Roles of DNA helicases in the mediation and regulation of homologous recombination

Abstract

Homologous recombination (HR) is an evolutionarily conserved process that eliminates DNA double-strand breaks from chromosomes, repairs injured DNA replication forks, and helps orchestrate meiotic chromosome segregation. Recent studies have shown that DNA helicases play multifaceted roles in HR mediation and regulation. In particular, the S. cerevisiae Sgs1 helicase and its human ortholog BLM helicase are involved in not only the resection of the primary lesion to generate single-stranded DNA to prompt the assembly of the HR machinery, but they also function in somatic cells to suppress the formation of chromosome arm crossovers during HR. On the other hand, the S. cerevisiae Mph1 and Srs2 helicases, and their respective functional equivalents in other eukaryotes, suppress spurious HR events and favor the formation of noncrossovers via distinct mechanisms. Thus, the functional integrity of the HR process and HR outcomes are dependent upon these helicase enzymes. Since mutations in some of these helicases lead to cancer predisposition in humans and mice, studies on them have clear relevance to human health and disease.

Figure 1. DNA break repair by either the Synthesis-Dependent Strand Annealing (SDSA) pathway or the Double Strand Break Repair (DSBR) pathway

Double strand breaks induced by DNA damage are first resected to produce 3’ DNA tails, which become coated by the ssDNA binding protein RPA. Recombination mediator proteins, such as Rad52 in yeast and BRCA2 in humans, promote the exchange of RPA by a helical filament of Rad51. DNA synthesis occurs after strand invasion catalyzed by the Rad51-ssDNA nucleoprotein filament. The resulting DNA joint can be processed through either the SDSA pathway or the DSBR pathway. In the former, the invading strand is ejected by Mph1/FANCM/Fml1 and then becomes annealed with the other ssDNA end. The SDSA pathway generates exclusively noncrossover recombinants. In the DSBR pathway, the second DSB end is captured to yield a double Holliday junction (dHJ), which is either resolved by a HJ resolvase to generate crossover or noncrossover products, or it can be dissolved by the concerted action of the Sgs1-Top3-Rmi1 complex in yeast and BLM-Topo IIIa-RMI1/2 complex in humans to generate noncrossover products exclusively.

Figure 2. Multiplicity of DNA end resection means

A limited amount of 5’ resection is catalyzed by the MRX-Sae2 complex in yeast or the MRN-CtIP complex in humans. Two parallel pathways are responsible for long range resection, with one being mediated by Exo1 and the other by the Sgs1 or BLM-associated protein ensemble. Abbreviations: MRX, Mre11-Rad50-Xrs2; MRN, MRE11-RAD50-NBS1; STR, Sgs1-Top3-Rmi1.