Dynamin 2 mutations in Charcot-Marie-Tooth neuropathy highlight the importance of clathrin-mediated endocytosis in myelination

Mutations in dynamin 2 (DNM2) lead to dominant intermediate Charcot-Marie-Tooth neuropathy type B, while a different set of DNM2 mutations cause autosomal dominant centronuclear myopathy. In this study, we aimed to elucidate the disease mechanisms in dominant intermediate Charcot-Marie-Tooth neuropathy type B and to find explanations for the tissue-specific defects that are associated with different DNM2 mutations in dominant intermediate Charcot-Marie-Tooth neuropathy type B versus autosomal dominant centronuclear myopathy. We used tissue derived from Dnm2-deficient mice to establish an appropriate peripheral nerve model and found that dominant intermediate Charcot-Marie-Tooth neuropathy type B-associated dynamin 2 mutants, but not autosomal dominant centronuclear myopathy mutants, impaired myelination. In contrast to autosomal dominant centronuclear myopathy mutants, Schwann cells and neurons from the peripheral nervous system expressing dominant intermediate Charcot-Marie-Tooth neuropathy mutants showed defects in clathrin-mediated endocytosis. We demonstrate that, as a consequence, protein surface levels are altered in Schwann cells. Furthermore, we discovered that myelination is strictly dependent on Dnm2 and clathrin-mediated endocytosis function. Thus, we propose that altered endocytosis is a major contributing factor to the disease mechanisms in dominant intermediate Charcot-Marie-Tooth neuropathy type

Fibulin-5 mutations link inherited neuropathies, age-related macular degeneration and hyperelastic skin

To identify the disease-causing gene responsible for an autosomal dominantly inherited Charcot-Marie-Tooth neuropathy subtype in a family excluded for mutations in the common Charcot-Marie-Tooth genes, we used array-based sequence capture to simultaneously analyse the disease-linked protein coding exome at chromosome 14q32. A missense mutation in fibulin-5, encoding a widely expressed constituent of the extracellular matrix that has an essential role in elastic fibre assembly and has been shown to cause cutis laxa, was detected as the only novel non-synonymous sequence variant within the disease interval. Screening of 112 index probands with unclassified Charcot-Marie-Tooth neuropathies detected two further fibulin-5 missense mutations in two families with Charcot-Marie-Tooth disease and hyperextensible skin. Since fibulin-5 mutations have been described in patients with age-related macular degeneration, an additional 300 probands with exudative age-related macular degeneration were included in this study. Two further fibulin-5 missense mutations were identified in six patients. A mild to severe peripheral neuropathy was detected in the majority of patients with age-related macular degeneration carrying mutations in fibulin-5. This study identifies fibulin-5 as a gene involved in Charcot-Marie-Tooth neuropathies and reveals heterozygous fibulin-5 mutations in 2% of our patients with age-related macular degeneration. Furthermore, it adumbrates a new syndrome by linking concurrent pathologic alterations affecting peripheral nerves, eyes and skin to mutations in the fibulin-5 gen

Expression of mitofusin 2R94Q in a transgenic mouse leads to Charcot-Marie-Tooth neuropathy type 2A

Charcot-Marie-Tooth disease type 2A is an autosomal dominant axonal form of peripheral neuropathy caused by mutations in the mitofusin 2 gene. Mitofusin 2 encodes a mitochondrial outer membrane protein that participates in mitochondrial fusion in mammalian cells. How mutations in this protein lead to Charcot-Marie-Tooth disease type 2A pathophysiology remains unclear. We have generated a transgenic mouse expressing either a mutated (R94Q) or wild-type form of human mitofusin 2 in neurons to evaluate whether the R94Q mutation was sufficient for inducing a Charcot-Marie-Tooth disease type 2A phenotype. Only mice expressing mitofusin 2R94Q developed locomotor impairments and gait defects thus mimicking the Charcot-Marie-Tooth disease type 2A neuropathy. In these animals, the number of mitochondria per axon was significantly increased in the distal part of the sciatic nerve axons with a diameter smaller than 3.5 μm. Importantly, the analysis of R94Q transgenic animals also revealed an age-related shift in the size of myelinated axons leading to an over-representation of axons smaller than 3.5 μm. Together these data suggest a link between an increased number of mitochondria in axons and a shift in axonal size distribution in mitofusin 2R94Q transgenic animals that may contribute to their neurological phenotyp

MRI biomarker assessment of neuromuscular disease progression: a prospective observational cohort study

BACKGROUND: A substantial impediment to progress in trials of new therapies in neuromuscular disorders is the absence of responsive outcome measures that correlate with patient functional deficits and are sensitive to early disease processes. Irrespective of the primary molecular defect, neuromuscular disorder pathological processes include disturbance of intramuscular water distribution followed by intramuscular fat accumulation, both quantifiable by MRI. In pathologically distinct neuromuscular disorders, we aimed to determine the comparative responsiveness of MRI outcome measures over 1 year, the validity of MRI outcome measures by cross-sectional correlation against functionally relevant clinical measures, and the sensitivity of specific MRI indices to early muscle water changes before intramuscular fat accumulation beyond the healthy control range. METHODS: We did a prospective observational cohort study of patients with either Charcot-Marie-Tooth disease 1A or inclusion body myositis who were attending the inherited neuropathy or muscle clinics at the Medical Research Council (MRC) Centre for Neuromuscular Diseases, National Hospital for Neurology and Neurosurgery, London, UK. Genetic confirmation of the chromosome 17p11·2 duplication was required for Charcot-Marie-Tooth disease 1A, and classification as pathologically or clinically definite by MRC criteria was required for inclusion body myositis. Exclusion criteria were concomitant diseases and safety-related MRI contraindications. Healthy age-matched and sex-matched controls were also recruited. Assessments were done at baseline and 1 year. The MRI outcomes-fat fraction, transverse relaxation time (T2), and magnetisation transfer ratio (MTR)-were analysed during the 12-month follow-up, by measuring correlation with functionally relevant clinical measures, and for T2 and MTR, sensitivity in muscles with fat fraction less than the 95th percentile of the control group. FINDINGS: Between Jan 19, 2010, and July 7, 2011, we recruited 20 patients with Charcot-Marie-Tooth disease 1A, 20 patients with inclusion body myositis, and 29 healthy controls (allocated to one or both of the 20-participant matched-control subgroups). Whole muscle fat fraction increased significantly during the 12-month follow-up at calf level (mean absolute change 1·2%, 95% CI 0·5-1·9, p=0·002) but not thigh level (0·2%, -0·2 to 0·6, p=0·38) in patients with Charcot-Marie-Tooth disease 1A, and at calf level (2·6%, 1·3-4·0, p=0·002) and thigh level (3·3%, 1·8-4·9, p=0·0007) in patients with inclusion body myositis. Fat fraction correlated with the lower limb components of the inclusion body myositis functional rating score (ρ=-0·64, p=0·002) and the Charcot-Marie-Tooth examination score (ρ=0·63, p=0·003). Longitudinal T2 and MTR changed consistently with fat fraction but more variably. In muscles with a fat fraction lower than the control group 95th percentile, T2 was increased in patients compared with controls (regression coefficients: inclusion body myositis thigh 4·0 ms [SE 0·5], calf 3·5 ms [0·6]; Charcot-Marie-Tooth 1A thigh 1·0 ms [0·3], calf 2·0 ms [0·3]) and MTR reduced compared with controls (inclusion body myositis thigh -1·5 percentage units [pu; 0·2], calf -1·1 pu [0·2]; Charcot-Marie-Tooth 1A thigh -0·3 pu [0·1], calf -0·7 pu [0·1]). INTERPRETATION: MRI outcome measures can monitor intramuscular fat accumulation with high responsiveness, show validity by correlation with conventional functional measures, and detect muscle water changes preceding marked intramuscular fat accumulation. Confirmation of our results in further cohorts with these and other muscle-wasting disorders would suggest that MRI biomarkers might prove valuable in experimental trials. FUNDING: Medical Research Council UK

Rehabilitation interventions for foot drop in neuromuscular disease

"Foot drop" or "Floppy foot drop" is the term commonly used to describe weakness or contracture of the muscles around the ankle joint. It may arise from many neuromuscular diseases

Mice Hemizygous for a Pathogenic Mitofusin-2 Allele Exhibit Hind Limb/Foot Gait Deficits and Phenotypic Perturbations in Nerve and Muscle.

Charcot-Marie-Tooth disease type 2A (CMT2A), the most common axonal form of hereditary sensory motor neuropathy, is caused by mutations of mitofusin-2 (MFN2). Mitofusin-2 is a GTPase required for fusion of mitochondrial outer membranes, repair of damaged mitochondria, efficient mitochondrial energetics, regulation of mitochondrial-endoplasmic reticulum calcium coupling and axonal transport of mitochondria. We knocked T105M MFN2 preceded by a loxP-flanked STOP sequence into the mouse Rosa26 locus to permit cell type-specific expression of this pathogenic allele. Crossing these mice with nestin-Cre transgenic mice elicited T105M MFN2 expression in neuroectoderm, and resulted in diminished numbers of mitochondria in peripheral nerve axons, an alteration in skeletal muscle fiber type distribution, and a gait abnormality

What's the Function of Connexin 32 in the Peripheral Nervous System?

Connexin 32 (Cx32) is a fundamental protein in the peripheral nervous system (PNS) as its mutations cause the X-linked form of Charcot-Marie-Tooth disease (CMT1X), the second most common form of hereditary motor and sensory neuropathy and a demyelinating disease for which there is no effective therapy. Since mutations of the GJB1 gene encoding Cx32 were first reported in 1993, over 450 different mutations associated with CMT1X including missense, frameshift, deletion and non-sense ones have been identified. Despite the availability of a sizable number of studies focusing on normal and mutated Cx32 channel properties, the crucial role played by Cx32 in the PNS has not yet been elucidated, as well as the molecular pathogenesis of CMT1X. Is Cx32 fundamental during a particular phase of Schwann cell (SC) life? Are Cx32 paired (gap junction, GJ) channels in myelinated SCs important for peripheral nerve homeostasis? The attractive hypothesis that short coupling of adjacent myelin layers by Cx32 GJs is required for efficient diffusion of K+ and signaling molecules is still debated, while a growing body of evidence is supporting other possible functions of Cx32 in the PNS, mainly related to Cx32 unpaired channels (hemichannels), which could be involved in a purinergic-dependent pathway controlling myelination. Here we review the intriguing puzzle of findings about Cx32 function and dysfunction, discussing possible directions for future investigation

Pregnancy and delivery in patients with Charcot-Marie-Tooth disease and related disorders

BACKGROUND: Charcot–Marie–Tooth disease is the most common inherited peripheral neuropathy and many patients with Charcot–Marie–Tooth are women of childbearing age. Guidelines for managing pregnancy in Charcot–Marie–Tooth are lacking. AIMS: To assess the impact of pregnancy on Charcot–Marie–Tooth and how Charcot–Marie–Tooth affects pregnancy, delivery and postnatal care. METHODS: A retrospective questionnaire exploring disease course during pregnancy, delivery, pregnancy complications, anaesthetic management and puerperium was administered to 92 patients with Charcot–Marie–Tooth and related disorders. RESULTS: Worsening of Charcot–Marie–Tooth symptoms were reported in 37% of pregnant patients which resolved after delivery in half of the patients. No significant increase in pregnancy, delivery and anaesthetic complications were observed and the type of delivery did not significantly differ from the normal population. CONCLUSIONS: While these results are reassuring, ideally an international prospective study should be done to confirm these results and to develop practice guidelines on the management of pregnancy in Charcot–Marie–Tooth

Complementation between mouse Mfn1 and Mfn2 protects mitochondrial fusion defects caused by CMT2A disease mutations

Mfn2, an oligomeric mitochondrial protein important for mitochondrial fusion, is mutated in Charcot-Marie-Tooth disease (CMT) type 2A, a peripheral neuropathy characterized by axonal degeneration. In addition to homooligomeric complexes, Mfn2 also associates with Mfn1, but the functional significance of such heterooligomeric complexes is unknown. Also unknown is why Mfn2 mutations in CMT2A lead to cell type–specific defects given the widespread expression of Mfn2. In this study, we show that homooligomeric complexes formed by many Mfn2 disease mutants are nonfunctional for mitochondrial fusion. However, wild-type Mfn1 complements mutant Mfn2 through the formation of heterooligomeric complexes, including complexes that form in trans between mitochondria. Wild-type Mfn2 cannot complement the disease alleles. Our results highlight the functional importance of Mfn1–Mfn2 heterooligomeric complexes and the close interplay between the two mitofusins in the control of mitochondrial fusion. Furthermore, they suggest that tissues with low Mfn1 expression are vulnerable in CMT2A and that methods to increase Mfn1 expression in the peripheral nervous system would benefit CMT2A patients