Late‐onset hereditary sensory and autonomic neuropathy expands the phenotypic spectrum of MFN2‐related diseases

Mutations in the Mitofusin 2 (MFN2) gene have been identified in patients with autosomal dominant axonal motor and sensory neuropathy or Charcot–Marie‐Tooth 2A (CMT2A). Here we describe clinical and pathological changes in an adult patient with sporadic hereditary sensory and autonomic neuropathy (HSAN) due to an MFN2 mutation. The patient was a 53‐year‐old man who had sensory involvement and anhidrosis in all limbs without motor features. The electrophysiological assessment documented severe axonal sensory neuropathy. The sural nerve biopsy confirmed the electrophysiological findings, revealing severe loss of myelinated and unmyelinated fibers with regeneration clusters. Genetic analysis revealed the previously identified mutation c.776 G > A in MFN2. Our report expands the phenotypic spectrum of MFN2‐related diseases. Sequencing of MFN2 should be considered in all patients presenting with late‐onset HSAN.     

Mutational mechanisms in MFN2‐related neuropathy: compound heterozygosity for recessive and semidominant mutations

Mitofusin‐2 (MFN2) mutations are the most common cause of autosomal dominant axonal Charcot‐Marie‐Tooth disease (CMT, type 2A), sometimes complicated by additional features such as optic atrophy (CMT6) and upper motor neuron involvement (CMT5). Several pathogenic mutations are reported, mainly acting in a dominant fashion, although few sequence variants behaved as recessive or semidominant in rare homozygous or compound heterozygous patients. We describe a 49‐year‐old woman with CMT5 associated with compound heterozygosity for two MFN2 variants, one already reported missense mutation (c.748C>T, p.R250W) and a novel nonsense sequence change (c.1426C>T, p.R476*). Her mother, carrying the p.R250W variant, had very late‐onset minimal axonal neuropathy, whilst the father harboring the nonsense sequence change had neither clinical nor electrophysiological neuropathy. The missense mutation is likely pathogenic according to in silico analyses and a previous report, while the nonsense variant is predicted to behave as a null allele. The p.R250W variant behaves as semidominant by causing only a mild, almost subclinical, neuropathy when heterozygous; the nonsense mutation in the father was phenotypically silent, suggesting that haploinsufficiency for MFN2 is not disease causative, but was deleterious in the daughter who had only one active mutated MFN2 allele.     

Clinical and genetic diversities of Charcot‐Marie‐Tooth disease with MFN2 mutations in a large case study

Charcot‐Marie‐Tooth disease (CMT) constitutes a heterogeneous group affecting motor and sensory neurons in the peripheral nervous system. MFN2 mutations are the most common cause of axonal CMT. We describe the clinical and mutational spectra of CMT patients harboring MFN2 mutations in Japan. We analyzed 1,334 unrelated patients with clinically suspected CMT referred by neurological and neuropediatric departments throughout Japan. We conducted mutation screening using a DNA microarray, targeted resequencing, and whole‐exome sequencing. We identified pathogenic or likely pathogenic MFN2 variants from 79 CMT patients, comprising 44 heterozygous and 1 compound heterozygous variants. A total of 15 novel variants were detected. An autosomal dominant family history was determined in 43 cases, and the remaining 36 cases were reported as sporadic with no family history. The mean onset age of CMT in these patients was 12 ± 14 (range 0–59) years. We observed neuropathic symptoms in all patients. Some had optic atrophy, vocal cord paralysis, or spasticity. We detected a compound heterozygous MFN2 mutation in a patient with a severe phenotype and the co‐occurrence of MFN2 and PMP22 mutations in a patient with an uncommon phenotype. MFN2 is the most frequent causative gene of CMT2 in Japan. We present 15 novel variants and broad clinical and mutational spectra of Japanese MFN2‐related CMT patients. Regardless of the onset age and inheritance pattern, MFN2 gene analysis should be performed. Combinations of causative genes should be considered to explain the phenotypic diversity.     

Exome sequencing reveals mutations in MFN2 and GDAP1 in severe Charcot–Marie–Tooth disease

The aim of our study was to characterize electrophysiologically and explain the genetic cause of severe Charcot–Marie–Tooth (CMT) in a 3.5‐year‐old with asymptomatic parents and a maternal grandfather with a history of mild adult‐onset axonal neuropathy. Severity of neuropathy was assessed by Charcot–Marie–Tooth neuropathy score (CMTNS). Whole‐exome sequencing was performed using an Illumina TruSeq Exome Enrichment Kit on the HiSeq 1500 with results followed up by Sanger sequencing on an ABI Prism 3500XL (Applied Biosystems, Foster City, CA, USA). Paternity was confirmed using a panel of 15 hypervariable markers. Electrophysiological studies demonstrated severe axonal sensory‐motor neuropathy in the proband, mild motor neuropathy in his mother, and mild sensory‐motor neuropathy in his grandfather. CMTNS in the proband, his mother, and grandfather was 21, 1, and 12, respectively. On genetic analysis, the boy was found to carry a heterozygous dominant MFN2 T236M mutation transmitted via the maternal line and a de novo GDAP1 H123R mutation. Our findings emphasize the need to search for more than one causative mutation when significant intrafamilial variability of CMT phenotype occurs and underline the role of whole‐exome sequencing in the diagnosis of compound forms of CMT disease.     

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     

Rigid spine syndrome associated with sensory‐motor axonal neuropathy resembling Charcot–Marie‐Tooth disease is characteristic of Bcl‐2‐associated athanogene‐3 gene mutations even without cardiac involvement

Introduction: Bcl‐2‐associated athanogene‐3 (BAG3) mutations have been described in rare cases of rapidly progressive myofibrillar myopathies. Symptoms begin in the first decade with axial involvement and contractures and are associated with cardiac and respiratory impairment in the second decade. Axonal neuropathy has been documented but usually not as a key clinical feature. Methods: We report a 24‐year‐old woman with severe rigid spine syndrome and sensory‐motor neuropathy resembling Charcot–Marie–Tooth disease (CMT). Results: Muscle MRI showed severe fat infiltration without any specific pattern. Deltoid muscle biopsy showed neurogenic changes and discrete myofibrillar abnormalities. Electrocardiogram and transthoracic echocardiography results were normal. Genetic analysis of a panel of 45 CMT genes showed no mutation. BAG3 gene screening identified the previously reported c.626C>T, pPro209Leu, mutation. Discussion: This case indicates that rigid spine syndrome and sensory‐motor axonal neuropathy are key clinical features of BAG3 mutations that should be considered even without cardiac involvement. Muscle Nerve, 57 : 330–334, 2018     

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.     

Autosomal recessive Charcot‐Marie‐Tooth disease: from genes to phenotypes

The prevalence of Charcot‐Marie‐Tooth (CMT) disease or hereditary motor and sensory neuropathy (HMSN) varies in different populations. While in some countries of Western Europe, the United States and Japan the dominant form of HMSN is the most frequent, in other countries such as those of the Mediterranean Basin, the autosomal recessive form (AR‐CMT) is more common. Autosomal recessive CMT cases are generally characterized by earlier onset, usually before the age of 2 or 3 years, and rapid clinical progression that results in severe polyneuropathy and more marked distal limb deformities such as pes equino‐varus, claw‐like hands, and often major spinal deformities. Recent clinical, morphological and molecular investigations of CMT families with autosomal recessive inheritance allowed the identification of many genes such as GDAP1, MTMR2, SBF2, NDRG1, EGR2, SH3TC2, PRX, FGD4, and FIG4, implicated in demyelinating forms (ARCMT1 or CMT4), and LMNA, MED25, HINT1, GDAP1, LRSAM1, NEFL, HSPB1 and MFN2 in axonal forms (ARCMT2). However, many patients remain without genetic diagnosis to date, prompting investigations into ARCMT families in order to help discover new genes and common pathways. This review summarizes recent advances regarding the genotypes and corresponding phenotypes of AR‐CMT.     

Spinal charcot‐marie‐tooth disease: A reappraisal

Introduction: Distal hereditary motor neuropathy (dHMN) is characterized by isolated distal muscle atrophy without sensory deficit. Nevertheless, clinical sensory loss has been reported despite preserved sensory nerve conduction in a few patients, thus differentiating these cases from the classical type 2 Charcot‐Marie‐Tooth disease (CMT2). Methods: We report 4 patients who presented with clinical sensory and motor neuropathy and normal peripheral sensory nerve conduction studies and were investigated with complete electrophysiological studies, including somatosensory evoked potentials (SEP). Results: These patients had a clinical presentation of classical CMT with isolated axonal motor neuropathy suggestive of dHMN. Interestingly, tibial nerve SEPs showed abnormalities suggestive of proximal involvement of dorsal roots that may explain the clinical somatosensory disturbances. Conclusions: These cases support the concept of spinal CMT that should be recognized as an intermediate form between dHMN and CMT2. SEP recording was helpful in defining a more precise phenotype of spinal CMT. Muscle Nerve 46: 603–607, 2012     

Mechanisms of disease and clinical features of mutations of the gene for mitofusin 2: an important cause of hereditary peripheral neuropathy with striking clinical variability in children and adults

Mitofusin 2, a large transmembrane GTPase located in the outer mitochondrial membrane, promotes membrane fusion and is involved in the maintenance of the morphology of axonal mitochondria. Mutations of the gene encoding mitofusin 2 (MFN2) have recently been identified as the cause of approximately one‐third of dominantly inherited cases of the axonal degenerative forms of Charcot–Marie–Tooth disease (CMT type 2A) and of rarer variants. The latter include a severe, early‐onset axonal neuropathy, which may occur in autosomal dominant or recessive forms, as well as some instances associated with pyramidal tract involvement (CMT type 5), with optic atrophy (CMT type 6), and, occasionally, with alterations of cerebral white matter. All individuals with a dominantly or recessively inherited or otherwise unexplained, chronic progressive axonal degenerative polyneuropathy should be tested for mutations of MFN2.     

A mutation in the heptad repeat 2 domain of MFN2 in a large CMT2A family

Dominant mutations in MFN2 cause a range of phenotypes, including severe, early‐onset axonal neuropathy, “classical CMT2,” and late‐onset axonal neuropathies. We report a large family with an axonal polyneuropathy, with clinical onset in the 20s, followed by slow progression.     

Clinical features of inherited neuropathy with BSCL2 mutations in Japan

Heterozygous mutations in the Berardinelli‐Seip congenital lipodystrophy 2 (BSCL2) gene have been reported with different clinical phenotypes including Silver syndrome (SS)/spastic paraplegia 17 (SPG17), distal hereditary motor neuropathy type V (dHMN‐V), and Charcot‐Marie‐Tooth (CMT) disease type 2. We screened 407 Japanese patients who were clinically suspected of having CMT by exome sequencing and searched mutations in BSCL2. As a result, we identified five patients with heterozygous mutations in BSCL2. We confirmed three cases of known mutations (p.N88S and p.S90L) and two cases of novel mutations (p.N88T and p.S141A). The clinical features of the cases with known mutations in Japan were similar to those previously reported in other countries. In particular, there were many cases with sensory disturbance. The case with p.N88T mutation showed severe phenotype such as early onset age and prominent vocal cord paresis. The case with p.S141A mutation showed characteristics of demyelinating neuropathy such as CMT disease type 1 by electrophysiological examination. In this article, we report the clinical features and spread of cases with BSCL2 mutation in a Japanese cohort.     

A novel pathogenic variant of NEFL responsible for deafness associated with peripheral neuropathy discovered through next‐generation sequencing and review of the literature

Neurofilaments are neuron‐specific intermediate filaments essential for the radial growth of axons during development and the maintenance of axonal diameter. Pathogenic variants of Neurofilament Light (NEFL) are associated with CMT1F, CMT2E, and CMTDIG and have been observed in less than 1% of Charcot‐Marie‐Tooth (CMT) cases, resulting in the reporting of 35 variants in 173 CMT patients to date. However, only six variants have been reported in 17 patients with impaired hearing. No genotype‐phenotype correlations have yet been established. Here, we report an additional case: a 69‐year‐old female, who originally presented with axonal sensory and motor neuropathy at the age of 45, associated with moderate sensorineural hearing loss, with a slight slope at high frequencies. Next‐generation sequencing identified a novel pathogenic variant: c.269A > G, p.(Glu90Gly). Hearing impairment is often linked to CMT due to pathogenic variants of NEFL, especially p.(Glu90Lys) and p.(Asn98Ser), and in our case p.(Glu90Gly). These pathogenic variants are all located at hot spots, in the head domain and the two ends of the rod domain of the protein.     

A novel NDRG1 mutation in a non‐Romani patient with CMT4D/HMSN‐Lom

Charcot‐Marie‐Tooth disease type 4D (CMT4D), also known as hereditary motor and sensory neuropathy Lom type (HMSNL), is an autosomal recessive, early onset, severe demyelinating neuropathy with hearing loss, caused by N‐Myc downstream‐regulated gene 1 (NDRG1) mutations. CMT4D is rare with only three known mutations, one of which (p.Arg148Ter) is found in patients of Romani ancestry and accounts for the vast majority of cases. We report a 38‐year‐old Italian female with motor development delay, progressive neuropathy, and sensorineural deafness. Magnetic resonance imaging showed slight atrophy of cerebellum, medulla oblongata, and upper cervical spinal cord. She had a novel homozygous NDRG1 frameshift mutation (c.739delC; p.His247ThrfsTer74). The identification of this NDRG1 mutation confirms that CMT4D is not a private Romani disease and should be considered in the differential diagnosis of recessive demyelinating CMT.     

Mutations in the mitofusin 2 gene are the most common cause of Charcot-Marie-Tooth type 2 disease

In contrast to Charcot-Marie-Tooth type 1 disease (CMT1), which is most commonly caused by 17p11.2-p12 duplication (in 70% of CMT1 cases), the axonal form of hereditary motor and sensory neuropathy (CMT2) seemed to be a genetically heterogeneous disease group, with no single gene playing a major pathogenetic role. In 2004, 10 mutations were identified in CMT2A families in the MFN2 gene coding for the mitochondrial protein mitofusin-2, previously mapped to the 1p35-36 locus. In the last two years, MFN2 gene mutations were shown to be the most common cause of autosomal dominant hereditary axonopathy. In addition, MFN2 gene mutations were also identified in CMT type 6 (axonal neuropathy with optic nerve atrophy). Recent reports indicate that some MFN2 gene mutations may by inherited as autosomal recessive traits. As MFN2 gene mutations are the most common cause of autosomal dominant CMT2 disease (33% of cases), MFN2 gene testing may be considered a diagnostic test for CMT2.     

Clinical and genetic spectra of Charcot‐Marie‐Tooth disease in Chinese Han patients

Charcot‐Marie‐Tooth disease (CMT) is a common hereditary motor and sensory neuropathy. Epidemiological data for Chinese CMT patients are few. This study aimed to analyze the electrophysiological and genetic characteristics of Chinese Han patients. A total of 106 unrelated patients with the clinical diagnosis of CMT were included. Clinical examination, nerve conduction studies (NCS), next‐generation sequencing (NGS), and bioinformatic analyses were performed. Genetic testing was performed for 82 patients; 27 (33%) patients carried known CMT‐associated gene mutations. PMP22 duplication was detected in 10 (12%) patients and GJB1 mutations in 9 (11%) patients. The mutation rate was higher in patients with a positive family history than in the sporadic cases (50% vs. 27%, p < 0.05). Six novel CMT‐associated gene mutations including BSCL2 (c.461C>T), LITAF (c.32C>G), MFN2 (c.497C>T), GARS (c.794C>T), NEFL (c.280C>T), and MPZ (c.440T>C) were discovered. All except the LITAF (c.32C>G) mutation were identified as “disease causing” via bioinformatic analyses. In this Chinese Han population, the frequency of PMP22 gene duplication in those with CMT1 was slightly (50% vs. 70%–80%) less than in Western/Caucasian populations. The novel CMT‐associated gene mutations broaden the mutation diversity of CMT1. NGS should be considered for genetic analyses in CMT patients.     

MFN2 deletion of exons 7 and 8: founder mutation in the UK population

Mitofusin 2 (MFN2) mutations are the most common cause of axonal Charcot‐Marie‐Tooth disease (CMT2). The majority are inherited in an autosomal dominant manner but recessive and semi‐dominant kindreds have also been described. We previously reported a deletion of exons 7 and 8 resulting in nonsense‐mediated decay, segregating with disease when present in trans with another pathogenic MFN2 mutation. Detailed clinical and electrophysiological data on a series of five affected patients from four kindreds and, when available, their parents and relatives were collected. MFN2 Sanger sequencing, multiplex ligation probe amplification, and haplotype analysis were performed. A severe early‐onset CMT phenotype was seen in all cases: progressive distal weakness, wasting, and sensory loss from infancy or early childhood. Optic atrophy (four of five) and wheelchair dependency in childhood were common (four of five). All were compound heterozygous for a deletion of exons 7 and 8 in MFN2 with another previously reported pathogenic mutation (Phe216Ser, Thr362Met, and Arg707Trp). Carrier parents and relatives were unaffected (age range: 24–82 years). Haplotype analysis confirmed that the deletion had a common founder in all families.     

Whole‐exome sequencing reveals a novel missense mutation in the MARS gene related to a rare Charcot‐Marie‐Tooth neuropathy type 2U

Charcot‐Marie‐Tooth (CMT) is a heterogeneous group of progressive disorders, characterized by chronic motor and sensory polyneuropathy. This hereditary disorder is related to numerous genes and varying inheritance patterns. Thus, many patients do not reach a final genetic diagnosis. We describe a 13‐year‐old girl presenting with progressive bilateral leg weakness and gait instability. Extensive laboratory studies and spinal magnetic resonance imaging scan were normal. Nerve conduction studies revealed severe lower limb peripheral neuropathy with prominent demyelinative component. Following presumptive diagnosis of chronic inflammatory demyelinating polyneuropathy, the patient received treatment with steroids and intravenous immunoglobulins courses for several months, with no apparent improvement. Whole‐exome sequencing revealed a novel heterozygous c.2209C>T (p.Arg737Trp) mutation in the MARS gene (OMIM 156560). This gene has recently been related to CMT type 2U. In‐silico prediction programs classified this mutation as a probable cause for protein malfunction. Allele frequency data reported this variant in 0.003% of representative Caucasian population. Family segregation analysis study revealed that the patient had inherited the variant from her 60‐years old mother, reported as healthy. Neurologic examination of the mother demonstrated decreased tendon reflexes, while nerve conduction studies were consistent with demyelinative and axonal sensory‐motor polyneuropathy. Our report highlights the importance of next‐generation sequencing approach to facilitate the proper molecular diagnosis of highly heterogeneous neurologic disorders. Amongst other numerous benefits, this approach might prevent unnecessary diagnostic testing and potentially harmful medical treatment.     

Mutation screening of <Emphasis Type="Italic">mitofusin 2</Emphasis> in Charcot-Marie-Tooth disease type 2

Charcot-Marie-Tooth (CMT) disease is among the most common inherited neurological disorders. Mutations in the gene mitofusin 2 (MFN2) cause the axonal subtype CMT2A, which has also been shown to be associated with optic atrophy, clinical signs of first motor neuron involvement, and early onset stroke. Mutations in MFN2 account for up to 20–30% of all axonal CMT type 2 cases. To further investigate the prevalence of MFN2 mutations and to add to the genotypic spectrum, we sequenced all exons of MFN2 in a cohort of 39 CMT2 patients. We identified seven variants, four of which are novel. One previously described change was co-inherited with a PMP22 duplication, which itself causes the demyelinating form CMT1A. Another mutation was a novel in frame deletion, which is a rare occurrence in the genotypic spectrum of MFN2 characterized mainly by missense mutations. Our results confirm a MFN2 mutation rate of ~15–20% in CMT2.