The miR-221-5p/RAD18/RAD51 Axis Regulates DNA Damage Tolerance and Homologous Recombination to Drive Platinum Resistance in Ovarian Cancer

Platinum resistance remains a major barrier in epithelial ovarian cancer (EOC) treatment, while upregulation of the DNA damage response (DDR) is a hallmark of chemoresistance, and the underlying epigenetic mechanisms driving this adaptation remain poorly understood. Here, we identify a novel post-transcriptional regulatory axis involving miR-221-5p that governs two critical DDR effectors: RAD18, which mediates DNA damage tolerance through translesion synthesis (TLS), and RAD51, the central recombinase for homologous recombination (HR). Although the miR-221/222 cluster has been traditionally categorized as oncogenic, we demonstrated that the miR-221-5p arm functions as a potent tumor suppressor in EOC. Bioinformatics and luciferase reporter assays confirmed that miR-221-5p directly targets the 3′UTRs of both RAD18 and RAD51 . In EOC clinical specimens and cell lines, miR-221-5p downregulation inversely correlated with RAD18/RAD51 expression. Functionally, miR-221-5p restoration suppressed platinum-induced PCNA mono-ubiquitination and HR, inducing a "functional BRCAness" that sensitized both established and patient-derived primary EOC cells to carboplatin and PARP inhibition. Furthermore, in xenograft models, stable miR-221-5p expression significantly reduced the tumor burden. Collectively, our results reveal a previously unrecognized miR-221-5p/ RAD18/RAD51 axis that may contribute to therapeutic responses in EOC and provide a rationale for therapeutic strategies targeting RAD18/RAD51-driven DNA repair pathways.