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Suppression of mismatched mutation by p53: a mechanism for guarding genomic integrity
Kalpana Ballal1, Wei Zhang, Tapas Mukhopadyay
1Department of Molecular Pathology, Box 89, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA.
Abstract:
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 alpha 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.
Insights
The tumor suppressor p53 guards genomic integrity by removing mismatched DNA nucleotides. This function reduces replication errors and mutations, and its defect may contribute to cancer.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- The tumor suppressor p53 is crucial for maintaining genomic stability.
- A newly identified 3'-->5' exonuclease activity of p53 excises mismatched DNA nucleotides.
- This activity enhances DNA replication fidelity in vitro.
Purpose of the Study:
- To investigate the role of p53's exonuclease activity in reducing cellular mutations.
- To determine if p53 mitigates replication errors under induced DNA stress.
Main Methods:
- Hydroxyurea was used to induce dNTP pool imbalance and replication errors in cells with varying p53 expression.
- Mutation rates at the hypoxanthine guanine phosphoribosyltransferase (HPRT) gene were measured.
- p53-null cells were transfected with a wild-type p53 expression vector.
Main Results:
- Cells lacking p53 exhibited significantly higher mutation rates under hydroxyurea-induced stress.
- Introducing wild-type p53 into p53-null cells reduced mutation frequency.
- Mutant clones lacked HPRT enzyme activity but transcribed HPRT mRNA, indicating post-transcriptional effects.
Conclusions:
- p53's exonuclease activity reduces mutations caused by deoxynucleotide misincorporation.
- Preferential removal of mismatched DNA nucleotides by p53 is a key mechanism for genomic integrity.
- Defects in p53's DNA repair function may drive genetic instability in cancer.