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 it is — and what it means — can make a meaningful difference in outcomes.

What Causes Lynch Syndrome?

Lynch syndrome is caused by inherited pathogenic variants in one of the mismatch repair (MMR) genes: MLH1, MSH2, MSH6, PMS2, or the EPCAM gene (which affects MSH2 expression).

Mismatch repair proteins act as proofreaders during DNA copying. When they are working normally, they identify and correct errors that occur when DNA is replicated. When one copy of an MMR gene carries a pathogenic variant, the repair system is less efficient. Over time, errors accumulate — particularly in short repeated sequences of DNA called microsatellites. This is called microsatellite instability (MSI).

Tumors from people with Lynch syndrome often show high microsatellite instability (MSI-H), which is one way Lynch syndrome is identified in cancer tissue.

Cancer Risks Associated With Lynch Syndrome

Lynch syndrome raises the risk of several cancers, with colorectal and endometrial cancer being the most common:

• Colorectal cancer: lifetime risk approximately 20–80%, depending on the specific gene
• Endometrial cancer: lifetime risk approximately 15–60% in women
• Ovarian cancer: modestly elevated risk
• Gastric cancer: elevated risk, particularly with MLH1 and MSH2 variants
• Urinary tract cancer: elevated risk with MSH2 variants
• Small bowel, biliary tract, brain, and sebaceous skin tumors: less common but associated

The specific gene involved matters. MLH1 and MSH2 variants generally carry higher cancer risks than MSH6 and PMS2 variants.

How Lynch Syndrome Is Diagnosed

Lynch syndrome can be identified through:

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

2. Germline genetic testing: A blood or saliva test that looks for pathogenic variants in the MMR genes directly.

3. Clinical criteria: Family history patterns (Amsterdam criteria, Bethesda guidelines) can identify people who should be offered testing.

Universal tumor testing of all colorectal and endometrial cancers is now recommended by many professional organizations, which has significantly improved Lynch syndrome detection.

Why Lynch Syndrome Is Manageable

Lynch syndrome is one of the most actionable hereditary cancer conditions because surveillance genuinely works.

Regular colonoscopy — typically starting at age 20–25 and repeated every 1–2 years — can detect colorectal cancer early or prevent it entirely by removing precancerous polyps. This surveillance has been shown to reduce colorectal cancer mortality in Lynch syndrome carriers.

For women, gynecologic surveillance and discussions about risk-reducing surgery (hysterectomy and bilateral salpingo-oophorectomy) are important parts of management.

Aspirin has also been studied as a chemopreventive agent in Lynch syndrome, with evidence suggesting it reduces colorectal cancer risk.

Implications for Family Members

Lynch syndrome follows autosomal dominant inheritance. Each first-degree relative of someone with a confirmed Lynch syndrome variant has a 50% chance of having inherited the same variant. Cascade testing — offering genetic testing to relatives — is strongly recommended.

At Genetic Insights, we help patients understand their Lynch syndrome results, discuss surveillance options, and navigate the process of informing and testing family members.

Key Takeaway: Lynch syndrome is caused by inherited MMR gene variants and raises the risk of colorectal, endometrial, and other cancers. It is one of the most manageable hereditary cancer conditions because regular surveillance significantly reduces cancer mortality.

Sources: NCCN Lynch Syndrome Guidelines; Lynch Syndrome International