Prostate specific membrane antigen and DNA damage repair in advanced prostate cancer

Abstract

Identification and implementation of novel therapeutic strategies for advanced prostate cancer (PCa) remains an urgent, unmet clinical need. Prostate Specific Membrane Antigen (PSMA), a PCa biomarker, is currently being studied to selectively deliver cytotoxic drugs and radioisotopes to PCa cells. The regulation and roles of PSMA remain controversial. In this thesis I described the heterogeneity of PSMA expression, through PSMA immunohistochemistry of patient samples, in castration resistant prostate cancer (CRPC). Both hormone sensitive PCa (HSPC) and CRPC demonstrated significant heterogeneity in membranous and cytoplasmic expression. Comparison of targeted sequencing data in "PSMA High" ans "PSMA Low" patient samples, showed a positive association between defective DNA damage repair (DDR) genes and PSMA expression. I next conducted a drug screen of 147 FDA approved compounds in cell lines to investigate potential combinatory therapies to induce PSMA expression and identified topoisomerase II inhibitors. I then investigated the impact of fractionated external beam radiation on PSMA expression in PCa cell lines due to interest in radionuclide therapy for PSMA targeted therapy. It similarly increased PSMA expression in a dose-dependent manner. To investigate the transcriptional regulation of PSMA, transcription factor motif enrichment analysis (TMEA) was conducted on ChIPseq data for the PSMA gene (FOLH1) identifying transcriptional enhancer factor TEF-3 (TEAD4) as a potential regulator of FOLH1. To elucidate possible functions and associated pathways of PSMA expression regulation, RNAseq of cell lines treated with siRNA knockdown of FOLH1 was also investigated. This revealed associations between FOLH1 and E2F targets as well as progression through G2/M. In conclusion, my work in this thesis has demonstrated a novel relationship between PSMA and DDR deleterious alterations, as well as between PSMA expression and induced double strand DNA breaks. I hypothesise that this can be utilised to drive PSMA protein expression, thus enhancing PSMA targeting therapy antitumour activity.