Heterozygous Seryl-tRNA Synthetase 1 Variants Cause Charcot-Marie-Tooth Disease.

OBJECTIVE: Despite the increasing number of genes associated with Charcot-Marie-Tooth (CMT) disease, many patients currently still lack appropriate genetic diagnosis for this disease. Autosomal dominant mutations in aminoacyl-tRNA synthetases (ARSs) have been implicated in CMT. Here, we describe causal missense mutations in the gene encoding seryl-tRNA synthetase 1 (SerRS) for 3 families affected with CMT. METHODS: Whole-exome sequencing was performed in 16 patients and 14 unaffected members of 3 unrelated families. The functional impact of the genetic variants identified was investigated using bioinformatic prediction tools and confirmed using cellular and biochemical assays. RESULTS: Combined linkage analysis for the 3 families revealed significant linkage (Zmax LOD = 6.9) between the genomic co-ordinates on chromosome 1: 108681600-110300504. Within the linkage region, heterozygous SerRS missense variants segregated with the clinical phenotype in the 3 families. The mutant SerRS proteins exhibited reduced aminoacylation activity and abnormal SerRS dimerization, which suggests the impairment of total protein synthesis and induction of eIF2α phosphorylation. INTERPRETATION: Our findings suggest the heterozygous SerRS variants identified represent a novel cause for autosomal dominant CMT. Mutant SerRS proteins are known to impact various molecular and cellular functions. Our findings provide significant advances on the current understanding of the molecular mechanisms associated with ARS-related CMT. ANN NEUROL 2023;93:244-256.

Mutations in C1orf194, encoding a calcium regulator, cause dominant Charcot-Marie-Tooth disease.

Charcot-Marie-Tooth disease is a hereditary motor and sensory neuropathy exhibiting great clinical and genetic heterogeneity. Here, the identification of two heterozygous missense mutations in the C1orf194 gene at 1p21.2-p13.2 with Charcot-Marie-Tooth disease are reported. Specifically, the p.I122N mutation was the cause of an intermediate form of Charcot-Marie-Tooth disease, and the p.K28I missense mutation predominately led to the demyelinating form. Functional studies demonstrated that the p.K28I variant significantly reduced expression of the protein, but the p.I122N variant increased. In addition, the p.I122N mutant protein exhibited the aggregation in neuroblastoma cell lines and the patient's peroneal nerve. Either gain-of-function or partial loss-of-function mutations to C1ORF194 can specify different causal mechanisms responsible for Charcot-Marie-Tooth disease with a wide range of clinical severity. Moreover, a knock-in mouse model confirmed that the C1orf194 missense mutation p.I121N led to impairments in motor and neuromuscular functions, and aberrant myelination and axonal phenotypes. The loss of normal C1ORF194 protein altered intracellular Ca2+ homeostasis and upregulated Ca2+ handling regulatory proteins. These findings describe a novel protein with vital functions in peripheral nervous systems and broaden the causes of Charcot-Marie-Tooth disease, which open new avenues for the diagnosis and treatment of related neuropathies.

[Classification and molecular diagnostic procedure for Chacort-Marie-Tooth disease].

Charcot-Marie-Tooth disease (CMT) is the most common form of hereditary neuropathy with significant clinical and genetic heterogeneity. So far 28 genes have been cloned. The main clinical manifestations of CMT include progressive distal muscle wasting and weakness, impaired distal sensation, and diminishing or loss of tendon reflex. Patients may be classified into demyelinating type (CMT1) and axonal type (CMT2) according to electrophysiological and pathological characteristics. Establishment of a standard diagnostic procedure based on clinical, electrophysiological and pathological findings will enable accurate diagnosis in most CMT patients and provide guidance for gene consulting and prognosis.

[Analysis of CX32 gene mutation and related clinical features in Chinese Han Charcot-Marie-Tooth families].

OBJECTIVE: To analyze the mutation of CX32 gene and related clinical features in Chinese Han patients with Charcot-Marie-Tooth (CMT) disease. METHODS: Thirty-four CMT families, from 2004 to 2011 at Departments of Neurology, Xiangya Hospital, Third Xiangya Hospital and National Key Laboratory of Medical Genetics, were selected for CX32 mutation screening after the exclusion of the PMP22 duplication and male-to-male transmission. Mutation analysis was carried out by polymerase chain reaction (PCR) plus direct sequencing. Analyses of clinical, electrophysiological and pathological features in 11 patients from 6 CMTX1 families were performed by 2 neurologists. RESULTS: Five CX32 gene mutations were detected in 6 CMT families: c.37G > A, c.65G > A, c.246C > G, c.256A > G and c.533A > G. Among them, c.246C > G and c.533A > G were firstly reported. The clinical manifestations included progressive distal muscle atrophy and weakness, areflexia, sensory abnormalities and pes vacus. Nerve conduction velocity ranged from 21.7 to 49.3 m/s. Both demyelination and axonal degeneration were detected in nerve biopsy. CONCLUSIONS: CMT1X has a frequency of around 9% in our study. The male patients tend to have more serious clinical features and their electrophysiological and pathological changes are intermediate. CX32 mutation analysis helps to confirm the genetic diagnosis of CMT so as to provide genetic counseling and reproductive guidance and elucidate its pathogenesis.

[The effect of HSPB8 gene mutation on cell viability in Charcot-Marie-Tooth disease type 2L].

OBJECTIVE: To study the effect of Charcot-Marie-Tooth 2L disease causing gene K141N mutation in heat shock protein B8 gene (HSPB8) on cell viability. METHODS: By using liposome transfection technique, (wt)HSPB8, (K141N)HSPB8 eukaryotic expression vector and green fluorescent protein (GFP) vector were transfected into SHSY-5Y cell, respectively. Twenty-four hours later, the cells were treated with 44 degree centigrade lethal heat shock for 40 minutes. The relative viability of SHSY-5Y cells in each group was tested by using tetrazole blue colorimetric method (methyl thiazolyl tetrazolium, MTT). RESULTS: There were significant differences among the light absorption value of GFP, pEGFP-(wt)HSPB8 and pEGFP-(K141N)HSPB8 transfected groups after heat shock (P<0.05), indicating that the relative viability of cells overexpressed with (wt)HSPB8 and (K141N)HSPB8 was different from that of control cells. The viability of cells overexpressing (wt)HSPB8 was highest, followed by cells overexpressed with (K141N)HSPB8. The viability of cells tranfected with GFP only was the lowest. CONCLUSION: HSPB8 may play an important role in the protection of cells under lethal heat shock treatment, and the K141N mutation can impair the protective effect.

[Cellular expression of (R127W)HSPB1 and its co-localization with neurofilament light chain].

OBJECTIVE: To observe the cellular expression of (R127W) HSPB1 and its influence on neurofilament light chain (NFL) self-assembly and co-localization with NFL. METHODS: Eukaryotic expression vectors pEGFPN1-(wt) HSPB1 and pEGFPN1- (R127W) HSPB1 were constructed. Hela cells were transiently transfected with pEGFPN1-(wt) HSPB1 or pEGFPN1- (R127W) HSPB1 and observed under a confocal microscope. Hela cells were also transiently co-transfected with Pcl-NFL and pEGFPN1-(wt)HSPB1, or pCL-NFL and pEGFPN1-(R127W)HSPB1. The self-assembly of NFL was observed and the co-localization study of HSPB1/ (R127W)HSPB1 with NFL was carried out in these two cell models by immunofluorescence technique. RESULTS: The aggregates formed by EGFP-(R127W)HSPB1 predominantly located around the nucleus, and EGFP-(wt)HSPB1 showed diffusion pattern in Hela cells. When co expressed with EGFP-(wt)HSPB1, NFL formed homogeneous structure in cytosol. When co-expressed with EGFP-(R127W)HSPB1, however, NFL had amorphous staining pattern predominantly consisting of NFL aggregates, and NFL co-localized with (R127W)HSPB1 in these aggregates. CONCLUSION: The R127W mutant of HSPB1 may have reduced capacity to serve as a chaperone to prevent aggregate formation, and fail to correctly organize the neurofilament network. Dysfunction of the axon cytoskeleton and axon transport may be the primary mechanism of R127W mutation of HSPB1 in the pathogenesis of Charcot-Marie-Tooth disease.

[Mutation analysis of LITAF, RAB7, LMNA and MTMR2 genes in Chinese Charcot-Marie-Tooth disease.].

The purpose of this study was to understand the mutation features of lipopolysaccharide-induced tumor necrosis factor-alpha factor (LITAF), ras-associated protein RAB7 (RAB7), lamin A/C (LMNA) and myotubularin-related protein 2 (MTMR2) genes in Chinese Charcot-Marie-Tooth disease (CMT) patients. Mutation analysis of LITAF gene was carried out using PCR combined with DNA sequencing, and mutation analysis of RAB7 gene by PCR-single strand conformation polymorphism (PCR-SSCP) combined with DNA sequencing in 33 CMT patients including 6 probands of autosomal domi-nated CMT families and 27 sporadic patients; mutation analysis of LMNA and MTMR2 genes was observed using PCR-SSCP combined with DNA sequencing in 41 CMT patients, including 14 probands of autosomal recessive CMT fami-lies and 27 sporadic patients. Two sequence variations c.269G-->A and c.274A-->G were detected in LITAF gene and two sequence variations c.1243G-->A and c.1910C-->T were detected in LMNA gene. No sequence variation was found in RAB7 and MTMR2 gene. Variations of c.269G-->A in LITAF gene and c.1243G-->A, c.1910C-->T in LMNA gene are newly found SNPs in this study. Variation of c.274A-->G in LITAF gene is known SNP reported in SNP database. Mutations in LITAF, RAB7, LMNA, and MTMR2 genes are rare in Chinese CMT patients.

[Mutation analysis of MFN2 gene in Chinese patients with Charcot-Marie-Tooth disease].

OBJECTIVE: To analyze MFN2 gene mutation in Chinese patients Charcot-Marie-Tooth disease (CMT) and to establish a quick and effective diagnostic method. METHODS: Through denaturing high-performance liquid chromatography (DHPLC) combined with DNA sequencing, MFN2 gene mutation analysis was carried out in 35 Chinese CMT2 patients including 9 probands of CMT2 pedigree and 26 sporadic CMT2 patients. RESULTS: The investigators found three abnormal sequence variations in MFN2 gene: c.281G-->A, c.395G-->A and c.408A-->T. c.395G-->A (C132T) was a novel causative missense mutation firstly reported while c.281G-->A (R94Q) a hotspot mutation and c.408A-->T (V136V) a single nucleotide polymorphism (SNP). The accuracy and specificity of DHPLC detection reached up to 100%. CONCLUSION: Through DHPLC combined with DNA sequencing, MFN2 mutations are detected in Chinese CMT2 patients. There are two causative missense mutations: c.395G-->A (C132T) and c.281G-->A (R94Q) and one SNP c.408A-->T (V136V). Such a method is an effective and economic diagnostic screening tool of MFN2 gene in CMT patients on a large scale.

Screening for SH3TC2, PMP2, and BSCL2 Variants in a Cohort of Chinese Patients with Charcot-Marie-Tooth.

BACKGROUND: SH3TC2, PMP2, and BSCL2 genes are related to autosomal recessive (AR) Charcot-Marie-Tooth (CMT) disease type 1, autosomal dominant (AD)-CMT1, and AD-CMT2, respectively. Pathogenic variants in these three genes were not well documented in Chinese CMT patients. Therefore, this study aims to detect SH3TC2, PMP2, and BSCL2 pathogenic variants in a cohort of 315 unrelated Chinese CMT families. METHODS: A total of 315 probands from 315 unrelated Chinese CMT families were recruited from the Department of Neurology of Third Xiangya Hospital and Xiangya Hospital. We screened for SH3TC2 pathogenic variants in 84 AR or sporadic CMT probands, PMP2 pathogenic variants in 39 AD or sporadic CMT1 probands, and BSCL2 pathogenic variants in 50 AD or sporadic CMT2 probands, using polymerase chain reaction and Sanger sequencing. All these patients were out of 315 unrelated Chinese CMT families and genetically undiagnosed after exclusion of pathogenic variants of PMP22, MFN2, MPZ, GJB1, GDAP1, HSPB1, HSPB8, EGR2, NEFL, and RAB7. Candidate variants were analyzed based on the standards and guidelines of American College of Medical Genetics and Genomics (ACMG). Clinical features were reevaluated. RESULTS: We identified three novel heterozygous variants such as p.L95V (c.283C>G), p.L1048P (c.3143T>C), and p.V1105M (c.3313G>A) of SH3TC2 gene and no pathogenic variants of PMP2 and BSCL2 genes. Although evaluation in silico and screening in the healthy control revealed that the three SH3TC2 variants were likely pathogenic, no second allele variants were discovered. According to the standards and guidelines of ACMG, the heterozygous SH3TC2 variants such as p.L95V, p.L1048P, and p.V1105M were considered to be of uncertain significance. CONCLUSIONS: SH3TC2, PMP2, and BSCL2 pathogenic variants might be rare in Chinese CMT patients. Further studies to confirm our findings are needed.

[Study on aggregate formation mechanism of HSPB8 gene mutation resulting in CMT2L].

OBJECTIVE: To study the possible mechanism of the intracellular aggregate formation of small heat shock protein HSPB8 (HSPB8)(K141N) mutation resulting in axonal Charcot-Marie-Tooth disease type 2L(CMT2L). METHODS: The cell models which transiently expressed pEGFPN1-HSPB8 and pEGFPN1-(K141N)HSPB8 were established. The immunofluorescent co-location study of EGFP-(K141N)HSPB8 and HSPB1, EGFP-(K141N)HSPB8 and neurofilament light chain (NEFL) was carried out in the SHSY5Y cell models. The aggregate formation of EGFP-(K141N)HSPB8 in cell models was investigated and the possible mechanism of cellular aggregate formation was analyzed by t test and analysis of variance between group(ANOVA). RESULTS: EGFP-(K141N)HSPB8 formed large aggregate which predominantly located around the nucleus in cell models. EGFP-(K141N)HSPB8 co-localized perfectly with HSPB1 and NEFL in the SHSY5Y cell models. The aggregate formation was different in different cell types, there were fewer aggregates formed in an sHSPs deficient milieu than in HEK293T cells. CONCLUSION: (K141N)HSPB8 formed aggregates predominantly locate around the nucleus in cells. (K141N)HSPB8 co-localizes perfectly with HSPB1 and NEFL. The aggregate formation may be due to (K141N)HSPB8 conformational change leading to self aggregation and its abnormal interaction with other sHSPs such as HSPB1.

[Cloning to rule out 10 candidate genes located in chromosome 12q24 for Charcot-Marie-Tooth disease type 2L].

OBJECTIVE: To clone the disease-causing genes possibly existing in 6.8 cM distance between microsatellite markers D12S1720 and D12S1611 in chromosome 12q24 for Charcot-Marie-Tooth disease type 2L (CMT2L). METHODS: Ten positional and functional candidate genes were chosen among all known genes in this locus region by bioinformatics inqury. Mutation detection was performed by sequencing the exons and intron-exon junctions of the candidate genes. RESULTS: Eleven sequence variations, that included 5 heterozygous and 6 homozygous variations, were detected in the exons and flanking areas of the 10 candidate genes. All the variations showed no co-segregation with disease phenotype. CONCLUSION: Ten candidate genes(TAOK3, RAB35, RPLP0, PXN, RNF10, RHOF, VPS33A, RSN, DENR, RNP24) were ruled out as the disease-causing gene for CMT2L. Ten single nucleotide polymorphisms (SNP) were reported for the first time.

[Mutation analysis of small heat shock protein 27 gene in Chinese patients with Charcot-Marie-Tooth disease].

OBJECTIVE: To investigate the features of small heat shock protein 27 (HSP27) gene mutation in Chinese patients with Charcot-Marie-Tooth disease (CMT). METHODS: DNA samples from 114 CMT probands were screened for mutations in HSP27 gene by polymerase chain reaction and direct sequencing, and haplotype analysis was further carried out on the mutation detected families. RESULTS: One missense mutation C379T was detected in 4 autosomal dominant CMT2 families. Haplotype analysis indicated that the 4 families probably had a common ancestor. CONCLUSION: To the authors' knowledge, this is the first report of HSP27 gene mutation in Chinese patients with CMT, but it may be not common(0.90%). The C379T mutation in HSP27 gene also causes CMT2 except for distal hereditary motor neuropathy, thus providing further evidence that even the same mutation in the same gene may lead to distinct phenotypes.

[Detection of duplications or deletions of the PMP22 gene using real-time quantitative PCR].

OBJECTIVE: To detect the duplication or deletion of peripheral myelin protein 22(PMP22) gene in Chinese patients with Charcot-Marie-Tooth disease(CMT) or hereditary neuropathy with liability to pressure palsies(HNPP) using real-time quantitative polymerase chain reaction. METHODS: Duplications or deletions of PMP22 gene were detected in 113 CMT cases, 4 HNPP cases and 50 normal controls by using real-time quantitative PCR. RESULTS: Thirty-six of 113 CMT cases had the PMP22 duplication, 4 HNPP cases had the PMP22 deletion. No duplication or deletion was found in 50 normal controls. CONCLUSION: The PMP22 duplication rate in Chinese patients with CMT is 31.9%(36/113). PMP22 deletion is the common cause of HNPP.

Mutation analysis of small heat-shock protein 22 gene in Chinese patients with Charcot-Marie-Tooth disease.

OBJECTIVE: To study the characteristics of the mutation of small heat-shock protein 22 (HSP22) gene in Chinese patients with Charcot-Marie-Tooth (CMT) disease. METHODS: A CMT2L proband with 423(G--> T) mutation in HSP22 gene had been studied and reported by the present authors. In this study, mutation analysis of HSP22 gene was performed using polymerase chain reaction and DNA direct sequencing in 114 CMT probands. RESULTS: In the 114 CMT probands, a 582(C--> T)(T194T)samesense mutation was found in two unrelated families. CONCLUSION: The rate of HSP22 gene mutation in Chinese patients with CMT is as low as 0.87%(1/115).

Mutation screening of Cx32 in Han Chinese patients with Charcot-Marie-Tooth disease.

OBJECTIVE: To investigate the Cx32 mutation features and the clinical manifestations of Chinese patients with Charcot-Marie-Tooth disease(CMT). METHODS: Twenty-four of 65 unrelated CMT patients were selected for Cx32 mutation screening after the exclusion of the CMT1A 1.5 Mb duplication and male-to-male transmission. The motor and sensory nerve conduction studies were performed in all probands and most of their affected family members to establish the clinical CMT1 ,CMT2 or CMT intermediate diagnosis. The presence of mutations in the coding region of Cx32 was detected by single-strand conformation polymorphism analysis combined with direct sequencing. RESULTS: We found 7 different point mutations in the coding region of Cx32 in a total of 7 families. All the patients were mildly to moderately affected with a clinical CMT1 or CMT intermediate diagnosis. The mutation Arg15Gln was inherited with X-linked recessive trait in family 1 involved in our study. The Arg75Trp mutation was detected in a family with X-linked dominant CMT and autosomal recessive nonsydromic hearing loss. The clinical phenotype of the Thr188Ala mutation was firstly reported. CONCLUSION: Seven different Cx32 point mutations were detected and the percentage of Chinese CMT families with Cx32 mutation is about 10% in our study. The inheritance model of CMT secondary to Cx32 mutation could be X-linked dominant, X-linked recessive or sporadic. Male patients are usually more severely affected than females with slower nerve conduction velocities. Cx32 mutation screening should be firstly performed in those CMT families without male-to-male transmission and CMT1A duplication.

[Analysis of the pathological features and gene mutations of Chinese patients with Charcot-Marie-Tooth disease].

OBJECTIVE: To analyze the relationship of the pathological features and the gene mutations of Chinese patients with Charcot-Marie-Tooth disease. METHODS: The clinical manifestations and pathological investigations of 26 Chinese patients with Charcot-Marie-Tooth disease, 17 males and 9 females, aged 19.0 (4 - 49), with an average disease course of 0.5 - 30 years, 16 being with CMT1 type and 10 being with CMT2 type. Biopsy of sural nerve was conducted in 26 cases, and gene diagnosis was carried out in 13 cases. RESULTS: Five patients were with peripheral myelin protein-22 (PMP22) duplication, 4 of which showed demyelination, 4 of which showed incrassation of myelin sheath, and two of which showed "onion bulb" change without axonal denaturation. Four cases were with connexin 32 (Cx32) point mutations, 3 of which showed demyelination and one of which showed incrassation of myelin sheath and absence of axonal denaturation. The 2 patients with heat shock protein 22 (Hsp22) and heat shock protein 27 (Hsp27) point mutations both showed axonal atrophy, axonal loss and axonal regeneration. CONCLUSION: The pathological findings of the Chinese CMT patients performed by mutation screening were not completely consistent with the pathological features reported abroad. The results of the mutation screening are consistent with the pathological features; mutation screening has the character of high accuracy, little harm and helps diagnose early, so it is suggested to be performed widely clinically, especially to the patients who has family history or to their lineal relatives.

[The characteristics of gene mutations in Chinese patients with Charcot-Marie-Tooth disease].

OBJECTIVE: To study the characteristics of gene mutations in Chinese patients with Charcot-Marie-Tooth disease (CMT). METHODS: Real-time quantitative PCR, PCR-SSCP, and/or direct sequencing were used to analyze the mutation of the pathogenic genes PMP22, MPZ, CX32, EGR2, GDAP1, NEFL, HSP22 and HSP27 in 113 probands of CMT families, 45 of which had family history, from different provinces in China. The whole family members of the subjects with abnormal electrophoretic bands and 50 normal controls underwent the same examination. RESULTS: Thirty-six cases of PMP22 duplication, 7 cases of CX32 mutation, 1 case of HSP22 mutation, 1 case of HSP27 mutation, 1 case of MPZ mutation, and 1 case of GDAP1 mutation were found in the 113 CMT probands. No point mutation was found in PMP22, EGR2 and NEFL genes. CONCLUSION: Among the Chinese CMT patients 31.9% are caused by PMP22 duplication, 6.2% by CX32, and 0.9% by HSP22, HSP27, MPZ and GDAP1. Point mutations of PMP22, EGR2 and NEFL are rare.

[Clinical, pathological and genetic studies in a Chinese Charcot-Marie-Tooth disease type 2 family].

OBJECTIVE: To report a Chinese Charcot-Marie-Tooth disease type 2 (CMT2) family. METHODS: All the members in the family were studied clinically,and 6 patients were studied electrophysiologically. Sural nerve biopsy was performed in the proband. PMP22 gene duplications were detected by highly polymorphic short tandem repeat. Point mutation analysis of PMP22, MPZ and NEFL gene was screened by PCR-SSCP combined with DNA direct sequencing. A genome-wide screening was carried out to the family. RESULT: Except 2 who had weakness and atrophy in both proximal and distal muscles of the lower limbs, all patients presented muscle wasting and a predominating weakness of distal parts of the lower limbs, and mild to moderate sensory impairments. In 6 patients who were subjected to elctrophysiological examinations, median-nerve conduction velocity (NCV) of the median nerve was normal. Electromyograms (EMGs) revealed signs of denervation with large motor unit potentials, fibrillation potentials and positive sharp waves. Sural nerve biopsy of the proband confirmed the presence of axonal neuropathy with an important loss of large myelinating fibers and a large number of clusters with mostly thinly myelinated axons. PMP22, MPZ and NEFL gene mutations were not found. The results of genome-wide screening revealed a linkage of CMT2 to a locus at chromosome 12q24. CONCLUSION: The results are consistent with the diagnosis of CMT2. This family represents a rare genetic type of CMT2 which can be designated as CMT2L.

[Mutation analysis of ganglioside-induced differentiation associated protein-1 gene in Chinese Charcot-Marie-Tooth disease].

OBJECTIVE: To study the mutation feature of ganglioside-induced differentiation associated protein-1 (GDAP1) gene in Chinese Charcot-Marie-Tooth disease(CMT) patients. METHODS: Mutation analysis was carried out by use of polymerase chain reaction-single strand conformation polymorphism(PCR-SSCP) combined with DNA direct sequencing of the six exons and their flanking regions of GDAP1 gene in twenty-three CMT patients, including 8 probands of autosomal recessive CMT families and 15 sporadic patients. RESULTS: A compound heterozygous mutation A533G and A767G were unveiled in one autosomal recessive CMT kindred. The homozygous and heterozygous T507G were common SNPs in Chinese population. CONCLUSION: A533G and A767G of GDAP1 gene were new mutations firstly reported.

Small heat-shock protein 22 mutated in autosomal dominant Charcot-Marie-Tooth disease type 2L.

Charcot-Marie-Tooth (CMT) disease is the most common inherited motor and sensory neuropathy. We have previously described a large Chinese CMT family and assigned the locus underlying the disease (CMT2L; OMIM 608673) to chromosome 12q24. Here, we report a novel c.423G-->T (Lys141Asn) missense mutation of small heat-shock protein 22-kDa protein 8 (encoded by HSPB8), which is also responsible for distal hereditary motor neuropathy type (dHMN) II. No disease-causing mutations have been identified in another 114 CMT families.