Suppression of Spontaneous Chromosomal Rearrangements by S Phase Checkpoint Functions in Saccharomyces cerevisiae

Abstract

Cancer cells show increased genome rearrangements, although it is unclear what defects cause these rearrangements. Mutations in Saccharomyces cerevisiae RFC5, DPB11, MEC1, DDC2 MEC3, RAD53, CHK1, PDS1, and DUN1 increased the rate of genome rearrangements up to 200-fold whereas mutations in RAD9, RAD17, RAD24, BUB3, and MAD3 had little effect. The rearrangements were primarily deletion of a portion of a chromosome arm along with TEL1-dependent addition of a new telomere. tel1 mutations increased the proportion of translocations observed, and in some cases showed synergistic interactions when combined with mutations that increased the genome rearrangement rate. These data suggest that one role of S phase checkpoint functions in normal cells is to suppress spontaneous genome rearrangements resulting from DNA replication errors.

Recombination and the Tel1 and Mec1 checkpoints differentially effect genome rearrangements driven by telomere dysfunction in yeast

“…Combining a mec1 mutation with tlc1 or est2 mutations did not change the GCR rate relative to that of a mec1 single mutant ( Table 1). But the GCR spectrum changed from all telomere additions in a mec1 mutant 11 to chromosome fusions, translocations and inversion translocations, a translocation structure we have not previously observed 4,10,11 , in the mec1 tlc1 double mutant ( Fig. 1 and Table 3).…”

“…The tel1 rad51 tlc1 strain accumulated mostly translocations, but chromosome fusions were formed at a higher rate than in the rad51 tlc1 strain (P = 0.02; Table 3). Because mutations in TEL1 are not known to cause defects in the Rad51 or Rad59 recombination pathways, this suggests that defects in a Tel1-dependent checkpoint function 3,11 or telomere-capping function 4,12,13 and expression of Rad51 (and Rad55) could both disrupt the protective function that prevents telomeres from being joined to broken DNAs.…”

“…In the absence of telomerase and either the Rad51 or Rad59 recombination pathways or Tel1, there are more broken chromosomes that escape repair owing to overburdening of the existing recombination capacity by telomere maintenance, less telomere protection or both, resulting in increased GCR rates even though there is sufficient recombination to maintain telomeres. There are three potential sources of broken chromosomes: replication errors that escape repair 3,11 , dicentric chromosomes due to telomere-telomere fusion followed by breakage of the dicentric chromosomes 1,2,15,19 and degradation of chromosome ends by exonucleases such as Exo1 (ref. 20).…”

“…The subsequent rearrangements are predominantly due to NHEJ; joining of broken DNAs to each other yields translocations and joining of broken DNAs to unprotected telomeres yields chromosome fusions. Finally, in the absence of Mec1-dependent DNA damage checkpoints 11,21,22 , broken chromosomes seem to replicate and then fuse by Rad51-or Rad59-dependent recombination to yield dicentric inversion translocations. This is in contrast to telomerase-defective mammalian cells, in which chromosome fusions and translocations in cells that escape cell cycle arrest and apoptosis because of a p53 mutation result from a ligase 4-dependent NHEJ reaction 17,23 .…”

DNA structures at chromosomal translocation sites

“…(100,101) Once a break occurs, breakage-induced replication (BIR) may occur, resulting in a non-reciprocal event. (102) Breaks that result in chromosomes with two centromeres can undergo breakage-fusion-bridge cycles.…”