Charcot-Marie-Tooth disease (CMT) is caused by inherited mutations in genes that build and maintain peripheral nerves, the long cables connecting your brain and spinal cord to your muscles and sensory organs. More than 130 different genes have been linked to CMT so far, making it one of the most genetically diverse inherited conditions known. Each gene affects peripheral nerves in a slightly different way, but the end result is similar: progressive weakness and sensation loss that typically starts in the feet and hands.
With a pooled global prevalence of roughly 17.7 per 100,000 people, CMT is the most common inherited nerve disorder. Understanding which gene is involved matters because it determines how the disease behaves, how it’s passed from parent to child, and which treatments may eventually help.
Two Ways Nerves Break Down
Peripheral nerves have two key parts: the axon (the long fiber that carries electrical signals) and the myelin sheath (an insulating wrapper that speeds those signals along). CMT subtypes are classified by which part the genetic mutation damages first.
In CMT type 1, the problem starts with the myelin sheath. Mutations cause the cells that produce myelin, called Schwann cells, to malfunction. Without healthy insulation, nerve signals slow dramatically. Doctors can detect this on a nerve conduction test: healthy upper-limb nerves conduct signals well above 38 meters per second, while CMT1 nerves fall below that threshold. Over time, the exposed axons themselves degenerate as a secondary consequence.
In CMT type 2, the axon is the primary target. Myelin stays relatively intact, so nerve conduction speed looks closer to normal, but the signal strength drops because fewer axon fibers survive. The clinical symptoms, weakness and numbness beginning in the feet and lower legs, overlap heavily between the two types. Most people notice them during their teen years or early adulthood, though onset can happen at any age. The weakness gradually spreads to the fingers, hands, and arms.
The Most Common Cause: A Duplicated Gene
About half of all CMT cases trace back to a single genetic event. In CMT1A, a roughly one-megabase stretch of chromosome 17 gets duplicated, giving a person three copies of the PMP22 gene instead of the usual two. PMP22 provides instructions for a protein that’s a structural component of the myelin sheath. Having an extra copy means Schwann cells produce too much of it.
That excess protein is surprisingly destructive. PMP22 is already hard for cells to fold correctly under normal conditions. When there’s too much of it, misfolded copies pile up inside the cell’s protein-processing machinery (the endoplasmic reticulum), triggering a stress response that can ultimately kill the Schwann cell. Excess PMP22 also raises calcium levels inside Schwann cells through specific calcium channels, and elevated calcium contributes to demyelination. On top of that, overproduction of PMP22 appears to keep Schwann cells stuck in an immature state, preventing them from properly forming myelin in the first place. Multiple studies have found that genes associated with immature, undifferentiated Schwann cells are abnormally active in CMT1A.
Axonal CMT and the Mitochondria Problem
The most common cause of CMT type 2 is a mutation in the MFN2 gene, which produces a protein responsible for fusing mitochondria together. Mitochondria are the energy-producing structures inside every cell, and in long nerve fibers they need to be transported from the cell body all the way to the tips of the axon. That journey can be over a meter in the nerves running to your feet.
When MFN2 is mutated, mitochondria can’t fuse properly. Instead, they cluster into small, fragmented clumps near the cell body and the base of the axon. Research has shown that these clumped mitochondria appear to get “stuck” because they form tethered intermediates that can’t complete the fusion process, and ongoing splitting then creates tangled clusters unable to attach to the transport machinery. The result is that the far ends of the longest axons are starved of mitochondria. Without a local energy supply, those distant stretches of nerve fiber degenerate.
This explains one of CMT2’s hallmark features: the longest nerves fail first. The feet and lower legs are affected before the hands because those nerves have the most territory to cover, and the mitochondria simply can’t get where they need to go. Interestingly, the neurons’ overall energy production and oxygen use remain normal. The problem isn’t generating energy; it’s delivering it.
X-Linked CMT: A Communication Breakdown
The second most common form of CMT overall is CMT1X, caused by mutations in the GJB1 gene on the X chromosome. This gene produces connexin 32, a protein that forms tiny channels called gap junctions in the Schwann cells that wrap around peripheral nerves. These channels sit at critical points along the myelin sheath and allow nutrients and signaling molecules to flow through shortcut pathways across the myelin layers.
Mutations in GJB1 disrupt this communication system in several ways. Some prevent connexin 32 from reaching the cell membrane at all, so gap junctions never form. Others allow the channels to assemble but lock them in a closed position or shrink the opening so much that molecules can’t pass through. At least one known mutation does the opposite, forcing channels to stay open in ways that damage cells by letting calcium flood in and disrupting the cell’s internal balance. There’s also evidence that connexin 32 has roles beyond forming channels: it helps regulate Schwann cell growth and may serve as a scaffolding protein. Losing those functions compounds the damage.
Because the gene sits on the X chromosome, the inheritance pattern differs from other forms. Males have only one X chromosome, so a single mutated copy causes disease. Females have two X chromosomes, so they may or may not develop symptoms depending on how the mutation interacts with their second, normal copy. When symptoms do appear in women, they tend to be milder.
How CMT Is Inherited
CMT follows three main inheritance patterns depending on the subtype, and the pattern determines the odds of passing it to your children.
- Autosomal dominant is the most common pattern, seen in CMT1A, most CMT2 subtypes, and others. Only one copy of the mutated gene is needed to cause disease. If one parent carries the mutation, each child has a 50% chance of inheriting it.
- Autosomal recessive forms, grouped as CMT4, require two copies of the mutated gene, one from each parent. When both parents are carriers, each child has a 25% chance of being affected, a 50% chance of being a carrier without symptoms, and a 25% chance of inheriting neither copy. These forms are rarer overall but show up more frequently in populations with higher rates of consanguinity or in geographically isolated communities. Clusters have been documented in Tunisian families, Amish communities, and populations in western North Carolina and Iran.
- X-linked inheritance applies to CMT1X. A mother carrying the mutation has a 50% chance of passing it to each child. Sons who inherit it will be affected. Daughters who inherit it may develop milder symptoms or none at all. An affected father passes the mutation to all of his daughters (who become carriers) and none of his sons.
A study from western Norway estimated the frequency of these patterns in the general population: autosomal dominant CMT occurred at 36 per 100,000, X-linked forms at 3.6 per 100,000, and autosomal recessive forms at 1.4 per 100,000.
Why So Many Genes Are Involved
Peripheral nerves depend on a complex supply chain. Schwann cells need to manufacture myelin proteins, transport them to the right location, wrap them tightly around axons, and maintain that insulation for decades. Axons need functioning mitochondria, intact structural scaffolding, and reliable transport systems running their entire length. A mutation in any gene involved in these processes can cause CMT.
The more than 130 identified genes span a wide range of cellular functions: structural proteins in the myelin sheath, enzymes that regulate cell signaling, proteins involved in mitochondrial dynamics, and components of the cellular recycling machinery. This genetic diversity is why CMT has so many subtypes and why two people with “the same disease” can have very different experiences. One person’s mutation might produce a mild foot drop in their 30s, while another’s causes severe disability in childhood.
De Novo Mutations
Not every case of CMT is inherited from an affected parent. Some people are the first in their family to carry the mutation, arising from a spontaneous genetic change that occurred in the egg or sperm cell that formed them. This is particularly relevant for CMT1A, where the chromosome 17 duplication can happen as a new event. If you’re diagnosed with CMT and neither parent shows symptoms, genetic testing can clarify whether you carry a de novo mutation or whether a parent is an unrecognized carrier with very mild or no symptoms.
No Approved Treatment Yet
Despite decades of research, no drug has received FDA or European approval for CMT. The most advanced candidate, PXT3003, is a combination therapy designed to reduce PMP22 overexpression in CMT1A. Its second phase III trial (called PREMIER) recently failed to meet its primary endpoints, a significant setback for the field. Gene silencing approaches that directly target PMP22 overproduction have shown promise in animal models, and early-phase human trials are being prepared for some of these. A gene therapy trial using a virus to deliver a nerve growth factor into leg muscles of CMT1A patients has also been completed, with results pending.
For now, management focuses on physical therapy, bracing, and in some cases surgery to correct foot deformities. Understanding the specific genetic cause of your CMT can help guide which clinical trials you might be eligible for as new therapies move through development.