A protective role for BRCA2 at stalled replication forks

The hereditary breast and ovarian cancer predisposition genes BRCA1 and BRCA2 have crucial roles in the control of double strand break (DSB) repair by homologous recombination (HR). BRCA1 functions early in HR, interacting with nucleases, including CtIP and the Mre11/Rad50/NBS1 (MRN) complex to coordinate DNA end resection to form single-stranded DNA [1]. BRCA1 also recruits BRCA2 to the break site. BRCA2 is a key repair component that loads the Rad51 recombinase onto single-stranded DNA of the processed DSB [2-4]. DSBs in cycling somatic cells are thought to arise predominantly during the DNA synthesis phase of the cell cycle, when a replicative DNA polymerase stalls on abnormal DNA structure. Some models invoke the use of HR to repair the broken fork using the undamaged neighboring sister chromatid as a template [5-7]. However, not all HR requires a DSB intermediate; a fork-stalling DNA adduct may be bypassed without complete breakage ('collapse') of the fork. Because the stalled fork is structurally different from an isolated DSB, the mechanisms governing HR in these two contexts may differ. Circumstantial evidence suggests that critical events underlying breast cancer risk, as well as the therapeutic action of poly(ADP) ribose polymerase inhibitors on BRCA-linked cancers, are played out at sites of DNA polymerase stalling [8-10]. Paradoxically, however, the experimental tools available for studying HR at sites of mammalian DNA polymerase stalling are quite limited.In a paper published recently in Cell, Schlacher and colleagues [11] have taken an interesting new approach to studying how BRCA2 affects stalled fork metabolism. The authors used single-molecule DNA fiber analysis to determine the fate of newly synthesized ('nascent') DNA strands just proximal to replication forks in cells treated with hydroxyurea (HU), an agent that depletes the nucleotide pool, causing genome-wide DNA polymerase stalling. In wild-type cells, the nascent strands at HU-arrested forks