Abstract 3626: Expression of BARD1 splice variant impairs homologous recombination and can predict PARP1 inhibitor therapy in colorectal cancer

Abstract

Abstract Background & Aims: BRCA1 associated RING domain protein 1 (BARD1) stabilizes BRCA1 protein by forming a heterodimeric RING-RING complex. Its alterations impact function of BRCA1, disabling HR (homologous recombination) repair leading to sensitivity to PARP-1 inhibition in cancer. In colorectal cancer (CRC), we observed expression of BARD1 splice variants (SVs) and loss of full-length is associated with poor prognosis despite wild type expression of BRCA1. Here, we examined the roles of BARD1 SV and its therapeutic impact on PARP-1 inhibition, oxaliplatin and irinotecan sensitivity in CRC cell lines. Methods: A panel of CRC cell lines was treated with PARP-1 inhibitor, ABT888, and HR proficiency assessed via RAD51 foci formation. BARD1 splice variant (SV B1/4) transcription was determined via real-time PCR, and protein expression assessed via immunoblotting. Tumorigenesis of CRC cells expressing SV B1/4 and wild type BARD1 was measured by epithelial-mesenchymal transitions (EMT), colony formation abilities, and migration assays. Results: Caco-2 CRC cells were significantly more sensitive to PARP-1 inhibition when compared to the other CRC cell lines. This sensitivity was not associated with loss of BRCA1 or full-length BARD1 expression but rather with expression of BARD1 SV 1/4. Caco-2 showed significantly impaired HR response following PARP-1 inhibition with ABT888 and characterized by decreased RAD51 foci formation. Exogenous expression of the BARD1 SV B1/4 in SW480 cells exhibits a more transformed phenotype, decreased RAD51 foci formation, reduced BRCA1 E3 ubiquitin ligase activity, and increased sensitivity to PARP-1 inhibitor. Addition of irinotecan to ABT888 pre-treated cells created higher specificity and sensitivity with respect to killing of SV B1/4 expressing SW480 cells. Further immunofluorescence and confocal microscopy studies are underway to investigate the effect of BARD1 SV B1/4 on BRCA1 homologous recombination DNA repair function. Conclusions: Expression of BARD1 SV might be a potential biomarker for therapeutic sensitivity to PARP-1 inhibitors in treatment of CRC. Citation Format: Ozkan Ozden, Faraz Bishehsari, Jessica Bauer, Seung Hyua Baik, Barbara Jung. Expression of BARD1 splice variant impairs homologous recombination and can predict PARP1 inhibitor therapy in colorectal cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3626. doi:10.1158/1538-7445.AM2015-3626