Mutation specificity of 8-methoxypsoralen plus two doses of uva irradiation in the hprt gene in diploid human fibroblasts

To investigate which specific kinds of base changes are induced by psoralen adducts in the genomic DNA of diploid human fibroblasts, cells were exposed to 8-methoxypsoralen (8-MOP) at 2 -12 μM followed by one dose of UVA (365 nm) irradiation (PUVA-I treatment) or two doses of UVA (PUVA-II treatment). While PUVA-I treatment produced little effect on the induction of cytotoxicity, PUVA-II treatment significantly reduced the fibroblasts' colony-forming ability and resulted in about 10-fold increases in mutation frequency at the Do dose. Mutations in the hypoxanthine (guanine) phosphoribosyltransferase (hprt) gene of 36 independent PUVA-II mutants were characterized by direct sequencing of cDNA amplified by the polymerase chain reaction (PCR). Seventeen mutants contained single base substitutions and the other 19 mutants either lacked one or more exons, or had deleted or gained nucleotides in the exon boundaries in their cDNA. The intron-exon boundaries of 10 of these 19 putative splicing mutants were further characterized by direct sequencing of the PCR-amplified hprt gene. The results showed that nine contained single base substitutions at the consensus splicing donor and acceptor sites. One splicing mutant possessed two base substitutions located at exon 8, whereas its splicing sites were intact. Most of the base substitutions occurred at TA base pairs (24/29). The majority of TA changes occurred at thymine of 5'TA and 5'ATA on the non-transcribed strand. Four of the five GC base substitutions were located at guanines of 5'TG sites adjacent 3' to AT or TA sequences. In addition, the occurrence of a specific type of mutation was highly correlated to the 5' flanking bases of TA sites. The mutagenesis of 13 of the 16 mutational events at 5'TA sites on the non-transcribed strand can be explained by the preferential incisions of the photoadducts on the transcribed strand followed by misalignment realignment during translesion repair synthesis of the bulky lesions on the non-transcribed strand.