Frequent mutations of the beta-catenin gene in mouse colon tumors induced by azoxymethane

The beta-catenin gene is frequently mutated at codons 33, 41 and 45 of the glycogen synthase kinase-3 beta phosphorylation motif in human colon cancer s in patients without APC mutations. Frequent mutations at codons 32 and 34 , as well as 33 and 41, have been detected in rat colon tumors induced by a zoxymethane (AOM), with the second G of CTGGA sequences being considered as a mutational hotspot. In the present study, exon 3 of the beta-catenin gen e in mouse colon tumors induced by AOM was amplified by PCR and mutations w ere detected by the single strand conformation polymorphism method, restric tion enzyme fragment length polymorphism and direct sequencing. All 10 colo n tumors tested were found to have beta-catenin mutations, four in codon 34 , three in codon 33, two in codon 41 and one in codon 37, nine being G:C--> A:T transitions. However, no mutations were found in codon 32 of the mouse beta-catenin gene. On immunostaining, beta-catenin was observed in the cyto plasm and nucleus of the tumor cells. The cytoplasmic staining was homogene ous, while both homogeneous and heterogeneous patterns were noted for the n uclei. Highly frequent mutations of the beta-catenin gene in AOM-induced mo use colon tumors suggest that consequent alterations in the stability and l ocalization of the protein may play an important role in this colon carcino genesis model.