Control of Translocations between Highly Diverged Genes by Sgs1, the Saccharomyces cerevisiae Homolog of the Bloom'sSyndrome Protein

Abstract

Sgs1 is a RecQ family DNA helicase required for genome stability in Saccharomyces cerevisiae whose human homologs BLM, WRN, and RECQL4 are mutated in Bloom's, Werner, and Rothmund Thomson syndromes, respectively. Sgs1 and mismatch repair (MMR) are inhibitors of recombination between similar but divergent (homeologous) DNA sequences. Here we show that SGS1, but not MMR, is critical for suppressing spontaneous, recurring translocations between diverged genes in cells with mutations in the genes encoding the checkpoint proteins Mec3, Rad24, Rad9, or Rfc5, the chromatin assembly factors Cac1 or Asf1, and the DNA helicase Rrm3. The S-phase checkpoint kinase and telomere maintenance factor Tel1, a homolog of the human ataxia telangiectasia (ATM) protein, prevents these translocations, whereas the checkpoint kinase Mec1, a homolog of the human ATM-related protein, and the Rad53 checkpoint kinase are not required. The translocation structures observed suggest involvement of a dicentric intermediate and break-induced replication with multiple cycles of DNA template switching.RecQ-like DNA helicases play important roles in the maintenance of genome stability from bacteria to humans. The only member of the RecQ family of 3Ј to 5Ј DNA helicases in the yeast Saccharomyces cerevisiae is Sgs1. Sgs1 has been implicated in the coordination between DNA replication and recombination, in the regulation of homologous recombination (HR) and the suppression of crossover products during HR, and in S-phase checkpoint activation as well as in transcription (16,26,38,49,61,96). As a consequence, cells that lack Sgs1 display a hyperrecombination phenotype, accumulate extrachromosomal ribosomal DNA (rDNA) circles, frequently missegregate chromosomes during mitosis and meiosis, have modestly increased rates of accumulating gross chromosomal rearrangements (GCRs), are sensitive to agents such as hydroxyurea and methyl-methanesulfonate, and show signs of premature aging (29,58,61,86,100,101,104).To date, five human genes encoding RecQ-like (RECQL) proteins have been identified. Mutations in RECQL2 (WRN) (105), RECQL3 (BLM) (23), and RECQL4 (41, 42) cause three rare, cancer-prone disorders, Werner syndrome (WS), Bloom's syndrome (BS), and a subset of Rothmund Thomson syndrome (Type II RTS) (97), respectively, while defects in RECQL1 (73, 74) and RECQL5 (41) have not been linked to a disease. Besides short stature, early onset of diabetes mellitus, and immunodeficiency, BS is characterized by extreme cancer risk, which has been estimated to be 150 to 300 times higher than the risk of malignancy in the unaffected population; in 168 BS patients, 100 cancers of many types had arisen at a mean age of 24.7 years, with many of the patients suffering from multiple primary cancers (31). Although WS patients share some of these symptoms, including early onset of diabetes mellitus and increased cancer susceptibility, they also show numerous other signs of accelerated aging not typical for BS. RTS patients also show a high prevalence of cancers, especially...

Defective Break-Induced Replication Leads to Half-Crossovers inSaccharomyces cerevisiae

“…The Sgs1p helicase has been implicated in many processes of DNA metabolism, including regulation of homologous recombination, suppression of crossing over in vegetative cells (Ira et al 2003), and prevention of aberrant crossing over during meiosis (Oh et al 2007). Also, Sgs1p suppresses homeologous recombination (Myung et al 2001;Spell and Jinks-Robertson 2004), and deletion of SGS1 leads to increased translocations between divergent DNA sequences that share only limited homology (Schmidt et al 2006). The latter function of Sgs1p could explain the increased GC observed in our experimental system.…”

Break-induced replication: What is it and what is it for?

“…An unusual class of translocations is detected between highly diverged sequences in the absence of the Sgs1 helicase and at least one other protein involved either in the DNA damage checkpoint response, chromatin assembly or the DNA unwinding. 38 Again, many of these translocations were complex because the initial event resulted in formation of dicentric chromosome that underwent a secondary rearrangement to form a stable product. These rearrangements between the diverged CAN1, LYP1 and ALP1 genes were dependent on RAD52 and partially dependent on RAD51, and the breakpoints were primarily within blocks of greatest sequence identity.…”

Smc5–Smc6 complex suppresses gross chromosomal rearrangements mediated by break-induced replications

“…Several studies using this approach have demonstrated that there are eight pathways for suppressing these chromosomal aberrations, while six pathways promote GCR formation. The suppression mechanisms include cell cycle checkpoints [7][8][9][10][11][12], post-replication [13,14] and mismatch repair [15,16], recombination pathways, an anti-de novo telomere addition mechanism [17,18], chromatin assembly factors [11,19], mechanisms that prevent end-to-end chromosome fusions [17,18,20] and a pathway detoxifying reactive oxygen species [14,21,22]. In contrast, the promoters of GCRs include telomerase-related factors [17,23], a mitotic checkpoint network [24], the Rad1-Rad10 endonuclease [25], non-homologous end-joining proteins including Lig4 and Nej1 [17], a pathway generating inappropriate recombination via sumoylation and the Srs2 helicase [13] and the Bre1 ubiquitin ligase [13].…”