High incidence of lung, bone, and lymphoid tumors in transgenic mice overexpressing mutant alleles of the p53 oncogene

p53 tetramerization: at the center of the dominant-negative effect of mutant p53

In this review, Gencel-Augusto and Lozano summarize the data on p53 mutants with a functional tetramerization domain that form mixed tetramers and in some cases have dominant-negative effects (DNE) that inactivate wild-type p53. They conclude that the DNE is mostly observed after DNA damage but fails in other contexts.

The p53 tumor suppressor functions as a tetrameric transcription factor to regulate hundreds of genes—many in a tissue-specific manner. Missense mutations in cancers in the p53 DNA-binding and tetramerization domains cement the importance of these domains in tumor suppression. p53 mutants with a functional tetramerization domain form mixed tetramers, which in some cases have dominant-negative effects (DNE) that inactivate wild-type p53. DNA damage appears necessary but not sufficient for DNE, indicating that upstream signals impact DNE. Posttranslational modifications and protein–protein interactions alter p53 tetramerization affecting transcription, stability, and localization. These regulatory components limit the dominant-negative effects of mutant p53 on wild-type p53 activity. A deeper understanding of the molecular basis for DNE may drive development of drugs that release WT p53 and allow tumor suppression.

The Role of Pre-Clinical 3-Dimensional Models of Osteosarcoma

Treatment for osteosarcoma (OS) has been largely unchanged for several decades, with typical therapies being a mixture of chemotherapy and surgery. Although therapeutic targets and products against cancer are being continually developed, only a limited number have proved therapeutically active in OS. Thus, the understanding of the OS microenvironment and its interactions are becoming more important in developing new therapies. Three-dimensional (3D) models are important tools in increasing our understanding of complex mechanisms and interactions, such as in OS. In this review, in vivo animal models, in vitro 3D models and in ovo chorioallantoic membrane (CAM) models, are evaluated and discussed as to their contribution in understanding the progressive nature of OS, and cancer research. We aim to provide insight and prospective future directions into the potential translation of 3D models in OS.

An update on the central nervous system manifestations of Li-Fraumeni syndrome

Li-Fraumeni syndrome (LFS), caused by the germline mutations in the TP53 gene, leads to significant lifetime risk to cancer in the central nervous system. Recognition of LFS, and elucidating its underlying cause has had a remarkable effect on our knowledge of the biology of brain tumors and represents a significant opportunity for cancer surveillance and screening. In this review, we discuss the historical context of the LFS with an emphasis on the clinicopathologic implications in clincal diagnosis, germline testing, and clinical management of brain tumor patients.

Alternative Mechanisms of p53 Action During the Unfolded Protein Response

The tumor suppressor protein p53 orchestrates cellular responses to a vast number of stresses, with DNA damage and oncogenic activation being some of the best described. The capacity of p53 to control cellular events such as cell cycle progression, DNA repair, and apoptosis, to mention some, has been mostly linked to its role as a transcription factor. However, how p53 integrates different signaling cascades to promote a particular pathway remains an open question. One way to broaden its capacity to respond to different stimuli is by the expression of isoforms that can modulate the activities of the full-length protein. One of these isoforms is p47 (p53/47, Δ40p53, p53ΔN40), an alternative translation initiation variant whose expression is specifically induced by the PERK kinase during the Unfolded Protein Response (UPR) following Endoplasmic Reticulum stress. Despite the increasing knowledge on the p53 pathway, its activity when the translation machinery is globally suppressed during the UPR remains poorly understood. Here, we focus on the expression of p47 and we propose that the alternative initiation of p53 mRNA translation offers a unique condition-dependent mechanism to differentiate p53 activity to control cell homeostasis during the UPR. We also discuss how the manipulation of these processes may influence cancer cell physiology in light of therapeutic approaches.

Expression of Hematopoietic Stem and Endothelial Cell Markers in Canine Hemangiosarcoma

Several chemicals and pharmaceuticals increase the incidence of hemangiosarcomas (HSAs) in mice, but the relevance to humans is uncertain. Recently, canine HSAs were identified as a powerful tool for investigating the pathogenesis of human HSAs. To characterize the cellular phenotype of canine HSAs, we evaluated immunoreactivity and/or messenger RNA (mRNA) expression of markers for hematopoietic stem cells (HSCs), endothelial cells (ECs), a tumor suppressor protein, and a myeloid marker in canine HSAs. Neoplastic canine cells expressed EC markers and a myeloid marker, but expressed HSC markers less consistently. The canine tumor expression results were then compared to previously published immunoreactivity results for these markers in human and mouse HSAs. There are 2 noteworthy differences across species: (1) most human HSAs had HSC marker expression, indicating that they were comprised of tumor cells that were less differentiated than those in canine and mouse tumors; and (2) human and canine HSAs expressed a late-stage EC maturation marker, whereas mouse HSAs were negative, suggesting that human and canine tumors may retain greater differentiation potential than mouse tumors. These results indicate that HSA development is variable across species and that caution is necessary when discussing translation of carcinogenic risk from animal models to humans.

BRCA1/P53: Two strengths in cancer chemoprevention

Increasing emphasis has been given to prevention as a feasible approach to reduce the cancer burden. However, for its clinical success, further advances are required to identify effective chemopreventive agents. This review affords a critical and up-to-date discussion of issues related to cancer prevention, including an in-depth knowledge on BRCA1 and p53 tumor suppressor proteins as key molecular players. Indeed, it compiles the most recent advances on the topic, highlighting the unique potential of BRCA1 and p53 germline mutations as molecular biomarkers for risk assessment and targets for chemoprevention. Relevant evidences are herein provided supporting the effectiveness of distinct pharmacological agents in cancer prevention, by targeting BRCA1 and p53. Moreover, the rationale for using germline mutant BRCA1- or p53-related cancer syndromes as model systems to investigate effective chemopreventive agents is also addressed. Altogether, this work provides an innovative conception about the dependence on p53 and BRCA1 co-inactivation in tumor formation and development, emphasizing the relationship between these two proteins as an encouraging direction for future personalized pharmacological interventions in cancer prevention.

Mutant p53 in cancer therapy-the barrier or the path

Since wild-type p53 is central for maintaining genomic stability and preventing oncogenesis, its coding gene TP53 is highly mutated in ~50% of human cancers, and its activity is almost abrogated in the rest of cancers. Approximately 80% of p53 mutations are single point mutations with several hotspot mutations. Besides loss of function and dominant-negative effect on the wild-type p53 activity, the hotspot p53 mutants also acquire new oncogenic functions, so-called 'gain-of-functions' (GOF). Because the GOF of mutant p53 is highly associated with late-stage malignance and drug resistance, these p53 mutants have become hot targets for developing novel cancer therapies. In this essay, we review some recent progresses in better understanding of the role of mutant p53 GOF in chemoresistance and the underlying mechanisms, and discuss the pros and cons of targeting mutant p53 for the development of anti-cancer therapies.

Drosophila p53 directs nonapoptotic programs in postmitotic tissue

TP53 is the most frequently mutated gene in human cancers, and despite intensive research efforts, genome-scale studies of p53 function in whole animal models are rare. The need for such in vivo studies is underscored by recent challenges to established paradigms, indicating that unappreciated p53 functions contribute to cancer prevention. Here we leveraged the Drosophila system to interrogate p53 function in a postmitotic context. In the developing embryo, p53 robustly activates important apoptotic genes in response to radiation-induced DNA damage. We recently showed that a p53 enhancer (p53RE^rpr) near the cell death gene reaper forms chromatin contacts and enables p53 target activation across long genomic distances. Interestingly, we found that this canonical p53 apoptotic program fails to activate in adult heads. Moreover, this failure to exhibit apoptotic responses was not associated with altered chromatin contacts. Instead, we determined that p53 does not occupy the p53RE^rpr enhancer in this postmitotic tissue as it does in embryos. Through comparative RNA-seq and chromatin immunoprecipitation-seq studies of developing and postmitotic tissues, we further determined that p53 regulates distinct transcriptional programs in adult heads, including DNA repair, metabolism, and proteolysis genes. Strikingly, in the postmitotic context, p53-binding landscapes were poorly correlated with nearby transcriptional effects, raising the possibility that p53 enhancers could be generally acting through long distances.

tp53 deficiency causes a wide tumor spectrum and increases embryonal rhabdomyosarcoma metastasis in zebrafish

The TP53 tumor-suppressor gene is mutated in >50% of human tumors and Li-Fraumeni patients with germ line inactivation are predisposed to developing cancer. Here, we generated tp53 deleted zebrafish that spontaneously develop malignant peripheral nerve-sheath tumors, angiosarcomas, germ cell tumors, and an aggressive Natural Killer cell-like leukemia for which no animal model has been developed. Because the tp53 deletion was generated in syngeneic zebrafish, engraftment of fluorescent-labeled tumors could be dynamically visualized over time. Importantly, engrafted tumors shared gene expression signatures with predicted cells of origin in human tissue. Finally, we showed that tp53^del/del enhanced invasion and metastasis in kRAS^G12D-induced embryonal rhabdomyosarcoma (ERMS), but did not alter the overall frequency of cancer stem cells, suggesting novel pro-metastatic roles for TP53 loss-of-function in human muscle tumors. In summary, we have developed a Li-Fraumeni zebrafish model that is amenable to large-scale transplantation and direct visualization of tumor growth in live animals.

p53 Isoforms and Their Implications in Cancer

In this review we focus on the major isoforms of the tumor-suppressor protein p53, dysfunction of which often leads to cancer. Mutations of the TP53 gene, particularly in the DNA binding domain, have been regarded as the main cause for p53 inactivation. However, recent reports demonstrating abundance of p53 isoforms, especially the N-terminally truncated ones, in the cancerous tissues suggest their involvement in carcinogenesis. These isoforms are ∆40p53, ∆133p53, and ∆160p53 (the names indicate their respective N-terminal truncation). Due to the lack of structural and functional characterizations the modes of action of the p53 isoforms are still unclear. Owing to the deletions in the functional domains, these isoforms can either be defective in DNA binding or more susceptive to altered ‘responsive elements’ than p53. Furthermore, they may exert a ‘dominant negative effect’ or induce more aggressive cancer by the ‘gain of function’. One possible mechanism of p53 inactivation can be through tetramerization with the ∆133p53 and ∆160p53 isoforms—both lacking part of the DNA binding domain. A recent report and unpublished data from our laboratory also suggest that these isoforms may inactivate p53 by fast aggregation—possibly due to ectopic overexpression. We further discuss the evolutionary significance of the p53 isoforms.

Genetic Events Responsible for Colorectal Tumorigenesis: Achievements and Challenges

Colorectal carcinogenesis is a multistep process that is accompanied by accumulation of changes in proto-oncogenes and tumor-suppressor genes. APC/MCC, RAS, DCC, p53 mutations and/or allelic losses, hyperexpression of c-MYC and RB genes, as well as other genomic alterations appear at characteristic stages of tumor development and are observed in most neoplasms. However, consideration of each of these abnormalities leaves many unanswered questions. The striking data on recurrent amplification of the RB tumor-suppressor gene as well as suppressive activities of protein kinase C and activated RAS genes, at least in some colon carcinoma cell lines, suggest the unusual effects of some signalling pathways in colonic epithelial cells. The results obtained to date indicate that distinct sets of genetic changes may underlie the development of colorectal tumors.

A General Overview of the Process of Carcinogenesis

A general synthetic overview of the process of carcinogenesis is presented. The following points are discussed: the uniqueness of tumor disease with respect to other pathologies; tumors viewed as a pathology of the transduction system of signals that regulate the communal life of the cells of multicell organisms; the tumor as a genetic disease of somatic cells; carcinogenesis as a multistage event; the fundamental role of physiologic and pathologic rhythms of cell proliferation in the modulation of tumor incidence; mechanisms entailed in the maintenance of genome integrity; mechanisms involved in the protection of genome integrity from exogenous and endogenous causes of degradation of the genetic message.

p53 Accumulation in Malignant Bone Tumors: An Immunohistochemical Analysis of 217 Cases

Using formalin-fixed paraffin-embedded tissue specimens, the authors studied the accumulation of p53 protein in various malignant bone tumors. p53 accumulation was detected in osteosarcoma (16.7%), malignant fibrous histiocytoma (MFH) of bone (30.4%), and chordoma (16.7%). In osteosarcoma, no difference was seen in the incidence of p53 accumulation according to the histologic grade of malignancy and any clinical factors including prognosis, but the incidence was significantly higher in conventional osteosarcoma than in other subtypes of osteosarcoma. In MFH of bone, the p53-positive group had a significantly worse prognosis compared with the p53-negative group. p53 accumulation is thus considered to be related to the prognosis of MFH of bone although it did not demonstrate any prognostic value for osteosarcoma in the current study.

Requirements for a Biologically Realistic Cancer Risk Assessment for Inorganic Arsenic

A remarkable feature of the carcinogenicity of inorganic arsenic (As,) is the observation that human exposures to Asi have been strongly associated with increases in skin, lung, and internal cancers, but As, does not typically cause tumors in standard laboratory animal test protocols. Considerable controversy has centered on whether there is epidemiological evidence of a “threshold” for the carcinogenic effects of Asi, or at least of a highly nonlinear dose–response. Saturation of metabolism in the dose-range associated with tumors does not appear to be adequate to produce a major impact on the dose-response for carcinogenicity. If there is a strong nonlinearity, it results from the nature of the carcinogenic mechanism(s) of Asi. However, no single hypothesis for the mechanism of Asi carcinogenicity has widespread support. A biologically realistic cancer risk assessment for Asi would requirea quantitative description of the dose of active arsenic species in target tissues, the interactions between active arsenic and tissue constituents, and the manner in which these interactions result in tumor formation in multiple organs in humans, but not in experimental animals. Although Asi has only infrequently been associated with tumors in animal studies, it has repeatedly been shown to act as a comutagen in vitro and as a cocarcinogen in vivo. Asi is clastogenic, producing chromatid aberrations, but does not produce point mutations at single gene loci. Of particular interest, Asi has been shown to inhibit repair of DNA single-strand breaks, a possible mechanism for its observed comutagenicity and cocarcinogenicity. We propose a cocarcinogenic mode of action in which Asi acts primarily on intermediate cells deficient in cell cycle control at a late stage in a preexisting carcinogenic process. This interaction enhances ge-nomic fragility and accelerates conversion of premalignant lesions to more aggressive, clinically observable tumors. An indirect effect of As, on DNA repair is consistent with the expectation of a nonlinear dose-response rather than the linear dose-response traditionally assumed for mutagenic carcinogens. However, defining the exact nature of this tumor dose-response will require further experimental data on the dose-response for the cellular effects of Asi. Because Asi carcinogenicity is unlikely to be observed in normal experimental animals not exposed to other carcinogens, studies in animals and cell lines deficient in cell cycle control should also be considered. Experimental studies specifically designed to address the key mechanistic and dose-response issues for Asi carcinogenicity are critically needed to support public health policy decisions regarding current environmental exposures to Asi.

Inactivation of p53 in Human Keratinocytes Leads to Squamous Differentiation and Shedding via Replication Stress and Mitotic Slippage

Tumor suppressor p53 is a major cellular guardian of genome integrity, and its inactivation is the most frequent genetic alteration in cancer, rising up to 80% in squamous cell carcinoma (SCC). By adapting the small hairpin RNA (shRNA) technology, we inactivated endogenous p53 in primary epithelial cells from the epidermis of human skin. We show that either loss of endogenous p53 or overexpression of a temperature-sensitive dominant-negative conformation triggers a self-protective differentiation response, resulting in cell stratification and expulsion. These effects follow DNA damage and exit from mitosis without cell division. p53 preserves the proliferative potential of the stem cell compartment and limits the power of proto-oncogene MYC to drive cell cycle stress and differentiation. The results provide insight into the role of p53 in self-renewal homeostasis and help explain why p53 mutations do not initiate skin cancer but increase the likelihood that cancer cells will appear.

Animal models in osteosarcoma

Osteosarcoma (OS) is the most common non-hematologic primary tumor of bone in children and adults. High-dose cytotoxic chemotherapy and surgical resection have improved prognosis, with long-term survival for non-metastatic disease approaching 70%. However, most OS tumors are high grade and tend to rapidly develop pulmonary metastases. Despite clinical advances, patients with metastatic disease or relapse have a poor prognosis. Toward a better understanding of the molecular pathogenesis of human OS, several genetically modified OS mouse models have been developed and will be reviewed here. However, better animal models that more accurately recapitulate the natural progression of the disease are needed for the development of improved prognostic and diagnostic markers as well as targeted therapies for both primary and metastatic OS.

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.

RUNX Family Participates in the Regulation of p53-Dependent DNA Damage Response

A proper DNA damage response (DDR), which monitors and maintains the genomic integrity, has been considered to be a critical barrier against genetic alterations to prevent tumor initiation and progression. The representative tumor suppressor p53 plays an important role in the regulation of DNA damage response. When cells receive DNA damage, p53 is quickly activated and induces cell cycle arrest and/or apoptotic cell death through transactivating its target genes implicated in the promotion of cell cycle arrest and/or apoptotic cell death such as p21^WAF1, BAX, and PUMA. Accumulating evidence strongly suggests that DNA damage-mediated activation as well as induction of p53 is regulated by posttranslational modifications and also by protein-protein interaction. Loss of p53 activity confers growth advantage and ensures survival in cancer cells by inhibiting apoptotic response required for tumor suppression. RUNX family, which is composed of RUNX1, RUNX2, and RUNX3, is a sequence-specific transcription factor and is closely involved in a variety of cellular processes including development, differentiation, and/or tumorigenesis. In this review, we describe a background of p53 and a functional collaboration between p53 and RUNX family in response to DNA damage.

p53 regulates mesenchymal stem cell-mediated tumor suppression in a tumor microenvironment through immune modulation

p53 is one of the most studied genes in cancer biology, and mutations in this gene may be predictive for the development of many types of cancer in humans and in animals. However, whether p53 mutations in non-tumor stromal cells can affect tumor development has received very little attention. In this study, we show that B16F0 melanoma cells form much larger tumors in p53-deficient mice than in wild-type mice, indicating a potential role of p53 deficiency in non-tumor cells of the microenvironment. As mesenchymal stem cells (MSCs) are attracted to tumors and form a major component of the tumor microenvironment, we examined the potential role of p53 status in MSCs in tumor development. We found that larger tumors resulted when B16F0 melanoma cells were co-injected with bone marrow MSCs derived from p53-deficient mice rather than MSCs from wild-type mice. Interestingly, this tumor-promoting effect by p53-deficient MSCs was not observed in non-obese diabetic/severe combined immunodeficiency mice, indicating the immune response has a critical role. Indeed, in the presence of inflammatory cytokines, p53-deficient MSCs expressed more inducible nitric oxide synthase (iNOS) and exhibited greater immunosuppressive capacity. Importantly, tumor promotion by p53-deficient MSCs was abolished by administration of S-methylisothiourea, an iNOS inhibitor. Therefore, our data demonstrate that p53 status in tumor stromal cells has a key role in tumor development by modulating immune responses.

Tumor protein p53 (TP53) testing and Li-Fraumeni syndrome : current status of clinical applications and future directions

Prevalent as an acquired abnormality in cancer, the role of tumor protein p53 (TP53) as a germline mutation continues to evolve. The clinical impact of a germline TP53 mutation is often dramatic and affects the full life course, with a propensity to develop rare tumors in childhood and multiple common cancers of unexpectedly early onset in adulthood. In this article, we review the clinical relevance of germline mutations in the TP53 tumor suppressor gene to current healthcare practice, including the optimal ways to identify patients with Li-Fraumeni syndrome (LFS), to recognize the core cancers associated with LFS, and to develop strategies for early detection of LFS-associated tumors. Several TP53-targeted approaches to improve outcomes in LFS patients are also reviewed. A case report is used to highlight special TP53 testing dilemmas and unique challenges associated with genetic testing decisions in the current age of rapidly advancing genomic technologies.

Senescence regulation by the p53 protein family

p53, a guardian of the genome, exerts its tumor suppression activity by regulating a large number of downstream targets involved in cell cycle arrest, DNA repair, apoptosis, and cellular senescence. Although p53-mediated apoptosis is able to kill cancer cells, a role for cellular senescence in p53-dependent tumor suppression is becoming clear. Mouse studies showed that activation of p53-induced premature senescence promotes tumor regression in vivo. However, p53-mediated cellular senescence also leads to aging-related phenotypes, such as tissue atrophy, stem cell depletion, and impaired wound healing. In addition, several p53 isoforms and two p53 homologs, p63 and p73, have been shown to play a role in cellular senescence and/or aging. Importantly, p53, p63, and p73 are necessary for the maintenance of adult stem cells. Therefore, understanding the dual role the p53 protein family in cancer and aging is critical to solve cancer and longevity in the future. In this chapter, we provide an overview on how p53, p63, p73, and their isoforms regulate cellular senescence and aging.

Genetically engineered mouse models and human osteosarcoma

Osteosarcoma is the most common form of bone cancer. Pivotal insight into the genes involved in human osteosarcoma has been provided by the study of rare familial cancer predisposition syndromes. Three kindreds stand out as predisposing to the development of osteosarcoma: Li-Fraumeni syndrome, familial retinoblastoma and RecQ helicase disorders, which include Rothmund-Thomson Syndrome in particular. These disorders have highlighted the important roles of P53 and RB respectively, in the development of osteosarcoma. The association of OS with RECQL4 mutations is apparent but the relevance of this to OS is uncertain as mutations in RECQL4 are not found in sporadic OS. Application of the knowledge or mutations of P53 and RB in familial and sporadic OS has enabled the development of tractable, highly penetrant murine models of OS. These models share many of the cardinal features associated with human osteosarcoma including, importantly, a high incidence of spontaneous metastasis. The recent development of these models has been a significant advance for efforts to improve our understanding of the genetics of human OS and, more critically, to provide a high-throughput genetically modifiable platform for preclinical evaluation of new therapeutics.

In vivo animal models of spinal metastasis

The vertebral column is the commonest site for skeletal metastases, with breast, prostate and lung cancers being the most common primary sources. The spine has structural and neural-protective properties thus involvement by metastatic cancer often causes bony instability and fracture, intractable pain and neurological deficit. In vivo animal models which resemble the human condition are essential in order to improve understanding of the pathophysiology behind the spread of metastatic cancer to the spine and its subsequent local growth and invasion, to enable in-depth analysis of the interaction between host and tumour cells and the molecular processes behind local cancer invasion and barriers to invasion as well as to allow assessment of novel treatment modalities for spinal metastases. This review summarizes the current status of the animal models specifically used for the study of spinal metastasis, their relevance, advantages and limitations, and important considerations for the development of future in vivo animal models.

Towards an understanding of the role of p53 in adrenocortical carcinogenesis

Adrenocortical carcinoma (ACC) is recognized to be a component tumor of the Li Fraumeni Syndrome (LFS), a familial cancer predisposition resulting from germline mutations in the p53 tumor-suppressor. p53 activity is tightly regulated by multiple post-translational mechanisms, disruption of which may lead to tumorigenesis. ACC is present in disproportionately high rates among p53-mutation carriers, suggesting tissue-specific manifestations of p53 deficiency. Additionally, p53-associated ACC demonstrates a strong predominance in infants and children. Several of the p53 alleles associated with pediatric ACC, however, retain significant wild-type activity and demonstrate incomplete penetrance, a finding distinct from other LFS-component tumors. In this review, we discuss the relationship between p53 and adrenocortical carcinogenesis, with specific focus on disease-specific alleles, tumorigenesis in the context of adrenal development and potential therapeutic approaches to p53-associated ACC.

Animal models of lung cancer characterization and use for chemoprevention research

Of the potential sites of cancer development, cancer of the lung accounts for the highest number of cancer deaths each year in the United States (Jemal et al., 2010(1)). Based on its histopathological features, lung cancer is grouped into small cell lung cancer (SCLC; ∼20%) and non-SCLC (NSCLC; ∼80%), which is further divided into three subtypes: squamous cell carcinoma (∼30%), adenocarcinoma (∼50%), and large cell lung carcinoma. Every subtype of lung cancer has a relatively low 5-year survival rate that is attributed, in part, to the fact that they are routinely diagnosed at later histologic stages. Due to this alarming statistic, it is necessary to develop not only new and effective means of treatment but also of prevention. One of the promising approaches is chemoprevention which is the use of synthetic or natural agents to inhibit the initial development of or further progression of early lung lesions (Hong and Sporn, 1997). Many compounds have been identified as potentially effective chemopreventive agents using animal models. Most chemopreventive studies have been performed using mouse models which were developed to study lung adenomas or adenocarcinomas. More recently, models of squamous cell lung cancer and small cell lung cancer have also been developed. This review seeks to highlight mouse models which we helped to develop and presents the results of recent chemopreventive studies that we have performed in models of lung adenocarcinoma, squamous cell carcinoma, and small cell lung cancer.

Designing phenotyping studies for genetically engineered mice

A phenotyping study records physiologic or morphologic changes in an experimental animal resulting from an intervention. In mice, this intervention is most frequently genetic, but it may be any type of experimental manipulation. Accurate representation of the human condition under study is essential if the model is to yield useful conclusions. In this review, general approaches to the design of phenotyping studies are considered. These approaches take into account major sources of reduced model validity, such as unexpected phenotypic variation in mice, evolutionary divergence between mice and humans, unanticipated sources of variation, and common design errors. As poor design is the most common reason why studies fail to yield enduring results, emphasis is placed on reduction of bias, sampling, controlled study design, and appropriate statistical analysis.

Transgenic and knockout mice models to reveal the functions of tumor suppressor genes

Cancer is caused by multiple genetic alterations leading to uncontrolled cell proliferation through multiple pathways. Malignant cells arise from a variety of genetic factors, such as mutations in tumor suppressor genes (TSGs) that are involved in regulating the cell cycle, apoptosis, or cell differentiation, or maintenance of genomic integrity. Tumor suppressor mouse models are the most frequently used animal models in cancer research. The anti-tumorigenic functions of TSGs, and their role in development and differentiation, and inhibition of oncogenes are discussed. In this review, we summarize some of the important transgenic and knockout mouse models for TSGs, including Rb, p53, Ink4a/Arf, Brca1/2, and their related genes.

TAp73 acts via the bHLH Hey2 to promote long-term maintenance of neural precursors

Increasing evidence suggests that deficits in adult stem cell maintenance cause aberrant tissue repair and premature aging [1]. While the mechanisms regulating stem cell longevity are largely unknown, recent studies have implicated p53 and its family member p63. Both proteins regulate organismal aging [2-4] as well as survival and self-renewal of tissue stem cells [5-9]. Intriguingly, haploinsufficiency for a third family member, p73, causes age-related neurodegeneration [10]. While this phenotype is at least partially due to loss of the ΔNp73 isoform, a potent neuronal prosurvival protein [11-16], a recent study showed that mice lacking the other p73 isoform, TAp73, have perturbations in the hippocampal dentate gyrus [17], a major neurogenic site in the adult brain. These findings, and the link between the p53 family, stem cells, and aging, suggest that TAp73 might play a previously unanticipated role in maintenance of neural stem cells. Here, we have tested this hypothesis and show that TAp73 ensures normal adult neurogenesis by promoting the long-term maintenance of neural stem cells. Moreover, we show that TAp73 does this by transcriptionally regulating the bHLH Hey2, which itself promotes neural precursor maintenance by preventing premature differentiation.

Myelodysplastic syndromes arising in patients with germline TP53 mutation and Li-Fraumeni syndrome

CONTEXT

Li-Fraumeni syndrome (LFS), characterized by predisposition to early onset of a variety of malignancies, is usually associated with germline mutation of the tumor-suppressor gene, TP53. Mutation carriers are at increased risk of multiple primary tumors, many of which arise in previous radiation-therapy sites. In patients with LFS, acute myeloid leukemia is uncommon and myelodysplastic syndrome (MDS) is rare.

OBJECTIVE

To evaluate the morphologic, cytogenetic, and molecular diagnostic findings of 3 unique cases of MDS arising in patients with germline TP53 mutation, 2 with classic LFS.

DESIGN

We searched the Li-Fraumeni Syndrome Registry in the Department of Genetics at the University of Texas M. D. Anderson Cancer Center (Houston, Texas) and identified 3 patients with documented germline TP53 mutations or LFS who had developed MDS during a period of 6 years (2000-2005). The clinical, cytogenetic, and molecular diagnostic data and bone marrow aspirate smears and biopsies on all patients were reviewed. Immunohistochemical staining with antibody to p53 was also performed.

RESULTS

Two patients met the criteria for classic LFS; one had no history of malignancy in first-degree relatives. The MDS followed chemotherapy and radiation therapy and progressed to acute myeloid leukemia in 2 patients. Cytogenetic analysis demonstrated chromosome 5 abnormalities in a complex karyotype in all cases. Two patients died, one of acute myeloid leukemia and one with glioblastoma multiforme, MDS, and persistent pancytopenia.

CONCLUSIONS

Patients with LFS may develop MDS, which is most likely therapy-related and is associated with cytogenetic markers of poor prognosis.

The role of TP53 and p21 gene polymorphisms in breast cancer biology in a well specified and characterized German cohort

OBJECTIVE

Abrogation of the function of TP53 gene is supposed to lead to a more aggressive breast cancer phenotype that produces a less favorable clinical outcome. The p21 gene on chromosome 6p21.2 can be stimulated by an activated TP53 gene. A product of transcription, the p21 protein, an inhibitor of cyclin-dependent kinases, has its function in gene repair and angiogenesis during cell division, and can regulate apoptosis. The purpose of this analysis was to examine for an association between the genotypes measured on two single nucleotide polymorphisms (SNPs) located within the TP53 and p21 genes.

METHODS

In a clinical epidemiological case-control study, 814 individuals were recruited. 550 samples (275 cases/275 control) of peripheral blood obtained from women (aged 22-87 years) with breast cancer and from healthy women (aged 23-87 years) were genotyped for frequencies of the following gene variances: R72P/rs1042522 (gene TP53) and S31R/ss4388499 (gene p21).

RESULTS

For the variance in gene TP53 no significant differences between the control group and women with breast cancer could be estimated. For the variance in gene p21 a statistically significant association between the SNP measured within p21 and breast cancer status was observed. The odds ratio for the increased risk for those carrying the CA genotype as opposed to the CC genotype is 1.74 (95% confidence ratio = 1.00-3.05).

CONCLUSION

Despite this finding p21 does not appear to act as an exclusive prognostic marker for breast cancer disease.

In vivo analysis of p53 tumor suppressor function using genetically engineered mouse models

p53 is a crucial tumor suppressor, as evidenced by the high propensity for p53 mutation during human cancer development. Already more than a decade ago, p53 knockout mice confirmed that p53 is critical for preventing tumorigenesis. More recently, a host of p53 knock-in mouse strains has been generated, with the aim of either more precisely modeling p53 mutations in human cancer or better understanding p53's regulation and downstream activities. In the first category, several mouse strains expressing mutant p53 proteins corresponding to human-tumor-derived mutants have demonstrated that mutant p53 is not equivalent to loss of p53 but additionally exhibits gain-of-function properties, promoting invasive and metastatic phenotypes. The second class of p53 knock-in mouse models expressing engineered p53 mutants has also provided new insight into p53 function. For example, mice expressing p53 mutants lacking specific posttranslational modification sites have revealed that these modifications serve to modulate p53 responses in vivo in a cell-type- and stress-specific manner rather than being absolutely required for p53 stabilization and activation as suggested by in vitro experiments. Additionally, studies of p53 mouse models have established that both p53-driven cell-cycle arrest and apoptosis responses contribute to tumor suppression and that activation of p53 by oncogenic stress imposes an important barrier to tumorigenesis. Finally, the use of mouse strains expressing temporally regulatable p53 has demonstrated that p53 loss is not only required for tumor development but also required for tumor maintenance, suggesting that p53 restoration in human cancer patients may be a promising therapeutic strategy. These sophisticated p53 mouse models have taught us important lessons, and new mouse models will certainly continue to reveal interesting and perhaps surprising aspects of p53's complex biology.

20 years studying p53 functions in genetically engineered mice

Cell and molecular biological studies of p53 functions over the past 30 years have been complemented in the past 20 years by studies that use genetically engineered mice. As expected, mice that have mutant Trp53 alleles usually develop cancers of various types more rapidly than their counterparts that have wild-type Trp53 genes. These mouse studies have been instrumental in providing important new insights into p53 tumour suppressor function. Such studies have been facilitated by the development of increasingly sophisticated genetic engineering approaches, which allow the more precise manipulation of p53 structure and function in a mammalian model.

Tumour suppression by p53: a role for the DNA damage response?

Loss of p53 function occurs during the development of most, if not all, tumour types. This paves the way for genomic instability, tumour-associated changes in metabolism, insensitivity to apoptotic signals, invasiveness and motility. However, the nature of the causal link between early tumorigenic events and the induction of the p53-mediated checkpoints that constitute a barrier to tumour progression remains uncertain. This Review considers the role of the DNA damage response, which is activated during the early stages of tumour development, in mobilizing the tumour suppression function of p53. The relationship between these events and oncogene-induced p53 activation through the ARF pathway is also discussed.

Cellular stress response pathway system as a sentinel ensemble in toxicological screening

High costs, long test times, and societal concerns related to animal use have required the development of in vitro assays for the rapid and cost-effective toxicological evaluation and characterization of compounds in both the pharmaceutical and environmental arenas. Although the pharmaceutical industry has developed very effective, high-throughput in vitro assays for determining the therapeutic potential of compounds, the application of this approach to toxicological screening has been limited. A primary reason for this is that while drug candidate screens are directed to a specific target/mechanism, xenobiotics can cause toxicity through any of a myriad of undefined interactions with cellular components and processes. Given that it is not practical to design assays that can interrogate each potential toxicological target, an integrative approach is required if there is to be a rapid and low-cost toxicological evaluation of chemicals. Cellular stress response pathways offer a viable solution to the creation of a set of integrative assays as there is a limited and hence manageable set (a small ensemble of 10 or less) of major cellular stress response pathways through which cells mount a homoeostatic response to toxicants and which also participate in cell fate/death decisions. Further, over the past decades, these pathways have been well characterized at a molecular level thereby enabling the development of high-throughput cell-based assays using the components of the pathways. Utilization of the set of cellular stress response pathway-based assays as indicators of toxic interactions of chemicals with basic cellular machinery will potentially permit the clustering of chemicals based on biological response profiles of common mode of action (MOA) and also the inference of the specific MOA of a toxicant. This article reviews the biochemical characteristics of the stress response pathways, their common architecture that enables rapid activation during stress, their participation in cell fate decisions, the essential nature of these pathways to the organism, and the biochemical basis of their cross-talk that permits an assay ensemble screening approach. Subsequent sections describe how the stress pathway ensemble assay approach could be applied to screening potentially toxic compounds and discuss how this approach may be used to derive toxicant MOA from the biological activity profiles that the ensemble strategy provides. The article concludes with a review of the application of the stress assay concept to noninvasive in vivo assessments of chemical toxicants.

Mutant mouse p53 transgene elevates the chemical induction of tumors that respond to gene silencing with siRNA

To study the role of mutant p53 in the induction and cure of tumors, we generated transgenic mice carrying mutant p53 (mp53) containing a 9 bp deletion in exon 6 in addition to wild-type p53, expressing both p53 and mp53. The mp53 cDNA was cloned from a radiation-induced mouse tumor and ligated to the chicken beta-actin promoter/CMV-IE enhancer in the expression vector. The presence of mp53 suppressed p21 expression in primary fibroblasts after ionizing irradiation, indicating the dominant-negative activity of mp53 in the mice. These mice developed fibrosarcomas after the subcutaneous injection of 3-methylcholanthrene with an incidence 1.7-fold higher than that of wild-type mice (42% excess). The tumors were then treated via a potent atelocollagen delivery system with small interfering RNA (siRNA), that targeted the promoter/enhancer of the expression vector, resulting in the suppression of tumor growth in 30% of 44 autochthonous tumors, including four cures, and their transplants, the total fraction corresponding to the tumor excess. This suppressive effect involved the induction of apoptosis. These results indicate that mp53 activity causes tumors that can be suppressed by subsequent silencing of mp53 in the presence of wild-type p53 alleles.

Antagonistic pleiotropy and p53

George Williams' antagonistic pleiotropy theory of aging proposes that cellular damage and organismal aging are caused by pleiotrophic genes, or genes with multiple phenotypic effects [Williams, G.C., 1957. Pleiotropy, natural selection, and the evolution of senescence. Evolution 11, 398-411]. According to this theory, genes that exhibit antagonistic pleiotropy increase the odds of successful reproduction early in life, but have deleterious effects later in life. The tumor suppressor p53 confers protection against cancer (and death) by interrupting the abnormal proliferation of cells. When control of proliferation is applied to normal stem cells, however, it can impair tissue homeostasis and accelerate aging. We use data from recently developed models of accelerated aging in mice to determine if the deleterious effects of p53 on aging reflect antagonistic pleiotropy of the p53 gene or are attributable to genes that can modify p53 activity but are evolving independently.

p73 regulates neurodegeneration and phospho-tau accumulation during aging and Alzheimer's disease

The genetic mechanisms that regulate neurodegeneration are only poorly understood. We show that the loss of one allele of the p53 family member, p73, makes mice susceptible to neurodegeneration as a consequence of aging or Alzheimer's disease (AD). Behavioral analyses demonstrated that old, but not young, p73+/- mice displayed reduced motor and cognitive function, CNS atrophy, and neuronal degeneration. Unexpectedly, brains of aged p73+/- mice demonstrated dramatic accumulations of phospho-tau (P-tau)-positive filaments. Moreover, when crossed to a mouse model of AD expressing a mutant amyloid precursor protein, brains of these mice showed neuronal degeneration and early and robust formation of tangle-like structures containing P-tau. The increase in P-tau was likely mediated by JNK; in p73+/- neurons, the activity of the p73 target JNK was enhanced, and JNK regulated P-tau levels. Thus, p73 is essential for preventing neurodegeneration, and haploinsufficiency for p73 may be a susceptibility factor for AD and other neurodegenerative disorders.

CARF: an emerging regulator of p53 tumor suppressor and senescence pathway

Replicative senescence, a major outcome of normal cells with finite lifespan, is a widely accepted in vitro model for ageing studies. Limited repair and defense mechanisms of normal cells, in addition to DNA alterations and oncogene inductions under stress, are believed to result in senescence as a protective mechanism to prevent undesirable proliferation of cells. The ARF/p53/p21(cip1/waf1) tumor suppression pathway acts as a molecular sensor and regulator of cellular stress, senescence, and immortalization. Understanding the molecular regulation of this pathway by intrinsic and extrinsic signals is extremely important to address unsolved questions in senescence and cancer. CARF was first discovered as a binding partner of ARF and has since been shown to have both ARF-dependent and -independent functions that converge to regulate p53 pathway. CARF directly binds to p53 and HDM2, and functions in a negative feedback pathway. Whereas CARF transcriptionally represses HDM2 to increase p53 activity, HDM2 in return degrades CARF. Thus, CARF may act as a novel key regulator of the p53 pathway at multiple checkpoints. The aim of this article is to discuss the current knowledge about functions of CARF and its impact on p53 pathway in regulation of senescence and carcinogenesis.

Cellular and organismal ageing: Role of the p53 tumor suppressor protein in the induction of transient and terminal senescence

In recent years, an impact of the p53 tumor suppressor protein in the processes of cellular and organismal ageing became evident. First hints were found in model organisms like Saccharomyces cerevisiae, Caenorhabditis elegans, and Drosophila melanogaster where a clear connection between ageing phenotypes and pathways that are regulated by p53, were found. Interestingly, pathways that are central to the ageing process are usually also involved in energy metabolism and are highly conserved throughout evolution. This also supports the long known empiric finding that caloric restriction has a positive impact on the life span of a wide variety of organisms. Within the last years, on the molecular level, an involvement of the insulin-like growth factor and of the histone deacetylase SRIT1 could be shown. Insight on the impact of p53 on ageing at the organismal level came from mice expressing aberrant forms of the p53 protein. Obviously, the balance of the full length p53 protein and of the shorter p44/DeltaNp53 isomer bear a strong impact on ageing. The shorter isoform regulates full length p53 and in cases where there is too much of the longer isoform, this leads to elevated apoptosis resulting in decreased tumor incidence but also in premature ageing due to exhaustion of the renewal potential. Therefore, modulating the expression of the truncated p53 isoform accordingly, might lead to increased health-span and elevated life-span.

p53: at the crossroad between cancer and ageing

The p53 tumour suppressor plays an undisputed role in cancer. p53's tumour suppressive activity stems from its ability to respond to a variety of stresses to trigger cell cycle arrest, apoptosis or senescence, thereby protecting against malignant transformation. An increasing body of evidence suggests that p53 also drives organismal ageing. Although genetic models with altered p53 function display age-related phenotypes and thus provide in vivo evidence that p53 contributes to the ageing process, p53's role in organismal ageing remains controversial. Anti-cancer therapies that target p53 and reactivate or enhance its activity are considered good alternatives for treating various neoplasms. Therefore, it is important to determine whether these clinical approaches compromise tissue homeostasis and contribute to ageing. This review presents a number of models with altered p53 function and discusses how these models implicate p53 as part of a molecular network that integrates tumour suppression and ageing.

Regenerative capacity of neural precursors in the adult mammalian brain is under the control of p53

The question of whether or not stem cell loss drives aging in the brain has not been fully resolved. Here, we used mice over-expressing the short isoform of p53 (DeltaNp53 or p44) as a model of aging to gain insight into the cellular mechanisms underlying age-related functional deficits in the brain. By BrdU labeling, we observed an accelerated decline in the number of subventricular zone proliferating cells with age in p44Tg mice compared to mice with normal p53 expression. A 2-3-fold reduction in the number of slowly dividing stem cells was evident in the subventricular zone of 9-12-month-old p44Tg mice, but not in younger p44Tg mice or in normal mice. Consequently, the supply of new olfactory bulb neurons was also reduced. The number and size of neurospheres generated from subventricular zone cells from p44Tg mice was significantly reduced, and cells derived from these neurospheres had limited self-renewal and amplification capacities. At the cellular level, p44 lengthened the cell cycle and affected cell cycle reentry properties, evident by an increased proportion of cells in G0. At the functional level, p44 expression resulted in impaired olfactory discrimination in 15-16-month-old mice. This phenotype is driven by constitutive activation of p53 and constitutive expression of p21(Cip1/waf1) in neural stem cells. Our results demonstrate that p53 plays a crucial role in the maintenance of the regenerative capacity of the brain by regulating the proliferation of stem and progenitor cells.

p53 alterations in human cancer: more questions than answers

The strongest and undisputed fact about p53 is the high frequency of p53 alterations in human cancer and that mutant p53 proteins constitute a complex family of several hundred proteins with heterogeneous properties. Beyond these observations, the p53 pathway and its regulation in a normal cell is like a desert trail, always moving with the wind of novel findings. The field is full of black boxes that are often ignored for sake of simplicity or because they do not fit with the current dominant view. Mutant p53 protein accumulation in tumours is the best example of a preconceived idea, as there is no experimental evidence to explain this observation. In this review, we will discuss several questions concerning the activity or selection of p53 mutations. The central domain of the p53 protein targeted by 80% of p53 mutations is associated with the DNA-binding activity of the p53 protein, but it is also the binding site for several proteins that play a key role in p53 regulation such as ASPP proteins or BclxL. The role of impaired DNA binding and/or protein interactions in tumour development has not been fully elucidated. Similarly, novel animal models carrying either missense p53 mutations or inducible p53 have provided abundant observations, some of which could challenge our view on p53 function as a tumour suppressor gene. Finally, the possible clinical applications of p53 will be discussed.

Hereditary cancer predisposition in children: Genetic basis and clinical implications

Although cancer predisposition syndromes are rare and malignancies arising in this context account for only 1-10% of childhood tumors, studies performed in affected patients and their families have been of unique value for the understanding of cancer development. Three classes of genes (tumor suppressor genes, oncogenes and stability genes) have been identified and shown to be involved in the pathogenesis of familial, as well as sporadic tumors. Cancer has long been recognized as a genetic disease of somatic cells. Despite improved understanding of the molecular basis of predisposition to cancer and better diagnostic tools, the care of these patients and their families remains a major challenge for the clinician. Medical, psychological, ethical and legal issues have to be considered. This review focuses on examples of each class of inherited cancer predisposition syndromes with special implications for patients in the pediatric age group, including retinoblastoma predisposition, Li-Fraumeni syndrome, multiple endocrine neoplasia disorders and Fanconi anemia. The genetic basis of cancer predisposition is discussed as well as the major concepts and controversies in the clinical management of these patients and their families.