Cancer progression is often associated with the accumulation of gross chromosomal rearrangements (GCRs), such as translocations, deletion of a chromosome arm, interstitial deletions or inversions. In many instances, GCRs inactivate tumour-suppressor genes or generate novel fusion proteins that initiate carcinogenesis. The mechanism underlying GCR formation appears to involve interactions between DNA sequences of little or no homology. We previously demonstrated that mutations in the gene encoding the largest subunit of the Saccharomyces cerevisiae single-stranded DNA binding protein (RFA1) increase microhomology-mediated GCR formation. To further our understanding of GCR formation, we have developed a novel mutator assay in S. cerevisiae that allows specific detection of such events. In this assay, the rate of GCR formation was increased 600-5, 000-fold by mutations in RFA1, RAD27, MRE11, XRS2 and RAD50, but was minimally affected by mutations in RAD51, RAD54, RAD57, YKU70, YKU80, LIG4 and POL30. Genetic analysis of these mutants suggested that at least three distinct pathways can suppress GCRs: two that suppress microhomology-mediated GCRs (RFA1 and RAD27) and one that suppresses non-homology-mediated GCRs (RAD50/MRE11/XRS2).
Gross chromosome rearrangements (GCRs), such as translocations, deletion of a chromosome arm, interstitial deletions and inversions, are often observed in cancer cells. Spontaneous GCRs are rare in Saccharomyces cerevisiae; however, the existence of mutator mutants with increased genome instability suggests that GCRs are actively suppressed. Here we show by genetic analysis that these genome rearrangements probably result from DNA replication errors and are suppressed by at least three interacting pathways or groups of proteins: S-phase checkpoint functions, recombination proteins and proteins that prevent de novo addition of telomeres at double-strand breaks (DSBs). Mutations that inactivate these pathways cause high rates of GCRs and show synergistic interactions, indicating that the pathways that suppress GCRs all compete for the same DNA substrates.
The Escherichia coli gene recQ was identified as a RecF recombination pathway gene. The gene SGS1, encoding the only RecQ-like DNA helicase in Saccharomyces cerevisiae, was identified by mutations that suppress the top3 slow-growth phenotype. Relatively little is known about the function of Sgs1p because single mutations in SGS1 do not generally cause strong phenotypes. Mutations in genes encoding RecQ-like DNA helicases such as the Bloom and Werner syndrome genes, BLM and WRN, have been suggested to cause increased genome instability. But the exact DNA metabolic defect that might underlie such genome instability has remained unclear. To better understand the cellular role of the RecQ-like DNA helicases, sgs1 mutations were analyzed for their effect on genome rearrangements. Mutations in SGS1 increased the rate of accumulating gross chromosomal rearrangements (GCRs), including translocations and deletions containing extended regions of imperfect homology at their breakpoints. sgs1 mutations also increased the rate of recombination between DNA sequences that had 91% sequence homology. Epistasis analysis showed that Sgs1p is redundant with DNA mismatch repair (MMR) for suppressing GCRs and for suppressing recombination between divergent DNA sequences. This suggests that defects in the suppression of rearrangements involving divergent, repeated sequences may underlie the genome instability seen in BLM and WRN patients and in cancer cases associated with defects in these genes.
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