Putting p53 in Context

Abstract

TP53 is the most frequently mutated gene in human cancer. Functionally, p53 is activated by a host of stress stimuli and, in turn, governs an exquisitely complex anti-proliferative transcriptional program that touches upon a bewildering array of biological responses. Despite the many unveiled facets of the p53 network, a clear appreciation of how and in what contexts p53 exerts its diverse effects remains unclear. How can we interpret p53's disparate activities and the consequences of its dysfunction to understand how cell type, mutation profile, and epigenetic cell state dictate outcomes, and how might we restore its tumor-suppressive activities in cancer?

Figure 1. The p53 network

A wide variety of regulators govern the activity of p53 (top), which, in turn, controls many distinct biological processes (bottom). Each node represents a gene and each line represents an interaction. Direct p53 inputs are indicated as blue lines and direct p53 outputs are indicated as red lines. Noticeably, p53 controls effector processes by activating multiple target genes. Downstream pathways are highly interconnected (gray lines). Interactions are annotated as positive (arrow), negative (T-bar), or modifying (solid circle).

Figure 2. Investigating mechanisms of tumor suppression

Defining the mechanism of p53-mediated tumor suppression has been interrogated in several ways: (A) knocking out p53 target genes and assessing tumor formation, (B) mutating p53 itself, such that it can activate some targets but not others, and (C) reconstituting p53 in p53 deficient cancer and determining the cell fate.

Figure 3. p53 alteration spectrum

(A) The TP53 mutation distribution for 16 cancer types with sufficient available data and frequency of TP53 alteration. Each histogram depicts the number of mutations found at each position along the p53 protein coding sequence, with the transactivation domain (TAD), DNA-binding domain (DBD), and oligomerization domain (OD) illustrated below. Symbol color indicates mutation type, including missense (green), nonsense (red), inframe indels (black), or multiple mutation types (purple). Data source: MSKCC cbio portal (Gao, Schultz, Sci Signal, 2013). (B) Multiple avenues to inactivating the second allele of TP53.

Figure 4. Mutant p53 gain-of-function

Several alternative mechanisms can lead to divergent phenotypes of p53 mutations. (A) wild-type, (B) loss or partial loss of function, (C) selection of function, or (D) neomorphic/gain-of-function.

Figure 5. Harnessing p53

(A) Stabilizing p53 in p53^WT cancer. Nutlin and other MDM2/MDMX inhibitors (RG7112, RO5503781, SAR405838, HDM201, MK4828, AMG232, and RG7388) allow for the accumulation and activity of p53 in cancer in which it is not mutated. (B) Cyclotherapy. Nutlin is used to transiently arrest p53^WT normal cells, while p53^MUT cancer cells continue to cycle and remain vulnerable to genotoxic chemotherapy. Sparing normal tissue allows for increased dosing and reduced toxicity. (C) Targeting p53^MUT cancer. PRIMA-1 and other agents (APR-246, RITA, PK7088, p53R3, and ZMC1) are used to support proper folding of mutant p53 and restore wild type-like structure and activity. p53 mutant protein is depleted through a number of indirect mechanisms including inhibition of HSP90 (17-AAG), HDAC (SAHA), and SIRT1 (YK-3-237). The aggregation and inactivation of mutant p53 and its family members is inhibited by ReACp53. Synthetic lethal interactions are dependencies in p53 mutant cancer but not in p53^WT cells. p53 deficient cells have a compromised DDR, leaving then vulnerable to even further genomic instability by inhibiting DDR-related kinases. Metabolic rewiring introduces druggable dependencies on PIP4K2, cholesterol biosynthesis/HMGCR (statins), and IAPP (pramlinitide). Some p53 mutations can result in recognizable neoantigens, which has led to the development of mutant p53-targeted immunotherapy. p53 ablation can also modify antigen presentation efficiency justifying the investigation of immune checkpoint inhibition, especially when combined with other strategies.