Mechanism and regulation of DNA end resection in eukaryotes

Abstract

The repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) is initiated by nucleolytic degradation of the 5′ terminated strands in a process termed end resection. End resection generates 3′ single-stranded DNA tails, substrates for Rad51 to catalyze homologous pairing and exchange of DNA strands, and for activation of the DNA damage checkpoint. The commonly accepted view is that end resection occurs by a two-step mechanism. In the first step, Sae2/CtIP activates the Mre11-Rad50-Xrs2/Nbs1 (MRX/N) complex to endonucleolytically cleave the 5′-terminated DNA strands close to break ends, and in the second step Exo1 and/or Dna2 nucleases extend the resected tracts to produce long 3′-ssDNA-tailed intermediates. Initiation of resection commits a cell to repair a DSB by HR because long ssDNA overhangs are poor substrates for non-homologous end joining (NHEJ). Thus, the initiation of end resection has emerged as a critical control point for repair pathway choice. Here, I review recent studies on the mechanism of end resection and how this process is regulated to ensure the most appropriate repair outcome.

Remarkably Long-Tract Gene Conversion Induced by Fragile Site Instability inSaccharomyces cerevisiae

“…Previously we demonstrated that BIR is strongly stimulated in cells under replication stress, and that many of these BIR events are initiated by breaks at FS2 . BIR events can initiate centromereproximal to the original break location due to extensive 59 to 39 end resection at DNA breaks to expose ssDNA (Chung et al 2010;Symington 2016).Both the extensive lengths and high abundance of bidirectional noncrossover tracts we observed can be explained if these tracts result not from canonical DSBR but from dBIR. The dBIR mechanism is also a better fit for our experimental system in which replication stress is likely to result in replication fork 124 S. A. Chumki et al…”

“…Previously we demonstrated that BIR is strongly stimulated in cells under replication stress, and that many of these BIR events are initiated by breaks at FS2 . BIR events can initiate centromereproximal to the original break location due to extensive 59 to 39 end resection at DNA breaks to expose ssDNA (Chung et al 2010;Symington 2016).…”

“…In the case of heterozygosity at a tumor-suppressor gene, an LOH event can result in loss of the functional allele, driving cancer progression. In general, homologous recombination begins with 59 to 39 end resection at the break to expose single-stranded DNA (ssDNA) (Symington 2016). Mediator proteins, such as Rad52p in yeast, nucleate onto the ssDNA and assist in recruiting Rad51p to form a filament that searches for homology.…”

Keep moving and stay in a good shape to find your homologous recombination partner

“…This initial binding helps recruit the NHEJ ligase, Lig4 which ligates DSB extremities even in the absence of significant homology. If not ligated by NHEJ, DSB is processed to generate 3′ single strand overhangs by partially redundant nucleases, which include Mre11/Sae2, Dna2 and Exo1 (Mimitou and Symington 2008;Cejka 2015;Symington 2016). The resulting 3′ss overhangs generated by the concerted action of MRX/Sae2, Exo1 and Sgs1/Dna2 proteins are rapidly stabilized by RPA.…”

DNA-dependent protein kinase promotes DNA end processing by MRN and CtIP

“…DNA-PK promotes the ligation of two DNA ends by ligase IV, aided by the accessory factors XLF, XRCC4, and APLF, and in some cases aided by limited end processing (1,2). The recognition and repair of DNA breaks by the end-joining machinery is generally considered to be the first and rapid phase of DNA repair, occurring within ~30 minutes of DNA damage, while homologous recombination is specific to the S and G 2 phases of the cell cycle and occurs over a longer time frame (3,4). DNA-PK is present at micromolar concentrations in cells (5) and binds DNA ends in all cell cycle phases, even during S phase at single-ended breaks (6).…”