Frequent recovery of triplet mutations in UVB-exposed skin epidermis of Xpc-knockout mice

Mutations of the Xpc gene cause a deficiency in global genome repair, a subpathway of nucleotide excision repair (NER), in mammalian cells. We used transgenic mice harboring the lambda-phage-based lacZ mutational reporter gene to study the effect of an Xpc null mutation (Xpc(-/-)) on damage induction, repair and mutagenesis in mouse skin epidermis after UVB irradiation. UVB induced equal amounts of cyclobutane pyrimidine dimers (CPDs) and pyrimidine(6-4)pyrimidone photoproducts (64PPs) in mouse skin epidermis of Xpc-/- and wild-type mice. CPDs were not significantly removed in either of the mouse genotypes by whereas removal of 64PPs was observed in the wild-type. Irradiation 12 h after irradiation, with 300 and 400J/m(2) UVB increased the lacZ mutant frequency in the Xpc(-/-) epidermis to at least twice as high as in the wild-type. Ninety-nine lacZ mutants isolated from the UVB-exposed epidermis of Xpc(-/-) mice were analyzed and compared with mutant sequences from irradiated wild-type mice. The spectra of the mutations in the two genotypes were both highly UV-specific and similar in the dominance of C -> T transitions at dipyrimidine sites; Xpc(-/-) mice had a higher frequency of two-base tandem substitutions, including however, CC -> TT mutations, three-base tandem substitutions and double base substitutions that were separated by one unchanged base in a three-base sequence (alternating mutations). These tandem/alternating mutations included a remarkably large number of triplet mutations, a recently reported, novel type of UV-specific mutation, characterized by multiple base substitutions or frameshifts within a three-nucleotide sequence containing a dipyrimidine. We concluded that the triplet mutation is a UV-specific mutation that preferably occurs in NER deficient genetic backgrounds. (c) 2006 Elsevier B.V. All rights reserved.