The molecular mechanism of BRPF proteins for the reversion of taxane resistance in prostate cancer

Prostate cancer (PCa) is a major cause of cancer-related death in men. Localized PCa can be treated with androgen suppression and surgery, nonetheless, many patients progress to the castration-resistant PCa (CRPCa), which is more aggressive and/or metastatic. The standard of care for CRPCa patients are taxanes (namely Docetaxel and Cabazitaxel), however resistance against these drugs develops over time. Taxane resistance makes clinical application of taxanes inefficient. Epigenetic regulation of cancer not only affects tumor development and progression but also resistance for chemotherapy. Hence, targeting these mechanisms appears as an alternative strategy to overcome drug resistance. Previous studies using an epigenetic drug library and epi-targeted CRISPR screens using taxane resistant CRPCa cells (Du145 and 22Rv1) have shown that inhibition of BRPF proteins (epigenetic reader proteins containing bromodomain group) efficiently reverses taxane resistance. To this end, we aimed to elucidate the molecular pathways involved in the reversion of taxane resistance through BRPFs in CRPCa cells. In this thesis, several approaches were combined including RNA sequencing, chromatin immunoprecipitation, bioinformatics and genome editing methods in order to characterize the biological significance of the BRPF proteins in taxane-resistant CRPCa. BRPF inhibition with small molecules resensitized taxane-resistant CRPCa cells to both taxanes under study. Efficient silencing of both BRPF1 and -2 were able to revert taxane resistance, albeit only mildly. On the other hand, knockout of BRPF2 effectively reversed taxane-resistance and induced cell death. BRPF1 was essential for the viability of resistant cells, hence BRPF1 knockout in resistant cells could not be generated despite several attempts. Furthermore, silencing or knocking out of BRPFs led to downregulation of ABCB1, which encodes a permeability glycoprotein (Pgp), and suppressed its function, potentially explaining how cells may be resensitized to taxane treatment. This supression seems to involve direct binding of BRPF1 to the ABCB1 promoter, as determined by ChIP-qPCR analysis. Furthermore, we observed that the promoter region of ABCB1 in taxane-resistant CRPCa cells exhibited occupancy by H3K27ac. Taken together, our results uncovered molecular players involved in BRPF mediated drug resistance. BRPF inhibition appears as a promising anticancer strategy in taxane resistant CRPCa and the mechanism seems to involve inhibition of drug efflux. BRPF inhibition can be utilized in the development of effective therapies against taxane resistant CRPCa. Keywords: BRPF, castration-resistant prostate cancer, drug resistance, epigenetics, taxane resistance.