Genotype–phenotype analysis in patients with giant axonal neuropathy (GAN)

Gigaxonin is required for intermediate filament transport

Gigaxonin is an adaptor protein for E3 ubiquitin ligase substrates. It is necessary for ubiquitination and degradation of intermediate filament (IF) proteins. Giant axonal neuropathy is a pathological condition caused by mutations in the GAN gene that encodes gigaxonin. This condition is characterized by abnormal accumulation of IFs in both neuronal and non-neuronal cells; however, it is unclear what causes IF aggregation. In this work, we studied the dynamics of IFs using their subunits tagged with a photoconvertible protein mEOS 3.2. We have demonstrated that the loss of gigaxonin dramatically inhibited transport of IFs along microtubules by the microtubule motor kinesin-1. This inhibition was specific for IFs, as other kinesin-1 cargoes, with the exception of mitochondria, were transported normally. Abnormal distribution of IFs in the cytoplasm can be rescued by direct binding of kinesin-1 to IFs, demonstrating that transport inhibition is the primary cause for the abnormal IF distribution. Another effect of gigaxonin loss was a more than 20-fold increase in the amount of soluble vimentin oligomers in the cytosol of gigaxonin knock-out cells. We speculate that these oligomers saturate a yet unidentified adapter that is required for kinesin-1 binding to IFs, which might inhibit IF transport along microtubules causing their abnormal accumulation.

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.

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.

Gene therapies for axonal neuropathies: Available strategies, successes to date, and what to target next

Nearly one-hundred loci in the human genome have been associated with different forms of Charcot-Marie-Tooth disease (CMT) and related inherited neuropathies. Despite this wealth of gene targets, treatment options are still extremely limited, and clear "druggable" pathways are not obvious for many of these mutations. However, recent advances in gene therapies are beginning to circumvent this challenge. Each type of CMT is a monogenic disorder, and the cellular targets are usually well-defined and typically include peripheral neurons or Schwann cells. In addition, the genetic mechanism is often also clear, with loss-of-function mutations requiring restoration of gene expression, and gain-of-function or dominant-negative mutations requiring silencing of the mutant allele. These factors combine to make CMT a good target for developing genetic therapies. Here we will review the state of relatively established gene therapy approaches, including viral vector-mediated gene replacement and antisense oligonucleotides for exon skipping, altering splicing, and gene knockdown. We will also describe earlier stage approaches for allele-specific knockdown and CRIPSR/Cas9 gene editing. We will next describe how these various approaches have been deployed in clinical and preclinical studies. Finally, we will evaluate various forms of CMT as candidates for gene therapy based on the current understanding of their genetics, cellular/tissue targets, validated animal models, and availability of patient populations and natural history data.

Two novel mutations in the GAN gene causing giant axonal neuropathy

BACKGROUND

Giant axonal neuropathy (GAN) is a rare neurodegenerative disease transmitted in an autosomal recessive mode. This disorder presents motor and sensitive symptoms with an onset in early childhood. Progressive neurodegeneration makes the patients wheelchair dependent by the end of the second decade of life. Affected individuals do not survive beyond the third decade of life. Molecular analysis has identified mutations in the gene GAN in patients with this disorder. This gene produces a protein called gigaxonin which is presumably involved in protein degradation via the ubiquitin-proteasome system. However, the underlying molecular mechanism is not clearly understood yet.

METHODS

Here we present the first patient from Mexico with clinical data suggesting GAN. Sequencing of the GAN gene was carried out. Changes in the nucleotide sequence were investigated for their possible impact on protein function and structure using the publicly available prediction tools PolyPhen-2 and PANTHER.

RESULTS

The patient is a compound heterozygous carrying two novel mutations in the GAN gene. The sequence analysis revealed two missense mutations in the Kelch repeats domain. In one allele, a C>T transition was found in exon 9 at the nucleotide position 55393 (g.55393C>T). In the other allele, a transversion G>T in exon 11 at the nucleotide position 67471 (g.67471G>T) was observed. Both of the bioinformatic tools predicted that these amino acid substitutions would have a negative impact on gigaxonin's function.

CONCLUSION

This work provides useful information for health professionals and expands the spectrum of disease-causing mutations in the GAN gene and it is the first documented case in Mexican population.

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

Background

CMT-2 is a clinically and genetically heterogeneous group of peripheral axonal neuropathies characterized by slowly progressive weakness and atrophy of distal limb muscles resulting from length-dependent motor and sensory neurodegeneration. Classical giant axonal neuropathy (GAN) is an autosomal recessively inherited progressive neurodegenerative disorder of the peripheral and central nervous systems, typically diagnosed in early childhood and resulting in death by the end of the third decade. Distinctive phenotypic features are the presence of “kinky” hair and long eyelashes. The genetic basis of the disease has been well established, with over 40 associated mutations identified in the gene GAN, encoding the BTB-KELCH protein gigaxonin, involved in intermediate filament regulation.

Methods

An Illumina Human CytoSNP-12 array followed by whole exome sequence analysis was used to identify the disease associated gene mutation in a large consanguineous family diagnosed with Charcot-Marie-Tooth disease type 2 (CMT-2) from which all but one affected member had straight hair.

Results

Here we report the identification of a novel GAN missense mutation underlying the CMT-2 phenotype observed in this family. Although milder forms of GAN, with and without the presence of kinky hair have been reported previously, a phenotype distinct from that was investigated in this study. All family members lacked common features of GAN, including ataxia, nystagmus, intellectual disability, seizures, and central nervous system involvement.

Conclusions

Our findings broaden the spectrum of phenotypes associated with GAN mutations and emphasize a need to proceed with caution when providing families with diagnostic or prognostic information based on either clinical or genetic findings alone.

A review of gigaxonin mutations in giant axonal neuropathy (GAN) and cancer

Gigaxonin, the product of GAN gene localized to chromosome 16, is associated with the early onset neuronal degeneration disease giant axonal neuropathy (GAN). Gigaxonin is an E3 ubiquitin ligase adaptor protein involved in intermediate filament processing in neural cells, and vimentin filaments in fibroblasts. Mutations of the gene cause pre-neural filaments to accumulate and form giant axons resulting in the inhibition of neural cell signaling. Analysis of the catalog of somatic mutations in cancer, driver DB and IDGC data portal databases containing 21,000 tumor genomic sequences has identified GAN patient mutations in cancer cell lines and primary tumors. The database search has also shown the presence of identical missense and nonsense gigaxonin mutations in GAN and colon cancer. These mutations frequently occur in the domains associated with protein homodimerization and substrate interaction such as Broad-Complex, Tramtrack and Bric a brac (BTB), BTB associated C-terminal KELCH (BACK), and KELCH repeats. Analysis of the International HapMap Project database containing 1200 normal genomic sequences has identified a single nucleotide polymorphism (SNP), rs2608555, in exon 8 of the gigaxonin sequence. While this SNP is present in >40 % of Caucasian population, it is present in less than 10 % of Japanese and Chinese populations. Although the role of gigaxonin polymorphism is not yet known, CFTR and MDR1 gene studies have shown that silent mutations play a role in the instability and aberrant splicing and folding of mRNAs. We believe that molecular and functional investigation of gigaxonin mutations including the exon 8 polymorphism could lead to an improved understanding of the relationship between GAN and cancer.

Measuring disease progression in giant axonal neuropathy: implications for clinical trial design

As part of a natural history study of giant axonal neuropathy, we hypothesized that the Friedreich Ataxia Rating Scale and the Gross Motor Function Measure would show a significant change over 6 months, reflecting subjects' decline in motor function. The Friedreich Ataxia Rating Scale was performed on 11 subjects and the Gross Motor Function Measure was performed on 10 subjects twice with a six-month interval. A paired two-tailed t-test was used to assess the difference in each subject's score. Significant changes were found over six months of 11.7 ± 11.0 (P = 0.006) for the Friedreich Ataxia Rating Scale and -10.0 ± 13.5 (P = 0.043) for the Gross Motor Function Measure, reflecting subjects' decline in motor function on examination and by report. These standardized assessments of clinical function are the first to be validated in giant axonal neuropathy and will be used in an upcoming gene therapy clinical trial.

Giant axonal neuropathy caused by a novel compound heterozygous mutation in the gigaxonin gene

Giant axonal neuropathy is a rare autosomal recessive disorder, which typically involves both central and peripheral nervous system. Yet the phenotypic-genotypic correlation remains obscure. We report a novel compound heterozygous mutation with the c. 805C>T in exon 4(Arg545His missense mutation) and the c. 1634G>A in exon 11(Arg269Trp missense mutation) in an 11-year-old Chinese giant axonal neuropathy case. This patient had an atypical giant axonal neuropathy phenotype rather similar to Charcot-Marie-Tooth disease, without tightly curled hair and mental retardation. The patient had a slowly progressive sensory motor neuropathy since age 3 years, and she also had nystagmus, feet deformities, scoliosis, and cerebellar tonsillar protrusion. Electrophysiological studies indicated a predominantly axonal sensory-motor neuropathy. The diagnosis was confirmed by sural nerve biopsy and direct sequencing of all the 11 gigaxonin exons. The proband's parents are heterozygotes of the disease without symptoms. Our findings extend the number of gigaxonin mutations that cause giant axonal neuropathy.

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.

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.

A novel mutation in the GAN gene causes an intermediate form of giant axonal neuropathy in an Arab-Israeli family

Giant axonal neuropathy is a severe autosomal recessive neurodegenerative disorder of childhood that affects both the peripheral and central nervous systems. It is caused by mutations in the GAN gene linked to chromosome 16q24.1 At least 45 distinct disease-causing mutations have been identified throughout the gene in families of various ethnic origins, with different symptomatologies and different clinical courses. To date, no characteristic mutation or phenotype-genotype correlation has been established. We describe a novel missense mutation in four siblings born to consanguineous parents of Arab original with clinical and molecular features compatible with giant axonal neuropathy. The phenotype was characterized by a predominant motor and sensory peripheral neuropathies and severe skeletal deformities.

Proteomic analysis in giant axonal neuropathy: new insights into disease mechanisms

INTRODUCTION

Giant axonal neuropathy (GAN) is a progressive hereditary disease that affects the peripheral and central nervous systems. It is characterized morphologically by aggregates of intermediate filaments in different tissues. Mutations have been reported in the gene that codes for gigaxonin. Nevertheless, the underlying molecular mechanism remains obscure.

METHODS

Cell lines from 4 GAN patients and 4 controls were analyzed by iTRAQ.

RESULTS

Among the dysregulated proteins were ribosomal protein L29, ribosomal protein L37, galectin-1, glia-derived nexin, and aminopeptidase N. Also, nuclear proteins linked to formin-binding proteins were found to be dysregulated. Although the major role of gigaxonin is reported to be degradation of cytoskeleton-associated proteins, the amount of 76 structural cytoskeletal proteins was unaltered.

CONCLUSIONS

Several of the dysregulated proteins play a role in cytoskeletal reorganization. Based on these findings, we speculate that disturbed cytoskeletal regulation is responsible for formation of aggregates of intermediate filaments.

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.

Modest loss of peripheral axons, muscle atrophy and formation of brain inclusions in mice with targeted deletion of gigaxonin exon 1

Mutations in the gigaxonin gene are responsible for giant axonal neuropathy (GAN), a progressive neurodegenerative disorder associated with abnormal accumulations of Intermediate Filaments (IFs). Gigaxonin is the substrate-specific adaptor for a new Cul3-E3-ubiquitin ligase family that promotes the proteasome dependent degradation of its partners MAP1B, MAP8 and tubulin cofactor B. Here, we report the generation of a mouse model with targeted deletion of Gan exon 1 (Gan(Deltaexon1;Deltaexon1)). Analyses of the Gan(Deltaexon1;Deltaexon1) mice revealed increased levels of various IFs proteins in the nervous system and the presence of IFs inclusion bodies in the brain. Despite deficiency of full length gigaxonin, the Gan(Deltaexon1;Deltaexon1) mice do not develop overt neurological phenotypes and giant axons reminiscent of the human GAN disease. Nonetheless, at 6 months of age the Gan(Deltaexon1;Deltaexon1) mice exhibit a modest hind limb muscle atrophy, a 10% decrease of muscle innervation and a 27% axonal loss in the L5 ventral roots. This new mouse model should provide a useful tool to test potential therapeutic approaches for GAN disease.