Abstract
Mismatch repair protein (MMR) immunohistochemistry is an important tool in screening for Lynch syndrome in colorectal cancer patients. Unusual staining patterns such as heterogeneous MSH6 staining have been reported in colorectal and endometrial cancers. We aim to better understand MSH6 staining heterogeneity in colorectal cancer by comparative sequencing of different tumor areas for MMR and DNA polymerase mutations. Whole-section slides of 1754 colorectal cancers were reviewed for heterogeneous MSH6 staining, defined as discrete tumor areas with abrupt loss of staining juxtaposed to tumor areas with retained staining. Nine cases (0.05%) demonstrated heterogeneous MSH6 staining; none received neoadjuvant therapy prior to the specimen collection. The area of tumor with loss of MSH6 expression ranged from 5% to 60% (average 22%). Four cases had enough tissue remaining in both retained and lost MSH6 areas to perform tumor sequencing on both areas. All 9 cases were negative for MSH6 germline mutation; MSH6 heterogeneous staining was seen in tumors with MLH1 or PMS2 abnormalities (6 cases of MLH1 methylation, 2 PMS2 germline mutation, 1 MLH1 germline mutation). In addition, case 1 also had a somatic POLD1 exonuclease domain mutation (p.Y405C) in the MSH6 loss area but not in the intact area. We recommend reporting MSH6 heterogeneous pattern as MSH6 staining is present with a comment stating that the heterogeneous pattern typically does not indicate germline mutation in MSH6 but is commonly associated with abnormality in another MMR gene such as MLH1 or PMS2, or even other DNA repair genes such as DNA polymerase.