Enhanced colon carcinogenesis induced by azoxymethane in min mice occurs via a mechanism independent of β-catenin mutation

The multiple intestinal neoplasia (min) mouse is a well-established cancer model in which loss of a single copy of the APC protein predisposes mice to the development of numerous tumors in the intestine. We have developed a novel variation of the min mouse model by using azoxymethane (AOM) to cause an increase in tumor incidence, number and size. Thus, treatment of min mice with AOM resulted in 2.6-, 6.3- and 5.9-fold increases in overall tumor incidence, multiplicity and size, respectively, when compared to wild type C57BL/6J mice treated with AOM. Furthermore, adenocarcinomas of the colon, which are otherwise relatively rare in min mice, increased in incidence (P<0.004), multiplicity (P<0.005), and size (P<0.02) in the AOM-treated min mice when compared to control untreated min mice. Of these adenocarcinomas, the number of poorly plus moderately differentiated adenocarcinomas was also significantly higher in the AOM-treated min mice (P<0.008). Thirty-seven histopathologically verified colon tumors (eight adenomas, five carcinoma in situ and 24 adenocarcinomas) induced in min mice and in C57BL/6J mice after treatment with or without AOM were analyzed for mutations in the β-catenin gene or de novo mutations in the Apc gene. No mutations in the β-catenin gene were found in any of colon tumors in min mice with or without treatment with AOM. However, mutations in either the β-catenin gene or the Apc gene were found in tumors induced in C57BL/6J mice by AOM. These results suggest that mutations in the β-catenin gene are less contributory to tumor development in min mice, as is the case in familial adenomatous polyposis (FAP) in humans. However, de novo mutations in either the Apc or β-catenin gene can play a role in tumor development in C57BL/6J mice treated with AOM. The differences in mutation status between min and C57BL/6J mice may indicate different genetic pathways for developing colon tumors. These two experimental systems may, therefore, be useful animal models of human colon carcinomas in patients with FAP and in patients with sporadic colon carcinomas.