by Kimberly Allen, RN
Charcot-Marie-Tooth or CMT is one of the most common inherited neurological disorders. CMT affects approximately 1 out of every 2,500 people in the US. CMT which is also known as hereditary motor and sensory neuropathy (HMSN) is a group of conditions that affect the peripheral nerves.
Charcot-Marie-Tooth disease is caused by mutations in genes that are responsible for the function and structure of the nerves in your extremities. Most disorders that are inherited can only be inherited as an autosomal dominant or an autosomal recessive disorder. CMT on the other hand can be inherited by both autosomal dominant and autosomal recessive patterns as well as an X-linked disorder. An X-linked disorder is one in which the mutated gene is on the X chromosome. In this type of CMT any child of a mother, boy or girl, that is a carrier of the CMT X-linked type has a 50% chance of inheriting the disease. However, boys are affected more often than girls, but girls tend to be carriers and not have any noticeable symptoms. On the other side all daughters of a father that has the mutated gene will inherit the disease, but any boys will not.
There are several types of CMT each results from a mutation on a different chromosome including the X chromosome. CMT1A is caused by an extra copy of the gene on chromosome 17. Chromosome 17 is responsible for providing instructions to produce the peripheral myelin protein-22. This protein is a crucial element in the myelin sheath. An excess of this gene results in the abnormal function and structure of the myelin sheath. People with this type of CMT start developing weakness in their legs during adolescence. As the disease progresses they also experience sensory loss and weakness in their hands. CMT1B is also an autosomal dominant disorder. It is caused by a mutation in a gene on chromosome 1 known as the PO gene. The PO gene is responsible for providing instructions to produce the myelin protein zero or PO. This protein is another crucial element in the myelin sheath. The majority of mutations in this type occur in only 1 letter of the DNA genetic code. So far researchers have discovered 30 different mutations that can cause this type. People with this form of CMT tend to experience the same symptoms as those with CMT1A. CMT2 is the result of abnormalities in the “tails” or axons of the peripheral nerve cells instead of problems with the myelin sheath. There are numerous subtypes of CMT2, each with it’s own pattern of inheritance and symptoms. CMT3 also known as Dejerine-Sottas disease is a severe form of demyelinating neuropathy that starts in infancy. It is caused by a specific mutation on the PO gene or a specific mutation in the PMP-22 gene. CMT4 consists of several subtypes of demyelinating sensory and motor neuropathies that are autosomal recessive. Each subtype of neuropathy is caused by it’s own genetic mutation, and each has their own distinct set of symptoms. Some may also affect certain ethnic groups as well. As a rule people with this type of CMT tend to develop weakness in their legs in childhood and frequently are unable to walk by adolescence. CMTX is a dominantly inherited disease that is caused by a specific mutation in the connexin-32 gene on the X chromosome. This protein is specific to schwan cells. The schwan cells are the cells that wrap around the axons of the nerve cells and is believed to be responsible for communication with the axon. Boys that inherit the mutated gene from their mothers usually manifest symptoms in late childhood or in adolescence. Girls that inherit 1 mutated gene from either parent and a normal gene from the other parent may develop a mild form of the disease and some don’t develop the disease at all.
Unfortunately, like most genetic disorders, there is no known cure for CMT at this time. However, there are a variety of physical and occupational therapies that can help manage the symptoms and improve the overall quality of life including muscle strength training and ligament stretching exercises. There are also a variety of assistive devices as well as orthopedic accessories that can be used to assist with your activities of daily living and help you to maintain mobility as well as prevent injury.
There are several potential treatments that are currently being investigated that researchers hope will be able to effectively treat CMT including gene therapy, though none are yet in human trials.
Kimberly Allen is a registered nurse with an AND in nursing. She has worked in ACF, LCF and psychiatric facilities, although she spent most of her career as a home health expert. She is now a regular contributor to HealthAndFitnessTalk.com, dispensing advice and knowledge about medical issues and questions. You can reach her with any comments or questions at firstname.lastname@example.org.