M Laura

Charcot–Marie–Tooth disease: frequency of genetic subtypes and guidelines for genetic testing

Background Charcot–Marie–Tooth disease (CMT) is a clinically and genetically heterogeneous group of diseases with approximately 45 different causative genes described. The aims of this study were to determine the frequency of different genes in a large cohort of patients with CMT and devise guidelines for genetic testing in practice. Methods The genes known to cause CMT were sequenced in 1607 patients with CMT (425 patients attending an inherited neuropathy clinic and 1182 patients whose DNA was sent to the authors for genetic testing) to determine the proportion of different subtypes in a UK population. Results A molecular diagnosis was achieved in 62.6% of patients with CMT attending the inherited neuropathy clinic; in 80.4% of patients with CMT1 (demyelinating CMT) and in 25.2% of those with CMT2 (axonal CMT). Mutations or rearrangements in PMP22, GJB1, MPZ and MFN2 accounted for over 90% of the molecular diagnoses while mutations in all other genes tested were rare. Conclusion Four commonly available genes account for over 90% of all CMT molecular diagnoses; a diagnostic algorithm is proposed based on these results for use in clinical practice. Any patient with CMT without a mutation in these four genes or with an unusual phenotype should be considered for referral for an expert opinion to maximise the chance of reaching a molecular diagnosis.

Charcot-Marie-Tooth disease

Charcot‐Marie‐Tooth (CMT) disease is the commonest inherited neuromuscular disorder affecting at least 1 in 2,500. Over the last two decades, there have been rapid advances in understanding the molecular basis for many forms of CMT with more than 30 causative genes now described. This has made obtaining an accurate genetic diagnosis possible but at times challenging for clinicians. This review aims to provide a simple, pragmatic approach to diagnosing CMT from a clinicians perspective.

Hereditary Sensory Neuropathy Type 1 Is Caused by the Accumulation of Two Neurotoxic Sphingolipids

HSAN1 is an inherited neuropathy found to be associated with several missense mutations in the SPTLC1 subunit of serine palmitoyltransferase (SPT). SPT catalyzes the condensation of serine and palmitoyl-CoA, the initial step in the de novo synthesis of sphingolipids. Here we show that the HSAN1 mutations induce a shift in the substrate specificity of SPT, which leads to the formation of the two atypical deoxy-sphingoid bases (DSBs) 1-deoxy-sphinganine and 1-deoxymethyl-sphinganine. Both metabolites lack the C1 hydroxyl group of sphinganine and can therefore neither be converted to complex sphingolipids nor degraded. Consequently, they accumulate in the cell, as demonstrated in HEK293 cells overexpressing mutant SPTLC1 and lymphoblasts of HSAN1 patients. Elevated DSB levels were also found in the plasma of HSAN1 patients and confirmed in three groups of HSAN1 patients with different SPTLC1mutations. The DSBs show pronounced neurotoxic effects on neurite formation in cultured sensory neurons. The neurotoxicity co-occurs with a disturbed neurofilament structure in neurites when cultured in the presence of DSBs. Based on these observations, we conclude that HSAN1 is caused by a gain of function mutation, which results in the formation of two atypical and neurotoxic sphingolipid metabolites.

Ascorbic acid in Charcot-Marie-Tooth disease type 1A (CMT-TRIAAL and CMT-TRAUK): a double-blind randomised trial

Summary Background Ascorbic acid reduced the severity of neuropathy in transgenic mice overexpressing peripheral myelin protein 22 (PMP22), a model of Charcot–Marie–Tooth disease type 1A (CMT1A) associated with the PMP22 duplication. However, in three 1-year trials, ascorbic acid had no benefit in human beings. We did a multicentre 2-year trial to test the efficacy and tolerability of ascorbic acid in patients with CMT1A. Methods Adult patients (aged 18–70 years) with symptomatic CMT1A were enrolled from nine centres in Italy and the UK, and were randomly assigned (1:1 ratio) to receive 1·5 g/day oral ascorbic acid or matching placebo for 24 months. The randomisation sequence was computer generated by block randomisation, stratified by centre and disease severity, and patients were allocated to treatment by telephone. The primary outcome was change in the CMT neuropathy score (CMTNS) at 24 months. Secondary outcomes were timed 10 m walk test, nine-hole peg test, overall neuropathy limitations scale, distal maximal voluntary isometric contraction, visual analogue scales for pain and fatigue, 36-item short-form questionnaire, and electrophysiological measurements. Patients, treating physicians, and physicians assessing outcome measures were masked to treatment allocation. Analysis of the primary outcome was done on all randomised patients who received at least one dose of study drug. This study is registered, numbers ISRCTN61074476 (CMT-TRAUK) and EudraCT 2006-000032-27 (CMT-TRIAAL). Findings We enrolled and randomly assigned 277 patients, of whom six (four assigned to receive ascorbic acid) withdrew consent before receiving treatment; 138 receiving ascorbic acid and 133 receiving placebo were eligible for analysis. Treatment was well tolerated: 241 of 271 patients (89% in each group) completed the study; 20 patients (nine receiving ascorbic acid) dropped out because of adverse events. Mean CMTNS at baseline with missing data imputed was 14·7 (SD 4·8) in the ascorbic acid group and 13·9 (4·2) in the placebo group. Mean worsening of CMTNS was 0·2 (SD 2·8, 95% CI −0·3 to 0·7) in the ascorbic acid group and 0·2 (2·7, −0·2 to 0·7) in the placebo group (mean difference 0·0, 95% CI −0·6 to 0·7; p=0·93). We recorded no differences between the groups for the secondary outcomes at 24 months. 21 serious adverse events occurred in 20 patients, eight in the ascorbic acid group and 13 in the placebo group. Interpretation Ascorbic acid supplementation had no significant effect on neuropathy compared with placebo after 2 years, suggesting that no evidence is available to support treatment with ascorbic acid in adults with CMT1A. Funding Telethon-UILDM and AIFA (Italian Medicines Agency) for CMT-TRIAAL, and Muscular Dystrophy Campaign for CMT-TRAUK.

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

Background: 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). Methods: 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. Results: We identified a novel homozygous mutation in the α-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. Conclusions: 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.

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. Nerveconduction 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.

MFN2 mutations cause severe phenotypes in most patients with CMT2A

Background: Charcot-Marie-Tooth disease type 2A (CMT2A), the most common form of CMT2, is caused by mutations in the mitofusin 2 gene (MFN2), a nuclear encoded gene essential for mitochondrial fusion and tethering the endoplasmic reticulum to mitochondria. Published CMT2A phenotypes have differed widely in severity. Methods: To determine the prevalence and phenotypes of CMT2A within our clinics we performed genetic testing on 99 patients with CMT2 evaluated at Wayne State University in Detroit and on 27 patients with CMT2 evaluated in the National Hospital for Neurology and Neurosurgery in London. We then preformed a cross-sectional analysis on our patients with CMT2A. Results: Twenty-one percent of patients had MFN2 mutations. Most of 27 patients evaluated with CMT2A had an earlier onset and more severe impairment than patients without CMT2A. CMT2A accounted for 91% of all our severely impaired patients with CMT2 but only 11% of mildly or moderately impaired patients. Twenty-three of 27 patients with CMT2A were nonambulatory prior to age 20 whereas just one of 78 non-CMT2A patients was nonambulatory after this age. Eleven patients with CMT2A had a pure motor neuropathy while another 5 also had profound proprioception loss. MFN2 mutations were in the GTPase domain, the coiled-coil domains, or the highly conserved R3 domain of the protein. Conclusions: We find MFN2 mutations particularly likely to cause severe neuropathy that may be primarily motor or motor accompanied by prominent proprioception loss. Disruption of functional domains of the protein was particularly likely to cause neuropathy.

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.

Validation of the Charcot–Marie–Tooth disease pediatric scale as an outcome measure of disability

Charcot–Marie–Tooth disease (CMT) is a common heritable peripheral neuropathy. There is no treatment for any form of CMT, although clinical trials are increasingly occurring. Patients usually develop symptoms during the first 2 decades of life, but there are no established outcome measures of disease severity or response to treatment. We identified a set of items that represent a range of impairment levels and conducted a series of validation studies to build a patient‐centered multi‐item rating scale of disability for children with CMT.

The phenotype of Charcot–Marie–Tooth disease type 4C due to SH3TC2 mutations and possible predisposition to an inflammatory neuropathy

Charcot-Marie-Tooth (CMT) disease is a heterogeneous group of inherited peripheral motor and sensory neuropathies. The locus responsible for CMT4C was previously assigned to the chromosome 5q23 region by homozygosity mapping and mutations in the SH3TC2 (KIAA1985) gene have been subsequently identified mainly in families around the Mediterranean basin but also frequently in European Gypsies. No English families have been reported to date. To determine the frequency, phenotype and neuropathology of CMT due to SH3TC2 mutations we screened 23 English autosomal recessive (AR) demyelinating CMT families. Five families with AR demyelinating CMT and SH3TC2 mutations were identified, four families were homozygous for the R954X mutation and the fifth family was compound heterozygous for the R954X and E657K mutations. There was significant clinical variation between these families with some cases presenting with a severe childhood onset neuropathy with respiratory and cranial nerve involvement, compared to other families with mild scoliosis and foot deformity. Characteristic sural nerve neuropathology was seen in three families with frequent demyelinating fibres surrounded by excess Schwann cell lamellae forming basal lamina onion bulbs and abnormally long and attenuated Schwann cell processes. One patient homozygous for the R954X mutation had a 20-year history of an inflammatory neuropathy that was superimposed onto the hereditary form, indicating that structural alterations to the SH3TC2 gene could possibly predispose to peripheral nerve inflammation.