Clinical, pathological and molecular findings in two siblings with giant axonal neuropathy (GAN): Report from India

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.

Calpain-mediated proteolysis of vimentin filaments is augmented in giant axonal neuropathy fibroblasts exposed to hypotonic stress

Giant Axonal Neuropathy (GAN) is a pediatric neurodegenerative disease caused by loss-of-function mutations in the E3 ubiquitin ligase adaptor gigaxonin, which is encoded by the KLHL16 gene. Gigaxonin regulates the degradation of multiple intermediate filament (IF) proteins, including neurofilaments, GFAP, and vimentin, which aggregate in GAN patient cells. Understanding how IFs and their aggregates are processed under stress can reveal new GAN disease mechanisms and potential targets for therapy. Here we tested the hypothesis that hypotonic stress-induced vimentin proteolysis is impaired in GAN. In both GAN and control fibroblasts exposed to hypotonic stress, we observed time-dependent vimentin cleavage that resulted in two prominent ∼40-45 kDa fragments. However, vimentin proteolysis occurred more rapidly and extensively in GAN cells compared to unaffected controls as both fragments were generated earlier and at 4-6-fold higher levels. To test enzymatic involvement, we determined the expression levels and localization of the calcium-sensitive calpain proteases-1 and -2 and their endogenous inhibitor calpastatin. While the latter was not affected, the expression of both calpains was 2-fold higher in GAN cells compared to control cells. Moreover, pharmacologic inhibition of calpains with MDL-28170 or MG-132 attenuated vimentin cleavage. Imaging analysis revealed striking colocalization between large perinuclear vimentin aggregates and calpain-2 in GAN fibroblasts. This colocalization was dramatically altered by hypotonic stress, where selective breakdown of filaments over aggregates occurred rapidly in GAN cells and coincided with calpain-2 cytoplasmic redistribution. Finally, mass spectrometry-based proteomics revealed that phosphorylation at Ser-412, located at the junction between the central "rod" domain and C-terminal "tail" domain on vimentin, is involved in this stress response. Over-expression studies using phospho-deficient and phospho-mimic mutants revealed that Ser-412 is important for filament organization, solubility dynamics, and vimentin cleavage upon hypotonic stress exposure. Collectively, our work reveals that osmotic stress induces calpain- and proteasome-mediated vimentin degradation and IF network breakdown. These effects are significantly augmented in the presence of disease-causing KLHL16 mutations that alter intermediate filament organization. While the specific roles of calpain-generated vimentin IF fragments in GAN cells remain to be defined, this proteolytic pathway is translationally-relevant to GAN because maintaining osmotic homeostasis is critical for nervous system function.

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.

Giant axonal neuropathy with novel GAN pathogenic variant in a patient of consanguineous origin from Poonch Jammu and Kashmir-India

Giant axonal neuropathy (GAN) is a severe and rare autosomal recessive neurodegenerative disorder of childhood affecting both the peripheral and central nervous systems (CNS). It is caused by mutations in the GAN (gigaxonin) gene linked to chromosome 16q24. Here, we present a 15-year-old male patient with GAN from a consanguineous family of Poonch, Jammu and Kashmir (J&K)-India. Whole-exome sequencing (WES) was employed to unravel the genetic cause of GAN in the proband. Pathogenic variant identified with WES was confirmed in other affected sibling using Sanger sequencing. Magnetic resonance imaging (MRI) and detailed clinical investigation was also carried out on proband. WES revealed a novel homozygous stopgain GAN mutation (NM_022041, c.C1028G, p.S343X) in the patient. MRI of brain displayed bilateral symmetrical confluent areas of deep white matter signal changes affecting periventricular regions (with sparing of subcortical U-fibers), posterior limbs of internal capsules, thalami, external capsules, and semioval centers. The patient was initially suspected to be a case of metachromatic leukodystrophy. However, WES analysis revealed a pathogenic variant in GAN gene as causative. No other pathogenic variant relevant to any other type of dystrophy was reported in WES. Our findings extend the geographical distribution of GAN to even a very remote region in India, extend the mutational and imaging spectrum of GAN and substantiate the need for introducing genetic testing and counselling in primary referral centers/district hospitals in India.

The rare Alus element-mediated chimerism of multiple de novo complex rearrangement sequences in GAN result in giant axonal neuropathy

Giant axonal neuropathy (GAN) is a rare and grievous autosomal recessive neurodegenerative disease due to loss-of-function mutation in GAN. However, the chimerism of complex rearrangement sequences of GAN has not been reported so far, and the mechanism for its complex rearrangements remains to be determined. We identified a family with clinical symptoms of GAN and aimed to reveal a genetic cause underlying this disease. By whole-exome sequencing in the patient we identified a novel homozygous frameshift mutation with 1 bp deletion (c.27delC) in GAN. However, when analyzed the patient's genomic DNA (gDNA) by quantitative real-time PCR and breakpoint DNA sequencing, we found the chimerism of multiple complex rearrangement sequences encompassing exon 1 of GAN in the patient's genome. The microhomology and localization of the breakpoint indicated that they may be caused by Alu repeat elements. We also found that the mRNA expression level of GAN in patient's lymphocyte was decreased, confirming the pathogenicity of these mutations. Our study is the first reported on many complex rearrangement sequences mosaic in GAN mediated by Alu element. The patient here is not a simple homozygous frameshift mutation, but a compound heterozygous paternal c.27delC mutation and the chimerism of multiple de novo complex rearrangement sequences in GAN. Our results may also provide new insights into the formation and pathogenicity of complex rearrangement in GAN, and may be helpful to genetic counseling and genetic testing. It also enriches the Alu-mediated disease-associated database which are important for correct clinical interpretation.

Giant axonal neuropathy

Giant axonal neuropathy is an autosomal recessive disorder of childhood with distinct morphological features. An 8-year-old boy presented with progressive walking difficulty and recurrent falls. Evaluation showed frizzy hair, characteristic facies, sensory motor neuropathy, and ataxia. Magnetic resonance imaging (MRI) showed bilateral symmetric white matter signal changes in the cerebellum and periventricular regions along with involvement of the posterior limb of the internal capsule. Sural nerve biopsy demonstrated giant axons with neurofilament accumulation. The clinicopathologic manifestations of giant axonal neuropathy are discussed along with the clinical and histologic differential diagnoses.

Whole-exome sequencing in patients with inherited neuropathies: outcome and challenges

Inherited peripheral neuropathies (IPN) are one of the most frequent inherited causes of neurological disability characterized by considerable phenotypic and genetic heterogeneity. Based on clinical and electrophysiological properties, they can be subdivided into three main groups: HMSN, dHMN, and HSN. At present, more than 50 IPN genes have been identified. Still, many patients and families with IPN have not yet received a molecular genetic diagnosis because clinical genetic testing usually only covers a subset of IPN genes. Moreover, a considerable proportion of IPN genes has to be identified. Here we present results of WES in 27 IPN patients excluded for mutations in many known IPN genes. Eight of the patients received a definite diagnosis. While six of these patients carried bona fide pathogenic mutations in known IPN genes, two patients had mutations in genes known to be involved in other types of neuromuscular disorders. A further group of eight patients carried sequence variations in IPN genes that could not unequivocally be classified as pathogenic. In addition, combining data of WES and linkage analysis identified SH3BP4, ITPR3, and KLHL13 as novel IPN candidate genes. Moreover, there was evidence that particular mutations in PEX12, a gene known to cause Zellweger syndrome, could also lead to an IPN phenotype. We show that WES is a useful tool for diagnosing IPN and we suggest an expanded phenotypic spectrum of some genes involved in other neuromuscular and neurodegenerative disorders. Nevertheless, interpretation of variants in known and potential novel disease genes has remained challenging.

Restoration of cytoskeleton homeostasis after gigaxonin gene transfer for giant axonal neuropathy

Giant axonal neuropathy (GAN) is caused by loss of function of the gigaxonin protein. On a cellular level GAN is characterized by intermediate filament (IF) aggregation, leading to a progressive and fatal peripheral neuropathy in humans. This study sought to determine if re-introduction of the GAN gene into GAN-deficient cells and mice would restore proper cytoskeleton IF homeostasis. Treatment of primary skin fibroblast cultures from three different GAN patients with an adeno-associated virus type 2 (AAV2) vector containing a normal human GAN transgene significantly reduced the number of cells displaying vimentin IF aggregates. A proteomic analysis of these treated cells was also performed, wherein the abundance of 32 of 780 identified proteins significantly changed in response to gigaxonin gene transfer. While 29 of these responding proteins have not been directly described in association with gigaxonin, three were previously identified as being disregulated in GAN and were now shifted toward normal levels. To assess the potential application of this approach in vivo and eventually in humans, GAN mice received an intracisternal injection of an AAV9/GAN vector to globally deliver the GAN gene to the brainstem and spinal cord. The treated mice showed a nearly complete clearance of peripherin IF accumulations at 3 weeks post-injection. These studies demonstrate that gigaxonin gene transfer can reverse the cellular IF aggregate pathology associated with GAN.

Clinicogenetical features of a Japanese patient with giant axonal neuropathy

Giant axonal neuropathy (GAN) is a rare autosomal recessive disorder that affects both the peripheral nerves and central nervous system. Since the discovery in 2000 of the gigaxonin gene on chromosome 16q24.1 to be causative, more than 40 GAN mutations have been reported from different racial backgrounds. We report the clinicogenetic findings of a 24-year-old Japanese man with GAN. He had consanguineous parents and showed the phenotype of classical severe GAN. We found a novel homozygous nonsense mutation (p.R162X) in the GAN gene. This is the first genetically-determined Japanese case of GAN, with a follow-up period of more than 15 years. In addition, this mutation is novel. We also reviewed previous reports of GAN to see whether there is any genotype-phenotype correlation.

Sensory-motor deficits and neurofilament disorganization in gigaxonin-null mice

Background

Giant Axonal Neuropathy (GAN) is a fatal neurodegenerative disorder with early onset characterized by a severe deterioration of the peripheral and central nervous system, involving both the motor and the sensory tracts and leading to ataxia, speech defect and intellectual disabilities. The broad deterioration of the nervous system is accompanied by a generalized disorganization of the intermediate filaments, including neurofilaments in neurons, but the implication of this defect in disease onset or progression remains unknown. The identification of gigaxonin, the substrate adaptor of an E3 ubiquitin ligase, as the defective protein in GAN allows us to now investigate the crucial role of the gigaxonin-E3 ligase in sustaining neuronal and intermediate filament integrity. To study the mechanisms controlled by gigaxonin in these processes and to provide a relevant model to test the therapeutic approaches under development for GAN, we generated a Gigaxonin-null mouse by gene targeting.

Results

We investigated for the first time in Gigaxonin-null mice the deterioration of the motor and sensory functions over time as well as the spatial disorganization of neurofilaments. We showed that gigaxonin depletion in mice induces mild but persistent motor deficits starting at 60 weeks of age in the 129/SvJ-genetic background, while sensory deficits were demonstrated in C57BL/6 animals. In our hands, another gigaxonin-null mouse did not display the early and severe motor deficits reported previously. No apparent neurodegeneration was observed in our knock-out mice, but dysregulation of neurofilaments in proximal and distal axons was massive. Indeed, neurofilaments were not only more abundant but they also showed the abnormal increase in diameter and misorientation that are characteristics of the human pathology.

Conclusions

Together, our results show that gigaxonin depletion in mice induces mild motor and sensory deficits but recapitulates the severe neurofilament dysregulation seen in patients. Our model will allow investigation of the role of the gigaxonin-E3 ligase in organizing neurofilaments and may prove useful in understanding the pathological processes engaged in other neurodegenerative disorders characterized by accumulation of neurofilaments and dysfunction of the Ubiquitin Proteasome System, such as Amyotrophic Lateral Sclerosis, Huntington's, Alzheimer's and Parkinson's diseases.

Gigaxonin controls vimentin organization through a tubulin chaperone-independent pathway

Gigaxonin mutations cause the fatal human neurodegenerative disorder giant axonal neuropathy (GAN). Broad deterioration of the nervous system in GAN patients is accompanied by massive disorganization of intermediate filaments (IFs) both in neurons and many non-neuronal cells. With newly developed antibodies, gigaxonin is now shown to be expressed at extremely low levels throughout the nervous system. In lymphoblast cell lines derived from severe and mild forms of GAN, mutations in gigaxonin are shown to yield highly unstable proteins, thereby permitting a rapid diagnostic test for the spectrum of GAN mutations as an alternative to invasive nerve biopsy or systematic sequencing of the GAN gene. Gigaxonin has been proposed as a substrate adaptor for an E3 ubiquitin ligase, which affects proteasome-dependent degradation of microtubule-related proteins including MAP1B, MAP8 and the tubulin folding chaperone TBCB. We demonstrate that, unlike its counterpart TBCE, TBCB only moderately destabilizes microtubules. Neither TBCB abundance nor microtubule organization or densities are altered in GAN mutant fibroblasts, thus demonstrating that altered TBCB levels are not primary determinants of IF disorganization in GAN. Characteristic GAN mutant-induced ovoid aggregates of vimentin are not produced in normal fibroblasts after disrupting microtubule assembly, either by TBCE overexpression or depolymerizing drugs. Thus, IF disorganization in GAN fibroblasts is independent of TBCB and microtubule loss and must be regulated by a yet unidentified mechanism.