Immunohistochemistry for hMLH1 and hMSH2: A practical test for DNA mismatch repair-deficient tumors

Inactivation of deoxyribonucleic acid (DNA) mismatch repair genes, most com monly human mutL homologue 1 (hMLH1) or human mutS homologue 2 (hMSH2), is a recently described alternate pathway in cancer development and progressio n. The resulting genetic instability is characterized by widespread somatic mutations in tumor DNA, and is termed high-frequency microsatellite instab ility (MSI-H). Although described in a variety of tumors, mismatch repair d eficiency has been studied predominantly in colorectal carcinoma. Most MSI- H colorectal carcinomas are sporadic, but some occur in patients with hered itary nonpolyposis colorectal cancer (HNPCC), and are associated with germl ine mutations in mismatch repair genes. Until now, the identification of MS I-H cancers has required molecular testing. To evaluate the role of immunoh istochemistry as a new screening tool for mismatch repair-deficient neoplas ms, the authors studied the expression of hMLH1 and hMSH2, using commercial ly available monoclonal antibodies, in 72 formalin-fixed, paraffin-embedded tumors that had been tested previously for microsatellite instability. The y compared immunohistochemical patterns of 38 MSI-H neoplasms, including 16 cases from HNPCC patients with known germline mutations in hMLH1 or hMSH2, with 34 neoplasms that did not show microsatellite instability. Thirty-sev en of 38 MSI-H neoplasms were predicted to have a mismatch repair gene defe ct, as demonstrated by the absence of hMLH1 and/or hMSH2 expression. This i ncluded correspondence with all 16 cases with germline mutations. All 34 mi crosatellite-stable cancers had intact staining with both antibodies. These findings clearly demonstrate that immunohistochemistry can discriminate ac curately between MSI-H and microsatellite-stable tumors, providing a practi cal new technique with important clinical and research applications.