A 28-year-old male with a family history of early-onset colorectal cancer and polyposis presents for genetic screening. Molecular analysis suggests a defect in mismatch repair (MMR) genes. Which investigation would be the most appropriate initial confirmatory test for Lynch Syndrome?

Explanation

## Diagnosis of Lynch Syndrome (Hereditary Non-Polyposis Colorectal Cancer) **Key Point:** Lynch Syndrome is an autosomal dominant predisposition to colorectal and other cancers caused by germline mutations in mismatch repair (MMR) genes (MLH1, MSH2, MSH6, PMS2). The hallmark molecular feature is **microsatellite instability (MSI)**. ### Investigation of Choice: Microsatellite Instability (MSI) Analysis **Microsatellite instability (MSI) by PCR** is the most appropriate initial confirmatory test: 1. **Principle:** Microsatellites are short tandem repeats (STRs) scattered throughout the genome. Defective MMR allows these repeats to accumulate mutations, creating a characteristic **MSI-High (MSI-H)** pattern. 2. **Method:** PCR amplification of a panel of microsatellite markers (typically 5 markers: BAT25, BAT26, D2S123, D5S346, D17S250) compares tumor DNA (or blood DNA in germline screening) to normal tissue. 3. **Interpretation:** - **MSI-H:** ≥2 of 5 markers show instability → highly suggestive of Lynch Syndrome - **MSI-L:** 1 of 5 markers unstable → intermediate risk - **MSS (Microsatellite Stable):** no instability → Lynch Syndrome less likely 4. **Advantages:** - Rapid (results in days) - Cost-effective - High sensitivity (~90%) and specificity for MMR defects - Can be performed on blood or tumor tissue **High-Yield:** MSI analysis is the **recommended first-line screening test** by ASCO, NCCN, and international guidelines (e.g., Bethesda criteria) before proceeding to genetic sequencing. ### Diagnostic Algorithm for Lynch Syndrome ```mermaid flowchart TD A[Clinical suspicion: early CRC + family history]:::outcome --> B[MSI analysis by PCR]:::action B --> C{MSI-H?}:::decision C -->|Yes| D[Immunohistochemistry for MMR proteins]:::action C -->|No| E[Lynch Syndrome unlikely]:::outcome D --> F{Protein loss?}:::decision F -->|Yes| G[Genetic sequencing of deficient MMR gene]:::action F -->|No| H[Possible Lynch Syndrome, consider sequencing]:::action G --> I[Confirm germline mutation]:::outcome H --> I ``` ### Comparison of Confirmatory Tests | Test | Principle | Timing | Cost | Sensitivity | When Used | |---|---|---|---|---|---| | **MSI-PCR** | Detects microsatellite repeat instability | 2–3 days | Low | ~90% | **First-line screening** | | **IHC for MMR proteins** | Identifies loss of MLH1, MSH2, MSH6, PMS2 | 3–5 days | Moderate | ~85% | Confirmatory after MSI-H | | **Genetic sequencing (WGS/targeted panel)** | Identifies specific germline mutations | 1–2 weeks | High | ~95% | Definitive diagnosis | | **SSCP analysis** | Detects single-strand conformational changes | 3–4 days | Low | ~70% | Outdated; rarely used now | **Clinical Pearl:** A positive MSI-H result should trigger **IHC for MMR proteins** to identify which protein is deficient, which then guides targeted genetic sequencing of the corresponding gene (e.g., MLH1 loss → sequence MLH1). **Mnemonic:** **MSI = Microsatellite Instability** — the molecular signature of mismatch repair defects; **MSI-H = Lynch Syndrome until proven otherwise**.