Intestinal tumours induced by the food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in multiple intestinal neoplasia mice have truncation mutations as well as loss of the wild-type Apc(+) allele

C57BL/6J-Min/+ (multiple intestinal neoplasia) is a murine model for famili al adenomatous polyposis (FAP), where the mice are heterozygous for a nonse nse Apc(Min) (adenomatous polyposis coli) mutation, and therefore develop n umerous spontaneous adenomas in the small intestine and colon, Neonatal exp osure of Min/+ mice to the food carcinogen 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP) (eight subcutaneous injections of 25 or 50 mg/kg PhI P to pups or 50 mg/kg PhIP to lactating dams) markedly increased (2-9-fold) the number of intestinal tumours, especially in the small intestine, We ex amined whether the Ape gene was affected in small intestinal and colonic tu mours induced by PhIP. In spontaneous tumours formed in these mice, the mai n mechanism for tumour induction is loss of the wild-type Apc(+) allele, i. e. loss of heterozygosity (LOH), Also in the PhIP-induced tumours, this is a major mechanism, since large fractions of PhIP-induced tumours had LOH in Ape. However, mechanisms other than LOH must also prevail, since a lower f requency of LOH was found in the small intestinal tumours from male mice ex posed to PhIP either via breast milk (65%) or by direct injection (68%), co mpared with the untreated controls (92%). Tumours that had retained the wil d-type Ape allele were further analysed for presence of truncated Ape prote ins with in vitro synthesized protein (IVSP) assay, Truncated Ape proteins, indicating truncation mutations in exon 15 of the Ape gene, were detected in 20% (8 of 40) of the tumours not showing LOH from the small intestine af ter PhIP exposure, all in segment 2 (codons 686-1217). Seventeen percent (2 of 12) of the colonic tumours had a truncated Ape protein in segment 3 (co dons 1099-1693). Importantly, no truncated proteins were detected in tumour s from unexposed mice with apparently retained wild-type Ape i allele, Thes e results show that PhIP induces intestinal tumours in the Min/+ mice both by causing LOH and truncation mutations in the wild-type Apc(+) allele.