Colorectal Cancer - Mismatch Repair and MSI Testing

Key Points

• MMR IHC and MSI testing are established biomarkers in colorectal cancer.
• MLH1/PMS2 and MSH2/MSH6 are functional protein pairs, so loss patterns provide etiologic clues.
• Internal positive controls are required to call protein loss by IHC.
• Combined MLH1/PMS2 loss often requires reflex assessment for sporadic MLH1 promoter methylation in colorectal cancer.
• MSH2/MSH6 loss, isolated MSH6 loss, or isolated PMS2 loss raises Lynch syndrome concern.
• BRAF V600E can support a sporadic MLH1-deficient colorectal cancer but is not a universal hereditary-cancer test.
• dMMR and MSI-high are closely related but discordant results occur.
• MSI-high/dMMR status predicts benefit from immune checkpoint inhibition in appropriate colorectal cancer settings.
• Tumor testing is a screening/biomarker step; Lynch syndrome is a germline diagnosis.
• Genetic counseling and cascade testing are important when Lynch syndrome is confirmed.

Algorithm

1. Perform validated MMR IHC or MSI testing on the colorectal carcinoma according to institutional universal-screening policy.
2. Verify adequate tumor and internal controls before interpreting IHC.
3. Classify intact versus lost expression for MLH1, PMS2, MSH2, and MSH6.
4. For MLH1/PMS2 loss, perform reflex MLH1 promoter methylation and/or BRAF V600E testing according to colorectal Lynch algorithm.
5. Refer patterns suspicious for Lynch syndrome to genetics for germline evaluation.
6. If germline testing is negative but dMMR remains unexplained, consider tumor sequencing for double-somatic alterations.
7. Report MMR/MSI result clearly for oncology treatment planning.
8. Resolve discordant IHC and MSI results through pathology review and orthogonal testing when clinically important.
9. Document implications for hereditary-cancer evaluation separately from treatment biomarker interpretation.

Clinical Synopsis & Reasoning

DNA mismatch repair corrects errors that arise during replication. Loss of MLH1, PMS2, MSH2, or MSH6 function allows insertion/deletion errors to accumulate at microsatellite sequences, producing microsatellite instability. In colorectal cancer, MMR status is commonly evaluated by immunohistochemistry for the four proteins or by a validated DNA-based MSI assay. The methods are related but not identical; discordant results require pathology review and sometimes repeat or orthogonal testing.

IHC interpretation relies on paired protein biology. MLH1 stabilizes PMS2, so loss of MLH1 usually causes combined MLH1/PMS2 loss. MSH2 stabilizes MSH6, so MSH2 loss usually causes combined MSH2/MSH6 loss. Isolated PMS2 loss points toward PMS2 dysfunction, and isolated MSH6 loss points toward MSH6 dysfunction. Intact internal control staining is mandatory; absent staining in tumor is uninterpretable if normal stromal or lymphocyte nuclei also fail to stain. Heterogeneous or weak patterns may reflect fixation, treatment, subclonal biology, or true abnormality and should be reviewed rather than forced into a simple category.

Combined MLH1/PMS2 loss in colorectal carcinoma is frequently sporadic due to MLH1 promoter hypermethylation, especially in older patients. Reflex MLH1 promoter methylation and/or BRAF V600E testing, according to local Lynch-screening algorithm, helps identify many sporadic tumors. BRAF V600E strongly favors a sporadic MLH1-deficient colorectal cancer and is unusual in Lynch syndrome, but methylation testing is more directly linked to the mechanism. These reflex rules are colorectal specific and should not be generalized to every tumor type.

Loss of MSH2/MSH6, isolated MSH6, isolated PMS2, or MLH1/PMS2 loss without a sporadic methylation explanation should prompt hereditary-cancer evaluation based on current criteria. Germline testing can identify Lynch syndrome, but some tumors have double-somatic MMR alterations rather than germline disease. Tumor sequencing can help resolve unexplained dMMR cases after appropriate genetic counseling.

For immunotherapy selection, CAP's MMR/MSI guideline supports validated methods for colorectal cancer. MSI-high/dMMR status is a predictive biomarker for immune checkpoint inhibitors in advanced disease and increasingly influences earlier-stage therapeutic strategies. Results must be clearly reported as proficient/intact versus deficient/lost or MSI-stable/low versus MSI-high according to assay validation.

Treatment Strategy & Disposition

Management implications differ by stage and by reason for testing. In localized colorectal cancer, dMMR/MSI-high status has prognostic and treatment-selection implications, particularly in stage II disease and in evolving neoadjuvant strategies. In metastatic colorectal cancer, MSI-high/dMMR status can make immune checkpoint blockade a preferred systemic approach in appropriate patients. Exact regimen selection follows current oncology guidelines rather than the pathology report alone.

A possible Lynch syndrome pattern triggers genetic counseling and germline testing, not prophylactic surgery based solely on tumor IHC. If Lynch syndrome is confirmed, surveillance extends beyond the treated colorectal cancer and can include intensified colonoscopy and screening/risk-reduction strategies for associated cancers, tailored to the specific gene, sex, family history, and guideline.

Family members should not be tested on the basis of tumor IHC alone when the proband's germline status is unresolved. Once a pathogenic germline variant is confirmed, cascade testing can identify relatives who would benefit from surveillance. Tumors with unexplained dMMR after negative germline testing may need somatic analysis to distinguish double-somatic disease from occult hereditary mechanisms.

Epidemiology / Risk Factors

• Approximately 10 to 15 percent of colorectal cancers show dMMR/MSI-high biology, with prevalence varying by stage and population.
• A substantial fraction of MLH1/PMS2-deficient colorectal cancers are sporadic rather than Lynch syndrome.
• Lynch syndrome is one of the most common hereditary cancer syndromes.
• MMR-deficient tumors are enriched in right-sided colorectal cancers and can have characteristic histologic features.
• Universal tumor screening has improved identification of Lynch syndrome compared with family-history criteria alone.

Investigations

Common Patterns

Pharmacology

Prognosis / Complications

• Localized dMMR colorectal cancers can have stage-specific favorable prognostic features, though prognosis depends on the full stage and tumor biology.
• MSI-high metastatic colorectal cancer can respond deeply and durably to immune checkpoint therapy.
• Identifying Lynch syndrome can prevent future cancers through surveillance and risk-reduction in the patient and relatives.
• Failure to recognize a hereditary pattern can lead to missed cascade testing opportunities.
• Double-somatic dMMR tumors generally do not confer the same inherited family risk as germline Lynch syndrome.

Patient Education / Counseling

• Explain that MMR/MSI testing serves two purposes: it can guide cancer treatment and can signal possible inherited Lynch syndrome.
• Clarify that an abnormal tumor test does not automatically mean the patient inherited a mutation.
• If genetics referral is recommended, explain how germline testing differs from testing the tumor itself.
• Patients with confirmed Lynch syndrome should be offered gene-specific surveillance and family cascade testing.
• Encourage patients to retain copies of both tumor biomarker and germline reports because the distinction matters for relatives and future treatment.

Notes

CAP's 2026 colorectal and multi-tumor quality measures continue to emphasize documented MMR or MSI status in primary colorectal carcinoma. For treatment selection, CAP's evidence-based guideline supports validated IHC and molecular approaches and highlights that assay performance can be tumor-type dependent.

Tumor heterogeneity, low tumor cellularity, poor fixation, and treatment effect can produce difficult cases. A pathologist should select the best block and review discordant findings before a clinically consequential "proficient" or "deficient" label is finalized.

The report should separate three concepts: the assay result, the treatment implication, and the hereditary-risk implication. Collapsing them into one sentence can lead patients and clinicians to mistake dMMR for a confirmed germline syndrome or to miss a treatment-relevant MSI result.

References

1. College of American Pathologists MMR/MSI Testing Guideline — Link
2. CAP 2026 MMR or MSI Quality Measure — Link