Ovarian Cancers Harbor Defects in Nonhomologous End Joining Resulting in Resistance to Rucaparib.

Abstract

<b>Purpose:</b> DNA damage defects are common in ovarian cancer and can be used to stratify treatment. Although most work has focused on homologous recombination (HR), DNA double-strand breaks are repaired primarily by nonhomologous end joining (NHEJ). Defects in NHEJ have been shown to contribute to genomic instability and have been associated with the development of chemoresistance.<b>Experimental Design:</b> NHEJ was assessed in a panel of ovarian cancer cell lines and 47 primary ascetic-derived ovarian cancer cultures, by measuring the ability of cell extracts to end-join linearized plasmid monomers into multimers. mRNA and protein expression of components of NHEJ was determined using RT-qPCR and Western blotting. Cytotoxicities of cisplatin and the PARP inhibitor rucaparib were assessed using sulforhodamine B (SRB) assays. HR function was assessed using γH2AX/RAD51 foci assay.<b>Results:</b> NHEJ was defective (D) in four of six cell lines and 20 of 47 primary cultures. NHEJ function was independent of HR competence (C). NHEJD cultures were resistant to rucaparib (<i>P</i> = 0.0022). When HR and NHEJ functions were taken into account, only NHEJC/HRD cultures were sensitive to rucaparib (compared with NHEJC/HRC <i>P</i> = 0.034, NHEJD/HRC <i>P</i> = 0.0002, and NHEJD/HRD <i>P</i> = 0.0045). The DNA-PK inhibitor, NU7441, induced resistance to rucaparib (<i>P</i> = 0.014) and HR function recovery in a BRCA1-defective cell line.<b>Conclusions:</b> This study has shown that NHEJ is defective in 40% of ovarian cancers, which is independent of HR function and associated with resistance to PARP inhibitors in <i>ex vivo</i> primary cultures. <i>Clin Cancer Res; 23(8); 2050-60. ©2016 AACR</i>.