Abstract C062: Defining the interplay between PARP and the RB/E2F axis in prostate cancer
Authors: Evans Latese 
Published: December 2023

Abstract
Prostate cancer (PCa) is the second leading cause of cancer-related death in American men. Non-Hispanic Black men are 1.8 times more likely to develop PCa and 2.3 times more likely to die from PCa. Tumor suppressor retinoblastoma (RB1) is frequently deleted in PCa. RB represses the transcriptional activity of E2Fs and the phosphorylation of RB by CDK4/6 results in dissociation of the complex and a subsequent increase in E2F transcriptional activity, leading to cell cycle progression. RB loss is associated with tumor aggressiveness occurring in 10-15% of castration-resistant prostate cancer (CRPC) tumors and corresponds to a decrease in overall survival. RB depletion leads to increased expression of DNA repair proteins due to elevated transcriptional activity of E2F1. PARP-1 is an enzyme that plays a role in DNA repair. Elevated PARP-1 enzymatic activity is associated with PCa progression and correlates with poor clinical outcomes in CRPC. E2F1 transcriptional activity is supported by PARP enzymatic activity, however the consequences of the functional interaction between the RB/E2F axis and PARP remains an open line of inquiry. Defining the interplay between PARP and RB/E2F is likely to contribute to the identification of novel treatment strategies and/or biomarker response. Canonically, E2F1-3 are transcription factors that positively regulate gene expression programs associated with pro-proliferative signals. Prior research by other groups has shown that E2F target genes expression is elevated in tumors excised from Black men compared to Asian and White men. There was an increase in germline RAD51 mutations observed. RAD51 is an E2F target gene involved in DDR, therefore attenuating PARP enzymatic activity may be a potential mode of targeting this axis. These data suggest that there may be differential DNA damage response (DDR) in African American men driven by aberrations in the RB/E2F signaling axis, however the mechanism is yet to be fully explored. Analysis of publicly available patient datasets were performed and demonstrate that RB1 deletions is more frequent than RB1/BRCA2 co-deletion in metastatic PCa. Isogenic models of RB1 knockdown were used to evaluate the impact of RB loss on PARP enzymatic activity. PARP-1 mRNA and PARP-1 protein are elevated upon RB1 depletion with a concomitant increase in PARP activity. Growth curve assays were conducted to examine the biological response to PARP inhibition in the isogenic models. In hormone-depleted conditions, results suggest a differential response between RB depleted models compared to the control when challenged with PARP inhibition. Furthermore, an IC50 dose of Olaparib elicits differential PARP enzymatic response in the RB1 depleted model. RB1 loss is associated with an enrichment in E2F1 binding at the PARP-1 locus. Future studies have been designed to determine the impact of the RB/E2F pathway alterations on downstream consequences PARP enzymatic function and establish the contribution of PARP and the RB/E2F axis on racial disparity in PCa.
Citation Format: Latese Evans. Defining the interplay between PARP and the RB/E2F axis in prostate cancer [abstract]. In: Proceedings of the 16th AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2023 Sep 29-Oct 2;Orlando, FL. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2023;32(12 Suppl):Abstract nr C062.