Abstract POSTER-BIOL-1325: Ovarian cancer-associated CDK12 mutations disable its kinase activity and cause defects in homologous recombination

Abstract

Abstract PURPOSE: The purpose of this study was to determine whether a) high-grade serous ovarian cancer-associated CDK12 kinase domain mutations affect catalytic activity, b) loss of CDK12 affects BRCA1 expression levels and Homologous Recombination (HR) in ovarian cancer, c) tumor-associated CDK12 mutations cause HR defects in ovarian cancer, and d) loss of CDK12 sensitizes ovarian cancer to chemotherapy and Poly(ADP-ribose) Polymerase [PARP] inhibitors. EXPERIMENTAL PROCEDURES: We compared the kinase activities of wild-type CDK12 and tumor-associated CDK12 kinase domain mutants using in-vitro kinase assays. To determine the impact of CDK12 activity on HR, we assessed RAD51 foci formation and HR using a DR-GFP construct. To analyze the effect of tumor-associated CDK12 mutations on HR, CDK12-depleted OVCAR-8 cells were reconstituted with either wild-type CDK12 or the tumor-associated CDK12 mutant (G909R), irradiated and stained for RAD51 foci. We performed clonogenic assays using ovarian cancer cell lines (OVCAR-8, OVCAR-5 and OVCAR-3) to measure sensitivity to DNA cross-linking agents (melphalan and cisplatin) and the PARP inhibitor, veliparib, in the presence or absence of CDK12 activity. CDK12 depletion was carried out using at least two independent short-interfering RNAs (siRNAs). RESULTS: We showed that many, but not all, ovarian cancer-associated CDK12 kinase domain mutations severely disrupted kinase activity. Moreover, we found that disabling CDK12 activity in ovarian cancer cells decreased BRCA1 levels and disrupted HR. Additionally, the tumor-associated CDK12 G909R mutant did not restore impaired RAD51 foci formation in CDK12-depleted OVCAR-8 cells. Finally we show that CDK12 depletion sensitized multiple ovarian cancer cells to the DNA cross-linking agents melphalan and cisplatin and to the PARP inhibitor veliparib. CONCLUSION: Ovarian cancer-associated CDK12 mutations disrupt kinase activity and cause defects in HR, suggesting that ovarian tumors harboring deleterious CDK12 mutations may predict sensitivity to chemotherapeutic agents, including PARP inhibitors. Citation Format: Poorval M. Joshi, Shari L. Sutor, Catherine J. Huntoon, Larry M. Karnitz. Ovarian cancer-associated CDK12 mutations disable its kinase activity and cause defects in homologous recombination [abstract]. In: Proceedings of the 10th Biennial Ovarian Cancer Research Symposium; Sep 8-9, 2014; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(16 Suppl):Abstract nr POSTER-BIOL-1325.