LONG-TERM ANALYSIS OF COLONIC ABERRANT CRYPT FORMATION AFTER TREATMENT OF SPRAGUE-DAWLEY RATS WITH AZOXYMETHANE

Human colon cancer is a multistage disease which has been shown to hav e a number of well-defined histological and genetic events. This knowl edge has identified a series of stages in the development of colon can cer in which dietary components and chemicals may play either a benefi cial or detrimental role. Azoxymethane-induced colon cancer in the rat represents a way of investigating such effects on the temporal develo pment of the disease. To assess the stages involved in the long-term d evelopment of colon cancer in this animal model, Sprague-Dawley rats w ere treated with either one or two (given 24 hours apart) doses of azo xymethane (15 mg/kg). These low doses were chosen in an attempt to mim ic the slow development of the human disease. At varying time interval s (5-84 weeks) after treatment, animals were killed and their colons w ere examined for lesions. Evidence was found in the distal region of t he colon of a progression from early alterations (aberrant crypt foci) to microadenomas and polyps. This progression occurs in the region wh ere carcinomas were found. The best correlation with tumorigenicity wa s the multiplicity of the crypts in each focus rather than simply the number of aberrant crypt foci. The aberrant crypts were microdissected from the colon and DNA was prepared. The following genes were screene d for mutation using polymerase chain reaction with single-strand conf ormation polymorphism, oligonucleotide hybridisation, restriction site changes and sequencing: Ki-ras (exons 1 and 2), p53 (exons 5, 6, and 7 which correspond to exons 5-8 in humans), and APC (exon 15 correspon ding to the mutation cluster region in humans). Extensive studies of t he aberrant crypt foci formed revealed no mutations in these lesions. These results suggest that the aberrant crypt focus may be a useful sh ort-term preneoplastic marker. However, it is clear from this and othe r studies that the genetic progression in the rat may vary according t o the treatment regimen used and differs from that found in human. Key genes in the development of colon cancer in the rat remain to be eluc idated. Teratogenesis Carcinog. Mutagen. 18:183-197 1998. (C) 1998 Wil ey-Liss, Inc.