Identification of mutant p53 inhibitors by high-content screening

The TP53 gene is the most commonly altered gene in human cancer. The majority of p53 mutations are missense and result in the accumulation of dysfunctional p53 protein in cancer cells. These mutant proteins frequently acquire neomorphic functions (defined as Gain of Function, GOF) including the induction malignant properties of cancer cells, such as uncontrolled cell proliferation, metastasis and drug resistance. A number of evidence reveals that stabilization of mutant p53 proteins in tumours is required for their GOF properties, while reduction of mutant p53 levels mitigates the malignant potential of cancer cells. Thus, targeting mutant p53 represents an attractive strategy for cancer therapy. Several small-molecule compounds that specifically target mutant p53 have been identified and are now in preclinical or clinical development. Some of them induce instability of mutant p53 proteins, leading to inhibition of multiple downstream pathways of GOF mutant p53. In this thesis, I tested a collection of FDA-approved drugs to identify molecules able to reduce the levels of mutant p53 in a triple negative breast cancer cell line. This screening allowed the identification of statins as class of drugs strongly inhibiting mutant p53 accumulation. Further investigation demonstrated that mutant p53 protein stability depends on the activation of the metabolic mevalonate pathway and that statins inhibit mutant p53 GOF in cancer cells. Mechanistically, the mevalonate pathway intermediate geranylgeranyl pyrophosphate inhibits the MDM2-dependent mutant p53 protein degradation. In particular, my data show that mutant p53 stability is controlled by geranylgeranylated proteins and that geranylgeranyl transferase inhibitors behave as statins. These results on a novel crosstalk between the metabolic mevalonate pathway and mutant p53, support the notion that these two signals are strongly intertwined and together concur to the malignant phenotype of different tumours. The data of this thesis provide the experimental-based rational for the use of mevalonate pathway inhibitors as adjuvant treatment in the therapy of tumors bearing sporadic or inherited mutations of p53.