search for




 

Axonal Charcot-Marie-Tooth case with a novel heterozygous variant in MFN2 assessed by the MutationDistiller
Journal of Genetic Medicine 2020;17:89-91
Published online December 31, 2020;  https://doi.org/10.5734/JGM.2020.17.2.89
© 2020 Korean Society of Medical Genetics and Genomics.

Ho-Sung Ryu1, Yun-Jeong Lee2, and Jong-Mok Lee1,3,*

1Department of Neurology, Kyungpook National University Hospital, Daegu, South Korea
2Department of Pediatrics, School of Medicine, Kyungpook National University, Kyungpook National University Hospital, Daegu, South Korea
3Department of Neurology, School of Medicine, Kyungpook National University, Daegu, South Korea
Jong-Mok Lee, M.D., Ph.D. https://orcid.org/0000-0002-2918-6166
Department of Neurology, School of Medicine, Kyungpook National University, 680 Gukchaebosang-ro, Jung-gu, Daegu 41944, South Korea.
Tel: +82-53-200-5765, Fax: +82-53-422-4265, E-mail: azulmar@gmail.com
Received July 3, 2020; Revised July 17, 2020; Accepted July 21, 2020.
cc This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Charcot-Marie-Tooth (CMT) disease can be divided mainly into demyelination and axonopathy based on the results of the electrophysiological study. Mitofusin 2, encoded by MFN2 gene, has a crucial role in the fusion of mitochondria, which is known to associate with CMT type 2A as one of the axonal forms. We describe a 44-year-old man with progressive weakness on bilateral legs after noticing foot drop in his early teen. When we examined him at 45 years of age, he presented atrophy on entire legs and with distal muscle weakness on limbs. The nerve conduction study revealed severely decreased amplitude on motor nerve ranging from 0.2 to 4.5 mV, while conduction velocity remained more than 30.4 m/s. The whole-exome sequencing revealed a novel variant c.2228G>T in MFN2 by efficient genetic analysis tool, MutationDistiller. This report will not only expand the mutation spectrum of CMT2A but also introduce a time-saving genetic analysis tool.
Keywords : Muscular atrophy, Whole exome sequencing, Hereditary sensory and motor neuropathy
Introduction

Charcot-Marie-Tooth (CMT) disease type 2A is known to be associated with mutations of MFN2, which is one of the leading causative genes for axonal CMT [1]. MFN2 encodes mitofusin 2 protein in the outer mitochondrial membrane, which has a crucial role in mitochondrial fusion by initiating the formation of antiparallel dimers between their second heptad repeat (HR2) domains [2]. Therefore, the loss-of-function of mitofusin 2 results in the failure of mitochondrial fusion and transport [2]. So far, more than 150 mutations in MFN2 have been identified (https://databases.lovd.nl/shared/genes/MFN2). Here, we report a CMT2A case with a novel mutation in MFN2. We believe this report may expand the mutation spectrum of CMT2A.

Case

The proband, a 44-year-old man, was born from non-consanguineous parents. He visited the department of neurology presenting with slowly progressive weakness of the limbs. At the age of 10, he noticed the dropping of feet while walking on a flat surface. At the age of 20, Achilles tendon lengthening was undergone to minimize ankle contracture. At the age of 37, he noticed atrophy on the distal part of bilateral limbs. At the age of 43, he experienced difficulties writing, using chopsticks, and zipping up the jacket. He denied family history.

A neurologic examination at 45 years of age revealed a weakness in the distal part of bilateral arms and legs. Individually, the patient was not able to move the ankle joint, including dorsi/plantar flexion at all. The motor power of flexion/extension in fingers and wrists and knee extension were four as the Medical Research Council (MRC) grade. The MRC grade of knee flexion was three. The power of the shoulder and hip joints remained normal. Strikingly, the atrophy was severe on the lower legs, in which the circumference was 23 cm at the point of 15 cm below the patella (Fig. 1A). Overall deep tendon reflexes were decreased. Vibration sense was disturbed below both ankles. The hammertoes on both feet were revealed, but pes cavus was not evident. The nerve conduction studies (NCS) demonstrated severely decreased amplitude of compound muscle action potential (CMAP) ranging from 0.2 to 4.5 mV in upper extremities and sensory nerve action potentials ranging from 2.0 to 3.6 mV in lower extremities. The CMAPs were not elucidated in the lower extremities. In contrast, the conduction velocity remained more than 30.4 m/s (Table 1). According to the findings of history, examination, and laboratory, this patient was graded as 14 by the CMT neuropathy score (CMTNS). Based on the results of NCS, we proceeded directly to whole-exome sequencing. To identify the pathogenic variant, we uploaded the variant call format file to the MutationDistiller website (https://www.mutationdistiller.org). Once uploaded, we queried “Charcot-Marie-Tooth” in the phenotype inquiry. The MutationDistiller system identified heterozygous variant c.2228G>T (p.Ser743Ile, NM_014874.3, Fig. 1B) of MFN2 in the proband, but is not listed in the Genome Aggregation Database (gnomAD) or the Exome Aggregation Consortium. And this variant was predicted to be “damaging” with a score of 0.018 by SIFT, “deleterious” with a score of -3.18 by PROVEAN, and “possibly damaging” with a score of 0.945 by PolyPhen-2 systems. Also, this variant is categorized as “likely pathogenic” satisfying two moderate and two supporting evidences based on ACMG/AMP guidelines [3].

Discussion

We recognized axonal CMT based on the electrophysiological findings, and the diagnosis was confirmed by detecting a novel mutation in MFN2 c.2228G>T (p.Ser743Ile). Moreover, we have discovered the pathogenic mutation efficiently through MutationDistiller [4]. Also, different amino acid substitutions at the same position (c.2229T>A, p.Ser743Arg) had been reported to be pathogenic, which can support the pathogenicity of a variant in our study [1].

WES is currently considered to be a powerful tool for genetic analysis. However, the more we perform WES, the more we get tons of variants to be sifted. To determine the pathogenic mutation from the results of WES, we have filtered allele frequency and the score of in-silico analysis such as SIFT, Proven, and Polyphen2 manually. MutationDistiller is a recently introduced online tool for the analysis of WES [4]. It combines MutationTaster’s pathogenic predictions with a phenotype database of the Human Phenotype Ontology and provides a list of potential disease mutations [4]. Therefore, this tool can reduce the burden of identifying possible pathogenic variants.

Human MFN2 protein consists of GTPase, first heptad repeat (HR1), transmembrane, and HR2 domains [2]. HR2 is an essential part of the initiation of mitochondrial fusion by tethering each other. And the dimerization of the GTPase domains is responsible for pulling the mitochondrial membrane [2]. The loss-of-function of a novel variant can lead to the failure of mitochondrial fusion by interferences of tethering.

In terms of phenotype-genotype correlation in CMT2A, the type or location of mutation was not associated with severe or mild phenotype [5,6]. The previous study could not reveal any relationship between the site or type of mutation and the age at onset, clinical severity, or additional features [6,7]. However, early-onset patients seem to present with severe phenotype with higher score of CMTNS [5,7].

In conclusion, this study demonstrated the CMT2A patient with a novel mutation c.2228G>T in MFN2, analyzed by an efficient tool of MutationDistiller.

Conflict of interest

The authors declare that they do not have any conflicts of interest.

Figures
Fig. 1. Photos and results of genetic analysis from the proband. (A) Atrophy of lower extremities, intrinsic muscles of hand, and the distal part of feet is evident. (B) A heterozygous variant c.2228G>T (p.Ser743Ile, NM_014874.3) in MFN2 is confirmed by Sanger sequencing in the proband. (C) Serine at the position 743 in mitofusin 2 protein is highly conserved across the species.
TABLES

Nerve conduction studies of the patient

Nerves Stimulation site Recording site Nerve conduction study
Motor Latency (msec) CMAP (mV) NCV (m/sec)
Median Wrist Abductor pollicis brevis 4.69 ↑ (<3.6) 0.4 ↓ (>5.0)
Elbow 0.2 ↓ (>5.0) 44.6 ↓ (>50)
Axilla 0.2 ↓ (>5.0) 34.3 ↓ (>55)
Ulnar Wrist Abductor digiti minimi 3.91 ↑ (<3.0) 2.6 ↓ (>5.0)
Below elbow 1.4 ↓ (>5.0) 52.6
Above elbow 1.3 ↓ (>5.0) 45.2
Axilla 0.7 ↓ (>5.0) 30.4 ↓ (>52)
Peroneal Ankle Extensor digitorum brevis NR
Knee NR
Tibial Ankle Abductor hallucis NR
Knee NR
Sensory SNAP (μV) NCV (m/sec)
Median Finger Wrist 2.0 ↓ (>10) 33.5 ↓ (>41)
Ulnar Finger Wrist 3.1 ↓ (>10) 33.1 ↓ (>39)
Sural Midcalf Lateral malleolus 3.6 ↓ (>6) 45.6

CMAP, compound muscle action potential; NCV, nerve conduction velocity; NR, no response; SNAP, sensory nerve action potential, normal ranges are described between parentheses.


References
  1. Bombelli F, Stojkovic T, Dubourg O, Echaniz-Laguna A, Tardieu S, Larcher K, et al. Charcot-Marie-Tooth disease type 2A: from typical to rare phenotypic and genotypic features. JAMA Neurol 2014;71:1036-42.
    Pubmed CrossRef
  2. Filadi R, Pendin D, Pizzo P. Mitofusin 2: from functions to disease. Cell Death Dis 2018;9:330.
    Pubmed KoreaMed CrossRef
  3. Richards S, Aziz N, Bale S, Bick D, Das S, Gastier-Foster J, et al; ACMG Laboratory Quality Assurance Committee. Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology. Genet Med 2015;17:405-24.
    Pubmed KoreaMed CrossRef
  4. Hombach D, Schuelke M, Knierim E, Ehmke N, Schwarz JM, Fischer-Zirnsak B, et al. MutationDistiller: user-driven identification of pathogenic DNA variants. Nucleic Acids Res 2019;47:W114-20.
    Pubmed KoreaMed CrossRef
  5. Choi BO, Nakhro K, Park HJ, Hyun YS, Lee JH, Kanwal S, et al. A cohort study of MFN2 mutations and phenotypic spectrums in Charcot-Marie-Tooth disease 2A patients. Clin Genet 2015;87:594-8.
    Pubmed CrossRef
  6. Verhoeven K, Claeys KG, Züchner S, Schröder JM, Weis J, Ceuterick C, et al. MFN2 mutation distribution and genotype/phenotype correlation in Charcot-Marie-Tooth type 2. Brain 2006;129(Pt 8):2093-102.
    Pubmed CrossRef
  7. Chung KW, Kim SB, Park KD, Choi KG, Lee JH, Eun HW, et al. Early onset severe and late-onset mild Charcot-Marie-Tooth disease with mitofusin 2 (MFN2) mutations. Brain 2006;129(Pt 8):2103-18.
    Pubmed CrossRef


June 2021, 18 (1)
Full Text(PDF) Free

Social Network Service
Services

Cited By Articles
  • CrossRef (0)

Author ORCID Information