Coexistence of Charcot-Marie-Tooth 1A and nondystrophic myotonia due to PMP22 duplication and SCN4A pathogenic variants: a case report

Background Charcot-Marie-Tooth disease (CMT) is a genetically heterogeneous hereditary neuropathy, and CMT1A is the most common form; it is caused by a duplication of the peripheral myelin protein 22 (PMP22) gene. Mutations in the transient sodium channel Nav1.4 alpha subunit (SCN4A) gene underlie a diverse group of dominantly inherited nondystrophic myotonias that run the spectrum from subclinical myopathy to severe muscle stiffness, disabling weakness, or frank episodes of paralysis. Case presentation We describe a Chinese family affected by both CMT1A and myotonia with concomitant alterations in both the PMP22 and SCN4A genes. In this family, the affected proband inherited the disease from his father in an autosomal dominant manner. Genetic analysis confirmed duplication of the PMP22 gene and a missense c.3917G > C (p. Gly1306Ala) mutation in SCN4A in both the proband and his father. The clinical phenotype in the proband showed the combined involvement of skeletal muscle and peripheral nerves. Electromyography showed myopathic changes, including myotonic discharges. MRI revealed the concurrence of neurogenic and myogenic changes in the lower leg muscles. Sural nerve biopsies revealed a chronic demyelinating and remyelinating process with onion bulb formations in the proband. The proband’s father presented with confirmed subclinical myopathy, very mild distal atrophy and proximal hypertrophy of the lower leg muscles, pes cavus, and areflexia. Conclusion This study reports the coexistence of PMP22 duplication and SCN4A mutation. The presenting features in this family suggested that both neuropathy and myopathy were inherited in an autosomal dominant manner. The proband had a typical phenotype of sodium channel myotonia (SCM) and CMT1A. However, his father with the same mutations presented a much milder clinical phenotype. Our study might expand the genetic and phenotypic spectra of neuromuscular disorders with concomitant mutations.

Treatment with IFB-088 improves neuropathy in CMT1A and CMT1B mice

Charcot Marie Tooth diseases type 1A (CMT1A), caused by duplication of Peripheral Myelin Protein 22 (PMP22) gene, and CMT1B, caused by mutations in myelin protein zero (MPZ) gene are the two most common forms of demyelinating CMT (CMT1) and no treatments are available for either. Prior studies of the MpzSer63del mouse model of CMT1B have demonstrated that protein misfolding, endoplasmic reticulum (ER) retention and activation of the unfolded protein response (UPR) contributed to the neuropathy. Heterozygous patients with an arginine to cysteine mutation in MPZ (MPZR98C) develop a severe infantile form of CMT1B which is modeled by MpzR98C/+ mice that also show ER-stress and an activated UPR. C3-PMP22 mice are considered to effectively model CMT1A. Altered proteostasis, ER-stress and activation of the UPR have been demonstrated in mice carrying Pmp22 mutations. To determine whether enabling the ER-stress/UPR and readjusting protein homeostasis would effectively treat these models of CMT1B and CMT1A we administered Sephin1/IFB-088/icerguestat, a UPR modulator which showed efficacy in the MpzS63del model of CMT1B, to heterozygous MpzR98C and C3-PMP22 mice. Mice were analyzed by behavioral, neurophysiological, morphological and biochemical measures. Both MpzR98C/+ and C3-PMP22 mice improved in motor function and neurophysiology. Myelination, as demonstrated by g-ratios and myelin thickness, improved in CMT1B and CMT1A mice and markers of UPR activation returned towards wild type values. Taken together our results demonstrate the capability of IFB-088 to treat a second mouse model of CMT1B and a mouse model of CMT1A, the most common form of CMT. Given the recent benefits of IFB-088 treatment in Amyotrophic Lateral Sclerosis and Multiple Sclerosis animal models, these data demonstrate its potential in managing UPR and ER-stress for multiple mutations in CMT1 as well as in other neurodegenerative diseases.

EPIDEMIOLOGICAL CHARACTERISTICS OF HEREDITARY MOTOR AND SENSORY NEUROPATHY IN THE KHARKOV REGION

Hereditary motor and sensory neuropathies are one of the most common diseases among monogenic hereditary diseases of the nervous system. Hereditary motor and sensory neuropathies are the group of clinically and genetically heterogeneous diseases characterized by peripheral nerve damage. Hereditary motor and sensory neuropathies have little effect on life expectancy, and this leads to their significant accumulation in individual families and in populations as a whole. The most common form of this disorder is hereditary motor and sensory neuropathy with an autosomal dominant type of inheritance - type 1A, caused by a mutation in the gene of peripheral myelin protein 22 (PMP22) on chromosome 17p11.2-12. According to various researches, the frequency of all hereditary motor and sensory neuropathies in the general population is 1:2500. The prevalence of hereditary motor and sensory neuropathies in different populations varies widely, therefore, the most appropriate at the initial stage of the genetic-epidemiological study of hereditary motor and sensory neuropathies is to determine the population frequency of this mutation in a specific region. The research of the territorial and ethnic distribution of hereditary motor and sensory neuropathies in the Kharkiv region was not carried out. That is why the aim of the research was to study and analyze the prevalence rates of hereditary motor and sensory neuropathies in the Kharkiv region in the context of administrative and territorial units and individual ethnic groups of the population. The epidemiological study of hereditary motor and sensory neuropathies in the Kharkiv region was carried out in the period from 2015 to 2020. The diagnosis of hereditary motor and sensory neuropathy was established in accordance with the recommendations of the WHO Research Group on neuromuscular diseases on the basis of diagnostic criteria.The prevalence rate of hereditary motor and sensory neuropathy was calculated both for various administrative and territorial units of the Kharkiv region and for individual ethnic groups of the population and expressed as the number of cases per 100,000 people. The results of the research showed that the prevalence rate of all forms of hereditary motor and sensory neuropathies in the Kharkiv region is 5.56 per 100,000 population and this indicator is unevenly distributed. The reason for the uneven distribution of hereditary motor and sensory neuropathies in the Kharkiv region may be the "effect of small samples" due to differences in population size both in individual administrative regions and in some ethnic groups of the population. In the Kharkiv region, the part of registered patients with hereditary motor and sensory neuropathies among the urban population (55.3%) is higher than among residents of country areas (44.7%). The heterogeneity of the prevalence rate of hereditary motor and sensory neuropathies in various ethnic groups of the Kharkiv region is due to the non-representativeness of these groups to the corresponding ethnic populations and such indicators cannot be transferred to the entire population as a whole. The high prevalence of hereditary motor and sensory neuropathies among certain ethnic groups is most likely due to the presence of ethnic isolates with a high degree of inbred members of the group.

Hereditary neuropathy with liability to pressure palsies: a case report

Hereditary neuropathy with liability to pressure palsies (HNPP) is a rare hereditary disorder characterized by recurrent episodes of nerve compression. The first attack usually occurs in the second or third decade of life. In the majority of cases, HNPP is associated with a mutation at chromosome 17 p11.2-12 comprising the gene encoding peripheral myelin protein 22 (PMP22). Here we present a case report of a 43-year-old male with HNPP confirmed by DNA testing. The patient complained of recurrent episodes of bilateral foot extensor muscles weakness and/or hyperesthesia on the outer surface of the hands and forearms, which started after a prolonged posture maintaining and without evident precipitating factors. We also describe typical clinical, electrophysiological, and nerve ultrasound characteristics of the disease.

A novel case of concurrent occurrence of demyelinating-polyneuropathy-causing PMP22 duplication and SOX10 gene mutation producing severe hypertrophic neuropathy

Background Hereditary motor and sensory neuropathy, also referred to as Charcot–Marie–Tooth disease (CMT), is most often caused by a duplication of the peripheral myelin protein 22 (PMP22) gene. This duplication causes CMT type 1A (CMT1A). CMT1A rarely occurs in combination with other hereditary neuromuscular disorders. However, such rare genetic coincidences produce a severe phenotype and have been reported in terms of “double trouble” overlapping syndrome. Waardenburg syndrome (WS) is the most common form of a hereditary syndromic deafness. It is primarily characterized by pigmentation anomalies and classified into four major phenotypes. A mutation in the SRY sex determining region Y-box 10 (SOX10) gene causes WS type 2 or 4 and peripheral demyelinating neuropathy, central dysmyelinating leukodystrophy, WS, and Hirschsprung disease. We describe a 11-year-old boy with extreme hypertrophic neuropathy because of a combination of CMT1A and WS type 2. This is the first published case on the co-occurrence of CMT1A and WS type 2. Case presentation The 11-year-old boy presented with motor developmental delay and a deterioration in unstable walking at 6 years of age. In addition, he had congenital hearing loss and heterochromia iridis. The neurological examination revealed weakness in the distal limbs with pes cavus. He was diagnosed with CMT1A by the fluorescence in situ hybridization method. His paternal pedigree had a history of CMT1A. However, no family member had congenital hearing loss. His clinical manifestation was apparently severe than those of his relatives with CMT1A. In addition, a whole-body magnetic resonance neurography revealed an extreme enlargement of his systemic cranial and spinal nerves. Subsequently, a genetic analysis revealed a heterozygous frameshift mutation c.876delT (p.F292Lfs*19) in the SOX10 gene. He was eventually diagnosed with WS type 2. Conclusions We described a patient with a genetically confirmed overlapping diagnoses of CMT1A and WS type 2. The double trouble with the genes created a significant impact on the peripheral nerves system. Severe phenotype in the proband can be attributed to the cumulative effect of mutations in both PMP22 and SOX10 genes, responsible for demyelinating neuropathy.

Glycosylation Limits Forward Trafficking of the Tetraspan Membrane Protein PMP22

Peripheral myelin protein 22 (PMP22) folds and traffics inefficiently, a phenomenon closely related to the mechanisms by which this tetraspan membrane protein causes Charcot-Marie-Tooth disease (CMTD). We report that elimination of N-glycosylation results in a 3-fold increase in the cell surface trafficking of wild type (WT) PMP22 and a 10-fold increase in trafficking of the unstable L16P disease mutant form. Studies of the interactions of PMP22 with oligosaccharyltransferases A and B as well as quantitative proteomic experiments established that critical endoplasmic reticulum (ER) quality control decisions occur earlier in the biogenesis to cell surface trafficking pathway for the L16P mutant than for WT. CRISPR knock-out cell lines for ER proteins calnexin, RER1, and UGGT1 illuminated the role of each protein in glycosylation dependent and independent surface trafficking of WT PMP22, as well as for a series of disease mutants of varying folding stabilities.One Sentence SummaryN-linked glycosylation was seen to dramatically limit the cell surface trafficking of PMP22, with some key quality control factors in PMP22 biogenesis being identified.

Direct relationship between increased expression and mistrafficking of the Charcot–Marie–Tooth–associated protein PMP22

Charcot–Marie–Tooth disease (CMT) is a neuropathy of the peripheral nervous system that afflicts ∼1:2500 people. The most common form of this disease (CMT1A, 1:4000) is associated with duplication of chromosome fragment 17p11.2-12, which results in a third WT PMP22 allele. In rodent models overexpressing the PMP22 (peripheral myelin protein 22) protein and in dermal fibroblasts from patients with CMT1A, PMP22 aggregates have been observed. This suggests that overexpression of PMP22 under CMT1A conditions overwhelms the endoplasmic reticulum quality control system, leading to formation of cytotoxic aggregates. In this work, we used a single-cell flow-cytometry trafficking assay to quantitatively examine the relationship between PMP22 expression and trafficking efficiency in individual cells. We observed that as expression of WT or disease variants of PMP22 is increased, the amount of intracellular PMP22 increases to a greater extent than the amount of surface-trafficked protein. This was true for both transiently transfected cells and PMP22 stable expressing cells. Our results support the notion that overexpression of PMP22 in CMT1A leads to a disproportionate increase in misfolding and mistrafficking of PMP22, which is likely a contributor to disease pathology and progression.

Peripheral myelin protein 22 preferentially partitions into ordered phase membrane domains

The ordered environment of cholesterol-rich membrane nanodomains is thought to exclude many transmembrane (TM) proteins. Nevertheless, some multispan helical transmembrane proteins have been proposed to partition into these environments. Here, giant plasma membrane vesicles (GPMVs) were employed to quantitatively show that the helical tetraspan peripheral myelin protein 22 (PMP22) exhibits a pronounced preference for, promotes the formation of, and stabilizes ordered membrane domains. Neither S-palmitoylation of PMP22 nor its putative cholesterol binding motifs are required for this preference. In contrast, Charcot–Marie–Tooth disease-causing mutations that disrupt the stability of PMP22 tertiary structure reduce or eliminate this preference in favor of the disordered phase. These studies demonstrate that the ordered phase preference of PMP22 derives from global structural features associated with the folded form of this protein, providing a glimpse at the structural factors that promote raft partitioning for multispan helical membrane proteins.