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Wood ML, Royle NJ. Chromosomally Integrated Human Herpesvirus 6: Models of Viral Genome Release from the Telomere and Impacts on Human Health. Viruses 2017; 9:E184. [PMID: 28704957 PMCID: PMC5537676 DOI: 10.3390/v9070184] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2017] [Revised: 06/30/2017] [Accepted: 07/05/2017] [Indexed: 12/11/2022] Open
Abstract
Human herpesvirus 6A and 6B, alongside some other herpesviruses, have the striking capacity to integrate into telomeres, the terminal repeated regions of chromosomes. The chromosomally integrated forms, ciHHV-6A and ciHHV-6B, are proposed to be a state of latency and it has been shown that they can both be inherited if integration occurs in the germ line. The first step in full viral reactivation must be the release of the integrated viral genome from the telomere and here we propose various models of this release involving transcription of the viral genome, replication fork collapse, and t-circle mediated release. In this review, we also discuss the relationship between ciHHV-6 and the telomere carrying the insertion, particularly how the presence and subsequent partial or complete release of the ciHHV-6 genome may affect telomere dynamics and the risk of disease.
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Affiliation(s)
- Michael L Wood
- Department of Genetics, University of Leicester, University Road, Leicester LE1 7RH, UK.
| | - Nicola J Royle
- Department of Genetics, University of Leicester, University Road, Leicester LE1 7RH, UK.
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2
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Dai X, Huang C, Chai W. CDK1 differentially regulates G-overhang generation at leading- and lagging-strand telomeres in telomerase-negative cells in G2 phase. Cell Cycle 2012; 11:3079-86. [PMID: 22871736 DOI: 10.4161/cc.21472] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
Human telomeres contain single-stranded 3' G-overhangs that function in telomere end protection and telomerase action. Previously we have demonstrated that multiple steps involving C-strand end resection, telomerase elongation and C-strand fill-in contribute to G-overhang generation in telomerase-positive cancer cells. However, how G-overhangs are generated in telomerase-negative human somatic cells is unknown. Here, we report that C-strand fill-in is present at lagging-strand telomeres in telomerase-negative human cells but not at leading-strand telomeres, suggesting that C-strand fill-in is independent of telomerase extension of G-strand. We further show that while cyclin-dependent kinase 1 (CDK1) positively regulates C-strand fill-in, CDK1 unlikely regulates G-overhang generation at leading-strand telomeres. In addition, DNA polymerase α (Polα) association with telomeres is not altered upon CDK1 inhibition, suggesting that CDK1 does not control the loading of Polα to telomeres during fill-in. In summary, our results reveal that G-overhang generation at leading- and lagging-strand telomeres are regulated by distinct mechanisms in human cells.
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Affiliation(s)
- Xueyu Dai
- School of Molecular Biosciences, Washington State University, Spokane, WA, USA
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Faucher D, Wellinger RJ. Methylated H3K4, a transcription-associated histone modification, is involved in the DNA damage response pathway. PLoS Genet 2010; 6:e1001082. [PMID: 20865123 PMCID: PMC2928815 DOI: 10.1371/journal.pgen.1001082] [Citation(s) in RCA: 115] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2010] [Accepted: 07/22/2010] [Indexed: 01/25/2023] Open
Abstract
Eukaryotic genomes are associated with a number of proteins such as histones that constitute chromatin. Post-translational histone modifications are associated with regulatory aspects executed by chromatin and all transactions on genomic DNA are dependent on them. Thus, it will be relevant to understand how histone modifications affect genome functions. Here we show that the mono ubiquitylation of histone H2B and the tri-methylation of histone H3 on lysine 4 (H3K4me3), both known for their involvement in transcription, are also important for a proper response of budding yeast cells to DNA damaging agents and the passage through S-phase. Cells that cannot methylate H3K4 display a defect in double-strand break (DSB) repair by non-homologous end joining. Furthermore, if such cells incur DNA damage or encounter a stress during replication, they very rapidly lose viability, underscoring the functional importance of the modification. Remarkably, the Set1p methyltransferase as well as the H3K4me3 mark become detectable on a newly created DSB. This recruitment of Set1p to the DSB is dependent on the presence of the RSC complex, arguing for a contribution in the ensuing DNA damage repair process. Taken together, our results demonstrate that Set1p and its substrate H3K4me3, which has been reported to be important for the transcription of active genes, also plays an important role in genome stability of yeast cells. Given the high degree of conservation for the methyltransferase and the histone mark in a broad variety of organisms, these results could have similar implications for genome stability mechanisms in vertebrate and mammalian cells.
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Affiliation(s)
- David Faucher
- Department of Microbiology and Infectious Diseases, Faculty of Medicine, Université de Sherbrooke, Sherbrooke, Québec, Canada
| | - Raymund J. Wellinger
- Department of Microbiology and Infectious Diseases, Faculty of Medicine, Université de Sherbrooke, Sherbrooke, Québec, Canada
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Paeschke K, McDonald KR, Zakian VA. Telomeres: structures in need of unwinding. FEBS Lett 2010; 584:3760-72. [PMID: 20637196 DOI: 10.1016/j.febslet.2010.07.007] [Citation(s) in RCA: 64] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2010] [Revised: 06/25/2010] [Accepted: 07/02/2010] [Indexed: 11/26/2022]
Abstract
Telomeres protect the ends of eukaryotic chromosomes from being recognized and processed as double strand breaks. In most organisms, telomeric DNA is highly repetitive with a high GC-content. Moreover, the G residues are concentrated in the strand running 3'-5' from the end of the chromosome towards its center. This G-rich strand is extended to form a 3' single-stranded tail that can form unusual secondary structures such as T-loops and G-quadruplex DNA. Both the duplex repeats and the single-stranded G-tail are assembled into stable protein-DNA complexes. The unique architecture, high GC content, and multi-protein association create particularly stable protein-DNA complexes that are a challenge for replication, recombination, and transcription. Helicases utilize the energy of nucleotide hydrolysis to unwind base paired nucleic acids and, in some cases, to displace proteins from them. The telomeric functions of helicases from the RecQ, Pifl, FANCJ, and DNA2 families are reviewed in this article. We summarize data showing that perturbation of their telomere activities can lead to telomere dysfunction and genome instability and in some cases human disease.
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Affiliation(s)
- Katrin Paeschke
- Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA
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5
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Arnoult N, Schluth-Bolard C, Letessier A, Drascovic I, Bouarich-Bourimi R, Campisi J, Kim SH, Boussouar A, Ottaviani A, Magdinier F, Gilson E, Londoño-Vallejo A. Replication timing of human telomeres is chromosome arm-specific, influenced by subtelomeric structures and connected to nuclear localization. PLoS Genet 2010; 6:e1000920. [PMID: 20421929 PMCID: PMC2858680 DOI: 10.1371/journal.pgen.1000920] [Citation(s) in RCA: 79] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2009] [Accepted: 03/22/2010] [Indexed: 12/12/2022] Open
Abstract
The mechanisms governing telomere replication in humans are still poorly understood. To fill this gap, we investigated the timing of replication of single telomeres in human cells. Using in situ hybridization techniques, we have found that specific telomeres have preferential time windows for replication during the S-phase and that these intervals do not depend upon telomere length and are largely conserved between homologous chromosomes and between individuals, even in the presence of large subtelomeric segmental polymorphisms. Importantly, we show that one copy of the 3.3 kb macrosatellite repeat D4Z4, present in the subtelomeric region of the late replicating 4q35 telomere, is sufficient to confer both a more peripheral localization and a later-replicating property to a de novo formed telomere. Also, the presence of β-satellite repeats next to a newly created telomere is sufficient to delay its replication timing. Remarkably, several native, non-D4Z4–associated, late-replicating telomeres show a preferential localization toward the nuclear periphery, while several early-replicating telomeres are associated with the inner nuclear volume. We propose that, in humans, chromosome arm–specific subtelomeric sequences may influence both the spatial distribution of telomeres in the nucleus and their replication timing. Functional telomeres are essential for genome stability. While replication of telomeres has been extensively studied in model organisms such as the baker's yeast, little is known about the mechanisms that govern the replication of human telomeres. In this study, we have determined the timing of replication of telomeres of individual human chromosomes and its association with potential modulating factors such as particular subtelomeric sequences, the presence of heterochromatic regions, and nuclear localization. We have found that native telomeres associated with D4Z4 sequences—a macrosatellite naturally located in the subtelomeric regions of 4q, 10q, and acrocentric chromosome extremities—replicate later than others. We also present descriptive and experimental evidence indicating that nuclear localization influences the timing of telomere replication. These results contribute to our understanding of telomere metabolism in humans.
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Affiliation(s)
- Nausica Arnoult
- Telomeres and Cancer Laboratory, Institut Curie, CNRS, UPMC University Paris 06, Paris, France
| | - Caroline Schluth-Bolard
- Epigenetics and Telomere Regulation, CNRS ENS UCBL IFR128, Ecole Normale Supérieure de Lyon, Lyon, France
| | - Anne Letessier
- Functional Organization and Plasticity of Mammalian Genomes, Institut Curie, UPMC University Paris 06, Paris, France
| | - Irena Drascovic
- Telomeres and Cancer Laboratory, Institut Curie, CNRS, UPMC University Paris 06, Paris, France
| | | | - Judith Campisi
- Lawrence Berkeley Laboratory, Berkeley, California, United States of America
- Buck Institute for Age Research, Novato, California, United States of America
| | - Sahn-ho Kim
- Lawrence Berkeley Laboratory, Berkeley, California, United States of America
| | - Amina Boussouar
- Epigenetics and Telomere Regulation, CNRS ENS UCBL IFR128, Ecole Normale Supérieure de Lyon, Lyon, France
| | - Alexandre Ottaviani
- Epigenetics and Telomere Regulation, CNRS ENS UCBL IFR128, Ecole Normale Supérieure de Lyon, Lyon, France
| | - Frédérique Magdinier
- Epigenetics and Telomere Regulation, CNRS ENS UCBL IFR128, Ecole Normale Supérieure de Lyon, Lyon, France
| | - Eric Gilson
- Epigenetics and Telomere Regulation, CNRS ENS UCBL IFR128, Ecole Normale Supérieure de Lyon, Lyon, France
| | - Arturo Londoño-Vallejo
- Telomeres and Cancer Laboratory, Institut Curie, CNRS, UPMC University Paris 06, Paris, France
- * E-mail:
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Kurth I, Gautier J. Origin-dependent initiation of DNA replication within telomeric sequences. Nucleic Acids Res 2009; 38:467-76. [PMID: 19906732 PMCID: PMC2811021 DOI: 10.1093/nar/gkp929] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
Replication of telomeres requires the action of telomerase, the semi-conservative replication machinery and the stabilization of the replication fork during passage through telomeric DNA. Whether vertebrate telomeres support initiation of replication has not been experimentally addressed. Using Xenopus cell free extracts we established a system to study replication initiation within linear telomeric DNA substrates. We show binding of TRF2 to telomeric DNA, indicating that exogenous DNA exclusively composed of telomeric repeats is recognized by shelterin components. Interaction with telomere binding proteins is not sufficient to prevent a DNA damage response. Notably, we observe regulated assembly of the pre-replicative complex proteins ORC2, MCM6 and Cdc6 to telomeric DNA. Most importantly, we detect origin-dependent replication of telomeric substrates under conditions that inhibit checkpoint activation. These results indicate that pre-replicative complexes assemble within telomeric DNA and can be converted into functional origins.
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Affiliation(s)
- Isabel Kurth
- Institute for Cancer Genetics, Department of Genetics and Development and Herbert Irving Comprehensive Cancer Center, Columbia University Medical Center, New York, NY 10032, USA
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Cooley C, Baird KM, Faure V, Wenner T, Stewart JL, Modino S, Slijepcevic P, Farr CJ, Morrison CG. Trf1 is not required for proliferation or functional telomere maintenance in chicken DT40 cells. Mol Biol Cell 2009; 20:2563-71. [PMID: 19321665 DOI: 10.1091/mbc.e08-10-1019] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
The telomere end-protection complex prevents the ends of linear eukaryotic chromosomes from degradation or inappropriate DNA repair. The homodimeric double-stranded DNA-binding protein, Trf1, is a component of this complex and is essential for mouse embryonic development. To define the requirement for Trf1 in somatic cells, we deleted Trf1 in chicken DT40 cells by gene targeting. Trf1-deficient cells proliferated as rapidly as control cells and showed telomeric localization of Trf2, Rap1, and Pot1. Telomeric G-strand overhang lengths were increased in late-passage Trf1-deficient cells, although telomere lengths were unaffected by Trf1 deficiency, as determined by denaturing Southern and quantitative FISH analysis. Although we observed some clonal variation in terminal telomere fragment lengths, this did not correlate with cellular Trf1 levels. Trf1 was not required for telomere seeding, indicating that de novo telomere formation can proceed without Trf1. The Pin2 isoform and a novel exon 4, 5-deleted isoform localized to telomeres in Trf1-deficient cells. Trf1-deficient cells were sensitive to DNA damage induced by ionizing radiation. Our data demonstrate that chicken DT40 B cells do not require Trf1 for functional telomere structure and suggest that Trf1 may have additional, nontelomeric roles involved in maintaining genome stability.
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Affiliation(s)
- Carol Cooley
- Centre for Chromosome Biology, National University of Ireland Galway, Department of Biochemistry and NCBES, Galway, Ireland
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Chavez A, Tsou AM, Johnson FB. Telomeres do the (un)twist: helicase actions at chromosome termini. Biochim Biophys Acta Mol Basis Dis 2009; 1792:329-40. [PMID: 19245831 DOI: 10.1016/j.bbadis.2009.02.008] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2008] [Revised: 02/12/2009] [Accepted: 02/12/2009] [Indexed: 12/17/2022]
Abstract
Telomeres play critical roles in protecting genome stability, and their dysfunction contributes to cancer and age-related degenerative diseases. The precise architecture of telomeres, including their single-stranded 3' overhangs, bound proteins, and ability to form unusual secondary structures such as t-loops, is central to their function and thus requires careful processing by diverse factors. Furthermore, telomeres provide unique challenges to the DNA replication and recombination machinery, and are particularly suited for extension by the telomerase reverse transcriptase. Helicases use the energy from NTP hydrolysis to track along DNA and disrupt base pairing. Here we review current findings concerning how helicases modulate several aspects of telomere form and function.
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Affiliation(s)
- Alejandro Chavez
- Department of Pathology and Laboratory Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA
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Faure V, Wenner T, Cooley C, Bourke E, Farr CJ, Takeda S, Morrison CG. Ku70 prevents genome instability resulting from heterozygosity of the telomerase RNA component in a vertebrate tumour line. DNA Repair (Amst) 2008; 7:713-24. [DOI: 10.1016/j.dnarep.2008.01.008] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2007] [Accepted: 01/16/2008] [Indexed: 10/22/2022]
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10
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Abstract
The replication of the ends of linear chromosomes, or telomeres, poses unique problems, which must be solved to maintain genome integrity and to allow cell division to occur. Here, we describe and compare the timing and specific mechanisms that are required to initiate, control and coordinate synthesis of the leading and lagging strands at telomeres in yeasts, ciliates and mammals. Overall, it emerges that telomere replication relies on a strong synergy between the conventional replication machinery, telomere protection systems, DNA-damage-response pathways and chromosomal organization.
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Affiliation(s)
- Eric Gilson
- Laboratoire de Biologie Moléculaire et Cellulaire, UMR5239, IFR 128, Centre National de la Recherche Scientifique, University Lyon 1, Faculty of Medicine Lyon-Sud, Hospices Civils de Lyon, Ecole Normale Supérieure de Lyon,France.
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Verdun RE, Karlseder J. The DNA damage machinery and homologous recombination pathway act consecutively to protect human telomeres. Cell 2006; 127:709-20. [PMID: 17110331 DOI: 10.1016/j.cell.2006.09.034] [Citation(s) in RCA: 234] [Impact Index Per Article: 13.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2006] [Revised: 07/26/2006] [Accepted: 09/29/2006] [Indexed: 10/23/2022]
Abstract
Telomeres protect chromosome ends from being detected as lesions and from triggering DNA damage checkpoints. Paradoxically, telomere function depends on checkpoint proteins such as ATM and ATR, but a molecular model explaining this seemingly contradictory relationship has been missing so far. Here we show that the DNA damage machinery acts on telomeres in at least two independent steps. First, the ATR-dependent machinery is recruited to telomeres before telomere replication is completed, likely in response to single-stranded DNA resulting from replication fork stalling. Second, after replication, telomeres attract ATM and the homologous recombination (HR) machinery. In vivo and in vitro results suggest that the HR machinery is required for formation of a telomere-specific structure at chromosome ends after replication. Our results suggest that telomere ends need to be recognized as DNA damage to complete end replication and to acquire a structure that is essential for function.
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Affiliation(s)
- Ramiro E Verdun
- The Salk Institute for Biological Studies, 10010 North Torrey Pines Rd., La Jolla, CA 92037, USA
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Verdun RE, Crabbe L, Haggblom C, Karlseder J. Functional human telomeres are recognized as DNA damage in G2 of the cell cycle. Mol Cell 2006; 20:551-61. [PMID: 16307919 DOI: 10.1016/j.molcel.2005.09.024] [Citation(s) in RCA: 197] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2005] [Revised: 09/08/2005] [Accepted: 09/29/2005] [Indexed: 01/13/2023]
Abstract
Telomeres have to be distinguished from DNA breaks that initiate a DNA damage response. Proteins involved in the DNA damage response have previously been found at telomeres in transformed cells; however, the importance of these factors for telomere function has not been understood. Here, we show that telomeres of telomerase-negative primary cells recruit Mre11, phosphorylated NBS1, and ATM in every G2 phase of the cell cycle. This recruitment correlates with a partial release of telomeric POT1; moreover, telomeres were found to be accessible to modifying enzymes at this time in the cell cycle, suggesting that they are unprotected. Degradation of the MRN complex, as well as inhibition of ATM, led to telomere dysfunction. Consequentially, we propose that a localized DNA damage response at telomeres after replication is essential for recruiting the processing machinery that promotes formation of a chromosome end protection complex.
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Affiliation(s)
- Ramiro E Verdun
- The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, California 92037, USA
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Abstract
A combination of classical genetic, biochemical, and molecular biological approaches have generated a rather detailed understanding of the structure and function of Saccharomyces telomeres. Yeast telomeres are essential to allow the cell to distinguish intact from broken chromosomes, to protect the end of the chromosome from degradation, and to facilitate the replication of the very end of the chromosome. In addition, yeast telomeres are a specialized site for gene expression in that the transcription of genes placed near them is reversibly repressed. A surprisingly large number of genes have been identified that influence either telomere structure or telomere function (or both), although in many cases the mechanism of action of these genes is poorly understood. This article reviews the recent literature on telomere biology and highlights areas for future research.
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Affiliation(s)
- V A Zakian
- Department of Molecular Biology, Princeton University, New Jersey 08544-1014, USA
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