Isolated loss of PMS2 immunohistochemical expression is frequently caused by heterogeneous MLH1 promoter hypermethylation in Lynch syndrome screening for endometrial cancer patients

Lynch syndrome (LS) is an autosomal dominant inherited disorder mainly caused by a germline mutation in the DNA mismatch repair (MMR) genes (MLH1, MSH2, MSH6, PMS2) and is associated with increased risk of various cancers, particularly colorectal cancer and endometrial cancer (EC). Women with LS account for 2–6% of EC patients; it is clinically important to identify LS in such individuals for predicting and/or preventing additional LS-associated cancers. PMS2 germline mutation (PMS2-LS) is the rarest contribution to LS etiology among the four LS- associated MMR germline mutations, and its detection is complicated. Therefore, prudent screening for PMS2-LS is important as it leads to an efficient LS-identification strategy. Immunohistochemistry (IHC) is recommended as a screening method for LS in EC. Isolated loss of PMS2 expression (IL- PMS2) is caused not only by PMS2-LS but also by MLH1 germline mutation or MLH1 promoter hypermethylation (MLH-PHM). This study aimed to determine the association between MLH1-PHM and IL-PMS2 to avoid inappropriate genetic analysis. We performed MLH1 methylation analysis and MLH1/PMS2 germline mutation testing for the IL-PMS2 cases. By performing MMR-IHC for 360 unselected ECs, we selected eight (2.2%) cases as IL-PMS2. Heterogeneous MLH1 staining and MLH1-PHM were detected in 4/8 (50%) IL-PMS2 tumors. Out of five IL-PMS2 patients who underwent genetic analysis, one had PMS2 germline mutation with normal MLH1 expression (without MLH1-PHM) and no MLH1 germline mutation was detected. We suggest that MLH1 promoter methylation analysis for IL-PMS2 EC should be performed to exclude sporadic cases prior to further PMS2 genetic testing.