Sequence divergence impedes crossover more than noncrossover events during mitotic gap repair in yeast

CNOT6: A Novel Regulator of DNA Mismatch Repair

DNA mismatch repair (MMR) is a highly conserved pathway that corrects both base–base mispairs and insertion-deletion loops (IDLs) generated during DNA replication. Defects in MMR have been linked to carcinogenesis and drug resistance. However, the regulation of MMR is poorly understood. Interestingly, CNOT6 is one of four deadenylase subunits in the conserved CCR4-NOT complex and it targets poly(A) tails of mRNAs for degradation. CNOT6 is overexpressed in acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML) and androgen-independent prostate cancer cells, which suggests that an altered expression of CNOT6 may play a role in tumorigenesis. Here, we report that a depletion of CNOT6 sensitizes human U2OS cells to N-methyl-N′nitro-N-nitrosoguanidine (MNNG) and leads to enhanced apoptosis. We also demonstrate that the depletion of CNOT6 upregulates MMR and decreases the mutation frequency in MMR-proficient cells. Furthermore, the depletion of CNOT6 increases the stability of mRNA transcripts from MMR genes, leading to the increased expression of MMR proteins. Our work provides insight into a novel CNOT6-dependent mechanism for regulating MMR.

Mismatch recognition and subsequent processing have distinct effects on mitotic recombination intermediates and outcomes in yeast

The post-replicative mismatch repair (MMR) system has anti-recombination activity that limits interactions between diverged sequences by recognizing mismatches in strand-exchange intermediates. In contrast to their equivalent roles during replication-error repair, mismatch recognition is more important for anti-recombination than subsequent mismatch processing. To obtain insight into this difference, ectopic substrates with 2% sequence divergence were used to examine mitotic recombination outcome (crossover or noncrossover; CO and NCO, respectively) and to infer molecular intermediates formed during double-strand break repair in Saccharomyces cerevisiae. Experiments were performed in an MMR-proficient strain, a strain with compromised mismatch-recognition activity (msh6Δ) and a strain that retained mismatch-recognition activity but was unable to process mismatches (mlh1Δ). While the loss of either mismatch binding or processing elevated the NCO frequency to a similar extent, CO events increased only when mismatch binding was compromised. The molecular features of NCOs, however, were altered in fundamentally different ways depending on whether mismatch binding or processing was eliminated. These data suggest a model in which mismatch recognition reverses strand-exchange intermediates prior to the initiation of end extension, while subsequent mismatch processing that is linked to end extension specifically destroys NCO intermediates that contain conflicting strand-discrimination signals for mismatch removal.

Role of the Srs2-Rad51 Interaction Domain in Crossover Control in Saccharomyces cerevisiae

Saccharomyces cerevisiae Srs2, in addition to its well-documented antirecombination activity, has been proposed to play a role in promoting synthesis-dependent strand annealing (SDSA). Here we report the identification and characterization of an SRS2 mutant with a single amino acid substitution (srs2-F891A) that specifically affects the Srs2 pro-SDSA function. This residue is located within the Srs2-Rad51 interaction domain and embedded within a protein sequence resembling a BRC repeat motif. The srs2-F891A mutation leads to a complete loss of interaction with Rad51 as measured through yeast two-hybrid analysis and a partial loss of interaction as determined through protein pull-down assays with purified Srs2, Srs2-F891A, and Rad51 proteins. Even though previous work has shown that internal deletions of the Srs2-Rad51 interaction domain block Srs2 antirecombination activity in vitro, the Srs2-F891A mutant protein, despite its weakened interaction with Rad51, exhibits no measurable defect in antirecombination activity in vitro or in vivo Surprisingly, srs2-F891A shows a robust shift from noncrossover to crossover repair products in a plasmid-based gap repair assay, but not in an ectopic physical recombination assay. Our findings suggest that the Srs2 C-terminal Rad51 interaction domain is more complex than previously thought, containing multiple interaction sites with unique effects on Srs2 activity.

Moving forward one step back at a time: reversibility during homologous recombination

DNA double-strand breaks are genotoxic lesions whose repair can be templated off an intact DNA duplex through the conserved homologous recombination (HR) pathway. Because it mainly consists of a succession of non-covalent associations of molecules, HR is intrinsically reversible. Reversibility serves as an integral property of HR, exploited and tuned at various stages throughout the pathway with anti- and pro-recombinogenic consequences. Here, we focus on the reversibility of displacement loops (D-loops), a central DNA joint molecule intermediate whose dynamics and regulation have recently been physically probed in somatic S. cerevisiae cells. From homology search to repair completion, we discuss putative roles of D-loop reversibility in repair fidelity and outcome.

Multi-Invasion-Induced Rearrangements as a Pathway for Physiological and Pathological Recombination

Cells mitigate the detrimental consequences of DNA damage on genome stability by attempting high fidelity repair. Homologous recombination templates DNA double-strand break (DSB) repair on an identical or near identical donor sequence in a process that can in principle access the entire genome. Other physiological processes, such as homolog recognition and pairing during meiosis, also harness the HR machinery using programmed DSBs to physically link homologs and generate crossovers. A consequence of the homology search process by a long nucleoprotein filament is the formation of multi-invasions (MI), a joint molecule in which the damaged ssDNA has invaded more than one donor molecule. Processing of MI joint molecules can compromise the integrity of both donor sites and lead to their rearrangement. Here, two mechanisms for the generation of rearrangements as a pathological consequence of MI processing are detailed and the potential relevance for non-allelic homologous recombination discussed. Finally, it is proposed that MI-induced crossover formation may be a feature of physiological recombination.

A Distinct Class of Genome Rearrangements Driven by Heterologous Recombination

Erroneous DNA repair by heterologous recombination (Ht-REC) is a potential threat to genome stability, but evidence supporting its prevalence is lacking. Here we demonstrate that recombination is possible between heterologous sequences and that it is a source of chromosomal alterations in mitotic and meiotic cells. Mechanistically, we find that the RTEL1 and HIM-6/BLM helicases and the BRCA1 homolog BRC-1 counteract Ht-REC in Caenorhabditis elegans, whereas mismatch repair does not. Instead, MSH-2/6 drives Ht-REC events in rtel-1 and brc-1 mutants and excessive crossovers in rtel-1 mutant meioses. Loss of vertebrate Rtel1 also causes a variety of unusually large and complex structural variations, including chromothripsis, breakage-fusion-bridge events, and tandem duplications with distant intra-chromosomal insertions, whose structure are consistent with a role for RTEL1 in preventing Ht-REC during break-induced replication. Our data establish Ht-REC as an unappreciated source of genome instability that underpins a novel class of complex genome rearrangements that likely arise during replication stress.

Sgs1 Binding to Rad51 Stimulates Homology-Directed DNA Repair in Saccharomyces cerevisiae

Accurate repair of DNA breaks is essential to maintain genome integrity and cellular fitness. Sgs1, the sole member of the RecQ family of DNA helicases in Saccharomyces cerevisiae, is important for both early and late stages of homology-dependent repair. Its large number of physical and genetic interactions with DNA recombination, repair, and replication factors has established Sgs1 as a key player in the maintenance of genome integrity. To determine the significance of Sgs1 binding to the strand-exchange factor Rad51, we have identified a single amino acid change at the C-terminal of the helicase core of Sgs1 that disrupts Rad51 binding. In contrast to an SGS1 deletion or a helicase-defective sgs1 allele, this new separation-of-function allele, sgs1-FD, does not cause DNA damage hypersensitivity or genome instability, but exhibits negative and positive genetic interactions with sae2Δ, mre11Δ, exo1Δ, srs2Δ, rrm3Δ, and pol32Δ that are distinct from those of known sgs1 mutants. Our findings suggest that the Sgs1-Rad51 interaction stimulates homologous recombination (HR). However, unlike sgs1 mutations, which impair the resection of DNA double-strand ends, negative genetic interactions of the sgs1-FD allele are not suppressed by YKU70 deletion. We propose that the Sgs1-Rad51 interaction stimulates HR by facilitating the formation of the presynaptic Rad51 filament, possibly by Sgs1 competing with single-stranded DNA for replication protein A binding during resection.

Regulation of hetDNA Length during Mitotic Double-Strand Break Repair in Yeast

Heteroduplex DNA (hetDNA) is a key molecular intermediate during the repair of mitotic double-strand breaks by homologous recombination, but its relationship to 5' end resection and/or 3' end extension is poorly understood. In the current study, we examined how perturbations in these processes affect the hetDNA profile associated with repair of a defined double-strand break (DSB) by the synthesis-dependent strand-annealing (SDSA) pathway. Loss of either the Exo1 or Sgs1 long-range resection pathway significantly shortened hetDNA, suggesting that these pathways normally collaborate during DSB repair. In addition, altering the processivity or proofreading activity of DNA polymerase δ shortened hetDNA length or reduced break-adjacent mismatch removal, respectively, demonstrating that this is the primary polymerase that extends both 3' ends. Data are most consistent with the extent of DNA synthesis from the invading end being the primary determinant of hetDNA length during SDSA.

Remarkably Long-Tract Gene Conversion Induced by Fragile Site Instability in Saccharomyces cerevisiae

Replication stress causes breaks at chromosomal locations called common fragile sites. Deletions causing loss of heterozygosity (LOH) in human tumors are strongly correlated with common fragile sites, but the role of gene conversion in LOH at fragile sites in tumors is less well studied. Here, we investigated gene conversion stimulated by instability at fragile site FS2 in the yeast Saccharomyces cerevisiae In our screening system, mitotic LOH events near FS2 are identified by production of red/white sectored colonies. We analyzed single nucleotide polymorphisms between homologs to determine the cause and extent of LOH. Instability at FS2 increases gene conversion 48- to 62-fold, and conversions unassociated with crossover represent 6-7% of LOH events. Gene conversion can result from repair of mismatches in heteroduplex DNA during synthesis-dependent strand annealing (SDSA), double-strand break repair (DSBR), and from break-induced replication (BIR) that switches templates [double BIR (dBIR)]. It has been proposed that SDSA and DSBR typically result in shorter gene-conversion tracts than dBIR. In cells under replication stress, we found that bidirectional tracts at FS2 have a median length of 40.8 kb and a wide distribution of lengths; most of these tracts are not crossover-associated. Tracts that begin at the fragile site FS2 and extend only distally are significantly shorter. The high abundance and long length of noncrossover, bidirectional gene-conversion tracts suggests that dBIR is a prominent mechanism for repair of lesions at FS2, thus this mechanism is likely to be a driver of common fragile site-stimulated LOH in human tumors.

Mismatch repair and homeologous recombination

DNA mismatch repair influences the outcome of recombination events between diverging DNA sequences. Here we discuss how mismatch repair proteins are active in different homologous recombination subpathways and specific reaction steps, resulting in differential modulation of these recombination events, with a focus on the mechanism of heteroduplex rejection during the inhibition of recombination between slightly diverged (homeologous) DNA sequences.

Mechanisms and regulation of mitotic recombination in Saccharomyces cerevisiae

Homology-dependent exchange of genetic information between DNA molecules has a profound impact on the maintenance of genome integrity by facilitating error-free DNA repair, replication, and chromosome segregation during cell division as well as programmed cell developmental events. This chapter will focus on homologous mitotic recombination in budding yeast Saccharomyces cerevisiae. However, there is an important link between mitotic and meiotic recombination (covered in the forthcoming chapter by Hunter et al. 2015) and many of the functions are evolutionarily conserved. Here we will discuss several models that have been proposed to explain the mechanism of mitotic recombination, the genes and proteins involved in various pathways, the genetic and physical assays used to discover and study these genes, and the roles of many of these proteins inside the cell.

Srs2 promotes Mus81-Mms4-mediated resolution of recombination intermediates

A variety of DNA lesions, secondary DNA structures or topological stress within the DNA template may lead to stalling of the replication fork. Recovery of such forks is essential for the maintenance of genomic stability. The structure-specific endonuclease Mus81–Mms4 has been implicated in processing DNA intermediates that arise from collapsed forks and homologous recombination. According to previous genetic studies, the Srs2 helicase may play a role in the repair of double-strand breaks and ssDNA gaps together with Mus81–Mms4. In this study, we show that the Srs2 and Mus81–Mms4 proteins physically interact in vitro and in vivo and we map the interaction domains within the Srs2 and Mus81 proteins. Further, we show that Srs2 plays a dual role in the stimulation of the Mus81–Mms4 nuclease activity on a variety of DNA substrates. First, Srs2 directly stimulates Mus81–Mms4 nuclease activity independent of its helicase activity. Second, Srs2 removes Rad51 from DNA to allow access of Mus81–Mms4 to cleave DNA. Concomitantly, Mus81–Mms4 inhibits the helicase activity of Srs2. Taken together, our data point to a coordinated role of Mus81–Mms4 and Srs2 in processing of recombination as well as replication intermediates.

Mismatch repair during homologous and homeologous recombination

Homologous recombination (HR) and mismatch repair (MMR) are inextricably linked. HR pairs homologous chromosomes before meiosis I and is ultimately responsible for generating genetic diversity during sexual reproduction. HR is initiated in meiosis by numerous programmed DNA double-strand breaks (DSBs; several hundred in mammals). A characteristic feature of HR is the exchange of DNA strands, which results in the formation of heteroduplex DNA. Mismatched nucleotides arise in heteroduplex DNA because the participating parental chromosomes contain nonidentical sequences. These mismatched nucleotides may be processed by MMR, resulting in nonreciprocal exchange of genetic information (gene conversion). MMR and HR also play prominent roles in mitotic cells during genome duplication; MMR rectifies polymerase misincorporation errors, whereas HR contributes to replication fork maintenance, as well as the repair of spontaneous DSBs and genotoxic lesions that affect both DNA strands. MMR suppresses HR when the heteroduplex DNA contains excessive mismatched nucleotides, termed homeologous recombination. The regulation of homeologous recombination by MMR ensures the accuracy of DSB repair and significantly contributes to species barriers during sexual reproduction. This review discusses the history, genetics, biochemistry, biophysics, and the current state of studies on the role of MMR in homologous and homeologous recombination from bacteria to humans.

High-resolution mapping of two types of spontaneous mitotic gene conversion events in Saccharomyces cerevisiae

Gene conversions and crossovers are related products of the repair of double-stranded DNA breaks by homologous recombination. Most previous studies of mitotic gene conversion events have been restricted to measuring conversion tracts that are <5 kb. Using a genetic assay in which the lengths of very long gene conversion tracts can be measured, we detected two types of conversions: those with a median size of ∼6 kb and those with a median size of >50 kb. The unusually long tracts are initiated at a naturally occurring recombination hotspot formed by two inverted Ty elements. We suggest that these long gene conversion events may be generated by a mechanism (break-induced replication or repair of a double-stranded DNA gap) different from the short conversion tracts that likely reflect heteroduplex formation followed by DNA mismatch repair. Both the short and long mitotic conversion tracts are considerably longer than those observed in meiosis. Since mitotic crossovers in a diploid can result in a heterozygous recessive deleterious mutation becoming homozygous, it has been suggested that the repair of DNA breaks by mitotic recombination involves gene conversion events that are unassociated with crossing over. In contrast to this prediction, we found that ∼40% of the conversion tracts are associated with crossovers. Spontaneous mitotic crossover events in yeast are frequent enough to be an important factor in genome evolution.

Mismatch repair inhibits homeologous recombination via coordinated directional unwinding of trapped DNA structures

Homeologous recombination between divergent DNA sequences is inhibited by DNA mismatch repair. In Escherichia coli, MutS and MutL respond to DNA mismatches within recombination intermediates and prevent strand exchange via an unknown mechanism. Here, using purified proteins and DNA substrates, we find that in addition to mismatches within the heteroduplex region, secondary structures within the displaced single-stranded DNA formed during branch migration within the recombination intermediate are involved in the inhibition. We present a model that explains how higher-order complex formation of MutS, MutL, and DNA blocks branch migration by preventing rotation of the DNA strands within the recombination intermediate. Furthermore, we find that the helicase UvrD is recruited to directionally resolve these trapped intermediates toward DNA substrates. Thus, our results explain on a mechanistic level how the coordinated action between MutS, MutL, and UvrD prevents homeologous recombination and maintains genome stability.

Roles of exonucleases and translesion synthesis DNA polymerases during mitotic gap repair in yeast

Transformation-based gap-repair assays have long been used to model the repair of mitotic double-strand breaks (DSBs) by homologous recombination in yeast. In the current study, we examine genetic requirements of two key processes involved in DSB repair: (1) the processive 5'-end resection that is required to efficiently engage a repair template and (2) the filling of resected ends by DNA polymerases. The specific gap-repair assay used allows repair events resolved as crossover versus noncrossover products to be distinguished, as well as the extent of heteroduplex DNA formed during recombination to be measured. To examine end resection, the efficiency and outcome of gap repair were monitored in the absence of the Exo1 exonuclease and the Sgs1 helicase. We found that either Exo1 or Sgs1 presence is sufficient to inhibit gap-repair efficiency over 10-fold, consistent with resection-mediated destruction of the introduced plasmid. In terms of DNA polymerase requirements for gap repair, we focused specifically on potential roles of the Pol ζ and Pol η translesion synthesis DNA polymerases. We found that both Pol ζ and Pol η are necessary for efficient gap repair and that each functions independently of the other. These polymerases may be involved either in the initiation of DNA synthesis from the an invading end, or in a gap-filling process that is required to complete recombination.

Removal of N-6-methyladenine by the nucleotide excision repair pathway triggers the repair of mismatches in yeast gap-repair intermediates

Gap-repair assays have been an important tool for studying the genetic control of homologous recombination in yeast. Sequence analysis of recombination products derived when a gapped plasmid is diverged relative to the chromosomal repair template additionally has been used to infer structures of strand-exchange intermediates. In the absence of the canonical mismatch repair pathway, mismatches present in these intermediates are expected to persist and segregate at the next round of DNA replication. In a mismatch repair defective (mlh1Δ) background, however, we have observed that recombination-generated mismatches are often corrected to generate gene conversion or restoration events. In the analyses reported here, the source of the aberrant mismatch removal during gap repair was examined. We find that most mismatch removal is linked to the methylation status of the plasmid used in the gap-repair assay. Whereas more than half of Dam-methylated plasmids had patches of gene conversion and/or restoration interspersed with unrepaired mismatches, mismatch removal was observed in less than 10% of products obtained when un-methylated plasmids were used in transformation experiments. The methylation-linked removal of mismatches in recombination intermediates was due specifically to the nucleotide excision repair pathway, with such mismatch removal being partially counteracted by glycosylases of the base excision repair pathway. These data demonstrate that nucleotide excision repair activity is not limited to bulky, helix-distorting DNA lesions, but also targets removal of very modest perturbations in DNA structure. In addition to its effects on mismatch removal, methylation reduced the overall gap-repair efficiency, but this reduction was not affected by the status of excision repair pathways. Finally, gel purification of DNA prior to transformation reduced gap-repair efficiency four-fold in a nucleotide excision repair-defective background, indicating that the collateral introduction of UV damage can potentially compromise genetic interpretations.

Heteroduplex DNA position defines the roles of the Sgs1, Srs2, and Mph1 helicases in promoting distinct recombination outcomes

The contributions of the Sgs1, Mph1, and Srs2 DNA helicases during mitotic double-strand break (DSB) repair in yeast were investigated using a gap-repair assay. A diverged chromosomal substrate was used as a repair template for the gapped plasmid, allowing mismatch-containing heteroduplex DNA (hDNA) formed during recombination to be monitored. Overall DSB repair efficiencies and the proportions of crossovers (COs) versus noncrossovers (NCOs) were determined in wild-type and helicase-defective strains, allowing the efficiency of CO and NCO production in each background to be calculated. In addition, the products of individual NCO events were sequenced to determine the location of hDNA. Because hDNA position is expected to differ depending on whether a NCO is produced by synthesis-dependent-strand-annealing (SDSA) or through a Holliday junction (HJ)-containing intermediate, its position allows the underlying molecular mechanism to be inferred. Results demonstrate that each helicase reduces the proportion of CO recombinants, but that each does so in a fundamentally different way. Mph1 does not affect the overall efficiency of gap repair, and its loss alters the CO-NCO by promoting SDSA at the expense of HJ-containing intermediates. By contrast, Sgs1 and Srs2 are each required for efficient gap repair, strongly promoting NCO formation and having little effect on CO efficiency. hDNA analyses suggest that all three helicases promote SDSA, and that Sgs1 and Srs2 additionally dismantle HJ-containing intermediates. The hDNA data are consistent with the proposed role of Sgs1 in the dissolution of double HJs, and we propose that Srs2 dismantles nicked HJs.

The many landscapes of recombination in Drosophila melanogaster

Recombination is a fundamental biological process with profound evolutionary implications. Theory predicts that recombination increases the effectiveness of selection in natural populations. Yet, direct tests of this prediction have been restricted to qualitative trends due to the lack of detailed characterization of recombination rate variation across genomes and within species. The use of imprecise recombination rates can also skew population genetic analyses designed to assess the presence and mode of selection across genomes. Here we report the first integrated high-resolution description of genomic and population variation in recombination, which also distinguishes between the two outcomes of meiotic recombination: crossing over (CO) and gene conversion (GC). We characterized the products of 5,860 female meioses in Drosophila melanogaster by genotyping a total of 139 million informative SNPs and mapped 106,964 recombination events at a resolution down to 2 kilobases. This approach allowed us to generate whole-genome CO and GC maps as well as a detailed description of variation in recombination among individuals of this species. We describe many levels of variation in recombination rates. At a large-scale (100 kb), CO rates exhibit extreme and highly punctuated variation along chromosomes, with hot and coldspots. We also show extensive intra-specific variation in CO landscapes that is associated with hotspots at low frequency in our sample. GC rates are more uniformly distributed across the genome than CO rates and detectable in regions with reduced or absent CO. At a local scale, recombination events are associated with numerous sequence motifs and tend to occur within transcript regions, thus suggesting that chromatin accessibility favors double-strand breaks. All these non-independent layers of variation in recombination across genomes and among individuals need to be taken into account in order to obtain relevant estimates of recombination rates, and should be included in a new generation of population genetic models of the interaction between selection and linkage.

The Rad1-Rad10 nuclease promotes chromosome translocations between dispersed repeats

Holliday junctions can be formed during homology-dependent repair of DNA double-strand breaks and their resolution is essential for chromosome segregation and generation of crossover products. The Mus81–Mms4 and Yen1 nucleases are required for mitotic crossovers between chromosome homologs in Saccharomyces cerevisiae; however, crossovers between dispersed repeats are still detected in their absence. Here we show the Rad1–Rad10 nuclease promotes formation of crossover and noncrossover recombinants between ectopic sequences. Crossover products were not recovered from the mus81Δ rad1Δ yen1Δ triple mutant indicating that all three nucleases participate in processing recombination intermediates that form between dispersed repeats. We suggest a novel mechanism for crossovers that involves Rad1–Rad10 clipping and resolution of a single Holliday junction-containing intermediate by Mus81–Mms4 or Yen1 cleavage, or by replication. Consistent with the model, we show the accumulation of Rad1 dependent joint molecules in the mus81Δ yen1Δ mutant.

The DNA damage checkpoint allows recombination between divergent DNA sequences in budding yeast

In the early steps of homologous recombination, single-stranded DNA (ssDNA) from a broken chromosome invades homologous sequence located in a sister or homolog donor. In genomes that contain numerous repetitive DNA elements or gene paralogs, recombination can potentially occur between non-allelic/divergent (homeologous) sequences that share sequence identity. Such recombination events can lead to lethal chromosomal deletions or rearrangements. However, homeologous recombination events can be suppressed through rejection mechanisms that involve recognition of DNA mismatches in heteroduplex DNA by mismatch repair factors, followed by active unwinding of the heteroduplex DNA by helicases. Because factors required for heteroduplex rejection are hypothesized to be targets and/or effectors of the DNA damage response (DDR), a cell cycle control mechanism that ensures timely and efficient repair, we tested whether the DDR, and more specifically, the RAD9 gene, had a role in regulating rejection. We performed these studies using a DNA repair assay that measures repair by single-strand annealing (SSA) of a double-strand break (DSB) using homeologous DNA templates. We found that repair of homeologous DNA sequences, but not identical sequences, induced a RAD9-dependent cell cycle delay in the G2 stage of the cell cycle. Repair through a divergent DNA template occurred more frequently in RAD9 compared to rad9Δ strains. However, repair in rad9Δ mutants could be restored to wild-type levels if a G2 delay was induced by nocodazole. These results suggest that cell cycle arrest induced by the Rad9-dependent DDR allows repair between divergent DNA sequences despite the potential for creating deleterious genome rearrangements, and illustrates the importance of additional cellular mechanisms that act to suppress recombination between divergent DNA sequences.

Interhomolog recombination and loss of heterozygosity in wild-type and Bloom syndrome helicase (BLM)-deficient mammalian cells

Genomic integrity often is compromised in tumor cells, as illustrated by genetic alterations leading to loss of heterozygosity (LOH). One mechanism of LOH is mitotic crossover recombination between homologous chromosomes, potentially initiated by a double-strand break (DSB). To examine LOH associated with DSB-induced interhomolog recombination, we analyzed recombination events using a reporter in mouse embryonic stem cells derived from F1 hybrid embryos. In this study, we were able to identify LOH events although they occur only rarely in wild-type cells (≤2.5%). The low frequency of LOH during interhomolog recombination suggests that crossing over is rare in wild-type cells. Candidate factors that may suppress crossovers include the RecQ helicase deficient in Bloom syndrome cells (BLM), which is part of a complex that dissolves recombination intermediates. We analyzed interhomolog recombination in BLM-deficient cells and found that, although interhomolog recombination is slightly decreased in the absence of BLM, LOH is increased by fivefold or more, implying significantly increased interhomolog crossing over. These events frequently are associated with a second homologous recombination event, which may be related to the mitotic bivalent structure and/or the cell-cycle stage at which the initiating DSB occurs.

Discontinuity and limited linkage in the homologous recombination system of a hyperthermophilic archaeon

Genetic transformation of Sulfolobus acidocaldarius by a multiply marked pyrE gene provided a high-resolution assay of homologous recombination in a hyperthermophilic archaeon. Analysis of 100 Pyr(+) transformants revealed that this recombination system could transfer each of 23 nonselected base pair substitutions to the recipient chromosome along with the selected marker. In 30% of the recombinants, donor markers were transferred as multiple blocks. In at least 40% of the recombinants, donor markers separated by 5 or 6 bp segregated from each other, whereas similar markers separated by 2 bp did not segregate. Among intermarker intervals, the frequency of recombination tract endpoints varied 40-fold, but in contrast to other recombination systems, it did not correlate with the length of the interval. The average length of donor tracts (161 bp) and the frequent generation of multiple tracts seemed generally consistent with the genetic properties observed previously in S. acidocaldarius conjugation. The efficiency with which short intervals of diverged pyrE sequence were incorporated into the genome raises questions about the threat of ectopic recombination in Sulfolobus spp. mediated by this apparently efficient yet permissive system.

Extensive DNA end processing by exo1 and sgs1 inhibits break-induced replication

Homology-dependent repair of DNA double-strand breaks (DSBs) by gene conversion involves short tracts of DNA synthesis and limited loss of heterozygosity (LOH). For DSBs that present only one end, repair occurs by invasion into a homologous sequence followed by replication to the end of the chromosome resulting in extensive LOH, a process called break-induced replication (BIR). We developed a BIR assay in Saccharomyces cerevisiae consisting of a plasmid with a telomere seeding sequence separated from sequence homologous to chromosome III by an I-SceI endonuclease recognition site. Following cleavage of the plasmid by I-SceI in vivo, de novo telomere synthesis occurs at one end of the vector, and the other end invades at the homologous sequence on chromosome III and initiates replication to the end of the chromosome to generate a stable chromosome fragment (CF). BIR was infrequent in wild-type cells due to degradation of the linearized vector. However, in the exo1Δ sgs1Δ mutant, which is defective in the 5′-3′ resection of DSBs, the frequency of BIR was increased by 39-fold. Extension of the invading end of the plasmid was detected by physical analysis two hours after induction of the I-SceI endonuclease in the wild-type exo1Δ, sgs1Δ, and exo1Δ sgs1Δ mutants, but fully repaired products were only visible in the exo1Δ sgs1Δ mutant. The inhibitory effect of resection was less in a plasmid-chromosome gene conversion assay, compared to BIR, and products were detected by physical assay in the wild-type strain. The rare chromosome rearrangements due to BIR template switching at repeated sequences were increased in the exo1Δ sgs1Δ mutant, suggesting that reduced resection can decrease the fidelity of homologous recombination.

DNA double-strand breaks (DSBs) can occur spontaneously in cells by defective DNA replication or are induced by various types of DNA damaging agents, such as those used in chemo- or radiation therapy. Failure to repair DSBs, or inappropriate repair, can result in chromosome loss or chromosome rearrangements, events associated with development of cancer cells. Typically, DNA DSBs have two ends that are reunited faithfully by copying from a homologous donor chromosome. For DSBs that present only one end, repair occurs by invasion into a homologous sequence followed by replication to the end of the chromosome, a process called break-induced replication (BIR). This repair pathway is thought to be suppressed at two-ended DSBs to prevent extensive loss of heterozygosity. Here, we describe a new assay to physically monitor BIR to see how this repair pathway differs from the repair of two-ended DSBs. We show this pathway is infrequent, but can be detected by eliminating factors (Exo1 and Sgs1) that degrade linear DNA. However, increased chromosome rearrangements were found in the resection-defective strain. We found the kinetics of strand invasion in two-ended and one-ended DSB repair were the same, suggesting that the distinction between these pathways occurs after initial repair steps have begun.

Molecular structures of crossover and noncrossover intermediates during gap repair in yeast: implications for recombination

The molecular structures of crossover (CO) and noncrossover (NCO) intermediates were determined by sequencing the products formed when a gapped plasmid was repaired using a diverged chromosomal template. Analyses were done in the absence of mismatch repair (MMR) to allow efficient detection of strand-transfer intermediates, and the results reveal striking differences in the extents and locations of heteroduplex DNA (hDNA) in NCO versus CO products. These data indicate that most NCOs are produced by synthesis-dependent strand annealing rather than by a canonical double-strand break repair pathway and that resolution of Holliday junctions formed as part of the latter pathway is highly constrained to generate CO products. We suggest a model in which the length of hDNA formed by the initiating strand invasion event determines susceptibility of the resulting intermediate to antirecombination and ultimately whether a CO- or a NCO-producing pathway is followed.

Cdk1 targets Srs2 to complete synthesis-dependent strand annealing and to promote recombinational repair

Cdk1 kinase phosphorylates budding yeast Srs2, a member of UvrD protein family, displays both DNA translocation and DNA unwinding activities in vitro. Srs2 prevents homologous recombination by dismantling Rad51 filaments and is also required for double-strand break (DSB) repair. Here we examine the biological significance of Cdk1-dependent phosphorylation of Srs2, using mutants that constitutively express the phosphorylated or unphosphorylated protein isoforms. We found that Cdk1 targets Srs2 to repair DSB and, in particular, to complete synthesis-dependent strand annealing, likely controlling the disassembly of a D-loop intermediate. Cdk1-dependent phosphorylation controls turnover of Srs2 at the invading strand; and, in absence of this modification, the turnover of Rad51 is not affected. Further analysis of the recombination phenotypes of the srs2 phospho-mutants showed that Srs2 phosphorylation is not required for the removal of toxic Rad51 nucleofilaments, although it is essential for cell survival, when DNA breaks are channeled into homologous recombinational repair. Cdk1-targeted Srs2 displays a PCNA–independent role and appears to have an attenuated ability to inhibit recombination. Finally, the recombination defects of unphosphorylatable Srs2 are primarily due to unscheduled accumulation of the Srs2 protein in a sumoylated form. Thus, the Srs2 anti-recombination function in removing toxic Rad51 filaments is genetically separable from its role in promoting recombinational repair, which depends exclusively on Cdk1-dependent phosphorylation. We suggest that Cdk1 kinase counteracts unscheduled sumoylation of Srs2 and targets Srs2 to dismantle specific DNA structures, such as the D-loops, in a helicase-dependent manner during homologous recombinational repair.

Broken DNA molecules can be repaired by copying a homologous DNA sequence located elsewhere in the genome. This process, called homologous recombination, needs to be carefully regulated, because unwanted DNA exchanges can lead to genome rearrangements and cell death. Cdk1 kinase is required for cell cycle progression and phosphorylates DNA repair factors, such as Srs2, a protein that can both translocate on single-stranded DNA and open the two strands of DNA double helix. DNA translocation activity of Srs2 is crucial to prevent unwanted recombination, while DNA unwinding activity might be important to promote recombination. In this study, we used two srs2 mutants that constitutively express the unphosphorylated or Cdk1-dependent phosphorylated Srs2 protein isoforms. We found that Srs2 performs genetically distinct functions in preventing or promoting homologous recombination. Cdk1 targets Srs2 to promote accurate repair of double-stranded DNA breaks, but is not essential for the removal of toxic recombination intermediates assembled at single-stranded DNA breaks. Further, Cdk1 counteracts sumoylation of Srs2, which is responsible for recombination defects due to the lack of Srs2 phosphorylation. In summary, Cdk1-dependent Srs2 phosphorylation prevents its unscheduled sumoylation and targets the helicase to promote accurate homologous recombinational repair.

Mechanisms of recombination between diverged sequences in wild-type and BLM-deficient mouse and human cells

Double-strand breaks (DSBs) are particularly deleterious DNA lesions for which cells have developed multiple mechanisms of repair. One major mechanism of DSB repair in mammalian cells is homologous recombination (HR), whereby a homologous donor sequence is used as a template for repair. For this reason, HR repair of DSBs is also being exploited for gene modification in possible therapeutic approaches. HR is sensitive to sequence divergence, such that the cell has developed ways to suppress recombination between diverged ("homeologous") sequences. In this report, we have examined several aspects of HR between homeologous sequences in mouse and human cells. We found that gene conversion tracts are similar for mouse and human cells and are generally < or =100 bp, even in Msh2(-)(/)(-) cells which fail to suppress homeologous recombination. Gene conversion tracts are mostly unidirectional, with no observed mutations. Additionally, no alterations were observed in the donor sequences. While both mouse and human cells suppress homeologous recombination, the suppression is substantially less in the transformed human cells, despite similarities in the gene conversion tracts. BLM-deficient mouse and human cells suppress homeologous recombination to a similar extent as wild-type cells, unlike Sgs1-deficient Saccharomyces cerevisiae.

Mph1 requires mismatch repair-independent and -dependent functions of MutSalpha to regulate crossover formation during homologous recombination repair

In budding yeast the DNA helicase Mph1 prevents genome rearrangements during ectopic homologous recombination (HR) by suppressing the formation of crossovers (COs). Here we show that during ectopic HR repair, the anti-CO function of Mph1 is intricately associated with the mismatch repair (MMR) factor, MutSα. In particular, during HR repair using a completely homologous substrate, we reveal an MMR-independent function of MutSα in generating COs that is specifically antagonized by Mph1, but not Sgs1. In contrast, both Mph1 and MutSα are required to efficiently suppress COs in the presence of a homeologous substrate. Mph1 acts redundantly with Sgs1 in this respect since mph1Δ sgs1Δ double mutant cells pheno-copy MutSα mutants and completely fail to discriminate homologous and homeologous sequences during HR repair. However, this defect of mph1Δ sgs1Δ cells is not due to an inability to carry out MMR but rather is accompanied by elevated levels of gene conversion (GC) and bi-directional GC tracts specifically in non-crossover products. Models describing how Mph1, MutSα and Sgs1 act in concert to suppress genome rearrangements during ectopic HR repair are discussed.

Regulation of homologous recombination in eukaryotes

Homologous recombination (HR) is required for accurate chromosome segregation during the first meiotic division and constitutes a key repair and tolerance pathway for complex DNA damage, including DNA double-strand breaks, interstrand crosslinks, and DNA gaps. In addition, recombination and replication are inextricably linked, as recombination recovers stalled and broken replication forks, enabling the evolution of larger genomes/replicons. Defects in recombination lead to genomic instability and elevated cancer predisposition, demonstrating a clear cellular need for recombination. However, recombination can also lead to genome rearrangements. Unrestrained recombination causes undesired endpoints (translocation, deletion, inversion) and the accumulation of toxic recombination intermediates. Evidently, HR must be carefully regulated to match specific cellular needs. Here, we review the factors and mechanistic stages of recombination that are subject to regulation and suggest that recombination achieves flexibility and robustness by proceeding through metastable, reversible intermediates.

From bacteria to plants: a compendium of mismatch repair assays

Mismatch repair (MMR) system maintains genome integrity by correcting mispaired or unpaired bases which have escaped the proofreading activity of DNA polymerases. The basic features of the pathway have been highly conserved throughout evolution, although the nature and number of the proteins involved in the mechanism vary from prokaryotes to eukaryotes and even between humans and plants. Cells deficient in MMR genes have been observed to display a mutator phenotype characterized by an increased rate in spontaneous mutation, instability of microsatellite sequences and illegitimate recombination between diverged DNA sequences. Studies of the mutator phenotype have demonstrated a critical role for the MMR system in mutation avoidance and genetic stability. Here, we briefly review our current knowledge of the MMR mechanism and then focus on the in vivo biochemical and genetic assays used to investigate the function of the MMR proteins in processing DNA mismatches generated during replication and mitotic recombination in Escherichia coli, Saccharomyces cerevisiae, Homo sapiens and Arabidopsis thaliana. An overview of the biochemical assays developed to study mismatch correction in vitro is also provided.