The distribution of UV photoproducts along the human p53 gene and its relation to mutations in skin cancer.

Mutations in the p53 gene have been found in a large proportion of human skin cancers. These mutations show the same characteristics as mutations induced by UV light in experimental systems. To establish correlations between formation of DNA adducts by a known carcinogen and incidence of mutations within a specific human gene, we have investigated the formation of UV-induced photoproducts along exons 5-9 of the p53 gene after UV irradiation of human cells. The two major types of DNA photoproducts, cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts [(6-4) photoproducts], were mapped at the DNA sequence level by strand cleavage at the sites of photoproducts. This was followed by ligation-mediated polymerase chain reaction (LMPCR) to amplify gene-specific fragments. In human skin cancers, mutations were most frequently found at codons 151/152, 245, 248, 278 and 286 of the p53 gene. The frequency of UV photoproducts is particularly high at codon 286, which is within a run of 12 adjacent pyrimidines. High levels of both photoproducts were also seen at codons 151 and 278. However, UV-induced DNA adducts are barely detectable at codons 245 and 248, which are mutation hotspots also for internal malignancies. At these positions, the frequency of photoproducts is much lower than at surrounding dipyrimidine sequences. These findings have some implications on molecular mechanisms of mutagenesis in the human genome.