title: Mutation spectrum in UVB-exposed skin epidermis of Xpa-knockout mice: frequent recovery of triplet mutations.
abstract: Knockout mutations in both alleles of the Xpa gene give rise to a complete deficiency in 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 Xpa null mutation (Xpa(-/-)) 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 Xpa(-/-) and wild-type mice. Neither photolesion was removed in the Xpa(-/-) epidermis by 12 hr after irradiation whereas removal of 64PPs was observed in the epidermis of wild-type mice. Irradiation with 200 and 300 J/m(2) UVB increased the lacZ mutant frequency in the epidermis of Xpa(-/-) mice, but the induced mutant frequencies were not significantly different from those previously determined for wild-type mice. One-hundred lacZ mutants isolated from the UVB-exposed epidermis of Xpa(-/-) mice were analyzed and compared with mutant sequences previously determined for irradiated wild-type mice. The distribution of the mutations along the lacZ transgene and the preferred dipyrimidine context of the UV-specific mutations were similar in mutants from the Xpa(-/-) and wild-type mice. The spectra of the mutations in the two genotypes were both highly UV-specific and similar in a dominance of C --> T transitions at dipyrimidine sites; however, Xpa(-/-) mice had a higher frequency than wild-type mice of two-base tandem substitutions, including CC --> TT mutations, three-base tandem mutations 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 conclude that the triplet mutation is a UV-specific mutation that preferably occurs in NER-deficient genetic backgrounds.
authors.affiliation: Department of Cell Biology, Graduate School of Medicine, Tohoku University, Seiryo-machi, Sendai 980-8575, Japan. ikehata@mail.tains.tohoku.ac.jp
authors.forename: Hironobu
authors.initials: H
authors.lastname: Ikehata
authors.forename: Fumitaka
authors.initials: F
authors.lastname: Yanase
authors.forename: Toshio
authors.initials: T
authors.lastname: Mori
authors.forename: Osamu
authors.initials: O
authors.lastname: Nikaido
authors.forename: Kiyoji
authors.initials: K
authors.lastname: Tanaka
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Ono
mesh_terms: D000818:Animals; D001483:Base Sequence; D004249:DNA Damage; D004260:DNA Repair; D004817:Epidermis / Q000378:metabolism / Q000528:radiation effects*; D007763:Lac Operon / Q000235:genetics; D051379:Mice; D018345:Mice, Knockout; D009154:Mutation / Q000528:radiation effects*; D011740:Pyrimidine Dimers / Q000235:genetics; D017422:Sequence Analysis, DNA; D012867:Skin / Q000378:metabolism / Q000528:radiation effects*; D014466:Ultraviolet Rays; D051760:Xeroderma Pigmentosum Group A Protein / Q000235:genetics*
chemical_list: D011740:Pyrimidine Dimers; D051760:Xeroderma Pigmentosum Group A Protein; C497315:Xpa protein, mouse