Fig. 1: Interpretation of atypical MLH1 immunohistochemistry can be facilitated by the addition of PMS2 immunohistochemistry. | Modern Pathology

Fig. 1: Interpretation of atypical MLH1 immunohistochemistry can be facilitated by the addition of PMS2 immunohistochemistry.

From: Detecting mismatch repair deficiency in solid neoplasms: immunohistochemistry, microsatellite instability, or both?

Fig. 1

In case 1 (A, B), a right colon adenocarcinoma, the staining for MLH1 (A) appears variable with most tumor cells having a staining intensity weaker than the internal control or no staining, whereas the staining for PMS2 (B) is lost. In this scenario, the loss of PMS2 serves as a surrogate marker for MLH1 abnormality. This patient was indeed found to carry a deleterious germline MLH1 mutation. In case 2 (C, D), another colonic adenocarcinoma, the variable staining of MLH1 (C) is accompanied by normal staining of PMS2 (D) as well as normal staining of MSH2 and MSH6 (not shown). The normal PMS2 staining suggests that the variable MLH1 staining is likely due to technical or other clinically inconsequential etiologies. This tumor was microsatellite stable.

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