Microsatellite Instability in Mouse Models of Colorectal Cancer.

Microsatellite instability (MSI) is caused by DNA mismatch repair deficiency and is an important prognostic and predictive biomarker in colorectal cancer but relatively few studies have exploited mouse models in the study of its clinical utility. Furthermore, most previous studies have looked at MSI in the small intestine rather than the colon of mismatch repair deficient Msh2-knockout (KO) mice. Here we compared Msh2-KO, p53-KO, and wild type (WT) mice that were treated with the carcinogen azoxymethane (AOM) and the nonsteroidal anti-inflammatory drug sulindac or received no treatment. The induced tumors and normal tissue specimens from the colon were analysed with a panel of five mononucleotide repeat markers. MSI was detected throughout the normal colon in untreated Msh2-KO mice and this involved contraction of the repeat sequences compared to WT. The markers with longer mononucleotide repeats (37-59) were the most sensitive for MSI while the markers with shorter repeats (24) showed only minor change. AOM exposure caused further contraction of the Bat37 and Bat59 repeats in the distal colon of Msh2-KO mice which was reversed by sulindac. Thus AOM-induced carcinogenesis is associated with increased instability of mononucleotide repeats in the colon of Msh2-KO mice but not in WT or p53-KO mice. Chemoprevention of these tumors by sulindac treatment reversed or prevented the increased MSI.

The NSAID sulindac is chemopreventive in the mouse distal colon but carcinogenic in the proximal colon.

BACKGROUND AND AIMS: The non-steroidal anti-inflammatory drug sulindac is an effective chemopreventive agent in sporadic colorectal cancer but its potential benefit in mismatch repair deficient cancers remains to be defined. We wanted to determine whether genetic defects that are relevant for colorectal cancer, such as Msh2 or p53 deficiency, would influence the efficiency of sulindac chemoprevention or increase the side effects. METHODS: Msh2 or p53 deficient and wild-type mice received feed containing 160-320 ppm sulindac for up to 25 weeks with or without a concurrent treatment with the carcinogen azoxymethane. Colon tissue was analysed by histopathology and molecular biology methods. RESULTS: We show that sulindac prevented azoxymethane-induced distal colon tumours in all mice. In the proximal colon, however, sulindac induced new inflammatory lesions on the mucosal folds, which further developed into adenocarcinoma in up to 18-25% of the p53 or Msh2 deficient mice but rarely in wild-type mice. This region in the proximal colon was characterised by a distinct profile of pro- and anti-inflammatory factors, which were modulated by the sulindac diet, including upregulation of hypoxia inducible factor 1α and macrophage inflammatory protein 2. CONCLUSIONS: These data show that the sulindac diet promotes carcinogenesis in the mouse proximal colon possibly through chronic inflammation. Sulindac has both beneficial and harmful effects in vivo, which are associated with different microenvironments within the colon of experimental mice. Deficiency for the Msh2 or p53 tumour suppressor genes increases the harmful side effects of long-term sulindac treatment in the mouse colon.

Low microsatellite instability is associated with poor prognosis in stage C colon cancer.

PURPOSE: The significance of low microsatellite instability (MSI-L) in colorectal cancer is poorly understood. No clear biologic distinction has been found between MSI-L and microsatellite stable (MSS) colorectal cancer, and these two phenotypes are usually combined when analyzed against the well-defined high MSI (MSI-H) phenotype. Evidence is emerging that an O(6)-methylguanine DNA methyltransferase (MGMT) gene defect is associated with MSI-L. Therefore, to further define this phenotype, we undertook a detailed analysis of the prognostic significance of MSI-L and loss of MGMT expression in colon cancer. PATIENTS AND METHODS: The study cohort was 183 patients with clinicopathologic stage C colon cancer who had not received adjuvant therapy. We analyzed MSI status, MGMT, and mismatch repair protein expression, as well as MGMT and p16 promoter hypermethylation. RESULTS: We showed that MSI-L defines a group of patients with poorer survival (P = .026) than MSS patients, and that MSI-L was an independent prognostic indicator (P = .005) in stage C colon cancer. Loss of MGMT protein expression was associated with the MSI-L phenotype but was not a prognostic factor for overall survival in colon cancer. p16 methylation was significantly less frequent in MSI-L than in MSI-H and MSS tumors and was not associated with survival. CONCLUSION: MSI-L characterizes a distinct subgroup of stage C colon cancer patients, including the MSI-L subset of proximal colon cancer, who have a poorer outcome. Neither the MGMT defect nor p16 methylation are likely to contribute to the worse prognosis of the MSI-L phenotype.

Susceptibility of Msh2-deficient mice to inflammation-associated colorectal tumors.

Patients with longstanding extensive ulcerative colitis have an increased risk of developing colorectal cancer (CRC). There are significant differences in the early pathogenesis of colitis-associated tumors compared with common CRC, whereas the frequency, degree, and significance of microsatellite instability (MSI) as a marker of mismatch repair deficiency in colitis tumors remain unclear. Here we describe the application of the DSS model of chronic colitis to mice with a defect in the Msh2 mismatch repair gene to discern these early events. These mice do not develop CRC spontaneously without an external trigger. The aim of this study was to determine the effect of the Msh2 defect on the frequency and grade of colitis-associated colorectal dysplasia and adenocarcinoma in Msh2-/-, Msh2+/-, and wild-type (Msh2+/+) mice and on the MSI status of the tumors. We show that in mice with chronic colitis, 60% of the Msh2-/- and 29% of the wild-type mice developed high-grade dysplasia or adenocarcinoma, but heterozygosity for the Msh2 defect did not increase tumor susceptibility over wild-type genotype. The largest difference between genotypes was in the frequency of high-grade dysplasia, with 46.7, 8, and 12.5% in Msh2-/-, Msh2+/-, and Msh2+/+ mice, respectively. The Msh2-/- mice developed MSI-high tumors, whereas the majority of the Msh2+/- and wild-type tumors had no MSI. In the Msh2-/- mice, MSI appeared early in non-neoplastic colon tissue, presumably as a result of markedly increased epithelial cell proliferation associated with inflammation. These observations suggest that a homozygous mismatch repair defect predisposes to tumors triggered by chronic inflammation but is not the only factor involved because tumors also developed in the wild-type mice. This model of colitis offers opportunities to characterize the different molecular pathways of carcinogenesis operating in chronic colitis.