Augmentation of Response to Chemotherapy by microRNA-506 Through Regulation of RAD51 in Serous Ovarian Cancers

Epithelial ovarian cancer remains the most lethal gynecological malignancy (1). The current standard of care consists of radical surgery and platinum-based chemotherapy. The five-year survival rate for patients with advanced ovarian cancer is only 30% to 40%, and acquired resistance to platinum is considered a major factor in disease relapse. Platinum-based drugs form intra- and interstrand adducts with DNA, which causes DNA double-strand breaks and triggers DNA damage and repair pathways. Homologous recombination is a critical pathway for DNA double-strand break repair (2) and is responsible for the resistance of high-grade serous ovarian cancer to frontline platinum-based chemotherapy (3). Cells with compromised homologous recombination machinery are highly sensitive to apoptosis triggered by platinum-induced DNA damage through a mechanism termed synthetic lethality (4). Thus, the ability to block homologous recombination-mediated repair is a focus of intense investigation as an approach to improve treatment outcomes in high-grade serous ovarian cancers. Recent studies demonstrated that BRCA2 mutations, and to a lesser extent BRCA1 mutations/methylation, are associated with improved survival and response to therapy in serous ovarian cancer (5,6). Whereas BRCA1 plays diverse roles in DNA damage pathways, the primary role of BRCA2 is to mediate homologous recombination by directly loading the RAD51 protein onto damage sites or stalled replication forks (7,8). RAD51 is a critical component of the homologous recombination-mediated double-strand DNA break repair machinery and assembles onto single-stranded DNA as a nucleoprotein filament and catalyzes the exchange of homologous DNA sequences (9). RAD51 suppression can sensitize cancer cells to DNA-damaging drugs (10–14), and RAD51 overexpression contributes to chemotherapy resistance in human soft tissue sarcoma cells (15). MicroRNAs (miRNAs) are a class of small noncoding RNAs (~22 nt) that regulate gene expression. MiRNAs bind to the 3′-untranslated region (3′-UTR) of target genes, which either leads to mRNA degradation or inhibits protein translation (16). Nearly 2578 miRNAs have been identified in the human genome and are thought to regulate 30% of the transcriptome (17). Increasing evidence has demonstrated that miRNA are highly deregulated in cancer, suggesting they may function as therapeutic tools (17–20). In a recent high-throughput miRNA signature screen, decreased expression of the chrXq27.3-miRNA cluster that included miR-506 was associated with early relapse in patients with advanced-stage epithelial ovarian cancer (21). Our studies established that miR-506 is a potent inhibitor of the epithelial-to-mesenchymal transition (EMT) (22,23), which is also associated with chemoresistance. In addition, we found that miR-506 could suppress proliferation and induce senescence by directly targeting the CDK4/6-FOXM1 axis in ovarian cancer (24). However, it is unknown whether miR-506 is involved in the chemotherapy response.