Genetic analysis of Charcot-Marie-Tooth disease in Denmark and the implementation of a next generation sequencing platform

Unleashing the Power of Multiomics: Unraveling the Molecular Landscape of Peripheral Neuropathy

Peripheral neuropathies (PNs) affect over 20 million individuals in the United States, manifesting as a wide range of sensory, motor, and autonomic nerve symptoms. While various conditions such as diabetes, metabolic disorders, trauma, autoimmune disease, and chemotherapy-induced neurotoxicity have been linked to PN, approximately one-third of PN cases remain idiopathic, underscoring a critical gap in our understanding of these disorders. Over the years, considerable efforts have focused on unraveling the complex molecular pathways underlying PN to advance diagnosis and treatment. Traditional methods such as linkage analysis, fluorescence in situ hybridization, polymerase chain reaction, and Sanger sequencing identified initial genetic variants associated with PN. However, the establishment and application of next-generation sequencing (NGS) and, more recently, long-read/single-cell sequencing have revolutionized the field, accelerating the discovery of novel disease-causing variants and challenging previous assumptions about pathogenicity. This review traces the evolution of genomic technologies in PN research, emphasizing the pivotal role of NGS in uncovering genetic complexities. We provide a comprehensive analysis of established genomic approaches such as genome-wide association studies, targeted gene panel sequencing, and whole-exome/genome sequencing, alongside emerging multiomic technologies including RNA sequencing and proteomics. Integrating these approaches promises holistic insights into PN pathophysiology, potentially revealing new biomarkers and therapeutic targets. Furthermore, we discuss the clinical implications of genomic and multiomic integration, highlighting their potential to enhance diagnostic accuracy, prognostic assessment, and personalized treatment strategies for PN. Challenges and questions in standardizing these technologies for clinical use are raised, underscoring the need for robust guidelines to maximize their clinical utility.

Genotype-phenotype characteristics and baseline natural history of Chinese myelin protein zero gene related neuropathy patients

BACKGROUND AND PURPOSE

The aim was to characterize the phenotypic and genotypic features of myelin protein zero (MPZ) related neuropathy and provide baseline data for longitudinal natural history studies or drug clinical trials.

METHOD

Clinical, neurophysiological and genetic data of 37 neuropathy patients with MPZ mutations were retrospectively collected.

RESULTS

Nineteen different MPZ mutations in 23 unrelated neuropathy families were detected, and the frequency of MPZ mutations was 5.84% in total. Mutations c.103_104InsTGGTTTACACCG, c.513dupG, c.521_557del and c.696_699delCAGT had not been reported previously. Hot spot mutation p.Thr124Met was detected in four unrelated families, and seven patients carried de novo mutations. The onset age indicated a bimodal distribution: prominent clustering in the first and fourth decades. The infantile-onset group included 12 families, the childhood-onset group consisted of two families and the adult-onset group included nine families. The Charcot-Marie-Tooth Disease Neuropathy Score ranged from 3 to 25 with a mean value of 15.85 ± 5.88. Mutations that changed the cysteine residue (p.Arg98Cys, p.Cys127Trp, p.Ser140Cys and p.Cys127Arg) in the extracellular region were more likely to cause severe early-onset Charcot-Marie-Tooth disease type 1B (CMT1B) or Dejerine-Sottas syndrome. Nonsense-mediated mRNA decay mutations p.Asp35delInsVVYTD, p.Leu174Argfs*66 and p.Leu172Alafs*63 were related to severe infantile-onset CMT1B or Dejerine-Sottas syndrome; however, mutation p.Val232Valfs*19 was associated with a relatively milder childhood-onset CMT1 phenotype.

CONCLUSION

Four novel MPZ mutations are reported that expand the genetic spectrum. De novo mutations accounted for 30.4% and were most related to a severe infantile-onset phenotype. Genetic and clinical data from this cohort will provide the baseline data necessary for clinical trials and natural history studies.

INF2 mutations in patients with a broad phenotypic spectrum of Charcot-Marie-Tooth disease and focal segmental glomerulosclerosis

Mutations in INF2 are associated with the complex symptoms of Charcot-Marie-Tooth disease (CMT) and focal segmental glomerulosclerosis (FSGS). To date, more than 100 and 30 genes have been reported to cause these disorders, respectively. This study aimed to identify INF2 mutations in Korean patients with CMT. This study was conducted with 743 Korean families with CMT who were negative for PMP22 duplication. In addition, a family with FSGS was included in this study. INF2 mutations were screened using whole exome sequencing (WES) and filtering processes. As the results, four pathogenic INF2 mutations were identified in families with different clinical phenotypes: p.L78P and p.L132P in families with symptoms of both CMT and FSGS; p.C104Y in a family with CMT; and p.R218Q in a family with FSGS. Moreover, different CMT types were observed in families with CMT symptoms: CMT1 in two families and Int-CMT in another family. Hearing loss was observed in two families with CMT1. Pathogenicity was predicted by in silico analyses, and considerable conformational changes were predicted in the mutant proteins. Two mutations (p.L78P and p.C104Y) were unreported, and three families showed de novo mutations that were putatively occurred from fathers. This study suggests that patients with INF2 mutations show a broad phenotypic spectrum: CMT1, CMT1 + FSGS, CMTDIE + FSGS, and FSGS. Therefore, the genotype-phenotype correlation may be more complex than previously recognized. We believe that this study expands the clinical spectrum of patients with INF2 mutations and will be helpful in the molecular diagnosis of CMT and FSGS.

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.

High diagnostic yield with algorithmic molecular approach on hereditary neuropathies

OBJECTIVE

Charcot-Marie-Tooth disease covers a group of inherited peripheral neuropathies. The aim of this study was to investigate the effect of targeted next-generation sequencing panels on the molecular diagnosis of Charcot-Marie-Tooth disease and its subtypes in routine clinical practice, and also to show the limitations and importance of next-generation sequencing in the diagnosis of Charcot-Marie-Tooth diseases.

METHODS

This is a retrospective study. Three different molecular methods (multiplex ligation probe amplification, next-generation sequencing, and whole-exome sequencing) were used to detect the mutations related to Charcot-Marie-Tooth disease.

RESULTS

In total, 64 patients (33 males and 31 females) with suspected Charcot-Marie-Tooth disease were analyzed for molecular etiology. In all, 25 (39%) patients were diagnosed by multiplex ligation probe amplification. With an extra 11 patients with normal PMP22 multiplex ligation probe amplification results that were consulted to our laboratory for further genetic analysis, a total of 50 patients underwent next-generation sequencing for targeted gene panels associated with Charcot-Marie-Tooth disease. Notably, 18 (36%) patients had pathogenic/likely pathogenic variants. Whole-exome sequencing was performed on five patients with normal next-generation sequencing results; the diagnostic yield by whole-exome sequencing was 80% and it was higher in the childhood group.

CONCLUSION

The molecular etiology in Charcot-Marie-Tooth disease patients can be determined according to pre-test evaluation, deciding the inheritance type with pedigree analysis, the clinical phenotype, and an algorithm for the genetic analysis. The presence of patients without a molecular diagnosis in all the literature suggests that there are new genes or mechanisms waiting to be discovered in the etiology of Charcot-Marie-Tooth disease.

[Genetic distribution in Chinese patients with hereditary peripheral neuropathy]

OBJECTIVE

To analyze the distribution characteristics of hereditary peripheral neuropathy (HPN) pathogenic genes in Chinese Han population, and to explore the potential pathogenesis and treatment prospects of HPN and related diseases.

METHODS

Six hundred and fifty-six index patients with HPN were enrolled in Peking University Third Hospital and China-Japan Friendship Hospital from January 2007 to May 2022. The PMP22 duplication and deletion mutations were screened and validated by multiplex ligation probe amplification technique. The next-generation sequencing gene panel or whole exome sequencing was used, and the suspected genes were validated by Sanger sequencing.

RESULTS

Charcot-Marie-Tooth (CMT) accounted for 74.3% (495/666) of the patients with HPN, of whom 69.1% (342/495) were genetically confirmed. The most common genes of CMT were PMP22 duplication, MFN2 and GJB1 mutations, which accounted for 71.3% (244/342) of the patients with genetically confirmed CMT. Hereditary motor neuropathy (HMN) accounted for 16.1% (107/666) of HPN, and 43% (46/107) of HPN was genetically confirmed. The most common genes of HMN were HSPB1, aminoacyl tRNA synthetases and SORD mutations, which accounted for 56.5% (26/46) of the patients with genetically confirmed HMN. Most genes associated with HMN could cause different phenotypes. HMN and CMT shared many genes (e.g. HSPB1, GARS, IGHMBP2). Some genes associated with dHMN-plus shared genes associated with amyotrophic lateral sclerosis (KIF5A, FIG4, DCTN1, SETX, VRK1), hereditary spastic paraplegia (KIF5A, ZFYVE26, BSCL2) and spinal muscular atrophy (MORC2, IGHMBP, DNAJB2), suggesting that HMN was a continuum rather than a distinct entity. Hereditary sensor and autosomal neuropathy (HSAN) accounted for a small proportion of 2.6% (17/666) in HPN. The most common pathogenic gene was SPTLC1 mutation. TTR was the main gene causing hereditary amyloid peripheral neuropathy. The most common types of gene mutations were p.A117S and p.V50M. The symptoms were characterized by late-onset and prominent autonomic nerve involvement.

CONCLUSION

CMT and HMN are the most common diseases of HPN. There is a large overlap between HMN and motor-CMT2 pathogenic genes, and some HMN pathogenic genes overlap with amyotrophic lateral sclerosis, hereditary spastic hemiplegia and spinal muscular atrophy, suggesting that there may be a potential common pathogenic pathway between different diseases.

Clinical characteristics and proteome modifications in two Charcot-Marie-Tooth families with the AARS1 Arg326Trp mutation

BACKGROUND

Aminoacyl tRNA-synthetases are ubiquitously-expressed enzymes that attach amino acids to their cognate tRNA molecules. Mutations in several genes encoding aminoacyl tRNA-synthetases, have been associated with peripheral neuropathy, i.e. AARS1, GARS1, HARS1, YARS1 and WARS1. The pathogenic mechanism underlying AARS1-related neuropathy is not known.

METHODS

From 2012 onward, all probands presenting at Telemark Hospital (Skien, Norway) with peripheral neuropathy were screened for variants in AARS1 using an "in-house" next-generation sequencing panel. DNA from patient's family members was examined by Sanger sequencing. Blood from affected family members and healthy controls were used for quantification of AARS1 mRNA and alanine. Proteomic analyses were conducted in peripheral blood mononuclear cells (PBMC) from four affected family members and five healthy controls.

RESULTS

Seventeen individuals in two Norwegian families affected by Charcot-Marie-Tooth disease (CMT) were characterized in this study. The heterozygous NM_001605.2:c.976C > T p.(Arg326Trp) AARS1 mutation was identified in ten affected family members. All living carriers had a mild to severe length-dependent sensorimotor neuropathy. Three deceased obligate carriers aged 74-98 were reported to be unaffected, but were not examined in the clinic. Proteomic studies in PBMC from four affected individuals suggest an effect on the immune system mediated by components of a systemic response to chronic injury and inflammation. Furthermore, altered expression of proteins linked to mitochondrial function/dysfunction was observed. Proteomic data are available via ProteomeXchange using identifier PXD023842.

CONCLUSION

This study describes clinical and neurophysiological features linked to the p.(Arg326Trp) variant of AARS1 in CMT-affected members of two Norwegian families. Proteomic analyses based on of PBMC from four CMT-affected individuals suggest that involvement of inflammation and mitochondrial dysfunction might contribute to AARS1 variant-associated peripheral neuropathy.

Comprehensive Genetic Analyses of Inherited Peripheral Neuropathies in Japan: Making Early Diagnosis Possible

Various genomic variants were linked to inherited peripheral neuropathies (IPNs), including large duplication/deletion and repeat expansion, making genetic diagnosis challenging. This large case series aimed to identify the genetic characteristics of Japanese patients with IPNs. We collected data on 2695 IPN cases throughout Japan, in which PMP22 copy number variation (CNV) was pre-excluded. Genetic analyses were performed using DNA microarrays, next-generation sequencing-based gene panel sequencing, whole-exome sequencing, CNV analysis, and RFC1 repeat expansion analysis. The overall diagnostic rate and the genetic spectrum of patients were summarized. We identified 909 cases with suspected IPNs, pathogenic or likely pathogenic variants. The most common causative genes were MFN2, GJB1, MPZ, and MME. MFN2 was the most common cause for early-onset patients, whereas GJB1 and MPZ were the leading causes of middle-onset and late-onset patients, respectively. Meanwhile, GJB1 and MFN2 were leading causes for demyelinating and axonal subtypes, respectively. Additionally, we identified CNVs in MPZ and GJB1 genes and RFC1 repeat expansions. Comprehensive genetic analyses explicitly demonstrated the genetic basis of our IPN case series. A further understanding of the clinical characteristics of IPN and genetic spectrum would assist in developing efficient genetic testing strategies and facilitate early diagnosis.

Analyzing Precision Medicine Utilization with Real-World Data: A Scoping Review

Precision medicine (PM), specifically genetic-based testing, is currently used in over 140,000 individual tests to inform the clinical management of disease. Though several databases (e.g., the NIH Genetic Testing Registry) demonstrate the availability of these sequencing-based tests, we do not currently understand the extent to which these tests are used. There exists a need to synthesize the body of real-world data (RWD) describing the use of sequencing-based tests to inform their appropriate use. To accomplish this, we performed a scoping review to examine what RWD sources have been used in studies of PM utilization between January 2015 and August 2021 to characterize the use of genome sequencing (GS), exome sequencing (ES), tumor sequencing (TS), next-generation sequencing-based panels (NGS), gene expression profiling (GEP), and pharmacogenomics (PGx) panels. We abstracted variables describing the use of these types of tests and performed a descriptive statistical analysis. We identified 440 articles in our search and included 72 articles in our study. Publications based on registry databases were the most common, followed by studies based on private insurer administrative claims. Slightly more than one-third (38%) used integrated datasets. Two thirds (67%) of the studies focused on the use of tests for oncological clinical applications. We summarize the RWD sources used in peer-reviewed literature on the use of PM. Our findings will help improve future study design by encouraging the use of centralized databases and registries to track the implementation and use of PM.

Genetic Workup for Charcot–Marie–Tooth Neuropathy: A Retrospective Single-Site Experience Covering 15 Years

Charcot–Marie–Tooth (CMT) disease is the most commonly inherited neurological disorder. This study includes patients affected by CMT during regular follow-ups at the CMT clinic in Genova, a neuromuscular university center in the northwest of Italy, with the aim of describing the genetic distribution of CMT subtypes in our cohort and reporting a peculiar phenotype. Since 2004, 585 patients (447 index cases) have been evaluated at our center, 64.9% of whom have a demyelinating neuropathy and 35.1% of whom have an axonal neuropathy. A genetic diagnosis was achieved in 66% of all patients, with the following distribution: CMT1A (48%), HNPP (14%), CMT1X (13%), CMT2A (5%), and P0-related neuropathies (7%), accounting all together for 87% of all the molecularly defined neuropathies. Interestingly, we observe a peculiar phenotype with initial exclusive lower limb involvement as well as lower limb involvement that is maintained over time, which we have defined as a “strictly length-dependent” phenotype. Most patients with this clinical presentation shared variants in either HSPB1 or MPZ genes. The identification of distinctive phenotypes such as this one may help to address genetic diagnosis. In conclusion, we describe our diagnostic experiences as a multidisciplinary outpatient clinic, combining a gene-by-gene approach or targeted gene panels based on clinical presentation.

Cohort Analysis of 67 Charcot-Marie-Tooth Italian Patients: Identification of New Mutations and Broadening of Phenotype Expression Produced by Rare Variants

Charcot-Marie-Tooth (CMT) disease is the most prevalent inherited motor sensory neuropathy, which clusters a clinically and genetically heterogeneous group of disorders with more than 90 genes associated with different phenotypes. The goal of this study is to identify the genetic features in the recruited cohort of patients, highlighting the role of rare variants in the genotype-phenotype correlation. We enrolled 67 patients and applied a diagnostic protocol including multiple ligation-dependent probe amplification for copy number variation (CNV) detection of PMP22 locus, and next-generation sequencing (NGS) for sequencing of 47 genes known to be associated with CMT and routinely screened in medical genetics. This approach allowed the identification of 26 patients carrying a whole gene CNV of PMP22. In the remaining 41 patients, NGS identified the causative variants in eight patients in the genes HSPB1, MFN2, KIF1A, GDAP1, MTMR2, SH3TC2, KIF5A, and MPZ (five new vs. three previously reported variants; three sporadic vs. five familial variants). Familial segregation analysis allowed to correctly interpret two variants, initially reported as "variants of uncertain significance" but re-classified as pathological. In this cohort is reported a patient carrying a novel familial mutation in the tail domain of KIF5A [a protein domain previously associated with familial amyotrophic lateral sclerosis (ALS)], and a CMT patient carrying a HSPB1 mutation, previously reported in ALS. These data indicate that combined tools for gene association in medical genetics allow dissecting unexpected phenotypes associated with previously known or unknown genotypes, thus broadening the phenotype expression produced by either pathogenic or undefined variants. Clinical trial registration: ClinicalTrials.gov (NCT03084224).

Charcot-Marie-Tooth disease and hereditary motor neuropathies – Update 2020

Inherited peripheral neuropathy is the most common hereditary neuromuscular disease with a prevalence of about 1:2,500. The most frequent form is Charcot-Marie-Tooth disease (CMT, or hereditary motor and sensory neuropathy [HMSN]). Other clinical entities are hereditary neuropathy with liability to pressure palsies (HNPP), distal hereditary motor neuropathies (dHMN), and hereditary sensory and autonomic neuropathies (HSAN). With the exception of HNPP, which is almost always caused by defects of the PMP22 gene, all other forms show genetic heterogeneity with altogether more than 100 genes involved. Mutation detection rates vary considerably, reaching up to 80 % in demyelinating CMT (CMT1) but are still as low as 10–30 % in axonal CMT (CMT2), dHMN, and HSAN. Based on current information, analysis of only four genes (PMP22, GJB1, MPZ, MFN2) identifies 80–90 % of CMT-causing mutations that can be detected in all known disease genes. For the remaining patients, parallel analysis of multiple neuropathy genes using next-generation sequencing is now replacing phenotype-oriented multistep gene-by-gene sequencing. Such approaches tend to generate a wealth of genetic information that requires comprehensive evaluation of the pathogenic relevance of identified variants. In this review, we present current classification systems, specific phenotypic clues, and diagnostic yields in the different subgroups of hereditary CMT and motor neuropathies.

Diagnostic yield of targeted sequential and massive panel approaches for inherited neuropathies

Diagnostic yield of genetic studies for Charcot-Marie-Tooth disease (CMT) is little known, with a lack of epidemiological data to build better diagnostic strategies outside the United States and Europe. We aimed to evaluate the performance of two molecular diagnostic strategies for patients with CMT, and to characterize epidemiological findings of these conditions in southern Brazil. We performed a single-center cross-sectional study, in which 94 patients (55 families) with CMT suspicion were evaluated. Overall, the diagnostic yield of the combined strategy of Multiplex-ligation-dependent-probe-amplification (MLPA) of PMP22/GJB1/MPZ and GJB1/MPZ/PMP22 Sanger sequencing was 63.6% (28/44) for index cases with demyelinating/intermediate CMT suspicion (21 CMT1A-PMP22, 5 CMTX1-GJB1 and 2 with probably CMT1B-MPZ diagnosis). Five of the 11 index cases (45.4%) with axonal CMT had at least a possible diagnosis with next generation sequencing (NGS) panel of 104 inherited neuropathies-related genes (one each with CMT1A-PMP22, CMT2A-MFN2, CMT2K-GDAP1, CMT2U-MARS, CMT2W-HARS1). Detailed clinical, neurophysiological and molecular data of families are provided. Sequential molecular diagnosis strategies with MLPA plus target Sanger sequencing for demyelinating/intermediate CMT had high diagnostic yield, and almost half of axonal CMT families had at least a possible diagnosis with the comprehensive NGS panel. Most frequent subtypes of CMT in our region are CMT1A-PMP22 and CMTX1-GJB1.

Comprehensive genetic sequence and copy number analysis for Charcot-Marie-Tooth disease in a Canadian cohort of 2517 patients

Charcot-Marie-Tooth disease (CMT) is one of the most common Mendelian disorders characterised by genetic heterogeneity, progressive distal muscle weakness and atrophy, foot deformities and distal sensory loss. In this report, we describe genetic testing data including comprehensive sequencing and copy number analysis of 34 CMT-related genes in a Canadian cohort of patients with suspected CMT. We have demonstrated a notable gender testing bias, with an overall diagnostic yield of 15% in males and 21% in females. We have identified a large number of novel pathogenic variants as well as variants of unknown clinical significance in CMT-related genes. In this largest to date analysis of gene CNVs in CMT, in addition to the common PMP22 deletion/duplication, we have described a significant contribution of pathogenic CNVs in several CMT-related genes. This study significantly expand the mutational spectrum of CMT genes, while demonstrating the clinical utility of a comprehensive sequence and copy number next-generation sequencing-based clinical genetic testing in patients with suspected diagnosis of CMT.

PMP22 Gene-Associated Neuropathies: Phenotypic Spectrum in a Cohort from India

Reports of spectrum of clinical manifestations in PMP22 gene-associated neuropathies (duplication/mutations) are scarce. To identify the frequency of PMP22 gene variations and establish their genotype-phenotype correlation. Patients with suspected genetic demyelinating neuropathy (n = 128) underwent evaluation for copy number variations and point mutations in PMP22 gene by multiplex ligation-dependent probe amplification (MLPA) and direct sequencing respectively. Of these, only 27 patients (M:F:19:8) from 18 families had PMP22 gene-associated neuropathy; they were subsequently analyzed for genotype-phenotype correlation. Twenty-five patients had PMP22 duplication while two patients had PMP22 missense mutations (p.A114V and p.L80P). Age at onset of neuropathy ranged from infancy to 63 years and symptom duration ranged from 2 to 32 years. Cranial nerve dysfunction in the form of ptosis, ophthalmoplegia, bifacial weakness, and sensorineural hearing loss was observed in addition to a number of systemic features. Three patients were asymptomatic. All except one patient were ambulant. Velocity of median nerve and amplitude of evoked motor responses from common peroneal nerve were significantly reduced in male patients. There was significantly worse disability in the late-onset group as compared with the early-onset group. Otherwise, the mean age at onset, frequency of skeletal deformities, patterns of motor weakness, muscle stretch reflexes, sensory impairment, disability rating scales, and electrophysiological parameters were comparable irrespective of gender, onset age, family history and ulnar nerve conduction velocities. The relatively low frequency of PMP22 duplication in the present cohort warrants a more comprehensive search to establish the genetic etiology. Further research into the role of other genetic variants as well as modifier genes and their effect on phenotypic heterogeneity is indicated.

Targeted next-generation sequencing panels in the diagnosis of Charcot-Marie-Tooth disease

OBJECTIVE

To investigate the effectiveness of targeted next-generation sequencing (NGS) panels in achieving a molecular diagnosis in Charcot-Marie-Tooth disease (CMT) and related disorders in a clinical setting.

METHODS

We prospectively enrolled 220 patients from 2 tertiary referral centers, one in London, United Kingdom (n = 120), and one in Iowa (n = 100), in whom a targeted CMT NGS panel had been requested as a diagnostic test. PMP22 duplication/deletion was previously excluded in demyelinating cases. We reviewed the genetic and clinical data upon completion of the diagnostic process.

RESULTS

After targeted NGS sequencing, a definite molecular diagnosis, defined as a pathogenic or likely pathogenic variant, was reached in 30% of cases (n = 67). The diagnostic rate was similar in London (32%) and Iowa (29%). Variants of unknown significance were found in an additional 33% of cases. Mutations in GJB1, MFN2, and MPZ accounted for 39% of cases that received genetic confirmation, while the remainder of positive cases had mutations in diverse genes, including SH3TC2, GDAP1, IGHMBP2, LRSAM1, FDG4, and GARS, and another 12 less common genes. Copy number changes in PMP22, MPZ, MFN2, SH3TC2, and FDG4 were also accurately detected. A definite genetic diagnosis was more likely in cases with an early onset, a positive family history of neuropathy or consanguinity, and a demyelinating neuropathy.

CONCLUSIONS

NGS panels are effective tools in the diagnosis of CMT, leading to genetic confirmation in one-third of cases negative for PMP22 duplication/deletion, thus highlighting how rarer and previously undiagnosed subtypes represent a relevant part of the genetic landscape of CMT.

Whole exome sequencing reveals a broader variant spectrum of Charcot-Marie-Tooth disease type 2

Charcot-Marie-Tooth disease type 2 (CMT2) is a clinically and genetically heterogeneous inherited neuropathy. Although new causative and disease-associated genes have been identified for CMT2 in recent years, molecular diagnoses are still lacking for a majority of patients. We here studied a cohort of 35 CMT2 patients of Chinese descent, using whole exome sequencing to investigate gene mutations and then explored relationships among genotypes, clinical features, and mitochondrial DNA levels in blood as assessed by droplet digital PCR. We identified pathogenic variants in 57% of CMT2 patients. The most common genetic causes in the cohort were MFN2 mutations. Two patients with typical CMT phenotype and neuromyotonia were detected to harbor compound heterozygous variations in the HINT1 gene. In conclusion, our work supports that the molecular diagnostic rate of CMT2 patients can be increased via whole exome sequencing, and our data suggest that assessment of possible HINT1 mutations should be undertaken for CMT2 patients with neuromyotonia.

Genetic profile and onset features of 1005 patients with Charcot-Marie-Tooth disease in Japan

OBJECTIVE : To identify the genetic characteristics in a large-scale of patients with Charcot-Marie-Tooth disease (CMT). METHODS: From May 2012 to August 2016, we collected 1005 cases with suspected CMT throughout Japan, whereas PMP22 duplication/deletion were excluded in advance for demyelinating CMT cases. We performed next-generation sequencing targeting CMT-related gene panels using Illumina MiSeq or Ion Proton, then analysed the gene-specific onset age of the identified cases and geographical differences in terms of their genetic spectrum. RESULTS : From 40 genes, we identified pathogenic or likely pathogenic variants in 301 cases (30.0%). The most common causative genes were GJB1 (n=66, 21.9%), MFN2 (n=66, 21.9%) and MPZ (n=51, 16.9%). In demyelinating CMT, variants were detected in 45.7% cases, and the most common reasons were GJB1 (40.3%), MPZ (27.1%), PMP22 point mutations (6.2%) and NEFL (4.7%). Axonal CMT yielded a relatively lower detection rate (22.9%), and the leading causes, occupying 72.4%, were MFN2 (37.2%), MPZ (9.0%), HSPB1 (8.3%), GJB1 (7.7%), GDAP1 (5.1%) and MME (5.1%). First decade of life was found as the most common disease onset period, and early-onset CMT cases were most likely to receive a molecular diagnosis. Geographical distribution analysis indicated distinctive genetic spectrums in different regions of Japan. CONCLUSIONS : Our results updated the genetic profile within a large-scale of Japanese CMT cases. Subsequent analyses regarding onset age and geographical distribution advanced our understanding of CMT, which would be beneficial for clinicians.

Identification of a pathogenic PMP2 variant in a multi-generational family with CMT type 1: Clinical gene panels versus genome-wide approaches to molecular diagnosis

Charcot-Marie-Tooth (CMT) disease type 1 is an inherited peripheral neuropathy characterized by demyelination and reduced nerve conduction velocities. We present a multi-generational family with peripheral neuropathy in whom clinical CMT panel testing failed to conclude a molecular diagnosis. We found a PMP2 pathogenic variant c.155T > C, p.(Ile52Thr) that segregates with disease suggesting that PMP2 variants should be considered in patients with neuropathy and that it may be prudent to include in clinical CMT gene panels.