Murine models of colorectal cancer

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.

Prevention of tumorigenesis in mice by exercise is dependent on strain background and timing relative to carcinogen exposure

Among cancer diagnoses, colorectal cancer (CRC) is prevalent, with a lifetime risk of developing CRC being approximately 5%. Population variation surrounding the mean risk of developing CRCs has been associated with both inter-individual differences in genomic architecture and environmental exposures. Decreased risk of CRC has been associated with physical activity, but protective responses are variable. Here, we utilized a series of experiments to examine the effects of genetic background (strain), voluntary exercise (wheel running), and their interaction on azoxymethane (AOM)-induced intestinal tumor number and size in mice. Additionally, we investigated how the timing of exercise relative to AOM exposure, and amount of exercise, affected tumor number and size. Our results indicated that voluntary exercise significantly reduced tumor number in a strain dependent manner. Additionally, among strains where exercise reduced tumor number (A/J, CC0001/Unc) the timing of voluntary exercise relative to AOM exposure was crucial. Voluntary exercise prior to or during AOM treatment resulted in a significant reduction in tumor number, but exercise following AOM exposure had no effect. The results indicate that voluntary exercise should be used as a preventative measure to reduce risk for environmentally induced CRC with the realization that the extent of protection may depend on genetic background.

The MYC 3' Wnt-Responsive Element Drives Oncogenic MYC Expression in Human Colorectal Cancer Cells

Mutations in components of the Wnt/β-catenin signaling pathway drive colorectal cancer (CRC) by deregulating expression of downstream target genes including the c-MYC proto-oncogene (MYC). The critical regulatory DNA enhancer elements that control oncogenic MYC expression in CRC have yet to be fully elucidated. In previous reports, we correlated T-cell factor (TCF) and β-catenin binding to the MYC 3′ Wnt responsive DNA element (MYC 3′ WRE) with MYC expression in HCT116 cells. Here we used CRISPR/Cas9 to determine whether this element is a critical driver of MYC. We isolated a clonal population of cells that contained a deletion of a single TCF binding element (TBE) within the MYC 3′ WRE. This deletion reduced TCF/β-catenin binding to this regulatory element and decreased MYC expression. Using RNA-Seq analysis, we found altered expression of genes that regulate metabolic processes, many of which are known MYC target genes. We found that 3′ WRE-Mut cells displayed a reduced proliferative capacity, diminished clonogenic growth, and a decreased potential to form tumors in vivo. These findings indicate that the MYC 3′ WRE is a critical driver of oncogenic MYC expression and suggest that this element may serve as a therapeutic target for CRC.

S100P, a calcium-binding protein, is preferentially associated with the growth of polypoid tumors in colorectal cancer

Colorectal cancer (CRC) is a genetically heterogeneous disease with distinct morphological patterns. It has been shown that polypoid and ulcerative CRC displays different genetic alterations. In the present study, we aimed to investigate genes with differential expression patterns between ulcerative and polypoid CRC. cDNA microarray analysis was performed to compare the gene expression profiles in samples of ulcerative and polypoid CRC with paired normal mucosa samples. Potential candidate genes were further validated using reverse transcription-quantitative polymerase chain reaction (RT-qPCR), western blot analysis and immunohistochemistry. The epigenetic regulation of gene expression was investigated using methylation-specific PCR (MSP). cDNA microarray analysis identified 11 upregulated and 14 downregulated genes which were differentially expressed in samples from both tumor types compared to the matched normal mucosa samples. Among these, S100P was the only upregulated gene preferentially associated with polypoid CRC (P=0.032). The samples of polypoid CRC displayed significantly higher S100P protein and mRNA expression levels than the samples of ulcerative CRC (P<0.05, respectively). Using semi-quantitative immunohistochemical analyses, S100P overexpression was found to be preferentially associated with polypoid CRC (24/30 vs. 14/40, P<0.001). The relative methylation level determined by MSP did not differ significantly between the samples of polypoid and ulcerative CRC (43.36 vs. 49.10%, P=0.168), indicating that promoter hypomethylation was not directly related to the upregulation of S100P mRNA. Our results demonstrate that the upregulation of S100P mRNA and protein expression is a predominant characteristic in polypoid CRC, whereas ulcerative CRC presents with a wide range of expression levels, indicating that S100P overexpression is not a key determinant in conferring invasion properties. The clinicopathological significance of S100P in CRC requires further investigation in well-controlled studies.

Induction of colorectal cancer in mice and histomorphometric evaluation of tumors

Colorectal cancer (CRC) originates from the epithelial cells lining the colon or rectum of the gastrointestinal tract and represents the third most common form of cancer worldwide. CRC is frequently associated with Colitis Ulcerosa or Crohn's Disease demonstrating the tumor-promoting role of inflammation. Colorectal tumor cells establish heterotypic interactions with inflammatory cells and cancer-associated fibroblasts in the tumor stroma that support tumor angiogenesis and are essential for tumor progression. Therefore, establishment of suitable mouse models mimicking the inflammatory etiology of CRC is important. Here we describe methods to induce CRC in mice, to quantify tumor parameters (multiplicity, tumor load, mean tumor size), and to analyze the cellular composition of the CRC tumor stroma.

Transformation of epithelial cells through recruitment leads to polyclonal intestinal tumors

Intestinal tumors from mice and humans can have a polyclonal origin. Statistical analyses indicate that the best explanation for this source of intratumoral heterogeneity is the presence of interactions among multiple progenitors. We sought to better understand the nature of these interactions. An initial progenitor could recruit others by facilitating the transformation of one or more neighboring cells. Alternatively, two progenitors that are independently initiated could simply cooperate to form a single tumor. These possibilities were tested by analyzing tumors from aggregation chimeras that were generated by fusing together embryos with unequal predispositions to tumor development. Strikingly, numerous polyclonal tumors were observed even when one genetic component was highly, if not completely, resistant to spontaneous tumorigenesis in the intestine. Moreover, the observed number of polyclonal tumors could be explained by the facilitated transformation of a single neighbor within 144 μm of an initial progenitor. These findings strongly support recruitment instead of cooperation. Thus, it is conceivable that these interactions are necessary for tumors to thrive, so blocking them might be a highly effective method for preventing the formation of tumors in the intestine and other tissues.

Development of a colon cancer GEMM-derived orthotopic transplant model for drug discovery and validation

PURPOSE

Effective therapies for KRAS-mutant colorectal cancer (CRC) are a critical unmet clinical need. Previously, we described genetically engineered mouse models (GEMM) for sporadic Kras-mutant and non-mutant CRC suitable for preclinical evaluation of experimental therapeutics. To accelerate drug discovery and validation, we sought to derive low-passage cell lines from GEMM Kras-mutant and wild-type tumors for in vitro screening and transplantation into the native colonic environment of immunocompetent mice for in vivo validation.

EXPERIMENTAL DESIGN

Cell lines were derived from Kras-mutant and non-mutant GEMM tumors under defined media conditions. Growth kinetics, phosphoproteomes, transcriptomes, drug sensitivity, and metabolism were examined. Cell lines were implanted in mice and monitored for in vivo tumor analysis.

RESULTS

Kras-mutant cell lines displayed increased proliferation, mitogen-activated protein kinase signaling, and phosphoinositide-3 kinase signaling. Microarray analysis identified significant overlap with human CRC-related gene signatures, including KRAS-mutant and metastatic CRC. Further analyses revealed enrichment for numerous disease-relevant biologic pathways, including glucose metabolism. Functional assessment in vitro and in vivo validated this finding and highlighted the dependence of Kras-mutant CRC on oncogenic signaling and on aerobic glycolysis.

CONCLUSIONS

We have successfully characterized a novel GEMM-derived orthotopic transplant model of human KRAS-mutant CRC. This approach combines in vitro screening capability using low-passage cell lines that recapitulate human CRC and potential for rapid in vivo validation using cell line-derived tumors that develop in the colonic microenvironment of immunocompetent animals. Taken together, this platform is a clear advancement in preclinical CRC models for comprehensive drug discovery and validation efforts.

More than two decades of Apc modeling in rodents

Mutation of tumor suppressor gene adenomatous polyposis coli (APC) is an initiating step in most colon cancers. This review summarizes Apc models in mice and rats, with particular concentration on those most recently developed, phenotypic variation among different models, and genotype/phenotype correlations.

Mapping six new susceptibility to colon cancer (Scc) loci using a mouse interspecific backcross

Colorectal cancer (CRC) has a complex etiology resulting from the combination of multiple genetic and environmental factors, each with small effects. Interactions among susceptibility modifier loci make many of the loci difficult to detect in human genome-wide association studies. Previous analyses in mice have used classical inbred strains, which share large portions of their genomes due to common ancestry. Herein, we used an interspecific backcross between the Mus musculus strain A/J and the Mus spretus strain SPRET/EiJ to map 6 additional CRC modifier loci (Scc16-21) and 2 suggestive loci. Three loci modify the location of tumors along the proximal-distal axis of the colon. Six CRC modifiers previously mapped in intraspecific crosses were also replicated. This work confirms genetic models suggesting that CRC is caused by many small effect alleles and brings the catalog of reported CRC modifier loci to 23 spread across 13 chromosomes. Furthermore, this work provides the foundation for large population-level epistatic interaction tests to identify combinations of low effect alleles that may have large effects on CRC susceptibility.

Epidermal growth factor receptor inhibits colitis-associated cancer in mice

Inflammatory bowel disease (IBD) is a chronic illness caused by complex interactions between genetic and environmental factors that propagate inflammation and damage to the gastrointestinal epithelium. This state of chronic inflammation increases the risk for development of colitis-associated cancer in IBD patients. Thus, the development of targeted therapeutics that can disrupt the cycle of inflammation and epithelial injury is highly attractive. However, such biological therapies, including those targeting epidermal growth factor receptor pathways, pose a risk of increasing cancer rates. Using two mouse models of colitis-associated cancer, we found that epidermal growth factor receptor inactivation accelerated the incidence and progression of colorectal tumors. By modulating inflammation and epithelial regeneration, epidermal growth factor receptor optimized the response to chronic inflammation and limited subsequent tumorigenesis. These findings provide important insights into the pathogenesis of colitis-associated cancer and suggest that epidermal growth factor-based therapies for IBD may reduce long-term cancer risk.

Histological and molecular evaluation of patient-derived colorectal cancer explants

Mouse models have been developed to investigate colorectal cancer etiology and evaluate new anti-cancer therapies. While genetically engineered and carcinogen-induced mouse models have provided important information with regard to the mechanisms underlying the oncogenic process, tumor xenograft models remain the standard for the evaluation of new chemotherapy and targeted drug treatments for clinical use. However, it remains unclear to what extent explanted colorectal tumor tissues retain inherent pathological features over time. In this study, we have generated a panel of 27 patient-derived colorectal cancer explants (PDCCEs) by direct transplantation of human colorectal cancer tissues into NOD-SCID mice. Using this panel, we performed a comparison of histology, gene expression and mutation status between PDCCEs and the original human tissues from which they were derived. Our findings demonstrate that PDCCEs maintain key histological features, basic gene expression patterns and KRAS/BRAF mutation status through multiple passages. Altogether, these findings suggest that PDCCEs maintain similarity to the patient tumor from which they are derived and may have the potential to serve as a reliable preclinical model that can be incorporated into future strategies to optimize individual therapy for patients with colorectal cancer.

Colorectal cancer chemoprevention by trans-resveratrol

trans-Resveratrol (trans-3,4',5-trihydroxystilbene) is a natural phytoalexin present in grapes, red wine, berries and peanuts with health protecting properties. The low oral bioavailability indicated for this polyphenol, with the intestine as a bottleneck to its absorption, has promoted the large intestine as a potential target site for its chemopreventive activity. This review recapitulates the current evidence of the effects of trans-resveratrol on colon cancer. First, we describe the studies conducted in vitro which show that the protective activity takes place by inhibition of proliferation and induction of apoptosis. Secondly, the chemopreventive activity in animal models of colon carcinogenesis is revised. trans-Resveratrol not only reduces the number of preneoplastic lesions but also the incidence and multiplicity of tumors. Lastly, the article also reviews the available data on clinical trials. Altogether, the present findings support the hypothesis that the oral administration of trans-resveratrol might contribute to the prevention of colon carcinogenesis.

Identification of Mom12 and Mom13, two novel modifier loci of Apc (Min) -mediated intestinal tumorigenesis

Colorectal cancer is a heterogeneous disease resulting from a combination of genetic and environmental factors. The C57BL/6J (B6) Apc (Min/+) mouse develops polyps throughout the gastrointestinal tract and has been a valuable model for understanding the genetic basis of intestinal tumorigenesis. Apc (Min/+) mice have been used to study known oncogenes and tumor suppressor genes on a controlled genetic background. These studies often utilize congenic knockout alleles, which can carry an unknown amount of residual donor DNA. The Apc (Min) model has also been used to identify modifer loci, known as Modifier of Min (Mom) loci, which alter Apc (Min) -mediated intestinal tumorigenesis. B6 mice carrying a knockout allele generated in WW6 embryonic stem cells were crossed to B6 Apc (Min/+) mice to determine the effect on polyp multiplicity. The newly generated colony developed significantly more intestinal polyps than Apc (Min/+) controls. Polyp multiplicity did not correlate with inheritance of the knockout allele, suggesting the presence of one or more modifier loci segregating in the colony. Genotyping of simple sequence length polymorphism (SSLP) markers revealed residual 129X1/SvJ genomic DNA within the congenic region of the parental knockout line. An analysis of polyp multiplicity data and SSLP genotyping indicated the presence of two Mom loci in the colony: 1) Mom12, a dominant modifier linked to the congenic region on chromosome 6, and 2) Mom13, which is unlinked to the congenic region and whose effect is masked by Mom12. The identification of Mom12 and Mom13 demonstrates the potential problems resulting from residual heterozygosity present in congenic lines.

Fem1b antigen in the stool of ApcMin mice as a biomarker of early Wnt signaling activation in intestinal neoplasia

BACKGROUND

Colorectal cancer is preventable by early detection and removal of precursor lesions. Central to early stages of colorectal neoplasia is activation of Wnt signaling, usually due to inactivation of the Apc tumor suppressor gene for which there is an established animal model, the Apc(Min) mouse. Immunodetection in stool of proteins up-regulated by aberrant Wnt signaling, within intestinal epithelial cells shed into the lumen, could be a rational approach to identify biomarkers of early intestinal neoplasia. Fem1b gene expression is up-regulated, following inactivation of Apc, in mouse intestinal epithelium.

METHODS

We initially screened pooled random stool samples by immunoblotting and found that we could detect, in Apc(Min) mice but not wild-type mice, a fragment of Fem1b protein with an antibody (Li-50) directed against an epitope near the middle of the protein, but not with antibodies directed against N-terminus or C-terminus epitopes. We then evaluated freshly voided individual stool samples collected on four consecutive days from four each of male and female Apc(Min) mice and their wild-type littermates.

RESULTS

The Fem1b antigen was detected with the Li-50 antibody in 15/16 samples from male Apc(Min) mice compared to 0/16 samples from male wild-type mice, and in 5/16 samples from female Apc(Min) mice compared to 0/16 samples from female wild-type mice.

CONCLUSIONS

This study provides proof-of-principle that fragments of proteins, whose expression is increased by aberrant Wnt signaling early in intestinal neoplasia, can be immunodetected in stool. Excreted Fem1b protein fragments may be a useful biomarker for epithelial Wnt signaling and early intestinal neoplasia.

The armadillo repeat domain of Apc suppresses intestinal tumorigenesis

The adenomatous polyposis coli (APC) gene is known to act as a tumor suppressor gene in both sporadic and hereditary colorectal cancer by negatively regulating WNT signaling. Familial adenomatous polyposis (FAP) patients develop intestinal polyps due to the presence of a single germline mutation in APC. The severity of the FAP phenotype is a function of the position of the APC mutation, indicating a complex role for APC that extends beyond the canonical WNT pathway. APC encodes a large protein with multiple functional domains, including an armadillo repeat domain that has been linked to protein-protein interactions. To determine the effect of the armadillo repeat domain on intestinal tumorigenesis, we generated a congenic mouse line (Apc ( Δ242 )) carrying a gene trap cassette between exons 7 and 8 of the murine Apc gene. Apc ( Δ242/+) mice express a truncated Apc product lacking the armadillo repeat domain as part of a fusion protein with β-geo. Expression of the fusion product was confirmed by X-gal staining, ensuring that Apc ( Δ242 ) is not a null allele. In contrast, Apc ( Min/+) mice produce a truncated Apc product that contains an intact armadillo repeat domain. On the C57BL/6J background, Apc ( Δ242/+) mice develop more polyps than do Apc ( Min/+) mice along the entire length of the small intestine; however, polyps were significantly smaller in Apc ( Δ242/+) mice. In addition, polyp multiplicity in Apc ( Δ242/+) mice is affected by polymorphisms between inbred strains. These data suggest that the armadillo repeat domain of the Apc protein suppresses tumor initiation in the murine intestine while also promoting tumor growth.

Enhanced intestinal tumor multiplicity and grade in vivo after HZE exposure: mouse models for space radiation risk estimates

Carcinogenesis induced by space radiation is considered a major risk factor in manned interplanetary and other extended missions. The models presently used to estimate the risk for cancer induction following deep space radiation exposure are based on data from A-bomb survivor cohorts and do not account for important biological differences existing between high-linear energy transfer (LET) and low-LET-induced DNA damage. High-energy and charge (HZE) radiation, the main component of galactic cosmic rays (GCR), causes highly complex DNA damage compared to low-LET radiation, which may lead to increased frequency of chromosomal rearrangements, and contribute to carcinogenic risk in astronauts. Gastrointestinal (GI) tumors are frequent in the United States, and colorectal cancer (CRC) is the third most common cancer accounting for 10% of all cancer deaths. On the basis of the aforementioned epidemiological observations and the frequency of spontaneous precancerous GI lesions in the general population, even a modest increase in incidence by space radiation exposure could have a significant effect on health risk estimates for future manned space flights. Ground-based research is necessary to reduce the uncertainties associated with projected cancer risk estimates and to gain insights into molecular mechanisms involved in space-induced carcinogenesis. We investigated in vivo differential effects of gamma-rays and HZE ions on intestinal tumorigenesis using two different murine models, ApcMin/+ and Apc1638N/+. We showed that gamma- and/or HZE exposure significantly enhances development and progression of intestinal tumors in a mutant-line-specific manner, and identified suitable models for in vivo studies of space radiation-induced intestinal tumorigenesis.

Mechanisms of intestinal inflammation and development of associated cancers: lessons learned from mouse models

Chronic inflammation is strongly associated with approximately 1/5th of all human cancers. Arising from combinations of factors such as environmental exposures, diet, inherited gene polymorphisms, infections, or from dysfunctions of the immune response, chronic inflammation begins as an attempt of the body to remove injurious stimuli; however, over time, this results in continuous tissue destruction and promotion and maintenance of carcinogenesis. Here we focus on intestinal inflammation and its associated cancers, a group of diseases on the rise and affecting millions of people worldwide. Intestinal inflammation can be widely grouped into inflammatory bowel diseases (ulcerative colitis and Crohn's disease) and celiac disease. Long-standing intestinal inflammation is associated with colorectal cancer and small-bowel adenocarcinoma, as well as extraintestinal manifestations, including lymphomas and autoimmune diseases. This article highlights potential mechanisms of pathogenesis in inflammatory bowel diseases and celiac disease, as well as those involved in the progression to associated cancers, most of which have been identified from studies utilizing mouse models of intestinal inflammation. Mouse models of intestinal inflammation can be widely grouped into chemically induced models; genetic models, which make up the bulk of the studied models; adoptive transfer models; and spontaneous models. Studies in these models have lead to the understanding that persistent antigen exposure in the intestinal lumen, in combination with loss of epithelial barrier function, and dysfunction and dysregulation of the innate and adaptive immune responses lead to chronic intestinal inflammation. Transcriptional changes in this environment leading to cell survival, hyperplasia, promotion of angiogenesis, persistent DNA damage, or insufficient repair of DNA damage due to an excess of proinflammatory mediators are then thought to lead to sustained malignant transformation. With regards to extraintestinal manifestations such as lymphoma, however, more suitable models are required to further investigate the complex and heterogeneous mechanisms that may be at play.