Abstract 1490: RAD51C-deficient cancer cells require DNA polymerase zeta to bypass cisplatin-induced lesion

RAD51C is a RAD51 paralog protein that mediates RAD51 filament formation on single-stranded DNA (ssDNA) in a canonical homologous recombination (HR) pathway. This step is vital for homology search to repair double strand breaks. RAD51C deficiency is linked to increased risks of developing breast and ovarian cancers. Fortunately, RAD51C-deficient cancer patients are generally responding well to chemotherapeutics drug like cisplatin. HR is one of the important pathways to repair cisplatin-induced lesion. Hence, cancer cells that lack RAD51C are more susceptible to cisplatin treatment. Nonetheless, high grade serous ovarian cancer patients are often relapse even though initial cisplatin treatment were successful. There is still incomplete knowledge on how different DNA repair factors contribute to cisplatin-induced damage repair in HR-deficient cancer cells. Our study is aimed to identify and study these factors to better understand how RAD51 paralog-deficient cancer cells evade cell death upon cisplatin treatment. Apart from HR, there are several DNA damage tolerance mechanisms in which cancer cells can adopt to bypass cisplatin-induced lesions. Based on our synthetic lethality screen in yeast, we observed that translesion synthesis (TLS) factors (rev1 and rev3) deletion, increase the cisplatin sensitivity in RAD51 paralog knockout (KO) cells. Cisplatin often causes the formation of bulky DNA lesions that provide structural barrier to DNA polymerase during DNA replication. TLS DNA polymerases are error-prone DNA polymerases that bypass these lesions to continue DNA replication and evade cell cycle arrest. We also observed the same scenario in human ovarian cancer cells. TLS depletion, that include REV3L of DNA pol zeta, (POL ξ), increase cisplatin sensitivity in RAD51C-deficient cancer cells. Using DNA fiber assay, we discovered that POL ξ limits the replication fork speed in RAD51C-KO cells upon cisplatin treatment. Based on S1 nuclease DNA fiber assay, we showed that POL ξ also prevents the accumulation of ssDNA in cisplatin-treated RAD51C-KO cells. These preliminary data suggest that POL ξ plays a role in DNA replication dynamics in RAD51C-deficient cells to ensure them to survive under cisplatin treatment. Uncharacterized RAD51C point mutations that found in cancer patients are often classified as variants of unknown significance (VUS). While cells that lack RAD51C are generally sensitive to cisplatin, some of these RAD51C VUS variant cells are resistant to it. In our clonogenic assay, we observed that REV3L depletion sensitizes several cisplatin-resistant RAD51C variants to cisplatin. Upon cisplatin treatment and REV3L depletion, we also observed an increase of ssDNA accumulation in RAD51C variant. Our data reveal the potential role of POL ξ in therapeutic avenue for cisplatin-resistant RAD51C variants. Citation Format: Mohd Azrin Jamalruddin,Emmanuel Tawiah,Isabela Contreras,McKenzie Grundy,Kara Bernstein. RAD51C-deficient cancer cells require DNA polymerase zeta to bypass cisplatin-induced lesion [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 1490.