Functional links between telomeres and proteins of the DNA-damage response

The linkage of chromatin remodeling to genome maintenance: contribution from a human disease gene BRIT1/MCPH1

Genomic DNA is packed into a highly condensed chromatin structure, which acts as natural barrier preventing accessibility of DNA. In various processes to maintain genomic integrity such as DNA replication, DNA repair, telomere regulation, proteins need to overcome the barrier of condensed chromatin to gain access to DNA. ATP-dependent chromatin remodeling is one of the fundamental mechanisms used by cells to relax chromatin. However, the chromatin remodeling complex does not contain intrinsic specificity for particular nuclear process, and the mechanism mediating its recruitment to DNA lesions remains to be an outstanding question. To address this question, in this review, we will discuss our current findings and future perspectives about how BRIT1/MCPH1, a human disease gene, specifies the function of chromatin remodelers and links chromatin remodeling to genome maintenance.

The MRT-1 nuclease is required for DNA crosslink repair and telomerase activity in vivo in Caenorhabditis elegans

The telomerase reverse transcriptase adds de novo DNA repeats to chromosome termini. Here we define Caenorhabditis elegans MRT-1 as a novel factor required for telomerase-mediated telomere replication and the DNA-damage response. MRT-1 is composed of an N-terminal domain homologous to the second OB-fold of POT1 telomere-binding proteins and a C-terminal SNM1 family nuclease domain, which confer single-strand DNA-binding and processive 3'-to-5' exonuclease activity, respectively. Furthermore, telomerase activity in vivo depends on a functional MRT-1 OB-fold. We show that MRT-1 acts in the same telomere replication pathway as telomerase and the 9-1-1 DNA-damage response complex. MRT-1 is dispensable for DNA double-strand break repair, but functions with the 9-1-1 complex to promote DNA interstrand cross-link (ICL) repair. Our data reveal MRT-1 as a dual-domain protein required for telomerase function and ICL repair, which raises the possibility that telomeres and ICL lesions may share a common feature that plays a critical role in de novo telomere repeat addition.

Human RAP1 inhibits non-homologous end joining at telomeres

Telomeres, the nucleoprotein structures at the ends of linear chromosomes, promote genome stability by distinguishing chromosome termini from DNA double-strand breaks (DSBs). Cells possess two principal pathways for DSB repair: homologous recombination and non-homologous end joining (NHEJ). Several studies have implicated TRF2 in the protection of telomeres from NHEJ, but the underlying mechanism remains poorly understood. Here, we show that TRF2 inhibits NHEJ, in part, by recruiting human RAP1 to telomeres. Heterologous targeting of hRAP1 to telomeric DNA was sufficient to bypass the need for TRF2 in protecting telomeric DNA from NHEJ in vitro. On expanding these studies in cells, we find that recruitment of hRAP1 to telomeres prevents chromosome fusions caused by the loss of TRF2/hRAP1 from chromosome ends despite activation of a DNA damage response. These results provide the first evidence that hRAP1 inhibits NHEJ at mammalian telomeres and identify hRAP1 as a mediator of genome stability.

Large liver cell change in hepatitis B virus-related liver cirrhosis

Large liver cell change (LLCC) refers to microscopic lesions often found in various chronic liver diseases; however, its nature is still controversial. Thirty-four formalin-fixed and 19 fresh frozen hepatitis B virus (HBV)-related cirrhosis samples were examined for the presence of LLCC, small liver cell change (SLCC), and hepatocellular carcinoma (HCC). The cell cycle checkpoint status (p21, p27, p16, Tp53), cell dynamics (proliferating cell nuclear antigen, Ki-67, terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling, M30), DNA damage (gamma-H2AX [H2A histone family, member X]), telomere lengths, chromosomal instability (micronuclei index), and senescence-associated beta-galactosidase (SA-beta-Gal) activity were evaluated using an in situ approach and compared to those in normal liver (n = 5) and liver with chronic cholestasis (34 cases of hepatolithiasis and three cases of primary biliary cirrhosis). In HBV-related cirrhosis, the p21, p27, and p16 cell cycle checkpoint markers were activated in normal-looking cirrhotic hepatocytes (NLCH), but diminished gradually from LLCC, SLCC, to HCC, with an increase in Tp53 expression. There was a general decrease in telomere length from NLCH, LLCC, SLCC, to HCC. Micronuclei, gamma-H2AX foci, and net cellular gain were significantly increased from normal hepatocytes, NLCH, LLCC, SLCC, to HCC. The SA-beta-Gal activity was weaker in LLCC compared to NLCH and absent in SLCC and HCC. In contrast, cholestatic LLCC showed retained expression of cell cycle checkpoint markers and decreased net cellular gain compared to adjacent normal-looking hepatocytes. HBV-related LLCC showed significantly higher Tp53 labeling index, gamma-H2AX labeling index, and micronuclei index; shorter telomere length; decreased SA-beta-Gal activity; and increased net cellular gain compared to cholestatic LLCC.

CONCLUSION

The nature of LLCC is rather heterogeneous depending on the biological setting. The characteristics of HBV-related LLCC are more consistent with dysplastic rather than merely reactive hepatocytes, whereas cholestatic LLCC more likely represents reactive change with more stringent cell cycle checkpoint control.

POT1 association with TRF2 regulates telomere length

Deleting the OB folds encoding the telomeric single-stranded DNA (ssDNA)-binding activity of the human telomeric protein POT1 induces significant telomere elongation, suggesting that at least one critical aspect of the regulation of telomere length is disrupted by this POT1(DeltaOB) mutant protein. POT1 is known to associate with two proteins through the protein interaction domain retained in POT1(DeltaOB)-the telomeric double-stranded DNA-binding protein TRF2 and the telomere-associated protein TPP1. We report that introducing a mutation that reduces association of POT1 with TRF2, but not a mutation that reduces the association with TPP1, abrogates the ability of POT1(DeltaOB) to promote telomere elongation. Mechanistically, expression of POT1(DeltaOB) reduced the association of TRF2 with POT1, RAP1, and TIN2; however, of these proteins, only ectopic expression of POT1 suppressed the telomere elongation induced by POT1(DeltaOB). Lastly, replacing endogenous POT1 with a full-length POT1 mutant defective in the association with TRF2 induced telomere elongation. Thus, we conclude that the association of POT1 with both ssDNA and TRF2 is critical for telomere length homeostasis.

New molecular markers in resistant B-CLL

B-chronic lymphocytic leukemia (B-CLL) is characterized by a highly variable clinical course which has long remained a stumbling block for clinicians. This variability appears to arise from complex molecular alterations identified in malignant cells from patient subsets. Recent studies have focused in particular on identifying new molecular markers to help predict the most effective and adapted treatments. In addition to the mutation status of immunoglobulin variable heavy-chain region (IgVH) genes, which is a well-established predictive factor in B-CLL, these new markers include defects of cell factors involved in the maintenance of genome stability, such as telomere function, DNA repair, ATM and p53. Other predictive factors, such as tyrosine kinase Zap-70 and soluble factors found in patient sera, may be associated with B-cell receptor signal transduction. Interestingly, an alteration of these factors fits closely, though not strikingly, with the absence of somatic mutations in IgVH genes, suggesting that the latter may be due either to epigenetic events leading to an unstable genome or to an inherited defect in the immune response of malignant B-cells. Recent lessons from Zap-70 expression/phosphorylation suggest that some of these markers may reflect the defective pathways in B-CLL cells rather than being markers of cell malignancy per se. Furthermore, specific subsets of markers are found in patient cells resistant to treatment. Current studies on gene expression profiling and proteomic analyses should soon lead to a better understanding of how these pathways are affected, especially in multi-drug resistant B-CLL.

G-quadruplex nucleic acids as therapeutic targets

Nucleic acid sequences containing several short runs of guanine nucleotides can form complex higher order structures, termed quadruplexes. Their occurrence has been most extensively characterised at the telomeric ends of eukaryotic chromosomes, whose DNA comprises such sequences, and where the extreme 3' ends are single-stranded. This enables relatively facile formation of quadruplex arrangements under the influence of a quadruplex-selective small molecule to compete effectively with telomeric protein-DNA interactions. Occurrences of quadruplexes within the human and other genomes have been mapped by bioinformatics surveys, which have revealed over-representations in promoter regions, especially of genes involved in replication, such as oncogenes, as well as in 5'UTR regions. The highly distinctive nature of quadruplex topologies suggests that they can act as novel therapeutic targets, for example in the selective inhibition of transcription of a given oncogene, using designed small molecules to stabilise a particular quadruplex. This offers the prospect of an alternative to, for example, direct kinase targeting with small molecules, without the attendant issues of active-site resistance. We survey here the basis of these approaches, together with current progress, and discuss the mechanistic issues posed by quadruplex targeting.

Translational reprogramming following UVB irradiation is mediated by DNA-PKcs and allows selective recruitment to the polysomes of mRNAs encoding DNA repair enzymes

UVB-induced lesions in mammalian cellular DNA can, through the process of mutagenesis, lead to carcinogenesis. However, eukaryotic cells have evolved complex mechanisms of genomic surveillance and DNA damage repair to counteract the effects of UVB radiation. We show that following UVB DNA damage, there is an overall inhibition of protein synthesis and translational reprogramming. This reprogramming allows selective synthesis of DDR proteins, such as ERCC1, ERCC5, DDB1, XPA, XPD, and OGG1 and relies on upstream ORFs in the 5' untranslated region of these mRNAs. Experiments with DNA-PKcs-deficient cell lines and a specific DNA-PKcs inhibitor demonstrate that both the general repression of mRNA translation and the preferential translation of specific mRNAs depend on DNA-PKcs activity, and therefore our data establish a link between a key DNA damage signaling component and protein synthesis.

Chromatin remodelling beyond transcription: the INO80 and SWR1 complexes

Chromatin-modifying factors have essential roles in DNA processing pathways that dictate cellular functions. The ability of chromatin modifiers, including the INO80 and SWR1 chromatin-remodelling complexes, to regulate transcriptional processes is well established. However, recent studies reveal that the INO80 and SWR1 complexes have crucial functions in many other essential processes, including DNA repair, checkpoint regulation, DNA replication, telomere maintenance and chromosome segregation. During these diverse nuclear processes, the INO80 and SWR1 complexes function cooperatively with their histone substrates, gamma-H2AX and H2AZ. This research reveals that INO80 and SWR1 ATP-dependent chromatin remodelling is an integral component of pathways that maintain genomic integrity.

The shelterin protein TRF2 inhibits Chk2 activity at telomeres in the absence of DNA damage

The shelterin complex [1] shapes and protects telomeric DNA from being processed as double strand breaks (DSBs) [2, 3]. Here we show that in human undamaged cells, a fraction of the kinase Chk2, a downstream target of ATM and mediator of checkpoint responses and senescence [4, 5], physically interacts with the shelterin subunit TRF2 and colocalizes with this complex at chromosome ends. This interaction, enhanced by TRF2 binding to telomeric DNA, inhibits the activation and senescence-induced function of Chk2 by a mechanism in which TRF2 binding to the N terminus of Chk2 surrounding Thr68 hinders the phosphorylation of this priming site. In response to radiation-induced DSBs, but not chromatin-remodelling agents, the telomeric Chk2-TRF2 binding dissociates in a Chk2 activity-dependent manner. Moreover, active Chk2 phosphorylates TRF2 and decreases its binding to telomeric DNA repeats, corroborating the evidences on the specific TRF2 relocalization in presence of DSBs [6]. Altogether, the capacity of TRF2 to locally repress Chk2 provides an additional level of control by which shelterin restrains the DNA damage response from an unwanted activation [6, 7] and may explain why TRF2 overexpression acts as a telomerase-independent oncogenic stimulus [8].

Trf1 is not required for proliferation or functional telomere maintenance in chicken DT40 cells

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.

Genetic dissection of the mechanisms underlying telomere-associated diseases: impact of the TRF2 telomeric protein on mouse epidermal stem cells

TRF2 is a telomere-binding protein involved in the protection of chromosome ends. Interestingly, TRF2 is overexpressed in a number of human cancers. Mice with increased TRF2 expression (K5TRF2 mice) display a severe skin phenotype including an increase in skin cancer and premature skin degeneration, which includes increased skin hyperpigmentation and skin dryness; these pathologies are concomitant with dramatic telomere shortening and increased chromosomal instability. Here, we show that K5TRF2 mice have a severe epidermal stem cell (ESC) dysfunction, which is reversed by abrogation of p53 in the absence of rescue of telomere length. Importantly, p53 deletion also rescues severe skin hyperpigmentation in these mice through regulation of alpha-melanocyte-stimulating hormone (alpha-MSH). In addition, skin carcinogenesis is accelerated in K5TRF2/p53(-/-)mice owing to attenuated p21 induction, which enables cell proliferation to resume. Altogether, these results reveal the existence of a DNA damage-dependent checkpoint that acts on ESCs with critically short telomeres and restricts skin proliferation, thereby increasing protection against skin cancer; however, the checkpoint also leads to premature skin aging phenotypes. Finally, the results described here are relevant to our understanding of the pathobiology of those human diseases that are characterized by the presence of critically short telomeres (hereafter referred to as 'telopathies'), such as dyskeratosis congenita which causes severe skin phenotypes including skin hyperpigmentation and skin cancer.

ERCC1/XPF protects short telomeres from homologous recombination in Arabidopsis thaliana

Many repair and recombination proteins play essential roles in telomere function and chromosome stability, notwithstanding the role of telomeres in "hiding" chromosome ends from DNA repair and recombination. Among these are XPF and ERCC1, which form a structure-specific endonuclease known for its essential role in nucleotide excision repair and is the subject of considerable interest in studies of recombination. In contrast to observations in mammalian cells, we observe no enhancement of chromosomal instability in Arabidopsis plants mutated for either XPF (AtRAD1) or ERCC1 (AtERCC1) orthologs, which develop normally and show wild-type telomere length. However, in the absence of telomerase, mutation of either of these two genes induces a significantly earlier onset of chromosomal instability. This early appearance of telomere instability is not due to a general acceleration of telomeric repeat loss, but is associated with the presence of dicentric chromosome bridges and cytologically visible extrachromosomal DNA fragments in mitotic anaphase. Such extrachromosomal fragments are not observed in later-generation single-telomerase mutant plants presenting similar frequencies of anaphase bridges. Extensive FISH analyses show that these DNAs are broken chromosomes and correspond to two specific chromosome arms. Analysis of the Arabidopsis genome sequence identified two extensive blocks of degenerate telomeric repeats, which lie at the bases of these two arms. Our data thus indicate a protective role of ERCC1/XPF against 3' G-strand overhang invasion of interstitial telomeric repeats. The fact that the Atercc1 (and Atrad1) mutants dramatically potentiate levels of chromosome instability in Attert mutants, and the absence of such events in the presence of telomerase, have important implications for models of the roles of recombination at telomeres and is a striking illustration of the impact of genome structure on the outcomes of equivalent recombination processes in different organisms.

Human flap endonuclease I is in complex with telomerase and is required for telomerase-mediated telomere maintenance

Studies from budding yeast and ciliates have suggested that telomerase extension of telomeres requires the conventional DNA replication machinery, yet little is known about how DNA replication proteins regulate telomerase action in higher eukaryotic cells. Here we investigate the role of one of the DNA replication factors, flap endonuclease I (FEN1), in regulating telomerase activity in mammalian cells. FEN1 is a nuclease that plays an important role in DNA replication, repair, and recombination. We show that FEN1 is in complex with telomerase in vivo via telomeric DNA. We further demonstrate that FEN1 deficiency in mouse embryonic fibroblasts leads to an increase in telomere end-to-end fusions. In cancer cells, FEN1 deficiency induces gradual shortening of telomeres but does not alter the single-stranded G-overhangs. This is, to our knowledge, the first evidence that FEN1 and telomerase physically co-exist as a complex and that FEN1 can regulate telomerase activity at telomeres in mammalian cells.

Cellular senescence: molecular mechanisms, in vivo significance, and redox considerations

Cellular senescence is recognized as a critical cellular response to prolonged rounds of replication and environmental stresses. Its defining characteristics are arrested cell-cycle progression and the development of aberrant gene expression with proinflammatory behavior. Whereas the mechanistic events associated with senescence are generally well understood at the molecular level, the impact of senescence in vivo remains to be fully determined. In addition to the role of senescence as an antitumor mechanism, this review examines cellular senescence as a factor in organismal aging and age-related diseases, with particular emphasis on aberrant gene expression and abnormal paracrine signaling. Senescence as an emerging factor in tissue remodeling, wound repair, and infection is considered. In addition, the role of oxidative stress as a major mediator of senescence and the role of NAD(P)H oxidases and changes to intracellular GSH/GSSG status are reviewed. Recent findings indicate that senescence and the behavior of senescent cells are amenable to therapeutic intervention. As the in vivo significance of senescence becomes clearer, the challenge will be to modulate the adverse effects of senescence without increasing the risks of other diseases, such as cancer. The uncoupled relation between cell-cycle arrest and the senescent phenotype suggests that this is an achievable outcome.

No attenuation of the ATM-dependent DNA damage response in murine telomerase-deficient cells

Inactivation of mammalian telomerase leads to telomere attrition, eventually culminating in uncapped telomeres, which elicit a DNA damage response and cell cycle arrest or death. In some instances, telomerase modulation evokes a response not obviously attributable to changes in telomere length. One such example is the suppression of the DNA damage response (DDR) and changes in histone modification that occur upon repression of the telomerase reverse transcriptase, TERT, in human primary cells [K. Masutomi, R. Possemato, J.M. Wong, J.L. Currier, Z. Tothova, J.B. Manola, S. Ganesan, P.M. Lansdorp, K. Collins and W.C. Hahn, The telomerase reverse transcriptase regulates chromatin state and DNA damage responses, Proc. Natl. Acad. Sci. U.S.A. 102 (2005) 8222-8227]. Here, we evaluate the contribution of TERT to the DDR in murine Tert(-/-) cells without critically shortened telomeres. We treated mTert(-/-) embryonic stem (ES) cells and murine embryonic fibroblasts (MEFs) with etoposide and irradiation, and assessed the status of p53(pS15), 53BP1, ATM(pS1981), SMC1(pS957), and gammaH2AX by indirect immunofluorescence or western blotting. In four independently derived mTert(-/-) ES cell lines, there was no significant difference in the induction of gammaH2AX, 53BP1 foci, or the phosphorylation of ATM targets (ATM, SMC1, p53) between wildtype and mTert(-/-) ES cells and MEFs. A slight difference in post-translational modification of histones H3 and H4 was observed in a subset of mTert(-/-) ES cells, however this difference was reflected in the cellular levels of H3 and H4. Thus, in contrast to previous studies in human cells, the absence of Tert does not overtly affect the ATM-dependent response to DNA damage in murine cells.

DNA damage and ageing: new-age ideas for an age-old problem

Loss of genome maintenance may causally contribute to ageing, as exemplified by the premature appearance of multiple symptoms of ageing in a growing family of human syndromes and in mice with genetic defects in genome maintenance pathways. Recent evidence revealed a similarity between such prematurely ageing mutants and long-lived mice harbouring mutations in growth signalling pathways. At first sight this seems paradoxical as they represent both extremes of ageing yet show a similar 'survival' response that is capable of delaying age-related pathology and extending lifespan. Understanding the mechanistic basis of this response and its connection with genome maintenance would open exciting possibilities for counteracting cancer or age-related diseases, and for promoting longevity.

Statins Use a Novel Nijmegen Breakage Syndrome-1–Dependent Pathway to Accelerate DNA Repair in Vascular Smooth Muscle Cells

Although the hydroxymethylglutaryl-coenzyme A reductase inhibitors (statins) are widely used in atherosclerosis to reduce serum cholesterol, statins have multiple other effects, including direct effects on cells of the vessel wall. Recently, DNA damage, including telomere shortening, has been identified in vascular smooth muscle cells (VSMCs) in human atherosclerosis. Although statins reduce DNA damage in vitro, the mechanisms by which they might protect DNA integrity in VSMCs are unknown. We show that human atherosclerotic plaque VSMCs exhibit increased levels of double-stranded DNA breaks and basal activation of DNA repair pathways involving ataxia telangiectasia–mutated (ATM) and the histone H2AX in vivo and in vitro. Oxidant stress induced DNA damage and activated DNA repair pathways in VSMCs. Statin treatment did not reduce oxidant stress or DNA damage but markedly accelerated DNA repair. Accelerated DNA repair required both the Nijmegen breakage syndrome (NBS)-1 protein and the human double minute protein Hdm2, accompanied by phosphorylation of Hdm2, dissociation of NBS-1 and Hdm2, inhibition of NBS-1 degradation, and accelerated phosphorylation of ATM. Statin treatment reduced VSMC senescence and telomere attrition in culture, accelerated DNA repair and reduced apoptosis in vivo after irradiation, and reduced ATM/ATR (ATM and Rad3-related) activity in atherosclerosis. We conclude that statins activate a novel mechanism of accelerating DNA repair, dependent on NBS-1 stabilization and Hdm2. Statin treatment may delay cell senescence and promote DNA repair in atherosclerosis.

RAP80 responds to DNA damage induced by both ionizing radiation and UV irradiation and is phosphorylated at Ser 205

Receptor-associated protein (RAP80), a nuclear protein containing two ubiquitin-interacting motifs (UIM), was recently found to be associated with breast cancer-1 (BRCA1) and to translocate to ionizing radiation-induced foci (IRIF). In this study, we show that the BRCT mutant of BRCA1, R1699W, which is associated with increased risk of breast cancer, is unable to interact with RAP80. Previously, we showed that ataxia-telangiectasia mutated protein kinase (ATM) can phosphorylate RAP80 in vitro at Ser(205), but whether this site is a target of ATM in whole cells was not established. To address this question, we generated an anti-RAP80Ser205(P) antibody that specifically recognizes RAP80 phosphorylated at Ser(205). Our data show that RAP80 becomes phosphorylated at Ser(205) in cells exposed to ionizing irradiation and that RAP80Ser205(P) translocates to IRIF. We show that this phosphorylation is mediated by ATM and does not require a functional BRCA1. The phosphorylation occurs within 5 minutes after irradiation, long before the translocation of RAP80 to IRIF. In addition, we show that UV irradiation induces translocation of RAP80 to DNA damage foci that colocalize with gamma-H2AX. We further show that this translocation is also dependent on the UIMs of RAP80 and that the UV-induced phosphorylation of RAP80 at Ser(205) is mediated by ATM- and RAD3-related kinase, not ATM. These findings suggest that RAP80 has a more general role in different types of DNA damage responses.

Telomere shortening is correlated with the DNA damage response and telomeric protein down-regulation in colorectal preneoplastic lesions

BACKGROUND

A relation between telomere attrition in early carcinogenesis and activation of DNA damage response (DDR) has been proposed. We explored telomere length and its link with DDR in colorectal multistep carcinogenesis.

PATIENTS AND METHODS

We studied normal mucosa, low-grade dysplasia (LGD) and high-grade dysplasia (HGD) and invasive carcinoma (IC) in matched human colon specimens by evaluating p-ataxia telangiectasia mutated (ATM), p-checkpoint kinase 2 (Chk2), c-H2AX, TRF1 and TRF2 expressions by immunohistochemistry. FISH was used to assess telomere length.

RESULTS

Telomeres shortened significantly from normal (N) to LGD and HGD (P < 0.0001; P = 0.012), then increased in length in IC (P = 0.006). TRF1 and TRF2 expressions were diminished from N to LGD and HGD (P = 0.004, P < 0.0001, ns) and were reexpressed at the invasive stage (P = 0.053 and P = 0.046). Phosphorylated ATM, Chk2 and H2AX appeared already in LGD (respectively, P = 0.001, P = 0.002 and P = 0.02). Their expression decreased from HGD to IC (respectively, P = 0.03, P = 0.02 and P = 0.37). These activating phosphorylations were inversely correlated with telomere length and TRF1/2 expression.

CONCLUSION

In a model of colon multistep carcinogenesis, our data indicate that telomeric length and protein expression levels are inversely correlated with the activation of the DDR pathway.

Living on a break: cellular senescence as a DNA-damage response

Cellular senescence is associated with ageing and cancer in vivo and has a proven tumour-suppressive function. Common to both ageing and cancer is the generation of DNA damage and the engagement of the DNA-damage response pathways. In this Review, the diverse mechanisms that lead to DNA-damage generation and the activation of DNA-damage-response signalling pathways are discussed, together with the evidence for their contribution to the establishment and maintenance of cellular senescence in the context of organismal ageing and cancer development.

RAP80 and RNF8, key players in the recruitment of repair proteins to DNA damage sites

Chromosomal double-strand breaks (DSBs) in eukaryotes provoke a rapid, extensive modification in chromatin flanking the breaks. The DNA damage response (DDR) coordinates activation of cell cycle checkpoints, apoptosis, and DNA repair networks, to ensure accurate repair and genomic integrity. The checkpoint kinase ATM plays a critical role in the initiation of DDR in response to DSBs. The early ATM-mediated phosphorylation of the histone variant H2AX proteins near DSBs leads to the subsequent binding of MDC1, which functions as a scaffold for the recruitment and assembly of many DDR mediators and effectors, including BRCA1. Recent studies have provided new insights into the mechanism by which BRCA1 and associated proteins are recruited to DNA damage foci and revealed key roles for the receptor-associated protein 80 (RAP80) and the E3 ligase RNF8 in this process. RAP80 is an ubiquitin-interaction motif (UIM) containing protein that is associated with a BRCA1/BARD1 complex through its interaction with CCDC98 (Abraxas). The UIMs of RAP80 are critical for targeting this protein complex to DSB sites. Additional studies revealed that after binding gamma-H2AX, ATM-phosphorylated MDC1 is recognized by the FHA domain of RNF8, which subsequently binds the E2 conjugating enzyme UBC13. This complex catalyzes K63-linked polyubiquitination of histones H2A and gamma-H2AX, which are then recognized by the UIMs of RAP80, thereby facilitating the recruitment of the BRCA1/BARD1/CCDC98/RAP80 protein complex to DSB sites. Depletion of RAP80 or RNF8 impairs the translocation of BRCA1 to DNA damage sites and results in defective cell cycle checkpoint control and DSB repair. In this review, we discuss this cascade of protein phosphorylation and ubiquitination and the role it plays in the control of cellular responses to genotoxic stress by regulating the interactions, localization, and function of DDR proteins.

ATM-like kinases and regulation of telomerase: lessons from yeast and mammals

Telomeres, the essential structures at the ends of eukaryotic chromosomes, are composed of G-rich DNA and asociated proteins. These structures are crucial for the integrity of the genome, because they protect chromosome ends from degradation and distinguish natural ends from chromosomal breaks. The complete replication of telomeres requires a telomere-dedicated reverse transcriptase called telomerase. Paradoxically, proteins that promote the very activities against which telomeres protect, namely DNA repair, recombination and checkpoint activation, are integral to both telomeric chromatin and telomere elongation. This review focuses on recent findings that shed light on the roles of ATM-like kinases and other checkpoint and repair proteins in telomere maintenance, replication and checkpoint signaling.

Potential biomarkers of ageing

Life span in individual humans is very heterogeneous.Thus, the ageing rate, measured as the decline of functional capacity and stress resistance, is different in every individual. There have been attempts made to analyse this individual age, the so-called biological age, in comparison to chronological age. Biomarkers of ageing should help to characterise this biological age and, as age is a major risk factor in many degenerative diseases,could be subsequently used to identify individuals at high risk of developing age-associated diseases or disabilities. Markers based on oxidative stress, protein glycation,inflammation, cellular senescence and hormonal deregulation are discussed.

Cellular senescence and organismal aging

Cellular senescence, first observed and defined using in vitro cell culture studies, is an irreversible cell cycle arrest which can be triggered by a variety of factors. Emerging evidence suggests that cellular senescence acts as an in vivo tumor suppression mechanism by limiting aberrant proliferation. It has also been postulated that cellular senescence can occur independently of cancer and contribute to the physiological processes of normal organismal aging. Recent data have demonstrated the in vivo accumulation of senescent cells with advancing age. Some characteristics of senescent cells, such as the ability to modify their extracellular environment, could play a role in aging and age-related pathology. In this review, we examine current evidence that links cellular senescence and organismal aging.

Contribution of telomerase RNA retrotranscription to DNA double-strand break repair during mammalian genome evolution

BACKGROUND

In vertebrates, tandem arrays of TTAGGG hexamers are present at both telomeres and intrachromosomal sites (interstitial telomeric sequences (ITSs)). We previously showed that, in primates, ITSs were inserted during the repair of DNA double-strand breaks and proposed that they could arise from either the capture of telomeric fragments or the action of telomerase.

RESULTS

An extensive comparative analysis of two primate (Homo sapiens and Pan troglodytes) and two rodent (Mus musculus and Rattus norvegicus) genomes allowed us to describe organization and insertion mechanisms of all the informative ITSs present in the four species. Two novel observations support the hypothesis of telomerase involvement in ITS insertion: in a highly significant fraction of informative loci, the ITSs were introduced at break sites where a few nucleotides homologous to the telomeric hexamer were exposed; in the rodent genomes, complex ITS loci are present in which a retrotranscribed fragment of the telomerase RNA, far away from the canonical template, was inserted together with the telomeric repeats. Moreover, mutational analysis of the TTAGGG arrays in the different species suggests that they were inserted as exact telomeric hexamers, further supporting the participation of telomerase in ITS formation.

CONCLUSION

These results strongly suggest that telomerase was utilized, in some instances, for the repair of DNA double-strand breaks occurring in the genomes of rodents and primates during evolution. The presence, in the rodent genomes, of sequences retrotranscribed from the telomerase RNA strengthens the hypothesis of the origin of telomerase from an ancient retrotransposon.

DNA damage signalling guards against activated oncogenes and tumour progression

DNA damage response (DDR), the guardian of genomic integrity, emerges as an oncogene-inducible biological barrier against progression of cancer beyond its early stages. Recent evidence from both cell culture and animal models as well as analyses of clinical specimens show that activation of numerous oncogenes and loss of some tumour suppressors result in DNA replication stress and DNA damage that alarm the cellular DDR machinery, a multifaceted response orchestrated by the ATR-Chk1 and ATM-Chk2 kinase signalling pathways. Such activation of the DDR network leads to cellular senescence or death of oncogene-transformed cells, resulting in delay or prevention of tumorigenesis. At the same time, the ongoing chronic DDR activation creates selective pressure that eventually favours outgrowth of malignant clones with genetic or epigenetic defects in the genome maintenance machinery, such as aberrations in the ATM-Chk2-p53 cascade and other DDR components. Furthermore, the executive DDR machinery is shared by at least two anticancer barriers, as both the oncogene-induced DNA replication stress and telomere shortening impact the cell fate decisions through convergence on DNA damage signalling. In this study, we highlight recent advances in this rapidly evolving area of cancer research, with particular emphasis on mechanistic insights, emerging issues of special conceptual significance and discussion of major remaining challenges and implications of the concept of DDR as a tumorigenesis barrier for experimental and clinical oncology.

Protection against chromosome degradation at the telomeres

Telomeres, the ends of linear chromosomes, contain repeated TG-rich sequences which, in dividing cells, must be constantly replenished in order to avoid chromosome erosion and, hence, genomic instability. Moreover, unprotected telomeres are prone to end-to-end fusions. Telomerase, a specialized reverse transcriptase with a built-in RNA template, or, in the absence of telomerase, alternative pathways of telomere maintenance are required for continuous cell proliferation in actively dividing cells as well as in cancerous cells emerging in deregulated somatic tissues. The challenge is to keep these free DNA ends masked from the nucleolytic attacks that will readily operate on any DNA double-strand break in the cell, while also allowing the recruitment of telomerase at intervals. Specialized telomeric proteins, as well as DNA repair and checkpoint proteins with a dual role in telomere maintenance and DNA damage signaling/repair, protect the telomere ends from degradation and some of them also function in telomerase recruitment or other aspects of telomere length homeostasis. Phosphorylation of some telomeric proteins by checkpoint protein kinases appears to represent a mode of regulation of telomeric mechanisms. Finally, recent studies have allowed starting to understand the coupling between progression of the replication forks through telomeric regions and the subsequent telomere replication by telomerase, as well as retroaction of telomerase in cis on the firing of nearby replication origins.

Recombination-based telomere maintenance is dependent on Tel1-MRN and Rap1 and inhibited by telomerase, Taz1, and Ku in fission yeast

Fission yeast cells survive loss of the telomerase catalytic subunit Trt1 (TERT) through recombination-based telomere maintenance or through chromosome circularization. Although trt1Delta survivors with linear chromosomes can be obtained, they often spontaneously circularize their chromosomes. Therefore, it was difficult to establish genetic requirements for telomerase-independent telomere maintenance. In contrast, when the telomere-binding protein Taz1 is also deleted, taz1Delta trt1Delta cells are able to stably maintain telomeres. Thus, taz1Delta trt1Delta cells can serve as a valuable tool in understanding the regulation of telomerase-independent telomere maintenance. In this study, we show that the checkpoint kinase Tel1 (ATM) and the DNA repair complex Rad32-Rad50-Nbs1 (MRN) are required for telomere maintenance in taz1Delta trt1Delta cells. Surprisingly, Rap1 is also essential for telomere maintenance in taz1Delta trt1Delta cells, even though recruitment of Rap1 to telomeres depends on Taz1. Expression of catalytically inactive Trt1 can efficiently inhibit recombination-based telomere maintenance, but the inhibition requires both Est1 and Ku70. While Est1 is essential for recruitment of Trt1 to telomeres, Ku70 is dispensable. Thus, we conclude that Taz1, TERT-Est1, and Ku70-Ku80 prevent telomere recombination, whereas MRN-Tel1 and Rap1 promote recombination-based telomere maintenance. Evolutionarily conserved proteins in higher eukaryotic cells might similarly contribute to telomere recombination.

A role for MRE11, NBS1, and recombination junctions in replication and stable maintenance of EBV episomes

Recombination-like structures formed at origins of DNA replication may contribute to replication fidelity, sister chromatid cohesion, chromosome segregation, and overall genome stability. The Epstein-Barr Virus (EBV) origin of plasmid replication (OriP) provides episomal genome stability through a poorly understood mechanism. We show here that recombinational repair proteins MRE11 and NBS1 are recruited to the Dyad Symmetry (DS) region of OriP in a TRF2- and cell cycle-dependent manner. Depletion of MRE11 or NBS1 by siRNA inhibits OriP replication and destabilized viral episomes. OriP plasmid maintenance was defective in MRE11 and NBS1 hypomorphic fibroblast cell lines and only integrated, non-episomal forms of EBV were detected in a lympoblastoid cell line derived from an NBS1-mutated individual. Two-dimensional agarose gel analysis of OriP DNA revealed that recombination-like structures resembling Holliday-junctions form at OriP in mid S phase. MRE11 and NBS1 association with DS coincided with replication fork pausing and origin activation, which preceded the formation of recombination structures. We propose that NBS1 and MRE11 promote replication-associated recombination junctions essential for EBV episomal maintenance and genome stability.

DEC1, a basic helix-loop-helix transcription factor and a novel target gene of the p53 family, mediates p53-dependent premature senescence

Cellular senescence plays an important role in tumor suppression. p53 tumor suppressor has been reported to be crucial in cellular senescence. However, the underlying mechanism is poorly understood. In this regard, a cDNA microarray assay was performed to identify p53 targets involved in senescence. Among the many candidates is DEC1, a basic helix-loop-helix transcription factor that has been recently shown to be up-regulated in K-ras-induced premature senescence. However, it is not clear whether DEC1 is capable of inducing senescence. Here, we found that DEC1 is a novel target gene of the p53 family and mediates p53-dependent premature senescence. Specifically, we showed that DEC1 is induced by the p53 family and DNA damage in a p53-dependent manner. We also found that the p53 family proteins bind to, and activate, the promoter of the DEC1 gene. In addition, we showed that overexpression of DEC1 induces G(1) arrest and promotes senescence. Moreover, we found that targeting endogenous DEC1 attenuates p53-mediated premature senescence in response to DNA damage. Furthermore, overexpression of DEC1 induces cellular senescence in p53-knockdown cells, albeit to a lesser extent. Finally, we showed that DEC1-induced senescence is p21-independent. Taken together, our data provided strong evidence that DEC1 is one of the effectors downstream of p53 to promote premature senescence.

ATM regulates the length of individual telomere tracts in Arabidopsis

Telomeres have the paradoxical ability of protecting linear chromosome ends from DNA damage sensors by using these same proteins as essential components of their maintenance machinery. We have previously shown that the absence of ataxia telangiectasia mutated (ATM), a central regulator of the DNA damage response, accelerates the onset of genome instability in telomerase-deficient Arabidopsis, without increasing the rate of bulk telomere shortening. Here, we examine individual telomere tracts through successive plant generations using both fluorescence situ in hybridization (FISH) and primer extension telomere repeat amplification (PETRA). Unexpectedly, we found that the onset of profound developmental defects and abundant end-to-end chromosome fusions in fifth generation (G(5)) atm tert mutants required the presence of only one critically shortened telomere. Parent progeny analysis revealed that the short telomere arose as a consequence of an unusually large telomere rapid deletion (TRD) event. The most dramatic TRD was detected in atm tert mutants that had undergone meiosis. Notably, in contrast to TRD, alternative lengthening of telomeres (ALT) was suppressed in the absence of ATM. Finally, we show that size differences between telomeres on homologous chromosome ends are greater for atm tert than tert plants. Altogether, these findings suggest a dual role for ATM in regulating telomere size by promoting elongation of short telomeres and by preventing the accumulation of cells that harbor large telomere deletions.

Tel1p preferentially associates with short telomeres to stimulate their elongation

In many organisms, telomeric DNA consists of long tracts of short repeats. Shorter tracts are preferentially lengthened by telomerase, suggesting a conserved mechanism that recognizes and elongates short telomeres. Tel1p, an ATM family checkpoint kinase, plays an important role in telomere elongation, as cells lacking Tel1p have short telomeres and show reduced recruitment of telomerase components to telomeres. We show that Tel1p association increased as telomeres shortened in vivo in the presence or absence of telomerase and that Tel1p preferentially associated with the shortest telomeres. Tel1p association was independent of Tel1p kinase activity and enhanced by Mre11p. Tel1p overexpression simultaneously stimulated telomerase-mediated elongation and Tel1p association with all telomeres. Thus, Tel1p preferentially associates with the shortest telomeres and stimulates their elongation by telomerase.

Telomere ResQue and preservation--roles for the Werner syndrome protein and other RecQ helicases

Werner syndrome is an autosomal recessive disorder resulting from loss of function of the RecQ helicase, WRN protein. WS patients prematurely develop numerous clinical symptoms and diseases associated with aging early in life and are predisposed to cancer. WRN protein and many other RecQ helicases in general, seem to function during DNA replication in the processing of stalled replication forks. Genetic, cellular and biochemical evidence support roles for WRN in proper replication and repair of telomeric DNA, and indicate that telomere dysfunction contributes to the WS disease pathology.

Telomere regulation and function during meiosis

Telomeres are essential for genomic stability and their dysfunction has been implicated in cancer and ageing. The most prominent function of the telomeres is to protect chromosome ends against degradation and fusion, which, in turn, requires maintenance of telomere DNA to a critical length that allows assembly of end-capping structures. During early meiosis, telomeres play the distinctive function of anchoring chromosomes to the inner nuclear membrane. Subsequently, as a consequence of the nuclear membrane polarization, telomeres cluster together into a bouquet configuration, which facilitates pairing and recombination of the homologous chromosomes. Here we review how the two fundamental aspects of telomere maintenance, elongation and protection, contribute to the essential functions performed by telomeres during meiosis.

Flow cytometry and FISH to measure the average length of telomeres (flow FISH)

Telomeres have emerged as crucial cellular elements in aging and various diseases including cancer. To measure the average length of telomere repeats in cells, we describe our protocols that use fluorescent in situ hybridization (FISH) with labeled peptide nucleic acid (PNA) probes specific for telomere repeats in combination with fluorescence measurements by flow cytometry (flow FISH). Flow FISH analysis can be performed using commercially available flow cytometers, and has the unique advantage over other methods for measuring telomere length of providing multi-parameter information on the length of telomere repeats in thousands of individual cells. The accuracy and reproducibility of the measurements is augmented by the automation of most pipetting (aspiration and dispensing) steps, and by including an internal standard (control cells) with a known telomere length in every tube. The basic protocol for the analysis of nucleated blood cells from 22 different individuals takes about 12 h spread over 2-3 days.

Cellular senescence: when bad things happen to good cells

Cells continually experience stress and damage from exogenous and endogenous sources, and their responses range from complete recovery to cell death. Proliferating cells can initiate an additional response by adopting a state of permanent cell-cycle arrest that is termed cellular senescence. Understanding the causes and consequences of cellular senescence has provided novel insights into how cells react to stress, especially genotoxic stress, and how this cellular response can affect complex organismal processes such as the development of cancer and ageing.

DNA damage, cellular senescence and organismal ageing: causal or correlative?

Cellular senescence has long been used as a cellular model for understanding mechanisms underlying the ageing process. Compelling evidence obtained in recent years demonstrate that DNA damage is a common mediator for both replicative senescence, which is triggered by telomere shortening, and premature cellular senescence induced by various stressors such as oncogenic stress and oxidative stress. Extensive observations suggest that DNA damage accumulates with age and that this may be due to an increase in production of reactive oxygen species (ROS) and a decline in DNA repair capacity with age. Mutation or disrupted expression of genes that increase DNA damage often result in premature ageing. In contrast, interventions that enhance resistance to oxidative stress and attenuate DNA damage contribute towards longevity. This evidence suggests that genomic instability plays a causative role in the ageing process. However, conflicting findings exist which indicate that ROS production and oxidative damage levels of macromolecules including DNA do not always correlate with lifespan in model animals. Here we review the recent advances in addressing the role of DNA damage in cellular senescence and organismal ageing.

Arabidopsis POT1A interacts with TERT-V(I8), an N-terminal splicing variant of telomerase

Chromosome integrity is maintained via the actions of ribonucleoprotein complexes that can add telomeric repeats or can protect the chromosome end from being degraded. POT1 (protection of telomeres 1), a class of single-stranded-DNA-binding proteins, is a regulator of telomeric length. The Arabidopsis genome contains three POT1 homologues: POT1A, POT1B and POT1C. Using yeast two-hybrid assays to identify components of a potential POT1A complex, we retrieved three interactors: the N-terminus of the telomerase, a protein kinase and a plant-specific protein. Further analysis of the interaction of POT1 proteins with telomerase showed that this interaction is specific to POT1A, suggesting a specific role for this paralogue. The interaction is specific to the N-terminal region of the telomerase, which can be encoded by splicing variants. This interaction indicates possible mechanisms for telomerase regulation by alternative splicing and by POT1 proteins.

Telomeres: hallmarks of radiosensitivity

Telomeres are the very ends of the chromosomes. They can be seen as natural double-strand breaks (DSB), specialized structures which prevent DSB repair and activation of DNA damage checkpoints. In somatic cells, attrition of telomeres occurs after each cell division until replicative senescence. In the absence of telomerase, telomeres shorten due to incomplete replication of the lagging strand at the very end of chromosome termini. Moreover, oxidative stress and accumulating reactive oxygen species (ROS) lead to an increased telomere shortening due to a less efficient repair of SSB in telomeres. The specialized structures at telomeres include proteins involved in both telomere maintenance and DNA repair. However when a telomere is damaged and has to be repaired, those proteins might fail to perform an accurate repair of the damage. This is the starting point of this article in which we first summarize the well-established relationships between DNA repair processes and maintenance of functional telomeres. We then examine how damaged telomeres would be processed, and show that irradiation alters telomere maintenance leading to possibly dramatic consequences. Our point is to suggest that those consequences are not restricted to the short term effects such as increased radiation-induced cell death. On the contrary, we postulate that the major impact of the loss of telomere integrity might occur in the long term, during multistep carcinogenesis. Its major role would be to act as an amplificator event unmasking in one single step recessive radiation-induced mutations among thousands of genes and providing cellular proliferative advantage. Moreover, the chromosomal instability generated by damaged telomeres will favour each step of the transformation from normal to fully transformed cells.

Dual-tagging system for the affinity purification of mammalian protein complexes

Although affinity purification coupled with mass spectrometry (MS) provides a powerful tool to study protein-protein interactions, this strategy has encountered numerous difficulties when adapted to mammalian cells. Here we describe a Gateway®-compatible dual-tag affinity purification system that integrates regulatable expression, tetracysteine motifs, and various combinations of affinity tags to facilitate the cloning, detection, and purification of bait proteins and their interacting partners. Utilizing the human telomere binding protein TRF2 as a benchmark, we demonstrate bait protein recoveries upwards of approximately 16% from as little as 1–7 × 107 cells and successfully identify known TRF2 interacting proteins, suggesting that our dual-tag affinity purification approach is a capable new tool for expanding the capacity to explore mammalian proteomic networks.

WRN at telomeres: implications for aging and cancer

Werner Syndrome (WS) is a premature aging syndrome characterized by early onset of age-related pathologies and cancer. Since WS is due to a single gene defect, it has attracted much interest from researchers seeking to understand pathways that contribute to cancer and aging at cellular and molecular levels. The protein mutated in WS, WRN, appears to play a major role in genome stability, particularly during DNA replication and telomere metabolism. Much of the pathophysiology associated with WS, including the rapid onset of cellular senescence, early cancer onset and premature aging, can be attributed to a defect in telomere maintenance. Recent genetic evidence from the mTerc(-/-) Wrn(-/-) mouse demonstrates that mice with critically shortened telomeres display aging phenotypes reminiscent of human WS, further reinforcing the notion that telomere dysfunction is required for the manifestation of aging pathophysiologies in the setting of WRN deficiency.

Mrc1 protects uncapped budding yeast telomeres from exonuclease EXO1

Mrc1 (Mediator of Replication Checkpoint 1) is a component of the DNA replication fork machinery and is necessary for checkpoint activation after replication stress. In this study, we addressed the role of Mrc1 at uncapped telomeres. Our experiments show that Mrc1 contributes to the vitality of both cdc13-1 and yku70Δ telomere capping mutants. Cells with telomere capping defects containing MRC1 or mrc1^AQ, a checkpoint defective allele, exhibit similar growth, suggesting growth defects of cdc13-1 mrc1Δ are not due to checkpoint defects. This is in accordance with Mrc1-independent Rad53 activation after telomere uncapping. Poor growth of cdc13-1 mutants in the absence of Mrc1 is a result of enhanced single stranded DNA accumulation at uncapped telomeres. Consistent with this, deletion of EXO1, encoding a nuclease that contributes to single stranded DNA accumulation after telomere uncapping, improves growth of cdc13-1 mrc1Δ strains and decreases ssDNA production. Our observations show that Mrc1, a core component of the replication fork, plays an important role in telomere capping, protecting from nucleases and checkpoint pathways.

A RAP1/TRF2 complex inhibits nonhomologous end-joining at human telomeric DNA ends

The mechanisms by which telomeres are distinguished from DNA double-strand breaks are poorly understood. Here we have defined the minimal requirements for the protection of telomeric DNA ends from nonhomologous end-joining (NHEJ). Neither long, single-stranded overhangs nor t loop formation is essential to prevent NHEJ-mediated ligation of telomeric ends in vitro. Instead, a tandem array of 12 telomeric repeats is sufficient to impede illegitimate repair in a highly directional manner at nearby DNA ends. The polarity of end protection is consistent with the orientation of naturally occurring telomeres and is well suited to minimize interference between chromosome capping and the repair of DNA double-strand breaks in subtelomeric sequences. Biochemical fractionation and reconstitution revealed that telomere protection is mediated by a RAP1/TRF2 complex, providing evidence for a direct role for human RAP1 in the protection of telomeric DNA from NHEJ.

MRX-dependent DNA damage response to short telomeres

Telomere structure allows cells to distinguish the natural chromosome ends from double-strand breaks (DSBs). However, DNA damage response proteins are intimately involved in telomere metabolism, suggesting that functional telomeres may be recognized as DNA damage during a time window. Here we show by two different systems that short telomeres are recognized as DSBs during the time of their replication, because they induce a transient MRX-dependent DNA damage checkpoint response during their prolonged elongation. The MRX complex, which is recruited at telomeres under these conditions, dissociates from telomeres concomitantly with checkpoint switch off when telomeres reach a new equilibrium length. We also show that MRX recruitment to telomeres is sufficient to activate the checkpoint independently of telomere elongation. We propose that MRX can signal checkpoint activation by binding to short telomeres only when they become competent for elongation. Because full-length telomeres are refractory to MRX binding and the shortest telomeres are elongated of only a few base pairs per generation, this limitation may prevent unscheduled checkpoint activation during an unperturbed S phase.

Spreading of mammalian DNA-damage response factors studied by ChIP-chip at damaged telomeres

Phosphorylated histone H2AX (gammaH2AX) is generated in nucleosomes flanking sites of DNA double-strand breaks, triggering the recruitment of DNA-damage response proteins such as MDC1 and 53BP1. Here, we study shortened telomeres in senescent human cells. We show that most telomeres trigger gammaH2AX formation, which spreads up to 570 kb into the subtelomeric regions. Furthermore, we reveal that the spreading patterns of 53BP1 and MDC1 are very similar to that of gammaH2AX, consistent with a structural link between these factors. Moreover, different subsets of telomeres signal in different cell lines, with those that signal tending to equate to the shortest telomeres of the corresponding cell line, thus linking telomere attrition with DNA-damage signalling. Notably, we find that, in some cases, gammaH2AX spreading is modulated in a manner suggesting that H2AX distribution or its ability to be phosphorylated is not uniform along the chromosome. Finally, we observe weak gammaH2AX signals at telomeres of proliferating cells, but not in hTERT immortalised cells, suggesting that low telomerase activity leads to telomere uncapping and senescence in proliferating primary cells.