Understanding Schwann cell-neurone interactions: the key to Charcot-Marie-Tooth disease?

A novel single-point mutation of NEFH and biallelic SACS mutation presenting as intermediate form Charcot-Marie-Tooth: A case report in Vietnam

Background

Charcot-Marie-Tooth disease (CMT) is among the most common group of inherited neuromuscular diseases. SACS mutations were demonstrated to cause autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS). However, there have been few case reports regarding to NEFH and SACS gene mutation to CMT in Vietnamese patients, and the diagnosis of CMT and ARSACS in the clinical setting still overlapped.

Case Description

We report two patients presenting with sensorimotor neuropathy without cerebellar ataxia, spasticity and other neurological features, being diagnosed with intermediate form CMT by electrophysiological and clinical examination and neuroimaging. By whole-exome sequencing panel of two affected members, and PCR Sanger on NEFH and SACS genes to confirm the presence of selected variants on their parents, we identified a novel missense variant NEFH c.1925C>T (inherited from the mother) in an autosomal dominant heterozygous state, and two recessive SACS variants (SACS c.13174C>T, causing missense variant, and SACS c.11343del, causing frameshift variant) (inherited one from the mother and another from the father) in these two patients. Clinical and electrophysiological findings on these patients did not match classical ARSACS. To the best of our knowledge, this is the first case report of two affected siblings diagnosed with CMT carrying both a novel NEFH variant and biallelic SACS variants.

Conclusion

We concluded that this novel NEFH variant is likely benign, and biallelic SACS mutation (c.13174C>T and c.11343del) is likely pathogenic for intermediate form CMT. This study is also expected to emphasize the current knowledge of intermediate form CMT, ARSACS, and the phenotypic spectrum of NEFH-related and SACS-related disorders. We expect to give a new understanding of CMT; however, further research should be conducted to provide a more thorough knowledge of the pathogenesis of CMT in the future.

Charcot-Marie-Tooth-1A and sciatic nerve crush rat models: insights from proteomics

The sensorimotor and histological aspects of peripheral neuropathies were already studied by our team in two rat models: the sciatic nerve crush and the Charcot-Marie-Tooth-1A disease. In this study, we sought to highlight and compare the protein signature of these two pathological situations. Indeed, the identification of protein profiles in diseases can play an important role in the development of pharmacological targets. In fact, Charcot-Marie-Tooth-1A rats develop motor impairments that are more severe in the hind limbs. Therefore, for the first time, protein expression in sciatic nerve of Charcot-Marie-Tooth-1A rats was examined. First, distal sciatic nerves were collected from Charcot-Marie-Tooth-1A and uninjured wild-type rats aged 3 months. After protein extraction, sequential window acquisition of all theoretical fragment ion spectra liquid chromatography and mass spectrometry was employed. 445 proteins mapped to Swiss-Prot or trEMBL Uniprot databases were identified and quantified. Of these, 153 proteins showed statistically significant differences between Charcot-Marie-Tooth-1A and wild-type groups. The majority of these proteins were overexpressed in Charcot-Marie-Tooth-1A. Hierarchical clustering and functional enrichment using Gene Ontology were used to group these proteins based on their biological effects concerning Charcot-Marie-Tooth-1A pathophysiology. Second, proteomic characterization of wild-type rats subjected to sciatic nerve crush was performed sequential window acquisition of all theoretical fragment ion spectra liquid chromatography and mass spectrometry. One month after injury, distal sciatic nerves were collected and analyzed as described above. Out of 459 identified proteins, 92 showed significant differences between sciatic nerve crush and the uninjured wild-type rats used in the first study. The results suggest that young adult Charcot-Marie-Tooth-1A rats (3 months old) develop compensatory mechanisms at the level of redox balance, protein folding, myelination, and axonogenesis. These mechanisms seem insufficient to hurdle the progress of the disease. Notably, response to oxidative stress appears to be a significant feature of Charcot-Marie-Tooth-1A, potentially playing a role in the pathological process. In contrast to the first experiment, the majority of the proteins that differed from wild-type were downregulated in the sciatic nerve crush group. Functional enrichment suggested that neurogenesis, response to axon injury, and oxidative stress were important biological processes. Protein analysis revealed an imperfect repair at this time point after injury and identified several distinguishable proteins. In conclusion, we suggest that peripheral neuropathies, whether of a genetic or traumatic cause, share some common pathological pathways. This study may provide directions for better characterization of these models and/or identifying new specific therapeutic targets.

Mitochondrial Phenotypes in Genetically Diverse Neurodegenerative Diseases and Their Response to Mitofusin Activation

Mitochondrial fusion is essential to mitochondrial fitness and cellular health. Neurons of patients with genetic neurodegenerative diseases often exhibit mitochondrial fragmentation, reflecting an imbalance in mitochondrial fusion and fission (mitochondrial dysdynamism). Charcot–Marie–Tooth (CMT) disease type 2A is the prototypical disorder of impaired mitochondrial fusion caused by mutations in the fusion protein mitofusin (MFN)2. Yet, cultured CMT2A patient fibroblast mitochondria are often reported as morphologically normal. Metabolic stress might evoke pathological mitochondrial phenotypes in cultured patient fibroblasts, providing a platform for the pre-clinical individualized evaluation of investigational therapeutics. Here, substitution of galactose for glucose in culture media was used to redirect CMT2A patient fibroblasts (MFN2 T105M, R274W, H361Y, R364W) from glycolytic metabolism to mitochondrial oxidative phosphorylation, which provoked characteristic mitochondrial fragmentation and depolarization and induced a distinct transcriptional signature. Pharmacological MFN activation of metabolically reprogrammed fibroblasts partially reversed the mitochondrial abnormalities in CMT2A and CMT1 and a subset of Parkinson’s and Alzheimer’s disease patients, implicating addressable mitochondrial dysdynamism in these illnesses.

Transmembrane protease serine 5: a novel Schwann cell plasma marker for CMT1A

OBJECTIVE

Development of biomarkers for Charcot-Marie-Tooth (CMT) disease is critical for implementing effective clinical trials. The most common form of CMT, type 1A, is caused by a genomic duplication surrounding the PMP22 gene. A recent report (Neurology 2018;90:e518-3524) showed elevation of neurofilament light (NfL) in plasma of CMT1A disease patients, which correlated with disease severity. However, no plasma/serum biomarker has been identified that is specific to Schwann cells, the most directly affected cells in CMT1A.

METHODS

We used the Olink immuno PCR platform to profile CMT1A patient (n = 47, 2 cohorts) and normal control plasma (n = 41, two cohorts) on five different Olink panels to screen 398 unique proteins.

RESULTS

The TMPRSS5 protein (Transmembrane protease serine 5) was elevated 2.07-fold (P = <0.0001) in two independent cohorts of CMT1A samples relative to controls. TMPRSS5 is most highly expressed in Schwann cells of peripheral nerve. Consistent with early myelination deficits in CMT1A, TMPRSS5 was not significantly correlated with disease score (CMTES-R, CMTNS-R), nerve conduction velocities (Ulnar CMAP, Ulnar MNCV), or with age. TMPRSS5 was not significantly elevated in smaller sample sets from patients with CMT2A, CMT2E, CMT1B, or CMT1X. The Olink immuno PCR assays confirmed elevated levels of NfL (average 1.58-fold, P < 0.0001), which correlated with CMT1A patient disease score.

INTERPRETATION

These data identify the first Schwann cell-specific protein that is elevated in plasma of CMT1A patients, and may provide a disease marker and a potentially treatment-responsive biomarker with good disease specificity for clinical trials.

Direct Conversion of Human Fibroblasts into Schwann Cells that Facilitate Regeneration of Injured Peripheral Nerve In Vivo

Schwann cells (SCs) play pivotal roles in the maintenance and regeneration of the peripheral nervous system. Although transplantation of SCs enhances repair of experimentally damaged peripheral and central nerve tissues, it is difficult to prepare a sufficient number of functional SCs for transplantation therapy without causing adverse events for the donor. Here, we generated functional SCs by somatic cell reprogramming procedures and demonstrated their capability to promote peripheral nerve regeneration. Normal human fibroblasts were phenotypically converted into SCs by transducing SOX10 and Krox20 genes followed by culturing for 10 days resulting in approximately 43% directly converted Schwann cells (dSCs). The dSCs expressed SC‐specific proteins, secreted neurotrophic factors, and induced neuronal cells to extend neurites. The dSCs also displayed myelin‐forming capability both in vitro and in vivo. Moreover, transplantation of the dSCs into the transected sciatic nerve in mice resulted in significantly accelerated regeneration of the nerve and in improved motor function at a level comparable to that with transplantation of the SCs obtained from a peripheral nerve. The dSCs induced by our procedure may be applicable for novel regeneration therapy for not only peripheral nerve injury but also for central nerve damage and for neurodegenerative disorders related to SC dysfunction. Stem Cells Translational Medicine2017;6:1207–1216

Morphology of Donor and Recipient Nerves Utilised in Nerve Transfers to Restore Upper Limb Function in Cervical Spinal Cord Injury

Loss of hand function after cervical spinal cord injury (SCI) impacts heavily on independence. Multiple nerve transfer surgery has been applied successfully after cervical SCI to restore critical arm and hand functions, and the outcome depends on nerve integrity. Nerve integrity is assessed indirectly using muscle strength testing and intramuscular electromyography, but these measures cannot show the manifestation that SCI has on the peripheral nerves. We directly assessed the morphology of nerves biopsied at the time of surgery, from three patients within 18 months post injury. Our objective was to document their morphologic features. Donor nerves included teres minor, posterior axillary, brachialis, extensor carpi radialis brevis and supinator. Recipient nerves included triceps, posterior interosseus (PIN) and anterior interosseus nerves (AIN). They were fixed in glutaraldehyde, processed and embedded in Araldite Epon for light microscopy. Eighty percent of nerves showed abnormalities. Most common were myelin thickening and folding, demyelination, inflammation and a reduction of large myelinated axon density. Others were a thickened perineurium, oedematous endoneurium and Renaut bodies. Significantly, very thinly myelinated axons and groups of unmyelinated axons were observed indicating regenerative efforts. Abnormalities exist in both donor and recipient nerves and they differ in appearance and aetiology. The abnormalities observed may be preventable or reversible.

Regulation of the PMP22 gene through an intronic enhancer

Successful myelination of the peripheral nervous system depends upon induction of major protein components of myelin, such as peripheral myelin protein 22 (PMP22). Myelin stability is also sensitive to levels of PMP22, as a 1.4 Mb duplication on human chromosome 17, resulting in three copies of PMP22, is the most common cause of the peripheral neuropathy Charcot-Marie-Tooth disease. The transcription factor Egr2/Krox20 is required for induction of high level expression of Pmp22 in Schwann cells but its activation elements have not yet been determined. Using chromatin immunoprecipitation analysis of the rat Pmp22 locus, we found a major peak of Egr2 binding within the large intron of the Pmp22 gene. Analysis of a 250 bp region within the largest intron showed that it is strongly activated by Egr2 expression in reporter assays. Moreover, this region contains conserved binding sites not only for Egr2 but also for Sox10, which is also required for Schwann cell development. Our analysis shows that Sox10 is required for optimal activity of the intronic site as well as PMP22 expression. Finally, mouse transgenic analysis revealed tissue-specific expression of this intronic sequence in peripheral nerve. Overall, these data show that Egr2 and Sox10 activity are directly involved in mediating the developmental induction of Pmp22 expression.

Motor axonal sprouting and neuromuscular junction loss in an animal model of Charcot-Marie-Tooth disease

Muscle weakness in Charcot-Marie-Tooth Type 1A disease (CMT1A) caused by mutations in peripheral myelin protein 22 (PMP22) has been attributed to an axonopathy that results in denervation and muscle atrophy. The underlying pathophysiological mechanisms involved are not understood. We investigated motor performance, neuromuscular junctions (NMJs), physiological parameters, and muscle morphometry of PMP22 transgenic mice. Neuromuscular junctions were progressively lost in hindlimb muscles of PMP22 transgenic mice, but their motor performance did not completely deteriorate during the observation period. There was considerable variability, including in laterality, in deficits among the animals. Cross-sectional areas and mean fiber size measurements indicated variable myofiber atrophy in hindlimb muscles. There was substantial concomitant axonal sprouting, and loss of neuromuscular junctions was inversely correlated with the accumulated length of axonal branches. Synaptic transmission studied in isolated nerve/muscle preparations indicated variable partial muscle denervation. Acetylcholine sensitivity was higher in the mutant muscles, and maximum tetanic force evoked by direct or indirect stimulation, specific force, and wet weights were markedly reduced in some mutant muscles. In summary, there is partial muscle denervation, and axons may retain some regenerative capacity but fail to reinnervate muscles in PMP22 transgenic mice.

Identification of the variant Ala335Val of MED25 as responsible for CMT2B2: molecular data, functional studies of the SH3 recognition motif and correlation between wild-type MED25 and PMP22 RNA levels in CMT1A animal models

Charcot-Marie-Tooth (CMT) disease is a clinically and genetically heterogeneous disorder. All mendelian patterns of inheritance have been described. We identified a homozygous p.A335V mutation in the MED25 gene in an extended Costa Rican family with autosomal recessively inherited Charcot-Marie-Tooth neuropathy linked to the CMT2B2 locus in chromosome 19q13.3. MED25, also known as ARC92 and ACID1, is a subunit of the human activator-recruited cofactor (ARC), a family of large transcriptional coactivator complexes related to the yeast Mediator. MED25 was identified by virtue of functional association with the activator domains of multiple cellular and viral transcriptional activators. Its exact physiological function in transcriptional regulation remains obscure. The CMT2B2-associated missense amino acid substitution p.A335V is located in a proline-rich region with high affinity for SH3 domains of the Abelson type. The mutation causes a decrease in binding specificity leading to the recognition of a broader range of SH3 domain proteins. Furthermore, Med25 is coordinately expressed with Pmp22 gene dosage and expression in transgenic mice and rats. These results suggest a potential role of this protein in the molecular etiology of CMT2B2 and suggest a potential, more general role of MED25 in gene dosage sensitive peripheral neuropathy pathogenesis.

Enhancement of Schwann cell myelin formation by K252a in the Trembler-J mouse dorsal root ganglion explant culture

The Trembler-J (TrJ) mouse, containing a point mutation in the peripheral myelin protein 22 gene, is characterized by severe hypomyelination and is a representative model of Charcot-Marie-Tooth 1A disease/Dejerine-Sottas Syndrome. Previous studies have shown that protein kinase inhibitor K252a enhances wild-type Schwann cell myelination in culture. We used a dorsal root ganglion (DRG) explant culture system from the heterozygous TrJ/+ mouse to investigate if myelination could be enhanced by K252a. The TrJ/+ DRG explant cultures replicated some important features of the TrJ/+ mouse, showing reduced myelin protein accumulation, thinner myelin sheaths, and shortened myelin internodes. K252a increased myelin protein accumulation and myelin sheath thickness but did not substantially increase myelin internode length. Furthermore, the TrJ/+ DRG explant culture and sciatic nerves continued to respond to K252a during the stage when myelination is complete in the wild type. A general tyrosine kinase inhibitor, genistein, but not inhibitors of serine/threonine protein kinase inhibitors, had a similar effect to K252a. K252a is therefore able to partially overcome hypomyelination by enhancing mutant Schwann cell myelin formation in the TrJ/+ mouse.

Charcot-Marie-Tooth disease type 1A: clinicopathological correlations in 24 patients

We examined nerve biopsies from 24 patients with Charcot-Marie-Tooth disease type 1A (CMT1A) and proven 17p11.2-12 duplication. There were seven males and 17 females with a mean age of 27.85 +/- 18.95 years at the time of nerve biopsy. A family history consistent with dominant inheritance was present in 17 patients. Clinical features were classical in 16 patients and were atypical in the other eight: one had calf hypertrophy; two had Roussy-Levy syndrome; one had had a subacute inflammatory demyelinating polyneuropathy 11 years earlier and presented a relapse on the form of a chronic inflammatory demyelinating polyneuropathy; one had carpal tunnel syndrome; one had a recent painful neuropathy in both legs; and two had chronic inflammatory demyelinating polyneuropathy. Onion bulb formations (OMFs) were present in every case and most of them were characteristic, whereas burnt-out or cluster-associated OMFs were less common. Depletion of myelinated fibers was severe in 20 cases (169-2927/mm2) and varied from 5187 to 3725/mm2 in three children (4-9 years old). In addition, features of macrophage-associated demyelination were observed in the last four atypical cases. Known for more than 20 years, inflammatory demyelination superimposed in the course of CMT1A has been reported in a few cases in the past few years, mainly concerning asymptomatic or atypical patients. Such an association deserves to be better known because corticotherapy improves weakness in most of these patients.

Mifepristone (RU 38486) influences expression of glycoprotein Po and morphological parameters at the level of rat sciatic nerve: in vivo observations

The observations here reported indicate that, in vivo, the expression of an important protein of peripheral myelin, the glycoprotein Po, is influenced by mifespristone (RU 38486), that is, an antagonist of progesterone (PR) and glucocorticoid (GR) receptor. In our experimental model, male rats have been treated at the first day of life with this antagonist and after repeated treatments, we have analyzed in the sciatic nerve of 20- (20d) and 30-day-old rats (30d) the mRNA and protein levels of Po. Moreover, expression of Po has also been analyzed in the sciatic nerve of animals treated during the first 30 days of postnatal life and then sacrificed at 90th day of life (90d). The results obtained have indicated that both mRNA and protein levels of Po decrease at 20d. Apparently, these effects seem to be transient because no changes are evident at the other two times of analysis. As shown by morphometric analysis, the treatment with RU 38486 is also able to induce morphological changes at the level of sciatic nerve. However, at variance to what is expected by an alteration of an important component of the myelin membranes like Po, no changes are evident at the level of the myelin compartment. On the contrary, a significant reduction of axon diameter in parallel to an increase in neurofilament (NF) density occurs since 30d. In conclusion, the present data seem to suggest that progestin and/or glucocorticoid signals are not only involved in the control of myelin compartment but also on the axon maintenance.

Distinct elements of the peripheral myelin protein 22 (PMP22) promoter regulate expression in Schwann cells and sensory neurons

Genetic disease mechanisms in the demyelinating peripheral neuropathies Charcot-Marie-Tooth disease type 1A (CMTA) and hereditary neuropathy with liability to pressure palsies (HNPP) as well as transgenic animals with altered PMP22 gene dosage revealed that alterations in PMP22 gene expression have profound effects on the development and maintenance of peripheral nerves. Consequently, the regulation of PMP22 is a crucial aspect in understanding the function of this protein in health and disease. In this study, we dissected and analyzed different cis-acting elements in the 5'-flanking region of the Pmp22 gene in vivo. We found two separate elements that contribute to different aspects of Pmp22 expression. The first is located 5' distally to promoter 1 and is involved in gene regulation during late phases of myelination in development ["late myelination Schwann cell-specific element" (LMSE)] and in remyelination after injury. The second element was identified upstream of promoter 2 and guides Pmp22 expression in sensory neurons. These results suggest that multiple distinct signaling pathways regulating Pmp22 expression in myelination as well as in neurons converge on distinct segments of the PMP22 promoter region. The underlying molecular mechanisms are likely to be crucially involved in the maintenance of the integrity of myelinated peripheral nerves.

Dominantly inherited peripheral neuropathies

Since 1886, the year that Charcot and Marie and Tooth described a genetic "peroneal muscular atrophy syndrome," electrophysiological and histological studies of the peripheral nervous system have greatly aided the characterization of this syndrome, which falls among the hereditary sensory-motor neuropathies. Two principal forms of Charcot-Marie-Tooth (CMT) disease have been distinguished: CMT 1, corresponding to a demyelinating type, and CMT 2, corresponding to an axonal type. The modes of transmission of these types are variable, recessive or dominant, autosomal, or X-linked. Our discussion here is confined to the dominant forms. In recent years, advances in molecular biology have greatly modified the approach to CMT disease and related neuropathies (such as hereditary neuropathy with liability to pressure palsies). With increased knowledge of responsible gene mutations and several other loci identified by linkage studies, our understanding of the pathophysiology of these neuropathies is increasing; however, with greater understanding, the classification of these disorders is becoming more complex. In this review we present and discuss the currently characterized subtypes, with emphasis on their known histological aspects. While nerve biopsy has lost its diagnostic importance in certain forms of the disease, such as CMT 1A, CMT 1B, and X-linked CMT (CMT X), it remains important for better characterizing the lesions of CMT 2 and forms of genetic peroneal atrophy in which DNA study is unrevealing.