Mechanism of functional inactivation of a Li-Fraumeni syndrome p53 that has a mutation outside of the DNA-binding domain
The majority of p53 mutations are located in the DNA-binding domain of the protein. However, recently a family suffering from Li-Fraumeni syndrome (LFS) has been discovered, some of whom harbor a p53 mutation in exon 4, outside of the core domain. How this mutation affects p53 function and subsequently leads to malignant transformation is not yet clear. Interestingly, the p53 mutation found in this LFS family is localized to the p53 region that we have recently identified as necessary for Mdm2-mediated p53 degradation. We therefore endeavored to study further the LFS-associated p53 mutation at the molecular level by creating an equivalent lesion in a p53 expression construct and functionally characterizing it. Here we demonstrate that a mutation in this region is associated not only with resistance of the mutant p53 to Mdm2-mediated degradation, but also with an impaired response of mutant protein to DNA damage. In addition, the p53(LFS) mutant was found to be defective in its transactivation function, which correlated with its inability to suppress cell growth and to induce apoptosis. The molecular basis for p53(LFS) functional impairment appears to be its predominantly cytoplasmic localization caused by faulty nuclear import mechanism, which, at least in part, resulted from the mutant's decreased affinity to importin.