Tumorigenic conversion of p53 deficient colon epithelial cells by an activated Ki-ras gene

Distinct genetic abnormalities (loss-of-function mutationsof APC and p53 and oncogenic activation of Ki-ras) are as-sociated with specific stages of the sporadic, most commontypes of colorectal tumors. However, the inability to main-tain primary colon epithelial cells in culture has hinderedthe analysis of the pathogenetic role of these abnormalitiesin colorectal tumorigenesis. We have now established pri-mary cultures of epithelial cells from the colon crypts ofp53-deficient mice; these cells are nontumorigenic as indi-cated by their failure to form colonies in soft agar and togrow as tumors in immunodeficient SCID mice and in im-munocompetent syngeneic hosts. Upon ectopic expressionof an activated Ki-ras gene, p53-deficient colon epithelialcells form colonies in soft agar and highly invasive subcuta-neous tumors in both immunodeficient and immunocompe-tent mice. Ectopic expression of wild-type p53, but not of aDNA-binding–deficient mutant, markedly suppressed thecolony-forming ability of the Ki-ras–transformed p53-defi-cient epithelial cells. Together, these findings establish afunctional synergism in colorectal tumorigenesis dependenton the effects of an oncogenic Ki-ras in a p53-deficientbackground. This model of tumorigenic conversion of colonepithelial cells might be useful to identify genetic changesassociated with disease progression and to evaluate thetherapeutic response to conventional and novel anticancerdrugs.