Quantitation of early clonal expansion of two mutant 61st codon c-Ha-ras alleles in DMBA/TPA treated mouse skin by nested PCR/RFLP

It has been hypothesized that tumor promotion in mouse skin involves clonal expansion of initiated cells with activated c-Harvey (Ha)-ras oncogene to give rise to benign tumors, We have used the two stage mouse skin carcinogenesis model using 7,12-dimethylbenz[a]anthracene (DMBA) as the initiator and 12-O-tetradecanoyl-phorbol-13-acetate (TPA) as the tumor promoter to quantitate the number of mutated c-Ha-ras alleles in mouse epidermal DNA, Epidermal samples were harvested over a 12-week period before the appearance of papillomas, Three 61st codon (i.e. CAA) c-Ha-ras mutations, CTA (T2), CGA (G2) and CAT (T3) were quantitated by newly developed nested PCR/RFLP assays, During TPA promotion the number of T2 mutant copies showed a progressive increase starting at 4 weeks after initiation and the number of T3 mutant alleles showed an increase starting at 6 weeks, By 12 weeks after initiation, TPA-promoted mouse epidermis averaged similar to 8 x 10(5) T2 mutant alleles per epidermis while the number of T3 mutant alleles averaged 3 x 10(4) per epidermis, The best-fit lines for the quantitation of mutant alleles derived from DMBA/TPA-treated mice from 4 to 12 weeks after initiation were exponential, These results were consistent with clonal expansion of epidermal cells carrying these mutations during tumor promotion, The slopes of the best-fit lines for the mutant copies indicated a trend in which cells with the T2 mutations had a growth advantage during TPA promotion over cells with the T3 mutation.