AR coactivators, CBP/p300, are critical mediators of DNA repair in prostate cancer.

Castration resistant prostate cancer (CRPC) remains an incurable disease stage with ineffective treatments options. Here, the androgen receptor (AR) coactivators CBP/p300, which are histone acetyltransferases, were identified as critical mediators of DNA damage repair (DDR) to potentially enhance therapeutic targeting of CRPC. Key findings demonstrate that CBP/p300 expression increases with disease progression and selects for poor prognosis in metastatic disease. CBP/p300 bromodomain inhibition enhances response to standard of care therapeutics. Functional studies, CBP/p300 cistrome mapping, and transcriptome in CRPC revealed that CBP/p300 regulates DDR. Further mechanistic investigation showed that CBP/p300 attenuation via therapeutic targeting and genomic knockdown decreases homologous recombination (HR) factors in vitro, in vivo, and in human prostate cancer (PCa) tumors ex vivo. Similarly, CBP/p300 expression in human prostate tissue correlates with HR factors. Lastly, targeting CBP/p300 impacts HR-mediate repair and patient outcome. Collectively, these studies identify CBP/p300 as drivers of PCa tumorigenesis and lay the groundwork to optimize therapeutic strategies for advanced PCa via CBP/p300 inhibition, potentially in combination with AR-directed and DDR therapies.

RNASEH2B loss and PARP inhibition in advanced prostate cancer.

BACKGROUND: Clinical trials have suggested antitumor activity from PARP inhibition beyond homologous recombination deficiency (HRD). RNASEH2B loss is unrelated to HRD and preclinically sensitizes to PARP inhibition. The current study reports on RNASEH2B protein loss in advanced prostate cancer and its association with RB1 protein loss, clinical outcome and clonal dynamics during treatment with PARP inhibition in a prospective clinical trial. METHODS: Whole tumor biopsies from multiple cohorts of patients with advanced prostate cancer were interrogated using whole-exome sequencing (WES), RNA sequencing (bulk and single nucleus) and immunohistochemistry (IHC) for RNASEH2B and RB1. Biopsies from patients treated with olaparib in the TOPARP-A and TOPARP-B clinical trials were used to evaluate RNASEH2B clonal selection during olaparib treatment. RESULTS: Shallow co-deletion of RNASEH2B and adjacent RB1, co-located at chromosome 13q14, was common, deep co-deletion infrequent, and gene loss associated with lower mRNA expression. In castration-resistant PC (CRPC) biopsies, RNASEH2B and RB1 mRNA expression correlated, but single nucleus RNA sequencing indicated discordant loss of expression. IHC studies showed that loss of the two proteins often occurred independently, arguably due to stochastic second allele loss. Pre- and post-treatment metastatic CRPC (mCRPC) biopsy studies from BRCA1/2 wildtype tumors, treated on the TOPARP phase II trial, indicated that olaparib eradicates RNASEH2B-loss tumor subclones. CONCLUSION: PARP inhibition may benefit men suffering from mCRPC by eradicating tumor subclones with RNASEH2B loss. CLINICALTRIALS: gov NCT01682772FUNDING. AstraZeneca; Cancer Research UK; Medical Research Council; Cancer Research UK; Prostate Cancer UK; Movember Foundation; Prostate Cancer Foundation.