Spontaneous tumorigenesis in mice overexpressing the p53-negative regulator Mdm4

Protecting the genome from mdm2 and mdmx

The contribution of Mdm2, and its recently identified family member Mdmx (Mdm4), to tumorigenesis has primarily focused on their negative regulation of the p53 tumor suppressor. Although Mdm2 and Mdmx clearly inhibit p53, which can lead to tumor development, both have also been shown to affect tumorigenesis independent of p53. Given that Mdm2 and/or Mdmx overexpression is common and likely underestimated in human cancers, understanding the functions of these proteins beyond p53 control is critical. In recent years, new functions of Mdm2 and Mdmx that lead to genome instability, a hallmark of malignancy, have emerged. Specifically, roles in the DNA damage response that are distinct from their regulation of p53 have been identified. Inhibition of p53 as well as other components of the DNA damage response by Mdm2 and Mdmx can result in delayed DNA repair and increased genome instability, making Mdm2 and Mdmx a danger to the genome when aberrantly expressed. However, the genome instability caused by altered levels of Mdm2 and Mdmx could be used therapeutically for the treatment of cancer. Specifically, drugs/small molecules that target the interaction between Mdm2 and p53 can stabilize Mdm2, resulting in negative consequences on the genome that could be exploited for cancer treatment, particularly malignancies lacking functional p53.

Using Mouse Models to Explore MDM-p53 Signaling in Development, Cell Growth, and Tumorigenesis

The p53 transcription factor regulates the expression of numerous genes whose products affect cell proliferation, senescence, cellular metabolism, apoptosis, and DNA repair. These p53-mediated effects can inhibit the growth of stressed or mutated cells and suppress tumorigenesis in the organism. However, the various growth-inhibitory properties of p53 must be kept in check in nondamaged cells in order to facilitate proper embryogenesis or the homeostatic maintenance of adult tissues. This requisite inhibition of p53 is performed primarily by the MDM oncoproteins, Mdm2 and MdmX. These p53-binding proteins limit p53 activity both in normal cells and in stressed cells seeking to promote resolution of their p53-stress response. Many mouse models bearing genetic alterations in Mdm2 or MdmX have been generated to explore the function and regulation of MDM-p53 signaling in development, in tissue homeostasis, in aging, and in cancer. These models not only have demonstrated a critical need for Mdm2 and MdmX in normal cell growth and in development but more recently have identified the MDM-p53 signaling axis as a key regulator of the cellular response to a wide variety of genetic or metabolic stresses. In this review, we discuss what has been learned from various studies of these Mdm2 and MdmX mouse models and highlight a few of the many important remaining questions.

Genome-wide characterization of pancreatic adenocarcinoma patients using next generation sequencing

Pancreatic adenocarcinoma (PAC) is among the most lethal malignancies. While research has implicated multiple genes in disease pathogenesis, identification of therapeutic leads has been difficult and the majority of currently available therapies provide only marginal benefit. To address this issue, our goal was to genomically characterize individual PAC patients to understand the range of aberrations that are occurring in each tumor. Because our understanding of PAC tumorigenesis is limited, evaluation of separate cases may reveal aberrations, that are less common but may provide relevant information on the disease, or that may represent viable therapeutic targets for the patient. We used next generation sequencing to assess global somatic events across 3 PAC patients to characterize each patient and to identify potential targets. This study is the first to report whole genome sequencing (WGS) findings in paired tumor/normal samples collected from 3 separate PAC patients. We generated on average 132 billion mappable bases across all patients using WGS, and identified 142 somatic coding events including point mutations, insertion/deletions, and chromosomal copy number variants. We did not identify any significant somatic translocation events. We also performed RNA sequencing on 2 of these patients' tumors for which tumor RNA was available to evaluate expression changes that may be associated with somatic events, and generated over 100 million mapped reads for each patient. We further performed pathway analysis of all sequencing data to identify processes that may be the most heavily impacted from somatic and expression alterations. As expected, the KRAS signaling pathway was the most heavily impacted pathway (P<0.05), along with tumor-stroma interactions and tumor suppressive pathways. While sequencing of more patients is needed, the high resolution genomic and transcriptomic information we have acquired here provides valuable information on the molecular composition of PAC and helps to establish a foundation for improved therapeutic selection.

Genome-wide characterization of pancreatic adenocarcinoma patients using next generation sequencing

Pancreatic adenocarcinoma (PAC) is among the most lethal malignancies. While research has implicated multiple genes in disease pathogenesis, identification of therapeutic leads has been difficult and the majority of currently available therapies provide only marginal benefit. To address this issue, our goal was to genomically characterize individual PAC patients to understand the range of aberrations that are occurring in each tumor. Because our understanding of PAC tumorigenesis is limited, evaluation of separate cases may reveal aberrations, that are less common but may provide relevant information on the disease, or that may represent viable therapeutic targets for the patient. We used next generation sequencing to assess global somatic events across 3 PAC patients to characterize each patient and to identify potential targets. This study is the first to report whole genome sequencing (WGS) findings in paired tumor/normal samples collected from 3 separate PAC patients. We generated on average 132 billion mappable bases across all patients using WGS, and identified 142 somatic coding events including point mutations, insertion/deletions, and chromosomal copy number variants. We did not identify any significant somatic translocation events. We also performed RNA sequencing on 2 of these patients' tumors for which tumor RNA was available to evaluate expression changes that may be associated with somatic events, and generated over 100 million mapped reads for each patient. We further performed pathway analysis of all sequencing data to identify processes that may be the most heavily impacted from somatic and expression alterations. As expected, the KRAS signaling pathway was the most heavily impacted pathway (P<0.05), along with tumor-stroma interactions and tumor suppressive pathways. While sequencing of more patients is needed, the high resolution genomic and transcriptomic information we have acquired here provides valuable information on the molecular composition of PAC and helps to establish a foundation for improved therapeutic selection.

The p53 pathway in hematopoiesis: lessons from mouse models, implications for humans

Aberrations in the p53 tumor suppressor pathway are associated with hematologic malignancies. p53-dependent cell cycle control, senescence, and apoptosis functions are actively involved in maintaining hematopoietic homeostasis under normal and stress conditions. Whereas loss of p53 function promotes leukemia and lymphoma development in humans and mice, increased p53 activity inhibits hematopoietic stem cell function and results in myelodysplasia. Thus, exquisite regulation of p53 activity is critical for homeostasis. Most of our understanding of p53 function in hematopoiesis is derived from genetically engineered mice. Here we summarize some of these models, the various mechanisms that disrupt the regulation of p53 activity, and their relevance to human disease.

Developing genetically engineered mouse models to study tumor suppression

Since the late 1980s, the tools to generate mice with deletions of tumor suppressors have made it possible to study such deletions in the context of a whole animal. Deletion of some tumor suppressors results in viable mice while deletion of others yield embryo lethal phenotypes cementing the concept that genes that often go awry in cancer are also of developmental importance. More sophisticated mouse models were subsequently developed to delete a gene in a specific cell type at a specific time point. Additionally, incorporation of point mutations in a specific gene as observed in human tumors has also revealed their contributions to tumorigenesis. On the other hand, some models never develop cancer unless combined with other deletions suggesting a modifying role in tumorigenesis. This review will describe the technical aspects of generating these mice and provide examples of the outcomes obtained from alterations of different tumor suppressors.

Abnormal MDMX degradation in tumor cells due to ARF deficiency

MDMX is a hetero dimeric partner of MDM2 and a critical regulator of p53. MDMX level is generally elevated in tumors with wild type p53 and contributes to p53 inactivation. MDMX degradation is controlled in part by MDM2-mediated ubiquitination. Here we show that MDMX turnover is highly responsive to changes in MDM2 level in non-transformed cells, but not in tumor cells. We found that loss of ARF expression, which occurs in most tumors with wild type p53, significantly reduces MDMX sensitivity to MDM2. Restoration of ARF expression in tumor cells enables MDM2 to degrade MDMX in a dose-dependent fashion. ARF binds to MDM2 and stimulates a second-site interaction between the central region of MDM2 and MDMX, thus increases MDMX-MDM2 binding and MDMX ubiquitination. These results reveal an important abnormality in the p53 regulatory pathway as a consequence of ARF deficiency. Loss of ARF during tumor development not only prevents p53 stabilization by proliferative stress, but also causes accumulation of MDMX that compromises p53 activity. This phenomenon may reduce the clinical efficacy of MDM2-specific inhibitors by preventing MDMX down regulation.

Oncogenic functions of hMDMX in in vitro transformation of primary human fibroblasts and embryonic retinoblasts

BACKGROUND

In around 50% of all human cancers the tumor suppressor p53 is mutated. It is generally assumed that in the remaining tumors the wild-type p53 protein is functionally impaired. The two main inhibitors of p53, hMDM2 (MDM2) and hMDMX (MDMX/MDM4) are frequently overexpressed in wild-type p53 tumors. Whereas the main activity of hMDM2 is to degrade p53 protein, its close homolog hMDMX does not degrade p53, but it represses its transcriptional activity. Here we study the role of hMDMX in the neoplastic transformation of human fibroblasts and embryonic retinoblasts, since a high number of retinoblastomas contain elevated hMDMX levels.

METHODS

We made use of an in vitro transformation model using a retroviral system of RNA interference and gene overexpression in primary human fibroblasts and embryonic retinoblasts. Consecutive knockdown of RB and p53, overexpression of SV40-small t, oncogenic HRasV12 and HA-hMDMX resulted in a number of stable cell lines representing different stages of the transformation process, enabling a comparison between loss of p53 and hMDMX overexpression. The cell lines were tested in various assays to assess their oncogenic potential.

RESULTS

Both p53-knockdown and hMDMX overexpression accelerated proliferation and prevented growth suppression induced by introduction of oncogenic Ras, which was required for anchorage-independent growth and the ability to form tumors in vivo. Furthermore, we found that hMDMX overexpression represses basal p53 activity to some extent. Transformed fibroblasts with very high levels of hMDMX became largely resistant to the p53 reactivating drug Nutlin-3. The Nutlin-3 response of hMDMX transformed retinoblasts was intact and resembled that of retinoblastoma cell lines.

CONCLUSIONS

Our studies show that hMDMX has the essential properties of an oncogene. Its constitutive expression contributes to the oncogenic phenotype of transformed human cells. Its main function appears to be p53 inactivation. Therefore, developing new drugs targeting hMDMX is a valid approach to obtain new treatments for a subset of human tumors expressing wild-type p53.

Coexpression of normally incompatible developmental pathways in retinoblastoma genesis

It is widely believed that the molecular and cellular features of a tumor reflect its cell of origin and can thus provide clues about treatment targets. The retinoblastoma cell of origin has been debated for over a century. Here, we report that human and mouse retinoblastomas have molecular, cellular, and neurochemical features of multiple cell classes, principally amacrine/horizontal interneurons, retinal progenitor cells, and photoreceptors. Importantly, single-cell gene expression array analysis showed that these multiple cell type-specific developmental programs are coexpressed in individual retinoblastoma cells, which creates a progenitor/neuronal hybrid cell. Furthermore, neurotransmitter receptors, transporters, and biosynthetic enzymes are expressed in human retinoblastoma, and targeted disruption of these pathways reduces retinoblastoma growth in vivo and in vitro.

Functions of MDMX in the modulation of the p53-response

The MDM family proteins MDM2 and MDMX are two critical regulators of the p53 tumor suppressor protein. Expression of both proteins is necessary for allowing the embryonal development by keeping the activity of p53 in check. Upon stresses that need to activate p53 to perform its function as guardian of the genome, p53 has to be liberated from these two inhibitors. In this review, we will discuss the various mechanisms by which MDMX protein levels are downregulated upon various types of stress, including posttranslational modifications of the MDMX protein and the regulation of mdmx mRNA expression, including alternative splicing. In addition, the putative function(s) of the described MDMX splice variants, particularly in tumor development, will be discussed. Lastly, in contrast to common belief, we have recently shown the existence of a p53-MDMX feedback loop, which is important for dampening the p53-response at later phases after genotoxic stress.