Novel homozygous missense mutation in GAN associated with Charcot-Marie-Tooth disease type 2 in a large consanguineous family from Israel

[Involvement of autonomic nervous system since middle age in elderly patient with giant axonal neuropathy caused by novel genetic mutation]

A 69-year-old man began to experience difficulty with walking at the age of 5 years and started use of a cane at around 13 years, then finally started using a wheelchair at 17 years old. A diagnosis of Charcot-Marie-Tooth disease was previously determined at another hospital, though neither peripheral nerve biopsy nor gene analysis was conducted. He visited our institution at the age of 54 years and irregular outpatient examinations were started, which indicated slowly progressive muscle weakness and sensory disturbance of the limbs, leading to a decline in activities of daily living. Gene analysis at 60 years old identified a novel homozygous missense mutation in the gigaxonin gene, c.1478A>C, p.E493A. Intellectual capacity was preserved and kinky hair was not present, though complications such as vocal cord paralysis, paralytic ileus, and dysarthria were noted starting at age 61. Based on these findings, the patient was diagnosed with a mild form of giant axonal neuropathy.

The mutational profile in a South African cohort with inherited neuropathies and spastic paraplegia

Introduction

Limited diagnostics are available for inherited neuromuscular diseases (NMD) in South Africa and (excluding muscle disease) are mainly aimed at the most frequent genes underlying genetic neuropathy (GN) and spastic ataxias in Europeans. In this study, we used next-generation sequencing to screen 61 probands with GN, hereditary spastic paraplegia (HSP), and spastic ataxias for a genetic diagnosis.

Methods

After identifying four GN probands with PMP22 duplication and one spastic ataxia proband with SCA1, the remaining probands underwent whole exome (n = 26) or genome sequencing (n = 30). The curation of coding/splice region variants using gene panels was guided by allele frequencies from internal African-ancestry control genomes (n = 537) and the Clinical Genome Resource's Sequence Variant Interpretation guidelines.

Results

Of 32 GN probands, 50% had African-genetic ancestry, and 44% were solved: PMP22 (n = 4); MFN2 (n = 3); one each of MORC2, ATP1A1, ADPRHL2, GJB1, GAN, MPZ, and ATM. Of 29 HSP probands (six with predominant ataxia), 66% had African-genetic ancestry, and 48% were solved: SPG11 (n = 3); KIF1A (n = 2); and one each of SPAST, ATL1, SPG7, PCYT2, PSEN1, ATXN1, ALDH18A1, CYP7B1, and RFT1. Structural variants in SPAST, SPG11, SPG7, MFN2, MPZ, KIF5A, and GJB1 were excluded by computational prediction and manual visualisation.

Discussion

In this preliminary cohort screening panel of disease genes using WES/WGS data, we solved ~50% of cases, which is similar to diagnostic yields reported for global cohorts. However, the mutational profile among South Africans with GN and HSP differs substantially from that in the Global North.

Expanding the genetic spectrum of giant axonal neuropathy: Two novel variants in Iranian families

BACKGROUND

Giant 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.

METHODS

Clinical 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.

RESULTS

Three 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.

CONCLUSIONS

One 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.

Genetic Approaches for the Treatment of Giant Axonal Neuropathy

Giant axonal neuropathy (GAN) is a pediatric, hereditary, neurodegenerative disorder that affects both the central and peripheral nervous systems. It is caused by mutations in the GAN gene, which codes for the gigaxonin protein. Gigaxonin plays a role in intermediate filament (IF) turnover hence loss of function of this protein leads to IF aggregates in various types of cells. These aggregates can lead to abnormal cellular function that manifests as a diverse set of symptoms in persons with GAN including nerve degeneration, cognitive issues, skin diseases, vision loss, and muscle weakness. GAN has no cure at this time. Currently, an adeno-associated virus (AAV) 9-mediated gene replacement therapy is being tested in a phase I clinical trial for the treatment of GAN. This review paper aims to provide an overview of giant axonal neuropathy and the current efforts at developing a treatment for this devastating disease.

Review of general and head and neck/oral and maxillofacial features of Charcot-Marie-Tooth disease and dental management considerations

Charcot-Marie-Tooth disease (CMTD) is an uncommon progressive neuromuscular disorder of the peripheral nervous system and primarily leads to distal extremity weakness and sensory deficits. Frequently, affected patients manifest pes cavus, drop foot, and digit contractures that may pose significant challenges in ambulation and grasping objects. Although there are numerous articles of this syndrome in the medical literature, there is a limited number of dental publications. The objective of this article is to review the general and head and neck/oral and maxillofacial features of CMTD. General guidelines for dental management are also provided.

Giant axonal neuropathy (GAN) in an 8-year-old girl caused by a homozygous pathogenic splicing variant in GAN gene

Giant axonal neuropathy (GAN) is a progressive disease that involves the peripheral and central nervous systems. This neurodegenerative disease is caused by variants in the GAN gene encoding gigaxonin, and is inherited in an autosomal recessive manner. Herein, we performed whole-exome sequencing on a 8-year-old child with dense, curly hair, weakness in both lower limbs, and abnormal MRI. The child was born to consanguineous parents. Our results revealed that the child carried the c.1373+1G>A homozygous pathogenic variant of the GAN gene, while both parents were heterozygous carriers. According to the validation at the cDNA levels, the splicing variant led to the skipping of exon 8 and affected the Kelch domain's formation. Unlike the previously reported cases of GAN, the child's clinical manifestations revealed peripheral nervous system involvement, no vertebral signs, cerebellar signs, and spasticity, but only MRI abnormalities. These results suggested that the patient's central nervous system was mildly involved, which may be related to the genotype. In order to further clarify the correlation between GAN genotype and phenotype, combined with this patient, 54 cases of reported homozygous variants of the GAN gene were merged for the analysis of genotype and phenotype. The results revealed a certain correlation between the GAN gene variant domain and the patient's clinical phenotype, such as central nervous system involvement and age of onset.

Giant axonal neuropathy: cross sectional analysis of a large natural history cohort

Giant axonal neuropathy (GAN) is an ultra-rare autosomal recessive, progressive neurodegenerative disease with early childhood onset that presents as a prominent sensorimotor neuropathy and commonly progresses to affect both the peripheral nervous system and central nervous system. The disease is caused by biallelic mutations in the GAN gene located on 16q23.2, leading to loss of functional gigaxonin, a substrate specific ubiquitin ligase adapter protein necessary for the regulation of intermediate filament turnover. Here, we report on cross-sectional data from the first study visit of a prospectively collected natural history study of 45 individuals, age range 3-21 years with genetically confirmed giant axonal neuropathy to describe and cross-correlate baseline clinical and functional cohort characteristics. We review causative variants distributed throughout the GAN gene in this cohort and identify a recurrent founder mutation in individuals with giant axonal neuropathy of Mexican descent as well as cases of recurrent uniparental isodisomy. Through cross correlation analysis of measures of strength, motor function, and electrophysiologic markers of disease severity, we identified the Motor Function Measure 32 (MFM-32) to have the strongest correlation across measures and age in individuals with giant axonal neuropathy. We analysed the Motor Function Measure 32 scores as they correspond to age and ambulatory status. Importantly, we identified and characterized a sub cohort of individuals with a milder form of giant axonal neuropathy and with a presentation similar to Charcot-Marie-Tooth disease. Such a clinical presentation is distinct from the classic presentation of giant axonal neuropathy, and we demonstrate how the two groups diverge in performance on the Motor Function Measure 32 and other functional motor scales. We further present data on the first systematic clinical analysis of autonomic impairment in giant axonal neuropathy as performed on a subset of the natural history cohort. Our cohort of individuals with genetically confirmed giant axonal neuropathy is the largest reported to date and highlights the clinical heterogeneity and the unique phenotypic and functional characteristics of giant axonal neuropathy in relation to disease state. The present work is designed to serve as a foundation for a prospective natural history study and functions in concert with the ongoing gene therapy trial for children with giant axonal neuropathy.

Identification of novel pathogenic copy number variations in Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a hereditary sensory-motor neuropathy characterized by a strong clinical and genetic heterogeneity. Over the past few years, with the occurrence of whole-exome sequencing (WES) or whole-genome sequencing (WGS), the molecular diagnosis rate has been improved by allowing the screening of more than 80 genes at one time. In CMT, except the recurrent PMP22 duplication accounting for about 60% of pathogenic variations, pathogenic copy number variations (CNVs) are rarely reported and only a few studies screening specifically CNVs have been performed. The aim of the present study was to screen for CNVs in the most prevalent genes associated with CMT in a cohort of 200 patients negative for the PMP22 duplication. CNVs were screened using the Exome Depth software on next generation sequencing (NGS) data obtained by targeted capture and sequencing of a panel of 81 CMT associated genes. Deleterious CNVs were identified in four patients (2%), in four genes: GDAP1, LRSAM1, GAN, and FGD4. All CNVs were confirmed by high-resolution oligonucleotide array Comparative Genomic Hybridization (aCGH) and/or quantitative PCR. By identifying four new CNVs in four different genes, we demonstrate that, although they are rare mutational events in CMT, CNVs might contribute significantly to mutational spectrum of Charcot-Marie-Tooth disease and should be searched in routine NGS diagnosis. This strategy increases the molecular diagnosis rate of patients with neuropathy.

Giant axonal neuropathy: a multicenter retrospective study with genotypic spectrum expansion

Giant axonal neuropathy (GAN) is an autosomal recessive disease caused by mutations in the GAN gene encoding gigaxonin. Patients develop a progressive sensorimotor neuropathy affecting peripheral nervous system (PNS) and central nervous system (CNS). Methods: In this multicenter observational retrospective study, we recorded French patients with GAN mutations, and 10 patients were identified. Mean age of patients was 9.7 years (2-18), eight patients were female (80%), and all patients met infant developmental milestones and had a family history of consanguinity. Mean age at disease onset was 3.3 years (1-5), and progressive cerebellar ataxia and distal motor weakness were the initial symptoms in all cases. Proximal motor weakness and bulbar symptoms appeared at a mean age of 12 years (8-14), and patients used a wheelchair at a mean age of 16 years (14-18). One patient died at age 18 years from aspiration pneumonia. In all cases, nerve conduction studies showed a mixed demyelinating and axonal sensorimotor neuropathy and MRI showed brain and cerebellum white matter abnormalities. Polyneuropathy and encephalopathy both aggravated during the course of the disease. Patients also showed a variety of associated findings, including curly hair (100% of cases), pes cavus (80%), ophthalmic abnormalities (30%), and scoliosis (30%). Five new GAN mutations were found, including the first synonymous mutation and a large intragenic deletion. Our findings expand the genotypic spectrum of GAN mutations, with relevant implications for molecular analysis of this gene, and confirm that GAN is an age-related progressive neurodegenerative disease involving PNS and CNS.

Novel gap junction protein beta-1 gene mutation associated with a stroke-like syndrome and central nervous system involvement in patients with X-linked Charcot-Marie-Tooth Type 1: A case report and literature review

Gap junction protein beta-1 (GJB1) gene mutations lead to X-linked Charcot-Marie-Tooth Type 1 (CMTX1). We studied a Chinese family with CMTX1 and identified a novel GJB1 point mutation. Our patient had a transient stroke-like clinical manifestations and magnetic resonance imaging (MRI) changes. An analysis of the genomic DNA of the proband showed a T to C hemizygous mutation in the GJB1 gene at nucleotide position 380, causing a predicted amino acid change from isoleucine to threonine at codon 127, which predicted structural alterations disrupting the function of the GJB1 protein. This novel point mutation expanded the spectrum of GJB1 mutations known to be associated with CMTX1. We performed a PubMed review of CMTX cases with central nervous system involvement in the English-language literature from the past 20 years, and summarized the demographic data, nucleotide and amino acid changes, clinical characteristics, clinical manifestations, and neuroimaging features.

Dimerization quality control ensures neuronal development and survival

Aberrant complex formation by recurrent interaction modules, such as BTB domains, leucine zippers, or coiled coils, can disrupt signal transduction, yet whether cells detect and eliminate complexes of irregular composition is unknown. By searching for regulators of the BTB family, we discovered a quality control pathway that ensures functional dimerization [dimerization quality control (DQC)]. Key to this network is the E3 ligase SCF^FBXL17, which selectively binds and ubiquitylates BTB dimers of aberrant composition to trigger their clearance by proteasomal degradation. Underscoring the physiological importance of DQC, SCF^FBXL17 is required for the differentiation, function, and survival of neural crest and neuronal cells. We conclude that metazoan organisms actively monitor BTB dimerization, and we predict that distinct E3 ligases similarly control complex formation by other recurrent domains.