A novel locus for dHMN with pyramidal features maps to chromosome 4q34.3-q35.2

The distal hereditary motor neuropathy (dHMN) is a rare genetically and clinically heterogeneous disorder characterized by weakness and wasting of distal limb muscles in absence of overt sensory abnormalities. Recently, pyramidal signs have been also described in some patients with dominant or recessive dHMN, and two different loci have been identified in families affected by dHMN complicated with pyramidal dysfunction. We investigated an Italian family affected by an autosomal dominant dHMN complicated by pyramidal signs in order to map a new gene locus. The disease maps to a novel locus in a 26-cM region flanked by D4S1552 and D4S2930 on chromosome 4q34.3-35.2. Three candidate genes ( SNX25 , CASP3 and TUBB4Q ) located in the critical region were screened for the presence of mutations by heteroduplex analysis. No mutations have been detected in the analyzed genes. In conclusion, the new private genetic locus we reported further confirms the wide heterogeneity of dHMN.     

Exome sequencing reveals HINT1 mutations as a cause of distal hereditary motor neuropathy

Distal hereditary motor neuropathies (dHMNs) are a heterogenous group of genetic disorders with length-dependent degeneration of motor axons. Obtaining a genetic diagnosis in patients with dHMN remains challenging. We performed exome sequencing in a diagnostic setting in 12 patients with a clinical diagnosis of dHMN. Potential disease-causing variants in genes associated with dHMN and other forms of inherited neuropathies/motor neuron diseases were validated using Sequenom. The coverage in the genes studied was >95% with an average coverage of >50 times. In none of the patients a mutations was found in genes previously reported to be associated with dHMN. However, in 2/12 patients a recessive mutation in histidine triad nucleotide binding protein 1 (HINT1, recently discovered as a cause of axonal neuropathy with neuromyotonia) was identified. Our results demonstrate the diagnostic value of exome sequencing for patients with inherited neuropathies. The phenotypic spectrum of recessive mutations in HINT1 includes dHMN. HINT1 should be added to the list of genes to check for in dHMN.     

A novel WARS mutation (p.Asp314Gly) identified in a Chinese distal hereditary motor neuropathy family

Distal hereditary motor neuropathy (dHMN) is a clinically and genetically heterogeneous group of inherited neuropathies characterized by distal limb muscle wasting and weakness with no or minimal sensory abnormalities. To investigate the clinical and genetic features of dHMN caused by WARS mutations in mainland China, we performed Sanger sequencing of the coding and untranslated region (UTR) regions of WARS in 160 unresolved dHMN and Charcot-Marie-Tooth (CMT) index patients. We detected a novel heterozygous variant c.941A>G (p.Asp314Gly) of WARS in an index patient from an autosomal dominant dHMN family including five affected members over three generations. The variant completely co-segregates with the dHMN phenotype in the family, and it was classified as likely pathogenic according to the American College of Medical Genetics and Genomics standards and guidelines. The clinical features included juvenile to adult onset (15-23 years), distal wasting and weakness, minimal sensory disturbance and length-dependent motor axonal degeneration with CMT examination score ranging from 6 to 10. Our report further confirms the role of WARS in dHMN and indicates that the variant c.941A>G (p.Asp314Gly) of WARS is related to a mild to moderate affected and later onset phenotype of dHMN.     

TDRKH is a candidate gene for an autosomal dominant distal hereditary motor neuropathy

Distal hereditary motor neuropathies (dHMNs) comprise a group of clinically and genetically heterogeneous inherited lower motor neuron syndromes mainly characterized by a distal-predominant pattern of progressive muscle atrophy, weakness and hyporeflexia, without sensory dysfunction. Although at least 21 causative genes for dHMN have been reported, mutational scanning of these genes often fails to identify the causative variants in dHMN cohorts, suggesting that additional causative genes remain to be identified. We studied a four-generation pedigree of a Japanese family with autosomal dominant dHMN to provide insight into the pathogenetic basis of the disease. Neurological examinations were performed on all six family members enrolled in this study. Whole-exome sequencing (WES) was used to identify the causative gene for dHMN. The clinical features of the patients included muscle weakness with distal extensor dominancy in the lower extremities, accompanied by facial and neck flexor muscle impairment, no sensory involvement, and areflexia. Nerve conduction studies demonstrated axonal changes mainly in the peroneal nerve. WES combined with rigorous filtering revealed three missense variants (NM_001083964: c.851G > A [p.Arg284His] in TDRKH, NM_002858: c.1654G > T [p.Gly552Cys] in ABCD3, NM_001005164: c.898A > T [p.Ile300Phe], in OR52E2). The variant in TDRKH is located in a conserved region of the tudor domain which is also present in the survival of motor neuron (SMN) protein, encoded by the SMN1 gene. Therefore, we concluded the variant in TDRKH is likely to be responsible for dHMN in our pedigree.     

X‐linked spinal muscular atrophy in mice caused by autonomous loss of ATP7A in the motor neuron

ATP7A is a copper‐transporting P‐type ATPase that is essential for cellular copper homeostasis. Loss‐of‐function mutations in the ATP7A gene result in Menkes disease, a fatal neurodegenerative disorder resulting in seizures, hypotonia and failure to thrive, due to systemic copper deficiency. Most recently, rare missense mutations in ATP7A that do not impact systemic copper homeostasis have been shown to cause X‐linked spinal muscular atrophy type 3 (SMAX3), a distal hereditary motor neuropathy. An understanding of the mechanistic and pathophysiological basis of SMAX3 is currently lacking, in part because the disease‐causing mutations have been shown to confer both loss‐ and gain‐of‐function properties to ATP7A, and because there is currently no animal model of the disease. In this study, the Atp7a gene was specifically deleted in the motor neurons of mice, resulting in a degenerative phenotype consistent with the clinical features in affected patients with SMAX3, including the progressive deterioration of gait, age‐dependent muscle atrophy, denervation of neuromuscular junctions and a loss of motor neuron cell bodies. Taken together, these data reveal autonomous requirements for ATP7A that reveal essential roles for copper in the maintenance and function of the motor neuron, and suggest that SMAX3 is caused by a loss of ATP7A function that specifically impacts the spinal motor neuron. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

Charcot�CMarie�CTooth disease

Charcot-Marie-Tooth (CMT) disease is a heterogeneous group of genetic disorders presenting with the phenotype of a chronic progressive neuropathy affecting both the motor and sensory nerves. During the last decade over two dozen genes have been identified in which mutations cause CMT. The disease illustrates a multitude of genetic principles, including diverse mutational mechanisms from point mutations to copy number variation (CNV), allelic heterogeneity, age-dependent penetrance and variable expressivity. Population based studies have determined the contributions of the various genes to disease burden enabling evidence-based approaches to genetic testing.     

Sensory neuropathy in progressive motor neuronopathy (pmn) mice is associated with defects in microtubule polymerization and axonal transport

Motor neuron diseases such as amyotrophic lateral sclerosis (ALS) are now recognized as multi‐system disorders also involving various non‐motor neuronal cell types. The precise extent and mechanistic basis of non‐motor neuron damage in human ALS and ALS animal models remain however unclear. To address this, we here studied progressive motor neuronopathy (pmn) mice carrying a missense loss‐of‐function mutation in tubulin binding cofactor E (TBCE). These mice manifest a particularly aggressive form of motor axon dying back and display a microtubule loss, similar to that induced by human ALS‐linked TUBA4A mutations. Using whole nerve confocal imaging of pmn × thy1.2‐YFP16 fluorescent reporter mice and electron microscopy, we demonstrate axonal discontinuities, bead‐like spheroids and ovoids in pmn suralis nerves indicating prominent sensory neuropathy. The axonal alterations qualitatively resemble those in phrenic motor nerves but do not culminate in the loss of myelinated fibers. We further show that the pmn mutation decreases the level of TBCE, impedes microtubule polymerization in dorsal root ganglion (DRG) neurons and causes progressive loss of microtubules in large and small caliber suralis axons. Live imaging of axonal transport using GFP‐tagged tetanus toxin C‐fragment (GFP‐TTC) demonstrates defects in microtubule‐based transport in pmn DRG neurons, providing a potential explanation for the axonal alterations in sensory nerves. This study unravels sensory neuropathy as a pathological feature of mouse pmn, and discusses the potential contribution of cytoskeletal defects to sensory neuropathy in human motor neuron disease.     

Altered intracellular localization and valosin-containing protein (p97 VCP) interaction underlie ATP7A-related distal motor neuropathy

ATP7A is a P-type ATPase that regulates cellular copper homeostasis by activity at the trans-Golgi network (TGN) and plasma membrane (PM), with the location normally governed by intracellular copper concentration. Defects in ATP7A lead to Menkes disease or its milder variant, occipital horn syndrome or to a newly discovered condition, ATP7A-related distal motor neuropathy (DMN), for which the precise pathophysiology has been obscure. We investigated two ATP7A motor neuropathy mutations (T994I, P1386S) previously associated with abnormal intracellular trafficking. In the patients' fibroblasts, total internal reflection fluorescence microscopy indicated a shift in steady-state equilibrium of ATP7A(T994I) and ATP7A(P1386S), with exaggerated PM localization. Transfection of Hek293T cells and NSC-34 motor neurons with the mutant alleles tagged with the Venus fluorescent protein also revealed excess PM localization. Endocytic retrieval of the mutant alleles from the PM to the TGN was impaired. Immunoprecipitation assays revealed an abnormal interaction between ATP7A(T994I) and p97/VCP, an ubiquitin-selective chaperone which is mutated in two autosomal dominant forms of motor neuron disease: amyotrophic lateral sclerosis and inclusion body myopathy with early-onset Paget disease and fronto-temporal dementia. Small-interfering RNA (SiRNA) knockdown of p97/VCP corrected ATP7A(T994I) mislocalization. Flow cytometry documented that non-permeabilized ATP7A(P1386S) fibroblasts bound a carboxyl-terminal ATP7A antibody, consistent with relocation of the ATP7A di-leucine endocytic retrieval signal to the extracellular surface and partially destabilized insertion of the eighth transmembrane helix. Our findings illuminate the mechanisms underlying ATP7A-related DMN and establish a link between p97/VCP and genetically distinct forms of motor neuron degeneration.     

Whole‐exome sequencing is a valuable diagnostic tool for inherited peripheral neuropathies: Outcomes from a cohort of 50 families

The inherited peripheral neuropathies (IPNs) are characterized by marked clinical and genetic heterogeneity and include relatively frequent presentations such as Charcot‐Marie‐Tooth disease and hereditary motor neuropathy, as well as more rare conditions where peripheral neuropathy is associated with additional features. There are over 250 genes known to cause IPN‐related disorders but it is estimated that in approximately 50% of affected individuals a molecular diagnosis is not achieved. In this study, we examine the diagnostic utility of whole‐exome sequencing (WES) in a cohort of 50 families with 1 or more affected individuals with a molecularly undiagnosed IPN with or without additional features. Pathogenic or likely pathogenic variants in genes known to cause IPN were identified in 24% (12/50) of the families. A further 22% (11/50) of families carried sequence variants in IPN genes in which the significance remains unclear. An additional 12% (6/50) of families had variants in novel IPN candidate genes, 3 of which have been published thus far as novel discoveries (KIF1A, TBCK, and MCM3AP). This study highlights the use of WES in the molecular diagnostic approach of highly heterogeneous disorders, such as IPNs, places it in context of other published neuropathy cohorts, while further highlighting associated benefits for discovery.     

Simplification of the Research Diagnosis of HIV-Associated Sensory Neuropathy

Abstract

Peripheral neuropathy (PN) is the most common neurological complication of HIV infection,affecting over one third of patients. The research diagnosis of PN is complicated by the need for expensive, time-consuming, and noxious diagnostic tests. We investigated whether nerve conduction studies (NSC) and quantitative sensory tests (QST) provide added value for the diagnosis of PN for research purposes or whether the easily obtainable clinical measures (sensory and motor symptoms, sensitivity to pain and vibration, tendon reflexes, motor function) are sufficient.     

多灶运动神经病病人的临床和神经电生理研究

目的：分析多灶运动神经病（multifocal motor neuropathy,MMN）的临床及神经电生理特点。方法：对13例MMN患者进行神经传导和常规肌电图（EMG）检查,采用微移（inching）技术判定是否有运动神经传导阻滞（CB）。结果：全部患者均隐袭起病,进展缓慢,临床上以单肢无力为首发症状,随病情发展,受累部位增加。常规神经传导检查可见所有明显萎缩肌肉的复合肌动作电位（CMAP）波幅下降;13例患者神经受累区域以下所支配肌肉EMG检查可见不同程度的神经原性损害。采用微移技术共测定出CB25处和拟诊CB6处。在3条临床及EMG测定均正常的神经发现5处CB,其余26处CB或拟诊CB均发生于轻度至中度无力的肌肉所对应的20条神经。结论：MMN是一种以远端神经受累为主的不对称性周围神经病,神经电生理检查对诊断和鉴别诊断MMN起重要作用,CB是MMN的主要神经电生理表现。     

Autosomal dominant sensory/motor neuropathy with Ataxia (SMNA): Linkage to chromosome 7q22‐q32

The autosomal dominant (AD) spinocerebellar ataxias (SCAs) and hereditary sensory neuropathies (HSN) are heterogeneous disorders characterized by variable clinical, electrophysiological, and neuropathological profiles. The SCAs are clinically characterized by slowly progressive incoordination of gait often associated with poor coordination of hands, speech, and eyes. Peripheral neuropathy is not a frequent part of the SCA syndrome. In contrast, the HSNs are primarily characterized by progressive sensory loss. There is substantial clinical overlap between the various SCAs and the various HSNs, and they often cannot be differentiated on the basis of clinical or neuro‐imaging studies. We have identified a five‐generation American family of Irish ancestry with a unique neurological disorder displaying an AD pattern of inheritance. There was variable expressivity and severity of symptoms including sensory loss, ataxia, pyramidal tract signs, and muscle weakness. Nerve conduction studies were consistent with a sensory axonal neuropathy. Muscle biopsy revealed neurogenic atrophy and brain MRI showed mild cerebellar atrophy. To identify the responsible locus we pursued a whole genome linkage analysis. After analyzing 114 markers, linkage to D7S486 was detected with a two point LOD score of 4.79 at θ = 0.00. Evaluation of additional markers in the region provided a maximum LOD score of 6.36 at θ = 0.00 for marker D7S2554. Haplotype analysis delimited an approximately 14‐cM region at 7q22‐q32 between markers D7S2418 and D7S1804 cosegregating with the disease. Because this disorder does not easily fall into either the SCA or HSN categories, it is designated sensory/motor neuropathy with ataxia (SMNA). © 2002 Wiley‐Liss, Inc.     

Refinement of a locus for autosomal dominant hereditary motor and sensory neuropathy with proximal dominancy (HMSN-P) and genetic heterogeneity

Hereditary motor and sensory neuropathy with proximal dominancy (HMSN-P) is an adult-onset peripheral neurodegenerative disorder which has been reported only in the Okinawa Islands, Japan. The disease locus of "Okinawa-type" HMSN-P has been previously mapped to 3q13.1, with all affected individuals sharing an identical haplotype around the locus, suggesting that the undiscovered causative mutation in HMSN-P originated from a single founder. We have newly found two large families from the western part of Japan within which multiple members developed symptoms similar to those exhibited by HMSN-P patients from Okinawa, with no record of affinal connection between the islands. Using these pedigrees with "Kansai-type" HMSN-P, we carried out a linkage study utilizing eight microsatellite markers and identified a candidate region on 3q13.1 cosegregating with the disease (maximum two-point LOD score of 8.44 at theta=0.0) overlapping with the Okinawa-type HMSN-P locus. However, the disease haplotype shared among all affected members in these families was different from that in the Okinawa kindred, suggesting allelic heterogeneity. Such allelic variation should aid in the identification of the disease-causative gene. Moreover, the allelic heterogeneity of HMSN-P in the Japanese population suggests that HMSN-P may be more common across other ethnic groups, but classified into other disease categories.     

A mutation in the small heat-shock protein HSPB1 leading to distal hereditary motor neuronopathy disrupts neurofilament assembly and the axonal transport of specific cellular cargoes

Distal hereditary motor neuronopathies (dHMNs) are a clinically and genetically heterogeneous group of disorders in which motor neurons selectively undergo age-dependant degeneration. Mutations in the small heat-shock protein HSPB1 (HSP27) are responsible for one form of dHMN. In this study, we have analysed the effect of expressing a form of mutant HSPB1 in primary neuronal cells in culture. Mutant (P182L) but not wild-type HSPB1 led to the formation of insoluble intracellular aggregates and to the sequestration in the cytoplasm of selective cellular components, including neurofilament middle chain subunit (NF-M) and p150 dynactin. These findings suggest a possible pathogenic mechanism for HSPB1 whereby the mutation may lead to preferential motor neuron loss by disrupting selective components essential for axonal structure and transport.     

Hereditary motor and sensory neuropathy with deafness,mental retardation,and absence of sensory large myelinated fibers: Confirmation of a new entity

We describe two brothers, 11 and 13 years old, respectively, with an early-onset hereditary motor and sensory neuropathy, deafness, and mental retardation. Electrophysiological studies showed marked reduction of motor and sensory conduction velocity and absence of sensory action potentials. Sural nerve biopsy, performed in both patients, showed absence of large myelinated fibers with normal density of small myelinated fibers without axonal degeneration. Signs of demyelination were found only in the younger patient. We suggest that motorsensory neuropathy associated with deafness and mental retardation with absence of large myelinated fibers on sural nerve biopsy represents a distinct clinicopathological entity, which is transmitted in families probably as an autosomal recessive trait. Am. J. Med. Genet. 75:309–313, 1998. © 1998 Wiley-Liss, Inc.     

Charcot–Marie–Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot–Marie–Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.     

A Loss‐of‐Function Variant in the Human Histidyl‐tRNA Synthetase (HARS) Gene is Neurotoxic In Vivo

Aminoacyl‐tRNA synthetases (ARSs) are ubiquitously expressed enzymes responsible for ligating amino acids to cognate tRNA molecules. Mutations in four genes encoding an ARS have been implicated in inherited peripheral neuropathy with an axonal pathology, suggesting that all ARS genes are relevant candidates for disease in patients with related phenotypes. Here, we present results from a mutation screen of the histidyl‐tRNA synthetase (HARS) gene in a large cohort of patients with peripheral neuropathy. These efforts revealed a rare missense variant (c.410G>A/p.Arg137Gln) that resides at a highly conserved amino acid, represents a loss‐of‐function allele when evaluated in yeast complementation assays, and is toxic to neurons when expressed in a worm model. In addition to the patient with peripheral neuropathy, p.Arg137Gln HARS was detected in three individuals by genome‐wide exome sequencing. These findings suggest that HARS is the fifth ARS locus associated with axonal peripheral neuropathy. Implications for identifying ARS alleles in human populations and assessing them for a role in neurodegenerative phenotypes are discussed.     

Mutation analysis of genes within the dynactin complex in a cohort of hereditary peripheral neuropathies

The cytoplasmic dynein–dynactin genes are attractive candidates for neurodegenerative disorders given their functional role in retrograde transport along neurons. The cytoplasmic dynein heavy chain (DYNC1H1) gene has been implicated in various neurodegenerative disorders, and dynactin 1 (DCTN1) genes have been implicated in a wide spectrum of disorders including motor neuron disease, Parkinson's disease, spinobulbar muscular atrophy and hereditary spastic paraplegia. However, the involvement of other dynactin genes with inherited peripheral neuropathies (IPN) namely, hereditary sensory neuropathy, hereditary motor neuropathy and Charcot–Marie–Tooth disease is under reported. We screened eight genes; DCTN1‐6 and ACTR1A and ACTR1B in 136 IPN patients using whole‐exome sequencing and high‐resolution melt (HRM) analysis. Eight non‐synonymous variants (including one novel variant) and three synonymous variants were identified. Four variants have been reported previously in other studies, however segregation analysis within family members excluded them from causing IPN in these families. No variants of disease significance were identified in this study suggesting the dynactin genes are unlikely to be a common cause of IPNs. However, with the ease of querying gene variants from exome data, these genes remain worthwhile candidates to assess unsolved IPN families for variants that may affect the function of the proteins.     

Emerging pathways for hereditary axonopathies

Motor neurons are affected in a number of neurological diseases. Their unifying pathological signature is degeneration of extended projecting axons and loss of motor neurons in the prefrontal cortex and/or the spinal cord. Based on clinical criteria, hereditary forms have been traditionally divided into distinct entities, such as familial amyotrophic lateral sclerosis, hereditary motor neuropathy, spinal muscular atrophy, familial spinal paraplegia, and Charcot–Marie–Tooth disease type 2, also known as hereditary motor and sensory neuropathy II. Genetic research of the last decade has revealed remarkable heterogeneity within these disorders. Most of the identified genes to date cause disease in a classic Mendelian inheritance pattern with a high phenotypic penetrance. This rich source of molecular genetic data has already provided insight into the underlying major pathways of these diseases and should continue to do so in the future. This review attempts to cross the traditional clinical classifications in order to draw an emerging picture of common pathways between causative genes, providing a different perspective of this rapidly growing scientific field.     

Mutant SOD1 causes motor neuron disease independent of copper chaperone-mediated copper loading

Copper-mediated oxidative damage is proposed to play a critical role in the pathogenesis of Cu/Zn superoxide dismutase (SOD1)-linked familial amyotrophic lateral sclerosis (FALS). We tested this hypothesis by ablating the gene encoding the copper chaperone for SOD1 (CCS) in a series of FALS-linked SOD1 mutant mice. Metabolic 64Cu labeling in SOD1-mutant mice lacking the CCS showed that the incorporation of copper into mutant SOD1 was significantly diminished in the absence of CCS. Motor neurons in CCS-/- mice showed increased rate of death after facial nerve axotomy, a response documented for SOD1-/- mice. Thus, CCS is necessary for the efficient incorporation of copper into SOD1 in motor neurons. Although the absence of CCS led to a significant reduction in the amount of copper-loaded mutant SOD1, however, it did not modify the onset and progression of motor neuron disease in SOD1-mutant mice. Hence, CCS-dependent copper loading of mutant SOD1 plays no role in the pathogenesis of motor neuron disease in these mouse models.