Microsatellite instability and mismatch repair testing evaluate whether a tumor has lost an important DNA error-correction system. Mismatch repair deficiency (dMMR) is usually measured by immunohistochemistry for the MLH1, PMS2, MSH2, and MSH6 proteins. Microsatellite instability-high (MSI-H) is measured by PCR or next-generation sequencing and reflects the DNA-length errors that accumulate when repair fails. The two results overlap strongly but are not identical. Testing is routine in colorectal and endometrial cancers and is used in many other tumors to identify candidates for immune checkpoint inhibitors. It also screens for possible Lynch syndrome, an inherited cancer predisposition caused by a germline variant in MLH1, MSH2, MSH6, PMS2, or an EPCAM deletion. A tumor MSI-H or dMMR result does not by itself diagnose Lynch syndrome because sporadic MLH1 promoter methylation and double somatic mutations are common. Conversely, a normal tumor screen cannot exclude every hereditary scenario. Correct interpretation requires the tumor type, protein-loss pattern, molecular method, family history, and appropriate reflex testing.
- MSI-H and dMMR are related but measured differently: MSI tests DNA instability, while MMR immunohistochemistry tests protein expression.
- MSI-H/dMMR can predict benefit from checkpoint immunotherapy in several advanced cancers.
- Tumor testing screens for Lynch syndrome but does not diagnose an inherited mutation by itself.
- MLH1/PMS2 loss in colorectal cancer often triggers BRAF V600E and/or MLH1 promoter methylation testing.
- MSS or pMMR means the assay did not find high instability or protein loss; it does not exclude all cancer risks or every immunotherapy indication.
Table of Contents
- How Mismatch Repair Works
- Who Should Be Tested
- IHC, PCR, and NGS Methods
- Interpreting dMMR and MSI Results
- Lynch Syndrome Reflex Testing
- Immunotherapy and Treatment
- Discordant and Difficult Results
- Follow-Up and Questions to Ask
How Mismatch Repair Works
DNA polymerase makes occasional mistakes when cells copy DNA. The mismatch repair system identifies single-base mismatches and small insertion/deletion loops, removes the incorrect segment, and fills it accurately. MSH2 pairs with MSH6 to recognize many errors; MLH1 pairs with PMS2 to coordinate repair. Loss of one partner can destabilize the other, producing characteristic immunohistochemistry patterns.
Microsatellites are short repeated DNA sequences such as one or two bases repeated many times. They are especially prone to copying slips. When mismatch repair fails, repeat lengths change across tumor cells. A tumor with instability at enough tested markers is classified MSI-H. Tumors without high instability are microsatellite stable (MSS); some PCR systems also report MSI-low, a category usually managed like MSS for current immunotherapy selection.
dMMR tumors accumulate many mutations, including frameshifts that create abnormal proteins called neoantigens. These can attract lymphocytes and make the cancer more visible to the immune system. The tumor may respond by activating PD-1/PD-L1 and other checkpoints, which helps explain sensitivity to checkpoint inhibitors.
MMR deficiency can arise from an inherited germline variant, an acquired epigenetic change such as MLH1 promoter methylation, or two acquired mutations affecting both copies of an MMR gene. The tumor phenotype is similar, but the family implications are very different. This is why tumor testing starts a diagnostic pathway rather than completing it.
Who Should Be Tested
Universal MMR or MSI screening is recommended for newly diagnosed colorectal cancer and endometrial cancer in many major guidelines. Testing identifies possible Lynch syndrome regardless of age or family history and provides prognostic and treatment information. Selective testing based only on young age misses families.
MSI/MMR testing is also important in gastric and gastroesophageal cancer, small-bowel cancer, pancreatic and biliary cancers, prostate cancer, ovarian cancer, and many other advanced tumors when immunotherapy eligibility or a hereditary syndrome is being considered. Broad NGS panels often report MSI status alongside tumor mutational burden and gene variants.
Common reasons include:
- universal Lynch screening in colorectal or endometrial cancer;
- treatment selection in unresectable, recurrent, or metastatic cancer;
- unusual personal or family history of Lynch-associated cancers;
- multiple primary tumors;
- a pathology pattern suggestive of MSI, such as prominent lymphocytes;
- clarification of an inherited MMR variant;
- evaluation of resistance or a new primary tumor.
In colorectal cancer, MMR status can influence adjuvant treatment discussions in stage II disease and immunotherapy in advanced disease. In rectal and colon cancers with localized dMMR disease, neoadjuvant checkpoint strategies have produced striking responses in trials, but organ-preserving management should follow specialized protocols rather than be inferred from a test alone.
Testing normal blood for Lynch syndrome and testing tumor for dMMR/MSI are different. Germline testing may be ordered directly when family history is compelling, but tumor results often help identify which gene and mechanism to investigate.
IHC, PCR, and NGS Methods
MMR immunohistochemistry (IHC) stains tumor nuclei for MLH1, PMS2, MSH2, and MSH6. Normal surrounding cells, lymphocytes, and stromal cells provide internal positive controls. Complete loss in tumor nuclei with intact control staining is abnormal. IHC suggests which protein pair is affected and can direct germline testing.
MSI-PCR compares repeat markers in tumor and often normal DNA. Modern mononucleotide panels are sensitive in colorectal cancer and many other tumors. The result is MSI-H, MSI-low, or MSS according to the validated marker threshold.
NGS-based MSI analyzes many microsatellite loci, sometimes without matched normal tissue. It can report MSI along with mutations, copy number, fusions, and TMB. Performance depends on tumor type, panel design, bioinformatics, and tumor purity.
| Method | Main output | Strength | Limitation |
|---|---|---|---|
| Four-protein MMR IHC | pMMR or loss pattern | Fast, inexpensive, and points to a gene | Missense proteins may stain despite loss of function |
| MSI-PCR | MSI-H, MSI-low, or MSS | Directly measures instability | Sensitivity can vary by tumor type and panel |
| NGS MSI | MSI status within a broad profile | Efficient when sequencing is already needed | Requires validated algorithms and enough tumor |
| Germline testing | Inherited pathogenic variant or other result | Diagnoses Lynch syndrome | Does not by itself prove the tumor’s second hit or phenotype |
IHC and MSI are often concordant, but neither is perfect. The College of American Pathologists provides tumor-specific recommendations on which assay is preferred for checkpoint-inhibitor selection. For colorectal cancer, either validated MMR IHC or MSI-PCR is widely used; NGS can be appropriate when validated.
Interpreting dMMR and MSI Results
A dMMR result means one or more MMR proteins are absent in tumor nuclei with valid controls. The loss pattern suggests the next step:
| IHC pattern | Common explanation | Typical follow-up |
|---|---|---|
| MLH1 and PMS2 loss | MLH1 is inactivated; often sporadic methylation | MLH1 promoter methylation and, in colorectal cancer, BRAF V600E testing |
| MSH2 and MSH6 loss | MSH2 is inactivated | Germline MSH2 and EPCAM evaluation, often multigene testing |
| Isolated MSH6 loss | MSH6 is inactivated | Germline MSH6 evaluation |
| Isolated PMS2 loss | PMS2 is inactivated | Germline PMS2 evaluation with methods suited to pseudogenes |
A pMMR result means all four proteins show retained nuclear staining. It does not prove every MMR gene is normal. Some pathogenic missense variants produce a stable but nonfunctional protein, and preanalytic or interpretive issues can occur.
An MSI-H result means instability exceeded the validated threshold. It commonly corresponds to dMMR and can be used for immunotherapy selection in appropriate cancers. An MSS result means no high-level instability was found. MSI-low is generally grouped with MSS for current treatment decisions unless a specific protocol says otherwise.
Results should not be described as “positive for Lynch syndrome.” The correct statement is that the tumor is dMMR/MSI-H and needs evaluation for an inherited or sporadic cause. The Lynch syndrome genetic test examines germline DNA and answers the hereditary question.
Lynch Syndrome Reflex Testing
When MLH1 and PMS2 are absent, sporadic MLH1 promoter methylation is common, especially in older adults with colorectal or endometrial cancer. In colorectal cancer, BRAF V600E strongly supports a sporadic methylated pathway because it is very uncommon in Lynch-associated colorectal tumors. BRAF is not a reliable substitute for methylation testing in endometrial cancer.
If MLH1 promoter methylation is absent, germline testing is usually recommended. If methylation is present but the person is young or has a striking family history, genetics review may still be appropriate because constitutional MLH1 epimutation and unusual scenarios exist.
MSH2/MSH6 loss directs attention to MSH2 and EPCAM. A deletion at the end of EPCAM can silence the neighboring MSH2 gene in tissues that express EPCAM. Isolated MSH6 or PMS2 loss generally points to that gene, but germline multigene panels are often used because patterns are not infallible.
If germline testing is negative and the tumor is dMMR, paired tumor-normal sequencing can look for two somatic hits. Finding a convincing double-somatic mechanism often explains a “Lynch-like” tumor and lowers the likelihood of an inherited syndrome, though family history still matters. If no cause is found, management may remain individualized.
A pathogenic germline result has implications for colonoscopy, gynecologic risk management, screening for other Lynch-associated cancers, and cascade testing of relatives. A variant of uncertain significance should not be used to diagnose Lynch syndrome or test relatives as if it were pathogenic.
Immunotherapy and Treatment
MSI-H/dMMR is a powerful predictive biomarker for PD-1 blockade. Pembrolizumab has tumor-agnostic indications for selected unresectable or metastatic MSI-H/dMMR solid tumors, and disease-specific approvals include colorectal, endometrial, gastric, and other cancers. Nivolumab, dostarlimab, and combination checkpoint regimens may be options in particular settings.
In metastatic colorectal cancer, checkpoint inhibitors can be used first line for MSI-H/dMMR disease under current standards. Responses can be durable, but not every tumor responds. BRAF, RAS, tumor burden, symptoms, organ function, autoimmune disease, transplant history, and need for rapid control influence treatment planning.
In endometrial cancer, MMR status helps select immunotherapy alone or with chemotherapy in advanced or recurrent disease. In gastric and other GI cancers, MSI-H may predict substantial benefit and can interact with PD-L1, HER2, and treatment line.
Localized dMMR rectal cancer has shown remarkable complete clinical responses to neoadjuvant PD-1 blockade in prospective studies, and dMMR colon cancer has shown high pathologic response rates. These results are changing specialist practice, but surveillance, duration, surgery decisions, and long-term outcomes require protocol-based multidisciplinary care.
Immune-related adverse effects can involve any organ. The biomarker predicts potential benefit, not safety. A person with MSI-H cancer can still experience severe colitis, hepatitis, pneumonitis, endocrinopathy, nephritis, myocarditis, or neurologic toxicity.
Discordant and Difficult Results
Discordance includes dMMR by IHC with MSS by PCR, retained proteins with MSI-H, or differing results between laboratories. Causes include low tumor content, poor fixation, heterogeneous or subclonal protein loss, an assay not optimized for that tumor type, recent treatment, interpretation error, and unusual missense variants.
The first step is pathology review. Confirm that internal controls were intact, the correct tumor area was tested, and the tissue was not severely damaged. Repeating IHC on another block and performing an orthogonal MSI method can resolve many cases. A next-generation sequencing test may identify MMR gene mutations, MSI, and TMB together.
MSH6 loss can appear after temozolomide treatment in glioma and may not produce classic MSI-H. Neoadjuvant therapy can alter staining. Endometrial and noncolorectal tumors may show lower levels or different patterns of instability than colorectal cancer, so tumor-specific validation matters.
A pathogenic germline MMR variant with a pMMR/MSS tumor may mean the cancer is sporadic and unrelated to Lynch syndrome, or it may reflect testing limitations. The germline diagnosis remains important for future cancer risk even if one tumor does not show the expected second hit.
When immunotherapy depends on the result, unresolved discordance should be reviewed by a molecular pathologist or tumor board. Choosing whichever result is more convenient can expose a patient to ineffective therapy or deny a useful treatment.
Follow-Up and Questions to Ask
Request the complete pathology and genetics pathway, not only “MSI positive.” Ask:
- Was testing done by four-protein IHC, PCR, NGS, or more than one method?
- Which MMR proteins were lost, and were internal controls valid?
- Is the tumor MSI-H, MSI-low, or MSS?
- If MLH1/PMS2 were lost, were MLH1 methylation and appropriate BRAF testing performed?
- Does the result meet the assay requirement for the proposed immunotherapy?
- Should I have germline genetic counseling and a Lynch syndrome panel?
- If germline testing is negative, would paired tumor-normal sequencing look for double-somatic changes?
- Do relatives need testing now, or only after a germline pathogenic variant is confirmed?
- How does MMR/MSI affect surgery, chemotherapy, immunotherapy, or surveillance in this cancer type?
- Are the IHC and MSI results discordant, and does the case need expert review?
Keep copies of tumor IHC, MSI, methylation, BRAF, germline, and paired tumor-normal reports. Each answers a different question. A complete chain of evidence prevents a sporadic tumor from being mislabeled inherited and prevents Lynch syndrome from being missed after an abnormal screen.
Examples of complete reflex pathways
For a colorectal cancer with MLH1/PMS2 loss, the laboratory or clinical team typically orders MLH1 promoter methylation and/or BRAF V600E. If BRAF V600E is present or MLH1 is methylated, a sporadic pathway is likely. If both are absent, germline testing becomes more important. The person’s age and family history can justify genetics review even when methylation is detected.
For an endometrial cancer with MLH1/PMS2 loss, MLH1 methylation is the preferred sporadic-cause test. BRAF V600E is not a useful substitute because the colorectal association does not carry over. If unmethylated, germline MLH1 testing is considered. Tumor morphology and age may add context but should not override the molecular pathway.
For MSH2/MSH6 loss, germline testing should include MSH2 and EPCAM deletion analysis, usually within a multigene hereditary cancer panel. If germline testing is negative, paired tumor-normal sequencing may show two acquired MSH2 hits. Isolated MSH6 and PMS2 loss similarly guide attention to their respective genes, while acknowledging occasional atypical patterns.
Why tumor and germline terminology must stay separate
“Positive MSI” describes tumor instability. “Pathogenic germline MSH6 variant” diagnoses inherited Lynch syndrome. “Double-somatic MSH2 mutations” explains a tumor phenotype without proving inherited risk. Keeping these phrases separate prevents unnecessary alarm for relatives and avoids missing the need for cascade testing when a germline result is truly positive.
A germline VUS is inconclusive. Relatives should not undergo predictive testing for the VUS as though it were pathogenic, and surgery or surveillance should not be escalated solely because of it. Management can still follow the family history while the variant is monitored for reclassification.
Immunotherapy response and resistance
MSI-H/dMMR tumors are enriched for checkpoint-inhibitor response, but primary resistance occurs. Loss of antigen presentation, beta-2 microglobulin alterations, JAK signaling defects, immunosuppressive microenvironment, and misclassified laboratory results can contribute. A patient whose cancer progresses should not be told the test was “wrong” without reviewing these possibilities.
Acquired resistance can develop after an initial durable response. A progressing lesion may be biopsied to confirm cancer, evaluate histologic transformation, and profile resistance. Oligoprogression can sometimes be treated locally while immunotherapy continues; widespread progression usually requires a new systemic strategy.
In localized disease, a complete clinical or pathologic response does not eliminate the need for disciplined follow-up. Nonoperative management of rectal cancer requires intensive endoscopy, MRI, examination, and imaging under an experienced program. The molecular result creates an opportunity but does not make surveillance optional.
Family communication and records
When Lynch syndrome is confirmed, relatives need a copy of the exact gene and variant, not the tumor MSI report. Adult first-degree relatives generally have a 50% chance of carrying an autosomal dominant pathogenic variant. Testing an unaffected relative for the known familial variant is more informative than ordering a broad tumor-style panel.
Keep a single file containing the IHC image or report, MSI method and result, MLH1 methylation, BRAF if relevant, germline report, paired tumor-normal report, and genetics letter. This chain explains both treatment eligibility and family risk and prevents duplicate or contradictory testing years later.
Preanalytic and pathology quality checks
For IHC, the tumor must be compared with intact internal controls. If lymphocytes and normal glands also lack staining, the run failed and the result is uninterpretable. Patchy weak staining can reflect fixation or a true subclonal pattern. Complete nuclear loss in a discrete tumor population may deserve repeat staining and molecular analysis rather than being averaged with retained areas.
Biopsies after chemotherapy or radiation can show reduced tumor and altered protein expression. Testing the pretreatment specimen may be more reliable, but a current sample can reveal acquired changes. The pathology team should know treatment history and select the block with the best viable tumor.
For PCR and NGS, tumor percentage matters. Too many normal cells can dilute unstable alleles and create an MSS call. Macrodissection, matched normal comparison, and a validated tumor-specific algorithm improve sensitivity. A report should not hide failed quality metrics behind a simple negative label.
Surveillance after a Lynch diagnosis
A confirmed germline result leads to gene- and sex-specific surveillance. Colonoscopy usually begins earlier and occurs more often than average-risk screening. Management of endometrial and ovarian risk can include symptom awareness, individualized screening discussions, and risk-reducing surgery after childbearing. Gastric, urinary tract, pancreatic, prostate, skin, and brain surveillance depend on gene, family history, and guideline.
Aspirin may reduce colorectal cancer risk in Lynch syndrome, but dose and duration should be discussed with a clinician because bleeding risk, age, and comorbidities matter. The tumor MSI result alone is not a reason to start aspirin.
Relatives should receive genetic counseling that explains autosomal dominant inheritance and the limits of testing minors. Most adult-onset Lynch testing is deferred until results would change surveillance. A negative relative test for the known familial variant returns that person to risk based on the remaining personal and family history rather than the Lynch mutation.
When the result is used for both family risk and treatment
One tumor can generate two parallel care pathways. Oncology uses dMMR/MSI-H to select systemic or neoadjuvant therapy. Genetics investigates whether the repair defect is inherited. These pathways should communicate but not wait unnecessarily for each other. A patient with advanced MSI-H cancer may need immunotherapy while germline testing is still pending.
Conversely, a person can have Lynch syndrome even when the current tumor does not qualify for an MSI-based immunotherapy. Treatment follows the tested tumor phenotype, while lifelong surveillance follows the germline result. Keeping both records avoids confusing inherited risk with drug eligibility.
References
- Mismatch Repair and Microsatellite Instability Testing for Immune Checkpoint Inhibitor Therapy 2022 (Guideline)
- Microsatellite instability: A 2024 update 2024 (Review)
- Mismatch repair deficiency and microsatellite instability in the era of immunotherapy 2024 (Review)
- Deficient Mismatch Repair and Microsatellite Instability in Solid Tumors 2025 (Review)
- Lynch Syndrome 2025 (Official Fact Sheet)
Disclaimer
This article provides general education and cannot diagnose Lynch syndrome or determine immunotherapy eligibility. Tumor IHC/MSI, reflex methylation or BRAF testing, germline analysis, pathology, and current treatment guidance must be integrated by oncology and genetics professionals. New severe symptoms during checkpoint therapy require prompt medical assessment.
