Endogenous 5-Methylcytosine Protects Neighboring Guanines from N7 and O⁶-Methylation and O⁶-Pyridyloxobutylation by the Tobacco Carcinogen 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone

All CG dinucleotides along exons 5-8 of the p53 tumor suppressor gene contain endogenous 5-methylcytosine (^MeC). These same sites (e.g., codons 157, 158, 245, 248, and 273) are mutational hot spots in smoking-induced lung cancer. Several groups used the UvrABC endonuclease incision assay to demonstrate that methylated CG dinucleotides of the p53 gene are the preferred binding sites for the diol epoxides of bay region polycyclic aromatic hydrocarbons (PAH). In contrast, effects of endogenous cytosine methylation on the distribution of DNA lesions induced by tobacco-specific nitrosamines, e.g., 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), have not been elucidated. In the work presented here, a stable isotope labeling HPLC-ESI-MS/MS approach was employed to analyze the reactivity of the N7 and O⁶ positions of guanines within hemimethylated and fully methylated CG dinucleotides toward NNK-derived methylating and pyridyloxobutylating species. ¹⁵N ₃-labeled guanine bases were placed within synthetic DNA sequences representing endogenously methylated p53 codons 154, 157, and 248, followed by treatment with acetylated precursors to NNK diazohydroxides. HPLC-ESI-MS/MS analysis was used to determine the relative yields of N7- and O ⁶-guanine adducts at the ¹⁵N₃-labeled position. In all cases, the presence of ^MeC inhibited the formation of N7-methylguanine, O⁶-methylguanine, and O ⁶-pyridyloxobutylguanine at a neighboring G, with the greatest decrease observed in fully methylated dinucleotides and at guanines preceded by ^MeC. Furthermore, the O⁶-Me-dG/N7-Me-G molar ratios were decreased in the presence of the 5′-neighboring ^MeC, suggesting that the observed decline in O⁶-alkylguanine adduct yields is, at least partially, a result of an altered reactivity pattern in methylated CG dinucleotides. These results indicate that, unlike N ²-guanine adducts of PAH diol epoxides, NNK-induced N7- and O ⁶-alkylguanine adducts are not preferentially formed at the endogenously methylated CG sites within the p53 tumor suppressor gene.