An axonal form of Charcot-Marie-Tooth disease showing distinctive features in association with mutations in the peripheral myelin protein zero gene (Thr124Met or Asp75Val)

Canine models of Charcot-Marie-Tooth: MTMR2, MPZ, and SH3TC2 variants in golden retrievers with congenital hypomyelinating polyneuropathy

Congenital hypomyelinating polyneuropathy (HPN) restricted to the peripheral nervous system was reported in 1989 in two Golden Retriever (GR) littermates. Recently, four additional cases of congenital HPN in young, unrelated GRs were diagnosed via neurological examination, electrodiagnostic evaluation, and peripheral nerve pathology. Whole-genome sequencing was performed on all four GRs, and variants from each dog were compared to variants found across >1,000 other dogs, all presumably unaffected with HPN. Likely causative variants were identified for each HPN-affected GR. Two cases shared a homozygous splice donor site variant in MTMR2, with a stop codon introduced within six codons following the inclusion of the intron. One case had a heterozygous MPZ isoleucine to threonine substitution. The last case had a homozygous SH3TC2 nonsense variant predicted to truncate approximately one-half of the protein. Haplotype analysis using 524 GR established the novelty of the identified variants. Each variant occurs within genes that are associated with the human Charcot-Marie-Tooth (CMT) group of heterogeneous diseases, affecting the peripheral nervous system. Testing a large GR population (n = >200) did not identify any dogs with these variants. Although these variants are rare within the general GR population, breeders should be cautious to avoid propagating these alleles.

Clinical genetics of Charcot-Marie-Tooth disease

Recent research in the field of inherited peripheral neuropathies (IPNs) such as Charcot-Marie-Tooth (CMT) disease has helped identify the causative genes provided better understanding of the pathogenesis, and unraveled potential novel therapeutic targets. Several reports have described the epidemiology, clinical characteristics, molecular pathogenesis, and novel causative genes for CMT/IPNs in Japan. Based on the functions of the causative genes identified so far, the following molecular and cellular mechanisms are believed to be involved in the causation of CMTs/IPNs: myelin assembly, cytoskeletal structure, myelin-specific transcription factor, nuclear related, endosomal sorting and cell signaling, proteasome and protein aggregation, mitochondria-related, motor proteins and axonal transport, tRNA synthetases and RNA metabolism, and ion channel-related mechanisms. In this article, we review the epidemiology, genetic diagnosis, and clinicogenetic characteristics of CMT in Japan. In addition, we discuss the newly identified novel causative genes for CMT/IPNs in Japan, namely MME and COA7. Identification of the new causes of CMT will facilitate in-depth characterization of the underlying molecular mechanisms of CMT, leading to the establishment of therapeutic approaches such as drug development and gene therapy.

Thr124Met myelin protein zero mutation mimicking motor neuron disease

Mutations in myelin protein zero (MPZ) are associated with heterogeneous manifestations. In this study, we report clinical, electrophysiological, pathological, and muscle MRI findings from two relatives with MPZ Thr124Met variants, disclosing different phenotypes. The proband was a 73-year-old female with a 12-year-story of atrophy, weakness, and fasciculations in her proximal and distal lower limbs. EMG examination showed neurogenic signs with active denervation together with reduced sensory action potentials, without sensory symptoms. The initial diagnosis was of a slowly progressive lower motor neuron disease (MND) with subclinical sensory axonal neuropathy. Two years later, the observation of her 60-year-old nephew, who had a distal sensory-motor neuropathy, prompted the analysis of inherited neuropathies-related genes and revealed a MPZ Thr124Met mutation in both cases. Our findings expand the clinical spectrum of MPZ-related neuropathy and highlight that Thr124Met mutation may cause a syndrome mimicking MND. The challenging issue to detect sensory features in the diagnostic MND work up is discussed.

New evidence for secondary axonal degeneration in demyelinating neuropathies

Development of peripheral nervous system (PNS) myelin involves a coordinated series of events between growing axons and the Schwann cell (SC) progenitors that will eventually ensheath them. Myelin sheaths have evolved out of necessity to maintain rapid impulse propagation while accounting for body space constraints. However, myelinating SCs perform additional critical functions that are required to preserve axonal integrity including mitigating energy consumption by establishing the nodal architecture, regulating axon caliber by organizing axonal cytoskeleton networks, providing trophic and potentially metabolic support, possibly supplying genetic translation materials and protecting axons from toxic insults. The intermediate steps between the loss of these functions and the initiation of axon degeneration are unknown but the importance of these processes provides insightful clues. Prevalent demyelinating diseases of the PNS include the inherited neuropathies Charcot-Marie-Tooth Disease, Type 1 (CMT1) and Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) and the inflammatory diseases Acute Inflammatory Demyelinating Polyneuropathy (AIDP) and Chronic Inflammatory Demyelinating Polyneuropathy (CIDP). Secondary axon degeneration is a common feature of demyelinating neuropathies and this process is often correlated with clinical deficits and long-lasting disability in patients. There is abundant electrophysiological and histological evidence for secondary axon degeneration in patients and rodent models of PNS demyelinating diseases. Fully understanding the involvement of secondary axon degeneration in these diseases is essential for expanding our knowledge of disease pathogenesis and prognosis, which will be essential for developing novel therapeutic strategies.

Genetic spectrum of Charcot-Marie-Tooth disease associated with myelin protein zero gene variants in Japan

We aimed to reveal the genetic features associated with MPZ variants in Japan. From April 2007 to August 2017, 64 patients with 23 reported MPZ variants and 21 patients with 17 novel MPZ variants were investigated retrospectively. Variation in MPZ variants and the pathogenicity of novel variants was examined according to the American College of Medical Genetics standards and guidelines. Age of onset, cranial nerve involvement, serum creatine kinase (CK), and cerebrospinal fluid (CSF) protein were also analyzed. We identified 64 CMT patients with reported MPZ variants. The common variants observed in Japan were different from those observed in other countries. We identified 11 novel pathogenic variants from 13 patients. Six novel MPZ variants in eight patients were classified as likely benign or uncertain significance. Cranial nerve involvement was confirmed in 20 patients. Of 30 patients in whom serum CK levels were evaluated, eight had elevated levels. Most of the patients had age of onset >20 years. In another subset of 30 patients, 18 had elevated CSF protein levels; four of these patients had spinal diseases and two had enlarged nerve root or cauda equina. Our results suggest genetic diversity across patients with MPZ variants.

Metabolic Interaction Between Schwann Cells and Axons Under Physiological and Disease Conditions

Recent research into axon-glial interactions in the nervous system has made a compelling case that glial cells have a relevant role in the metabolic support of axons, and that, in the case of myelinating cells, this role is independent of myelination itself. In this mini-review article, we summarize some of those observations and focus on Schwann cells (SC), drawing parallels between glia of the central and peripheral nervous systems (PNS), pointing out limitations in current knowledge, and discussing its potential clinical relevance. First, we introduce SC, their development and main roles, and follow with an evolutionary perspective of glial metabolic function. Then we provide evidence of the myelin-independent aspects of axonal support and their coupling to neuronal metabolism. Finally, we address the opportunity to use SC-axon metabolic interactions as therapeutic targets to treat peripheral neuropathies.

Histopathology of the Inner Ear in Charcot-Marie-Tooth Syndrome Caused by a Missense Variant (p.Thr65Ala) in the MPZ Gene

Charcot-Marie-Tooth (CMT) syndrome is a clinically and genetically heterogeneous group of neuropathies affecting both peripheral motor and sensory nerves. Progressive sensorineural hearing loss, vestibular abnormalities, and dysfunction of other cranial nerves have been described. This is the second case report of otopathology in a patient with CMT syndrome. Molecular genetic testing of DNA obtained at autopsy revealed a missense variant in the MPZ gene (p.Thr65Ala), pathogenic for an autosomal-dominant form of CMT1B. The temporal bones were also prepared for light microscopy by hematoxylin and eosin and Gömöri trichome stains, and immunostaining for anti-myelin protein zero. Pathology was consistent with a myelinopathy of the auditory, vestibular, and facial nerves bilaterally. The pathophysiology of cranial nerve dysfunction in CMT is unknown. Findings in the current case suggested, at least in cranial nerves 7 and 8, that a myelinopathy may be causative.

The spectrum of Charcot-Marie-Tooth disease due to myelin protein zero: An electrodiagnostic, nerve ultrasound and histological study

OBJECTIVE

Nerve ultrasound (US) data on myelin protein zero (MPZ)-related Charcot-Marie-Tooth disease (CMT) are lacking. To offer a comprehensive perspective on MPZ-related CMTs, we combined nerve US with clinics, electrodiagnosis and histopathology.

METHODS

We recruited 36 patients (12 MPZ mutations), and correlated nerve US to clinical, electrodiagnostic measures, and sural nerve biopsy.

RESULTS

According to motor nerve conduction velocity (MNCV) criteria, nine patients were categorized as "demyelinating" CMT1B, 17 as "axonal" CMT2I/J, and 10 as dominant "intermediate" CMTDID. Sural nerve biopsy showed hypertrophic de-remyelinating neuropathy with numerous complex onion bulbs in one patient, de-remyelinating neuropathy with scanty/absent onion bulbs in three, axonal neuropathy in two, mixed demyelinating-axonal neuropathy in five. Electrodiagnosis significantly differed in CMT1B vs. CMT2I/J and CMTDID subgroups. CMT1B had slightly enlarged nerve cross sectional area (CSA) especially at proximal upper-limb (UL) sites. CSA was negatively correlated to UL MNCV and not increased at entrapment sites. Major sural nerve pathological patterns were uncorrelated to UL nerve US and MNCV.

CONCLUSIONS

Sural nerve biopsy confirmed the wide pathological spectrum of MPZ-CMT. UL nerve US identified two major patterns corresponding to the CMT1B and CMT2I/J-CMTDID subgroups.

SIGNIFICANCE

Nerve US phenotype of MPZ-CMT diverged from those in other demyelinating peripheral neuropathies and may have diagnostic value.

A Novel Asp121Asn Mutation of Myelin Protein Zero Is Associated with Late-Onset Axonal Charcot-Marie-Tooth Disease, Hearing Loss and Pupil Abnormalities

Myelin protein zero (MPZ) is a major component of compact myelin in peripheral nerves. Mutations in MPZ have been associated with different Charcot-Marie-Tooth disease (CMT) phenotypes (CMT1B, CMT2I/J, CMTDI), Dejerine-Sottas syndrome, and congenital hypomyelination neuropathy. Here, we report phenotypic variability in a four-generation Chinese family with the MPZ mutation Asp121Asn. Genetic testing was performed on nine family members and 200 controls. Clinical, electrophysiological and skeletal muscle MRI assessments were available for review in six family members. A novel heterozygous missense mutation, Asp121Asn, was observed in five affected members of the family. Unaffected relatives and 200 normal controls were without the mutation. Four of the affected members of the family displayed late-onset, predominantly axonal sensory and motor neuropathy, pupil abnormalities, and progressive sensorineural hearing loss. One young affected member presented with Argyll-Robertson pupils and diminished deep tendon reflexes in the lower limbs. The MPZ mutation Asp121Asn may be associated with late-onset axonal neuropathy, early onset hearing loss and pupil abnormalities. Our report expands the number and phenotypic spectrum of MPZ mutations.

Genotype-phenotype characteristics and baseline natural history of heritable neuropathies caused by mutations in the MPZ gene

We aimed to characterize genotype-phenotype correlations and establish baseline clinical data for peripheral neuropathies caused by mutations in the myelin protein zero (MPZ) gene. MPZ mutations are the second leading cause of Charcot-Marie-Tooth disease type 1. Recent research makes clinical trials for patients with MPZ mutations a realistic possibility. However, the clinical severity varies with different mutations and natural history data on progression is sparse. We present cross-sectional data to begin to define the phenotypic spectrum and clinical baseline of patients with these mutations. A cohort of patients with MPZ gene mutations was identified in 13 centres of the Inherited Neuropathies Consortium - Rare Disease Clinical Research Consortium (INC-RDCRC) between 2009 and 2012 and at Wayne State University between 1996 and 2009. Patient phenotypes were quantified by the Charcot-Marie-Tooth disease neuropathy score version 1 or 2 and the Charcot-Marie-Tooth disease paediatric scale outcome instruments. Genetic testing was performed in all patients and/or in first- or second-degree relatives to document mutation in MPZ gene indicating diagnosis of Charcot-Marie-Tooth disease type 1B. There were 103 patients from 71 families with 47 different MPZ mutations with a mean age of 40 years (range 3-84 years). Patients and mutations were separated into infantile, childhood and adult-onset groups. The infantile onset group had higher Charcot-Marie-Tooth disease neuropathy score version 1 or 2 and slower nerve conductions than the other groups, and severity increased with age. Twenty-three patients had no family history of Charcot-Marie-Tooth disease. Sixty-one patients wore foot/ankle orthoses, 19 required walking assistance or support, and 10 required wheelchairs. There was hearing loss in 21 and scoliosis in 17. Forty-two patients did not begin walking until after 15 months of age. Half of the infantile onset patients then required ambulation aids or wheelchairs for ambulation. Our results demonstrate that virtually all MPZ mutations are associated with specific phenotypes. Early onset (infantile and childhood) phenotypes likely represent developmentally impaired myelination, whereas the adult-onset phenotype reflects axonal degeneration without antecedent demyelination. Data from this cohort of patients will provide the baseline data necessary for clinical trials of patients with Charcot-Marie-Tooth disease caused by MPZ gene mutations.

Parasympathetic Dominant Autonomic Dysfunction in Charcot-Marie-Tooth Disease Type 2J with the MPZ Thr124Met Mutation

We herein report the case of a 69-year-old woman with Charcot-Marie-Tooth Disease type 2J (CMT2J) who presented with Adie's pupil, deafness, and urinary disturbance in addition to motor symptoms. On autonomic investigation, the coefficient of variation of the R-R intervals was decreased, and a urodynamic analysis showed a hypotonic bladder. A heart rate variability analysis revealed a decreased high frequency component and low frequency/high frequency ratio. Orthostatic hypotension was not present, and the sympathetic skin response and cardiac scintigraphy using (123)I-metaiodobenzylguanidine were normal. A gene analysis showed a known heterozygous mutation associated with CMT2J in myelin protein zero exon 3, resulting in the substitution of threonine to methionine at position 124. Our case suggests that mainly the parasympathetic autonomic function is disturbed in CMT2J.

A review of genetic counseling for Charcot Marie Tooth disease (CMT)

Charcot Marie Tooth disease (CMT) encompasses the inherited peripheral neuropathies. While four genes have been found to cause over 90 % of genetically identifiable causes of CMT (PMP22, GJB1, MPZ, MFN2), at least 51 genes and loci have been found to cause CMT when mutated, creating difficulties for clinicians to find a genetic subtype for families. Here, the classic features of CMT as well as characteristic features of the most common subtypes of CMT are described, as well as methods for narrowing down the possible subtypes. Psychosocial concerns particular to the CMT population are identified. This is the most inclusive publication for CMT-specific genetic counseling.

CFTR-deficient pigs display peripheral nervous system defects at birth

Peripheral nervous system abnormalities, including neuropathy, have been reported in people with cystic fibrosis. These abnormalities have largely been attributed to secondary manifestations of the disease. We tested the hypothesis that disruption of the cystic fibrosis transmembrane conductance regulator (CFTR) gene directly influences nervous system function by studying newborn CFTR(-/-) pigs. We discovered CFTR expression and activity in Schwann cells, and loss of CFTR caused ultrastructural myelin sheath abnormalities similar to those in known neuropathies. Consistent with neuropathic changes, we found increased transcripts for myelin protein zero, a gene that, when mutated, can cause axonal and/or demyelinating neuropathy. In addition, axon density was reduced and conduction velocities of the trigeminal and sciatic nerves were decreased. Moreover, in vivo auditory brainstem evoked potentials revealed delayed conduction of the vestibulocochlear nerve. Our data suggest that loss of CFTR directly alters Schwann cell function and that some nervous system defects in people with cystic fibrosis are likely primary.

Dominant Charcot-Marie-Tooth syndrome and cognate disorders

Charcot-Marie-Tooth neuropathy (CMT) is a group of genetically heterogeneous disorders sharing a similar phenotype, characterized by wasting and weakness mainly involving the distal muscles of lower and upper limbs, variably associated with distal sensory loss and skeletal deformities. This chapter deals with dominantly transmitted CMT and related disorders, namely hereditary neuropathy with liability to pressure palsies (HNPP) and hereditary neuralgic amyotrophy (HNA). During the last 20 years, several genes have been uncovered associated with CMT and our understanding of the underlying molecular mechanisms has greatly improved. Consequently, a precise genetic diagnosis is now possible in the majority of cases, thus allowing proper genetic counseling. Although, unfortunately, treatment is still unavailable for all types of CMT, several cellular and animal models have been developed and some compounds have proved effective in these models. The first trials with ascorbic acid in CMT type 1A have been completed and, although negative, are providing relevant information on disease course and on how to prepare for future trials.

Murine therapeutic models for Charcot-Marie-Tooth (CMT) disease

INTRODUCTION OR BACKGROUND

Charcot-Marie-Tooth (CMT) disease represents a broad group of inherited motor and sensory neuropathies which can originate from various genetic aberrations, e.g. mutations, deletions and duplications.

SOURCES OF DATA

We performed a literature review on murine animal models of CMT disease with regard to experimental therapeutic approaches. Hereby, we focussed on the demyelinating subforms of CMT (CMT1). PubMed items were CMT, animal model, demyelination and therapy.

AREAS OF AGREEMENT

Patients affected by CMT suffer from slowly progressive, distally pronounced muscle atrophy caused by an axonal loss. The disease severity is highly variable and impairments may result in wheelchair boundness. No therapy is available yet.

AREAS OF CONTROVERSY

Numerous rodent models for the various CMT subtypes are available today. The selection of the correct animal model for the specific CMT subtype provides an important prerequisite for the successful translation of experimental findings in patients.

GROWING POINTS

Despite more than 20 years of remarkable progress in CMT research, the disease is still left untreatable. There is a growing number of experimental therapeutic strategies that may be translated into future clinical trials in patients with CMT.

AREAS TIMELY FOR DEVELOPING RESEARCH

The slow disease progression and insensitive outcome measures hamper clinical therapy trials in CMT. Biomarkers may provide powerful tools to monitor therapeutic efficacy. Recently, we have shown that transcriptional profiling can be utilized to assess and predict the disease severity in a transgenic rat model and in affected humans.

Novel MPZ mutations and congenital hypomyelinating neuropathy

We report two new MPZ mutations causing congenital hypomyelinating neuropathies; c.368_382delGCACGTTCACTTGTG (in-frame deletion of five amino acids) and c.392A>G, Asn131Ser. Each child had clinical and electrodiagnostic features consistent with an inherited neuropathy, confirmed by sural nerve biopsy. The cases illustrate the clinically heterogeneity that exists even within early-onset forms of this disease. They also lend additional support to the emerging clinical and laboratory evidence that impaired intracellular protein trafficking may represent the cause of some congenital hypomyelinating neuropathies.

Mpz gene suppression by shRNA increases Schwann cell apoptosis in vitro

We investigated the effects of short hairpin RNA (shRNA) on myelin protein zero (MPZ) gene expression in Schwann cells (SCs) in vitro and determined the effects of the MPZ gene suppression on the survival of SCs. The MPZ-specific shRNA was introduced into a lentiviral vector for expression under the U6 promoter, and the viral vector-based shRNAs were used to infect cultured SCs. The efficiency of MPZ knockdown was analyzed by real time-PCR (RT-PCR) and western blotting. Flow cytometric analysis and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) were used to determine the cell cycle and the amount of apoptosis of SCs. We found that MPZ shRNAs significantly inhibited the expression of the MPZ gene and induced SC apoptosis in vitro. These results provided interesting experimental evidence for understanding the mechanism of demyelinating neuropathies caused by MPZ gene malfunction.

Cellular characterization of MPZ mutations presenting with diverse clinical phenotypes

Mutations in MPZ, which encodes myelin protein zero (P(0)), may lead to different subtypes of Charcot-Marie-Tooth disease (CMT). The aim of this study was to characterize the cellular manifestations of various MPZ mutations associated with CMT1, Dejerine-Sottas syndrome (DSS) and CMT2, and to correlate their cellular and clinical phenotypes. Nine P(0) mutants associated with CMT1 (P(0)S63F, R98H, R277S, and S233fs), DSS (P(0) I30T and R98C), and CMT2 (P(0)S44F, D75V, and T124M), were investigated. Wild-type and mutant P(0) fused with fluorescent proteins were expressed in vitro to monitor their intracellular localization. An adhesiveness assay was used to evaluate the adhesiveness of the transfected cells. Protein localization and cell adhesiveness of each mutant protein were compared and correlated with their clinical phenotypes. Three different intracellular localization patterns of the mutant P(0) were observed. Wild-type P(0), P(0)I30T, S44F, S63F, D75V, T124M, and R227S were mostly localized on the cell membrane, P(0)R98H, and R98C were found in the endoplasmic reticulum (ER) or Golgi apparatus, and P(0)S233fs formed aggregates within the ER. Cells expressing mutant P(0), as compared with those expressing wild-type P(0), demonstrated variable degrees of reduction in the cell adhesiveness. The molecular patho-mechanisms of MPZ mutations are likely very complex and the clinical phenotype must be influenced by many genetic or environmental factors. This complexity may contribute to the highly variable clinical manifestations resulting from different MPZ mutations.

[Predominant parasympathetic involvement in a patient with Charcot-Marie-Tooth disease caused by the MPZ Thr124Met mutation]

Erectile dysfunction, dysuria, photophobia, and chronic cough developed insidiously in a 49-year-old man from his third decade. Severe difficulty of urination resulted in intermittent catheterization. He had six family members who had suffered similar autonomic symptoms with or without motor deficits. He presented asymmetrical tonic pupils, a neurogenic bladder, and mild sensory impairment in the distal parts of the bilateral lower limbs without orthostatic hypotension and motor deficits. Nerve conduction studies revealed mild axonal changes with slightly reduced conduction velocities in the lower limbs. His left pupil over-responded to instillation with 0.125% pilocarpine. Functional bladder tests showed an atonic bladder, suggesting postganglionic parasympathetic involvement. Autonomic evaluation for sympathetic components including head-up tilt, beat to beat responses to Valsalva's maneuver, cardiac MIBG imaging, plasma catecholamine levels and sweat tests were all normal. A genetic test disclosed a heterozygous mutation of myelin protein zero (MPZ); p.Thr124Met. Selectively distributed dysautonomia in this pedigree may indicate parasympathetic postganglionic components including the ganglion as the primary target of this mutated MPZ in the autonomic nervous system.

Charcot-Marie-Tooth disease type 1B: marked phenotypic variation of the Ser78Leu mutation in five Italian families

OBJECTIVES

To describe clinical, electrophysiological and genetic data of five unrelated Sicilian pedigrees harbouring a heterozygous Ser78Leu mutation in the myelin protein zero (MPZ) extracellular domain.

MATERIALS AND METHODS

Clinical, electrophysiological and genetic findings of 16 patients were reported. Polymorphic markers flanking the coding sequence of MPZ gene were also analysed.

RESULTS

A wide range of age at onset was observed in families 1 and 3, with a clinical heterogeneity, in terms of severity of the disease, within the same family (families 1 and 3), and among families. A markedly unsteady gait was a distinctive feature of many members of family 1. All patients in family 2 complained of severe cramps and painful paresthesia. Molecular genetic analysis showed that all affected subjects shared a common haplotype at three microsatellite loci D1S2858, D1S2624 and D1S484.

CONCLUSIONS

Our study provides further evidence that phenotypic features of MPZ mutations can vary within and among different families. High frequency of Ser78Leu mutation in Sicily as well as the results of haplotype analyses suggest that the mutation may have been inherited from a common ancestor.

Late onset autosomal dominant Charcot-Marie-Tooth 2 neuropathy in a Costa Rican family

OBJECTIVE

To describe the clinical, electrophysiologic and morphologic features of a Costa Rican family with an autosomal dominant inherited Charcot-Marie-Tooth (CMT) neuropathy.

METHODS

The field study took place in Costa Rica, Central America. Seven patients underwent neurological examinations and standard electrodiagnostic tests, and a sural nerve biopsy was taken from one patient. Fifteen family members were screened for gene defects associated with CMT disease.

RESULTS

Characteristic features of this family were a late age of onset (35-56 years), positive sensory symptoms and muscle cramps. Based on electrodiagnostic and morphologic data, the patients were classified as having a CMT2 neuropathy. The CMT1A duplication/HNPP deletion and point mutations in genes PMP22, MPZ, Cx32 and EGR2 implicated in the most common types of CMT disease were excluded. Subsequently, almost all known CMT loci were excluded by linkage analysis.

DISCUSSION

Features of this family were a late age of onset and positive sensory symptoms. This new autosomal dominant CMT neuropathy is associated with an unknown gene defect.

Auditory nerve is affected in one of two different point mutations of the neurofilament light gene

OBJECTIVE

To define auditory nerve and cochlear functions in two families with autosomal dominant axonal Charcot-Marie-Tooth (CMT).

METHODS

Affected members in two families with different point mutations of NF-L gene were screened with auditory brainstem responses (ABRs). Those with abnormal ABRs were further investigated with clinical, neurophysiological and audiological procedures. The point mutations of NF-L gene involved were Glu397Lys in 8 affected members of the family with AN, and Pro22Ser in 9 affected members of the family without AN.

RESULTS

ABRs and stapedial muscle reflexes were absent or abnormal in affected members of only one family consistent with auditory neuropathy (AN). In them, audiograms, otoacoustic emissions, and speech comprehension were normal. Absent or abnormal ABRs were consistent with slowing of conduction along auditory nerve and/or brainstem auditory pathway. Wave I when present was of normal latency.

CONCLUSIONS

Auditory nerve involvement in the presence of normal cochlear outer hair cell activity is asymptomatic in one of two families with CMT disorder with different point mutations of the NF-L gene. The nerve disorder is consistent with altered synchrony and slowed conduction.

SIGNIFICANCE

The absence of "deafness" may reflect the ability of central mechanisms to compensate for the slowly developing auditory nerve abnormalities.

Rapid progression of late onset axonal Charcot-Marie-Tooth disease associated with a novel MPZ mutation in the extracellular domain

Myelin protein zero (MPZ) is a major component of compact myelin in peripheral nerves where it plays an essential role in myelin formation and adhesion. MPZ gene mutations are usually responsible for demyelinating neuropathies, namely Charcot-Marie-Tooth (CMT) type 1B, Déjèrine-Sottas neuropathy and congenital hypomyelinating neuropathy. Less frequently, axonal CMT (CMT2) associated with MPZ mutations has been described. We report six patients (one sporadic case and five subjects from two apparently unrelated families) with a late onset, but rapidly progressive, axonal peripheral neuropathy. In all patients, molecular analysis demonstrated a novel heterozygous missense mutation (208C>T) in MPZ exon 2, causing the Pro70Ser substitution in the extracellular domain. The diagnosis of CMT2 associated with MPZ mutations should be considered in both sporadic and familial cases of late onset, progressive polyneuropathy. The mechanism whereby compact myelin protein mutations cause axonal neuropathy remains to be elucidated.

Human cytomegalovirus (HCMV) and hearing impairment: infection of fibroblast cells with HCMV induces chromosome breaks at 1q23.3, between loci DFNA7 and DFNA49 -- both involved in dominantly inherited, sensorineural, hearing impairment

Human cytomegalovirus (HCMV) infection is the most common congenital infection in developed countries and is responsible for a substantial fraction of sensorineural hearing impairment (SNHI) in children. The risk of hearing impairment is associated with viral load in urine and blood collected during the first postnatal month. However, although inner ear abnormalities are observed in some children with HCMV-induced SNHI, the exact mechanism whereby congenital HCMV infection causes hearing impairment is unknown. Earlier studies using standard cytogenetic mapping techniques showed that infection of S-phase human fibroblast cells with HCMV resulted in two specific, site-directed, chromosome breaks at band positions 1q21 and 1q42 which include loci involved in dominantly and recessively inherited hearing impairment, respectively. These findings suggested that cells infected with HCMV might provide a reservoir for genetic damage and, in a clinical perspective, a scenario could be envisioned whereby hearing impairment could result from early DNA damage of dividing fetal cells rather than viral replication and cell lysis. In this work we demonstrate, using fine mapping techniques, that HCMV infection in S-phase fibroblast cells induces genetic damage at 1q23.3, within a maximal region of 37 kb, containing five low copy repeat (LCR) elements. The breakpoint is situated between two hearing impairment (HI) loci, DFNA49 and DFNA7, and in close proximity to the MPZ gene previously shown to be involved in autosomal dominant Charcot-Marie-Tooth syndrome (CMT1B) with auditory neuropathy.

Molecular genetics of autosomal-dominant axonal Charcot-Marie-Tooth disease

The autosomal-dominant axonal peripheral neuropathies comprise a genetically heterogeneous group of disorders that are clinically subsumed under Charcot-Marie-Tooth disease type 2 (CMT2). A significant increase in the number of genes underlying major forms of CMT2 has improved the classification of specific CMT phenotypes. The molecular dissection of cellular functions of the related gene products has only begun and detailed pathophysiological models are still missing, but already the biological scope of genes linked to CMT2 is more diversified than CMT1. The known CMT2 genes present key players in these pathways and will likely prove as powerful tools in identifying eventual future targets for therapeutic intervention.

Molecular diagnostics of Charcot-Marie-Tooth disease and related peripheral neuropathies

DNA diagnostics plays an important role in the characterization and management of patients manifesting inherited peripheral neuropathies. We describe the clinical integration of molecular diagnostics with medical history, physical examination, and electrophysiological studies. Molecular testing can help establish a secure diagnosis, enable genetic counseling regarding recurrence risk, potentially provide prognostic information, and in the near future may be important for the choice of therapies.

Mutation in Mpzl3, a novel [corrected] gene encoding a predicted [corrected] adhesion protein, in the rough coat (rc) mice with severe skin and hair abnormalities

The rough coat (rc), an autosomal-recessive mutation, arose spontaneously in C57BL/6J mice. Homozygous rc mice develop severe skin and hair abnormalities, including cyclic and progressive hair loss and sebaceous gland hypertrophy. The rc locus was previously mapped to Chromosome 9. To elucidate the genetic basis underlying the rc phenotype development, we carried out positional cloning, and mapped the rc locus to a 246-kb interval. We identified a missense mutation within a novel open reading frame in the rc/rc mice, which is predicted to encode a cell adhesion molecule with the highest homology to myelin protein zero (MPZ) and myelin protein zero-like 2 (MPZL2, also called epithelial V-like antigen). We therefore named this gene Mpzl3 (myelin protein zero-like 3). The mutation in the rc/rc mice occurred at a highly conserved residue within the conserved Ig-like V-type domain, thus likely altering the MPZL3 protein function. Reverse transcriptase-PCR and Western blot analyses revealed expression of the Mpzl3 gene in various adult organs, including the skin. Using indirect immunofluorescence, we detected MPZL3 protein in the keratinocytes and sebocytes in the skin. Results from this study identified a novel gene encoding a predicted adhesion protein whose mutation in the rc/rc mice likely caused the rc phenotype.

Major myelin protein gene (P0) mutation causes a novel form of axonal degeneration

Mutations in the major peripheral nervous system (PNS) myelin protein, myelin protein zero (MPZ), cause Charcot-Marie-Tooth Disease type 1B (CMT1B), typically thought of as a demyelinating peripheral neuropathy. Certain MPZ mutations, however, cause adult onset neuropathy with minimal demyelination but pronounced axonal degeneration. Mechanism(s) for this phenotype are unknown. We performed an autopsy of a 73-year-old woman with a late-onset neuropathy caused by an H10P MPZ mutation whose nerve conduction studies suggested severe axonal loss but no demyelination. The autopsy demonstrated axonal loss and reorganization of the molecular architecture of the axolemma. Segmental demyelination was negligible. In addition, we identified focal nerve enlargements containing MPZ and ubiquitin either in the inner myelin intralaminar and/or periaxonal space that separates axons from myelinating Schwann cells. Taken together, these data confirmed that a mutation in MPZ can cause axonal neuropathy, in the absence of segmental demyelination, thus uncoupling the two pathological processes. More important, it also provided potential molecular mechanisms as to how the axonal degeneration occurred: either by disruption of glial-axon interaction by protein aggregates or by alterations in the molecular architecture of internodes and paranodes. This report represents the first study in which the molecular basis of axonal degeneration in the late-onset CMT1B has been explored in human tissue.

Novel C59T leader peptide mutation in the MPZ gene associated with late-onset, axonal, sensorimotor polyneuropathy

The objective of this study was to report a novel exon-1 mutation in the myelin protein zero (MPZ) gene, resulting in axonal Charcot-Marie-Tooth neuropathy with recurrent hyper-CK-emia. In a 64-year-old woman slowly progressive distal lower limb weakness, muscle cramps in the lower limb muscles, and stocking-type numbness had developed from the age of 61. Neurologic examination revealed discrete hip flexor weakness, weakness for foot extension, diffuse wasting of the distal lower limb muscles, reduced patella tendon reflexes, and absent Achilles tendon reflexes. There was recurrently elevated creatine kinase with a maximum of 607 U/l (n, <145 U/l). Stimulation of the peroneal and tibial nerves did not evoke a muscular response. Electromyography was neurogenic. Biopsy of the right sural nerve showed diffuse axonal degeneration and loss of axons of all diameters. Muscle biopsy showed increased fiber-size variability, angulated fibers, internalized nuclei, accumulations of nuclei, grouped atrophic muscle fibers, and fiber splitting. Molecular genetic analysis by PCR and direct nucleotide sequencing revealed the heterozygous C59T exon-1 MPZ gene mutation, resulting in the amino acid exchange S20F of the MPZ signal protein domain (leader peptide). The novel C59T mutation in the leader peptide of the MPZ gene is pathogenic and manifests as severe, late-onset, axonal, symmetric sensorimotor polyneuropathy (CMT2) and hyper-CK-emia.

Clinical and electrophysiological aspects of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a genetically heterogeneous group of disorders sharing the same clinical phenotype, characterized by distal limb muscle wasting and weakness, usually with skeletal deformities, distal sensory loss, and abnormalities of deep tendon reflexes. Mutations of genes involved in different functions eventually lead to a length-dependent axonal degeneration, which is the likely basis of the distal predominance of the CMT phenotype. Nerve conduction studies are important for classification, diagnosis, and understanding of pathophysiology. The subdivision into demyelinating CMT1 and axonal CMT2 types was a milestone and is still valid for the majority of patients. However, exceptions to this partition are increasing. Intermediate conduction velocities are often found in males with X-linked CMT (CMTX), and different intermediate CMT types have been identified. Moreover, for some genes, different mutations may result either in demyelinating CMT with slow conduction, or in axonal CMT. Nerve conduction slowing is uniform and diffuse in the most common CMT1A associated with the 17p12 duplication, whereas it is often asymmetric and nonhomogeneous in CMTX, sometimes rendering difficult the differential diagnosis with acquired inflammatory neuropathies. The demyelinating recessive forms, termed CMT4, usually have early onset and run a more severe course than the dominant types. Pure motor CMT types are now classified as distal hereditary motor neuronopathy. The diagnostic approach to the identification of the CMT subtype is complex and cannot be based on the clinical phenotype alone, as different forms are often clinically indistinguishable. However, there are features that may be of help in addressing molecular investigation in a single patient. Late onset, prominent or peculiar sensory manifestations, autonomic nervous system dysfunction, cranial nerve involvement, upper limb predominance, subclinical central nervous system abnormalities, severe scoliosis, early-onset glaucoma, neutropenia are findings helpful for diagnosis.

Peripheral neuropathies caused by mutations in the myelin protein zero

Charcot-Marie-Tooth disease type 1B (CMT1B) is caused by mutations in the major PNS myelin protein myelin protein zero (MPZ). MPZ is a member of the immunoglobulin supergene family and functions as an adhesion molecule helping to mediate compaction of PNS myelin. Mutations in MPZ appear to either disrupt myelination during development, leading to severe early onset neuropathies, or to disrupt axo-glial interactions leading to late onset neuropathies in adulthood. Identifying molecular pathways involved in early and late onset CMT1B will be crucial to understand how MPZ mutations cause CMT1B so that rational therapies for both early and late onset neuropathies can be developed.

Marked phenotypic variation in a family with a new myelin protein zero mutation

Myelin protein zero (MPZ) is a member of the immunoglobulin gene superfamily, which has a role in myelin compaction. MPZ gene mutations cause mostly demyelinating neuropathies of the Charcot-Marie-Tooth 1B type (CMT1B), but axonal CMT have been described as well. There is a broad spectrum of phenotypic manifestation of neuropathies caused by MPZ mutations. Some mutations of MPZ cause severe early-onset neuropathies such as Dejerine-Sottas disease, while others cause the classical CMT phenotype with normal early milestones but development of disability during the first two decades of life. We describe a family in which five members of three consecutive generations had a heterozygous mutation in nucleotide position 143 with a T-C transition in exon 2 of the MPZ gene. The resulting substitution of Leu48 with proline has not been previously described. The age of onset of symptoms varied from 8 months to 41 years. The marked variation of the age of disease onset and clinical phenotype in this one family, related to the same MPZ mutation, suggests that in addition to the type and intragenic location of the mutation, other putative modifying gene(s) are regulating MPZ gene expression, mRNA stability and posttranslational protein modification and may have an important effect on the ultimate clinical phenotype.

A new locus for autosomal dominant Charcot-Marie-Tooth disease type 2 (CMT2L) maps to chromosome 12q24

Charcot-Marie-Tooth disease (CMT) is one of the most common inherited neurological disorders with a prevalence estimated at 1/2500. The axonal form of this disorder is referred to as Charcot-Marie-Tooth type 2 disease (CMT2). Recently, a large Chinese family with CMT2 was found in the Hunan and Hubei provinces of China. The known loci for CMT1A, CMT2D, CMT1B (the same locus is also responsible for CMT2I and CMT2J), CMT2A, CMT2E, and CMT2F were excluded in this family by linkage analysis. A genome-wide screening was then carried out, and the results revealed linkage of CMT2 to a locus at chromosome 12q24. Haplotype construction and analyses localized this novel locus to a 6.8-cM interval between microsatellite markers D12S366 and D12S1611. The maximal two-point LOD score of 6.35 and multipoint LOD score of 8.08 for marker D12S76 at a recombination fraction (theta) of 0 strongly supported linkage to this locus. Thus, CMT2 neuropathy in this family represents a novel genetic entity that we have designated as CMT2L.

A novel mutation of myelin protein zero associated with an axonal form of Charcot-Marie-Tooth disease

OBJECTIVE

To report a new mutation in the MPZ gene which encodes myelin protein zero (P0), associated with an axonal form of Charcot-Marie-Tooth disease (CMT).

METHODS

Three patients from an Italian family with a mild, late onset axonal peripheral neuropathy are described clinically and electrophysiologically. To detect point mutation in MPZ gene the whole coding sequence was examined. The structure of the mutated protein was investigated using the three dimensional model of P0.

RESULTS

All patients showed a relatively mild CMT phenotype characterised by late onset and heterogeneity of the clinical and electrophysiological features. Molecular analysis demonstrated a novel heterozygous T/A transversion in the exon 3 of MPZ gene that predicts an Asp109Glu amino acid substitution in the extracellular domain of the P0. Asp109 is found at the protein surface, on beta strand E, in the interior of the P0 tetramer.

CONCLUSIONS

The identification of Asp109Glu mutation confirms the pivotal role of P0 in axonal neuropathies and stresses the phenotypic heterogeneity associated with MPZ mutations. This study suggests the value of screening for MPZ mutations in CMT family members with minor clinical and electrophysiological signs of peripheral neuropathy.

The novel neurofilament light (NEFL) mutation Glu397Lys is associated with a clinically and morphologically heterogeneous type of Charcot-Marie-Tooth neuropathy

Charcot-Marie-Tooth disease comprises a heterogeneous group of hereditary neuropathies which fall into two main groups: demyelinating CMT1 with reduced nerve conduction velocity and axonal CMT2 with normal nerve conduction velocity. The neuropathological features correspond in most cases to this classification. Four genes were recently identified to cause autosomal dominant CMT2, including the neurofilament light gene. Thus far, only few mutations have been reported in neurofilament light involving eight amino acids of the gene. We identified a novel mutation, Glu397Lys, in a conserved motive signaling the end of the rod domain. The affected family members from three generations showed strikingly different clinical phenotypes, including weakness of the lower extremities, foot deformities, and deafness. The mutation was associated with nerve conduction velocities ranging from 27 m/s in a 25-year-old female to 43 m/s in an 82-year-old male in the lower extremity motor nerves. Sural nerve biopsies of two affected subjects were analyzed by light and electron microscopy. The pathological changes consisted of a reduction of predominantly large myelinated nerve fibers and various stages of onion bulb formation as typically seen in CMT1. This correlative study further confirms that neurofilament light gene mutations cause a wide clinical spectrum. Thus, analysis of the neurofilament light gene should not be restricted to pure axonal neuropathies.

Late onset Charcot–Marie–Tooth 2 syndrome caused by two novel mutations in the MPZ gene

MPZ gene mutations cause demyelinating and axonal Charcot–Marie–Tooth (CMT) disease. Two novel MPZ mutations are reported in very late onset and progressive CMT syndrome. The N60H caused axonal CMT in a large family, whereas the I62M occurred in a single patient presenting with a primary axonal neuropathy. Previously, chronic polyradiculoneuritis was assumed in two patients. Molecular genetic testing and particularly screening for MPZ mutations in late onset neuropathies are important to differentiate acquired and inherited neuropathies.

Autonomic and respiratory dysfunction in Charcot-Marie-Tooth disease due to Thr124Met mutation in the myelin protein zero gene

OBJECTIVE

To report the clinical and electrophysiological characteristics of a family presenting Charcot-Marie-Tooth disease (CMT) associated with autonomic nervous system disturbances.

METHODS

We studied nerve conduction values, postural adaptation, sympathetic skin reflex, the variation in heart rate by the Valsalva ratio and pupillometry in 7 members of a French family in which CMT due to a Thr124Met mutation in the myelin protein zero (MPZ) gene was diagnosed.

RESULTS

Clinical and laboratory evidence of autonomic nervous system disturbances were found in the affected individuals. The clinical phenotype was characterized by sensorimotor peripheral neuropathy, defined as axonal type by electrophysiological studies, and was associated with severe pain, bladder dysfunction, sudorimotor disturbances and abolished pupillary reflex to light. Moreover, two patients had severe restrictive respiratory insufficiency requiring noninvasive mechanical ventilation.

CONCLUSIONS

Our study demonstrates that autonomic disturbances may be one of the major clinical signs associated with CMT secondary to MPZ gene mutation in codon 124. Testing of pupillary reflex allows the discrimination of affected and unaffected subjects in our family. However, involvement of the autonomic nervous system in this type of neuropathy is unclear and further studies are required to elucidate the role of the MPZ gene in the autonomic nervous system.

Molecular Mechanisms, Diagnosis, and Rational Approaches to Management of and Therapy for Charcot-Marie-Tooth Disease and Related Peripheral Neuropathies

During the last decade, 18 genes and 11 additional loci harboring candidate genes have been associated with Charcot-Marie-Tooth disease (CMT) and related peripheral neuropathies. Ten of these 18 genes have been identified in the last 2 years. This phenomenal pace of CMT gene discovery has fomented an unprecedented explosion of information regarding peripheral nerve biology and its pathologic manifestations in CMT. This review integrates molecular genetics with the clinical phenotypes and provides a flowchart for molecular-based diagnostics. In addition, we discuss rational approaches to molecular therapeutics, including novel biologic molecules (eg, small interfering ribonucleic acid [siRNA], antisense RNA, and ribozymes) that potentially could be used as drugs in the future. These may be applicable in attempts to normalize gene expression in cases of CMT type 1A, wherein a 1.5 Mb genomic duplication causes an increase in gene dosage that is associated with the majority of CMT cases. Aggresome formation by the PMP22 gene product, the disease-associated gene in the duplication cases, could thus be avoided. We also discuss alternative therapeutics, in light of other neurodegenerative disorders, to disrupt such aggresomes. Finally, we review rational therapeutic approaches, including the use of antioxidants such as vitamin E, coenzyme Q10, or lipoic acid to relax potential oxidative stress in peripheral nerves, for CMT management.

The phenotypic manifestations of autosomal recessive axonal Charcot-Marie-Tooth due to a mutation in Lamin A/C gene

Charcot-Marie-Tooth disease constitutes a genetically heterogeneous group of hereditary motor and sensory peripheral neuropathies. The axonal type of Charcot-Marie-Tooth is designated type 2. Six loci for autosomal dominant and three for recessive Charcot-Marie-Tooth type 2 have been reported so far. In this study we report the phenotype of autosomal recessive axonal Charcot-Marie-Tooth type 2 due to a recently-described mutation (c.892C>T-p.R298C) in a gene encoding Lamin A/C nuclear envelope proteins and the first gene in which a mutation leads to autosomal recessive Charcot-Marie-Tooth type 2. We have explored eight patients from four Algerian families. The onset is usually in the second decade and the course is rapid, involving upper limbs and proximal muscles, leading to a severe condition in less than 4 years. Many different mutations in Lamin A/C have been identified as causing variable phenotypes, such as limb girdle muscular dystrophy type 1B, autosomal dominant and recessive Emery-Dreyfuss muscular dystrophy, dilated cardiomyopathy with atrioventricular conduction defect, and Dunnigan-type familial partial lipodystrophy should prompt us to fully investigate the skeletal and cardiac muscles in patients affected with autosomal recessive Charcot-Marie-Tooth type 2 carrying a mutation in LMNA.

Demyelinating and axonal features of Charcot-Marie-Tooth disease with mutations of myelin-related proteins (PMP22, MPZ and Cx32): a clinicopathological study of 205 Japanese patients

Three genes commonly causing Charcot-Marie-Tooth disease (CMT) encode myelin-related proteins: peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ) and connexin 32 (Cx32). Demyelinating versus axonal phenotypes are major issues in CMT associated with mutations of these genes. We electrophysiologically, pathologically and genetically evaluated demyelinating and axonal features of 205 Japanese patients with PMP22 duplication, MPZ mutations or Cx32 mutations. PMP22 duplication caused mainly demyelinating phenotypes with slowed motor nerve conduction velocity (MCV) and demyelinating histopathology, while axonal features were variably present. Two distinctive phenotypic subgroups were present in patients with MPZ mutations: one showed preserved MCV and exclusively axonal pathological features, while the other was exclusively demyelinating. These axonal and demyelinating phenotypes were well concordant among siblings in individual families, and MPZ mutations did not overlap among these two subgroups, suggesting that the nature and position of the MPZ mutations mainly determine the axonal and demyelinating phenotypes. Patients with Cx32 mutations showed intermediate slowing of MCV, predominantly axonal features and relatively mild demyelinating pathology. These axonal and demyelinating features were present concomitantly in individual patients to a variable extent. The relative severity of axonal and demyelinating features was not associated with particular Cx32 mutations. Median nerve MCV and overall histopathological phenotype changed little with disease advancement. Axonal features of diminished amplitudes of compound muscle action potentials (CMAPs), axonal loss, axonal sprouting and neuropathic muscle wasting all changed as disease advanced, especially in PMP22 duplication and Cx32 mutations. Median nerve MCVs were well maintained independently of age, disease duration and the severity of clinical and pathological abnormalities, confirming that median nerve MCV is an excellent marker for the genetically determined neuropathic phenotypes. Amplitude of CMAPs was correlated significantly with distal muscle strength in PMP22 duplication, MPZ mutations and Cx32 mutations, while MCV slowing was not, indicating that clinical weakness results from reduced numbers of functional large axons, not from demyelination. Thus, the three major myelin-related protein mutations induced varied degrees of axonal and demyelinating phenotypic features according to the specific gene mutation as well as the stage of disease advancement, while clinically evident muscle wasting was attributable to loss of functioning large axons.

Molecular analysis in Japanese patients with Charcot-Marie-Tooth disease: DGGE analysis for PMP22, MPZ, and Cx32/GJB1 mutations

Charcot-Marie-Tooth disease (CMT) is a heterogeneous disorder and is traditionally classified into two major types, CMT type 1 (CMT1) and CMT type 2 (CMT2). Most CMT1 patients are associated with the duplication of 17p11.2-p12 (CMT1A duplication) and small numbers of patients have mutations of the peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ), connexin 32 (Cx32/GJB1), and early growth response 2 (EGR2) genes. Some mutations of MPZ and Cx32 were also associated with the clinical CMT2 phenotype. We constructed denaturing gradient gel electrophoresis (DGGE) analysis as a screening method for PMP22, MPZ, and Cx32 mutations and studied 161 CMT patients without CMT1A duplication. We detected 27 mutations of three genes including 15 novel mutations; six of PMP22, three of MPZ, and six of Cx32. We finally identified 21 causative mutations in 22 unrelated patients and five polymorphic mutations. Eighteen of 22 patients carrying PMP22, MPZ, or Cx32 mutations presented with CMT1 and four of them with MPZ or Cx32 mutations presented with the CMT2 phenotype. DGGE analysis was sensitive for screening for those gene mutations, but causative gene mutation was not identified in many of the Japanese patients with CMT, especially with CMT1. Other candidate genes should be studied to elucidate the genetic basis of Japanese CMT patients.

Charcot-Marie-Tooth neuropathy: clinical phenotypes of four novel mutations in the MPZ and Cx 32 genes

Charcot-Marie-Tooth Hereditary Neuropathy is a heterogeneous syndrome associated with mutations in several different genes including peripheral myelin protein 22, myelin P0, connexin 32, and early growth response 2. There is considerable variability in the phenotypic expression of this syndrome and the relationship of this variability to mutation genotypes requires extensive analysis. Here we describe the phenotypes and genotypes of four new mutations underlying the Charcot-Marie-Tooth syndrome and document segregation with disease. Four families with Charcot-Marie-Tooth were ascertained, examined, and evaluated electrophysiologically. Each family had peripheral blood DNA screened for mutations in myelin protein 22, myelin P0, and connexin 32. Two families were found with new mutations in the myelin P0 gene: S140T in the extracellular domain and K236del in the cytoplasmic domain. All families showed segregation of the mutations with the Charcot-Marie-Tooth phenotype as did a new family with the rare G163R mutation in the membrane domain. A 49-year-old man with the S140T mutation demonstrated conduction block on electrophysiological testing. A family with a novel S49P mutation in the connexin 32 gene had a neuropathy with very slow nerve conduction. These new mutations in the myelin P0 and connexin 32 genes help to clarify the pathophysiology of the clinical Charcot-Marie-Tooth syndrome. The S140T mutation in myelin P0 can be associated with conduction block and Charcot-Marie-Tooth should be part of the differential diagnosis of that phenomenon. Mutations in the cytoplasmic domain of myelin P0 can cause clinical neuropathy. The S49P mutation in the connexin 32 gene can produce aspects of a demyelinating type of X-linked hereditary neuropathy.

Inherited neuropathies

Inherited neuropathies are common and are usually caused by mutations in genes that are expressed by myelinating Schwann cells or neurons, which is the biological basis for long-standing distinction between primary demyelinating and axonal neuropathies. Neuropathies can be isolated, the primary manifestation of a more complex syndrome, or overshadowed by other aspects of the inherited disease. Increasing knowledge of the molecular-genetic causes of inherited neuropathies facilitates faster, more accurate diagnosis, and sets the stage for development of specific therapeutic interventions.