Mitotic cyclins regulate telomeric recombination in telomerase-deficient yeast cells

Upregulation of dNTP Levels After Telomerase Inactivation Influences Telomerase-Independent Telomere Maintenance Pathway Choice in Saccharomyces cerevisiae

In 10–15% of cancers, telomere length is maintained by a telomerase-independent, recombination-mediated pathway called alternative lengthening of telomeres (ALT). ALT mechanisms were first seen, and have been best studied, in telomerase-null Saccharomyces cerevisiae cells called “survivors”. There are two main types of survivors. Type I survivors amplify Y′ subtelomeric elements while type II survivors, similar to the majority of human ALT cells, amplify the terminal telomeric repeats. Both types of survivors require Rad52, a key homologous recombination protein, and Pol32, a non-essential subunit of DNA polymerase δ. A number of additional proteins have been reported to be important for either type I or type II survivor formation, but it is still unclear how these two pathways maintain telomeres. In this study, we performed a genome-wide screen to identify novel genes that are important for the formation of type II ALT-like survivors. We identified 23 genes that disrupt type II survivor formation when deleted. 17 of these genes had not been previously reported to do so. Several of these genes (DUN1, CCR4, and MOT2) are known to be involved in the regulation of dNTP levels. We find that dNTP levels are elevated early after telomerase inactivation and that this increase favors the formation of type II survivors.

Rad6-Bre1-mediated H2B ubiquitination regulates telomere replication by promoting telomere-end resection

Rad6 and Bre1, ubiquitin-conjugating E2 and E3 enzymes respectively, are responsible for histone H2B lysine 123 mono-ubiquitination (H2Bub1) in Saccharomyces cerevisiae. Previous studies have shown that Rad6 and Bre1 regulate telomere length and recombination. However, the underlying molecular mechanism remains largely unknown. Here we report that H2BK123 mutation results in telomere shortening, while inactivation of Ubp8 and/or Ubp10, deubiquitinases of H2Bub1, leads to telomere lengthening in Rad6–Bre1-dependent manner. In telomerase-deficient cells, inactivation of Rad6–Bre1 pathway retards telomere shortening rate and the onset of senescence, while deletion of UBP8 and/or UBP10 accelerates senescence. Thus, Rad6–Bre1 pathway regulates both telomere length and recombination through its role in H2Bub1. Additionally, inactivation of both Rad6–Bre1–H2Bub1 and Mre11–Rad50–Xrs2 (MRX) pathways causes synthetic growth defects and telomere shortening in telomerase-proficient cells, and significantly accelerates senescence and eliminates type II telomere recombination in telomerase-deficient cells. Furthermore, RAD6 or BRE1 deletion, or H2BK123R mutation decreases the accumulation of ssDNA at telomere ends. These results support the model that Rad6–Bre1–H2Bub1 cooperates with MRX to promote telomere-end resection and thus positively regulates both telomerase- and recombination-dependent telomere replication. This study provides a mechanistic link between histone H2B ubiquitination and telomere replication.

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.

Cdk1-dependent regulation of the Mre11 complex couples DNA repair pathways to cell cycle progression

Homologous recombination (HR) and non-homologous end joining (NHEJ) are the main pathways ensuring the repair of DNA double-stranded breaks (DSBs) in eukaryotes. It has long been known that cell cycle stage is a major determinant of the type of pathway used to repair DSBs in vivo. However, the mechanistic basis for the cell cycle regulation of the DNA damage response is still unclear. Here we show that a major DSB sensor, the Mre11-Rad50-Xrs2 (MRX) complex, is regulated by cell cycle-dependent phosphorylation specifically in mitosis. This modification depends on the cyclin-dependent kinase Cdc28/Cdk1, and abrogation of Xrs2 and Mre11 phosphorylation results in a marked preference for DSB repair through NHEJ. Importantly, we show that phosphorylation of the MRX complex after DNA damage and during mitosis are regulated independently of each other by Tel1/ATM and Cdc28/Cdk1 kinases. Collectively, our results unravel an intricate network of phosphoregulatory mechanisms that act through the MRX complex to modulate DSB repair efficiency during mitosis.

Telomerase-null survivor screening identifies novel telomere recombination regulators

Telomeres are protein–DNA structures found at the ends of linear chromosomes and are crucial for genome integrity. Telomeric DNA length is primarily maintained by the enzyme telomerase. Cells lacking telomerase will undergo senescence when telomeres become critically short. In Saccharomyces cerevisiae, a very small percentage of cells lacking telomerase can remain viable by lengthening telomeres via two distinct homologous recombination pathways. These “survivor” cells are classified as either Type I or Type II, with each class of survivor possessing distinct telomeric DNA structures and genetic requirements. To elucidate the regulatory pathways contributing to survivor generation, we knocked out the telomerase RNA gene TLC1 in 280 telomere-length-maintenance (TLM) gene mutants and examined telomere structures in post-senescent survivors. We uncovered new functional roles for 10 genes that affect the emerging ratio of Type I versus Type II survivors and 22 genes that are required for Type II survivor generation. We further verified that Pif1 helicase was required for Type I recombination and that the INO80 chromatin remodeling complex greatly affected the emerging frequency of Type I survivors. Finally, we found the Rad6-mediated ubiquitination pathway and the KEOPS complex were required for Type II recombination. Our data provide an independent line of evidence supporting the idea that these genes play important roles in telomere dynamics.

Homologous recombination is a means for an organism or a cell to repair damaged DNA in its genome. Eukaryotic chromosomes have a linear configuration with two ends that are special DNA–protein structures called telomeres. Telomeres can be recognized by the cell as DNA double-strand breaks and subjected to repair by homologous recombination. In the baker's yeast Saccharomyces cerevisiae, cells that lack the enzyme telomerase, which is the primary factor responsible for telomeric DNA elongation, are able to escape senescence and cell death when telomeres undergo repair via homologous recombination. In this study, we have performed genetic screens to identify genes that affect telomeric DNA recombination. By examining the telomere structures in 280 mutants, each of which lacks both a telomere-length-maintenance gene and telomerase RNA gene, we identified 32 genes that were not previously known to be involved in telomere recombination. These genes have functions in a variety of cellular processes, and our work provides new insights into the regulation of telomere recombination in the absence of telomerase.

A network-based approach on elucidating the multi-faceted nature of chronological aging in S. cerevisiae

BACKGROUND

Cellular mechanisms leading to aging and therefore increasing susceptibility to age-related diseases are a central topic of research since aging is the ultimate, yet not understood mechanism of the fate of a cell. Studies with model organisms have been conducted to ellucidate these mechanisms, and chronological aging of yeast has been extensively used as a model for oxidative stress and aging of postmitotic tissues in higher eukaryotes.

METHODOLOGY/PRINCIPAL FINDINGS

The chronological aging network of yeast was reconstructed by integrating protein-protein interaction data with gene ontology terms. The reconstructed network was then statistically "tuned" based on the betweenness centrality values of the nodes to compensate for the computer automated method. Both the originally reconstructed and tuned networks were subjected to topological and modular analyses. Finally, an ultimate "heart" network was obtained via pooling the step specific key proteins, which resulted from the decomposition of the linear paths depicting several signaling routes in the tuned network.

CONCLUSIONS/SIGNIFICANCE

The reconstructed networks are of scale-free and hierarchical nature, following a power law model with γ  =  1.49. The results of modular and topological analyses verified that the tuning method was successful. The significantly enriched gene ontology terms of the modular analysis confirmed also that the multifactorial nature of chronological aging was captured by the tuned network. The interplay between various signaling pathways such as TOR, Akt/PKB and cAMP/Protein kinase A was summarized in the "heart" network originated from linear path analysis. The deletion of four genes, TCB3, SNA3, PST2 and YGR130C, was found to increase the chronological life span of yeast. The reconstructed networks can also give insight about the effect of other cellular machineries on chronological aging by targeting different signaling pathways in the linear path analysis, along with unraveling of novel proteins playing part in these pathways.

Cdc28/Cdk1 positively and negatively affects genome stability in S. cerevisiae

We studied the function of the cyclin-dependent kinase Cdc28 (Cdk1) in the DNA damage response and maintenance of genome stability using Saccharomyces cerevisiae. Reduced Cdc28 activity sensitizes cells to chronic DNA damage, but Cdc28 is not required for cell viability upon acute exposure to DNA-damaging agents. Cdc28 is also not required for activation of the DNA damage and replication checkpoints. Chemical–genetic analysis reveals that CDC28 functions in an extensive network of pathways involved in maintenance of genome stability, including homologous recombination, sister chromatid cohesion, the spindle checkpoint, postreplication repair, and telomere maintenance. In addition, Cdc28 and Mre11 appear to cooperate to prevent mitotic catastrophe after DNA replication arrest. We show that reduced Cdc28 activity results in suppression of gross chromosomal rearrangements (GCRs), indicating that Cdc28 is required for formation or recovery of GCRs. Thus, we conclude that Cdc28 functions in a genetic network that supports cell viability during DNA damage while promoting the formation of GCRs.

Telomerase- and Rad52-independent immortalization of budding yeast by an inherited-long-telomere pathway of telomeric repeat amplification

In the absence of telomerase, telomeres erode, provoking accumulation of DNA damage and death by senescence. Rare survivors arise, however, due to Rad52-based amplification of telomeric sequences by homologous recombination. The present study reveals that in budding yeast cells, postsenescence survival relying on amplification of the TG(1-3) telomeric repeats can take place in the absence of Rad52 when overelongated telomeres are present during senescence (hence its designation ILT, for inherited-long-telomere, pathway). By growth competition, the Rad52-independent pathway was almost as efficient as the Rad51- and Rad52-dependent pathway that predominates in telomerase-negative cells. The ILT pathway could also be triggered by increased telomerase accessibility before telomerase removal, combined with loss of telomere protection, indicating that prior accumulation of recombination proteins was not required. The ILT pathway was dependent on Rad50 and Mre11 but not on the Rad51 recombinase and Rad59, thus making it distinct from both the type II (budding yeast ALT [alternative lengthening of telomeres]) and type I pathways amplifying the TG(1-3) repeats and subtelomeric sequences, respectively. The ILT pathway also required the Rad1 endonuclease and Elg1, a replication factor C (RFC)-like complex subunit, but not Rad24 or Ctf18 (two subunits of two other RFC-like complexes), the Dnl4 ligase, Yku70, or Nej1. Possible mechanisms for this Rad52-independent pathway of telomeric repeat amplification are discussed. The effects of inherited long telomeres on Rad52-dependent recombination are also reported.

Protection against chromosome degradation at the telomeres

Telomeres, the ends of linear chromosomes, contain repeated TG-rich sequences which, in dividing cells, must be constantly replenished in order to avoid chromosome erosion and, hence, genomic instability. Moreover, unprotected telomeres are prone to end-to-end fusions. Telomerase, a specialized reverse transcriptase with a built-in RNA template, or, in the absence of telomerase, alternative pathways of telomere maintenance are required for continuous cell proliferation in actively dividing cells as well as in cancerous cells emerging in deregulated somatic tissues. The challenge is to keep these free DNA ends masked from the nucleolytic attacks that will readily operate on any DNA double-strand break in the cell, while also allowing the recruitment of telomerase at intervals. Specialized telomeric proteins, as well as DNA repair and checkpoint proteins with a dual role in telomere maintenance and DNA damage signaling/repair, protect the telomere ends from degradation and some of them also function in telomerase recruitment or other aspects of telomere length homeostasis. Phosphorylation of some telomeric proteins by checkpoint protein kinases appears to represent a mode of regulation of telomeric mechanisms. Finally, recent studies have allowed starting to understand the coupling between progression of the replication forks through telomeric regions and the subsequent telomere replication by telomerase, as well as retroaction of telomerase in cis on the firing of nearby replication origins.

Mdt1 facilitates efficient repair of blocked DNA double-strand breaks and recombinational maintenance of telomeres

DNA recombination plays critical roles in DNA repair and alternative telomere maintenance. Here we show that absence of the SQ/TQ cluster domain-containing protein Mdt1 (Ybl051c) renders Saccharomyces cerevisiae particularly hypersensitive to bleomycin, a drug that causes 3'-phospho-glycolate-blocked DNA double-strand breaks (DSBs). mdt1Delta also hypersensitizes partially recombination-defective cells to camptothecin-induced 3'-phospho-tyrosyl protein-blocked DSBs. Remarkably, whereas mdt1Delta cells are unable to restore broken chromosomes after bleomycin treatment, they efficiently repair "clean" endonuclease-generated DSBs. Epistasis analyses indicate that MDT1 acts in the repair of bleomycin-induced DSBs by regulating the efficiency of the homologous recombination pathway as well as telomere-related functions of the KU complex. Moreover, mdt1Delta leads to severe synthetic growth defects with a deletion of the recombination facilitator and telomere-positioning factor gene CTF18 already in the absence of exogenous DNA damage. Importantly, mdt1Delta causes a dramatic shift from the usually prevalent type II to the less-efficient type I pathway of recombinational telomere maintenance in the absence of telomerase in liquid senescence assays. As telomeres resemble protein-blocked DSBs, the results indicate that Mdt1 acts in a novel blocked-end-specific recombination pathway that is required for the efficiency of both drug-induced DSB repair and telomerase-independent telomere maintenance.

Break-induced replication and recombinational telomere elongation in yeast

When a telomere becomes unprotected or if only one end of a chromosomal double-strand break succeeds in recombining with a template sequence, DNA can be repaired by a recombination-dependent DNA replication process termed break-induced replication (BIR). In budding yeasts, there are two BIR pathways, one dependent on the Rad51 recombinase protein and one Rad51 independent; these two repair processes lead to different types of survivors in cells lacking the telomerase enzyme that is required for normal telomere maintenance. Recombination at telomeres is triggered by either excessive telomere shortening or disruptions in the function of telomere-binding proteins. Telomere elongation by BIR appears to often occur through a "roll and spread" mechanism. In this process, a telomeric circle produced by recombination at a dysfunctional telomere acts as a template for a rolling circle BIR event to form an elongated telomere. Additional BIR events can then copy the elongated sequence to all other telomeres.

When CDK1 rides the telomere cycle

Reconstitution of telomeric DNA at each cell division implies the coordination of DNA semiconservative replication with several processing events still poorly understood. Two reports published recently in Molecular Cell show that a cell-cycle cyclin-dependent kinase, Cdk1p, is required to create the cell-cycle-regulated overhang ().

Control of the yeast telomeric senescence survival pathways of recombination by the Mec1 and Mec3 DNA damage sensors and RPA

Saccharomyces cerevisiae telomerase-negative cells undergo homologous recombination on subtelomeric or TG_1–3 telomeric sequences, thus allowing Type I or Type II post-senescence survival, respectively. Here, we find that the DNA damage sensors, Mec1, Mec3 and Rad24 control Type II recombination, while the Rad9 adaptor protein and the Rad53 and Chk1 effector kinases have no effect on survivor type selection. Therefore, the Mec1 and Mec3 checkpoint complexes control telomeric recombination independently of their roles in generating and amplifying the Mec1-Rad53-Chk1 kinase cascade. rfa1-t11 mutant cells, bearing a mutation in Replication Protein A (RPA) conferring a defect in recruiting Mec1-Ddc2, were also deficient in both types of telomeric recombination. Importantly, expression of an Rfa1-t11-Ddc2 hybrid fusion protein restored checkpoint-dependent arrest, but did not rescue defective telomeric recombination. Therefore, the Rfa1-t11-associated defect in telomeric recombination is not solely due to its failure to recruit Mec1. We have also isolated novel alleles of RFA1 that were deficient in Type I but not in Type II recombination and proficient in checkpoint control. Therefore, the checkpoint and recombination functions of RPA can be genetically separated, as can the RPA-mediated control of the two types of telomeric recombination.

Regulation of telomere elongation by the cyclin-dependent kinase CDK1

Telomere elongation is cell-cycle regulated and requires the coordinated activity of proteins involved in the DNA damage response. We used an assay that detects de novo telomere addition to examine the role of the cyclin-dependent kinase Cdk1 (Cdc28) in cell-cycle-specific telomere elongation. Inhibition of an ATP analog-sensitive allele of Cdk1 completely blocked the addition of telomere repeats. Mutations in Rif2 and DNA polymerase alpha that cause increased telomere elongation were unable to compensate for the loss of Cdk1 activity, suggesting Cdk1 activity is required for an early step in telomere addition. Mutations in DNA repair proteins that act with Cdk1 at double-strand breaks also prevented telomere elongation. Cdk1 activity was required for the generation of 3' single-strand overhangs at both native and de novo telomeres. We propose Cdk1 activity controls the timing of telomere elongation by regulating the single-strand overhang at chromosome ends.

Functional genomic analysis of CAF-1 mutants in Arabidopsis thaliana

Duplication of chromatin following DNA replication requires spatial reorganization of chromatin domains assisted by chromatin assembly factor CAF-1. Here, we tested the genomic consequences of CAF-1 loss and the function of chromatin assembly factor CAF-1 in heterochromatin formation. Genes located in heterochromatic regions are usually silent, and we found that this transcriptional repression persists in the absence of CAF-1 in Arabidopsis. However, using microarrays we observed that genes that are active during late S-phase, when heterochromatin is duplicated, were up-regulated in CAF-1 mutants. Arabidopsis CAF-1 mutants also have reduced cytological heterochromatin content; however, DNA methylation of pericentromeric repeats was normal, demonstrating that CAF-1 is not required for maintenance of DNA methylation. Instead, hypomethylation of the genome, which has only mild effects on the development of wild-type plants, completely arrested development of CAF-1 mutants. These results suggest that CAF-1 functions in heterochromatin formation. CAF-1 and DNA methylation, which is also needed for heterochromatin formation, have partially redundant functions that are essential for cell proliferation. Interestingly, transcriptional repression and heterochromatin compaction can be genetically separated, and CAF-1 is required only for the complete compaction of heterochromatin but not to maintain transcriptional repression of heterochromatic genes.

Telomere dynamics in genome stability

The past several years have seen an increasing interest in telomere recombinational interactions that provide many functions in telomere capping, in telomere size homeostasis and in overcoming the catastrophic effects of telomerase deficiency. Several key recombination mechanisms have emerged from recent investigations. In the yeasts, these mechanisms include exchange between subtelomeric regions and telomere sequences, rapid telomere expansion and telomere deletion. These processes proceed by pathways that use both the cellular recombination machinery and novel mechanisms such as rolling circle replication. The insights gained from recent studies extend our understanding of similar processes in higher eukaryotes and suggest that the recombinational dynamics of telomeres have additional roles that contribute to genomic stability and instability.

Srs2 and Sgs1 DNA helicases associate with Mre11 in different subcomplexes following checkpoint activation and CDK1-mediated Srs2 phosphorylation

Mutations in the genes encoding the BLM and WRN RecQ DNA helicases and the MRE11-RAD50-NBS1 complex lead to genome instability and cancer predisposition syndromes. The Saccharomyces cerevisiae Sgs1 RecQ helicase and the Mre11 protein, together with the Srs2 DNA helicase, prevent chromosome rearrangements and are implicated in the DNA damage checkpoint response and in DNA recombination. By searching for Srs2 physical interactors, we have identified Sgs1 and Mre11. We show that Srs2, Sgs1, and Mre11 form a large complex, likely together with yet unidentified proteins. This complex reorganizes into Srs2-Mre11 and Sgs1-Mre11 subcomplexes following DNA damage-induced activation of the Mec1 and Tel1 checkpoint kinases. The defects in subcomplex formation observed in mec1 and tel1 cells can be recapitulated in srs2-7AV mutants that are hypersensitive to intra-S DNA damage and are altered in the DNA damage-induced and Cdk1-dependent phosphorylation of Srs2. Altogether our observations indicate that Mec1- and Tel1-dependent checkpoint pathways modulate the functional interactions between Srs2, Sgs1, and Mre11 and that the Srs2 DNA helicase represents an important target of the Cdk1-mediated cellular response induced by DNA damage.

A Tel1/MRX-dependent checkpoint inhibits the metaphase-to-anaphase transition after UV irradiation in the absence of Mec1

In Saccharomyces cerevisiae, Mec1/ATR plays a primary role in sensing and transducing checkpoint signals in response to different types of DNA lesions, while the role of the Tel1/ATM kinase in DNA damage checkpoints is not as well defined. We found that UV irradiation in G(1) in the absence of Mec1 activates a Tel1/MRX-dependent checkpoint, which specifically inhibits the metaphase-to-anaphase transition. Activation of this checkpoint leads to phosphorylation of the downstream checkpoint kinases Rad53 and Chk1, which are required for Tel1-dependent cell cycle arrest, and their adaptor Rad9. The spindle assembly checkpoint protein Mad2 also partially contributes to the G(2)/M arrest of UV-irradiated mec1Delta cells independently of Rad53 phosphorylation and activation. The inability of UV-irradiated mec1Delta cells to undergo anaphase can be relieved by eliminating the anaphase inhibitor Pds1, whose phosphorylation and stabilization in these cells depend on Tel1, suggesting that Pds1 persistence may be responsible for the inability to undergo anaphase. Moreover, while UV irradiation can trigger Mec1-dependent Rad53 phosphorylation and activation in G(1)- and G(2)-arrested cells, Tel1-dependent checkpoint activation requires entry into S phase independently of the cell cycle phase at which cells are UV irradiated, and it is decreased when single-stranded DNA signaling is affected by the rfa1-t11 allele. This indicates that UV-damaged DNA molecules need to undergo structural changes in order to activate the Tel1-dependent checkpoint. Active Clb-cyclin-dependent kinase 1 (CDK1) complexes also participate in triggering this checkpoint and are required to maintain both Mec1- and Tel1-dependent Rad53 phosphorylation, suggesting that they may provide critical phosphorylation events in the DNA damage checkpoint cascade.