RIPOR2/FAM65b promotes multinucleation of melanoma cells downstream of the RAS/ERK oncogenic pathway
One-third of skin melanoma arise from melanocytic nevi, benign skin lesions composed of clustered melanocytes. Benign nevi are associated with overactivation of mitogen-activated protein kinase RAS/ERK pathway, a consequence of driver mutations, most often in the BRAF or NRAS genes. However, this overactivation in melanocytes is not sufficient to induce melanoma formation, as only a minority of benign nevi give rise to melanoma. Overactivation of the RAS/ERK pathway promotes the appearance of genetic and epigenetic alterations by inducing aneuploidy, but the process of evolution of the nevus towards melanoma, downstream of RAS/ERK pathway that induces aneuploidy is only partially understood. Using single-nucleus RNA sequencing analysis after overactivation of the RAS/ERK pathway in the chick embryo, we discovered that RIPOR2/FAM65B is a positive transcriptional target of RAS/ERK overactivation, including in embryonic melanocyte precursors. Similar transcriptional control of RIPOR2 by RAS/ERK is conserved in human melanoma cells. RIPOR2 emerged as an attractive target because it codes for an atypical RHOA inhibitory protein, involved in the development of physiologically multinucleated cell types. Multinucleation in cancer has been shown to promote aneuploidy, which correlates with lesion aggressiveness. We discovered that RIPOR2 is ectopically expressed in human nevi and melanoma and functionally promotes multinucleation, both in an animal model and in cultured cells from human tumors, including melanoma cell lines. Our results suggest that RIPOR2/FAM65B expression downstream of RAS/ERK overactivation in the skin promotes the emergence of multinucleated cells, an overlooked step in melanoma formation.