MUTANT P53 IS NOT FULLY DOMINANT OVER ENDOGENOUS WILD-TYPE P53 IN A COLORECTAL ADENOMA CELL-LINE AS DEMONSTRATED BY INDUCTION OF MDM2 PROTEIN AND RETENTION OF A P53 DEPENDENT G1 ARREST AFTER GAMMA-IRRADIATION

To determine whether a single mutational event in one p53 gene is suff icient to confer a significant growth advantage on a colonic epithelia l cell, the 143(Ala) p53 mutation was previously expressed in the huma n colonic adenoma derived cell line AA/C1 (which is wild type for p53) and shown to have no effect on it's in vitro or in vivo growth charac teristics. In this investigation, by expressing the 175(His), 248(Trp) or 273(His) mutations in the same AA/C1 cell line, we have shown that this failure to affect the growth of the cells was not mutant specifi c. We have also demonstrated, using induction of MDM2 protein and the ability of the cells to undergo a p53 dependent G1 arrest, that the 14 3(Ala), 175(His) or 248(Trp) transfected cells retain functional endog enous wild type p53 activity, and suggest that these p53 mutations wou ld not have a fully dominant negative mode of action in vivo. In contr ast, one of the two AA/C1 cell lines transfected with the 273(His) mut ation did fail to cell cycle arrest after gamma irradiation, indicatin g that this mutation can act as a dominant negative. However even loss of wild type p53 function in this fell line was insufficient to direc tly effect the growth rate of the AA/C1 cells, suggesting that acquisi tion of the 273(His) mutation may contribute to malignant progression through genomic instability (by inhibiting the G1 arrest) and that oth er mutations are required before outgrowth of the cell population cont aining the p53 mutation.