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CMT Term Glossary

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Affected: A person who has the condition in question.

Amino acid: The building block of proteins. Amino acids are determined by the sequence of bases found in a person's genetic code (DNA). In general, if a mutation in a gene does not cause an amino acid to change, it is unlikely that it is the cause of a person's condition.

Autosomes/autosomal: One of the first 22 pairs of chromosomes (all excepting the sex chromosomes - X and Y). If a person has a condition that is found on an autosome, there is an equal chance of it affecting males and females, and an equal chance of passing on the condition, no matter the gender of the person affected.

Axon: The central part of a nerve through which signals are sent.

Axonal: Due to the axon.

 

Carrier: A carrier is a term used when talking about autosomal recessive inheritance, and is a person who has a copy of a gene with a mutation but does not have CMT or symptoms. In autosomal recessive inheritance, both copies of the gene need to have a mutation. Carriers only have one copy with a mutation. If two carriers of a condition have a child, there is a 1 in 4 or 25% chance of that child having the condition in question. 

Central nervous system: The brain and the spinal cord. These are not typically affected in CMT. 

Charcot Marie Tooth disease (CMT): Any type of a hereditary peripheral neuropathy. 

Chromosome: A structure that holds genetic information. Humans typically have 46 chromosomes.

Demyelinating: If a person has a demyelinating neuropathy, there is a problem with the myelin that protects the axon. This is defined as nerve conduction velocities less than 38 meters per second in the median or ulnar nerves (both found in the arms). 

DNA: The main unit of heredity. The instructions for the inner workings of the body are found within the DNA. 

Dominant: In a dominant condition, only one mutation is needed for a person to have a condition. For an autosomal dominant condition, there is a fifty percent chance of passing on the condition in each pregnancy, and a fifty percent chance of not passing on the condition.

 
 

Gene: A smaller piece of genetic information. Genes are found on the chromosomes. There are about 25,000 genes in a human.

Genetic testing: Genetic testing involves looking at specific parts of the genetic code to see if there is a change from what is expected. If a person has positive genetic testing, a change has been found that is causing the disorder in question. If a person has negative genetic testing, that person does not have a change in the gene tested. It does not necessarily mean that the person does not have the disorder in question.

Hereditary: Capable of being passed on from generation to generation.

 

 

 
 

Myelin/Myelin Sheath: The protective insulation that surrounds an axon in a nerve.

Natural history: The way a condition changes over time.

Neuropathy: A problem with a nerve.

 

Peripheral nervous system: The system of nerves that branch out from the central nervous system to the feet and hands (the periphery of the body). These nerves include motor (which control movement) and sensory (which allow a person to feel temperature and proprioception) nerves.

Proprioception: The ability to know where one's limbs are in space.

 

Recessive: In recessive inheritance, a person needs to have two copies of a gene that both have a mutation and are not working properly in order to be affected. A person who has a mutation in one copy of the gene but not the other are called “carriers,” and do not have CMT or symptoms of CMT. If two carriers have children, there is a 1 in 4 or 25% chance in each pregnancy of having a child affected with CMT. A person with a recessive form of CMT will pass on one copy of the gene with the mutation to each child.

 
 

 

Variant of uncertain significance: A change was found during genetic testing, but the laboratory does not have enough information at the time to say if the change is the cause of the disorder in question. The laboratory may in time reclassify the change as positive or negative, and a literature search by a physician may be able to provide more information. Often, other family members need to be tested in order to determine if a variant is causing the condition.

 

X-linked:  X-linked refers to any condition in which the gene that has a mutation is found on the X chromosome. In X-linked inheritance, the sex of the person who is affected matters in terms of the risk to the children of that person’s chance of having inherited the condition. Females have two X chromosomes, and males have an X and a Y chromosome. Females have a 50% risk of passing on the condition to her children in each pregnancy. Males will pass on their X chromosome to all of his daughters, and thus all his daughters will inherit the mutation. Males will pass on their Y chromosome to all of his sons, and thus all his sons will not inherit the mutation and will not have CMT.