MLH1, MSH2, MSH6 and PMS2 are related by function, but they are not interchangeable labels on a genetic report. EPCAM contributes through a different mechanism. Understanding those relationships helps explain why a tumor pattern suggests a line of investigation without proving a particular inherited variant.
In this guide: What is mismatch repair? · A guide to the five gene names · What can tumor staining tell us? · How to read an actual sequence finding · A worked research-reading example · Questions worth taking to a specialist
What is mismatch repair?
When DNA is copied, occasional mismatches and small insertion/deletion errors occur. Mismatch repair helps identify and correct such errors. MSH2 can partner with MSH6 to recognize relevant mismatches; MLH1 and PMS2 form a complex involved in coordinating repair. These pairs participate in a larger system rather than working as isolated proteins. [1] [2] [3]
A pathogenic germline finding affecting one copy of an appropriate repair gene can cause Lynch syndrome predisposition. Changes affecting both copies of a mismatch repair gene can instead cause constitutional mismatch repair deficiency, a distinct disorder associated with cancers beginning in childhood. These situations need separate clinical evaluation and should not be merged under a generic “Lynch gene positive” label. [4] [3]
A guide to the five gene names
- MLH1: encodes a repair protein that partners with PMS2. Sequence changes and gene-regulation mechanisms must be distinguished when investigating absent tumor staining. [3]
- MSH2: encodes a protein involved in recognizing DNA replication errors as part of a protein complex. Its relationship with MSH6 helps explain some paired staining-loss patterns. [1]
- MSH6: forms a mismatch-recognition complex with MSH2. A specific finding still requires variant-level evidence; the gene name alone is not a classification. [2]
- PMS2: encodes a repair partner of MLH1. One-copy and two-copy disease-causing findings have different implications. [4]
- EPCAM: does not encode a mismatch repair protein. Certain deletions at its end can cause methylation and silencing of neighboring MSH2. Not every EPCAM variant has this Lynch syndrome mechanism. [5]
What can tumor staining tell us?
Immunohistochemistry (IHC) examines protein staining in tissue. Common patterns include paired MLH1/PMS2 loss, paired MSH2/MSH6 loss and isolated loss of MSH6 or PMS2. They help guide investigation, but exceptions occur. Microsatellite instability testing measures a related consequence of repair dysfunction, not the inherited sequence itself. [6]
Loss of MLH1/PMS2 can reflect tumor MLH1 methylation. MSH2/MSH6 loss may prompt consideration of MSH2 and relevant EPCAM mechanisms. Tumor-only changes can also produce repair deficiency. Appropriate follow-up may include methylation analysis, germline testing or paired tumor/normal assessment; the tumor type and clinical setting determine the pathway. [6]
Do not use a staining diagram as an automatic gene diagnosis. Ask the pathology and genetics teams whether staining controls were adequate, whether results are concordant and what the unresolved alternatives are. The companion Lynch syndrome guide explains what a confirmed inherited finding can mean for a family.
How to read an actual sequence finding
- Establish sample context. Was the result from a tumor, a constitutional sample or paired testing?
- Match the variant exactly. Preserve gene, transcript/version, DNA and protein notation, and any copy-number description.
- Read the classification. Separate pathogenic/likely pathogenic from uncertain significance and from benign findings.
- Match the mechanism. Ask whether the evidence applies to Lynch syndrome, another condition or a somatic tumor interpretation.
- Check the report’s limitations. Identify unresolved regions, variant classes, methylation mechanisms or technical challenges that matter to this question.
- Connect the evidence to the clinical assessment. Do not treat a database result as a substitute for the laboratory and genetics team’s interpretation.
The NCI distinguishes germline testing from tumor profiling and explains why a variant of uncertain significance generally should not determine clinical management. Keep uncertain findings visible as questions rather than converting them into a diagnosis. [7]
A worked research-reading example
Hypothetical case description: a study calls a tumor “MMR deficient” and reports loss of two proteins. Before classifying participants as having inherited Lynch syndrome, look for the germline methods, methylation work-up where relevant, and handling of tumor-only findings. If those details are missing, record “inherited status unresolved,” not “Lynch syndrome confirmed.”
Build a paper-extraction sheet with columns for tumor type, IHC result, MSI result, germline finding, relevant additional tests and the study’s final case definition. Keep missing data separate from negative tests. This is an original organizational exercise for reading evidence; it is not a diagnostic algorithm.
Use tumor versus germline testing for the basic distinction and the ACMG/AMP framework for variant-level interpretation. Our ClinVar discordance guide explains how to investigate conflicting assertions.
Questions worth taking to a specialist
- What did the tumor test demonstrate, and what did it leave unresolved?
- Has an inherited causative variant actually been established?
- Does the testing strategy cover the relevant gene and mechanism?
- What result, if any, can be used for targeted testing of relatives?
- Who will review an uncertain result or a discrepancy between tests?
The variant research assistant helps organize a precise database query. The literature tools support a documented search; the care finder links to professional directories. These tools do not determine whether a patient has Lynch syndrome.
Educational information. The gene map is a simplified conceptual illustration, not an exhaustive account of repair biology or a clinical testing protocol.