Underestimated associated features in CMT neuropathies: clinical indicators for the causative gene?

Electrophysiological and radiological diagnosis of hereditary motor and sensory polyneuropathy

Hereditary motor and sensory neuropathy (HMSN), also known as Charcot-Marie-Tooth disease (CMT), is a member of the inherited neuropathy family with specific clinical and genetical manifestations. More than twenty genes have been linked to HMSN, and the number might increase. Regarding diagnosis, a healthcare provider should be suspicious if the patient is young with a family history. Integrative diagnosis, which includes electrophysiological, radiological, and genetic screening, is of great value to exclude metabolic, nutritive-toxic, infectious, and inflammatory or autoimmunological causes and to reach the exact subtype of hereditary neuropathy. Nowadays, next-generation sequencing-based analysis is becoming a routine diagnostic tool for inherited neuropathy, but if this facility is not available, electrophysiological and radiological diagnoses are the best diagnostic tools to be used. Differentiation between hereditary neuropathy and diabetic neuropathy is essential for primary care physicians to have the right plan.

Clinical practice guidelines for the diagnosis and management of Charcot-Marie-Tooth disease

INTRODUCTION

Charcot-Marie-Tooth (CMT) disease is classified considering the neurophysiological and histological findings, the inheritance pattern and the underlying genetic defect. In recent years, with the advent of next generation sequencing, genetic complexity has increased exponentially, expanding the knowledge about disease pathways, and having an impact in clinical management. The aim of this guide is to offer recommendations for the diagnosis, prognosis, monitoring and treatment of this disease in Spain.

MATERIAL AND METHODS

This consensus guideline has been developed by a multidisciplinary panel encompassing a broad group of professionals including neurologists, neuropediatricians, geneticists, rehabilitators, and orthopaedic surgeons.

RECOMMENDATIONS

The diagnosis is based in the clinical characterization, usually presenting with a common phenotype. It should be followed by an appropriate neurophysiological study that allows for a correct classification, specific recommendations are established for the parameters that should be included. Genetic diagnosis must be approached in sequentially, once the PMP22 duplication has been ruled out if appropriate, a next generation sequencing should be considered taking into account the limitations of the available techniques. To date, there is no pharmacological treatment that modifies the course of the disease, but symptomatic management is important, as are the rehabilitation and orthopaedic considerations. The latter should be initiated early to identify and improve the patient's functional impairments, including individualised exercise guidelines, orthotic adaptation, and assessment of conservative surgeries such as tendon transpositions. The follow-up of patients with CMT is exclusively clinical, ancillary testing are not necessary in routine clinical practice.

An integrative analysis of genotype-phenotype correlation in Charcot Marie Tooth type 2A disease with MFN2 variants: A case and systematic review

Dominant genetic variants in the mitofusin 2 (MFN2) gene lead to Charcot-Marie-Tooth type 2A (CMT2A), a neurodegenerative disease caused by genetic defects that directly damage axons. In this study, we reported a proband with a pathogenic variant in the GTPase domain of MFN2, c.494A > G (p.His165Arg). To date, at least 184 distinct MFN2 variants identified in 944 independent probands have been reported in 131 references. However, the field of medical genetics has long been challenged by how genetic variation in the MFN2 gene is associated with disease phenotypes. Here, by collating the MFN2 variant data and patient clinical information from Leiden Open Variant Database 3.0, NCBI clinvar database, and available related references in PubMed, we determined the mutation frequency, age of onset, sex ratio, and geographical distribution. Furthermore, the results of an analysis examining the relationship between variants and phenotypes from multiple genetic perspectives indicated that insertion and deletions (indels), copy number variants (CNVs), duplication variants, and nonsense mutations in single nucleotide variants (SNVs) tend to be pathogenic, and the results emphasized the importance of the GTPase domain to the structure and function of MFN2. Overall, three reliable classification methods of MFN2 genotype-phenotype associations provide insights into the prediction of CMT2A disease severity. Of course, there are still many MFN2 variants that have not been given clear clinical significance, which requires clinicians to make more accurate clinical diagnoses.

Evaluation of Pathogenicity and Causativity of Variants in the MPZ and SH3TC2 Genes in a Family Case of Hereditary Peripheral Neuropathy

The implementation of NGS methods into clinical practice allowed researchers effectively to establish the molecular cause of a disorder in cases of a genetically heterogeneous pathology. In cases of several potentially causative variants, we need additional analysis that can help in choosing a proper causative variant. In the current study, we described a family case of hereditary motor and sensory neuropathy (HMSN) type 1 (Charcot-Marie-Tooth disease). DNA analysis revealed two variants in the SH3TC2 gene (c.279G>A and c.1177+5G>A), as well as a previously described variant c.449-9C>T in the MPZ gene, in a heterozygous state. This family segregation study was incomplete because of the proband's father's unavailability. To evaluate the variants' pathogenicity, minigene splicing assay was carried out. This study showed no effect of the MPZ variant on splicing, but the c.1177+5G>A variant in the SH3TC2 gene leads to the retention of 122 nucleotides from intron 10 in the RNA sequence, causing a frameshift and an occurrence of a premature stop codon (NP_078853.2:p.Ala393GlyfsTer2).

Concomitant MPZ and MFN2 Gene Variants and Charcot Marie Tooth Disease in a Boy: Clinical and Genetic Analysis—Literature Review

Charcot- Marie- Tooth (CMT) disease includes a group of clinically and genetically heterogeneous neuropathic disorders with an estimated frequency of 1 on 2.500 individuals. CMTs are differently classified according to the age of onset, type of inheritance, and type of inheritance plus clinical features. For these disorders, more than 100 genes have been implicated as causal factors, with mutations in the PMP22 being one of the most common. The demyelinating type (CMT1) affects more than 30% of the CMTs patients and manifests with motor and sensory dysfunctions of the peripheral nervous system mainly starting with slow progressive weakness of the lower extremities. We report here a 12 year- old boy presenting with typical features of CMT1 type, hearing impairment, and inguinal hernia who at the next-generation sequence analysis displayed a concomitant presence of two variants: the c.233 C>T p.Ser 78Leu of the MPZ gene (NM_000530.6) characterized as pathogenetic and the c.1403 G>A p.Arg 468His of the MFN2 gene (NM_014874.3) characterized as VUS. Concomitant variant mutations in CMTs have been uncommonly reported. The role of these gene mutations on the clinical expression and a literature review on this topic is discussed.

Morc2a p.S87L mutant mice develop peripheral and central neuropathies associated with neuronal DNA damage and apoptosis

The microrchidia (MORC)-family CW-type zinc finger 2 (MORC2) gene is related to DNA repair, adipogenesis and epigenetic silencing via the human silencing hub (HUSH) complex. MORC2 missense mutation is known to cause peripheral neuropathy of Charcot-Marie-Tooth disease type 2 Z (CMT2Z). However, there have been reports of peripheral and central neuropathy in patients, and the disease has been co-categorized with developmental delay, impaired growth, dysmorphic facies and axonal neuropathy (DIGFAN). The etiology of MORC2 mutation-mediated neuropathy remains uncertain. Here, we established and analyzed Morc2a p.S87L mutant mice. Morc2a p.S87L mice displayed the clinical symptoms expected in human CMT2Z patients, such as axonal neuropathy and skeletal muscle weakness. Notably, we observed severe central neuropathy with cerebella ataxia, cognition disorder and motor neuron degeneration in the spinal cord, and this seemed to be evidence of DIGFAN. Morc2a p.S87L mice exhibited an accumulation of DNA damage in neuronal cells, followed by p53/cytochrome c/caspase 9/caspase 3-mediated apoptosis. This study presents a new mouse model of CMT2Z and DIGFAN with a Morc2a p.S87L mutation. We suggest that neuronal apoptosis is a possible target for therapeutic approach in MORC2 missense mutation. This article has an associated First Person interview with the first author of the paper.

A review and analysis of the clinical literature on Charcot-Marie-Tooth disease caused by mutations in neurofilament protein L

Charcot-Marie-Tooth disease (CMT) is one of the most common inherited neurological disorders and can be caused by mutations in over 100 different genes. One of the causative genes is NEFL on chromosome 8 which encodes neurofilament light protein (NEFL), one of five proteins that co-assemble to form neurofilaments. At least 34 different CMT-causing mutations in NEFL have been reported which span the head, rod, and tail domains of the protein. The majority of these mutations are inherited dominantly, but some are inherited recessively. The resulting disease is classified variably in clinical reports based on electrodiagnostic studies as either axonal (type 2; CMT2E), demyelinating (type 1; CMT1F), or a form intermediate between the two (dominant intermediate; DI-CMTG). In this article, we first present a brief introduction to CMT and neurofilaments. We then collate and analyze the data from the clinical literature on the disease classification, age of onset and electrodiagnostic test results for the various mutations. We find that mutations in the head, rod, and tail domains can all cause disease with early onset and profound neurological impairment, with a trend toward greater severity for head domain mutations. We also find that the disease classification does not correlate with specific mutation or domain. In fact, different individuals with the same mutation can be classified as having axonal, demyelinating, or dominant intermediate forms of the disease. This suggests that the classification of the disease as CMT2E, CMT1F or DI-CMTG has more to do with variable disease presentation than to differences in the underlying disease mechanism, which is most likely primarily axonal in all cases.

Association between restless legs syndrome and peripheral neuropathy: A systematic review and meta-analysis

BACKGROUND AND PURPOSE

The coexistence of peripheral neuropathy (PN) and restless legs syndrome (RLS) or Willis-Ekbom disease is relatively frequent, but its prevalence has shown a high variability across studies. In addition, several reports have shown data suggesting the presence of PN in patients with idiopathic RLS.

METHODS

A search was undertaken using the PubMed, Embase and Web of Science Databases, from 1966 to 6 December 2020, crossing the search term 'restless legs syndrome' with 'neuropathy', 'polyneuropathy' (PNP) and 'peripheral neuropathy', and the references of interest for this topic were identified; a meta-analysis was performed, according to PRISMA guidelines, and a calculation of pooled prevalences, where appropriate, was made using standard methods.

RESULTS

Restless legs syndrome has been reported in 5.2%-53.7% of patients with PN (average 21.5%; 95% confidence interval 18.6%-24.5%), and PN has been reported in 0%-87.5% of patients with RLS (average 41.8%; 95% confidence interval 39.9%-43.6%), both being significantly more frequent than in controls. The heterogeneity across studies could be due to differences in the diagnostic criteria used for both RLS and PN. RLS is a frequent clinical complaint in patients with PN of different aetiologies, mainly diabetic PN, uraemic PNP, familial amyloid PNP, Charcot-Marie-Tooth disease and chronic dysimmune inflammatory PNP. Recent neurophysiological findings suggest the presence of small sensory fibre loss in patients diagnosed with idiopathic RLS, but it remains to be determined whether RLS associated with small sensory fibre loss and idiopathic RLS are different clinical entities.

CONCLUSIONS

Future studies including clinical and neurophysiological assessment and skin biopsy involving a large series of patients with PN and RLS are needed for a better understanding of the association between these two entities.

A Search for Undiagnosed Charcot-Marie-Tooth Disease Among Patients Registered with Unspecified Polyneuropathy in the Danish National Patient Registry

Purpose

In a recent study based on data from the Danish National Patients Registry (DNPR), we reported the prevalence of Charcot-Marie-Tooth disease (CMT) in Denmark to be 22.5 per 100.000. This prevalence is most likely a minimum estimate, as many cases of CMT may be misdiagnosed or remain undiagnosed due to the heterogeneous nature of the disorder. The aim of this study was to investigate the possible number of undiagnosed CMT cases among patients registered with unspecified polyneuropathy (UP) diagnoses in the DNPR.

Patients and Methods

From the DNPR we extracted data on all patients given an UP diagnosis in the period 1977 to 2012. We selected all patients diagnosed with a primary UP diagnosis before age 40 at a department of neurology, neurophysiology, clinical genetics or pediatrics, and excluded all patients with a specified polyneuropathy diagnosis or with diagnostic codes related to alcohol and diabetes mellitus. To assess the proportion of possible CMT patients, we performed medical record review in a random sample of patients diagnosed in the Central Denmark Region. To further investigate the possible overlap between UP and CMT in the DNPR, we performed a series of searches for ICD-8 and ICD-10 codes related to CMT.

Results

Between 1977 and 2012, 30.903 patients were diagnosed with UP without also being diagnosed with CMT. A total of 940 patients fulfilled the selection criteria. We found that 21.5% (95% CI 13.1%–32.2%) of the cases in the random sample fulfilled our criteria for CMT. This estimate increases the prevalence of CMT in Denmark with 3.6 per 100,000 (95% CI 2.4%–5.5%).

Conclusion

This study illustrates how hitherto undiagnosed CMT patients may be identified in the DNPR and further reports the number of possible CMT cases. Our results support the hypothesis that the true prevalence of CMT is higher than recently reported.

Impaired Mitochondrial Mobility in Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth (CMT) disease is a progressive, peripheral neuropathy and the most commonly inherited neurological disorder. Clinical manifestations of CMT mutations are typically limited to peripheral neurons, the longest cells in the body. Currently, mutations in at least 80 different genes are associated with CMT and new mutations are regularly being discovered. A large portion of the proteins mutated in axonal CMT have documented roles in mitochondrial mobility, suggesting that organelle trafficking defects may be a common underlying disease mechanism. This review will focus on the potential role of altered mitochondrial mobility in the pathogenesis of axonal CMT, highlighting the conceptional challenges and potential experimental and therapeutic opportunities presented by this "impaired mobility" model of the disease.

Developmental demands contribute to early neuromuscular degeneration in CMT2D mice

Dominantly inherited, missense mutations in the widely expressed housekeeping gene, GARS1, cause Charcot-Marie-Tooth type 2D (CMT2D), a peripheral neuropathy characterised by muscle weakness and wasting in limb extremities. Mice modelling CMT2D display early and selective neuromuscular junction (NMJ) pathology, epitomised by disturbed maturation and neurotransmission, leading to denervation. Indeed, the NMJ disruption has been reported in several different muscles; however, a systematic comparison of neuromuscular synapses from distinct body locations has yet to be performed. We therefore analysed NMJ development and degeneration across five different wholemount muscles to identify key synaptic features contributing to the distinct pattern of neurodegeneration in CMT2D mice. Denervation was found to occur along a distal-to-proximal gradient, providing a cellular explanation for the greater weakness observed in mutant Gars hindlimbs compared with forelimbs. Nonetheless, muscles from similar locations and innervated by axons of equivalent length showed significant differences in neuropathology, suggestive of additional factors impacting on site-specific neuromuscular degeneration. Defective NMJ development preceded and associated with degeneration, but was not linked to a delay of wild-type NMJ maturation processes. Correlation analyses indicate that muscle fibre type nor synaptic architecture explain the differential denervation of CMT2D NMJs, rather it is the extent of post-natal synaptic growth that predisposes to neurodegeneration. Together, this work improves our understanding of the mechanisms driving synaptic vulnerability in CMT2D and hints at pertinent pathogenic pathways.

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.

HDAC6 inhibition promotes α-tubulin acetylation and ameliorates CMT2A peripheral neuropathy in mice

Charcot-Marie-Tooth type 2A (CMT2A) peripheral neuropathy, the most common axonal form of CMT, is caused by dominantly inherited point mutations in the Mitofusin 2 (Mfn2) gene. It is characterized by progressive length-dependent degeneration of motor and sensory nerves with corresponding clinical features of motor and sensory impairment. There is no cure for CMT, and therapeutic approaches are limited to physical therapy, orthopedic devices, surgery, and analgesics. In this study we focus on histone deacetylase 6 (HDAC6) as a therapeutic target in a mouse model of mutant MFN2 (MFN2^R94Q)-induced CMT2A. We report that these mice display progressive motor and sensory dysfunction as well as a significant decrease in α-tubulin acetylation in distal segments of long peripheral nerves. Treatment with a new, highly selective HDAC6 inhibitor, SW-100, was able to restore α-tubulin acetylation and ameliorate motor and sensory dysfunction when given either prior to or after the onset of symptoms. To confirm HDAC6 is the target for ameliorating the CMT2A phenotype, we show that genetic deletion of Hdac6 in CMT2A mice prevents the development of motor and sensory dysfunction. Our findings suggest α-tubulin acetylation defects in distal parts of nerves as a pathogenic mechanism and HDAC6 as a therapeutic target for CMT2A.

Identification of a rare SEPT9 variant in a family with autosomal dominant Charcot-Marie-Tooth disease

Background

Charcot-Marie-Tooth disease (CMT) is one of the most commonly inherited neurological disorders. A growing number of genes, involved in glial and neuronal functions, have been associated with different subtypes of CMT leading to improved diagnostics and understanding of pathophysiological mechanisms. However, some patients and families remain genetically unsolved.

Methods

We report on a German family including four affected members over three generations with a CMT phenotype accompanied by cognitive deficits, predominantly with regard to visual abilities and episodic memory.

Results

A comprehensive clinical characterization followed by a sequential diagnostic approach disclosed a heterozygous rare SEPT9 missense variant c.1406 T > C, p.(Val469Ala), that segregates with disease. SEPT9 has been linked to various intracellular functions, such as cytokinesis and membrane trafficking. Interestingly, SEPT9-mutations are known to cause hereditary neuralgic amyotrophy (HNA), a recurrent focal peripheral neuropathy.

Conclusion

We, for the first time, present a SEPT9 variant associated to a CMT phenotype and suggest SEPT9 as new sufficient candidate gene in CMT.

PMP22 Gene-Associated Neuropathies: Phenotypic Spectrum in a Cohort from India

Reports of spectrum of clinical manifestations in PMP22 gene-associated neuropathies (duplication/mutations) are scarce. To identify the frequency of PMP22 gene variations and establish their genotype-phenotype correlation. Patients with suspected genetic demyelinating neuropathy (n = 128) underwent evaluation for copy number variations and point mutations in PMP22 gene by multiplex ligation-dependent probe amplification (MLPA) and direct sequencing respectively. Of these, only 27 patients (M:F:19:8) from 18 families had PMP22 gene-associated neuropathy; they were subsequently analyzed for genotype-phenotype correlation. Twenty-five patients had PMP22 duplication while two patients had PMP22 missense mutations (p.A114V and p.L80P). Age at onset of neuropathy ranged from infancy to 63 years and symptom duration ranged from 2 to 32 years. Cranial nerve dysfunction in the form of ptosis, ophthalmoplegia, bifacial weakness, and sensorineural hearing loss was observed in addition to a number of systemic features. Three patients were asymptomatic. All except one patient were ambulant. Velocity of median nerve and amplitude of evoked motor responses from common peroneal nerve were significantly reduced in male patients. There was significantly worse disability in the late-onset group as compared with the early-onset group. Otherwise, the mean age at onset, frequency of skeletal deformities, patterns of motor weakness, muscle stretch reflexes, sensory impairment, disability rating scales, and electrophysiological parameters were comparable irrespective of gender, onset age, family history and ulnar nerve conduction velocities. The relatively low frequency of PMP22 duplication in the present cohort warrants a more comprehensive search to establish the genetic etiology. Further research into the role of other genetic variants as well as modifier genes and their effect on phenotypic heterogeneity is indicated.

Charcot-Marie-Tooth disease Type 2E/1F mutant neurofilament proteins assemble into neurofilaments

Charcot-Marie-Tooth disease Type 2E/1F (CMT2E/1F) is a peripheral neuropathy caused by mutations in neurofilament protein L (NFL), which is one of five neurofilament subunit proteins that co-assemble to form neurofilaments in vivo. Prior studies on cultured cells have shown that CMT2E/1F mutations disrupt neurofilament assembly and lead to protein aggregation, suggesting a possible disease mechanism. However, electron microscopy of axons in peripheral nerve biopsies from patients has revealed accumulations of neurofilament polymers of normal appearance and no evidence of protein aggregates. To reconcile these observations, we reexamined the assembly of seven CMT2E/1F NFL mutants in cultured cells. None of the mutants assembled into homopolymers in SW13vim- cells, but P8R, P22S, L268/269P, and P440/441L mutant NFL assembled into heteropolymers in the presence of neurofilament protein M (NFM) alone, and N98S, Q332/333P, and E396/397K mutant NFL assembled in the presence of NFM and peripherin. P8R, P22S, N98S, L268/269P, E396/397K, and P440/441L mutant NFL co-assembled into neurofilaments with endogenous NFL, NFM, and α-internexin in cultured neurons, although the N98S and E396/397K mutants showed reduced filament incorporation, and the Q332/333P mutant showed limited incorporation. We conclude that all the mutants are capable of assembling into neurofilaments, but for some of the mutants this was dependent on the identity of the other neurofilament proteins available for co-assembly, and most likely also their relative expression level. Thus, caution should be exercised when drawing conclusions about the assembly capacity of CMT2E/1F mutants based on transient transfections in cultured cells.

Hereditary Neuropathies

BACKGROUND

Hereditary peripheral neuropathies constitute a large group of genetic diseases, with an overall prevalence of 1:2500. In recent years, the use of so-called next-generation sequencing (NGS) has led to the identification of many previously unknown involved genes and genetic defects that cause neuropathy. In this article, we review the procedures and utility of genetic evaluation for hereditary neurop - athies, while also considering the implications of the fact that causally directed treatment of these disorders is generally unavailable.

METHODS

This review is based on pertinent publications retrieved by a PubMed search employing the search terms "hereditary neuropathy," "Charcot-Marie-Tooth disease," "hereditary sensory neuropathy," and "hereditary motor neuropathy."

RESULTS

With rare exceptions, the diagnostic evaluation for hereditary neuropathies proceeds in stepwise fashion, beginning with the study of individual genes. If this fails to detect any abnormality, NGS analysis, which involves the sequencing of many different genes in parallel and has now become available for routine diagnosis, should be performed early on in the diagnostic work-up. Exome and genome analyses are currently performed only when considered to be indicated in the individual case. Whenever a hereditary neuropathy is suspected, other (including potentially treatable) causes of neuropathy should be ruled out. Mutations in neurop athy-associated genes may also be associated with other clinical entities such as spastic paraplegia or myopathy. Thus, interdisciplinary assessment is necessary.

CONCLUSION

The molecular diagnosis of neuropathies has become much more successful through the use of NGS. Although causally directed treatment approaches still need to be developed, the correct diagnosis puts an end to the often highly stressful search for a cause and enables determination of the risk of disease in other members of the patient's family.

Charcot-Marie-Tooth: From Molecules to Therapy

Charcot-Marie-Tooth (CMT) is the most prevalent category of inherited neuropathy. The most common inheritance pattern is autosomal dominant, though there also are X-linked and autosomal recessive subtypes. In addition to a variety of inheritance patterns, there are a myriad of genes associated with CMT, reflecting the heterogeneity of this disorder. Next generation sequencing (NGS) has expanded and simplified the diagnostic yield of genes/molecules underlying and/or associated with CMT, which is of paramount importance in providing a substrate for current and future targeted disease-modifying treatment options. Considerable research attention for disease-modifying therapy has been geared towards the most commonly encountered genetic mutations (PMP22, GJB1, MPZ, and MFN2). In this review, we highlight the clinical background, molecular understanding, and therapeutic investigations of these CMT subtypes, while also discussing therapeutic research pertinent to the remaining less common CMT subtypes.

Inverted formins: A subfamily of atypical formins

Formins are a family of regulators of actin and microtubule dynamics that are present in almost all eukaryotes. These proteins are involved in many cellular processes, including cytokinesis, stress fiber formation, and cell polarization. Here we review one subfamily of formins, the inverted formins. Inverted formins as a group break several formin stereotypes, having atypical biochemical properties and domain organization, and they have been linked to kidney disease and neuropathy in humans. In this review, we will explore recent research on members of the inverted formin sub-family in mammals, zebrafish, fruit flies, and worms.

Genotypic and phenotypic spectrum of the most common causative genes of Charcot-Marie-Tooth disease in Hungarian patients

Charcot-Marie-Tooth neuropathy (CMT) is a genetically and clinically heterogeneous group of neuromuscular disorders with an overall prevalence of 1 per 2500. Here we report the first comprehensive genetic epidemiology study of Hungarian CMT patients. 409 CMT1 and 122 CMT2 patients were enrolled and genetic testing of PMP22, GJB1, MPZ, EGR2 and MFN2 genes were performed routinely. NDRG1 and CTDP1 genes were screened only for founder mutations in Roma patients. Causative genetic mutations were identified in 67.2% of the CMT1 and in 33.6% of the CMT2 cases, which indicates an overall success rate of 59.9% in the study population. Considering all affected individuals, alterations were most frequently found in PMP22 (40.5%), followed by GJB1 (9.2%), MPZ (4.5%), MFN2 (2.5%), NDRG1 (1.5%), EGR2 (0.8%) and CTDP1 (0.8%). The phenotypic spectrum and the disease severity of the studied patients also varied broadly. Deafness and autoimmune disorders were more often associated with PMP22 duplication, while MFN2 and GJB1 mutations were frequently present with central nervous system abnormalities. Our study may be helpful in determining the strategy of genetic diagnostics in Hungarian CMT patients.

Genetic and clinical characteristics of NEFL-related Charcot-Marie-Tooth disease

OBJECTIVES

To analyse and describe the clinical and genetic spectrum of Charcot-Marie-Tooth disease (CMT) caused by mutations in the neurofilament light polypeptide gene (NEFL).

METHODS

Combined analysis of newly identified patients with NEFL-related CMT and all previously reported cases from the literature.

RESULTS

Five new unrelated patients with CMT carrying the NEFL mutations P8R and N98S and the novel variant L311P were identified. Combined data from these cases and 62 kindreds from the literature revealed four common mutations (P8R, P22S, N98S and E396K) and three mutational hotspots accounting for 37 (55%) and 50 (75%) kindreds, respectively. Eight patients had de novo mutations. Loss of large-myelinated fibres was a uniform feature in a total of 21 sural nerve biopsies and 'onion bulb' formations and/or thin myelin sheaths were observed in 14 (67%) of them. The neurophysiological phenotype was broad but most patients with E90K and N98S had upper limb motor conduction velocities <38 m/s. Age of onset was ≤3 years in 25 cases. Pyramidal tract signs were described in 13 patients and 7 patients were initially diagnosed with or tested for inherited ataxia. Patients with E90K and N98S frequently presented before age 3 years and developed hearing loss or other neurological features including ataxia and/or cerebellar atrophy on brain MRI.

CONCLUSIONS

NEFL-related CMT is clinically and genetically heterogeneous. Based on this study, however, we propose mutational hotspots and relevant clinical-genetic associations that may be helpful in the evaluation of NEFL sequence variants and the differential diagnosis with other forms of CMT.

Identification of a Large DNAJB2 Deletion in a Family with Spinal Muscular Atrophy and Parkinsonism

In this study, we described the identification of a large DNAJB2 (HSJ1) deletion in a family with recessive spinal muscular atrophy and Parkinsonism. After performing homozygosity mapping and whole genome sequencing, we identified a 3.8 kb deletion, spanning the entire DnaJ domain of the HSJ1 protein, as the disease-segregating mutation. By performing functional assays, we showed that HSJ1b-related DnaJ domain deletion leads to loss of HSJ1b mRNA and protein levels, increased HSJ1a mRNA and protein expressions, increased cell death, protein aggregation, and enhanced autophagy. Given the role of HSJ1 proteins in the degradation of misfolded proteins, we speculated that enhanced autophagy might be promoted by the elevated HSJ1a expression seen in HSJ1b-deficient cells. We also observed a significant reduction in both tau and brain-derived neurotrophic factor levels, which may explain the dopaminergic deficits seen in one of the affected siblings. We concluded that HSJ1b deficiency leads to a complex neurological phenotype, possibly due to the accumulation of misfolded proteins, caused by the lack of the DnaJ domain activity. We thus expand the phenotypic and genotypic spectrums associated with DNAJB2 disease and suggest relevant disease-associated mechanisms.

Underestimated associated features in CMT neuropathies: clinical indicators for the causative gene?

INTRODUCTION

Charcot-Marie-Tooth neuropathy (CMT) is a genetically heterogeneous group of peripheral neuropathies. In addition to the classical clinical phenotype, additional features can occur.

METHODS

We studied a wide range of additional features in a cohort of 49 genetically confirmed CMT patients and performed a systematic literature revision.

RESULTS

Patients harbored a PMP22 gene alteration (n = 28) or a mutation in MPZ (n = 11), GJB1 (n = 4), LITAF (n = 2), MFN2 (n = 2), INF2 (n = 1), NEFL (n = 1). We identified four novel mutations (3 MPZ, 1 GJB1). A total of 88% presented at least one additional feature. In MPZ patients, we detected hypertrophic nerve roots in 3/4 cases that underwent spinal MRI, and pupillary abnormalities in 27%. In our cohort, restless legs syndrome (RLS) was present in 18%. We describe for the first time RLS associated with LITAF or MFN2 and predominant upper limb involvement with LITAF. Cold-induced hand cramps occurred in 10% (PMP22,MPZ,MFN2), and autonomous nervous system involvement in 18% (PMP22,MPZ, LITAF,MFN2). RLS and respiratory insufficiency were mostly associated with severe neuropathy, and pupillary abnormalities with mild to moderate neuropathy.

CONCLUSIONS

In CMT patients, additional features occur frequently. Some of them might be helpful in orienting genetic diagnosis. Our data broaden the clinical spectrum and genotype-phenotype associations with CMT.