| Summary: | Although the incidence is quite low, epithelial ovarian cancer (EOC) is the most lethal gynaecologic malignancy. The unfavourable prognosis and the high mortality rate associated with EOC are mainly owed to late diagnosis, frequent relapse and development of chemoresistance. Indeed, most of the patients who achieve a complete response to first-line platinum-based treatment eventually relapse often developing platinum-resistant disease. Due to their master regulatory role, miRNAs are considered powerful tools to obtain representative molecular portraits of specific tumour characteristics and behaviours. My laboratory performed microRNA expression profiles on advanced stage EOC patients and a cluster of miRNAs, including miR-506, located on ChrXq27.3 was identified as down-regulated in EOC early relapsing patients. Since I observed that expression of miR-506 was associated with EOC patients’ sensitivity to platinum treatment, the overall aim of this thesis was to better characterize the role of this miRNA in regulating response to chemotherapy. Among the miR-506 predicted targets, I identified several genes involved in DNA damage repair (DDR) pathway, like RAD51, RAD17, CHEK1 and WEE1 and I concentrated on genes not previously studied in EOC. I validated RAD17 as a direct target of miR-506 and identified the miR-506-RAD17 axis as relevant in chemosensitising EOC cells to different treatments. I demonstrated that miR-506 expression, by targeting RAD17, was able to mediate sensitisation to platinum treatment and accordingly RAD17 silencing exerted the same effect. MiR-506 expression in EOC cells led to a reduced ability to properly sense DNA damage following platinum treatment causing mitotic defects, micronuclei formation, and impairing the signalling cascade responsible for G2/M checkpoint activation upon DNA damage insults. This behaviour, recapitulating a BRCAness phenotype, would allow propagation of cells with unrepaired DNA damage with the subsequent sensitisation to DNA damaging drugs. Furthermore, miR-506 expression, by regulating RAD17, impairs ATM signalling pathway, sensitising EOC cells to PARP inhibitor olaparib. Acting in the same way, miR-506 expression was synthetic lethal with Chk1 and Wee1 checkpoint kinases inhibitors in agreement with recent data reporting RAD17 depletion to be synthetically lethal with these small molecules. Accordingly, RAD17 down-modulation phenocopied the effect of miR-506 expression. Also combination treatments of Checkpoint kinases inhibitors with platinum resulted to be synergistic. Together the findings presented in this thesis support miR-506 as a key node in regulating DDR pathway in response to drug treatments and provide the rationale for its use to select EOC patients with BRCAness phenotype for efficient personalized therapeutic treatments.
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