Tumorigenic Conversion of p53-deficient Colon Epithelial Cells by an Activated

Distinct genetic abnormalities (loss-of-function mutations of APC and p53 and oncogenic activation of Ki-ras) are as-sociated with specific stages of the sporadic, most common types of colorectal tumors. However, the inability to main-tain primary colon epithelial cells in culture has hindered the analysis of the pathogenetic role of these abnormalities in colorectal tumorigenesis. We have now established pri-mary cultures of epithelial cells from the colon crypts of p53-deficient mice; these cells are nontumorigenic as indi-cated by their failure to form colonies in soft agar and to grow as tumors in immunodeficient SCID mice and in im-munocompetent syngeneic hosts. Upon ectopic expression of an activated Ki-ras gene, p53-deficient colon epithelial cells 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 a DNA-binding–deficient mutant, markedly suppressed the colony-forming ability of the Ki-ras–transformed p53-defi-cient epithelial cells. Together, these findings establish a functional synergism in colorectal tumorigenesis dependent on the effects of an oncogenic Ki-ras in a p53-deficient background. This model of tumorigenic conversion of colon epithelial cells might be useful to identify genetic changes associated with disease progression and to evaluate the therapeutic response to conventional and novel anticancer drugs.