/2023]
Abstract
One of the hallmarks of cancer consists in the ability of tumor cells to divide indefinitely, and to maintain stable telomere lengths throughout the activation of specific telomere maintenance mechanisms (TMM). Therefore in the last fifteen years, researchers proposed to target telomerase or telomeric structure in order to block limitless replicative potential of cancer cells providing a fascinating strategy for a broad-spectrum cancer therapy. In the present review, we report in vitro and in vivo evidence regarding the use of chemical agents targeting both telomerase or telomere structure and showing promising antitumor effects when used in combination with ionizing radiation (IR). RNA interference, antisense oligonucleotides (e.g., GRN163L), non-nucleoside inhibitors (e.g., BIBR1532) and nucleoside analogs (e.g., AZT) represent some of the most potent strategies to inhibit telomerase activity used in combination with IR. Furthermore, radiosensitizing effects were demonstrated also for agents acting directly on the telomeric structure such as G4-ligands (e.g., RHPS4 and Telomestatin) or telomeric-oligos (T-oligos). To date, some of these compounds are under clinical evaluation (e.g., GRN163L and KML001). Advantages of Telomere/Telomerase Targeting Compounds (T/TTCs) coupled with radiotherapy may be relevant in the treatment of radioresistant tumors and in the development of new optimized treatment plans with reduced dose adsorbed by patients and consequent attenuation of short- end long-term side effects. Pros and cons of possible future applications in cancer therapy based on the combination of T/TCCs and radiation treatment are discussed.
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Affiliation(s)
- F Berardinelli
- Dipartimento di Scienze, Università Roma Tre, Rome Italy; Istituto Nazionale di Fisica Nucleare, INFN, Sezione di Roma Tre, Rome, Italy.
| | - E Coluzzi
- Dipartimento di Scienze, Università Roma Tre, Rome Italy
| | - A Sgura
- Dipartimento di Scienze, Università Roma Tre, Rome Italy; Istituto Nazionale di Fisica Nucleare, INFN, Sezione di Roma Tre, Rome, Italy
| | - A Antoccia
- Dipartimento di Scienze, Università Roma Tre, Rome Italy; Istituto Nazionale di Fisica Nucleare, INFN, Sezione di Roma Tre, Rome, Italy
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16
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Legartová S, Sehnalová P, Malyšková B, Küntziger T, Collas P, Cmarko D, Raška I, Sorokin DV, Kozubek S, Bártová E. Localized Movement and Levels of 53BP1 Protein Are Changed by γ-irradiation in PML Deficient Cells. J Cell Biochem 2016; 117:2583-96. [PMID: 27526954 DOI: 10.1002/jcb.25551] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2016] [Accepted: 03/23/2016] [Indexed: 01/07/2023]
Abstract
We studied epigenetics, distribution pattern, kinetics, and diffusion of proteins recruited to spontaneous and γ-radiation-induced DNA lesions. We showed that PML deficiency leads to an increased number of DNA lesions, which was accompanied by changes in histone signature. In PML wt cells, we observed two mobile fractions of 53BP1 protein with distinct diffusion in spontaneous lesions. These protein fractions were not detected in PML-deficient cells, characterized by slow-diffusion of 53BP1. Single particle tracking analysis revealed limited local motion of 53BP1 foci in PML double null cells and local motion 53BP1 foci was even more reduced after γ-irradiation. However, radiation did not change co-localization between 53BP1 nuclear bodies and interchromatin granule-associated zones (IGAZs), nuclear speckles, or chromocenters. This newly observed interaction pattern imply that 53BP1 protein could be a part of not only DNA repair, but also process mediated via components accumulated in IGAZs, nuclear speckles, or paraspeckles. Together, PML deficiency affected local motion of 53BP1 nuclear bodies and changed composition and a number of irradiation-induced foci. J. Cell. Biochem. 117: 2583-2596, 2016. © 2016 Wiley Periodicals, Inc.
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Affiliation(s)
- Soňa Legartová
- Institute of Biophysics, Academy of Sciences of the Czech Republic, v.v.i., Královopolská 135, Brno, 612 65, Czech Republic
| | - Petra Sehnalová
- Institute of Biophysics, Academy of Sciences of the Czech Republic, v.v.i., Královopolská 135, Brno, 612 65, Czech Republic
| | - Barbora Malyšková
- Institute of Biophysics, Academy of Sciences of the Czech Republic, v.v.i., Královopolská 135, Brno, 612 65, Czech Republic
| | | | - Philippe Collas
- Department of Biochemistry, Institute of Basic Medical Sciences, University of Oslo, Norwegian Center for Stem Cell Research, Oslo, Norway
| | - Dušan Cmarko
- Institute of Cellular Biology and Pathology, the First Faculty of Medicine, Charles University in Prague, Albertov 4, Prague, 128 01, Czech Republic
| | - Ivan Raška
- Institute of Cellular Biology and Pathology, the First Faculty of Medicine, Charles University in Prague, Albertov 4, Prague, 128 01, Czech Republic
| | - Dmitry V Sorokin
- Institute of Biophysics, Academy of Sciences of the Czech Republic, v.v.i., Královopolská 135, Brno, 612 65, Czech Republic.,Faculty of Informatics, Masaryk University, Botanická 68a, Brno, 602 00, Czech Republic
| | - Stanislav Kozubek
- Institute of Biophysics, Academy of Sciences of the Czech Republic, v.v.i., Královopolská 135, Brno, 612 65, Czech Republic
| | - Eva Bártová
- Institute of Biophysics, Academy of Sciences of the Czech Republic, v.v.i., Královopolská 135, Brno, 612 65, Czech Republic. .,Institute of Cellular Biology and Pathology, the First Faculty of Medicine, Charles University in Prague, Albertov 4, Prague, 128 01, Czech Republic.
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17
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Root H, Larsen A, Komosa M, Al-Azri F, Li R, Bazett-Jones DP, Stephen Meyn M. FANCD2 limits BLM-dependent telomere instability in the alternative lengthening of telomeres pathway. Hum Mol Genet 2016; 25:3255-3268. [DOI: 10.1093/hmg/ddw175] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2016] [Revised: 05/02/2016] [Accepted: 06/06/2016] [Indexed: 11/12/2022] Open
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18
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Wilson FR, Ho A, Walker JR, Zhu XD. Cdk-dependent phosphorylation regulates TRF1 recruitment to PML bodies and promotes C-circle production in ALT cells. J Cell Sci 2016; 129:2559-72. [PMID: 27185864 DOI: 10.1242/jcs.186098] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2016] [Accepted: 05/06/2016] [Indexed: 12/26/2022] Open
Abstract
TRF1, a duplex telomeric DNA binding protein, is implicated in homologous-recombination-based alternative lengthening of telomeres, known as ALT. However, how TRF1 promotes ALT activity has yet to be fully characterized. Here we report that Cdk-dependent TRF1 phosphorylation on T371 acts as a switch to create a pool of TRF1, referred to as (pT371)TRF1, which is recruited to ALT-associated PML bodies (APBs) in S and G2 phases independently of its binding to telomeric DNA. We find that phosphorylation of T371 is essential for APB formation and C-circle production, both of which are hallmarks of ALT. We show that the interaction of (pT371)TRF1 with APBs is dependent upon ATM and homologous-recombination-promoting factors Mre11 and BRCA1. In addition, (pT371)TRF1 interaction with APBs is sensitive to transcription inhibition, which also reduces DNA damage at telomeres. Furthermore, overexpression of RNaseH1 impairs (pT371)TRF1 recruitment to APBs in the presence of campothecin, an inhibitor that prevents topoisomerase I from resolving RNA-DNA hybrids. These results suggest that transcription-associated DNA damage, perhaps arising from processing RNA-DNA hybrids at telomeres, triggers (pT371)TRF1 recruitment to APBs to facilitate ALT activity.
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Affiliation(s)
- Florence R Wilson
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
| | - Angus Ho
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
| | - John R Walker
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
| | - Xu-Dong Zhu
- Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
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19
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Tsai RYL. Balancing self-renewal against genome preservation in stem cells: How do they manage to have the cake and eat it too? Cell Mol Life Sci 2016; 73:1803-23. [PMID: 26886024 PMCID: PMC5040593 DOI: 10.1007/s00018-016-2152-y] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2015] [Revised: 01/18/2016] [Accepted: 01/28/2016] [Indexed: 01/22/2023]
Abstract
Stem cells are endowed with the awesome power of self-renewal and multi-lineage differentiation that allows them to be major contributors to tissue homeostasis. Owing to their longevity and self-renewal capacity, they are also faced with a higher risk of genomic damage compared to differentiated cells. Damage on the genome, if not prevented or repaired properly, will threaten the survival of stem cells and culminate in organ failure, premature aging, or cancer formation. It is therefore of paramount importance that stem cells remain genomically stable throughout life. Given their unique biological and functional requirement, stem cells are thought to manage genotoxic stress somewhat differently from non-stem cells. The focus of this article is to review the current knowledge on how stem cells escape the barrage of oxidative and replicative DNA damage to stay in self-renewal. A clear statement on this subject should help us better understand tissue regeneration, aging, and cancer.
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Affiliation(s)
- Robert Y L Tsai
- Center for Translational Cancer Research, Institute of Biosciences and Technology, Texas A&M University Health Science Center, 2121 W. Holcombe Blvd, Houston, TX, 77030, USA.
- Department of Molecular and Cellular Medicine, Texas A&M University Health Science Center, College Station, TX, 77843, USA.
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20
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Cox KE, Maréchal A, Flynn RL. SMARCAL1 Resolves Replication Stress at ALT Telomeres. Cell Rep 2016; 14:1032-1040. [PMID: 26832416 PMCID: PMC5051350 DOI: 10.1016/j.celrep.2016.01.011] [Citation(s) in RCA: 55] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2015] [Revised: 11/25/2015] [Accepted: 12/29/2015] [Indexed: 11/20/2022] Open
Abstract
Cancer cells overcome replicative senescence by exploiting mechanisms of telomere elongation, a process often accomplished by reactivation of the enzyme telomerase. However, a subset of cancer cells lack telomerase activity and rely on the alternative lengthening of telomeres (ALT) pathway, a recombination-based mechanism of telomere elongation. Although the mechanisms regulating ALT are not fully defined, chronic replication stress at telomeres might prime these fragile regions for recombination. Here, we demonstrate that the replication stress response protein SMARCAL1 is a critical regulator of ALT activity. SMARCAL1 associates with ALT telomeres to resolve replication stress and ensure telomere stability. In the absence of SMARCAL1, persistently stalled replication forks at ALT telomeres deteriorate into DNA double-strand breaks promoting the formation of chromosome fusions. Our studies not only define a role for SMARCAL1 in ALT telomere maintenance, but also demonstrate that resolution of replication stress is a crucial step in the ALT mechanism.
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Affiliation(s)
- Kelli E Cox
- Department of Pharmacology and Experimental Therapeutics, Boston University School of Medicine, Boston, MA 02118, USA; Department of Medicine, Cancer Center, Boston University School of Medicine, Boston, MA 02118, USA
| | - Alexandre Maréchal
- Department of Biology, Université de Sherbrooke, Sherbrooke, QC J1K 2R1, Canada
| | - Rachel Litman Flynn
- Department of Pharmacology and Experimental Therapeutics, Boston University School of Medicine, Boston, MA 02118, USA; Department of Medicine, Cancer Center, Boston University School of Medicine, Boston, MA 02118, USA.
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21
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Abstract
ATRX was identified over 20 years ago as the gene responsible for a rare developmental disorder characterized by α-thalassemia and intellectual disability. Similarities to the sucrose nonfermentable SNF2 type chromatin remodelers initially suggested a role in transcriptional regulation. However, over the last years, our knowledge of the epigenetic activities of ATRX has expanded steadily. Recent exciting discoveries have propelled ATRX into the limelight of chromatin and telomere biology, development and cancer research. This review summarizes recent breakthroughs in understanding ATRX function in heterochromatin structure, genome stability and its frequent dysregulation in a variety of cancers.
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Affiliation(s)
- L Ashley Watson
- Departments of Paediatrics, Biochemistry & Oncology, University of Western Ontario, Victoria Research Laboratories, 800 Commissioners Road East, London, Canada.,Children's Health Research Institute, London, Canada.,Lawson Health Research Institute, London, Canada
| | - Hannah Goldberg
- Departments of Paediatrics, Biochemistry & Oncology, University of Western Ontario, Victoria Research Laboratories, 800 Commissioners Road East, London, Canada.,Children's Health Research Institute, London, Canada.,Lawson Health Research Institute, London, Canada
| | - Nathalie G Bérubé
- Departments of Paediatrics, Biochemistry & Oncology, University of Western Ontario, Victoria Research Laboratories, 800 Commissioners Road East, London, Canada.,Children's Health Research Institute, London, Canada.,Lawson Health Research Institute, London, Canada
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22
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Guan D, Kao HY. The function, regulation and therapeutic implications of the tumor suppressor protein, PML. Cell Biosci 2015; 5:60. [PMID: 26539288 PMCID: PMC4632682 DOI: 10.1186/s13578-015-0051-9] [Citation(s) in RCA: 62] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2015] [Accepted: 10/28/2015] [Indexed: 12/21/2022] Open
Abstract
The tumor suppressor protein, promyelocytic leukemia protein (PML), was originally identified in acute promyelocytic leukemia due to a chromosomal translocation between chromosomes 15 and 17. PML is the core component of subnuclear structures called PML nuclear bodies (PML-NBs), which are disrupted in acute promyelocytic leukemia cells. PML plays important roles in cell cycle regulation, survival and apoptosis, and inactivation or down-regulation of PML is frequently found in cancer cells. More than 120 proteins have been experimentally identified to physically associate with PML, and most of them either transiently or constitutively co-localize with PML-NBs. These interactions are associated with many cellular processes, including cell cycle arrest, apoptosis, senescence, transcriptional regulation, DNA repair and intermediary metabolism. Importantly, PML inactivation in cancer cells can occur at the transcriptional-, translational- or post-translational- levels. However, only a few somatic mutations have been found in cancer cells. A better understanding of its regulation and its role in tumor suppression will provide potential therapeutic opportunities. In this review, we discuss the role of PML in multiple tumor suppression pathways and summarize the players and stimuli that control PML protein expression or subcellular distribution.
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Affiliation(s)
- Dongyin Guan
- Department of Biochemistry, School of Medicine, Case Western Reserve University, and Comprehensive Cancer Center of Case Western Reserve University, Cleveland, 10900 Euclid Avenue, Cleveland, OH 44106 USA
| | - Hung-Ying Kao
- Department of Biochemistry, School of Medicine, Case Western Reserve University, and Comprehensive Cancer Center of Case Western Reserve University, Cleveland, 10900 Euclid Avenue, Cleveland, OH 44106 USA
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23
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Reddel RR. Telomere maintenance mechanisms in cancer: clinical implications. Curr Pharm Des 2015; 20:6361-74. [PMID: 24975603 PMCID: PMC4262939 DOI: 10.2174/1381612820666140630101047] [Citation(s) in RCA: 57] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2014] [Accepted: 06/26/2014] [Indexed: 01/20/2023]
Abstract
The presence of immortal cell populations with an up-regulated telomere maintenance mechanism (TMM) is an almost universal characteristic of cancers, whereas normal somatic cells are unable to prevent proliferation-associated telomere shortening and have a limited proliferative potential. TMMs and related aspects of telomere structure and function therefore appear to be ideal targets for the development of anticancer therapeutics. Such treatments would be targeted to a specific cancer-related molecular abnormality, and also be broad-spectrum in that they would be expected to be potentially applicable to most cancers. However, the telomere biology of normal and malignant human cells is a relatively young research field with large numbers of unanswered questions, so the optimal design of TMM-targeted therapeutic approaches remains unclear. This review outlines the opportunities and challenges presented by telomeres and TMMs for clinical management of cancer.
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Affiliation(s)
- Roger R Reddel
- Children's Medical Research Institute, 214 Hawkesbury Road, Westmead, New South Wales 2145, Australia.
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24
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Lajud SA, Nagda DA, Yamashita T, Zheng J, Tanaka N, Abuzeid WM, Civantos A, Bezpalko O, O'Malley BW, Li D. Dual disruption of DNA repair and telomere maintenance for the treatment of head and neck cancer. Clin Cancer Res 2014; 20:6465-78. [PMID: 25324139 DOI: 10.1158/1078-0432.ccr-14-0176] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
PURPOSE Poly(ADP-ribose) polymerases (PARP) and the Mre11, Rad50, and Nbs1 (MRN) complex are key regulators of DNA repair, and have been recently shown to independently regulate telomere length. Sensitivity of cancers to PARPi is largely dependent on the BRCAness of the cells. Unfortunately, the vast majority of cancers are BRCA-proficient. In this study, therefore, we investigated whether a targeted molecular "hit" on the MRN complex, which is upstream of BRCA, can effectively sensitize BRCA-proficient head and neck squamous cell carcinoma (HNSCC) to PARP inhibitor (PARPi). EXPERIMENTAL DESIGN Human HNSCC cell lines and a mouse model with HNSCC xenografts were used in this study. In vitro and in vivo studies were conducted to evaluate the effects and underlying mechanisms of dual molecular disruption of PARP and the MRN complex, using a pharmacologic inhibitor and a dominant-negative Nbs1 expression vector, respectively. RESULTS Our findings demonstrate that downregulation of the MRN complex disrupts homologous recombination, and, when combined with PARPi, leads to accumulation of lethal DNA double-strand breaks. Moreover, we show that PARPi and MRN complex disruption induces significantly shortening telomere length. Together, our results demonstrate that dual disruption of these pathways causes significant cell death in BRCA-proficient tumor cells both in vitro and in vivo. CONCLUSION Our study, for the first time, elucidates a novel mechanism for MRN complex and PARP inhibition beyond DNA repair, demonstrating the feasibility of a dual disruption approach that extends the utility of PARPi to the treatment of BRCA-proficient cancers.
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Affiliation(s)
- Shayanne A Lajud
- Department of Otorhinolaryngology-Head and Neck Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
| | - Danish A Nagda
- Department of Otorhinolaryngology-Head and Neck Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
| | - Taku Yamashita
- Department of Otorhinolaryngology-Head and Neck Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania. Department of Otolaryngology-Head and Neck Surgery, National Defense Medical College, Tokorozawa, Saitama, Japan
| | - Jun Zheng
- Department of Otorhinolaryngology-Head and Neck Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
| | - Nobuaki Tanaka
- Department of Otorhinolaryngology-Head and Neck Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania. Department of Otolaryngology-Head and Neck Surgery, National Defense Medical College, Tokorozawa, Saitama, Japan
| | - Waleed M Abuzeid
- Department of Otorhinolaryngology-Head and Neck Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania. Department of Otolaryngology-Head and Neck Surgery, Stanford University School of Medicine, Stanford, California
| | - Alyssa Civantos
- Department of Otorhinolaryngology-Head and Neck Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
| | - Orysia Bezpalko
- Department of Otorhinolaryngology-Head and Neck Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
| | - Bert W O'Malley
- Department of Otorhinolaryngology-Head and Neck Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
| | - Daqing Li
- Department of Otorhinolaryngology-Head and Neck Surgery, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania.
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25
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Deng Z, Kim ET, Vladimirova O, Dheekollu J, Wang Z, Newhart A, Liu D, Myers JL, Hensley SE, Moffat J, Janicki SM, Fraser NW, Knipe DM, Weitzman MD, Lieberman PM. HSV-1 remodels host telomeres to facilitate viral replication. Cell Rep 2014; 9:2263-78. [PMID: 25497088 DOI: 10.1016/j.celrep.2014.11.019] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2014] [Revised: 10/12/2014] [Accepted: 11/11/2014] [Indexed: 12/23/2022] Open
Abstract
Telomeres protect the ends of cellular chromosomes. We show here that infection with herpes simplex virus 1 (HSV-1) results in chromosomal structural aberrations at telomeres and the accumulation of telomere dysfunction-induced DNA damage foci (TIFs). At the molecular level, HSV-1 induces transcription of telomere repeat-containing RNA (TERRA), followed by the proteolytic degradation of the telomere protein TPP1 and loss of the telomere repeat DNA signal. The HSV-1-encoded E3 ubiquitin ligase ICP0 is required for TERRA transcription and facilitates TPP1 degradation. Small hairpin RNA (shRNA) depletion of TPP1 increases viral replication, indicating that TPP1 inhibits viral replication. Viral replication protein ICP8 forms foci that coincide with telomeric proteins, and ICP8-null virus failed to degrade telomere DNA signal. These findings suggest that HSV-1 reorganizes telomeres to form ICP8-associated prereplication foci and to promote viral genomic replication.
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Affiliation(s)
- Zhong Deng
- The Wistar Institute, Philadelphia, PA 19104, USA
| | - Eui Tae Kim
- Department of Pathology and Laboratory Medicine, University of Pennsylvania Perelman School of Medicine and The Children's Hospital of Philadelphia, Philadelphia, PA 19104, USA
| | | | | | - Zhuo Wang
- The Wistar Institute, Philadelphia, PA 19104, USA
| | | | - Dongmei Liu
- Department of Microbiology and Immunology, SUNY Upstate Medical University, Syracuse, NY 13210, USA
| | | | | | - Jennifer Moffat
- Department of Microbiology and Immunology, SUNY Upstate Medical University, Syracuse, NY 13210, USA
| | | | - Nigel W Fraser
- Department of Microbiology, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA 19104, USA
| | - David M Knipe
- Department of Microbiology and Immunobiology, Harvard Medical School, Boston, MA 02115, USA
| | - Matthew D Weitzman
- Department of Pathology and Laboratory Medicine, University of Pennsylvania Perelman School of Medicine and The Children's Hospital of Philadelphia, Philadelphia, PA 19104, USA
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Imani-Saber Z, Ghafouri-Fard S. Promyelocytic Leukemia Gene Functions and Roles in Tumorigenesis. Asian Pac J Cancer Prev 2014. [DOI: 10.7314/apjcp.2014.15.19.8019] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022] Open
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27
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Böhm S, Bernstein KA. The role of post-translational modifications in fine-tuning BLM helicase function during DNA repair. DNA Repair (Amst) 2014; 22:123-32. [PMID: 25150915 DOI: 10.1016/j.dnarep.2014.07.007] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/13/2014] [Accepted: 07/14/2014] [Indexed: 12/12/2022]
Abstract
RecQ-like helicases are a highly conserved family of proteins which are critical for preserving genome integrity. Genome instability is considered a hallmark of cancer and mutations within three of the five human RECQ genes cause hereditary syndromes that are associated with cancer predisposition. The human RecQ-like helicase BLM has a central role in DNA damage signaling, repair, replication, and telomere maintenance. BLM and its budding yeast orthologue Sgs1 unwind double-stranded DNA intermediates. Intriguingly, BLM functions in both a pro- and anti-recombinogenic manner upon replicative damage, acting on similar substrates. Thus, BLM activity must be intricately controlled to prevent illegitimate recombination events that could have detrimental effects on genome integrity. In recent years it has become evident that post-translational modifications (PTMs) of BLM allow a fine-tuning of its function. To date, BLM phosphorylation, ubiquitination, and SUMOylation have been identified, in turn regulating its subcellular localization, protein-protein interactions, and protein stability. In this review, we will discuss the cellular context of when and how these different modifications of BLM occur. We will reflect on the current model of how PTMs control BLM function during DNA damage repair and compare this to what is known about post-translational regulation of the budding yeast orthologue Sgs1. Finally, we will provide an outlook toward future research, in particular to dissect the cross-talk between the individual PTMs on BLM.
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Affiliation(s)
- Stefanie Böhm
- Department of Microbiology and Molecular Genetics, University of Pittsburgh School of Medicine, Pittsburgh, PA 15213, United States
| | - Kara Anne Bernstein
- Department of Microbiology and Molecular Genetics, University of Pittsburgh School of Medicine, Pittsburgh, PA 15213, United States.
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Gocha ARS, Acharya S, Groden J. WRN loss induces switching of telomerase-independent mechanisms of telomere elongation. PLoS One 2014; 9:e93991. [PMID: 24709898 PMCID: PMC3977986 DOI: 10.1371/journal.pone.0093991] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2014] [Accepted: 03/11/2014] [Indexed: 12/24/2022] Open
Abstract
Telomere maintenance can occur in the presence of telomerase or in its absence, termed alternative lengthening of telomeres (ALT). ALT adds telomere repeats using recombination-based processes and DNA repair proteins that function in homologous recombination. Our previous work reported that the RecQ-like BLM helicase is required for ALT and that it unwinds telomeric substrates in vitro. WRN is also a RecQ-like helicase that shares many biochemical functions with BLM. WRN interacts with BLM, unwinds telomeric substrates, and co-localizes to ALT-associated PML bodies (APBs), suggesting that it may also be required for ALT processes. Using long-term siRNA knockdown of WRN in three ALT cell lines, we show that some, but not all, cell lines require WRN for telomere maintenance. VA-13 cells require WRN to prevent telomere loss and for the formation of APBs; Saos-2 cells do not. A third ALT cell line, U-2 OS, requires WRN for APB formation, however WRN loss results in p53-mediated apoptosis. In the absence of WRN and p53, U-2 OS cells undergo telomere loss for an intermediate number of population doublings (50-70), at which point they maintain telomere length even with the continued loss of WRN. WRN and the tumor suppressor BRCA1 co-localize to APBs in VA-13 and U-2 OS, but not in Saos-2 cells. WRN loss in U-2 OS is associated with a loss of BRCA1 from APBs. While the loss of WRN significantly increases telomere sister chromatid exchanges (T-SCE) in these three ALT cell lines, loss of both BRCA1 and WRN does not significantly alter T-SCE. This work demonstrates that ALT cell lines use different telomerase-independent maintenance mechanisms that variably require the WRN helicase and that some cells can switch from one mechanism to another that permits telomere elongation in the absence of WRN. Our data suggest that BRCA1 localization may define these mechanisms.
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Affiliation(s)
- April Renee Sandy Gocha
- Department of Molecular Virology, Immunology and Medical Genetics, The Ohio State University College of Medicine, Columbus, Ohio, United States of America
| | - Samir Acharya
- Department of Molecular Virology, Immunology and Medical Genetics, The Ohio State University College of Medicine, Columbus, Ohio, United States of America
| | - Joanna Groden
- Department of Molecular Virology, Immunology and Medical Genetics, The Ohio State University College of Medicine, Columbus, Ohio, United States of America
- * E-mail:
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Kamranvar S, Masucci M. Detection of ALT Associated Promyelocytic Leukemia Nuclear Bodies (APBs) by Immunofluorescence-FISH (IF-FISH). Bio Protoc 2014. [DOI: 10.21769/bioprotoc.1303] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022] Open
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30
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Queisser A, Heeg S, Thaler M, von Werder A, Opitz OG. Inhibition of telomerase induces alternative lengthening of telomeres during human esophageal carcinogenesis. Cancer Genet 2013; 206:374-86. [DOI: 10.1016/j.cancergen.2013.10.001] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2013] [Revised: 09/25/2013] [Accepted: 10/02/2013] [Indexed: 12/28/2022]
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31
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McKerlie M, Walker JR, Mitchell TRH, Wilson FR, Zhu XD. Phosphorylated (pT371)TRF1 is recruited to sites of DNA damage to facilitate homologous recombination and checkpoint activation. Nucleic Acids Res 2013; 41:10268-82. [PMID: 23997120 PMCID: PMC3905873 DOI: 10.1093/nar/gkt775] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
TRF1, a duplex telomeric DNA-binding protein, plays an important role in telomere metabolism. We have previously reported that a fraction of endogenous TRF1 can stably exist free of telomere chromatin when it is phosphorylated at T371 by Cdk1; however, the role of this telomere-free (pT371)TRF1 has yet to be fully characterized. Here we show that phosphorylated (pT371)TRF1 is recruited to sites of DNA damage, forming damage-induced foci in response to ionizing radiation (IR), etoposide and camptothecin. We find that IR-induced (pT371)TRF1 foci formation is dependent on the ATM- and Mre11/Rad50/Nbs1-mediated DNA damage response. While loss of functional BRCA1 impairs the formation of IR-induced (pT371)TRF1 foci, depletion of either 53BP1 or Rif1 stimulates IR-induced (pT371)TRF1 foci formation. In addition, we show that TRF1 depletion or the lack of its phosphorylation at T371 impairs DNA end resection and repair of nontelomeric DNA double-strand breaks by homologous recombination. The lack of TRF1 phosphorylation at T371 also hampers the activation of the G2/M checkpoint and sensitizes cells to PARP inhibition, IR and camptothecin. Collectively, these results reveal a novel but important function of phosphorylated (pT371)TRF1 in facilitating DNA double-strand break repair and the maintenance of genome integrity.
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Affiliation(s)
- Megan McKerlie
- Department of Biology, McMaster University, 1280 Main St. West, Hamilton, Ontario L8S4K1, Canada
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32
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Dynamic length changes of telomeres and their nuclear organization in chronic myeloid leukemia. Cancers (Basel) 2013; 5:1086-102. [PMID: 24202335 PMCID: PMC3795380 DOI: 10.3390/cancers5031086] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2013] [Revised: 08/08/2013] [Accepted: 08/16/2013] [Indexed: 01/11/2023] Open
Abstract
Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm characterized by the t(9;22) translocation. As in most cancers, short telomeres are one of the features of CML cells, and telomere shortening accentuates as the disease progresses from the chronic phase to the blastic phase. Although most individual telomeres are short, some of them are lengthened, and long individual telomeres occur non-randomly and might be associated with clonal selection. Telomerase is the main mechanism used to maintain telomere lengths, and its activity increases when CML evolves toward advanced stages. ALT might be another mechanism employed by CML cells to sustain the homeostasis of their telomere lengths and this mechanism seems predominant at the early stage of leukemogenesis. Also, telomerase and ALT might jointly act to maintain telomere lengths at the chronic phase, and as CML progresses, telomerase becomes the major mechanism. Finally, CML cells display an altered nuclear organization of their telomeres which is characterized by the presence of high number of telomeric aggregates, a feature of genomic instability, and differential positioning of telomeres. CML represents a good model to study mechanisms responsible for dynamic changes of individual telomere lengths and the remodeling of telomeric nuclear organization throughout cancer progression.
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Feng X, Luo Z, Jiang S, Li F, Han X, Hu Y, Wang D, Zhao Y, Ma W, Liu D, Huang J, Songyang Z. The telomere-associated homeobox-containing protein TAH1/HMBOX1 participates in telomere maintenance in ALT cells. J Cell Sci 2013; 126:3982-9. [PMID: 23813958 DOI: 10.1242/jcs.128512] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
The majority of cancer cells rely on elevated telomerase expression and activity for rapid growth and proliferation. Telomerase-negative cancer cells, by contrast, often employ the alternative lengthening of telomeres (ALT) pathway to maintain telomeres. ALT cells are characterized by long and dynamic telomeres and the presence of ALT-associated promyelocytic leukemia (PML) bodies (APBs). Previous work has shown the importance of APBs to the ALT pathway, but their formation and precise role remain unclear. Here, we demonstrate that a homeobox-containing protein known as HMBOX1 can directly bind telomeric double-stranded DNA and associate with PML nuclear bodies. Hence, we renamed this protein TAH1 for telomere-associated homeobox-containing protein 1. TAH1 knockdown significantly reduced the number of APBs and led to an increase in DNA damage response signals at telomeres. Importantly, TAH1 inhibition also notably reduced the presence of telomere C-circles, indicating altered ALT activity. Our findings point to TAH1 as a novel link between pathways that regulate DNA damage responses, PML nuclear bodies, and telomere homeostasis in ALT cells, and provide insight into how ALT cells may achieve sustained growth and proliferation independent of the telomerase.
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Affiliation(s)
- Xuyang Feng
- Key Laboratory of Gene Engineering of the Ministry of Education, School of Life Sciences, Sun Yat-Sen University, Guangzhou 510006, China
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Kong CM, Lee XW, Wang X. Telomere shortening in human diseases. FEBS J 2013; 280:3180-93. [PMID: 23647631 DOI: 10.1111/febs.12326] [Citation(s) in RCA: 130] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/29/2012] [Revised: 04/12/2013] [Accepted: 04/30/2013] [Indexed: 01/22/2023]
Abstract
The discovery of telomeres dates back to the early 20th century. In humans, telomeres are heterochromatic structures with tandem DNA repeats of 5'-TTAGGG-3' at the chromosomal ends. Telomere length varies greatly among species and ranges from 10 to 15 kb in humans. With each cell division, telomeres shorten progressively because of the 'end-replication problem'. Short or dysfunctional telomeres are often recognized as DNA DSBs, triggering cell-cycle arrest and result in cellular senescence or apoptotic cell death. Therefore, telomere shortening serves as an important tumor-suppressive mechanism by limiting cellular proliferative capacity by regulating senescence checkpoint activation. Although telomeres serve as a mitotic clock to cells, they also confer capping on chromosomes, with help from telomere-associated proteins. Over the past decades, many studies of telomere biology have demonstrated that telomeres and telomere-associated proteins are implicated in human genetic diseases. In addition, it has become more apparent that accelerated telomere erosion is associated with a myriad of metabolic and inflammatory diseases. Moreover, critically short or unprotected telomeres are likely to form telomeric fusions, leading to genomic instability, the cornerstone for carcinogenesis. In light of these, this minireview summarizes studies on telomeres and telomere-associated proteins in human diseases. Elucidating the roles of telomeres involved in the mechanisms underlying pathogenesis of these diseases may open up new possibilities for novel molecular targets as well as provide important diagnostic and therapeutic implications.
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Affiliation(s)
- Chiou Mee Kong
- Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
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35
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PML-mediated signaling and its role in cancer stem cells. Oncogene 2013; 33:1475-84. [PMID: 23563177 DOI: 10.1038/onc.2013.111] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2012] [Revised: 02/06/2013] [Accepted: 02/09/2013] [Indexed: 02/08/2023]
Abstract
The promyelocytic leukemia (PML) protein, initially discovered as a part of the PML/retinoic acid receptor alpha fusion protein, has been found to be a critical player in oncogenesis and tumor progression. Multiple cellular activities, including DNA repair, alternative lengthening of telomeres, transcriptional control, apoptosis and senescence, are regulated by PML and its featured subcellular structure, the PML nuclear body. In correspondence with its role in many important life processes, PML mediates several complex downstream signaling pathways. The determinant function of PML in tumorigenesis and cancer progression raises the interest in its involvement in cancer stem cells (CSCs), a subpopulation of cancer cells that share properties with stem cells and are critical for tumor propagation. Recently, there are exciting discoveries concerning the requirement of PML in CSC maintenance. Growing evidences strongly suggest a positive role of PML in regulating CSCs in both hematopoietic cancers and solid tumors, whereas the underlying mechanisms may be different and remain elusive. Here we summarize and discuss the PML-mediated signaling pathways in cancers and their potential roles in regulating CSCs.
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36
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Galati A, Micheli E, Cacchione S. Chromatin structure in telomere dynamics. Front Oncol 2013; 3:46. [PMID: 23471416 PMCID: PMC3590461 DOI: 10.3389/fonc.2013.00046] [Citation(s) in RCA: 54] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2012] [Accepted: 02/21/2013] [Indexed: 11/13/2022] Open
Abstract
The establishment of a specific nucleoprotein structure, the telomere, is required to ensure the protection of chromosome ends from being recognized as DNA damage sites. Telomere shortening below a critical length triggers a DNA damage response that leads to replicative senescence. In normal human somatic cells, characterized by telomere shortening with each cell division, telomere uncapping is a regulated process associated with cell turnover. Nevertheless, telomere dysfunction has also been associated with genomic instability, cell transformation, and cancer. Despite the essential role telomeres play in chromosome protection and in tumorigenesis, our knowledge of the chromatin structure involved in telomere maintenance is still limited. Here we review the recent findings on chromatin modifications associated with the dynamic changes of telomeres from protected to deprotected state and their role in telomere functions.
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Affiliation(s)
- Alessandra Galati
- Dipartimento di Biologia e Biotecnologie, Istituto Pasteur - Fondazione Cenci Bolognetti, Sapienza Università di Roma Rome, Italy
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37
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Chang FTM, McGhie JD, Chan FL, Tang MC, Anderson MA, Mann JR, Andy Choo KH, Wong LH. PML bodies provide an important platform for the maintenance of telomeric chromatin integrity in embryonic stem cells. Nucleic Acids Res 2013; 41:4447-58. [PMID: 23444137 PMCID: PMC3632112 DOI: 10.1093/nar/gkt114] [Citation(s) in RCA: 49] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
We have previously shown that α-thalassemia mental retardation X-linked (ATRX) and histone H3.3 are key regulators of telomeric chromatin in mouse embryonic stem cells. The function of ATRX and H3.3 in the maintenance of telomere chromatin integrity is further demonstrated by recent studies that show the strong association of ATRX/H3.3 mutations with alternative lengthening of telomeres in telomerase-negative human cancer cells. Here, we demonstrate that ATRX and H3.3 co-localize with the telomeric DNA and associated proteins within the promyelocytic leukemia (PML) bodies in mouse ES cells. The assembly of these telomere-associated PML bodies is most prominent at S phase. RNA interference (RNAi)-mediated knockdown of PML expression induces the disassembly of these nuclear bodies and a telomere dysfunction phenotype in mouse ES cells. Loss of function of PML bodies in mouse ES cells also disrupts binding of ATRX/H3.3 and proper establishment of histone methylation pattern at the telomere. Our study demonstrates that PML bodies act as epigenetic regulators by serving as platforms for the assembly of the telomeric chromatin to ensure a faithful inheritance of epigenetic information at the telomere.
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Affiliation(s)
- Fiona T M Chang
- Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria 3800, Australia
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38
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Rezazadeh S. On BLM helicase in recombination-mediated telomere maintenance. Mol Biol Rep 2012; 40:3049-64. [DOI: 10.1007/s11033-012-2379-0] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2012] [Accepted: 12/17/2012] [Indexed: 11/29/2022]
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39
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Hao SY, Yu JC. Shelterin complex and digestive system tumor. Shijie Huaren Xiaohua Zazhi 2012; 20:3124-3129. [DOI: 10.11569/wcjd.v20.i32.3124] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
The Shelterin complex is the crucial components of telomere binding proteins. The regulation of this complex, together with telomerase and the alterative lengthening of telomeres (ALT mechanism), plays a critical role in maintaining telomere functions. Telomeres are DNA-protein complexes that contain short repeat sequences added on to the ends of chromosome by the telomerase for protecting the ends of chromosome and preventing chromosome fusion. The loss of protective function of telomeres is closely related to genome instability, and this is the molecular basis for tumor development. Thus, telomeres play key roles in the process of malignant tumor development. Many studies have shown that telomere binding proteins are associated with gastric, colorectal and liver cancers, and other digestive system tumors. This review will focus on the role of the shelterin complex in digestive system neoplasms to provide an insight into prevention and targeted therapy of these malignancies.
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40
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Hsu JK, Lin T, Tsai RYL. Nucleostemin prevents telomere damage by promoting PML-IV recruitment to SUMOylated TRF1. ACTA ACUST UNITED AC 2012; 197:613-24. [PMID: 22641345 PMCID: PMC3365494 DOI: 10.1083/jcb.201109038] [Citation(s) in RCA: 43] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
A novel telomere-protection mechanism relies upon nucleostemin-mediated recruitment of PML-IV to telomeres and subsequent recruitment of RAD51 in both ALT and telomerase-active cells. Continuously dividing cells must be protected from telomeric and nontelomeric DNA damage in order to maintain their proliferative potential. Here, we report a novel telomere-protecting mechanism regulated by nucleostemin (NS). NS depletion increased the number of telomere damage foci in both telomerase-active (TA+) and alternative lengthening of telomere (ALT) cells and decreased the percentage of damaged telomeres associated with ALT-associated PML bodies (APB) and the number of APB in ALT cells. Mechanistically, NS could promote the recruitment of PML-IV to SUMOylated TRF1 in TA+ and ALT cells. This event was stimulated by DNA damage. Supporting the importance of NS and PML-IV in telomere protection, we demonstrate that loss of NS or PML-IV increased the frequency of telomere damage and aberration, reduced telomeric length, and perturbed the TRF2ΔBΔM-induced telomeric recruitment of RAD51. Conversely, overexpression of either NS or PML-IV protected ALT and TA+ cells from telomere damage. This work reveals a novel mechanism in telomere protection.
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Affiliation(s)
- Joseph K Hsu
- Center for Cancer and Stem Cell Biology, Institute of Biosciences and Technology, Texas A&M Health Science Center, Houston, TX 77030, USA
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41
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Chung I, Osterwald S, Deeg KI, Rippe K. PML body meets telomere: the beginning of an ALTernate ending? Nucleus 2012; 3:263-75. [PMID: 22572954 PMCID: PMC3414403 DOI: 10.4161/nucl.20326] [Citation(s) in RCA: 75] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023] Open
Abstract
The unlimited proliferation potential of cancer cells requires the maintenance of their telomeres. This is frequently accomplished by reactivation of telomerase. However, in a significant fraction of tumors an alternative lengthening of telomeres (ALT) mechanism is active. The molecular mechanism of the ALT pathway remains elusive. In particular, the role of characteristic complexes of promyelocytic leukemia nuclear bodies (PML-NBs) with telomeres, the ALT-associated PML-NBs (APBs), is currently under investigation. Here, we review recent findings on the assembly, structure and functions of APBs. It is discussed how genomic aberrations in ALT-positive cancer cells could result in the formation of APBs and in ALT activity. We conclude that they are important functional intermediates in what is considered the canonical ALT pathway and discuss deregulations of cellular pathways that contribute to the emergence of the ALT phenotype.
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Affiliation(s)
- Inn Chung
- Research Group Genome Organization & Function, Deutsches Krebsforschungszentrum (DKFZ) and BioQuant, Heidelberg, Germany
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42
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Slatter TL, Tan X, Yuen YC, Gunningham S, Ma SS, Daly E, Packer S, Devenish C, Royds JA, Hung NA. The alternative lengthening of telomeres pathway may operate in non-neoplastic human cells. J Pathol 2012; 226:509-18. [DOI: 10.1002/path.2981] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2010] [Revised: 07/27/2011] [Accepted: 08/04/2011] [Indexed: 01/20/2023]
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43
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Warren DT, Shanahan CM. Defective DNA-damage repair induced by nuclear lamina dysfunction is a key mediator of smooth muscle cell aging. Biochem Soc Trans 2011; 39:1780-5. [PMID: 22103525 DOI: 10.1042/bst20110703] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
Accumulation of DNA damage is a major driving force of normal cellular aging and has recently been demonstrated to hasten the development of vascular diseases such as atherosclerosis. VSMCs (vascular smooth muscle cells) are essential for vessel wall integrity and repair, and maintenance of their proliferative capacity is essential for vascular health. The signalling pathways that determine VSMC aging remain poorly defined; however, recent evidence implicates persistent DNA damage and the A-type nuclear lamins as key regulators of this process. In the present review, we discuss the importance of the nuclear lamina in the spatial organization of nuclear signalling events, including the DNA-damage response. In particular, we focus on the evidence suggesting that prelamin A accumulation interferes with nuclear spatial compartmentalization by disrupting chromatin organization and DNA-damage repair pathways to promote VSMC aging and senescence.
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Affiliation(s)
- Derek T Warren
- BHF Centre of Research Excellence, Cardiovascular Division, King's College London, James Black Centre, 125 Coldharbour Lane, London SE5 9NU, UK.
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44
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Chung I, Leonhardt H, Rippe K. De novo assembly of a PML nuclear subcompartment occurs through multiple pathways and induces telomere elongation. J Cell Sci 2011; 124:3603-18. [PMID: 22045732 DOI: 10.1242/jcs.084681] [Citation(s) in RCA: 69] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
Telomerase-negative tumor cells use an alternative lengthening of telomeres (ALT) pathway that involves DNA recombination and repair to maintain their proliferative potential. The cytological hallmark of this process is the accumulation of promyelocytic leukemia (PML) nuclear protein at telomeric DNA to form ALT-associated PML bodies (APBs). Here, the de novo formation of a telomeric PML nuclear subcompartment was investigated by recruiting APB protein components. We show that functionally distinct proteins were able to initiate the formation of bona fide APBs with high efficiency in a self-organizing and self-propagating manner. These included: (1) PML and Sp100 as the constituting components of PML nuclear bodies, (2) telomere repeat binding factors 1 and 2 (TRF1 and TRF2, respectively), (3) the DNA repair protein NBS1 and (4) the SUMO E3 ligase MMS21, as well as the isolated SUMO1 domain, through an interacting domain of another protein factor. By contrast, the repair factors Rad9, Rad17 and Rad51 were less efficient in APB nucleation but were recruited to preassembled APBs. The artificially created APBs induced telomeric extension through a DNA repair mechanism, as inferred from their colocalization with sites of non-replicative DNA synthesis and histone H2A.X phosphorylation, and an increase of the telomere repeat length. These activities were absent after recruitment of the APB factors to a pericentric locus and establish APBs as functional intermediates of the ALT pathway.
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Affiliation(s)
- Inn Chung
- German Cancer Research Center & BioQuant, Research Group Genome Organization & Function, Im Neuenheimer Feld 280, 69120 Heidelberg, Germany
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45
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Osterwald S, Wörz S, Reymann J, Sieckmann F, Rohr K, Erfle H, Rippe K. A three-dimensional colocalization RNA interference screening platform to elucidate the alternative lengthening of telomeres pathway. Biotechnol J 2011; 7:103-16. [DOI: 10.1002/biot.201000474] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2010] [Revised: 03/03/2011] [Accepted: 04/04/2011] [Indexed: 01/17/2023]
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46
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Grach AA. Alternative telomere-lengthening mechanisms. CYTOL GENET+ 2011. [DOI: 10.3103/s0095452711020046] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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47
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Jiang WQ, Nguyen A, Cao Y, Chang ACM, Reddel RR. HP1-mediated formation of alternative lengthening of telomeres-associated PML bodies requires HIRA but not ASF1a. PLoS One 2011; 6:e17036. [PMID: 21347226 PMCID: PMC3039646 DOI: 10.1371/journal.pone.0017036] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2010] [Accepted: 01/11/2011] [Indexed: 12/20/2022] Open
Abstract
Approximately 10% of cancers use recombination-mediated Alternative Lengthening of Telomeres (ALT) instead of telomerase to prevent telomere shortening. A characteristic of cells that utilize ALT is the presence of ALT-associated PML nuclear bodies (APBs) containing (TTAGGG)n DNA, telomere binding proteins, DNA recombination proteins, and heterochromatin protein 1 (HP1). The function of APBs is unknown and it is possible that they are functionally heterogeneous. Most ALT cells lack functional p53, and restoration of the p53/p21 pathway in these cells results in growth arrest/senescence and a substantial increase in the number of large APBs that is dependent on two HP1 isoforms, HP1α and HP1γ. Here we investigated the mechanism of HP1-mediated APB formation, and found that histone chaperones, HIRA and ASF1a, are present in APBs following activation of the p53/p21 pathway in ALT cells. HIRA and ASF1a were also found to colocalize inside PML bodies in normal fibroblasts approaching senescence, providing evidence for the existence of a senescence-associated ASF1a/HIRA complex inside PML bodies, consistent with a role for these proteins in induction of senescence in both normal and ALT cells. Moreover, knockdown of HIRA but not ASF1a significantly reduced p53-mediated induction of large APBs, with a concomitant reduction of large HP1 foci. We conclude that HIRA, in addition to its physical and functional association with ASF1a, plays a unique, ASF1a-independent role, which is required for the localization of HP1 to PML bodies and thus for APB formation.
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Affiliation(s)
- Wei-Qin Jiang
- Cancer Research Unit, Children's Medical Research Institute, Westmead, New South Wales, Australia
- Sydney Medical School, University of Sydney, Sydney, New South Wales, Australia
| | - Akira Nguyen
- Cancer Research Unit, Children's Medical Research Institute, Westmead, New South Wales, Australia
- Sydney Medical School, University of Sydney, Sydney, New South Wales, Australia
| | - Ying Cao
- Cancer Research Unit, Children's Medical Research Institute, Westmead, New South Wales, Australia
- Sydney Medical School, University of Sydney, Sydney, New South Wales, Australia
| | - Andy C.-M. Chang
- Cancer Research Unit, Children's Medical Research Institute, Westmead, New South Wales, Australia
- Sydney Medical School, University of Sydney, Sydney, New South Wales, Australia
| | - Roger R. Reddel
- Cancer Research Unit, Children's Medical Research Institute, Westmead, New South Wales, Australia
- Sydney Medical School, University of Sydney, Sydney, New South Wales, Australia
- * E-mail:
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48
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Slatter T, Gifford-Garner J, Wiles A, Tan X, Chen YJ, MacFarlane M, Sullivan M, Royds J, Hung N. Pilocytic astrocytomas have telomere-associated promyelocytic leukemia bodies without alternatively lengthened telomeres. THE AMERICAN JOURNAL OF PATHOLOGY 2010; 177:2694-700. [PMID: 21037079 DOI: 10.2353/ajpath.2010.100468] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
Abstract
Telomere maintenance by either telomerase activity or the recombination-mediated alternative lengthening of telomeres (ALT) mechanism is a hallmark of cancer. Tumors that use ALT as their telomere maintenance mechanism are characterized by long telomeres of great heterogeneity in length and by specific nuclear structures of co-localized promyelocytic leukemia protein and telomere DNA, called ALT-associated promyelocytic leukemia bodies (APBs). Recent advances have revealed a direct role for APBs in telomere recombination in ALT-positive cells. In this study, we investigated the possibility that APBs could occur before the long 'alternatively' lengthened telomeres arise, particularly in low-grade tumors. We measured APBs, telomere length, and telomerase activity in 64 astrocytomas inclusive of grade 1-4 tumors. Almost all grade 1-3 tumors (93%) were APB-positive using published criteria. Grade 2-3 APB-positive tumors also had long telomeres and were confirmed as ALT positive. However, grade 1 tumors lacked long telomeres and were therefore classified as ALT negative, but positive for telomere-associated promyelocytic leukemia bodies (TPB). This is the first report of a TPB-positive but ALT-negative tumor, and suggests that low-grade tumors have the foundation for recombinational telomere repair, as in ALT. Further work is warranted to characterize the TPB-positive phenotype in other early malignancies, as well as to determine whether TPBs predispose to telomere maintenance by ALT.
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Affiliation(s)
- Tania Slatter
- Department of Pathology, Dunedin School of Medicine, PO Box 913, University of Otago, Dunedin, New Zealand
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49
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Wong LH. Epigenetic regulation of telomere chromatin integrity in pluripotent embryonic stem cells. Epigenomics 2010; 2:639-55. [DOI: 10.2217/epi.10.49] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Telomeres are protective chromosomal structures highly conserved from primitive organisms to humans. The evolutionary conservation of telomere DNA implicates the importance of telomeric structure for basic cellular functions. Loss of telomere function causes chromosomal fusion, activation of DNA damage checkpoint responses, genome instability and impaired stem cell function. In human cells, the telomeric chromatin consists of TTAGGG repeats associated with a complex of proteins known as Shelterin. It is also organized in nucleosomes enriched with epigenetic modifications of ‘closed’ or ‘silenced’ chromatin states, including DNA hypermethylation and trimethylation of H3K9 and H4K20. These heterochromatin marks serve as a higher-order level of control of telomere length and structural integrity. Recent studies have shown that the telomere nucleosome in pluripotent embryonic stem cells is characterized by a more ‘open’ chromatin state that switches to become more repressive during differentiation. Conversely, the reprogramming of adult somatic cells into induced pluripotent cells results in the switch in telomeric chromatin from a repressive to a more open embryonic stem cell-like state, coupled with the restoration of telomere length. These findings indicate that telomeric chromatin is dynamic and reprogrammable, and has a fundamental role in the maintenance of embryonic stem cell pluripotency.
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Affiliation(s)
- Lee H Wong
- Chromosome & Chromatin Research, Murdoch Children’s Research Institute, Flemington Road, Parkville, Victoria 3052, Australia
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50
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Sizing the ends: normal length of human telomeres. Ann Anat 2010; 192:284-91. [PMID: 20732797 DOI: 10.1016/j.aanat.2010.07.005] [Citation(s) in RCA: 83] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2010] [Accepted: 07/18/2010] [Indexed: 01/14/2023]
Abstract
The ends of human chromosomes are constituted of telomeres, a nucleoprotein complex. They are mainly formed by the entanglement of repeat DNA and telomeric and non-telomeric proteins. Telomeric sequences are lost in each cell division and this loss happens in vitro as well as in vivo. The diminution of telomere length over the cell cycle has led to the consideration of telomeres as a 'mitotic clock'. Telomere lengths are heterogeneous because they differ among tissues, cells, and chromosome arms. Cell proliferation capacity, cellular environment, and epigenetic factors are some elements that affect this telomere heterogeneity. Also, genetic and environmental factors modulate the difference in telomere lengths between individuals. Telomere length is regulated by telomere structure, telomerase, the enzyme that elongates the 3'-end of telomeres, and alternative lengthening of telomeres (ALT) used exclusively in immortalized and cancer cells. The understanding of telomere length dynamic in the normal population is essential to develop a deeper insight into the role of telomere function in pathological settings.
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