X-linked Charcot-Marie-Tooth disease with connexin 32 mutations: clinical and electrophysiologic study

Demyelinating prenatal and infantile developmental neuropathies

The prenatal and infantile neuropathies are an uncommon and complex group of conditions, most of which are genetic. Despite advances in diagnostic techniques, approximately half of children presenting in infancy remain without a specific diagnosis. This review focuses on inherited demyelinating neuropathies presenting in the first year of life. We clarify the nomenclature used in these disorders, review the clinical features of demyelinating forms of Charcot-Marie-Tooth disease with early onset, and discuss the demyelinating infantile neuropathies associated with central nervous system involvement. Useful clinical, neurophysiologic, and neuropathologic features in the diagnostic work-up of these conditions are also presented.

Genetic epidemiology of Charcot-Marie-Tooth disease

BACKGROUND

Charcot-Marie-Tooth disease (CMT) is the most common inherited disorder of the peripheral nervous system. The frequency of different CMT genotypes has been estimated in clinic populations, but prevalence data from the general population is lacking. Point mutations in the mitofusin 2 (MFN2) gene has been identified exclusively in Charcot-Marie-Tooth disease type 2 (CMT2), and in a single family with intermediate CMT. MFN2 point mutations are probably the most common cause of CMT2. The CMT phenotype caused by mutation in the myelin protein zero (MPZ) gene varies considerably, from early onset and severe forms to late onset and milder forms. The mechanism is not well understood. The myelin protein zero (P(0) ) mediates adhesion in the spiral wraps of the Schwann cell's myelin sheath. X-linked Charcot-Marie Tooth disease (CMTX) is caused by mutations in the connexin32 (cx32) gene that encodes a polypeptide which is arranged in hexameric array and form gap junctions.

AIMS

Estimate prevalence of CMT. Estimate frequency of Peripheral Myelin Protein 22 (PMP22) duplication and point mutations, insertions and deletions in Cx32, Early growth response 2 (EGR2), MFN2, MPZ, PMP22 and Small integral membrane protein of lysosome/late endosome (SIMPLE) genes. Description of novel mutations in Cx32, MFN2 and MPZ. Description of de novo mutations in MFN2.

MATERIAL AND METHODS

Our population based genetic epidemiological survey included persons with CMT residing in eastern Akershus County, Norway. The participants were interviewed and examined by one geneticist/neurologist, and classified clinically, neurophysiologically and genetically. Two-hundred and thirty-two consecutive unselected and unrelated CMT families with available DNA from all regions in Norway were included in the MFN2 study. We screened for point mutations in the MFN2 gene. We describe four novel mutations, two in the connexin32 gene and two in the MPZ gene.

RESULTS

A total of 245 affected from 116 CMT families from the general population of eastern Akershus county were included in the genetic epidemiological survey. In the general population 1 per 1214 persons (95% CI 1062-1366) has CMT. Charcot-Marie-Tooth disease type 1 (CMT1), CMT2 and intermediate CMT were found in 48.2%, 49.4% and 2.4% of the families, respectively. A mutation in the investigated genes was found in 27.2% of the CMT families and in 28.6% of the affected. The prevalence of the PMP22 duplication and mutations in the Cx32, MPZ and MFN2 genes was found in 13.6%, 6.2%, 1.2%, 6.2% of the families, and in 19.6%, 4.8%, 1.1%, 3.2% of the affected, respectively. None of the families had point mutations, insertions or deletions in the EGR2, PMP22 or SIMPLE genes. Four known and three novel mitofusin 2 (MFN2) point mutations in 8 unrelated Norwegian CMT families were identified. The novel point mutations were not found in 100 healthy controls. This corresponds to 3.4% (8/232) of CMT families having point mutations in MFN2. The phenotypes were compatible with CMT1 in two families, CMT2 in four families, intermediate CMT in one family and distal hereditary motor neuronopathy (dHMN) in one family. A point mutation in the MFN2 gene was found in 2.3% of CMT1, 5.5% of CMT2, 12.5% of intermediate CMT and 6.7% of dHMN families. Two novel missense mutations in the MPZ gene were identified. Family 1 had a c.368G>A (Gly123Asp) transition while family 2 and 3 had a c.103G>A (Asp35Asn) transition. The affected in family 1 had early onset and severe symptoms compatible with Dejerine-Sottas syndrome (DSS), while affected in family 2 and 3 had late onset, milder symptoms and axonal neuropathy compatible with CMT2. Two novel connexin32 mutations that cause early onset X-linked CMT were identified. Family 1 had a deletion c.225delG (R75fsX83) which causes a frameshift and premature stop codon at position 247 while family 2 had a c.536G>A (Cys179Tyr) transition which causes a change of the highly conserved cysteine residue, i.e. disruption of at least one of three disulfide bridges. The mean age at onset was in the first decade and the nerve conduction velocities were in the intermediate range.

DISCUSSION

Charcot-Marie-Tooth disease is the most common inherited neuropathy. At present 47 hereditary neuropathy genes are known, and an examination of all known genes would probably only identify mutations in approximately 50% of those with CMT. Thus, it is likely that at least 30-50 CMT genes are yet to be identified. The identified known and novel point mutations in the MFN2 gene expand the clinical spectrum from CMT2 and intermediate CMT to also include possibly CMT1 and the dHMN phenotypes. Thus, genetic analyses of the MFN2 gene should not be restricted to persons with CMT2. The phenotypic variation caused by different missense mutations in the MPZ gene is likely caused by different conformational changes of the MPZ protein which affects the functional tetramers. Severe changes of the MPZ protein cause dysfunctional tetramers and predominantly uncompacted myelin, i.e. the severe phenotypes congenital hypomyelinating neuropathy and DSS, while milder changes cause the phenotypes CMT1 and CMT2. The two novel mutations in the connexin32 gene are more severe than the majority of previously described mutations possibly due to the severe structural change of the gap junction they encode.

CONCLUSION

Charcot-Marie-Tooth disease is the most common inherited disorder of the peripheral nervous system with an estimated prevalence of 1 in 1214. CMT1 and CMT2 are equally frequent in the general population. The prevalence of PMP22 duplication and of mutations in Cx32, MPZ and MFN2 is 19.6%, 4.8%, 1.1% and 3.2%, respectively. The ratio of probable de novo mutations in CMT families was estimated to be 22.7%. Genotype- phenotype correlations for seven novel mutations in the genes Cx32 (2), MFN2 (3) and MPZ (2) are described. Two novel phenotypes were ascribed to the MFN2 gene, however further studies are needed to confirm that MFN2 mutations can cause CMT1 and dHMN.

Pes cavus and hereditary neuropathies: when a relationship should be suspected

The hereditary peripheral neuropathies are a clinically and genetically heterogeneous group of diseases of the peripheral nervous system. Foot deformities, including the common pes cavus, but also hammer toes and twisting of the ankle, are frequently present in patients with hereditary peripheral neuropathy, and often represent one of the first signs of the disease. Pes cavus in hereditary peripheral neuropathies is caused by imbalance between the intrinsic muscles of the foot and the muscles of the leg. Accurate clinical evaluation in patients with pes cavus is necessary to exclude or confirm the presence of peripheral neuropathy. Hereditary peripheral neuropathies should be suspected in those cases with bilateral foot deformities, in the presence of family history for pes cavus and/or gait impairment, and in the presence of neurological symptoms or signs, such as distal muscle hypotrophy of limbs. Herein, we review the hereditary peripheral neuropathies in which pes cavus plays a key role as a "spy sign," discussing the clinical and molecular features of these disorders to highlight the importance of pes cavus as a helpful clinical sign in these rare diseases.

[Novel mutation in X-linked Charcot-Marie-tooth (CMTXI) disease associated with central conduction slowing on brainstem auditory evoked potential (BAEP)]

CMTX1, the second most common type of inherited hereditary motor and sensory neuropathy (HMSN), is associated with mutations of the gene for the gap junction protein connexin 32 (Cx32). In this condition, central conduction velocity is known to be delayed, presumably because mutated Cx32 is expressed in oligodendrocytes. A 45-year-old man presented with a 5-year history of progressive gait disturbance due to leg muscle weakness. The family history revealed that the mother had also progressive gait disturbance in her early 40s, and the younger sister could not walk faster than before at the age of 41. On neurological assessment, the patient exhibited pes cavus, distal muscle atrophy and weakness, and absence of the knee and ankle jerks. Touch sensation was impaired in the both feet. Motor and sensory nerve conduction velocities were reduced to 30-36 m/s with mild temporal dispersion. Sural nerve biopsy revealed diffuse loss of large myelinated fibers with the remaining large and intermediate nerve fibers being frequently surrounded by a thin myelin sheath. Onion bulb formation was only occasional and mild in degree. His hearing acuity was normal on pure-tone audiometry, but BAEP test demonstrated prolonged central conduction time (-I wave 1.8 milliseconds, I-V wave 6.4 milliseconds). The BAEP findings prompted us to choose Cx32 gene to analyze first to find a novel mutation of two (A and T) base pairs deletion at codons 277 and 278 (Met93fs). Thus, the present case indicates that Cx32 gene mutation should be targeted first in case of HMSN with abnormal BAEP.

Myotonic dystrophy type I combined with X-linked dominant Charcot-Marie-Tooth neuropathy

Both the myotonic dystrophy type 1 (DM1) and the X-linked dominant Charcot-Marie-Tooth disease (CMTX1) are well-established inherited neuromuscular disorders characterized by progressive weakness and atrophy of the distal limb muscles. The underlying causes of the DM1 and CMTX1 are mutations in the DMPK and GJB1 gene, respectively. A patient with both DM1 and CMTX1 inherited these from his father and mother, respectively. Histopathological and electrodiagnostic studies revealed both chronic neuropathic and myopathic features. Physical disabilities were more severe than seen with either DM1 or CMTX1 alone. In addition, the present case reveals an asymmetric atrophy (22%) of the right calf muscle compared to the left side.

Glia modulation of the extracellular milieu as a factor in central CO2 chemosensitivity and respiratory control

We discuss the influence of astrocytes on respiratory function, particularly central CO2 chemosensitivity. Fluorocitrate (FC) poisons astrocytes, and studies in intact animals using FC provide strong evidence that disrupting astrocytic function can influence CO2 chemosensitivity and ventilation. Gap junctions interconnect astrocytes and contribute to K+ homeostasis in the extracellular fluid (ECF). Blocking gap junctions alters respiratory control, but proof that this is truly an astrocytic effect is lacking. Intracellular pH regulation of astrocytes has reciprocal effects on extracellular pH. Electrogenic sodium-bicarbonate transport (NBCe) is present in astrocytes. The activity of NBCe alkalinizes intracellular pH and acidifies extracellular pH when activated by depolarization (and a subset of astrocytes are depolarized by hypercapnia). Thus, to the extent that astrocytic intracellular pH regulation during hypercapnia lowers extracellular pH, astrocytes will amplify the hypercapnic stimulus and may influence central chemosensitivity. However, the data so far provide only inferential support for this hypothesis. A lactate shuttle from astrocytes to neurons seems to be active in the retrotrapezoid nucleus (RTN) and important in setting the chemosensory stimulus in the RTN (and possibly other chemosensory nuclei). Thus astrocytic processes, so vital in controlling the constituents of the ECF in the central nervous system, may profoundly influence central CO2 chemosensitivity and respiratory control.

-459C>T point mutation in 5' non-coding region of human GJB1 gene is linked to X-linked Charcot-Marie-Tooth neuropathy

Charcot-Marie-Tooth (CMT) neuropathy is inherited with genetic and clinical heterogeneity. The X-linked form (CMTX) is linked to mutations in the GJB1 gene. However, the genotype-phenotype correlation between variants in the non-coding region of GJB1 gene and CMTX is unclear. We found two structural variants (-459C>T and -713G>A) in the 5' non-coding region of a transcript (Ref seq ID: NM_000166) of the GJB1 gene and explored its association with CMTX in two Chinese families. All family members who carried the -459C>T variant either were symptomatic or had abnormal electrophysiological studies compatible with CMTX, whereas all the non-symptomatic family members who had normal electrophysiological studies and 10 healthy unrelated controls did not have this variant. The other variant in the 5'-flanking region of the gene was found to be a benign polymorphism, although it had been earlier reported to be associated with CMTX in a Taiwanese family. Secondary structure prediction analysis of mutant mRNA using M fold and RNA structure softwares indicates that the -459C>T mutation may reduce translation efficiency of the GJB1 gene by changing its 5'-untranslated region secondary structure and abolishing the internal ribosome entry site at the initialization of its translation in Schwann cells. Our study can help clarify the causal mutations of CMTX in the non-protein coding region of GJB1.

[Charcot-Marie-Tooth disease]

Charcot-Marie-Tooth (CMT) disease, also known as peroneal muscular atrophy or hereditary motor and sensory neuropathy, is among the most frequent hereditary disorders of the nervous system. The relatively homogeneous clinical phenotype involves mainly progressive weakness and wasting of distal muscles; it starts and predominates in the peroneal muscles. Electrophysiological and pathology data distinguish two principal forms of CMT: demyelinating and axonal. More than 20 distinct genetic subtypes have been identified to date and other new loci and genes remain to be discovered, thus demonstrating wide genetic heterogeneity and a number of different pathophysiological mechanisms. The classification of these different forms is based on both the mode of inheritance--autosomal dominant, recessive or X-linked--and the neuropathy type--demyelinating or axonal or "intermediate". The principal dominant forms are CMT1A, due to a duplication or point mutation in the PMP22 gene, and CMTX, due to mutations in the connexin 32 gene. Autosomal recessive forms are more frequent in North Africa. The most common involve mutations of GDAP1 or lamin A/C and generally lead to more severe phenotypes than the dominant forms. The great genetic heterogeneity necessitates a strategy for genetic diagnosis. It is based in part on the classification of the different genetic forms and in part on the phenotypic particularities and the frequency of the responsible genes in the population under study.

Two novel connexin32 mutations cause early onset X-linked Charcot-Marie-Tooth disease

BACKGROUND

X-linked Charcot-Marie Tooth (CMT) is caused by mutations in the connexin32 gene that encodes a polypeptide which is arranged in hexameric array and form gap junctions.

METHODS

We describe two novel mutations in the connexin32 gene in two Norwegian families.

RESULTS

Family 1 had a c.225delG (R75fsX83) which causes a frameshift and premature stop codon at position 247. This probably results in a shorter non-functional protein structure. Affected individuals had an early age at onset usually in the first decade. The symptoms were more severe in men than women. All had severe muscle weakness in the legs. Several abortions were observed in this family. Family 2 had a c.536 G>A (C179Y) transition which causes a change of the highly conserved cysteine residue, i.e. disruption of at least one of three disulfide bridges. The mean age at onset was in the first decade. Muscle wasting was severe and correlated with muscle weakness in legs. The men and one woman also had symptom from their hands. The neuropathy is demyelinating and the nerve conduction velocities were in the intermediate range (25-49 m/s). Affected individuals had symmetrical clinical findings, while the neurophysiology revealed minor asymmetrical findings in nerve conduction velocity in 6 of 10 affected individuals.

CONCLUSION

The two novel mutations in the connexin32 gene are more severe than the majority of previously described mutations possibly due to the severe structural change of the gap junction they encode.

CMT1X phenotypes represent loss of GJB1 gene function

OBJECTIVE

To investigate possible genotype-phenotype correlations and to evaluate the natural history of patients with Charcot-Marie-Tooth disease type 1X (CMT1X).

BACKGROUND

CMT1X is caused by over 260 distinct mutations in the gap junction beta 1 (GJB1) gene, located on the X chromosome, which encodes the gap junction protein connexin 32 (Cx32). The natural history of CMT1X is poorly understood, and it remains unknown whether particular mutations cause more severe neuropathies through abnormal gain-of-function mechanisms.

METHODS

We evaluated 73 male patients with CMT1X, who each have 1 of 28 different GJB1 mutations predicted to affect nearly all domains of Cx32. Disability was evaluated quantitatively by the CMT Neuropathy Score (CMTNS) as well as by the CMT Symptom Score (CMTSS) and the CMT Examination Score (CMTES), which are both based on the CMTNS. Patients were also evaluated by neurophysiology.

RESULTS

In all patients, disability increased with age, and the degree of disability was comparable with that observed in patients with a documented GJB1 deletion. Disability correlated with a loss of motor units as assessed by motor unit number estimates.

CONCLUSIONS

Taken together, these data suggest that most GJB1 mutations cause neuropathy by a loss of normal connexin 32 function. Therefore, treatment of male patients with Charcot-Marie-Tooth disease type 1X may prove amenable to gene replacement strategies.

The Inherited Neuropathies

Neuropathy is one of the most common referrals to neurologic clinics. Patients often undergo extensive testing for acquired etiologies; inherited causes are common. Increasingly, genetic causes are becoming known and commercial testing available. The rate of recent discovery has been rapid and relates to the extent of single gene disorders of nerve, the ease of peripheral nervous system functional examination, and readily accessible pathologic tissue. Foremost in the rate of recent discoveries is the work and tools of the human genome project. the rapidity of the ongoing discovery requires clinicians to be familiar with molecular biologic discoveries and consider wisely which testing should be performed.

Effects of exercise and creatine on myosin heavy chain isoform composition in patients with Charcot-Marie-Tooth disease

It is not known whether myosin heavy chain (MHC) content changes in response to exercise training or creatine supplementation in subjects with Charcot-Marie-Tooth disease (CMT). Based on previous data, we hypothesized that resistance exercise and creatine would increase the percentage of type I MHC composition in the vastus lateralis muscle and that myosin isoform changes would correlate with improved chair rise-time in CMT subjects. To test this hypothesis, 18 CMT subjects were randomly assigned to either a placebo or creatine group. All subjects performed a 12-week, home-based, moderate-intensity resistance training program. Chair rise-time was measured before and after the training program. Muscle biopsies were obtained from the vastus lateralis before and after the 12-week program. Gel electrophoresis showed a significant decrease (approximately 30%) in MHC type I in CMT subjects given creatine supplementation when compared with placebo. There was a nonsignificant increase in both MHC type IIa (approximately 23%) and MHC type IIx (approximately 7%) in CMT subjects given creatine. Reduced MHC type I content and increased MHC type IIa content correlated with faster chair rise-times (i.e., improved muscle performance). The training-induced change in MHC IIa content was inversely correlated with chair rise-time in CMT subjects given creatine. When the two subject groups were combined, there was a linear, negative relationship between the change in MHC type IIa content and chair rise-time after training and a positive relationship between the training-induced change in MHC type I content and chair rise-time. These data suggest that improved function (chair rise-time) was associated with a lower level of MHC type I and increased MHC type IIa composition. Furthermore, the data are consistent with the hypothesis that creatine supplementation alters MHC composition in CMT patients undergoing resistance training and that MHC changes associated with creatine supplementation can improve muscle function.

Proof of genetic heterogeneity in X-linked Charcot-Marie-Tooth disease

OBJECTIVE

To characterize a large family with X-linked Charcot-Marie-Tooth (CMT) neuropathy without mutations in the gap junction protein B1 (GJB1) gene, which has an unusual phenotype that is different in some aspects from classic CMTX1.

METHODS

We tested CMT families consistent with X-linked inheritance for GJB1 mutations. We compared the largest family (CMT623) without GJB1 mutation and with linkage excluding the CMTX1 locus to CMTX1 and normal individuals.

RESULTS

Only 51% of probable X-linked CMT families had mutations in GJB1. Family CMT623 shows linkage to Xq26.3-q27.1 (lod score z = 6.58), a region within the previously identified locus for CMTX3, Xq26-q28. Unlike CMTX1, affected males in family CMT623 report pain and paraesthesia before the onset of sensory loss, and women are usually asymptomatic. As in CMTX1, affected males have widely ranging intermediate motor conduction velocities. The coding regions of 14 positional candidate genes within the narrowed CMTX3 locus have been excluded for a pathogenic role in the disease.

CONCLUSION

This study is the first to confirm the CMTX3 locus and to refine the genetic interval to a 5.7-Mb region flanked by the markers DXS1041 and DXS8106. GJB1 mutation-negative forms of X-linked CMT, such as CMTX3, may account for a significant proportion of X-linked CMT.

Charcot-marie-tooth disease: seventeen causative genes

Charcot-Marie-Tooth disease (CMT) is the most common form of inherited motor and sensory neuropathy. Moreover, CMT is a genetically heterogeneous disorder of the peripheral nervous system, with many genes identified as CMT-causative. CMT has two usual classifications: type 1, the demyelinating form (CMT1); and type 2, the axonal form (CMT2). In addition, patients are classified as CMTX if they have an X-linked inheritance pattern and CMT4 if the inheritance pattern is autosomal recessive. A large amount of new information on the genetic causes of CMT has become available, and mutations causing it have been associated with more than 17 different genes and 25 chromosomal loci. Advances in our understanding of the molecular basis of CMT have revealed an enormous diversity in genetic mechanisms, despite a clinical entity that is relatively uniform in presentation. In addition, recent encouraging studies - shown in CMT1A animal models - concerning the therapeutic effects of certain chemicals have been published; these suggest potential therapies for the most common form of CMT, CMT1A. This review focuses on the inherited motor and sensory neuropathy subgroup for which there has been an explosion of new molecular genetic information over the past decade.

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.

Phenotypic and cellular expression of two novel connexin32 mutations causing CMT1X

OBJECTIVE

To determine the phenotypic and cellular expression of two novel connexin32 (Cx32) mutations causing X-linked Charcot-Marie-Tooth disease (CMT1X).

METHODS

The authors evaluated several members of two families with CMT1X clinically, electrophysiologically, pathologically, and by genetic testing. The Cx32 mutations were expressed in vitro and studied by immunocytochemistry.

RESULTS

In both families, men were more severely affected than women with onset in the second decade of life. In the first family, the phenotype was that of demyelinating polyneuropathy with variable involvement of peripheral nerves. There was clinical evidence of CNS involvement in at least three of the patients, with extensor plantar responses and brisk reflexes. In the second family, the affected man presented with symmetric polyneuropathy and intermediate slowing of conduction velocities, whereas affected women had prominent asymmetric atrophy of the leg muscles. The authors identified two novel missense mutations resulting in L143P amino acid substitution in the first family and in V140E substitution in the second family, both located in the third transmembrane domain of Cx32. Expression of these Cx32 mutations in communication-incompetent HeLa cells and immunocytochemical analysis revealed that both mutants were retained intracellularly and were localized in the Golgi apparatus. In contrast to wild-type protein, they did not form gap junctions.

CONCLUSION

These novel connexin32 (Cx32) mutations cause a spectrum of clinical manifestations characteristic of Charcot-Marie-Tooth disease (CMT1X), including demyelinating or intermediate polyneuropathy, which is often asymmetric, and CNS involvement in one family. The position and cellular expression of Cx32 mutations alone cannot fully predict these phenotypic variations in CMT1X.

Early clinical and electrophysiologic features of the two most common autosomal recessive forms of Charcot-Marie-Tooth disease in the Roma (Gypsies)

Our recent studies of the genetic epidemiology of neuromuscular disorders in Gypsies in Bulgaria have revealed that two private disorders, hereditary motor and sensory neuropathy type Lom and hereditary motor and sensory neuropathy type Russe, account for most cases of Charcot-Marie-Tooth disease in this population. In this study, we examined the clinical and electrophysiologic manifestations of the two disorders in childhood, aiming to identify the distinctive features that allow early differential diagnosis. The study included 13 patients, aged between 2 and 15 years. The childhood clinical manifestations of both neuropathies were similar, although they tended to be more severe in hereditary motor and sensory neuropathy type Lom. The nerve conduction velocities in hereditary motor and sensory neuropathy type Lom were lower than in hereditary motor and sensory neuropathy type Russe. Brainstem auditory evoked potentials were abnormal in hereditary motor and sensory neuropathy type Lom, even at an early age, and normal in hereditary motor and sensory neuropathy type Russe. Although electrophysiologic data provide a more reliable differentiation than clinical data, the definitive diagnosis should rely on genetic testing. (J Child Neurol 2006;21:20-25).

Molecular genetics of X-linked Charcot-Marie-Tooth disease

The X-linked form of Charcot-Marie-Tooth disease (CMT1X) is the second most common molecularly designated form of hereditary motor and sensory neuropathy. The clinical phenotype is characterized by progressive distal muscle atrophy and weakness, areflexia, and variable sensory abnormalities. Affected males have moderate-to-severe symptoms, whereas heterozygous females are usually mildly affected or even asymptomatic. Several patients also have manifestations of central nervous system involvement or hearing impairment. Electrophysiological and pathological studies of peripheral nerves show evidence of demyelinating neuropathy with prominent axonal degeneration. A large number of mutations in the GJB1 gene encoding the gap junction (GJ) protein connexin32 (Cx32) cause CMT1X. Cx32 is expressed by Schwann cells and oligodendrocytes, as well as by other tissues, and the GJ formed by Cx32 play an important role in the homeostasis of myelinated axons. The reported CMT1X mutations are diverse and affect both the promoter region as well as the coding region of GJB1. Many Cx32 mutants fail to form functional GJ, or form GJ with abnormal biophysical properties. Furthermore, Cx32 mutants are often retained intracellularly either in the endoplasmic reticulum or Golgi in which they could potentially have additional dominant-negative effects. Animal models of CMT1X demonstrate that loss of Cx32 in myelinating Schwann cells causes a demyelinating neuropathy. No definite phenotype-genotype correlation has yet been established for CMT1X and effective molecular based therapeutics for this disease, remain to be developed.

[The role of the immune system in hereditary demyelinating neuropathies]

Hereditary neuropathies, e.g., Charcot-Marie-Tooth (CMT) disease, are inherited diseases of the peripheral nervous system causing chronic progressive motor and sensory dysfunction. Most neuropathies are due to mutations in myelin genes such as PMP22, P0, and the gap junction protein Cx32. Myelin mutant mice are regarded as suitable animal models for several forms of hereditary neuropathies and are important neurobiological tools for the evaluation of pathogenetic and therapeutic concepts in hereditary neuropathies. Using these animal models we could recently show that the immune system is involved in the pathogenesis of hereditary neuropathies. Due to the phenotypic similarities we also consider the immune system important for human inherited neuropathies, in particular since several case reports demonstrate a beneficial effect of immune therapies in patients with hereditary neuropathies. In this review we compare findings from animal models and human disease to elucidate the role of the immune system in hereditary neuropathies.

Transgenic expression of human connexin32 in myelinating Schwann cells prevents demyelination in connexin32-null mice

Mutations in Gap Junction beta1 (GJB1), the gene encoding the gap junction protein connexin32 (Cx32), cause the X-linked form of Charcot-Marie-Tooth disease (CMT1X), an inherited demyelinating neuropathy. We investigated the possibility that the expression of mutant Cx32 in other cells besides myelinating Schwann cells contributes to the development of demyelination. Human Cx32 was expressed in transgenic mice using a rat myelin protein zero (Mpz) promoter, which is exclusively expressed by myelinating Schwann cells. Male mice expressing the human transgene were crossed with female Gjb1/cx32-null mice; the resulting male offspring were all cx32-null (on the X chromosome), and one-half were transgene positive. In these transgenic mice, all of the Cx32 was derived from the expression of the transgene and was found in the sciatic nerve but not in the spinal cord or the liver. Furthermore, the Cx32 protein was properly localized (within incisures and paranodes) in myelinating Schwann cells. Finally, the expression of human Cx32 protein "rescued" the phenotype of cx32-null mice, because the transgenic mice have significantly fewer demyelinated or remyelinated axons than their nontransgenic littermates. These results indicate that the loss of Schwann-cell-autonomous expression of Cx32 is sufficient to account for demyelination in CMT1X.

X-linked Charcot-Marie-Tooth disease: phenotypic expression of a novel mutation Ile127Ser in the GJB1 (connexin 32) gene

We report a family with X-linked dominant Charcot-Marie-Tooth disease (CMTX1). Three affected family members are described, who underwent detailed clinical, electrophysiological, molecular genetic, and histopathological studies. A novel isoleucine at position 127 with serine (Ile127Ser) mutation in the gap junction protein beta 1 (GJB1) gene was detected. The electrophysiological findings were consistent with a primary demyelinating neuropathy with secondary axonal loss and support this model of disease progression. All patients having the CMT phenotype and intermediate conduction velocities who are negative for CMT1A duplication/hereditary neuropathy with liability to pressure palsies (HNPP) deletion, and whose family shows a dominant trait without male-to-male transmission, should be screened for CMTX1.

[Molecular diagnosis of hereditary neuropathies such as Charcot-Marie-Tooth disease]

During the last decade, molecular biology has demonstrated the extraordinary heterogeneity of genetic abnormalities in Charcot-Marie-Tooth disease (CMT). The main phenotypes are either of the demyelinating or axonal type, transmitted with dominant or recessive autosomal inheritance. X-linked CMT is less rare than it was initially described and is often misdiagnosed as autosomal dominant type. Linked phenotypes are Dejerine-Sottas disease, congenital hypomyelinization and hereditary neuropathy with susceptibility to pressure palsies. Each phenotype can be due to different genotypes and concerned genes are numerous. Conversely, each genotype can express different phenotypes. Molecular diagnostic strategy of CMT is mainly baised on three elements: - phenotypic expertise which is based on the analysis of the inheritance mode and on electrophysiological data, which are peculiar in CMTX - knowledge of respective occurrence of the different genotypes and phenotypes which is increasing - technical feasibility of molecular biology methods which is important to consider, even though progress are fastly coming. According to these considerations, a strategy is proposed for molecular diagnosis of CMT.

Inter-nerves and intra-nerve conduction heterogeneity in CMTX with Arg(15)Gln mutation

OBJECTIVE

In X-linked Charcot-Marie-Tooth disease (CMTX), electrophysiological and histopathological studies have suggested either a demyelinating or an axonal polyneuropathy. We report a CMTX family with a striking heterogeneity of nerve conductions between and within nerves.

METHODS

Two men and one woman have been studied by conduction velocities, sural nerve biopsy with morphometry (one man) and DNA analysis.

RESULTS

In both men motor conduction velocities were slowed in the demyelinating range, conduction velocity differences among nerves in the same subject varied from 13 to 24 m/s, and distal median compound muscle action potential (CMAP) amplitudes were 3-5 times reduced compared to ulnar CMAPs. Abnormal area reduction or excessive temporal dispersion of proximal CMAP was present in at least two nerves in all patients. Sural nerve biopsy showed reduction of large myelinated fibres, cluster formations, occasional onion bulbs. Teased fibres study revealed short internodes for fibre diameter, enlarged Ranvier nodes but no evidence of segmental demyelination and remyelination. DNA analysis showed an Arg(15)Gln mutation in connexin32 gene in all patients.

CONCLUSIONS

In this family conduction slowing and segmental conduction abnormalities, in absence of morphological evidence of de-remyelination, may be related to short internodes, widened Ranvier nodes and the specific effect of the mutation. The occurrence in some CMTX patients of a non uniform involvement between and within nerves, as in acquired demyelinating neuropathies, should be kept in mind to avoid misdiagnoses.

Dominant intermediate Charcot-Marie-Tooth type C maps to chromosome 1p34-p35

Dominant intermediate Charcot-Marie-Tooth (DI-CMT) neuropathy is a genetic and phenotypic variant of classical CMT, characterized by intermediate nerve conduction velocities and histological evidence of both axonal and demyelinating features. We report two unrelated families with intermediate CMT linked to a novel locus on chromosome 1p34-p35 (DI-CMTC). The combined haplotype analysis in both families localized the DI-CMTC gene within a 6.3-cM linkage interval flanked by markers D1S2787 and D1S2830. The functional and positional candidate genes, Syndecan 3 (SDC3), and lysosomal-associated multispanning membrane protein 5 (LAPTM5) were excluded for pathogenic mutations.

Dominant intermediate Charcot-Marie-Tooth type C maps to chromosome 1p34-p35

Dominant intermediate Charcot-Marie-Tooth (DI-CMT) neuropathy is a genetic and phenotypic variant of classical CMT, characterized by intermediate nerve conduction velocities and histological evidence of both axonal and demyelinating features. We report two unrelated families with intermediate CMT linked to a novel locus on chromosome 1p34-p35 (DI-CMTC). The combined haplotype analysis in both families localized the DI-CMTC gene within a 6.3-cM linkage interval flanked by markers D1S2787 and D1S2830. The functional and positional candidate genes, Syndecan 3 (SDC3), and lysosomal-associated multispanning membrane protein 5 (LAPTM5) were excluded for pathogenic mutations.

Motor unit number estimate of distal and proximal muscles in Charcot-Marie-Tooth disease

In order to determine the utility of motor unit number estimation (MUNE) in assessing axonal loss in chronic inherited neuropathies, we determined MUNEs in 54 patients with Charcot-Marie-Tooth (CMT) disease (29 patients with CMT-1A, 13 with CMT-X, and 12 with CMT-2) by using spike-triggered averaging (STA) of the ulnar-innervated abductor digiti minimi/hypothenar muscles (ADM) and the musculo-cutaneous innervated biceps/brachialis (BB) muscles. MUNEs were analyzed in relationship to the corresponding compound muscle action potential (CMAP) amplitudes as well as to clinical strength. Proximal muscles, which appeared strong clinically, had evidence of chronic denervation/reinnervation, although to a lesser extent than weak distal hand muscles, supporting the concept that axonal loss in CMT occurs in a length-dependent fashion. The reduction in ADM-MUNE strongly correlated with clinical weakness in the hand. Both the ADM-MUNE and BB-MUNE were abnormal more often than CMAP amplitude, probably reflecting extensive motor unit reconfiguration and enlargement that maintains CMAP amplitude despite severe motor unit loss. This study suggests that MUNE can assess motor unit loss in CMT and may better reflect axonal loss than CMAP amplitude. The STA technique of MUNE may be useful in longitudinal studies of proximal and distal motor unit changes in CMT.

Electrodiagnostic evaluation of hereditary motor and sensory neuropathies

Electrodiagnosis can classify hereditary motor and sensory neuropathies (HMSN) into two basic types: primarily demyelinating with secondary axonal loss and primarily axonal. For the most part, the various forms of HMSN show uniform symmetric nerve conduction slowing, in contrast to acquired neuropathies, which may be multifocal with nonuniform conduction velocity slowing and temporal dispersion. Nevertheless, there are exceptions. This article reviews the available literature and describes the electrodiagnostic approach to HMSN, detailing potential sources of error that can lead to misinterpretation of data.

Phenotypic expression of a Pro 87 to Leu mutation in the connexin 32 gene in a large Swiss family with Charcot-Marie-Tooth neuropathy

BACKGROUND

The clinical manifestations of CMTX have been well described but the natural history has not yet been studied in detail. We studied phenotype variability in a family with a Pro 87 to Leu mutation of the connexin 32 (Cx32) gene.

METHODS

A total of 32 family members, of which 19 patients were affected, underwent clinical, electrophysiological, and genetic studies.

RESULTS

Onset was in the second decade. Clinical features were similar in both sexes when quantitative scores were compared, but more males had a steppage gait and skeletal deformities. All adult patients had a predominant involvement of the thenar muscles. The median values of nerve conduction velocities (NCVs) were not statistically different in men and in women. The correlation coefficients were low between motor NCVs within the same extremities, indicating nonuniform slowing between nerves, the ulnar nerve being the least affected. When disability was rated, a strong correlation was seen in male patients between severity of motor axonal loss and duration of the disease. The main pathological features were axonal loss, clusters of regenerating fibers and paranodal demyelination, the hallmark of a Schwann cell pathology.

CONCLUSIONS

Our data support the hypothesis that clinical disability in CMTX is caused by loss of large myelinated axons in men. Furthermore, this study shows that the nerves are not uniformly affected in terms of axonal loss. Preventing axonal degeneration and promoting axonal regeneration in the most affected nerves might be the best therapeutic approaches to ameliorate disability in CMTX.

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.

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.

Connexin mutations in hearing loss, dermatological and neurological disorders

Gap junctions are important structures in cell-to-cell communication. Connexins, the protein units of gap junctions, are involved in several human disorders. Mutations in beta-connexin genes cause hearing, dermatological and peripheral nerve disorders. Recessive mutations in the gene encoding connexin 26 (GJB2) are the most common cause of childhood-onset deafness. The combination of mutations in the GJB2 and GJB6 (Cx30) genes also cause childhood hearing impairment. Although both recessive and dominant connexin mutants are functionally impaired, dominant mutations might have in addition a dominant-negative effect on wild-type connexins. Some dominant mutations in beta-connexin genes have a pleiotropic effect at the level of the skin, the auditory system and the peripheral nerves. Understanding the genotype-phenotype correlations in diseases caused by mutations in connexin genes might provide important insight into the mechanisms that lead to these disorders.

Disease mechanisms and potential therapeutic strategies in Charcot-Marie-Tooth disease

Until 10 years ago, the genetic basis of Charcot-Marie-Tooth (CMT) disease was largely unknown. With the finding of an intrachromosomal duplication on chromosome 17 in 1991, associated with the most commonly found subtype CMT1A, and the discovery of a point mutation in the peripheral myelin protein-22 (pmp22) gene in the Trembler mouse in 1992, the groundwork was laid down for a novel chapter in the elucidation of the molecular basis of this large group of peripheral neuropathies. In the meantime, several different genes have been found to be associated with different forms of demyelinating and axonal forms of CMT. In this review, we will summarize what is known today about the genetics of this group of disease which constitute the most common known monogenetic disorder affecting the nervous system in man, the animal models that have been generated, and what we have learned about the underlying disease mechanisms. Furthermore, we will review how this gain of knowledge about CMT may open new avenues to the development of novel treatment strategies.

Localization of the gene for the intermediate form of Charcot-Marie-Tooth to chromosome 10q24.1-q25.1

Intermediate Charcot-Marie-Tooth neuropathy (CMT) is an inherited sensory motor neuropathy characterized by motor median nerve conduction velocities of 25-45 m/s. We performed a genomewide search in an Italian family with autosomal dominant intermediate CMT and mapped the locus on chromosome 10q. Analysis of key recombinants maps the gene for autosomal dominant intermediate CMT to a 10.7-Mb interval on chromosome 10q24.1-q25.1, between simple tandem repeat markers D10S1709 and D10S1795.

Progress in clinical neurosciences: Charcot-Marie-Tooth disease and related inherited peripheral neuropathies

The classification of Charcot-Marie-Tooth disease and related hereditary motor and sensory neuropathies has evolved to incorporate clinical, electrophysiological and burgeoning molecular genetic information that characterize the many disorders. For several inherited neuropathies, the gene product abnormality is known and for others, candidate genes have been identified. Genetic testing can pinpoint a specific inherited neuropathy for many patients. However, clinical and electrophysiological assessments continue to be essential tools for diagnosis and management of this disease group. This article reviews clinical, electrophysiological, pathological and molecular aspects of hereditary motor and sensory neuropathies.