Data from Triapine Disrupts CtIP-Mediated Homologous Recombination Repair and Sensitizes Ovarian Cancer Cells to PARP and Topoisomerase Inhibitors

Abstract

<div>Abstract<p>PARP inhibitors exploit synthetic lethality to target epithelial ovarian cancer (EOC) with hereditary BRCA mutations and defects in homologous recombination repair (HRR). However, such an approach is limited to a small subset of EOC patients and compromised by restored HRR due to secondary mutations in BRCA genes. Here, it was demonstrated that triapine, a small-molecule inhibitor of ribonucleotide reductase, enhances the sensitivity of BRCA wild-type EOC cells to the PARP inhibitor olaparib and the topoisomerase II inhibitor etoposide. Triapine abolishes olaparib-induced BRCA1 and Rad51 foci, and disrupts the BRCA1 interaction with the Mre11–Rad50–Nbs1 (MRN) complex in BRCA1 wild-type EOC cells. It has been shown that phosphorylation of CtIP (RBBP8) is required for the interaction with BRCA1 and with MRN to promote DNA double-strand break (DSB) resection during S and G<sub>2</sub> phases of the cell cycle. Mechanistic studies within reveal that triapine inhibits cyclin-dependent kinase (CDK) activity and blocks olaparib-induced CtIP phosphorylation through Chk1 activation. Furthermore, triapine abrogates etoposide-induced CtIP phosphorylation and DSB resection as evidenced by marked attenuation of RPA32 phosphorylation. Concurrently, triapine obliterates etoposide-induced BRCA1 foci and sensitizes BRCA1 wild-type EOC cells to etoposide. Using a GFP-based HRR assay, it was determined that triapine suppresses HRR activity induced by an I-SceI–generated DSB. These results suggest that triapine augments the sensitivity of BRCA wild-type EOC cells to drug-induced DSBs by disrupting CtIP-mediated HRR.</p><p><b>Implications:</b> These findings provide a strong rationale for combining triapine with PARP or topoisomerase inhibitors to target HRR-proficient EOC cells. <i>Mol Cancer Res; 12(3); 381–93. ©2014 AACR</i>.</p></div>