giant axonal neuropathy
Giant axonal neuropathy is an inherited condition characterized by abnormally large and dysfunctional axons called giant axons. Axons are specialized extensions of nerve cells (neurons) that transmit nerve impulses. The giant axons occur in the peripheral nervous system, which connects the brain and spinal cord (central nervous system) to muscles and to sensory cells that detect sensations such as touch, pain, heat, and sound. However, axons in the central nervous system can be affected as well.
The signs and symptoms of giant axonal neuropathy generally begin in early childhood and get worse over time. Most individuals with giant axonal neuropathy first have problems with walking. Later they may lose sensation, strength, and reflexes in their limbs; experience difficulty coordinating movements (ataxia); and require wheelchair assistance. Visual problems may also occur. Many affected individuals have extremely kinky hair as compared to others in their family.
As the disorder worsens and the central nervous system becomes involved, paralysis, seizures, and a gradual decline in mental function (dementia) can also occur. Most people with giant axonal neuropathy do not survive past their 30s.
Giant axonal neuropathy is a very rare disorder; only about 50 affected families have been described in the medical literature. The condition is thought to be under-diagnosed because its early symptoms resemble those of other, more common disorders affecting the peripheral nervous system, such as Charcot-Marie-Tooth disease.
Giant axonal neuropathy is caused by mutations in the GAN gene, which provides instructions for making a protein called gigaxonin. Gigaxonin is part of the ubiquitin-proteasome system, which is a process that identifies and gets rid of excess or damaged proteins within cells. In particular, gigaxonin plays a role in the breakdown of neurofilaments, which comprise the structural framework that establishes the size and shape of axons.
The GAN gene mutations that have been identified in people with giant axonal neuropathy result in an unstable gigaxonin protein that breaks down more easily than normal, resulting in much less gigaxonin in cells. In neurons without enough functional gigaxonin, neurofilaments that would otherwise have been broken down by the ubiquitin-proteasome system accumulate. The neurofilaments become densely packed in the giant axons of people with giant axonal neuropathy. These giant axons do not transmit signals properly and eventually deteriorate, resulting in the death of neurons. The loss of nerve cells leads to problems with walking and sensation in people with giant axonal neuropathy.
This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.
These resources address the diagnosis or management of giant axonal neuropathy:
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- giant axonal disease