Inherited duplications of PPP2R3B promote naevi and melanoma via a novel C21orf91-driven proliferative phenotype

Much of the heredity of melanoma remains unexplained. Using an unbiased whole genome screening approach for copy number we identify here a novel melanoma predisposing factor, familial duplications of gene PPP2R3B, encoding a regulatory unit of critical phosphatase PP2A. Significant correlation between expression of PPP2R3B in tumour tissue and survival in a large melanoma cohort was confirmed, and demonstrated to be via a non-immunological mechanism. Mechanistically, construction and extensive characterization of a stable, inducible cellular model for PPP2R3B overexpression revealed induction of pigment cell switching towards proliferation and away from migration. Importantly, this was independent of the known microphthalmia-associated transcription factor (MITF)-controlled pigment cell phenotype switch, and was instead driven by uncharacterised gene C21orf91. Bioinformatic studies point to C21orf91 as a novel target of MITF, and therefore a missing hub in the control of phenotype switching in melanoma.