In the previous 500 2-year chemical bioassays within the National Toxicology Program, large intestinal
tumors (cecal
carcinomas) related to chemical exposure have not been observed in B6C3F1 mice. The recently completed
o-nitrotoluene study provided the first cecal
tumor response and an opportunity to evaluate the morphology and molecular profile of oncogenes and tumor suppressor genes that are relevant to humans. Morphologically, the
carcinomas were gland-forming
tumors lined by tall columnar epithelial cells that were positive for
cytokeratin 20 and negative for
cytokeratin 7. Using immunohistochemistry
beta-catenin (encoded by Catnb)
protein accumulation was detected in 80% (8/10) of the cecal
carcinomas, while increased
cyclin D1 and p53
protein expression was detected in 73% (8/11), respectively. There was no difference in adenomatous polyposis
protein expression between normal colon and cecal
carcinomas. All
tumors examined exhibited mutations in exon 2 (corresponds to exon 3 in humans) in the Catnb gene. Mutations in p53 were identified in nine of 11
carcinomas, and all were in exon 7. Analysis of the K-ras gene revealed mutations in 82% (9/11) of
carcinomas; all had specific G --> T transversions (
Gly --> Val) at
codons 10 or 12. The alterations in cancer genes and
proteins found in the mouse large intestinal
tumors included mutations that activate signal transduction pathways (K-ras and Catnb) and changes that disrupt the cell-cycle and bypass G(1) arrest (p53,
cyclin D1). These alterations, which are hallmarks of human
colon cancer, probably contributed to the pathogenesis of the large intestinal
carcinomas in mice following
o-nitrotoluene exposure.