Frequent beta-catenin gene mutations and accumulations of the protein in the putative preneoplastic lesions lacking macroscopic aberrant crypt foci appearance, in rat colon carcinogenesis

Activating mutations in the beta-catenin gene is thought to be responsible for the excessive beta-catenin signaling involved in the majority of carcin ogen-induced colonic carcinomas. To determine whether beta-catenin signalin g is involved in the initial stage of colon carcinogenesis, mutational anal ysis of the gene and immunohistochemistry for beta-catenin protein were per formed in the early appearing lesions, including aberrant crypt foci (ACF), of colonic mucosa in rats given azoxymethane. Male F344 rats received s,c. injections of azoxymethane at a dose of 15 mg/kg body weight, once weekly for 3 weeks, and they were sacrificed 10 weeks after the carcinogen treatme nt. The colonic mucosa was examined in en face preparations and in serial s ections after the observation in whole mount preparations, Microscopical ob servations in the cross sections have shown two populations of histological ly altered crypts, The first type had a macroscopic feature resembling ACF [histologically altered crypts with ACF appearance (HACAs)]. The second typ e of altered crypts did not have the ACP-like appearance and could not be c learly distinguished from adjacent normal crypts in whole mount preparation s [histologically altered crypts with macroscopically normal-like appearanc e (HACNs)]. The beta-catenin gene mutations were recognized in 10 of 15 HAC Ns (67%) and 3 of 15 HACAs (20%). Frequent immunoreactivity of beta-catenin protein was seen in the cytoplasm of HACNs (13 of 15 cases), whereas appar ent accumulation was not confirmed in any HACAs analyzed. These results sug gest that (a) there are two types of putative preneoplastic lesions in canc er-predisposed colonic mucosa, and beta-catenin signaling may contribute to the initial stage of colon carcinogenesis; and (b) HACNs are more likely t o he direct precursors of colon tumors than HACAs in the rat colon carcinog enesis.