Novel gain of function activity of p53 mutants: activation of the dUTPase gene expression leading to resistance to 5-fluorouracil.

Mutated forms of p53 are often expressed in a variety of human tumors. In addition to loss of function of the p53 tumor suppressor, mutant p53s contribute to malignant process by acquisition of novel functions that enhance transformed properties of cells and resistance to anticancer therapy in vitro, and increase tumorigenecity, invasiveness and metastatic ability in vivo. Searching for genes that change expression in response to p53 gain of function mutants may give a clue to the mechanisms underlying their oncogenic effects. Recently by subtraction hybridization cloning we found that the dUTPase gene is transcriptionally upregulated in p53-null mouse fibroblasts expressing the exogenous human tumor-derived His175 p53 mutant. Here we show that conditional expression of His175 and Trp248 hot-spot p53 mutants in p53-negative mouse 10(1) fibroblasts and human SK-OV3 and H1299 tumor cells results in increase in dUTPase gene transcription, an important marker predicting the efficacy of cancer therapy with fluoropyrimidine drugs. Using tetracycline-regulated retroviral vectors for conditional expression of p53 mutants, we found that transcription of the dUTPase gene is increased within 24 h after tetracycline withdrawal, and the cells acquire higher resistance to 5-FU. Additional inactivation of the N-terminal transcription activation domain of mutant p53 (substitutions in amino-acid residues 22 and 23) results in abrogation of both induction of dUTPase transcripts and 5-FU resistance.