The effect of dietary folate on Apc and p53 mutations in the dimethylhydrazine rat model of colorectal cancer.

Abstract

Dietary inadequacy of folate enhances and folate supplementation suppresses colorectal carcinogenesis in the dimethylhydrazine rat model. Folate is an essential factor for DNA methylation and the de novo biosynthesis of nucleotides, aberrations of which play important roles in mutagenesis. This study investigated whether the mutational hot spots of the Apc and p53 genes for human colorectal cancer are mutated in dimethylhydrazine-induced colorectal neoplasms and whether dietary folate can modulate mutations in these regions. Rats were fed diets containing 0, 2 (basal requirement), 8 or 40 mg folate/kg diet. Five weeks after diet initiation, dimethylhydrazine was injected weekly for 15 weeks. Mutations were determined by direct sequencing in 11 low and seven high grade dysplasias and 13 invasive adenocarcinomas. A total of six Apc mutations were found in four dysplastic and carcinomatous lesions: two in two low grade dysplasias, two in one high grade dysplasia and two in one adenocarcinoma. All mutations were single base substitutions, four of which were A:T-->G:C transitions. Five of the six mutations were located upstream from the region corresponding to the human APC mutation cluster region. Dietary folate had no effect on the frequency and type of Apc mutations. No mutations were detected in exons 5-9 of the p53 gene in neoplastic lesions. These data suggest that in the dimethylhydrazine rat model of colorectal cancer, the Apc gene is mutated in early stages, albeit to a lesser degree than observed in human colorectal cancer, whereas the mutational hot spot of the p53 gene for human colorectal cancer is not commonly mutated. Although the low frequency of Apc mutations and the small number of neoplasms studied in this study might have precluded our ability to observe modulatory effects of folate, dietary folate appears to have no significant effect on Apc and p53 mutations.