Increased mitochondrial fusion in a autosomal recessive CMT2A family with mitochondrial GTPase mitofusin 2 mutations

Charcot‐Marie‐Tooth type 2A disease (CMT2A) is an inherited peripheral neuropathy mainly caused by mutations in the MFN2 gene coding for the mitochondrial fusion protein mitofusin 2. Although the disease is mainly inherited in a dominant fashion, few cases of early‐onset autosomal recessive CMT2A (AR‐CMT2A) have been reported in recent years. In this study, we characterized the structure of the mitochondrial network in cultured primary fibroblasts obtained from AR‐CMT2A family members. The patient‐derived cells showed an increase of the mitochondrial fusion with large connected networks and an increase of the mitochondrial volume. Interestingly, fibroblasts derived from the two asymptomatic parents showed similar changes to a lesser extent. These results support the hypothesis that AR‐CMT2A‐related MFN2 mutations acts through a semi‐dominant negative mechanism and suggest that other biological parameters might show mild alterations in asymptomatic heterozygote AR‐CMT2A patients. Such alterations could be useful biomarkers helping to distinguish MFN2 mutations from variants, a growing challenge with the advent of next generation sequencing into routine clinical practice.     

Sonography of the median nerve in CMT1A,CMT2A,CMTX, and HNPP

Introduction: In this study we compare the ultrasound features in the median nerve in patients with different types of Charcot–Marie–Tooth (CMT) disease and hereditary neuropathies with liability to pressure palsies (HNPP) as a typical entrapment neuropathy. Methods: Median nerve ultrasound and conduction studies were performed in patients with CMT1A (n = 12), MFN2‐associated CMT2A (n = 7), CMTX (n = 5), and HNPP (n = 5), and in controls (n = 28). Results: Median nerve cross‐sectional area (CSA) was significantly increased in CMT1A, whereas, in axonal CMT2A, fascicle diameter (FD) was enlarged. CSA correlated with nerve conduction slowing in CMT1A and with axonal loss, as shown by motor and sensory nerve amplitudes in both CMT1A and CMT2A. A relatively low wrist‐to‐forearm‐ratio (WFR <0.8) or a relatively high WFR (>1.8) appeared to be unlikely in MFN2 and Cx32 mutations of CMT2A and CMTX, respectively. Conclusion: Differences in CSA, FD, and WFR of the median nerve can be helpful in defining subtypes of hereditary neuropathies. Muscle Nerve 47:385‐395, 2013     

Selected items from the Charcot-Marie-Tooth (CMT) Neuropathy Score and secondary clinical outcome measures serve as sensitive clinical markers of disease severity in CMT1A patients

This study evaluates primary and secondary clinical outcome measures in Charcot-Marie-Tooth disease type 1A (CMT1A) with regard to their contribution towards discrimination of disease severity. The nine components of the composite Charcot-Marie-Tooth disease Neuropathy Score and six additional secondary clinical outcome measures were assessed in 479 adult patients with genetically proven CMT1A and 126 healthy controls. Using hierarchical clustering, we identified four significant clusters of patients according to clinical severity. We then tested the impact of each of the CMTNS components and of the secondary clinical parameters with regard to their power to differentiate these four clusters. The CMTNS components ulnar sensory nerve action potential (SNAP), pin sensibility, vibration and strength of arms did not increase the discriminant value of the remaining five CMTNS components (Ulnar compound motor action potential [CMAP], leg motor symptoms, arm motor symptoms, leg strength and sensory symptoms). However, three of the six additional clinical outcome measures – the 10 m-timed walking test (T10MW), 9 hole-peg test (9HPT), and foot dorsal flexion dynamometry – further improved discrimination between severely and mildly affected patients. From these findings, we identified three different composite measures as score hypotheses and compared their discriminant power with that of the CMTNS. A composite of eight components CMAP, Motor symptoms legs, Motor symptoms arms, Strength of Legs, Sensory symptoms), displayed the strongest power to discriminate between the clusters. As a conclusion, five items from the CMTNS and three secondary clinical outcome measures improve the clinical assessment of patients with CMT1A significantly and are beneficial for upcoming clinical and therapeutic trials.     

Screening for SH3TC2 gene mutations in a series of demyelinating recessive Charcot‐Marie‐Tooth disease (CMT4)

Charcot‐Marie‐Tooth disease type 4C (CMT4C) is an autosomal recessive (AR) demyelinating neuropathy associated to SH3TC2 mutations, characterized by early onset, spine deformities, and cranial nerve involvement. We screened 43 CMT4 patients (36 index cases) with AR inheritance, demyelinating nerve conductions, and negative testing for PMP22 duplication, GJB1 and MPZ mutations, for SH3TC2 mutations. Twelve patients (11 index cases) had CMT4C as they carried homozygous or compound heterozygous mutations in SH3TC2. We found six mutations: three nonsense (p.R1109*, p.R954*, p.Q892*), one splice site (c.805+2T>C), one synonymous variant (p.K93K) predicting altered splicing, and one frameshift (p.F491Lfs*32) mutation. The splice site and the frameshift mutations are novel. Mean onset age was 7 years (range: 1–14). Neuropathy was moderate‐to‐severe. Scoliosis was present in 11 patients (severe in 4), and cranial nerve deficits in 9 (hearing loss in 7). Scoliosis and cranial nerve involvement are frequent features of this CMT4 subtype, and their presence should prompt the clinician to look for SH3TC2 gene mutations. In our series of undiagnosed CMT4 patients, SH3TC2 mutation frequency is 30%, confirming that CMT4C may be the most common AR‐CMT type.     

Characterization of the mitofusin 2 R94W mutation in a knock‐in mouse model

Charcot‐Marie‐Tooth disease (CMT) comprises a group of heterogeneous peripheral axonopathies affecting 1 in 2,500 individuals. As mutations in several genes cause axonal degeneration in CMT type 2, mutations in mitofusin 2 (MFN2) account for approximately 90% of the most severe cases, making it the most common cause of inherited peripheral axonal degeneration. MFN2 is an integral mitochondrial outer membrane protein that plays a major role in mitochondrial fusion and motility; yet the mechanism by which dominant mutations in this protein lead to neurodegeneration is still not fully understood. Furthermore, future pre‐clinical drug trials will be in need of validated rodent models. We have generated a Mfn2 knock‐in mouse model expressing Mfn2^R94W, which was originally identified in CMT patients. We have performed behavioral, morphological, and biochemical studies to investigate the consequences of this mutation. Homozygous inheritance leads to premature death at P1, as well as mitochondrial dysfunction, including increased mitochondrial fragmentation in mouse embryonic fibroblasts and decreased ATP levels in newborn brains. Mfn2^R94W heterozygous mice show histopathology and age‐dependent open‐field test abnormalities, which support a mild peripheral neuropathy. Although behavior does not mimic the severity of the human disease phenotype, this mouse can provide useful tissues for studying molecular pathways associated with MFN2 point mutations.     

A patient with both Charcot-Marie-Tooth disease (CMT 1A) and mild spinal muscular atrophy (SMA 3)

In the present study, we report a single Polish SMA family in which the 17p11.2–p12 duplication causative for the Charcot-Marie-Tooth type 1A disease (CMT1A) was found in addition to a deletion of exons 7 and 8 of the SMN1 gene. A patient harboring both SMA and CMT1A mutations manifested with SMA3 phenotype and foot deformity. Her electrophysiological testing showed chronic neurogenic changes in proximal muscles that are typical for SMA, but also slowed conduction velocity in motor and sensory fibers that is typical for demyelinating neuropathy.     

腓骨肌萎缩症2A2A型家系的神经电生理及基因突变分析

目的探讨一个腓骨肌萎缩症(charcot-marie-tooth,CMT)家系的临床表现、神经电生理学和基因突变特点。方法收集家系先证者及其他成员的临床资料,对先证者进行神经电生理学检查和全外显子组基因测序,用Sanger测序技术对先证者及其家系进行突变位点验证。应用计算机软件预测突变位点氨基酸进化保守性和突变可能导致的蛋白质结构和功能变化,分析突变位点的性质。结果先证者儿童期发病,出现双下肢对称性肌肉无力伴跟腱反射消失及足部畸形,其母亲有类似症状。先证者神经电生理检查示运动和感觉神经纤维脱髓鞘及轴索性改变。基因检测发现先证者和母亲MFN2基因第11个外显子均检出c.1066A>G(p.T356A)杂合错义突变;先证者姐姐和父亲未检测到该突变。用PolyPhen-2和MutationTaster软件预测该突变为致病性,突变区域序列在不同物种间高度保守。结论儿童CMT2A2A患者的神经电生理、临床特点、发病机制及相关基因表型均有改变,此可为儿童CMT的临床诊断提供依据。     

A novel homozygous mutation in ATP13A2 gene causing pure hereditary spastic paraplegia

SPG78 is a subtype of hereditary spastic paraplegia(HSP) caused by ATP13A2 gene mutations. SPG78 was reported as complicated HSP in several cases, but was never associated with pure HSP. Here we report the first Chinese patient carrying a novel homozygous nonsense mutation in ATP13A2 presenting with pure HSP.     

Phenotypical features of the p.R120W mutation in the GDAP1 gene causing autosomal dominant Charcot-Marie-Tooth disease

Mutations in the ganglioside-induced-differentiation-associated protein 1 gene (GDAP1) can cause Charcot-Marie-Tooth (CMT) disease with demyelinating (CMT4A) or axonal forms (CMT2K and ARCMT2K). Most of these mutations present a recessive inheritance, but few autosomal dominant GDAP1 mutations have also been reported. We performed a GDAP1 gene screening in a clinically well-characterized series of 81 index cases with axonal CMT neuropathy, identifying 17 patients belonging to 4 unrelated families in whom the heterozygous p.R120W was found to be the only disease-causing mutation. The main objective was to fully characterize the neuropathy caused by this mutation. The clinical picture included a mild-moderate phenotype with onset around adolescence, but great variability. Consistently, ankle dorsiflexion and plantar flexion were impaired to a similar degree. Nerve conduction studies revealed an axonal neuropathy. Muscle magnetic resonance imaging studies demonstrated selective involvement of intrinsic foot muscles in all patients and a uniform pattern of fatty infiltration in the calf, with distal and superficial posterior predominance. Pathological abnormalities included depletion of myelinated fibers, regenerative clusters and features of axonal degeneration with mitochondrial aggregates. Our findings highlight the relevance of dominantly transmitted p.R120W GDAP1 gene mutations which can cause an axonal CMT with a wide clinical profile.     

Developmental defects and neuromuscular alterations due to mitofusin 2 gene (MFN2) silencing in zebrafish: a new model for Charcot-Marie-Tooth type 2A neuropathy

The development of new animal models is a crucial step in determining the pathological mechanism underlying neurodegenerative diseases and is essential for the development of effective therapies. We have investigated the zebrafish (Danio rerio) as a new model to study CMT2A, a peripheral neuropathy characterized by the selective loss of motor neurons, caused by mutations of mitofusin 2 gene. Using a knock-down approach, we provide evidence that during embryonic development, mitofusin 2 loss of function is responsible of several morphological defects and motility impairment. Immunohistochemical investigations, revealing the presence of severe alterations in both motor neurons and muscles fibres, indicated the central role played by MFN2 in axonal and neuromuscular development. Finally, we demonstrated the ability of human MFN2 to balance the downregulation of endogenous mfn2 in zebrafish, further supporting the conserved function of the MFN2 gene. These results highlight the essential role of mitofusin 2 in the motor axon development and demonstrate the potential of zebrafish as a suitable and complementary platform for dissecting pathogenetic mechanisms of MFN2 mutations in vivo.     

Characterizing the phenotypic manifestations of MFN2 R104W mutation in Charcot-Marie-Tooth type 2

Mutations of the mitofusin 2 (MFN2) gene have been reported to be the most common cause of the axonal form of Charcot-Marie-Tooth disease (CMT). The aim of this study was to describe a de novo MFN2 p.R104W mutation and characterize the associated phenotype. We screened the entire coding region of MFN2 gene and characterized its clinical phenotype, nerve conduction studies and sural nerve biopsy. Neuropsychological tests and brain MRI were also performed. A de novo mutation was found in exon 4 (c.310C > T; p.R104W). In addition to a severe and early onset axonal neuropathy, the patient presented learning problems, obesity, glucose intolerance, leukoencephalopathy, brain atrophy and evidence of myelin involvement and mitochondrial structural changes on sural nerve biopsy. These results suggest that MFN2 p.R104W mutation is as a hot-spot for MFN2 gene associated to a large and complex range of phenotypes.     

Charcot-Marie-Tooth disease: a novel Tyr145Ser mutation in the myelin protein zero (MPZ,P0) gene causes different phenotypes in homozygous and heterozygous carriers within one family

Charcot-Marie-Tooth disease type 1B (CMT 1B) is caused by mutations in the gene coding for peripheral myelin protein zero (MPZ, P0) that plays a fundamental role in adhesion and compaction of peripheral myelin. Here we report a Costa Rican family with a hereditary peripheral neuropathy due to a novel Tyr145Ser MPZ mutation. Four family members were heterozygously affected; two siblings of two heterozygous carriers were homozygous for this mutation. On neurological examination the heterozygous parents and their homozygous children both showed distal sensory deficits. The mother and the siblings displayed impaired deep tendon reflexes and mild sensory ataxia. The homozygous individuals were more severely affected with an earlier age of onset, distal motor weakness, and pupillary abnormalities. Electrophysiological studies revealed both signs of demyelination and axonal nerve degeneration. The sural nerve biopsy of one sibling showed thinly myelinated nerve fibers, onion bulb formation, and clusters of regenerating fibers. On electron microscopy axonal degeneration and decompaction of inner myelin layers were found. This Costa Rican family shows phenotypic variability depending on the homozygous or heterozygous state of the Tyr145Ser mutation carriers.A. Leal and C. Berghoff contributed equally to this work.     

Co-occurrence of amyotrophic lateral sclerosis and Charcot-Marie-Tooth disease type 2A in a patient with a novel mutation in the mitofusin-2 gene

Mitofusin-2 gene (MFN2) mutations cause Charcot-Marie-Tooth type 2A (CMT2A), sometimes complicated by additional features such as optic atrophy, hearing loss, upper motor neuron signs and cerebral white-matter abnormalities. Here we report, for the first time, the occurrence of motor neuron disease, consistent with amyotrophic lateral sclerosis (ALS), in a 62-year-old woman affected by early-onset slowly progressive CMT2A, due to a novel MFN2 mutation. After age 60, rate of disease progression changed and she rapidly developed generalised muscle wasting, weakness, and fasciculations, together with dysarthria and dysphagia. Clinical features, EMG findings, and fast progression were consistent with ALS superimposed on CMT.     

A cryptic splicing mutation in the INF2 gene causing Charcot‐Marie‐Tooth disease with minimal glomerular dysfunction

Heterozygous mutations in the inverted formin‐2 (INF2) gene provoke focal segmental glomerulosclerosis (FSGS) and intermediate Charcot‐Marie‐Tooth (CMT) disease with FSGS. Here, we report four patients from a three‐generation family with a new cryptic splicing INF2 mutation causing autosomal dominant intermediate CMT with minimal glomerular dysfunction. Three males and one female with a mean age of 51 years (26‐87) presented with a slowly progressive sensorimotor polyneuropathy, pes cavus, and kyphoscoliosis. Mean age at CMT disease onset was 11.5 years (3‐17), and electrophysiological studies showed demyelinating and axonal features consistent with intermediate CMT. Plasma albumin and creatinine were normal in all four cases, and urine protein was normal in one case and mildly raised in three patients (mean: 0.32 g/L [0.18‐0.44], N < 0.14). Genetic analysis found a c.271C > G (p. Arg91Gly) variation in INF2 exon 2, and in vitro splicing assays showed the deletion of the last 120 nucleotides of INF2 exon 2 leading to a 40 amino acids in‐frame deletion (p. Arg91_p. Gln130del). This report expands the genetic spectrum of INF2‐associated disorders and demonstrates that INF2 mutations may provoke isolated CMT with no clinically relevant kidney involvement. Consequently, INF2 mutation analysis should not be restricted to individuals with coincident neuropathy and renal disease.     

Small heat shock protein B3 (HSPB3) mutation in an axonal Charcot‐Marie‐Tooth disease family

Heat shock protein B3 (HSPB3) gene encodes a small heat‐shock protein 27‐like protein which has a high sequence homology with HSPB1. A mutation in the HSPB3 was reported as the putative underlying cause of distal hereditary motor neuropathy 2C (dHMN2C) in 2010. We identified a heterozygous mutation (c.352T>C, p.Tyr118His) in the HSPB3 from a Charcot‐Marie‐Tooth disease type 2 (CMT2) family by the method of targeted next generation sequencing. The mutation was located in the well conserved alpha‐crystalline domain, and several in silico predictions indicated a pathogenic effect of the mutation. Clinical and electrophysiological features of the patients indicated the axonal type of CMT. Clinical symptoms without sensory involvements were similar between the present family and the previous family. Mutations in the HSPB1 and HSPB8 genes have been reported to be relevant with both types of CMT2 and dHMN. Our findings will help in the molecular diagnosis of CMT2 by expanding the phenotypic range due to the HSPB3 mutations.     

A novel HSPB1 mutation associated with a late onset CMT2 phenotype: Case presentation and systematic review of the literature

Mutations in the HSPB1 gene are associated with Charcot‐Marie‐Tooth (CMT) disease type 2F (CMT2F) and distal hereditary motor neuropathy type 2 (dHMN2). More than 18 pathogenic mutations spanning across the whole HSPB1 gene have been reported. Three family members with a novel p.P57S (c.169C>T) HSPB1 mutation resulting in a late onset axonal neuropathy with heterogeneous clinical and electrophysiological features are detailed. We systematically reviewed published case reports and case series on HSPB1 mutations. While a genotype‐phenotype correlation was not obvious, we identified a common phenotype, which included adult onset, male predominance, motor more frequently than sensory involvement, distal and symmetric distribution with preferential involvement of plantar flexors, and a motor and axonal electrophysiological picture.     

Mitochondrial complex I deficiency in GDAP1-related autosomal dominant Charcot-Marie-Tooth disease (CMT2K)

Mutations in GDAP1, an outer mitochondrial membrane protein responsible for recessive Charcot-Marie-Tooth disease (CMT4A), have also been associated with CMT2K, a dominant form of the disease. The three CMT2K patients we studied carried a novel dominant GDAP1 mutation, C240Y (c.719G > A). Mitochondrial respiratory chain complex I activity in fibroblasts from CMT2K patients was 40% lower than in controls, whereas the tubular mitochondria were 33% larger in diameter and the mitochondrial mass was 20% greater. Thus, besides the regulatory role GDAP1 plays in mitochondrial network dynamics, it may also be involved in energy production and in the control of mitochondrial volume. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Julien Cassereau and Arnaud Chevrollier contributed equally to this work.     

A novel presenilin 2 gene mutation (D439A) in a patient with early-onset Alzheimer's disease.

The authors describe a novel missense mutation in the presenilin 2 (PSEN2) gene at residue 439 that predicts an aspartate-to-alanine substitution (D439A). This mutation was found in a 58-year old patient who displayed a progressive dementia at the age of 52. The mutation was absent in his cognitively normal relatives. Haplotype analysis indicated that his affected mother was the most probable mutation carrier. The D439A mutation is located near the C-terminal end of the PS2 protein, a region critical for endoproteolytic processing.