Charcot Marie-Tooth disease: What is the contribution of cellular and molecular research?

Charcot-Marie-Tooth disease (CMT) is a genetic disease that manifests itself from childhood or adulthood by muscle weakness and a decrease in the sensitivity of the feet and hands (deformities of the feet , falls, lack of dexterity …). Research in CMT is both fundamental and genetic (understanding of the physiology of the nerve, identification of the genetic causes and mechanisms of each of the CMT subtypes, etc.), clinical (natural history of the disease, database, etc.) and therapeutic (pharmacology, gene therapy, pleotherapy …).

This document presents a summary of medical-scientific research in Charcot-MarieTooth disease: mechanisms involved, clinical studies, therapeutic approaches, organization of research … It can in no way replace the opinion of a doctor, even if he can facilitate dialogue with your healthcare team. Aimed at people with the disease and their entourage, it is available from the AFM-Telethon Regional Service in your region and downloadable from the AFM-Telethon website where other information is also available in scientific fields, medical, psychological, social or technical related to Charcot-Marie-Tooth disease.

What is Charcot-Marie-Tooth disease?

Charcot-Marie-Tooth1 disease (CMT) is a rare disease of genetic origin. It affects between 30,000 people and 50,000 people in France.

  1. It manifests itself by a decrease in muscle strength and a melting of the muscles at the ends of the legs (feet and calves) and later, of the hands and forearms. Balance disorders and a decrease in sensitivity (to touch, heat, cold …) in the feet and hands are frequently associated.
  2. These manifestations are the consequence of damage to the nerves of the legs and arms (peripheral nerves).
  3. They begin most of the time in childhood or in young adults with walking difficulties and / or deformities of the feet. The age at which the first symptoms are felt, their intensity and their evolution vary greatly from one person to another, including among members of the same family.
  4. The medical care aims essentially at preventing complications, in particular at the level of muscles and joints (physiotherapy, foot care, cane, ergotherapy, foot lift, rehabilitation of balance disorders, even orthopedic surgery .. .). The regular practice of a sporting activity adapted to motor skills maintains muscular strength and endurance; it decreases the feelings of fatigue and pain in people with CMT.
  5. To find out more about treatment in Charcot-Marie-Tooth disease, Zoom on… Charcot-Marie-Tooth disease presents general, medical, psychological and social information on CMT: Is it frequent? How is it manifested? How is it evolving? How is it transmitted? What medical care? Where to consult? When? What administrative procedures? …

Charcot-Marie-Tooth disease (CMT) is a hereditary genetic disease. It is transmitted from parents to children through genes. Many genes are involved in the onset and transmission of CMT. Depending on the genes involved, the mode of transmission of the disease is different (autosomal or linked to the X chromosome, dominant or recessive).

Some doctors prefer to speak of “Charcot-Marie-Tooth diseases” in the plural, because if all these forms manifest themselves in a similar way, the mechanisms involved and the future innovative treatments under study are specific to each genetic form of the disease.

Different forms of Charcot-Marie-Tooth disease

There are more than sixty different forms of Charcot-Marie-Tooth, classified according to 3 criteria:

  1. the nature of the damage to the peripheral nerve, determined according to the nerve conduction speeds:
    • axonal shape: nerve conduction speed> 40 m / s, it is normal or slightly slowed down (it is mainly the intensity (amplitude) of the nerve impulse which is reduced),
    • demyelinating form (or myelin): conduction speed nerve <35 m / s, • intermediate form: nerve conduction speed between 25 m / s and 45 m / s;
  2. the mode of genetic transmission: autosomal dominant, autosomal recessive and X-linked;
  3. the genetic defect in question.

There are thus 6 main types of CMT: CMT1, CMT2, CMT42, CMTX DI-CMT and RI-CMT. These 6 main types are divided into subtypes (1A, 1B, 1C …, 2A, 2B …) which each correspond to a gene abnormality. For example, CMT1A corresponds to the autosomal dominant demyelinating form linked to the PMP22 gene, while CMT1B corresponds to the autosomal dominant demyelinating form linked to the MPZ gene. This complex classification is primarily intended for doctors and researchers studying CMT.

It allows them to have a system that can account for the great genetic variety encountered in CMT.

  1. It facilitates the diagnostic process and guides the genetic tests to be performed as a priority (depending on the speed of nerve conduction and family history, the doctor chooses one or a few genes to be analyzed first).
  2. It also makes it possible to envisage new therapeutic avenues aimed at improving the disorders observed inside the cell which are specific to a specific gene abnormality, and therefore to a particular CMT subtype.
  3. A proposal to make it evolve towards a simpler, more practical and more comprehensible classification by all (patients, neurologists, researchers…) is under discussion.

What is Charcot-Marie-Tooth disease due to?

Charcot-Marie-Tooth disease (CMT) is due to a modification of DNA: it is a genetic disease. The appearance of this DNA anomaly is due to chance.

A peripheral nerve that transmits nerve impulses less well

The genetic abnormalities involved in CMT disrupt the functioning of the peripheral nerves. The peripheral nerves connect the peripheral cells (muscle cells, sensory cells of the skin, sensory cells of the joints …) to the spinal cord, itself connected to the brain (central nervous system).

The peripheral nerves are made up of nerve fibers, or axons. Axons carry an electrical signal: nerve impulses. Many axons are surrounded by an insulating sheath, myelin, which allows faster signal transmission. This is called myelinated nerve fibers.

It is the Schwann cells that produce myelin. They thus provide electrical isolation of the axons which allows the rapid conduction of nerve impulses. They are also involved in the growth and nutrition of axons, the survival of neurons, the repair of nerve damage, etc.

Nerve conduction speed

The speed of circulation of the nerve impulse along the nerves (nerve conduction speed or PNT) is measured when performing an electroneuromyogram (or ENMG). It depends on the diameter of the nerve fiber and whether or not myelin is present. Normally, it is greater than 40 m / s.

The electroneuromyogram makes it possible to distinguish different forms of CMT:

  1. demyelinating (or myelinated) forms, in which the speed of nerve conduction is less than 35 m / s; they are due to genetic abnormalities that alter myelin or the Schwann cells that make it.
  2. axonal forms, in which the speed of nerve conduction is greater than 40 m / s; they are due to genetic anomalies leading to degeneration of the axons. As there are fewer axons, which participate in the transmission of nerve impulses inside the nerve, the signal is less intense (it is said to have a lower amplitude).
  3. intermediate forms, in which the nerve conduction speed is between 25 m / s <PNV

The longer the nerve, the more likely it is to be damaged

Whether the signal is less intense or less rapid, the peripheral nerves are less efficient at transmitting their message. This message is either a sensation like touch (superficial sensitivity), or information on the position of the body in space, necessary for balance (deep sensitivity), or an order of contraction intended for a muscle for perform a movement (muscle strength).

In CharcotMarie-Tooth disease (CMT), damage to the peripheral nerves mainly results in sensory disturbances and a lack of stress on the muscles at the extremities of the limbs; we are talking about distal muscle damage. Insufficiently stressed muscles tend to decrease in size (muscle wasting), lose their strength (muscle weakness) and shorten (muscle retraction).

What is the difference between the central nervous system and the peripheral nervous system?

The central nervous system includes the brain (brain, cerebellum, brainstem) and its extension, the spinal cord. It is protected by a bone structure (the skull for the brain and the spine for the spinal cord). It analyzes sensory information, programs movement and transmits orders of contraction to the muscle.

The peripheral nervous system extends and works closely with the central nervous system. It is formed by all of the peripheral nerves, motor or sensory, which connect the central nervous system to the rest of the body (organs, muscles, sensory receptors, etc.). The motor nerves send the muscles the order to contract; the sensory nerves transmit sensations – touch, heat, body position … – to the central nervous system.

Proteins with various functions

Knowledge of the functions of proteins encoded by the genes involved in the various forms of CMT has improved in recent years.

Myelin proteins

Myelin is made up of Schwann cells that wrap around the axons in layers. The superposition of these layers constitutes an insulating sheath (myelin sheath) which allows the nerve impulses to propagate very quickly. Certain proteins that make up myelin, such as the proteins PMP22, P0 or connexin 32, are involved in forms of Charcot-Marie-Tooth disease.

The P0 protein The P0 protein

Encoded by the MPZ gene (for myelin protein zero), is the most important protein in myelin, since it represents 50% of myelin proteins. The P0 protein is abnormal in CMT1B, CMT2I, CMT2J and DI-CMTD. The P0 protein is an essential protein for the compaction and stability of myelin because it constitutes a link between two layers of myelin formed by Schwann cells.

PMP22 protein

The peripheral myelin protein 22 or PMP22 is encoded by the PMP22 gene. It represents 2 to 5% of myelin proteins. The precise role of the PMP22 protein is still poorly understood, but studies suggest that it is involved in the growth and differentiation of Schwann cells.

The PMP22 protein is mainly expressed in the peripheral nervous system, but it is also found in small quantities in the central nervous system (brain).

Connexin 32

Connexin 32 is a protein found only in myelin and is encoded by the GJB1 gene (also called Cx32), which, when it has abnormalities, causes CMTX1. Connexins 32 come together in pairs to form a channel between two layers of myelin which allows the passage of molecules of a layer of myelin on the other.

In the vast majority of cases, the genetic abnormalities involved affect the functioning of the canal, which disrupts myelin and causes a decrease in the speed of propagation of nerve impulses.

Axon proteins

Certain axonal proteins are involved in one form or another of CMT:

  1. proteins that play a role in regulating the functioning of mitochondria (such as mitofusin 2, the protein GDAP1, etc.),
  2. proteins involved in intracellular traffic (such as dynamin 2, kinesin-1 beta, etc.),
  3. other proteins with various roles (such as hexokinase 1, an enzyme which participates in the transformation of glucose into energy …).

Proteins involved in the dynamics of nerve cell mitochondria

 The mitochondria are the energy centers of the cell that produce most of the energy directly usable by the cell. The number of mitochondria within a cell is variable; it is a function of the cell’s energy needs. The mitochondria are constantly undergoing fusion and fission (separation into two mitochondria) in order to form new mitochondria, to eliminate altered mitochondria and to exchange material and information between them. This fusion / fission mechanism is therefore important for the growth of mitochondria, their distribution and the maintenance of a mitochondrial network in good condition.

  1. In CMT, anomalies in the fusion / fission dynamics of mitochondria in neurons are responsible for poor transmission of nerve impulses and can impair neuron survival.
  2. Among the proteins involved in the dynamics of mitochondria, mitofusin (altered in CMT2A) plays a role in the process of fusion of mitochondria by making the link between two mitochondria to allow them to attach to one another.
  3. The GDAP1 protein (altered in CMT2H, CMT2K and CMT4A) plays a role in the fission of mitochondria, but its exact role is still poorly understood.

Proteins involved in intracellular nerve cell traffic

Intracellular traffic is the set of mechanisms that circulate material (proteins, vesicles, organelles, etc.) inside a cell. In neurons, intracellular traffic is important for the growth, reshaping and functional maintenance of axons. It is also necessary for the myelination process by bringing the components of myelin to Schwann cells.

Several proteins involved in different forms of CMT play a role in the intracellular trafficking of nerve cells:

  • Dynamin 2 (CMT2M, DI-CMTB) regulates the stability of microtubules which participate in the movement of vesicles inside cells.
  • Kinesin-1 beta (CMT2A1) makes it possible to transport the mitochondria along microtubules, in the axon.
  •  The LITAF protein (CMT1C) is normally found in vesicles which participate in intracellular traffic (endosome, lysosome, etc.). When abnormal, the protein is delocalized and found in the mitochondria.

Proteins involved in protein synthesis

During protein synthesis, amino acids are assembled one after the other to make a protein. Aminoacyl-tRNA synthetases are enzymes, which attach an amino acid to the corresponding transfer RNA.

In CMT, certain aminoacyl-tRNA synthetases are abnormal:

  1. glycyl-tRNA synthetase (GARS),
  2. alanyl-tRNA synthetase (AARS),
  3. methionyl-tRNA synthetase (MARS), – histidyl-tRNA synthetase ( HARS),
  4. tyrosyl-tRNA synthetase (YARS) – lysyl-tRNA synthetase (KARS).

A large number of genes involved

The discovery of the genes involved in CMT was made gradually.

The PMP22 gene

It was in 1992 that the first gene involved in a CMT was identified: it is the PMP22 gene (located on chromosome 17) which codes for the PMP22 protein.
▪ CMT1A is linked to a duplication of the PMP22 gene: there is an additional copy of the PMP22 gene, resulting in higher production of the PMP22 protein and a slowing down of the speed of propagation of nerve impulses.
▪ Two other forms of disease are linked to the PMP22 gene:

  1. CMT1E due to mutations in the PMP22 gene, also causing a slowdown in the rate of conduction of nerve impulses.
  2. hereditary neuropathy with hypersensitivity to pressure (HNPP) or tomacular neuropathy due to a deletion of the PMP22 gene (that is to say that there is one copy of the PMP22 gene missing); the protein PMP22 is then produced in a smaller quantity which weakens the myelin sheath. The nerve then becomes sensitive to prolonged mechanical compression.

Over seventy genes identified

▪ In 1993, two other genes were identified: the MPZ gene (located on chromosome1) which codes for protein P0 and the GJB1 gene (located on chromosome X) which codes for connexin 32.
▪ Since then, many genes have been identified as being involved in the appearance of a form of CMT. Even if there is still more to be discovered, each year researchers find new genes involved in CMT. The list of genes involved in CMT is therefore likely to be enriched as new genes are discovered.

For example, in CMTX3, there is no known gene involved, but a chromosomal abnormality has recently been located in the X chromosome (this is a large insertion of a piece of chromosome 8 into the region q26.3-q27.3 of chromosome X).