Mouse models with modified p53 sequences to study cancer and ageing

Murine Models of Chronic Viral Infections and Associated Cancers

Viruses are now recognized as bona fide etiologic factors of human cancer. Carcinogenic viruses include Epstein– Barr virus (EBV), high-risk human papillomaviruses (HPVs), hepatitis B virus (HBV), hepatitis C virus (HCV), human T-cell leukemia virus type 1 (HTLV-1), human immunodeficiency virus type 1 (HIV-1, indirectly), and several candidate human cancer viruses. It is estimated that 15% of all human tumors worldwide are caused by viruses. Tumor viruses establish long-term persistent infections in humans, and cancer is an accidental side effect of viral replication strategies. Viruses are usually not complete carcinogens, supporting the concept that cancer results from the accumulation of multiple cooperating events, in which human cancer viruses display different, often opposing roles. The laboratory mouse Mus musculus is one of the best in vivo experimental systems for modeling human pathology, including viral infections and cancer. However, mice are unsusceptible to infection with the known carcinogenic viruses. Many murine models were developed to overcome this limitation and to address various aspects of virus-associated carcinogenesis, from tumors resulting from xenografts of human tissues and cells, including cancerous and virus infected, to genetically engineered mice susceptible to viral infections and associated cancer. The review considers the main existing models, analyzes their advantages and drawbacks, describes their applications, outlines the prospects of their further development.

Mammalian cells acquire epigenetic hallmarks of human cancer during immortalization

Progression to malignancy requires that cells overcome senescence and switch to an immortal phenotype. Thus, exploring the genetic and epigenetic changes that occur during senescence/immortalization may help elucidate crucial events that lead to cell transformation. In the present study, we have globally profiled DNA methylation in relation to gene expression in primary, senescent and immortalized mouse embryonic fibroblasts. Using a high-resolution genome-wide mapping technique, followed by extensive locus-specific validation assays, we have identified 24 CpG islands that display significantly higher levels of CpG methylation in immortalized cell lines as compared to primary murine fibroblasts. Several of these hypermethylated CpG islands are associated with genes involved in the MEK–ERK pathway, one of the most frequently disrupted pathways in cancer. Approximately half of the hypermethylated targets are developmental regulators, and bind to the repressive Polycomb group (PcG) proteins, often in the context of bivalent chromatin in mouse embryonic stem cells. Because PcG-associated aberrant DNA methylation is a hallmark of several human malignancies, our methylation data suggest that epigenetic reprogramming of pluripotency genes may initiate cell immortalization. Consistent with methylome alterations, global gene expression analysis reveals that the vast majority of genes dysregulated during cell immortalization belongs to gene families that converge into the MEK–ERK pathway. Additionally, several dysregulated members of the MAP kinase network show concomitant hypermethylation of CpG islands. Unlocking alternative epigenetic routes for cell immortalization will be paramount for understanding crucial events leading to cell transformation. Unlike genetic alterations, epigenetic changes are reversible events, and as such, can be amenable to pharmacological interventions, which makes them appealing targets for cancer therapy when genetic approaches prove inadequate.

Creation and preliminary characterization of a Tp53 knockout rat

The tumor suppressor TP53 plays a crucial role in cancer biology, and the TP53 gene is the most mutated gene in human cancer. Trp53 knockout mouse models have been widely used in cancer etiology studies and in search for a cure of cancer with some limitations that other model organisms might help overcome. Via pronuclear microinjection of zinc finger nucleases (ZFNs), we created a Tp53 knockout rat that contains an 11-bp deletion in exon 3, resulting in a frameshift and premature terminations in the open reading frame. In cohorts of 25 homozygous (Tp53^Δ11/Δ11), 37 heterozygous (Tp53^Δ11/+) and 30 wild-type rats, the Tp53^Δ11/Δ11 rats lived an average of 126 days before death or removal from study because of clinical signs of abnormality or formation of tumors. Half of Tp53^Δ11/+ were removed from study by 1 year of age because of tumor formation. Both Tp53^Δ11/+ and Tp53^Δ11/Δ11 rats developed a wide spectrum of tumors, most commonly sarcomas. Interestingly, there was a strikingly high incidence of brain lesions, especially in Tp53^Δ11/Δ11 animals. We believe that this mutant rat line will be useful in studying cancer types rarely observed in mice and in carcinogenicity assays for drug development.

The expanding universe of p53 targets

The p53 tumour suppressor is modified through mutation or changes in expression in most cancers, leading to the altered regulation of hundreds of genes that are directly influenced by this sequence-specific transcription factor. Central to the p53 master regulatory network are the target response element (RE) sequences. The extent of p53 transactivation and transcriptional repression is influenced by many factors, including p53 levels, cofactors and the specific RE sequences, all of which contribute to the role that p53 has in the aetiology of cancer. This Review describes the identification and functionality of REs and highlights the inclusion of non-canonical REs that expand the universe of genes and regulation mechanisms in the p53 tumour suppressor network.

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.

Gene expression profiling of aging reveals activation of a p53-mediated transcriptional program

BACKGROUND

Aging has been associated with widespread changes at the gene expression level in multiple mammalian tissues. We have used high density oligonucleotide arrays and novel statistical methods to identify specific transcriptional classes that may uncover biological processes that play a central role in mammalian aging.

RESULTS

We identified 712 transcripts that are differentially expressed in young (5 month old) and old (25-month old) mouse skeletal muscle. Caloric restriction (CR) completely or partially reversed 87% of the changes in expression. Examination of individual genes revealed a transcriptional profile indicative of increased p53 activity in the older muscle. To determine whether the increase in p53 activity is associated with transcriptional activation of apoptotic targets, we performed RT-PCR on four well known mediators of p53-induced apoptosis: puma, noxa, tnfrsf10b and bok. Expression levels for these proapoptotic genes increased significantly with age (P < 0.05), while CR significantly lowered expression levels for these genes as compared to control fed old mice (P < 0.05). Age-related induction of p53-related genes was observed in multiple tissues, but was not observed in young SOD2+/- and GPX4+/- mice, suggesting that oxidative stress does not induce the expression of these genes. Western blot analysis confirmed that protein levels for both p21 and GADD45a, two established transcriptional targets of p53, were higher in the older muscle tissue.

CONCLUSION

These observations support a role for p53-mediated transcriptional program in mammalian aging and suggest that mechanisms other than reactive oxygen species are involved in the age-related transcriptional activation of p53 targets.

Silencing of APAF-1 in B-CLL results in poor prognosis in the case of concomitant p53 mutation

Apoptosis protease-activating factor 1 (APAF-1), a transcriptional target of p53, is a cytosolic adaptor protein that links the mitochondrial apoptosis pathway to the caspase cascade. Here, we aimed to study the impact of APAF-1 expression levels on cell death induced by anticancer drugs or ionizing irradiation (IR) and disease prognosis in B-type chronic lymphocytic leukemia (B-CLL) patients. Samples from 138 patients with B-CLL were investigated for APAF-1 expression and p53 mutations. The results were related to survival data, in vitro cytotoxicity of various cytotoxic drugs and IR and clinico-pathological data. Variable APAF-1 expression was observed in all investigated B-CLL samples. Reduction in APAF-1 expression was observed at both mRNA and protein level indicating transcriptional silencing whereas mutation of p53 or the immunoglobulin heavy chain variable genes (IgH(V)) had no impact on APAF-1 expression. Surprisingly, APAF-1 loss did not result in resistance to cytotoxic therapies. Likewise, APAF-1 downregulation on its own showed no impact on disease prognosis. Nevertheless, a poor prognosis was observed in patients with loss of APAF-1 expression and additional p53 mutation. Thus, loss of APAF-1 may become relevant when additional core apoptosis signaling components are disrupted.

p73, the “Assistant” Guardian of the Genome?

Although p53 is clearly involved in the salvage pathway to DNA damage, its frequent mutations do not explain the efficacy of radiotherapy and chemotherapy. Indeed, around 50% of all human cancers show mutations in p53, and a further fraction show a functional inactivation of the protein. Nevertheless, patients seem to respond to therapy that would otherwise require a functional p53. At least in part, these responses could be explained by the pathway mediated by p73. This mechanism is parallel to, but independent of the p53 pathway. Several pieces of evidence show a significant interaction between these two proteins. Therefore, while p53 can be rightly defined as the guardian of the genome, we could think of p73 as the "assistant" guardian of the genome!

Nucleotide excision repair- and p53-deficient mouse models in cancer research

Cancer is caused by the loss of controlled cell growth due to mutational (in)activation of critical genes known to be involved in cell cycle regulation. Three main mechanisms are known to be involved in the prevention of cells from becoming cancerous; DNA repair and cell cycle control, important to remove DNA damage before it will be fixed into mutations and apoptosis, resulting in the elimination of cells containing severe DNA damage. Several human syndromes are known to have (partially) deficiencies in these pathways, and are therefore highly cancer prone. Examples are xeroderma pigmentosum (XP) caused by an inborn defect in the nucleotide excision repair (NER) pathway and the Li-Fraumeni syndrome, which is the result of a germ line mutation in the p53 gene. XP patients develop skin cancer on sun exposed areas at a relatively early age, whereas Li-Fraumeni patients spontaneously develop a wide variety of early onset tumors, including sarcomas, leukemia's and mammary gland carcinomas. Several mouse models have been generated to mimic these human syndromes, providing us information about the role of these particular gene defects in the tumorigenesis process. In this review, spontaneous phenotypes of mice deficient for nucleotide excision repair and/or the p53 gene will be described, together with their responses upon exposure to either chemical carcinogens or radiation. Furthermore, possible applications of these and newly generated mouse models for cancer will be given.

New approaches for modelling cancer mechanisms in the mouse

Mouse models of human cancer are vital to our understanding of the neoplastic process, and to advances in both basic and clinical research. Indeed, models of many of the major human tumours are now available and are subject to constant revision to more faithfully recapitulate human disease. Despite these advances, it is important to recognize that limitations do exist to the current range of models. The principal approach to modelling has relied upon the use of constitutive gene knockouts, which can often result in embryonic lethality, can potentially be affected by developmental compensation, and which do not mimic the sporadic development of a tumour expanding from a single cell in an otherwise normal environment. Furthermore, simple knockouts are usually designed to lead to loss of protein function, whereas a subset of cancer-causing mutations clearly results in gain of function. These drawbacks are well recognized and this review describes some of the approaches used to address these issues. Key amongst these is the development of conditional alleles that precisely mimic the mutations found in vivo, and which can be spatially and tissue-specifically controlled using 'smart' systems such as the tetracycline system and Cre-Lox technology. Examples of genes being manipulated in this way include Ki-Ras, Myc, and p53. These new developments in modelling mean that any mutant allele can potentially be turned on or off, or over- or under-expressed, in any tissue at any stage of the life-cycle of the mouse. This will no doubt lead to ever more accurate and powerful mouse models to dissect the genetic pathways that lead to cancer.

DNA damage-induced apoptosis: insights from the mouse

The availability of murine models with precisely defined genetic lesions has greatly increased our understanding of the genetic control of cell death, with functional dependence established for a wide range of genes including (amongst others) the p53 and Bcl-2 gene family members, the mismatch repair (MMR) genes and the methyl binding domain family member Mbd4. These studies raised the attractive hypotheses that tumour predisposition may be explained in terms of failed cell death, and also that tumour regression may be initiated through activation of an apoptotic programme. The studies that have addressed these notions have revealed complex consequences of a failed death programme, such that these simple hypotheses have not always been supported. Remarkably, however, some tissues show more predictable responses than others, most apparent in the contrast between the intestine and the haematopoietic system. This review will focus upon a discussion of these relationships, and will also consider the relevance of some of these findings to tumour predisposition and regression.

Human tumor p53 mutations are selected for in mouse embryonic fibroblasts harboring a humanized p53 gene

To date, there has been no way to examine induced human p53 gene mutations in cell cultures exposed to mutagenic factors, other than by restriction site analysis. Here, we used embryonic cells from our Hupki (human p53 knock-in) mouse strain to generate human p53 DNA-binding domain (DBD) mutations experimentally. Twenty cultures of untreated primary mouse Hupki fibroblasts and 20 short-wavelength UV light (UVC)-treated cultures (20J/m(2)) were passaged >20 times. Established Hupki embryonic fibroblast cell lines (HUFs) were genotyped by dideoxy DNA sequencing of p53 exons 4-9. Seven of the HUFs harbored point mutations in the humanized p53 DBD. Of the 9 mutations (6 single- and 1 triple-site mutation), 2 were at the most frequently mutated codons in human cancers (c.248 and c.273). The Affymetrix p53 GeneChip assay also readily identified the 6 single-base substitutions. All mutations in HUFs from UV-treated cultures were at dipyrimidine sites, including 3 nontranscribed strand C -->T transitions. The mutant HUFs were deficient in p53 transactivation function, and missense mutants had high levels of nuclear p53 protein. In a second experiment, primary Hupki cells were exposed to the carcinogen aristolochic acid I (AAI). Five of 10 cultures that became established within 2 months harbored p53 DBD mutations. All were transversions, including 4 A --> T substitutions on the nontranscribed strand, a hallmark of DNA mutation by AAI. We conclude that establishment of Hupki mouse fibroblasts in culture readily selects for p53 DBD mutations found in human tumors, providing a basis for generating experimental mutation patterns in human p53.