Myelin abnormality in Charcot-Marie-Tooth type 4J recapitulates features of acquired demyelination

A Homoplasmic MT-TV Mutation Associated with Mitochondrial Inheritance of Hereditary Spastic Paraplegia

BACKGROUND

Hereditary spastic paraplegia (HSP) is characterized by progressive lower limb weakness and spasticity, with unknown genetic cause in many cases.

OBJECTIVES

To identify novel genetic causes of HSP.

METHODS

Phenotypic characterization, genetic screening, transcriptome sequencing, and peroneal nerve biopsy were conducted in a Chinese HSP family.

RESULTS

We found a homoplasmic MT-TV (mitochondrial tRNAVal) mutation, m.1661A > G, present in all affected individuals across four generations of a family with complex HSP. Fourth-generation affected individuals displayed earlier onset, likely due to presumptive anticipation, and greater symptom severity, potentially caused by decreased mitochondrial DNA (mtDNA) copy number. Upregulation of mitochondrial autophagy genes in these patients suggested that MT-TV mutations could lead to reduced mtDNA copy number. Neural biopsies revealed ultrastructural abnormalities in myelin and mitochondria.

CONCLUSIONS

The rare MT-TV m.1661A > G mutation is associated with HSP. Variations in mtDNA copy number may play a causal role in differences among clinical phenotypes. © 2024 International Parkinson and Movement Disorder Society.

FIG4-Related Parkinsonism and the Particularities of the I41T Mutation: A Review of the Literature

Background/Objectives: The genetic underpinnings of Parkinson's disease (PD) and parkinsonism have drawn increasing attention in recent years. Mutations in the Factor-Induced Gene 4 (FIG4) have been implicated in various neurological disorders, including Charcot-Marie-Tooth disease type 4J (CMT4J), amyotrophic lateral sclerosis (ALS), and Yunis-Varón syndrome. This review aims to explore the association between FIG4 mutations and parkinsonism, with a specific focus on the rare missense mutation p.Ile41Thr (I41T). Methods: We identified 12 cases from 10 different families in which parkinsonism was reported in conjunction with CMT4J polyneuropathy. All cases involved the I41T mutation in a compound heterozygous state, combined with a FIG4 loss-of-function mutation. Data from clinical observations, neuroimaging studies, and genetic analyses were evaluated to understand the characteristics of parkinsonism in these patients. Results: In all 12 cases, parkinsonism developed either concurrently or following the onset of CMT4J neuropathy, but was never observed in isolation. Cases of both early- and late-onset parkinsonism were identified, reflecting similarities to genetic forms of parkinsonism with autosomal recessive inheritance. Imaging studies, including Dopamine transporter Single Photon Emission Computed Tomography (DaTscan) and brain magnetic resonance imaging (MRI), revealed abnormalities indicative of neurodegeneration, consistent with findings in other neurodegenerative disorders. Conclusions: The co-occurrence of parkinsonism with CMT4J in patients carrying the I41T mutation suggests an expanded spectrum of FIG4-related disorders, potentially implicating the same molecular mechanisms seen in other neurodegenerative disorders. Further research into FIG4-mediated pathways may offer valuable insights into potential therapeutic targets for disorders of both the central and peripheral nervous systems.

Clinical Characteristics of Charcot-Marie-Tooth Disease Type 4J

BACKGROUND AND OBJECTIVES

Charcot-Marie-Tooth disease type 4J (CMT4J) is caused by autosomal recessive variants in the Factor-Induced Gene 4 (FIG4) gene. Recent preclinical work has demonstrated the feasibility of adeno-associated virus serotype 9-FIG4 gene therapy. This study aimed to further characterize the CMT4J phenotype and evaluate feasibility of validated CMT-related outcome measures for future clinical trials.

METHODS

This cross-sectional study enrolled children and adults with genetically confirmed CMT4J, with 2 documented disease-causing variants in the FIG4 gene. Patients were recruited through the Inherited Neuropathy Consortium network. Disease severity was assessed using standardized CMT-specific outcome measures and exploratory biomarkers including muscle MRI fat fraction, electrophysiology, and neurofilament light chain levels. Descriptive statistics and correlation analyses were conducted to explore relationships between variables.

RESULTS

We recruited a total of 19 patients, including 14 pediatric patients (mean age 10.9 ± 3.9 years) and 5 adults (mean age 40.0 ± 13.9 years). The most frequent symptoms were gross motor delay and distal more than proximal muscle weakness, which were observed in 14 of 19 patients. The most common non-neuromuscular symptoms were cognitive and respiratory deficits, each seen in 8 of 19 patients. We denoted asymmetric weakness in 2 patients and nonuniform slowing of conduction velocities in 6 patients. Charcot-Marie-Tooth Disease Pediatric Scale (CMTPedS), Pediatric Quality of Life Inventory, and Vineland Adaptive Behavior Scale scores were affected in most patients. We observed a significant positive correlation between neurofilament light chain levels and CMTPedS, but the study was underpowered to observe a correlation between CMTPedS and MRI fat fraction.

DISCUSSION

We obtained baseline clinical and biomarker data in a broad cohort with CMT4J in pediatric and adult patients. Motor delay, muscle weakness, and respiratory and cognitive difficulties were the most common clinical manifestations of CMT4J. Many patients had nerve conduction studies with nonuniform slowing, and 2 had an asymmetric pattern of muscle weakness. We observed that the neurofilament light chain levels correlated with the CMTPedS in the pediatric population. This study showed feasibility of clinical outcomes including CMTPedS in assessment of disease severity in the pediatric patient population and provided baseline characteristics of exploratory biomarkers, neurofilament light chain levels, and muscle MRI fat fraction. The coronavirus disease 2019 pandemic affected some of the visits, resulting in a reduced number of some of the assessments.

Conduction slowing, conduction block and temporal dispersion in demyelinating, dysmyelinating and axonal neuropathies: Electrophysiology meets pathology

Nerve conduction studies are usually the first diagnostic step in peripheral nerve disorders and their results are the basis for planning further investigations. However, there are some commonplaces in the interpretation of electrodiagnostic findings in peripheral neuropathies that, although useful in the everyday practice, may be misleading: (1) conduction block and abnormal temporal dispersion are distinctive features of acquired demyelinating disorders; (2) hereditary neuropathies are characterized by uniform slowing of conduction velocity; (3) axonal neuropathies are simply diagnosed by reduced amplitude of motor and sensory nerve action potentials with normal or slightly slow conduction velocity. In this review, we reappraise the occurrence of uniform and non-uniform conduction velocity slowing, conduction block and temporal dispersion in demyelinating, dysmyelinating and axonal neuropathies attempting, with a translational approach, a correlation between electrophysiological and pathological features as derived from sensory nerve biopsy in patients and animal models. Additionally, we provide some hints to navigate in this complex field.

Clinical and genetic features of patients suffering from CMT4J

Mutations in the FIG4 gene have been identified in various diseases, including amyotrophic lateral sclerosis, Parkinson's disease, and Charcot-Marie-Tooth 4 J (CMT4J), with a wide range of phenotypic manifestations. We present eight cases of CMT4J patients carrying the p.Ile41Thr mutation of FIG4. The patients were categorized according to their phenotype. Six patients had a pure CMT; whereas, two patients had a CMT associated with parkinsonism. Three patients had an early onset and exhibited more severe forms of the disease. Three others experienced symptoms in their teenage years and had milder forms. Two patients had a late onset in adulthood. Four patients showed electrophysiological evidence of conduction blocks, typically associated with acquired neuropathies. Consequently, two of them received intravenous immunoglobulin treatment without a significant objective response. Interestingly, two heterozygous patients with the same mutations exhibited contrasting phenotypes, one having a severe early-onset form and the other experiencing a slow disease progression starting at the age of 49. Notably, although 7 out of 8 patients in this study were compound heterozygous for the p.Ile41Thr mutation, only one individual was found to be homozygous for this genetic variant and exhibited an early-onset, severe form of the disease. Additionally, one patient who developed the disease in his youth was also diagnosed with hereditary neuropathy with pressure palsies. Our findings provide insights into the CMT4J subtype by reporting on eight heterogeneous patient cases and highlight the potential for misdiagnosis when conduction blocks or asymmetrical nerve conduction study results are observed in patients with FIG4 mutations.

Gene therapy and other novel treatment approaches for Charcot-Marie-Tooth disease

There is still no effective drug treatment available for Charcot-Marie-Tooth disease (CMT). Current management relies on rehabilitation therapy, surgery for skeletal deformities, and symptomatic treatment. The challenge is to find disease-modifying therapies. Several approaches, including gene silencing (by means of ASO, siRNA, shRNA, miRNA, CRISPR-Cas9 editing), to counteract the PMP22 gene overexpression in the most frequent CMT1A type are under investigation. PXT3003 is the compound in the most advanced phase for CMT1A, as a second phase-III trial is ongoing. Gene therapy to substitute defective genes (particularly in recessive forms associated with loss-of-function mutations) or insert novel ones (e.g., NT3 gene) are being developed and tested in animal models and in still exceptional cases have reached the clinical trial phase in humans. Novel treatment approaches are also aimed at developing compounds acting on pathways important for different CMT types. Modulation of the neuregulin pathway determining myelin thickness is promising for both hypo-demyelinating and hypermyelinating neuropathies; intervention on Unfolded Protein Response seems effective for rescuing misfolded myelin proteins such as MPZ in CMT1B. HDAC6 inhibitors improved axonal transport and ameliorated phenotypes in different CMT models. Other potential therapeutic strategies include targeting macrophages, lipid metabolism, and Nav1.8 sodium channel in demyelinating CMT and the P2×7 receptor, which regulates calcium influx into Schwann cells, in CMT1A. Further approaches are aimed at correcting metabolic abnormalities, including the accumulation of sorbitol caused by biallelic mutations in the sorbitol dehydrogenase (SORD) gene and of neurotoxic glycosphingolipids in HSN1.

Case report and literature review: Novel compound heterozygous FIG4 variants causing both of peripheral and central nervous system defects

BACKGROUND

Pathogenic variants in the FIG4 gene have been described to be associated with a diverse spectrum of syndromes, such as autosomal recessive bilateral temporooccipital polymicrogyria (OMIM 612691), autosomal dominant amyotrophic lateral sclerosis-11 (ALS11; OMIM 612577), autosomal recessive Charcot-Marie-Tooth disease, type 4J (CMT4J; OMIM 611228), and autosomal recessive Yunis-Varon syndrome (YVS; OMIM 216340). Heterozygous FIG4 variants are responsible for ALS11 characterized by progressive muscular weakness, atrophy, and bulbar palsy. CMT4J is a disorder of peripheral nervous system defects mainly presenting with a highly variable onset of proximal and/or distal muscle weakness. YVS is a disorder of severe neurological involvement with central nervous system (CNS) dysfunction and extensive skeletal anomalies.

CASE PRESENTATION

We reported two Chinese siblings born with a weakness in all limbs. They experienced rapidly progressive weakness in distal limbs. At the age of 6 years, the elder brother presented with severe scoliosis and cervical kyphosis. They both had global developmental delay and a CNS involvement with cognitive deficits and swallowing problems. Genetic screening in the patients' family for inherited diseases was recommended. Novel compound heterozygous variants in the FIG4 gene (c.2148delTinsAA and c.317A > G) were found by whole-exome sequencing in the patients. These variants were confirmed by Sanger sequencing in family members.

CONCLUSIONS

Herein, we reported two Chinese male patients with CMT4J who presented with abnormal CNS features. CMT4J with CNS involvement has been very rarely reported. We hoped this study could expand the phenotypic and genetic spectrum of FIG4-related diseases. And we helped physicians to understand the genotype-phenotype correlation.

Mechanisms and Treatments in Demyelinating CMT

Demyelinating forms of Charcot-Marie-Tooth disease (CMT) are genetically and phenotypically heterogeneous and result from highly diverse biological mechanisms including gain of function (including dominant negative effects) and loss of function. While no definitive treatment is currently available, rapid advances in defining the pathomechanisms of demyelinating CMT have led to promising pre-clinical studies, as well as emerging clinical trials. Especially promising are the recently completed pre-clinical genetic therapy studies in PMP-22, GJB1, and SH3TC2-associated neuropathies, particularly given the success of similar approaches in humans with spinal muscular atrophy and transthyretin familial polyneuropathy. This article focuses on neuropathies related to mutations in PMP-22, MPZ, and GJB1, which together comprise the most common forms of demyelinating CMT, as well as on select rarer forms for which promising treatment targets have been identified. Clinical characteristics and pathomechanisms are reviewed in detail, with emphasis on therapeutically targetable biological pathways. Also discussed are the challenges facing the CMT research community in its efforts to advance the rapidly evolving biological insights to effective clinical trials. These considerations include the limitations of currently available animal models, the need for personalized medicine approaches/allele-specific interventions for select forms of demyelinating CMT, and the increasing demand for optimal clinical outcome assessments and objective biomarkers.

Animal Models as a Tool to Design Therapeutical Strategies for CMT-like Hereditary Neuropathies

Since ancient times, animal models have provided fundamental information in medical knowledge. This also applies for discoveries in the field of inherited peripheral neuropathies (IPNs), where they have been instrumental for our understanding of nerve development, pathogenesis of neuropathy, molecules and pathways involved and to design potential therapies. In this review, we briefly describe how animal models have been used in ancient medicine until the use of rodents as the prevalent model in present times. We then travel along different examples of how rodents have been used to improve our understanding of IPNs. We do not intend to describe all discoveries and animal models developed for IPNs, but just to touch on a few arbitrary and paradigmatic examples, taken from our direct experience or from literature. The idea is to show how strategies have been developed to finally arrive to possible treatments for IPNs.

Hereditary neuropathies: A pathological perspective

Hereditary neuropathies may result from mutations in genes expressed by Schwann cells or neurons that affect selectively the peripheral nervous system (PNS) or may represent a minor or major component of complex inherited diseases that involve also the central nervous system and/or other organs and tissues. The chapter is constantly expanding and reworking, thanks to advances of molecular genetics; next-generation sequencing is identifying a plethora of new genes and is revolutionizing the diagnostic approach. In the past, diagnostic sural nerve biopsies paved the way to the discovery and elucidation of major genes and molecular pathways associated to most frequent hereditary motor-sensory neuropathies. Nowadays, a sural nerve biopsy may prove useful in selected cases for the differential diagnosis of an acquired neuropathy when clinical examination, nerve conduction studies, and molecular tests are not sufficiently informative. Skin biopsy has emerged as a minimally invasive window on the PNS, which may provide biomarkers of progression and clues to the physiopathology and molecular pathology of inherited neuropathies. The aim of our review is to illustrate the pathological features of more frequent and paradigmatic hereditary neuropathies and to highlight their correlations with the roles of the involved genes and functional consequences of related molecular defects.

AAV9-mediated FIG4 delivery prolongs life span in Charcot-Marie-Tooth disease type 4J mouse model

Charcot-Marie-Tooth disease type 4J (CMT4J) is caused by recessive, loss-of-function mutations in FIG4, encoding a phosphoinositol(3,5)P2-phosphatase. CMT4J patients have both neuron loss and demyelination in the peripheral nervous system, with vacuolization indicative of endosome/lysosome trafficking defects. Although the disease is highly variable, the onset is often in childhood and FIG4 mutations can dramatically shorten life span. There is currently no treatment for CMT4J. Here, we present the results of preclinical studies testing a gene-therapy approach to restoring FIG4 expression. A mouse model of CMT4J, the Fig4-pale tremor (plt) allele, was dosed with a single-stranded adeno-associated virus serotype 9 (AAV9) to deliver a codon-optimized human FIG4 sequence. Untreated, Fig4plt/plt mice have a median survival of approximately 5 weeks. When treated with the AAV9-FIG4 vector at P1 or P4, mice survived at least 1 year, with largely normal gross motor performance and little sign of neuropathy by neurophysiological or histopathological evaluation. When mice were treated at P7 or P11, life span was still significantly prolonged and peripheral nerve function was improved, but rescue was less complete. No unanticipated adverse effects were observed. Therefore, AAV9-mediated delivery of FIG4 is a well-tolerated and efficacious strategy in a mouse model of CMT4J.

Adeno-associated virus gene therapy to the rescue for Charcot-Marie-Tooth disease type 4J

The genetic peripheral neuropathy known as Charcot-Marie-Tooth disease type 4J (CMT4J) is caused by recessive mutations in the FIG4 gene. The transformational success of adeno-associated virus (AAV) gene therapy for spinal muscular atrophy has generated substantial interest in using this approach to create similar treatments for CMT. In this issue of the JCI, Presa et al. provide a preclinical demonstration of efficacy using AAV-directed gene therapy for CMT4J. The study showed a dramatic improvement in both survival and neuropathy symptoms in a severe mouse model of CMT4J after administration of AAV gene therapy at several time points. The authors' approach advances the technique for delivering treatments to individuals with CMT, for which FDA-approved therapies have not yet come to the clinic.

The Complex Clinical and Genetic Landscape of Hereditary Peripheral Neuropathy

Hereditary peripheral neuropathy (HPN) is a complex group of neurological disorders caused by mutations in genes expressed by neurons and Schwann cells. The inheritance of a single mutation or multiple mutations in several genes leads to disease phenotype. Patients exhibit symptoms during development, at an early age or later in adulthood. Most of the mechanistic understanding about these neuropathies comes from animal models and histopathological analyses of postmortem human tissues. Diagnosis is often very complex due to the heterogeneity and overlap in symptoms and the frequent overlap between various genes and different mutations they possess. Some symptoms in HPN are common through different subtypes such as axonal degeneration, demyelination, and loss of motor and sensory neurons, leading to similar physiologic abnormalities. Recent advances in gene-targeted therapies, genetic engineering, and next-generation sequencing have augmented our understanding of the underlying pathogenetic mechanisms of HPN.

CIDP mimics: a case series

Background

To report our experience with a group of patients referred for refractory CIDP who fulfilled “definite” electrodiagnostic EFNS criteria for CIDP but were found to have an alternate diagnosis.

Methods

Patients who were seen between 2017 and 2019 for refractory CIDP that fulfilled “definite” electrodiagnostic ENFS criteria for CIDP, but had an alternate diagnosis, were included. Patients who correctly had CIDP, anti MAG neuropathy, or MMN with conduction block, were excluded from the study. Demographics, clinical and electrophysiological characteristics, pertinent workup, final alternate diagnoses, and outcomes were collected.

Results

Seven patients were included: POEMS (n = 5), CANOMAD (n = 1), and neurolymphomatosis (n = 1). Most patients reported neuropathic pain and leg swelling (n = 6) or significant weight loss (n = 4). All patients had a monoclonal protein, and most patients who were tested had an elevated VEGF and CSF cyto-albuminologic dissociation. Electrophysiology showed pronounced intermediate more than distal demyelination, and axonal loss in the lower extremities. Response to steroids or IVIG varied, but some patients did respond to these treatments, especially early in the disease.

Conclusion

Pain, systemic symptoms, suggestive electrophysiological findings, and/or a serum monoclonal protein should raise suspicion for CIDP mimics. Initial response to steroids or IVIG, over reliance on CSF, and electrophysiology findings can all be misleading.

Clinical features of homozygous FIG4-p.Ile41Thr Charcot-Marie-Tooth 4J patients

We describe the clinical, electrodiagnostic, and genetic findings of three homozygous FIG4‐c.122T>C patients suffering from Charcot‐Marie‐Tooth disease type 4J (AR‐CMT‐FIG4). This syndrome usually involves compound heterozygosity associating FIG4‐c.122T>C, a hypomorphic allele coding an unstable FIG4‐p.Ile41Thr protein, and a null allele. While the compound heterozygous patients presenting with early onset usually show rapid progression, the homozygous patients described here show the signs of relative clinical stability. As FIG4 activity is known to be dose dependent, these patients’ observations could suggest that the therapeutic perspective of increasing levels of the protein to improve the phenotype of AR‐CMT‐FIG4‐patients might be efficient.

Reducing monocarboxylate transporter MCT1 worsens experimental diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is one of the most common complications in diabetic patients. Though the exact mechanism for DPN is unknown, it clearly involves metabolic dysfunction and energy failure in multiple cells within the peripheral nervous system. Lactate is an alternate source of metabolic energy that is increasingly recognized for its role in supporting neurons. The primary transporter for lactate in the nervous system, monocarboxylate transporter-1 (MCT1), has been shown to be critical for peripheral nerve regeneration and metabolic support to neurons/axons. In this study, MCT1 was reduced in both sciatic nerve and dorsal root ganglia in wild-type mice treated with streptozotocin (STZ), a common model of type-1 diabetes. Heterozygous MCT1 null mice that developed hyperglycemia following STZ treatment developed a more severe DPN compared to wild-type mice, as measured by greater axonal demyelination, decreased peripheral nerve function, and increased numbness to innocuous low-threshold mechanical stimulation. Given that MCT1 inhibitors are being developed as both immunosuppressive and chemotherapeutic medications, our results suggest that clinical development in patients with diabetes should proceed with caution. Collectively, our findings uncover an important role for MCT1 in DPN and provide a potential lead toward developing novel treatments for this currently untreatable disease.

Charcot-Marie-Tooth disease type 4J with spastic quadriplegia, epilepsy and global developmental delay: a tale of three siblings

Charcot-Marie-Tooth (CMT) disease is mainly a disease of peripheral nervous system and patients typically present with features of demyelinating neuropathy or axonal neuropathy or both. Rarely patients present with features of central nervous system involvement. Parkinsonism, aphemia and familial epilepsy syndrome have previously come up as case reports in association with CMT type 4 J.We hereby describe a family with 3 siblings affected with CMT4J with homozygous FIG4 mutation who presented with global developmental delay, epilepsy and spastic quadriparesis.

Recent Advances in Drosophila Models of Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth disease (CMT) is one of the most common inherited peripheral neuropathies. CMT patients typically show slowly progressive muscle weakness and sensory loss in a distal dominant pattern in childhood. The diagnosis of CMT is based on clinical symptoms, electrophysiological examinations, and genetic testing. Advances in genetic testing technology have revealed the genetic heterogeneity of CMT; more than 100 genes containing the disease causative mutations have been identified. Because a single genetic alteration in CMT leads to progressive neurodegeneration, studies of CMT patients and their respective models revealed the genotype-phenotype relationships of targeted genes. Conventionally, rodents and cell lines have often been used to study the pathogenesis of CMT. Recently, Drosophila has also attracted attention as a CMT model. In this review, we outline the clinical characteristics of CMT, describe the advantages and disadvantages of using Drosophila in CMT studies, and introduce recent advances in CMT research that successfully applied the use of Drosophila, in areas such as molecules associated with mitochondria, endosomes/lysosomes, transfer RNA, axonal transport, and glucose metabolism.

Unusual electrophysiological findings in a Chinese ALS 4 family with SETX-L389S mutation: a three-year follow-up

Amyotrophic lateral sclerosis type 4 (ALS4) is a familial form of ALS caused by mutations in the SETX gene. To date, there are seven unrelated ALS4 families with four missense mutations (L389S, T31I, R2136H, and M386T) in SETX. ALS4 is characterized by early onset, distal muscle weakness and atrophy, pyramidal signs, and the absence of sensory deficits. Motor conduction studies often present normality or reduced amplitudes of compound muscle action potential (CMAP). The conduction blocks (CBs) are rare and only observed in one male of an Italian ALS4 family. Our study showed that seven symptomatic patients presented the classical ALS4 phenotype with two asymptomatic females in a Chinese family spanning three generations. Sequencing analysis revealed a heterozygous c.1166T > C/p.L389S mutation in SETX that co-segregated with disease phenotype in the family. The same mutation has been identified previously in three ALS4 families from the United States and Italy, respectively. Specifically, three young males presented multiple CBs and abnormal temporal dispersions (TD) in the median, ulnar and tibial nerves over the three-year follow-up period. Moreover, for the first time, we found that senataxin was also expressed in the myelin sheath of peripheral nerves besides axons. The study indicates that CBs and abnormal TD are the characteristics in the ALS4 family, providing pivotal familial evidence of CBs and TD of motor nerves in ALS4. The unusual electrophysiological features may be associated with the expression of senataxin in peripheral nerves.

Demyelination in hereditary sensory neuropathy type-1C

OBJECTIVE

Sphingolipids are enriched in the nerves. Serine-palmitoyltransferase (SPT) catalyzes the key step of sphingolipids biosynthesis. Mutations in SPT subunits (SPTLC) lead to the excessive production of neurotoxic deoxysphingolipids (DoxSLs) in patients with Hereditary Sensory Neuropathy Type-1C (HSN1C). HSN1C is an autosomal dominant peripheral neuropathy characterized by sensory loss and distal muscle weakness. In this study, by leveraging a HSN1C family with a previously reported N177D mutation in SPTLC2, we aim to further define the spectrum of DoxSL species and the peripheral neve pathology of the disease.

METHODS

Next-generation sequencing along with Sanger confirmation was performed for family members and healthy controls. LC-MS was used for lipidomic analysis in participants' plasma. Quantitative magnetic resonance imaging (qMRI) was performed to study sciatic nerve pathologies.

RESULTS

A heterozygous N177D mutation in SPTLC2 was co-segregated in individuals with sensory-motor deficits in the limbs. Nerve conduction studies (NCS) revealed nonuniform slowing of conduction velocities. In line with the NCS, qMRI detected a pattern of nerve changes similar to those in acquired demyelinating polyneuropathies. Additionally, we detected a significant increase in multiple species of deoxysphingoid bases and deoxyceramides in patients' plasma.

INTERPRETATION

Mutations in the SPTLC2 cause a demyelinating phenotype resembling those in acquired demyelinating polyneuropathy. The species of increased DoxSLs in HSN1C may be more diverse than originally thought.

Clinical and radiological characterization of novel FIG4-related combined system disease with neuropathy

Variants in the FIG4 gene, which encodes a phosphatidylinositol-3,5-bisphosphatase lead to obstruction of endocytic trafficking, causing accumulation of enlarged vesicles in murine peripheral neurons and fibroblasts. Bi-allelic pathogenic variants in FIG4 are associated with neurological disorders including Charcot-Marie-Tooth disease type-4J (CMT4J) and Yunis-Varón syndrome (YVS). We present four probands from three unrelated families, all homozygous for a recurrent FIG4 missense variant c.506A>C p.(Tyr169Ser), with a novel phenotype involving features of both CMT4J and YVS. Three presented with infant-onset dystonia and one with hypotonia. All have depressed lower limb reflexes and distal muscle weakness, two have nerve conduction studies (NCS) consistent with severe sensorimotor demyelinating peripheral neuropathy and one had NCS showing patchy intermediate/mildly reduced motor conduction velocities. All have cognitive impairment and three have swallowing difficulties. MRI showed cerebellar atrophy and bilateral T2 hyperintense medullary swellings in all patients. These children represent a novel clinicoradiological phenotype and suggest that phenotypes associated with FIG4 missense variants do not neatly fall into previously described diagnoses but can present with variable features. Analysis of this gene should be considered in patients with central and peripheral neurological signs and medullary radiological changes, providing earlier diagnosis and informing reproductive choices.

FIG4 mutations leading to parkinsonism and a phenotypical continuum between CMT4J and Yunis Varón syndrome

BACKGROUND

Charcot-Marie-Tooth disease type 4J (CMT4J) originates from mutations in the FIG4 gene and leads to distal muscle weakness. Two null alleles of FIG4 cause Yunis Varón syndrome with severe central nervous system involvement, cleidocranial dysmorphism, absent thumbs and halluces and early death.

OBJECTIVES

To analyse the phenotypic spectrum of FIG4-related disease and explore effects of residual FIG4 protein.

METHODS

Phenotyping of five new patients with FIG4-related disease. Western Blot analyses of FIG4 from patient fibroblasts.

RESULTS

Next generation sequencing revealed compound heterozygous variants in FIG4 in five patients. All five patients presented with peripheral neuropathy, various degree of dysmorphism and a central nervous system involvement comprising Parkinsonism in 3/5 patients, cerebellar ataxia (1/5), spasticity of lower limbs (1/5), epilepsy (1/5) and/or cognitive deficits (2/5). Onset varied between the first and the seventh decade. There was no residual FIG4 protein detectable in fibroblasts of the four analysed patients.

CONCLUSION

This study extends the phenotypic spectrum of FIG4-related disease to Parkinsonism as a feature and demonstrates new phenotypes on a continuum between CMT4J and Yunis Varón syndrome.

Whole exome sequencing establishes diagnosis of Charcot-Marie-Tooth 4J, 1C, and X1 subtypes

BACKGROUND

Charcot-Marie-Tooth (CMT) hereditary polyneuropathies pose a diagnostic challenge. Our aim here is to describe CMT patients diagnosed by whole exome sequencing (WES) following years of fruitless testing.

METHODS/RESULTS

Three patients with polyneuropathy suspected to be genetic in origin, but not harboring PMP22 gene deletion/duplication, were offered WES. The first patient, a 66-year-old man, had been suffering from progressive weakness and atrophies in the lower and upper extremities for 20 years. Due to ambiguous electrophysiological findings, immune therapies were administered to no avail. Twelve years after PMP22 deletion/duplication testing, WES revealed two pathogenic variants in the FIG4 gene (p.Ile41Thr and p.Phe598fs, respectively), as a cause of CMT 4J. The second patient, a 19-year-old man, had been suffering from hearing and gait impairment since at least his infancy, and recently presented with weakness and dystonia of the lower extremities. In this patient, WES identified the p.Leu122Val LITAF gene variant in heterozygous state, suggesting the diagnosis of CMT 1C, several years after initial genetic analyses. The third patient, a 44-year-old man, presented with progressive weakness and atrophies of the lower and upper extremities since the age of 17 years old. In this patient, WES identified the hemizygous p.Arg164Gln pathogenic variant in the GJB1 gene, establishing the diagnosis of CMT X1, 8 years after testing for PMP22 deletion/duplication.

CONCLUSION

Novel diagnostic techniques, such as WES, offer the possibility to decipher the cause of CMT subtypes, ending the diagnostic Odyssey of the patients and sparing them from unnecessary and potentially harmful treatments.

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.

Severe Consequences of SAC3/FIG4 Phosphatase Deficiency to Phosphoinositides in Patients with Charcot-Marie-Tooth Disease Type-4J

Charcot-Marie-Tooth disease type-4J (CMT4J), an autosomal recessively inherited peripheral neuropathy characterized by neuronal degeneration, segmental demyelination, and limb muscle weakness, is caused by compound heterozygous mutations in the SAC3/FIG4 gene, resulting in SAC3/FIG4 protein deficiency. SAC3/FIG4 is a phosphatase that not only turns over PtdIns(3,5)P2 to PtdIns3P but also promotes PtdIns(3,5)P2 synthesis by activating the PIKFYVE kinase that also makes PtdIns5P. Whether CMT4J patients have alterations in PtdIns(3,5)P2, PtdIns5P or in other phosphoinositides (PIs), and if yes, in what direction these changes might be, has never been examined. We performed PI profiling in primary fibroblasts from a cohort of CMT4J patients. Subsequent to myo-[2-3H]inositol cell labeling to equilibrium, steady-state levels of PIs were quantified by HPLC under conditions concurrently detecting PtdIns5P, PtdIns(3,5)P2, and the other PIs. Immunoblotting verified SAC3/FIG4 depletion in CMT4J fibroblasts. Compared to normal human controls (n = 9), both PtdIns(3,5)P2 and PtdIns5P levels were significantly decreased in CMT4J fibroblasts (n = 13) by 36.4 ± 3.6% and 43.1 ± 4.4%, respectively (p < 0.0001). These reductions were independent of patients' gender or disease onset. Although mean values for PtdIns3P in the CMT4J cohort remained unchanged, there were high variations in PtdIns3P among individual patients. Aberrant endolysosomal vacuoles, typically seen under PtdIns(3,5)P2 reduction, were apparent but not in fibroblasts from all patients. The subset of patients without aberrant vacuoles exhibited especially low PtdIns3P levels. Concomitant decreases in PtdIns5P and PtdIns(3,5)P2 and the link between PtdIns3P levels and cellular vacuolization are novel insights shedding further light into the molecular determinants in CMT4J polyneuropathy.

Peripheral nerve magnetic resonance imaging

Magnetic resonance imaging (MRI) has been used extensively in revealing pathological changes in the central nervous system. However, to date, MRI is very much underutilized in evaluating the peripheral nervous system (PNS). This underutilization is generally due to two perceived weaknesses in MRI: first, the need for very high resolution to image the small structures within the peripheral nerves to visualize morphological changes; second, the lack of normative data in MRI of the PNS and this makes reliable interpretation of the data difficult. This article reviews current state-of-the-art capabilities in in vivo MRI of human peripheral nerves. It aims to identify areas where progress has been made and those that still require further improvement. In particular, with many new therapies on the horizon, this review addresses how MRI can be used to provide non-invasive and objective biomarkers in the evaluation of peripheral neuropathies. Although a number of techniques are available in diagnosing and tracking pathologies in the PNS, those techniques typically target the distal peripheral nerves, and distal nerves may be completely degenerated during the patient’s first clinic visit. These techniques may also not be able to access the proximal nerves deeply embedded in the tissue. Peripheral nerve MRI would be an alternative to circumvent these problems. In order to address the pressing clinical needs, this review closes with a clinical protocol at 3T that will allow high-resolution, high-contrast, quantitative MRI of the proximal peripheral nerves.