MicroRNA-103 and 107 target RAD51 and RAD51D to regulate homologous recombination and enhance cellular sensitivity to DNA damaging agents
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Homologous recombination mediates the error-free repair of DNA double-strand breaks. RAD51 is a protein that is essential for homologous recombination and its recruitment to DNA double-strand breaks is mediated by several factors including the products of the breast and ovarian cancer susceptibility genes, such as BRCA1 and BRCA2, and the RAD51 paralogs, such as RAD51D. Deregulation of these factors leads to genomic instability and cellular sensitivity to ionizing radiation, DNA interstrand cross-linking agents, poly(ADP-ribose) polymerase (PARP) inhibitors and other DNA damaging agents. The regulation of homologous recombination by microRNAs, which are non-coding RNAs that post-transcriptionally downregulate gene expression, is not well characterized. To systematically identify microRNAs with the potential to regulate homologous recombination, we performed overexpression screens of human microRNA libraries using the efficiency of ionizing radiation-induced RAD51 foci formation as a readout. Three microRNAs (miR-103, miR-107 and miR-221) consistently reduced ionizing radiation-induced RAD51 foci formation in screens performed in both HeLa and U2OS cells. Accordingly, these microRNAs potently impaired homologous recombination repair efficiency. Furthermore, miR-103 and miR-107 enhanced cellular sensitivity to various DNA damaging agents, including cisplatin and a PARP inhibitor (AZD2281/olaparib), <italic>in vitro</italic> and <italic>in vivo</italic>. Importantly, we identified RAD51 and RAD51D as direct targets of miR-103 and miR-107 and demonstrated that their repression was critical for miR-103/107-mediated inhibition of RAD51 foci formation as well as chemosensitivity to DNA damaging agents. Altogether, we identified novel microRNA players in the regulation of homologous recombination and chemoresistance to DNA damaging agents. MiR-103, miR-107 and other hits from our screens provide a more complete picture of the regulatory crosstalk that exists between microRNA and DNA repair pathways to maintain genomic stability. Furthermore, these microRNAs may potentially be useful as prognostic indicators of chemotherapy response or as therapeutic agents for the chemosensitization of tumors.