Abstract B022: PARP1 complex composition as a predictor of response to PARP inhibitors in BRCA-linked ovarian carcinoma

Background: About 40-50% of epithelial ovarian cancers (EOC) show defects in DNA repair by homologous recombination (HR), which are mostly associated with BRCA1/2 loss-of-function mutations. The PARP inhibitors (PARPis) olaparib, niraparib and rucaparib were recently approved for treatment of ovarian cancer patients with platinum-sensitivity and recurrent ovarian cancer who carry inactivating BRCA1/2 mutations. These targeted drugs produce significant response rates ranging from 40-60% in patients with BRCA-linked advanced EOCs, but resistance is a continuing challenge. Whereas several studies have reported various mechanisms of acquired resistance to PARPis, the mechanisms of primary resistance are still poorly understood. Our goal is to develop predictors of PARPi response and to identify new targets for combination therapy to overcome primary resistance. We apply a novel integrated proteomics approach to develop mechanism-based biomarkers of response or primary resistance and to identify new therapeutic targets for rational combination approaches that can overcome resistance to single agent PARPi therapy. Methods: Primary patient-derived EOC cell lines and an isogenic EOC cell line pair with BRCA1 deletion and reconstituted with ectopic BRCA1 was used. Primary cell lines were genomically characterized by BROCA next generation sequencing. The effects of the FDA approved PARPis olaparib, rucaparib, and niraparib on BRCA1-null and BRCA1-reconstituted EOC cell lines regarding short- and long-term cell viability were determined by CellTiterGlo and crystal violet assays. Chemical and phosphoproteomics approach was used to identify the components of PARP1-based multiprotein complexes as well as protein post-translational modifications in the DNA damage signaling network in BRCA1/2-linked EOC cells. Specific PARP1-engaged protein complexes were further determined by immunoblotting. Frozen BRCA1-proficient and deficient ovarian cancer patient tumor samples collected at the time of debulking were also characterized by chemical proteomics. Results: Cell viability and clonogenic assays confirmed the expected correlation between PARPi response and BRCA1/2 status. Chemical proteomics with rucaparib and olaparib detected important elements of DNA damage repair, as components of PARP1 protein complexes, which are differentially represented in EOC cells that are sensitive to PARPi treatment compared to cells that show primary resistance to PARPi. In addition, phosphoproteomics analysis further identified significant and actionable changes in the DNA damage response signaling network in these cells. Most interestingly, these PARP1 protein complex differences are also observed in PARPi drug pulldowns (olaparib and rucaparib) from BRCA1-proficient compared to BRCA1-deficient tumor samples. Conclusion: Ovarian cancers that do not respond to PARPi displayed significant changes in PARPi-engaged protein complexes as well as protein modifications. We expect to identify new potential targets for combination therapy BRCA-linked EOC by further validation of our data. Citation Format: Ou Deng, Sweta Dash, Thales Nepomuceno, Ming D Han, Bin Fang, Doug Marchion, Alvaro N Monteiro, Uwe Rix. PARP1 complex composition as a predictor of response to PARP inhibitors in BRCA-linked ovarian carcinoma [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr B022. doi:10.1158/1535-7163.TARG-19-B022