Mutation spectrum in sunlight-exposed mouse skin epidermis: small but appreciable contribution of oxidative stress-mediated mutagenesis

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We studied the mutations induced in skin by sunlight using transgenic Muta™ mice. Noon sunlight during summer at Sendai, Japan induced mutations efficiently in both epidermis and dermis. The mutant frequency (MF) in epidermis reached nearly 0.5% during the first 40 min irradiation but became saturated at this level with the appearance of skin inflammation after further irradiation. At the equivalent inflammatory dose, sunlight was twice as genotoxic as 313 nm-peak UVB. The 81 mutations detected in 80 lacZ transgene mutants isolated from the sunlight-exposed epidermis were dominated by C → T transitions (89%), occurring exclusively at dipyrimidine sites, and also included a CC → TT tandem substitution. Thus, the sunlight-induced mutation spectrum is highly UV-specific, quite similar to that induced by UVB but significantly different from that induced by UVA. Although oxidative damage-related C → A transversions were detected only in five mutants (6%), their frequency was elevated to at least 15 times the background level, suggesting that the contribution of UVA-mediated oxidative stress is comparatively small but considerable. An analysis of bases adjacent to the mutated cytosines revealed that the sunlight-induced mutations prefer 5′-TC-3′ dipyrimidine sites to 5′-CC-3′ and 5′-CT-3′. The distribution of the frequent C → T transition sites in the transgene was well associated with the CpG motif, which is known to be completely methylated in the gene, and quite similar to that induced by UVB rather than that by UVA. Thus, the UVB component contributes to the sunlight-induced mutations in the mammalian skin much more than the UVA component, whose influence through reactive oxygen species (ROS)-mediated mutagenesis is still appreciable.