Abstract LB-120: Investigation of PARP inhibitor resistance in BRCA1-mutant ovarian tumors

Patients with familial high-grade serous ovarian carcinoma (HGSOC) associated with germline BRCA1/2 mutations exhibit improved outcome and high sensitivity to double strand DNA break (DSB)-inducing agents [i.e. platinum and Poly(ADP-ribose) polymerase (PARP) inhibitors] due to an underlying defect in DNA repair via homologous recombination (HR). Importantly, a subset of patients with sporadic HGSOCs also exhibit improved outcome and responsiveness to these cytotoxic agents, possibly due to defective HR caused by mechanisms unrelated to germline mutation in BRCA-1 or 2. In this regard, in The Cancer Genome Atlas (TCGA) dataset, approximately 50% of HGSOC harbored the genetic or epigenetic alterations involving HR pathway. Interestingly, even though these tumors harbored an underlying defect in the HR pathway, a significant percentage of them unexpectedly exhibited resistance to platinum chemotherapy resulting in poor prognosis. In contrast to the relative promiscuity of platinum-based therapy, the PARP inhibitor (PARPi)s have emerged as an exciting new therapeutic opportunity for HGSOCs with BRCA mutations. However, several resistance mechanisms are emerging, including reversion mutations in the BRCA genes, and loss of 53BP1. A systematic investigation of the underlying cause of PARPi resistance is necessary to identify novel drug targets to potentially overcome resistance. The bacterial clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 system for genome editing has greatly expanded the toolbox for mammalian genetics, enabling the rapid generation of isogenic cell lines with disrupted genes. We utilized this elegant system in the form of a genome-scale lenti-viral CRISPR-Cas9 knockout (GeCKO) library targeting 18,080 genes with 64,751 unique guide sequences to screen a panel of patient-derived BRCA1-deficient HGSOC lines for resistance to clinical grade PARPi and platinum drugs. The goal was to identify specific genes and signaling pathways that influence the process of resistance in these tumors. The ‘top’ hit was DYNLL1 and another hit was its binding partner ATMIN. In our validation screen, targeted disruption of ATMIN or DYNLL1 in multiple BRCA1-mutant HGSOC lines led to increase of Cisplatin IC50 up to 200% and Olaparib IC50 up to 400% along with the restoration of HR. Analysis of TCGA revealed that ATMIN is under-expressed in about 30% ovarian cancer patients and low ATMIN or DYNLL1 expression correlated with poor prognosis specifically for patients with BRCA1-mutant ovarian carcinomas. Here we explore the mechanism by which loss of ATMIN/DYNLL1 causes restoration of HR and consequent resistance to PARPi and platinum drugs in BRCA1-mutant tumors. We found ATMIN/DYNLL1 loss lead to increased end resection and Rad51 foci in the absence of BRCA1. This increase is due to increased expression of end resection enzyme in ATMIN/DYNLL1 deficient cells. We have preliminary evidence that ATMIN/DYNLL1 impedes the expression of end resection enzymes via specific transcription factors to further restrict HR in the BRCA1-mutant cells. Our study identifies a new role for ATMIN/DYNLL1 in regulating HR and provide insight on novel mechanisms of PARPi resistance that may occur in BRCA1-mutant HGSOCs. Citation Format: Yizhou J. He, Dipanjan Chowdhury, Khyati Meghani. Investigation of PARP inhibitor resistance in BRCA1-mutant ovarian tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-120. doi:10.1158/1538-7445.AM2017-LB-120