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Granular dot-like staining with MLH1 immunohistochemistry is a clone-dependent artefact
S Dasgupta1, P C Ewing-Graham1, F H Groenendijk1
1Department of Pathology, Erasmus MC, University Medical Centre Rotterdam, the Netherlands.
Pathology, Research and Practice
|August 13, 2019
Summary
A specific MLH1 antibody clone can cause a false-positive result in microsatellite instability (MSI) testing for colorectal and endometrial cancers. This immunohistochemistry artifact can lead to misdiagnosis and unnecessary genetic testing referrals.
Area of Science:
- Oncology
- Pathology
- Molecular Diagnostics
Background:
- Immunohistochemistry (IHC) for DNA mismatch repair (MMR) proteins MLH1, PMS2, MSH2, and MSH6 is crucial for microsatellite instability (MSI) screening in colorectal carcinoma (CRC) and endometrial carcinoma (EC).
- Loss of PMS2 with retained MLH1 staining in IHC is a key indicator for potential germline PMS2 gene mutations, necessitating genetic testing.
Purpose of the Study:
- To report a significant pitfall in immunohistochemical interpretation during MSI screening.
- To highlight a clone-dependent artifact in MLH1 staining that can mimic true positivity.
Main Methods:
- Retrospective review of MLH1 immunohistochemistry (IHC) staining in endometrial carcinoma (EC) and colorectal carcinoma (CRC) cases.
- Comparison of MLH1 IHC results using different antibody clones (M1-clone and ES05-clone).
- Correlation of IHC findings with molecular testing for MLH1 promoter methylation.
Main Results:
- An initial MLH1-IHC interpretation in an EC case showed a granular, dot-like, nuclear staining with the M1-clone, initially suggesting MLH1 positivity.
- Repeating MLH1 IHC with a different clone (ES05-clone) revealed complete negativity.
- Molecular testing confirmed MLH1 promoter methylation in the EC case.
- The same dot-like MLH1 staining artifact using the M1-clone was identified in two archived CRC cases.
Conclusions:
- The granular, dot-like MLH1 staining pattern observed with the M1-clone is a clone-dependent artifact, not indicative of true MLH1 expression.
- Awareness of this specific immunohistochemistry artifact is essential to prevent reporting errors in MSI screening.
- Recognizing this pitfall can avoid unnecessary genetic testing referrals for germline mutations.
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