Mutagenesis of the N-(Deoxyguanosin-8-yl)-2-amino-1-methyl-6- phenylimidazo[4,5-b]pyridine DNA adduct in mammalian cells. Sequence context effects

Site-specifically modified oligodeoxynucleotides were used to investigate the mutagenic properties of a major cooked food mutagen-derived DNA adduct, N-(deoxyguanosin-8-yl)-2-amino-1-methyl-6-phenylimidazo[4,5- b]pyridine (dG-C8-PhIP). dG-C8-PhIP-modified oligodeoxynucleotides were prepared by reacting an oligodeoxynucleotide containing a single dG (5'- TCCTC-CTXGCCTCTC, where X = C, A, G, or T) with N-acetoxy-PhIP. The unmodified and dG-C8-PhIP-modified oligomers were inserted into single- stranded phagemid vectors. These single-stranded vectors were transfected into simian kidney (COS-7) cells. The progeny plasmid obtained was used to transform Escherichia coli DH10B. When dC was at the 5'-flanking position to dG-C8-PhIP, preferential incorporation of dCMP, the correct base, was observed opposite the dG-C8-PhIP. Targeted G → T transversions were detected, along with lesser amounts of G → A transitions and G → C transversions. No mutations were detected for the unmodified vector. The influence of sequence context on the dG-C8-PhIP mutation frequency and spectrum was also explored. When the dC 5'-flanking base was replaced by dT, dA, or dG, the mutational spectra were similar to that observed with dC- flanking base. Higher mutational frequencies (28-30%) were observed when dC or dG was 5' to dG-C8-PhIP. A lower mutational frequency (13%) was observed when dA was at the 5' to the lesion. Single-base deletions were detected only then dG or dT flanked the adduct. We conclude that dG-C8-PhIP is mutagenic, generating primarily G → T transversions in mammalian cells. The mutational frequency and specificity of dG-C8-PhIP vary depending on the neighboring sequence context.