Mutant K-ras promotes carcinogen-induced murine colorectal tumourigenesis, but does not alter tumour chromosome stability

K-ras (KRAS) mutations are observed in around 40% of human colorectal adenomas and carcinomas. Previously, we developed and characterized a strain of transgenic mice with inducible intestinal epithelial expression of K-ras(Val12) via a Cre/LoxP system. To evaluate the influence of mutant K-ras on carcinogen-induced colorectal tumourigenesis, we induced neoplastic alterations in the large intestines of wild-type and K-ras(Val12) mice using the colon-selective carcinogen 1,2-dimethylhydrazine (DMH), which has been widely used to induce colorectal tumours that are histopathologically similar to those observed in humans. K-ras(Val12) expression significantly promoted DMH-induced colorectal tumourigenesis: the average lifespan of the mice decreased from 38.52 +/- 1.97 weeks for 40 control mice to 32.42 +/- 2.17 weeks for 26 K-ras(Val12) mice (mean +/- SEM, p < 0.05) and the abundance of large intestinal tumours increased from 2.27 +/- 0.15 per control mouse to 3.85 +/- 0.20 in K-ras(Val12) mice (mean +/- SEM, p < 0.01). Adenomas from DMH-treated K-ras(Val12) mice showed significantly higher proportions of Ki-67-positive proliferating cells (10.9 +/- 0.69%) compared with those from DMH-treated wild-type mice (7.77 +/- 0.47%) (mean +/- SEM, p < 0.01) and a mild increase in apoptotic nuclei staining for cleaved caspase-3 (1.94 +/- 0.21% compared with 1.15 +/- 0.14%, mean +/- SEM, p < 0.01). In the adenomas from DMH-treated K-ras(Val12) mice, K-ras(Val12) transgene recombination and expression were confirmed, with immunohistochemical evidence of strong Erk/MapK and mild PI3K/Akt pathway activation compared with adenomas from DMH-treated wild-type mice. Microarray hybridization and clustering analysis demonstrated different expression profiles in adenomas from DMH-treated wild-type and DMH-treated K-ras(Val12) mice, indicating involvement of different molecular mechanisms including Erk/MapK and PI3K/Akt signalling in K-ras(Val12)-expressing adenomas. Array-comparative genomic hybridization analysis showed chromosome stability in both cohorts, with only a very few tiny alterations observed in one adenoma from a DMH-treated K-ras(Val12) mouse. Taken together, these data show that mutant K-ras significantly promotes DMH-induced colorectal tumourigenesis, resulting in distinct changes in cell signalling and proliferation, but does not alter chromosome stability in the tumours. Copyright (C) 2010 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.