Soo Hyun Nam

SET binding factor 1 (SBF1) mutation causes Charcot-Marie-Tooth disease type 4B3

Objective: To identify the genetic cause of an autosomal recessive demyelinating Charcot-Marie-Tooth disease type 4B (CMT4B) family. Methods: We enrolled 14 members of a Korean family in which 3 individuals had demyelinating CMT4B phenotype and obtained distal sural nerve biopsies from all affected participants. We conducted exome sequencing on 6 samples (3 affected and 3 unaffected individuals). Results: One pair of heterozygous missense mutations in the SET binding factor 1 (SBF1) gene (22q13.33), also called MTMR5, was identified as the underlying cause of the CMT4B family illness. Clinical phenotypes of affected study participants with CMT4B were similar, to some extent, to patients with CMT4B1 and CMT4B2. We found a similar loss of large myelinated fibers and focally folded myelin sheaths in our patients, but the actual number of myelinated fibers was different from CMT4B1 and CMT4B2. Conclusions: We suggest that the compound heterozygous mutations in SBF1 are the underlying causes of a novel CMT4B subtype, designated as CMT4B3. We believe that this study will lead to mechanistic studies to discover the function of SBF1 and to the development of molecular diagnostics for CMT disease.

Identification of Genetic Causes of Inherited Peripheral Neuropathies by Targeted Gene Panel Sequencing.

Inherited peripheral neuropathies (IPN), which are a group of clinically and genetically heterogeneous peripheral nerve disorders including Charcot-Marie-Tooth disease (CMT), exhibit progressive degeneration of muscles in the extremities and loss of sensory function. Over 70 genes have been reported as genetic causatives and the number is still growing. We prepared a targeted gene panel for IPN diagnosis based on next generation sequencing (NGS). The gene panel was designed to detect mutations in 73 genes reported to be genetic causes of IPN or related peripheral neuropathies, and to detect duplication of the chromosome 17p12 region, the major genetic cause of CMT1A. We applied the gene panel to 115 samples from 63 non-CMT1A families, and isolated 15 pathogenic or likely-pathogenic mutations in eight genes from 25 patients (17 families). Of them, eight mutations were unreported variants. Of particular interest, this study revealed several very rare mutations in the SPTLC2, DCTN1, and MARS genes. In addition, the effectiveness of the detection of CMT1A was confirmed by comparing five 17p12-nonduplicated controls and 15 CMT1A cases. In conclusion, we developed a gene panel for one step genetic diagnosis of IPN. It seems that its time- and cost-effectiveness are superior to previous tiered-genetic diagnosis algorithms, and it could be applied as a genetic diagnostic system for inherited peripheral neuropathies.

Rare variants in methionyl- and tyrosyl-tRNA synthetase genes in late-onset autosomal dominant Charcot–Marie–Tooth neuropathy

Fig. 1. Pedigrees and sequencing analysis of methionyl-tRNA synthetase (MARS ) and tyrosyl-tRNA synthetase (YARS ) variants in the FC433 and FC415 families. (a, b) Pedigrees and sequencing chromatograms. Genotypes of MARS (a) and YARS (b) variants were indicated at the bottom of each examined individual. Arrows in pedigrees indicate probands whose DNA were used for whole exome sequencing ( , : unaffected; , : affected). (c) Conservation analysis of amino acid sequences between different species. Two substitution sites and their neighboring sequences are highly conserved between different species. have been reported as the cause of CMT by 2012: GARS (CMT2D), YARS (DI-CMT), AARS (CMT2N), and KARS (RI-CMT). Recently, mutations in two other ARS genes, MARS and HARS have been reported to be associated with CMT2 (2, 3). Exome sequencing (ES) has been recently applied to CMT as a powerful strategy of identifying causative genes (4). We performed ES in 166 Korean CMT

DGAT2 Mutation in a Family with Autosomal-Dominant Early-Onset Axonal Charcot-Marie-Tooth Disease

Charcot‐Marie‐Tooth disease (CMT) is the most common inherited peripheral neuropathy and is a genetically and clinically heterogeneous disorder. We examined a Korean family in which two individuals had an autosomal‐dominant axonal CMT with early‐onset, sensory ataxia, tremor, and slow disease progression. Pedigree analysis and exome sequencing identified a de novo missense mutation (p.Y223H) in the diacylglycerol O‐acyltransferase 2 (DGAT2) gene. DGAT2 encodes an endoplasmic reticulum‐mitochondrial‐associated membrane protein, acyl‐CoA:diacylglycerol acyltransferase, which catalyzes the final step of the triglyceride (TG) biosynthesis pathway. The patient showed consistently decreased serum TG levels, and overexpression of the mutant DGAT2 significantly inhibited the proliferation of mouse motor neuron cells. Moreover, the variant form of human DGAT2 inhibited the axonal branching in the peripheral nervous system of zebrafish. We suggest that mutation of DGAT2 is the novel underlying cause of an autosomal‐dominant axonal CMT2 neuropathy. This study will help provide a better understanding of the pathophysiology of axonal CMT and contribute to the molecular diagnostics of peripheral neuropathies.

Exome Sequencing Reveals a Novel PRPS1 Mutation in a Family with CMTX5 without Optic Atrophy

Background X-linked Charcot-Marie-Tooth disease type 5 (CMTX5) is caused by mutations in the gene encoding phosphoribosyl pyrophosphate synthetase I (PRPS1). There has been only one case report of CMTX5 patients. The aim of this study was to identify the causative gene in a family with CMTX with peripheral neuropathy and deafness. Case Report A Korean family with X-linked recessive CMT was enrolled. The age at the onset of hearing loss of the male proband was 5 months, and that of steppage gait was 6 years; he underwent cochlear surgery at the age of 12 years. In contrast to what was reported for the first patients with CMTX5, this patient did not exhibit optic atrophy. Furthermore, there was no cognitive impairment, respiratory dysfunction, or visual disturbance. Assessment of his family history revealed two male relatives with very similar clinical manifestations. Electrophysiological evaluations disclosed sensorineural hearing loss and peripheral neuropathy. Whole-exome sequencing identified a novel p.Ala121Gly (c.362C>G) PRPS1 mutation as the underlying genetic cause of the clinical phenotype. Conclusions A novel mutation of PRPS1 was identified in a CMTX5 family in which the proband had a phenotype of peripheral neuropathy with early-onset hearing loss, but no optic atrophy. The findings of this study will expand the clinical spectrum of X-linked recessive CMT and will be useful for the molecular diagnosis of clinically heterogeneous peripheral neuropathies.

A Mutation in PMP2 Causes Dominant Demyelinating Charcot-Marie-Tooth Neuropathy

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of peripheral neuropathies with diverse genetic causes. In this study, we identified p.I43N mutation in PMP2 from a family exhibiting autosomal dominant demyelinating CMT neuropathy by whole exome sequencing and characterized the clinical features. The age at onset was the first to second decades and muscle atrophy started in the distal portion of the leg. Predominant fatty replacement in the anterior and lateral compartment was similar to that in CMT1A caused by PMP22 duplication. Sural nerve biopsy showed onion bulbs and degenerating fibers with various myelin abnormalities. The relevance of PMP2 mutation as a genetic cause of dominant CMT1 was assessed using transgenic mouse models. Transgenic mice expressing wild type or mutant (p.I43N) PMP2 exhibited abnormal motor function. Electrophysiological data revealed that both mice had reduced motor nerve conduction velocities (MNCV). Electron microscopy revealed that demyelinating fibers and internodal lengths were shortened in both transgenic mice. These data imply that overexpression of wild type as well as mutant PMP2 also causes the CMT1 phenotype, which has been documented in the PMP22. This report might expand the genetic and clinical features of CMT and a further mechanism study will enhance our understanding of PMP2-associated peripheral neuropathy.

Early-onset severe hereditary sensory and autonomic neuropathy type 1 with S331F SPTLC1 mutation

Hereditary sensory and autonomic neuropathy type I (HSAN I) is an autosomal dominant disease characterized by prominent sensory impairment, resulting in foot ulcers or amputations and has a juvenile to adult onset. The major underlying causes of HSAN I are mutations in SPTLC1, which encodes the first subunit of serine palmitoyltransferase (SPT). To date, there have been no reports with regard to an HSAN patient of Korean origin. In this report we discussed an HSAN I patient with a missense mutation in SPTLC1 (c.992C>T: p.S331F). The patient had noticed frequent falls, lower leg weakness and hand tremors at age five. The patient also presented with foot ulcers, muscle hypotrophy, cataracts, hoarseness, vocal cord palsy and respiratory difficulties and succumbed to the condition at the age of 28 years. In accordance with previous reports, a mutation in Ser331 in the present patient was associated with early-onset and a severe phenotype. Therefore, Ser331 in SPTLC1 is a crucial amino acid, which characterizes the HSAN I phenotype.

Novel Compound Heterozygous Nonsense PRX Mutations in a Korean Dejerine-Sottas Neuropathy Family

Background Mutations in the gene encoding periaxin (PRX) are known to cause autosomal recessive Dejerine-Sottas neuropathy (DSN) or Charcot-Marie-Tooth disease type 4F. However, there have been no reports describing Korean patients with these mutations. Case Report We examined a Korean DSN patient with an early-onset, slowly progressive, demyelinating neuropathy with prominent sensory involvement. Whole-exome sequencing and subsequent capillary sequencing revealed novel compound heterozygous nonsense mutations (p.R392X and p.R679X) in PRX. One mutation was transmitted from each of the patients parents. No unaffected family member had both mutations, and the mutations were not found in healthy controls. Conclusions We believe that these novel compound heterozygous nonsense mutations are the underlying cause of DSN. The clinical, electrophysiologic, and pathologic phenotypes in this family were similar to those described previously for patients with PRX mutations. We have identified the first PRX mutation in a Korean patient with DSN.

Ser135Phe mutation in HSPB1 ( HSP27 ) from Charcot–Marie–Tooth disease type 2F families

Charcot–Marie–Tooth disease (CMT) is a group of clinically and genetically heterogeneous peripheral neuropathies. We identified two axonal CMT type 2F (CMT2F) families presented with distally predominant weakness in upper and lower extremities with sensory involvement. This study identified a c.404C>T (p.Ser135Phe) mutation in HSPB1 gene as the underlying cause of the both families by applying of whole exome sequencing. The p.Ser135Phe mutation was completely cosegregated with the affected members in the both families, and it was not found in 300 healthy controls. This mutation has been previously reported as the causes of CMT2F or hereditary motor neuropathy 2B (dHMN2B). The mutation was located in the highly conserved alpha-crystallin domain, and several in silico analyses also predicted that the mutation is likely to be pathogenic. HSPB1 encodes heat shock protein 27 (HSP27) which belongs to the superfamily of small stress induced proteins. These results suggest that the HSPB1 mutation is underlying cause of CMT2F phenotype shown in the present families. We believe that this study will be useful for the molecular diagnosis of peripheral neuropathies.

Axonal Charcot-Marie-Tooth neuropathy concurrent with distal and proximal weakness by translational elongation of the 3’ UTR in NEFH

Mutations in the NEFH gene encoding the heavy neurofilament protein are usually associated with neuronal damage and susceptibility to amyotrophic lateral sclerosis (ALS). Recently, frameshift variants in NEFH (p.Asp1004Glnfs*58 and p.Pro1008Alafs*56) have been reported to be the underlying cause of axonal Charcot‐Marie‐Tooth disease type 2CC (CMT2CC). The frameshift mutation resulted in a stop loss and translation of a cryptic amyloidogenic element (CAE) encoded by the 3′ untranslated region (UTR). This study also identified a de novo c.3015_3027dup frameshift mutation predicting p.Lys1010Glnfs*57 in NEFH from a CMT2 family with an atypical clinical symptom of prominent proximal weakness. This mutation is located near the previously reported frameshift mutations, suggesting a mutational hotspot. Lower limb magnetic resonance imaging (MRI) revealed marked hyperintense signal changes in the thigh muscles compared with those in the calf muscles. Therefore, this study suggests that the stop loss and translational elongations by the 3′ UTR of the NEFH mutations may be a relatively frequent genetic cause of axonal peripheral neuropathy with the specific characteristics of proximal dominant weakness.