Induction of skin papillomas, carcinomas, and sarcomas in mice in which the connexin 43 gene is heterologously deleted

It has been suggested that blocked gap junctional intercellular communication plays a crucial part in multistage carcinogenesis. The mouse skin tumor-promoting phorbol esters are potent inhibitors of gap junctional intercellular communication and this inhibition is considered to be a mechanism by which clonal expansion of "initiated" cells is promoted. We examined whether mice in which the gene for a gap junction protein, connexin 43, is heterozygously deleted are more susceptible to chemical carcinogenesis; connexin 43 is expressed in the basal cell layer and the dermis of the skin. When the back skin was painted with 7,12-dimethylbenz[a]anthracene and 12-O-tetradecanoylphorbol 13-acetate, the incidence and yields of both papillomas and carcinomas were similar in connexin 43+/- and connexin 43+/+ mice; for this experiment, the original mice with C57BL/6 genetic background was crossed with CD1 strain for three generations. Subcutaneous injection of 7, 12-dimethylbenz[a]anthracene resulted in induction of fibrosarcomas in connexin 43+/- and connexin 43+/+ mice to a similar extent. All papillomas and carcinomas induced with 7, 12-dimethylbenz[a]anthracene and 12-O-tetradecanoylphorbol 13-acetate contained the 7,12-dimethylbenz[a] anthracene-specific mutation in the ras gene (A to T transversion at the 61st codon). About 50% of fibrosarcomas also contained this mutation, but in the Ki-ras gene; there was no difference in the prevalence of this mutation in tumors from connexin 43+/- and connexin 43+/+ mice. None of the tumors examined, however, showed any mutation in the connexin 43 gene. These results suggest that the deletion of one allele of the connexin 43 gene does not significantly contribute to, nor alter, the molecular events involved in skin carcinogenesis. These results are compatible with previous observations that nongenetic disruption of function rather than mutations of connexins, commonly occurs in cancer cells.

G-->A mutations in p53 and Ha-ras genes in esophageal papillomas induced by N-nitrosomethylbenzylamine in two strains of rats

In human esophageal cancers, no ras gene mutations but a relatively high prevalence of p53 gene mutations have been reported. We found a high prevalence of point mutations in Ha-ras and p53 genes in N-nitrosomethylbenzylamine (NMBA)-induced esophageal tumors in two strains of rats (BD VI and F344). Our analysis showed the point mutation GGA-->GAA (expected from the known mechanisms of action of NMBA) at Ha-ras codon 12 in 22 of 46 (48%) and 22 of 38 (58%) papillomas from BD VI and F344 rats, respectively. There was no significant difference in the prevalence of ras mutations in tumors induced by high doses (5.0 mg/kg) and low doses (2.5 mg/kg) of NMBA. Eleven papillomas from each strain were analyzed for p53 mutations. The prevalent mutations found were G-->A and C-->T transitions. The frequency of p53 mutation was 36% (four of 11) for each strain. No apparent hot-spot codon or exon was found in the p53 gene, and two papillomas contained double mutations in this gene. The high prevalence of G-->A mutations in the rat Ha-ras gene contrasts with that in the human gene, in which no ras mutations have been found in primary tumors, and suggests either that the biology of esophageal carcinogenesis differs in humans and rats or that nitrosamines are not the major etiological risk factor for human esophageal cancers.