title: Remarkable induction of UV-signature mutations at the 3'-cytosine of dipyrimidine sites except at 5'-TCG-3' in the UVB-exposed skin epidermis of xeroderma pigmentosum variant model mice.
abstract: The human POLH gene is responsible for the variant form of xeroderma pigmentosum (XP-V), a genetic disease highly susceptible to cancer on sun-exposed skin areas, and encodes DNA polymerase η (polη), which is specialized for translesion DNA synthesis (TLS) of UV-induced DNA photolesions. We constructed polη-deficient mice transgenic with lacZ mutational reporter genes to study the effect of Polh null mutation (Polh(-/-)) on mutagenesis in the skin after UVB irradiation. UVB induced lacZ mutations with remarkably higher frequency in the Polh(-/-) epidermis and dermis than in the wild-type (Polh(+/+)) and heterozygote. DNA sequences of a hundred lacZ mutants isolated from the epidermis of four UVB-exposed Polh(-/-) mice were determined and compared with mutant sequences from irradiated Polh(+)(/)(+) mice. The spectra of the mutations in the two genotypes were both highly UV-specific and dominated by C→T transitions at dipyrimidines, namely UV-signature mutations. However, sequence preferences of the occurrence of UV-signature mutations were quite different between the two genotypes: the mutations occurred at a higher frequency preferentially at the 5'-TCG-3' sequence context than at the other dipyrimidine contexts in the Polh(+/+) epidermis, whereas the mutations were induced remarkably and exclusively at the 3'-cytosine of almost all dipyrimidine contexts with no preference for 5'-TCG-3' in the Polh(-/-) epidermis. In addition, in Polh(-/-) mice, a small but remarkable fraction of G→T transversions was also observed exclusively at the 3'-cytosine of dipyrimidine sites, strongly suggesting that these transversions resulted not from oxidative damage but from UV photolesions. These results would reflect the characteristics of the error-prone TLS functioning in the bypass of UV photolesions in the absence of polη, which would be mediated by mechanisms based on the two-step model of TLS. On the other hand, the deamination model would explain well the mutation spectrum in the Polh(+/+) genotype.
authors.affiliation: Department of Cell Biology, Tohoku University Graduate School of Medicine, Sendai 980-8575, Japan; Department of Physiological Sciences, Tohoku University Graduate School of Medicine, Sendai 980-8575, Japan. Electronic address: ikehata@med.tohoku.ac.jp.
authors.forename: Hironobu
authors.initials: H
authors.lastname: Ikehata
authors.affiliation: Department of Cell Biology, Tohoku University Graduate School of Medicine, Sendai 980-8575, Japan.
authors.forename: Yumin
authors.initials: Y
authors.lastname: Chang
authors.affiliation: Department of Life Science, Faculty of Science, Gakushuin University, Tokyo 171-8588, Japan.
authors.forename: Masayuki
authors.initials: M
authors.lastname: Yokoi
authors.affiliation: Department of Physiological Sciences, Tohoku University Graduate School of Medicine, Sendai 980-8575, Japan.
authors.forename: Masayuki
authors.initials: M
authors.lastname: Yamamoto
authors.affiliation: Department of Life Science, Faculty of Science, Gakushuin University, Tokyo 171-8588, Japan.
authors.forename: Fumio
authors.initials: F
authors.lastname: Hanaoka
mesh_terms: D000818:Animals; D004259:DNA-Directed DNA Polymerase / Q000235:genetics*; D004817:Epidermis / Q000528:radiation effects*; D051379:Mice; D059372:Nucleotide Motifs; D017354:Point Mutation; D014466:Ultraviolet Rays; D000099270:Y-Family DNA Polymerases
chemical_list: D004259:DNA-Directed DNA Polymerase; C112338:Rad30 protein; D000099270:Y-Family DNA Polymerases
keywords: DNA polymerase η; Deamination model; Translesion DNA synthesis; Two-step model; UVB-induced mutation; Xeroderma pigmentosum variant