Involvement of 5-methylcytosine in sunlight-induced mutagenesis

In human skin cancers, more than 30% of all mutations in the p53 gene are t ransitions at dipyrimidines within the sequence context CpG, i.e. 5'-TCG an d 5'-CCG, found-at several mutational hotspots. Since CpGs are methylated a long the p53 gene,, these mutations may be derived from solar UV-induced py rimidine dimers forming at sequences that contain 5-methylcytosine. in Xord er(to define the contribution of 5-methylcytosine to sunlight-induced mutat ions, we have used mouse fibroblasts containing the CpG-methylated lacI tra nsgene as a mutational target. We sequenced 182 UVC (254 nm UV)-induced mut ations and 170 mutations induced by a solar UV simulator, along with 75 mut ations in untreated cells. Only a few of the mutations in untreated cells w ere transitions at dipyrimidines, but more than 95% of the UVC and solar ir radiation-induced mutations were targeted to dipyrimidine sites, the majori ty being transitions. After UVC irradiation, 6% of the base substitutions w ere at dipyrimidines containing 5-methylcytosine and only 2.2% of all mutat ions were transitions within this sequence context. However, 24% of the sol ar light-induced mutations were at dipyrimidines that contain 5-methylcytos ine and most of them were transitions. Two sunlight-induced mutational hots pots at methylated CpGs correlated with sequences that form the highest lev els of cyclobutane pyrimidine dimers after irradiation with sunlight but no t with UVC. The data indicate that dipyrimidines that contain 5-methylcytos ine are preferential targets for sunlight-induced mutagenesis in cultured m ammalian cells, thus explaining the large proportion of p53 mutations at su ch sites in skin tumors in vivo. (C) 1999 Academic Press.