Molecular basis of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal disorder of motor neuron degeneration with unclear etiology and no effective treatment to date. ALS is, however, increasingly recognized as a multisystem disorder associated with impaired cognition. The overlap between ALS and dementia at clinical, genetic and neuropathologic levels indicates a spectrum of clinical phenotypes that may include features of frontotemporal lobar degeneration (FTLD). Most cases of ALS are sporadic (SALS), but approximately 10% of all ALS cases are familial ALS (FALS). Mutations in the Cu/Zn superoxide dismutase-1 gene (SOD-1) occur in about 20% of FALS cases. Mutations in the TAR DNA-binding protein 43 gene (TARDBP or TDP-43) may occur in 3-4% of FALS cases, and less frequently, in FTLD. Recently, mutations in the fused in sarcoma/translation in liposarcoma gene (FUS/TLS) were identified as causing about 4-5% of FALS, SALS, and FTLD cases, but not SOD-1 ALS cases, indicating a pathogenic role of FUS, together with TDP-43, in possibly all types of ALS, except for SOD-1 linked ALS. TDP-43 and FUS have striking structural and functional similarities, most likely implicating altered RNA processing as a major event in ALS pathogenesis. Thus, TARDBP and FUS/TLS mutations define a novel class of neurodegenerative diseases called TDP-43- and FUS-proteinopathies, in which both misfolded proteins are novel targets for the development of therapeutics in this spectrum of diseases. However, SOD-1 linked ALS may have a pathogenic pathway distinct from other types of ALS.     

A Japanese patient with familial ALS and a p.K510M mutation in the gene for FUS (FUS) resulting in the totally locked‐in state

We describe a Japanese patient with familial amyotrophic lateral sclerosis (ALS) and a p.K510M mutation in the fused in sarcoma gene (FUS). The patient's condition was characterized clinically by an early onset and rapid progression. The patient eventually required mechanical ventilation and progressed to the totally locked‐in state. Neuropathologically, multiple system degeneration with many FUS‐immunoreactive structures was observed. The involvement of the globus pallidus, subthalamic nucleus, substantia nigra, cerebellar efferent system, and both upper and lower motor neurons in the present patient was comparable to that described for ALS patients with different mutations in FUS, all of whom progressed to the totally locked‐in state. However, the patient also exhibited degeneration of the cerebellar afferent system and posterior column. Furthermore, the appearance of non‐compact FUS‐immunoreactive neuronal cytoplasmic inclusions and many FUS‐immunoreactive glial cytoplasmic inclusions were unique to the present patient. These features suggest that the morphological characteristics of the FUS‐immunoreactive structures and distribution of the lesions vary with the diversity of mutations in FUS.     

Novel miR-b2122 regulates several ALS-related RNA-binding proteins

Common pathological features of amyotrophic lateral sclerosis (ALS) include cytoplasmic aggregation of several RNA-binding proteins. Out of these RNA-binding proteins, TDP-43, FUS/TLS and RGNEF have been shown to co-aggregate with one another within motor neurons of sporadic ALS (sALS) patients, suggesting that there may be a common regulatory network disrupted. MiRNAs have been a recent focus in ALS research as they have been identified to be globally down-regulated in the spinal cord of ALS patients. The objective of this study was to identify if there are miRNA(s) dysregulated in sALS that are responsible for regulating the TDP-43, FUS/TLS and RGNEF network. In this study, we identify miR-194 and miR-b2122 to be significantly down-regulated in sALS patients, and were predicted to regulate TARDBP, FUS/TLS and RGNEF expression. Reporter gene assays and RT-qPCR revealed that miR-b2122 down-regulates the reporter gene through direct interactions with either the TARDBP, FUS/TLS, or RGNEF 3’UTR, while miR-194 down-regulates firefly expression when it contained either the TARDBP or FUS/TLS 3’UTR. Further, we showed that miR-b2122 regulates endogenous expression of all three of these genes in a neuronal-derived cell line. Also, an ALS-associated mutation in the FUS/TLS 3’UTR ablates the ability of miR-b2122 to regulate reporter gene linked to FUS/TLS 3’UTR, and sALS samples which showed a down-regulation in miR-b2122 also showed an increase in FUS/TLS protein expression. Overall, we have identified a novel miRNA that is down-regulated in sALS that appears to be a central regulator of disease-related RNA-binding proteins, and thus its dysregulation likely contributes to TDP-43, FUS/TLS and RGNEF pathogenesis in sALS.     

Mutational analysis of the Cu/Zn superoxide dismutase gene in a Catalan ALS population: should all sporadic ALS cases also be screened for SOD1?

BACKGROUND: SOD1 gene mutations are the most common identified cause of ALS, accounting for approximately 20% of familial ALS cases and around 4% of sporadic ALS cases. However, the prevalence of SOD1 varies in different ethnic groups. No previous epidemiological studies have been carried out in Catalonia. OBJECTIVE: To determine the prevalence of SOD1 gene mutations in a Catalan ALS population, and to analyze the genotype-phenotype relationship. MATERIALS AND METHODS: 30 different FALS pedigrees and 94 sporadic ALS patients were screened for SOD1 mutations using direct sequence analysis. RESULTS: Five of the 30 FALS pedigrees (16.6%) carried a SOD1 mutant. The mutations identified in this group were G37R, D76V, S105L, I112M and N139H. Four SOD1 mutants (4.25%) were found in the sporadic ALS group (SALS). The overall frequency (FALS plus SALS) of SOD1 mutations in our series was 6.45%. In the SALS group, D90A was identified in a patient presenting the typical Scandinavian phenotype. A 53-year-old woman with no family history of ALS carried the N139H mutation. Two unrelated sporadic ALS cases carried the A140A SOD1 mutant. CONCLUSIONS: The prevalence of the SOD1 mutation in FALS in Catalonia is similar to levels in other Mediterranean countries, but lower than those in reports studying the Belgian, Japanese, and Scottish populations. The prevalence of the SOD1 mutation was 4.25% in patients with no family history of ALS. These results may have significant repercussions on genetic counseling, and screening for the SOD1 mutation in sporadic ALS cases must therefore be considered.     

Xp22.3 genomic deletions involving the CDKL5 gene in girls with early onset epileptic encephalopathy

Purpose : Mutations of the X‐linked gene cyclin‐dependent kinase‐like 5 (CDKL5) cause an X‐linked encephalopathy with early onset intractable epilepsy, including infantile spasms and other seizure types, and a Rett syndrome (RTT)–like phenotype. Very limited information is available on the frequency and phenotypic spectrum associated with CDKL5 deletions/duplications. We investigated the role of CDKL5 deletions/duplications in causing early onset intractable epilepsy of unknown etiology in girls. Methods : We studied 49 girls with early onset intractable epilepsy, with or without infantile spasms, and developmental impairment, for whom no etiologic factors were obvious after clinical examination, brain magnetic resonance imaging (MRI) and expanded screening for inborn errors of metabolism. We performed CDKL5 gene mutation analysis in all and multiplex ligation dependent probe amplification assay (MLPA) in those who were mutation negative. Custom Array‐comparative genomic hybridization (CGH), breakpoint polymerase chain reaction (PCR) analysis, and X‐inactivation studies were performed in patients in whom MLPA uncovered a genomic alteration. Results : We found CDKL5 mutations in 8.2% (4 of 49) of patients and genomic deletions in 8.2% (4 of 49). Overall, abnormalities of the CDKL5 gene accounted for 16.3% (8 of 49) of patients. Discussion : CDKL5 gene deletions are an under‐ascertained cause of early onset intractable epilepsy in girls. Genetic testing of CDKL5, including both mutation and deletion/duplication analysis, should be considered in this clinical subgroup.     

Transportin 1 colocalization with Fused in Sarcoma (FUS) inclusions is not characteristic for amyotrophic lateral sclerosis‐FUS confirming disrupted nuclear import of mutant FUS and distinguishing it from frontotemporal lobar degeneration with FUS inclusions

C. Troakes, T. Hortobágyi, C. Vance, S. Al‐Sarraj, B. Rogelj and C. E. Shaw (2013) Neuropathology and Applied Neurobiology 39, 553–561 Transportin 1 colocalization with Fused in Sarcoma (FUS) inclusions is not characteristic for amyotrophic lateral sclerosis‐FUS confirming disrupted nuclear import of mutant FUS and distinguishing it from frontotemporal lobar degeneration with FUS inclusions Aims: Transportin 1 (TNPO 1) is an abundant component of the Fused in Sarcoma (FUS)‐immunopositive inclusions seen in a subgroup of frontotemporal lobar degeneration (FTLD‐FUS). TNPO 1 has been shown to bind to the C‐terminal nuclear localizing signal (NLS) of FUS and mediate its nuclear import. Amyotrophic lateral sclerosis (ALS)‐linked C‐terminal mutants disrupt TNPO 1 binding to the NLS and impair nuclear import in cell culture. If this held true for human ALS then we predicted that FUS inclusions in patients with C‐terminal FUS mutations would not colocalize with TNPO 1. Methods: Expression of TNPO 1 and colocalization with FUS was studied in the frontal cortex of FTLD‐FUS (n = 3) and brain and spinal cord of ALS‐FUS (n = 3), ALS‐C9orf72 (n = 3), sporadic ALS (n = 7) and controls (n = 7). Expression levels and detergent solubility of TNPO 1 was measured by Western blot. Results: Aggregates of TNPO 1 were abundant and colocalized with FUS inclusions in the cortex of all FTLD‐FUS cases. In contrast, no TNPO 1‐positive aggregates or FUS colocalization was evident in two‐thirds, ALS‐FUS cases and was rare in one ALS‐FUS case. Nor were they present in C9orf72 or sporadic ALS. No increase in the levels of TNPO 1 was seen in Western blots of spinal cord tissues from all ALS cases compared with controls. Conclusions: These findings confirm that C‐terminal FUS mutations prevent TNPO 1 binding to the NLS, inhibiting nuclear import and promoting cytoplasmic aggregation. The presence of TNPO 1 in wild‐type FUS aggregates in FTLD‐FUS distinguishes the two pathologies and implicates different disease mechanisms.     

A novel ferritin light chain gene mutation in a Japanese family with neuroferritinopathy: Description of clinical features and implications for genotype–phenotype correlations

Neuroferritinopathy is a hereditary neurodegenerative disorder caused by mutations in the ferritin light chain gene (FTL1). The cardinal features are progressive movement disturbance, hypoferritinemia, and iron deposition in the brain. To date, five mutations have been described in Caucasian and Japanese families, but the genotype–phenotype correlations remain to be established. We identified a novel FTL1 mutation (exon 4, c.641/642, 4‐nucletotide duplication) in a Japanese family and compared the clinical traits with those previously reported. All mutations but one are insertions in exon 4, resulting in frameshifts. Clinical features are similar among patients with the same mutations. Middle‐age onset chorea is common in patients with insertions in the 5′ portion of exon 4 including our cases, whereas patients with insertions in the 3′ portion of exon 4 develop early‐onset tremor, suggesting genotype–phenotype correlations. In this family, male predominance and normal serum ferritin levels are characteristic. © 2008 Movement Disorder Society     

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.     

Recent advances in the genetics of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder with a low survival rate beyond 5 years from symptom onset. Although the genes that cause most cases of ALS are still unknown, several important genetic discoveries have been made recently that will bring substantial insight into some of the mechanisms involved in ALS. Mutations in two genes with related functions were recently reported in patients with familial ALS: the FUS/TLS gene at the ALS6 locus on chromosome 16 and the TARDBP gene at the ALS10 locus on chromosome 1. In addition, the first wave of genomewide association studies in ALS has been published. While these studies clearly show that there is no definitive and common highly penetrant allele that causes ALS, some interesting candidate genes emerged from these studies. The findings help to better delineate the types of genes and genetic variants that are involved in ALS and provide substantial material for future research.     

The occurrence of mutations in FUS in a Belgian cohort of patients with familial ALS

Background and purpose: Mutations in fused in sarcoma (FUS) were recently identified as a cause of familial amyotrophic lateral sclerosis (ALS). The frequency of occurrence of mutations in FUS in sets of patients with familial ALS remains to be established. Methods: We sequenced the FUS gene in a cohort of patients with familial ALS seen at the neuromuscular clinic in Leuven. A total of 28 patients with SOD1‐negative ALS from 22 families were analyzed. Results: We identified a R521H mutation in 4 patients, belonging to a kindred of dominantly inherited classical ALS. The mutation segregated with disease. Mutations in FUS were observed in 2.9% of ALS pedigrees in our cohort. Conclusions: These results show that mutations in FUS are also a significant cause of familial ALS in Belgium.     

Japanese familial amyotrophic lateral sclerosis family with a two‐base deletion in the superoxide dismutase‐1 gene

The clinical characteristics of members of a familial amyotrophic lateral sclerosis (FALS) family from Oki Island, whose members have a 2‐bp deletion at codon 126 of Cu/Zn superoxide dismutase (SOD1) gene, are presented here. Mean age of the onset in the members was 42 years. Mean disease duration among the members who had not been placed on a respirator was approximately 2 years. Long‐term survivors with respiratory support presented disturbances in eye movement and urination toward the end stages of the disease. They predominantly exhibited lower motor neuron symptoms. In addition, the authors focused on frameshift, nonsense and non‐amino‐acid‐altering mutations. Frameshift and nonsense mutations were all found within exon 4, exon 5 and intron 4. These amyotrophic lateral sclerosis cases were likely to have shorter disease duration than the FALS patients with single substitution. Several hypotheses were presented on the pathogenesis of FALS with SOD1 mutation.     

Broad clinical phenotypes associated with TAR-DNA binding protein (TARDBP) mutations in amyotrophic lateral sclerosis

The finding of TDP-43 as a major component of ubiquitinated protein inclusions in amyotrophic lateral sclerosis (ALS) has led to the identification of 30 mutations in the transactive response-DNA binding protein (TARDBP) gene, encoding TDP-43. All but one are in exon 6, which encodes the glycine-rich domain. The aim of this study was to determine the frequency of TARDBP mutations in a large cohort of motor neurone disease patients from Northern England (42 non-superoxide dismutase 1 (SOD1) familial ALS (FALS), nine ALS-frontotemporal dementia, 474 sporadic ALS (SALS), 45 progressive muscular atrophy cases). We identified four mutations, two of which were novel, in two familial (FALS) and two sporadic (SALS) cases, giving a frequency of TARDBP mutations in non-SOD1 FALS of 5% and SALS of 0.4%. Analysis of clinical data identified that patients had typical ALS, with limb or bulbar onset, and showed considerable variation in age of onset and rapidity of disease course. However, all cases had an absence of clinically overt cognitive dysfunction.     

FUS/TLS-immunoreactive neuronal and glial cell inclusions increase with disease duration in familial amyotrophic lateral sclerosis with an R521C FUS/TLS mutation

Basophilic inclusions (BIs) are pathological features of a subset of frontotemporal lobar degeneration disorders, including sporadic amyotrophic lateral sclerosis (ALS) and familial ALS (FALS). Mutations in the fused in sarcoma/translocated in liposarcoma (FUS/TLS) gene have recently been identified as a cause of FALS. The FUS/TLS-immunoreactive inclusions are consistently found in cases of frontotemporal lobar degeneration with BIs; however, the association between ALS cases with BIs and FUS/TLS accumulation is not well understood. We used immunohistochemistry to analyze 3 autopsy cases of FALS with the FUS/TLS mutation and with BIs using anti-FUS/TLS antibodies. The disease durations were 1, 3, and 9 years. As the disease duration becomes longer, there were broader distributions of neuronal and glial FUS/TLS-immunoreactive inclusions. As early as 1 year after the onset, BIs, neuronal cytoplasmic inclusions and glial cytoplasmic inclusions were found in the substantia nigra in addition to the anterior horn of the spinal cord. Glial cytoplasmic inclusions are found earlier and in a wider distribution than neuronal cytoplasmic inclusions. The distribution of FUS/TLS-immunoreactive inclusions in FUS/TLS-mutated FALS with BIs was broader than that of BIs alone, suggesting that the pathogenetic mechanism may have originated from the FUS/TLS proteinopathy.     

Genetic bases and phenotypes of autosomal recessive Parkinson disease in a Turkish population

Background and purpose: To evaluate the phenotype and the frequencies of mutations in PRKN, DJ1 and PINK1 genes in patients with Parkinson disease (PD) in Turkey. Methods: Eighty‐six patients from 77 PD families participated in the study. Seventy‐four families were originating from Turkey, two families from Greece and one family from Bulgaria. All patients underwent detailed neurological examination. PRKN, PINK1 and DJ1 genes were sequenced, and dosage analysis was performed by multiplex ligation‐dependent probe amplification. Results: Sixteen patients with PD were found to carry homozygous (n = 14) or compound heterozygous (n = 2) PRKN mutations. We identified exon rearrangements, three point mutations and one new point mutation in exon 2 (p.K27del). In two families, two new PINK1 point mutations (L31X and P416L) were identified. No pathogenic mutations were found in DJ1 gene. Clinical phenotypes of PRKN patients were comparable to previously described features, but only in four of 13 families, the pedigree structure was clearly consistent with an autosomal recessive (AR) mode of inheritance in comparison with nine families where also different pattern of transmission could have been possible. Conclusions: Our data suggest that the PRKN gene mutation is the most frequent form of ARPD in Turkey. The proportion of mutations with regard to the age of onset in our population is in the range of those previously described, but our pedigrees are characterized by high rate of consanguinity, which might explain the high proportion of families with homozygous mutations and of patients in more than one generation. Pathogenic DJ1 mutations do not seem to play a major role in Turkey.     

A family with autosomal dominant leukodystrophy linked to 5q23.2–q23.3 without lamin B1 mutations

Background and purpose: Duplications of lamin B1 (LMNB1) at 5q23 are implicated in adult‐onset autosomal dominant leukodystrophy (ADLD) having been described in six families with diverse ethnic background but with a homogeneous phenotype. In a large Italian family, we recently identified a variant form of ADLD characterized clinically by absence of the autonomic dysfunction at onset described in ADLD and, on MRI, by milder cerebellar involvement with sparing of hemispheric white matter. Aim of this study was to investigate the genetic basis of this variant form of ADLD. Methods: We carried out a genome‐wide linkage analysis using microsatellite markers, and the genes in the candidate region were screened for point mutations. LMNB1 was also screened for deletions/duplications by real‐time PCR, multiplex ligation‐dependent probe amplification and Southern blot. Results: We mapped the variant ADLD locus to 5q23.2–q23.3, a genomic region containing 11 genes including LMNB1. Neither gene copy‐number defects nor point mutations in the LMNB1 gene were found. We also excluded point mutations in the coding exons of the other ten genes in the candidate region. However, expression of lamin B1 evaluated in lymphoblastoid cells was higher in patients than in healthy controls, and was similar to the lamin B1 expression levels found in a patient with LMNB1 duplication. Conclusions: This observation suggests that a mutation in an LMNB1 regulatory sequence underlies the variant ADLD phenotype. Thus, adult forms of ADLD linked to 5q23 appear to be more heterogeneous clinically and genetically than previously thought.     

Mutations in CHMP2B are not a cause of frontotemporal lobar degeneration in Finnish patients

Background: Frontotemporal lobar degeneration (FTLD) is a genetically complex disorder. The majority of mutations linked to FTLD families are found in the microtubule‐associated protein tau (MAPT) and progranulin (PGRN) genes. Mutations in the chromatin‐modifying protein 2B gene (CHMP2B) have been identified in a few families. However, CHMP2B has been showed to be a rare cause of FTLD. Our aim was to determine the frequency of CHMP2B mutations in a clinical series of patients with FTLD in Northern Finland. Patients and methods: We examined 72 (36 men) Finnish patients with FTLD. The mean age at onset was 58.9 (range 43–80). Symptoms of motor neuron disease (FTLD‐MND) were present in 12 patients (17%). Positive family history was detected in 28% of the patients. Mutations in MAPT and PGRN were excluded from these patients. All exons and exon–intron boundaries of the CHMP2B gene were sequenced. Results: No pathogenic CHMP2B mutations were found. A rare polymorphism in the non‐coding region of exon 1 (rs36098294) and three other previously reported polymorphisms were detected. Conclusions: Our results confirm that mutations in CHMP2B are not a common cause of FTLD. MAPT and PGRN mutations are also rare in Finnish population, suggesting that other, still unknown genetic factors may play a role in the pathogenesis of FTLD in Finnish population.     

X‐linked Charcot–Marie–Tooth disease predominates in a cohort of multiethnic Malaysian patients

Introduction: Data regarding Charcot–Marie–Tooth disease is lacking in Southeast Asian populations. We investigated the frequency of the common genetic mutations in a multiethnic Malaysian cohort. Methods: Patients with features of Charcot–Marie–Tooth disease or hereditary liability to pressure palsies were investigated for PMP22 duplication, deletion, and point mutations and GJB1, MPZ, and MFN2 point mutations. Results: Over a period of 3 years, we identified 25 index patients. A genetic diagnosis was reached in 60%. The most common were point mutations in GJB1, accounting for X‐linked Charcot–Marie–Tooth disease (24% of the total patient population), followed by PMP22 duplication causing Charcot–Marie–Tooth disease type 1A (20%). We also discovered 2 novel GJB1 mutations, c.521C>T (Proline174Leucine) and c.220G>A (Valine74Methionine). Conclusions: X‐linked Charcot–Marie–Tooth disease was found to predominate in our patient cohort. We also found a better phenotype/genotype correlation when applying a more recently recommended genetic approach to Charcot–Marie–Tooth disease. Muscle Nerve 49 : 198–201, 2014     

Glial fibrillary acidic protein mutations in adult‐onset Alexander disease: clinical features observed in 12 Japanese patients

Yoshida T, Sasayama H, Mizuta I, Okamoto Y, Yoshida M, Riku Y, Hayashi Y, Yonezu T, Takata Y, Ohnari K, Okuda S, Aiba I, Nakagawa M. Glial fibrillary acidic protein mutations in adult‐onset Alexander disease: clinical features observed in 12 Japanese patients.
Acta Neurol Scand: 2011: 124: 104–108.
© 2010 John Wiley & Sons A/S. Objective – To clarify the clinical manifestations of adult‐onset Alexander disease (AOAD) in Japanese patients with glial fibrillary acidic protein (GFAP) gene mutations. Methods and materials – Twelve patients of AOAD with GFAP mutations detected in our centre were examined for neurological and magnetic resonance imaging (MRI) findings. Results – Major symptoms were pyramidal and bulbar signs. In addition, three patients presented abnormal behaviour and/or memory disturbance. Two of the three patients also had Parkinsonism and had been diagnosed with fronto‐temporal dementia or progressive supranuclear palsy until GFAP mutations were detected. Abnormalities of the medulla oblongata and cervical spinal cord were observed on MRI in all patients. Conclusions – Patients presenting with pyramidal and/or bulbar signs with abnormalities of the medulla oblongata and cervical spinal cord on MRI should be considered for GFAP analysis as this is the typical presentation of AOAD. Abnormal behaviour and cognitive disorders including deterioration of memory were rare symptoms but could be an obstacle to diagnosing Alexander disease.     

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.