Abstract 1342: Double knockdown of RAD51 and RAD54 overcame the radioresistance in bladder cancer

Abstract Introduction and Objective: Radical cystectomy remains a main treatment for muscle-invasive bladder cancer (MIBC) because outcomes of radiotherapy (RT) have still been insufficient compared to surgery. Recent evidence indicated the involvement of DNA repair pathways with radio-resistance in MIBC. Previous study showed that RAD51 and RAD54 which promoted DNA repair cooperatively, were significantly associated with radio-sensitivity. More recently, our study demonstrated the significant association between estrogen receptor β signaling which was involved in radio-sensitivity in bladder cancer (BC), and their expression in it. However, there has been no study evaluating the relationship between RAD51/54 and radio-sensitivity in it. Therefore, we evaluated this association in BC cells. Methods: We compared the inhibitory effect of RT on BC cell viability between control and RAD51 and/or RAD54 knockdown BC cells (5637 and T24) cells. Moreover, we evaluated it on tumor growth in a mouse xenograft model between irradiated control and their knockdown cells. We also compared DNA double-strand breaks (DSB) via γ-H2AX foci formation, and DNA repair gene expressions in these cells after RT. Results: We then established radio-resistant cells and confirmed considerable elevation of the expressions of RAD51 and RAD54. In vitro experiments showed that RT reduced the viability in RAD51 or RAD54 knockdown cells more significantly than control cells. Moreover, double knockdown cells significantly reduced the cell viability more than each knockdown cell after irradiation in vitro and in vivo. Double knockdown of RAD51 and RAD54 resulted in a delay in DSB repair after RT. Conclusions: These results suggest that inhibition of both RAD51 and RAD54 has a potential of enhancing the cytotoxic effects of radiation in BC cells. Citation Format: Hiroki Ide, Tetsushi Murakami, Eiji Kashiwagi, Sotaro Kitaoka, Takemi Shishido, Mayuko Yamaguchi, Masahiro Katsui, Shinsuke Funakoshi, Hiroshi Miyamoto, Mototsugu Oya. Double knockdown of RAD51 and RAD54 overcame the radioresistance in bladder cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1342.