Radiotherapy (RT) is a central treatment for prostate cancer (PCa), acting by inducing DNA double-strand breaks (DSBs). Tumor ability to repair these breaks limits RT efficacy, making DSB repair inhibitors potential radiosensitizers. Therefore, tumor-specific radiosensitization strategies are critically needed for PCa. Approximately 50% of PCa cases harbor the TMPRSS2-ERG gene fusion, leading to overexpression of the ERG transcription factor (ERG+). We demonstrate that ERG+ tumors shift DSB repair toward the PARP1-dependent end-joining (PARP1-EJ) pathway. Proteomic and western blot analyses revealed elevated PARP1, XRCC1, and LIG3 in ERG+ cells. PARP inhibition with olaparib increased residual γH2AX/53BP1 foci post-irradiation in ERG+ cells, indicating enhanced radiosensitization. In tissue-slice-cultures (TSCs) from 53 tumors of 40 high-risk PCa patients, olaparib selectively increased H2AX/53BP1 foci selectively in ERG+ samples. ERG+ patient-derived organoids also showed significantly delayed growth and survival when treated with olaparib plus RT compared to either treatment alone. Interestingly, ERG-negative cells within ERG+ TSCs were similarly radiosensitized by olaparib, likely through bystander effect, with residual 53BP1 foci levels comparable to ERG+ cells, confirmed by medium exchange experiments. These findings suggest that ERG expression promotes dependency on PARP1-EJ, rendering ERG+ PCa more susceptible to PARP inhibition. Combining PARP inhibitors with RT may offer a tumor-selective radiosensitization for ERG+ PCa patients.
Köcher et al. (Tue,) studied this question.