Immunohistochemical demonstration of mutant p53 tumour suppressor gene product in aberrant crypt foci.

Mutation of the p53 gene is the most common genetic change accompanying the sequential development of colon cancer, but it has not been studied in the early stages of colon cancer particularly at the single and multiple crypt levels. The expression of the mutant p53 gene product in aberrant crypt foci and in adenocarcinomas induced by azoxymethane was investigated immunohistochemically, using the rat model system. Aberrant crypt foci, which may be the premalignant lesions of colon cancer, are one of the earliest recognizable lesions evident in the stepwise development of colon carcinogenesis. Immunohistochemistry was performed with three mouse monoclonal antibodies to p53 proteins. These antibodies included PAB1620 specific for the wild-type p53 protein, PAB240 specific for the mutant p53 protein and PAB421 specific for both the wild-type and mutant p53 proteins. Positive reactivity with PAB240 and PAB421 was observed in 27 of 65 (42%) aberrant crypt foci and in six of eight (75%) adenocarcinomas. No reactivity with these antibodies was present in normal adjacent crypts and in colons from untreated animals. Immunohistochemical staining with PAB240 and PAB421 was present mainly in the cytoplasm and occasionally in the nucleus of cells. This is consistent with the known preferential location of the mutant p53 protein. In adenocarcinomas, uniform staining was present throughout the tissue. Reactivity with PAB1620 in premalignant and malignant tissue was not observed. The results indicate that a p53 gene mutation occurs in aberrant crypt foci as an early genetic event in colon carcinogenesis.