Mechanism of homologous recombination: mediators and helicases take on regulatory functions

Abstract

Homologous recombination (HR) is an important mechanism for the repair of damaged chromosomes, for preventing the demise of damaged replication forks, and for several other aspects of chromosome maintenance. As such, HR is indispensable for genome integrity, but it must be regulated to avoid deleterious events. Mutations in the tumour-suppressor protein BRCA2, which has a mediator function in HR, lead to cancer formation. DNA helicases, such as Bloom's syndrome protein (BLM), regulate HR at several levels, in attenuating unwanted HR events and in determining the outcome of HR. Defects in BLM are also associated with the cancer phenotype. The past several years have witnessed dramatic advances in our understanding of the mechanism and regulation of HR.

BRCA2: a universal recombinase regulator

“…RAD51 loading is facilitated by the ssDNA-binding protein RPA and by RAD52. The RAD51 nucleoprotein filament mediates a search for homology within the sister chromatid, which in turn serves as a template for DNA synthesis to restore any lost sequences at the break site and ultimately regenerate intact DNA (West, 2003;Sung and Klein, 2006). The tumour suppressor and breast cancer-susceptibility gene BRCA2 is required for normal levels of HR-mediated DSB repair (Yu et al, 2000;Moynahan et al, 2001), providing an important link between the ability of a cell to maintain genome stability and tumorigenesis.…”

Two distinct conformational states define the interaction of human RAD 51‐ ATP with single‐stranded DNA

“…The NPF is responsible for DNA sequence homology recognition in a duplex DNA, usually the sister chromatid, and formation of a joint intermediate that will serve as a priming site for DNA synthesis needed to copy the missing information (Benson et al , 1994). In the cell, HR is tightly regulated, for example by the tumour‐suppressor protein BRCA2 (Sung & Klein, 2006), which is known to mediate the loading of hRAD51 onto RPA‐coated ssDNA, making use of its ability to bind hRAD51 with its BRC‐repeat domain (Wong et al , 1997; Chen et al , 1998; Carreira et al , 2009). …”

Cdk-mediated phosphorylation of Chk1 is required for efficient activation and full checkpoint proficiency in response to DNA damage

“…Non-homologous end-joining can occur throughout the cell cycle, however, because HRR requires a sister chromatid to serve as a template, this mechanism is utilized preferentially in the S and G2 phases when chromosome duplication is ongoing or complete. Unlike non-homologous endjoining, HRR requires extensive DNA damage processing to generate tracts of single-stranded DNA (ssDNA) which, once coated with Rad51 recombinase, invade the homologous DNA duplex to initiate repair (Sung and Klein, 2006;San Filippo et al, 2008). It is well established in yeasts that cyclin-dependent kinases (Cdks) have a major role in controlling DNA strand resection (Wohlbold and Fisher, 2009;Yata and Esashi, 2009).…”