miR-579-3p restrains tumor heterogeneity to overcome resistance to target therapies in metastatic melanoma

My PhD thesis has been directed to the study of the mechanisms responsible for the development of drug resistance in melanoma. A large subset of melanoma patients harbors activating mutations in the BRAF oncogene at position V600. These mutations sensitize tumors to inhibition by inhibitors of BRAF in combination with inhibitors of the downstream kinase MEK. This has led in the past years to approval of combo therapies composed of a BRAF and a MEK inhibitor for these patients. These combo therapies give rise to strong objective responses and provide significant improvements in overall survival. However their duration in time is strongly limited by the development of drug resistance. While initial studies focused mainly in genetic mechanisms at the basis of de novo drug resistance, in recent years several groups, including my PhD supervisor, focused their attention of non genetic mechanisms and in particular to phenotypic changes underlying drug adaptation. In this context several microRNAs have been shown to play an important role, in particular miR-579-3p which was discovered some years ago as an oncosuppressor and antagonist of drug resistance in the lab of my supervisor. In order to assess the role of this microRNA my PhD thesis has been directed initially to build and characterize a human melanoma cell line engineered to express miR-579-3p in a transcriptionally inducible manner. After the initial characterization the cell line has been subjected to a series of studies which have led to the demonstration that miR-579 is able to severely affect the development of drug resistance. A particular emphasis has been given to bulk RNA sequencing studies as well as to single cell mass cytometry which have shown that induction of expression of miR-579-3p is able to impair drug adaptive mechanism and to strongly diminish the degree of the heterogeneity in a isogenic cell population of melanoma cells when these cells are exposed to the selective pressure of BRAF and MEK inhibitors.