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Difficulty: MediumGenetic and Molecular Mechanisms

A 45-year-old man undergoes a right hemicolectomy for a colon adenocarcinoma. His family history is notable for a father who died of colorectal cancer at age 49 and a sister diagnosed with endometrial cancer at age 46. The physician suspects Lynch syndrome and plans a comprehensive molecular and genetic evaluation.

Assuming this patient's cancer is ultimately found to be caused by a germline mutation in the MLH1 gene, arrange the following diagnostic and management steps in the most appropriate chronological order.

  1. 1Perform immunohistochemical (IHC) staining or microsatellite instability (MSI) testing on the resected tumor tissue
  2. 2Evaluate the tumor tissue for a BRAF V600E mutation and MLH1 promoter hypermethylation
  3. 3Obtain a peripheral blood sample for germline sequencing and deletion/duplication analysis of the MLH1 gene
  4. 4Offer targeted, single-site predictive genetic testing to the patient's asymptomatic first-degree relatives

Answer

The correct sequence is: Tumor IHC/MSI testing -> Tumor BRAF/MLH1 methylation testing -> Germline MLH1 sequencing -> Cascade testing of relatives.
The correct sequence follows the standard molecular diagnostic algorithm for Lynch syndrome. Initial evaluation begins with tumor tissue screening via immunohistochemistry (IHC) or microsatellite instability (MSI) testing. Because loss of MLH1 is frequently caused by sporadic somatic hypermethylation of the MLH1 promoter (often correlated with a BRAF V600E mutation), tumor tissue must be tested for these somatic markers to rule out sporadic disease. Once a sporadic etiology is excluded, a peripheral blood sample is used for germline sequencing of the specific mismatch repair gene to confirm the hereditary diagnosis. Only after a specific pathogenic germline variant is definitively identified in the index patient can targeted predictive cascade testing be offered to at-risk family members.

Step-by-Step Solution

1
Determine the initial screening test for a suspected hereditary cancer syndrome when tumor tissue is available.
Tumor immunohistochemistry (IHC) or microsatellite instability (MSI) testing is indicated.
Tumor screening identifies if the mismatch repair pathway is defective and which specific protein is involved, guiding further testing.
2
Identify the next step if MLH1 expression is lost on IHC.
Test the tumor for BRAF V600E mutation and MLH1 promoter hypermethylation.
Loss of MLH1 is most commonly due to sporadic somatic promoter hypermethylation. Testing for this (and the associated BRAF mutation) differentiates sporadic cases from true suspected Lynch syndrome.
3
Select the appropriate test to confirm the hereditary diagnosis once sporadic causes are ruled out.
Germline sequencing and deletion/duplication analysis of the MLH1 gene from a blood sample.
Germline testing confirms the presence of an inherited pathogenic variant, establishing the diagnosis of Lynch syndrome.
4
Determine the management of family members after a germline mutation is confirmed.
Targeted predictive cascade testing for the specific variant in first-degree relatives.
Once the familial mutation is known, relatives can be tested for that specific variant to determine their cancer risk and screening needs.

Key Concept

The diagnostic algorithm for Lynch syndrome requires sequential molecular testing: initial tumor screening (IHC/MSI), exclusion of sporadic causes (BRAF/methylation if MLH1 is absent), confirmatory germline sequencing, and finally, targeted cascade testing for at-risk relatives.
Estimated Time:1m 30s
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Topics

Genetic and Molecular Mechanisms

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