Human Nerve Pathology Caused by Different Mutational Mechanisms of thePMP22Gene

Loss of YAP in Schwann cells improves HNPP pathophysiology

Rapid nerve conduction in the peripheral nervous system (PNS) is facilitated by the multilamellar myelin sheath encasing many axons of peripheral nerves. Charcot-Marie-Tooth type 1A (CMT1A), and hereditary neuropathy with liability to pressure palsy (HNPP) are common demyelinating inherited peripheral neuropathies and are caused by mutations in the peripheral myelin protein 22 (PMP22) gene. Duplication of PMP22 leads to its overexpression and causes CMT1A, while its deletion results in PMP22 under expression and causes HNPP. Here, we investigated novel targets for modulating the protein level of PMP22 in HNPP. We found that genetic attenuation of the transcriptional coactivator Yap in Schwann cells reduces p-TAZ levels, increased TAZ activity, and increases PMP22 in peripheral nerves. Based on these findings, we ablated Yap alleles in Schwann cells of the Pmp22-haploinsufficient mouse model of HNPP and identified fewer tomacula on morphological assessment and improved nerve conduction in peripheral nerves. These findings suggest YAP modulation may be a new avenue for treatment of HNPP.

Nerve Sonography in Charcot-Marie-Tooth Disease: A Systematic Review and Meta-analysis of 6061 Measured Nerves

Because of the insidious character and variations in presenting symptoms, Charcot-Marie-Tooth (CMT) disease is challenging to diagnose in children. Diagnosis is based on clinical and nerve conduction studies, as well as genetic examination. Therefore, competent nerve imaging techniques and non-invasive alternatives to nerve conduction studies are a necessity, especially in children. We performed a systematic review and meta-analysis to evaluate the current evidence and effectiveness of ultrasound in investigating nerve cross-sectional area (CSA) in those with CMT compared with healthy controls and to pool the CSA measurements. We included studies published in international peer-reviewed journals that measured nerve CSA by ultrasound in patients with CMT. We implemented double-arm meta-analyses to compare the mean CSA of nerves between patients with CMT and healthy controls by calculating the pooled mean difference in CSA. Moreover, we performed subgroup analyses by stratifying the studies according to the site of CSA measurement and examined the difference in nerve CSA between CMT1A and other CMT types. The included studies provide measurements of 12 nerve roots and nerves (vagus, C3, C4, C5, C6, greater auricular, phrenic, median, ulnar, fibular, tibial and sural nerves) in 628 patients with CMT and 586 healthy controls with a total of 6061 measured nerves. Meta-analyses of sonographic nerve CSA are provided to express nerve ultrasonography in the diagnosis of CMT patient.

Utility of carpal tunnel release and ulnar decompression in CMT1A and HNPP

INTRODUCTION/AIMS

Carpal and cubital tunnel syndrome (CTS, CuTS) are common among patients with hereditary neuropathy with liability to pressure-palsies (HNPP) and Charcot-Marie-Tooth-1A (CMT1A), and may impact quality of life. We aimed to evaluate the utility of nerve decompression surgeries in these patients.

METHODS

Medical records were reviewed for patients with PMP22 mutation confirmed in Mayo Clinic laboratories from January 1999 to December 2020, who had CTS and CuTS and underwent surgical decompression.

RESULTS

CTS occurred in 53.3% of HNPP and 11.5% of CMT1A, while CuTS was present in 43.3% of HNPP and 5.8% of CMT1A patients. CTS decompression occurred in 10-HNPP and 5-CMT1A patients, and CuTS decompression with/without transposition was performed in 5-HNPP and 1-CMT1A patients. In HNPP, electrodiagnostic studies identified median neuropathy at the wrist in 9/10 patients and ultrasound showed focal enlargements at carpal and cubital tunnels. In CMT1A, median and ulnar sensory responses were all absent, and the nerves were diffusely enlarged. After CTS surgery, pain, sensory loss, and strength improved in 4/5 CMT1A, and 6/10 HNPP patients. Of clinical, electrophysiologic and ultrasound findings, only activity-provoked features significantly correlated with CTS surgical benefit in HNPP patients (odds ratio=117.0:95% CI 1.94>999.99, p=0.01). One CMT1A and one HNPP patient improved with CuTS surgery while 2 HNPP patients worsened.

DISCUSSION

CTS symptom improvement post-surgery can be seen in CMT1A and (less frequent) in HNPP patients. CuTS surgery commonly worsened course in HNPP. Activity-provoked symptoms in HNPP best informed benefits from CTS surgery.

Counter-movement jump characteristics in children with Charcot-Marie-Tooth type 1a disease

BACKGROUND

Poor performance in sports, especially activities that require explosive movements, is a common reason for initial presentation of children with Charcot-Marie-Tooth type 1a (CMT1a) to the paediatric neuromuscular specialist.

RESEARCH QUESTION

The aim of this descriptive, retrospective study was to analyse counter-movement jump characteristics in children with CMT1a in comparison to those in typically developing children (TDC).

METHODS

This retrospective study included seven patients with CMT1a and 44 TDC from our data pool. All the participants performed counter-movement jumps, and jump height, peak force, time to peak force, average and peak rate of force development and net vertical impulse were then calculated. For statistical comparison by means of an independent Student's t-test, children with CMT1a were compared to seven sex- and age-matched TDC. Correlation coefficients were calculated to determine the relationship between the force-time variables and jump height.

RESULTS

Peak force, net vertical impulse and jump height values in the CMT1a group were significantly lower than those in the TDC group. There were no between-group differences in the time to peak force or average and peak rate of force development. In terms of task symmetry, the correlation between the time-force curve of the left and right leg in the CMT1a group was reduced as compared with that in the TDC group. In both groups, among the parameters measured, there was a significant correlation between jump height and net vertical impulse.

SIGNIFICANCE

This study showed that reduced jump performance in children with CMT1a, as demonstrated by decreased counter-movement jump height, was due to a reduced net impulse during this explosive movement task. This finding is critical for children with CMT1a and has to be considered in clinical management and activities of daily living (e.g. sports lessons in school).

Codon Usage Analysis of Pro-Apoptotic Bim Gene Isoforms

Background: Bim is a Bcl-2 homology 3 (BH3)-only proteins, a group of pro-apoptotic proteins involved in physiological and pathological conditions. Both the overexpression and under-expression of Bim protein are associated with the diseased condition, and various isoforms of Bim protein are present with differential apoptotic potential. Objective: The present study attempted to envisage the association of various molecular signatures with the codon choices of Bim isoforms. Methods: Molecular signatures like composition, codon usage, nucleotide skews, the free energy of mRNA transcript, physical properties of proteins, codon adaptation index, relative synonymous codon usage, and dinucleotide odds ratio were determined and analyzed for their associations with codon choices of Bim gene. Results: Skew analysis of the Bim gene indicated the preference of C nucleotide over G, A, and T and preference of G over T and A nucleotides was observed. An increase in C content at the first and third codon position increased gene expression while it decreased at the second codon position. Compositional constraints on nucleotide C at all three codon positions affected gene expression. The analysis revealed an exceptionally high usage of CpC dinucleotide in all the envisaged 31 isoforms of Bim. We correlated it with the requirement of rapid demethylation machinery to fine-tune the Bimgene expression. Also, mutational pressure played a dominant role in shaping codon usage bias in Bim isoforms. Conclusion: An exceptionally high usage of CpC dinucleotide in all the envisaged 31 isoforms of Bim indicates a high order selectional force to fine tune Bim gene expression.

Treating PMP22 gene duplication-related Charcot-Marie-Tooth disease: the past, the present and the future

Charcot-Marie-Tooth (CMT) disease is the most frequent inherited neuropathy, affecting 1/1500 to 1/10000. CMT1A represents 60%-70% of all CMT and is caused by a duplication on chromosome 17p11.2 leading to an overexpression of the Peripheral Myelin Protein 22 (PMP22). PMP22 gene is under tight regulation and small changes in its expression influences myelination and affect motor and sensory functions. To date, CMT1A treatment is symptomatic and classic pharmacological options have been disappointing. Here, we review the past, present, and future treatment options for CMT1A, with a special emphasis on the highly promising potential of PMP22-targeted small interfering RNA and antisense oligonucleotides.

Charcot-Marie-Tooth disease type 1A: Longitudinal change in nerve ultrasound parameters

BACKGROUND

This study aimed to elucidate the longitudinal changes in nerve ultrasound parameters of adult Charcot-Marie-Tooth disease type 1A (CMT1A) patients.

METHODS

Fifteen adult patients with CMT1A prospectively underwent nerve ultrasound and clinical assessment (CMT neuropathy score [CMTNS]) at baseline and 5 y later. Nerve cross-sectional area (CSA) and echogenicity were measured in the median and sural nerves. Changes in ultrasound parameters and CMTNS and correlation between changes of ultrasound parameters and CMTNS were analyzed.

RESULTS

Median and sural nerve CSAs did not change over 5 y, although CMTNS increased (P < .01). Nerve echogenicity in the sural nerve decreased over 5 y (P = .045). No correlations between changes in nerve ultrasound parameters and CMTNS were identified.

CONCLUSIONS

No longitudinal changes in nerve size was detected in adult CMT1A. Exploring the factors that determine nerve size in childhood CMT1A may lead to the development of treatments.

Pmp22 super-enhancer deletion causes tomacula formation and conduction block in peripheral nerves

Copy number variation of the peripheral nerve myelin gene Peripheral Myelin Protein 22 (PMP22) causes multiple forms of inherited peripheral neuropathy. The duplication of a 1.4 Mb segment surrounding this gene in chromosome 17p12 (c17p12) causes the most common form of Charcot-Marie-Tooth disease type 1A, whereas the reciprocal deletion of this gene causes a separate neuropathy termed hereditary neuropathy with liability to pressure palsies (HNPP). PMP22 is robustly induced in Schwann cells in early postnatal development, and several transcription factors and their cognate regulatory elements have been implicated in coordinating the gene's proper expression. We previously found that a distal super-enhancer domain was important for Pmp22 expression in vitro, with particular impact on a Schwann cell-specific alternative promoter. Here, we investigate the consequences of deleting this super-enhancer in vivo. We find that loss of the super-enhancer in mice reduces Pmp22 expression throughout development and into adulthood, with greater impact on the Schwann cell-specific promoter. Additionally, these mice display tomacula formed by excessive myelin folding, a pathological hallmark of HNPP, as have been previously observed in heterozygous Pmp22 mice as well as sural biopsies from patients with HNPP. Our findings demonstrate a mechanism by which smaller copy number variations, not including the Pmp22 gene, are sufficient to reduce gene expression and phenocopy a peripheral neuropathy caused by the HNPP-associated deletion encompassing PMP22.

Peripheral myelin protein 22 modulates store-operated calcium channel activity, providing insights into Charcot-Marie-Tooth disease etiology

Charcot-Marie-Tooth (CMT) disease is a peripheral neuropathy associated with gene duplication and point mutations in the peripheral myelin protein 22 (PMP22) gene. However, the role of PMP22 in Schwann cell physiology and the mechanisms by which PMP22 mutations cause CMT are not well-understood. On the basis of homology between PMP22 and proteins associated with modulation of ion channels, we hypothesized that PMP22 alters ion channel activity. Using whole-cell electrophysiology, we show here that heterologous PMP22 expression increases the amplitude of currents similar to those ascribed to store-operated calcium (SOC) channels, particularly those involving transient receptor canonical channel 1 (TrpC1). These channels help replenish Ca2+ in the endoplasmic reticulum (ER) following stimulus-induced depletion. Currents with similar properties were recorded in WT but not pmp22-/- mouse Schwann cells. Heterologous expression of the CMT-associated PMP22_L16P variant, which fails to reach the plasma membrane and localizes to the ER, led to larger currents than WT PMP22. Similarly, Schwann cells isolated from Trembler J (TrJ; PMP22_L16P) mice had larger currents than WT littermates. Calcium imaging in live nerves and cultured Schwann cells revealed elevated intracellular Ca2+ in TrJ mice compared with WT. Moreover, we found that PMP22 co-immunoprecipitated with stromal interaction molecule 1 (STIM1), the Ca2+ sensor SOC channel subunit in the ER. These results suggest that in the ER, PMP22 interacts with STIM1 and increases Ca2+ influx through SOC channels. Excess or mutant PMP22 in the ER may elevate intracellular Ca2+ levels, which could contribute to CMT pathology.

Rodent models with expression of PMP22: Relevance to dysmyelinating CMT and HNPP

BACKGROUND

Charcot-Marie-Tooth diseases (CMT) are due to abnormalities of many genes, the most frequent being linked to PMP22 (Peripheral Myelin Protein 22). In the past, only spontaneous genetic anomalies occurring in mouse mutants such as Trembler (Tr) mice were available; more recently, several rodent models have been generated for exploration of the pathophysiological mechanisms underlying these neuropathies.

METHODS

Based on the personal experience of our team, we describe here the pathological hallmarks of most of these animal models and compare them to the pathological features observed in some CMT patient nerves (CMT types 1A and E; hereditary neuropathy with liability to pressure palsies, HNPP).

RESULTS

We describe clinical data and detailed pathological analysis mainly by electron microscopy of the sciatic nerves of these animal models conducted in our laboratory; lesions of PMP22 deficient animals (KO and mutated PMP22) and PMP22 overexpressed models are described and compared to ultrastructural anomalies of nerve biopsies from CMT patients due to PMP22 gene anomalies. It is of note that while there are some similarities, there are also significant differences between the lesions in animal models and human cases. Such observations highlight the complex roles played by PMP22 in nerve development.

CONCLUSION

It should be borne in mind that we require additional correlations between animal models of hereditary neuropathies and CMT patients to rationalize the development of efficient drugs.

Involvement of Aryl hydrocarbon receptor in myelination and in human nerve sheath tumorigenesis

Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor involved in xenobiotic metabolism. Plexiform neurofibromas (PNFs) can transform into malignant peripheral nerve sheath tumors (MPNSTs) that are resistant to existing therapies. These tumors are primarily composed of Schwann cells. In addition to neurofibromatosis type 1 (NF1) gene inactivation, further genetic lesions are required for malignant transformation. We have quantified the mRNA expression levels of AHR and its associated genes in 38 human samples. We report that AHR and the biosynthetic enzymes of its endogenous ligand are overexpressed in human biopsies of PNFs and MPNSTs. We also detect a strong nuclear AHR staining in MPNSTs. The inhibition of AHR by siRNA or antagonists, CH-223191 and trimethoxyflavone, induces apoptosis in human MPNST cells. Since AHR dysregulation is observed in these tumors, we investigate AHR involvement in Schwann cell physiology. Hence, we studied the role of AHR in myelin structure and myelin gene regulation in Ahr^-/- mice during myelin development. AHR ablation leads to locomotion defects and provokes thinner myelin sheaths around the axons. We observe a dysregulation of myelin gene expression and myelin developmental markers in Ahr^-/- mice. Interestingly, AHR does not directly bind to myelin gene promoters. The inhibition of AHR in vitro and in vivo increased β-catenin levels and stimulated the binding of β-catenin on myelin gene promoters. Taken together, our findings reveal an endogenous role of AHR in peripheral myelination and in peripheral nerve sheath tumors. Finally, we suggest a potential therapeutic approach by targeting AHR in nerve tumors.

Diseases of the peripheral nerves

This chapter reviews the diseases of the peripheral nerves from a neuropathologic point of view, with a special focus on specific morphologic changes, and includes a summary of the histopathologic methods available for their diagnosis. As the rate of obesity and the prevalence of type 2 diabetes increase, diabetic neuropathy is the most common cause of peripheral neuropathy. Many systemic disorders with metabolic origin, like amyloidosis, hepatic failure, vitamin deficiencies, uremia, lipid metabolism disorders, and others, can also cause axonal or myelin alterations in the peripheral nervous system. The most notable causes of toxic neuropathies are chemotherapeutic agents, alcohol consumption, and exposure to heavy metals and other environmental or biologic toxins. Inflammatory neuropathies cover infectious neuropathies (Lyme disease, human immunodeficiency virus, leprosy, hepatitis) and neuropathies of autoimmune origin (sarcoidosis, Guillain-Barré syndrome/acute inflammatory demyelinating polyneuropathy, chronic inflammatory demyelinating polyneuropathy, and diverse forms of vasculitis. The increasing number of known diseases causing gene mutations in hereditary peripheral neuropathies requires precise characterization, which includes histopathology.

Caveats in the Established Understanding of CMT1A

Charcot‐Marie‐Tooth disease type‐1A (CMT1A) is one of the most common types of inherited peripheral nerve diseases. It is caused by the trisomy of chromosome 17p12 (c17p12), a large DNA segment of 1.4 Mb containing PMP22 plus eight other genes. The size of c17p12 is formidable for any cloning technique to manipulate, and thus precludes production of models in vitro and in vivo that can precisely recapitulate the genetic alterations in humans with CMT1A. This limitation and other factors have led to several assumptions, which have yet been carefully scrutinized, serving as key principles in our understanding of the disease. For instance, one extra copy of c17p12 in patients with CMT1A results in a higher gene dosage of PMP22, thereby expected to produce a higher level of PMP22 mRNA/proteins that cause the disease. However, there has been increasing evidence that PMP22 levels are highly variable among patients with CMT1A and may fall into the normal range at a given time point. This raises an alternative mechanism causing the disease by dysregulation of PMP22 expression or excessive fluctuation of PMP22 levels, not the absolute increase of PMP22. This has become a pressing issue since recent clinical trials using ascorbic acid failed to alter the clinical outcome of CMT1A patients, leaving no effective therapy for the disease. In this article, we will discuss how this fundamental issue might be investigated. In addition, several other key issues in CMT1A will be discussed, including potential mechanisms responsible for the uniform slowing of conduction velocities. A clear understanding of these issues could radically change how therapies should be developed against CMT1A.

Towards a functional pathology of hereditary neuropathies

A growing number of hereditary neuropathies have been assigned to causative gene defects in recent years. The study of human nerve biopsy samples has contributed substantially to the discovery of many of these neuropathy genes. Genotype-phenotype correlations based on peripheral nerve pathology have provided a comprehensive picture of the consequences of these mutations. Intriguingly, several gene defects lead to distinguishable lesion patterns that can be studied in nerve biopsies. These characteristic features include the loss of certain nerve fiber populations and a large spectrum of distinct structural changes of axons, Schwann cells and other components of peripheral nerves. In several instances the lesion patterns are directly or indirectly linked to the known functions of the mutated gene. The present review is designed to provide an overview on these characteristic patterns. It also considers other aspects important for the manifestation and pathology of hereditary neuropathies including the role of inflammation, effects of chemotherapeutic agents and alterations detectable in skin biopsies.

Neuroregenerative Effect of Oxandrolone: A Case Report

Patient: Male, 25

Final Diagnosis: Charcot-Marie-Tooth 1

Symptoms: Muscular • spasticity

Medication: Oxandrolone

Clinical Procedure: Neural and muscle biopsies

Specialty: Neurology

Molecular regulators of nerve conduction - Lessons from inherited neuropathies and rodent genetic models

Myelinated nerve fibers are highly compartmentalized. Helically wrapped lipoprotein membranes of myelin are integrated with subsets of proteins specifically in each compartment to shape the physiological behavior of these nerve fibers. With the advance of molecular biology and genetics, many functions of these proteins have been revealed over the past decade. In this review, we will first discuss how action potential propagation has been understood by classical electrophysiological studies. In particular, the discussion will be concentrated on how the geometric dimensions of myelinated nerve fibers (such as internodal length and myelin thickness) may affect nerve conduction velocity. This discussion will then extend into how specific myelin proteins may shape these geometric parameters, thereby regulating action potential propagation. For instance, periaxin may specifically affect the internodal length, but not other parameters. In contrast, neuregulin-1 may affect myelin thickness, but not axon diameter or internodal length. Finally, we will discuss how these basic neurobiological observations can be applied to inherited peripheral nerve diseases.

Distinctive patterns of sonographic nerve enlargement in Charcot-Marie-Tooth type 1A and hereditary neuropathy with pressure palsies

OBJECTIVE

The extent of sonomorphologic differences of peripheral nerves between CMT and HNPP is unknown.

METHODS

We recruited 9 patients with CMT-1A and 9 with HNPP. Patients underwent a standardized sonographic protocol, which evaluated nerve size and vascularization. We quantitatively assessed fascicle size and echogenicity.

RESULTS

All 18 patients demonstrated nerve enlargement, but no increased vascularization. HNPP demonstrated larger nerves at sites of entrapment (median nerve at the carpal tunnel p=0.049, ulnar nerve at the sulcus p<0.001), greater swelling ratios of median (p<0.001), ulnar (p=0.017) and fibular nerve (p=0.005) than CMT-1A. CMT-1A revealed larger nerves proximal to sites of entrapment (median and fibular nerve, brachial plexus p<0.001). Nerve fascicles where larger (p<0.001) and more hypo-echogenic in CMT-1A. Nerve, fascicle size nor echogenicity correlated with age, gender or MRC sum-score.

CONCLUSIONS

Ultrasonography of nerves reveals specific phenotypes differentiating CMT-1A from HNPP. In CMT-1A enlargement of nerves and fascicles is multifocal among multiple nerves, whereas in HNPP nerve enlargement is restricted to sites of entrapment.

SIGNIFICANCE

Our findings of specific sonomorphological phenotypes, differentiating CMT-1A from HNPP, may help to improve our pathophysiological insights in CMT and HNPP.

Pathophysiology of HNPP explored using axonal excitability

OBJECTIVE

Hereditary liability to pressure palsies (HNPP) is an autosomal dominant disorder of myelination resulting in susceptibility to pressure palsies from compression or stretching of peripheral nerves.

PATIENTS AND METHODS

This study examined axonal excitability at two sites (one distal and one proximal) in five patients with biopsy and genetically proven HNPP to understand the pathophysiology of the disease. Comparisons were made with age-matched control subjects as well as five Charcot-Marie-Tooth type 1A patients to contrast the findings and explain the different phenotypes of diseases affecting the same gene.

RESULTS

Changes in axonal excitability were found in HNPP subjects, but these were not uniform along the nerve: at the wrist there were prominent alterations in threshold electrotonus, whereas at the elbow there were only subtle alterations in the recovery cycle and the response to strong long-lasting hyperpolarisation. Threshold was raised at both sites, but the nerves were probably not hyperpolarised. Not unexpectedly, changes in CMT1A subjects were more marked than those in HNPP subjects and were uniform along the nerve.

CONCLUSIONS

Structural abnormalities at the node of Ranvier are sufficient to explain the changes in axonal excitability in HNPP, and these abnormalities would predispose the nerves to conduction block when subjected to pressure or stretch.

Sonography of the median nerve in CMT1A, CMT2A, CMTX, and HNPP

INTRODUCTION

In this study we compare the ultrasound features in the median nerve in patients with different types of Charcot-Marie-Tooth (CMT) disease and hereditary neuropathies with liability to pressure palsies (HNPP) as a typical entrapment neuropathy.

METHODS

Median nerve ultrasound and conduction studies were performed in patients with CMT1A (n = 12), MFN2-associated CMT2A (n = 7), CMTX (n = 5), and HNPP (n = 5), and in controls (n = 28).

RESULTS

Median nerve cross-sectional area (CSA) was significantly increased in CMT1A, whereas, in axonal CMT2A, fascicle diameter (FD) was enlarged. CSA correlated with nerve conduction slowing in CMT1A and with axonal loss, as shown by motor and sensory nerve amplitudes in both CMT1A and CMT2A. A relatively low wrist-to-forearm-ratio (WFR <0.8) or a relatively high WFR (>1.8) appeared to be unlikely in MFN2 and Cx32 mutations of CMT2A and CMTX, respectively.

CONCLUSION

Differences in CSA, FD, and WFR of the median nerve can be helpful in defining subtypes of hereditary neuropathies.

The PMP22 gene and its related diseases

Peripheral myelin protein-22 (PMP22) is primarily expressed in the compact myelin of the peripheral nervous system. Levels of PMP22 have to be tightly regulated since alterations of PMP22 levels by mutations of the PMP22 gene are responsible for >50 % of all patients with inherited peripheral neuropathies, including Charcot-Marie-Tooth type-1A (CMT1A) with trisomy of PMP22, hereditary neuropathy with liability to pressure palsies (HNPP) with heterozygous deletion of PMP22, and CMT1E with point mutations of PMP22. While overexpression and point-mutations of the PMP22 gene may produce gain-of-function phenotypes, deletion of PMP22 results in a loss-of-function phenotype that reveals the normal physiological functions of the PMP22 protein. In this article, we will review the basic genetics, biochemistry and molecular structure of PMP22, followed by discussion of the current understanding of pathogenic mechanisms involving in the inherited neuropathies with mutations in PMP22 gene.

Shortened internodal length of dermal myelinated nerve fibres in Charcot-Marie-Tooth disease type 1A

Charcot-Marie-Tooth disease type 1A is the most common inherited neuropathy and is caused by duplication of chromosome 17p11.2 containing the peripheral myelin protein-22 gene. This disease is characterized by uniform slowing of conduction velocities and secondary axonal loss, which are in contrast with non-uniform slowing of conduction velocities in acquired demyelinating disorders, such as chronic inflammatory demyelinating polyradiculoneuropathy. Mechanisms responsible for the slowed conduction velocities and axonal loss in Charcot-Marie-Tooth disease type 1A are poorly understood, in part because of the difficulty in obtaining nerve samples from patients, due to the invasive nature of nerve biopsies. We have utilized glabrous skin biopsies, a minimally invasive procedure, to evaluate these issues systematically in patients with Charcot-Marie-Tooth disease type 1A (n = 32), chronic inflammatory demyelinating polyradiculoneuropathy (n = 4) and healthy controls (n = 12). Morphology and molecular architecture of dermal myelinated nerve fibres were examined using immunohistochemistry and electron microscopy. Internodal length was uniformly shortened in patients with Charcot-Marie-Tooth disease type 1A, compared with those in normal controls (P < 0.0001). Segmental demyelination was absent in the Charcot-Marie-Tooth disease type 1A group, but identifiable in all patients with chronic inflammatory demyelinating polyradiculoneuropathy. Axonal loss was measurable using the density of Meissner corpuscles and associated with an accumulation of intra-axonal mitochondria. Our study demonstrates that skin biopsy can reveal pathological and molecular architectural changes that distinguish inherited from acquired demyelinating neuropathies. Uniformly shortened internodal length in Charcot-Marie-Tooth disease type 1A suggests a potential developmental defect of internodal lengthening. Intra-axonal accumulation of mitochondria provides new insights into the pathogenesis of axonal degeneration in Charcot-Marie-Tooth disease type 1A.

Claudins and epithelial paracellular transport

Tight junctions form continuous intercellular contacts controlling solute movement through the paracellular pathway across epithelia. Paracellular barriers vary among epithelia in electrical resistance and behave as if they are lined with pores that have charge and size selectivity. Recent evidence shows that claudins, a large family (at least 24 members) of intercellular adhesion molecules, form the seal and its variable pore-like properties. This evidence comes from the study of claudins expressed in cultured epithelial cell models, genetically altered mice, and human mutants. We review information on the structure, function, and transcriptional and posttranslational regulation of the claudin family as well as of their evolutionarily distant relatives called the PMP22/EMP/MP20/claudin, or pfam00822, superfamily.

Alterations in degradative pathways and protein aggregation in a neuropathy model based on PMP22 overexpression

Charcot-Marie-Tooth disease type 1A (CMT1A) is commonly associated with duplication of the peripheral myelin protein 22 (PMP22) gene. Mice expressing seven copies of the human PMP22, termed C22, suffer from a demyelinating neuropathy and display phenotypic traits of CMT1A. In this article, we investigate whether protein aggregates play a role in the CMT1A-like pathology of C22 mice. Utilizing biochemical and immunochemical tools, we found slowed turnover rate of the newly-synthesized PMP22 and the presence of cytoplasmic protein aggregates in affected nerves. The formation of these aggregates correlates with reduced proteasome activity and the accumulation of detergent-insoluble ubiquitinated substrates. A fraction of the aggregates associates with autophagosomes and lysosomes. Together, these data indicate that as a result of missorting and inefficient proteasomal degradation, the aggregation of PMP22 and recruitment of autophagosomes and lysosomes are key factors in the subcellular pathogenesis of CMT1A neuropathies.

Molecular alterations resulting from frameshift mutations in peripheral myelin protein 22: implications for neuropathy severity

Alterations in peripheral myelin protein 22 (PMP22) expression are associated with a heterogeneous group of hereditary demyelinating peripheral neuropathies. Two mutations at glycine 94, a single guanine insertion or deletion in PMP22, result in different reading frameshifts and, consequently, an extended G94fsX222 or a truncated G94fsX110 protein, respectively. Both of these autosomal dominant mutations alter the second half of PMP22 and yet are linked to clinical phenotypes with distinct severities. The G94fsX222 is associated with hereditary neuropathy with liability to pressure palsies, whereas G94fsX110 causes severe neuropathy diagnosed as Dejerine-Sottas disease or Charcot-Marie-Tooth disease type IA. To investigate the subcellular changes associated with the G94 frameshift mutations, we expressed epitope-tagged forms in primary rat Schwann cells. Biochemical and immunolabeling studies indicate that, unlike the wild-type protein, which is targeted for the plasma membrane, frameshift PMP22s are retained in the cell, prior to reaching the medial Golgi compartment. Similar to Wt-PMP22, both frameshift mutants are targeted for proteasomal degradation and accumulate in detergent-insoluble, ubiquitin-containing aggregates upon inhibition of this pathway. The extended frameshift PMP22 shows the ability to form spontaneous aggregates in the absence of proteasome inhibition. On the other hand, Schwann cells expressing the truncated protein proliferate at a significantly higher rate than Schwann cells expressing the wild-type or the extended PMP22. In summary, these results suggest that a greater potential for PMP22 aggregation is associated with a less severe phenotype, whereas dysregulation of Schwann cell proliferation is linked to severe neuropathy.

Recurrent polyradiculoneuropathy and PMP22 defects

BACKGROUND

Although immunologic factors play an important role in the pathogenesis of the inflammatory neuropathies, the mechanisms of recurrent episodes of Guillain-Barré syndrome (GBS) and chronic relapsing polyneuropathies (CRP) are not known. Hereditary neuropathy with liability to pressure palsy (HNPP) is an inherited disease caused by a deletion or point mutation in the peripheral myelin protein 22 (PMP22) gene, which may manifest as a recurrent polyradiculoneuropathy. This study tried to elucidate the relationship between PMP22 and recurrent GBS and CRP.

METHODS

Between 1993 and 2003, we saw 114 patients with polyradiculoneuropathies or their variants. Only 4 patients had recurrent episodes: 2 had recurrent GBS and 2 had CRP. We analyzed the PMP22 gene to determine its genetic role in these 4 patients. Genomic DNA was extracted from peripheral lymphocytes of all 4 patients using a previously described procedure, and molecular detection of PMP22 deletion was performed.

RESULTS

The results showed no duplication, deletion or point mutation in the PMP22 gene.

CONCLUSION

PMP22 gene deletion did not play a role in our patients with recurrent GBS and CRP.

Skin biopsies in myelin-related neuropathies: bringing molecular pathology to the bedside

Skin biopsy is a minimally invasive procedure and has been used in the evaluation of non-myelinated, but not myelinated nerve fibres, in sensory neuropathies. We therefore evaluated myelinated nerves in skin biopsies from normal controls and patients with Charcot-Marie-Tooth (CMT) disease caused by mutations in myelin proteins. Light microscopy, electron microscopy and immunohistochemistry routinely identified myelinated dermal nerves in glabrous skin that appeared similar to myelinated fibres in sural and sciatic nerve. Myelin abnormalities were observed in all patients with CMT. Moreover, skin biopsies detected potential pathogenic abnormalities in the axolemmal molecular architecture previously undetected in human neuropathies. Finally, myelin gene expression at both mRNA and protein levels was evaluated by real-time PCR and immunoelectron microscopy. Peripheral myelin protein 22 (PMP22) was increased in CMT1A (PMP22 duplication) and decreased in patients with hereditary neuropathy with liability to pressure palsies (PMP22 deletion). Taken together, our data suggest that skin biopsy may in certain circumstances replace the more invasive sural nerve biopsy in the morphological and molecular evaluation of inherited and other demyelinating neuropathies.

A newly identified chromosomal microdeletion of the rapsyn gene causes a congenital myasthenic syndrome

The objective is mutation analysis of the RAPSN gene in a patient with sporadic congenital myasthenic syndrome (CMS). Mutations in various genes encoding proteins expressed at the neuromuscular junction may cause CMS. Most mutations affect the epsilon subunit gene of the acetylcholine receptor (AChR) leading to endplate AChR deficiency. Recently, mutations in the RAPSN gene have been identified in several CMS patients with AChR deficiency. In most patients, RAPSN N88K was identified, either homozygously or heteroallelic to a second missense mutation. A sporadic CMS patient from Germany was analyzed for RAPSN mutations by RFLP, long-range PCR and sequence analysis. Clinically, the patient presents with an early onset CMS, associated with arthrogryposis multiplex congenita, recurrent episodes of respiratory insufficiency provoked by infections, and a moderate general weakness, responsive to anticholinesterase treatment. The mutation RAPSN N88K was found heterozygously to a large deletion of about 4.5 kb disrupting the RAPSN gene. Interestingly, an Alu-mediated unequal homologous recombination may have caused the deletion. We hypothesize that numerous interspersed Alu elements may predispose the RAPSN locus for genetic rearrangements.

The temporospatial expression of peripheral myelin protein 22 at the developing blood-nerve and blood-brain barriers

Peripheral myelin protein 22 (PMP22), also known as growth arrest-specific gene 3 (gas3), is a tetraspan membrane protein whose misexpression is associated with demyelinating peripheral neuropathies. Although the function of PMP22 in Schwann cells is unknown, the protein is found at intercellular junctions of various epithelia and endothelia. To begin to elucidate the role of PMP22 at cell junctions, we examined the temporal expression and protein localization during development and maturation of the rat blood-nerve barrier (BNB) and blood-brain barrier (BBB). Developing and adult rat sciatic nerves and brains were coimmunostained for PMP22 and known junctional proteins including zonula occludens-1 (ZO-1), occludin, and claudin-5. Prior to the maturation of the BNB and BBB and detection of the tight junction protein occludin, PMP22 is present at ZO-1 positive endothelial junctions of the sciatic nerve and brain cortex. The subcellular localization of PMP22 in cultured brain endothelia was confirmed by internalization with ZO-1 after EGTA-induced disruption of cell junctions. In choroid epithelia, PMP22 is detected along with occludin and ZO-1 as early as embryonic day 15 (E15). In agreement, PMP22 message is elevated in P1 rat brain microvasculature and choroid epithelia, compared with total cortex. Additionally, neuroepithelial cell junctions in the embryonic rat brain are immunoreactive for PMP22, ZO-1, and beta-catenin but not occludin. Together, these studies identify PMP22 as an early constituent of intercellular junctions in the developing and mature rat BNB and BBB.

Loss-of-function phenotype of hereditary neuropathy with liability to pressure palsies

Hereditary neuropathy with liability to pressure palsies (HNPP) provides a human model to investigate the role of PMP22 in myelinated peripheral nerve, since the disease is caused by a deletion of one of the two PMP22 alleles. To systematically characterize the phenotype of HNPP, we prospectively evaluated the clinical features and electrophysiological findings in 17 genetically confirmed patients, 7 men and 10 women, ranging in age from 9 to 66 years (mean, 41 +/- 13). Fifteen symptomatic patients presented with episodes of transient focal weakness or sensory loss that were usually related to particular activities causing nerve compression, including stretching or minor repetitive focal trauma. No patient sought medical attention for symptoms of a symmetric polyneuropathy. Neurological examinations were either normal or mildly abnormal. Neither focal slowing of nerve conduction studies, nor reduction in compound muscle action potential (CMAP) or sensory nerve action potential (SNAP) amplitudes consistently predicted the site of symptoms. We conclude that the majority of patients with HNPP present with transient, recurrent, focal symptoms of weakness or sensory loss in the distribution of individual nerves or plexus, and that a diffuse symmetric sensorimotor polyneuropathy is an unusual presentation of HNPP. These studies suggest that the function of PMP22, at least in part, is to stabilize myelin so that it will be protected from injuries resulting from repetitive, minor trauma.

Sonographic detection of diffuse peripheral nerve enlargement in hereditary neuropathy with liability to pressure palsies

Hereditary neuropathy with liability to pressure palsies (HNPP) is a disorder characterized by a tendency to develop focal neuropathies after trivial traumas. On teased nerve fiber studies, sausage-shaped myelin sheath swellings (tomacula) are found. We report the sonographic findings in a patient with genetically proven HNPP. We were able to demonstrate enlargement of several peripheral nerves, even nerves that were clinically unaffected. Enlargement was found not only at typical nerve entrapment sites but also outside these sites. This diffuse nerve enlargement may play an important role in the pathogenesis of entrapment neuropathies in HNPP patients.

Disease mechanisms and potential therapeutic strategies in Charcot-Marie-Tooth disease

Until 10 years ago, the genetic basis of Charcot-Marie-Tooth (CMT) disease was largely unknown. With the finding of an intrachromosomal duplication on chromosome 17 in 1991, associated with the most commonly found subtype CMT1A, and the discovery of a point mutation in the peripheral myelin protein-22 (pmp22) gene in the Trembler mouse in 1992, the groundwork was laid down for a novel chapter in the elucidation of the molecular basis of this large group of peripheral neuropathies. In the meantime, several different genes have been found to be associated with different forms of demyelinating and axonal forms of CMT. In this review, we will summarize what is known today about the genetics of this group of disease which constitute the most common known monogenetic disorder affecting the nervous system in man, the animal models that have been generated, and what we have learned about the underlying disease mechanisms. Furthermore, we will review how this gain of knowledge about CMT may open new avenues to the development of novel treatment strategies.

Progress in clinical neurosciences: Charcot-Marie-Tooth disease and related inherited peripheral neuropathies

The classification of Charcot-Marie-Tooth disease and related hereditary motor and sensory neuropathies has evolved to incorporate clinical, electrophysiological and burgeoning molecular genetic information that characterize the many disorders. For several inherited neuropathies, the gene product abnormality is known and for others, candidate genes have been identified. Genetic testing can pinpoint a specific inherited neuropathy for many patients. However, clinical and electrophysiological assessments continue to be essential tools for diagnosis and management of this disease group. This article reviews clinical, electrophysiological, pathological and molecular aspects of hereditary motor and sensory neuropathies.