Mechanisms of the Gain-of-functions of p53 mutants and the effects by USP7 PROTAC

Icahn School Of Medicine At Mount Sinai, New York NY

Investigators

Abstract

Project Summary TP53 (also called p53) is the most frequently mutated gene in human cancers, and p53 missense mutant alleles are present in more than 40% of all human tumors. In addition to loss of the wild-type p53 function, some missense substitutions enable its mutant forms to gain a selective advantage for cancers such as increased tumor growth and metastatic potential. Interestingly, however, accumulating evidence suggests that different p53 mutants show different gain-of-functions in vivo, challenging the conventional thinking that the p53 mutants share the same GOFs through interacting with the same cellular factors. In a proteomic screen, we identified a heme binding transcriptional factor BACH1 as a major cellular factor that recognizes the DNA binding domain of p53 in a manner dependent on its mutation status. BACH1 specifically interacts with p53R175H but fails to bind either wild type p53 or several other hot-spot p53 mutants. Interestingly, p53R175H acts as a repressor for BACH1- dependent ferroptosis by abrogating BACH1-mediated downregulation of SLC7A11 expression to promote tumor growth; conversely, p53R175H functions as a coactivator for BACH1-dependent tumor metastasis by upregulating pro-metastatic gene expression. Furthermore, we found that the stability of p53R175H is tightly regulated by USP7; by using PU7-1, a small-molecule USP7 PROTAC degrader, we found that the levels of p53R175H were significantly reduced upon USP7 degradation. Moreover, USP7 also interacts with other p53 hot-spot mutants, suggesting that PU7-1-mediated effects on protein stability is a potential general mechanism applied to other p53 mutants. The central hypothesis to be tested here is whether this p53R175H and BACH1 interaction is crucial for p53R175H-mediated GOFs in promoting both tumor growth and metastasis and whether destabilization of p53 mutants by PU7-1, effectively abrogates the GOFs of p53R175H and potentially, other p53 hot-spot mutants for cancer therapy. In Aim 1, we will elucidate mechanistic insights into p53R175H–mediated gain of functions by interacting BACH1 and USP7. In Aim 2, we will dissect the role of the gain of functions by using different mouse models and effects by the USP7 degrader in vivo.