Oncogenomic Approaches in Exploring Gain of Function of Mutant p53

P63 and P73 Activation in Cancers with p53 Mutation

The members of the p53 family comprise p53, p63, and p73, and full-length isoforms of the p53 family have a tumor suppressor function. However, p53, but not p63 or p73, has a high mutation rate in cancers causing it to lose its tumor suppressor function. The top and second-most prevalent p53 mutations are missense and nonsense mutations, respectively. In this review, we discuss possible drug therapies for nonsense mutation and a missense mutation in p53. p63 and p73 activators may be able to replace mutant p53 and act as anti-cancer drugs. Herein, these p63 and p73 activators are summarized and how to improve these activator responses, particularly focusing on p53 gain-of-function mutants, is discussed.

NAMPT Inhibitor and P73 Activator Represses P53 R175H Mutated HNSCC Cell Proliferation in a Synergistic Manner

The p53 family has the following three members: p53, p63 and p73. p53 is a tumor suppressor gene that frequently exhibits mutation in head and neck cancer. Most p53 mutants are loss-of-function (LoF) mutants, but some acquire some oncogenic function, such as gain of function (GoF). It is known that the aggregation of mutant p53 can induce p53 GoF. The p73 activators RETRA and NSC59984 have an anti-cancer effect in p53 mutation cells, but we found that p73 activators were not effective in all head and neck squamous cell carcinoma (HNSCC) cell lines, with different p53 mutants. A comparison of the gene expression profiles of several regulator(s) in mutant HNSCC cells with or without aggregation of p53 revealed that nicotinamide phosphoribosyltransferase (NAMPT) is a key regulator of mutant p53 aggregation. An NAMPT inhibitor, to reduce abnormal aggregation of mutant p53, used in combination with a p73 activator, was able to effectively repress growth in HNSCC cells with p53 GoF mutants. This study, therefore, suggests a potential combination therapy approach for HNSCC with a p53 GoF mutation.

A negative feedback regulatory loop between miR-138 and TP53 is mediated by USP10

TP53 is a critical tumor suppressor. In approximately 50% of human cancers the TP53 gene is either lost or mutated. The expression level of TP53 in the cells is tightly controlled by a fine-tune machinery, mainly through the Mdm2-mediated ubiquitination pathway. On the other side, the ubiquitinated TP53 could be reversed and stabilized by USP7 and USP10, to keep the amount of TP53 in check. MicroRNAs can negatively regulate TP53 expression levels through direct targeting or positively regulate TP53 function through the repression of negative regulators of TP53. Here we report that microRNA-138 controls TP53 expression by directly targeting USP10. Furthermore, TP53 represses microRNA-138 expression, forming a negative feedback regulatory loop. This finding adds another layer of complexity to the TP53 network.

How mutations shape p53 interactions with the genome to promote tumorigenesis and drug resistance

The tumor suppressive transcription factor p53 regulates a wide array of cellular processes that confer upon cells an essential protection against cancer development. Wild-type p53 regulates gene expression by directly binding to DNA in a sequence-specific manner. p53 missense mutations are the most common mutations in malignant cells and can be regarded as synonymous with anticancer drug resistance and poor prognosis. The current review provides an overview of how the extraordinary variety of more than 2000 different mutant p53 proteins, known as the p53 mutome, affect the interaction of p53 with DNA. We discuss how the classification of p53 mutations to loss of function (LOF), gain of function (GOF), and dominant-negative (DN) inhibition of a remaining wild-type allele, hides a complex p53 mutation spectrum that depends on the distinctive nature of each mutant protein, requiring different therapeutic strategies for each mutant p53 protein. We propose to regard the different mutant p53 categories as continuous variables, that may not be independent of each other. In particular, we suggest here to consider GOF mutations as a special subset of LOF mutations, especially when mutant p53 binds to DNA through cooperation with other transcription factors, and we present a model for GOF mechanism that consolidates many observations on the GOF phenomenon. We review how novel mutant p53 targeting approaches aim to restore a wild-type-like DNA interaction and to overcome resistance to cancer therapy.

TP53: an oncogene in disguise

The standard classification used to define the various cancer genes confines tumor protein p53 (TP53) to the role of a tumor suppressor gene. However, it is now an indisputable fact that many p53 mutants act as oncogenic proteins. This statement is based on multiple arguments including the mutation signature of the TP53 gene in human cancer, the various gains-of-function (GOFs) of the different p53 mutants and the heterogeneous phenotypes developed by knock-in mouse strains modeling several human TP53 mutations. In this review, we will shatter the classical and traditional image of tumor protein p53 (TP53) as a tumor suppressor gene by emphasizing its multiple oncogenic properties that make it a potential therapeutic target that should not be underestimated. Analysis of the data generated by the various cancer genome projects highlights the high frequency of TP53 mutations and reveals that several p53 hotspot mutants are the most common oncoprotein variants expressed in several types of tumors. The use of Muller's classical definition of mutations based on quantitative and qualitative consequences on the protein product, such as ‘amorph', ‘hypomorph', ‘hypermorph' ‘neomorph' or ‘antimorph', allows a more meaningful assessment of the consequences of cancer gene modifications, their potential clinical significance, and clearly demonstrates that the TP53 gene is an atypical cancer gene.

A novel PTEN/mutant p53/c-Myc/Bcl-XL axis mediates context-dependent oncogenic effects of PTEN with implications for cancer prognosis and therapy

Phosphatase and tensin homolog located on chromosome 10 (PTEN) is one of the most frequently mutated tumor suppressors in human cancer including in glioblastoma. Here, we show that PTEN exerts unconventional oncogenic effects in glioblastoma through a novel PTEN/mutant p53/c-Myc/Bcl-XL molecular and functional axis. Using a wide array of molecular, genetic, and functional approaches, we demonstrate that PTEN enhances a transcriptional complex containing gain-of-function mutant p53, CBP, and NFY in human glioblastoma cells and tumor tissues. The mutant p53/CBP/NFY complex transcriptionally activates the oncogenes c-Myc and Bcl-XL, leading to increased cell proliferation, survival, invasion, and clonogenicity. Disruption of the mutant p53/c-Myc/Bcl-XL axis or mutant p53/CBP/NFY complex reverses the transcriptional and oncogenic effects of PTEN and unmasks its tumor-suppressive function. Consistent with these data, we find that PTEN expression is associated with worse patient survival than PTEN loss in tumors harboring mutant p53 and that a small molecule modulator of p53 exerts greater antitumor effects in PTEN-expressing cancer cells. Altogether, our study describes a new signaling pathway that mediates context-dependent oncogenic/tumor-suppressive role of PTEN. The data also indicate that the combined mutational status of PTEN and p53 influences cancer prognosis and anticancer therapies that target PTEN and p53.

More targets, more pathways and more clues for mutant p53

Mutations in the transcription factor p53 are among the most common genetic alterations in human cancer, and missense p53 mutations in cancer cells can lead to aggressive phenotypes. So far, only few studies investigated transcriptional reprogramming under mutant p53 expression as a means to identify deregulated targets and pathways. A review of the literature was carried out focusing on mutant p53-dependent transcriptome changes with the aims of (i) verifying whether different p53 mutations can be equivalent for their effects, or whether there is a mutation-specific transcriptional reprogramming of target genes, (ii) understanding what is the main mechanism at the basis of upregulation or downregulation of gene expression under the p53 mutant background, (iii) identifying novel candidate target genes of WT and/or mutant p53 and (iv) defining cellular pathways affected by the mutant p53-dependent gene expression reprogramming. Nearly 600 genes were consistently found upregulated or downregulated upon ectopic expression of mutant p53, regardless of the specific p53 mutation studied. Promoter analysis and the use of ChIP-seq data indicate that, for most genes, the expression changes could be ascribed to a loss both of WT p53 transcriptional activation and repressor functions. Pathway analysis indicated changes in the metabolism/catabolism of amino acids such as aspartate, glutamate, arginine and proline. Novel p53 candidate target genes were also identified, including ARID3B, ARNT2, CLMN, FADS1, FTH1, KPNA2, LPHN2, PARD6B, PDE4C, PIAS2, PRPF40A, PYGL and RHOBTB2, involved in the metabolism, xenobiotic responses and cell differentiation.

Current understanding of the role and targeting of tumor suppressor p53 in glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most common primary malignancy in the brain and confers a uniformly poor prognosis. Despite decades of research on the topic, limited progress has been made to improve the poor survival associated with this disease. GBM arises de novo (primary GBM) or via dedifferentiation of lower grade glioma (secondary GBM). While distinct mutations are predominant in each subtype, alterations of tumor suppressor p53 are the most common, seen in 25-30 % of primary GBM and 60-70 % of secondary GBM. Various roles of p53 that protect against neoplastic transformation include modulation of cell cycle, DNA repair, apoptosis, senescence, angiogenesis, and metabolism, resulting in an extremely complex signaling network. Mutations of p53 in GBM are most common in the DNA-binding domain, namely within six hotspot mutation sites (codons 175, 245, 248, 249, 273, and 282). These alterations generally result in loss-of-function, gain-of-function, and dominant-negative mutational effects for p53, however, the distinct effect of these mutation types in GBM pathogenesis remain unclear. Signaling alterations downstream from p53 (e.g., MDM2, MDM4, INK4/ARF), p53 isoforms (e.g., p63, p73), and microRNAs (e.g., miR-34) also play critical roles in modulating the p53 pathway. Despite novel mouse models of GBM showing that p53 combined with other mutation generate tumors de novo, the role of p53 as a molecular marker of GBM remains controversial with most studies failing to show an association with prognosis. Regarding treatment in GBM, p53 targeted-gene therapy and vaccinations have reached phase I clinical trials while therapeutic drugs are still in preclinical development. This review aims to discuss the most recent findings regarding the impact of p53 mutations on GBM pathogenesis, prognosis, and treatment.

p53 mutations in cancer

In the past fifteen years, it has become apparent that tumour-associated p53 mutations can provoke activities that are different to those resulting from simply loss of wild-type tumour-suppressing p53 function. Many of these mutant p53 proteins acquire oncogenic properties that enable them to promote invasion, metastasis, proliferation and cell survival. Here we highlight some of the emerging molecular mechanisms through which mutant p53 proteins can exert these oncogenic functions.

Silencing of mutant p53 by siRNA induces cell cycle arrest and apoptosis in human bladder cancer cells

BACKGROUND

p53 is the most frequently mutated tumor-suppressor gene in human cancers. It has been reported that mutations in p53 result not only in the loss of its ability as a tumor suppressor, but also in the gain of novel cancer-related functions that contribute to oncogenesis. The present study evaluated the potential of silencing of mutant p53 by small interfering RNA in the treatment of bladder cancer cells in vitro.

METHODS

We used the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay to assess cell viability and flow cytometry to detect cell cycle alterations and apoptosis. The related molecular mechanisms were assessed by western blotting. We also used the MTT assay and flow cytometry to investigate if silencing of mutant p53 by knockdown with small interfering (si)RNA would change the sensitivity to cisplatin treatment.

RESULTS

Using 5637 and T24 human bladder cancer cell lines characterized by mutations in p53, we found that silencing of the mutant p53 by RNA interference induced evident inhibition of cell proliferation and viability, which was related to the induction of G2 phase cell cycle arrest and apoptosis. Moreover, our study also showed that the p53-targeting siRNA cooperated with cisplatin in the inhibition of bladder cancer cells.

CONCLUSIONS

These findings suggest that RNA interference targeting mutant p53 may be a promising therapeutic strategy for the treatment of bladder cancer.

Resistance and gain-of-resistance phenotypes in cancers harboring wild-type p53

Chemotherapy is the bedrock for the clinical management of cancer, and the tumor suppressor p53 has a central role in this therapeutic modality. This protein facilitates favorable antitumor drug response through a variety of key cellular functions, including cell cycle arrest, senescence, and apoptosis. These functions essentially cease once p53 becomes mutated, as occurs in ∼50% of cancers, and some p53 mutants even exhibit gain-of-function effects, which lead to greater drug resistance. However, it is becoming increasingly evident that resistance is also seen in cancers harboring wild-type p53. In this review, we discuss how wild-type p53 is inactivated to render cells resistant to antitumor drugs. This may occur through various mechanisms, including an increase in proteasomal degradation, defects in post-translational modification, and downstream defects in p53 target genes. We also consider evidence that the resistance seen in wild-type p53 cancers can be substantially greater than that seen in mutant p53 cancers, and this poses a far greater challenge for efforts to design strategies that increase drug response in resistant cancers already primed with wild-type p53. Because the mechanisms contributing to this wild-type p53 "gain-of-resistance" phenotype are largely unknown, a concerted research effort is needed to identify the underlying basis for the occurrence of this phenotype and, in parallel, to explore the possibility that the phenotype may be a product of wild-type p53 gain-of-function effects. Such studies are essential to lay the foundation for a rational therapeutic approach in the treatment of resistant wild-type p53 cancers.

TP53 mutations in nonsmall cell lung cancer

The tumor suppressor gene TP53 is frequently mutated in human cancers. Abnormality of the TP53 gene is one of the most significant events in lung cancers and plays an important role in the tumorigenesis of lung epithelial cells. Human lung cancers are classified into two major types, small cell lung cancer (SCLC) and nonsmall cell lung cancer (NSCLC). The latter accounts for approximately 80% of all primary lung cancers, and the incidence of NSCLC is increasing yearly. Most clinical studies suggest that NSCLC with TP53 alterations carries a worse prognosis and may be relatively more resistant to chemotherapy and radiation. A deep understanding of the role of TP53 in lung carcinogenesis may lead to a more reasonably targeted clinical approach, which should be exploited to enhance the survival rates of patients with lung cancer. This paper will focus on the role of TP53 in the molecular pathogenesis, epidemiology, and therapeutic strategies of TP53 mutation in NSCLC.

Mutant p53 gain-of-function in cancer

In its wild-type form, p53 is a major tumor suppressor whose function is critical for protection against cancer. Many human tumors carry missense mutations in the TP53 gene, encoding p53. Typically, the affected tumor cells accumulate excessive amounts of the mutant p53 protein. Various lines of evidence indicate that, in addition to abrogating the tumor suppressor functions of wild-type p53, the common types of cancer-associated p53 mutations also endow the mutant protein with new activities that can contribute actively to various stages of tumor progression and to increased resistance to anticancer treatments. Collectively, these activities are referred to as mutant p53 gain-of-function. This article addresses the biological manifestations of mutant p53 gain-of-function, the underlying molecular mechanisms, and their possible clinical implications.

Interference with the p53 family network contributes to the gain of oncogenic function of mutant p53 in hepatocellular carcinoma

Whereas the hallmark of wild-type p53 is its tumor suppressor activity, tumor-associated mutant p53 proteins can exert novel anti-apoptotic gain-of-function activities, which confer a selective advantage upon tumor cells harboring such mutations. We investigated the molecular mechanisms of mutant p53 gain-of-function in hepatocellular carcinoma with special emphasis on the interaction of mutant p53 gain-of-function proteins with the p53 family members p63 and p73. Mutant forms of p53, namely the hot-spot mutants p53R143A, p53R175D, p53R175H, p53R248W, and p53R273H, acquire anti-apoptotic gain-of-function in hepatocellular carcinoma by repressing the activity of genes regulating both, the extrinsic apoptosis pathway initiated by ligation of death receptors and the intrinsic/mitochondrial apoptosis pathway. In the presence of mutated p53, the CD95L-CD95 apoptotic pathway is markedly attenuated. This is due to repression of CD95 gene transcription by mutant p53. In addition, these mutants repress the expression of the Bax gene and attenuate mitochondria-mediated apoptosis signaling. Furthermore, and of clinical relevance, these gain-of-function mutants are anti-apoptotic due to their inhibitory interaction with the pro-apoptotic p53 family members TAp63 and TAp73. p53 gain-of-function mutants significantly decrease activation of pro-apoptotic target genes by wild-type p53, TAp63, and TAp73. This contributes to the ability of cancer cells to withstand DNA damage-induced apoptosis. Interference with the interaction of p53 gain-of-function mutants with TAp63 or TAp73 may thus sensitize hepatocellular carcinoma to elimination by therapy.

When mutants gain new powers: news from the mutant p53 field

Ample data indicate that mutant p53 proteins not only lose their tumour suppressive functions, but also gain new abilities that promote tumorigenesis. Moreover, recent studies have modified our view of mutant p53 proteins, portraying them not as inert mutants, but rather as regulated proteins that influence the cancer cell transcriptome and phenotype. This influence is clinically manifested as association of TP53 mutations with poor prognosis and drug resistance in a growing array of malignancies. Here, we review recent studies on mutant p53 regulation, gain-of-function mechanisms, transcriptional effects and prognostic association, with a focus on the clinical implications of these findings.