UVA induces C→T transitions at methyl-CpG-associated dipyrimidine sites in mouse skin epidermis more frequently than UVB

We studied the kinetics of mutation induction in skin epidermis and dermis of UVA-irradiated transgenic Muta™ mice and analyzed the sequence changes in 80 lacZ transgene mutants from the irradiated epidermis. The mutant frequency increased linearly in both the epidermis and dermis up to 240 kJ/m² UVA, twice as efficiently in the epidermis as in the dermis, without provoking any inflammatory reactions in the exposed skin. The 83 mutations detected in the UVA-exposed epidermis were dominated by C→T transitions (88%), found almost exclusively at dipyrimidine sites, and specified by four occurrences of CC→TT tandem substitutions, suggesting that UV-specific photoproducts induced in DNA have a major role in the genotoxicity. No T→G transversions, which have been considered as a UVA signature mutation, and few mutations suggesting the relevance of oxidative damage were recovered in the present study. An analysis of the bases adjacent to the mutated cytosines revealed that the 3′-cytosine of dipyrimidine sites is the preferred target of UVA-induced C→T transition. Moreover, C→T transitions were induced at dipyrimidine sites associated with CpG much more frequently by UVA than by UVB, forming hotspots at several of these sites. These results suggest that UVA contributes more to the formation of recurrent or hotspot mutations at methylated CpG sites in the mammalian genome than UVB, since methylation of the CpG motif is observed entirely in the lacZ transgenes and is known to enhance the formation of cyclobutane pyrimidine dimers by longer wavelength UV.