SENCAR MOUSE SKIN TUMORS PRODUCED BY PROMOTION ALONE HAVE A-MUTATION TO G-MUTATION IN CODON-61 OF THE C-RAS(HA) GENE

SENCAR mice, developed by selective breeding for high susceptibility t o skin carcinogenesis by initiation with 7,12-dimethylbenz[a]anthracen e and promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA), form squamous papillomas in similar to 20% of animals treated repeatedly wi th TPA, without chemical initiation. DNA from eight skin tumors produc ed by a TPA-only protocol and four cell lines derived from these tumor s was amplified by polymerase chain reaction and analyzed by discrimin ative oligonucleotide hybridization using oligomers specific for vario us c-ras(Ha) gene codon 61 sequences. Five tumors and three cell lines had CAA (wild-type) to CGA mutations. In addition, one tumor had a CA A to CTA mutation, for a total of six of eight tumors having an activa ting mutation at this codon. Two tumors and one cell line had no codon 61 mutations detectable by this method. Since tumors derived from pro motion-only protocols presumably originated from constitutively initia ted cells, we examined tumor-free skins of untreated newborn and eight -month-old retired breeders and of 78-88-week-old SENCAR mice of both sexes, which were treated with TPA for 10 weeks starting at age 16-28 weeks and were untreated thereafter. Only the wild-type c-ras(Ha) gene codon 61 sequence was seen, suggesting that the constitutively initia ted cell population, if present, is below the limit of detection by th is method.