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Giant axonal neuropathy: The first Iranian case with a variation in the gigaxonin gene and a glance to the other cases

ABSTRACT: Background: Giant axonal neuropathy (GAN) is a very rare fatal neurodegenerative disorder with clinical and allelic heterogeneity. The disease is caused by mutations in the GAN (gigaxonin) gene. Herein, we reported the clinical presentations and results of genetic analysis of the first Iranian GAN case. Methods: Phenotypic data were obtained by neurologic examination, brain magnetic resonance imaging (MRI), electromyography (EMG), electroencephalography (EEG), and sonography from the proband. Deoxyribonucleic acid (DNA) was isolated from peripheral blood leucocytes and whole exome sequencing (WES) was performed. The candidate variant was screened by Sanger sequencing in the proband and her family members. Results: The proband was a 7-year-old girl who was admitted with a chief complaint of ataxia, muscle weakness, delayed developmental milestones, and history of psychiatric disorders. She was very moody and had clumsy gait, decreased deep tendon reflexes (DTRs) of lower limbs, and kinky hair. The brain MRI revealed white matter abnormality. The EMG revealed that her disease was compatible with the chronic axonal type of sensorimotor polyneuropathy; however, her EEG was normal. Results of the WES revealed a homozygous variant; c.G778T:p.E260* in the GAN gene, indicating the GAN disorder. Conclusion: The present study affirmed GAN allelic heterogeneity and resulted in the expansion of the phenotypic spectrum of GAN pathogenic variants. Identification of more families with mutations in GAN gene helps to further understand the molecular basis of the disease and provides an opportunity for genetic counseling especially in the populations with a high degree of consanguineous marriage such as the Iranian population.

SUBMITTER: Vafaee-Shahi M

PROVIDER: S-EPMC8236428 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BackgroundGiant axonal neuropathy (GAN) is a progressive childhood hereditary polyneuropathy that affects both the peripheral and central nervous systems. Disease-causing variants in the gigaxonin gene (GAN) cause autosomal recessive giant axonal neuropathy. Facial weakness, nystagmus, scoliosis, kinky or curly hair, pyramidal and cerebellar signs, and sensory and motor axonal neuropathy are the main symptoms of this disorder. Here, we report two novel variants in the GAN gene from two unrelated Iranian families.MethodsClinical and imaging data of patients were recorded and evaluated, retrospectively. Whole-exome sequencing (WES) was undertaken in order to detect disease-causing variants in participants. Confirmation of a causative variant in all three patients and their parents was carried out using Sanger sequencing and segregation analysis. In addition, for comparing to our cases, we reviewed all relevant clinical data of previously published cases of GAN between the years 2013-2020.ResultsThree patients from two unrelated families were included. Using WES, we identified a novel nonsense variant [NM_022041.3:c.1162del (p.Leu388Ter)], in a 7-year-old boy of family 1, and a likely pathogenic missense variant [NM_022041.3:c.370T>A (p.Phe124Ile)], in two affected siblings of the family 2. Clinical examination revealed typical features of GAN-1 in all three patients, including walking difficulties, ataxic gait, kinky hair, sensory-motor polyneuropathy, and nonspecific neuroimaging abnormalities. Review of 63 previously reported cases of GAN indicated unique kinky hair, gait problem, hyporeflexia/areflexia, and sensory impairment were the most commonly reported clinical features.ConclusionsOne homozygous nonsense variant and one homozygous missense variant in the GAN gene were discovered for the first time in two unrelated Iranian families that expand the mutation spectrum of GAN. Imaging findings are nonspecific, but the electrophysiological study in addition to history is helpful to achieve the diagnosis. The molecular test confirms the diagnosis.

Project description:The BTB-KELCH protein Gigaxonin plays key roles in sustaining neuron survival and cytoskeleton architecture. Indeed, recessive mutations in the Gigaxonin-encoding gene cause Giant Axonal Neuropathy (GAN), a severe neurodegenerative disorder characterized by a wide disorganization of the Intermediate Filament network. Growing evidences suggest that GAN is a continuum with the peripheral neuropathy Charcot-Marie-Tooth diseases type 2 (CMT2). Sharing similar sensory-motor alterations and aggregation of Neurofilaments, few reports have revealed that GAN and some CMT2 forms can be misdiagnosed on clinical and histopathological examination. The goal of this study is to propose a new differential diagnostic test for GAN/CMT2. Moreover, we aim at identifying the mechanisms causing the loss-of-function of Gigaxonin, which has been proposed to bind CUL3 and substrates as part of an E3 ligase complex.We establish that determining Gigaxonin level constitutes a very valuable diagnostic test in discriminating new GAN cases from clinically related inherited neuropathies. Indeed, in a set of seven new families presenting a neuropathy resembling GAN/CMT2, only five exhibiting a reduced Gigaxonin abundance have been subsequently genetically linked to GAN. Generating the homology modeling of Gigaxonin, we suggest that disease mutations would lead to a range of defects in Gigaxonin stability, impairing its homodimerization, BTB or KELCH domain folding, or CUL3 and substrate binding. We further demonstrate that regardless of the mutations or the severity of the disease, Gigaxonin abundance is severely reduced in all GAN patients due to both mRNA and protein instability mechanisms.In this study, we developed a new penetrant and specific test to diagnose GAN among a set of individuals exhibiting CMT2 of unknown etiology to suggest that the prevalence of GAN is probably under-evaluated among peripheral neuropathies. We propose to use this new test in concert with the clinical examination and prior to the systematic screening of GAN mutations that has shown strong limitations for large deletions. Combining the generation of the structural modeling of Gigaxonin to an analysis of Gigaxonin transcripts and proteins in patients, we provide the first evidences of the instability of this E3 ligase adaptor in disease.

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Giant axonal neuropathy: The first Iranian case with a variation in the gigaxonin gene and a glance to the other cases

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