Mapping the Landscape of Acquired Vulnerabilities in Ovarian Cancer

Recent undertakings to identify the genetic lesions associated with ovarian cancer have noted the striking diversity of mutations occurring in this disease. This genetic diversity has complicated the search for novel therapies. However, recent data has suggested that one commonality of ovarian tumors might be ablation of miRNA biogenesis. Here I conducted a broad-scale gain-of-function microRNA (miRNA) screen in 16 ovarian cancer cell lines to annotate the functional landscape present in such a chaotic genetic background. miRNAs function as multigenic perturbations allowing for interrogation of maximal gene space with few experiments. This screen identified multiple miRNAs reducing cell viability with the majority of hits being toxic in only one or two lines screened. This surprising finding reflected the commonality of altered miRNA function in ovarian tumors while also suggesting that specifics of this alteration in function are unique to each tumor. To investigate more public vulnerabilities, I focused mechanistic studies on miRNAs displaying penetrance in greater than 5 cell lines. miR-517a reduced cell viability in over 30% of the panel and also reduced tumor burden in vivo. Functional analysis of the predicted targets of miR-517a revealed that expression of this miRNA reduced protein levels of ARCN1, a member of the coatamer complex, and that knockdown of ARCN1 reduced cell viability similar to miR-517a. Another penetrant miRNA, miR-124a, reduced cell viability in 37.5% of the panel and functional analysis of this miRNA revealed it promoted a cell differentiation program. Analysis of predicted targets revealed that expression of miR-124a reduced expression the homeodomain transcription factor SIX4, resulting in increased signaling along the tumor suppressive AMPK pathway and epithelial differentiation. Furthermore, SIX4 displayed increased expression in ovarian tumors and depletion of SIX4 expression reduced tumor cell viability in vitro and in vivo. Therefore, SIX4 overexpression might function to deflect cell differentiation in tumors. Thus, the common loss of miRNA function observed in ovarian tumors might serve to maintain an undifferentiated state, and engagement of cell fate determination programs via re-expression of miRNAs can result in catastrophic consequences for cancer cell viability.