Involvement of 5-methylcytosine in sunlight-induced mutagenesis

In human skin cancers, more than 30 % of all mutations in the p53 gene are transitions at dipyrimidines within the sequence context CpG, i.e. 5'-TCG and 5'-CCG, found at several mutational hotspots. Since CpGs are methylated along the p53 gene, these mutations may be derived from solar UV-induced pyrimidine dimers forming at sequences that contain 5-methylcytosine. In Xorder to define the contribution of 5-methylcytosine to sunlight-induced mutations, we have used mouse fibroblasts containing the CpG-methylated lacI transgene as a mutational target. We sequenced 182 UVC (254 nm UV)-induced mutations and 170 mutations induced by a solar UV simulator, along with 75 mutations in untreated cells. Only a few of the mutations in untreated cells were transitions at dipyrimidines, but more than 95% of the UVC and solar irradiation-induced mutations were targeted to dipyrimidine sites, the majority being transitions. After UVC irradiation, 6% of the base substitutions were at dipyrimidines containing 5-methylcytosine and only 2.2% of all mutations were transitions within this sequence context. However, 24% of the solar light-induced mutations were at dipyrimidines that contain 5-methylcytosine and most of them were transitions. Two sunlight-induced mutational hotspots at methylated CpGs correlated with sequences that form the highest levels of cyclobutane pyrimidine dimers after irradiation with sunlight but not with UVC. The data indicate that dipyrimidines that contain 5-methylcytosine are preferential targets for sunlight-induced mutagenesis in cultured mammalian cells, thus explaining the large proportion of p53 mutations at such sites in skin tumors in vivo.