Deciphering the Role of Oncogenic MITFE318K in Senescence Delay and Melanoma Progression

Background: MITF encodes an oncogenic lineage-specific transcription factor in which a germline mutation (MITFE318K) was identified in human patients predisposed to both nevus formation and, among other tumor types, melanoma. The molecular mechanisms underlying the oncogenic activity of MITFE318K remained uncharacterized. Methods: Here, we compared the SUMOylation status of endogenous MITF by proximity ligation assay in melanocytes isolated from wild-type (n = 3) or E318K (n = 4) MITF donors. We also used a newly generated Mitf(E318K) knock-in (KI) mouse model to assess the role of Mitf(E318K) (n = 7 to 13 mice per group) in tumor development in vivo and performed transcriptomic analysis of the tumors to identify the molecular mechanisms. Finally, using immortalized or normal melanocytes (wild-type or E318K MITF, n = 2 per group), we assessed the role of MITFE318K on the induction of senescence mediated by BRAF(V600E). All statistical tests were two-sided. Results: We demonstrated a decrease in endogenous MITF SUMOylation in melanocytes from MITFE318K patients (mean of cells with hypoSUMOylated MITF, MITFE318K vs MITFWT, 94% vs 44%, difference = 50%, 95% CI = 21.8% to 67.2%, P = .004). The Mitf(E318K) mice were slightly hypopigmented (mean melanin content Mitf(WT) vs Mitf(E318K/+), 0.54 arbitrary units [AU] vs 0.36 AU, difference = -0.18, 95% CI = -0.36 to -0.007, P = .04). We provided genetic evidence that Mitf(E318K) enhances BRaf(V600E)-induced nevus formation in vivo (mean nevus number for Mitf(E318K), BRaf(V600E) vs Mitf(WT), BRaf(V600E), 68 vs 44, difference = 24, 95% CI = 9.1 to 38.9, P = .006). Importantly, although Mitf(E318K) was not sufficient to cooperate with BRaf(V600E) alone in promoting metastatic melanoma, it accelerated tumor formation on a BRaf(V600E), Pten-deficient background (median survival, Mitf(E318K/+) = 42 days, 95% CI = 31 to 46 vs Mitf(WT) = 51 days, 95% CI = 50 to 55, P < .001). Transcriptome analysis suggested a decrease in senescence in tumors from Mitf(E318K) mice. We confirmed this hypothesis by in vitro experiments, demonstrating that Mitf(E318K) impaired the ability of human melanocytes to undergo BRAF(V600E)-induced senescence. Conclusions: We characterized the functions of melanoma-associated MITFE318K mutations. Our results demonstrate that MITFE318K reduces the program of senescence to potentially favor melanoma progression in vivo.