Impact of BRAF, MLH1 on the incidence of microsatellite instability high colorectal cancer in populations based study.

We have identified an alternative pathway of tumorigenesis in sporadic colon cancer, involving microsatellite instability due to mismatched repair methylation, which may be driven by mutations in the BRAF gene (V600E). Colorectal cancer (CRC) is the most common cancer in the world, and African Americans show a higher incidence than other populations in the United States. We analyzed sporadic CRCs in Omani (of African origin, N = 61), Iranian (of Caucasian origin, N = 53) and African American (N = 95) patients for microsatellite instability, expression status of mismatched repair genes (hMLH1, hMSH2) and presence of the BRAF (V600E) mutation. In the Omani group, all tumors with BRAF mutations were located in the left side of the colon, and for African Americans, 88% 7 of tumors with BRAF mutations were found in the right side of the colon. In African Americans, 31% of tumors displayed microsatellite instability at two or more markers (MSI-H), while this rate was 26% and 13% for tumors in the Iranian and Omani groups, respectively. A majority of these MSI-H tumors were located in the proximal colon (right side) in African American and Iranian subjects, whereas most were located in the distal colon (left side) in Omani subjects. Defects in hMLH1 gene expression were found in 77% of MSI-H tumors in both African Americans and Iranians and in 38% of tumors in Omanis. BRAF mutations were observed in all subjects: 10% of tumors in African Americans (8/82), 2% of tumors in Iranians (1/53), and 19% of tumors in Omanis (11/59). Our findings suggest that CRC occurs at a younger age in Omani and Iranian patients, and these groups showed a lower occurrence of MSI-H than did African American patients. Our multivariate model suggests an important and significant role of hMLH1 expression and BRAF mutation in MSI-H CRC in these populations. The high occurrence of MSI-H tumors in African Americans may have significant implications for treatment, since patients with MSI-H lesions display a different response to chemotherapeutic agents such as 5-fluorouracil.

Sporadic colon cancer: mismatch repair immunohistochemistry and microsatellite instability in Omani subjects.

BACKGROUND: Colorectal carcinoma (CRC) is the most common gastrointestinal malignancy in the world, and there are suggestions of a particularly high incidence in the Middle East, including those of African origin. Defects in DNA mismatch repair (MMR) systems are involved in the carcinogenesis of both sporadic and inherited human cancers. We assessed colonic cancers in an attempt to identify tumors with DNA MMR deficiency and microsatellite instability (MSI). Additionally, we tested the ability of cell cycle regulator p16 that effects cell proliferation and can be abrogated by hypermethylation of the promoter region. METHODS: We reviewed the charts of 756 patients who were referred to the Oman major colonoscopy unit of the Sultan Qaboos University Hospital and Royal Hospital from the years 2000 to 2004. Colon cancer tissue was assayed using immunohistochemistry for expression of hMLH1 and hMSH2, and a panel of five pairs of microsatellite primers (NR21, NR22, NR24, BAT25, and BAT26) for MSI-H analysis and additional dinucleotide markers (D17S250, D5S346, and D2S123) used for MSI-L. The expression status of MMR genes and MSI was correlated with cancer stage, location, and histology. A total of 49 tumors were analyzed for histopathology, MSI, and hMLH1/hMSH2 protein expression analysis. The methylation status of the p16 promoter was determined by methylation-specific polymerase chain reaction (PCR). RESULTS: The mean age for the carcinomas was 52.2 years and 53% of the patients were male. The majority of the tumors were left-sided. The information currently available indicates that there is an incidence of 4.7% colon cancer (49/1036) and 12.1% (126/1290) colon adenoma among the cases who underwent colonoscopy at these centers. The rate of MSI-H was 12.2% (n = 6), which appears to be the same as previously reported in literature. Eight of 49 tumors (16.3%) were MMR defective by IHC. Defects in the mismatch repair genes hMLH1 and hMSH2 were found in four (66.7%) and two (33.3%) of CRCs MSI-H cases, respectively. Defects in hMLH1 expression in tumors were commonly associated with moderate differentiation. The p16 promoter was methylated in 4% of tumors. CONCLUSION: This is the first genetic study of CRC in this region of the world to demonstrate the incidence of MSI, p16 methylation, and hMLH1 and MSH2 expression in the Omani population. In addition, a relatively high frequency of CRC in younger age groups was noted, which is an important observation. The left-sided preponderance of MMR defective tumors was mostly associated with hMLH1, and with possible loss of hMSH2 expression, an observation that differs from studies on other populations. In conclusion, although the overall rate of CRC is unknown in this region, the frequency of MSI in CRC in this region appears to be the same as in Caucasians in the USA.