5-methylcytosine in CpG sites and the reactivity of nearest neighboring guanines toward the carcinogen aflatoxin B-1-8,9-epoxide

The reactivity of guanines in an oligonueleotide containing mutational hot spots within the p53 gene (codons 248 and 249), 5'-CCG(1)G(2)AG(4)CCCA-3', toward dimethyl sulfate (DMS) and aflatoxin B-1-8,9-epoxide (AFB(1)-8,9-epo xide) was investigated by a modified Maxam-Gilbert technique. 5-Methylcytos ine in the CpG; site of codon 248 did not appear to modulate the reactivity of target guanines G(1), G(2), G(3), and G(4) toward either genotoxin when compared to the sequence containing a nonmethylated CpG site. A similar ex periment was conducted in which a 0.5-kb fragment of the human HPRT gene co ntaining exon 1 and several CpG; sites was treated with UV-activated aflato xin B-1. Results showed that guanine adduct formation was independent of th e methylation status of the CpG site. These findings are discussed in relat ion to other studies that have shown that cytosine methylation has an inhib iting effect, an enhancing effect, or no effect on adduct formation with ne arby guanine nucleotides. (C) 1999 Academic Press.