Charcot-Marie-Tooth Type Research Articles

A newly identified Thr99fsX110 mutation in the PMP22 gene associated with an atypical phenotype of the hereditary neuropathy with liability to pressure palsies.

Hereditary neuropathy with liability to pressure palsies (HNPP) is manifested by a spectrum of phenotypes, from the classical HNPP course associated with intermittent nerve palsies to a neuropathy resembling Charcot-Marie-Tooth type 1 (CMT1) disease. The majority of HNPP cases are associated with submicroscopical deletions in the 17p11.2-p12 region containing the PMP22 gene, while PMP22 point mutations are rare, representing about 15% of HNPP cases. In this study, we present a patient manifesting with atypical HNPP phenotype associated with a new Thr99fsX110 mutation in the PMP22 gene. We conclude that all patients who fulfill the electrophysiological criteria of HNPP, even if they lack the typical HNPP phenotype, should be tested for point mutations in the PMP22 gene.

Rab7 and the CMT2B disease

The CMT2B (Charcot-Marie-Tooth type 2B) disease is an autosomal dominant axonal neuropathy. Sensory loss, distal muscle weakness and wasting, frequent foot ulcers and amputations of the toes due to frequent infections characterize this neuropathy. Four missense mutations in the rab7 gene have been identified as causative of the disease. Rab7 is a small G-protein of the Rab family that controls vesicular transport to late endosomes and lysosomes in the endocytic pathway. The CMT2B-associated mutant Rab7 proteins show altered nucleotide dissociation rates and impaired GTPase activity. In addition, these mutant proteins are predominantly in the GTP-bound form when expressed in human cells and they are able to rescue Rab7 function in Rab7-depleted cells. Thus these mutations generate activated forms of Rab7 that are responsible for the development of the disease. In spite of these results, there are still important gaps in our understanding of the mechanism underlying CMT2B. Indeed, how these mutations in the rab7 gene affect specifically peripheral neurons leading to an axonal pathology in CMT2B is not clear, and it is a particularly puzzling and challenging issue in view of the fact that Rab7 is a ubiquitous protein. The present review discusses possible molecular mechanisms underlying CMT2B.

Fast Responses to Stepping on an Unexpected Surface Height Depend on Intact Large-Diameter Nerve Fibers: A Study on Charcot–Marie–Tooth Type 1A Disease

The contribution of reflexes from the large myelinated afferents in the control of normal and perturbed gait in humans is a highly debated issue. One way to investigate this topic is by studying normal and perturbed gait in patients lacking large myelinated fibers in the distal limb (Charcot-Marie-Tooth [CMT] type 1A disease). Such patients should have delayed and decreased reflexes if the latter depend on these large myelinated fibers. To elicit the reflexes, both patients and controls had to step on a platform that was either at the same level or lowered by 5 cm. In control subjects, landing on a level surface induced short-latency responses in the biceps femoris and tibialis anterior muscles, whereas such responses were largely absent in the patients. Similarly, stepping down unexpectedly induced a very fast muscle synergy, leading to a brake of the forward propulsion in the controls, which was significantly reduced and delayed (on average 32 ms) in the patients. The observed changes correlated with both sensory and motor deficits. Nevertheless, it is concluded that the results are primarily related to the sensory deficits, since the delayed or absent responses appeared in both upper and lower leg muscles, whereas only the latter showed motor deficits. The data are taken as evidence that large-diameter afferents from the distal leg are essential for fast reflex activations induced by stepping on a level or lowered surface unexpectedly.

Adenine nucleotide translocase is involved in a mitochondrial coupling defect in MFN2-related Charcot–Marie–Tooth type 2A disease

Charcot-Marie-Tooth type 2A disease (CMT2A), a dominantly inherited peripheral neuropathy, is caused by mutations in MFN2, a mitochondrial fusion protein. Having previously demonstrated a mitochondrial coupling defect in CMT2A patients' fibroblasts, we here investigate mitochondrial oxygen consumption and the expression of adenine nucleotide translocase (ANT) and uncoupling proteins from eight other patients with the disease. The mitochondrial uncoupling was associated with a higher respiratory rate, essentially involving complex II proteins. Furthermore, a twofold increase in the expression of ANT led to the reduced efficiency of oxidative phosphorylation in CMT2A cells, suggesting that MFN2 plays a role in controlling ATP/ADP exchanges.

Mitochondrial Dynamics in Mammalian Health and Disease

The meaning of the word mitochondrion (from the Greek mitos, meaning thread, and chondros, grain) illustrates that the heterogeneity of mitochondrial morphology has been known since the first descriptions of this organelle. Such a heterogeneous morphology is explained by the dynamic nature of mitochondria. Mitochondrial dynamics is a concept that includes the movement of mitochondria along the cytoskeleton, the regulation of mitochondrial architecture (morphology and distribution), and connectivity mediated by tethering and fusion/fission events. The relevance of these events in mitochondrial and cell physiology has been partially unraveled after the identification of the genes responsible for mitochondrial fusion and fission. Furthermore, during the last decade, it has been identified that mutations in two mitochondrial fusion genes (MFN2 and OPA1) cause prevalent neurodegenerative diseases (Charcot-Marie Tooth type 2A and Kjer disease/autosomal dominant optic atrophy). In addition, other diseases such as type 2 diabetes or vascular proliferative disorders show impaired MFN2 expression. Altogether, these findings have established mitochondrial dynamics as a consolidated area in cellular physiology. Here we review the most significant findings in the field of mitochondrial dynamics in mammalian cells and their implication in human pathologies.

Diagnostic nerve ultrasound in Charcot–Marie–Tooth disease type 1B

Ultrasound is emerging as a useful tool for evaluation of neuromuscular conditions, because it can provide high-resolution anatomic information to complement electrodiagnostic data. There have been few studies in which ultrasound was used to assess the peripheral nerves of individuals with Charcot-Marie-Tooth (CMT) disease and none involving CMT type 1B. In this study we compared nerve cross-sectional area in individuals from a single large family with CMT 1B with normal, healthy controls. We also assessed for cranial nerve enlargement in those with CMT 1B with cranial neuropathies compared to those with CMT 1B without cranial neuropathies. Individuals with CMT 1B have significantly larger median and vagus nerves than healthy controls, but no difference was seen in cranial nerve size between those with versus those without cranial neuropathies. This is the first study to characterize the ultrasonographic findings in the peripheral nerves of individuals with CMT 1B.

Neurofibromatosis type 1 associated with Charcot–Marie–Tooth type 1A

Neurofibromatosis and Charcot-Marie-Tooth are genetic disorders of the nervous system affecting the development and growth of nerve cells and demyelination of peripheral neurons, respectively. We report a 22-year-old man who presented clinical manifestations of both neurofibromatosis type 1 (NF1) and Charcot-Marie-Tooth type 1A. The simultaneous occurrence of neurofibromatosis and Charcot-Marie-Tooth disease has rarely been reported. More extensive reports and further investigations of this combination will certainly provide a better understanding of this linkage in the near future.

Intermittent fasting alleviates the neuropathic phenotype in a mouse model of Charcot–Marie–Tooth disease

Charcot–Marie–Tooth type 1A (CMT1A) neuropathies linked to the misexpression of peripheral myelin protein 22 (PMP22) are progressive demyelinating disorders of the peripheral nervous system. In this study we asked whether dietary restriction by intermittent fasting (IF) could alleviate the neuropathic phenotype in the Trembler J (TrJ) mouse model of CMT1A. Our results show that neuropathic mice kept on a five month long IF regimen had improved locomotor performance compared to ad libitum (AL) fed littermates. The functional benefits of this dietary intervention are associated with an increased expression of myelin proteins combined with a thicker myelin sheath, less redundant basal lamina, and a reduction in aberrant Schwann cell proliferation. These morphological improvements are accompanied by a decrease in PMP22 protein aggregates, and enhanced expression of cytosolic chaperones and constituents of the autophagy–lysosomal pathway. These results indicate that dietary restriction is beneficial for peripheral nerve function in TrJ neuropathic mice, as it promotes the maintenance of locomotor performance.

Two Spanish families with Charcot–Marie–Tooth type 2A: Clinical, electrophysiological and molecular findings

Mutations in the Mitofusin 2 (MFN2) gene have been related to the axonal type of Charcot–Marie–Tooth type 2 (CMT 2A). We report the first two Spanish families with CMT 2 and mutations in MFN2 gene. Molecular studies of one family with late onset revealed the novel mutation Arg364Gln. The affected family members presented mild clinical and electrophysiological worsening after 14 years of follow-up. The other family presented an early onset and optic atrophy. Molecular studies revealed the Arg94Gln mutation. This is the first report of a family in which this mutation is related to optic atrophy. Molecular analysis aimed at detecting mutations of MFN2 could be extremely useful in mild axonal neuropathies with slow evolution and indispensable in cases of dominant inheritance or optic atrophy. Population studies of mutations in MFN2 should be undertaken to discover the real frequencies in the Mediterranean area.

Motor axon loss is associated with hand dysfunction in Charcot-Marie-Tooth disease 1a

Charcot Marie Tooth type 1a (CMT1a) is a primarily demyelinating neuropathy, characterized by slowly progressive muscle weakness, atrophy, and sensory loss, and is most pronounced in both feet and hands. There is increasing evidence that muscle weakness is determined by motor axonal dysfunction. To investigate in patients with CMT1a whether motor axon loss, as estimated with motor unit number estimation (MUNE) and compound muscle action potential (CMAP), is related to hand function and manual dexterity. Hand function, manual dexterity, and axon loss were studied in 48 patients with proven CMT1a. Using high-density surface EMG on the thenar muscles, MUNE was determined and CMAPs were measured. Pinch strength, clawing of the fingers, and manual dexterity correlated significantly with MUNE and CMAP (amplitude and area), while sensory impairments did not. Grip strength correlated significantly with CMAP amplitude but did not become significant with MUNE and CMAP area. Neurophysiologic variables were particularly associated with fine motor function of the hand. Motor axon loss is likely to be the major cause of hand dysfunction and impaired manual dexterity in Charcot Marie Tooth type 1a (CMT1a). In a clinical setting, the evaluation of the hands of patients with CMT1a should thus be mainly directed toward the evaluation of fine motor functions.

Mutations in the HSP27 ( HSPB1 ) gene cause dominant, recessive, and sporadic distal HMN/CMT type 2

Charcot-Marie-Tooth disease (CMT) is the most common inherited neuromuscular disorder and is characterized by significant clinical and genetic heterogeneity. Recently, mutations in both the small heat shock protein 27 (HSP27 or HSPB1) and 22 (HSP22 or HSPB8) genes have been reported to cause autosomal dominant CMT with minimal sensory involvement (CMT 2F/CMT2L) and autosomal dominant distal hereditary motor neuropathy type II (dHMN II). We analyzed the HSPB1 and HSPB8 genes in a large clinically well-characterized series of dHMN and CMT type 2 (CMT2) cases and families using linkage analysis and direct sequencing of these genes. We identified a novel homozygous mutation in the alpha-crystallin domain of HSPB1 segregating in an autosomal recessive fashion in a family with distal HMN/CMT2. A further four heterozygous HSPB1 mutations were identified in four autosomal dominant families dHMN/CMT2, and two sporadic cases were identified with probable de novo mutations. In the autosomal dominant and autosomal recessive families, there were no clinical sensory findings, but reduced sural nerve action potential amplitudes were found in some affected individuals, indicating that long sensory axons are mildly affected in this predominantly motor disorder. This extends the clinical and electrophysiologic spectrum of HSPB1 mutations and identifies four unreported dominant HSPB1 mutations and the first family where the HSPB1 mutation acts in a recessive way to cause distal HMN.

Compound heterozygous deletions of PMP22 causing severe Charcot‐Marie‐Tooth disease of the Dejerine‐Sottas disease phenotype

Dejerine-Sottas disease (DSD) is a particular phenotype of the Charcot-Marie-Tooth (CMT) disease spectrum that is genetically heterogeneous. It represents a severe form of hypertrophic axonal and demyelinating neuropathy. Although it is predominantly inherited as an autosomal recessive condition, autosomal dominant inheritance has also been described. To date, the autosomal recessive forms of DSD are classified into several CMT type 4 (CMT4) subclasses based on allelic heterogeneity. We present a 7-year-old boy with a severe form of CMT disease consistent with the autosomal recessive phenotype of DSD. He was found to be a compound heterozygote for mutations in the PMP22 gene resulting in homozygous deletion of exons 2 and 3. The maternally inherited allele was the typical 1.5 Mb deletion involving PMP22 seen with hereditary neuropathy with liability to pressure palsy (HNPP). The paternally inherited allele was a deletion of exons 2 and 3. Both parents presented with a typical clinical picture of HNPP. To our knowledge, this is the first patient reported with large deletions involving both PMP22 alleles. Our patient has also developed severe gastroesophageal reflux disease (GERD), a clinical feature not previously reported with CMT or DSD. The correlation of the phenotype and the molecular defects observed in this patient may set a new subcategory in the classification of DSD.

Extreme phenotypic diversity and nonpenetrance in families with the LMNA gene mutation R644C

Mutations in the LMNA gene result in diverse phenotypes including Emery Dreifuss muscular dystrophy, limb girdle muscular dystrophy, dilated cardiomyopathy with conduction system disease, Dunnigan type familial partial lipodystrophy, mandibulo acral dysplasia, Hutchinson Gilford progeria syndrome, restrictive dermopathy and autosomal recessive Charcot Marie Tooth type 2. The c.1930C > T (R644C) missense mutation has previously been reported in eight unrelated patients with variable features including left ventricular hypertrophy, limb girdle muscle weakness, dilated cardiomyopathy and atypical progeria. Here we report on the details of nine additional patients in eight families with this mutation. Patients 1 and 2 presented with lipodystrophy and insulin resistance, Patient 1 having in addition focal segmental glomerulosclerosis. Patient 3 presented with motor neuropathy, Patient 4 with arthrogryposis and dilated cardiomyopathy with left ventricular non-compaction, Patient 5 with severe scoliosis and contractures, Patient 6 with limb girdle weakness and Patient 7 with hepatic steatosis and insulin resistance. Patients 8 and 9 are brothers with proximal weakness and contractures. Nonpenetrance was observed frequently in first degree relatives. This report provides further evidence of the extreme phenotypic diversity and low penetrance associated with the R644C mutation. Possible explanations for these observations are discussed.

Capillary electrophoresis for the detection of PMP22 gene duplication: Study in Mexican patients

Charcot-Marie-Tooth (CMT) disease is the most common inherited disorder of the human peripheral nerve, with an estimated overall prevalence of 17-40/10 000 [1]. The typical phenotype presents peroneal muscular atrophy and pes cavus [2]. CMT is usually divided into two large types, about two-thirds of the patients have CMT type 1 (CMT1), that affects the layer of myelin (demyelination). In type 2 (CMT2) the nerve fibers are affected (axonal). CMT diseases have autosomal dominant, autosomal recessive, and X-linked inheritance [1]. The most frequent subtype is 1A (CMT1A) with autosomal dominant transmission, secondary in most cases to a tandem duplication of a 1.5 Mb DNA fragment on chromosome 17p11.2-p12 [4-7]. In this region, the codification of the peripheral myelin protein 22 (PMP22) takes place. The severity of the disease varies among patients, even within the same family, from almost no symptoms to severe foot-drop and sensory loss. The PMP22 gene has four exons and is regulated by two promoters located toward the extreme 5'. The origin of the duplication that causes the disease is an uneven exchange of the chromatids during the meiosis. This unequal recombination occurs between two regions that limit the PMP22 gene, described as REP places of 24 kb, proximal and distal [3, 4].

Functional Characterization of Rab7 Mutant Proteins Associated with Charcot-Marie-Tooth Type 2B Disease

Charcot-Marie-Tooth (CMT) type 2 neuropathies are a group of autosomal-dominant axonal disorders genetically and clinically heterogeneous. In particular, CMT type 2B (CMT2B) neuropathies are characterized by severe sensory loss, often complicated by infections, arthropathy, and amputations. Recently, four missense mutations in the small GTPase Rab7 associated with the Charcot-Marie Tooth type 2B phenotype have been identified. These mutations target highly conserved amino acid residues. However, nothing is known about whether and how these mutations affect Rab7 function. We investigated the biochemical and functional properties of three of the mutant proteins. Interestingly, all three proteins exhibited higher nucleotide exchange rates and hydrolyzed GTP slower than the wild-type protein. In addition, whereas 23% of overexpressed wild-type Rab7 was GTP bound in HeLa cells, the large majority of the mutant proteins (82-89%) were in the GTP-bound form, consistent with the data on GTP hydrolysis and exchange rates. The CMT2B-associated Rab7 proteins were also able to bind the Rab7 effector RILP (Rab-interacting lysosomal protein) and to rescue Rab7 function after silencing. Altogether, these data demonstrate that all tested CMT2B-associated Rab7 mutations are mechanistically similar, suggesting that activated forms of the Rab7 are responsible for CMT2B disease.

Mitochondrial fusion and function in Charcot–Marie–Tooth type 2A patient fibroblasts with mitofusin 2 mutations

Charcot–Marie–Tooth Type 2A is a dominantly inherited peripheral neuropathy characterized by axonal degeneration of sensory and motor nerves. The disease is caused by mutations in the mitochondrial fusion gene MFN2. Mfn2 is an integral outer mitochondrial membrane protein composed of a large GTPase domain and two heptad repeat (HR) domains that face the cytoplasm. Mitochondrial membrane fusion and division are balanced processes that are necessary to maintain tubular mitochondrial morphology, respiratory function, and uniform distribution of the organelle throughout the cell. We have utilized primary fibroblasts from CMT2A patients to survey mitochondrial phenotypes associated with heterozygous MFN2 alleles expressed at physiological levels. Our results indicate that, in fibroblasts, mitofusin expression, mitochondrial morphology, ultrastructure, mtDNA content, and respiratory capacity are not affected by the presence of mutant Mfn2 protein. Consistent with a lack of mitochondrial dysfunction, we also show that mitochondrial fusion occurs efficiently in CMT2A patient-derived fibroblasts. Our observations are in agreement with the neuronal specificity of the disease and are consistent with a recent finding that mitochondrial fusion can be maintained in cells that express mutant Mfn2 protein due to complementation by a second mitofusin, Mfn1. We discuss our results and those of others in terms of a comprehensive model for the mechanism(s) by which mutations in MFN2 may lead to CMT2A disease.

Hindlimb gait defects due to motor axon loss and reduced distal muscles in a transgenic mouse model of Charcot–Marie–Tooth type 2A

Charcot-Marie-Tooth (CMT) disease type 2A is a progressive, neurodegenerative disorder affecting long peripheral motor and sensory nerves. The most common clinical sign is weakness in the lower legs and feet, associated with muscle atrophy and gait defects. The axonopathy in CMT2A is caused by mutations in Mitofusin 2 (Mfn2), a mitochondrial GTPase necessary for the fusion of mitochondria. Most Mfn2 disease alleles dominantly aggregate mitochondria upon expression in cultured fibroblasts and neurons. To determine whether this property is related to neuronal pathogenesis, we used the HB9 promoter to drive expression of a pathogenic allele, Mfn2(T105M), in the motor neurons of transgenic mice. Transgenic mice develop key clinical signs of CMT2A disease in a dosage-dependent manner. They have a severe gait defect due to an inability to dorsi-flex the hindpaws. Consequently, affected animals drag their hindpaws while walking and support themselves on the hind knuckles, rather than the soles. This distal muscle weakness is associated with reduced numbers of motor axons in the motor roots and severe reduction of the anterior calf muscles. Many motor neurons from affected animals show improper mitochondrial distribution, characterized by tight clusters of mitochondria within axons. This transgenic line recapitulates key motor features of CMT2A and provides a system to dissect the function of mitochondria in the axons of mammalian motor neurons.

A novel founder mutation in the MFN2 gene associated with variable Charcot–Marie–Tooth type 2 phenotype in two families from Southern Italy

Charcot–Marie–Tooth (CMT) disease is the most common hereditary neuropathy. CMT falls into two main forms: the demyelinating CMT type 1 with decreased nerve conduction velocities and the axonal CMT type...

Charcot–Marie–Tooth type 4B demyelinating neuropathy: deciphering the role of MTMR phosphatases

Charcot-Marie-Tooth type 4B (CMT4B) is a severe autosomal recessive neuropathy with demyelination and myelin outfoldings of the nerve. This disorder is genetically heterogeneous, but thus far, mutations in myotubularin-related 2 (MTMR2) and MTMR13 genes have been shown to underlie CMT4B1 and CMT4B2, respectively. MTMR2 and MTMR13 belong to a family of ubiquitously expressed proteins sharing homology with protein tyrosine phosphatases (PTPs). The MTMR family, which has 14 members in humans, comprises catalytically active proteins, such as MTMR2, and catalytically inactive proteins, such as MTMR13. Despite their homology with PTPs, catalytically active MTMR phosphatases dephosphorylate both PtdIns3P and PtdIns(3,5)P2 phosphoinositides. Thus, MTMR2 and MTMR13 may regulate vesicular trafficking in Schwann cells. Loss of these proteins could lead to uncontrolled folding of myelin and, ultimately, to CMT4B. In this review, we discuss recent findings on this interesting protein family with the main focus on MTMR2 and MTMR13 and their involvement in the biology of Schwann cell and CMT4B neuropathies.

Channels, currents and mechanisms of accommodative processes in simulated cases of systematic demyelinating neuropathies

To clarify the mechanisms of accommodative processes in systematic demyelinating neuropathies, this study presents the kinetics of the ionic, transaxonal and transmyelin currents defining the electrotonic potentials in different segments of human demyelinated motor fibres. The electrotonic potentials are obtained for fibres, which are in simulated cases of internodal, paranodal and simultaneously of paranodal internodal demyelinations, each of them systematic. The computations used our previous double-cable model of the fibre. The results show that the slow components of the electrotonic potentials depend on the activation of the channel types in the nodal or internodal axolemma, whereas the fast components of the potentials are determined mainly by the passive cable responses, i.e. by the capacitances and resistances of the corresponding different segments along the fibre. In the nodal segments, the depolarizing electrotonus is determined mainly by the activation of the K + slow nodal channels, whereas in the paranodal and internodal segments the potentials depend on the activation of the K + fast and slow internodal channels. For the hyperpolarizing electrotonus, the contribution of the activating inward rectifier (IR) and leak (Lk) channels in the internodal axolemma dominates in the total ionic currents. The results show that the greater polarizing electrotonic potentials and their defining currents in the mild systematic demyelinations abnormally increase when these demyelinations are severe. The study summarizes the insights gained from these modeling investigations of the accommodative mechanisms underlying the threshold electrotonus abnormalities observed in demyelinating neuropathies such as Charcot–Marie–Tooth type 1A and chronic inflammatory demyelinating polyneuropathy.