Mutational signature of the proximate bladder carcinogen N-hydroxy-4-acetylaminobiphenyl: inconsistency with the p53 mutational spectrum in bladder cancer.

We studied the mutagenicity of the proximate bladder carcinogen, N-hydroxy-4-acetylaminobiphenyl (N-OH-AABP) in embryonic fibroblasts of the Big Blue mouse. Treatment of these cells with increasing concentrations of N-OH-AABP for 24 h resulted in a dose-dependent increase in mutation frequency of the cII transgene up to 12.8-fold over the background. Single base substitutions comprised 86% of the N-OH-AABP-induced mutations and 74% of the spontaneous cII mutations (sequenced number of mutant plaques, 141 and 145, respectively). Of these, 63 and 36%, respectively, occurred at guanine residues along the cII gene. Whereas G to T transversions predominated in the induced cII mutations (47%), insertion was the most spontaneously derived cII mutation (19%). Mapping of N-OH-AABP-induced DNA adducts along the cII gene by terminal transferase-dependent PCR showed the formation of DNA adducts at specific nucleotide positions. Five preferential DNA adduction sites were established, of which four were major mutation sites for N-OH-AABP, especially for G to T transversions. This unique mutational signature of N-OH-AABP in the cII gene was, however, in sharp contrast with the mutational spectrum of the p53 gene in human bladder cancer. G to A transitions are the dominant type of p53 mutations (53%), being also prevalent in almost all of its five mutational hotspots (codons 175, 248, 273, 280, and 285). In addition, the majority of mutations in three of these hotspots (codons 175, 248, and 273) are at a methylated CpG site, whereas in the cII gene neither the preferential N-OH-AABP DNA adduction sites nor the induced mutational hotspots are biased toward methylated CpG dinucleotides. We conclude that N-OH-AABP leaves a characteristic mutational signature in the cII transgene, which is consistent with its preferential DNA adduction profile. However, the pattern of mutation induced by N-OH-AABP in the cII gene is largely at odds with the mutational spectrum of the p53 gene in human bladder cancer.