Charcot-Marie-Tooth disease (CMT) is the name given to a group of rare, heritable genetic disorders, which affect the peripheral nervous system, causing a loss of movement and sensation. Patients suspected of having one of these disorders should meet with a genetic counselor to determine the best genetic test for them and get help interpreting the results of the test.
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, can occur even in families with no history of the disease, the majority of cases are inherited.
Types of CMT and their inheritance pattern
While more than 100 mutations have been linked to CMT, the mutations fall into broad groups with similar symptoms and inheritance patterns. A genetic counselor can use genetic test information and family history to distinguish between different inheritance patterns.
The most common form of CMT, CMT type 1, is inherited in an autosomal dominant manner. This means that inheriting a single copy of the disease-causing mutation is enough for the disease to develop. If a patient has one copy of the disease-causing gene, his or her children would have a 50 percent risk of inheriting the disease-causing gene. Dominant intermediate type CMT is also inherited in this way.
Some other forms of the disease are inherited in an autosomal recessive manner. This means that patients would have to inherit two copies of the disease-causing mutation to develop CMT. Both parents would have to carry the mutated gene and transmit it to their child for him or her to develop the disease. There is a 25 percent risk that a child whose parents are both carriers of the same mutation will inherit the disease.
CMT type 2, type 3, and type 4 are all inherited in either an autosomal dominant or autosomal recessive pattern, so family histories are often required to determine which inheritance pattern the disease is following.
X-linked CMT occurs when the disease-causing mutation is located on the X-chromosome. The X-chromosome is one of two “sex” chromosomes responsible for determining biological gender. People with two X-chromosomes are biologically female, while those with both an X- and Y-chromosome are biologically male. This form of the disease is often more severe in male patients because the second X-chromosome that females have usually compensates for the mutated gene. A woman would not need to show signs of CMT herself to transmit the disease to her children — but it would be likely that only her sons would inherit the disease. For a daughter to inherit X-linked CMT, both parents would have to transmit an X-chromosome carrying the mutation. In such a case, the father would have symptoms of X-linked CMT.
How to find a genetic counselor
In the U.S. and Canada, patients can search for genetic counselors in their region on the National Society of Genetic Counselors website.
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