Telomeres avoid end detection by severing the checkpoint signal transduction pathway

Targeting Telomere Dynamics as an Effective Approach for the Development of Cancer Therapeutics

Telomere is a protective structure located at the end of chromosomes of eukaryotes, involved in maintaining the integrity and stability of the genome. Telomeres play an essential role in cancer progression; accordingly, targeting telomere dynamics emerges as an effective approach for the development of cancer therapeutics. Targeting telomere dynamics may work through multifaceted molecular mechanisms; those include the activation of anti-telomerase immune responses, shortening of telomere lengths, induction of telomere dysfunction and constitution of telomerase-responsive drug release systems. In this review, we summarize a wide variety of telomere dynamics-targeted agents in preclinical studies and clinical trials, and reveal their promising therapeutic potential in cancer therapy. As shown, telomere dynamics-active agents are effective as anti-cancer chemotherapeutics and immunotherapeutics. Notably, these agents may display efficacy against cancer stem cells, reducing cancer stem levels. Furthermore, these agents can be integrated with the capability of tumor-specific drug delivery by the constitution of related nanoparticles, antibody drug conjugates and HSA-based drugs.

Pot1 promotes telomere DNA replication via the Stn1-Ten1 complex in fission yeast

Telomeres are nucleoprotein complexes that protect the chromosome-ends from eliciting DNA repair while ensuring their complete duplication. Pot1 is a subunit of telomere capping complex that binds to the G-rich overhang and inhibits the activation of DNA damage checkpoints. In this study, we explore new functions of fission yeast Pot1 by using a pot1-1 temperature sensitive mutant. We show that pot1 inactivation impairs telomere DNA replication resulting in the accumulation of ssDNA leading to the complete loss of telomeric DNA. Recruitment of Stn1 to telomeres, an auxiliary factor of DNA lagging strand synthesis, is reduced in pot1-1 mutants and overexpression of Stn1 rescues loss of telomeres and cell viability at restrictive temperature. We propose that Pot1 plays a crucial function in telomere DNA replication by recruiting Stn1-Ten1 and Polα-primase complex to telomeres via Tpz1, thus promoting lagging-strand DNA synthesis at stalled replication forks.

High-throughput screen to identify compounds that prevent or target telomere loss in human cancer cells

Chromosome instability (CIN) is an early step in carcinogenesis that promotes tumor cell progression and resistance to therapy. Using plasmids integrated adjacent to telomeres, we have previously demonstrated that the sensitivity of subtelomeric regions to DNA double-strand breaks (DSBs) contributes to telomere loss and CIN in cancer. A high-throughput screen was created to identify compounds that affect telomere loss due to subtelomeric DSBs introduced by I-SceI endonuclease, as detected by cells expressing green fluorescent protein (GFP). A screen of a library of 1832 biologically-active compounds identified a variety of compounds that increase or decrease the number of GFP-positive cells following activation of I-SceI. A curated screen done in triplicate at various concentrations found that inhibition of classical nonhomologous end joining (C-NHEJ) increased DSB-induced telomere loss, demonstrating that C-NHEJ is functional in subtelomeric regions. Compounds that decreased DSB-induced telomere loss included inhibitors of mTOR, p38 and tankyrase, consistent with our earlier hypothesis that the sensitivity of subtelomeric regions to DSBs is a result of inappropriate resection during repair. Although this assay was also designed to identify compounds that selectively target cells experiencing telomere loss and/or chromosome instability, no compounds of this type were identified in the current screen.

Fission yeast Stn1 maintains stability of repetitive DNA at subtelomere and ribosomal DNA regions

Telomere binding protein Stn1 forms the CST (Cdc13/CTC1-STN1-TEN1) complex in budding yeast and mammals. Likewise, fission yeast Stn1 and Ten1 form a complex indispensable for telomere protection. We have previously reported that stn1-1, a high-temperature sensitive mutant, rapidly loses telomere DNA at the restrictive temperature due to frequent failure of replication fork progression at telomeres and subtelomeres, both containing repetitive sequences. It is unclear, however, whether Stn1 is required for maintaining other repetitive DNAs such as ribosomal DNA. In this study, we have demonstrated that stn1-1 cells, even when grown at the permissive temperature, exhibited dynamic rearrangements in the telomere-proximal regions of subtelomere and ribosomal DNA repeats. Furthermore, Rad52 and γH2A accumulation was observed at ribosomal DNA repeats in the stn1-1 mutant. The phenotypes exhibited by the stn1-1 allele were largely suppressed in the absence of Reb1, a replication fork barrier-forming protein, suggesting that Stn1 is involved in the maintenance of the arrested replication forks. Collectively, we propose that Stn1 maintains the stability of repetitive DNAs at subtelomeres and rDNA regions.

Monitoring Schizosaccharomyces pombe genome stress by visualizing end-binding protein Ku

Studies of genome stability have exploited visualization of fluorescently tagged proteins in live cells to characterize DNA damage, checkpoint, and repair responses. In this report, we describe a new tool for fission yeast, a tagged version of the end-binding protein Pku70 which is part of the KU protein complex. We compare Pku70 localization to other markers upon treatment to various genotoxins, and identify a unique pattern of distribution. Pku70 provides a new tool to define and characterize DNA lesions and the repair response.

Summary: The authors describe a fluorescently tagged Ku70 protein to monitor replication stress in live S. pombe cells.

RNA-DNA Hybrids Support Recombination-Based Telomere Maintenance in Fission Yeast

A subset of cancers rely on telomerase-independent mechanisms to maintain their chromosome ends. The predominant "alternative lengthening of telomeres" pathway appears dependent on homology-directed repair (HDR) to maintain telomeric DNA. However, the molecular changes needed for cells to productively engage in telomeric HDR are poorly understood. To gain new insights into this transition, we monitored the state of telomeres during serial culture of fission yeast (Schizosaccharomyces pombe) lacking the telomerase recruitment factor Ccq1. Rad52 is loaded onto critically short telomeres shortly after germination despite continued telomere erosion, suggesting that recruitment of recombination factors is not sufficient to maintain telomeres in the absence of telomerase function. Instead, survivor formation coincides with the derepression of telomeric repeat-containing RNA (TERRA). In this context, degradation of TERRA associated with the telomere in the form of R-loops drives a severe growth crisis, ultimately leading to a novel type of survivor with linear chromosomes and altered cytological telomere characteristics, including the loss of the shelterin component Rap1 (but not the TRF1/TRF2 ortholog, Taz1) from the telomere. We demonstrate that deletion of Rap1 is protective in this context, preventing the growth crisis that is otherwise triggered by degradation of telomeric R-loops in survivors with linear chromosomes. These findings suggest that upregulation of telomere-engaged TERRA, or altered recruitment of shelterin components, can support telomerase-independent telomere maintenance.

Telomeres, Telomerase and Ageing

Telomeres are specialised structures at the end of linear chromosomes. They consist of tandem repeats of the hexanucleotide sequence TTAGGG, as well as a protein complex called shelterin. Together, they form a protective loop structure against chromosome fusion and degradation. Shortening or damage to telomeres and opening of the loop induce an uncapped state that triggers a DNA damage response resulting in senescence or apoptosis.Average telomere length, usually measured in human blood lymphocytes, was thought to be a biomarker for ageing, survival and mortality. However, it becomes obvious that regulation of telomere length is very complex and involves multiple processes. For example, the "end replication problem" during DNA replication as well as oxidative stress are responsible for the shortening of telomeres. In contrast, telomerase activity can potentially counteract telomere shortening when it is able to access and interact with telomeres. However, while highly active during development and in cancer cells, the enzyme is down-regulated in most human somatic cells with a few exceptions such as human lymphocytes. In addition, telomeres can be transcribed, and the transcription products called TERRA are involved in telomere length regulation.Thus, telomere length and their integrity are regulated at many different levels, and we only start to understand this process under conditions of increased oxidative stress, inflammation and during diseases as well as the ageing process.This chapter aims to describe our current state of knowledge on telomeres and telomerase and their regulation in order to better understand their role for the ageing process.

Ssu72 phosphatase is a conserved telomere replication terminator

Telomeres, the protective ends of eukaryotic chromosomes, are replicated through concerted actions of conventional DNA polymerases and elongated by telomerase, but the regulation of this process is not fully understood. Telomere replication requires (Ctc1/Cdc13)‐Stn1‐Ten1, a telomeric ssDNA‐binding complex homologous to RPA. Here, we show that the evolutionarily conserved phosphatase Ssu72 is responsible for terminating the cycle of telomere replication in fission yeast. Ssu72 controls the recruitment of Stn1 to telomeres by regulating Stn1 phosphorylation at Ser74, a residue located within its conserved OB‐fold domain. Consequently, ssu72∆ mutants are defective in telomere replication and exhibit long 3′‐ssDNA overhangs, indicative of defective lagging‐strand DNA synthesis. We also show that hSSU72 regulates telomerase activation in human cells by controlling recruitment of hSTN1 to telomeres. These results reveal a previously unknown yet conserved role for the phosphatase SSU72, whereby this enzyme controls telomere homeostasis by activating lagging‐strand DNA synthesis, thus terminating the cycle of telomere replication.

A subtelomeric region affects telomerase-negative replicative senescence in Saccharomyces cerevisiae

In eukaryotes, telomeres determine cell proliferation potential by triggering replicative senescence in the absence of telomerase. In Saccharomyces cerevisiae, senescence is mainly dictated by the first telomere that reaches a critically short length, activating a DNA-damage-like response. How the corresponding signaling is modulated by the telomeric structure and context is largely unknown. Here we investigated how subtelomeric elements of the shortest telomere in a telomerase-negative cell influence the onset of senescence. We found that a 15 kb truncation of the 7L subtelomere widely used in studies of telomere biology affects cell growth when combined with telomerase inactivation. This effect is likely not explained by (i) elimination of sequence homology at chromosome ends that would compromise homology-directed DNA repair mechanisms; (ii) elimination of the conserved subtelomeric X-element; (iii) elimination of a gene that would become essential in the absence of telomerase; and (iv) heterochromatinization of inner genes, causing the silencing of an essential gene in replicative senescent cells. This works contributes to better delineate subtelomere functions and their impact on telomere biology.

The inhibition of checkpoint activation by telomeres does not involve exclusion of dimethylation of histone H4 lysine 20 (H4K20me2)

DNA double-strand (DSBs) breaks activate the DNA damage checkpoint machinery to pause or halt the cell cycle.  Telomeres, the specific DNA-protein complexes at linear eukaryotic chromosome ends, are capped DSBs that do not activate DNA damage checkpoints.  This "checkpoint privileged" status of telomeres was previously investigated in the yeast Schizosaccharomyces pombe lacking the major double-stranded telomere DNA binding protein Taz1.  Telomeric DNA repeats in cells lacking Taz1 are 10 times longer than normal and contain single-stranded DNA regions.  DNA damage checkpoint proteins associate with these damaged telomeres, but the DNA damage checkpoint is not activated.  This severing of the DNA damage checkpoint signaling pathway was reported to stem from exclusion of histone H4 lysine 20 dimethylation (H4K20me2) from telomeric nucleosomes in both wild type cells and cells lacking Taz1.  However, experiments to identify the mechanism of this exclusion failed, prompting our re-evaluation of H4K20me2 levels at telomeric chromatin.  In this short report, we used an extensive series of controls to identify an antibody specific for the H4K20me2 modification and show that the level of this modification is the same at telomeres and internal loci in both wild type cells and those lacking Taz1.  Consequently, telomeres must block activation of the DNA Damage Response by another mechanism that remains to be determined.

The inhibition of checkpoint activation by telomeres does not involve exclusion of dimethylation of histone H4 lysine 20 (H4K20me2)

DNA double-strand breaks (DSBs) activate the DNA damage checkpoint machinery to pause or halt the cell cycle.  Telomeres, the specific DNA-protein complexes at linear eukaryotic chromosome ends, are capped DSBs that do not activate DNA damage checkpoints.  This "checkpoint privileged" status of telomeres was previously investigated in the yeast  Schizosaccharomyces pombelacking the major double-stranded telomere DNA binding protein Taz1. Telomeric DNA repeats in cells lacking Taz1 are 10 times longer than normal and contain single-stranded DNA regions. DNA damage checkpoint proteins associate with these damaged telomeres, but the DNA damage checkpoint is not activated. This severing of the DNA damage checkpoint signaling pathway was reported to stem from exclusion of histone H4 lysine 20 dimethylation (H4K20me2) from telomeric nucleosomes in both wild type cells and cells lacking Taz1.  However, experiments to identify the mechanism of this exclusion failed, prompting our re-evaluation of H4K20me2 levels at telomeric chromatin.  In this short report, we used an extensive series of controls to identify an antibody specific for the H4K20me2 modification and show that the level of this modification is the same at telomeres and internal loci in both wild type cells and those lacking Taz1.  Consequently, telomeres must block activation of the DNA Damage Response by another mechanism that remains to be determined.

Fundamental mechanisms of telomerase action in yeasts and mammals: understanding telomeres and telomerase in cancer cells

Aberrant activation of telomerase occurs in 85–90% of all cancers and underpins the ability of cancer cells to bypass their proliferative limit, rendering them immortal. The activity of telomerase is tightly controlled at multiple levels, from transcriptional regulation of the telomerase components to holoenzyme biogenesis and recruitment to the telomere, and finally activation and processivity. However, studies using cancer cell lines and other model systems have begun to reveal features of telomeres and telomerase that are unique to cancer. This review summarizes our current knowledge on the mechanisms of telomerase recruitment and activation using insights from studies in mammals and budding and fission yeasts. Finally, we discuss the differences in telomere homeostasis between normal cells and cancer cells, which may provide a foundation for telomere/telomerase targeted cancer treatments.

Analysis of DNA Metabolism in Fission Yeast

The fission yeast Schizosaccharomyces pombe is an excellent model organism to study DNA metabolism, in which the DNA replication and repair mechanisms are evolutionarily conserved. In this introduction we describe a range of methods commonly used to study aspects of DNA metabolism in fission yeast, focusing on approaches used for the analysis of genome stability, DNA replication, and DNA repair. We describe the use of a minichromosome, Ch¹⁶, for monitoring different aspects of genome stability. We introduce two-dimensional gel electrophoresis and immunofluorescent visualization of combed DNA molecules for the analysis of DNA replication. Further, we introduce a pulsed field gel electrophoresis (PFGE) assay to physically monitor chromosome integrity, which can be used in conjunction with a DNA double-strand break (DSB) repair assay to genetically quantitate different DSB repair and misrepair outcomes, including gross chromosomal rearrangements, in fission yeast.

Structure of the fission yeast S. pombe telomeric Tpz1-Poz1-Rap1 complex

Telomeric shelterin complex caps chromosome ends and plays a crucial role in telomere maintenance and protection. In the fission yeast Schizosaccharomyces pombe, shelterin is composed of telomeric single- and double-stranded DNA-binding protein subcomplexes Pot1-Tpz1 and Taz1-Rap1, which are bridged by their interacting protein Poz1. However, the structure of Poz1 and how Poz1 functions as an interaction hub in the shelterin complex remain unclear. Here we report the crystal structure of Poz1 in complex with Poz1-binding motifs of Tpz1 and Rap1. The crystal structure shows that Poz1 employs two different binding surfaces to interact with Tpz1 and Rap1. Unexpectedly, the structure also reveals that Poz1 adopts a dimeric conformation. Mutational analyses suggest that proper interactions between Tpz1, Poz1, and Rap1 in the shelterin core complex are required for telomere length homeostasis and heterochromatin structure maintenance at telomeres. Structural resemblance between Poz1 and the TRFH domains of other shelterin proteins in fission yeast and humans suggests a model for the evolution of shelterin proteins.

The telomere bouquet facilitates meiotic prophase progression and exit in fission yeast

During meiotic prophase, chromosome arrangement and oscillation promote the pairing of homologous chromosomes for meiotic recombination. This dramatic movement involves clustering of telomeres at the nuclear membrane to form the so-called telomere bouquet. In fission yeast, the telomere bouquet is formed near the spindle pole body (SPB), which is the microtubule organising centre, functionally equivalent to the metazoan centrosome. Disruption of bouquet configuration impedes homologous chromosome pairing, meiotic recombination and spindle formation. Here, we demonstrate that the bouquet is maintained throughout meiotic prophase and promotes timely prophase exit in fission yeast. Persistent DNA damages, induced during meiotic recombination, activate the Rad3 and Chk1 DNA damage checkpoint kinases and extend the bouquet stage beyond the chromosome oscillation period. The auxin-inducible degron system demonstrated that premature termination of the bouquet stage leads to severe extension of prophase and consequently spindle formation defects. However, this delayed exit from meiotic prophase was not caused by residual DNA damage. Rather, loss of chromosome contact with the SPB caused delayed accumulation of CDK1-cyclin B at the SPB, which correlated with impaired SPB separation. In the absence of the bouquet, CDK1-cyclin B localised near the telomeres but not at the SPB at the later stage of meiotic prophase. Thus, bouquet configuration is maintained throughout meiotic prophase, by which this spatial organisation may facilitate local and timely activation of CDK1 near the SPB. Our findings illustrate that chromosome contact with the nuclear membrane synchronises meiotic progression of the nucleoplasmic chromosomes with that of the cytoplasmic SPB.

Microscopic Observation of Living Cells Stained with Fluorescent Probes

Fluorescence imaging of living cells provides a unique opportunity to follow dynamic behavior of specific molecules under physiological conditions. In the fission yeast Schizosaccharomyces pombe, expression of a target protein genetically fused with a fluorescent protein such as the jellyfish green fluorescent protein (GFP) is widely used. In addition, fluorescent chemical reagents are also used to stain specific molecules (e.g., Hoechst 33324 to stain DNA). Specimens of S. pombe cells for live cell imaging are prepared by either of two methods: sandwiching the cells between glass coverslips and by mounting the cells on a glass-bottom culture dish. For time-lapse observation, it is necessary to immobilize fission yeast cells on the glass surface of the glass-bottom dish because they are nonadherent and tend to move easily as a result of stage movement, convection flow of culture medium, and the contact and pushing of neighboring cells during cell growth. Either concanavalin A or soybean lectin, which bind to S. pombe cell walls, can be used for immobilization. Considerations for sample preparations and observation conditions are described.

Systematic analysis of human telomeric dysfunction using inducible telosome/shelterin CRISPR/Cas9 knockout cells

CRISPR/Cas9 technology enables efficient loss-of-function analysis of human genes using somatic cells. Studies of essential genes, however, require conditional knockout (KO) cells. Here, we describe the generation of inducible CRISPR KO human cell lines for the subunits of the telosome/shelterin complex, TRF1, TRF2, RAP1, TIN2, TPP1 and POT1, which directly interact with telomeres or can bind to telomeres through association with other subunits. Homozygous inactivation of several subunits is lethal in mice, and most loss-of-function studies of human telomere regulators have relied on RNA interference-mediated gene knockdown, which suffers its own limitations. Our inducible CRISPR approach has allowed us to more expediently obtain large numbers of KO cells in which essential telomere regulators have been inactivated for biochemical and molecular studies. Our systematic analysis revealed functional differences between human and mouse telomeric proteins in DNA damage responses, telomere length and metabolic control, providing new insights into how human telomeres are maintained.

The Drosophila telomere-capping protein Verrocchio binds single-stranded DNA and protects telomeres from DNA damage response

Drosophila telomeres are sequence-independent structures maintained by transposition to chromosome ends of three specialized retroelements rather than by telomerase activity. Fly telomeres are protected by the terminin complex that includes the HOAP, HipHop, Moi and Ver proteins. These are fast evolving, non-conserved proteins that localize and function exclusively at telomeres, protecting them from fusion events. We have previously suggested that terminin is the functional analogue of shelterin, the multi-protein complex that protects human telomeres. Here, we use electrophoretic mobility shift assay (EMSA) and atomic force microscopy (AFM) to show that Ver preferentially binds single-stranded DNA (ssDNA) with no sequence specificity. We also show that Moi and Ver form a complex in vivo. Although these two proteins are mutually dependent for their localization at telomeres, Moi neither binds ssDNA nor facilitates Ver binding to ssDNA. Consistent with these results, we found that Ver-depleted telomeres form RPA and γH2AX foci, like the human telomeres lacking the ssDNA-binding POT1 protein. Collectively, our findings suggest that Drosophila telomeres possess a ssDNA overhang like the other eukaryotes, and that the terminin complex is architecturally and functionally similar to shelterin.

Biosemantics guided gene expression profiling of Sjögren's syndrome: a comparative analysis with systemic lupus erythematosus and rheumatoid arthritis

BACKGROUND

Sjögren's syndrome (SS) shares many clinical and pathological similarities with systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). These autoimmune diseases mostly affect women. In this study, concept profile analysis (CPA) and gene expression meta-analysis were used to identify genes potentially involved in SS pathogenesis.

METHODS

Human genes associated with SS, SLE, and RA were identified using the CPA tool, Anni 2.1. The differential mRNA expression of genes common to SS and SLE (SS-SLE) was determined in female peripheral blood mononuclear cells (PBMCs) using NCBI-GEO2R. Differentially expressed (DE) SS-SLE PBMC genes in common with the SS-SLE CPA-identified genes were analyzed for differential expression in salivary glands or synovial biopsies, and for genes common to SS and RA and SLE and RA, analyzing differential expression in salivary glands in SS, synovial fibroblasts in RA, and synovial fluid in SLE. Among common genes, DE genes found in salivary gland mRNA expression in patients with SS were used for gene enrichment and SS molecular network construction. Secondary analysis was performed to identify DE genes unique to the disease site tissues, by excluding PBMC and CPA common DE genes to complement the SS network.

RESULTS

We identified 22 DE genes in salivary gland datasets in SS that have not previously been clearly associated with SS pathogenesis. Among these, higher levels of checkpoint kinase 1 (CHEK1), V-Ets avian erythroblastosis virus E26 oncogene homolog 1 (ETS1), and lymphoid enhancer binding factor 1 (LEF1) were significantly correlated with higher matrix metalloproteinase 9 (MMP9) levels. Higher MMP9 levels have been implicated in degradation of salivary gland structural integrity, leading to hypo-salivation in patients with SS. Salivary gland mRNA expression of MMP9 and the expression of cytokine CXCL10 were higher in patients with SS. CXCL10 has been shown to increase MMP9 expression and therefore may also play an important role in SS pathogenesis.

CONCLUSION

Using CPA and gene expression analysis, we identified factors targeting MMP9 expression and/or function, namely CHEK1, CXCL10, ETS1, LEF1, and tissue inhibitor of metalloproteinase 1; altered mRNA expression of these could increase expression/activity of MMP9 in a concerted manner, thereby potentially impacting SS pathogenesis.

Sequential phosphorylation of CST subunits by different cyclin-Cdk1 complexes orchestrate telomere replication

Telomeres are nucleoprotein structures that cap the ends of linear chromosomes. Telomere homeostasis is central to maintaining genomic integrity. In budding yeast, Cdk1 phosphorylates the telomere-specific binding protein, Cdc13, promoting the recruitment of telomerase to telomere and thereby telomere elongation. Cdc13 is also an integral part of the CST (Cdc13-Stn1-Ten1) complex that is essential for telomere capping and counteracting telomerase-dependent telomere elongation. Therefore, telomere length homeostasis is a balance between telomerase-extendable and CST-unextendable states. In our earlier work, we showed that Cdk1 also phosphorylates Stn1 which occurs sequentially following Cdc13 phosphorylation during cell cycle progression. This stabilizes the CST complex at the telomere and results in telomerase inhibition. Hence Cdk1-dependent phosphorylations of Stn1 acts like a molecular switch that drives Cdc13 to complex with Stn1-Ten1 rather than with telomerase. However, the underlying mechanism of how a single cyclin-dependent kinase phosphorylates Cdc13 and Stn1 in temporally distinct windows is largely unclear. Here, we show that S phase cyclins are necessary for telomere maintenance. The S phase and mitotic cyclins facilitate Cdc13 and Stn1 phosphorylation respectively, to exert opposing outcomes at the telomere. Thus, our results highlight a previously unappreciated role for cyclins in telomere replication.

Exposing the G-quadruplex to electric fields: the role played by telomeres in the propagation of DNA errors

We use quantum calculations to assess the impact of external electric fields on the stability of G-quadruplex, a key structure in telomere functionality.

Screening and Treating Prostate Cancer in the Older Patient: Decision Making Across the Clinical Spectrum

Treatment of the growing geriatric patient population is increasingly being recognized as a necessary priority of the oncology community. As the most common cancer among men in developed countries, prostate cancer afflicts a sizable portion of elderly men. Caring for this population requires knowledge of aspects of disease presentation, screening strategies, treatment approaches, and survivorship care considerations unique to the geriatric population. In this article, we review characteristics of prostate cancer screening and treatment decision making for localized disease in elderly men, including a discussion of the biology of disease in the elderly population. We also review best practices for providing treatment for localized and recurrent disease in an elderly population, including engaging in a basic geriatric assessment to determine fitness for treatment, eliciting information about patient preferences and support systems, and balancing treatment decisions in the context of these factors using the resources of a multidisciplinary care team. We then consider complications of prostate cancer survivorship related to systemic treatment in the elderly population of men with this disease. Finally, we emphasize the importance of engaging patients in treatment decision making across the spectrum of disease to personalize treatment plans and provide optimal care.

Ablation of human telomerase reverse transcriptase (hTERT) induces cellular senescence in gastric cancer through a galectin-3 dependent mechanism

The human Telomerase Reverse Transcriptase (hTERT) gene encodes a rate-limiting catalytic subunit of telomerase that maintains genomic integrity. Suppression of hTERT expression could induce cellular senescence and is considered a potent approach for gastric cancer therapy. However, control of hTERT expression and function remains poorly understood in gastric cancer. In this study, we demonstrated that high expression levels of hTERT in malignant tissues are correlated with poor survival probability in gastric cancer patients. Knockdown of hTERT expression retarded cell proliferation and cellular senescence, which was confirmed by increased protein expression levels of p21cip1 and p27kip1, and decreased phosphorylation of Rb. In contrast, overexpression of hTERT increased cell proliferation and decreased cellular senescence. Remarkably, the down-regulation of hTERT expression was detected in lgals3-/- mouse embryo fibroblasts (MEFs). Knockdown of galectin-3 decreased the expression of hTERT in gastric cancer cells. Galectin-3 ablation-induced cellular senescence was rescued by concomitant overexpression of hTERT. hTERT ablation-induced cellular senescence and p21cip1 and p27kip1 expression was rescued by concomitant overexpression of galectin-3. The size of tumor burdens was increased in hTERT-overexpressed gastric cancer cells xenografted mice, whereas it was repressed by concomitant depletion of galectin-3. Additionally, we determined that the N-terminal domain of galectin-3 directly interacted with hTERT. The telomeric activity of hTERT was also decreased by galectin-3 ablation. Taken together, ablation of hTERT induces cellular senescence and inhibits the growth of gastric cancer cells, suggesting that it could be a potent target in gastric cancer therapy. We also propose that galectin-3 is an important regulator of hTERT expression and telomeric activity in gastric tumorigenesis.

Rif1 Regulates the Fate of DNA Entanglements during Mitosis

Clearance of entangled DNA from the anaphase mid-region must accurately proceed in order for chromosomes to segregate with high fidelity. Loss of Taz1 (fission yeast ortholog of human TRF1/TRF2) leads to stalled telomeric replication forks that trigger telomeric entanglements; the resolution of these entanglements fails at ≤20°C. Here, we investigate these entanglements and their promotion by the conserved replication/repair protein Rif1. Rif1 plays no role in taz1Δ fork stalling. Rather, Rif1 localizes to the anaphase mid-region and regulates the resolution of persisting DNA structures. This anaphase role for Rif1 is genetically separate from the role of Rif1 in S/G2, though both roles require binding to PP1 phosphatase, implying spatially and temporally distinct Rif1-regulated phosphatase substrates. Rif1 thus acts as a double-edged sword. Although it inhibits the resolution of taz1Δ telomere entanglements, it promotes the resolution of non-telomeric ultrafine anaphase bridges at ≤20°C. We suggest a unifying model for Rif1's seemingly diverse roles in chromosome segregation in eukaryotes.

The proper connection between shelterin components is required for telomeric heterochromatin assembly

Telomeric regions contain prominent sites of heterochromatin, which is associated with unique histone modification profiles such as the methylation of histone H3 at Lys9 (H3K9me). In fission yeast, the conserved telomeric shelterin complex recruits the histone H3K9 methyltransferase complex CLRC to establish subtelomeric heterochromatin. Although many shelterin mutations affect subtelomeric heterochromatin assembly, the mechanism remains elusive due to the diverse functions of shelterin. Through affinity purification, we found that shelterin directly associates with CLRC through the Ccq1 subunit. Surprisingly, mutations that disrupt interactions between shelterin subunits compromise subtelomeric heterochromatin without affecting CLRC interaction with shelterin component Pot1, located at chromosome ends. We further discovered that telomeric repeats are refractory to heterochromatin spreading and that artificial restoration of shelterin connections or increased heterochromatin spreading rescued heterochromatin defects in these shelterin mutants. Thus, subtelomeric heterochromatin assembly requires both the recruitment of CLRC by shelterin to chromosome ends and the proper connection of shelterin components, which allows CLRC to skip telomeric repeats to internal regions.

Hypothesis: Paralog Formation from Progenitor Proteins and Paralog Mutagenesis Spur the Rapid Evolution of Telomere Binding Proteins

Through elegant studies in fungal cells and complex organisms, we propose a unifying paradigm for the rapid evolution of telomere binding proteins (TBPs) that associate with either (or both) telomeric DNA and telomeric proteins. TBPs protect and regulate telomere structure and function. Four critical factors are involved. First, TBPs that commonly bind to telomeric DNA include the c-Myb binding proteins, OB-fold single-stranded binding proteins, and G-G base paired Hoogsteen structure (G4) binding proteins. Each contributes independently or, in some cases, cooperatively, to provide a minimum level of telomere function. As a result of these minimal requirements and the great abundance of homologs of these motifs in the proteome, DNA telomere-binding activity may be generated more easily than expected. Second, telomere dysfunction gives rise to genome instability, through the elevation of recombination rates, genome ploidy, and the frequency of gene mutations. The formation of paralogs that diverge from their progenitor proteins ultimately can form a high frequency of altered TBPs with altered functions. Third, TBPs that assemble into complexes (e.g., mammalian shelterin) derive benefits from the novel emergent functions. Fourth, a limiting factor in the evolution of TBP complexes is the formation of mutually compatible interaction surfaces amongst the TBPs. These factors may have different degrees of importance in the evolution of different phyla, illustrated by the apparently simpler telomeres in complex plants. Selective pressures that can utilize the mechanisms of paralog formation and mutagenesis to drive TBP evolution along routes dependent on the requisite physiologic changes.

Concepts of Chronic Lymphocytic Leukemia Pathogenesis: DNA Damage Response and Tumor Microenvironment

Pathogenesis of chronic lymphocytic leukemia (CLL) is characterized by specific genetic aberrations and alterations of cellular signaling pathways. In particular, a disturbed DNA damage response (DDR) and an activated B-cell receptor signaling pathway play a major role in promoting CLL cell survival. External stimuli are similarly essential for CLL cell survival and lead to activation of the PI3K/AKT and MAPK pathways. Activation of nuclear factor-kappa B (NFkB) influences the disturbed anti-apoptotic balance of CLL cells. Losses or disabling mutations in TP53 and ATM are frequent events in chemotherapy-naïve patients and are further enriched in chemotherapy-resistant patients. As these lesions define key regulatory elements of the DDR pathway, they also determine treatment response to genotoxic therapy. Novel therapeutic strategies therefore try to circumvent defective DDR signaling and to suppress the pro-survival stimuli received from the tumor microenvironment. With increasing knowledge on specific genetic alterations of CLL, we may be able to target CLL cells more efficiently even in the situation of mutated DDR pathways or protection by microenvironmental stimuli.

Replication stress in early S phase generates apparent micronuclei and chromosome rearrangement in fission yeast

Unable to complete S phase, a fission yeast MCM mutant evades the mitotic checkpoint, causing aneuploidy, chromosome fragments, and bridges. The formation of apparent yeast micronuclei that are membrane bound is shown in real time; they develop DNA damage signals and may rejoin the parent nucleus.

DNA replication stress causes genome mutations, rearrangements, and chromosome missegregation, which are implicated in cancer. We analyze a fission yeast mutant that is unable to complete S phase due to a defective subunit of the MCM helicase. Despite underreplicated and damaged DNA, these cells evade the G2 damage checkpoint to form ultrafine bridges, fragmented centromeres, and uneven chromosome segregations that resembles micronuclei. These micronuclei retain DNA damage markers and frequently rejoin with the parent nucleus. Surviving cells show an increased rate of mutation and chromosome rearrangement. This first report of micronucleus-like segregation in a yeast replication mutant establishes underreplication as an important factor contributing to checkpoint escape, abnormal chromosome segregation, and chromosome instability.

A nucleosome turnover map reveals that the stability of histone H4 Lys20 methylation depends on histone recycling in transcribed chromatin

Nucleosome composition actively contributes to chromatin structure and accessibility. Cells have developed mechanisms to remove or recycle histones, generating a landscape of differentially aged nucleosomes. This study aimed to create a high-resolution, genome-wide map of nucleosome turnover in Schizosaccharomyces pombe. The recombination-induced tag exchange (RITE) method was used to study replication-independent nucleosome turnover through the appearance of new histone H3 and the disappearance or preservation of old histone H3. The genome-wide location of histones was determined by chromatin immunoprecipitation-exonuclease methodology (ChIP-exo). The findings were compared with diverse chromatin marks, including histone variant H2A.Z, post-translational histone modifications, and Pol II binding. Finally, genome-wide mapping of the methylation states of H4K20 was performed to determine the relationship between methylation (mono, di, and tri) of this residue and nucleosome turnover. Our analysis showed that histone recycling resulted in low nucleosome turnover in the coding regions of active genes, stably expressed at intermediate levels. High levels of transcription resulted in the incorporation of new histones primarily at the end of transcribed units. H4K20 was methylated in low-turnover nucleosomes in euchromatic regions, notably in the coding regions of long genes that were expressed at low levels. This transcription-dependent accumulation of histone methylation was dependent on the histone chaperone complex FACT. Our data showed that nucleosome turnover is highly dynamic in the genome and that several mechanisms are at play to either maintain or suppress stability. In particular, we found that FACT-associated transcription conserves histones by recycling them and is required for progressive H4K20 methylation.

The 9-1-1 checkpoint clamp coordinates resection at DNA double strand breaks

DNA-end resection, the generation of single-stranded DNA at DNA double strand break (DSB) ends, is critical for controlling the many cellular responses to breaks. Here we show that the conserved DNA damage checkpoint sliding clamp (the 9-1-1 complex) plays two opposing roles coordinating DSB resection in budding yeast. We show that the major effect of 9-1-1 is to inhibit resection by promoting the recruitment of Rad9(53BP1) near DSBs. However, 9-1-1 also stimulates resection by Exo1- and Dna2-Sgs1-dependent nuclease/helicase activities, and this can be observed in the absence of Rad9(53BP1). Our new data resolve the controversy in the literature about the effect of the 9-1-1 complex on DSB resection. Interestingly, the inhibitory role of 9-1-1 on resection is not observed near uncapped telomeres because less Rad9(53BP1) is recruited near uncapped telomeres. Thus, 9-1-1 both stimulates and inhibits resection and the effects of 9-1-1 are modulated by different regions of the genome. Our experiments illustrate the central role of the 9-1-1 checkpoint sliding clamp in the DNA damage response network that coordinates the response to broken DNA ends. Our results have implications in all eukaryotic cells.

Real-time imaging of DNA damage in yeast cells using ultra-short near-infrared pulsed laser irradiation

Analysis of accumulation of repair and checkpoint proteins at repair sites in yeast nuclei has conventionally used chemical agents, ionizing radiation or induction of endonucleases to inflict localized damage. In addition to these methods, similar studies in mammalian cells have used laser irradiation, which has the advantage that damage is inflicted at a specific nuclear region and at a precise time, and this allows accurate kinetic analysis of protein accumulation at DNA damage sites. We show here that it is feasible to use short pulses of near-infrared laser irradiation to inflict DNA damage in subnuclear regions of yeast nuclei by multiphoton absorption. In conjunction with use of fluorescently-tagged proteins, this allows quantitative analysis of protein accumulation at damage sites within seconds of damage induction. PCNA accumulated at damage sites rapidly, such that maximum accumulation was seen approximately 50 s after damage, then levels declined linearly over 200-1000 s after irradiation. RPA accumulated with slower kinetics such that hardly any accumulation was detected within 60 s of irradiation, and levels subsequently increased linearly over the next 900 s, after which levels were approximately constant (up to ca. 2700 s) at the damage site. This approach complements existing methodologies to allow analysis of key damage sensors and chromatin modification changes occurring within seconds of damage inception.

A histone H3K36 chromatin switch coordinates DNA double-strand break repair pathway choice

DNA double-strand break (DSB) repair is a highly regulated process performed predominantly by non-homologous end joining (NHEJ) or homologous recombination (HR) pathways. How these pathways are coordinated in the context of chromatin is unclear. Here we uncover a role for histone H3K36 modification in regulating DSB repair pathway choice in fission yeast. We find Set2-dependent H3K36 methylation reduces chromatin accessibility, reduces resection and promotes NHEJ, while antagonistic Gcn5-dependent H3K36 acetylation increases chromatin accessibility, increases resection and promotes HR. Accordingly, loss of Set2 increases H3K36Ac, chromatin accessibility and resection, while Gcn5 loss results in the opposite phenotypes following DSB induction. Further, H3K36 modification is cell cycle regulated with Set2-dependent H3K36 methylation peaking in G1 when NHEJ occurs, while Gcn5-dependent H3K36 acetylation peaks in S/G2 when HR prevails. These findings support an H3K36 chromatin switch in regulating DSB repair pathway choice.

PinX1-siRNA/mPEG-PEI-SPION combined with doxorubicin enhances the inhibition of glioma growth

Resistance to chemotherapy and the side effects of anticancer drugs are the major obstacles for glioma treatment. The aim of the present study was to develop a novel approach for the treatment of gliomas that improved the therapeutic effect; the anticancer drug, doxorubicin (DOX), was combined with short interfering (si)RNA and monomethoxy polyethylene glycol polyethylenimine superparamagnetic iron oxide nanoparticle (mPEG-PEI-SPION), a magnetic resonance imaging (MRI)-visible nanoparticle. Specific siRNA molecules, delivered by mPEG-PEI-SPION, were employed to knockdown the PIN2-interacting protein 1 (PinX1) gene in C6 glioma cells. PinX1 is a nucleolar protein associated with telomere and telomerase. C6 cells were treated with DOX and/or PinX1-siRNA. The results of the transfection experiments revealed that siRNA/mPEG-PEI-SPION was transfected into C6 cells with high efficiency. PinX1-siRNA was unable to inhibit C6 cells, while in the PinX1-siRNA + DOX group, the same dose of DOX caused an increased loss of cell viability. Therefore, mPEG-PEI-SPION was shown to be viable for siRNA delivery into C6 cells and coadministration of DOX with PinX1-siRNA may be a potential therapeutic method for inhibiting gliomas.

The telomere deprotection response is functionally distinct from the genomic DNA damage response

Loss of chromosome end protection through telomere erosion is a hallmark of aging and senescence. Here we developed an experimental system that mimics physiological telomere deprotection in human cells and discovered that the telomere deprotection response is functionally distinct from the genomic DNA damage response. We found that, unlike genomic breaks, deprotected telomeres that are recognized as DNA damage but remain in the fusion-resistant intermediate state activate differential ataxia telangiectasia mutated (ATM) signaling where CHK2 is not phosphorylated. Also unlike genomic breaks, we found that deprotected telomeres do not contribute to the G2/M checkpoint and are instead passed through cell division to induce p53-dependent G1 arrest in the daughter cells. Telomere deprotection is therefore an epigenetic signal passed between cell generations to ensure that replication-associated telomere-dependent growth arrest occurs in stable diploid G1 phase cells before genome instability can occur.

DNA repair at telomeres: keeping the ends intact

The molecular era of telomere biology began with the discovery that telomeres usually consist of G-rich simple repeats and end with 3' single-stranded tails. Enormous progress has been made in identifying the mechanisms that maintain and replenish telomeric DNA and the proteins that protect them from degradation, fusions, and checkpoint activation. Although telomeres in different organisms (or even in the same organism under different conditions) are maintained by different mechanisms, the disparate processes have the common goals of repairing defects caused by semiconservative replication through G-rich DNA, countering the shortening caused by incomplete replication, and postreplication regeneration of G tails. In addition, standard DNA repair mechanisms must be suppressed or modified at telomeres to prevent their being recognized and processed as DNA double-strand breaks. Here, we discuss the players and processes that maintain and regenerate telomere structure.

DNA break site at fragile subtelomeres determines probability and mechanism of antigenic variation in African trypanosomes

Antigenic variation in African trypanosomes requires monoallelic transcription and switching of variant surface glycoprotein (VSG) genes. The transcribed VSG, always flanked by ‘70 bp’-repeats and telomeric-repeats, is either replaced through DNA double-strand break (DSB) repair or transcriptionally inactivated. However, little is known about the subtelomeric DSBs that naturally trigger antigenic variation in Trypanosoma brucei, the subsequent DNA damage responses, or how these responses determine the mechanism of VSG switching. We found that DSBs naturally accumulate close to both transcribed and non-transcribed telomeres. We then induced high-efficiency meganuclease-mediated DSBs and monitored DSB-responses and DSB-survivors. By inducing breaks at distinct sites within both transcribed and silent VSG transcription units and assessing local DNA resection, histone modification, G₂/M-checkpoint activation, and both RAD51-dependent and independent repair, we reveal how breaks at different sites trigger distinct responses and, in ‘active-site’ survivors, different switching mechanisms. At the active site, we find that promoter-adjacent breaks typically failed to trigger switching, 70 bp-repeat-adjacent breaks almost always triggered switching through 70 bp-repeat recombination (∼60% RAD51-dependent), and telomere-repeat-adjacent breaks triggered switching through loss of the VSG expression site (25% of survivors). Expression site loss was associated with G₂/M-checkpoint bypass, while 70 bp-repeat-recombination was associated with DNA-resection, γH2A-focus assembly and a G₂/M-checkpoint. Thus, the probability and mechanism of antigenic switching are highly dependent upon the location of the break. We conclude that 70 bp-repeat-adjacent and telomere-repeat-adjacent breaks trigger distinct checkpoint responses and VSG switching pathways. Our results show how subtelomere fragility can generate the triggers for the major antigenic variation mechanisms in the African trypanosome.

Previous studies on antigenic variation in African trypanosomes relied upon positive or negative selection, yielding only cells that underwent variation. This made it difficult to define individual switched clones as independent, potentially introduced bias in the relative contribution of each switching mechanism and precluded analysis of cells undergoing switching. We show that DNA double-strand breaks (DSBs) naturally accumulate close to Trypanosoma brucei telomeres. Using the I-SceI meganuclease, we then established a system to trigger breaks in all cells in a population. The specificity, temporal constraint and efficiency of cleavage facilitated the application of a quantitative approach to dissecting subtelomeric break responses and their consequences. Accordingly, we show that the DSB-site determines probability and mechanism of antigenic switching, that DSBs can trigger switching via recombination or transcription inactivation and that a checkpoint-bypass mechanism can explain switching via VSG expression site deletion. Our results provide major new insights into the mechanisms underlying antigenic variation and provide a new model to explain how the repeats flanking VSG genes serve distinct roles in fragility and recombination. The findings are also relevant to telomeric gene rearrangements that control immune evasion in other protozoal, fungal and bacterial pathogens such as Plasmodium, Pneumocystis and Borrelia species, respectively.

A two-step mechanism for TRF2-mediated chromosome-end protection

Mammalian telomeres repress DNA damage activation at natural chromosome ends by recruiting specific inhibitors of the DNA damage machinery that form a protective complex termed shelterin. Within this complex, TRF2 plays a crucial role in end-protection as it is required to suppress ATM activation and the formation of end-to-end chromosome fusions^{1, 2}. Here, we address the molecular properties of TRF2 that are both necessary and sufficient to protect chromosome ends. Our data support a two-step mechanism for TRF2-mediated end protection. First, the dimerization domain of TRF2 is required to inhibit ATM activation, the key initial step involved in activation of a DNA damage response. Next, TRF2 independently suppresses the propagation of DNA damage signaling downstream of ATM activation. This novel modulation of the DNA damage response at telomeres occurs at the level of the E3 ubiquitin ligase RNF168 ³. Inhibition of RNF168 at telomeres involves the de-ubiquitinating enzyme BRCC3 and the ubiquitin ligase UBR5 and is sufficient to suppress chromosome end-to-end fusions. This two-step mechanism for TRF2-mediated end protection helps to explain the apparent paradox of frequent localization of DNA damage response proteins at functional telomeres without concurrent induction of detrimental DNA repair activities.

The fission yeast MRN complex tethers dysfunctional telomeres for NHEJ repair

Telomeres protect the natural ends of chromosomes from being repaired as deleterious DNA breaks. In fission yeast, absence of Taz1 (homologue of human TRF1 and TRF2) renders telomeres vulnerable to DNA repair. During the G1 phase, when non-homologous end joining (NHEJ) is upregulated, taz1Δ cells undergo telomere fusions with consequent loss of viability. Here, we show that disruption of the fission yeast MRN (Rad23(MRE11)-Rad50-Nbs1) complex prevents NHEJ at telomeres and, as a result, rescues taz1Δ lethality in G1. Neither Tel1(ATM) activation nor 5'-end resection was required for telomere fusion. Nuclease activity of Rad32(MRE11) was also dispensable for NHEJ. Mutants unable to coordinate metal ions required for nuclease activity were proficient in NHEJ repair. In contrast, Rad32(MRE11) mutations that affect binding and/or positioning of DNA ends leaving the nuclease function largely unaffected also impaired NHEJ at telomeres and restored the viability of taz1Δ in G1. Consistently, MRN structural integrity but not nuclease function is also required for NHEJ of independent DNA ends in a novel split-molecule plasmid assay. Thus, MRN acts to tether unlinked DNA ends, allowing for efficient NHEJ.

Aging, cellular senescence, and cancer

For most species, aging promotes a host of degenerative pathologies that are characterized by debilitating losses of tissue or cellular function. However, especially among vertebrates, aging also promotes hyperplastic pathologies, the most deadly of which is cancer. In contrast to the loss of function that characterizes degenerating cells and tissues, malignant (cancerous) cells must acquire new (albeit aberrant) functions that allow them to develop into a lethal tumor. This review discusses the idea that, despite seemingly opposite characteristics, the degenerative and hyperplastic pathologies of aging are at least partly linked by a common biological phenomenon: a cellular stress response known as cellular senescence. The senescence response is widely recognized as a potent tumor suppressive mechanism. However, recent evidence strengthens the idea that it also drives both degenerative and hyperplastic pathologies, most likely by promoting chronic inflammation. Thus, the senescence response may be the result of antagonistically pleiotropic gene action.

Nucleostemin prevents telomere damage by promoting PML-IV recruitment to SUMOylated TRF1

A novel telomere-protection mechanism relies upon nucleostemin-mediated recruitment of PML-IV to telomeres and subsequent recruitment of RAD51 in both ALT and telomerase-active cells.

Continuously dividing cells must be protected from telomeric and nontelomeric DNA damage in order to maintain their proliferative potential. Here, we report a novel telomere-protecting mechanism regulated by nucleostemin (NS). NS depletion increased the number of telomere damage foci in both telomerase-active (TA⁺) and alternative lengthening of telomere (ALT) cells and decreased the percentage of damaged telomeres associated with ALT-associated PML bodies (APB) and the number of APB in ALT cells. Mechanistically, NS could promote the recruitment of PML-IV to SUMOylated TRF1 in TA⁺ and ALT cells. This event was stimulated by DNA damage. Supporting the importance of NS and PML-IV in telomere protection, we demonstrate that loss of NS or PML-IV increased the frequency of telomere damage and aberration, reduced telomeric length, and perturbed the TRF2^ΔBΔM-induced telomeric recruitment of RAD51. Conversely, overexpression of either NS or PML-IV protected ALT and TA⁺ cells from telomere damage. This work reveals a novel mechanism in telomere protection.

Genome-wide profiles of H2AX and γ-H2AX differentiate endogenous and exogenous DNA damage hotspots in human cells

Phosphorylation of the histone variant H2AX forms γ-H2AX that marks DNA double-strand break (DSB). Here, we generated the sequencing-based maps of H2AX and γ-H2AX positioning in resting and proliferating cells before and after ionizing irradiation. Genome-wide locations of possible endogenous and exogenous DSBs were identified based on γ-H2AX distribution in dividing cancer cells without irradiation and that in resting cells upon irradiation, respectively. γ-H2AX-enriched regions of endogenous origin in replicating cells included sub-telomeres and active transcription start sites, apparently reflecting replication- and transcription-mediated stress during rapid cell division. Surprisingly, H2AX itself, prior to phosphorylation, was specifically located at these endogenous hotspots. This phenomenon was only observed in dividing cancer cells but not in resting cells. Endogenous H2AX was concentrated on the transcription start site of actively transcribed genes but was irrelevant to pausing of RNA polymerase II (pol II), which precisely coincided with γ-H2AX of endogenous origin. γ-H2AX enrichment upon irradiation also coincided with actively transcribed regions, but unlike endogenous γ-H2AX, it extended into the gene body and was not specifically concentrated on the pausing site of pol II. Sub-telomeres were less responsive to external DNA damage than to endogenous stress. Our findings provide insight into DNA repair programs of cancer and may have implications for cancer therapy.

Tel1(ATM) and Rad3(ATR) phosphorylate the telomere protein Ccq1 to recruit telomerase and elongate telomeres in fission yeast

In fission yeast, the DNA damage sensor kinases Tel1(ATM) and Rad3(ATR) exist at telomeres and are required for telomere maintenance, but the biological role they play at telomeres is not known. Here we show that the telomere protein Ccq1 is phosphorylated at Thr 93 (threonine residue at amino acid 93) by Tel1(ATM) and Rad3(ATR) both in vitro and in vivo. A ccq1 mutant in which alanine was substituted for Thr 93 failed to recruit telomerase to telomeres and showed gradual shortening of telomeres. These results indicate that the direct phosphorylation of Ccq1 Thr 93 by Tel1 and Rad3 is involved in the recruitment of telomerase to elongate telomeres.

Anticheckpoint pathways at telomeres in yeast

Telomeres hide (or 'cap') chromosome ends from DNA-damage surveillance mechanisms that arrest the cell cycle and promote repair, but the checkpoint status of telomeres is not well understood. Here we characterize the response in Saccharomyces cerevisiae to DNA double-strand breaks (DSBs) flanked by varying amounts of telomeric repeat sequences (TG(1-3)). We show that even short arrays of TG(1-3) repeats do not induce G2/M arrest. Both Rif1 and Rif2 are required for capping at short, rapidly elongating ends, yet are largely dispensable for protection of longer telomeric arrays. Rif1 and Rif2 act through parallel pathways to block accumulation of both RPA and Rad24, activators of checkpoint kinase Mec1 (ATR). Finally, we show that Rif function is correlated with an 'anticheckpoint' effect, in which checkpoint recovery at an adjacent unprotected end is stimulated, and we provide insight into the molecular mechanism of this phenomenon.

A novel checkpoint and RPA inhibitory pathway regulated by Rif1

Cells accumulate single-stranded DNA (ssDNA) when telomere capping, DNA replication, or DNA repair is impeded. This accumulation leads to cell cycle arrest through activating the DNA-damage checkpoints involved in cancer protection. Hence, ssDNA accumulation could be an anti-cancer mechanism. However, ssDNA has to accumulate above a certain threshold to activate checkpoints. What determines this checkpoint-activation threshold is an important, yet unanswered question. Here we identify Rif1 (Rap1-Interacting Factor 1) as a threshold-setter. Following telomere uncapping, we show that budding yeast Rif1 has unprecedented effects for a protein, inhibiting the recruitment of checkpoint proteins and RPA (Replication Protein A) to damaged chromosome regions, without significantly affecting the accumulation of ssDNA at those regions. Using chromatin immuno-precipitation, we provide evidence that Rif1 acts as a molecular "band-aid" for ssDNA lesions, associating with DNA damage independently of Rap1. In consequence, small or incipient lesions are protected from RPA and checkpoint proteins. When longer stretches of ssDNA are generated, they extend beyond the junction-proximal Rif1-protected regions. In consequence, the damage is detected and checkpoint signals are fired, resulting in cell cycle arrest. However, increased Rif1 expression raises the checkpoint-activation threshold to the point it simulates a checkpoint knockout and can also terminate a checkpoint arrest, despite persistent telomere deficiency. Our work has important implications for understanding the checkpoint and RPA-dependent DNA-damage responses in eukaryotic cells.

Tel1ATM and Rad3ATR kinases promote Ccq1-Est1 interaction to maintain telomeres in fission yeast

The shelterin complex plays both positive and negative roles in telomerase regulation. While shelterin prevents the checkpoint kinases ATM and ATR from fully activating DNA damage responses at telomeres, those kinases are also required for telomere maintenance. In fission yeast, cells lacking both Tel1 (ATM ortholog) and Rad3 (ATR ortholog) fail to recruit telomerase to telomeres, and survive by circularizing chromosomes. However, the critical telomere substrate(s) of Tel1^ATM/Rad3^ATR remained unknown. Here, we show that Tel1^ATM/Rad3^ATR-dependent phosphorylation of the shelterin subunit Ccq1 on Thr93 is essential for telomerase association with telomeres. In addition, we show that the telomerase subunit Est1 interacts directly with the phosphorylated Thr93 of Ccq1 to ensure telomere maintenance. The shelterin subunits Taz1, Rap1 and Poz1 (previously established inhibitors of telomerase) were also found to negatively regulate Ccq1 phosphorylation. These findings establish Tel1^ATM/Rad3^ATR-dependent Ccq1 Thr93 phosphorylation as a critical regulator of telomere maintenance in fission yeast.

Fission yeast Pot1 and RecQ helicase are required for efficient chromosome segregation

Pot1 is a single-stranded telomere-binding protein that is conserved from fission yeast to mammals. Deletion of Schizosaccharomyces pombe pot1(+) causes immediate telomere loss. S. pombe Rqh1 is a homolog of the human RecQ helicase WRN, which plays essential roles in the maintenance of genomic stability. Here, we demonstrate that a pot1Δ rqh1-hd (helicase-dead) double mutant maintains telomeres that are dependent on Rad51-mediated homologous recombination. Interestingly, the pot1Δ rqh1-hd double mutant displays a "cut" (cell untimely torn) phenotype and is sensitive to the antimicrotubule drug thiabendazole (TBZ). Moreover, the chromosome ends of the double mutant do not enter the pulsed-field electrophoresis gel. These results suggest that the entangled chromosome ends in the pot1Δ rqh1-hd double mutant inhibit chromosome segregation, signifying that Pot1 and Rqh1 are required for efficient chromosome segregation. We also found that POT1 knockdown, WRN-deficient human cells are sensitive to the antimicrotubule drug vinblastine, implying that some of the functions of S. pombe Pot1 and Rqh1 may be conserved in their respective human counterparts POT1 and WRN.