Sequence distribution of acetaldehyde-derived N²-Ethyl-dG Adducts along Duplex DNA

Acetaldehyde (AA) is the major metabolite of ethanol and may be responsible for an increased gastrointestinal cancer risk associated with alcohol beverage consumption. Furthermore, AA is one of the most abundant carcinogens in tobacco smoke and induces tumors of the respiratory tract in laboratory animals. AA binding to DNA induces Schiff base adducts at the exocyclic amino group of dG, N²-ethylidene-dG, which are reversible on the nucleoside level but can be stabilized by reduction to N²-ethyl-dG. Mutagenesis studies in the HPRT reporter gene and in the p53 tumor suppressor gene have revealed the ability of AA to induce G-A transitions and A-T transversions, as well as frameshift and splice mutations. AA-induced point mutations are most prominent at 5′-AGG-3′ trinucleotides, possibly a result of sequence specific adduct formation, mispairing, and/or repair. However, DNA sequence preferences for the formation of acetaldehyde adducts have not been previously examined. In the present work, we employed a stable isotope labeling-HPLC-ESI ⁺-MS/MS approach developed in our laboratory to analyze the distribution of acetaldehyde-derived N²-ethyl-dG adducts along double-stranded oligodeoxynucleotides representing two prominent lung cancer mutational "hotspots" and their surrounding DNA sequences. 1,7,NH ₂-¹⁵N-2-¹³C-dG was placed at defined positions within DNA duplexes derived from the K-ras protooncogene and the p53 tumor suppressor gene, followed by AA treatment and NaBH₃CN reduction to convert N²-ethylidene-dG to N²-ethyl-dG. Capillary HPLC-ESI⁺-MS/MS was used to quantify N²-ethyl-dG adducts originating from the isotopically labeled and unlabeled guanine nucleobases and to map adduct formation along DNA duplexes. We found that the formation of N²-ethyl-dG adducts was only weakly affected by the local sequence context and was slightly increased in the presence of 5-methylcytosine within CG dinucleotides. These results are in contrast with sequence-selective formation of other tobacco carcinogen-DNA adducts along K-ras- and p53-derived duplexes and the preferential modification of endogenously methylated CG dinucleotides by benzo[a]pyrene diol epoxide and acrolein.