Study Of MSI Detection In Sporadic Colorectal Cancer And The Correlation With Clinicopathological Features

Colorectal carcinoma (CRC) is the most common gastrointestinal malignancy in the world,and there are suggestions of a particularly high incidence with the change of lifestyle in China. Of all cancer types, the molecular changes leading to malignant growth are perhaps best known in CRC. The malignant transformation associated with colorectal carcinogenesis involves the stepwise accumulation of multiple genetic alterations that allow the growth of neoplastic cells. Currently, adenocarinomas of the colon and rectum are believed to arise from two distinct molecular genetic pathways: one involving chromosomal instability (CIN) and the other involving microsatellite instability (MSI). Identification of colorectal carcinomas with high levels of DNA microsatellite instability (MSI-H) is important because of the suggested prognostic and therapeutic significance associated with MSI. The expression of DNA mismatch repair (MMR) protein could reflect the MSI status, and the utility of immunohistochemistry (IHC) as a screening method for the identification of persons with mutations in the DNA mismatch repair (MMR) genes in hereditar y nonpolyposis colorectal cancer (HNPCC) remains to be defined. In this study, we analyzed the value of IHC versus that of MSI testing in predicting mutation status of the MLH1, PMS2, MSH2, and MSH6genes in colorectal carcinomas and adenomas, and explored the frequency and significance of immunohistochemical staining variability. The current research project is comprised of the following two parts.PartⅠPurpose:To analysis the mismatch repair (MMR) gene expression in Chinese sporadic colorectal cancers (SCRCs) and the correlations with clinicopathological features and prognoses of the cases.Methods and materials:By auto-immunohistochemical staining, four panels of antibodies, including MLH1, MSH2, PMS2and MSH6, were detected in the paraffin embedded tissue samples from452SCRC patients. Clinical and pathological features were comparative analyzed between the cases with different phenotypes.Results:Defects in the MMR proteins was found in9.51%(43/452) cases analyzed. Among them, MLH1expresison was absent in29cases,MSH2absence in5cases, PMS2absence in29cases, and MSH6absence in11cases. Double protein absence was found in29cases, MLH1/PMS2in25and MSH2/MSH6in4cases. The absence of all the4MMR proteins was only detected in one case. SCRCs with the MMR defects showed distinct clinicopathological features, such as mostly with the location in the proximal colon, poor differentiation and relatively good prognosis.Part ⅡPurpose:To detect the MSI status by PCR-capillary electrophoresis system, compare the results with the correspondent MMRP expression of the tumor cells, and evaluate the practicability of IHC detection of MMRP as a useful screening way of MSI status of SCRCs.Methods and materials:144patients who had been assessed for expression of MMRP by IHC (43MMR-D and101MMR-I) was selected to be tested for MSI status. MSI was analyzed by PCR amplification, using fluorescent dye-labeled primers for the microsatellite loci recommended by the National Cancer Institute, including BAT25, BAT26, D2S123, D5S346and D17S250.Results:Of144tumors tested,100were microsatellite stable (MSS),41were MSI-H, and3were MSI-L. In all,29tumors showed absence of hMLHl expression,of which24were MSI-H,1was MSI-L and4were MSS;29tumors showed absence of hPMS2expression:25were MSI-H and4were MSS;5showed absence of hMSH2expression: all were MSI-H;11showed absence of hMSH6expression:10were MSI-H and1was MSS. The OS and DFS were all significantly better in patients tested as MSI-H when compared to patients of MSS/MSI-L (P<0.05). IHC detection of MMRP using4panels of antibodies targeting the products of hMLH1, hPMS2, hMSH2and hMSH6was an extremely sensitive (90.2%) and specific (97%) method for screening for DNA mismatch repair defects.Conclusions:Our studies showed that IHC detection with antibodies targeted MLH1, MSH2, PMS2and MSH6is as sensitive as MSI testing, given that IHC is easily available and generally inexpensive and, importantly, identifies the affected gene, it is reasonable to regard IHC as a more optimal first-line screening tool than MSI testing for identifying MSI-H SCRC. MSI testing can provide a fallback position in equivocal situations, while remaining an important research tool. MMR-D tumors more often arise in the right colon and are more likely to occur in individuals with poor differentiated, tumor size>5cm but no distant metastasis.And just like MSI status,MMR status did seem also to be a viable predictive marker to evaluate prognosis in SCRC.