How to Diagnose Lynch Syndrome
Diagnosis of Lynch syndrome requires a systematic three-tiered approach: initial risk assessment using clinical criteria or family history, followed by tumor tissue testing (immunohistochemistry and/or microsatellite instability testing), and confirmation with germline genetic testing of mismatch repair genes. 1
Step 1: Initial Risk Assessment and Screening
Clinical Criteria for Identifying At-Risk Individuals
Use Amsterdam II criteria or revised Bethesda Guidelines to identify individuals who warrant further testing. 1
Amsterdam II Criteria (3-2-1 rule): 1
- 3 relatives with Lynch syndrome-associated cancer (colorectal, endometrial, small bowel, ureter, or renal pelvis)
- At least 2 successive generations affected
- At least 1 diagnosed before age 50 years
- One relative must be a first-degree relative of the other two
- Familial adenomatous polyposis excluded
- Tumors verified by pathologic examination
Important caveat: Amsterdam II criteria miss up to 68% of patients with Lynch syndrome due to their stringent requirements. 1
Revised Bethesda Guidelines (broader, more sensitive): 1
- Colorectal cancer diagnosed before age 50 years
- Synchronous or metachronous colorectal or other Lynch syndrome-associated tumor
- Colorectal cancer with MSI-high histology (tumor-infiltrating lymphocytes, Crohn's-like lymphocytic reaction, mucinous/signet-ring differentiation, or medullary growth pattern) diagnosed before age 60 years
- Colorectal cancer with family history of Lynch syndrome-associated cancer diagnosed before age 50 years
- Colorectal cancer in more than 1 relative with Lynch syndrome-associated cancer, regardless of age
Alternative Approach: Universal Tumor Screening
Consider universal tumor screening for all newly diagnosed colorectal and endometrial cancers, as this approach improves detection rates compared to clinical criteria alone. 1 This strategy bypasses the limitations of family history-based criteria and is increasingly recommended in current practice. 2
Risk Prediction Models
When tumor tissue is unavailable, use risk prediction models such as PREMM(1,2,6) to estimate the probability of Lynch syndrome and guide decisions about germline testing. 1 Your doctor will need: 1
- Personal history of cancer and type(s)
- Age at which each family member was diagnosed with cancer
Step 2: Tumor Tissue Testing
When tumor tissue is available from a Lynch syndrome-associated cancer, perform both immunohistochemistry (IHC) and microsatellite instability (MSI) testing. 1
Immunohistochemistry (IHC)
IHC testing evaluates the presence or absence of the four mismatch repair proteins: MLH1, MSH2, MSH6, and PMS2. 1
- Absence of one or more proteins indicates which gene should be targeted for germline testing 1
- Approximately 10% of Lynch syndrome colorectal cancers show intact MMR protein staining, so MSI testing and clinical judgment must supplement IHC results 1
- IHC requires less tissue than MSI-PCR, making it preferable when tissue is limited 1
Microsatellite Instability (MSI) Testing
MSI-PCR assesses somatic changes in microsatellite length in tumor tissue, with high-level MSI (MSI-H) being the hallmark of mismatch repair deficiency. 1
- More than 90% of Lynch syndrome colorectal cancers show MSI-H 3
- However, 10-15% of sporadic colorectal cancers also demonstrate MSI-H, requiring additional testing to distinguish Lynch syndrome from sporadic cases 1
Critical Additional Testing for MLH1 Loss
When IHC reveals loss of MLH1 and PMS2 proteins, perform BRAF V600E mutation testing and/or MLH1 promoter hypermethylation analysis before proceeding to germline testing. 1, 3
- BRAF mutations are rare or absent in Lynch syndrome tumors 3
- MLH1 promoter hypermethylation indicates sporadic cancer and essentially excludes Lynch syndrome in most patients 1
- This step prevents unnecessary germline testing and false reassurance
Tissue Type Considerations
Colorectal or endometrial tumor tissue is preferred for Lynch syndrome evaluation, as these have been prospectively validated. 1 For upper tract urothelial carcinoma (UTUC): 1
- Consider testing in patients diagnosed before age 60 years
- Consider testing with family history of UTUC, colon cancer diagnosed before age 60, or endometrial cancer
- Prioritize IHC over MSI-PCR when tissue is limited from endoscopic procedures
Step 3: Germline Genetic Testing
Germline genetic testing of MMR genes (MLH1, MSH2, MSH6, PMS2) and EPCAM confirms the diagnosis when a pathogenic mutation is identified. 1, 3
When Tumor Testing Guides Genetic Testing
IHC results indicate which specific gene to test: 1
- Loss of MLH1/PMS2: Test MLH1 (after excluding BRAF mutation and MLH1 hypermethylation)
- Loss of MSH2/MSH6: Test MSH2 and evaluate for EPCAM deletions
- Loss of MSH6 alone: Test MSH6
- Loss of PMS2 alone: Test PMS2
When Tumor Tissue Is Unavailable
If tissue is not available but clinical suspicion remains high (family meets Amsterdam II criteria or high-risk features present), proceed directly to comprehensive germline genetic testing of all MMR genes. 1
Genetic Counseling
Refer to genetic counseling before or concurrent with germline testing to discuss: 1
- Risks and benefits of genetic testing
- Implications for family members (50% risk for offspring)
- Surveillance and management strategies
- Interpretation of results including variants of uncertain significance
Interpretation of Results
Positive Germline Testing
A pathogenic mutation in MLH1, MSH2, MSH6, PMS2, or EPCAM deletion confirms Lynch syndrome diagnosis. 1, 3
Negative or Inconclusive Results
When tumor shows MSI-H or loss of MMR proteins but no germline mutation is found: 1
- Consider "Lynch-like syndrome" diagnosis
- Maintain suspicion for Lynch syndrome
- Manage based on personal and family history
- Consider repeat testing as new genetic technologies emerge
Variants of uncertain significance are inconclusive findings that require periodic reassessment as laboratories update variant classifications. 1
Special Populations
Known Family Mutation
When a pathogenic mutation is already identified in the family, perform mutation-specific germline testing in at-risk relatives. 1
- Positive test confirms Lynch syndrome
- Negative test indicates average population risk
Patients with Upper Tract Urothelial Carcinoma
Consider Lynch syndrome testing in patients with UTUC who: 1
- Are diagnosed at age 60 years or younger
- Have family history of UTUC, colon cancer diagnosed before age 60, or endometrial cancer
- Have personal history of colon or endometrial cancer
Common Pitfalls to Avoid
Do not rely solely on Amsterdam criteria, as they miss up to 68% of Lynch syndrome cases. 1 The revised Bethesda Guidelines or universal tumor screening approaches are more sensitive. 1
Do not skip BRAF and MLH1 promoter testing when MLH1/PMS2 loss is detected, as this can lead to unnecessary germline testing and misdiagnosis. 1, 3
Do not assume normal tumor studies (intact IHC and MSI-stable) in non-colorectal or non-endometrial cancers exclude Lynch syndrome. 1 Colorectal or endometrial tissue is preferred for evaluation.
Do not forget that approximately 10% of Lynch syndrome colorectal cancers show intact MMR protein staining on IHC, requiring both IHC and MSI-PCR for complete evaluation. 1
Remember that only 5% of individuals with Lynch syndrome in the UK are aware of their diagnosis, emphasizing the importance of systematic screening approaches. 2