Suppression of mismatched mutation by p53: A mechanism for guarding genomic integrity

The tumor suppressor p53 plays an important role in guarding the genomic integrity of the cells. The 3′→5′ exonuclease activity of p53 has recently been recognized as a novel biochemical function of this molecule, and has been shown to preferentially excise mismatched nucleotides from DNA and enhance the DNA replication fidelity of polymerase α in vitro. The present study further investigated the role of this biochemical function in whole cells by testing the possibility that p53 may reduce mismatched mutations in cells under a stress of DNA replication errors. Cells with different states of p53 expression, either endogenously or ectopically, were exposed to hydroxyurea to induce an imbalance of cellular dNTP pools and cause replication errors. The rates of mutation at the hypoxanthine guanine phosphoribosyltransferase (HPRT) gene were determined by selecting colonies of HPRT^- mutants. Incubation of cells with hydroxyurea induced a similar degree of dNTP pool imbalance in each cell line, but caused significantly more mutations in cells lacking p53 protein expression. The mutation frequency was significantly reduced by introduction of a wild-type p53 expression vector into the p53-null cells. Analysis of the mutants demonstrated that the clones were devoid of HPRT enzyme activity, but appeared to transcribe full-length HPRT mRNA. These data suggest that p53 is able to reduce mutations caused by misincorporation of deoxynucleotides. Thus, the preferential removal of mismatched nucleotides from DNA by p53 may be a mechanism to maintain genomic integrity. Defect in this biochemical function of p53 may contribute to genetic instability associated with cancer development and progression.