Links between replication, recombination and genome instability in eukaryotes

Abstract

Double-strand breaks in DNA can be repaired by homologous recombination including breakinduced replication. In this reaction, the end of a broken DNA invades an intact chromosome and primes DNA replication resulting in the synthesis of an intact chromosome. Break-induced replication has also been suggested to cause different types of genome rearrangements.The ability of cells to repair double-strand breaks (DSBs) is critical for cell viability. Failure to correct such damage can result in cell cycle arrest, cell death and, if repaired incorrectly, the loss of genetic information or the accumulation of mutations. DSBs can arise in cells by exposure to ionizing radiation 1 and other types of DNA-damaging agents or through mechanical stress. In addition, some cellular processes, like DNA replication, can generate DSBs. These could arise by the action of nucleases at unprotected sites where replication forks stall, as a consequence of DNA polymerases (Pols) replicating across nicked chromosomes, as well as by the active processing of stalled replication forks by specific enzyme systems [2][3][4] . DSBs can also occur naturally and play important roles in processes such as meiosis 5 , mating-type switching 6 and mammalian V(D)J recombination 7 .Cells have developed a number of mechanisms to repair such lesions. In Saccharomyces cerevisiae, most of the repair of DSBs is carried out by mechanisms that promote homologous recombination. This requires the existence in the cell of sequences that are homologous to those affected by the DSB. Non-homologous end-joining also rejoins DSBs in S. cerevisiae albeit less efficiently than homologous recombination; non-homologous endjoining appears to be of greater importance in mammalian cells 8 . General models of recombinationSeveral recent reviews have discussed recombination and the repair of DSBs in detail 4,9-14 . Given the increasing evidence that translocations and other types of genome rearrangements arise from inappropriate repair of DSBs, we will focus on the repair of DSBs by a homologous recombination pathway that involves extensive DNA synthesis. Such a process, termed break-induced replication (BIR), has been suggested based on the observations by 16 . In this study, activation of the transcription enhancer element HOT1 creates a recombination hotspot suggested to act by generating DSBs that are processed resulting in strains with distal markers converted to those present in the homologous chromosome. The resulting gene conversion, which could extend over regions as long as 75 kb, was proposed to arise by replication of a primer structure generated by the centromere-containing fragment invading the homologous chromosome in a process analogous to that described for bacteriophage T4 (Ref. 17 (Fig. 1a). In both cases the ends of the broken DNA molecule are degraded by exonucleases creating 3′ overhangs that invade the homologous template. The result is the formation of a D-loop in which the annealed invading end serves as a primer that is extended by the DNA syn...

Association of p53 and MSH2 with recombinative repair complexes during S phase

“…In the same cells, we observed at least a partial overlap for almost every distinct Rad50 focus with p53 staining, although more than half of the nuclear p53 foci were localized elsewhere due to the higher number of p53 foci compared to the few confined Rad50 foci. Mre11-Rad50 complexes are necessary for the initiation of HR, which represents the predominant DSB repair pathway in replicating cells, as opposed to NHEJ (Haber, 1998;Lambert et al, 1999;Flores-Rozas and Kolodner, 2000). Rad51 is required for initial and continued strand exchange during HR (Baumann and West, 1998).…”

DNA replication meets genetic exchange: Chromosomal damage and its repair by homologous recombination

“…The major chromosomal repair pathway operates via homologous recombination. Richard Kolodner describes that the search for mutants with dramatically increased frequency of gross chromosomal rearrangements in yeast yields defects in DNA metabolism (which contribute to one-strand DNA damage) as well as mutants in recombinational repair (127). Thus, an increased frequency of one-strand damage contributes to chromosomal damage, whereas unrepaired chromosomal damage in the best-case scenario leads to genome rearrangements.…”

Transcriptional response of Saccharomyces cerevisiae to DNA-damaging agents does not identify the genes that protect against these agents

“…With the exception of UV irradiation, it is unlikely that the richness of the highly conserved DNA-repair pathways in prokaryotes and eukaryotes evolved to cope with the precise lesions produced by the DNA-damaging agents used in the present study. However, in recent years the crucial involvement of recombination to repair broken DNA during its replication as well as to maintain genomic stability has become apparent (28). Thus, in addition to their widely recognized role in the proper segregation and shuff ling of genetic material during meiosis, the enzymes of recombination are now seen to play a vital role in repairing damage similar to that introduced by IR that occurs normally during DNA synthesis as a result of stalled replication forks or other processes that break DNA (29 -31).…”