UVA and UVB elicit distinct mutational signatures in melanoma

BRAF-mutant melanomas are more likely than NRAS-mutant melanomas to arise in anatomical locations protected from chronic sun damage. We hypothesized that this discrepancy in tumor location is a consequence of the differential sensitivity of BRAF and NRAS-mutant melanocytes to ultraviolet light (UV)-mediated carcinogenesis. We tested this hypothesis by comparing the mutagenic consequences of a single, narrow band ultraviolet-A (UVA; 320-400nm) or ultraviolet-B (UVB; 280-320nm) exposure in mouse models predisposed to Braf- or Nras-mutant melanoma. Exposures approximated the amount of UVA or UVB energy contained in ~40 minutes of summer sunlight. Tumor onset was accelerated in all UVB-, but only half of UVA- irradiated mice as compared to unirradiated controls. Melanomas from both mouse models, harbored recurrent mutations affecting the RING domain of MAP3K1 and Actin-binding domain of Filamin A irrespective of UV status. Melanomas from UVB-irradiated, Braf-mutant mice averaged twice as many SNVs (1,025 vs. 435) and five times as many dipyrimidine variants (33.3 vs. 5.7) than tumors from similarly irradiated Nras-mutant mice. We identified a mutational signature enriched in UVB-accelerated tumors which mirrored COSMIC signatures associated with human skin cancer. Notably, this signature was enriched to a greater extent in Braf- than Nras-mutant murine melanomas. These data suggest that oncogenic BRAF may enhance UVB carcinogenesis to promote melanoma formation at anatomic sites with low or intermittent sun exposure.