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1.  ERCC1/XPF Protects Short Telomeres from Homologous Recombination in Arabidopsis thaliana 
PLoS Genetics  2009;5(2):e1000380.
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.
Author Summary
Telomeres are the specialised nucleoprotein structures evolved to avoid progressive replicative shortening and recombinational instability of the ends of linear chromosomes. Notwithstanding this role of telomeres in “hiding” chromosome ends from DNA repair and recombination, many repair and recombination proteins play essential roles in telomere function and chromosome stability. Among these are XPF and ERCC1, which form a structure-specific endonuclease known for its essential role in nucleotide excision repair and that is the subject of considerable interest in studies of recombination. In this study, we analyse the roles of the XPF/ERCC1 in telomere function and chromosome stability in the plant Arabidopsis thaliana, which, with its remarkable tolerance to genomic instability and sequenced genome, is an excellent higher eukaryotic model for these studies. Surprisingly, and in striking contrast to observations in mammalian cells, we observe no enhancement of chromosomal instability in Arabidopsis plants lacking either of these two proteins, which develop normally and show wild-type telomere length. However, Atercc1 (and Atrad1) mutants profoundly affect the recombination of de-protected telomeres, dramatically potentiating chromosome instability. These results provide a striking illustration of the different outcomes and genomic impacts of the same recombination processes in different organisms.
PMCID: PMC2632759  PMID: 19214203
2.  POT1a and Components of CST Engage Telomerase and Regulate Its Activity in Arabidopsis 
PLoS Genetics  2014;10(10):e1004738.
Protection of Telomeres 1 (POT1) is a conserved nucleic acid binding protein implicated in both telomere replication and chromosome end protection. We previously showed that Arabidopsis thaliana POT1a associates with the TER1 telomerase RNP, and is required for telomere length maintenance in vivo. Here we further dissect the function of POT1a and explore its interplay with the CST (CTC1/STN1/TEN1) telomere complex. Analysis of pot1a null mutants revealed that POT1a is not required for telomerase recruitment to telomeres, but is required for telomerase to maintain telomere tracts. We show that POT1a stimulates the synthesis of long telomere repeat arrays by telomerase, likely by enhancing repeat addition processivity. We demonstrate that POT1a binds STN1 and CTC1 in vitro, and further STN1 and CTC1, like POT1a, associate with enzymatically active telomerase in vivo. Unexpectedly, the in vitro interaction of STN1 with TEN1 and POT1a was mutually exclusive, indicating that POT1a and TEN1 may compete for the same binding site on STN1 in vivo. Finally, unlike CTC1 and STN1, TEN1 was not associated with active telomerase in vivo, consistent with our previous data showing that TEN1 negatively regulates telomerase enzyme activity. Altogether, our data support a two-state model in which POT1a promotes an extendable telomere state via contacts with the telomerase RNP as well as STN1 and CTC1, while TEN1 opposes these functions.
Author Summary
Telomeres are required to stabilize the ends of linear chromosomes, and thus ensure genome integrity. Telomeric DNA is maintained though the action of both conventional and non-conventional DNA replication mechanisms. To ensure that chromosome ends are fully protected and fully replicated, telomeres dynamically oscillate between a closed (non-extendable) and an open (extendable) conformation throughout the cell cycle. The telomerase reverse transcriptase engages telomeres when they are in an extendable conformation. How this conversion occurs, how telomerase is recruited to the chromosome terminus and how telomerase action is terminated are unanswered questions. Here we provide evidence that POT1a, a telomerase accessory protein from the flowering plant Arabidopsis, helps to convert the telomere into a telomerase-extendable state through dynamic interactions with a critical telomere binding protein complex, and through stimulation of telomerase enzyme activity. The results of this study provide new insight into the regulation of telomeric DNA replication.
PMCID: PMC4199523  PMID: 25329641
3.  The Role of ATM in the Deficiency in Nonhomologous End-Joining near Telomeres in a Human Cancer Cell Line 
PLoS Genetics  2013;9(3):e1003386.
Telomeres distinguish chromosome ends from double-strand breaks (DSBs) and prevent chromosome fusion. However, telomeres can also interfere with DNA repair, as shown by a deficiency in nonhomologous end joining (NHEJ) and an increase in large deletions at telomeric DSBs. The sensitivity of telomeric regions to DSBs is important in the cellular response to ionizing radiation and oncogene-induced replication stress, either by preventing cell division in normal cells, or by promoting chromosome instability in cancer cells. We have previously proposed that the telomeric protein TRF2 causes the sensitivity of telomeric regions to DSBs, either through its inhibition of ATM, or by promoting the processing of DSBs as though they are telomeres, which is independent of ATM. Our current study addresses the mechanism responsible for the deficiency in repair of DSBs near telomeres by combining assays for large deletions, NHEJ, small deletions, and gross chromosome rearrangements (GCRs) to compare the types of events resulting from DSBs at interstitial and telomeric DSBs. Our results confirm the sensitivity of telomeric regions to DSBs by demonstrating that the frequency of GCRs is greatly increased at DSBs near telomeres and that the role of ATM in DSB repair is very different at interstitial and telomeric DSBs. Unlike at interstitial DSBs, a deficiency in ATM decreases NHEJ and small deletions at telomeric DSBs, while it increases large deletions. These results strongly suggest that ATM is functional near telomeres and is involved in end protection at telomeric DSBs, but is not required for the extensive resection at telomeric DSBs. The results support our model in which the deficiency in DSB repair near telomeres is a result of ATM-independent processing of DSBs as though they are telomeres, leading to extensive resection, telomere loss, and GCRs involving alternative NHEJ.
Author Summary
The ends of chromosomes, called telomeres, prevent chromosome ends from appearing as DNA double-strand breaks (DSBs) and prevent chromosome fusion by forming a specialized nucleo-protein complex. The critical function of telomeres in end protection has a downside, in that it interferes with the repair of DSBs that occur near telomeres. DSBs are critical DNA lesions, because if they are not repaired correctly they can result in gross chromosome rearrangements (GCRs). As a result, the deficiency in DSB repair near telomeres has now been implicated in ageing, by promoting cell senescence, and cancer, by promoting telomere dysfunction due to oncogene-induced replication stress. The studies presented here demonstrate that DSBs near telomeres commonly result in GCRs in a human tumor cell line. Moreover, our results demonstrate that the mechanism of repair of telomeric DSBs is very different from the mechanism of repair of DSBs at other locations, supporting our hypothesis that the deficiency in repair of DSBs near telomeres is a result of the abnormal processing of DSBs due to the presence of telomeric proteins. Understanding the mechanism responsible for the deficiency in DSB repair near telomeres will provide important insights into critical human disease pathways.
PMCID: PMC3610639  PMID: 23555296
4.  Ku Must Load Directly onto the Chromosome End in Order to Mediate Its Telomeric Functions 
PLoS Genetics  2011;7(8):e1002233.
The Ku heterodimer associates with the Saccharomyces cerevisiae telomere, where it impacts several aspects of telomere structure and function. Although Ku avidly binds DNA ends via a preformed channel, its ability to associate with telomeres via this mechanism could be challenged by factors known to bind directly to the chromosome terminus. This has led to uncertainty as to whether Ku itself binds directly to telomeric ends and whether end association is crucial for Ku's telomeric functions. To address these questions, we constructed DNA end binding–defective Ku heterodimers by altering amino acid residues in Ku70 and Ku80 that were predicted to contact DNA. These mutants continued to associate with their known telomere-related partners, such as Sir4, a factor required for telomeric silencing, and TLC1, the RNA component of telomerase. Despite these interactions, we found that the Ku mutants had markedly reduced association with telomeric chromatin and null-like deficiencies for telomere end protection, length regulation, and silencing functions. In contrast to Ku null strains, the DNA end binding defective Ku mutants resulted in increased, rather than markedly decreased, imprecise end-joining proficiency at an induced double-strand break. This result further supports that it was the specific loss of Ku's telomere end binding that resulted in telomeric defects rather than global loss of Ku's functions. The extensive telomere defects observed in these mutants lead us to propose that Ku is an integral component of the terminal telomeric cap, where it promotes a specific architecture that is central to telomere function and maintenance.
Author Summary
The telomeric cap modulates telomere replication and prevents natural chromosome ends from being processed as DNA double-strand breaks (DSBs). In multiple species, including budding yeast, a detailed picture exists of the factors that comprise the telomeric cap and how they associate with telomeric DNA. It is less clear where to place Ku, a conserved heterodimer involved in multiple aspects of telomere biology and DSB repair. Although Ku avidly binds DNA ends, its access to telomeric ends might be restricted by telomere binding proteins and/or higher-order telomere structure. Ku might also be recruited to telomeres via its telomere-associated binding partners. Here, we address whether Ku loads directly onto telomeric ends and whether direct DNA binding is crucial for its telomeric functions. Using structure-guided mutagenesis, we generated end binding–defective yeast Ku heterodimers that retained the ability to associate with Ku's known telomeric binding partners. These end binding–defective heterodimers showed a dramatic reduction in telomere association and were defective for all of Ku's telomeric functions. Our findings indicate that Ku is indeed a component of the telomere cap and that its loading onto telomeric ends is crucial for its telomeric functions and, perhaps, a specific telomere architecture.
PMCID: PMC3154960  PMID: 21852961
5.  Telomerase-Null Survivor Screening Identifies Novel Telomere Recombination Regulators 
PLoS Genetics  2013;9(1):e1003208.
Telomeres are protein–DNA structures found at the ends of linear chromosomes and are crucial for genome integrity. Telomeric DNA length is primarily maintained by the enzyme telomerase. Cells lacking telomerase will undergo senescence when telomeres become critically short. In Saccharomyces cerevisiae, a very small percentage of cells lacking telomerase can remain viable by lengthening telomeres via two distinct homologous recombination pathways. These “survivor” cells are classified as either Type I or Type II, with each class of survivor possessing distinct telomeric DNA structures and genetic requirements. To elucidate the regulatory pathways contributing to survivor generation, we knocked out the telomerase RNA gene TLC1 in 280 telomere-length-maintenance (TLM) gene mutants and examined telomere structures in post-senescent survivors. We uncovered new functional roles for 10 genes that affect the emerging ratio of Type I versus Type II survivors and 22 genes that are required for Type II survivor generation. We further verified that Pif1 helicase was required for Type I recombination and that the INO80 chromatin remodeling complex greatly affected the emerging frequency of Type I survivors. Finally, we found the Rad6-mediated ubiquitination pathway and the KEOPS complex were required for Type II recombination. Our data provide an independent line of evidence supporting the idea that these genes play important roles in telomere dynamics.
Author Summary
Homologous recombination is a means for an organism or a cell to repair damaged DNA in its genome. Eukaryotic chromosomes have a linear configuration with two ends that are special DNA–protein structures called telomeres. Telomeres can be recognized by the cell as DNA double-strand breaks and subjected to repair by homologous recombination. In the baker's yeast Saccharomyces cerevisiae, cells that lack the enzyme telomerase, which is the primary factor responsible for telomeric DNA elongation, are able to escape senescence and cell death when telomeres undergo repair via homologous recombination. In this study, we have performed genetic screens to identify genes that affect telomeric DNA recombination. By examining the telomere structures in 280 mutants, each of which lacks both a telomere-length-maintenance gene and telomerase RNA gene, we identified 32 genes that were not previously known to be involved in telomere recombination. These genes have functions in a variety of cellular processes, and our work provides new insights into the regulation of telomere recombination in the absence of telomerase.
PMCID: PMC3547846  PMID: 23390378
6.  At Short Telomeres Tel1 Directs Early Replication and Phosphorylates Rif1 
PLoS Genetics  2014;10(10):e1004691.
The replication time of Saccharomyces cerevisiae telomeres responds to TG1–3 repeat length, with telomeres of normal length replicating late during S phase and short telomeres replicating early. Here we show that Tel1 kinase, which is recruited to short telomeres, specifies their early replication, because we find a tel1Δ mutant has short telomeres that nonetheless replicate late. Consistent with a role for Tel1 in driving early telomere replication, initiation at a replication origin close to an induced short telomere was reduced in tel1Δ cells, in an S phase blocked by hydroxyurea. The telomeric chromatin component Rif1 mediates late replication of normal telomeres and is a potential substrate of Tel1 phosphorylation, so we tested whether Tel1 directs early replication of short telomeres by inactivating Rif1. A strain lacking both Rif1 and Tel1 behaves like a rif1Δ mutant by replicating its telomeres early, implying that Tel1 can counteract the delaying effect of Rif1 to control telomere replication time. Proteomic analyses reveals that in yku70Δ cells that have short telomeres, Rif1 is phosphorylated at Tel1 consensus sequences (S/TQ sites), with phosphorylation of Serine-1308 being completely dependent on Tel1. Replication timing analysis of a strain mutated at these phosphorylation sites, however, suggested that Tel1-mediated phosphorylation of Rif1 is not the sole mechanism of replication timing control at telomeres. Overall, our results reveal two new functions of Tel1 at shortened telomeres: phosphorylation of Rif1, and specification of early replication by counteracting the Rif1-mediated delay in initiation at nearby replication origins.
Author Summary
The ends of chromosomes are protected by specialized structures called telomeres, which prevent their recognition as DNA breaks and enable recruitment of telomerase, the reverse transcriptase that maintains telomere length by replacing terminal TG-repeat sequences lost during successive rounds of DNA replication. Chromosomal DNA is replicated from initiation sites called origins, which are activated in a reproducible temporal sequence. Replication origins close to telomeres are subject to specialized temporal control that contributes to telomere stabilization: origins close to normal-length telomeres initiate replication late, while those close to shortened telomeres initiate early. Here we uncover the control mechanism that links telomere length with replication timing. Rif1, one of the components of telomeric chromatin, directs late replication of normal telomeres by delaying the activation of nearby origins. Our experiments show that a kinase called Tel1, which is recruited to shortened telomeres, neutralizes the origin-delaying activity of Rif1. We also find that Tel1 phosphorylates Rif1 at short telomeres, although our investigation shows this phosphorylation is not the sole mechanism through which Tel1 prevents Rif1-mediated replication delay. Since correct telomere replication timing control is important for telomerase-mediated length maintenance, this discovery represents an important step towards understanding the molecular mechanisms that ensure proper long-term stabilization of chromosome ends, as well as the controls over the DNA replication temporal program.
PMCID: PMC4199499  PMID: 25329891
7.  Mechanisms of telomere loss and their consequences for chromosome instability 
Frontiers in Oncology  2012;2:135.
The ends of chromosomes in mammals, called telomeres, are composed of a 6-bp repeat sequence, TTAGGG, which is added on by the enzyme telomerase. In combination with a protein complex called shelterin, these telomeric repeat sequences form a cap that protects the ends of chromosomes. Due to insufficient telomerase expression, telomeres shorten gradually with each cell division in human somatic cells, which limits the number of times they can divide. The extensive cell division involved in cancer cell progression therefore requires that cancer cells must acquire the ability to maintain telomeres, either through expression of telomerase, or through an alternative mechanism involving recombination. It is commonly thought that the source of many chromosome rearrangements in cancer cells is a result of the extensive telomere shortening that occurs prior to the expression of telomerase. However, despite the expression of telomerase, tumor cells can continue to show chromosome instability due to telomere loss. Dysfunctional telomeres in cancer cells can result from oncogene-induced replication stress, which results in double-strand breaks (DSBs) at fragile sites, including telomeres. DSBs near telomeres are especially prone to chromosome rearrangements, because telomeric regions are deficient in DSB repair. The deficiency in DSB repair near telomeres is also an important mechanism for ionizing radiation-induced replicative senescence in normal human cells. In addition, DSBs near telomeres can result in chromosome instability in mouse embryonic stem cells, suggesting that telomere loss can contribute to heritable chromosome rearrangements. Consistent with this possibility, telomeric regions in humans are highly heterogeneous, and chromosome rearrangements near telomeres are commonly involved in human genetic disease. Understanding the mechanisms of telomere loss will therefore provide important insights into both human cancer and genetic disease.
PMCID: PMC3463808  PMID: 23061048
telomere; gross chromosomal rearrangement; sister chromatid fusion; chromosome instability; double-strand break repair
8.  Telomere dysfunction and chromosome instability 
Mutation Research  2011;730(1-2):28-36.
The ends of chromosomes are composed of a short repeat sequence and associated proteins that together form a cap, called a telomere, that keeps the ends from appearing as double-strand breaks (DSBs) and prevents chromosome fusion. The loss of telomeric repeat sequences or deficiencies in telomeric proteins can result in chromosome fusion and lead to chromosome instability. The similarity between chromosome rearrangements resulting from telomere loss and those found in cancer cells implicates telomere loss as an important mechanism for the chromosome instability contributing to human cancer. Telomere loss in cancer cells can occur through gradual shortening due to insufficient telomerase, the protein that maintains telomeres. However, cancer cells often have a high rate of spontaneous telomere loss despite the expression of telomerase, which has been proposed to result from a combination of oncogene-mediated replication stress and a deficiency in DSB repair in telomeric regions. Chromosome fusion in mammalian cells primarily involves nonhomologous end joining (NHEJ), which is the major form of DSB repair. Chromosome fusion initiates chromosome instability involving breakage-fusion-bridge (B/F/B) cycles, in which dicentric chromosomes form bridges and break as the cell attempts to divide, repeating the process in subsequent cell cycles. Fusion between sister chromatids results in large inverted repeats on the end of the chromosome, which amplify further following additional B/F/B cycles. B/F/B cycles continue until the chromosome acquires a new telomere, most often by translocation of the end of another chromosome. The instability is not confined to a chromosome that loses its telomere, the instability is transferred to the chromosome donating a translocation. Moreover, the amplified regions are unstable and form extrachromosomal DNA that can reintegrate at new locations. Knowledge concerning the factors promoting telomere loss and its consequences is therefore important for understanding chromosome instability in human cancer.
PMCID: PMC3178001  PMID: 21575645
Chromosome fusion; Chromosome healing; Chromosome instability; Double-strand break; Intrastrand annealing; Nonhomologous end joining; Telomere
9.  Role of STN1 and DNA Polymerase α in Telomere Stability and Genome-Wide Replication in Arabidopsis 
PLoS Genetics  2014;10(10):e1004682.
The CST (Cdc13/CTC1-STN1-TEN1) complex was proposed to have evolved kingdom specific roles in telomere capping and replication. To shed light on its evolutionary conserved function, we examined the effect of STN1 dysfunction on telomere structure in plants. STN1 inactivation in Arabidopsis leads to a progressive loss of telomeric DNA and the onset of telomeric defects depends on the initial telomere size. While EXO1 aggravates defects associated with STN1 dysfunction, it does not contribute to the formation of long G-overhangs. Instead, these G-overhangs arise, at least partially, from telomerase-mediated telomere extension indicating a deficiency in C-strand fill-in synthesis. Analysis of hypomorphic DNA polymerase α mutants revealed that the impaired function of a general replication factor mimics the telomeric defects associated with CST dysfunction. Furthermore, we show that STN1-deficiency hinders re-replication of heterochromatic regions to a similar extent as polymerase α mutations. This comparative analysis of stn1 and pol α mutants suggests that STN1 plays a genome-wide role in DNA replication and that chromosome-end deprotection in stn1 mutants may represent a manifestation of aberrant replication through telomeres.
Author Summary
Telomeres form an elaborate nucleoprotein structure that may represent an obstacle for replication machinery and renders this region prone to fork stalling. CST is an evolutionary conserved complex that was originally discovered to specifically act at telomeres. Interestingly, the function of CST seems to have diverged in the course of evolution; in yeast it is required for telomere protection, while in mammals it was proposed to facilitate replication through telomeres. In plants, inactivation of CST leads to telomere deprotection and genome instability. Here we show that the telomere deprotection in Arabidopsis deficient in STN1, one of the CST components, is consistent with defects in telomere replication and that STN1 phenotypes can be partially phenocopied by an impairment of a general replication factor, DNA polymerase α. In addition, we provide evidence that STN1 facilitates re-replication at non-telomeric loci. This suggests a more general role of CST in genome maintenance and further infers that its seemingly specific function(s) in telomere protection may rather represent unique requirements for efficient replication of telomeric DNA.
PMCID: PMC4191939  PMID: 25299252
10.  Break-Induced Replication Requires DNA Damage-Induced Phosphorylation of Pif1 and Leads to Telomere Lengthening 
PLoS Genetics  2014;10(10):e1004679.
Broken replication forks result in DNA breaks that are normally repaired via homologous recombination or break induced replication (BIR). Mild insufficiency in the replicative ligase Cdc9 in budding yeast Saccharomyces cerevisiae resulted in a population of cells with persistent DNA damage, most likely due to broken replication forks, constitutive activation of the DNA damage checkpoint and longer telomeres. This telomere lengthening required functional telomerase, the core DNA damage signaling cascade Mec1-Rad9-Rad53, and the components of the BIR repair pathway – Rad51, Rad52, Pol32, and Pif1. The Mec1-Rad53 induced phosphorylation of Pif1, previously found necessary for inhibition of telomerase at double strand breaks, was also important for the role of Pif1 in BIR and telomere elongation in cdc9-1 cells. Two other mutants with impaired DNA replication, cdc44-5 and rrm3Δ, were similar to cdc9-1: their long telomere phenotype was dependent on the Pif1 phosphorylation locus. We propose a model whereby the passage of BIR forks through telomeres promotes telomerase activity and leads to telomere lengthening.
Author Summary
Telomeres are the ends of linear eukaryotic chromosomes maintained by an enzyme called telomerase. Non-telomeric DNA ends are often generated as a result of broken replication forks and are usually repaired by break-induced replication (BIR) or homologous recombination to avoid genomic instability. However, telomerase can interfere with the repair by adding a new telomere to a broken DNA end, or the break can be ligated to a telomere, thereby inducing genome re-arrangements that are often found in human genetic disorders and cancer. To understand how cells avoid erroneous repair, we studied cdc9-1 yeast mutant cells that generate broken replication forks with high frequency. We discovered that, in cells with DNA damage, a helicase called Pif1 is phosphorylated and this phosphorylation enables Pif1 not only to inhibit telomerase at broken DNA ends but also stimulate the break repair by BIR, which in turn leads to additional telomere lengthening. Thus, a new regulatory pathway stimulates accurate break repair by BIR and at the same time promotes telomerase activity at telomeres.
PMCID: PMC4199488  PMID: 25329304
11.  Telomeres in cancer and ageing 
Telomeres protect the chromosome ends from unscheduled DNA repair and degradation. Telomeres are heterochromatic domains composed of repetitive DNA (TTAGGG repeats) bound to an array of specialized proteins. The length of telomere repeats and the integrity of telomere-binding proteins are both important for telomere protection. Furthermore, telomere length and integrity are regulated by a number of epigenetic modifications, thus pointing to higher order control of telomere function. In this regard, we have recently discovered that telomeres are transcribed generating long, non-coding RNAs, which remain associated with the telomeric chromatin and are likely to have important roles in telomere regulation. In the past, we showed that telomere length and the catalytic component of telomerase, Tert, are critical determinants for the mobilization of stem cells. These effects of telomerase and telomere length on stem cell behaviour anticipate the premature ageing and cancer phenotypes of telomerase mutant mice. Recently, we have demonstrated the anti-ageing activity of telomerase by forcing telomerase expression in mice with augmented cancer resistance. Shelterin is the major protein complex bound to mammalian telomeres; however, its potential relevance for cancer and ageing remained unaddressed to date. To this end, we have generated mice conditionally deleted for the shelterin proteins TRF1, TPP1 and Rap1. The study of these mice demonstrates that telomere dysfunction, even if telomeres are of a normal length, is sufficient to produce premature tissue degeneration, acquisition of chromosomal aberrations and initiation of neoplastic lesions. These new mouse models, together with the telomerase-deficient mouse model, are valuable tools for understanding human pathologies produced by telomere dysfunction.
PMCID: PMC3001316  PMID: 21115533
telomeres; telomerase; cancer; ageing; shelterins; stem cells
12.  DNA-Dependent Protein Kinase Catalytic Subunit Is Not Required for Dysfunctional Telomere Fusion and Checkpoint Response in the Telomerase-Deficient Mouse▿  
Molecular and Cellular Biology  2006;27(6):2253-2265.
Telomeres are key structural elements for the protection and maintenance of linear chromosomes, and they function to prevent recognition of chromosomal ends as DNA double-stranded breaks. Loss of telomere capping function brought about by telomerase deficiency and gradual erosion of telomere ends or by experimental disruption of higher-order telomere structure culminates in the fusion of defective telomeres and/or the activation of DNA damage checkpoints. Previous work has implicated the nonhomologous end-joining (NHEJ) DNA repair pathway as a critical mediator of these biological processes. Here, employing the telomerase-deficient mouse model, we tested whether the NHEJ component DNA-dependent protein kinase catalytic subunit (DNA-PKcs) was required for fusion of eroded/dysfunctional telomere ends and the telomere checkpoint responses. In late-generation mTerc−/− DNA-PKcs−/− cells and tissues, chromosomal end-to-end fusions and anaphase bridges were readily evident. Notably, nullizygosity for DNA Ligase4 (Lig4)—an additional crucial NHEJ component—was also permissive for chromosome fusions in mTerc−/− cells, indicating that, in contrast to results seen with experimental disruption of telomere structure, telomere dysfunction in the context of gradual telomere erosion can engage additional DNA repair pathways. Furthermore, we found that DNA-PKcs deficiency does not reduce apoptosis, tissue atrophy, or p53 activation in late-generation mTerc−/− tissues but rather moderately exacerbates germ cell apoptosis and testicular degeneration. Thus, our studies indicate that the NHEJ components, DNA-PKcs and Lig4, are not required for fusion of critically shortened telomeric ends and that DNA-PKcs is not required for sensing and executing the telomere checkpoint response, findings consistent with the consensus view of the limited role of DNA-PKcs in DNA damage signaling in general.
PMCID: PMC1820500  PMID: 17145779
13.  Tpz1-Ccq1 and Tpz1-Poz1 Interactions within Fission Yeast Shelterin Modulate Ccq1 Thr93 Phosphorylation and Telomerase Recruitment 
PLoS Genetics  2014;10(10):e1004708.
In both fission yeast and humans, the shelterin complex plays central roles in regulation of telomerase recruitment, protection of telomeres against DNA damage response factors, and formation of heterochromatin at telomeres. While shelterin is essential for limiting activation of the DNA damage checkpoint kinases ATR and ATM at telomeres, these kinases are required for stable maintenance of telomeres. In fission yeast, Rad3ATR and Tel1ATM kinases are redundantly required for telomerase recruitment, since Rad3ATR/Tel1ATM-dependent phosphorylation of the shelterin subunit Ccq1 at Thr93 promotes interaction between Ccq1 and the telomerase subunit Est1. However, it remained unclear how protein-protein interactions within the shelterin complex (consisting of Taz1, Rap1, Poz1, Tpz1, Pot1 and Ccq1) contribute to the regulation of Ccq1 Thr93 phosphorylation and telomerase recruitment. In this study, we identify domains and amino acid residues that are critical for mediating Tpz1-Ccq1 and Tpz1-Poz1 interaction within the fission yeast shelterin complex. Using separation of function Tpz1 mutants that maintain Tpz1-Pot1 interaction but specifically disrupt either Tpz1-Ccq1 or Tpz1-Poz1 interaction, we then establish that Tpz1-Ccq1 interaction promotes Ccq1 Thr93 phosphorylation, telomerase recruitment, checkpoint inhibition and telomeric heterochromatin formation. Furthermore, we demonstrate that Tpz1-Poz1 interaction promotes telomere association of Poz1, and loss of Poz1 from telomeres leads to increases in Ccq1 Thr93 phosphorylation and telomerase recruitment, and telomeric heterochromatin formation defect. In addition, our studies establish that Tpz1-Poz1 and Tpz1-Ccq1 interactions redundantly fulfill the essential telomere protection function of the shelterin complex, since simultaneous loss of both interactions caused immediate loss of cell viability for the majority of cells and generation of survivors with circular chromosomes. Based on these findings, we suggest that the negative regulatory function of Tpz1-Poz1 interaction works upstream of Rad3ATR kinase, while Tpz1-Ccq1 interaction works downstream of Rad3ATR kinase to facilitate Ccq1 Thr93 phosphorylation and telomerase recruitment.
Author Summary
Proper maintenance of telomeres is essential for maintaining genomic stability, and genomic instability caused by dysfunctional telomeres could lead to accumulation of mutations that may drive tumor formation. Telomere dysfunction has also been linked to premature aging caused by depletion of stem cells. Therefore, it is important to understand how cells ensure proper maintenance of telomeres. Mammalian cells and fission yeast cells utilize an evolutionarily conserved multi-subunit telomere protection complex called shelterin to ensure protection against telomere fusions by DNA repair factors and cell cycle arrest by DNA damage checkpoint kinases. However, previous studies have not yet fully established how protein-protein interactions within the shelterin complex contribute to the regulation of DNA damage checkpoint signaling and telomerase recruitment. By utilizing separation of function mutations that specifically disrupt either Tpz1-Ccq1 or Tpz1-Poz1 interaction within the fission yeast shelterin, we establish that Tpz1-Ccq1 interaction is essential for phosphorylation of Ccq1 by the DNA damage checkpoint kinases Rad3ATR and Tel1ATM that is needed for telomerase recruitment to telomeres, while Tpz1-Poz1 interaction prevents Ccq1 phosphorylation by promoting Poz1 association with telomeres. These findings thus establish for the first time how protein-protein interactions within the shelterin complex modulate checkpoint kinase-dependent phosphorylation essential for telomerase recruitment.
PMCID: PMC4199508  PMID: 25330395
14.  Defective Repair of Oxidative Base Lesions by the DNA Glycosylase Nth1 Associates with Multiple Telomere Defects 
PLoS Genetics  2013;9(7):e1003639.
Telomeres are chromosome end structures and are essential for maintenance of genome stability. Highly repetitive telomere sequences appear to be susceptible to oxidative stress-induced damage. Oxidation may therefore have a severe impact on telomere integrity and function. A wide spectrum of oxidative pyrimidine-derivatives has been reported, including thymine glycol (Tg), that are primarily removed by a DNA glycosylase, Endonuclease III-like protein 1 (Nth1). Here, we investigate the effect of Nth1 deficiency on telomere integrity in mice. Nth1 null (Nth1−/−) mouse tissues and primary MEFs harbor higher levels of Endonuclease III-sensitive DNA lesions at telomeric repeats, in comparison to a non-telomeric locus. Furthermore, oxidative DNA damage induced by acute exposure to an oxidant is repaired slowly at telomeres in Nth1−/− MEFs. Although telomere length is not affected in the hematopoietic tissues of Nth1−/− adult mice, telomeres suffer from attrition and increased recombination and DNA damage foci formation in Nth1−/− bone marrow cells that are stimulated ex vivo in the presence of 20% oxygen. Nth1 deficiency also enhances telomere fragility in mice. Lastly, in a telomerase null background, Nth1−/− bone marrow cells undergo severe telomere loss at some chromosome ends and cell apoptosis upon replicative stress. These results suggest that Nth1 plays an important role in telomere maintenance and base repair against oxidative stress-induced base modifications. The fact that telomerase deficiency can exacerbate telomere shortening in Nth1 deficient mouse cells supports that base excision repair cooperates with telomerase to maintain telomere integrity.
Author Summary
Oxidative stress causes DNA base damage that is mainly repaired by base excision repair pathway, where a DNA glycosylase initiates the recognition and removal of specific base damage. Mammalian telomeres are composed of repetitive purine and pyrimidine bases, TTAGGG, which are prone to damage by oxidation. Though previously we have shown that oxidative damage to a purine base, guanine (G), affects telomere integrity, damage to other telomere bases, e.g. a pyrimidine base, thymine (T), may also occur and potentially disrupt telomere maintenance. In order to test this hypothesis, we utilize a mouse model lacking Endonuclease III-like protein 1 (Nth1), a DNA glycosylase that primarily recognizes and excises oxidative thymine and other pyrimidine damage. We show that Nth1 deficient mouse cells have higher levels of oxidative base damage at telomeres and display multiple telomere defects including telomere loss. Our studies support that besides oxidative guanine damage, other oxidative base damage can interfere with telomere maintenance. These results may be relevant to understanding how oxidative base damage and inefficient DNA repair contribute to telomere loss, aging and cancer susceptibility in humans and other mammals.
PMCID: PMC3715427  PMID: 23874233
15.  Surprises from the Chromosome Front: Lessons from Arabidopsis on Telomeres and Telomerase 
Cold Spring Harbor symposia on quantitative biology  2013;77:10.1101/sqb.2013.77.017053.
Telomeres serve two vital functions: They act as a buffer against the end-replication problem, and they prevent chromosome ends from being recognized as double-strand DNA (dsDNA) breaks. These functions are orchestrated by the telomerase reverse transcriptase and a variety of telomere protein complexes. Here, we discuss our recent studies with Arabidopsis thaliana that uncovered a new and highly conserved telomere complex called CST (Cdc13/CTC1, STN1, TEN1). Formerly believed to be yeast specific, CST has now been identified as a key component of both plant and vertebrate telomeres, which is essential for genome integrity and stem cell viability. We also describe the unexpected discovery of alternative telomerase ribonucleoprotein complexes in Arabidopsis. Fueled by duplication and diversification of the telomerase RNA subunit and telomerase accessory proteins, these telomerase complexes act in concert to maintain genome stability. In addition to the canonical telomerase enzyme, one of two alternative telomerase ribonucleoprotein (RNP) complexes functions as a novel negative regulator of enzyme activity in response to genotoxic stress. These contributions highlight the immense potential of Arabidopsis in probing the depths of the chromosome end.
PMCID: PMC3836370  PMID: 23460576
16.  Rif1 Supports the Function of the CST Complex in Yeast Telomere Capping 
PLoS Genetics  2011;7(3):e1002024.
Telomere integrity in budding yeast depends on the CST (Cdc13-Stn1-Ten1) and shelterin-like (Rap1-Rif1-Rif2) complexes, which are thought to act independently from each other. Here we show that a specific functional interaction indeed exists among components of the two complexes. In particular, unlike RIF2 deletion, the lack of Rif1 is lethal for stn1ΔC cells and causes a dramatic reduction in viability of cdc13-1 and cdc13-5 mutants. This synthetic interaction between Rif1 and the CST complex occurs independently of rif1Δ-induced alterations in telomere length. Both cdc13-1 rif1Δ and cdc13-5 rif1Δ cells display very high amounts of telomeric single-stranded DNA and DNA damage checkpoint activation, indicating that severe defects in telomere integrity cause their loss of viability. In agreement with this hypothesis, both DNA damage checkpoint activation and lethality in cdc13 rif1Δ cells are partially counteracted by the lack of the Exo1 nuclease, which is involved in telomeric single-stranded DNA generation. The functional interaction between Rif1 and the CST complex is specific, because RIF1 deletion does not enhance checkpoint activation in case of CST-independent telomere capping deficiencies, such as those caused by the absence of Yku or telomerase. Thus, these data highlight a novel role for Rif1 in assisting the essential telomere protection function of the CST complex.
Author Summary
Protection of chromosome ends is crucial for maintaining chromosome stability and genome integrity, and its failure leads to genome rearrangements that may facilitate carcinogenesis. This protection is achieved by the packaging of chromosome ends into protective structures called telomeres that prevent DNA repair/recombination activities. Telomeric DNA is bound and stabilized by two protein complexes named CST and shelterin, which are present in a wide range of multicellular organisms. Whether structural and functional connections exist between these two capping complexes is an important issue in telomere biology. Here, we investigate this topic by analyzing the consequences of disabling the two Saccharomyces cerevisiae shelterin-like components, Rif1 and Rif2, in different hypomorphic mutants defective in CST components. We demonstrate that Rif1 plays a previously unanticipated role in assisting the essential telomere protection function of the CST complex, indicating a tight coupling between CST and Rif1. As CST complexes have been recently identified also in other organisms, including humans, which all rely on shelterin for telomere protection, this functional link between CST and shelterin might be an evolutionarily conserved common feature to ensure telomere integrity.
PMCID: PMC3060071  PMID: 21437267
17.  Sensitivity of Yeast Strains with Long G-Tails to Levels of Telomere-Bound Telomerase 
PLoS Genetics  2007;3(6):e105.
The Saccharomyces cerevisiae Pif1p helicase is a negative regulator of telomere length that acts by removing telomerase from chromosome ends. The catalytic subunit of yeast telomerase, Est2p, is telomere associated throughout most of the cell cycle, with peaks of association in both G1 phase (when telomerase is not active) and late S/G2 phase (when telomerase is active). The G1 association of Est2p requires a specific interaction between Ku and telomerase RNA. In mutants lacking this interaction, telomeres were longer in the absence of Pif1p than in the presence of wild-type PIF1, indicating that endogenous Pif1p inhibits the active S/G2 form of telomerase. Pif1p abundance was cell cycle regulated, low in G1 and early S phase and peaking late in the cell cycle. Low Pif1p abundance in G1 phase was anaphase-promoting complex dependent. Thus, endogenous Pif1p is unlikely to act on G1 bound Est2p. Overexpression of Pif1p from a non-cell cycle-regulated promoter dramatically reduced viability in five strains with impaired end protection (cdc13–1, yku80Δ, yku70Δ, yku80–1, and yku80–4), all of which have longer single-strand G-tails than wild-type cells. This reduced viability was suppressed by deleting the EXO1 gene, which encodes a nuclease that acts at compromised telomeres, suggesting that the removal of telomerase by Pif1p exposed telomeres to further C-strand degradation. Consistent with this interpretation, depletion of Pif1p, which increases the amount of telomere-bound telomerase, suppressed the temperature sensitivity of yku70Δ and cdc13–1 cells. Furthermore, eliminating the pathway that recruits Est2p to telomeres in G1 phase in a cdc13–1 strain also reduced viability. These data suggest that wild-type levels of telomere-bound telomerase are critical for the viability of strains whose telomeres are already susceptible to degradation.
Author Summary
Telomeres, the ends of linear chromosomes, are essential for chromosome stability. Telomerase is the enzyme that is responsible for lengthening telomeres in most organisms, including humans. One mechanism of survival for many human cancers is increased expression of telomerase. In baker's yeast, telomerase acts only late in the cell cycle, even though the catalytic subunit of telomerase is telomere bound throughout most of the cell cycle. Pif1p is a yeast helicase that limits telomerase by removing it from DNA ends. We demonstrate that Pif1p abundance is cell cycle regulated with its highest expression at the same time when telomerase acts. Consistent with this expression pattern, Pif1p is able to remove the active form of telomerase from DNA ends. Reducing the amount of telomere-bound telomerase either by Pif1p overexpression or by mutation in strains with defective telomere end protection causes death. Moreover, reducing Pif1p levels in the same end protection mutants improves their growth. These experiments suggest that compared to wild-type cells, strains with defective end protection require more telomere-bound telomerase for the proper replication or proper protection of their chromosome ends.
PMCID: PMC1892048  PMID: 17590086
18.  Epigenetic Telomere Protection by Drosophila DNA Damage Response Pathways 
PLoS Genetics  2006;2(5):e71.
Analysis of terminal deletion chromosomes indicates that a sequence-independent mechanism regulates protection of Drosophila telomeres. Mutations in Drosophila DNA damage response genes such as atm/tefu, mre11, or rad50 disrupt telomere protection and localization of the telomere-associated proteins HP1 and HOAP, suggesting that recognition of chromosome ends contributes to telomere protection. However, the partial telomere protection phenotype of these mutations limits the ability to test if they act in the epigenetic telomere protection mechanism. We examined the roles of the Drosophila atm and atr-atrip DNA damage response pathways and the nbs homolog in DNA damage responses and telomere protection. As in other organisms, the atm and atr-atrip pathways act in parallel to promote telomere protection. Cells lacking both pathways exhibit severe defects in telomere protection and fail to localize the protection protein HOAP to telomeres. Drosophila nbs is required for both atm- and atr-dependent DNA damage responses and acts in these pathways during DNA repair. The telomere fusion phenotype of nbs is consistent with defects in each of these activities. Cells defective in both the atm and atr pathways were used to examine if DNA damage response pathways regulate telomere protection without affecting telomere specific sequences. In these cells, chromosome fusion sites retain telomere-specific sequences, demonstrating that loss of these sequences is not responsible for loss of protection. Furthermore, terminally deleted chromosomes also fuse in these cells, directly implicating DNA damage response pathways in the epigenetic protection of telomeres. We propose that recognition of chromosome ends and recruitment of HP1 and HOAP by DNA damage response proteins is essential for the epigenetic protection of Drosophila telomeres. Given the conserved roles of DNA damage response proteins in telomere function, related mechanisms may act at the telomeres of other organisms.
Organisms with linear chromosomes must distinguish between the naturally occurring ends of chromosomes (telomeres) and chromosome breaks due to DNA damage. Many eukaryotic cells use DNA binding proteins that specifically recognize telomeric DNA sequences to protect telomeric DNA ends from the inappropriate action of DNA repair enzymes. In Drosophila melanogaster, however, chromosomes that lack telomere-specific sequences can be isolated and stably maintained. Thus, telomere protection can be inherited via a sequence-independent or epigenetic mechanism. Oikemus et al. demonstrate that two groups of genes that help cells respond to DNA damage are also required to localize a telomere protection protein to chromosome ends. Two experiments are described to support the model that these DNA damage detection genes help maintain telomere protection regardless of telomere sequence. First, mutations in these genes lead to loss of telomere protection without loss of telomeric DNA. Second, telomeres that lack all telomere-specific sequences still require these genes for protection. Combined, these experiments suggest that recognition of DNA ends is required for sequence-independent protection of Drosophila melanogaster telomeres. Since the same genes also promote telomere protection in yeast, plants, and mammals, these observations may be relevant to chromosome function in many organisms.
PMCID: PMC1463044  PMID: 16710445
19.  Defective Resection at DNA Double-Strand Breaks Leads to De Novo Telomere Formation and Enhances Gene Targeting 
PLoS Genetics  2010;6(5):e1000948.
The formation of single-stranded DNA (ssDNA) at double-strand break (DSB) ends is essential in repair by homologous recombination and is mediated by DNA helicases and nucleases. Here we estimated the length of ssDNA generated during DSB repair and analyzed the consequences of elimination of processive resection pathways mediated by Sgs1 helicase and Exo1 nuclease on DSB repair fidelity. In wild-type cells during allelic gene conversion, an average of 2–4 kb of ssDNA accumulates at each side of the break. Longer ssDNA is formed during ectopic recombination or break-induced replication (BIR), reflecting much slower repair kinetics. This relatively extensive resection may help determine sequences involved in homology search and prevent recombination within short DNA repeats next to the break. In sgs1Δ exo1Δ mutants that form only very short ssDNA, allelic gene conversion decreases 5-fold and DSBs are repaired by BIR or de novo telomere formation resulting in loss of heterozygosity. The absence of the telomerase inhibitor, PIF1, increases de novo telomere pathway usage to about 50%. Accumulation of Cdc13, a protein recruiting telomerase, at the break site increases in sgs1Δ exo1Δ, and the requirement of the Ku complex for new telomere formation is partially bypassed. In contrast to this decreased and alternative DSB repair, the efficiency and accuracy of gene targeting increases dramatically in sgs1Δ exo1Δ cells, suggesting that transformed DNA is very stable in these mutants. Altogether these data establish a new role for processive resection in the fidelity of DSB repair.
Author Summary
Chromosomal breaks occur spontaneously or are induced by ionizing radiation and many chemotherapeutic drugs. DNA double-strand breaks are processed by nucleases and helicases in yeast and human to generate single-stranded DNA that is then used for repair by recombination with homologous chromosome. Single-stranded DNA at chromosomal breaks also constitutes a signal for cells to arrest cell cycle progression until the DNA damage is repaired. This study examines the consequences of elimination of enzymes that process chromosomal breaks to single-stranded DNA on the fidelity of repair and genome stability in the model organism yeast. Mutants deficient in these enzymes often fail to repair the breaks by homologous recombination and instead add new telomeres at the breaks. Formation of new telomeres is associated with partial loss of the chromosome arm distal from the break. Such chromosomal aberrations were frequently observed in tumor cells and are responsible for about 10% of human genomic disorders resulting from chromosomal abnormalities. We also observed that elimination of enzymes that process chromosomal breaks into single-stranded DNA greatly stimulates genome manipulation by gene targeting, suggesting that transformed DNA is also a substrate for degradation by these enzymes. We discuss the possibility of using a similar approach in mammalian cells where gene targeting is inaccurate and less efficient when compared to yeast.
PMCID: PMC2869328  PMID: 20485519
20.  Progressive Telomere Dysfunction Causes Cytokinesis Failure and Leads to the Accumulation of Polyploid Cells 
PLoS Genetics  2012;8(4):e1002679.
Most cancer cells accumulate genomic abnormalities at a remarkably rapid rate, as they are unable to maintain their chromosome structure and number. Excessively short telomeres, a known source of chromosome instability, are observed in early human-cancer lesions. Besides telomere dysfunction, it has been suggested that a transient phase of polyploidization, in most cases tetraploidization, has a causative role in cancer. Proliferation of tetraploids can gradually generate subtetraploid lineages of unstable cells that might fire the carcinogenic process by promoting further aneuploidy and genomic instability. Given the significance of telomere dysfunction and tetraploidy in the early stages of carcinogenesis, we investigated whether there is a connection between these two important promoters of chromosomal instability. We report that human mammary epithelial cells exhibiting progressive telomere dysfunction, in a pRb deficient and wild-type p53 background, fail to complete the cytoplasmatic cell division due to the persistence of chromatin bridges in the midzone. Flow cytometry together with fluorescence in situ hybridization demonstrated an accumulation of binucleated polyploid cells upon serial passaging cells. Restoration of telomere function through hTERT transduction, which lessens the formation of anaphase bridges by recapping the chromosome ends, rescued the polyploid phenotype. Live-cell imaging revealed that these polyploid cells emerged after abortive cytokinesis due to the persistence of anaphase bridges with large intervening chromatin in the cleavage plane. In agreement with a primary role of anaphase bridge intermediates in the polyploidization process, treatment of HMEC-hTERT cells with bleomycin, which produces chromatin bridges through illegimitate repair, resulted in tetraploid binucleated cells. Taken together, we demonstrate that human epithelial cells exhibiting physiological telomere dysfunction engender tetraploid cells through interference of anaphase bridges with the completion of cytokinesis. These observations shed light on the mechanisms operating during the initial stages of human carcinogenesis, as they provide a link between progressive telomere dysfunction and tetraploidy.
Author Summary
Chromosome instability leads to the accumulation of chromosome number and structure aberrations that have been suggested as necessary for neoplastic transformation. Telomeres, specialized DNA–protein complexes localized at the physical ends of linear chromosomes, are crucial for maintaining chromosome stability. Massive chromosomal instability may occur when cells continuously proliferate in the absence of specific telomere elongation mechanisms. Besides telomere dysfunction, it has been suggested that a transient phase of tetraploidization has a causative role in cancer. This study provides a link between dysfunctional telomeres and the generation of tetraploids. Using a human mammary epithelial cell model, we show that diploid cells exhibiting progressive telomere dysfunction, in a p53 proficient background, engender tetraploid cells through cytokinesis failure. Our studies give new insights into the mechanisms that may facilitate the evolution of malignant phenotypes: telomere-dependent chromosome instability would engender tetraploid intermediates that, on division, would promote further cellular genome remodelling, which is needed at the early stages of tumour development in order for cells to become neoplasic.
PMCID: PMC3343085  PMID: 22570622
21.  Recognition of Double Strand Breaks by a Mutator Protein (MU2) in Drosophila melanogaster 
PLoS Genetics  2009;5(5):e1000473.
Telomere capture, a rare event that stabilizes chromosome breaks, is associated with certain genetic abnormalities in humans. Studies pertaining to the generation, maintenance, and biological effects of telomere formation are limited in metazoans. A mutation, mu2a, in Drosophila melanogaster decreases the rate of repair of double strand DNA breaks in oocytes, thus leading to chromosomes that have lost a natural telomere and gained a new telomere. Amino acid sequence, domain architecture, and protein interactions suggest that MU2 is an ortholog of human MDC1. The MU2 protein is a component of meiotic recombination foci and localizes to repair foci in S2 cells after irradiation in a manner similar to that of phosphorylated histone variant H2Av. Domain searches indicated that the protein contains an N-terminal FHA domain and a C-terminal tandem BRCT domain. Peptide pull-down studies showed that the BRCT domain interacts with phosphorylated H2Av, while the FHA domain interacts with the complex of MRE11, RAD50, and NBS. A frameshift mutation that eliminates the MU2 BRCT domain decreases the number and size of meiotic phospho-H2Av foci. MU2 is also required for the intra-S checkpoint in eye-antennal imaginal discs. MU2 participates at an early stage in the recognition of DNA damage at a step that is prerequisite for both DNA repair and cell cycle checkpoint control. We propose a model suggesting that neotelomeres may arise when radiation-induced chromosome breaks fail to be repaired, fail to arrest progression through meiosis, and are deposited in the zygote, where cell cycle control is absent and rapid rounds of replication and telomere formation ensue.
Author Summary
Telomeres are structures at the ends of eukaryotic chromosomes required for chromosome stability. If unrepaired, a single chromosome end without a telomere is sufficient to kill a cell, but new telomere formation is rare. Previously, we described a gene in Drosophila whose mutants, after irradiation, produced many progeny with chromosomes lacking a natural telomere. The new broken chromosome ends, however, bound telomeric proteins and behaved as telomeres. Here, we show that the protein encoded by this gene, a homolog of the human MDC1 gene, is a component of the repair foci that form at double strand DNA breaks and are prerequisite for both cell cycle arrest and DNA repair. The protein acts as a scaffold, connecting a phosphorylated histone that marks the site of the break to a protein complex necessary for repair. These results suggest a model for formation of neotelomeres in which DNA breaks induced in mutant oocytes evade repair and are deposited into embryos, which contain an abundance of maternally deposited telomeric proteins. In this context a chromosome end not recognized as broken may be treated as a telomere. These results may provide a basis to understand neotelomere formation.
PMCID: PMC2672170  PMID: 19424425
22.  ATR cooperates with CTC1 and STN1 to maintain telomeres and genome integrity in Arabidopsis 
Molecular Biology of the Cell  2012;23(8):1558-1568.
Telomeres protect chromosome ends from DNA damage. CTC1/STN1/TEN1 (CST), a core telomere-capping complex in plant and vertebrates, suppresses an ATR-dependent DNA damage response in Arabidopsis. Protracted ATR inactivation inhibits telomerase, hastening the onset of telomere dysfunction in CST mutants.
The CTC1/STN1/TEN1 (CST) complex is an essential constituent of plant and vertebrate telomeres. Here we show that CST and ATR (ataxia telangiectasia mutated [ATM] and Rad3-related) act synergistically to maintain telomere length and genome stability in Arabidopsis. Inactivation of ATR, but not ATM, temporarily rescued severe morphological phenotypes associated with ctc1 or stn1. Unexpectedly, telomere shortening accelerated in plants lacking CST and ATR. In first-generation (G1) ctc1 atr mutants, enhanced telomere attrition was modest, but in G2 ctc1 atr, telomeres shortened precipitously, and this loss coincided with a dramatic decrease in telomerase activity in G2 atr mutants. Zeocin treatment also triggered a reduction in telomerase activity, suggesting that the prolonged absence of ATR leads to a hitherto-unrecognized DNA damage response (DDR). Finally, our data indicate that ATR modulates DDR in CST mutants by limiting chromosome fusions and transcription of DNA repair genes and also by promoting programmed cell death in stem cells. We conclude that the absence of CST in Arabidopsis triggers a multifaceted ATR-dependent response to facilitate maintenance of critically shortened telomeres and eliminate cells with severe telomere dysfunction.
PMCID: PMC3327312  PMID: 22357613
23.  Quantitative Fitness Analysis Shows That NMD Proteins and Many Other Protein Complexes Suppress or Enhance Distinct Telomere Cap Defects 
PLoS Genetics  2011;7(4):e1001362.
To better understand telomere biology in budding yeast, we have performed systematic suppressor/enhancer analyses on yeast strains containing a point mutation in the essential telomere capping gene CDC13 (cdc13-1) or containing a null mutation in the DNA damage response and telomere capping gene YKU70 (yku70Δ). We performed Quantitative Fitness Analysis (QFA) on thousands of yeast strains containing mutations affecting telomere-capping proteins in combination with a library of systematic gene deletion mutations. To perform QFA, we typically inoculate 384 separate cultures onto solid agar plates and monitor growth of each culture by photography over time. The data are fitted to a logistic population growth model; and growth parameters, such as maximum growth rate and maximum doubling potential, are deduced. QFA reveals that as many as 5% of systematic gene deletions, affecting numerous functional classes, strongly interact with telomere capping defects. We show that, while Cdc13 and Yku70 perform complementary roles in telomere capping, their genetic interaction profiles differ significantly. At least 19 different classes of functionally or physically related proteins can be identified as interacting with cdc13-1, yku70Δ, or both. Each specific genetic interaction informs the roles of individual gene products in telomere biology. One striking example is with genes of the nonsense-mediated RNA decay (NMD) pathway which, when disabled, suppress the conditional cdc13-1 mutation but enhance the null yku70Δ mutation. We show that the suppressing/enhancing role of the NMD pathway at uncapped telomeres is mediated through the levels of Stn1, an essential telomere capping protein, which interacts with Cdc13 and recruitment of telomerase to telomeres. We show that increased Stn1 levels affect growth of cells with telomere capping defects due to cdc13-1 and yku70Δ. QFA is a sensitive, high-throughput method that will also be useful to understand other aspects of microbial cell biology.
Author Summary
Telomeres, specialized structures at the end of linear chromosomes, ensure that chromosome ends are not mistakenly treated as DNA double-strand breaks. Defects in the telomere cap contribute to ageing and cancer. In yeast, defects in telomere capping proteins can cause telomeres to behave like double-strand breaks. To better understand the telomere and responses to capping failure, we have combined a systematic yeast gene deletion library with mutations affecting important yeast telomere capping proteins, Cdc13 or Yku70. Quantitative Fitness Analysis (QFA) was used to accurately measure the fitness of thousands of different yeast strains containing telomere capping defects and additional deletion mutations. Interestingly, we find that many gene deletions suppress one type of telomere capping defect while enhancing another. Through QFA, we can begin to define the roles of different gene products in contributing to different aspects of the telomere cap. Strikingly, mutations in nonsense-mediated mRNA decay pathways, which degrade many RNA molecules, suppress the cdc13-1 defect while enhancing the yku70Δ defect. QFA is widely applicable and will be useful for understanding other aspects of yeast cell biology.
PMCID: PMC3072368  PMID: 21490951
24.  Analysis of Poly(ADP-Ribose) Polymerases in Arabidopsis Telomere Biology 
PLoS ONE  2014;9(2):e88872.
Maintaining the length of the telomere tract at chromosome ends is a complex process vital to normal cell division. Telomere length is controlled through the action of telomerase as well as a cadre of telomere-associated proteins that facilitate replication of the chromosome end and protect it from eliciting a DNA damage response. In vertebrates, multiple poly(ADP-ribose) polymerases (PARPs) have been implicated in the regulation of telomere length, telomerase activity and chromosome end protection. Here we investigate the role of PARPs in plant telomere biology. We analyzed Arabidopsis thaliana mutants null for PARP1 and PARP2 as well as plants treated with the PARP competitive inhibitor 3-AB. Plants deficient in PARP were hypersensitive to genotoxic stress, and expression of PARP1 and PARP2 mRNA was elevated in response to MMS or zeocin treatment or by the loss of telomerase. Additionally, PARP1 mRNA was induced in parp2 mutants, and conversely, PARP2 mRNA was induced in parp1 mutants. PARP3 mRNA, by contrast, was elevated in both parp1 and parp2 mutants, but not in seedlings treated with 3-AB or zeocin. PARP mutants and 3-AB treated plants displayed robust telomerase activity, no significant changes in telomere length, and no end-to-end chromosome fusions. Although there remains a possibility that PARPs play a role in Arabidopsis telomere biology, these findings argue that the contribution is a minor one.
PMCID: PMC3923816  PMID: 24551184
25.  Evidence That a RecQ Helicase Slows Senescence by Resolving Recombining Telomeres  
PLoS Biology  2007;5(6):e160.
RecQ helicases, including Saccharomyces cerevisiae Sgs1p and the human Werner syndrome protein, are important for telomere maintenance in cells lacking telomerase activity. How maintenance is accomplished is only partly understood, although there is evidence that RecQ helicases function in telomere replication and recombination. Here we use two-dimensional gel electrophoresis (2DGE) and telomere sequence analysis to explore why cells lacking telomerase and Sgs1p (tlc1 sgs1 mutants) senesce more rapidly than tlc1 mutants with functional Sgs1p. We find that apparent X-shaped structures accumulate at telomeres in senescing tlc1 sgs1 mutants in a RAD52- and RAD53-dependent fashion. The X-structures are neither Holliday junctions nor convergent replication forks, but instead may be recombination intermediates related to hemicatenanes. Direct sequencing of examples of telomere I-L in senescing cells reveals a reduced recombination frequency in tlc1 sgs1 compared with tlc1 mutants, indicating that Sgs1p is needed for tlc1 mutants to complete telomere recombination. The reduction in recombinants is most prominent at longer telomeres, consistent with a requirement for Sgs1p to generate viable progeny following telomere recombination. We therefore suggest that Sgs1p may be required for efficient resolution of telomere recombination intermediates, and that resolution failure contributes to the premature senescence of tlc1 sgs1 mutants.
Author Summary
Because telomeres are situated at the ends of chromosomes, they are both essential for chromosome integrity and particularly susceptible to processes that lead to loss of their own DNA sequences. The enzyme telomerase can counter these losses, but there are also other means of telomere maintenance, some of which depend on DNA recombination. The RecQ family of DNA helicases process DNA recombination intermediates and also help ensure telomere integrity, but the relationship between these activities is poorly understood. Family members include yeast Sgs1p and human WRN and BLM, which are deficient in the Werner premature aging syndrome and the Bloom cancer predisposition syndrome, respectively. We have found that the telomeres of yeast cells lacking both telomerase and Sgs1p accumulate structures that resemble recombination intermediates. Further, we provide evidence that the inability of cells lacking Sgs1p to process these telomere recombination intermediates leads to the premature arrest of cell division. We predict that similar defects in the processing of recombination intermediates may contribute to telomere defects in human Werner and Bloom syndrome cells.
Yeast cells lacking the RecQ helicase Sgs1p show an accumulation of telomere recombination intermediates associated with premature senescence.
PMCID: PMC1885831  PMID: 17550308

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