Mapping of UV photoproducts within ras proto-oncogenes in UV-irradiated cells: correlation with mutations in human skin cancer.

Mutations in ras proto-oncogenes have been found in human skin cancers. Since ultraviolet light is implicated in the development of skin cancers, we have investigated the formation of UV-induced photoproducts along exons 1 and 2 of the three ras proto-oncogenes, H-ras, K-ras, and N-ras, in UV-irradiated 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 ligation-mediated polymerase chain reaction (LMPCR). No significant differences were seen between irradiated purified DNA and irradiated cells, implying that local chromatin structure does not influence the distribution of photoproducts along exons 1 and 2 of the three ras genes. We find that the transcribed strand near codon 61 in H-ras, K-ras and N-ras shows a high frequency of potentially mutagenic cyclobutane dimers and (6-4) photoproducts. Codon 12 of H-ras, K-ras and N-ras displays only barely detectable photoproducts at a CpC dinucleotide. In human skin cancers, mutations were most frequently detected at codon 12 of H-ras and K-ras. These results imply that the initial frequency distribution of a mutagenic DNA adduct may not correlate with mutation spectra in human tumors.