Exploring miR-29 in melanoma progression and prevention

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PROJECT SUMMARY The miR-29 family of microRNAs (miRNAs), consisting of miR-29a, miR-29b1, miR-29b2, and miR-29c, is encoded by two miRNA clusters (miR-29a~b1 and miR-29b2~c) and has tumor suppressive properties in a variety of human cancers. We found that, paradoxically, oncogenic BRAFV600E induces the transcription of both clusters in melanocytes through either MAPK pathway activation (miR-29a~b1) or oncogenic stress-mediated p53 activation (miR-29b2~c). This creates a barrier that prevents melanoma development, and we showed in a genetically engineered mouse model that inactivation of the miR-29 family with a sponge construct promotes melanoma progression. Interestingly, engineered human cell line models and human expression data from nevi and melanomas show that melanoma progression is associated with decreased expression of miR-29b2~c. Based on these findings we hypothesize that de-repression and overexpression of critical miR-29 targets promotes melanoma development and that such targets are vulnerabilities of melanoma that can be exploited for therapy. We have established a high-throughput genetically engineered mouse modeling platform to evaluate the role of miR-29 targets in melanoma and to assess their potential as therapeutic targets. In this proposal, we will extensively use this platform to first test if restoration of miR-29 expression and, thus, repression of all miR- 29 targets impairs melanoma development. We will further evaluate if silencing of a single miR-29 target elicits significant effects on melanomagenesis. We have identified the oncogenic transcription factor MYBL2 as a bona fide miR-29 target whose silencing reduces the aggressiveness of melanoma cell lines in vitro. We will test if de- repressed MYBL2 mediates the effects of miR-29 loss by silencing MYBL2 in a melanoma mouse model driven by miR-29 inactivation. Moreover, we will silence MYBL2 in a model with normal miR-29 activity to determine if MYBL2 is a general vulnerability of melanoma. To ensure relevance to human melanoma, we will complement these genetically engineered mouse models with experiments in human melanoma PDXs. Our proposed studies will use our newly developed mouse modeling platform as a pipeline to systematically evaluate if miR-29 targets in general, and MYBL2 specifically, have the potential to be further explored as therapeutic targets in melanoma.