Prevalence and orthopedic management of foot and ankle deformities in Charcot-Marie-Tooth disease

Introduction: Foot deformities are frequent complications in Charcot–Marie–Tooth disease (CMT) patients, often requiring orthopedic surgery. However, there are no prospective, randomized studies on surgical management, and there is variation in the approaches among centers both within and between countries. Methods: In this study we assessed the frequency of foot deformities and surgery among patients recruited into the Inherited Neuropathies Consortium (INC). We also designed a survey addressed to orthopedic surgeons at INC centers to determine whether surgical approaches to orthopedic complications in CMT are variable. Results: Foot deformities were reported in 71% of CMT patients; 30% of the patients had surgery. Survey questions were answered by 16 surgeons working in different specialized centers. Most of the respondents were foot and ankle surgeons. There was marked variation in surgical management. Discussion: Our findings confirm that the approaches to orthopedic management of CMT are varied. We identify areas that require further research. Muscle Nerve57: 255–259, 2018

Loss of Coupling Distinguishes GJB1 Mutations Associated with CNS Manifestations of CMT1X from Those Without CNS Manifestations

CMT1X, an X-linked inherited neuropathy, is caused by mutations in GJB1, which codes for Cx32, a gap junction protein expressed by Schwann cells and oligodendrocytes. Many GJB1 mutations cause central nervous system (CNS) abnormality in males, including stable subclinical signs and, less often, short-duration episodes characterized by motor difficulties and altered consciousness. However, some mutations have no apparent CNS effects. What distinguishes mutations with and without CNS manifestations has been unclear. Here we studied a total of 14 Cx32 mutations, 10 of which are associated with florid episodic CNS clinical syndromes in addition to peripheral neuropathy. The other 4 mutations exhibit neuropathy without clinical or subclinical CNS abnormalities. These "PNS-only" mutations (Y151C, V181M, R183C and L239I) form gap junction plaques and produce levels of junctional coupling similar to those for wild-type Cx32. In contrast, mutants with CNS manifestations (F51L, E102del, V139M, R142Q, R142W, R164W T55I, R164Q and C168Y) either form no morphological gap junction plaques or, if they do, produce little or no detectable junctional coupling. Thus, PNS and CNS abnormalities may involve different aspects of connexin function.

Target-enrichment sequencing and copy number evaluation in inherited polyneuropathy

OBJECTIVE

To assess the efficiency of target-enrichment next-generation sequencing (NGS) with copy number assessment in inherited neuropathy diagnosis.

METHODS

A 197 polyneuropathy gene panel was designed to assess for mutations in 93 patients with inherited or idiopathic neuropathy without known genetic cause. We applied our novel copy number variation algorithm on NGS data, and validated the identified copy number mutations using CytoScan (Affymetrix). Cost and efficacy of this targeted NGS approach was compared to earlier evaluations.

RESULTS

Average coverage depth was ∼760× (median = 600, 99.4% > 100×). Among 93 patients, 18 mutations were identified in 17 cases (18%), including 3 copy number mutations: 2 PMP22 duplications and 1 MPZ duplication. The 2 patients with PMP22 duplication presented with bulbar and respiratory involvement and had absent extremity nerve conductions, leading to axonal diagnosis. Average onset age of these 17 patients was 25 years (2-61 years), vs 45 years for those without genetic discovery. Among those with onset age less than 40 years, the diagnostic yield of targeted NGS approach is high (27%) and cost savings is significant (∼20%). However, the cost savings for patients with late onset age and without family history is not demonstrated.

CONCLUSIONS

Incorporating copy number analysis in target-enrichment NGS approach improved the efficiency of mutation discovery for chronic, inherited, progressive length-dependent polyneuropathy diagnosis. The new technology is facilitating a simplified genetic diagnostic algorithm utilizing targeted NGS, clinical phenotypes, age at onset, and family history to improve diagnosis efficiency. Our findings prompt a need for updating the current practice parameters and payer guidelines.

De novo PMP2 mutations in families with type 1 Charcot-Marie-Tooth disease

We performed whole exome sequencing on a patient with Charcot-Marie-Tooth disease type 1 and identified a de novo mutation in PMP2, the gene that encodes the myelin P2 protein. This mutation (p.Ile52Thr) was passed from the proband to his one affected son, and segregates with clinical and electrophysiological evidence of demyelinating neuropathy. We then screened a cohort of 136 European probands with uncharacterized genetic cause of Charcot-Marie-Tooth disease and identified another family with Charcot-Marie-Tooth disease type 1 that has a mutation affecting an adjacent amino acid (p.Thr51Pro), which segregates with disease. Our genetic and clinical findings in these kindred demonstrate that dominant PMP2 mutations cause Charcot-Marie-Tooth disease type 1.

Activated immune response in an inherited leukodystrophy disease caused by the loss of oligodendrocyte gap junctions

Oligodendrocyte:oligodendrocyte (O:O) gap junction (GJ) coupling is a widespread and essential feature of the CNS, and is mediated by connexin47 (Cx47) and Cx32. Loss of function mutations affecting Cx47 results in a severe leukodystrophy, Pelizeus-Merzbacher-like disease (also known as Hypomyelinating Leukodystrophy 2), which can be reproduced in mice lacking both Cx47 and Cx32. Here we report the gene expression profile of the cerebellum--an affected brain region--in mice lacking both Cx47 and Cx32. Of the 43,174 mRNA probes examined, we find decreased expression of 23 probes (corresponding to 23 genes) and increased expression of 545 probes (corresponding to 348 genes). Many of the genes with reduced expression map to oligodendrocytes, and two of them (Fa2h and Ugt8a) are involved in the synthesis of myelin lipids. Many of the genes with increased expression map to lymphocytes and microglia, and involved in leukotrienes/prostaglandins synthesis and chemokines/cytokines interactions and signaling pathways. In accord, immunostaining showed T- and B-cells in the cerebella of mutant mice as well as activated microglia and astrocytes. Thus, in addition to the loss of GJ coupling, there is a prominent immune response in mice lacking both Cx47 and Cx32.

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.

CMT subtypes and disease burden in patients enrolled in the Inherited Neuropathies Consortium natural history study: a cross-sectional analysis

BACKGROUND

The international Inherited Neuropathy Consortium (INC) was created with the goal of obtaining much needed natural history data for patients with Charcot-Marie-Tooth (CMT) disease. We analysed clinical and genetic data from patients in the INC to determine the distribution of CMT subtypes and the clinical impairment associated with them.

METHODS

We analysed data from 1652 patients evaluated at 13 INC centres. The distribution of CMT subtypes and pathogenic genetic mutations were determined. The disease burden of all the mutations was assessed by the CMT Neuropathy Score (CMTNS) and CMT Examination Score (CMTES).

RESULTS

997 of the 1652 patients (60.4%) received a genetic diagnosis. The most common CMT subtypes were CMT1A/PMP22 duplication, CMT1X/GJB1 mutation, CMT2A/MFN2 mutation, CMT1B/MPZ mutation, and hereditary neuropathy with liability to pressure palsy/PMP22 deletion. These five subtypes of CMT accounted for 89.2% of all genetically confirmed mutations. Mean CMTNS for some but not all subtypes were similar to those previously reported.

CONCLUSIONS

Our findings confirm that large numbers of patients with a representative variety of CMT subtypes have been enrolled and that the frequency of achieving a molecular diagnosis and distribution of the CMT subtypes reflects those previously reported. Measures of severity are similar, though not identical, to results from smaller series. This study confirms that it is possible to assess patients in a uniform way between international centres, which is critical for the planned natural history study and future clinical trials. These data will provide a representative baseline for longitudinal studies of CMT.

CLINICAL TRIAL REGISTRATION

ID number NCT01193075.

Intraneural GJB1 gene delivery improves nerve pathology in a model of X-linked Charcot-Marie-Tooth disease

OBJECTIVE

X-linked Charcot-Marie-Tooth disease (CMT1X) is a common inherited neuropathy caused by mutations in the GJB1 gene encoding the gap junction protein connexin32 (Cx32). Clinical studies and disease models indicate that neuropathy mainly results from Schwann cell autonomous, loss-of-function mechanisms; therefore, CMT1X may be treatable by gene replacement.

METHODS

A lentiviral vector LV.Mpz-GJB1 carrying the GJB1 gene under the Schwann cell-specific myelin protein zero (Mpz) promoter was generated and delivered into the mouse sciatic nerve by a single injection immediately distal to the sciatic notch. Enhanced green fluorescent protein (EGFP) reporter gene expression was quantified and Cx32 expression was examined on a Cx32 knockout (KO) background. A gene therapy trial was performed in a Cx32 KO model of CMT1X.

RESULTS

EGFP was expressed throughout the length of the sciatic nerve in up to 50% of Schwann cells starting 2 weeks after injection and remaining stable for up to 16 weeks. Following LV.Mpz-GJB1 injection into Cx32 KO nerves, we detected Cx32 expression and correct localization in non-compact myelin areas where gap junctions are normally formed. Gene therapy trial by intraneural injection in groups of 2-month-old Cx32 KO mice, before demyelination onset, significantly reduced the ratio of abnormally myelinated fibers (p = 0.00148) and secondary inflammation (p = 0.0178) at 6 months of age compared to mock-treated animals.

INTERPRETATION

Gene delivery using a lentiviral vector leads to efficient gene expression specifically in Schwann cells. Restoration of Cx32 expression ameliorates nerve pathology in a disease model and provides a promising approach for future treatments of CMT1X and other inherited neuropathies.

Caspr2 autoantibodies target multiple epitopes

Objective:

To better understand the mechanisms of autoantibodies to the axonal protein contactin-associated protein-like 2 (Caspr2) by studying their target epitopes.

Methods:

A plasmid for expressing Caspr2 was modified so that the various extracellular subdomains were deleted individually and in groups. Cultured cells were transfected to express these constructs and assayed by immunofluorescence staining with a commercial Caspr2 antibody and a panel of patient sera known to react with Caspr2. Western blotting was also performed. The role of glycosylation in immunogenicity was tested with tunicamycin and PNGase F treatment.

Results:

Patient antibodies bound to the extracellular domain of Caspr2. Neither native protein structure nor glycosylation was required for immunoreactivity. Caspr2 constructs with single or multidomain deletions were expressed on the plasma membrane. All deletion constructs were recognized by patients' sera, although reactivity was significantly reduced with deletion of the discoidin-like subdomain and strongly reduced or abolished with larger deletions of multiple N-terminal subdomains. Caspr2 with all subdomains deleted except the discoidin-like domain was still recognized by the antibodies.

Conclusion:

Caspr2 autoantibodies recognize multiple target epitopes in the extracellular domain of Caspr2, including one in the discoidin-like domain. Reactivity for some epitopes is not dependent on glycosylation or native protein structure.

A novel AARS mutation in a family with dominant myeloneuropathy

OBJECTIVE

To determine the genetic cause of neurodegeneration in a family with myeloneuropathy.

METHODS

We studied 5 siblings in a family with a mild, dominantly inherited neuropathy by clinical examination and electrophysiology. One patient had a sural nerve biopsy. After ruling out common genetic causes of axonal Charcot-Marie-Tooth disease, we sequenced 3 tRNA synthetase genes associated with neuropathy.

RESULTS

All affected family members had a mild axonal neuropathy, and 3 of 4 had lower extremity hyperreflexia, evidence of a superimposed myelopathy. A nerve biopsy showed evidence of chronic axonal loss. All affected family members had a heterozygous missense mutation c.304G>C (p.Gly102Arg) in the alanyl-tRNA synthetase (AARS) gene; this allele was not identified in unaffected individuals or control samples. The equivalent change in the yeast ortholog failed to complement a strain of yeast lacking AARS function, suggesting that the mutation is damaging.

CONCLUSION

A novel mutation in AARS causes a mild myeloneuropathy, a novel phenotype for patients with mutations in one of the tRNA synthetase genes.

Defects of mutant DNMT1 are linked to a spectrum of neurological disorders

We report a broader than previously appreciated clinical spectrum for hereditary sensory and autonomic neuropathy type 1E (HSAN1E) and a potential pathogenic mechanism for DNA methyltransferase (DNMT1) mutations. The clinical presentations and genetic characteristics of nine newly identified HSAN1E kinships (45 affected subjects) were investigated. Five novel mutations of DNMT1 were discovered; p.C353F, p.T481P, p.P491L, p.Y524D and p.I531N, all within the target-sequence domain, and two mutations (p.T481P, p.P491L) arising de novo. Recently, HSAN1E has been suggested as an allelic disorder of autosomal dominant cerebellar ataxia, deafness and narcolepsy. Our results indicate that all the mutations causal for HSAN1E are located in the middle part or N-terminus end of the TS domain, whereas all the mutations causal for autosomal dominant cerebellar ataxia, deafness and narcolepsy are located in the C-terminus end of the TS domain. The impact of the seven causal mutations in this cohort was studied by cellular localization experiments. The binding efficiency of the mutant DNMT proteins at the replication foci and heterochromatin were evaluated. Phenotypic characterizations included electromyography, brain magnetic resonance and nuclear imaging, electroencephalography, sural nerve biopsies, sleep evaluation and neuropsychometric testing. The average survival of HSAN1E was 53.6 years. [standard deviation = 7.7, range 43-75 years], and mean onset age was 37.7 years. (standard deviation = 8.6, range 18-51 years). Expanded phenotypes include myoclonic seizures, auditory or visual hallucinations, and renal failure. Hypersomnia, rapid eye movement sleep disorder and/or narcolepsy were identified in 11 subjects. Global brain atrophy was found in 12 of 14 who had brain MRI. EEGs showed low frequency (delta waves) frontal-predominant abnormality in five of six patients. Marked variability in cognitive deficits was observed, but the majority of patients (89%) developed significant cognitive deficit by the age of 45 years. Cognitive function decline often started with personality changes and psychiatric manifestations. A triad of hearing loss, sensory neuropathy and cognitive decline remains as the stereotypic presentation of HSAN1E. Moreover, we show that mutant DNMT1 proteins translocate to the cytoplasm and are prone to form aggresomes while losing their binding ability to heterochromatin during the G2 cell cycle. Our results suggest mutations in DNMT1 result in imbalanced protein homeostasis through aggresome-induced autophagy. This mechanism may explain why mutations in the sole DNA maintenance methyltransferase lead to selective central and peripheral neurodegeneration.

Kv7.2 regulates the function of peripheral sensory neurons

The Kv7 (KCNQ) family of voltage-gated K(+) channels regulates cellular excitability. The functional role of Kv7.2 has been hampered by the lack of a viable Kcnq2-null animal model. In this study, we generated homozygous Kcnq2-null sensory neurons using the Cre-Lox system; in these mice, Kv7.2 expression is absent in the peripheral sensory neurons, whereas the expression of other molecular components of nodes (including Kv7.3), paranodes, and juxtaparanodes is not altered. The conditional Kcnq2-null animals exhibit normal motor performance but have increased thermal hyperalgesia and mechanical allodynia. Whole-cell patch recording technique demonstrates that Kcnq2-null sensory neurons have increased excitability and reduced spike frequency adaptation. Taken together, our results suggest that the loss of Kv7.2 activity increases the excitability of primary sensory neurons.

Activated microglia do not form functional gap junctions in vivo

We investigated whether microglia form gap junctions with themselves, or with astrocytes, oligodendrocytes, or neurons in vivo in normal mouse brains, and in pathological conditions that induce microglial activation - brain injury and a model of Alzheimer's disease. Although microglia are in close physical proximity to glia and neurons, they do not form functional gap junctions under these pathological conditions.

Treatable childhood neuronopathy caused by mutations in riboflavin transporter RFVT2

Childhood onset motor neuron diseases or neuronopathies are a clinically heterogeneous group of disorders. A particularly severe subgroup first described in 1894, and subsequently called Brown-Vialetto-Van Laere syndrome, is characterized by progressive pontobulbar palsy, sensorineural hearing loss and respiratory insufficiency. There has been no treatment for this progressive neurodegenerative disorder, which leads to respiratory failure and usually death during childhood. We recently reported the identification of SLC52A2, encoding riboflavin transporter RFVT2, as a new causative gene for Brown-Vialetto-Van Laere syndrome. We used both exome and Sanger sequencing to identify SLC52A2 mutations in patients presenting with cranial neuropathies and sensorimotor neuropathy with or without respiratory insufficiency. We undertook clinical, neurophysiological and biochemical characterization of patients with mutations in SLC52A2, functionally analysed the most prevalent mutations and initiated a regimen of high-dose oral riboflavin. We identified 18 patients from 13 families with compound heterozygous or homozygous mutations in SLC52A2. Affected individuals share a core phenotype of rapidly progressive axonal sensorimotor neuropathy (manifesting with sensory ataxia, severe weakness of the upper limbs and axial muscles with distinctly preserved strength of the lower limbs), hearing loss, optic atrophy and respiratory insufficiency. We demonstrate that SLC52A2 mutations cause reduced riboflavin uptake and reduced riboflavin transporter protein expression, and we report the response to high-dose oral riboflavin therapy in patients with SLC52A2 mutations, including significant and sustained clinical and biochemical improvements in two patients and preliminary clinical response data in 13 patients with associated biochemical improvements in 10 patients. The clinical and biochemical responses of this SLC52A2-specific cohort suggest that riboflavin supplementation can ameliorate the progression of this neurodegenerative condition, particularly when initiated soon after the onset of symptoms.

Exome sequencing identification of a GJB1 missense mutation in a kindred with X-linked spinocerebellar ataxia (SCA-X1)

We undertook a gene identification and molecular characterization project in a large kindred originally clinically diagnosed with SCA-X1. While presenting with ataxia, this kindred also had some unique peripheral nervous system features. The implicated region on the X chromosome was delineated using haplotyping. Large deletions and duplications were excluded by array comparative genomic hybridization. Exome sequencing was undertaken in two affected subjects. The single identified X chromosome candidate variant was then confirmed to co-segregate appropriately in all affected, carrier and unaffected family members by Sanger sequencing. The variant was confirmed to be novel by comparison with dbSNP, and filtering for a minor allele frequency of <1% in 1000 Genomes project, and was not present in the NHLBI Exome Sequencing Project or a local database at the BCM HGSC. Functional experiments on transfected cells were subsequently undertaken to assess the biological effect of the variant in vitro. The variant identified consisted of a previously unidentified non-synonymous variant, GJB1 p.P58S, in the Connexin 32/Gap Junction Beta 1 gene. Segregation studies with Sanger sequencing confirmed the presence of the variant in all affected individuals and one known carrier, and the absence of the variant in unaffected members. Functional studies confirmed that the p.P58S variant reduced the number and size of gap junction plaques, but the conductance of the gap junctions was unaffected. Two X-linked ataxias have been associated with genetic loci, with the first of these recently characterized at the molecular level. This represents the second kindred with molecular characterization of X-linked ataxia, and is the first instance of a previously unreported GJB1 mutation with a dominant and permanent ataxia phenotype, although different CNS deficits have previously been reported. This pedigree has also been relatively unique in its phenotype due to the presence of central and peripheral neural abnormalities. Other X-linked SCAs with unique features might therefore also potentially represent variable phenotypic expression of other known neurological entities.

How do mutations in GJB1 cause X-linked Charcot-Marie-Tooth disease?

The X-linked form of Charcot-Marie-Tooth disease (CMT1X) is the second most common form of hereditary motor and sensory neuropathy. The clinical phenotype is characterized by progressive weakness, atrophy, and sensory abnormalities that are most pronounced in the distal extremities. Some patients have CNS manifestations. Affected males have moderate to severe symptoms, whereas heterozygous females are usually less affected. Neurophysiology shows intermediate slowing of conduction and length-dependent axonal loss. Nerve biopsies show more prominent axonal degeneration than de/remyelination. Mutations in GJB1, the gene that encodes the gap junction (GJ) protein connexin32 (Cx32) cause CMT1X; more than 400 different mutations have been described. Many Cx32 mutants fail to form functional GJs, or form GJs with abnormal biophysical properties. Schwann cells and oligodendrocytes express Cx32, and the GJs formed by Cx32 play an important role in the homeostasis of myelinated axons. Animal models of CMT1X demonstrate that loss of Cx32 in myelinating Schwann cells causes a demyelinating neuropathy. Effective therapies remain to be developed. This article is part of a Special Issue entitled Electrical Synapses.

X-linked Charcot-Marie-Tooth disease

The X-linked form of Charcot-Marie-Tooth disease (CMT1X) is the second most common form of hereditary motor and sensory neuropathy. The clinical phenotype is characterized by progressive muscle atrophy and weakness, areflexia, and variable sensory abnormalities; central nervous system manifestations occur, too. Affected males have moderate to severe symptoms, whereas heterozygous females are usually less affected. Neurophysiology shows intermediate slowing of conduction and distal axonal loss. Nerve biopsies show more prominent axonal degeneration than de/remyelination. More than 400 different mutations in GJB1, the gene that encodes the gap junction (GJ) protein connexin32 (Cx32), cause CMT1X. Many Cx32 mutants fail to form functional GJs, or form GJs with abnormal biophysical properties. Schwann cells and oligodendrocytes express Cx32, and the GJs formed by Cx32 play an important role in the homeostasis of myelinated axons. Animal models of CMT1X demonstrate that loss of Cx32 in myelinating Schwann cells causes a demyelinating neuropathy. An effective therapy remains to be developed.

Identification of novel cell-adhesion molecules in peripheral nerves using a signal-sequence trap

The development and maintenance of myelinated nerves in the PNS requires constant and reciprocal communication between Schwann cells and their associated axons. However, little is known about the nature of the cell-surface molecules that mediate axon-glial interactions at the onset of myelination and during maintenance of the myelin sheath in the adult. Based on the rationale that such molecules contain a signal sequence in order to be presented on the cell surface, we have employed a eukaryotic-based, signal-sequence-trap approach to identify novel secreted and membrane-bound molecules that are expressed in myelinating and non-myelinating Schwann cells. Using cDNA libraries derived from dbcAMP-stimulated primary Schwann cells and 3-day-old rat sciatic nerve mRNAs, we generated an extensive list of novel molecules expressed in myelinating nerves in the PNS. Many of the identified proteins are cell-adhesion molecules (CAMs) and extracellular matrix (ECM) components, most of which have not been described previously in Schwann cells. In addition, we have identified several signaling receptors, growth and differentiation factors, ecto-enzymes and proteins that are associated with the endoplasmic reticulum and the Golgi network. We further examined the expression of several of the novel molecules in Schwann cells in culture and in rat sciatic nerve by primer-specific, real-time PCR and in situ hybridization. Our results indicate that myelinating Schwann cells express a battery of novel CAMs that might mediate their interactions with the underlying axons.

Kv7.5 is the primary Kv7 subunit expressed in C-fibers

Kv7 (KCNQ) potassium channel openers (enhancers) decrease neuropathic pain in experimental models. Here we show that C-fibers, and their associated small-diameter neurons in the dorsal root ganglia (both IB4- and TrkA-positive), expressed Kv7.5. In contrast, C-fibers did not express detectable levels of Kv7.2 or Kv7.3, which are instead localized to nodes of Ranvier and the cell bodies of large sensory neurons. These data suggest that Kv7.5 provides the primary M current in nociceptive neurons.

Gap junctions in inherited human disorders of the central nervous system

CNS glia and neurons express connexins, the proteins that form gap junctions in vertebrates. We review the connexins expressed by oligodendrocytes and astrocytes, and discuss their proposed physiologic roles. Of the 21 members of the human connexin family, mutations in three are associated with significant central nervous system manifestations. For each, we review the phenotype and discuss possible mechanisms of disease. Mutations in GJB1, the gene for connexin 32 (Cx32) cause the second most common form of Charcot-Marie-Tooth disease (CMT1X). Though the only consistent phenotype in CMT1X patients is a peripheral demyelinating neuropathy, CNS signs and symptoms have been found in some patients. Recessive mutations in GJC2, the gene for Cx47, are one cause of Pelizaeus-Merzbacher-like disease (PMLD), which is characterized by nystagmus within the first 6 months of life, cerebellar ataxia by 4 years, and spasticity by 6 years of age. MRI imaging shows abnormal myelination. A different recessive GJC2 mutation causes a form of hereditary spastic paraparesis, which is a milder phenotype than PMLD. Dominant mutations in GJA1, the gene for Cx43, cause oculodentodigital dysplasia (ODDD), a pleitropic disorder characterized by oculo-facial abnormalities including micropthalmia, microcornia and hypoplastic nares, syndactyly of the fourth to fifth fingers and dental abnormalities. Neurologic manifestations, including spasticity and gait difficulties, are often but not universally seen. Recessive GJA1 mutations cause Hallermann-Streiff syndrome, a disorder showing substantial overlap with ODDD. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and functions.

Neuronal cadherin (NCAD) increases sensory neurite formation and outgrowth on astrocytes

We examined the neurite outgrowth of sensory neurons on astrocytes following the genetic deletion of N-cadherin (NCAD). Deletion abolished immunostaining for NCAD and the other classical cadherins, indicating that NCAD is likely the only classical cadherin expressed by astrocytes. Only 38% of neurons grown on NCAD-deficient astrocytes for 24 h produced neurites, as compared to 74% of neurons grown on NCAD-expressing astrocytes. Of the neurons that produced neurites, those grown on NCAD-deficient astrocytes had a mean total length of 378 μm, as compared to 1093 μm for neurons grown on NCAD-expressing astrocytes. Thus, the loss of NCAD greatly impairs the formation and extension neurites on astrocytes.

Reliability of the CMT neuropathy score (second version) in Charcot-Marie-Tooth disease

The Charcot-Marie-Tooth neuropathy score (CMTNS) is a reliable and valid composite score comprising symptoms, signs, and neurophysiological tests, which has been used in natural history studies of CMT1A and CMT1X and as an outcome measure in treatment trials of CMT1A. Following an international workshop on outcome measures in Charcot-Marie-Tooth disease (CMT), the CMTNS was modified to attempt to reduce floor and ceiling effects and to standardize patient assessment, aiming to improve its sensitivity for detecting change over time and the effect of an intervention. After agreeing on the modifications made to the CMTNS (CMTNS2), three examiners evaluated 16 patients to determine inter-rater reliability; one examiner evaluated 18 patients twice within 8 weeks to determine intra-rater reliability. Three examiners evaluated 63 patients using the CMTNS and the CMTNS2 to determine how the modifications altered scoring. For inter- and intra-rater reliability, intra-class correlation coefficients (ICCs) were ≥0.96 for the CMT symptom score and the CMT examination score. There were small but significant differences in some of the individual components of the CMTNS compared with the CMTNS2, mainly in the components that had been modified the most. A longitudinal study is in progress to determine whether the CMTNS2 is more sensitive than the CMTNS for detecting change over time.

The debut of a rational treatment for an inherited neuropathy?

Hereditary neuropathies are common neurological conditions characterized by progressive loss of motor and/or sensory function. There are no effective treatments. Among the many causes of hereditary neuropathies are dominant mutations in serine palmitoyltransferase, long chain base subunit 1 (SPTLC1), which cause hereditary sensory and autonomic neuropathy type 1 (HSAN1). By incorporating L-alanine in place of L-serine, the mutant HSAN1–associated serine palmitoyltransferase generates deoxysphingolipids, which are thought to be neurotoxic. In this issue of the JCI, Garofalo and colleagues report that oral L-serine reverses the accumulation of deoxysphingolipids in humans with HSAN1 and in a transgenic mouse model. As oral L-serine reduces the severity of neuropathy in the mouse model of HSAN1, these data suggest a rational candidate therapy for this devastating condition.