Mucin-depleted foci (
MDF) are microscopic dysplastic lesions induced in the colon of rodents by specific colon
carcinogens. Most
MDF show Wnt pathway activation, whereas only a subset shows mutations in the Ctnnb1 gene, coding for
beta-catenin. Because Apc is a member of the Wnt pathway and the most frequent mutated gene in human
colon cancer, we tested whether
MDF harbor Apc mutations. F344 rats were treated twice with 150 mg/kg of
1,2-dimethylhydrazine. After 15 or 28 weeks,
MDF,
aberrant crypt foci (ACF), and
tumors were collected. We screened a segment of the Apc gene comprising the region homologous to the mutation cluster region (MCR) of human APC, which frequently shows mutations in experimental colon
tumors. Mutations were identified by PCR amplification and sequencing in 6:24
MDF (25%), 7:23
tumors (30%), 0:24 ACF (0%). Most of the mutations (92%) in
MDF and
tumors were localized in a region upstream from the MCR. All mutations were single-base substitutions and mainly formed by G:C-->A:T and C:G-->T:A transitions. The pattern of
nucleotide changes was similar in
MDF and
tumors, and, interestingly, the same mutation in
codon 1047 was found in two
MDF and in three
tumors. Four out of the six mutations found in
MDF were
nonsense mutations, and two were missense. All mutations in
tumors determined a
protein truncation. These results show that Apc mutations are present in
MDF with a frequency similar to that of
tumors, strengthening the evidence that they are precancerous lesions in colon
carcinogenesis.