Tel1/ATM Signaling to the Checkpoint Contributes to Replicative Senescence in the Absence of Telomerase

Exo1 cooperates with Tel1/ATM in promoting recombination events at DNA replication forks

Tel1/ataxia telangiectasia mutated (ATM) kinase plays multiple functions in response to DNA damage, promoting checkpoint-mediated cell-cycle arrest and repair of broken DNA. In addition, Saccharomyces cerevisiae Tel1 stabilizes replication forks that arrest upon the treatment with the topoisomerase poison camptothecin (CPT). We discover that inactivation of the Exo1 nuclease exacerbates the sensitivity of Tel1-deficient cells to CPT and other agents that hamper DNA replication. Furthermore, cells lacking both Exo1 and Tel1 activities exhibit sustained checkpoint activation in the presence of CPT, indicating that Tel1 and Exo1 limit the activation of a Mec1-dependent checkpoint. The absence of Tel1 or its kinase activity enhances recombination between inverted DNA repeats induced by replication fork blockage in an Exo1-dependent manner. Thus, we propose that Exo1 processes intermediates arising at stalled forks in tel1 mutants to promote DNA replication recovery and cell survival.

To Fix or Not to Fix: Maintenance of Chromosome Ends Versus Repair of DNA Double-Strand Breaks

Early work by Muller and McClintock discovered that the physical ends of linear chromosomes, named telomeres, possess an inherent ability to escape unwarranted fusions. Since then, extensive research has shown that this special feature relies on specialized proteins and structural properties that confer identity to the chromosome ends, thus allowing cells to distinguish them from intrachromosomal DNA double-strand breaks. Due to the inability of conventional DNA replication to fully replicate the chromosome ends and the downregulation of telomerase in most somatic human tissues, telomeres shorten as cells divide and lose this protective capacity. Telomere attrition causes the activation of the DNA damage checkpoint that leads to a cell-cycle arrest and the entering of cells into a nondividing state, called replicative senescence, that acts as a barrier against tumorigenesis. However, downregulation of the checkpoint overcomes this barrier and leads to further genomic instability that, if coupled with re-stabilization of telomeres, can drive tumorigenesis. This review focuses on the key experiments that have been performed in the model organism Saccharomyces cerevisiae to uncover the mechanisms that protect the chromosome ends from eliciting a DNA damage response, the conservation of these pathways in mammals, as well as the consequences of their loss in human cancer.

The regulation of the DNA damage response at telomeres: focus on kinases

The natural ends of linear chromosomes resemble those of accidental double-strand breaks (DSBs). DSBs induce a multifaceted cellular response that promotes the repair of lesions and slows down cell cycle progression. This response is not elicited at chromosome ends, which are organized in nucleoprotein structures called telomeres. Besides counteracting DSB response through specialized telomere-binding proteins, telomeres also prevent chromosome shortening. Despite of the different fate of telomeres and DSBs, many proteins involved in the DSB response also localize at telomeres and participate in telomere homeostasis. In particular, the DSB master regulators Tel1/ATM and Mec1/ATR contribute to telomere length maintenance and arrest cell cycle progression when chromosome ends shorten, thus promoting a tumor-suppressive process known as replicative senescence. During senescence, the actions of both these apical kinases and telomere-binding proteins allow checkpoint activation while bulk DNA repair activities at telomeres are still inhibited. Checkpoint-mediated cell cycle arrest also prevents further telomere erosion and deprotection that would favor chromosome rearrangements, which are known to increase cancer-associated genome instability. This review summarizes recent insights into functions and regulation of Tel1/ATM and Mec1/ATR at telomeres both in the presence and in the absence of telomerase, focusing mainly on discoveries in budding yeast.

Modified chromosome structure caused by phosphomimetic H2A modulates the DNA damage response by increasing chromatin mobility in yeast

In budding yeast and mammals, double-strand breaks (DSBs) trigger global chromatin mobility together with rapid phosphorylation of histone H2A over an extensive region of the chromatin. To assess the role of H2A phosphorylation in this response to DNA damage, we have constructed strains where H2A has been mutated to the phosphomimetic H2A-S129E. We show that mimicking H2A phosphorylation leads to an increase in global chromatin mobility in the absence of DNA damage. The intrinsic chromatin mobility of H2A-S129E is not due to downstream checkpoint activation, histone degradation or kinetochore anchoring. Rather, the increased intrachromosomal distances observed in the H2A-S129E mutant are consistent with chromatin structural changes. Strikingly, in this context the Rad9-dependent checkpoint becomes dispensable. Moreover, increased chromatin dynamics in the H2A-S129E mutant correlates with improved DSB repair by non-homologous end joining and a sharp decrease in interchromosomal translocation rate. We propose that changes in chromosomal conformation due to H2A phosphorylation are sufficient to modulate the DNA damage response and maintain genome integrity.This article has an associated First Person interview with the first author of the paper.

Origin, Regulation, and Fitness Effect of Chromosomal Rearrangements in the Yeast Saccharomyces cerevisiae

Chromosomal rearrangements comprise unbalanced structural variations resulting in gain or loss of DNA copy numbers, as well as balanced events including translocation and inversion that are copy number neutral, both of which contribute to phenotypic evolution in organisms. The exquisite genetic assay and gene editing tools available for the model organism Saccharomyces cerevisiae facilitate deep exploration of the mechanisms underlying chromosomal rearrangements. We discuss here the pathways and influential factors of chromosomal rearrangements in S. cerevisiae. Several methods have been developed to generate on-demand chromosomal rearrangements and map the breakpoints of rearrangement events. Finally, we highlight the contributions of chromosomal rearrangements to drive phenotypic evolution in various S. cerevisiae strains. Given the evolutionary conservation of DNA replication and recombination in organisms, the knowledge gathered in the small genome of yeast can be extended to the genomes of higher eukaryotes.

Telomere instability initiates and then boosts carcinogenesis by the butterfly effect

Telomeres are composed of DNA repeat sequences at the ends of chromosomes that recruit a multitude of proteins to form a complex loop structure at each extremity. The integrity of this structure is critical and correct conformation of the loop is essential for the protection of chromosome ends from DDR signaling. The properties of telomere composition and synthesis result in telomere shortening at each cell division, programming cellular lifespan by driving aged cells towards death. Indeed, many external factors, such as cellular stress, trigger cell-cycle dysfunction and, in some cases, enable the survival of cells with dysfunctionally short telomeres. Destabilized loops at chromosome ends can then lead to dramatic consequences, via a butterfly effect such as multiple chromosomal fusions and rearrangements causing large chromosomal deletions, XXL-LOH (loss of heterozygoty due to very large chromosome deletions, up to whole chromosome arm), the expression of recessive mutations, and potential cell transformation.