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

Abstract

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 MitfE318K knock-in (KI) mouse model to assess the role of MitfE318K (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 BRAFV600E. 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 MitfE318K mice were slightly hypopigmented (mean melanin content MitfWT vs MitfE318K/+, 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 MitfE318K enhances BRafV600E-induced nevus formation in vivo (mean nevus number for MitfE318K, BRafV600E vs MitfWT, BRafV600E, 68 vs 44, difference=24, 95% CI=9.1 to 38.9, P=.006). Importantly, although MitfE318K was not sufficient to cooperate with BRafV600E alone in promoting metastatic melanoma, it accelerated tumor formation on a BRafV600E, Pten-deficient background (median survival, MitfE318K/+=42 days, 95% CI=31 to 46 vs MitfWT=51 days, 95% CI=50 to 55, P