Swi1Timeless Prevents Repeat Instability at Fission Yeast Telomeres

The TIMELESS Roles in Genome Stability and Beyond

TIMELESS protein (TIM) protects replication forks from stalling at difficult-to-replicate regions and plays an important role in DNA damage response, including checkpoint signaling, protection of stalled replication forks and DNA repair. Loss of TIM causes severe replication stress, while its overexpression is common in various types of cancer, providing protection from DNA damage and resistance to chemotherapy. Although TIM has mostly been studied for its part in replication stress response, its additional roles in supporting genome stability and a wide variety of other cellular pathways are gradually coming to light. This review discusses the diverse functions of TIM and its orthologs in healthy and cancer cells, open questions, and potential future directions.

The Adaptive Mechanisms and Checkpoint Responses to a Stressed DNA Replication Fork

DNA replication is a tightly controlled process that ensures the faithful duplication of the genome. However, DNA damage arising from both endogenous and exogenous assaults gives rise to DNA replication stress associated with replication fork slowing or stalling. Therefore, protecting the stressed fork while prompting its recovery to complete DNA replication is critical for safeguarding genomic integrity and cell survival. Specifically, the plasticity of the replication fork in engaging distinct DNA damage tolerance mechanisms, including fork reversal, repriming, and translesion DNA synthesis, enables cells to overcome a variety of replication obstacles. Furthermore, stretches of single-stranded DNA generated upon fork stalling trigger the activation of the ATR kinase, which coordinates the cellular responses to replication stress by stabilizing the replication fork, promoting DNA repair, and controlling cell cycle and replication origin firing. Deregulation of the ATR checkpoint and aberrant levels of chronic replication stress is a common characteristic of cancer and a point of vulnerability being exploited in cancer therapy. Here, we discuss the various adaptive responses of a replication fork to replication stress and the roles of ATR signaling that bring fork stabilization mechanisms together. We also review how this knowledge is being harnessed for the development of checkpoint inhibitors to trigger the replication catastrophe of cancer cells.

The TIMELESS effort for timely DNA replication and protection

Accurate replication of the genome is fundamental to cellular survival and tumor prevention. The DNA replication fork is vulnerable to DNA lesions and damages that impair replisome progression, and improper control over DNA replication stress inevitably causes fork stalling and collapse, a major source of genome instability that fuels tumorigenesis. The integrity of the DNA replication fork is maintained by the fork protection complex (FPC), in which TIMELESS (TIM) constitutes a key scaffold that couples the CMG helicase and replicative polymerase activities, in conjunction with its interaction with other proteins associated with the replication machinery. Loss of TIM or the FPC in general results in impaired fork progression, elevated fork stalling and breakage, and a defect in replication checkpoint activation, thus underscoring its pivotal role in protecting the integrity of both active and stalled replication forks. TIM is upregulated in multiple cancers, which may represent a replication vulnerability of cancer cells that could be exploited for new therapies. Here, we discuss recent advances on our understanding of the multifaceted roles of TIM in DNA replication and stalled fork protection, and how its complex functions are engaged in collaboration with other genome surveillance and maintenance factors.

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.

Alternative paths to telomere elongation

Overcoming cellular senescence that is induced by telomere shortening is critical in tumorigenesis. A majority of cancers achieve telomere maintenance through telomerase expression. However, a subset of cancers takes an alternate route for elongating telomeres: recombination-based alternative lengthening of telomeres (ALT). Current evidence suggests that break-induced replication (BIR), independent of RAD51, underlies ALT telomere synthesis. However, RAD51-dependent homologous recombination is required for homology search and inter-chromosomal telomere recombination in human ALT cancer cell maintenance. Accumulating evidence suggests that the breakdown of stalled replication forks, the replication stress, induces BIR at telomeres. Nevertheless, ALT research is still in its early stage and a comprehensive view is still unclear. Here, we review the current findings regarding the genesis of ALT, how this recombinant pathway is chosen, the epigenetic regulation of telomeres in ALT, and perspectives for clinical applications with the hope that this overview will generate new questions.

Maf1-dependent transcriptional regulation of tRNAs prevents genomic instability and is associated with extended lifespan

Maf1 is the master repressor of RNA polymerase III responsible for transcription of tRNAs and 5S rRNAs. Maf1 is negatively regulated via phosphorylation by the mTOR pathway, which governs protein synthesis, growth control, and lifespan regulation in response to nutrient availability. Inhibiting the mTOR pathway extends lifespan in various organisms. However, the downstream effectors for the regulation of cell homeostasis that are critical to lifespan extension remain elusive. Here we show that fission yeast Maf1 is required for lifespan extension. Maf1's function in tRNA repression is inhibited by mTOR-dependent phosphorylation, whereas Maf1 is activated via dephosphorylation by protein phosphatase complexes, PP4 and PP2A. Mutational analysis reveals that Maf1 phosphorylation status influences lifespan, which is correlated with elevated tRNA and protein synthesis levels in maf1∆ cells. However, mTOR downregulation, which negates protein synthesis, fails to rescue the short lifespan of maf1∆ cells, suggesting that elevated protein synthesis is not a cause of lifespan shortening in maf1∆ cells. Interestingly, maf1∆ cells accumulate DNA damage represented by formation of Rad52 DNA damage foci and Rad52 recruitment at tRNA genes. Loss of the Rad52 DNA repair protein further exacerbates the shortened lifespan of maf1∆ cells. Strikingly, PP4 deletion alleviates DNA damage and rescues the short lifespan of maf1∆ cells even though tRNA synthesis is increased in this condition, suggesting that elevated DNA damage is the major cause of lifespan shortening in maf1∆ cells. We propose that Maf1-dependent inhibition of tRNA synthesis controls fission yeast lifespan by preventing genomic instability that arises at tRNA genes.

Alternative Lengthening of Telomeres in Pediatric Cancer: Mechanisms to Therapies

Achieving replicative immortality is a crucial step in tumorigenesis and requires both bypassing cell cycle checkpoints and the extension of telomeres, sequences that protect the distal ends of chromosomes during replication. In the majority of cancers this is achieved through the enzyme telomerase, however a subset of cancers instead utilize a telomerase-independent mechanism of telomere elongation-the Alternative Lengthening of Telomeres (ALT) pathway. Recent work has aimed to decipher the exact mechanism that underlies this pathway. To this end, this pathway has now been shown to extend telomeres through exploitation of DNA repair machinery in a unique process that may present a number of druggable targets. The identification of such targets, and the subsequent development or repurposing of therapies to these targets may be crucial to improving the prognosis for many ALT-positive cancers, wherein mean survival is lower than non-ALT counterparts and the cancers themselves are particularly unresponsive to standard of care therapies. In this review we summarize the recent identification of many aspects of the ALT pathway, and the therapies that may be employed to exploit these new targets.

At the Beginning of the End and in the Middle of the Beginning: Structure and Maintenance of Telomeric DNA Repeats and Interstitial Telomeric Sequences

Tandem DNA repeats derived from the ancestral (TTAGGG)n run were first detected at chromosome ends of the majority of living organisms, hence the name telomeric DNA repeats. Subsequently, it has become clear that telomeric motifs are also present within chromosomes, and they were suitably called interstitial telomeric sequences (ITSs). It is well known that telomeric DNA repeats play a key role in chromosome stability, preventing end-to-end fusions and precluding the recurrent DNA loss during replication. Recent data suggest that ITSs are also important genomic elements as they confer its karyotype plasticity. In fact, ITSs appeared to be among the most unstable microsatellite sequences as they are highly length polymorphic and can trigger chromosomal fragility and gross chromosomal rearrangements. Importantly, mechanisms responsible for their instability appear to be similar to the mechanisms that maintain the length of genuine telomeres. This review compares the mechanisms of maintenance and dynamic properties of telomeric repeats and ITSs and discusses the implications of these dynamics on genome stability.

Dependency of Heterochromatin Domains on Replication Factors

Chromatin structure regulates both genome expression and dynamics in eukaryotes, where large heterochromatic regions are epigenetically silenced through the methylation of histone H3K9, histone deacetylation, and the assembly of repressive complexes. Previous genetic screens with the fission yeast Schizosaccharomyces pombe have led to the identification of key enzymatic activities and structural constituents of heterochromatin. We report here on additional factors discovered by screening a library of deletion mutants for silencing defects at the edge of a heterochromatic domain bound by its natural boundary-the IR-R+ element-or by ectopic boundaries. We found that several components of the DNA replication progression complex (RPC), including Mrc1/Claspin, Mcl1/Ctf4, Swi1/Timeless, Swi3/Tipin, and the FACT subunit Pob3, are essential for robust heterochromatic silencing, as are the ubiquitin ligase components Pof3 and Def1, which have been implicated in the removal of stalled DNA and RNA polymerases from chromatin. Moreover, the search identified the cohesin release factor Wpl1 and the forkhead protein Fkh2, both likely to function through genome organization, the Ssz1 chaperone, the Fkbp39 proline cis-trans isomerase, which acts on histone H3P30 and P38 in Saccharomyces cerevisiae, and the chromatin remodeler Fft3. In addition to their effects in the mating-type region, to varying extents, these factors take part in heterochromatic silencing in pericentromeric regions and telomeres, revealing for many a general effect in heterochromatin. This list of factors provides precious new clues with which to study the spatiotemporal organization and dynamics of heterochromatic regions in connection with DNA replication.

Alternative Lengthening of Telomeres Mediated by Mitotic DNA Synthesis Engages Break-Induced Replication Processes

Alternative lengthening of telomeres (ALT) is a telomerase-independent telomere maintenance mechanism that occurs in a subset of cancers. By analyzing telomerase-positive cells and their human TERC knockout-derived ALT human cell lines, we show that ALT cells harbor more fragile telomeres representing telomere replication problems. ALT-associated replication defects trigger mitotic DNA synthesis (MiDAS) at telomeres in a RAD52-dependent, but RAD51-independent, manner. Telomeric MiDAS is a conservative DNA synthesis process, potentially mediated by break-induced replication, similar to type II ALT survivors in Saccharomyces cerevisiae Replication stresses induced by ectopic oncogenic expression of cyclin E, G-quadruplexes, or R-loop formation facilitate the ALT pathway and lead to telomere clustering, a hallmark of ALT cancers. The TIMELESS/TIPIN complex suppresses telomere clustering and telomeric MiDAS, whereas the SMC5/6 complex promotes them. In summary, ALT cells exhibit more telomere replication defects that result in persistent DNA damage responses at telomeres, leading to the engagement of telomeric MiDAS (spontaneous mitotic telomere synthesis) that is triggered by DNA replication stress, a potential driver of genomic duplications in cancer.

Role of recombination and replication fork restart in repeat instability

Eukaryotic genomes contain many repetitive DNA sequences that exhibit size instability. Some repeat elements have the added complication of being able to form secondary structures, such as hairpin loops, slipped DNA, triplex DNA or G-quadruplexes. Especially when repeat sequences are long, these DNA structures can form a significant impediment to DNA replication and repair, leading to DNA nicks, gaps, and breaks. In turn, repair or replication fork restart attempts within the repeat DNA can lead to addition or removal of repeat elements, which can sometimes lead to disease. One important DNA repair mechanism to maintain genomic integrity is recombination. Though early studies dismissed recombination as a mechanism driving repeat expansion and instability, recent results indicate that mitotic recombination is a key pathway operating within repetitive DNA. The action is two-fold: first, it is an important mechanism to repair nicks, gaps, breaks, or stalled forks to prevent chromosome fragility and protect cell health; second, recombination can cause repeat expansions or contractions, which can be deleterious. In this review, we summarize recent developments that illuminate the role of recombination in maintaining genome stability at DNA repeats.

Regulation of DNA Replication through Natural Impediments in the Eukaryotic Genome

All living organisms need to duplicate their genetic information while protecting it from unwanted mutations, which can lead to genetic disorders and cancer development. Inaccuracies during DNA replication are the major cause of genomic instability, as replication forks are prone to stalling and collapse, resulting in DNA damage. The presence of exogenous DNA damaging agents as well as endogenous difficult-to-replicate DNA regions containing DNA–protein complexes, repetitive DNA, secondary DNA structures, or transcribing RNA polymerases, increases the risk of genomic instability and thus threatens cell survival. Therefore, understanding the cellular mechanisms required to preserve the genetic information during S phase is of paramount importance. In this review, we will discuss our current understanding of how cells cope with these natural impediments in order to prevent DNA damage and genomic instability during DNA replication.

Solving the Telomere Replication Problem

Telomeres are complex nucleoprotein structures that protect the extremities of linear chromosomes. Telomere replication is a major challenge because many obstacles to the progression of the replication fork are concentrated at the ends of the chromosomes. This is known as the telomere replication problem. In this article, different and new aspects of telomere replication, that can threaten the integrity of telomeres, will be reviewed. In particular, we will focus on the functions of shelterin and the replisome for the preservation of telomere integrity.

Transient structural variations have strong effects on quantitative traits and reproductive isolation in fission yeast

Large structural variations (SVs) within genomes are more challenging to identify than smaller genetic variants but may substantially contribute to phenotypic diversity and evolution. We analyse the effects of SVs on gene expression, quantitative traits and intrinsic reproductive isolation in the yeast Schizosaccharomyces pombe. We establish a high-quality curated catalogue of SVs in the genomes of a worldwide library of S. pombe strains, including duplications, deletions, inversions and translocations. We show that copy number variants (CNVs) show a variety of genetic signals consistent with rapid turnover. These transient CNVs produce stoichiometric effects on gene expression both within and outside the duplicated regions. CNVs make substantial contributions to quantitative traits, most notably intracellular amino acid concentrations, growth under stress and sugar utilization in winemaking, whereas rearrangements are strongly associated with reproductive isolation. Collectively, these findings have broad implications for evolution and for our understanding of quantitative traits including complex human diseases.

Centromere Stability: The Replication Connection

The fission yeast centromere, which is similar to metazoan centromeres, contains highly repetitive pericentromere sequences that are assembled into heterochromatin. This is required for the recruitment of cohesin and proper chromosome segregation. Surprisingly, the pericentromere replicates early in the S phase. Loss of heterochromatin causes this domain to become very sensitive to replication fork defects, leading to gross chromosome rearrangements. This review examines the interplay between components of DNA replication, heterochromatin assembly, and cohesin dynamics that ensures maintenance of genome stability and proper chromosome segregation.

Timeless protection of telomeres

The DNA replication machinery encounters problems at numerous genomic regions that are inherently difficult to replicate. These genomic regions include telomeres, which contain repetitive DNA and telomere-binding proteins. If not properly regulated, replication of such genomic regions can result in DNA damage, leading to genomic instability. Studies implicated a role of Timeless-related proteins at difficult-to-replicate genomic regions, including telomeres. However, how these proteins maintain telomeres was elusive. In a recent report, we described the role of Swi1, a Timeless-related protein, in telomere maintenance in fission yeast. We demonstrated that Swi1 is required for proper replication of repeat DNA sequences at telomeres. We also showed that Swi1-deficient cells utilize recombination-based ALT (alternative lengthening of telomeres)-like mechanisms to maintain telomeres in the absence of telomerase. Here, we highlight these findings and present additional data to discuss the role of Swi1^Timeless in telomere protection and ALT prevention.