miR-876-3p is a tumor suppressor on 9p21 that is inactivated in melanoma and targets ERK.

Background: While melanomas commonly harbor losses of 9p21, on which CDKN2A resides, the presence of additional tumor suppressor elements at this locus is incompletely characterized. Here we assess the expression levels and functional role of microRNA-876-3p (miR-876), whose gene also maps to 9p21.

Methods: Expression of miR-876 was assessed in human tissues and cell lines using quantitative miRNA reverse transcriptase polymerase chain reaction (qRT-PCR). MIR876 copy number was determined in The Cancer Genome Atlas (TCGA) melanoma cohort. The consequences of regulation of miR-876 expression were assessed on melanoma cell colony formation, migration, invasion, apoptosis, cell cycle progression, and drug sensitivity in culture, and on in vivo tumor growth in a xenograft model. Genome-wide transcriptomic changes induced by miR-876 overexpression were determined using RNA sequencing (RNA-Seq).

Results: miR-876 expression was significantly decreased in primary melanoma samples when compared with nevi, and in human melanoma cell lines when compared with human melanocytes. Analysis of the TCGA cohort revealed deletions in MIR876 in > 50% of melanomas. miR-876 overexpression resulted in decreased melanoma cell colony formation, migration, and invasion, which was accompanied by cell cycle arrest and increased apoptosis. Intra-tumoral injections of miR-876 significantly suppressed melanoma growth in vivo. RNA-Seq analysis of miR-876-treated tumors revealed downregulation of several growth-promoting genes, along with upregulation of tumor suppressor genes, which was confirmed by qRT-PCR analysis. Computational analyses identified MAPK1 (or ERK2) as a possible target of miR-876 action. Overexpression of miR-876 significantly suppressed luciferase expression driven by the MAPK1/ERK2 3' UTR, and resulted in decreased ERK protein expression in melanoma cells. MAPK1/ERK2 cDNA overexpression rescued the effects of miR-876 on melanoma colony formation. miR-876 overexpression sensitized melanoma cells to treatment with the BRAF inhibitor vemurafenib.

Conclusions: These studies identify miR-876 as a distinct tumor suppressor on 9p21 that is inactivated in melanoma and suggest miR-876 loss as an additional mechanism to activate ERK and the mitogen activated protein kinase (MAPK) pathway in melanoma. In addition, they suggest the therapeutic potential of combining miR-876 overexpression with BRAF inhibition as a rational therapeutic strategy for melanoma.