Genomic imbalance in rat mammary gland carcinomas induced by 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine

2-Amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhlP), a compound found in cooked meat, is a mammary gland carcinogen in female Sprague-Dawley rats. PhlP-induced rat mammary gland carcinomas were examined for mutations in se veral genes (exons) known to regulate cell growth and apoptosis, including p53 (4-8), p21(Waf1) (coding region), Apc(14, 15), B-catenin (3), E-cadheri n (9,13,15), Bcl-x(coding region), Bar (3), IGFIIR (28), and TGFBIIR (3). D NA from 30 carcinomas was examined by single-strand conformation polymorphi sm analysis, but no mutations were detected in these genes or gene regions. DNA from carcinomas and matching normal tissue were further screened for a llelic imbalance by using a polymerase chain reaction-based approach with p rimers to known microsatellite regions located throughout the rat genome. O f 53 markers examined, 12 revealed allelic imbalance. Microsatellite instab ility (MSI) was detected at two markers, one an chromosome 4 and one on chr omosome 6. Sixty-five percent and 96% of all carcinomas examined (N=23) sho wed MSI at these loci on chromosomes 4 and 6, respectively, supporting the notion that MSI plays a role in PhlP-induced mammary carcinogenesis. Loss o f heterozygosity (LOH), an indication of a possible tumor suppressor gene, was observed at 10 markers distributed on chromosomes 3, 10, 11, 14, and X. The frequency of LOH at these markers was 75-94%, supporting that the regi ons of allelic imbalance were largely similar for the PhlP-induced carcinom as examined in this study. When PhlP-induced carcinomas from rats placed on high-fat and low-fat diet were compared, no unique regions of allelic imba lance or statistical differences in the frequency of allelic imbalance were observed. Therefore, the high-fat diet, known to be a promoter of PhlP-ind uced rat mammary carcinogenesis, did not appear to influence allelic imbala nce in the carcinomas. Interestingly, 7,12-dimethylbenz[a]anthracene-induce d mammary carcinomas did not show allelic imbalance at 11 of the 12 loci th at showed allelic imbalance in PhlP-induced carcinomas. These findings sugg est that distinct chemical carcinogens induce different patterns of allelic imbalance during rat mammary carcinogenesis. Since several of the known ge nes involved in carcinogenesis did not harbor mutations in PhlP-induced car cinomas, further studies are needed to clarify the critical genes involved in PhlP-induced mammary carcinogenesis and to determine whether regions of LOH harbor potentially novel tumor suppressor genes involved in this diseas e. (C) 2000 Wiley-Liss, Inc.