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

Telomerase Repairs Collapsed Replication Forks at Telomeres

Telomeres are difficult-to-replicate sites whereby replication itself may threaten telomere integrity. We investigate, in fission yeast, telomere replication dynamics in telomerase-negative cells to unmask problems associated with telomere replication. Two-dimensional gel analysis reveals that replication of telomeres is severely impaired and correlates with an accumulation of replication intermediates that arises from stalled and collapsed forks. In the absence of telomerase, Rad51, Mre11-Rad50-Nbs1 (MRN) complex, and its co-factor CtIP^Ctp1 become critical to maintain telomeres, indicating that homologous recombination processes these intermediates to facilitate fork restart. We further show that a catalytically dead mutant of telomerase prevents Ku recruitment to telomeres, suggesting that telomerase and Ku both compete for the binding of telomeric-free DNA ends that are likely to originate from a reversed fork. We infer that Ku removal at collapsed telomeric forks allows telomerase to repair broken telomeres, thereby shielding telomeres from homologous recombination.

Tpz1TPP1 prevents telomerase activation and protects telomeres by modulating the Stn1-Ten1 complex in fission yeast

In both mammalian and fission yeast cells, conserved shelterin and CST (CTC1-STN1-TEN1) complexes play critical roles in protection of telomeres and regulation of telomerase, an enzyme required to overcome the end replication problem. However, molecular details that govern proper coordination among shelterin, CST, and telomerase have not yet been fully understood. Here, we establish a conserved SWSSS motif, located adjacent to the Lys242 SUMOylation site in the fission yeast shelterin subunit Tpz1, as a new functional regulatory element for telomere protection and telomere length homeostasis. The SWSSS motif works redundantly with Lys242 SUMOylation to promote binding of Stn1-Ten1 at telomere and sub-telomere regions to protect against single-strand annealing (SSA)-dependent telomere fusions, and to prevent telomerase accumulation at telomeres. In addition, we provide evidence that the SWSSS motif defines an unanticipated role of Tpz1 in limiting telomerase activation at telomeres to prevent uncontrolled telomere elongation.

A telomerase with novel non-canonical roles: TERT controls cellular aggregation and tissue size in Dictyostelium

Telomerase, particularly its main subunit, the reverse transcriptase, TERT, prevents DNA erosion during eukaryotic chromosomal replication, but also has poorly understood non-canonical functions. Here, in the model social amoeba Dictyostelium discoideum, we show that the protein encoded by tert has telomerase-like motifs, and regulates, non-canonically, important developmental processes. Expression levels of wild-type (WT) tert were biphasic, peaking at 8 and 12 h post-starvation, aligning with developmental events, such as the initiation of streaming (~7 h) and mound formation (~10 h). In tert KO mutants, however, aggregation was delayed until 16 h. Large, irregular streams formed, then broke up, forming small mounds. The mound-size defect was not induced when a KO mutant of countin (a master size-regulating gene) was treated with TERT inhibitors, but anti-countin antibodies did rescue size in the tert KO. Although, conditioned medium (CM) from countin mutants failed to rescue size in the tert KO, tert KO CM rescued the countin KO phenotype. These and additional observations indicate that TERT acts upstream of smlA/countin: (i) the observed expression levels of smlA and countin, being respectively lower and higher (than WT) in the tert KO; (ii) the levels of known size-regulation intermediates, glucose (low) and adenosine (high), in the tert mutant, and the size defect's rescue by supplemented glucose or the adenosine-antagonist, caffeine; (iii) the induction of the size defect in the WT by tert KO CM and TERT inhibitors. The tert KO's other defects (delayed aggregation, irregular streaming) were associated with changes to cAMP-regulated processes (e.g. chemotaxis, cAMP pulsing) and their regulatory factors (e.g. cAMP; acaA, carA expression). Overexpression of WT tert in the tert KO rescued these defects (and size), and restored a single cAMP signaling centre. Our results indicate that TERT acts in novel, non-canonical and upstream ways, regulating key developmental events in Dictyostelium.

Life and cancer without telomerase: ALT and other strategies for making sure ends (don't) meet

While most cancer cells rely on telomerase expression/re-activation for linear chromosome maintenance and sustained proliferation, a significant population of cancers (10-15%) employs telomerase-independent strategies, collectively dubbed Alternative Lengthening of Telomeres (ALT). Most ALT cells relax the usual role of telomeres as inhibitors of local homologous recombination while maintaining the ability of telomeres to prohibit local non-homologous end joining reactions. Here we review current concepts surrounding how ALT telomeres achieve this new balance via alterations in chromatin landscape, DNA damage repair processes and handling of telomeric transcription. We also discuss telomerase independent end maintenance strategies utilized by other organisms, including fruitflies and yeasts, to draw parallels and contrasts and highlight additional modes, beyond ALT, that may be available to telomerase-minus cancers. We conclude by commenting on promises and challenges in the development of effective anti-ALT cancer therapies.

TeloPCR-seq: a high-throughput sequencing approach for telomeres

We have developed a high-throughput sequencing approach that enables us to determine terminal telomere sequences from tens of thousands of individual Schizosaccharomyces pombe telomeres. This method provides unprecedented coverage of telomeric sequence complexity in fission yeast. S. pombe telomeres are composed of modular degenerate repeats that can be explained by variation in usage of the TER1 RNA template during reverse transcription. Taking advantage of this deep sequencing approach, we find that 'like' repeat modules are highly correlated within individual telomeres. Moreover, repeat module preference varies with telomere length, suggesting that existing repeats promote the incorporation of like repeats and/or that specific conformations of the telomerase holoenzyme efficiently and/or processively add repeats of like nature. After the loss of telomerase activity, this sequencing and analysis pipeline defines a population of telomeres with altered sequence content. This approach will be adaptable to study telomeric repeats in other organisms and also to interrogate repetitive sequences throughout the genome that are inaccessible to other sequencing methods.

Telomere recombination pathways: tales of several unhappy marriages

All happy families are alike; each unhappy family is unhappy in its own way.-Leo Tolstoy, Anna Karenina.

Swi1Timeless Prevents Repeat Instability at Fission Yeast Telomeres

Genomic instability associated with DNA replication stress is linked to cancer and genetic pathologies in humans. If not properly regulated, replication stress, such as fork stalling and collapse, can be induced at natural replication impediments present throughout the genome. The fork protection complex (FPC) is thought to play a critical role in stabilizing stalled replication forks at several known replication barriers including eukaryotic rDNA genes and the fission yeast mating-type locus. However, little is known about the role of the FPC at other natural impediments including telomeres. Telomeres are considered to be difficult to replicate due to the presence of repetitive GT-rich sequences and telomere-binding proteins. However, the regulatory mechanism that ensures telomere replication is not fully understood. Here, we report the role of the fission yeast Swi1^Timeless, a subunit of the FPC, in telomere replication. Loss of Swi1 causes telomere shortening in a telomerase-independent manner. Our epistasis analyses suggest that heterochromatin and telomere-binding proteins are not major impediments for telomere replication in the absence of Swi1. Instead, repetitive DNA sequences impair telomere integrity in swi1Δ mutant cells, leading to the loss of repeat DNA. In the absence of Swi1, telomere shortening is accompanied with an increased recruitment of Rad52 recombinase and more frequent amplification of telomere/subtelomeres, reminiscent of tumor cells that utilize the alternative lengthening of telomeres pathway (ALT) to maintain telomeres. These results suggest that Swi1 ensures telomere replication by suppressing recombination and repeat instability at telomeres. Our studies may also be relevant in understanding the potential role of Swi1^Timeless in regulation of telomere stability in cancer cells.

In every round of the cell cycle, cells must accurately replicate their full genetic information. This process is highly regulated, as defects during DNA replication cause genomic instability, leading to various genetic disorders including cancers. To thwart these problems, cells carry an array of complex mechanisms to deal with various obstacles found across the genome that can hamper DNA replication and cause DNA damage. Understanding how these mechanisms are regulated and orchestrated is of paramount importance in the field. In this report, we describe how Swi1, a Timeless-related protein in fission yeast, regulates efficient replication of telomeres, which are considered to be difficult to replicate due to the presence of repetitive DNA and telomere-binding proteins. We show that Swi1 prevents telomere damage and maintains telomere length by protecting integrity of telomeric repeats. Swi1-mediated telomere maintenance is independent of telomerase activity, and loss of Swi1 causes hyper-activation of recombination-based telomere maintenance, which generates heterogeneous telomeres. Similar telomerase-independent and recombination-dependent mechanism is utilized by approximately 15% of human cancers, linking telomere replication defects with cancer development. Thus, our study may be relevant in understanding the role of telomere replication defects in the development of cancers in humans.

Acentric chromosome ends are prone to fusion with functional chromosome ends through a homology-directed rearrangement

The centromeres of many eukaryotic chromosomes are established epigenetically on potentially variable tandem repeats; hence, these chromosomes are at risk of being acentric. We reported previously that artificially created acentric chromosomes in the fission yeast Schizosaccharomyces pombe can be rescued by end-to-end fusion with functional chromosomes. Here, we show that most acentric/functional chromosome fusion events in S. pombe cells harbouring an acentric chromosome I differed from the non-homologous end-joining-mediated rearrangements that result in deleterious dicentric fusions in normal cells, and were elicited by a previously unidentified homologous recombination (HR) event between chromosome end-associated sequences. The subtelomere repeats associated with the non-fusogenic ends were also destabilized in the surviving cells, suggesting a causal link between general subtelomere destabilization and acentric/functional chromosome fusion. A mutational analysis indicated that a non-canonical HR pathway was involved in the rearrangement. These findings are indicative of a latent mechanism that conditionally induces general subtelomere instability, presumably in the face of accidental centromere loss events, resulting in rescue of the fatal acentric chromosomes by interchromosomal HR.

The many facets of homologous recombination at telomeres

The ends of linear chromosomes are capped by nucleoprotein structures called telomeres. A dysfunctional telomere may resemble a DNA double-strand break (DSB), which is a severe form of DNA damage. The presence of one DSB is sufficient to drive cell cycle arrest and cell death. Therefore cells have evolved mechanisms to repair DSBs such as homologous recombination (HR). HR-mediated repair of telomeres can lead to genome instability, a hallmark of cancer cells, which is why such repair is normally inhibited. However, some HR-mediated processes are required for proper telomere function. The need for some recombination activities at telomeres but not others necessitates careful and complex regulation, defects in which can lead to catastrophic consequences. Furthermore, some cell types can maintain telomeres via telomerase-independent, recombination-mediated mechanisms. In humans, these mechanisms are called alternative lengthening of telomeres (ALT) and are used in a subset of human cancer cells. In this review, we summarize the different recombination activities occurring at telomeres and discuss how they are regulated. Much of the current knowledge is derived from work using yeast models, which is the focus of this review, but relevant studies in mammals are also included.

Fungal Ku prevents permanent cell cycle arrest by suppressing DNA damage signaling at telomeres

The Ku heterodimer serves in the initial step in repairing DNA double-strand breaks by the non-homologous end-joining pathway. Besides this key function, Ku also plays a role in other cellular processes including telomere maintenance. Inactivation of Ku can lead to DNA repair defects and telomere aberrations. In model organisms where Ku has been studied, inactivation can lead to DNA repair defects and telomere aberrations. In general Ku deficient mutants are viable, but a notable exception to this is human where Ku has been found to be essential. Here we report that similar to the situation in human Ku is required for cell proliferation in the fungus Ustilago maydis. Using conditional strains for Ku expression, we found that cells arrest permanently in G2 phase when Ku expression is turned off. Arrest results from cell cycle checkpoint activation due to persistent signaling via the DNA damage response (DDR). Our results point to the telomeres as the most likely source of the DNA damage signal. Inactivation of the DDR makes the Ku complex dispensable for proliferation in this organism. Our findings suggest that in U. maydis, unprotected telomeres arising from Ku depletion are the source of the signal that activates the DDR leading to cell cycle arrest.

Tpz1-Ccq1 and Tpz1-Poz1 interactions within fission yeast shelterin modulate Ccq1 Thr93 phosphorylation and telomerase recruitment

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, Rad3^ATR and Tel1^ATM kinases are redundantly required for telomerase recruitment, since Rad3^ATR/Tel1^ATM-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 Rad3^ATR kinase, while Tpz1-Ccq1 interaction works downstream of Rad3^ATR kinase to facilitate Ccq1 Thr93 phosphorylation and telomerase recruitment.

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 Rad3^ATR and Tel1^ATM 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.

Role and mechanism of radiological protection cream in treating radiation dermatitis in rats

OBJECTIVE

To explore the role and mechanism of a radiation protection cream (Rp) in the treatment of radiation dermatitis, and to accumulate necessary technical information for a new drug report on Rp.

METHODS

High-performance liquid chromatography was used to establish the method of measuring the main effective ingredients of sovereign and adjuvant herbs of Rp drugs, and to formulate the draft quality standards of Rp. A total of 48 Sprague-Dawley male rats were randomly divided into the Model, Trolamine cream (Tc), Rp and Blank groups according to a random number table method. The skin of each rat's buttocks was irradiated using an electron linear accelerator to establish an acute radiation dermatitis model. The histological changes were observed under light microscopy and electron microscopy during wound healing and the effect of Rp on rat fibroblast Ku70/80 gene expression was detected at the transcriptional level.

RESULTS

Pathological examination revealed that Rp protected the cellular and subcellular structures of skin after irradiation, promoting the proliferation and restoration of collagen fibers. Ku70/80 mRNA expression levels in the Rp and Tc groups were higher than that in the model group (P < 0.05). Moreover, The majority of grade radiation dermatitis relative to the Model, Rp and Tc groups for reducing grade III and IV dermatitis efficiency were 85.7% and 69.2% (P < 0.05), respectively. The efficacy of Rp group in treating radiation dermatitis was better than the Trolamine cream group by 16.5% (P < 0.05).

CONCLUSION

Compared with Tc, Rp had certain advantages in the efficacy and performance to price ratio. Thus, Rp is considered an effective alternative formulation for the prevention and treatment of radiation dermatitis.

Fission yeast shelterin regulates DNA polymerases and Rad3(ATR) kinase to limit telomere extension

Studies in fission yeast have previously identified evolutionarily conserved shelterin and Stn1-Ten1 complexes, and established Rad3^ATR/Tel1^ATM-dependent phosphorylation of the shelterin subunit Ccq1 at Thr93 as the critical post-translational modification for telomerase recruitment to telomeres. Furthermore, shelterin subunits Poz1, Rap1 and Taz1 have been identified as negative regulators of Thr93 phosphorylation and telomerase recruitment. However, it remained unclear how telomere maintenance is dynamically regulated during the cell cycle. Thus, we investigated how loss of Poz1, Rap1 and Taz1 affects cell cycle regulation of Ccq1 Thr93 phosphorylation and telomere association of telomerase (Trt1^TERT), DNA polymerases, Replication Protein A (RPA) complex, Rad3^ATR-Rad26^ATRIP checkpoint kinase complex, Tel1^ATM kinase, shelterin subunits (Tpz1, Ccq1 and Poz1) and Stn1. We further investigated how telomere shortening, caused by trt1Δ or catalytically dead Trt1-D743A, affects cell cycle-regulated telomere association of telomerase and DNA polymerases. These analyses established that fission yeast shelterin maintains telomere length homeostasis by coordinating the differential arrival of leading (Polε) and lagging (Polα) strand DNA polymerases at telomeres to modulate Rad3^ATR association, Ccq1 Thr93 phosphorylation and telomerase recruitment.

Stable maintenance of telomeres is critical to maintain a stable genome and to prevent accumulation of undesired mutations that may lead to formation of tumors. Telomere dysfunction can also lead to premature aging due to depletion of the stem cell population, highlighting the importance of understanding the regulatory mechanisms that ensure stable telomere maintenance. Based on careful analysis of cell cycle-regulated changes in telomere association of telomerase, DNA polymerases, Replication Protein A, checkpoint kinases, telomere protection complex shelterin, and Stn1-Ten1 complex, we will provide here a new and dynamic model of telomere length regulation in fission yeast, which suggests that shelterin-dependent regulation of differential arrival of leading and lagging strand DNA polymerase at telomeres is responsible for modulating Rad3^ATR checkpoint kinase accumulation and Rad3^ATR-dependent phosphorylation of shelterin subunit Ccq1 to control telomerase recruitment to telomeres.

Chromosome end protection by blunt-ended telomeres

Single-stranded telomeric DNA protrusions are considered to be evolutionarily conserved structural elements essential for chromosome end protection. Their formation at telomeres replicated by the leading strand mechanism is thought to involve poorly understood post-replicative processing of blunt ends. Unexpectedly, we found that angiosperm plants contain blunt-ended and short (1- to 3-nucleotide) G-overhang-containing telomeres that are stably retained in post-mitotic tissues, revealing a novel mechanism of chromosome end protection. The integrity of blunt-ended telomeres depends on the Ku70/80 heterodimer but not on another telomere capping protein, STN1. Curiously, Ku-depleted telomeres are fully functional. They are resected by exonuclease 1, promoting intrachromatid recombination, which may facilitate formation of an alternative capping structure. These data challenge the view that telomeres require ssDNA protrusions for forming a functional capping structure and demonstrate flexibility in solutions to the chromosome end protection problem.

Structural identity of telomeric complexes

A major issue in telomere research is to understand how the integrity of chromosome ends is controlled. Although several nucleoprotein complexes have been described at the telomeres of different organisms, it is still unclear how they confer a structural identity to chromosome ends in order to mask them from DNA repair and to ensure their proper replication. In this review, we describe how telomeric nucleoprotein complexes are structured, comparing different organisms and trying to link these structures to telomere biology. It emerges that telomeres are formed by a complex and specific network of interactions between DNA, RNA and proteins. The fact that these interactions and associated activities are reinforcing each other might help to guaranty the robustness of telomeric functions across the cell cycle and in the event of cellular perturbations. We propose that telomeric nucleoprotein complexes orient cell fate through dynamic transitions in their structures and their organization.

Fission yeast telomeres forecast the end of the crisis

Recent years have placed fission yeast at the forefront of telomere research, as this organism combines a high level of conservation with human telomeres and precise genetic manipulability. Here we highlight some of the latest knowledge of fission yeast telomere maintenance and dysfunction, and illustrate how principles arising from fission yeast research are raising novel questions about telomere plasticity and function in all eukaryotes.

To fuse or not to fuse: how do checkpoint and DNA repair proteins maintain telomeres?

DNA damage checkpoint and DNA repair mechanisms play critical roles in the stable maintenance of genetic information. Various forms of DNA damage that arise inside cells due to common errors in normal cellular processes, such as DNA replication, or due to exposure to various DNA damaging agents, must be quickly detected and repaired by checkpoint signaling and repair factors. Telomeres, the natural ends of linear chromosomes, share many features with undesired "broken" DNA, and are recognized and processed by various DNA damage checkpoint and DNA repair proteins. However, their modes of action at telomeres must be altered from their actions at other DNA damage sites to avoid telomere fusions and permanent cell cycle arrest. Interestingly, accumulating evidence indicates that DNA damage checkpoint and DNA repair proteins are essential for telomere maintenance. In this article, we review our current knowledge on various mechanisms by which DNA damage checkpoint and DNA repair proteins are modulated at telomeres and how they might contribute to telomere maintenance in eukaryotes.

Protection and replication of telomeres in fission yeast

Telomeres, the natural ends of linear chromosomes, must be protected and completely replicated to guarantee genomic stability in eukaryotic cells. However, the protected state of telomeres is not compatible with recruitment of telomerase, an enzyme responsible for extending telomeric G-rich repeats during S-phase; thus, telomeres must undergo switches from a protected state to an accessible state during the cell cycle. In this minireview, we will summarize recent advances in our understanding of proteins involved in the protection and replication of telomeres, and the way these factors are dynamically recruited to telomeres during the cell cycle. We will focus mainly on recent results from fission yeast Schizosaccharomyces pombe, and compare them with results from budding yeast Saccharomyces cerevisiae and mammalian cell studies. In addition, a model for the way in which fission yeast cells replicate telomeres will be presented.

Roles of heterochromatin and telomere proteins in regulation of fission yeast telomere recombination and telomerase recruitment

When the telomerase catalytic subunit (Trt1/TERT) is deleted, a majority of fission yeast cells survives by circularizing chromosomes. Alternatively, a small minority survives by maintaining telomeric repeats through recombination among telomeres. The recombination-based telomere maintenance in trt1Delta cells is inhibited by the telomere protein Taz1. In addition, catalytically inactive full-length Trt1 (Trt1-CI) and truncated Trt1 lacking the T-motif and reverse transcriptase (RT) domain (Trt1-DeltaT/RT) can strongly inhibit recombination-based survival. Here, we investigated the effects of deleting the heterochromatin proteins Swi6 (HP1 ortholog) and Clr4 (Suv39 family of histone methyltransferases) and the telomere capping complex subunits Poz1 and Ccq1 on Taz1- and Trt1-dependent telomere recombination inhibition. The ability of Taz1 to inhibit telomere recombination did not require Swi6, Clr4, Poz1, or Ccq1. Although Swi6, Clr4, and Poz1 were dispensable for the inhibition of telomere recombination by Trt1-CI, Ccq1 was required for efficient telomere recruitment of Trt1 and Trt1-CI-dependent inhibition of telomere recombination. We also found that Swi6, Clr4, Ccq1, the checkpoint kinase Rad3 (ATR ortholog), and the telomerase regulatory subunit Est1 are all required for Trt1-DeltaT/RT to inhibit telomere recombination. However, because loss of Swi6, Clr4, Rad3, Ccq1, or Est1 did not significantly alter the recruitment efficiency of Trt1-DeltaT/RT to telomeres, these factors are likely to enhance the ability of Trt1-DeltaT/RT to inhibit recombination-based survival by contributing to the negative regulation of telomere recombination.

Rap1 in Candida albicans: an unusual structural organization and a critical function in suppressing telomere recombination

Rap1 (repressor activator protein 1) is a conserved multifunctional protein initially identified as a transcriptional regulator of ribosomal protein genes in Saccharomyces cerevisiae but subsequently shown to play diverse functions at multiple chromosomal loci, including telomeres. The function of Rap1 appears to be evolutionarily plastic, especially in the budding yeast lineages. We report here our biochemical and molecular genetic characterizations of Candida albicans Rap1, which exhibits an unusual, miniaturized domain organization in comparison to the S. cerevisiae homologue. We show that in contrast to S. cerevisiae, C. albicans RAP1 is not essential for cell viability but is critical for maintaining normal telomere length and structure. The rap1 null mutant exhibits drastic telomere-length dysregulation and accumulates high levels of telomere circles, which can be largely attributed to aberrant recombination activities at telomeres. Analysis of combination mutants indicates that Rap1 and other telomere proteins mediate overlapping but nonredundant roles in telomere protection. Consistent with the telomere phenotypes of the mutant, C. albicans Rap1 is localized to telomeres in vivo and recognizes the unusual telomere repeat unit with high affinity and sequence specificity in vitro. The DNA-binding Myb domain of C. albicans Rap1 is sufficient to suppress most of the telomere aberrations observed in the null mutant. Notably, we were unable to detect specific binding of C. albicans Rap1 to gene promoters in vivo or in vitro, suggesting that its functions are more circumscribed in this organism. Our findings provide insights on the evolution and mechanistic plasticity of a widely conserved and functionally critical telomere component.

How telomeres solve the end-protection problem

The ends of eukaryotic chromosomes have the potential to be mistaken for damaged or broken DNA and must therefore be protected from cellular DNA damage response pathways. Otherwise, cells might permanently arrest in the cell cycle, and attempts to "repair" the chromosome ends would have devastating consequences for genome integrity. This end-protection problem is solved by protein-DNA complexes called telomeres. Studies of mammalian cells have recently uncovered the mechanism by which telomeres disguise the chromosome ends. Comparison to unicellular eukaryotes reveals key differences in the DNA damage response systems that inadvertently threaten chromosome ends. Telomeres appear to be tailored to these variations, explaining their variable structure and composition.

Fission yeast Tel1(ATM) and Rad3(ATR) promote telomere protection and telomerase recruitment

The checkpoint kinases ATM and ATR are redundantly required for maintenance of stable telomeres in diverse organisms, including budding and fission yeasts, Arabidopsis, Drosophila, and mammals. However, the molecular basis for telomere instability in cells lacking ATM and ATR has not yet been elucidated fully in organisms that utilize both the telomere protection complex shelterin and telomerase to maintain telomeres, such as fission yeast and humans. Here, we demonstrate by quantitative chromatin immunoprecipitation (ChIP) assays that simultaneous loss of Tel1(ATM) and Rad3(ATR) kinases leads to a defect in recruitment of telomerase to telomeres, reduced binding of the shelterin complex subunits Ccq1 and Tpz1, and increased binding of RPA and homologous recombination repair factors to telomeres. Moreover, we show that interaction between Tpz1-Ccq1 and telomerase, thought to be important for telomerase recruitment to telomeres, is disrupted in tel1Delta rad3Delta cells. Thus, Tel1(ATM) and Rad3(ATR) are redundantly required for both protection of telomeres against recombination and promotion of telomerase recruitment. Based on our current findings, we propose the existence of a regulatory loop between Tel1(ATM)/Rad3(ATR) kinases and Tpz1-Ccq1 to ensure proper protection and maintenance of telomeres in fission yeast.

Kinase-independent functions of TEL1 in telomere maintenance

TEL1 is important in Saccharomyces cerevisiae telomere maintenance, and its kinase activity is required. Tel1p associates with telomeres in vivo, is enriched at short telomeres, and enhances the binding of telomerase components to short telomeres. However, it is unclear how the kinase activity and telomere association contribute to Tel1p's overall function in telomere length maintenance. To investigate this question, we generated a set of single point mutants and a double point mutant (tel1(KD)) of Tel1p that were kinase deficient and two Xrs2p mutants that failed to bind Tel1p. Using these separation-of-function alleles in a de novo telomere elongation assay, we found, surprisingly, that the tel1(KD) allele and xrs2 C-terminal mutants were both partially functional. Combining the tel1(KD) and xrs2 C-terminal mutants had an additive effect and resembled the TEL1 null (tel1Delta) phenotype. These data indicate that Tel1p has two separate functions in telomere maintenance and that the Xrs2p-dependent recruitment of Tel1p to telomeres plays an important role even in the absence of its kinase activity.

Genome stability roles of SUMO-targeted ubiquitin ligases

Post-translational modification of the cell's proteome by ubiquitin and ubiquitin-like proteins provides dynamic functional regulation. Ubiquitin and SUMO are well-studied post-translational modifiers that typically impart distinct effects on their targets. The recent discovery that modification by SUMO can target proteins for ubiquitination and proteasomal degradation sets a new paradigm in the field, and offers insights into the roles of SUMO and ubiquitin in genome stability.

Differential arrival of leading and lagging strand DNA polymerases at fission yeast telomeres

To maintain genomic integrity, telomeres must undergo switches from a protected state to an accessible state that allows telomerase recruitment. To better understand how telomere accessibility is regulated in fission yeast, we analysed cell cycle-dependent recruitment of telomere-specific proteins (telomerase Trt1, Taz1, Rap1, Pot1 and Stn1), DNA replication proteins (DNA polymerases, MCM, RPA), checkpoint protein Rad26 and DNA repair protein Nbs1 to telomeres. Quantitative chromatin immunoprecipitation studies revealed that MCM, Nbs1 and Stn1 could be recruited to telomeres in the absence of telomere replication in S-phase. In contrast, Trt1, Pot1, RPA and Rad26 failed to efficiently associate with telomeres unless telomeres are actively replicated. Unexpectedly, the leading strand DNA polymerase epsilon (Polepsilon) arrived at telomeres earlier than the lagging strand DNA polymerases alpha (Polalpha) and delta (Poldelta). Recruitment of RPA and Rad26 to telomeres matched arrival of DNA Polepsilon, whereas S-phase specific recruitment of Trt1, Pot1 and Stn1 matched arrival of DNA Polalpha. Thus, the conversion of telomere states involves an unanticipated intermediate step where lagging strand synthesis is delayed until telomerase is recruited.

Tdp1 protects against oxidative DNA damage in non-dividing fission yeast

In humans, a mutation in the tyrosyl-DNA phosphodiesterase (Tdp1) is responsible for the recessively inherited syndrome spinocerebellar ataxia with axonal neuropathy (SCAN1). Tdp1 is a well-conserved DNA repair enzyme, which processes modified 3' phospho-DNA adducts in vitro. Here, we report that in the yeast Schizosaccharomyces pombe, tdp1 mutant cells progressively accumulate DNA damage and rapidly lose viability in a physiological G0/quiescent state. Remarkably, this effect is independent of topoisomerase I function. Moreover, we provide evidence that Tdp1, with the polynucleotide kinase (Pnk1), processes the same naturally occurring 3'-ends, produced from oxidative DNA damage in G0. We also found that one half of the dead cells lose their nuclear DNA. Nuclear DNA degradation is genetically programmed and mainly depends on the two DNA damage checkpoint responses, ATM/Tel1 and ATR/Rad3, reminiscent to programmed cell death. Diminishing the respiration rate or treating cells with a low concentration of antioxidants rescues the quiescent tdp1 mutant cells. These findings suggest that mitochondrial respiration causes neuronal cell death in the SCAN1 syndrome and in other neurological disorders.

Fission yeast Ccq1 is telomerase recruiter and local checkpoint controller

Telomeres recruit telomerase and differentiate chromosome ends from sites of DNA damage. Although the DNA damage checkpoint PI3-kinases ATM and ATR localize to telomeres and promote telomerase activation, activation of their downstream checkpoint pathway targets is inhibited. Here, we show that the fission yeast telomeric protein Ccq1 is required for telomerase recruitment and inhibition of ATR target activation at telomeres. The loss of Ccq1 results in progressive telomere shortening and persistent ATR-dependent activation of Chk1. Unlike the checkpoint activation that follows loss of telomerase, this checkpoint activation occurs prior to detectable levels of critically short telomeres. When ccq1Delta telomeres do become critically short, activated Chk1 promotes an unusual homologous recombination-based telomere maintenance process. We find that the previously reported meiotic segregation defects of cells lacking Ccq1 stem from its role in telomere maintenance rather than from a role in formation of the meiotic bouquet. These findings demonstrate the existence of a novel telomerase recruitment factor that also serves to suppress local checkpoint activation.

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

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