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 have a number of well-defined histological and genetic events. This knowledge has identified a series of stages in the development of colon cancer in which dietary components and chemicals may play either a beneficial or detrimental role. Azoxymethane-induced colon cancer in the rat represents a way of investigating such effects on the temporal development of the disease. To assess the stages involved in the long-term development of colon cancer in this animal model, Sprague-Dawley rats were treated with either one or two (given 24 hours apart) doses of azoxymethane (15 mg/kg). These low doses were chosen in an attempt to mimic the slow development of the human disease. At varying time intervals (5-84 weeks) after treatment, animals were killed and their colons were examined for lesions. Evidence was found in the distal region of the colon of a progression from early alterations (aberrant crypt foci) to microadenomas and polyps. This progression occurs in the region where carcinomas were found. The best correlation with tumorigenicity was 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 screened for mutation using polymerase chain reaction with single-strand conformation 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 corresponding to the mutation cluster region in humans). Extensive studies of the aberrant crypt foci formed revealed no mutations in these lesions. These results suggest that the aberrant crypt focus may be a useful short-term preneoplastic marker. However, it is clear from this and other studies that the genetic progression in the rat may vary according to 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 elucidated.