Spontaneous tumorigenesis in mice overexpressing the p53-negative regulator Mdm4

Cellular carcinogenesis in preleukemic conditions:drivers and defenses

Acute myeloid leukemia (AML) arises from preleukemic conditions. We have investigated the pathogenesis of typical preleukemia, myeloproliferative neoplasms, and clonal hematopoiesis. Hematopoietic stem cells in both preleukemic conditions harbor recurrent driver mutations; additional mutation provokes further malignant transformation, leading to AML onset. Although genetic alterations are defined as the main cause of malignant transformation, non-genetic factors are also involved in disease progression. In this review, we focus on a non-histone chromatin protein, high mobility group AT-hook2 (HMGA2), and a physiological p53 inhibitor, murine double minute X (MDMX). HMGA2 is mainly overexpressed by dysregulation of microRNAs or mutations in polycomb components, and provokes expansion of preleukemic clones through stem cell signature disruption. MDMX is overexpressed by altered splicing balance in myeloid malignancies. MDMX induces leukemic transformation from preleukemia via suppression of p53 and p53-independent activation of WNT/β-catenin signaling. We also discuss how these non-genetic factors can be targeted for leukemia prevention therapy.

Review: MDMX plays a central role in leukemic transformation and may be a promising target for leukemia prevention strategies

Acute myeloid leukemia (AML) is a fatal disease resulting from preleukemic hematopoietic conditions including asymptomatic clonal hematopoiesis. The accumulation of genetic changes is one of the causes of leukemic transformation. However, nongenetic factors including the overexpression of specific genes also contribute to preleukemic to leukemic transition. Among them, the p53 inhibitor Murine Double Minute X (MDMX) plays crucial roles especially in leukemia initiation. MDMX is broadly overexpressed in vast majority of AML cases, including in hematopoietic stem/progenitor cell (HSPC) level. Recently, high expression of MDMX in HSPC has been shown to be associated with leukemic transformation in patients with myelodysplastic syndromes, and preclinical studies demonstrated that MDMX overexpression accelerates the transformation of preleukemic murine models, including models of clonal hematopoiesis. MDMX inhibition, through activation of cell-intrinsic p53 activity, shows antileukemic effects. However, the molecular mechanisms of MDMX in provoking leukemic transformation are complicated. Both p53-dependent and independent mechanisms are involved in the progression of the disease. This review discusses the canonical and noncanonical functions of MDMX and how these functions are involved in the maintenance, expansion, and progression to malignancy of preleukemic stem cells. Moreover, strategies on how leukemic transformation could possibly be prevented by targeting MDMX in preleukemic stem cells are discussed.

MDM4: What do we know about the association between its polymorphisms and cancer?

MDM4 is an important p53-negative regulator, consequently, it is involved in cell proliferation, DNA repair, and apoptosis regulation. MDM4 overexpression and amplification are described to lead to cancer formation, metastasis, and poor disease prognosis. Several MDM4 SNPs are in non-coding regions, and some affect the MDM4 regulation by disrupting the micro RNA binding site in 3'UTR (untranslated region). Here, we gathered several association studies with different MDM4 SNPs and populations to understand the relationship between its SNPs and solid tumor risk. Many studies failed to replicate their results regarding different populations, cancer types, and risk genotypes, leading to conflicting conclusions. We suggested that distinct haplotype patterns in different populations might affect the association between MDM4 SNPs and cancer risk. Thus, we propose to investigate some linkage SNPs in specific haplotypes to provide informative MDM4 markers for association studies with cancer.

MDM2 E3 ligase activity is essential for p53 regulation and cell cycle integrity

MDM2 and MDM4 are key regulators of p53 and function as oncogenes when aberrantly expressed. MDM2 and MDM4 partner to suppress p53 transcriptional transactivation and polyubiquitinate p53 for degradation. The importance of MDM2 E3-ligase-mediated p53 regulation remains controversial. To resolve this, we generated mice with an Mdm2 L466A mutation that specifically compromises E2 interaction, abolishing MDM2 E3 ligase activity while preserving its ability to bind MDM4 and suppress p53 transactivation. Mdm2^L466A/L466A mice exhibit p53-dependent embryonic lethality, demonstrating MDM2 E3 ligase activity is essential for p53 regulation in vivo. Unexpectedly, cells expressing Mdm2^L466A manifest cell cycle G2-M transition defects and increased aneuploidy even in the absence of p53, suggesting MDM2 E3 ligase plays a p53-independent role in cell cycle regulation and genome integrity. Furthermore, cells bearing the E3-dead MDM2 mutant show aberrant cell cycle regulation in response to DNA damage. This study uncovers an uncharacterized role for MDM2’s E3 ligase activity in cell cycle beyond its essential role in regulating p53’s stability in vivo.

The most frequently mutated protein in human cancer, the p53 tumor suppressor protein, is negatively regulated by the potentially oncogenic proteins MDM2 and MDM4. MDM2/MDM4 regulates p53 through two mechanisms, MDM2 E3 ubiquitin ligase activity marks p53 for degradation while MDM2/MDM4 can bind p53 to inhibit its ability to promote RNA transcription. Whether these mechanisms contribute to normal p53 regulation in vivo remains controversial. Using a newly developed mouse model that genetically separates these two mechanisms, we find that mice expressing MDM2 deficient specifically for E3 ubiquitin ligase activity do not survive embryonic development because unregulated p53 is lethal. In contrast to prior reports, MDM2 E3 ubiquitin ligase activity is thus required for p53 regulation during embryonic development. In addition, cells lacking MDM2 E3 ubiquitin ligase activity have cell cycle defects regardless of p53 status, uncovering a p53-independent function for MDM2 in regulating the cell cycle. Activating p53 by blocking physical interaction with MDM2/MDM4 is one currently pursued approach for cancer therapy, but this approach does not account for cancer-promoting activities of MDM2/MDM4 independent of p53. Findings reported here suggest targeting MDM2 E3 ligase activity directly may be advantageous as it would inhibit both p53-dependent and p53-independent oncogenic mechanisms.

MDM4 polymorphisms associated with the risk but not the prognosis of esophageal cancer in Cixian high-incidence region from northern China

AIM

The mouse double minute 4 (MDM4) may contribute to tumorgenesis by inhibiting p53 tumor suppressor activity. This study was designed to investigate whether MDM4 polymorphisms could affect susceptibility to esophageal squamous cell carcinoma (ESCC) and the survival of ESCC patients in a population from Cixian high-incidence region of northern China, which has not been explored.

METHODS

MDM4 rs1380576 and rs4245739 were genotyped by polymerase chain reaction-ligase detection reaction (PCR-LDR) in 568 ESCC patients and 578 controls.

RESULTS

Compared to rs1380576 C/C genotype, C/G genotype was associated with decreased risk of ESCC (odds ratio [OR] = 0.761, 95% confidence interval [CI] = 0.595-0.973). Compared to rs4245739 A/A genotype, A/C or C/C genotype was related to increased susceptibility to ESCC (OR = 1.551, 95% CI = 1.001-2.402). Individuals with GC haplotype had significantly higher risk of ESCC than those with CA or GA haplotype (OR = 1.598, 95% CI = 1.048-2.438). Neither rs1380576 nor rs4245739 influenced the survival of ESCC patients.

CONCLUSION

rs1380576 and rs4245739 may be used to predict susceptibility to ESCC for population in Cixian high-incidence region.

Inhibition of the mTOR pathway and reprogramming of protein synthesis by MDM4 reduce ovarian cancer metastatic properties

Epithelial ovarian cancer (EOC) is a highly heterogeneous disease with a high death rate mainly due to the metastatic spread. The expression of MDM4, a well-known p53-inhibitor, is positively associated with chemotherapy response and overall survival (OS) in EOC. However, the basis of this association remains elusive. We show that in vivo MDM4 reduces intraperitoneal dissemination of EOC cells, independently of p53 and an immune-competent background. By 2D and 3D assays, MDM4 impairs the early steps of the metastatic process. A 3D-bioprinting system, ad hoc developed by co-culturing EOC spheroids and endothelial cells, showed reduced dissemination and intravasation into vessel-like structures of MDM4-expressing cells. Consistent with these data, high MDM4 levels protect mice from ovarian cancer-related death and, importantly, correlate with increased 15 y OS probability in large data set analysis of 1656 patients. Proteomic analysis of EOC 3D-spheroids revealed decreased protein synthesis and mTOR signaling, upon MDM4 expression. Accordingly, MDM4 does not further inhibit cell migration when its activity towards mTOR is blocked by genetic or pharmacological approaches. Importantly, high levels of MDM4 reduced the efficacy of mTOR inhibitors in constraining cell migration. Overall, these data demonstrate that MDM4 impairs EOC metastatic process by inhibiting mTOR activity and suggest the usefulness of MDM4 assessment for the tailored application of mTOR-targeted therapy.

MDMX acts as a pervasive preleukemic-to-acute myeloid leukemia transition mechanism

MDMX is overexpressed in the vast majority of patients with acute myeloid leukemia (AML). We report that MDMX overexpression increases preleukemic stem cell (pre-LSC) number and competitive advantage. Utilizing five newly generated murine models, we found that MDMX overexpression triggers progression of multiple chronic/asymptomatic preleukemic conditions to overt AML. Transcriptomic and proteomic studies revealed that MDMX overexpression exerts this function, unexpectedly, through activation of Wnt/β-Catenin signaling in pre-LSCs. Mechanistically, MDMX binds CK1α and leads to accumulation of β-Catenin in a p53-independent manner. Wnt/β-Catenin inhibitors reverse MDMX-induced pre-LSC properties, and synergize with MDMX-p53 inhibitors. Wnt/β-Catenin signaling correlates with MDMX expression in patients with preleukemic myelodysplastic syndromes and is associated with increased risk of progression to AML. Our work identifies MDMX overexpression as a pervasive preleukemic-to-AML transition mechanism in different genetically driven disease subtypes, and reveals Wnt/β-Catenin as a non-canonical MDMX-driven pathway with therapeutic potential for progression prevention and cancer interception.

Association of MDM4 Gene rs4245739 Polymorphism with the Risk and Clinical Characteristics of Colorectal Cancer in a Chinese Han Population

Background

Studies show that MDM4 may play a pivotal role in colorectal cancer (CRC). Recently, a host of studies suggest that MDM4 gene rs4245739 polymorphism may modify the risk of different cancers.

Methods

In this study, we were interested whether MDM4 gene rs4245739 polymorphism correlated with the risk and clinical characteristics of CRC. Logistic regression was adopted to estimate the association of rs4245739 polymorphism and CRC risk.

Results

We enrolled 444 CRC patients and 530 controls and found MDM4 gene rs4245739 polymorphism may decrease the risk of CRC. Stratified analyses uncovered that this variant was connected to a less risk of CRC in females, non-drinkers, non-smokers, and people under 60 years old. Additionally, rs4245739 polymorphism was related to TNM staging, pathological type, tumor size, and location of CRC. Furthermore, this polymorphism was significantly linked with the survival of CRC.

Conclusion

Totally, this study suggests that MDM4 rs4245739 polymorphism is linked with the risk and clinical characteristics of CRC.

Regulation of the p53 Family Proteins by the Ubiquitin Proteasomal Pathway

The tumor suppressor p53 and its homologues, p63 and p73, play a pivotal role in the regulation of the DNA damage response, cellular homeostasis, development, aging, and metabolism. A number of mouse studies have shown that a genetic defect in the p53 family could lead to spontaneous tumor development, embryonic lethality, or severe tissue abnormality, indicating that the activity of the p53 family must be tightly regulated to maintain normal cellular functions. While the p53 family members are regulated at the level of gene expression as well as post-translational modification, they are also controlled at the level of protein stability through the ubiquitin proteasomal pathway. Over the last 20 years, many ubiquitin E3 ligases have been discovered that directly promote protein degradation of p53, p63, and p73 in vitro and in vivo. Here, we provide an overview of such E3 ligases and discuss their roles and functions.

Emerging treatment options for patients with p53-pathway-deficient CLL

Over the past 40 years, p53 has been the most widely studied protein in cancer biology. Originally thought to be an oncogene due to its stabilization in many cancers, it is now considered to be one of the most critical tumor suppressors in a cell's ability to combat neoplastic transformation. Due to its critical roles in apoptosis, cell-cycle arrest, and senescence, TP53 deletions and mutations are commonly observed and are often a portent of treatment failures and poor clinical outcomes. This is particularly true in chronic lymphocytic leukemia (CLL), as patients with p53 alterations have historically had dismal outcomes. As such, the tremendous efforts made to better understand the functions of p53 in CLL have contributed substantially to recent advances in treating patients with p53-pathway-deficient CLL.

Dual inhibition of MDMX and MDM2 as a therapeutic strategy in leukemia

The tumor suppressor p53 is often inactivated via its interaction with endogenous inhibitors mouse double minute 4 homolog (MDM4 or MDMX) or mouse double minute 2 homolog (MDM2), which are frequently overexpressed in patients with acute myeloid leukemia (AML) and other cancers. Pharmacological disruption of both of these interactions has long been sought after as an attractive strategy to fully restore p53-dependent tumor suppressor activity in cancers with wild-type p53. Selective targeting of this pathway has thus far been limited to MDM2-only small-molecule inhibitors, which lack affinity for MDMX. We demonstrate that dual MDMX/MDM2 inhibition with a stapled α-helical peptide (ALRN-6924), which has recently entered phase I clinical testing, produces marked antileukemic effects. ALRN-6924 robustly activates p53-dependent transcription at the single-cell and single-molecule levels and exhibits biochemical and molecular biological on-target activity in leukemia cells in vitro and in vivo. Dual MDMX/MDM2 inhibition by ALRN-6924 inhibits cellular proliferation by inducing cell cycle arrest and apoptosis in cell lines and primary AML patient cells, including leukemic stem cell-enriched populations, and disrupts functional clonogenic and serial replating capacity. Furthermore, ALRN-6924 markedly improves survival in AML xenograft models. Our study provides mechanistic insight to support further testing of ALRN-6924 as a therapeutic approach in AML and other cancers with wild-type p53.

Estrogens Counteract Platinum-Chemosensitivity by Modifying the Subcellular Localization of MDM4

Estrogen activity towards cancer-related pathways can impact therapeutic intervention. Recent omics data suggest possible crosstalk between estrogens/gender and MDM4, a key regulator of p53. Since MDM4 can either promote cell transformation or enhance DNA damage-sensitivity, we analysed in vivo impact of estrogens on both MDM4 activities. In Mdm4 transgenic mouse, Mdm4 accelerates the formation of fibrosarcoma and increases tumor sensitivity to cisplatin as well, thus confirming in vivo Mdm4 dual mode of action. Noteworthy, Mdm4 enhances chemo- and radio-sensitivity in male but not in female animals, whereas its tumor-promoting activity is not affected by mouse gender. Combination therapy of transgenic females with cisplatin and fulvestrant, a selective estrogen receptor degrader, was able to recover tumor cisplatin-sensitivity, demonstrating the relevance of estrogens in the observed sexual dimorphism. Molecularly, estrogen receptor-α alters intracellular localization of MDM4 by increasing its nuclear fraction correlated to decreased cell death, in a p53-independent manner. Importantly, MDM4 nuclear localization and intra-tumor estrogen availability correlate with decreased platinum-sensitivity and apoptosis and predicts poor disease-free survival in high-grade serous ovarian carcinoma. These data demonstrate estrogen ability to modulate chemo-sensitivity of MDM4-expressing tumors and to impinge on intracellular trafficking. They support potential usefulness of combination therapy involving anti-estrogenic drugs.

Context-dependent roles of MDMX (MDM4) and MDM2 in breast cancer proliferation and circulating tumor cells

INTRODUCTION

Many human breast cancers overexpress the E3 ubiquitin ligase MDM2 and its homolog MDMX. Expression of MDM2 and MDMX occurs in estrogen receptor α-positive (ERα+) breast cancer and triple-negative breast cancer (TNBC). There are p53-independent influences of MDM2 and MDMX, and 80% of TNBC express mutant p53 (mtp53). MDM2 drives TNBC circulating tumor cells (CTCs) in mice, but the context-dependent influences of MDM2 and MDMX on different subtypes of breast cancers expressing mtp53 have not been determined.

METHODS

To assess the context-dependent roles, we carried out MDM2 and MDMX knockdown in orthotopic tumors of TNBC MDA-MB-231 cells expressing mtp53 R280K and MDM2 knockdown in ERα+ T47D cells expressing mtp53 L194F. The corresponding cell proliferation was scored in vitro by growth curves and in vivo by orthotopic tumor volumes. Cell migration was assessed in vitro by wound-healing assays and cell intravasation in vivo by sorting GFP-positive CTCs by flow cytometry. The metastasis gene targets were determined by an RT-PCR array card screen and verified by qRT-PCR and Western blot analysis.

RESULTS

Knocking down MDMX or MDM2 in MDA-MB-231 cells reduced cell migration and CTC detection, but only MDMX knockdown reduced tumor volumes at early time points. This is the first report of MDMX overexpression in TNBC enhancing the CTC phenotype with correlated upregulation of CXCR4. Experiments were carried out to compare MDM2-knockdown outcomes in nonmetastatic ERα+ T47D cells. The knockdown of MDM2 in ERα+ T47D orthotopic tumors decreased primary tumor volumes, supporting our previous finding that estrogen-activated MDM2 increases cell proliferation.

CONCLUSIONS

This is the first report showing that the expression of MDM2 in ERα+ breast cancer and TNBC can result in different tumor-promoting outcomes. Both MDMX and MDM2 overexpression in TNBC MDA-MB-231 cells enhanced the CTC phenotype. These data indicate that both MDM2 and MDMX can promote TNBC metastasis and that it is important to consider the context-dependent roles of MDM2 family members in different subtypes of breast cancer.

MDMX acidic domain inhibits p53 DNA binding in vivo and regulates tumorigenesis

The MDM2 homolog MDMX oncoprotein is indispensable for inhibition of p53 during normal embryonic development and malignant transformation, yet how MDMX harnesses p53 functions is unclear. In addition to a canonical N-terminal p53-binding domain, recent work suggests the central acidic domain of MDMX regulates p53 interaction through intramolecular mimicry and engages in second-site interaction with the p53 core domain in vitro. To test the physiological relevance of these interactions, we generated an MDMX knockin mouse having substitutions in a conserved WW motif necessary for these functions (W201S/W202G). Notably, MDMX^SG cells have normal p53 level but increased p53 DNA binding and target gene expression, and rapidly senesce. In vivo, MDMX^SG inhibits early-phase disease in Eµ-Myc transgenic mice but accelerates the onset of lethal lymphoma and shortens overall survival. Therefore, MDMX is an important regulator of p53 DNA binding, which complements the role of MDM2 in regulating p53 level. Furthermore, the results suggest that the WW motif has dual functions that regulate p53 and inhibit Myc-driven lymphomas independent of p53.

Tumorigenesis promotes Mdm4-S overexpression

Disruption of the p53 tumor suppressor pathway is a primary cause of tumorigenesis. In addition to mutation of the p53 gene itself, overexpression of major negative regulators of p53, MDM2 and MDM4, also act as drivers for tumor development. Recent studies suggest that expression of splice variants of Mdm2 and Mdm4 may be similarly involved in tumor development. In particular, multiple studies show that expression of a splice variant of MDM4, MDM4-S correlates with tumor aggressiveness and can be used as a prognostic marker in different tumor types. However, in the absence of prospective studies, it is not clear whether expression of MDM4-S in itself is oncogenic or is simply an outcome of tumorigenesis. Here we have examined the role of Mdm4-S in tumor development in a transgenic mouse model. Our results suggest that splicing of Mdm4 does not promote tumor development and does not cooperate with other oncogenic insults to alter tumor latency or aggressiveness. We conclude that Mdm4-S overexpression is a consequence of splicing defects in tumor cells rather than a cause of tumor evolution.

The associations between MDM4 gene polymorphisms and cancer risk

Considerable studies have investigated the associations between MDM4 gene polymorphisms and cancer risk recently, but with contradictory results. The aim of this meta-analysis was to evaluate the associations between MDM4 gene polymorphisms and cancer risk. Relevant studies were identified by a systematic search of PubMed, Embase, and CNKI databases. Crude odds ratios (ORs) and 95% confidence intervals (CIs) were used to describe the strength of the associations. Fifty-six studies published in 11 publications involving 18,910 cases and 51,609 controls were included in this meta-analysis. Five MDM4 gene polymorphisms were evaluated: rs4245739, rs1563828, rs11801299, rs10900598, and rs1380576. Our analyses suggested that the rs4245739 polymorphism was significantly associated with overall cancer risk. Furthermore, stratification analyses of ethnicity indicated that rs4245739 decreased the risk of cancer among the Asian population, and stratification analyses of smoking status indicated that rs4245739 decreased the risk of cancer among nonsmokers. However, stratification analyses of cancer type and sex suggested that rs4245739 was not related to cancer risk. There were no associations of rs1563828, rs11801299, rs10900598, or rs1380576 with overall cancer risk. In conclusion, our analyses indicated that rs4245739 polymorphism in the MDM4 gene may play an important role in the etiology of cancer.

Lack of Associations of the MDM4 rs4245739 Polymorphism with Risk of Thyroid Cancer among Iranian-Azeri Patients: a Case-Control Study

Background and Aim:

MDM4, a negative regulator of the p53 tumor suppression pathway, has been demonstrated to be overexpressed in a variety of human cancers. Research has revealed that the rs4245739 A>C polymorphism of MDM4 in the 3’-untranslated region makes it a miR-191 target site, leading to lower MDM4 expression. This study aimed to detect if the rs4245739 single nucleotide polymorphism (SNP) impacts on thyroid cancer (TC) development in Iranian-Azeri patients.

Materials and Method:

Blood samples were taken from 232 healthy controls and 130 TC patients of Iranian-Azeri ethnicity. For genotyping, Tetra-ARMS PCR was performed. SPSS for Windows (version 22.0, IBM SPSS Inc., USA) and the SHEsis online software were used for data analysis.

Results:

Alleles of MDM4 rs4245739 SNP demonstrated no significant different in frequencies between patients and controls (p>0.05). Additionally, genotypes of MDM4 rs4245739 SNP did not increase or decrease TC risk in patients compared with healthy subjects.

Conclusion:

Considering the lack of any observed association between the MDM4 rs4245739 polymorphism and TC, we conclude no significant role in the pathophysiology of the disease.

MDM4 actively restrains cytoplasmic mTORC1 by sensing nutrient availability

BACKGROUND

Many tumor-related factors have shown the ability to affect metabolic pathways by paving the way for cancer-specific metabolic features. Here, we investigate the regulation of mTORC1 by MDM4, a p53-inhibitor with oncogenic or anti-survival activities depending on cell growth conditions.

METHOD

MDM4-mTOR relationship was analysed through experiments of overexpression or silencing of endogenous proteins in cell culture and using purified proteins in vitro. Data were further confirmed in vivo using a transgenic mouse model overexpressing MDM4. Additionally, the Cancer Genome Atlas (TCGA) database (N = 356) was adopted to analyze the correlation between MDM4 and mTOR levels and 3D cultures were used to analyse the p53-independent activity of MDM4.

RESULTS

Following nutrient deprivation, MDM4 impairs mTORC1 activity by binding and inhibiting the kinase mTOR, and contributing to maintain the cytosolic inactive pool of mTORC1. This function is independent of p53. Inhibition of mTORC1 by MDM4 results in reduced phosphorylation of the mTOR downstream target p70S6K1 both in vitro and in vivo in a MDM4-transgenic mouse. Consistently, MDM4 reduces cell size and proliferation, two features controlled by p70S6K1, and, importantly, inhibits mTORC1-mediated mammosphere formation. Noteworthy, MDM4 transcript levels are significantly reduced in breast tumors characterized by high mTOR levels.

CONCLUSION

Overall, these data identify MDM4 as a nutrient-sensor able to inhibit mTORC1 and highlight its metabolism-related tumor-suppressing function.

p53: Multiple Facets of a Rubik's Cube

The p53 tumor suppressor has been studied for decades, and still there are many questions left unanswered. In this review, we first describe the current understanding of the wild-type p53 functions that determine cell survival or death, and regulation of the protein, with a particular focus on the negative regulators, the murine double minute family of proteins. We also summarize tissue-, stress-, and age-specific p53 activities and the potential underlying mechanisms. Among all p53 gene alterations identified in human cancers, p53 missense mutations predominate, suggesting an inherent biological advantage. Numerous gain-of-function activities of mutant p53 in different model systems and contexts have been identified. The emerging theme is that mutant p53, which retains a potent transcriptional activation domain, also retains the ability to modify gene transcription, albeit indirectly. Lastly, because mutant p53 stability is necessary for its gain of function, we summarize the mechanisms through which mutant p53 is specifically stabilized. A deeper understanding of the multiple pathways that impinge upon wild-type and mutant p53 activities and how these, in turn, regulate cell behavior will help identify vulnerabilities and therapeutic opportunities.

Role of Mdm2 and Mdmx in DNA repair

Mdm2 and Mdmx are critical regulators of the p53 tumour suppressor and are overexpressed in many human malignancies. However, in recent years, their impact on genome instability was shown to be at least, in part, independent of p53. Both Mdm2 and Mdmx inhibit DNA break repair through their association with the Mre11/Rad50/Nbs1 DNA repair complex. Recent evidence indicates that harnessing Mdm2 and/or Mdmx-mediated inhibition of DNA break repair in cancer cells could provide a therapeutic opportunity, particularly for those malignancies that have lost functional p53.

The p53 inhibitor Mdm4 cooperates with multiple genetic lesions in tumourigenesis

The p53 inhibitor Mdm4 is present at high levels in multiple human cancers. Overexpression of Mdm4 in mice drives the spontaneous development of mostly lymphomas and sarcomas. In this study, we explored the ability of Mdm4 to cooperate with lesions in tumour development. The Mdm4 transgene contributed to mammary tumour development in a BALB/cJ background. High levels of Mdm4 enhanced tumour development in a mutant p53R172H heterozygous background, and reduced the need to lose the wild-type p53 allele, as compared with mice heterozygous only for the p53R172H mutation. Additionally, high levels of Mdm4 cooperated with an oncogenic K-ras mutation to drive lung tumourigenesis in vivo. Finally, we examined p53-independent functions of Mdm4 by studying the contribution of Mdm4 to tumour development in the absence of p53. Whereas the overall survival times of p53-null mice with and without the Mdm4 transgene were similar, male mice with both alterations showed significantly shorter survival than p53-null male mice, and showed differences in tumour spectrum, demonstrating a p53-independent function of Mdm4 in tumourigenesis. Furthermore, p53-null mice with the highest level of Mdm4 tended to have multiple tumours. Thus, a detailed analysis of Mdm4 transgenic mice in various genetic backgrounds shows synergy in tumour development in vivo. Mdm4 may thus serve as a therapeutic target in cancers. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

The Enigma of p53

This perspective will focus on the physiological impact of wild-type and mutant p53 activities. In particular, the tissue-specific nature of activation of p53 targets and their subsequent effects on cell behavior will be discussed. Because mutations in p53 are common in human cancers, the regulation and physiological consequences of mutant p53 proteins will also be discussed.

Attenuating the p53 Pathway in Human Cancers: Many Means to the Same End

The p53 pathway is perturbed in the majority of human cancers. Although this most frequently occurs through the direct mutation or deletion of p53 itself, there are a number of other alterations that can attenuate the pathway and contribute to tumorigenesis. For example, amplification of important negative regulators, MDM2 and MDM4, occurs in a number of cancers. In this work, we will review both the normal regulation of the p53 pathway and the different mechanisms of pathway inhibition in cancer, discuss these alterations in the context of the global genomic analyses that have been conducted across tumor types, and highlight the translational implications for cancer diagnosis and treatment.

Pharmacological activation of wild-type p53 in the therapy of leukemia

The tumor suppressor p53 is inactivated by mutations in the majority of human solid tumors. Conversely, p53 mutations are rare in leukemias and are only observed in a small fraction of the patient population, predominately in patients with complex karyotype acute myeloid leukemia or hypodiploid acute lymphoblastic leukemia. However, the loss of p53 function in leukemic cells is often caused by abnormalities in p53-regulatory proteins, including overexpression of MDM2/MDMX, deletion of CDKN2A/ARF, and alterations in ATM. For example, MDM2 inhibits p53-mediated transcription, promotes its nuclear export, and induces proteasome-dependent degradation. The MDM2 homolog MDMX is another direct regulator of p53 that inhibits p53-mediated transcription. Several small-molecule inhibitors and stapled peptides targeting MDM2 and MDMX have been developed and have recently entered clinical trials. The clinical trial results of the first clinically used MDM2 inhibitor, RG7112, illustrated promising p53 activation and apoptosis induction in leukemia cells as proof of concept. Side effects of RG7112 were most prominent in suppression of thrombopoiesis and gastrointestinal symptoms in leukemia patients. Predictive biomarkers for response to MDM2 inhibitors have been proposed, but they require further validation both in vitro and in vivo so that the accumulated knowledge concerning pathological p53 dysregulation in leukemia and novel molecular-targeted strategies to overcome this dysregulation can be translated safely and efficiently into novel clinical therapeutics.

Genome Stability Requires p53

It is now clear that functional p53 is critical to protect the genome from alterations that lead to tumorigenesis. However, with the myriad of cellular stresses and pathways linked to p53 activation, much remains unknown about how p53 maintains genome stability and the proteins involved. The current understanding of the multiple ways p53 contributes to genome stability and how two of its negative regulators, Mdm2 and Mdmx, induce genome instability will be described.

The importance of p53 pathway genetics in inherited and somatic cancer genomes

Decades of research have shown that mutations in the p53 stress response pathway affect the incidence of diverse cancers more than mutations in other pathways. However, most evidence is limited to somatic mutations and rare inherited mutations. Using newly abundant genomic data, we demonstrate that commonly inherited genetic variants in the p53 pathway also affect the incidence of a broad range of cancers more than variants in other pathways. The cancer-associated single nucleotide polymorphisms (SNPs) of the p53 pathway have strikingly similar genetic characteristics to well-studied p53 pathway cancer-causing somatic mutations. Our results enable insights into p53-mediated tumour suppression in humans and into p53 pathway-based cancer surveillance and treatment strategies.

The Roles of MDM2 and MDMX in Cancer

For more than 25 years, MDM2 and its homolog MDMX (also known as MDM4) have been shown to exert oncogenic activity. These two proteins are best understood as negative regulators of the p53 tumor suppressor, although they may have additional p53-independent roles. Understanding the dysregulation of MDM2 and MDMX in human cancers and how they function either together or separately in tumorigenesis may improve methods of diagnosis and for assessing prognosis. Targeting the proteins themselves, or their regulators, may be a promising therapeutic approach to treating some forms of cancer.

The MDM4 SNP34091 (rs4245739) C-allele is associated with increased risk of ovarian-but not endometrial cancer

The MDM4 protein (also known as MDMX or HDMX) is a negative regulator of p53, not only by direct interaction but also through its interaction with MDM2. Further, MDM4 overexpression and amplification have been observed in several cancer forms. Recently, a single nucleotide polymorphism (SNP) in the 3’ untranslated region of the MDM4 gene, SNP34091A > C (rs4245739) was reported to alter MDM4 messenger RNA (mRNA) stability by modulating a microRNA binding site, thereby leading to decreased MDM4 levels. In this case-control study, we aimed to evaluate the possible association between MDM4 SNP34091 status and cancer risk by comparing the genotype frequencies in large hospital-based cohorts of endometrial- (n = 1404) and ovarian (n = 1385) cancer patients with healthy female controls (n = 1870). Genotype frequencies were compared by odds ratio (OR) estimates and Fisher exact tests. We found that individuals harboring the MDM4 SNP34091AC/CC genotypes had a significantly elevated risk for serous ovarian cancer (SOC) in general and high-grade serous ovarian cancer (HGSOC) in particular (SOC: OR = 1.18., 95 % CI = 1.01–1.39; HGSOC: OR = 1.25, CI = 1.02–1.53). No association between SNP34091 genotypes and endometrial cancer risk was observed. Our data indicate the MDM4 SNP34091AC/CC genotypes to be associated with an elevated risk for SOC and in particular the HGSOC type.

p53 Activity Dominates That of p73 upon Mdm4 Loss in Development and Tumorigenesis

Mdm4 negatively regulates the p53 tumor suppressor. Mdm4 loss in mice leads to an embryonic lethal phenotype that is p53-dependent. Biochemical studies indicate that Mdm4 also binds p73, a member of the p53 family, with higher affinity than p53. In this study, the significance of the Mdm4 and p73 interaction in vivo during embryogenesis and tumorigenesis was examined. The data revealed that p73 loss did not rescue either the early Mdm4-deficient embryonic lethality or the runted phenotype of Mdm4(Δ2/Δ2) p53(+/-) embryos. Furthermore, studies in the developing central nervous system wherein both genes have prominent roles indicated that loss of p73 also did not rescue the Mdm4-null brain phenotype as did p53 loss. This p53 dependency occurred despite evidence for p73-specific transcriptional activity. In tumor studies, the combination of Mdm4 overexpression and p73 loss did not alter survival of mice or the tumor spectrum as compared with Mdm4 overexpression alone. In summary, these data demonstrate that the Mdm4-p73 axis cannot override the dominant role of p53 in development and tumorigenesis.

IMPLICATIONS

Genetic characterization of the Mdm4 and p73 interaction during development and tumorigenesis suggests new insight into the role of p53 family members, which may influence treatment options for patients.

MDM4 SNP34091 (rs4245739) and its effect on breast-, colon-, lung-, and prostate cancer risk

The MDM4 protein plays an important part in the negative regulation of the tumor suppressor p53 through its interaction with MDM2. In line with this, MDM4 amplification has been observed in several tumor forms. A polymorphism (rs4245739 A>C; SNP34091) in the MDM4 3′ untranslated region has been reported to create a target site for hsa‐miR‐191, resulting in decreased MDM4 mRNA levels. In this population‐based case–control study, we examined the potential association between MDM4 SNP34091, alone and in combination with the MDM2 SNP309T>G (rs2279744), and the risk of breast‐, colon‐, lung‐, and prostate cancer in Norway. SNP34091 was genotyped in 7,079 cancer patients as well as in 3,747 gender‐ and age‐matched healthy controls. MDM4 SNP34091C was not associated with risk for any of the tumor forms examined, except for a marginally significant association with reduced risk for breast cancer in a recessive model (OR = 0.77: 95% CI = 0.59–0.99). Stratifying according to MDM2 SNP309 status, we observed a reduced risk for breast cancer related to MDM4 SNP34091CC among individuals harboring the MDM2 SNP309GG genotype (OR = 0.41; 95% CI = 0.21–0.82). We conclude, MDM4 SNP34091 status to be associated with reduced risk of breast cancer, in particular in individuals carrying the MDM2 SNP309GG genotype, but not to be associated with either lung‐, colon‐ or prostate cancer.

A Regulatory MDM4 Genetic Variant Locating in the Binding Sequence of Multiple MicroRNAs Contributes to Susceptibility of Small Cell Lung Cancer

A functional rs4245739 A>C single nucleotide polymorphism (SNP) locating in the MDM43’-untranslated (3’-UTR) region creates a miR-191-5p or miR-887-3p targeting sites. This change results in decreased expression of oncogene MDM4. Therefore, we examined the association between this SNP and small cell lung cancer (SCLC) risk as well as its regulatory function in SCLC cells. Genotypes were determined in two independent case-control sets consisted of 520SCLC cases and 1040 controls from two regions of China. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated by logistic regression. The impact of the rs4245739 SNP on miR-191-5p/miR-887-3p mediated MDM4 expression regulation was investigated using luciferase reporter gene assays. We found that the MDM4 rs4245739AC and CC genotypes were significantly associated with decreased SCLC susceptibility compared with the AA genotype in both case-control sets (Shandong set: OR = 0.53, 95% CI = 0.32–0.89, P = 0.014; Jiangsu set: OR = 0.47, 95% CI = 0.26–0.879, P = 0.017). Stratified analyses indicated that there was a significantly multiplicative interaction between rs4245739 and smoking (P_interactioin = 0.048). After co-tranfection of miRNAs and different allelic-MDM4 reporter constructs into SCLC cells, we found that the both miR-191-5p and miR-887-3p can lead to significantly decreased MDM4 expression activities in the construct with C-allelic 3’-UTR but not A-allelic 3’-UTR, suggesting a consistent genotype-phenotype correlation. Our data illuminate that the MDM4rs4245739SNP contributes to SCLC risk and support the notion that gene 3’-UTR genetic variants, impacting miRNA-binding, might modify SCLC susceptibility.

MDM4/HIPK2/p53 cytoplasmic assembly uncovers coordinated repression of molecules with anti-apoptotic activity during early DNA damage response

The p53 inhibitor, MDM4 (MDMX) is a cytoplasmic protein with p53-activating function under DNA damage conditions. Particularly, MDM4 promotes phosphorylation of p53 at Ser46, a modification that precedes different p53 activities. We investigated the mechanism by which MDM4 promotes this p53 modification and its consequences in untransformed mammary epithelial cells and tissues. In response to severe DNA damage, MDM4 stimulates p53Ser46^P by binding and stabilizing serine–threonine kinase HIPK2. Under these conditions, the p53-inhibitory complex, MDM4/MDM2, dissociates and this allows MDM4 to promote p53/HIPK2 functional interaction. Comparative proteomic analysis of DNA damage-treated cells versus -untreated cells evidenced a diffuse downregulation of proteins with anti-apoptotic activity, some of which were targets of p53Ser46^P/HIPK2 repressive activity. Importantly, MDM4 depletion abolishes the downregulation of these proteins indicating the requirement of MDM4 to promote p53-mediated transcriptional repression. Consistently, MDM4-mediated HIPK2/p53 activation precedes HIPK2/p53 nuclear translocation and activity. Noteworthy, repression of these proteins was evident also in mammary glands of mice subjected to γ-irradiation and was significantly enhanced in transgenic mice overexpressing MDM4. This study evidences the flexibility of MDM2/MDM4 heterodimer, which allows the development of a positive activity of cytoplasmic MDM4 towards p53-mediated transcriptional function. Noteworthy, this activity uncovers coordinated repression of molecules with shared anti-apoptotic function which precedes active cell apoptosis and that are frequently overexpressed and/or markers of tumour phenotype in human cancer.

Mdmx promotes genomic instability independent of p53 and Mdm2

The oncogene Mdmx is overexpressed in many human malignancies, and together with Mdm2, negatively regulates the p53 tumor suppressor. However, a p53-independent function of Mdmx that impacts genome stability has been described, but this function is not well understood. In the present study, we determined that of the 13 different cancer types evaluated, 6-90% of those that had elevated levels of Mdmx had concurrent inactivation (mutated or deleted) of p53. We show elevated levels of Mdmx-inhibited double-strand DNA break repair and induced chromosome and chromatid breaks independent of p53, leading to genome instability. Mdmx impaired early DNA damage-response signaling, such as phosphorylation of the serine/threonine-glutamine motif, mediated by the ATM kinase. Moreover, we identified Mdmx associated with Nbs1 of the Mre11-Rad50-Nbs1 (MRN) DNA repair complex, and this association increased upon DNA damage and was detected at chromatin. Elevated Mdmx levels also increased cellular transformation in a p53-independent manner. Unexpectedly, all Mdmx-mediated phenotypes also occurred in cells lacking Mdm2 and were independent of the Mdm2-binding domain (RING) of Mdmx. Therefore, Mdmx-mediated inhibition of the DNA damage response resulted in delayed DNA repair and increased genome instability and transformation independent of p53 and Mdm2. Our results reveal a novel p53- and Mdm2-independent oncogenic function of Mdmx that provides new insight into the many cancers that overexpress Mdmx.

Genetically engineered mouse and orthotopic human tumor xenograft models of retinoblastoma

Retinoblastoma is a rare pediatric cancer of the developing retina that initiates with biallelic inactivation of the RB1 gene. Murine models of retinoblastoma provide excellent tools for preclinical studies as well as for the study of the biological processes that drive tumorigenesis following Rb loss. In this chapter, we describe several genetically engineered mouse and orthotopic human xenograft models of retinoblastoma and discuss the advantages and disadvantages of these models.

The functional TP53 rs1042522 and MDM4 rs4245739 genetic variants contribute to Non-Hodgkin lymphoma risk

As a heterogeneous kind of malignances, Non-Hodgkin lymphoma (NHL) is the most common hematologic cancer worldwide with the significantly increased morbidity in China. Accumulated evidences demonstrated that oncoprotein MDM4 plays a crucial role in the TP53 tumor suppressor signaling pathway. An rs4245739 A>C polymorphism locating in the MDM4 3′-untranslated region creates a miR-191 target site and results in allele-specific MDM4 expression. In this study, we examined the association between this polymorphism as well as the TP53 Arg72Pro (rs1042522 G>C) genetic variant and Non-Hodgkin Lymphoma (NHL) risk in a Chinese Han population. Genotypes were determined in 200 NHL cases and 400 controls. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated by logistic regression. We found significantly increased NHL risk among carriers of the TP53 72Pro allele compared with those with the 72Arg allele (P = 0.002 for the Pro/Pro genotype). We also observed a significantly decreased NHL risks among carriers of the MDM4 rs4245739 C allele compared with those with the A allele in Chinese (P = 0.014 for the AC genotype). Stratified analyses revealed the associations between these SNPs and NHL risk are especially noteworthy in young or male individuals. Additionally, the associations are much pronounced in NHL patients with B-cell lymphomas or grade 3 or 4 disease. Our results indicate that the TP53 Arg72Pro and the MDM4 rs4245739 polymorphisms contribute to NHL susceptibility and support the hypothesis that genetic variants in the TP53 pathway genes can act as important modifiers of NHL risk.

Stress-induced alternative splice forms of MDM2 and MDMX modulate the p53-pathway in distinct ways

MDM2 and MDMX are the chief negative regulators of the tumor-suppressor protein p53 and are essential for maintaining homeostasis within the cell. In response to genotoxic stress and also in several cancer types, MDM2 and MDMX are alternatively spliced. The splice variants MDM2-ALT1 and MDMX-ALT2 lack the p53-binding domain and are incapable of negatively regulating p53. However, they retain the RING domain that facilitates dimerization of the full-length MDM proteins. Concordantly, MDM2-ALT1 has been shown to lead to the stabilization of p53 through its interaction with and inactivation of full-length MDM2. The impact of MDM2-ALT1 expression on the p53 pathway and the nature of its interaction with MDMX remain unclear. Also, the role of the architecturally similar MDMX-ALT2 and its influence of the MDM2-MDMX-p53 axis are yet to be elucidated. We show here that MDM2-ALT1 is capable of binding full-length MDMX as well as full-length MDM2. Additionally, we demonstrate that MDMX-ALT2 is able to dimerize with both full-length MDMX and MDM2 and that the expression of MDM2-ALT1 and MDMX-ALT2 leads to the upregulation of p53 protein, and also of its downstream target p21. Moreover, MDM2-ALT1 expression causes cell cycle arrest in the G1 phase in a p53 and p21 dependent manner, which is consistent with the increased levels of p21. Finally we present evidence that MDM2-ALT1 and MDMX-ALT2 expression can activate subtly distinct subsets of p53-transcriptional targets implying that these splice variants can modulate the p53 tumor suppressor pathway in unique ways. In summary, our study shows that the stress-inducible alternative splice forms MDM2-ALT1 and MDMX-ALT2 are important modifiers of the p53 pathway and present a potential mechanism to tailor the p53-mediated cellular stress response.

Tissue-specific and age-dependent effects of global Mdm2 loss

Mdm2, an E3 ubiquitin ligase, negatively regulates the tumour suppressor p53. In this study we utilized a conditional Mdm2 allele, Mdm2(FM) , and a CAG-CreER tamoxifen-inducible recombination system to examine the effects of global Mdm2 loss in adult mice. Two different tamoxifen injection regimens caused 100% lethality of Mdm2(FM) (/-) ;CAG-CreER mice; both radio-sensitive and radio-insensitive tissues were impaired. Strikingly, a large number of radio-insensitive tissues, including the kidney, liver, heart, retina and hippocampus, exhibited various pathological defects. Similar tamoxifen injections in older (16-18 month-old) Mdm2(FM) (/-) ;CAG-CreER mice yielded abnormalities only in the kidney. In addition, transcriptional activation of Cdkn1a (p21), Bbc3 (Puma) and multiple senescence markers in young (2-4 month-old) mice following loss of Mdm2 was dampened in older mice. All phenotypes were p53-dependent, as Mdm2(FM) (/-) ;Trp53(-/-) ;CAG-CreER mice subjected to the same tamoxifen regimens were normal. Our findings implicate numerous possible toxicities in many normal tissues upon use of cancer therapies that aim to inhibit Mdm2 in tumours with wild-type p53.

The Mdm network and its regulation of p53 activities: a rheostat of cancer risk

The potent transcriptional activity of p53 (Trp53, TP53) must be kept in check for normal cell growth and survival. Tumors, which drastically deviate from these parameters, have evolved multiple mechanisms to inactivate TP53, the most prevalent of which is the emergence of TP53 missense mutations, some of which have gain-of-function activities. Another important mechanism by which tumors bypass TP53 functions is via increased levels of two TP53 inhibitors, MDM2, and MDM4. Studies in humans and in mice reveal the complexity of TP53 regulation and the exquisite sensitivity of this pathway to small changes in regulation. Here, we summarize the factors that impinge on TP53 activity and thus cell death/arrest or tumor development.

Illuminating p53 function in cancer with genetically engineered mouse models

The key role of the p53 protein in tumor suppression is highlighted by its frequent mutation in human cancers and by the completely penetrant cancer predisposition of p53 null mice. Beyond providing definitive evidence for the critical function of p53 in tumor suppression, genetically engineered mouse models have offered numerous additional insights into p53 function. p53 knock-in mice expressing tumor-derived p53 mutants have revealed that these mutants display gain-of-function activities that actively promote carcinogenesis. The generation of p53 knock-in mutants with alterations in different domains of p53 has helped further elucidate the cellular and biochemical activities of p53 that are most fundamental for tumor suppression. In addition, modulation of p53 post-translational modification (PTM) status by generating p53 knock-in mouse strains with mutations in p53 PTM sites has revealed a subtlety and complexity to p53 regulation. Analyses of mouse models perturbing upstream regulators of p53 have solidified the notion that the p53 pathway can be compromised by means other than direct p53 mutation. Finally, switchable p53 models that allow p53 reactivation in tumors have helped evaluate the potential of p53 restoration therapy for cancer treatment. Collectively, mouse models have greatly enhanced our understanding of physiological p53 function and will continue to provide new biological and clinical insights in future investigations.

Mdm2 and MdmX involvement in human cancer

Discovered in 1987 and 1997 respectively, Mdm2 and MdmX represent two critical cellular regulators of the p53 tumor suppressor. This chapter reviews each from initial discovery to our current understanding of their deregulation in human cancer with a focus on how each regulator impacts p53 function. While p53 independent activities of Mdm2 and MdmX are noted the reader is directed to other reviews on this topic. The chapter concludes with an examination of the various mechanisms of Mdm-deregulation and an assessment of the current therapeutic approaches to target Mdm2 and MdmX overexpression.

Haploinsufficiency of del(5q) genes, Egr1 and Apc, cooperate with Tp53 loss to induce acute myeloid leukemia in mice

An interstitial deletion of chromosome 5, del(5q), is the most common structural abnormality in primary myelodysplastic syndromes (MDS) and therapy-related myeloid neoplasms (t-MNs) after cytotoxic therapy. Loss of TP53 activity, through mutation or deletion, is highly associated with t-MNs with a del(5q). We previously demonstrated that haploinsufficiency of Egr1 and Apc, 2 genes lost in the 5q deletion, are key players in the progression of MDS with a del(5q). Using genetically engineered mice, we now show that reduction or loss of Tp53 expression, in combination with Egr1 haploinsufficiency, increased the rate of development of hematologic neoplasms and influenced the disease spectrum, but did not lead to overt myeloid leukemia, suggesting that altered function of additional gene(s) on 5q are likely required for myeloid leukemia development. Next, we demonstrated that cell intrinsic loss of Tp53 in hematopoietic stem and progenitor cells haploinsufficient for both Egr1 and Apc led to the development of acute myeloid leukemia (AML) in 17% of mice. The long latency (234-299 days) and clonal chromosomal abnormalities in the AMLs suggest that additional genetic changes may be required for full transformation. Thus, loss of Tp53 activity in cooperation with Egr1 and Apc haploinsufficiency creates an environment that is permissive for malignant transformation and the development of AML.

Functional MDM4 rs4245739 genetic variant, alone and in combination with P53 Arg72Pro polymorphism, contributes to breast cancer susceptibility

The oncoprotein MDM4 plays an essential role in P53 tumor suppressor pathway through negative regulation of P53 function. It has been reported that the rs4245739 A > C polymorphism located in the MDM4 3'-untranslated region creates a miR-191 target site and results in decreased MDM4 expression. Therefore, we investigated the association of the MDM4 rs4245739 polymorphism as well as the P53 Arg72Pro genetic variant with the breast cancer risk. Genotypes were determined in two independent case-control sets consisting of 1100 breast cancer cases and 1400 controls from two regions of China. Odds ratios (ORs) and 95 % confidence intervals (CIs) were estimated by logistic regression. Our results demonstrated that the MDM4 rs4245739 AC and CC genotypes were significantly associated with decreased breast cancer risk compared to the AA genotype in both the case-control sets (Jinan set: OR = 0.55, 95 % CI 0.40-0.76, P = 2.3 × 10(-4); Huaian set: OR = 0.41, 95 % CI 0.25-0.67, P = 3.1 × 10(-4)). The P53 Arg/Pro genotype or Pro/Pro genotype was significantly associated with an increased risk of developing breast cancer, compared to the P53 Arg/Arg genotype in both the case-control sets (all P < 0.05). Interestingly, we observed a combinational effect between MDM4 rs4245739 and P53 Arg72Pro variants in attenuating breast cancer risk, highlighting the importance of the P53 tumor suppressor pathway genes during malignant transformation. Our results also support the hypothesis that genetic variants interrupting miRNA-mediated gene regulation might be important genetic modifiers of breast cancer risk.

Association of a genetic variation in a miR-191 binding site in MDM4 with risk of esophageal squamous cell carcinoma

As an oncoprotein, MDM4 plays a key part in P53 tumor suppressor pathway through negatively regulating P53 function. It has been reported that an rs4245739 A>C polymorphism locating in the MDM4 3′-untranslated region creates a miR-191 target site and results in decreased MDM4 expression. Therefore, we investigated the association between this polymorphism and esophageal squamous cell carcinoma (ESCC) risk as well as its biological function in vivo. Genotypes were determined in two independent case-control sets consisted of 1128 ESCC cases and 1150 controls from two regions of China. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated by logistic regression. The impact of the polymorphism on MDM4 expression was examined with esophagus tissues. Our results demonstrated that the MDM4 rs4245739 AC and CC genotypes were significantly associated with decreased ESCC risk compared with the AA genotype in both case-control sets (Jinan set: OR = 0.54, 95% CI = 0.35–0.82, P = 0.004; Huaian set: OR = 0.68, 95% CI = 0.45–0.99, P = 0.049). Stratified analyses revealed that a multiplicative interaction between rs4245739 and smoking or drinking was evident (Gene-smoking: P_interactioin = 0.022; gene-drinking: P_interactioin = 0.032). After detecting In vivo MDM4 mRNA expression, we found that the rs4245739 AC and CC genotype carriers had significantly decreased MDM4 expression in normal esophagus tissues compared with AA genotype carriers, indicating a consistent genotype-phenotype correlation. Our results elucidate that the MDM4 rs4245739 polymorphism contributes to susceptibility of ESCC and support the hypothesis that genetic variants, interrupting miRNA-mediated gene regulation, may modify cancer risk.

p53 dysfunction precedes the activation of nuclear factor-κB during disease progression in mice expressing Tax, a human T-cell leukemia virus type 1 oncoprotein

Transgenic (Tg) mice expressing Tax, a human T-cell leukemia virus type 1 (HTLV-1) oncoprotein, develop mature T-cell leukemia/lymphoma. The leukemic cells in Tg mice expressing Tax show p53 dysfunction and nuclear factor-κB (NF-κB) activation, similar to that seen in adult T-cell leukemia/lymphoma (ATLL) cells from patients infected with HTLV-1. However, it is unclear when these effects occur in HTLV-1 carriers during the development of ATLL. Here, we examined p53 function and NF-κB activity before the onset of leukemia in Tax-expressing Tg (Tax-Tg) mice between 4 and 25 months of age. At 4-10 months of age, 71% of mice showed p53 inactivation, without evidence for NF-κB activation, even though tax expression was consistent from 4 to 25 months of age. The decline in p53 function resulted from decreased p53 accumulation after DNA damage. From 11 months of age onward, 75% of mice showed p53 dysfunction and 37.5% showed constitutive NF-κB activation with the components of p50 and RelB. An NF-κB inhibitor, dehydroxymethylepoxyquinomicin (DHMEQ), reduced NF-κB activity (i.e. p50/RelB) but did not restore p53 function. In vivo, treatment with DHMEQ until 24 months of age prevented the onset of T-cell leukemia in Tax-Tg mice. These results suggest that the Tax-induced decline in p53 function, which is independent of NF-κB activation in the early stage, might be the first stage in the onset of ATLL. NF-κB activity is involved in the later stages of ATLL onset.

MDM2, MDMX and p53 in oncogenesis and cancer therapy

The MDM2 and MDMX (also known as HDMX and MDM4) proteins are deregulated in many human cancers and exert their oncogenic activity predominantly by inhibiting the p53 tumour suppressor. However, the MDM proteins modulate and respond to many other signalling networks in which they are embedded. Recent mechanistic studies and animal models have demonstrated how functional interactions in these networks are crucial for maintaining normal tissue homeostasis, and for determining responses to oncogenic and therapeutic challenges. This Review highlights the progress made and pitfalls encountered as the field continues to search for MDM-targeted antitumour agents.

Mouse models of Mdm2 and Mdm4 and their clinical implications

Mdm2 and Mdm4 are two key negative regulators of the tumor suppressor p53. Deletion of either Mdm2 or Mdm4 induces p53-dependent early embryonic lethality in knockout mouse models. The tissue-specific deletion of Mdm2 induces p53-dependent apoptosis, whereas the deletion of Mdm4 induces both p53-dependent apoptosis and cell cycle arrest. Compared to Mdm4 deletion, Mdm2 deletion causes more severe phenotypic defects. Disrupting the Mdm2 and Mdm4 interaction using knockin mice models causes embryonic lethality that can be completely rescued by the concomitant loss of p53, suggesting that Mdm2 and Mdm4 heterodimerization is critical to inhibit p53 activity during embryogenesis. Overexpression of Mdm2 and Mdm4 in mice induces spontaneous tumorigenesis, which clearly indicates that Mdm2 and Mdm4 are bona fide oncogenes. Studies from these mouse models strongly suggest that blocking Mdm2- and Mdm4-mediated p53 inhibition is an appealing therapeutic strategy for cancer patients with wild-type p53 alleles.

Molecular pathways: targeting Mdm2 and Mdm4 in cancer therapy

The p53 tumor suppressor is activated in response to cellular stresses to induce cell-cycle arrest, cellular senescence, and apoptosis. The p53 gene is inactivated by mutations in more than 50% of human tumors. In addition, tumor cells dampen p53 activities via overexpression of p53-negative regulators, in particular 2 structurally related proteins, Mdm2 and Mdm4. And yet, Mdm2 and Mdm4 possess p53-independent activities, which also contribute to tumor formation and progression. Given that Mdm2 and Mdm4 inhibit p53 activities to promote tumor development, small molecules and peptides were developed to abrogate the inhibition of p53 by Mdm proteins. Antitumor activities of these molecules have already been confirmed in preclinical studies and early-phase clinical trials. These research endeavors and clinical advances constitute the main focus of this review.

p53, SKP2, and DKK3 as MYCN Target Genes and Their Potential Therapeutic Significance

Neuroblastoma is the most common extra-cranial solid tumor of childhood. Despite significant advances, it currently still remains one of the most difficult childhood cancers to cure, with less than 40% of patients with high-risk disease being long-term survivors. MYCN is a proto-oncogene implicated to be directly involved in neuroblastoma development. Amplification of MYCN is associated with rapid tumor progression and poor prognosis. Novel therapeutic strategies which can improve the survival rates whilst reducing the toxicity in these patients are therefore required. Here we discuss genes regulated by MYCN in neuroblastoma, with particular reference to p53, SKP2, and DKK3 and strategies that may be employed to target them.