Interaction of Wwox with Brca1 and associated complex proteins prevents premature resection at double-strand breaks and aberrant homologous recombination.

ABSTRACT

Down regulation of Wwox protein expression occurs in many cancers, contributing to insensitivity to ionizing radiation (IR) and platinum drug treatments. Patients with reduced Wwox expression in their cancer tissue show decreased overall survival following these treatments, in accord with our earlier finding that reduced Wwox protein expression in cancers is associated with changes in choice of DNA double-strand break (DSB) repair pathway. Our current investigation of mechanisms underlying the initial choice of repair by homologous recombination/single-strand annealing (HR/SSA) in Wwox-deficient cells, showed immediate DNA end-resection at DSBs following IR, abrogating initial repair by the expected non-homologous end-joining (NHEJ) pathway. Mechanisms supporting the expected choice of DSB repair by NHEJ in Wwox-sufficient cells are 1) direct recruitment of Wwox protein binding to Brca1 through the Brca1 981PPLF984 Wwox-binding motif; 2) possible Wwox blocking of Brca1-Rad50 interaction and of Brca1 activation by Chk2 phosphorylation of Brca1 S988; 3) Wwox suppression of Brca1 interaction with the B and C complex proteins, Brip1 and CtIP, thereby delaying the process of DSB end-resection post-IR. Wwox deficiency, instead, leads to early formation of the Brca1-CtIP/MRN complex at induced DSBs, stimulating immediate post-IR end-resection. This premature resection at DNA DSBs leads to inappropriate HR/SSA repair not restricted to late S/G2 cell cycle phases, and increases mutations in genomes of radiation or platinum-resistant colonies. Prevention of premature initiation of end-resection, by combining Chk2 inhibition with IR or carboplatin treatment, successfully sensitized IR and platinum-resistant Wwox-deficient cells by synthetic lethality, but did not alter response of Wwox-sufficient cells. Our results establish Wwox as a biomarker for treatment response and provide potential targets, such as Chk2, for reversal of treatment resistance.