TARGETING BROMODOMAIN PROTEINS AS A THERAPEUTIC STRATEGY FOR CLEAR CELL CARCINOMA OF THE OVARY

Background and Objectives: Clear cell carcinoma of the ovary (OCCC) is more resistant to platinum-based chemotherapy than high-grade serous ovarian cancer (HGSOC). Also, the indication for PARP inhibitors seems to be limited in OCCC because of the lower frequency of homologous recombination deficiency. As genomic background significantly differs among subtypes, therapeutic strategies should be optimized in response to each subtype or unique gene alterations. This study was designed to find novel therapeutic targets for OCCC by utilizing siRNA screens and unique patient-derived ovarian cancer cells that phenocopy original tumors. Materials and Methods: Novel patient-derived ovarian cancer cells were established and kindly provided by Dr. Tan Ince at University of Miami. Genomic alterations of these cells were determined by DNA sequencing. siRNA screens targeting 2,172 unique genes were performed with ARID1A-mutated OCCC cell line OCI C5x, and high-grade serous ovarian cancer cell line OCI P5x. Genes inhibition of which significantly suppressed cell proliferation were selected as hits for each subtype independently. The difference of cell viability between OCI C5x and P5x was also compared, and subtype-specific hits were additionally determined. Hits were applied to Gene Ontology (GO) enrichment analysis to comprehend the gene set enrichment. Several prioritized targets for OCCC were validated with additional RNAi experiments and small molecule inhibitors. Results: 114 and 108 genes were selected as hits specific to OCI C5x and OCI P5x, respectively. GO enrichment analysis revealed OCCC is potentially more vulnerable to epigenetic interventions than HOGSC. The following inspection revealed bromodomain-containing proteins were enriched in the hits for OCI C5x. Among these, we highlighted bromodomain and extra-terminal domain (BET) family proteins BRD2 and BRD3. Subsequent RNAi experiments validated BRD2 and BRD3 are highly potent therapeutic targets for OCCC. Conclusions: siRNA screening results suggest different approaches are required according to subtypes for the targeted therapy of ovarian cancer. We propose that epigenetic intervention such as BET protein inhibition as a potential therapeutic option for OCCC, 50-60% of which harbor ARID1A mutation. Citation Format: Shogo Shigeta, Reid Shaw, Russell Moser, Kay Gurley, Goldie Lui, Carla Grandori, Christopher Kemp. TARGETING BROMODOMAIN PROTEINS AS A THERAPEUTIC STRATEGY FOR CLEAR CELL CARCINOMA OF THE OVARY [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr NT-108.