Punctate MLH1 mismatch repair immunostaining in colorectal cancer.

We read with interest the description by Niu et al of punctate immunostaining for the mismatch repair (MMR) protein MLH1 in a small series of cases of endometrial cancer, in association with complete loss of PMS2 immunostaining. (1) The authors conclude that unawareness of this rare pattern of aberrant MLH1 staining can result in erroneous reporting of such cases as isolated loss of PMS2 staining, interpreted as representing likely Lynch syndrome due to germline mutation, whereas in fact somatic hypermethylation of the MLH1 promoter is the likely underlying molecular mechanism leading to defective mismatch repair in most such cases. We have observed this phenomenon similarly in a series of colon cancers, to the best of our knowledge not previously described in peer-reviewed literature, and herein describe our series, which support the conclusions of Niu et al. This article is protected by copyright. All rights reserved.