title: Frequent recovery of triplet mutations in UVB-exposed skin epidermis of Xpc-knockout mice.
abstract: 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 12h after irradiation, whereas removal of 64PPs was observed in the wild-type. Irradiation with 300 and 400J/m2 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; however, Xpc-/- mice had a higher frequency of two-base tandem substitutions, including 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.
authors.affiliation: Department of Cell Biology, Graduate School of Medicine, Tohoku University, Sendai 980-8575, Japan. ikehata@mail.tains.tohoku.ac.jp
authors.forename: Hironobu
authors.initials: H
authors.lastname: Ikehata
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Saito
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: Tetsuya
authors.initials: T
authors.lastname: Ono
mesh_terms: D000818:Animals; D004260:DNA Repair; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D004817:Epidermis / Q000378:metabolism / Q000528:radiation effects*; D051379:Mice; D018345:Mice, Knockout; D008822:Mice, Transgenic; D023421:Models, Animal; D009154:Mutation; D012867:Skin / Q000378:metabolism / Q000528:radiation effects*; D014466:Ultraviolet Rays
chemical_list: D004268:DNA-Binding Proteins; C501002:Xpc protein, mouse