At loose ends: resecting a double-strand break

Abstract

Double-strand break (DSB) repair is critical for maintaining genomic integrity and requires the processing of the 5' DSB ends. Recent studies have shed light on the mechanism and regulation of DNA end processing during DSB repair by homologous recombination.

Figure 1. DNA End Resection during Double-Strand Break Repair

Recent studies characterize a two-step mechanism for the processing of double-strand breaks (DSBs) at the 5′ ends to expose the 3′ single-stranded DNA (ssDNA) overhangs. Depending upon the cell-cycle phase and the type of DNA lesion, a DSB is processed by either the nonhomologous end joining pathway or the homologous recombination pathway. The names of the human homologs of the yeast proteins depicted are indicated in magenta. Following a DSB, the MRX/MRN complex is loaded onto the DNA ends at the break site. If cells are in the G1 phase of the cell cycle, nonhomologous end joining is used to repair the break. If the cells are in S/G2 phases, phosphorylation of Sae2/CtIP by cyclin-dependent kinases (CDKs) favors homologous recombination-mediated repair. During the first step of the homologous recombination repair pathway, the initial resection of the DSB is promoted by MRX/MRN and Sae2/CtIP, resulting in 50–100 nucleotide ssDNA 3′ overhangs. In the second step, these fragments can serve as templates for long-range DNA end resection. This processive reaction can occur by two independent mechanisms: one that utilizes Sgs1/BLM and Dna2 (left) and the other using Exo1/hEXO1 (right). Following resection, the exposed ssDNA is coated by replication protein A (RPA), which recruits the Rad52 epistasis group of proteins (Rad52, Rad55, Rad57, Rad59, Rad54, Rdh54) to enable Rad51 filament formation. The DNA ends can be repaired using different mechanisms, such as single-strand annealing, break-induced replication, or gene conversion. DSB repair using the homologous chromosome is depicted here. After the search for homology, a joint DNA structure is formed. The resulting double Holliday junctions are then resolved and the lost bases are restored at the break site. The resolution of the junctions by the Sgs1-Top3-Rmi1/BLM-TOP3α-RMI1 (BLAP75) complex depicted here leads to a non-crossover product of DSB repair by homologous recombination.