What Is Lynch Syndrome and Why Is It Important?

What Is Lynch Syndrome?

Lynch syndrome (LS) is a hereditary syndrome that causes a marked increased risk of colorectal and other cancers. It is inherited in an autosomal dominant pattern, which means that it is due to a mutation in one copy of a gene (in LS, a DNA mismatch repair gene). A LS mutation carrier would typically have inherited the mutation from one of their parents and each of their children would have a 50% chance of inheriting the mutation.

How Common Is LS?

LS is among the most common of the hereditary cancer syndromes. The precise frequency of LS is uncertain, but it is estimated to occur in about 1/350 to 1/1000 individuals (about as common as BRCA 1/2 mutations which cause hereditary breast and ovarian cancers). The two most common cancers in patients with LS are colorectal and endometrial cancer; overall it causes about 3%-5% of colorectal cancers and 2% -5% of endometrial cancers in the United States.

Why Is LS Important?

Individuals with LS mutations and many of their close relatives are at very high risk for cancer. LS is the most common hereditary colorectal cancer syndrome but it is much more than that. LS mutation carriers are at increased risk of other cancers including cancers of the uterus, ovary, renal collecting system (renal pelvis and ureter), small bowel, stomach, and bile ducts, among others. Many of the LS-associated cancers can be prevented or detected early. Colonoscopy screening and properly timed prophylactic surgery to remove the uterus and ovaries (after childbearing) have been shown to dramatically decrease the risk of colorectal and uterine/ovarian cancer in LS mutation carriers.

When Should LS Be Suspected?

In general, hereditary cancer syndromes are characterized by cancer that occurs at an early age, multiple cancers of the same type in one individual, multiple different types of cancer in one individual, multiple close blood relatives with the same type of cancer or with different cancers that are known to cluster in a hereditary syndrome.

LS should be suspected in any family that has three or more close relatives with any LS-associated cancer (CRC, endometrial, ovary, small bowel, ureter/renal pelvis) involving at least two generations with one or more occurring under the age of 50; these are called the Amsterdam II criteria for Lynch syndrome.

Only about half of the families with established Lynch syndrome will meet these criteria, however. A broader set of guidelines for suspecting Lynch syndrome include:

• any individual with colorectal (CRC) or endometrial cancer under the age of 50 (CRC under the age of 60 if the CRC has histologic features suggestive of LS)
• any individual with multiple CRCs or with CRC and another LS-associated cancer regardless of age
• anyone with CRC who also has a first degree relative (parent, sibling or child) with any LS-associated cancer under the age of 50
• anyone with more than 2 FDRs with CRC or endometrial cancer.

In addition to suspecting LS on the basis of personal and family history, LS cancers have a characteristic genetic feature called microsatellite instability (MSI). Testing CRCs for MSI is a useful way to try to identify LS; in fact, the broader clinical criteria described above are modifications of the Bethesda guidelines that were developed to identify patients with CRC who should have MSI testing of their CRCs.

Recently some guidelines have recommended testing all CRCs (or all those occurring in individuals under age 70) for MSI in order to increase the detection of individuals with LS.

What to Do Next?

If LS is suspected on the basis of a family history or detection of a CRC with MSI, a good next step is to consult a genetic counselor or other expert in LS and other hereditary CRC syndromes. To find a genetic counselor in your region contact The National Society of Genetic Counselors www.nsgc.org or the National Cancer Institute: Cancer Genetics Services Directory at www.cancer.gov/cancertopics/.

Dennis J. Ahnen MD, AGAF, FACG, is professor of medicine and gastroenterology at the University of Colorado School of Medicine and director of the Gastroenterology of the Rockies GI Genetics Clinic.