Quality control of homologous recombination

Abstract

Exogenous and endogenous genotoxic agents, such as ionizing radiation and numerous chemical agents, cause DNA double-strand breaks (DSBs), which are highly toxic and lead to genomic instability or tumorigenesis if not repaired accurately and efficiently. Cells have over evolutionary time developed certain repair mechanisms in response to DSBs to maintain genomic integrity. Major DSB repair mechanisms include non-homologous end joining and homologous recombination (HR). Using sister homologues as templates, HR is a high-fidelity repair pathway that can rejoin DSBs without introducing mutations. However, HR execution without appropriate guarding may lead to more severe gross genome rearrangements. Here we review current knowledge regarding the factors and mechanisms required for accomplishment of accurate HR.

Fig. 1

Models of double-strand break repair. Upon induction of DSBs, the broken DNA ends are either coated by Ku complex which initiates the canonical non-homologous end-joining (C-NHEJ) pathway or MRN/X complex which processes the DNA ends into short 3′-single-stranded DNA (ssDNA) tails. The first step of resection by MRN/X complex could trigger either the alternative end-joining (Alt-NHEJ) pathway or homologous recombination (HR) pathway. In HR pathway, the short ssDNA tails that are initially coated by the replication protein A (RPA) complex can be further resected into longer ssDNA tails. In a subsequent step, recombination mediator proteins such as BRCA2 and RAD51 paralogs catalyze the replacement of RPA with RAD51, resulting in the formation of ssDNA-RAD51 nucleoprotein filament. The ssDNA-RAD51 nucleoprotein filament then catalyzes strand invasion into homologous duplex DNA, leading to the formation of D-loop. After DNA synthesis primed by the invading strand, the repair can bifurcate into two alternative sub-pathways referred to as synthesis-dependent strand annealing (SDSA) and double-strand break repair (DSBR). In SDSA, the extended D-loop can be dissolved by specialized DNA helicases and the newly synthesized strand is annealed to the ssDNA tail on the other break end, which is followed by gap-filling DNA synthesis and ligation. The repair products from SDSA are always non-crossover. In DSBR, the second DSB end is captured to form an intermediate with two Holliday junctions, called double Holliday junction (dHJ). dHJ a central intermediate of HR that can be processed to yield crossover or non-crossover recombination products