Mice expressing a mammary gland-specific R270H mutation in the p53 tumor suppressor gene mimic human breast cancer development

Impact of Simulated Rotating Shift Work on Mammary Tumor Development in the p53R270H©/+WAPCre Mouse Model for Breast Cancer

Epidemiological studies associate night shift work with increased breast cancer risk. However, the underlying mechanisms are not clearly understood. To better understand these mechanisms, animal models that mimic the human situation of different aspects of shift work are needed. In this study, we used "timed sleep restriction" (TSR) cages to simulate clockwise and counterclockwise rotating shift work schedules and investigated predicted sleep patterns and mammary tumor development in breast tumor-prone female p53R270H©/+WAPCre mice. We show that TSR cages are effective in disturbing normal activity and estimated sleep patterns. Although circadian rhythms were not shifted, we observed effects of the rotating schedules on sleep timing and sleep duration. Sleep loss during a simulated shift was partly compensated after the shift and also partly during the free days. No effects were observed on body weight gain and latency time of breast cancer development. In summary, our study shows that the TSR cages can be used to model shift work in mice and affect patterns of activity and sleep. The effect of disturbing sleep patterns on carcinogenesis needs to be further investigated.

Is loss of p53 a driver of ductal carcinoma in situ progression?

Ductal carcinoma in situ (DCIS) is a non-obligate precursor of invasive carcinoma. Multiple studies have shown that DCIS lesions typically possess a driver mutation associated with cancer development. Mutation in the TP53 tumour suppressor gene is present in 15-30% of pure DCIS lesions and in ~30% of invasive breast cancers. Mutations in TP53 are significantly associated with high-grade DCIS, the most likely form of DCIS to progress to invasive carcinoma. In this review, we summarise published evidence on the prevalence of mutant TP53 in DCIS (including all DCIS subtypes), discuss the availability of mouse models for the study of DCIS and highlight the need for functional studies of the role of TP53 in the development of DCIS and progression from DCIS to invasive disease.

Use of RNA-Seq and a Transgenic Mouse Model to Identify Genes Which May Contribute to Mutant p53-Driven Prostate Cancer Initiation

Simple Summary

We use RNA-seq analysis to identify genes that may contribute to mutant p53-mediated prostate cancer initiation in a genetically engineered mouse model (B6.129S4-Trp53tm3.1Tyj/J). A total of 1378 differentially expressed genes, including wildtype p53 target genes (e.g. Cdkn1a, Bax, Bcl2, Kras, Mdm2), p53 gain-of-function-related genes (Mgmt, Id4), and prostate cancer-related genes (Cav-1, Raf1, Kras), were identified. Mice that were homozygous or heterozygous for the Trp53 R270H mutation developed grade one PIN lesions at 3 months and 5 months, respectively, whereas wildtype mice did not develop PIN. Immunohistochemical analysis revealed decreased levels of irradiation-mediated apoptosis in homozygous and heterozygous mice when compared to wildtype counterparts, and this aligned with observed differences in apoptosis-related gene expression.

Abstract

We previously demonstrated that the Trp53-R270H mutation can drive prostate cancer (CaP) initiation using the FVB.129S4 (Trp53^tm3Tyj/wt); FVB.129S (Nkx3-1^{tm3(cre)Mmswt}) genetically engineered mouse model (GEM). We now validate this finding in a different model (B6.129S4-Trp53^tm3.1Tyj/J mice) and use RNA-sequencing (RNA-Seq) to identify genes which may contribute to Trp53 R270H-mediated prostate carcinogenesis. Wildtype (Trp53^WT/WT), heterozygous (Trp53^R270H/WT), and homozygous mice (Trp53^R270H/R270H) were exposed to 5 Gy irradiation to activate and stabilize p53, and thereby enhance our ability to identify differences in transcriptional activity between the three groups of mice. Mouse prostates were harvested 6 h post-irradiation and processed for histological/immunohistochemistry (IHC) analysis or were snap-frozen for RNA extraction and transcriptome profiling. IHC analyses determined that presence of the Trp53-R270H mutation impacts apoptosis (lower caspase 3 activity) but not cell proliferation (Ki67). RNA-Seq analysis identified 1378 differentially expressed genes, including wildtype p53 target genes (E.g., Cdkn1a, Bax, Bcl2, Kras, Mdm2), p53 gain-of-function (GOF)-related genes (Mgmt, Id4), and CaP-related genes (Cav-1, Raf1, Kras). Further understanding the mechanisms which contribute to prostate carcinogenesis could allow for the development of improved preventive methods, diagnostics, and treatments for CaP.

Mutant p53 oncogenicity: dominant-negative or gain-of-function?

The p53 protein is mutated in about 50% of human cancers. Aside from losing its tumor-suppressive activities, mutant p53 may acquire pro-oncogenic activity, which is facilitated by two underlying mechanisms. The first mechanism is the inhibition of co-expressed wild-type p53 (WTp53) activity, dubbed the dominant-negative effect (DNE). The second mechanism is a neomorphic pro-oncogenic activity that does not involve the inhibition of WTp53, termed gain-of-function (GOF). Throughout the years, both mechanisms were demonstrated in a plethora of in vitro and in vivo models. However, whether both account for protumorigenic activities of mutant p53 and in which contexts is still a matter of ongoing debate. Here, we discuss evidence for both DNE and GOF in a variety of models. These models suggest that both GOF and DNE can be relevant, but are highly dependent on the specific mutation type, genetic and cellular context and even the phenotype that is being assessed. In addition, we discuss how mutant and WTp53 might not exist as two separate entities, but rather as a continuum that may involve a balance between the two forms in the same cells, which could be tilted by various factors and drugs. Further elucidation of the factors that dictate the balance between the WT and mutant p53 states, as well as the factors that govern the impact of DNE and GOF in different cancer types, may lead to the development of more effective treatment regimens for cancer patients.

Dimethylbenz(a)anthracene-induced mammary tumorigenesis in mice

Every year, over 2 million women are diagnosed with breast cancer. Although considerable progress was made within the last years in cancer prevention, diagnosis and treatment, breast cancer is still responsible for over 600,000 of deaths per year. Over the years, numerous mouse models have been developed to understand breast cancer etiology and progression. Among those, mammary carcinomas induced by carcinogen, such as 7,12-dimethylbenz[a]anthracene (DMBA), has been widely used. Generally, 30-70% of mice exposed to 4-6 weekly doses of 1mg of DMBA during the peripubertal period (4-10 weeks of age) will develop mammary tumors within 150-200 days after the first exposure, that sometime metastasize to the lungs. As a result, DMBA-induced tumorigenesis is thought to be an accurate and relevant model to study breast cancer as it closely mimics this multistep process. This chapter presents the typical protocol used in mice to induce mammary gland tumors using DMBA. The influence of the number of doses and the total burden of DMBA given, as well as of the age and strain of the mice on mammary gland incident and on tumor onset are discussed. The current knowledge regarding mechanisms involved in DMBA-induced tumorigenesis is also presented.

Upregulation of fibronectin following loss of p53 function is a poor prognostic factor in ovarian carcinoma with a unique immunophenotype

Background

We previously demonstrated that ovarian high grade serous carcinomas (OHGSeCa) and ovarian clear cell carcinomas (OCCCa) with an HNF-1β+/p53+/ARID1A+ immunophenotype were associated with the worst unfavorable prognosis. To clarify the molecular mechanisms underlying this finding, we focused on alterations in the p53 signaling pathway in these tumors.

Methods

Changes in cell phenotype and function following knockdown of wild-type p53 (p53-KD) were assessed using OCCCa cells expressing endogenous HNF-1β and ARID1A. The prognostic significance of molecules that were deregulated following p53-KD was also examined using 129 OCCCa/OHGSeCa cases.

Results

p53-KD cells had increased expression of Snail, phospho-Akt (pAkt), and pGSK3β, and decreased E-cadherin expression, leading to epithelial-mesenchymal transition (EMT)/cancer stem cell (CSC) features. The cells also exhibited acceleration of cell motility and inhibition of cell proliferation and apoptosis. Next generation sequencing revealed that fibronectin (FN) expression was significantly increased in the p53 KD-cells, in line with our observation that wild-type p53 (but not mutant p53) repressed FN1 promoter activity. In addition, treatment of OCCCa cells with FN significantly increased cell migration capacity and decreased cell proliferation rate, independent of induction of EMT features. In clinical samples, FN/p53 scores were significantly higher in OCCCa/OHGSeCa with the HNF-1β+/p53+/ARID1A+ immunophenotype when compared to others. Moreover, high FN/high p53 expression was associated with the worst overall survival and progression-free survival in OCCCa/OHGSeCa patients.

Conclusion

These findings suggest that upregulation of FN following loss of p53 function may impact the biological behavior of OCCCa/OHGSeCa, particularly in tumors with an HNF-1β+/p53+/ARID1A+ immunophenotype, through alterations in cell mobility and cell proliferation. The accompanying induction of EMT/CSC properties and inhibition of apoptosis due to p53 abnormalities also contribute to the establishment and maintenance of tumor phenotypic characteristics.

Mutant p53 Gain-of-Function: Role in Cancer Development, Progression, and Therapeutic Approaches

Frequent p53 mutations (mutp53) not only abolish tumor suppressor capacities but confer various gain-of-function (GOF) activities that impacts molecules and pathways now regarded as central for tumor development and progression. Although the complete impact of GOF is still far from being fully understood, the effects on proliferation, migration, metabolic reprogramming, and immune evasion, among others, certainly constitute major driving forces for human tumors harboring them. In this review we discuss major molecular mechanisms driven by mutp53 GOF. We present novel mechanistic insights on their effects over key functional molecules and processes involved in cancer. We analyze new mechanistic insights impacting processes such as immune system evasion, metabolic reprogramming, and stemness. In particular, the increased lipogenic activity through the mevalonate pathway (MVA) and the alteration of metabolic homeostasis due to interactions between mutp53 and AMP-activated protein kinase (AMPK) and Sterol regulatory element-binding protein 1 (SREBP1) that impact anabolic pathways and favor metabolic reprograming. We address, in detail, the impact of mutp53 over metabolic reprogramming and the Warburg effect observed in cancer cells as a consequence, not only of loss-of-function of p53, but rather as an effect of GOF that is crucial for the imbalance between glycolysis and oxidative phosphorylation. Additionally, transcriptional activation of new targets, resulting from interaction of mutp53 with NF-kB, HIF-1α, or SREBP1, are presented and discussed. Finally, we discuss perspectives for targeting molecules and pathways involved in chemo-resistance of tumor cells resulting from mutp53 GOF. We discuss and stress the fact that the status of p53 currently constitutes one of the most relevant criteria to understand the role of autophagy as a survival mechanism in cancer, and propose new therapeutic approaches that could promote the reduction of GOF effects exercised by mutp53 in cancer.

Amphiregulin deletion strongly attenuates the development of estrogen receptor-positive tumors in p53 mutant mice

PURPOSE

The epidermal growth factor receptor ligand, Amphiregulin, is a transcriptional target of estrogen receptor alpha and is required for pubertal mammary gland development. Previous studies using immortalized human breast cancer cell line xenografts have suggested that Amphiregulin may be an important effector of estrogen receptor alpha during breast cancer development, at least in immune-compromised animals. Here, we evaluate the requirement for Amphiregulin in an immune-competent mouse model which is prone to developing estrogen receptor-positive tumors.

METHODS

We have intercrossed mice with mammary-specific mutation of p53 with mice deficient in Amphiregulin in order to assess the requirement for Amphiregulin in the initiation and progression of both estrogen receptor-positive and estrogen receptor-negative mammary tumors.

RESULTS

Deletion of Amphiregulin significantly delayed the onset of palpable mammary tumors and also strongly reduced the proportion of estrogen receptor alpha-positive tumors formed. Upon necropsy, no substantial differences in the prevalence of non-palpable lesions were observed between cohorts, suggesting that the importance of Amphiregulin in mammary tumorigenesis is limited to the post-initiation phase.

CONCLUSIONS

This study underlines the importance of the EGFR ligand, Amphiregulin, as a key mediator of estrogen receptor action in breast cancer.

Exogenous ERα Expression in the Mammary Epithelium Decreases Over Time and Does Not Contribute to p53-Deficient Mammary Tumor Formation in Mice

Approximately 75% of all breast cancers express the nuclear hormone receptor estrogen receptor α (ERα). However, the majority of mammary tumors from genetically engineered mouse models (GEMMs) are ERα-negative. To model ERα-positive breast cancer in mice, we exogenously introduced expression of mouse and human ERα in an existing GEMM of p53-deficient breast cancer. After initial ERα expression during mammary gland development, expression was reduced or lost in adult glands and p53-deficient mammary tumors. Chromatin immunoprecipitation (ChIP)-sequencing analysis of primary mouse mammary epithelial cells (MMECs) derived from these models, in which expression of the ERα constructs was induced in vitro, confirmed interaction of ERα with the DNA. In human breast and endometrial cancer, and also in healthy breast tissue, DNA binding of ERα is facilitated by the pioneer factor FOXA1. Surprisingly, the ERα binding sites identified in primary MMECs, but also in mouse mammary gland and uterus, showed an high enrichment of ERE motifs, but were devoid of Forkhead motifs. Furthermore, exogenous introduction of FOXA1 and GATA3 in ERα-expressing MMECs was not sufficient to promote ERα-responsiveness of these cells. Together, this suggests that species-specific differences in pioneer factor usage between mouse and human are dictated by the DNA sequence, resulting in ERα-dependencies in mice that are not FOXA1 driven. These species-specific differences in ERα-biology may limit the utility of mice for in vivo modeling of ERα-positive breast cancer.

Somatic Trp53 mutations differentially drive breast cancer and evolution of metastases

TP53 mutations are the most frequent genetic alterations in breast cancer and are associated with more aggressive disease and worse overall survival. We have created two conditional mutant Trp53 alleles in the mouse that allow expression of Trp53R172H or Trp53R245W missense mutations in single cells surrounded by a normal stroma and immune system. Mice with Trp53 mutations in a few breast epithelial cells develop breast cancers with high similarity to human breast cancer including triple negative. p53R245W tumors are the most aggressive and exhibit metastases to lung and liver. Development of p53R172H breast tumors with some metastases requires additional hits. Sequencing of primary tumors and metastases shows p53R245W drives a parallel evolutionary pattern of metastases. These in vivo models most closely simulate the genesis of human breast cancer and will thus be invaluable in testing novel therapeutic options.

Mutations in TP53 gene are very common in cancer development. Here the authors take advantage of murine models to show that somatic Trp53 mutations differentially drive breast cancer and evolution of metastases.

Establishment of basal cell carcinoma animal model in Chinese tree shrew (Tupaia belangeri chinensis)

Basal cell carcinoma (BCC) is the most common skin cancer worldwide, with incidence rates continuing to increase. Ultraviolet radiation is the major environmental risk factor and dysregulation of the Hedgehog (Hh) signaling pathway has been identified in most BCCs. The treatment of locally advanced and metastatic BBCs is still a challenge and requires a better animal model than the widely used rodents for drug development and testing. Chinese tree shrews (Tupaia belangeri chinensis) are closely related to primates, bearing many physiological and biochemical advantages over rodents for characterizing human diseases. Here, we successfully established a Chinese tree shrew BCC model by infecting tail skins with lentiviral SmoA1, an active form of Smoothened (Smo) used to constitutively activate the Hh signaling pathway. The pathological characteristics were verified by immunohistochemical analysis. Interestingly, BCC progress was greatly enhanced by the combined usage of lentiviral SmoA1 and shRNA targeting Chinese tree shrew p53. This work provides a useful animal model for further BCC studies and future drug discoveries.

The DNA resection protein CtIP promotes mammary tumorigenesis

Many DNA repair factors act to suppress tumor formation by preserving genomic stability. Similarly, the CtIP protein, which interacts with the BRCA1 tumor suppressor, is also thought to have tumor suppression activity. Through its role in DNA end resection, CtIP facilitates DNA double-strand break (DSB) repair by homologous recombination (DSBR-HR) and microhomology-mediated end joining (MMEJ). In addition, however, CtIP has also been implicated in the formation of aberrant chromosomal rearrangements in an MMEJ-dependent manner, an activity that could potentially promote tumor development by increasing genome instability. To clarify whether CtIP acts in vivo to suppress or promote tumorigenesis, we have examined its oncogenic potential in mouse models of human breast cancer. Surprisingly, mice heterozygous for a null Ctip allele did not display an increased susceptibility to tumor formation. Moreover, mammary-specific biallelic CtIP ablation did not elicit breast tumors in a manner reminiscent of BRCA1 loss. Instead, CtIP inactivation dramatically reduced the kinetics of mammary tumorigenesis in mice bearing mammary-specific lesions of the p53 gene. Thus, unlike other repair factors, CtIP is not a tumor suppressor, but has oncogenic properties that can promote tumorigenesis, consistent with its ability to facilitate MMEJ-dependent chromosomal instability. Consequently, inhibition of CtIP-mediated MMEJ may prove effective against tumor types, such as human breast cancer, that display MMEJ-dependent chromosomal rearrangements.

How do we fit ferroptosis in the family of regulated cell death?

In the last few years many new cell death modalities have been described. To classify different types of cell death, the term 'regulated cell death' was introduced to discriminate it from 'accidental cell death'. Regulated cell death involves the activation of genetically encoded molecular machinery that couples the presence of some signal to cell death. These forms of cell death, like apoptosis, necroptosis and pyroptosis have important physiological roles in development, tissue repair, and immunity. Accidental cell death occurs in response to physical or chemical insults and occurs independently of molecular signalling pathways. Ferroptosis, an emerging and recently (re)discovered type of regulated cell death occurs through Fe(II)-dependent lipid peroxidation when the reduction capacity of a cell is insufficient. Ferroptosis is coined after the requirement for free ferrous iron. Here, we will consider the extent to which ferroptosis is similar to other regulated cell deaths and explore emerging ideas about the physiological role of ferroptosis.

Osteosarcoma: Molecular Pathogenesis and iPSC Modeling

Rare hereditary disorders provide unequivocal evidence of the importance of genes in human disease pathogenesis. Familial syndromes that predispose to osteosarcomagenesis are invaluable in understanding the underlying genetics of this malignancy. Recently, patient-derived induced pluripotent stem cells (iPSCs) have been successfully utilized to model Li-Fraumeni syndrome (LFS)-associated bone malignancy, demonstrating that iPSCs can serve as an in vitro disease model to elucidate osteosarcoma etiology. We provide here an overview of osteosarcoma predisposition syndromes and review recently established iPSC disease models for these familial syndromes. Merging molecular information gathered from these models with the current knowledge of osteosarcoma biology will help us to gain a deeper understanding of the pathological mechanisms underlying osteosarcomagenesis and will potentially aid in the development of future patient therapies.

Chronically Alternating Light Cycles Increase Breast Cancer Risk in Mice

Although epidemiological studies in shift workers and flight attendants have associated chronic circadian rhythm disturbance (CRD) with increased breast cancer risk, causal evidence for this association is lacking. Several scenarios have been proposed to contribute to the shift work-cancer connection: (1) internal desynchronization, (2) light at night (resulting in melatonin suppression), (3) sleep disruption, (4) lifestyle disturbances, and (5) decreased vitamin D levels due to lack of sunlight. The confounders inherent in human field studies are less problematic in animal studies, which are therefore a good approach to assess the causal relation between circadian disturbance and cancer. However, the experimental conditions of many of these animal studies were far from the reality of human shift workers. For example, some involved xenografts (addressing tumor growth rather than cancer initiation and/or progression), chemically induced tumor models, or continuous bright light exposure, which can lead to suppression of circadian rhythmicity. Here, we have exposed breast cancer-prone p53(R270H/+)WAPCre conditional mutant mice (in a FVB genetic background) to chronic CRD by subjecting them to a weekly alternating light-dark (LD) cycle throughout their life. Animals exposed to the weekly LD inversions showed a decrease in tumor suppression. In addition, these animals showed an increase in body weight. Importantly, this study provides the first experimental proof that CRD increases breast cancer development. Finally, our data suggest internal desynchronization and sleep disturbance as mechanisms linking shift work with cancer development and obesity.

Overview of Genetically Engineered Mouse Models of Breast Cancer Used in Translational Biology and Drug Development

Breast cancer is a heterogeneous condition with no single standard of treatment and no definitive method for determining whether a tumor will respond to therapy. The development of murine models that faithfully mimic specific human breast cancer subtypes is critical for the development of patient-specific treatments. While the artificial nature of traditional in vivo xenograft models used to characterize novel anticancer treatments has limited clinical predictive value, the development of genetically engineered mouse models (GEMMs) makes it possible to study the therapeutic responses in an intact microenvironment. GEMMs have proven to be an experimentally tractable platform for evaluating the efficacy of novel therapeutic combinations and for defining the mechanisms of acquired resistance. Described in this overview are several of the more popular breast cancer GEMMs, including details on their value in elucidating the molecular mechanisms of this disorder.

Comparison of effects of p53 null and gain-of-function mutations on salivary tumors in MMTV-Hras transgenic mice

p53 is an important tumor suppressor gene which is mutated in ~50% of all human cancers. Some of these mutants appear to have acquired novel functions beyond merely losing wild-type functions. To investigate these gain-of-function effects in vivo, we generated mice of three different genotypes: MMTV-Hras/p53(+/+), MMTV-Hras/p53(-/-), and MMTV-Hras/p53R172H/R172H. Salivary tumors from these mice were characterized with regard to age of tumor onset, tumor growth rates, cell cycle distribution, apoptotic levels, tumor histopathology, as well as response to doxorubicin treatment. Microarray analysis was also performed to profile gene expression. The MMTV-Hras/p53(-/-) and MMTV-Hras/p53R172H/R172H mice displayed similar properties with regard to age of tumor onset, tumor growth rates, tumor histopathology, and response to doxorubicin, while both groups were clearly distinct from the MMTV-Hras/p53(+/+) mice by these measurements. In addition, the gene expression profiles of the MMTV-Hras/p53(-/-) and MMTV-Hras/p53(R172H/R172H) tumors were tightly clustered, and clearly distinct from the profiles of the MMTV-Hras/p53(+/+) tumors. Only a small group of genes showing differential expression between the MMTV-Hras/p53(-/-) and MMTV-Hras/p53(R172H/R172H) tumors, that did not appear to be regulated by wild-type p53, were identified. Taken together, these results indicate that in this MMTV-Hras-driven salivary tumor model, the major effect of the p53 R172H mutant is due to the loss of wild-type p53 function, with little or no gain-of-function effect on tumorigenesis, which may be explained by the tissue- and tumor type-specific properties of this gain-of-function mutant of p53.

Mammary gland tumor promotion by chronic administration of IGF1 and the insulin analogue AspB10 in the p53R270H/⁺WAPCre mouse model

INTRODUCTION

Insulin analogues are structurally modified molecules with altered pharmaco-kinetic and -dynamic properties compared to regular human insulin used by diabetic patients. While these compounds are tested for undesired mitogenic effects, an epidemiological discussion is ongoing regarding an association between insulin analogue therapy and increased cancer incidence, including breast cancer. Standard in vivo rodent carcinogenesis assays do not pick up this possible increased carcinogenic potential.

METHODS

Here we studied the role of insulin analogues in breast cancer development. For this we used the human relevant mammary gland specific p53R270H/⁺WAPCre mouse model. Animals received life long repeated treatment with four different insulin (-like) molecules: normal insulin, insulin glargine, insulin X10 (AspB10) or insulin-like growth factor 1 (IGF1).

RESULTS

Insulin-like molecules with strong mitogenic signaling, insulin X10 and IGF1, significantly decreased the time for tumor development. Yet, insulin glargine and normal insulin, did not significantly decrease the latency time for (mammary gland) tumor development. The majority of tumors had an epithelial to mesenchymal transition phenotype (EMT), irrespective of treatment condition. Enhanced extracellular signaling related kinase (Erk) or serine/threonine kinase (Akt) mitogenic signaling was in particular present in tumors from the insulin X10 and IGF1 treatment groups.

CONCLUSIONS

These data indicate that insulin-like molecules with enhanced mitogenic signaling increase the risk of breast cancer development. Moreover, the use of a tissue specific cancer model, like the p53R270H/⁺WAPCre mouse model, is relevant to assess the intrinsic pro-carcinogenic potential of mitogenic and non-mitogenic biologicals such as insulin analogues.

Conditional knockout mouse models of cancer

In 2007, three scientists, Drs. Mario R. Capecchi, Martin J. Evans, and Oliver Smithies, received the Nobel Prize in Physiology or Medicine for their contributions of introducing specific gene modifications into mice. This technology, commonly referred to as gene targeting or knockout, has proven to be a powerful means for precisely manipulating the mammalian genome and has generated great impacts on virtually all phases of mammalian biology and basic biomedical research. Of note, germline mutations of many genes, especially tumor suppressors, often result in lethality during embryonic development or at developmental stages before tumor formation. This obstacle has been effectively overcome by the use of conditional knockout technology in conjunction with Cre-LoxP- or Flp-Frt-mediated temporal and/or spatial systems to generate genetic switches for precise DNA recombination. Currently, numerous conditional knockout mouse models have been successfully generated and applied in studying tumor initiation, progression, and metastasis. This review summarizes some conditional mutant mouse models that are widely used in cancer research and our understanding of the possible mechanisms underlying tumorigenesis.

Genetically engineered ERα-positive breast cancer mouse models

The majority of human breast cancers are estrogen receptor-positive (ER+), but this has proven challenging to model in genetically engineered mice. This review summarizes information on 21 mouse models that develop ER+ mammary cancer. Where available, information on cancer pathology and gene expression profiles is referenced to assist in understanding which histological subtype of ER+ human cancer each model might represent. ESR1, CCDN1, prolactin, TGFα, AIB1, ESPL1, and WNT1 overexpression, PIK3CA gain of function, as well as loss of P53 (Trp53) or STAT1 are associated with ER+ mammary cancer. Treatment with the PPARγ agonist efatutazone in a mouse with Brca1 and p53 deficiency and 7,12-dimethylbenz(a)anthracene exposure in combination with an activated myristoylated form of AKT1 also induce ER+ mammary cancer. A spontaneous mutant in nude mice that develops metastatic ER+ mammary cancer is included. Age of cancer development ranges from 3 to 26 months and the percentage of cancers that are ER+ vary from 21 to 100%. Not all models are characterized as to their estrogen dependency and/or response to anti-hormonal therapy. Strain backgrounds include C57Bl/6, FVB, BALB/c, 129S6/SvEv, CB6F1, and NIH nude. Most models have only been studied on one strain background. In summary, while a range of models are available for studies of pathogenesis and therapy of ER+ breast cancers, many could benefit from further characterization, and opportunity for development of new models remains.

Mammary gland-specific ablation of focal adhesion kinase reduces the incidence of p53-mediated mammary tumour formation

Background:

Elevated expression of focal adhesion kinase (FAK) occurs in numerous human cancers including colon-, cervix- and breast cancer. Although several studies have implicated FAK in mammary tumour formation induced by ectopic oncogene expression, evidence supporting a role for FAK in spontaneous mammary tumour development caused by loss of tumour suppressor genes such as p53 is lacking. Alterations in the tumour suppressor gene p53 have been implicated in over 50% of human breast cancers. Given that elevated FAK expression highly correlates with p53 mutation status in human breast cancer, we set out to investigate the importance of FAK in p53-mediated spontaneous mammary tumour development.

Methods:

To directly assess the role of FAK, we generated mice with conditional inactivation of FAK and p53. We generated female p53^lox/lox/FAK^+/+/WapCre, p53^lox/lox/FAK^flox/+/WapCre and p53^lox/lox/FAK^flox/−/WapCre mice, and mice with WapCre-mediated conditional expression of p53^R270H, the mouse equivalent of human p53^R273H hot spot mutation, together with conditional deletion of FAK, P53^R270H/+/FAK^lox/+/WapCre and p53^R270H/+/FAK^flox/−/WapCre mice. All mice were subjected to one pregnancy to induce WapCre-mediated deletion of p53 or expression of p53 R270H, and Fak genes flanked by two loxP sites, and subsequently followed the development of mammary tumours.

Results:

Using this approach, we show that FAK is important for p53-induced mammary tumour development. In addition, mice with the mammary gland-specific conditional expression of p53 point mutation R270H, the mouse equivalent to human R273H, in combination with conditional deletion of Fak showed reduced incidence of p53^R270H-induced mammary tumours. In both models these effects of FAK were related to reduced proliferation in preneoplastic lesions in the mammary gland ductal structures.

Conclusions:

Mammary gland-specific ablation of FAK hampers p53-regulated spontaneous mammary tumour formation. Focal adhesion kinase deletion reduced proliferative capacity of p53 null and p53^R270H mammary epithelial cells but did not lead to increased apoptosis in vivo. Our data identify FAK as an important regulator in mammary epithelial cell proliferation in p53-mediated and p53^R270H-induced mammary tumour development.

Cell-type, dose, and mutation-type specificity dictate mutant p53 functions in vivo

The specific roles of mutant p53's dominant-negative (DN) or gain-of-function (GOF) properties in regulating acute response and long-term tumorigenesis is unclear. Using "knockin" mouse strains expressing varying R246S mutant levels, we show that the DN effect on transactivation is universally observed after acute p53 activation, whereas the effect on cellular outcome is cell-type specific. Reducing mutant p53 levels abrogated the DN effect. Mutant p53's DN effect protected against radiation-induced death but did not accentuate tumorigenesis. Furthermore, the R246S mutant did not promote tumorigenesis compared to p53(-/-) mice in various models, even when MDM2 is absent, unlike the R172H mutant. Together, these data demonstrate that mutant p53's DN property only affects acute responses, whereas GOF is not universal, being mutation-type specific.

Human breast adenocarcinoma cytotoxicity and modulation of 7,12-dimethylbenz[a]anthracene-induced mammary carcinoma in Balb/c mice by Acacia catechu (L.f.) Wild heartwood

OBJECTIVE

The chemopreventive potential of (+)-catechin-rich extract of Acacia catechu (L.f.) Willd. heartwood (AQCE) was evaluated against human breast adenocarcinoma cell line (MCF-7) and 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary carcinoma in Balb/c mice.

METHODS

Cell cytotoxicity was investigated using different colorimetric assays. Apoptosis was observed using diphenylamine assay and fluorescent microscopy. AQCE was further evaluated for antitumor activity against DMBA-induced mammary carcinoma. The levels of tumor markers and oxidative stress were measured. Furthermore, level of transcription factors was measured by enzyme-linked immunosorbent assay.

RESULTS

The results showed that administration of AQCE showed a dose-dependent growth inhibition response and DNA fragmentation in MCF-7 cells. Tumor multiplicity was significantly decreased to 42.91% with AQCE when compared with DMBA-treated animals. The levels of tumor markers such as total sialic acid and lipid-associated sialic acid were substantially increased after DMBA treatment. However, AQCE treatment restored tumor markers level. AQCE also significantly reduced elevated levels of nitrite and malondialdehyde in DMBA-treated animals. Additionally, AQCE also increased the activities of antioxidant enzymes, viz., catalase, superoxide dismutase, total thiol, reduced glutathione, protein thiol, glutathione peroxidase, glutathione reductase, and glutathione-S-transferase in the mammary tissue and liver mitochondria of DMBA-administered animals. Significant increase in the protein levels of p53, c-jun, and p65 were observed in DMBA-treated mice, whereas less expression was observed in AQCE-treated animals. Eventually, AQCE also significantly improved body weight and maintained the mammary tissue architecture in normal range.

CONCLUSIONS

The present data strongly suggest that anticancer potentiality of (+)-catechin-rich AQCE may be attributable to its ability to positively modulate tumor markers as well as the antioxidant system that could decompose the peroxides and, thereby, offer a protection against lipid peroxidation and linked to the expression of transcription factors during DMBA-induced mammary carcinoma.

A bead-based multiplexed immunoassay to evaluate breast cancer biomarkers for early detection in pre-diagnostic serum

This study investigates whether a set of ten potential breast cancer serum biomarkers and cancer antigens (osteopontin (OPN), haptoglobin, cancer antigen 15-3 (CA15-3), carcinoembryonic antigen (CEA), cancer antigen 125 (CA-125), prolactin, cancer antigen 19-9 (CA19-9), α-fetoprotein (AFP), leptin and migration inhibitory factor (MIF)) can predict early stage breast cancer in samples collected before clinical diagnosis (phase III samples). We performed a nested case-control study within the Prospect-EPIC (European Prospective Investigation into Cancer and nutrition) cohort. We examined to what extent the biomarker panel could discriminate between 68 women diagnosed with breast cancer up to three years after enrollment and 68 matched healthy controls (all 56-64 years at baseline). Using a quantitative bead-based multiplexed assay, we determined protein concentrations in serum samples collected at enrollment. Principal Component Analysis (PCA) and Random Forest (RF) analysis revealed that on the basis of all ten proteins, early cases could not be separated from controls. When we combined serum protein concentrations and subject characteristics related to breast cancer risk in the RF analysis, this did not result in classification accuracy scores that could correctly classify the samples (sensitivity: 50%, specificity: 50%). Our findings indicate that this panel of selected tumor markers cannot be used for diagnosis of early breast cancer.

The rebel angel: mutant p53 as the driving oncogene in breast cancer

Breast cancer is the most frequent invasive tumor diagnosed in women, causing over 400 000 deaths yearly worldwide. Like other tumors, it is a disease with a complex, heterogeneous genetic and biochemical background. No single genomic or metabolic condition can be regarded as decisive for its formation and progression. However, a few key players can be pointed out and among them is the TP53 tumor suppressor gene, commonly mutated in breast cancer. In particular, TP53 mutations are exceptionally frequent and apparently among the key driving factors in triple negative breast cancer -the most aggressive breast cancer subgroup-whose management still represents a clinical challenge. The majority of TP53 mutations result in the substitution of single aminoacids in the central region of the p53 protein, generating a spectrum of variants ('mutant p53s', for short). These mutants lose the normal p53 oncosuppressive functions to various extents but can also acquire oncogenic properties by gain-of-function mechanisms. This review discusses the molecular processes translating gene mutations to the pathologic consequences of mutant p53 tumorigenic activity, reconciling cell and animal models with clinical outcomes in breast cancer. Existing and speculative therapeutic methods targeting mutant p53 are also discussed, taking into account the overlap of mutant and wild-type p53 regulatory mechanisms and the crosstalk between mutant p53 and other oncogenic pathways in breast cancer. The studies described here concern breast cancer models and patients-unless it is indicated otherwise and justified by the importance of data obtained in other models.

Initiation of prostate cancer in mice by Tp53R270H: evidence for an alternative molecular progression

Tp53 mutations are common in human prostate cancer (CaP), occurring with a frequency of ∼30% and ∼70% in localized and metastatic disease, respectively. In vitro studies have determined several common mutations of Tp53 that have specific gain-of-function properties in addition to loss of function, including the ability to promote castration-resistant (CR) growth of CaP cells in some contexts. To date, a lack of suitable mouse models has prohibited investigation of the role played by Tp53 mutations in mediating CaP progression in vivo. Here, we describe the effects of conditional expression of a mutant Tp53 (Tp53^R270H; equivalent to the human hotspot mutant R273H) in the prostate epithelium of mice. Heterozygous “Tp53^LSL-R270H/+” [129S4(Trp53^tm3Tyj)] and “Nkx3.1-Cre” [129S(Nkx3-1^tm3(cre)Mms)] mice with prostate-specific expression of the Tp53^R270H mutation (p53^R270H/+Nkx3.1-Cre mice) were bred onto an FVB/N background via speed congenesis to produce strain FVB.129S4(Trp53^tm3Tyj/wt); FVB.129S(Nkx3-1^{tm3(cre)Mms/wt}) and littermate genotype negative control mice. These mutant mice had significantly increased incidences of prostatic intraepithelial neoplasia (PIN) lesions, and these appeared earlier, compared with the Nkx3.1 haploinsufficient (Nkx3.1-Cre het) littermate mice, which did not express the Tp53 mutation. PIN lesions in these mice showed consistent progression and some developed into invasive adenocarcinoma with a high grade, sarcomatoid or epithelial-mesenchymal transition (EMT) phenotype. PIN lesions were similar to those seen in PTEN conditional knockout mice, with evidence of AKT activation concomitant with neoplastic proliferation. However, the invasive tumor phenotype is rarely seen in previously described mouse models of prostatic neoplasia. These data indicate that the Tp53^R270H mutation plays a role in CaP initiation. This finding has not previously been reported. Further characterization of this model, particularly in a setting of androgen deprivation, should allow further insight into the mechanisms by which the Tp53^R270H mutation mediates CaP progression.

Mouse models of estrogen receptor-positive breast cancer

Breast cancer is the most frequent malignancy and second leading cause of cancer-related deaths among women. Despite advances in genetic and biochemical analyses, the incidence of breast cancer and its associated mortality remain very high. About 60 – 70% of breast cancers are Estrogen Receptor alpha (ER-α) positive and are dependent on estrogen for growth. Selective estrogen receptor modulators (SERMs) have therefore provided an effective targeted therapy to treat ER-α positive breast cancer patients. Unfortunately, development of resistance to endocrine therapy is frequent and leads to cancer recurrence. Our understanding of molecular mechanisms involved in the development of ER-α positive tumors and their resistance to ER antagonists is currently limited due to lack of experimental models of ER-α positive breast cancer. In most mouse models of breast cancer, the tumors that form are typically ER-negative and independent of estrogen for their growth. However, in recent years more attention has been given to develop mouse models that develop different subtypes of breast cancers, including ER-positive tumors. In this review, we discuss the currently available mouse models that develop ER-α positive mammary tumors and their potential use to elucidate the molecular mechanisms of ER-α positive breast cancer development and endocrine resistance.

BRCA1 tumor suppression depends on BRCT phosphoprotein binding, but not its E3 ligase activity

Germline mutations of the breast cancer 1 (BRCA1) gene are a major cause of familial breast and ovarian cancer. The BRCA1 protein displays E3 ubiquitin ligase activity, and this enzymatic function is thought to be required for tumor suppression. To test this hypothesis, we generated mice that express an enzymatically defective Brca1. We found that this mutant Brca1 prevents tumor formation to the same degree as does wild-type Brca1 in three different genetically engineered mouse (GEM) models of cancer. In contrast, a mutation that ablates phosphoprotein recognition by the BRCA C terminus (BRCT) domains of BRCA1 elicits tumors in each of the three GEM models. Thus, BRCT phosphoprotein recognition, but not the E3 ligase activity, is required for BRCA1 tumor suppression.

Choosing a mouse model: experimental biology in context--the utility and limitations of mouse models of breast cancer

Genetically engineered mice are critical experimental models for the study of breast cancer biology. Transgenic mice, employing strong mammary epithelial promoters to drive oncogenes, develop carcinomas with phenotypes corresponding to the molecular pathway activated. Gene-targeted (knockout) mice, in which tumor suppressors are deleted, develop mammary neoplasms with phenotypes primarily including patterns seen in spontaneous mouse mammary tumors, albeit at higher rates. Improved genetic engineering, using inducible gene expression, somatic gene transduction, conditional alleles, and crossbreeding for combined/compound genetic engineering yields precise molecular models with exquisite experimental control and phenotypes with comparative pathologic validity. Mammary gland transplantation technology adds a practical and validated method for assessing biologic behavior of selected mammary tissues. Overall, the many mouse models available are a rich resource for experimental biology with phenocopies of breast cancer subtypes, and a variety of practical advantages. The challenge is matching the model to the experimental question.

Transgenic animal models in toxicology: historical perspectives and future outlook

Transgenic animal models are powerful tools for developing a more detailed understanding on the roles of specific genes in biological pathways and systems. Applications of these models have been made within the field of toxicology, most notably for the screening of mutagenic and carcinogenic potential and for the characterization of toxic mechanisms of action. It has long been a goal of research toxicologists to use the data from these models to refine hazard identification and characterization to better inform human health risk assessments. This review provides an overview on the applications of transgenic animal models in the assessment of mutagenicity and carcinogenicity, their use as reporter systems, and as tools for understanding the roles of xenobiotic-metabolizing enzymes and biological receptors in the etiology of chemical toxicity. Perspectives are also shared on the future outlook for these models in toxicology and risk assessment and how transgenic technologies are likely to be an integral tool for toxicity testing in the 21st century.

Dominant-negative features of mutant TP53 in germline carriers have limited impact on cancer outcomes

Germline TP53 mutations result in cancer proneness syndromes known as Li-Fraumeni, Li-Fraumeni-like, and nonsyndromic predisposition with or without family history. To explore genotype/phenotype associations, we previously adopted a functional classification of all germline TP53 mutant alleles based on transactivation. Severe deficiency (SD) alleles were associated with more severe cancer proneness syndromes, and a larger number of tumors, compared with partial deficiency (PD) alleles. Because mutant p53 can exert dominant-negative (DN) effects, we addressed the relationship between DN and clinical manifestations. We reasoned that DN effects might be stronger in familial cancer cases associated with germline TP53 mutations, where mutant alleles coexist with the wild-type allele since conception. We examined 104 p53 mutant alleles with single amino acid substitutions described in the IARC germline database for (i) transactivation capability and (ii) capacity to reduce the activity of the wild-type allele (i.e., DN effect) using a quantitative yeast-based assay. The functional classifications of p53 alleles were then related to clinical variables. We confirmed that a classification based on transactivation alone can identify familial cancer cases with more severe clinical features. Classification based on DN effects allowed us to highlight similar associations but did not reveal distinct clinical subclasses of SD alleles, except for a correlation with tumor tissue prevalence. We conclude that in carriers of germline TP53 mutations transactivation-based classification of TP53 alleles appears more important for genotype/phenotype correlations than DN effects and that haplo-insufficiency of the TP53 gene is an important factor in cancer proneness in humans.

Genotoxic exposure: novel cause of selection for a functional ΔN-p53 isoform

The p53 gene is frequently mutated in cancers and it is vital for cell cycle control, homeostasis and carcinogenesis. We describe a novel p53 mutational spectrum, different to those generally observed in human and murine tumors. Our study shows a high prevalence of nonsense mutations in the p53 N terminus of 2-acetylaminofluorene (2-AAF)-induced urinary bladder tumors. These nonsense mutations forced downstream translation initiation at codon 41 of Trp53, resulting in the aberrant expression of the p53 isoform ΔN-p53 (or p44). We propose a novel mechanism for the origination and the selection for this isoform. We show that chemical exposure can act as a novel cause of selection for this truncated protein. In addition, our data suggest that the occurrence of ΔN-p53 accounts, at least in mice, for a cancer phenotype. We also show that gene expression profiles of embryonic stem (ES) cells carrying the ΔN-p53 isoform in a p53-null background are divergent from p53 knockout ES cells, and therefore postulate that ΔN-p53 itself has functional transcriptional properties.

Neonatal experiences differentially influence mammary gland morphology, estrogen receptor {α} protein levels, and carcinogenesis in BALB/c mice

Prevention of breast cancer can be achieved with a better understanding of the factors contributing to normal breast development. Because the breast develops postnatally, alterations in the development and lifetime activity of the neuroendocrine system may set up an environment that increases cancer risk. The present study examined how two neonatal experiences over the first 3 weeks of life influence normal and malignant mammary gland development in female BALB/c mice. Following puberty, both brief (15 minutes) and prolonged (4 hours) daily maternal separations of newborn mice accelerated mammary gland development relative to nonseparated mice. Despite similar mammary gland morphologies between mice exposed to these two neonatal separation experiences, only mice exposed to prolonged maternal separation bouts showed a higher incidence and faster onset of mammary tumorigenesis following adulthood carcinogen [7,12-dimethylbenz(a)anthracene] administration. Molecular analysis of estrogen receptor α (ERα) and p53, two proteins that have been implicated in breast cancer, revealed that for mice exposed to prolonged neonatal maternal separation bouts, mammary gland ERα protein levels were upregulated in a transcription-independent manner. On the other hand, p53 expression in mammary glands of adult mice was not differentially influenced by neonatal experiences. Our findings show that chronic, moderate psychosocial stress during the neonatal period increases the expression of ERα protein and promotes mammary tumorigenesis in adulthood.

Rb inactivation accelerates neoplastic growth and substitutes for recurrent amplification of cIAP1, cIAP2 and Yap1 in sporadic mammary carcinoma associated with p53 deficiency

Genetically defined mouse models offer an important tool to identify critical secondary genetic alterations with relevance to human cancer pathogenesis. We used newly generated MMTV-Cre105Ayn mice to inactivate p53 and/or Rb strictly in the mammary epithelium, and to determine recurrent genomic changes associated with deficiencies of these genes. p53 inactivation led to formation of estrogen receptor-positive raloxifene-responsive mammary carcinomas with features of luminal subtype B. Rb deficiency was insufficient to initiate carcinogenesis but promoted genomic instability and growth rate of neoplasms associated with p53 inactivation. Genome-wide analysis of mammary carcinomas identified a recurrent amplification at chromosome band 9A1, a locus orthologous to human 11q22, which contains protooncogenes cIAP1 (Birc2), cIAP2 (Birc3) and Yap1. It is interesting that this amplicon was preferentially detected in carcinomas carrying wild-type Rb. However, all three genes were overexpressed in carcinomas with p53 and Rb inactivation, likely due to E2F-mediated transactivation, and cooperated in carcinogenesis according to gene knockdown experiments. These findings establish a model of luminal subtype B mammary carcinoma, identify critical role of cIAP1, cIAP2 and Yap1 co-expression in mammary carcinogenesis and provide an explanation for the lack of recurrent amplifications of cIAP1, cIAP2 and Yap1 in some tumors with frequent Rb deficiency, such as mammary carcinoma.

Identification of breast cancer biomarkers in transgenic mouse models: A proteomics approach

PURPOSE

Transgenic mouse models for cancer circumvent many challenges that hamper human studies aimed at biomarker discovery. Lower biological variances among mice combined with controllable factors such as food uptake and health status may enable the detection of more subtle protein expression differences. This is envisioned to result in the identification of biomarkers better discriminating cancer cases from controls.

EXPERIMENTAL DESIGN

The current study used two innovative mouse models for breast-cancer to identify new serum biomarkers. Multi-analyte profiling technique was used to analyze 70 proteins in individual serum samples of non-tumor and mammary tumor-bearing Tg.NK (MMTV/c-neu) mice.

RESULTS

A small set of proteins fully differentiated tumor samples from controls. These comprised osteopontin, interleukin-18, cystatin C and CD40 antigen. Comparison of protein expression in another breast-cancer mouse model, the humanized p53.R270H mice, showed common discriminatory expression of osteopontin. However, other biomarkers showed distinct expression in the two different breast-cancer models, indicating that different mammary tumor sub-types with respect to molecular and estrogen receptor status reveal divergent serum biomarker sets.

CONCLUSIONS AND CLINICAL RELEVANCE

The current study supports the concept that serum proteins can discriminate mammary tumor cases from controls, and yielded interesting biomarkers that need further testing and validation in human studies.

Animal models of breast cancer for the study of pathogenesis and therapeutic insights

Activation of oncogenes and inactivation of tumour suppressor genes are common events during breast cancer initiation and progression and often determine treatment responsiveness. Indeed, these events need to be recreated in in vitro systems and in mouse cancer models in order to unravel the molecular mechanisms involved in breast cancer initiation and metastasis and assess their possible impact on responses to anticancer drugs. Optical-based imaging models are used to investigate and to follow important tumour progression processes. Moreover, the development of novel anticancer strategies requires more sensitive and less invasive methods to detect and monitor in vivo drug responses in breast cancer models. This review highlights some of the current strategies for modelling breast cancer in vitro and in the mouse, in order to answer biological or translational questions about human breast malignancies.

Pathways contributing to development of spontaneous mammary tumors in BALB/c-Trp53+/- mice

Mutation and loss of function in p53 are common features among human breast cancers. Here we use BALB/c-Trp53+/- mice as a model to examine the sequence of events leading to mammary tumors. Mammary gland proliferation rates were similar in both BALB/c-Trp53+/- mice and wild-type controls. In addition, sporadic mammary hyperplasias were rare in BALB/c-Trp53+/- mice and not detectably different from those of wild-type controls. Among the 28 mammary tumors collected from BALB/c-Trp53+/- mice, loss of heterozygosity for Trp53 was detected in more than 90% of invasive mammary tumors. Transplantation of Trp53+/- ductal hyperplasias also indicated an association between loss of the wild-type allele of Trp53 and progression to invasive carcinomas. Therefore, loss of p53 function seems to be a rate-limiting step in progression. Moreover, expression of biomarkers such as estrogen receptor alpha, progesterone receptor, Her2/Neu, and activated Notch1 varied among mammary tumors, suggesting that multiple oncogenic lesions collaborate with loss of p53 function. Expression of biomarkers was retained when tumor fragments were transplanted to syngeneic hosts. Tumors expressing solely luminal or basal keratins were also observed (27 and 11%, respectively), but the largest class of tumors expressed both luminal and basal keratins (62%). Overall, this panel of transplantable tumors provides a resource for detailed evaluation of the cell lineages undergoing transformation and preclinical testing of therapeutic agents targeting a variety of oncogenic pathways including cancer stem cells.

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.

Heterozygosity for hypoxia inducible factor 1alpha decreases the incidence of thymic lymphomas in a p53 mutant mouse model

Hypoxia inducible factors (HIF) are critical mediators of the cellular response to decreased oxygen tension and are overexpressed in a number of tumors. Although HIF1alpha and HIF2alpha share a high degree of sequence homology, recent work has shown that the two alpha subunits can have contrasting and tissue-specific effects on tumor growth. To directly compare the role of each HIFalpha subunit in spontaneous tumorigenesis, we bred a mouse model of expanded HIF2alpha expression and Hif1alpha(+/-) mice to homozygotes for the R270H mutation in p53. Here, we report that p53(R270H/R270H) mice, which have not been previously described, develop a unique tumor spectrum relative to p53(R270H/-) mice, including a high incidence of thymic lymphomas. Heterozygosity for Hif1alpha significantly reduced the incidence of thymic lymphomas observed in this model. Moreover, reduced Hif1alpha levels correlated with decreased stabilization of activated Notch1 and expression of the Notch target genes, Dtx1 and Nrarp. These observations uncover a novel role for HIF1alpha in Notch pathway activation during T-cell lymphomagenesis.

A mouse mammary epithelial cell model to identify molecular mechanisms regulating breast cancer progression

Breast cancer, like any other human cancer, results from the accumulation of mutations that deregulate critical cellular processes, such as cell proliferation and death. Activation of oncogenes and inactivation of tumor suppressor genes are common events during cancer initiation and progression and often determine treatment responsiveness. Thus, recapitulating these events in mouse cancer models is critical for unraveling the molecular mechanisms involved in tumorigenesis and for interrogating their possible impact on response to anticancer drugs. We have developed a novel mouse mammary epithelial cell model, which replicates the steps of epithelial tumor progression and takes advantage of the power of mouse genetics and the ability to assess three-dimensional morphogenesis in the presence of extracellular matrix to model human breast cancer.

Identification of conserved gene expression features between murine mammary carcinoma models and human breast tumors

BACKGROUND

Although numerous mouse models of breast carcinomas have been developed, we do not know the extent to which any faithfully represent clinically significant human phenotypes. To address this need, we characterized mammary tumor gene expression profiles from 13 different murine models using DNA microarrays and compared the resulting data to those from human breast tumors.

RESULTS

Unsupervised hierarchical clustering analysis showed that six models (TgWAP-Myc, TgMMTV-Neu, TgMMTV-PyMT, TgWAP-Int3, TgWAP-Tag, and TgC3(1)-Tag) yielded tumors with distinctive and homogeneous expression patterns within each strain. However, in each of four other models (TgWAP-T121, TgMMTV-Wnt1, Brca1Co/Co;TgMMTV-Cre;p53+/- and DMBA-induced), tumors with a variety of histologies and expression profiles developed. In many models, similarities to human breast tumors were recognized, including proliferation and human breast tumor subtype signatures. Significantly, tumors of several models displayed characteristics of human basal-like breast tumors, including two models with induced Brca1 deficiencies. Tumors of other murine models shared features and trended towards significance of gene enrichment with human luminal tumors; however, these murine tumors lacked expression of estrogen receptor (ER) and ER-regulated genes. TgMMTV-Neu tumors did not have a significant gene overlap with the human HER2+/ER- subtype and were more similar to human luminal tumors.

CONCLUSION

Many of the defining characteristics of human subtypes were conserved among the mouse models. Although no single mouse model recapitulated all the expression features of a given human subtype, these shared expression features provide a common framework for an improved integration of murine mammary tumor models with human breast tumors.

Role of autophagy in breast cancer

Autophagy is an evolutionarily conserved process of cytoplasm and cellular organelle degradation in lysosomes. Autophagy is a survival pathway required for cellular viability during starvation; however, if it proceeds to completion, autophagy can lead to cell death. In neurons, constitutive autophagy limits accumulation of polyubiquitinated proteins and prevents neuronal degeneration. Therefore, autophagy has emerged as a homeostatic mechanism regulating the turnover of long-lived or damaged proteins and organelles, and buffering metabolic stress under conditions of nutrient deprivation by recycling intracellular constituents. Autophagy also plays a role in tumorigenesis, as the essential autophagy regulator beclin1 is monoallelically deleted in many human ovarian, breast, and prostate cancers, and beclin1(+/-) mice are tumor-prone. We found that allelic loss of beclin1 renders immortalized mouse mammary epithelial cells susceptible to metabolic stress and accelerates lumen formation in mammary acini. Autophagy defects also activate the DNA damage response in vitro and in mammary tumors in vivo, promote gene amplification, and synergize with defective apoptosis to accelerate mammary tumorigenesis. Thus, loss of the prosurvival role of autophagy likely contributes to breast cancer progression by promoting genome damage and instability. Exploring the yet unknown relationship between defective autophagy and other breast cancer promoting functions may provide valuable insight into the pathogenesis of breast cancer and may have significant prognostic and therapeutic implications for breast cancer patients.

Dominant-negative but not gain-of-function effects of a p53.R270H mutation in mouse epithelium tissue after DNA damage

p53 alterations in human tumors often involve missense mutations that may confer dominant-negative or gain-of-function properties. Dominant-negative effects result in inactivation of wild-type p53 protein in heterozygous mutant cells and as such in a p53 null phenotype. Gain-of-function effects can directly promote tumor development or metastasis through antiapoptotic mechanisms or transcriptional activation of (onco)genes. Here, we show, using conditional mouse technology, that epithelium-specific heterozygous expression of mutant p53 (i.e., the p53.R270H mutation that is equivalent to the human hotspot R273H) results in an increased incidence of spontaneous and UVB-induced skin tumors. Expression of p53.R270H exerted dominant-negative effects on latency, multiplicity, and progression status of UVB-induced but not spontaneous tumors. Surprisingly, gain-of-function properties of p53.R270H were not detected in skin epithelium. Apparently, dominant-negative and gain-of-function effects of mutant p53 are highly tissue specific and become most manifest upon stabilization of p53 after DNA damage.

Tumor models for efficacy determination

The first in vivo tumor models were developed in the mid-1960s. These models were mouse leukemia models grown as ascites. The growth pattern was like that of bacteria in vivo and therefore it was possible to apply similar mathematics of growth and response to these tumors as had been worked out for bacteria. Since the development of the murine leukemia models, investigators have devoted a large effort to modeling solid tumors in mice. There are now a variety of models including syngeneic mouse tumors and human tumor xenografts grown as s.c. nodules, syngeneic mouse tumors and human tumor xenografts grown in orthotopic sites, models of disseminated disease, "labeled" tumor models that can be visualized using varied technologies, and transgenic tumor models. Each of these types of models has advantages and disadvantages to the "drug hunter" searching for improved treatments.

Using genetically engineered mouse models of cancer to aid drug development: an industry perspective

Recent developments in the generation and characterization of genetically engineered mouse models of human cancer have resulted in notable improvements in these models as platforms for preclinical target validation and experimental therapeutics. In this review, we enumerate the criteria used to assess the accuracy of various models with respect to human disease and provide some examples of their prognostic and therapeutic utility, focusing on models for cancers that affect the largest populations. Technological advancements that allow greater exploitation of genetically engineered mouse models, such as RNA interference in vivo, are described in the context of target and drug validation. Finally, this review discusses stratagems for, and obstacles to, the application of these models in the drug development process.