Lynch syndrome is one of those Step “free points” if you can recognize the pattern fast: early colon cancer + right-sided tumors + endometrial cancer—all tied together by DNA mismatch repair failure. This post is built for quick recall and easy sharing, with a comparison table and a clean mnemonic.
The 10-second one-liner (memorize this)
Lynch syndrome = autosomal dominant mismatch repair defect (MLH1/MSH2/MSH6/PMS2) → microsatellite instability → early, right-sided colon cancer + endometrial cancer (and more).
Visual / mnemonic device
“LoST in MIcrosatellites”
Picture a colon map where the right side is a maze labeled MSI (microsatellite instability). A person gets LoST in it:
- L = Lynch
- S = Start young (often < 50)
- T = Two big ones: colon + endometrial
- Bonus: “MI” in MIcrosatellites reminds you: Mismatch repair → MSI
If you prefer gene-first: “M&M’s Protect DNA”
MLH1, MSH2, MSH6, PMS2 = mismatch repair proteins (the “DNA proofreaders”).
Core pathophysiology (what Step wants you to say)
- Inheritance: Autosomal dominant
- Defect: DNA mismatch repair (MMR) genes → inability to correct replication errors
- Molecular signature: Microsatellite instability (MSI-high)
- Tumor progression concept: Often follows the adenoma → carcinoma sequence, but driven by MMR failure rather than APC first.
Comparison table: Lynch vs FAP vs sporadic MSI colon cancer
| Feature | Lynch syndrome (HNPCC) | FAP | Sporadic MSI colon cancer |
|---|---|---|---|
| Inheritance | AD | AD | Not inherited (usually) |
| Key gene(s) | MMR genes: MLH1, MSH2, MSH6, PMS2 | APC tumor suppressor | Often MLH1 silenced by promoter hypermethylation |
| Mechanism | Mismatch repair defect → MSI | Wnt pathway dysregulation → innumerable polyps | MSI from epigenetic MLH1 loss |
| Polyps | Few polyps; cancer risk high anyway | Hundreds–thousands | Variable |
| Typical colon location | Right-sided (proximal) predominance | Classically left-sided (can be diffuse) | Often right-sided |
| Age of CRC | Earlier (<50) common | Very early if untreated | Older |
| Extra-colonic cancers | Endometrial (classic), ovarian, gastric, small bowel, hepatobiliary, pancreas, urothelial, brain, sebaceous tumors | Duodenal/ampullary, desmoid tumors, thyroid (papillary), hepatoblastoma | Not a syndrome pattern |
| Classic board clue | CRC + endometrial, “young + right-sided,” MSI-high | Teen with innumerable colon polyps | Older patient, MSI-high without family history |
High-yield cancer associations (Lynch “shopping list”)
Think: Colon + uterus, then add “GI + GU + oddballs.”
- Colon (often proximal/right)
- Endometrial (often the sentinel cancer)
- Ovarian
- Gastric
- Small bowel
- Hepatobiliary tract
- Pancreas
- Urothelial (ureter/renal pelvis > bladder in classic teaching)
- Brain (glioblastoma; historically in “Turcot variants”)
- Sebaceous tumors (Muir–Torre variant: sebaceous adenomas/carcinomas, keratoacanthomas)
How it shows up on exams (classic vignettes)
Step-style colon vignette
- 45-year-old with iron deficiency anemia and right-sided colon cancer, plus a first-degree relative with endometrial cancer → think Lynch.
Step-style uterus vignette
- Endometrial carcinoma at 41, family history of early colon cancer → screen for Lynch.
Diagnostic testing: what to do and what results mean
Tumor screening tests (common workflow)
- IHC (immunohistochemistry) for MMR proteins:
- Loss of staining suggests which gene is affected
- Example: Loss of MLH1/PMS2 often pairs together; MSH2/MSH6 often pair together
- MSI testing (PCR-based):
- MSI-high supports MMR deficiency
Key Step nuance: Lynch vs sporadic MLH1 loss
- If MLH1 is absent, check for:
- BRAF V600E mutation or MLH1 promoter hypermethylation → supports sporadic origin (classically in older patients)
- No BRAF mutation / no hypermethylation → more suspicious for Lynch
Why MSI matters clinically (one-liner you can use)
MSI-high tumors generate lots of neoantigens → often respond well to immune checkpoint inhibitors (anti–PD-1).
(You don’t need oncology-level detail for Step 1, but Step 2 loves the “MSI-high → immunotherapy-sensitive” association.)
Rapid-fire USMLE facts (printable)
- Inheritance: AD
- Mechanism: MMR defect → MSI
- Colon location: Right-sided predominance
- Biggest extra-colonic cancer: Endometrial
- Notable variant: Muir–Torre = sebaceous tumors + visceral malignancy
- Testing: IHC for MMR proteins + MSI testing
- Sporadic mimic: MLH1 hypermethylation (often BRAF+)
Mini “comparison table” for memory: left vs right colon cancer patterns (tie-in)
| Feature | Right-sided (proximal) | Left-sided (distal) |
|---|---|---|
| Presentation | Occult bleeding → iron deficiency anemia | Obstruction, change in stool caliber |
| Lynch association | Yes (high-yield) | Less classic |
| Typical gross | Exophytic, polypoid | Annular “napkin-ring” lesion |
Take-home shareable summary (tweet-length)
Lynch = AD mismatch repair defect (MLH1/MSH2/MSH6/PMS2) → MSI-high → early RIGHT-sided colon cancer + endometrial cancer (+ ovarian, gastric, urothelial, sebaceous).