MFN2-related genetic and clinical features in a cohort of Chinese CMT2 patients

An integrative analysis of genotype-phenotype correlation in Charcot Marie Tooth type 2A disease with MFN2 variants: A case and systematic review

Dominant genetic variants in the mitofusin 2 (MFN2) gene lead to Charcot-Marie-Tooth type 2A (CMT2A), a neurodegenerative disease caused by genetic defects that directly damage axons. In this study, we reported a proband with a pathogenic variant in the GTPase domain of MFN2, c.494A > G (p.His165Arg). To date, at least 184 distinct MFN2 variants identified in 944 independent probands have been reported in 131 references. However, the field of medical genetics has long been challenged by how genetic variation in the MFN2 gene is associated with disease phenotypes. Here, by collating the MFN2 variant data and patient clinical information from Leiden Open Variant Database 3.0, NCBI clinvar database, and available related references in PubMed, we determined the mutation frequency, age of onset, sex ratio, and geographical distribution. Furthermore, the results of an analysis examining the relationship between variants and phenotypes from multiple genetic perspectives indicated that insertion and deletions (indels), copy number variants (CNVs), duplication variants, and nonsense mutations in single nucleotide variants (SNVs) tend to be pathogenic, and the results emphasized the importance of the GTPase domain to the structure and function of MFN2. Overall, three reliable classification methods of MFN2 genotype-phenotype associations provide insights into the prediction of CMT2A disease severity. Of course, there are still many MFN2 variants that have not been given clear clinical significance, which requires clinicians to make more accurate clinical diagnoses.

Clinical genetics of Charcot-Marie-Tooth disease

Recent research in the field of inherited peripheral neuropathies (IPNs) such as Charcot-Marie-Tooth (CMT) disease has helped identify the causative genes provided better understanding of the pathogenesis, and unraveled potential novel therapeutic targets. Several reports have described the epidemiology, clinical characteristics, molecular pathogenesis, and novel causative genes for CMT/IPNs in Japan. Based on the functions of the causative genes identified so far, the following molecular and cellular mechanisms are believed to be involved in the causation of CMTs/IPNs: myelin assembly, cytoskeletal structure, myelin-specific transcription factor, nuclear related, endosomal sorting and cell signaling, proteasome and protein aggregation, mitochondria-related, motor proteins and axonal transport, tRNA synthetases and RNA metabolism, and ion channel-related mechanisms. In this article, we review the epidemiology, genetic diagnosis, and clinicogenetic characteristics of CMT in Japan. In addition, we discuss the newly identified novel causative genes for CMT/IPNs in Japan, namely MME and COA7. Identification of the new causes of CMT will facilitate in-depth characterization of the underlying molecular mechanisms of CMT, leading to the establishment of therapeutic approaches such as drug development and gene therapy.

Mitofusin 2 mutation drives cell proliferation in Charcot-Marie-Tooth 2A fibroblasts

Dominant mutations in ubiquitously expressed mitofusin 2 gene (MFN2) cause Charcot-Marie-Tooth type 2A (CMT2A; OMIM 609260), an inherited sensory-motor neuropathy that affects peripheral nerve axons. Mitofusin 2 protein has been found to take part in mitochondrial fusion, mitochondria-endoplasmic reticulum tethering, mitochondrial trafficking along axons, mitochondrial quality control and various types of cancer, in which MFN2 has been indicated as a tumor suppressor gene. Discordant data on the mitochondrial altered phenotypes in patient-derived fibroblasts harboring MFN2 mutations and in animal models have been reported. We addressed some of these issues by focusing on mitochondria behavior during autophagy and mitophagy in fibroblasts derived from a CMT2AMFN2 patient with an MFN2650G > T/C217F mutation in the GTPase domain. This study investigated mitochondrial dynamics, respiratory capacity and autophagy/mitophagy, to tackle the multifaceted MFN2 contribution to CMT2A pathogenesis. We found that MFN2 mutated fibroblasts showed impairment of mitochondrial morphology, bioenergetics capacity, and impairment of the early stages of autophagy, but not mitophagy. Unexpectedly, transcriptomic analysis of mutated fibroblasts highlighted marked differentially expressed pathways related to cell population proliferation and extracellular matrix organization. We consistently found the activation of mTORC2/AKT signaling and accelerated proliferation in the CMT2AMFN2 fibroblasts. In conclusion, our evidence indicates that MFN2 mutation can positively drive cell proliferation in CMT2AMFN2 fibroblasts.

Mitochondria Clumping vs. Mitochondria Fusion in CMT2A Diseases

Phenotypic variations in Charcot-Marie-Tooth disease type 2A (CMT2A) result from the many mutations in the mitochondrial fusion protein, mitofusin 2 (MFN2). While the GTPase domain mutations of MFN2 lack the ability to hydrolyze GTP and complete mitochondrial fusion, the mechanism of dysfunction in HR1 domain mutations has yet to be explored. Using Mfn1/Mfn2 double null cells and Mfn2 knock out (KO) fibroblasts, we measured the ability of this variant protein to change conformations and hydrolyze GTP. We found that a mutation in the HR1 domain (M376A) of MFN2 results in conformational change dysfunction while maintaining GTPase ability. Prolonged exposure to mitofusin agonist MiM 111 reverses mitochondrial fusion dysfunction in the HR1 mutant through encouraging an open conformation, resulting in a potential therapeutic model in this variant. Herein, we describe a novel mechanism of dysfunction in MFN2 variants through exploring domain-specific mitochondrial characteristics leading to CMT2A.

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.

MFN2-related Charcot-Marie-Tooth Disease with Atypical Ocular Manifestations

We herein describe a Charcot-Marie-Tooth disease (CMT) family with a MFN2 mutation with atypical ocular manifestations. The proband, his mother, his third daughter, and his deceased maternal grandfather all had symptoms of CMT and a visual impairment (either cataracts or severe astigmatism). On whole-exome sequencing for the proband having CMT and congenital cataracts, we identified a c.314C>T (p.Thr105Met) mutation in MFN2, but no mutation in the causative genes associated with cataracts. This missense mutation in MFN2 co-segregated with CMT and the atypical ocular manifestations in this family. The findings of this study might help to expand the clinical phenotype of heterogeneous MFN2-related CMT.

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.

Clinical Phenotype in an Early-Onset French Pediatric Population: Charcot-Marie-Tooth's Disease Type 2A

Charcot-Marie-Tooth's disease type 2A (MCT2A), induced by mutation of the mitofusin 2 (MFN2) gene represents the main cause of MCT2. The aim of this study is to provide details of the clinical and electromyographic phenotype of MCT2A in a pediatric population. We conducted a French multicenter retrospective study, including all children with a genetic diagnosis of MCT2A. Thirteen MCT2A children were included with a beginning of symptoms before the age of 10 years ("early-onset group"). We report two new mutations: c.1070 A → T (p.Lys357.Met) and c.280 C → G (p.Arg94Gly). The evolution of the disease is marked by a fast worsening for three patients with loss of motor autonomy, while the evolution is relatively stable for eight patients. The group of early-onset MCT2A seems more heterogeneous than previously described, with a nonconstant severe phenotype.

Charcot-Marie-Tooth disease: Genetic profile of patients from a large Brazilian neuromuscular reference center

This study aimed to describe the clinical, genetic, and epidemiological features of Charcot-Marie-Tooth disease (CMT) in Brazilian patients from a tertiary center, and to compare our data with previously published findings. This retrospective observational study conducted between February 2015 and July 2020 evaluated 503 patients (94 families and 192 unrelated individuals), diagnosed with CMT. Clinical and neurophysiological data were obtained from electronic medical records and blood samples were used for genetic analyses. Multiplex ligation-dependent probe amplification was used to assess duplications/deletions in PMP22. Sanger sequencing of GJB1 was performed in cases of suspected demyelinating CMT. Targeted gene panel sequencing was used for the remaining negative demyelinating cases and all axonal CMT cases. The first decade of life was the most common period of disease onset. In all, 353 patients had demyelinating CMT, 39 had intermediate CMT, and 111 had axonal CMT. Pathogenic or likely pathogenic variants were identified in 197 index cases. The most common causative genes among probands were PMP22 (duplication) (n = 116, 58.88%), GJB1 (n = 23, 11.67%), MFN2 (n = 12, 6.09%), GDAP1 (n = 7, 3.55%), MPZ (n = 6, 3.05%), PMP22 (point mutation) (n = 6, 3.05%), NEFL (n = 3, 1.52%), SBF2 (n = 3, 1.52%), and SH3TC2 (n = 3, 1.52%). Other identified variants were ≤1% of index cases. This study provides further data on the frequency of CMT subtypes in a Brazilian clinical-based population and highlights the importance of rarer and previously undiagnosed variants in clinical practice.

Impaired Mitochondrial Mobility in Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth (CMT) disease is a progressive, peripheral neuropathy and the most commonly inherited neurological disorder. Clinical manifestations of CMT mutations are typically limited to peripheral neurons, the longest cells in the body. Currently, mutations in at least 80 different genes are associated with CMT and new mutations are regularly being discovered. A large portion of the proteins mutated in axonal CMT have documented roles in mitochondrial mobility, suggesting that organelle trafficking defects may be a common underlying disease mechanism. This review will focus on the potential role of altered mitochondrial mobility in the pathogenesis of axonal CMT, highlighting the conceptional challenges and potential experimental and therapeutic opportunities presented by this "impaired mobility" model of the disease.

Early-onset cerebellar ataxia in a patient with CMT2A2

A 9-yr 8-mo-old right-handed female presented with a history of gait difficulties, which first became apparent at age 9 mo of age, along with slurred speech and hand tremors while holding a tray. Her past medical history was significant for global developmental delay, and she was attending fourth grade special education classes. On examination, she had an ataxic gait, dysarthria, absent deep tendon reflexes, and flexor plantar responses. There were no signs of optic atrophy or hearing loss. Nerve conduction studies were consistent with an axonal neuropathy. A fascicular sural nerve biopsy showed a marked decrease of myelinated fibers larger than 6 µm in diameter as compared with an age-matched control. By electron microscopy, clusters of degenerating axonal mitochondria in both myelinated and unmyelinated fibers were frequently found. Whole-exome sequencing revealed a heterozygous c.314C > T (p.Thr105Met) missense variant in MFN2 in the patient but not in her mother. The father was unavailable for testing. The phenotypes with MFN2 variants can be quite variable, including intellectual disability, optic atrophy, auditory impairment, spinal atrophy with or without hydromyelia, and hydrocephalus. We report here that early onset ataxia with intellectual disability can also be associated with MFN2-related Charcot–Marie–Tooth, Type 2A2A diagnosis, the most common type of autosomal dominant axonal neuropathy.

Exome sequencing reveals a novel MFN2 missense mutation in a Chinese family with Charcot-Marie-Tooth type 2A

Charcot-Marie-Tooth (CMT) is a group of inherited peripheral neuropathies. To date, mutations in >80 genes are reportedly associated with CMT. Protein mitofusin 2 encoded by MFN2 serves an essential role in mitochondrial fusion and regulation of apoptosis, which has previously been reported to be highly associated with an axonal form of neuropathy (CMT2A). In the present study, a large Chinese family with severe CMT was reported and a genetic analysis of the disease was performed. A detailed physical examination for CMT was performed in 13 family members and electrophysiological examinations were performed in 3 affected family members. Whole-exome sequencing was performed on the proband, and the suspected variants were identified by Sanger sequencing. The pathogenicity of mutation was verified by restriction fragment length polymorphism analysis in the family followed by a bioinformatics analysis. A novel c.1190G>C; p.(R397P) mutation in the MFN2 gene was identified in the proband, and co-segregated between genotype and phenotype in the family. The substituted amino acid changed the hydrophobicity and charge characteristics of the mitofusin 2 coiled-coiled domain; thus it may affect its biological function. In summary, a novel pathogenic mutation was identified in a Chinese family with CMT, which expands the phenotypic and mutational spectrum of CMT2A, and provides evidence for prenatal interventions and more precise pharmacological treatments to this family.

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.

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.