Peripheral neuropathy in complex inherited diseases: an approach to diagnosis

Genetics of inherited peripheral neuropathies and the next frontier: looking backwards to progress forwards

Inherited peripheral neuropathies (IPNs) encompass a clinically and genetically heterogeneous group of disorders causing length-dependent degeneration of peripheral autonomic, motor and/or sensory nerves. Despite gold-standard diagnostic testing for pathogenic variants in over 100 known associated genes, many patients with IPN remain genetically unsolved. Providing patients with a diagnosis is critical for reducing their 'diagnostic odyssey', improving clinical care, and for informed genetic counselling. The last decade of massively parallel sequencing technologies has seen a rapid increase in the number of newly described IPN-associated gene variants contributing to IPN pathogenesis. However, the scarcity of additional families and functional data supporting variants in potential novel genes is prolonging patient diagnostic uncertainty and contributing to the missing heritability of IPNs. We review the last decade of IPN disease gene discovery to highlight novel genes, structural variation and short tandem repeat expansions contributing to IPN pathogenesis. From the lessons learnt, we provide our vision for IPN research as we anticipate the future, providing examples of emerging technologies, resources and tools that we propose that will expedite the genetic diagnosis of unsolved IPN families.

Peripheral Nervous System (PNS) Myelin Diseases

This is a review of inherited and acquired causes of human demyelinating neuropathies and a subset of disorders that affect axon-Schwann cell interactions. Nearly all inherited demyelinating neuropathies are caused by mutations in genes that are expressed by myelinating Schwann cells, affecting diverse functions in a cell-autonomous manner. The most common acquired demyelinating neuropathies are Guillain-Barré syndrome and chronic, inflammatory demyelinating polyneuropathy, both of which are immune-mediated. An additional group of inherited and acquired disorders affect axon-Schwann cell interactions in the nodal region. Overall, these disorders affect the formation of myelin and its maintenance, with superimposed axonal loss that is clinically important.

Early Onset Inherited Peripheral Neuropathies: The Experience of a Specialized Referral Center for Genetic Diagnosis Achievement

BACKGROUND

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of inherited peripheral neuropathies. Although the typical disease onset is reported in the second decade, earlier onsets are not uncommon. To date, few studies on pediatric populations have been conducted and the achievement of molecular diagnosis remains challenging.

METHODS

During the last 24 years we recruited 223 patients with early-onset hereditary peripheral neuropathies (EOHPN), negative for PMP22 duplication, 72 of them referred by a specialized pediatric hospital. Genetic testing for CMT-associated genes has been carried out with a range of different techniques.

RESULTS

Of the 223 EOHPN cases, 43% were classified as CMT1 (demyelinating), 49% as CMT2 (axonal), and 8% as CMTi (intermediate). Genetic diagnosis was reached in 51% of patients, but the diagnostic yield increased to 67% when focusing only on cases from the specialized pediatric neuromuscular centers. Excluding PMP22 rearrangements, no significant difference in diagnostic rate between demyelinating and axonal forms was identified. De novo mutations account for 38% of cases.

CONCLUSIONS

This study describes an exhaustive picture of EOHPN in an Italian referral genetic center and analyzes the molecular diagnostic rate of a heterogeneous cohort compared with one referred by a specialized pediatric center. Our data identify MPZ, MFN2, GDAP1, and SH3TC2 genes as the most frequent players in EOHPN. Our study underlines the relevance of a specific neurological pediatric expertise to address the genetic testing and highlights its importance to clarify possible unexpected results when neuropathy is only a secondary clinical sign of a more complex phenotype.

Molecular mechanisms and therapeutic strategies for neuromuscular diseases

Neuromuscular diseases encompass a heterogeneous array of disorders characterized by varying onset ages, clinical presentations, severity, and progression. While these conditions can stem from acquired or inherited causes, this review specifically focuses on disorders arising from genetic abnormalities, excluding metabolic conditions. The pathogenic defect may primarily affect the anterior horn cells, the axonal or myelin component of peripheral nerves, the neuromuscular junction, or skeletal and/or cardiac muscles. While inherited neuromuscular disorders have been historically deemed not treatable, the advent of gene-based and molecular therapies is reshaping the treatment landscape for this group of condition. With the caveat that many products still fail to translate the positive results obtained in pre-clinical models to humans, both the technological development (e.g., implementation of tissue-specific vectors) as well as advances on the knowledge of pathogenetic mechanisms form a collective foundation for potentially curative approaches to these debilitating conditions. This review delineates the current panorama of therapies targeting the most prevalent forms of inherited neuromuscular diseases, emphasizing approved treatments and those already undergoing human testing, offering insights into the state-of-the-art interventions.

Recurrent "outsider" intronic variation in the SLC5A6 gene causes severe mixed axonal and demyelinating neuropathy, cyclic vomiting and optic atrophy in 3 families from Maghreb

Sodium dependent multivitamin transporter (SMVT) deficiency is a very rare autosomal recessive disorder characterized by multisystemic clinical manifestations due to combined biotin, panthotenic acid and lipoic acid deficiency. About 10 families have been described so far. Accurate diagnosis is crucial because of the possibility of a supplementation treatment with proven efficacy. Here we describe 4 new patients (3 additional families) originating from the same world region (Algeria, Maghreb). All patients, born form consanguineous parents, were homozygous carriers of the same intronic variation, outside of canonical sites, in the SLC5A6 gene encoding SMVT. RNA study in one family allowed confirming the pathogenic effect of the variation and re-classifying this variant of uncertain significance as pathogenic, opening the possibility of genetic counseling and treatment. The identification of the same variation in three distinct and apparently unrelated families is suggestive of a founder effect. The phenotype of all patients was very similar, with systematic optic atrophy (initially considered as a very rare sign), severe cyclic vomiting, and rapidly progressive mixed axonal and demyelinating sensory motor neuropathy.

Charcot-Marie-Tooth disease: from historical landmarks in Brazil to current care perspectives

Hereditary motor and sensory neuropathy, also known as Charcot-Marie-Tooth disease (CMT), traditionally refers to a group of genetic disorders in which neuropathy is the main or sole feature. Its prevalence varies according to different populations studied, with an estimate between 1:2,500 to 1:10,000. Since the identification of PMP22 gene duplication on chromosome 17 by Vance et al., in 1989, more than 100 genes have been related to this group of disorders, and we have seen advances in the care of patients, with identification of associated conditions and better supportive treatments, including clinical and surgical interventions. Also, with discoveries in the field of genetics, including RNA interference and gene editing techniques, new treatment perspectives begin to emerge. In the present work, we report the most import landmarks regarding CMT research in Brazil and provide a comprehensive review on topics such as frequency of different genes associated with CMT in our population, prevalence of pain, impact on pregnancy, respiratory features, and development of new therapies.

The genetic and clinical spectrum in a cohort of 39 families with complex inherited peripheral neuropathies

With complicated conditions and a large number of potentially causative genes, the diagnosis of a patient with complex inherited peripheral neuropathies (IPNs) is challenging. To provide an overview of the genetic and clinical features of 39 families with complex IPNs from central south China and to optimize the molecular diagnosis approach to this group of heterogeneous diseases, a total of 39 index patients from unrelated families were enrolled, and detailed clinical data were collected. TTR Sanger sequencing, hereditary spastic paraplegia (HSP) gene panel, and dynamic mutation detection in spinocerebellar ataxia (SCAs) were performed according to the respective additional clinical features. Whole-exome sequencing (WES) was used in patients with negative or unclear results. Dynamic mutation detection in NOTCH2NLC and RCF1 was applied as a supplement to WES. As a result, an overall molecular diagnosis rate of 89.7% was achieved. All 21 patients with predominant autonomic dysfunction and multiple organ system involvement carried pathogenic variants in TTR, among which nine had c.349G > T (p.A97S) hotspot variants. Five out of 7 patients (71.4%) with muscle involvement harbored biallelic pathogenic variants in GNE. Five out of 6 patients (83.3%) with spasticity reached definite genetic causes in SACS, KIF5A, BSCL2, and KIAA0196, respectively. NOTCH2NLC GGC repeat expansions were identified in all three cases accompanied by chronic coughing and in one patient accompanied by cognitive impairment. The pathogenic variants, p.F284S and p.G111R in GNE, and p.K4326E in SACS, were first reported. In conclusion, transthyretin amyloidosis with polyneuropathy (ATTR-PN), GNE myopathy, and neuronal intranuclear inclusion disease (NIID) were the most common genotypes in this cohort of complex IPNs. NOTCH2NLC dynamic mutation testing should be added to the molecular diagnostic workflow. We expanded the genetic and related clinical spectrum of GNE myopathy and ARSACS by reporting novel variants.

Novel Therapeutic Approaches in Inherited Neuropathies: A Systematic Review

The management of inherited neuropathies relies mostly on the treatment of symptoms. In recent years, a better understanding of the pathogenic mechanisms that underlie neuropathies has allowed for the development of disease-modifying therapies. Here, we systematically review the therapies that have emerged in this field over the last five years. An updated list of diseases with peripheral neuropathy as a clinical feature was created based on panels of genes used clinically to diagnose inherited neuropathy. This list was extended by an analysis of published data by the authors and verified by two experts. A comprehensive search for studies of human patients suffering from one of the diseases in our list yielded 28 studies that assessed neuropathy as a primary or secondary outcome. Although the use of various scales and scoring systems made comparisons difficult, this analysis identified diseases associated with neuropathy for which approved therapies exist. An important finding is that the symptoms and/or biomarkers of neuropathies were assessed only in a minority of cases. Therefore, further investigation of treatment efficacy on neuropathies in future trials must employ objective, consistent methods such as wearable technologies, motor unit indexes, MRI or sonography imaging, or the use of blood biomarkers associated with consistent nerve conduction studies.

Trials for Slowly Progressive Neurogenetic Diseases Need Surrogate Endpoints

Heritable neurological disorders provide insights into disease mechanisms that permit development of novel therapeutic approaches including antisense oligonucleotides, RNA interference, and gene replacement. Many neurogenetic diseases are rare and slowly progressive making it challenging to measure disease progression within short time frames. We share our experience developing clinical outcome assessments and disease biomarkers in the inherited peripheral neuropathies. We posit that carefully developed biomarkers from imaging, plasma, or skin can predict meaningful progression in functional and patient reported outcome assessments such that clinical trials of less than 2 years will be feasible for these rare and ultra-rare disorders. ANN NEUROL 2023;93:906-910.

Comprehensive association analysis of speech recognition thresholds after cisplatin-based chemotherapy in survivors of adult-onset cancer

PURPOSE

Deficits in speech understanding constitute one of the most severe consequences of hearing loss. Here we investigate the clinical and genetic risk factors for symmetric deterioration of speech recognition thresholds (SRT) among cancer survivors treated with cisplatin.

METHODS

SRT was measured using spondaic words and calculating the mean of measurements for both ears with symmetric SRT values. For clinical associations, SRT-based hearing disability (SHD) was defined as SRT≥15 dB hearing loss and clinical variables were derived from the study dataset. Genotyped blood samples were used for GWAS with rank-based inverse normal transformed SRT values as the response variable. Age was used as a covariate in association analyses.

RESULTS

SHD was inversely associated with self-reported health (p = 0.004). Current smoking (p = 0.002), years of smoking (p = 0.02), BMI (p < 0.001), and peripheral motor neuropathy (p = 0.003) were positively associated with SHD, while physical activity was inversely associated with SHD (p = 0.005). In contrast, cumulative cisplatin dose, peripheral sensory neuropathy, hypertension, and hypercholesterolemia were not associated with SHD. Although no genetic variants had an association p value < 5 × 10-8 , 22 genetic variants were suggestively associated (p < 10-5 ) with SRT deterioration. Three of the top variants in 10 respective linkage disequilibrium regions were either positioned within the coding sequence or were eQTLs for genes involved in neuronal development (ATE1, ENAH, and ZFHX3).

CONCLUSION

Current results improve our understanding of risk factors for SRT deterioration in cancer survivors. Higher BMI, lower physical activity, and smoking are associated with SHD. Larger samples would allow for expansion of the current findings on the genetic architecture of SRT.

Strategy for genetic analysis in hereditary neuropathy

Inherited neuropathies are a heterogeneous group of slowly progressive disorders affecting either motor, sensory, and/or autonomic nerves. Peripheral neuropathy may be the major component of a disease such as Charcot-Marie-Tooth disease or a feature of a more complex multisystemic disease involving the central nervous system and other organs. The goal of this review is to provide the clinical clues orientating the genetic diagnosis in a patient with inherited peripheral neuropathy. This review focuses on primary inherited neuropathies, amyloidosis, inherited metabolic diseases, while detailing clinical, neurophysiological and potential treatment of these diseases.

A National French Consensus on Gene List for the Diagnosis of Charcot–Marie–Tooth Disease and Related Disorders Using Next-Generation Sequencing

Next generation sequencing (NGS) is strategically used for genetic diagnosis in patients with Charcot–Marie–Tooth disease (CMT) and related disorders called non-syndromic inherited peripheral neuropathies (NSIPN) in this paper. With over 100 different CMT-associated genes involved and ongoing discoveries, an important interlaboratory diversity of gene panels exists at national and international levels. Here, we present the work of the French National Network for Rare Neuromuscular Diseases (FILNEMUS) genetic diagnosis section which coordinates the seven French diagnosis laboratories using NGS for peripheral neuropathies. This work aimed to establish a unique, simple and accurate gene classification based on literature evidence. In NSIPN, three subgroups were usually distinguished: (1) HMSN, Hereditary Motor Sensory Neuropathy, (2) dHMN, distal Hereditary Motor Neuropathy, and (3) HSAN, Hereditary Sensory Autonomic Neuropathy. First, we reported ClinGen evaluation, and second, for the genes not evaluated yet by ClinGen, we classified them as “definitive” if reported in at least two clinical publications and associated with one report of functional evidence, or “limited” otherwise. In total, we report a unique consensus gene list for NSIPN including the three subgroups with 93 genes definitive and 34 limited, which is a good rate for our gene’s panel for molecular diagnostic use.

Genetic spectrum of inherited neuropathies in India

Background and Objectives:

Charcot-Marie-Tooth (CMT) disease is the commonest inherited neuromuscular disorder and has heterogeneous manifestations. Data regarding genetic basis of CMT from India is limited. This study aims to report the variations by using high throughput sequencing in Indian CMT cohort.

Methods:

Fifty-five probands (M:F 29:26) with suspected inherited neuropathy underwent genetic testing (whole exome: 31, clinical exome: 17 and targeted panel: 7). Their clinical and genetic data were analysed.

Results:

Age at onset ranged from infancy to 54 years. Clinical features included early-onset neuropathy (n=23), skeletal deformities (n=45), impaired vision (n=8), impaired hearing (n=6), facial palsy (n=8), thickened nerves (n=4), impaired cognition (n=5), seizures (n=5), pyramidal signs (n=7), ataxia (n=8) and vocal cord palsy, slow tongue movements and psychosis in one patient each. Twenty-eight patients had demyelinating electrophysiology. Abnormal visual and auditory evoked potentials were noted in 60.60% and 37.5% respectively. Sixty two variants were identified in 37 genes including variants of uncertain significance (n=34) and novel variants (n=45). Eleven patients had additional variations in genes implicated in CMTs/ other neurological disorders. Ten patients did not have variations in neuropathy associated genes, but had variations in genes implicated in other neurological disorders. In seven patients, no variations were detected.

Conclusion:

In this single centre cohort study from India, genetic diagnosis could be established in 87% of patients with inherited neuropathy. The identified spectrum of genetic variations adds to the pool of existing data and provides a platform for validation studies in cell culture or animal model systems.

Peripheral Nerve Development and the Pathogenesis of Peripheral Neuropathy: the Sorting Point

Nerve development requires a coordinated sequence of events and steps to be accomplished for the generation of functional peripheral nerves to convey sensory and motor signals. Any abnormality during development may result in pathological structure and function of the nerve, which evolves in peripheral neuropathy. In this review, we will briefly describe different steps of nerve development while we will mostly focus on the molecular mechanisms involved in radial sorting of axons, one of these nerve developmental steps. We will summarize current knowledge of molecular pathways so far reported in radial sorting and their possible interactions. Finally, we will describe how disruption of these pathways may result in human neuropathies.

Axonal Charcot-Marie-Tooth Disease: from Common Pathogenic Mechanisms to Emerging Treatment Opportunities

Inherited peripheral neuropathies are a genetically and phenotypically diverse group of disorders that lead to degeneration of peripheral neurons with resulting sensory and motor dysfunction. Genetic neuropathies that primarily cause axonal degeneration, as opposed to demyelination, are most often classified as Charcot-Marie-Tooth disease type 2 (CMT2) and are the focus of this review. Gene identification efforts over the past three decades have dramatically expanded the genetic landscape of CMT and revealed several common pathological mechanisms among various forms of the disease. In some cases, identification of the precise genetic defect and/or the downstream pathological consequences of disease mutations have yielded promising therapeutic opportunities. In this review, we discuss evidence for pathogenic overlap among multiple forms of inherited neuropathy, highlighting genetic defects in axonal transport, mitochondrial dynamics, organelle-organelle contacts, and local axonal protein translation as recurrent pathological processes in inherited axonal neuropathies. We also discuss how these insights have informed emerging treatment strategies, including specific approaches for single forms of neuropathy, as well as more general approaches that have the potential to treat multiple types of neuropathy. Such therapeutic opportunities, made possible by improved understanding of molecular and cellular pathogenesis and advances in gene therapy technologies, herald a new and exciting phase in inherited peripheral neuropathy.

Not so Shocking: Electromyography in Pediatrics Remains Feasible and Diagnostically Useful

BACKGROUND

Electrodiagnostic testing, including nerve conduction studies (NCS) and electromyography (EMG), assists with localizing lesions within the peripheral nervous system. NCS/EMG in children can be technically challenging and its relevance has been questioned in the era of affordable genetic testing. NCS/EMG provides information that may not be available in the examination of a young or developmentally delayed child. Our goal was to review the volume and referral sources of NCS/EMG studies and evaluate its feasibility and diagnostic yield at a pediatric tertiary care hospital.

METHODS

Retrospective chart review of NCS/EMG studies done in pediatric patients at one center from 2014 to 2019.

RESULTS

A total of 725 studies were performed, with a median age of 13.2 years (range 0-18 years). The annual number of studies remained constant throughout the study period. Neurologists and surgeons were the most common referral sources, but an increased number of referrals from geneticists was observed. Most (94.5%) NCS/EMG were done on awake patients, with only 5.5% of studies being terminated early due to tolerability of the patient. Of all studies, 326/725 (44%) demonstrated a neuromuscular abnormality, of which 63.5% (207/326) were acquired conditions. Mononeuropathies and polyneuropathies were the most common electrophysiologic diagnoses.

DISCUSSION

Our study indicates that NCS/EMG remains a useful diagnostic tool, both for the diagnosis of acquired neuromuscular conditions but also as an adjunct for interpreting genetic results, as indicated by the recent increase in referrals from geneticists. Overall NCS/EMG is well tolerated and able to be performed without sedation in children of all ages.

The Intersection Between Cerebellar Ataxia and Neuropathy: a Proposed Classification and a Diagnostic Approach

Neuropathy is a common associated feature of different types of genetic or sporadic cerebellar ataxias. The pattern of peripheral nerve involvement and its associated clinical features can be an invaluable aspect for narrowing the etiologic diagnosis in the investigation of cerebellar ataxias. In this review, we discuss the differential diagnosis of the intersection between peripheral nerve and cerebellar involvement, and classify them in accordance with the predominant features. Genetics, clinical features, neuroimaging, and neurophysiologic characteristics are discussed. Furthermore, a diagnostic approach for cerebellar ataxia with neuropathy is proposed according to the different clinical characteristics. This is an Educational and Descriptive review with the aim of medical education for the approach to the patients with cerebellar ataxia and neuropathy. The diagnostic approach to the patient with cerebellar ataxia with neuropathy requires a detailed medical history, phenotyping, characterization of disease progression and family history. Neuroimaging features and the neurophysiological findings play pivotal roles in defining the diagnosis. Establishing an organized classification method for the disorders based on the clinical features may be very helpful, and could be divided as those with predominant cerebellar features, predominant neuropathic feature, or conditions with both cerebellar ataxia and neuropathy. Second, determining the mode of inheritance is critical on cerebellar ataxias: autosomal dominant and recessive cerebellar ataxias, mitochondrial or sporadic types. Third, one must carefully assess neurophysiologic findings in order to better characterize the predominant pattern of involvement: damage location, mechanism of lesion (axonal or demyelinating), motor, sensory or sensory motor compromise, large or small fibers, and autonomic system abnormalities.

Differential Diagnosis of Acquired and Hereditary Neuropathies in Children and Adolescents-Consensus-Based Practice Guidelines

Disorders of the peripheral nerves can be caused by a broad spectrum of acquired or hereditary aetiologies. The objective of these practice guidelines is to provide the reader with information about the differential diagnostic workup for a target-oriented diagnosis. Following an initiative of the German-speaking Society of Neuropaediatrics, delegates from 10 German societies dedicated to neuroscience worked in close co-operation to write this guideline. Applying the Delphi methodology, the authors carried out a formal consensus process to develop practice recommendations. These covered the important diagnostic steps both for acquired neuropathies (traumatic, infectious, inflammatory) and the spectrum of hereditary Charcot-Marie-Tooth (CMT) diseases. Some of our most important recommendations are that: (i) The indication for further diagnostics must be based on the patient's history and clinical findings; (ii) Potential toxic neuropathy also has to be considered; (iii) For focal and regional neuropathies of unknown aetiology, nerve sonography and MRI should be performed; and (iv) For demyelinated hereditary neuropathy, genetic diagnostics should first address PMP22 gene deletion: once that has been excluded, massive parallel sequencing including an analysis of relevant CMT-genes should be performed. This article contains a short version of the guidelines. The full-length text (in German) can be found at the Website of the "Arbeitsgemeinschaft der Wissenschftlichen Medizinischen Fachgesellschaften e.V. (AWMF), Germany.

Diagnostic yield of advanced genetic testing in patients with hereditary neuropathies: A retrospective single-site study

INTRODUCTION/AIMS

Advanced genetic testing including next-generation sequencing (AGT/NGS) has facilitated DNA testing in the clinical setting and greatly expanded new gene discovery for the Charcot-Marie-Tooth neuropathies and other hereditary neuropathies (CMT/HN). Herein, we report AGT/NGS results, clinical findings, and diagnostic yield in a cohort of CMT/HN patients evaluated at our neuropathy care center.

METHODS

We reviewed the medical records of all patients with suspected CMT/HN who underwent AGT/NGS at the Hospital for Special Care from January 2017 through January 2020. Patients with variants reported as pathogenic or likely pathogenic were included for further clinical review.

RESULTS

We ordered AGT/NGS on 108 patients with suspected CMT/HN. Of these, pathogenic or likely pathogenic variants were identified in 17 patients (diagnostic yield, 15.7%), including 6 (35%) with PMP22 duplications; 3 (18%) with MPZ variants; 2 (12%) with MFN2 variants; and 1 each with NEFL, IGHMBP2, GJB1, BSCL2, DNM2, and TTR variants. Diagnostic yield increased to 31.0% for patients with a positive family history.

DISCUSSION

AGT/NGS panels can provide specific genetic diagnoses for a subset of patients with CMT/HN disorders, which improves disease and genetic counseling and prepares patients for disease-focused therapies. Despite these advancements, many patients with known or suspected CMT/HN disorders remain without a specific genetic diagnosis. Continued advancements in genetic testing, such as multiomic technology and better understanding of genotype-phenotype correlation, will further improve detection rates for patients with suspected CMT/HN disorders.

Optic Neuropathy in Charcot-Marie-Tooth Disease

BACKGROUND

Charcot-Marie-Tooth disease Type 2A (CMT2A) presents with optic atrophy in a subset of patients, but the prevalence and severity of optic nerve involvement in relation to other CMT subtypes has not been explored.

METHODS

Patients with genetically confirmed CMT2A (n = 5), CMT1A (n = 9) and CMTX1 (n = 10) underwent high- and low-contrast acuity testing using Sloan letter charts, and circumpapillary retinal nerve fiber layer (RNFL) and macular total retinal, RNFL, and ganglion cell layer/inner plexiform layer thickness was measured using spectral domain optical coherence tomography (OCT). We used age- and gender-adjusted linear regression to compare contrast acuity and retinal thickness between CMT groups.

RESULTS

One of 5 patients with CMT2A had optic nerve atrophy (binocular high-contrast acuity equivalent 20/160, mean circumpapillary RNFL 47.5 μm). The other patients with CMT2A had normal high- and low-contrast acuity and retinal thickness, and there were no significant differences between patients with CMT2A, CMT1A, and CMTX1.

CONCLUSIONS

Optic atrophy occurs in some patients with CMT2A, but in others, there is no discernible optic nerve involvement. This suggests that optic neuropathy is specific to certain MFN2 mutations in CMT2A and that low-contrast acuity or OCT is of limited value as a disease-wide biomarker.

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.

Genetic Neuropathy Due to Impairments in Mitochondrial Dynamics

Mitochondria are dynamic organelles capable of fusing, dividing, and moving about the cell. These properties are especially important in neurons, which in addition to high energy demand, have unique morphological properties with long axons. Notably, mitochondrial dysfunction causes a variety of neurological disorders including peripheral neuropathy, which is linked to impaired mitochondrial dynamics. Nonetheless, exactly why peripheral neurons are especially sensitive to impaired mitochondrial dynamics remains somewhat enigmatic. Although the prevailing view is that longer peripheral nerves are more sensitive to the loss of mitochondrial motility, this explanation is insufficient. Here, we review pathogenic variants in proteins mediating mitochondrial fusion, fission and transport that cause peripheral neuropathy. In addition to highlighting other dynamic processes that are impacted in peripheral neuropathies, we focus on impaired mitochondrial quality control as a potential unifying theme for why mitochondrial dysfunction and impairments in mitochondrial dynamics in particular cause peripheral neuropathy.

Distal hereditary motor neuropathies: Mutation spectrum and genotype-phenotype correlation

BACKGROUND AND PURPOSE

Distal hereditary motor neuropathies (dHMNs) are a heterogeneous group of disorders characterized by degeneration of the motor component of peripheral nerves. Currently, only 15% to 32.5% of patients with dHMN are characterized genetically. Additionally, the prevalence of these genetic disorders is not well known. Recently, biallelic mutations in the sorbitol dehydrogenase gene (SORD) have been identified as a cause of dHMN, with an estimated frequency in undiagnosed cases of up to 10%.

METHODS

In the present study, we included 163 patients belonging to 108 different families who were diagnosed with a dHMN and who underwent a thorough genetic screening that included next-generation sequencing and subsequent Sanger sequencing of SORD.

RESULTS

Most probands were sporadic cases (62.3%), and the most frequent age of onset of symptoms was 2 to 10 years (28.8%). A genetic diagnosis was achieved in 37/108 (34.2%) families and 78/163 (47.8%) of all patients. The most frequent cause of distal hereditary motor neuropathies were mutations in HSPB1 (10.4%), GARS1 (9.8%), BICD2 (8.0%), and DNAJB2 (6.7%) genes. In addition, 3.1% of patients were found to be carriers of biallelic mutations in SORD. Mutations in another seven genes were also identified, although they were much less frequent. Eight new pathogenic mutations were detected, and 17 patients without a definite genetic diagnosis carried variants of uncertain significance. The calculated minimum prevalence of dHMN was 2.3 per 100,000 individuals.

CONCLUSIONS

This study confirms the genetic heterogeneity of dHMN and that biallelic SORD mutations are a cause of dHMN in different populations.

Genetic mechanisms of peripheral nerve disease

Peripheral neuropathies of genetic etiology are a very diverse group of disorders manifesting either as non-syndromic inherited neuropathies without significant manifestations outside the peripheral nervous system, or as part of a systemic or syndromic genetic disorder. The former and most frequent group is collectively known as Charcot-Marie-Tooth disease (CMT), with prevalence as high as 1:2,500 world-wide, and has proven to be genetically highly heterogeneous. More than 100 different genes have been identified so far to cause various CMT forms, following all possible inheritance patterns. CMT causative genes belong to several common functional pathways that are essential for the integrity of the peripheral nerve. Their discovery has provided insights into the normal biology of axons and myelinating cells, and has highlighted the molecular mechanisms including both loss of function and gain of function effects, leading to peripheral nerve degeneration. Demyelinating neuropathies result from dysfunction of genes primarily affecting myelinating Schwann cells, while axonal neuropathies are caused by genes affecting mostly neurons and their long axons. Furthermore, mutation in genes expressed outside the nervous system, as in the case of inherited amyloid neuropathies, may cause peripheral neuropathy resulting from accumulation of β-structured amyloid fibrils in peripheral nerves in addition to various organs. Increasing insights into the molecular-genetic mechanisms have revealed potential therapeutic targets. These will enable the development of novel therapeutics for genetic neuropathies that remain, in their majority, without effective treatment.

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.

Charcot-Marie-Tooth Disease and Other Hereditary Neuropathies

PURPOSE OF REVIEW

This article provides an overview of Charcot-Marie-Tooth disease (CMT) and other inherited neuropathies. These disorders encompass a broad spectrum with variable motor, sensory, autonomic, and other organ system involvement. Considerable overlap exists, both phenotypically and genetically, among these separate categories, all eventually exhibiting axonal injury and neurologic impairment. Depending on the specific neural and non-neural localizations, patients experience varying morbidity and mortality. Neurologic evaluations, including neurophysiologic testing, can help diagnose and predict patient disabilities. Diagnosis is often complex, especially when genetic and acquired components overlap.

RECENT FINDINGS

Next-generation sequencing has greatly improved genetic diagnosis, with many third-party reimbursement parties now embracing phenotype-based panel evaluations. Through the advent of comprehensive gene panels, symptoms previously labeled as idiopathic or atypical now have a better chance to receive a specific diagnosis. A definitive molecular diagnosis affords patients improved care and counsel. The new classification scheme for inherited neuropathies emphasizes the causal gene names. A specific genetic diagnosis is important as considerable advances are being made in gene-specific therapeutics. Emerging therapeutic approaches include small molecule chaperones, antisense oligonucleotides, RNA interference, and viral gene delivery therapies. New therapies for hereditary transthyretin amyloidosis and Fabry disease are discussed.

SUMMARY

Comprehensive genetic testing through a next-generation sequencing approach is simplifying diagnostic algorithms and affords significantly improved decision-making processes in neuropathy care. Genetic diagnosis is essential for pathogenic understanding and for gene therapy development. Gene-targeted therapies have begun entering the clinic. Currently, for most inherited neuropathy categories, specific symptomatic management and family counseling remain the mainstays of therapy.

Ataxia pancytopenia syndrome due to SAMD9L mutation presenting as demyelinating neuropathy

Ataxia pancytopenia (ATXPC) syndrome due to gain-of-function pathogenic variants in the SAMD9L gene has been described in 38 patients to date. It is characterized by variable neurological and hematological phenotypes including ataxia, pyramidal signs, cytopenias, and hematological malignancies. Peripheral neuropathy with slowing of conduction velocities has been reported in only two affected individuals. We describe a female with childhood onset neuropathy diagnosed as Charcot-Marie-Tooth disease type 1 with onset of cerebellar ataxia in her 50s. Cerebellar, pyramidal, and neuropathic features were found on examination. Additionally, she also had conjunctival telangiectasia. Nerve conduction studies confirmed a demyelinating neuropathy. MRI brain showed cerebellar atrophy with diffuse white matter hyperintensities. OCT demonstrated global thinning of the retinal nerve fiber layer (RNFL). Full blood count has always been normal. A previously described pathogenic variant in SAMD9L [c.2956C>T p.(Arg986Cys)] was identified on whole exome sequencing. This case extends the previously described phenotype to include conjunctival telangiectasia and RNFL thinning and suggests that ATXPC syndrome should be considered in the differential for inherited demyelinating neuropathies.

Paternal gender specificity and mild phenotypes in Charcot-Marie-Tooth type 1A patients with de novo 17p12 rearrangements

Background

Charcot–Marie–Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) are developed by duplication and deletion of the 17p12 (PMP22) region, respectively.

Methods

De novo rates were determined in 211 CMT1A or HNPP trio families, and then, analyzed gender‐specific genetic features and clinical phenotypes of the de novo cases.

Results

This study identified 40 de novo cases (19.0%). Paternal origin was highly frequent compared to maternal origin (p = .005). Most de novo CMT1A rearrangements occurred between non‐sister chromatids (p = .003), but it was interesting that three of the four sister chromatids exchange cases were observed in the less frequent maternal origin. Paternal ages at the affected child births were slightly higher in the de novo CMT1A group than in the non‐de novo CMT1A control group (p = .0004). For the disability score of CMTNS, the de novo CMT1A group had a slightly lower value compared to the control group (p = .005). Electrophysiological studies showed no significant differences between the two groups.

Conclusion

This study suggests that de novo CMT1A patients tend to have milder symptoms and that the paternal ages at child births in the de novo group are higher than those of the non‐de novo group.

Charcot-Marie-Tooth disease and related disorders: an evolving landscape

PURPOSE OF REVIEW

Charcot-Marie-Tooth (CMT) disease and related disorders are the commonest group of inherited neuromuscular diseases and represent a heterogeneous group of disorders. This review will cover recent advances in genetic diagnosis and the evolving genetic and phenotype landscape of this disease group. We will review recent evidence of the increasingly recognized phenotypic overlap with other neurodegenerative conditions including hereditary spastic paraplegia, hereditary ataxias and mitochondrial diseases and highlight the importance of deep phenotyping to inform genetic diagnosis and prognosis.

RECENT FINDINGS

Through whole exome sequencing and multicentre collaboration new genes are being identified as causal for CMT expanding the genetic heterogeneity of this condition. In addition, an increasing number of variants have been identified in genes known to cause complex inherited diseases in which the peripheral neuropathy is part of the disorder and may be the presenting feature. The recent discovery of a repeat expansion in the RFC1 gene in cerebellar ataxia, neuropathy, vestibular areflexia syndrome highlights the prevalence of late-onset recessive conditions which have historically been considered to cause early-onset disease.

SUMMARY

CMT is an evolving field with considerable phenotypic and genetic heterogeneity and deep phenotyping remains a cornerstone in contemporary CMT diagnostics.

Alanyl-tRNA synthetase 1 (AARS1) gene mutation in a family with intermediate Charcot-Marie-Tooth neuropathy

BACKGROUND

Alanyl-tRNA synthetase 1 (AARS1) gene encodes a ubiquitously expressed class II enzyme that catalyzes the attachment of alanine to the cognate tRNA. AARS1 mutations are frequently responsible for autosomal dominant Charcot-Marie-Tooth disease type 2N (CMT2N).

OBJECTIVE

To identify pathogenic mutation in the Korean patients with CMT and distal hereditary motor neuronopathy (dHMN).

METHODS

We screened AARS1 mutations in 373 unrelated CMT families including 318 axonal CMT, 36 dHMN, and 19 intermediate CMT (Int-CMT) who were negative for 17p12 (PMP22) duplication or deletion using whole exome sequencing and targeted sequencing of CMT-related genes.

RESULTS

This study identified an early onset Int-CMT family harboring an AARS1 p.Arg329His mutation which was previously reported as pathogenic in French and Australian families. The mutation was located in the highly conserved tRNA binding domain and several in silico analyses suggested pathogenic prediction of the mutations. The patients harboring p.Arg329His showed clinically similar phenotypes of the early onset and electrophysiological intermediate type as those in Australian patients with same mutation. We also found a novel c.2564A>G (p.Gln855Arg) in a CMT2 patient, but its' pathogenic role was uncertain (variant of uncertain significance).

CONCLUSION

This study suggests that the frequency of the AARS1 mutations appears to be quite low in Korean CMT. This is the first report of the AARS1 mutation in Korean CMT patients and will be helpful for the exact molecular diagnosis and treatment of Int-CMT patients.

A recessive Trim2 mutation causes an axonal neuropathy in mice

We analyzed Trim2^A/A mice, generated by CRISPR-Cas9, which have a recessive, null mutation of Trim2. Trim2^A/A mice develop ataxia that is associated with a severe loss of cerebellar Purkinje cells and a peripheral neuropathy. Myelinated axons in the CNS, including those in the deep cerebellar nuclei, have focal enlargements that contain mitochondria and neurofilaments. In the PNS, there is a loss of myelinated axons, particularly in the most distal nerves. The pathologically affected neuronal populations - primary sensory and motor neurons as well as cerebellar Purkinje cells - express TRIM2, suggesting that loss of TRIM2 in these neurons results in cell autonomous effects on their axons. In contrast, these pathological findings were not found in a second strain of Trim2 mutant mice (Trim2^C/C), which has a partial deletion in the RING domain that is needed for ubiquitin ligase activity. Both the Trim2^Aand the Trim2^C alleles encode mutant TRIM2 proteins with reduced ubiquitination activity. In sum, Trim2^A/A mice are a genetically authentic animal model of a recessive axonal neuropathy of humans, apparently for a function that does not depend on the ubiquitin ligase activity.

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.

Distal motor neuropathy associated with novel EMILIN1 mutation

Elastin microfibril interface-located proteins (EMILINs) are extracellular matrix glycoproteins implicated in elastogenesis and cell proliferation. Recently, a missense mutation in the EMILIN1 gene has been associated with autosomal dominant connective tissue disorder and motor-sensory neuropathy in a single family. We identified by whole exome sequencing a novel heterozygous EMILIN1 mutation c.748C>T [p.R250C] located in the coiled coil forming region of the protein, in four affected members of an autosomal dominant family presenting a distal motor neuropathy phenotype. In affected patient a sensory nerve biopsy showed slight and unspecific changes in the number and morphology of myelinated fibers. Immunofluorescence study of a motor nerve within a muscle biopsy documented the presence of EMILIN-1 in nerve structures. Skin section and skin derived fibroblasts displayed a reduced extracellular deposition of EMILIN-1 protein with a disorganized network of poorly ramified fibers in comparison with controls. Downregulation of emilin1a in zebrafish displayed developmental delay, locomotion defects, and abnormal axonal arborization from spinal cord motor neurons. The phenotype was complemented by wild-type zebrafish emilin1a, and partially the human wild-type EMILIN1 cRNA, but not by the cRNA harboring the novel c.748C>T [p.R250C]. These data suggest a role of EMILIN-1 in the pathogenesis of diseases affecting the peripheral nervous system.

Yield of next-generation neuropathy gene panels in axonal neuropathies

The use and utility of targeted gene panels for diagnosing the type of Charcot-Marie-Tooth have grown rapidly because commercial gene panels that contain most of the relevant genes are available and affordable for many patients. We used a targeted gene panel to analyze 175 patients who had an unexplained axonal polyneuropathy affecting large myelinated axons, 86 of whom reported a family history of neuropathy, and 89 of whom did not. In patients reporting a family history, the panel identified a pathogenic variant causing the neuropathy in six cases (7%); in patients not reporting a family history, the gene panel identified pathogenic variants causing neuropathy in two patients (2%). Interpretation in a tertiary referral setting, current gene panels identify the genetic cause of neuropathy in a small minority of patients who have an unexplained axonal neuropathy, even in those reporting a family history.

Regulating PMP22 expression as a dosage sensitive neuropathy gene

Structural variation in the human genome has emerged as a major cause of disease as genomic data have accumulated. One of the most common structural variants associated with human disease causes the heritable neuropathy known as Charcot-Marie-Tooth (CMT) disease type 1A. This 1.4 Mb duplication causes nearly half of the CMT cases that are genetically diagnosed. The PMP22 gene is highly induced in Schwann cells during development, although its precise role in myelin formation and homeostasis is still under active investigation. The PMP22 gene can be considered as a nucleoprotein complex with enzymatic activity to produce the PMP22 transcript, and the complex is allosterically regulated by transcription factors that respond to intracellular signals and epigenomic modifications. The control of PMP22 transcript levels has been one of the major therapeutic targets of therapy development, and this review summarizes those approaches as well as efforts to characterize the regulation of the PMP22 gene.

Next-generation sequencing in Charcot-Marie-Tooth disease: opportunities and challenges

Charcot-Marie-Tooth disease and the related disorders hereditary motor neuropathy and hereditary sensory neuropathy, collectively termed CMT, are the commonest group of inherited neuromuscular diseases, and they exhibit wide phenotypic and genetic heterogeneity. CMT is usually characterized by distal muscle atrophy, often with foot deformity, weakness and sensory loss. In the past decade, next-generation sequencing (NGS) technologies have revolutionized genomic medicine and, as these technologies are being applied to clinical practice, they are changing our diagnostic approach to CMT. In this Review, we discuss the application of NGS technologies, including disease-specific gene panels, whole-exome sequencing, whole-genome sequencing (WGS), mitochondrial sequencing and high-throughput transcriptome sequencing, to the diagnosis of CMT. We discuss the growing challenge of variant interpretation and consider how the clinical phenotype can be combined with genetic, bioinformatic and functional evidence to assess the pathogenicity of genetic variants in patients with CMT. WGS has several advantages over the other techniques that we discuss, which include unparalleled coverage of coding, non-coding and intergenic areas of both nuclear and mitochondrial genomes, the ability to identify structural variants and the opportunity to perform genome-wide dense homozygosity mapping. We propose an algorithm for incorporating WGS into the CMT diagnostic pathway.

POLG mutations presenting as Charcot-Marie-Tooth disease

We report on two patients, with different POLG mutations, in whom axonal neuropathy dominated the clinical picture. One patient presented with late onset sensory axonal neuropathy caused by a homozygous c.2243G>C (p.Trp748Ser) mutation that resulted from uniparental disomy of the long arm of chromosome 15. The other patient had a complex phenotype that included early onset axonal Charcot-Marie-Tooth disease (CMT) caused by compound heterozygous c.926G>A (p.Arg309His) and c.2209G>C (p.Gly737Arg) mutations.

Autosomal dominant optic atrophy and cataract "plus" phenotype including axonal neuropathy

OBJECTIVE

To characterize the phenotype in individuals with OPA3-related autosomal dominant optic atrophy and cataract (ADOAC) and peripheral neuropathy (PN).

METHODS

Two probands with multiple affected relatives and one sporadic case were referred for evaluation of a PN. Their phenotype was determined by clinical ± neurophysiological assessment. Neuropathologic examination of sural nerve and skeletal muscle, and ultrastructural analysis of mitochondria in fibroblasts were performed in one case. Exome sequencing was performed in the probands.

RESULTS

The main clinical features in one family (n = 7 affected individuals) and one sporadic case were early-onset cataracts (n = 7), symptoms of gastrointestinal dysmotility (n = 8), and possible/confirmed PN (n = 7). Impaired vision was an early-onset feature in another family (n = 4 affected individuals), in which 3 members had symptoms of gastrointestinal dysmotility and 2 developed PN and cataracts. The less common features among all individuals included symptoms/signs of autonomic dysfunction (n = 3), hearing loss (n = 3), and recurrent pancreatitis (n = 1). In 5 individuals, the neuropathy was axonal and clinically asymptomatic (n = 1), sensory-predominant (n = 2), or motor and sensory (n = 2). In one patient, nerve biopsy revealed a loss of large and small myelinated fibers. In fibroblasts, mitochondria were frequently enlarged with slightly fragmented cristae. The exome sequencing identified OPA3 variants in all probands: a novel variant (c.23T>C) and the known mutation (c.313C>G) in OPA3.

CONCLUSIONS

A syndromic form of ADOAC (ADOAC+), in which axonal neuropathy may be a major feature, is described. OPA3 mutations should be included in the differential diagnosis of complex inherited PN, even in the absence of clinically apparent optic atrophy.

The Italian neuromuscular registry: a coordinated platform where patient organizations and clinicians collaborate for data collection and multiple usage

BACKGROUND

The worldwide landscape of patient registries in the neuromuscular disease (NMD) field has significantly changed in the last 10 years, with the international TREAT-NMD network acting as strong driver. At the same time, the European Medicines Agency and the large federations of rare disease patient organizations (POs), such as EURORDIS, contributed to a great cultural change, by promoting a paradigm shift from product-registries to patient-centred registries. In Italy, several NMD POs and Fondazione Telethon undertook the development of a TREAT-NMD linked patient registry in 2009, with the referring clinical network providing input and support to this initiative through the years. This article describes the outcome of this joint effort and shares the experience gained.

METHODS

The Italian NMD registry is based on an informatics technology platform, structured according to the most rigorous legal national and European requirements for management of patient sensitive data. A user-friendly web interface allows both direct patients and clinicians' participation. The platform's design permits expansion to incorporate new modules and new registries, and is suitable of interoperability with other international efforts.

RESULTS

When the Italian NMD Registry was initiated, an ad hoc legal entity (NMD Registry Association) was devised to manage registries' data. Currently, several disease-specific databases are hosted on the platform. They collect molecular and clinical details of individuals affected by Duchenne or Becker muscular dystrophy, Charcot-Marie-Tooth disease, transthyretin type-familial amyloidotic polyneuropathy, muscle glycogen storage disorders, spinal and bulbar muscular atrophy, and spinal muscular atrophy. These disease-specific registries are at different stage of development, and the NMD Registry itself has gone through several implementation steps to fulfil different technical and governance needs. The new governance model is based on the agreement between the NMD Registry Association and the professional societies representing the Italian NMD clinical network. Overall, up to now the NMD registry has collected data on more than 2000 individuals living with a NMD condition.

CONCLUSIONS

The Italian NMD Registry is a flexible platform that manages several condition-specific databases and is suitable to upgrade. All stakeholders participate in its management, with clear roles and responsibilities. This governance model has been key to its success. In fact, it favored patient empowerment and their direct participation in research, while also engaging the expert clinicians of the Italian network in the collection of accurate clinical data according to the best clinical practices.