DNA adduct formation and assessment of aberrant crypt foci in vivo in the rat colon mucosa after treatment with N-methyl-N-nitrosourea

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.

Animal models of colorectal cancer

Colorectal cancer is a heterogeneous disease that afflicts a large number of people in the USA. The use of animal models has the potential to increase our understanding of carcinogenesis, tumor biology, and the impact of specific molecular events on colon biology. In addition, animal models with features of specific human colorectal cancers can be used to test strategies for cancer prevention and treatment. In this review, we provide an overview of the mechanisms driving human cancer, we discuss the approaches one can take to model colon cancer in animals, and we describe a number of specific animal models that have been developed for the study of colon cancer. We believe that there are many valuable animal models to study various aspects of human colorectal cancer. However, opportunities for improving upon these models exist.

Aberrant crypt foci: the case for inclusion as a biomarker for colon cancer

Aberrant crypt foci (ACF) are one of the earliest histopathological manifestations of colon cancer. In this review, we critically present the molecular, cellular, histopathological, and chemopreventive evidence that ACF are relevant biomarkers for colon cancer. The laboratory and clinical evidence are highly suggestive that ACF are in the pathway leading to colon cancer, but not all ACF will do so. The possible fate and outcome of ACF in the progression toward colon cancer may be dependent on a number of features that define their predictive power for the prevention or progression of cancer.

A noncarcinogenic small dose of n-methylnitrosourea enhances colon tumorigenesis in F344 rats

Very small amount of carcinogens such as nitroso compounds and heterocyclic amines has been found in human feces. This study was designed to investigate in F344 rats whether an intrarectal noncarcinogenic small dose of n-methylnitrosourea (0.2 mg, 3 times weekly) affects colon carcinogenesis initiated with an intrarectal large dose (2 mg, 3 times weekly) of this carcinogen. The colon tumor yield at week 50 in 12 rats receiving the large dose for 2 weeks and then the small dose for the next 30 weeks significantly increased as compared to 23 rats receiving only the large dose for 2 weeks: 100% vs. 70% in tumor incidence and 4.3 vs. 1.2 in mean number of tumors per rat. No tumors were found in 12 rats receiving only the small dose for 30 weeks. The results indicate that a very small amount of carcinogens could participate in a tumor-enhancing effect on the high-risk colon induced with a large dose of carcinogens, even though their less amounts insufficient to complete carcinogenesis process. Further study is needed to elucidate if administration of such a small dose could induce epigenetic alterations or further genetic changes in the initiated colonic cells.

Inhibition by ginseng of 1,2-dimethylhydrazine induction of aberrant crypt foci in the rat colon

The modifying effects of dietary administration of ginseng on the induction and development of 1,2-dimethylhydrazine (DMH)-induced aberrant crypt foci (ACF) were investigated in Fischer 344 (F-344) rats. In Experiment 1, starting at six weeks of age, 65 rats were injected with DMH or saline alone once a week for four weeks. Rats in Groups 1 and 2 were fed diets containing 1% ginseng for five weeks, starting one week before the first dose of DMH. Animals in Groups 3 and 4 received ginseng for eight weeks after DMH treatment; Group 5 served as a carcinogen control group. In Experiment 2, 60 rats were injected with DMH or saline alone four times at one-week intervals. They were also fed diets containing 1% ginseng or the control diet throughout the 30 days of the experiment. In Experiment 1, numbers of foci with at least four crypts were significantly reduced in Group 2 treated with red ginseng during the initiation phase (p < 0.005). In Experiment 2, treatment with red ginseng also resulted in a decrease in the total number of DMH-induced ACF accompanied by a reduction in 5-bromo-2'-deoxyuridine labeling indexes in colonic crypts comprising ACF (p < 0.005 and p < 0.05, respectively). These findings suggest that dietary administration of red ginseng in combination with DMH suppresses colon carcinogenesis of rats, and the inhibition may be associated, in part, with suppression of cell proliferation in the colonic mucosa.

Heterozygous DNA mismatch repair gene PMS2-knockout mice are susceptible to intestinal tumor induction with N-methyl-N-nitrosourea

PMS2-deficient (PMS2(-/-)) mice are hypersensitive to N-methyl-N-nitrosourea (MNU)-induced thymic lymphomas based on the failure to initiate mismatch repair (MMR) at O(6)-methylguanine:T mismatches formed after MNU exposure. However, heterozygous PMS2 knockout (PMS2(+/-)) mice do not develop spontaneous tumors, suggesting that they have sufficient MMR function to prevent genomic instability. We hypothesized that in PMS2(+/-) mice, exogenous carcinogens may either mutationally knockout the remaining normal allele leading cells to develop tumors or introduce sufficient DNA adducts and mismatches to overload the lower capacity for MMR, leading in either case to an increased rate of tumor production. In the present study, PMS2(+/-) mice and their littermate PMS2(+/+) mice were monitored for tumor incidence following MNU treatment. Mice were given 50 mg MNU/kg i.p. when 5 weeks old. They demonstrated a similar incidence of thymic lymphomas, suggesting that expression of the single normal PMS2 allele is sufficient to protect the thymus and implying that a single dose of MNU may not efficiently knock out the remaining PMS2 allele in the thymus. Surprisingly, PMS2(+/-) mice were significantly more likely to develop intestinal tumors-both adenomas and adenocarcinomas-after MNU than were PMS2(+/+) mice (2.34 +/- 0.34 tumors per mouse versus 1.34 +/- 0.25 tumors per mouse; P < 0.05). The intestinal tumors were located mainly in the small intestine. However, these tumors in both the PMS2(+/-) mice and PMS2(+/+) mice did not show microsatellite instability characteristic of loss of MMR. These results suggest that a single normal PMS2 allele can protect thymus but not intestine from MNU carcinogenesis. Organ-specific factors might influence MMR- mediated resistance to methylating agents. Heterozygous PMS2 knockout mice may be used as a promising animal model for intestinal tumorigenesis studies involving environmental carcinogens.

ras gene mutations in oral cancer in eastern India

Oral tumor specimens (n = 50) from eastern Indian population were studied for the presence of mutations in the H-, K- and N-ras genes using selective oligodeoxynucleotide hybridization and restriction fragment length polymorphism analysis of polymerase chain reaction-amplified products. Mutations in H- and K-ras genes were observed at a frequency of 28 and 33%, respectively, whereas no N-ras mutation was noticed.

Mice defective in the DNA mismatch gene PMS2 are hypersensitive to MNU induced thymic lymphoma and are partially protected by transgenic expression of human MGMT

DNA mismatch repair (MMR) stabilizes the cellular genome. Mice defective in the MMR gene PMS2 are susceptible to spontaneous thymic lymphoma and sarcomas. To determine the sensitivity of PMS2 knockout mice to environmental carcinogens and the protective effect of O6-methylguanine DNA methyltransferase (MGMT), heterozygous PMS2 knockout mice and human MGMT (hMGMT) transgenic mice were mated and the PMS2-/- and PMS2+/+ with or without hMGMT offspring were treated at 5 weeks of age with 50 mg/kg N-methyl-N-nitrosourea (MNU). MNU produces carcinogenic O6-methylguanine (O6-meG) adducts, resulting in thymic lymphoma in mice, which can be prevented in normal mice by overexpression of hMGMT. A significantly higher incidence of thymic lymphomas was observed in MNU-treated PMS2-/- mice, compared to wildtype PMS2+/+ mice (100 vs 52%; P < 0.001). The mean latency of lymphomas was also significantly shortened in PMS2-/- mice (81 vs 102 days, P < 0.01). Transgenic expression of hMGMT significantly but incompletely blocked MNU lymphomagenesis in PMS2-/- mice. The incidence of lymphomas in PMS2-/-/hMGMT+ mice was reduced to 80% (P < 0.01) and mean latency increased to 91 days (P < 0.05). Thymic lymphomagenesis was efficiently blocked in PMS2+/+/hMGMT+ mice with rapid repair of O6-meG. Since O6-meG:T mismatches in MMR+ cells may trigger mismatch repair resulting in abortive repair and cell death whereas in the absence of MMR, these mismatches are converted to A:T, we predicted that G to A point mutations in codon 12 of the K-ras gene would occur. In this study, we found G to A point mutations in codon 12 of the K-ras gene in many tumors. Thus, in MMR deficient tissues, methylating agents induce point mutations in cells with a higher rate of cell survival which together are potently carcinogenic in the thymus. These data suggest that PMS2 defective lymphomas may arise by the concerted action of environmental and perhaps endogenous methylation of DNA coupled to genomic instability.

Short- and intermediate-term carcinogenicity testing--a review. Part 2: available experimental models

Numerous experimental protocols for short- and intermediate-term carcinogenicity assays have been available for many years. This paper surveys various of these test systems in rodents, fish species, non-vertebrates and avian embryos in ovo. The mouse skin tumour assay and the rat liver foci assay were used to introduce the basic concepts of short- and intermediate-term carcinogenicity testing in the previous part of the review. The focus of this second part of the review is on rodent assays for carcinogenicity testing in the lung, kidney, urinary bladder, pancreas, stomach, oral cavity, small intestine, colon, and on the possibility to combine several target organs in multi-organ models. The potential use of various fish species, non-vertebrates and hatching eggs for carcinogenicity testing is outlined and the advantages and limitations are discussed. This review also presents the problem of validation of any carcinogenicity test system and proposes a strategy for contemporary safety assessment of chemicals with regard to the detection and evaluation of carcinogenicity.

Relationship between the nature of mucus and crypt multiplicity in aberrant crypt foci in the rat colon

Aberrant crypt foci (ACF) induced in the distal colon of F344 male rats, 4, 8, 12 and 35 weeks after the first administration of 1, 2-dimethylhydrazine-2HCl (DMH) were examined to determine whether a correlation exists between the nature of goblet cell mucin and the number of crypts (crypt multiplicity) comprising the ACF. According to the ACF score calculated from the results of the qualitative observation of sulfomucins (SuMs) and sialomucins (SiMs), the ACF in the 4th week showed a weak correlation between the nature of the mucus and crypt multiplicity, and the ACF of each class showed similar mucous profiles. From the 8th week, a significant difference (P < 0.01) was recognized between the ACF consisting of 3 crypts or less and those consisting of 4 crypts or more. The proportion of crypts with SiM predominance showed a decrease in the 8th week in the ACF consisting of 1 crypt and in the 12th week in the ACF consisting of 2 or 3 crypts, implying a recovery tendency. The ACF consisting of more than 4 crypts showed little change over time, retaining the tendency of SiM predominance. Ulex europaeus agglutinin-I (UEA-I) lectin-positive crypts appeared in the ACF. This finding was significantly more prominent (P < 0.001) in the ACF with SiM predominance than in the ACF with SuM predominance at each experimental period, and in the 12th week after the first administration of DMH, the incidence of ACF with UEA-I-reactive mucin was decreased in the ACF groups consisting of 3 crypts or less, compared with the ACF groups consisting of 4 or more crypts. These results suggest that the biological quality of mucus in ACF consisting of 4 or more crypts is different from that in ACF consisting of 3 crypts or less. This difference should be considered when ACF are used as an intermediate biomarker of colon cancer.

Evaluation of the potential carcinogenic activity of Senna and Cascara glycosides for the rat colon

Anthraquinone glycosides of Senna and Cascara were investigated for their ability to induce aberrant crypt foci (ACF) in the rat colon mucosa, which are considered putative preneoplastic lesions. Dietary exposure to high doses of these glycosides for 56 successive days did not cause the appearance of ACF or increase in incidence of ACF induced by 1,2-dimethyl-hydrazine (DMH). However, in rats treated with both DMH and the highest dose of glycosides, the average number of aberrant crypts per focus, considered a consistent predictor of tumor outcome, was higher than in rats given DMH alone. These findings suggest that Senna and Cascara glycoside might behave as weak promoters in rat colon carcinogenesis.

Studies on the role of specific dietary fibres in protection against colorectal cancer

Although dietary fibre is generally thought to protect against the development of colorectal cancer, some of the results of animal and epidemiological studies are equivocal. We believe that this may be because the term dietary fibre covers a range of complex materials and some may protect but others may not. Dietary fibre is mainly composed of plant cell walls which vary in composition and properties according cell type and plant species. In addition to polysaccharides, the walls of some plant cell types contain the hydrophobic polymers lignin or suberin. Two groups of mechanisms have been proposed for the way dietary fibres may protect against colorectal cancer: those in which the dietary fibre may act directly and those in which the dietary fibre may have an indirect effect as a consequence of it being degraded by colonic bacterial enzymes and the products fermented. Direct mechanisms include the adsorption of carcinogens onto undegraded dietary fibres which pass out of the intestinal tract in the faeces. we have shown that different types of plant cell walls adsorbed a range of carcinogens, including heterocyclic aromatic amines, to different extents. Cell walls that contained lignin or suberin adsorbed hydrophobic carcinogens particularly well. Furthermore, the presence of lignin, and probably suberin, in the walls makes them resistant to degradation in the colon. Wheat bran, which is a good source of dietary fibre, contains some cell types with lignified walls. We used Fischer-344 rats to test the ability of wheat bran to protect against the formation of aberrant crypts (which are considered to be precursors to colon cancer) caused by the heterocyclic aromatic amine, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). Our results indicate that wheat bran protects and probably does so by a direct mechanism.