Genome-wide maps of UVA and UVB mutagenesis in yeast reveal distinct causative lesions and mutational strand asymmetries.

Ultraviolet (UV) light primarily causes C > T substitutions in lesion-forming dipyrimidine sequences. However, many of the key driver mutations in melanoma do not fit this canonical UV signature, but are instead caused by T > A, T > C, or C > A substitutions. To what extent exposure to the UVB or UVA spectrum of sunlight can induce these noncanonical mutation classes, and the molecular mechanism involved is unclear. Here, we repeatedly exposed wild-type or repair-deficient yeast (Saccharomyces cerevisiae) to UVB or UVA light and characterized the resulting mutations by whole genome sequencing. Our data indicate that UVB induces C > T and T > C substitutions in dipyrimidines, and T > A substitutions that are often associated with thymine-adenine (TA) sequences. All of these mutation classes are induced in nucleotide excision repair-deficient cells and show transcriptional strand asymmetry, suggesting they are caused by helix-distorting UV photoproducts. In contrast, UVA exposure induces orders of magnitude fewer mutations with a distinct mutation spectrum. UVA-induced mutations are elevated in Ogg1-deficient cells, and the resulting spectrum consists almost entirely of C > A/G > T mutations, indicating they are likely derived from oxidative guanine lesions. These mutations show replication asymmetry, with elevated G > T mutations on the leading strand, suggesting there is a strand bias in the removal or bypass of guanine lesions during replication. Finally, we develop a mutation reporter to show that UVA induces a G > T reversion mutation in yeast that mimics the oncogenic NRAS Q61K mutation in melanoma. Taken together, these findings indicate that UVA and UVB exposure can induce many of the noncanonical mutation classes that cause driver mutations in melanoma.

Contributions of replicative and translesion DNA polymerases to mutagenic bypass of canonical and atypical UV photoproducts.

UV exposure induces a mutation signature of C > T substitutions at dipyrimidines in skin cancers. We recently identified additional UV-induced AC > TT and A > T substitutions that could respectively cause BRAF V600K and V600E oncogenic mutations. The mutagenic bypass mechanism past these atypical lesions, however, is unknown. Here, we whole genome sequenced UV-irradiated yeast and used reversion reporters to delineate the roles of replicative and translesion DNA polymerases in mutagenic bypass of UV-lesions. Our data indicates that yeast DNA polymerase eta (pol η) has varied impact on UV-induced mutations: protecting against C > T substitutions, promoting T > C and AC > TT substitutions, and not impacting A > T substitutions. Surprisingly, deletion rad30Δ increased novel UV-induced C > A substitutions at CA dinucleotides. In contrast, DNA polymerases zeta (pol ζ) and epsilon (pol ε) participated in AC > TT and A > T mutations. These results uncover lesion-specific accurate and mutagenic bypass of UV lesions, which likely contribute to key driver mutations in melanoma.