Genetic Tests and Genetic Counseling

Charcot-Marie-Tooth disease (CMT) is a group of genetic disorders that cause defects in the nerves responsible for movement and sensation.

Over a thousand genetic mutations have been linked to CMT. Most of these mutations are found in four genes: PMP22, GJB1, MPZ, and MFN2.

These genetic changes lead to the degeneration of either the nerve fibers themselves or myelin, the protective coating that insulates nerve fibers.

When is genetic testing done?

Genetic testing is done to confirm the diagnosis and type of CMT. It can also exclude other conditions affecting nerve health.

This test may give patients information they need for family planning, as it can determine if two people are carriers of a CMT-causing mutation that they could pass to their biological children.

Genetic tests may also be part of CMT prenatal screening, a set of tests that are performed during pregnancy to check if the developing fetus will have the disorder.

Prenatal screening can be done by either chorionic villus sampling (CVS) or amniocentesis. CVS is usually performed during weeks 11 to 14 of pregnancy, and it consists of removing a small sample of placenta from the womb and testing for known CMT genes. Amniocentesis is usually done during weeks 15 to 20 of pregnancy and involves taking a sample of amniotic fluid — the fluid that surrounds the developing fetus in the womb — for testing.

A test called pre-implantation genetic diagnosis (PGD) also may be an option for couples at risk of having a child with CMT. it uses in vitro fertilization (IVF), a lab method in which eggs are taken from a woman’s ovaries to be fertilized with sperm. The resulting embryos are then tested for CMT, and up to two healthy embryos can be transferred to the uterus.

Types of genetic testing

The test can be in the form of genetic sequencing, where the DNA sequence of a whole gene is determined, along with any mutations. This method is slow and time-consuming, as it must be done one gene at a time. It can also be expensive.

An alternative is next-generation sequencing, a newer type of sequencing that makes it possible to sequence many genes at the same time. This method has been used to identify previously unknown genetic mutations in patients with several subtypes of CMT.

Another testing method is called chromosomal microarray. It uses a chip that contains complementary probes to specific chromosome regions. Although this method can detect chromosomal abnormalities, small changes in single genes will not be evident.

Patients should talk with their neurologists and genetic counselors to decide what type of genetic test is right for them.

How is the test performed?

Genetic testing requires a small blood sample drawn at a hospital or clinic, which is then sent to a laboratory for analysis. Patients and caregivers will meet with their care team and a genetic counselor to discuss the results.

What do the results mean?

Genetic testing results are interpreted considering the patient’s symptoms, physical exam, and family history, as well as the results of other laboratory tests.

Although genetic tests are able to scan many of the CMT-causing mutations, they cannot find them all. A positive genetic test can confirm diagnosis in most people with CMT. But a negative result does not exclude the disease, as an unidentified gene may be missed by DNA sampling.

In prenatal screening, the results don’t show the severity of CMT the child will have, as symptoms and disease progression are variable even within a family with the same type of CMT.

What is genetic counseling?

Patients diagnosed with CMT will work with a genetic counselor to understand the type of CMT they have and what they can expect as the disease progresses.

A genetic counselor may also work with patients to plot their family tree and determine whether other family members may have inherited the disease.

Patients should also discuss with a genetic counselor their risk of transmitting the disease to their children. While de novo, or new disease-causing mutations that occur during conception, can occur even in families with no history of the disease, the vast majority of cases are inherited.

In the U.S. and Canada, patients can search for genetic counselors in their region on the National Society of Genetic Counselors’ website.

Other information

For someone who has not been diagnosed with CMT, especially those without symptoms and not planning on children, a positive genetic test could mean no longer being eligible for certain supplemental insurance, such as long-term care, disability, or life insurance. It is important to consider the long-term implications of a genetic test before deciding to be tested.


Last updated: Nov. 10, 2021


Charcot-Marie-Tooth News is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.