Tumorigenesis in the multiple intestinal neoplasia mouse: Redundancy of negative regulators and specificity of modifiers

p53/MDM2 signaling pathway in aging, senescence and tumorigenesis

A wealth of evidence has emerged that there is an association between aging, senescence and tumorigenesis. Senescence, a biological process by which cells cease to divide and enter a status of permanent cell cycle arrest, contributes to aging and aging-related diseases, including cancer. Aging populations have the higher incidence of cancer due to a lifetime of exposure to cancer-causing agents, reduction of repairing DNA damage, accumulated genetic mutations, and decreased immune system efficiency. Cancer patients undergoing cytotoxic therapies, such as chemotherapy and radiotherapy, accelerate aging. There is growing evidence that p53/MDM2 (murine double minute 2) axis is critically involved in regulation of aging, senescence and oncogenesis. Therefore, in this review, we describe the functions and mechanisms of p53/MDM2-mediated senescence, aging and carcinogenesis. Moreover, we highlight the small molecular inhibitors, natural compounds and PROTACs (proteolysis targeting chimeras) that target p53/MDM2 pathway to influence aging and cancer. Modification of p53/MDM2 could be a potential strategy for treatment of aging, senescence and tumorigenesis.

Mouse models in colon cancer, inferences, and implications

Mouse models of colorectal cancer (CRC) have been crucial in the identification of the role of genes responsible for the full range of pathology of the human disease and have proved to be dependable for testing anti-cancer drugs. Recent research points toward the relevance of tumor, angiogenic, and immune microenvironments in CRC progression to late-stage disease, as well as the treatment of it. This study examines important mouse models in CRC, discussing inherent strengths and weaknesses disclosed during their construction. It endeavors to provide both a synopsis of previous work covering how investigators have defined various models and to evaluate critically how researchers are most likely to use them in the future. Accumulated evidence regarding the metastatic process and the hope of using checkpoint inhibitors and immunological inhibitor therapies points to the need for a genetically engineered mouse model that is both immunocompetent and autochthonous.

Experimental Murine Models for Colorectal Cancer Research

Colorectal cancer (CRC) is the third most prevalent malignancy worldwide and in both sexes. Numerous animal models for CRC have been established to study its biology, namely carcinogen-induced models (CIMs) and genetically engineered mouse models (GEMMs). CIMs are valuable for assessing colitis-related carcinogenesis and studying chemoprevention. On the other hand, CRC GEMMs have proven to be useful for evaluating the tumor microenvironment and systemic immune responses, which have contributed to the discovery of novel therapeutic approaches. Although metastatic disease can be induced by orthotopic injection of CRC cell lines, the resulting models are not representative of the full genetic diversity of the disease due to the limited number of cell lines suitable for this purpose. On the other hand, patient-derived xenografts (PDX) are the most reliable for preclinical drug development due to their ability to retain pathological and molecular characteristics. In this review, the authors discuss the various murine CRC models with a focus on their clinical relevance, benefits, and drawbacks. From all models discussed, murine CRC models will continue to be an important tool in advancing our understanding and treatment of this disease, but additional research is required to find a model that can correctly reflect the pathophysiology of CRC.

Lentiviral in situ targeting of stem cells in unperturbed intestinal epithelium

BACKGROUND

Methods for the long-term in situ transduction of the unperturbed murine intestinal epithelium have not been developed in past research. Such a method could speed up functional studies and screens to identify genetic factors influencing intestinal epithelium biology. Here, we developed an efficient method achieving this long-sought goal.

RESULTS

We used ultrasound-guided microinjections to transduce the embryonic endoderm at day 8 (E8.0) in utero. The injection procedure can be completed in 20 min and had a 100% survival rate. By injecting a small volume (0.1-0.2 μl) of concentrated virus, single shRNA constructs as well as lentiviral libraries can successfully be transduced. The new method stably and reproducibly targets adult intestinal epithelium, as well as other endoderm-derived organs such as the lungs, pancreas, liver, stomach, and bladder. Postnatal analysis of young adult mice indicates that single transduced cells at E8.0 gave rise to crypt fields that were comprised of 20-30 neighbouring crypts per crypt-field at 90 days after birth. Lentiviral targeting of ApcMin/+ mutant and wildtype mice revealed that heterozygous loss of Apc function suppresses the developmental normal growth pattern of intestinal crypt fields. This suppression of crypt field sizes did not involve a reduction of the crypt number per field, indicating that heterozygous Apc loss impaired the growth of individual crypts within the fields. Lentiviral-mediated shRNA knockdown of p53 led to an approximately 20% increase of individual crypts per field in both Apc+/+ and ApcMin/+ mice, associating with an increase in crypt size in ApcMin/+ mice but a slight reduction in crypt size in Apc+/+ mice. Overall, p53 knockdown rescued the reduction in crypt field size in Apc-mutant mice but had no effect on crypt field size in wildtype mice.

CONCLUSIONS

This study develops a novel technique enabling robust and reproducible in vivo targeting of intestinal stem cells in situ in the unperturbed intestinal epithelium across different regions of the intestine. In vivo somatic gene editing and genetic screening of lentiviral libraries has the potential to speed up discoveries and mechanistic understanding of genetic pathways controlling the biology of the intestinal epithelium during development and postnatal life. The here developed method enables such approaches.

Natural Marine Products: Anti-Colorectal Cancer In Vitro and In Vivo

Colorectal cancer, a malignant tumor with high mortality, has a poor prognosis due to drug resistance and toxicity in clinical surgery and chemotherapy. Thus, finding safer and more efficient drugs for clinical trials is vital and urgent. Natural marine compounds, with rich resources and original chemical structures, are applied widely in anticancer treatments. We provide a systematic overview of recently reported marine compounds such as alkaloids, peptides, terpenoids, polysaccharides, and carotenoids from in vitro, in vivo, and clinical studies. The in vitro studies summarized the marine origins and pharmacological mechanisms, including anti-proliferation, anti-angiogenesis, anti-migration, anti-invasion, the acceleration of cycle arrest, and the promotion of tumor apoptosis, of various compounds. The in vivo studies outlined the antitumor effects of marine compounds on colorectal cancer model mice and evaluated their efficacy in terms of tumor inhibition, hepatotoxicity, and nephrotoxicity. The clinical studies summarized the major chemical classifications and targets of action of the clinical drugs that have entered clinical approval and completed approval for marine anticancer. In summary, we present the current situation regarding the application of natural anti-colorectal cancer marine compounds and prospects for their clinical application.

The inflammatory pathogenesis of colorectal cancer

The mutational landscape of colorectal cancer (CRC) does not enable predictions to be made about the survival of patients or their response to therapy. Instead, studying the polarization and activation profiles of immune cells and stromal cells in the tumour microenvironment has been shown to be more informative, thus making CRC a prototypical example of the importance of an inflammatory microenvironment for tumorigenesis. Here, we review our current understanding of how colon cancer cells interact with their microenvironment, comprised of immune cells, stromal cells and the intestinal microbiome, to suppress or escape immune responses and how inflammatory processes shape the immune pathogenesis of CRC.

14-3-3σ Functions as an Intestinal Tumor Suppressor

Although the 14-3-3σ gene was initially identified as a p53 target gene in colorectal cancer cells, its potential role in intestinal tumorigenesis has remained unknown. Here we determined that 14-3-3σ expression is significantly downregulated in primary human colorectal cancer when compared with adjacent normal colonic tissue in patient samples. Downregulation of 14-3-3σ in primary colorectal cancers was significantly associated with p53 mutation, increasing tumor stage, distant metastasis, and poor patient survival. Poor survival was more significantly associated with decreased 14-3-3σ expression in p53 wild-type than in p53-mutant colorectal cancers. 14-3-3σ expression was detected in enterocytes of the transit amplifying zone and gradually increased towards the apical villi in the small intestinal epithelium. In small and large intestinal epithelia and adenomas, 14-3-3σ expression was upregulated in differentiated areas. Deletion of 14-3-3σ in Apc^Min mice increased the number and size of adenomas in the small intestine and colon, shortening the median survival by 64 days. 14-3-3σ-deficient adenomas displayed increased proliferation and decreased apoptosis, as well as increased dysplasia. In adenomas, loss of 14-3-3σ promoted acquisition of a mesenchymal-like gene expression signature, which was also found in colorectal cancers from patients with poor relapse-free survival. The transcriptional programs controlled by the 14-3-3σ-interacting factors SNAIL, c-JUN, YAP1, and FOXO1 were activated by deletion of 14-3-3σ, potentially contributing to the enhanced tumor formation and growth. Taken together, these results provide genetic evidence of a tumor-suppressor function of 14-3-3σ in the intestine. SIGNIFICANCE: Downregulation of 14-3-3σ in colorectal cancer is associated with metastasis and poor survival of patients, and its inactivation in a murine tumor model drives intestinal tumor formation and epithelial-mesenchymal transition.

Rational Treatment of Metastatic Colorectal Cancer: A Reverse Tale of Men, Mice, and Culture Dishes

Stratification of colorectal cancer into subgroups with different response to therapy was initially guided by descriptive associations between specific biomarkers and treatment outcome. Recently, preclinical models based on propagatable patient-derived tumor samples have yielded an improved understanding of disease biology, which has facilitated the functional validation of correlative information and the discovery of novel response determinants, therapeutic targets, and mechanisms of tumor adaptation and drug resistance. We review the contribution of patient-derived models to advancing colorectal cancer characterization, discuss their influence on clinical decision-making, and highlight emerging challenges in the interpretation and clinical transferability of results obtainable with such approaches. SIGNIFICANCE: Association studies in patients with colorectal cancer have led to the identification of response biomarkers, some of which have been implemented as companion diagnostics for therapeutic decisions. By enabling biological investigation in a clinically relevant experimental context, patient-derived colorectal cancer models have proved useful to examine the causal role of such biomarkers in dictating drug sensitivity and are providing fresh knowledge on new actionable targets, dynamics of tumor evolution and adaptation, and mechanisms of drug resistance.

Pre-clinical modelling of rectal cancer to develop novel radiotherapy-based treatment strategies

Pre-operative chemoradiotherapy reduces local recurrence rates in locally advanced rectal cancer. 10-20% of patients undergo complete response to chemoradiotherapy, however, many patients show no response. The mechanisms underlying this are poorly understood; identifying molecular and immunological factors underpinning heterogeneous responses to chemoradiotherapy, will promote development of treatment strategies to improve responses and overcome resistance mechanisms. This review describes the advances made in pre-clinical modelling of colorectal cancer, including genetically engineered mouse models, transplantation models, patient derived organoids and radiotherapy platforms to study responses to chemoradiotherapy. Relevant literature was identified through the PubMed and MEDLINE databases, using the following keywords: rectal cancer; mouse models; organoids; neo-adjuvant treatment; radiotherapy; chemotherapy. By delineating the advantages and disadvantages of available models, we discuss how modelling techniques can be utilized to address current research priorities in locally advanced rectal cancer. We provide unique insight into the potential application of pre-clinical models in the development of novel neo-adjuvant treatment strategies, which will hopefully guide future clinical trials.

Deficiency of Splicing Factor 1 (SF1) Reduces Intestinal Polyp Incidence in ApcMin/+ Mice

BACKGROUND

Splicing factor 1 (SF1) is a conserved alternative splicing factor expressed in many different mammalian cell types. The genetically modified Sf1+/- (or Sf1^β-geo/+) mice express reduced levels of SF1 protein in mouse tissues, including in cells of the intestines. Mutational inactivation of human adenomatous polyposis coli (APC) gene deregulates the Wnt signaling pathway and is a frequent genetic event in colon cancers. Mice with a point mutation in the Apc gene (Apc^Min/+) also develop numerous intestinal polyps at a young age. Our aim was to determine the effect of reduced SF1 levels on polyp development due to the strong driver Apc^Min/+ mutation.

METHODS

We utilized mice genetically deficient for expression of SF1 to assess how SF1 levels affect intestinal tumorigenesis. We crossed Apc^Min/+ to Sf1+/- mice to generate a cohort of heterozygous mutant ApcMin/+;Sf1+/- mice and compared intestinal polyp development in these mice to that in a control cohort of sibling Apc^Min/+ mice. We compared total polyp numbers, sizes of polyps and gender differences in polyp numbers between ApcMin/+;Sf1+/- and Apc^Min/+ mice.

RESULTS

Our results showed that Apc^Min/+ mice with lower SF1 expression developed 25-30% fewer intestinal polyps compared to their Apc^Min/+ siblings with normal SF1 levels. Interestingly, this difference was most significant for females (ApcMin/+;Sf1+/- and Apc^Min/+ females developed 39 and 55 median number of polyps, respectively). Furthermore, the difference in polyp numbers between ApcMin/+;Sf1+/- and Apc^Min/+ mice was significant for smaller polyps with a size of 2 mm or less, whereas both groups developed similar numbers of larger polyps.

CONCLUSIONS

Our results suggest that lower SF1 levels likely inhibit the rate of initiation of polyp development due to Apc^Min/+ driver mutation in the mouse intestine. Thus, therapeutic lowering of SF1 levels in the intestine could attenuate intestinal polyp development.

Driver mutations of the adenoma-carcinoma sequence govern the intestinal epithelial global translational capacity

Deregulated global mRNA translation is an emerging feature of cancer cells. Oncogenic transformation in colorectal cancer (CRC) is driven by mutations in APC, KRAS, SMAD4, and TP53, known as the adenoma-carcinoma sequence (ACS). Here we introduce each of these driver mutations into intestinal organoids to show that they are modulators of global translational capacity in intestinal epithelial cells. Increased global translation resulting from loss of Apc expression was potentiated by the presence of oncogenic Kras ^G12D Knockdown of Smad4 further enhanced global translation efficiency and was associated with a lower 4E-BP1-to-eIF4E ratio. Quadruple mutant cells with additional P53 loss displayed the highest global translational capacity, paralleled by high proliferation and growth rates, indicating that the proteome is heavily geared toward cell division. Transcriptional reprogramming facilitating global translation included elevated ribogenesis and activation of mTORC1 signaling. Accordingly, interfering with the mTORC1/4E-BP/eIF4E axis inhibited the growth potential endowed by accumulation of multiple drivers. In conclusion, the ACS is characterized by a strongly altered global translational landscape in epithelial cells, exposing a therapeutic potential for direct targeting of the translational apparatus.

High Canonical Wnt/β-Catenin Activity Sensitizes Murine Hematopoietic Stem and Progenitor Cells to DNA Damage

Aging is characterized by the accumulation of DNA damage and a decrease in stem cell functionality, yet molecular mechanisms that limit the maintenance of stem cells in response to DNA damage remain to be delineated. Here we show in mouse models that DNA damage leads to a transient over-activation of Wnt signaling in hematopoietic stem cells (HSCs), and that high activity of canonical Wnt/β-catenin signaling sensitizes HSCs to DNA damage induced by X-irradiation which results in preferential maintenance of HSCs with low levels of Wnt signaling. The study shows that genetic or chemical activation of canonical Wnt signaling enhances radiosensitivity of HSCs while inhibition of Wnt signaling decreases it. Together, these results indicate that levels of Wnt signaling activity mediate heterogeneity in the sensitivity of HSCs to DNA damage induced depletion. These findings could be relevant for molecular alterations and selection of stem cells in the context of DNA damage accumulation during aging and cancer formation.

The gut microbiome switches mutant p53 from tumour-suppressive to oncogenic

Somatic mutations in p53, which inactivate the tumour-suppressor function of p53 and often confer oncogenic gain-of-function properties, are very common in cancer^1,2. Here we studied the effects of hotspot gain-of-function mutations in Trp53 (the gene that encodes p53 in mice) in mouse models of WNT-driven intestinal cancer caused by Csnk1a1 deletion^3,4 or Apc^Min mutation⁵. Cancer in these models is known to be facilitated by loss of p53^3,6. We found that mutant versions of p53 had contrasting effects in different segments of the gut: in the distal gut, mutant p53 had the expected oncogenic effect; however, in the proximal gut and in tumour organoids it had a pronounced tumour-suppressive effect. In the tumour-suppressive mode, mutant p53 eliminated dysplasia and tumorigenesis in Csnk1a1-deficient and Apc^Min/+ mice, and promoted normal growth and differentiation of tumour organoids derived from these mice. In these settings, mutant p53 was more effective than wild-type p53 at inhibiting tumour formation. Mechanistically, the tumour-suppressive effects of mutant p53 were driven by disruption of the WNT pathway, through preventing the binding of TCF4 to chromatin. Notably, this tumour-suppressive effect was completely abolished by the gut microbiome. Moreover, a single metabolite derived from the gut microbiota–gallic acid–could reproduce the entire effect of the microbiome. Supplementing gut-sterilized p53-mutant mice and p53-mutant organoids with gallic acid reinstated the TCF4–chromatin interaction and the hyperactivation of WNT, thus conferring a malignant phenotype to the organoids and throughout the gut. Our study demonstrates the substantial plasticity of a cancer mutation and highlights the role of the microenvironment in determining its functional outcome.

Mutant p53 in colon cancer

The accumulation of genetic alterations in driver genes is responsible for the development and malignant progression of colorectal cancer. Comprehensive genome analyses have revealed the driver genes, including APC, KRAS, TGFBR2, and TP53, whose mutations are frequently found in human colorectal cancers. Among them, the p53 mutation is found in ~60% of colorectal cancers, and a majority of mutations are missense-type at ‘hot spots’, suggesting an oncogenic role of mutant p53 by ‘gain-of-function’ mechanisms. Mouse model studies have shown that one of these missense-type mutations, p53 R270H (corresponding to human R273H), causes submucosal invasion of intestinal tumors, while the loss of wild-type p53 has a limited effect on the invasion process. Furthermore, the same mutant p53 promotes metastasis when combined with Kras activation and TGF-β suppression. Importantly, either missense-type p53 mutation or loss of wild-type p53 induces NF-κB activation by a variety of mechanisms, such as increasing promoter accessibility by chromatin remodeling, which may contribute to progression to epithelial–mesenchymal transition. These results indicate that missense-type p53 mutations together with loss of wild-type p53 accelerate the late stage of colorectal cancer progression through the activation of both oncogenic and inflammatory pathways. Accordingly, the suppression of the mutant p53 function via the inhibition of nuclear accumulation is expected to be an effective strategy against malignant progression of colorectal cancer.

Human Colorectal Cancer from the Perspective of Mouse Models

Colorectal cancer (CRC) is a heterogeneous disease that includes both hereditary and sporadic types of tumors. Tumor initiation and growth is driven by mutational or epigenetic changes that alter the function or expression of multiple genes. The genes predominantly encode components of various intracellular signaling cascades. In this review, we present mouse intestinal cancer models that include alterations in the Wnt, Hippo, p53, epidermal growth factor (EGF), and transforming growth factor β (TGFβ) pathways; models of impaired DNA mismatch repair and chemically induced tumorigenesis are included. Based on their molecular biology characteristics and mutational and epigenetic status, human colorectal carcinomas were divided into four so-called consensus molecular subtype (CMS) groups. It was shown subsequently that the CMS classification system could be applied to various cell lines derived from intestinal tumors and tumor-derived organoids. Although the CMS system facilitates characterization of human CRC, individual mouse models were not assigned to some of the CMS groups. Thus, we also indicate the possible assignment of described animal models to the CMS group. This might be helpful for selection of a suitable mouse strain to study a particular type of CRC.

Activated Signaling Pathways and Targeted Therapies in Desmoid-Type Fibromatosis: A Literature Review

Desmoid-type fibromatosis (DTF) is a rare, soft tissue tumor of mesenchymal origin which is characterized by local infiltrative growth behavior. Besides “wait and see,” surgery and radiotherapy, several systemic treatments are available for symptomatic patients. Recently, targeted therapies are being explored in DTF. Unfortunately, effective treatment is still hampered by the limited knowledge of the molecular mechanisms that prompt DTF tumorigenesis. Many studies focus on Wnt/β-catenin signaling, since the vast majority of DTF tumors harbor a mutation in the CTNNB1 gene or the APC gene. The established role of the Wnt/β-catenin pathway in DTF forms an attractive therapeutic target, however, drugs targeting this pathway are still in an experimental stage and not yet available in the clinic. Only few studies address other signaling pathways which can drive uncontrolled growth in DTF such as: JAK/STAT, Notch, PI3 kinase/AKT, mTOR, Hedgehog, and the estrogen growth regulatory pathways. Evidence for involvement of these pathways in DTF tumorigenesis is limited and predominantly based on the expression levels of key pathway genes, or on observed clinical responses after targeted treatment. No clear driver role for these pathways in DTF has been identified, and a rationale for clinical studies is often lacking. In this review, we highlight common signaling pathways active in DTF and provide an up-to-date overview of their therapeutic potential.

Conserved serum protein biomarkers associated with growing early colorectal adenomas

A major challenge for the reduction of colon cancer is to detect patients carrying high-risk premalignant adenomas with minimally invasive testing. As one step, we have addressed the feasibility of detecting protein signals in the serum of patients carrying an adenoma as small as 6-9 mm in maximum linear dimension. Serum protein biomarkers, discovered in two animal models of early colonic adenomagenesis, were studied in patients using quantitative mass-spectrometric assays. One cohort included patients bearing adenomas known to be growing on the basis of longitudinal computed tomographic colonography. The other cohort, screened by optical colonoscopy, included both patients free of adenomas and patients bearing adenomas whose risk status was judged by histopathology. The markers F5, ITIH4, LRG1, and VTN were each elevated both in this patient study and in the studies of the Pirc rat model. The quantitative study in the Pirc rat model had demonstrated that the elevated level of each of these markers is correlated with the number of colonic adenomas. However, the levels of these markers in patients were not significantly correlated with the total adenoma volume. Postpolypectomy blood samples demonstrated that the elevated levels of these four conserved markers persisted after polypectomy. Two additional serum markers rapidly renormalized after polypectomy: growth-associated CRP levels were enhanced only with high-risk adenomas, while PI16 levels, not associated with growth, were reduced regardless of risk status. We discuss biological hypotheses to account for these observations, and ways for these signals to contribute to the prevention of colon cancer.

Shortcuts to intestinal carcinogenesis by genetic engineering in organoids

Inactivation of the Adenomatous polyposis coli (APC) gene is an initiating and the most relevant event in most sporadic cases of colorectal cancer, providing a rationale for using Apc‐mutant mice as the disease model. Whereas carcinogenesis has been observed only at the organism level, the recent development of the organoid culture technique has enabled long‐term propagation of intestinal stem cells in a physiological setting, raising the possibility that organoids could serve as an alternative platform for modeling colon carcinogenesis. Indeed, it is demonstrated in the present study that lentivirus‐based RNAi‐mediated knockdown of Apc in intestinal organoids gave rise to subcutaneous tumors upon inoculation in immunodeficient mice. Reconstitution of common genetic aberrations in organoids resulted in development of various lesions, ranging from aberrant crypt foci to full‐blown cancer, recapitulating multi‐step colorectal tumorigenesis. Due to its simplicity and utility, similar organoid‐based approaches have been applied to both murine and human cells in many investigations, to gain mechanistic insight into tumorigenesis, to validate putative tumor suppressor genes or oncogenes, and to establish preclinical models for drug discovery. In this review article, we provide a multifaceted overview of these types of approaches that will likely accelerate and advance research on colon cancer.

Cancer pharmacoprevention: Targeting polyamine metabolism to manage risk factors for colon cancer

Cancer is a set of diseases characterized by uncontrolled cell growth. In certain cancers of the gastrointestinal tract, the adenomatous polyposis coli (APC) tumor suppressor gene is altered in either germline or somatic cells and causes formation of risk factors, such as benign colonic or intestinal neoplasia, which can progress to invasive cancer. APC is a key component of the WNT pathway, contributing to normal GI tract development, and APC alteration results in dysregulation of the pathway for production of polyamines, which are ubiquitous cations essential for cell growth. Studies with mice have identified nonsteroidal anti-inflammatory drugs (NSAIDs) and difluoromethylornithine (DFMO), an inhibitor of polyamine synthesis, as potent inhibitors of colon carcinogenesis. Moreover, gene expression profiling has uncovered that NSAIDs activate polyamine catabolism and export. Several DFMO-NSAID combination strategies are effective and safe methods for reducing risk factors in clinical trials with patients having genetic or sporadic risk of colon cancer. These strategies affect cancer stem cells, inflammation, immune surveillance, and the microbiome. Pharmacotherapies consisting of drug combinations targeting the polyamine pathway provide a complementary approach to surgery and cytotoxic cancer treatments for treating patients with cancer risk factors. In this Minireview, we discuss the role of polyamines in colon cancer and highlight the mechanisms of select pharmacoprevention agents to delay or prevent carcinogenesis in humans.

Dual gene deficient models of ApcMin/+ mouse in assessing molecular mechanisms of intestinal carcinogenesis

The Apc^Min/+ mouse, carrying an inactivated allele of the adenomatous polyposis coli (Apc) gene, is a widely used animal model of human colorectal tumorigenesis. While crossed with other gene knockout or knock-in mice, these mice possess advantages in investigation of human intestinal tumorigenesis. Intestinal tumor pathogenesis involves multiple gene alterations; thus, various double gene deficiency models could provide novel insights into molecular mechanisms of tumor biology, as well as gene-gene interactions involved in intestinal tumor development and assessment of novel strategies for preventing and treating intestinal cancer. This review discusses approximately 100 double gene deficient mice and their associated intestinal tumor development and progression phenotypes. The dual gene knockouts based on the Apc mutation background consist of inflammation and immune-related, cell cycle-related, Wnt/β-catenin signaling-related, tumor growth factor (TGF)-signaling-related, drug metabolism-related, and transcription factor genes, as well as some oncogenes and tumor suppressors. Future studies should focus on conditional or inducible dual or multiple mouse gene knockout models to investigate the molecular mechanisms underlying intestinal tumor development, as well as potential drug targets.

The conserved protective cyclic AMP-phosphodiesterase function PDE4B is expressed in the adenoma and adjacent normal colonic epithelium of mammals and silenced in colorectal cancer

Conservation over three mammalian genera-the mouse, rat, and human-has been found for a subset of the transcripts whose level differs between the adenoma and normal epithelium of the colon. Pde4b is one of the triply conserved transcripts whose level is enhanced both in the colonic adenoma and in the normal colonic epithelium, especially adjacent to adenomas. It encodes the phosphodiesterase PDE4B, specific for cAMP. Loss of PDE4B function in the ApcMin/+ mouse leads to a significant increase in the number of colonic adenomas. Similarly, Pde4b-deficient ApcMin/+ mice are hypersensitive to treatment by the inflammatory agent DSS, becoming moribund soon after treatment. These observations imply that the PDE4B function protects against ApcMin-induced adenomagenesis and inflammatory lethality. The paradoxical enhancement of the Pde4b transcript in the adenoma versus this inferred protective function of PDE4B can be rationalized by a feedback model in which PDE4B is first activated by early oncogenic stress involving cAMP and then, as reported for frank human colon cancer, inactivated by epigenetic silencing.

Defining key concepts of intestinal and epithelial cancer biology through the use of mouse models

Over the past 20 years, huge advances have been made in modelling human diseases such as cancer using genetically modified mice. Accurate in vivo models are essential to examine the complex interaction between cancer cells, surrounding stromal cells, tumour-associated inflammatory cells, fibroblast and blood vessels, and to recapitulate all the steps involved in metastasis. Elucidating these interactions in vitro has inherent limitations, and thus animal models are a powerful tool to enable researchers to gain insight into the complex interactions between signalling pathways and different cells types. This review will focus on how advances in in vivo models have shed light on many aspects of cancer biology including the identification of oncogenes, tumour suppressors and stem cells, epigenetics, cell death and context dependent cell signalling.

Daikenchuto (TU-100) Suppresses Tumor Development in the Azoxymethane and APCmin/+ Mouse Models of Experimental Colon Cancer

Chemopreventative properties of traditional medicines and underlying mechanisms of action are incompletely investigated. This study demonstrates that dietary daikenchuto (TU-100), comprised of ginger, ginseng, and Japanese pepper effectively suppresses intestinal tumor development and progression in the azoxymethane (AOM) and APCmin/+ mouse models. For the AOM model, TU-100 was provided after the first of six biweekly AOM injections. Mice were sacrificed at 30 weeks. APCmin/+ mice were fed diet without or with TU-100 starting at 6 weeks, and sacrificed at 24 weeks. In both models, dietary TU-100 decreased tumor size. In APC min/+ mice, the number of small intestinal tumors was significantly decreased. In the AOM model, both TU-100 and Japanese ginseng decreased colon tumor numbers. Decreased Ki-67 and β-catenin immunostaining and activation of numerous transduction pathways involved in tumor initiation and progression were observed. EGF receptor expression and stimulation/phosphorylation in vitro were investigated in C2BBe1 cells. TU-100, ginger, and 6-gingerol suppressed EGF receptor induced Akt activation. TU-100 and ginseng and to a lesser extent ginger or 6-gingerol inhibited EGF ERK1/2 activation. TU-100 and some of its components and metabolites of these components inhibit tumor progression in two mouse models of colon cancer by blocking downstream pathways of EGF receptor activation. Copyright © 2016 John Wiley & Sons, Ltd.

Disruption of the RP-MDM2-p53 pathway accelerates APC loss-induced colorectal tumorigenesis

Inactivation of the adenomatous polyposis coli (APC) tumor suppressor is frequently found in colorectal cancer. Loss of APC function results in deregulation of the Wnt/β-catenin signaling pathway causing overexpression of the c-MYC oncogene. In lymphoma, both p19ARF and ribosomal proteins RPL11 and RPL5 respond to c-MYC activation to induce p53. Their role in c-MYC-driven colorectal carcinogenesis is unclear, as p19ARF deletion does not accelerate APC loss-triggered intestinal tumorigenesis. To determine the contribution of the RP-MDM2-p53 pathway to APC loss-induced tumorigenesis, we crossed mice bearing MDM2^C305F mutation, which disrupts RPL11- and RPL5-MDM2 binding, with Apc^min/+ mice, which are prone to intestinal tumor formation. Interestingly, loss of RP-MDM2 binding significantly accelerated colorectal tumor formation while having no discernable effect on small intestinal tumor formation. Mechanistically, APC loss leads to overexpression of c-MYC, RPL11 and RPL5 in mouse colonic tumor cells irrespective of MDM2^C305F mutation. However, notable p53 stabilization and activation were observed only in Apc^min/+;Mdm2^+/+ but not Apc^min/+;Mdm2^C305F/C305F colon tumors. These data establish that the RP-MDM2-p53 pathway, in contrast to the p19ARF-MDM2-p53 pathway, is a critical mediator of colorectal tumorigenesis following APC loss.

Advanced Intestinal Cancers often Maintain a Multi-Ancestral Architecture

A widely accepted paradigm in the field of cancer biology is that solid tumors are uni-ancestral being derived from a single founder and its descendants. However, data have been steadily accruing that indicate early tumors in mice and humans can have a multi-ancestral origin in which an initiated primogenitor facilitates the transformation of neighboring co-genitors. We developed a new mouse model that permits the determination of clonal architecture of intestinal tumors in vivo and ex vivo, have validated this model, and then used it to assess the clonal architecture of adenomas, intramucosal carcinomas, and invasive adenocarcinomas of the intestine. The percentage of multi-ancestral tumors did not significantly change as tumors progressed from adenomas with low-grade dysplasia [40/65 (62%)], to adenomas with high-grade dysplasia [21/37 (57%)], to intramucosal carcinomas [10/23 (43%]), to invasive adenocarcinomas [13/19 (68%)], indicating that the clone arising from the primogenitor continues to coexist with clones arising from co-genitors. Moreover, neoplastic cells from distinct clones within a multi-ancestral adenocarcinoma have even been observed to simultaneously invade into the underlying musculature [2/15 (13%)]. Thus, intratumoral heterogeneity arising early in tumor formation persists throughout tumorigenesis.

Mouse models of intestinal cancer

Murine models of intestinal cancer are powerful tools to recapitulate human intestinal cancer, understand its biology and test therapies. With recent developments identifying the importance of the tumour microenvironment and the potential for immunotherapy, autochthonous genetically engineered mouse models (GEMMs) will remain an important part of preclinical studies for the foreseeable future. This review will provide an overview of the current mouse models of intestinal cancer, from the Apc(Min/+) mouse, which has been used for over 25 years, to the latest 'state-of-the-art' organoid models. We discuss here how these models have been used to define fundamental processes involved in tumour initiation and the attempts to generate metastatic models, which is the ultimate cause of cancer mortality. Together these models will provide key insights to understand this complex disease and hopefully will lead to the discovery of new therapeutic strategies.

Targeted Deletion of p53 in Lgr5-Expressing Intestinal Stem Cells Promotes Colon Tumorigenesis in a Preclinical Model of Colitis-Associated Cancer

p53 has been shown to mediate cancer stem-like cell function by suppressing pluripotency and cellular dedifferentiation. However, there have been no studies to date that have addressed the specific effects of p53 loss in colonic adult stem cells. In this study, we investigated the consequences of conditionally ablating p53 in the highly relevant Lgr5(+) stem cell population on tumor initiation and progression in the colon. In a mouse model of carcinogen (AOM)-induced colon cancer, tamoxifen-inducible Lgr5-driven deletion of p53 reduced apoptosis and increased proliferation of crypt stem cells, but had no effect on tumor incidence or size. Conversely, in a mouse model of colitis-associated cancer, in which mice are exposed to AOM and the potent inflammation inducer DSS, stem cell-specific p53 deletion greatly enhanced tumor size and incidence in the colon. These novel findings suggest that the loss of p53 function in stem cells enables colonic tumor formation only when combined with DNA damage and chronic inflammation. Furthermore, we propose that stem cell targeting approaches are valuable for interrogating prevention and therapeutic strategies that aim to specifically eradicate genetically compromised stem cells.

Colon Tumors with the Simultaneous Induction of Driver Mutations in APC, KRAS, and PIK3CA Still Progress through the Adenoma-to-carcinoma Sequence

Human colorectal cancers often possess multiple mutations, including three to six driver mutations per tumor. The timing of when these mutations occur during tumor development and progression continues to be debated. More advanced lesions carry a greater number of driver mutations, indicating that colon tumors might progress from adenomas to carcinomas through the stepwise accumulation of mutations following tumor initiation. However, mutations that have been implicated in tumor progression have been identified in normal-appearing epithelial cells of the colon, leaving the possibility that these mutations might be present before the initiation of tumorigenesis. We utilized mouse models of colon cancer to investigate whether tumorigenesis still occurs through the adenoma-to-carcinoma sequence when multiple mutations are present at the time of tumor initiation. To create a model in which tumors could concomitantly possess mutations in Apc, Kras, and Pik3ca, we developed a novel minimally invasive technique to administer an adenovirus expressing Cre recombinase to a focal region of the colon. Here, we demonstrate that the presence of these additional driver mutations at the time of tumor initiation results in increased tumor multiplicity and an increased rate of progression to invasive adenocarcinomas. These cancers can even metastasize to retroperitoneal lymph nodes or the liver. However, despite having as many as three concomitant driver mutations at the time of initiation, these tumors still proceed through the adenoma-to-carcinoma sequence.

Knockdown of type I insulin-like growth factor receptor inhibits human colorectal cancer cell growth and downstream PI3K/Akt, WNT/β-catenin signal pathways

Type I insulin-like growth factor receptor (IGF1R) signal is involved in normal physiology and many disease progressions. In this study, we presented the role of IGF1R in colorectal cancer cell lines. Results showed that knockdown of IGF1R using small interfering RNA in HT-29, SW620 cells strongly inhibited cell proliferation, arrested cell cycle and also promoted cell apoptosis. Western blotting results indicated that the downstream PI3K/Akt and canonical WNT signal pathways were blocked. In addition, we observed that reduction of IGF1R suppressed the expression of many inflammatory factors, such as NF-κB, p-NF-κB, COX-2 and iNOS. Together, this study demonstrate that knockdown of IGF1R inhibits CRC cells growth and provides an additional evidence for further clarifying the mechanism of IGF1R involved in CRC and inflammation-induced tumorigenesis.

Association of aberrant DNA methylation in Apc(min/+) mice with the epithelial-mesenchymal transition and Wnt/β-catenin pathways: genome-wide analysis using MeDIP-seq

BACKGROUND

Aberrant DNA methylation at the 5-carbon on cytosine residues (5mC) in CpG dinucleotides is probably the most extensively characterized epigenetic modification in colon cancer. It has been suggested that the loss of adenomatous polyposis coli (APC) function initiates tumorigenesis and that additional genetic and epigenetic events are involved in colon cancer progression. We aimed to study the genome-wide DNA methylation profiles of intestinal tumorigenesis in Apc(min/+) mice.

RESULTS

Methylated DNA immunoprecipitation (MeDIP) followed by next-generation sequencing was used to determine the global profile of DNA methylation changes in Apc(min/+) mice. DNA was extracted from adenomatous polyps from Apc(min/+) mice and from normal intestinal tissue from age-matched Apc(+/+) littermates, and the MeDIP-seq assay was performed. Ingenuity Pathway Analysis (IPA) software was used to analyze the data for gene interactions. A total of 17,265 differentially methylated regions (DMRs) displayed a ≥ 2-fold change (log2) in methylation in Apc(min/+) mice; among these DMRs, 9,078 (52.6 %) and 8,187 (47.4 %) exhibited increased and decreased methylation, respectively. Genes with altered methylation patterns were mainly mapped to networks and biological functions associated with cancer and gastrointestinal diseases. Among these networks, several canonical pathways, such as the epithelial-mesenchymal transition (EMT) and Wnt/β-catenin pathways, were significantly associated with genome-wide methylation changes in polyps from Apc(min/+) mice. The identification of certain differentially methylated molecules in the EMT and Wnt/β-catenin pathways, such as APC2 (adenomatosis polyposis coli 2), SFRP2 (secreted frizzled-related protein 2), and DKK3 (dickkopf-related protein 3), was consistent with previous publications.

CONCLUSIONS

Our findings indicated that Apc(min/+) mice exhibited extensive aberrant DNA methylation that affected certain signaling pathways, such as the EMT and Wnt/β-catenin pathways. The genome-wide DNA methylation profile of Apc(min/+) mice is informative for future studies investigating epigenetic gene regulation in colon tumorigenesis and the prevention of colon cancer.

Wnt activity and basal niche position sensitize intestinal stem and progenitor cells to DNA damage

Aging and carcinogenesis coincide with the accumulation of DNA damage and mutations in stem and progenitor cells. Molecular mechanisms that influence responses of stem and progenitor cells to DNA damage remain to be delineated. Here, we show that niche positioning and Wnt signaling activity modulate the sensitivity of intestinal stem and progenitor cells (ISPCs) to DNA damage. ISPCs at the crypt bottom with high Wnt/β-catenin activity are more sensitive to DNA damage compared to ISPCs in position 4 with low Wnt activity. These differences are not induced by differences in cell cycle activity but relate to DNA damage-dependent activation of Wnt signaling, which in turn amplifies DNA damage checkpoint activation. The study shows that instructed enhancement of Wnt signaling increases radio-sensitivity of ISPCs, while inhibition of Wnt signaling decreases it. These results provide a proof of concept that cell intrinsic levels of Wnt signaling modulate the sensitivity of ISPCs to DNA damage and heterogeneity in Wnt activation in the stem cell niche contributes to the selection of ISPCs in the context of DNA damage.

Cdx1 and Cdx2 function as tumor suppressors

In humans, colorectal cancer is often initiated through APC loss of function, which leads to crypt hyperplasia and polyposis driven by unrestricted canonical Wnt signaling. Such polyps typically arise in the colorectal region and are at risk of transforming to invasive adenocarcinomas. Although colorectal cancer is the third most common cause of cancer-related death worldwide, the processes impacting initiation, transformation, and invasion are incompletely understood. Murine APC(Min/+) mutants are often used to model colorectal cancers; however, they develop nonmetastatic tumors confined largely to the small intestine and are thus not entirely representative of the human disease. APC(Min/+) alleles can collaborate with mutations impacting other pathways to recapitulate some aspects of human colorectal cancer. To this end, we assessed APC(Min/+)-induced polyposis following somatic loss of the homeodomain transcription factor Cdx2, alone or with a Cdx1 null allele, in the adult gastrointestinal tract. APC(Min/+)-Cdx2 mutants recapitulated several aspects of human colorectal cancer, including an invasive phenotype. Notably, the concomitant loss of Cdx1 led to a significant increase in the incidence of tumors in the distal colon, relative to APC(Min/+)-Cdx2 offspring, demonstrating a previously unrecognized role for this transcription factor in colorectal tumorigenesis. These findings underscore previously unrecognized roles for Cdx members in intestinal tumorigenesis.

Concomitant Notch activation and p53 deletion trigger epithelial-to-mesenchymal transition and metastasis in mouse gut

Epithelial-to-mesenchymal transition-like (EMT-like) is a critical process allowing initiation of metastases during tumour progression. Here, to investigate its role in intestinal cancer, we combine computational network-based and experimental approaches to create a mouse model with high metastatic potential. Construction and analysis of this network map depicting molecular mechanisms of EMT regulation based on the literature suggests that Notch activation and p53 deletion have a synergistic effect in activating EMT-like processes. To confirm this prediction, we generate transgenic mice by conditionally activating the Notch1 receptor and deleting p53 in the digestive epithelium (NICD/p53(-/-)). These mice develop metastatic tumours with high penetrance. Using GFP lineage tracing, we identify single malignant cells with mesenchymal features in primary and metastatic tumours in vivo. The development of such a model that recapitulates the cellular features observed in invasive human colorectal tumours is appealing for innovative drug discovery.

Unbiased metabolite profiling indicates that a diminished thymidine pool is the underlying mechanism of colon cancer chemoprevention by alpha-difluoromethylornithine

The ornithine decarboxylase inhibitor α-difluoromethylornithine (DFMO) is a highly effective chemopreventive agent for colorectal cancer thought to act via polyamine depletion. However, in DFMO-treated patients, mucosal polyamine levels do not directly correlate with colorectal cancer risk. Untargeted metabolite profiling was used to broadly survey DFMO actions on colon cancer cell metabolism. We found that DFMO treatment of Apc(Min) intestinal tumors and human colorectal cancer cells is associated with reduced levels of folate-dependent metabolites, including S-adenosylmethionine (SAM), thymidine pools, and related pathway intermediates. We hypothesized that unrestrained SAM consumption/regeneration constitutes a futile DFMO-triggered cascade that can steal tetrahydrofolate from thymidylate synthase and thereby diminish thymidine pools. In accord with this hypothesis, DFMO treatment altered the folate cofactor balance and thymidine supplementation prevented DFMO-elicited cytostasis without restoring polyamine levels. These findings suggest that thymidine metabolite pool insufficiency is a fundamental mechanism of DFMO cytostatic activity.

SIGNIFICANCE

A previously unappreciated metabolic linkage between polyamine and thymidine biosynthesis is revealed, based on the competing requirement of these pathways for a limited pool of tetrahydrofolate cofactor. This study identifies the fi rst shared mechanism for colorectal cancer chemoprevention and chemotherapy, suggesting a common metabolic target for both premalignant and malignant colon cells.

Genetic reconstitution of tumorigenesis in primary intestinal cells

Animal models for human colorectal cancer recapitulate multistep carcinogenesis that is typically initiated by activation of the Wnt pathway. Although potential roles of both genetic and environmental modifiers have been extensively investigated in vivo, it remains elusive whether epithelial cells definitely require interaction with stromal cells or microflora for tumor development. Here we show that tumor development could be simply induced independently of intestinal microenvironment, even with WT murine primary intestinal cells alone. We developed an efficient method for lentiviral transduction of intestinal organoids in 3D culture. Despite seemingly antiproliferative effects by knockdown of adenomatous polyposis coli (APC), we managed to reproducibly induce APC-inactivated intestinal organoids. As predicted, these organoids were constitutively active in the Wnt signaling pathway and proved tumorigenic when injected into nude mice, yielding highly proliferative tubular epithelial glands accompanied by prominent stromal tissue. Consistent with cellular transformation, tumor-derived epithelial cells acquired sphere formation potential, gave rise to secondary tumors on retransplantation, and highly expressed cancer stem cell markers. Inactivation of p53 or phosphatase and tensin homolog deleted from chromosome 10, or activation of Kras, promoted tumor development only in the context of APC suppression, consistent with earlier genetic studies. These findings clearly indicated that genetic cooperation for intestinal tumorigenesis could be essentially recapitulated in intestinal organoids without generating gene-modified mice. Taken together, this in vitro model for colon cancer described herein could potentially provide unique opportunities for carcinogenesis studies by serving as a substitute or complement to the currently standard approaches.

Recent progress in mouse models for tumor suppressor genes and its implications in human cancer

Gain-of-function mutations in oncogenes and loss-of-function mutations in tumor suppressor genes (TSG) lead to cancer. In most human cancers, these mutations occur in somatic tissues. However, hereditary forms of cancer exist for which individuals are heterozygous for a germline mutation in a TSG locus at birth. The second allele is frequently inactivated by gene deletion, point mutation, or promoter methylation in classical TSGs that meet Knudson's two-hit hypothesis. Conversely, the second allele remains as wild-type, even in tumors in which the gene is haplo-insufficient for tumor suppression. This article highlights the importance of PTEN, APC, and other tumor suppressors for counteracting aberrant PI3K, β-catenin, and other oncogenic signaling pathways. We discuss the use of gene-engineered mouse models (GEMM) of human cancer focusing on Pten and Apc knockout mice that recapitulate key genetic events involved in initiation and progression of human neoplasia. Finally, the therapeutic potential of targeting these tumor suppressor and oncogene signaling networks is discussed.

Loss of p53 in enterocytes generates an inflammatory microenvironment enabling invasion and lymph node metastasis of carcinogen-induced colorectal tumors

Loss of p53 is considered to allow progression of colorectal tumors from the adenoma to the carcinoma stage. Using mice with an intestinal epithelial cell (IEC)-specific p53 deletion, we demonstrate that loss of p53 alone is insufficient to initiate intestinal tumorigenesis but markedly enhances carcinogen-induced tumor incidence and leads to invasive cancer and lymph node metastasis. Whereas p53 controls DNA damage and IEC survival during the initiation stage, loss of p53 during tumor progression is associated with increased intestinal permeability, causing formation of an NF-κB-dependent inflammatory microenvironment and the induction of epithelial-mesenchymal transition. Thus, we propose a p53-controlled tumor-suppressive function that is independent of its well-established role in cell-cycle regulation, apoptosis, and senescence.

WNT signalling pathways as therapeutic targets in cancer

Since the initial discovery of the oncogenic activity of WNT1 in mouse mammary glands, our appreciation for the complex roles for WNT signalling pathways in cancer has increased dramatically. WNTs and their downstream effectors regulate various processes that are important for cancer progression, including tumour initiation, tumour growth, cell senescence, cell death, differentiation and metastasis. Although WNT signalling pathways have been difficult to target, improved drug-discovery platforms and new technologies have facilitated the discovery of agents that can alter WNT signalling in preclinical models, thus setting the stage for clinical trials in humans.

Signal transducer and activator of transcription 3 (STAT3) protein suppresses adenoma-to-carcinoma transition in Apcmin/+ mice via regulation of Snail-1 (SNAI) protein stability

STAT3 was recently reported to suppress tumor invasion in Apc(min)(/+) mice. We investigated the mechanisms by which STAT3 inhibits intestinal epithelial tumors using Apc(min)(/+)/Stat3(IEC-KO) mice (intestinal epithelial cell (IEC)-specific deletion of STAT3 in the Apc(min)(/+) background) to determine the role of STAT3 in carcinogenesis in vivo as well as colorectal cancer cell lines in vitro. To inhibit invasion of IEC tumors, STAT3 functions as a molecular adaptor rather than a transcription factor. Accordingly, the tumors in Apc(min)(/+)/Stat3(IEC-KO) mice undergo adenoma-to-carcinoma transition and acquire an invasive phenotype. Similarly, STAT3 knockdown in a colorectal cell line enhances IEC invasion. We demonstrate that STAT3 down-regulates SNAI (Snail-1) expression levels and hence suppresses epithelial-mesenchymal transition of colorectal cancer cells. Mechanistically, STAT3 facilitates glycogen synthase kinase (GSK) 3β-mediated degradation of SNAI by regulating phosphorylation of GSK3β. Our data identified a new role for STAT3 in the adenoma-to-carcinoma sequence of intestinal tumors.

Supplementation by vitamin D compounds does not affect colonic tumor development in vitamin D sufficient murine models

Epidemiological studies indicate that sunlight exposure and vitamin D are each associated with a lower risk of colon cancer. The few controlled supplementation trials testing vitamin D in humans reported to date show conflicting results. We have used two genetic models of familial colon cancer, the Apc(Pirc/+) (Pirc) rat and the Apc(Min/+) (Min) mouse, to investigate the effect of 25-hydroxyvitamin D(3) [25(OH)D(3)] and two analogs of vitamin D hormone on colonic tumors. Longitudinal endoscopic monitoring allowed us to test the efficacy of these compounds in preventing newly arising colonic tumors and in affecting established colonic tumors. 25(OH)D(3) and two analogs of vitamin D hormone each failed to reduce tumor multiplicities or alter the growth patterns of colonic tumors in the Pirc rat or the Min mouse.

Mammalian target of rapamycin is a therapeutic target for murine ovarian endometrioid adenocarcinomas with dysregulated Wnt/β-catenin and PTEN

Despite the fact that epithelial ovarian cancers are the leading cause of death from gynecological cancer, very little is known about the pathophysiology of the disease. Mutations in the WNT and PI3K pathways are frequently observed in the human ovarian endometrioid adenocarcinomas (OEAs). However, the role of WNT/β-catenin and PTEN/AKT signaling in the etiology and/or progression of this disease is currently unclear. In this report we show that mice with a gain-of-function mutation in β-catenin that leads to dysregulated nuclear accumulation of β-catenin expression in the ovarian surface epithelium (OSE) cells develop indolent, undifferentiated tumors with both mesenchymal and epithelial characteristics. Combining dysregulated β-catenin with homozygous deletion of PTEN in the OSE resulted in development of significantly more aggressive tumors, which was correlated with inhibition of p53 expression and cellular senescence. Induced expression of both mTOR kinase, a master regulator of proliferation, and phosphorylation of its downstream target, S6Kinase was also observed in both the indolent and aggressive mouse tumors, as well as in human OEA with nuclear β-catenin accumulation. Ectopic allotransplants of the mouse ovarian tumor cells with a gain-of-function mutation in β-catenin and PTEN deletion developed into tumors with OEA histology, the growth of which were significantly inhibited by oral rapamycin treatment. These studies demonstrate that rapamycin might be an effective therapeutic for human ovarian endometrioid patients with dysregulated Wnt/β-catenin and Pten/PI3K signaling.

Expression analysis of proline rich 15 (Prr15) in mouse and human gastrointestinal tumors

Proline rich 15 (Prr15), which encodes a protein of unknown function, is expressed almost exclusively in postmitotic cells both during fetal development and in adult tissues, such as the intestinal epithelium and the testis. To determine if this specific expression is lost in intestinal neoplasias, we examined Prr15 expression by in situ hybridization (ISH) on mouse intestinal tumors caused by different gene mutations, and on human colorectal cancer (CRC) samples. Prr15/PRR15 expression was consistently observed in mouse gastrointestinal (GI) tumors caused by mutations in the Apc gene, as well as in several advanced stage human CRCs. In contrast, no Prr15 expression was detected in intestinal tumors derived from mice carrying mutations in the Smad3, Smad4, or Cdkn1b genes. These findings, combined with the fact that a majority of sporadic human CRCs carry APC mutations, strongly suggest that the expression of Prr15/PRR15 in mouse and human GI tumors is linked, directly or indirectly, to the absence of the APC protein or, more generally, to the disruption of the Wnt signaling pathway.

Long-lived Min mice develop advanced intestinal cancers through a genetically conservative pathway

C57BL/6J mice carrying the Min allele of Adenomatous polyposis coli (Apc) develop numerous adenomas along the entire length of the intestine and consequently die at an early age. This short lifespan would prevent the accumulation of somatic genetic mutations or epigenetic alterations necessary for tumor progression. To overcome this limitation, we generated F(1) Apc(Min/+) hybrids by crossing C57BR/cdcJ and SWR/J females to C57BL/6J Apc(Min/+) males. These hybrids developed few intestinal tumors and often lived longer than 1 year. Many of the tumors (24-87%) were invasive adenocarcinomas, in which neoplastic tissue penetrated through the muscle wall into the mesentery. In a few cases (3%), lesions metastasized by extension to regional lymph nodes. The development of these familial cancers does not require chromosomal gains or losses, a high level of microsatellite instability, or the presence of Helicobacter. To test whether genetic instability might accelerate tumor progression, we generated Apc(Min/+) mice homozygous for the hypomorphic allele of the Nijmegen breakage syndrome gene (Nbs1(DeltaB)) and also treated Apc(Min/+) mice with a strong somatic mutagen. These imposed genetic instabilities did not reduce the time required for cancers to form nor increase the percentage of cancers nor drive progression to the point of distant metastasis. In summary, we have found that the Apc(Min/+) mouse model for familial intestinal cancer can develop frequent invasive cancers in the absence of overt genomic instability. Possible factors that promote invasion include age-dependent epigenetic changes, conservative somatic recombination, or direct effects of alleles in the F(1) hybrid genetic background.

PUMA suppresses intestinal tumorigenesis in mice

Defective apoptosis contributes to tumorigenesis, although the critical molecular targets remain to be fully characterized. PUMA, a BH3-only protein essential for p53-dependent apoptosis, has been shown to suppress lymphomagenesis. In this study, we investigated the role of PUMA in intestinal tumorigenesis using two animal models. In the azoxymethane (AOM)/dextran sulfate sodium salt model, PUMA deficiency increased the multiplicity and size of colon tumors but reduced the frequency of beta-catenin hotspot mutations. The absence of PUMA led to a significantly elevated incidence of precursor lesions induced by AOM. AOM was found to induce p53-dependent PUMA expression and PUMA-dependent apoptosis in the colonic crypts and stem cell compartment. Furthermore, PUMA deficiency significantly enhanced the formation of spontaneous macroadenomas and microadenomas in the distal small intestine and colon of APC(Min/+) mice. These results show an essential role of PUMA-mediated apoptosis in suppressing intestinal tumorigenesis in mice.

Ku80 deletion suppresses spontaneous tumors and induces a p53-mediated DNA damage response

Ku80 facilitates DNA repair and therefore should suppress cancer. However, ku80(-/-) mice exhibit reduced cancer, although they age prematurely and have a shortened life span. We tested the hypothesis that Ku80 deletion suppresses cancer by enhancing cellular tumor-suppressive responses to inefficiently repaired DNA damage. In support of this hypothesis, Ku80 deletion ameliorated tumor burden in APC(MIN) mice and increased a p53-mediated DNA damage response, DNA lesions, and chromosomal rearrangements. Thus, contrary to its assumed role as a caretaker tumor suppressor, Ku80 facilitates tumor growth most likely by dampening baseline cellular DNA damage responses.

The pleiotropic phenotype of Apc mutations in the mouse: allele specificity and effects of the genetic background

Familial adenomatous polyposis (FAP) is a human cancer syndrome characterized by the development of hundreds to thousands of colonic polyps and extracolonic lesions including desmoid fibromas, osteomas, epidermoid cysts, and congenital hypertrophy of the pigmented retinal epithelium. Afflicted individuals are heterozygous for mutations in the APC gene. Detailed investigations of mice heterozygous for mutations in the ortholog Apc have shown that other genetic factors strongly influence the phenotype. Here we report qualitative and quantitative modifications of the phenotype of Apc mutants as a function of three genetic variables: Apc allele, p53 allele, and genetic background. We have found major differences between the Apc alleles Min and 1638N in multiplicity and regionality of intestinal tumors, as well as in incidence of extracolonic lesions. By contrast, Min mice homozygous for either of two different knockout alleles of p53 show similar phenotypic effects. These studies illustrate the classic principle that functional genetics is enriched by assessing penetrance and expressivity with allelic series. The mouse permits study of an allelic gene series on multiple genetic backgrounds, thereby leading to a better understanding of gene action in a range of biological processes.