Supplementary Material for: Immunohistochemical Analysis of Mismatch Repair Gene Proteins in Early Gastric Cancer Based on Microsatellite Status

<b><i>Background:</i></b> Microsatellite instability (MSI) is a major pathway involved in gastric carcinogenesis and is observed in 10–20% of early gastric cancers (EGCs). Early detection of EGCs with an MSI-high phenotype would be useful for elucidating the mechanisms of gastric carcinogenesis and improving outcomes in patients with GC. <b><i>Objective:</i></b> We explored the usefulness of immunohistochemical expression of mismatch repair (MMR) proteins, including MLH1, PMS2, MSH2, and MSH6 in EGC. <b><i>Methods:</i></b> We examined the expression of 4 MMR proteins using immunohistochemistry in 119 patients with EGC based on MS status, as determined by polymerase chain reaction-microsatellite analysis. In addition, methylation of the <i>MLH1</i> gene was quantified by pyrosequencing. <b><i>Results:</i></b> EGCs were classified into 46 MSI-high phenotypes and 73 microsatellite stable (MSS) phenotypes. Although loss of MLH1 expression was associated with loss of PMS2 expression in the MSI-high phenotype, discordant cases of loss of expression between MLH1 and PMS2 were found (MLH1 [−]/PMS2 [+], 3 cases). Loss of MLH1/PMS2 expression was observed in 2 of 73 MSS phenotypes. Loss of MSH2/MSH6 expression was found in 4 of 46 MSI-high phenotypes, whereas loss of MSH2/MSH6 expression was not detected in the MSS phenotype. In addition, loss of MLH1 expression was correlated with methylation of <i>MLH1</i>. However, there were discordant cases in which loss of MLH1 expression was not accompanied by methylation of <i>MLH1</i>. <b><i>Conclusion:</i></b> Although immunostaining of MMR proteins could help predict MSI in EGCs, immunostaining did not have the same value as genetic testing for determination of MSI.