| Abstract: |
Reciprocal crosses were made between AKR/J, a 1,2-dimethylhydrazine (DMH)-resistant mouse strain, and SWR/J, a sensitive strain. The F1 hybrids were tested with DMH and methylazoxymethanol (MAM), two colon carcinogens. Either DMH (20 mg/kg body weight) or MAM (35 mg/kg body weight), a metabolic derivative of DMH, was injected weekly for 10 weeks. In each group of 35 mice, 10 were injected with tritiated thymidine (25 μCi) 1 week after the sixth injection of DMH and MAM for the evaluation of proliferative characteristics and the number of foci of dysplasia occuring in 325 μm of distal colonic mucosa. At 27 weeks after the first injection of the carcinogen, the colons of remaining mice were opened longitudinally and the number of tumors enumerated. Compared with DMH-treated mice, the number of foci of dysplasia per mouse, the percentage of tumor-bearing mice, the number of tumors per animal, and the number of tumors per tumor-bearing animal induced by MAM were severalfold higher. This would suggest the presence of a gene(s) repressing metabolism of DMH to MAM. Moreover, differences in response to the carcinogens were observed between the sexes. In contrast to males, females treated with both DMH and MAM had significantly greater numbers of tumors per animal, tumors per tumor-bearing mice, and a greater proliferative response with extension of S-phase cells to the upper third and luminal surface of crypts. Among males, those with the XAKR/YSWR heritage appeared more resistant than XSWR/YAKR males, particularly in their response to MAM. A twofold difference in the number of foci of dysplasia per mouse, tumors per animal, and the number of tumors per tumor-bearing animals was seen. Analyses of the response to DMH and MAM by F1 reciprocal hybrids of the AKR and SWR strains have shown a complex inheritance pattern governing susceptibility to DMH. Resistance to the carcinogen is provided by at least two specific repressor genes, one governing metabolism of carcinogen from DMH to MAM, and the other controlled by gender. Genetic factors contributed by the AKR female appear to convey additional resistance to male progeny, suggesting more than one gender-related gene. © 1989 Springer-Verlag. |
