Our previous studies have shown that human
skin cancers occurring on sun-exposed body sites frequently contain G----T mutations at the second position of Ha-ras
codon 12. In this study, we investigated whether the c-Ha-ras-1 proto-oncogene could be activated by in vitro UV-irradiation of pEC plasmid
DNA, which contains the 6.6 kb BamHI fragment of the human c-Ha-ras-1 proto-oncogene. Focus formation and nude mouse tumorigenicity assays showed that UV-irradiated pEC
DNA induced morphologic and
tumorigenic transformation of NIH3T3 cells in multiple cycles of transfection, whereas unirradiated pEC
DNA did not. DNAs from secondary cycle foci and tertiary cycle
tumors were analyzed for mutations in Ha-ras
codons 12 and 61 using the polymerase chain reaction and synthetic
oligonucleotide probes. Eleven of 11 secondary cycle foci analyzed possessed a G----T mutation at the second position of Ha-ras
codon 12. However, the nude mouse
tumors exhibited a G----A mutation at position 1 of the Ha-ras
codon 12. These results suggest that in vitro UV irradiation of the c-Ha-ras-1 proto-oncogene
DNA can induce mutations that are similar to those found in human
skin cancers that originated on sun-exposed body sites.