Giant axonal neuropathy: clinical and genetic study in six cases.

BACKGROUND: Giant axonal neuropathy (GAN) is a severe recessive disorder characterised by variable combination of progressive sensory motor neuropathy, central nervous system (CNS) involvement, and "frizzly" hair. The disease is caused by GAN gene mutations on chromosome 16q24.1. AIMS: To search for GAN gene mutations in Turkish patients with GAN and characterise the phenotype associated with them. METHODS: Linkage and mutation analyses were performed in six affected patients from three consanguineous families. These patients were also investigated by cranial magnetic resonance imaging (MRI) and electroencephalography (EEG). Electromyography (EMG) was performed in heterozygous carriers from family 1 and family 3. RESULTS: Linkage to 16q24.1 was confirmed by haplotype analysis. GAN mutations were identified in all families. Family 1 had the R293X mutation, previously reported in another Turkish family. Families 2 and 3, originating from close geographical areas, shared a novel mutation, 1502+1G>T, at the donor splice site of exon 9. All patients displayed a common phenotype, including peripheral neuropathy, cerebellar ataxia, and frizzly hair. Cranial MRI showed diffuse white matter abnormalities in two patients from family 1 and the patient from family 3, and minimal white matter involvement in the patient from family 2. EMG of a heterozygous R293X mutation carrier showed signs of mild axonal neuropathy, whereas a 1502+1G>T mutation carrier had normal EMG. EEG abnormalities were found in three patients. CONCLUSION: These findings highlight the association of CNS involvement, in particular white matter abnormalities, with peripheral neuropathy in GAN. The phenotypical consequences of both mutations (when homozygous) were similar.

Identification of seven novel mutations in the GAN gene.

Giant axonal neuropathy (GAN) is a severe early onset neurodegenerative disorder affecting both the peripheral nerves and the central nervous system. The diagnosis is based on the presence of characteristic giant axons on nerve biopsy. In GAN, the integrity of the intermediate filament network is altered. Indeed, abnormal accumulation of the intermediate filaments has been reported in different cell types, including in the swollen axons, which are filled with neurofilaments. We identified the defective protein, gigaxonin, of unknown function, and reported fourteen distinct mutations in twelve families of various origins. Two additional mutations have been recently reported. In the present study, we analysed the GAN gene in 6 families, and identified seven novel mutations: three nonsense and two missense mutations and two deletions. In addition, the molecular result for an already reported family was re-evaluated. In this family, the R269Q "polymorphism" is in fact the pathogenic mutation.

Genetic heterogeneity in giant axonal neuropathy: an Algerian family not linked to chromosome 16q24.1.

Giant axonal neuropathy is a rare severe autosomal recessive childhood disorder affecting both the peripheral nerves and the central nervous system. Peripheral nerves characteristically show giant axonal swellings filled with neurofilaments. The giant axonal neuropathy gene was localised by homozygosity mapping to chromosome 16q24.1 and identified as encoding a novel, ubiquitously expressed cytoskeletal protein named gigaxonin.We describe a consanguineous Algerian family with three affected sibs aged 16, 14 and 12 years who present a mild demyelinating sensory motor neuropathy, hypoacousia and kyphoscoliosis which was moderate in the two elder patients, severe in the third one, with no sign of central nervous system involvement and normal cerebral magnetic resonance imaging. This clinical picture is different from the classical severe form, with kinky hairs and early onset of central nervous system involvement and from the less severe form, with protracted course and late involvement of central nervous system. Nerve biopsy showed a moderate loss of myelinated fibers and several giant axons with thin or absent myelin, filled with neurofilaments. This neuropathological aspect is similar to the previously described families linked to the gigaxonin gene. Genetic study in this family showed absence of linkage to chromosome 16q24.1, indicating for the first time, a genetic heterogeneity in giant axonal neuropathy. We propose to call this form of giant axonal neuropathy giant axonal neuropathy 2, and to use the name of giant axonal neuropathy 1 for the form linked to 16q24.1.

The gene encoding gigaxonin, a new member of the cytoskeletal BTB/kelch repeat family, is mutated in giant axonal neuropathy.

Disorganization of the neurofilament network is a prominent feature of several neurodegenerative disorders including amyotrophic lateral sclerosis (ALS), infantile spinal muscular atrophy and axonal Charcot-Marie-Tooth disease. Giant axonal neuropathy (GAN, MIM 256850), a severe, autosomal recessive sensorimotor neuropathy affecting both the peripheral nerves and the central nervous system, is characterized by neurofilament accumulation, leading to segmental distension of the axons. GAN corresponds to a generalized disorganization of the cytoskeletal intermediate filaments (IFs), to which neurofilaments belong, as abnormal aggregation of multiple tissue-specific IFs has been reported: vimentin in endothelial cells, Schwann cells and cultured skin fibroblasts, and glial fibrillary acidic protein (GFAP) in astrocytes. Keratin IFs also seem to be alterated, as most patients present characteristic curly or kinky hairs. We report here identification of the gene GAN, which encodes a novel, ubiquitously expressed protein we have named gigaxonin. We found one frameshift, four nonsense and nine missense mutations in GAN of GAN patients. Gigaxonin is composed of an amino-terminal BTB (for Broad-Complex, Tramtrack and Bric a brac) domain followed by a six kelch repeats, which are predicted to adopt a beta-propeller shape. Distantly related proteins sharing a similar domain organization have various functions associated with the cytoskeleton, predicting that gigaxonin is a novel and distinct cytoskeletal protein that may represent a general pathological target for other neurodegenerative disorders with alterations in the neurofilament network.

Giant axonal neuropathy locus refinement to a < 590 kb critical interval.

Giant axonal neuropathy (GAN) is a rare autosomal recessive neurodegenerative disorder, characterised clinically by the development of chronic distal polyneuropathy during childhood, mental retardation, kinky or curly hair, skeletal abnormalities and, ultrastructurally, by axons in the central and peripheral nervous systems distended by masses of tightly woven neurofilaments. We recently localised the GAN locus in 16q24.1 to a 5-cM interval between the D16S507 and D16S511 markers by homozygosity mapping in three consanguineous Tunisian families. We have now established a contig-based physical map of the region comprising YACs and BACs where we have placed four genes, ten ESTs, three STSs and two additional microsatellite markers, and where we have identified six new SSCP polymorphisms and six new microsatellite markers. Using these markers, we have refined the position of our previous flanking recombinants. We also identified a shared haplotype between two Tunisian families and a small region of homozygosity in a Turkish family with distant consanguinity, both suggesting the occurrence of historic recombinations and supporting the conclusions based on the phase-known recombinations. Taken together, these results allow us to establish a transcription map of the region, and to narrow down the GAN position to a < 590 kb critical interval, an important step toward the identification of the defective gene.

Homozygosity mapping of spinocerebellar ataxia with cerebellar atrophy and peripheral neuropathy to 9q33-34, and with hearing impairment and optic atrophy to 6p21-23.

With the availability of a simple molecular test that distinguishes Friedreich ataxia, the most frequent form of inherited ataxia, from other recessive ataxias, it now becomes possible to unravel the genetic heterogeneity of the latter. We have now localised two genes causing autosomal recessive spinocerebellar ataxia in two consanguineous families. In the first family, the four affected Japanese sibs had spinocerebellar ataxia associated with elevated levels of serum creatine kinase, gamma-globulin, and alpha-foetoprotein. Homozygosity over a 20 cM region allowed to demonstrate linkage at 9q33.3-34.3 with a lod score of 3.0. Genotyping two unrelated Japanese patients from first degree consanguineous parents revealed that one was homozygous for the same region but did not share the biochemical features. In the second family, an Israeli uncle and a niece were affected by an early-onset recessive ataxia and subsequently developed hearing impairment and optic atrophy. Homozygosity over a 17 cM region allowed demonstration of linkage at 6p21-23 with a lod score of 3.25. These two localisations of autosomal recessive ataxia genes represent a first step toward the identification of genetically homogenous, non-Friedreich, ataxic patients and subsequent cloning of the genes.