Testing for Microsatellite Instability in Colorectal Cancer - a Comparative Evaluation of Immunohistochemical and Molecular Methods.

Introduction: The majority of colorectal cancers (CRC) develop through the chromosomal instability pathway and approximately 15% display microsatellite instability (MSI) as a carcinogenic event. CRCs with microsatellite instability status have a characteristic phenotype. We aimed to assess the clinico-pathological and MSI profiles of sixty-one cases of CRC through immunohistochemical (IHC) staining for the mismatch repair(MMR) proteins and DNA based Polymerase Chain Reaction (PCR) assay for microsatellite markers.Haematoxylin & Eosin stained sections of the tumor were evaluated for various histopathologic features. Immunohistochemistry was performed for the four MMR proteins, MLH1, MSH2, MSH6 and PMS2. NCI recommended panel of five nucleotide repeat markers was amplified from tumor DNA.The majority of the patients were males above fifty years of age. Around 61% of tumors were in the leftsided colon. Adenocarcinoma NOS (55, 90%) was the most common histological type. A total of 18 (29.5 %) cases showed dMMR by immunohistochemistry. Loss of PMS2 protein and combined loss of MSH2 & MSH6 were the most common findings in low and high MSI respectively. Of the 13 cases selected for PCR analysis, nine cases had high MSI (at least two markers unstable) and four cases had low MSI (one marker unstable) Results of PCR based DNA assay showed good concordance with IHC. No significant statistical association could be identified between the status of MSI by either methods and sociodemographic or clinical features.MSI constitutes 12%-20% and 6%-13% of CRCs in Western and Eastern countries respectively. In our series IHC staining revealed that 29.5% of cases showed dMMR. This was similar to other Indian studies which reported a prevalence of 22-27%. The combined loss of MSH2 & MSH6 (78%) was the most common type of dMMR. There was good concordance between IHC and PCR results. The issue of heterogenous or weak staining is a limiting factor in IHC interpretation and few cases of dMMR may be missed.To conclude, IHC can be a very useful screening tool to detect microsatellite instability and triage cases of dMMR for MSI biomarker testing. The MSI status also serves as a prognostic and predictive tool.Colorectal cancer, microsatellite instability, immunohistochemistry, Polymerase chain reaction.