Lynch syndrome is the most common hereditary colorectal cancer syndrome, affecting an estimated 1 in 279 people in the United States. Despite its prevalence, it is significantly underdiagnosed. Understanding what Lynch syndrome is — and what can be done about it — is one of the most important applications of hereditary cancer genetic testing.

What Causes Lynch Syndrome?

Lynch syndrome is caused by inherited pathogenic variants in the mismatch repair (MMR) genes: MLH1, MSH2, MSH6, PMS2, and EPCAM.

These genes encode proteins that proofread newly copied DNA and correct errors that occur during replication. When mismatch repair function is lost in a cell, copying errors accumulate — particularly in short repeated DNA sequences called microsatellites. This leads to a pattern called microsatellite instability (MSI), which can be detected in tumor tissue.

Lynch syndrome follows an autosomal dominant inheritance pattern: one pathogenic variant in one copy of an MMR gene is sufficient to significantly raise cancer risk.

Cancer Risks Associated With Lynch Syndrome

The cancer risks associated with Lynch syndrome depend on which gene is affected. MLH1 and MSH2 variants generally carry higher risks than MSH6 or PMS2 variants.

Colorectal cancer: Lifetime risk ranges from approximately 10–80% depending on the gene, compared to approximately 4–5% in the general population.

Endometrial (uterine) cancer: Lifetime risk ranges from approximately 15–60% for women with Lynch syndrome.

Other associated cancers include:

• Ovarian cancer
• Gastric cancer
• Urinary tract cancers (ureter, renal pelvis)
• Small bowel cancer
• Hepatobiliary cancers
• Certain brain tumors (Turcot syndrome variant)
• Sebaceous skin tumors (Muir-Torre syndrome variant)

Why Lynch Syndrome Is Manageable

Despite the elevated cancer risks, Lynch syndrome is one of the most actionable hereditary cancer conditions because:

Colonoscopy is highly effective. Colorectal cancer in Lynch syndrome often develops from polyps that can be detected and removed before they become cancerous. Guidelines generally recommend colonoscopy every 1–2 years beginning at age 20–25 for people with Lynch syndrome.

Endometrial surveillance is available. Annual endometrial sampling and gynecologic evaluation are recommended for women with Lynch syndrome.

Aspirin may reduce risk. The CAPP2 trial demonstrated that aspirin significantly reduced colorectal cancer incidence in Lynch syndrome carriers. Current guidelines discuss aspirin as a potential chemopreventive option.

Immunotherapy works differently in Lynch syndrome cancers. Tumors with mismatch repair deficiency (dMMR) or high microsatellite instability (MSI-H) respond particularly well to immune checkpoint inhibitors. This has treatment implications if a Lynch syndrome carrier does develop cancer.

How Lynch Syndrome Is Diagnosed

Lynch syndrome can be identified through:

Germline genetic testing: A blood or saliva sample is tested for pathogenic variants in the MMR genes.

Tumor testing: Colorectal and endometrial tumors are often tested for MMR protein expression (by immunohistochemistry) or microsatellite instability. Abnormal tumor results prompt germline testing.

Universal tumor screening: Many hospitals now test all colorectal and endometrial cancers for MMR deficiency, regardless of family history, to identify Lynch syndrome cases that would otherwise be missed.

Cascade Testing

Because Lynch syndrome is autosomal dominant, first-degree relatives of a person with a confirmed pathogenic MMR variant have a 50% chance of carrying the same variant. Cascade testing — offering genetic testing to at-risk relatives — is one of the most important steps after a Lynch syndrome diagnosis.

At Genetic Insights, we help patients understand their Lynch syndrome results, coordinate cascade testing for family members, and connect with appropriate surveillance and prevention programs.

Key Takeaway: Lynch syndrome is caused by inherited variants in mismatch repair genes and raises the risk of colorectal, endometrial, and other cancers. It is one of the most manageable hereditary cancer conditions because effective surveillance and prevention strategies exist.

Sources: NCCN Guidelines — Lynch Syndrome; NCI Lynch Syndrome