A novel de novo MFN2 mutation causing CMT2A with upper motor neuron signs

The phenotyping dilemma in VRK1-related motor neuron disease: a Turkish family with young-onset amyotrophic lateral sclerosis caused by a novel mutation

Objective: Vaccinia-related kinase 1 (VRK1)-related disease is an extremely rare autosomal recessive disorder primarily affecting the peripheral and/or central nervous system. In this report, we describe the genetic and clinical features of two siblings from a Turkish family presenting with an amyotrophic lateral sclerosis (ALS) phenotype due to a novel homozygous VRK1 mutation, and discuss the broad phenotypic spectrum associated with pathogenic variants in this gene. Methods: We analyzed the demographic data, clinical histories, neurological examinations, laboratory findings, and genetic results of 53 patients, including our cases, derived from 27 different reports. Results: Whole-exome sequencing identified a novel homozygous missense mutation, c.700A > G (p.Asn234Asp), in the VRK1 gene in two affected siblings. The characteristic features of the ALS phenotype included a recessive inheritance pattern, motor deficits with onset in the lower limbs, pyramidal tract signs, and a muscle magnetic resonance imaging (MRI) pattern demonstrating preferential involvement of the posterior compartments of the leg and thigh. The most common phenotypes associated with VRK1 mutations were ALS (18/53, 34%) and distal hereditary motor neuropathy (dHMN) (14/53, 26.4%), followed by pontocerebellar hypoplasia type 1 (7/53, 13.2%), hereditary motor and sensory neuropathy (5/53, 9.4%), autosomal recessive primary microcephaly with brain malformations (4/53, 7.5%), and spastic paraplegia (2/53, 3.8%). The ALS phenotype exhibited a significantly earlier mean age of onset compared to the dHMN phenotype (p = 0.015; 15.3 ± 11.5 and 27 ± 15.5 years, respectively). Conclusion: Our findings highlight the importance of investigating VRK1 mutations in patients with young-onset familial ALS. Furthermore, this report provides a systematic classification of the phenotype definitions associated with VRK1 mutations.

The Role of Impaired Mitochondrial Dynamics in MFN2-Mediated Pathology

The Mitofusin 2 protein (MFN2), encoded by the MFN2 gene, was first described for its role in mediating mitochondrial fusion. However, MFN2 is now recognized to play additional roles in mitochondrial autophagy (mitophagy), mitochondrial motility, lipid transfer, and as a tether to other organelles including the endoplasmic reticulum (ER) and lipid droplets. The tethering role of MFN2 is an important mediator of mitochondrial-ER contact sites (MERCs), which themselves have many important functions that regulate mitochondria, including calcium homeostasis and lipid metabolism. Exemplifying the importance of MFN2, pathogenic variants in MFN2 are established to cause the peripheral neuropathy Charcot-Marie-Tooth Disease Subtype 2A (CMT2A). However, the mechanistic basis for disease is not clear. Moreover, additional pathogenic phenotypes such as lipomatosis, distal myopathy, optic atrophy, and hearing loss, can also sometimes be present in patients with CMT2A. Given these variable patient phenotypes, and the many cellular roles played by MFN2, the mechanistic underpinnings of the cellular impairments by which MFN2 dysfunction leads to disease are likely to be complex. Here, we will review what is known about the various functions of MFN2 that are impaired by pathogenic variants causing CMT2A, with a specific emphasis on the ties between MFN2 variants and MERCs.

The Genotype and Phenotype Features in a Large Chinese MFN2 Mutation Cohort

Introduction: Charcot–Marie–Tooth disease type 2A (CMT2A) is a group of clinically and genetically heterogeneous disorders, which is mostly caused by mutations of the mitofusin2 (MFN2) gene. As the genotype–phenotype characteristics of CMT2A were still incompletely understood, we further explored the spectrum of CMT2A variants in China and demonstrated their phenotypic diversities.

Methods: A total of 402 index patients/families with CMT throughout Mainland China were enrolled in this study. Among them, we analyzed 20 unrelated index cases with CMT2A by Sanger sequencing, next-generation sequencing, or whole-exome sequencing. Detailed clinical and genetic features of CMT2A patients were collected and analyzed. Of note, de novo mutations were not rare in MFN2 gene; we compared the clinical features of patients from the de novo group with those from the non-de novo group.

Results: We identified 20 MFN2 variants, occupying 5.0% of CMT. Most patients presented with early onset and moderate phenotype with abnormal gait and foot drop as the main complaints at onset. Pyramidal signs accounts for 31.6% (6/19) in all patients, which is not uncommon. Four novel variants (p.Tyr752^*, c.475-2A>G, p.Val99Met, and p.Arg275_Gln276insArg) were identified in the cohort. Besides, de novo variants occupied 35.0% (7/20) in our study with a much earlier age at onset compared with those in the non-de novo group (p = 0.021).

Conclusion: Chinese CMT2A is a predominant typical pure CMT2A, with early onset and mild to moderate phenotype. Given the high frequency of de novo MFN2 mutations, genetic study should be considered for patients with early onset and severe idiopathic axonal neuropathy.

Natural history of Charcot-Marie-Tooth disease type 2A: a large international multicentre study

Mitofusin-2 (MFN2) is one of two ubiquitously expressed homologous proteins in eukaryote cells, playing a critical role in mitochondrial fusion. Mutations in MFN2 (most commonly autosomal dominant) cause Charcot-Marie-Tooth disease type 2A (CMT2A), the commonest axonal form of CMT, with significant allelic heterogeneity. Previous, moderately-sized, cross sectional genotype-phenotype studies of CMT2A have described the phenotypic spectrum of the disease, but longitudinal natural history studies are lacking. In this large multicentre prospective cohort study of 196 patients with dominant and autosomal recessive CMT2A, we present an in-depth genotype-phenotype study of the baseline characteristics of patients with CMT2A and longitudinal data (1–2 years) to describe the natural history. A childhood onset of autosomal dominant CMT2A is the most predictive marker of significant disease severity and is independent of the disease duration. When compared to adult onset autosomal dominant CMT2A, it is associated with significantly higher rates of use of ankle-foot orthoses, full-time use of wheelchair, dexterity difficulties and also has significantly higher CMT Examination Score (CMTESv2) and CMT Neuropathy Score (CMTNSv2) at initial assessment. Analysis of longitudinal data using the CMTESv2 and its Rasch-weighted counterpart, CMTESv2-R, show that over 1 year, the CMTESv2 increases significantly in autosomal dominant CMT2A (mean change 0.84 ± 2.42; two-tailed paired t-test P = 0.039). Furthermore, over 2 years both the CMTESv2 (mean change 0.97 ± 1.77; two-tailed paired t-test P = 0.003) and the CMTESv2-R (mean change 1.21 ± 2.52; two-tailed paired t-test P = 0.009) increase significantly with respective standardized response means of 0.55 and 0.48. In the paediatric CMT2A population (autosomal dominant and autosomal recessive CMT2A grouped together), the CMT Pediatric Scale increases significantly both over 1 year (mean change 2.24 ± 3.09; two-tailed paired t-test P = 0.009) and over 2 years (mean change 4.00 ± 3.79; two-tailed paired t-test P = 0.031) with respective standardized response means of 0.72 and 1.06. This cross-sectional and longitudinal study of the largest CMT2A cohort reported to date provides guidance for variant interpretation, informs prognosis and also provides natural history data that will guide clinical trial design.

Clinical, Genetic, and Disability Profile of Pediatric Distal Hereditary Motor Neuropathy

OBJECTIVE

To describe the clinical, genetic, and disability profile of pediatric distal hereditary motor neuropathy (dHMN) and to determine the utility of an outcome measure validated for children with Charcot-Marie-Tooth disease (CMT) in assessing disability in this cohort.

METHODS

We reviewed the clinical, neurophysiologic, and disability data on individuals with dHMN, evaluated before the age of 20 years, at 2 tertiary neuromuscular clinics in Australia and Spain. Disability was assessed annually with the CMT Pediatric Scale (CMTPedS) in a subset of individuals.

RESULTS

Twenty-two children (13 female) from 19 families were included. Fourteen individuals were symptomatic in the first year of life. Intellectual disability was present in 6 individuals; upper motor neuron signs were seen in 8. Pathogenic variants were found in 9 families, more frequently in BICD2 (BICD2-4, DYNC1H1-2, MFN2-2, GARS-1). A novel pathogenic variant in the GARS gene was detected and characterized phenotypically. Disability was moderate on the CMTPedS (mean [SD] 18.2 [6.3], n = 16), with balance and long jump being the most affected and sensation items and grip strength the least affected. Over 1 year, the CMTPedS total score deteriorated, on average 1.5 points (SD 3.7) or 9% (n = 12), with significant variability in the rate of progression within the cohort.

CONCLUSIONS

The genetic profile of pediatric dHMN is different from that identified in adult cohorts. This study has identified distinct functional limitations for the CMTPedS in children and adolescents with dHMN.

Molecular modelling of mitofusin 2 for a prediction for Charcot-Marie-Tooth 2A clinical severity

Charcot-Marie-Tooth disease type 2A (CMT2A) is an autosomal dominant neuropathy caused by mutations in the mitofusin 2 gene (MFN2). More than 100 MFN2 gene mutations have been reported so far, with majority located within the GTPase domain encoding region. These domain-specific mutations present wide range of symptoms with differences associated with distinct amino acid substitutions in the same position. Due to the lack of conclusive phenotype-genotype correlation the predictive value of genetic results remains still limited. We have explored whether changes in the protein structure caused by MFN2 mutations can help to explain diseases phenotypes. Using a stable protein model, we evaluated the effect of 26 substitutions on the MFN2 structure and predicted the molecular consequences of such alterations. The observed changes were correlated with clinical features associated with a given mutation. Of all tested mutations positive correlation of molecular modelling with the clinical features reached 73%. Our analysis revealed that molecular modelling of mitofusin 2 mutations is a powerful tool, which predicts associated pathogenic impacts and that these correlate with clinical outcomes. This approach may aid an early diagnosis and prediction of symptoms severity in CMT2A patients.

MFN2-related neuropathies: Clinical features, molecular pathogenesis and therapeutic perspectives

Mitofusin 2 (MFN2) is a GTPase dynamin-like protein of the outer mitochondrial membrane, encoded in the nuclear genome by the MFN2 gene located on the short (p) arm of chromosome 1. MFN2 protein is involved in several intracellular pathways, but is mainly involved in a network that has an essential role in several mitochondrial functions, including fusion, axonal transport, interorganellar communication and mitophagy. Mutations in the gene encoding MFN2 are associated with Charcot-Marie-Tooth disease type 2A (CMT2A), a neurological disorder characterized by a wide clinical phenotype that involves the central and peripheral nervous system. Here, we present the clinical, genetic and neuropathological features of human diseases associated with MFN2 mutations. We also report proposed pathogenic mechanisms through which MFN2 mutations likely contribute to the development of neurodegeneration. MFN2-related disorders may occur more frequently than previously considered, and they may represent a paradigm for the study of the defective mitochondrial dynamics that seem to play a significant role in the molecular and cellular pathogenesis of common neurodegenerative diseases; thus they may also lead to the identification of related therapeutic targets.

Acute optic neuropathy associated with a novel MFN2 mutation

Mutations in the mitofusin 2 (MFN2) gene cause CMT2A the most common form of autosomal dominant axonal Charcot-Marie-Tooth (CMT). In addition, mutations in MFN2 have been shown to be responsible for Hereditary Motor Sensory Neuropathy type VI (HSMN VI), a rare early-onset axonal CMT associated with optic neuropathy. Most reports of HMSN VI presented with a sub-acute form of optic neuropathy. Herein, we report a CMT2A patient, who developed very rapidly progressing severe optic neuropathy. A 40-year-old Caucasian man was evaluated for gait disturbance and lower limbs weakness, slowly progressed over the last 2 years. Due to clinical data and family history, a diagnosis of CMT2 was made. The novel heterozygous c.775C > T (p.Arg259Cys) mutation in MFN2 was detected in the patient and his clinical affected mother. Interestingly, the patient developed a severe sudden bilateral visual deterioration few years early, with clinical and instrumental picture suggestive of acute bilateral optic neuropathy. Our report expands the spectrum of MFN2-related manifestation because it indicates that visual symptoms of HMSN VI may enter in the differential with acquired or hereditary acute optic neuropathies, and that severe optic neuropathy is not invariably an early manifestation of the disease but may occur as disease progressed. This report could have an impact on clinicians who evaluate patients with otherwise unexplainable bilateral acute-onset optic neuropathy, especially if associated with a motor and sensory axonal neuropathy.

Mitochondrial dynamics and inherited peripheral nerve diseases

Peripheral nerves have peculiar energetic requirements because of considerable length of axons and therefore correct mitochondria functioning and distribution along nerves is fundamental. Mitochondrial dynamics refers to the continuous change in size, shape, and position of mitochondria within cells. Abnormalities of mitochondrial dynamics produced by mutations in proteins involved in mitochondrial fusion (mitofusin-2, MFN2), fission (ganglioside-induced differentiation-associated protein-1, GDAP1), and mitochondrial axonal transport usually present with a Charcot-Marie-Tooth disease (CMT) phenotype. MFN2 mutations cause CMT type 2A by altering mitochondrial fusion and trafficking along the axonal microtubule system. CMT2A is an axonal autosomal dominant CMT type which in most cases is characterized by early onset and rather severe course. GDAP1 mutations also alter fission, fusion and transport of mitochondria and are associated either with recessive demyelinating (CMT4A) and axonal CMT (AR-CMT2K) and, less commonly, with dominant, milder, axonal CMT (CMT2K). OPA1 (Optic Atrophy-1) is involved in fusion of mitochondrial inner membrane, and its heterozygous mutations lead to early-onset and progressive dominant optic atrophy which may be complicated by other neurological symptoms including peripheral neuropathy. Mutations in several proteins fundamental for the axonal transport or forming the axonal cytoskeleton result in peripheral neuropathy, i.e., CMT, distal hereditary motor neuropathy (dHMN) or hereditary sensory and autonomic neuropathy (HSAN), as well as in hereditary spastic paraplegia. Indeed, mitochondrial transport involves directly or indirectly components of the kinesin superfamily (KIF5A, KIF1A, KIF1B), responsible of anterograde transport, and of the dynein complex and related proteins (DYNC1H1, dynactin, dynamin-2), implicated in retrograde flow. Microtubules, neurofilaments, and chaperones such as heat shock proteins (HSPs) also have a fundamental role in mitochondrial transport and mutations in some of related encoding genes cause peripheral neuropathy (TUBB3, NEFL, HSPB1, HSPB8, HSPB3, DNAJB2). In this review, we address the abnormalities in mitochondrial dynamics and their role in determining CMT disease and related neuropathies.