Mutational specificity of 2‐amino‐3‐methylimidazo‐[4,5‐f]quinoline in the hprt locus of CHO‐K1 cells

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ), a food carcinogen formed in cooked meats, can induce gene mutation at the hprt locus of CHO-K1 cells in the presence of hepatic S9 mix. To elucidate the molecular nature of IQ-induced mutation, we characterized the entire coding region of the hypoxanthine phosphoribosyltransferase gene of 23 independent mutants derived from IQ-treated CHO cells by direct sequencing of polymerase chain reaction-amplified cDNA. Ten of the 23 IQ-induced mutants examined contained single base substitutions; one mutant had three single-base substitutions. Among the base substitutions, G·C→C·G (six of 13) and A·T→C·G (three of 13) transversions predominated. Most of the base-substitution mutations occurred preferentially at a middle G and had a dA in their 3 ends. Of the 13 other mutations (56.5%), 12 missing one or more complete exons were splice-site mutations, and one mutant had a partial deletion of an exon. A high frequency of complete exon deletion (11 of 12) in exons 2.5 was observed. Interestingly, 75% of the mutants (nine of 12) with splice-site mutations were induced by IQ only at higher concentrations (300-500 μM). This was probably due to the occurrence of GC base-substitution mutations that affected hprt mRNA splicing, especially at the intron-exon boundaries.