Evaluating Mutant p53 Gain-of-function Phenotypes and Mechanisms in Cancer

Abstract

p53 is mutated in over half of human cancers. In addition to losing wild-type (WT) tumor-suppressive function, mutant p53 proteins are proposed to acquire gain-of-function (GOF) activity, leading to novel oncogenic phenotypes. Further, mutant p53 protein frequently accumulates to high levels in transformed cells. The elevation of mutant p53 protein has been proposed to underlie mutant p53 GOF activity, making the mutant protein an attractive therapeutic target. To study mutant p53 GOF mechanisms and phenotypes, we genetically engineered non-transformed, and tumor-derived WT p53 cell line models to either express endogenous missense mutant p53 (R175H and R273H) or be deficient for p53 protein (null). Characterization of the models, which initially differed only by TP53 genotype, revealed that aneuploidy frequently occurred in mutant p53-expressing cells following p53 loss-of-function (LOF). However, mutant p53 did not contribute to the development of aneuploidy through modulation of p73 activity. GOF phenotypes occurred clonally in vitro and in vivo, were independent of p53 alteration, and correlated with increased aneuploidy. Acquisition of aneuploidy also contributed to an accumulation of mutant p53 protein. Highly aneuploid cells displayed increased mutant p53 ubiquitination and cytoplasmic localization, and mutant p53 degradation occurred through proteasomal and non-proteasomal mechanisms, independent of MDM2. Finally, a comparison of clinical outcome data from patients with tumors having differing p53 genotypes revealed that individuals with aneuploid-high tumors displayed unfavorable prognoses, regardless of the TP53 genotype. Our results indicate that mutant p53 proteins have LOF, but not GOF activity and that genetic variation resulting from aneuploidy accounts for the diversity of previously reported mutant p53 GOF phenotypes. These findings are significant given the extent of pre-clinical and clinical investigation currently underway to target the GOF activities of mutant p53 for cancer therapy.