Giant axonal neuropathy: An updated perspective on its pathology and pathogenesis.

Giant axonal neuropathy (GAN) is a rare pediatric neurodegenerative disease. It is best known for the "giant" axons caused by accumulations of intermediate filaments. The disease is progressive, with onset around age 3 years and death by the third decade of life. GAN results from recessive mutations in the GAN gene encoding gigaxonin, and our analysis of all reported mutations shows that they are distributed throughout the protein structure. Precisely how these mutations cause the disease remains to be determined. In addition to changes in peripheral nerves that are similar to those seen in neuropathies such as Charcot-Marie-Tooth type 2, GAN patients exhibit a wide range of central nervous system signs. These features, corroborated by degeneration of central tracts apparent from postmortem pathology, indicate that GAN is also a progressive neurodegenerative disease. To reflect this phenotype more precisely, we therefore propose that the disease should be more appropriately referred to as "giant axonal neurodegeneration."

The absence of curly hair is associated with a milder phenotype in Giant Axonal Neuropathy.

Giant Axonal Neuropathy is a pediatric neurodegenerative disorder caused by autosomal recessive mutations in the GAN gene on chromosome 16q24.1. Mutations in the GAN gene lead to functional impairment of the cytoskeletal protein gigaxonin and a generalized disorder of intermediate filaments, including neurofilaments in axons. Tightly curled hair is a common but not universal feature of Giant Axonal Neuropathy. The pathogenesis of curly hair is unknown, although disruption of keratin architecture is thought to play a role. As part of a broader natural history study of Giant Axonal Neuropathy, we found that the absence of curly hair is correlated with superior motor function (p=0.013) when controlling for age, as measured by the Gross Motor Function Measure. Theoretically, higher levels of functional gigaxonin protein or compensatory mechanisms could produce fewer abnormalities of neurofilaments and keratin, accounting for this phenotype. We suggest that straight-haired patients with Giant Axonal Neuropathy are potentially underdiagnosed due to their divergence from the classic phenotype of the disease. Due to their non-specific features of an axonal neuropathy, these patients may be misdiagnosed with Charcot-Marie-Tooth Disease type 2. Genetic testing for Giant Axonal Neuropathy should be considered in relevant cases of Charcot-Marie-Tooth Disease type 2.