Abstract 6285: Defining androgen receptor function and the effects of PARP inhibition in BRCA1/2-deficient vs. proficient prostate cancer

Abstract Mutation or loss of the tumor suppressor genes BRCA1 and BRCA2 promotes the development of aggressive prostate cancer with poor outcomes and a high risk of metastatic relapse. Approximately 15% of metastatic prostate tumors are BRCA1/2-deficient. These tumors are frequently resistant to standard-of-care treatments that block androgen receptor (AR) function, although the mechanisms underlying this effect remain unclear. PARP inhibitors are synthetically lethal with BRCA1/2-deficiency and are commonly used as a late-line treatment in metastatic prostate cancer; however, both preclinical and clinical data have provided conflicting results on whether PARP inhibitors can be effective in BRCA1/2-proficient prostate tumors. BRCA1, BRCA2, PARP-1, and PARP-2 all have proposed roles in regulating transcription and the chromatin landscape. We therefore hypothesized that loss of BRCA1, BRCA2, and/or inhibition of PARP modulates AR function through changes in global chromatin structure and transcriptional pathways. We used patient-derived xenograft models and next-generation sequencing techniques to profile AR function in BRCA1/2-deficient vs. proficient prostate tumors. We found that loss of BRCA1 and BRCA2 is associated with chromatin accessibility changes, and differentially accessible regions correlate with DNA binding motifs for AR and AR cofactors. ChIP-seq for AR in these tumors demonstrated that BRCA1/2-deficiency is associated with an altered AR cistrome and a global decrease in AR genomic binding. Corresponding RNA-seq data confirmed a downregulated androgen response in BRCA1/2-deficient tumors. In cell line models, inhibition of PARP1 and PARP1/2 inhibited AR-driven transcription. Combining PARP inhibition with either androgen deprivation or the AR antagonist enzalutamide dampened the growth-inhibitory effects of PARP inhibitor treatment. Together, these results indicate that BRCA1/2-deficiency and PARP inhibition modulate AR function in prostate cancer. Citation Format: Nicole A. Traphagen, Esme Wheeler, Rong Li, Xintao Qiu, Myles Brown. Defining androgen receptor function and the effects of PARP inhibition in BRCA1/2-deficient vs. proficient prostate cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 6285.