PMP22 messenger RNA levels in skin biopsies: testing the effectiveness of a Charcot-Marie-Tooth 1A biomarker

Hereditary neuropathy

The hereditary neuropathies, collectively referred as Charcot-Marie-Tooth disease (CMT) and related disorders, are heterogeneous genetic peripheral nerve disorders that collectively comprise the commonest inherited neurological disease with an estimated prevalence of 1:2500 individuals. The field of hereditary neuropathies has made significant progress in recent years with respect to both gene discovery and treatment as a result of next-generation sequencing (NGS) approach. These investigations which have identified over 100 causative genes and new mutations have made the classification of CMT even more challenging. Despite so many different mutated genes, the majority of CMT forms share a similar clinical phenotype, and due to this phenotypic homogeneity, genetic testing in CMT is increasingly being performed through the use of NGS panels. The majority of patients still have a mutation in one the four most common genes (PMP22 duplication-CMT1A, MPZ-CMT1B, GJB1-CMTX1, and MFN2-CMT2A). This chapter focuses primarily on these four forms and their potential therapeutic approaches.

Mechanisms and Treatments in Demyelinating CMT

Demyelinating forms of Charcot-Marie-Tooth disease (CMT) are genetically and phenotypically heterogeneous and result from highly diverse biological mechanisms including gain of function (including dominant negative effects) and loss of function. While no definitive treatment is currently available, rapid advances in defining the pathomechanisms of demyelinating CMT have led to promising pre-clinical studies, as well as emerging clinical trials. Especially promising are the recently completed pre-clinical genetic therapy studies in PMP-22, GJB1, and SH3TC2-associated neuropathies, particularly given the success of similar approaches in humans with spinal muscular atrophy and transthyretin familial polyneuropathy. This article focuses on neuropathies related to mutations in PMP-22, MPZ, and GJB1, which together comprise the most common forms of demyelinating CMT, as well as on select rarer forms for which promising treatment targets have been identified. Clinical characteristics and pathomechanisms are reviewed in detail, with emphasis on therapeutically targetable biological pathways. Also discussed are the challenges facing the CMT research community in its efforts to advance the rapidly evolving biological insights to effective clinical trials. These considerations include the limitations of currently available animal models, the need for personalized medicine approaches/allele-specific interventions for select forms of demyelinating CMT, and the increasing demand for optimal clinical outcome assessments and objective biomarkers.

Circulating Biomarkers in Neuromuscular Disorders: What Is Known, What Is New

The urgent need for new therapies for some devastating neuromuscular diseases (NMDs), such as Duchenne muscular dystrophy or amyotrophic lateral sclerosis, has led to an intense search for new potential biomarkers. Biomarkers can be classified based on their clinical value into different categories: diagnostic biomarkers confirm the presence of a specific disease, prognostic biomarkers provide information about disease course, and therapeutic biomarkers are designed to predict or measure treatment response. Circulating biomarkers, as opposed to instrumental/invasive ones (e.g., muscle MRI or nerve ultrasound, muscle or nerve biopsy), are generally easier to access and less “time-consuming”. In addition to well-known creatine kinase, other promising molecules seem to be candidate biomarkers to improve the diagnosis, prognosis and prediction of therapeutic response, such as antibodies, neurofilaments, and microRNAs. However, there are some criticalities that can complicate their application: variability during the day, stability, and reliable performance metrics (e.g., accuracy, precision and reproducibility) across laboratories. In the present review, we discuss the application of biochemical biomarkers (both validated and emerging) in the most common NMDs with a focus on their diagnostic, prognostic/predictive and therapeutic application, and finally, we address the critical issues in the introduction of new biomarkers.

Updated review of therapeutic strategies for Charcot-Marie-Tooth disease and related neuropathies

Introduction: Charcot-Marie-Tooth disease (CMT) and related neuropathies represent the most prevalent inherited neuromuscular disorders. Nonetheless, there is still no pharmacological treatment available for any CMT type. However, the landscape is rapidly evolving and several novel approaches are providing encouraging results in preclinical studies and leading to clinical trials.Areas covered: The authors review the most promising therapies under study and the ongoing/planned clinical trials. Several approaches to address PMP22 overexpression underlying CMT1A, the most frequent subtype, are being tested. Gene silencing, targeting PMP22, and gene therapy, to introduce specific genes or to substitute or modulate defective ones, are being experimented in animal models. Compounds acting on ER stress, unfolded protein response, neuregulin pathways, phosphoinositides metabolism, axonal transport and degeneration, inflammation, polyol pathway, deoxysphingolipid metabolism, purine nucleotide pool are potential therapeutic candidates for different forms of CMT and related neuropathies.Expert opinion: We are getting closer to find effective therapies for CMT, but are far behind the exciting examples of other genetic neuromuscular disorders. The authors analyze the possible reasons for this gap and the way to fill it. Preclinical and clinical research is ongoing with coordinated efforts and they are confident that in the next few years we will see the first effective treatments.

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.

Genetic mechanisms of peripheral nerve disease

Peripheral neuropathies of genetic etiology are a very diverse group of disorders manifesting either as non-syndromic inherited neuropathies without significant manifestations outside the peripheral nervous system, or as part of a systemic or syndromic genetic disorder. The former and most frequent group is collectively known as Charcot-Marie-Tooth disease (CMT), with prevalence as high as 1:2,500 world-wide, and has proven to be genetically highly heterogeneous. More than 100 different genes have been identified so far to cause various CMT forms, following all possible inheritance patterns. CMT causative genes belong to several common functional pathways that are essential for the integrity of the peripheral nerve. Their discovery has provided insights into the normal biology of axons and myelinating cells, and has highlighted the molecular mechanisms including both loss of function and gain of function effects, leading to peripheral nerve degeneration. Demyelinating neuropathies result from dysfunction of genes primarily affecting myelinating Schwann cells, while axonal neuropathies are caused by genes affecting mostly neurons and their long axons. Furthermore, mutation in genes expressed outside the nervous system, as in the case of inherited amyloid neuropathies, may cause peripheral neuropathy resulting from accumulation of β-structured amyloid fibrils in peripheral nerves in addition to various organs. Increasing insights into the molecular-genetic mechanisms have revealed potential therapeutic targets. These will enable the development of novel therapeutics for genetic neuropathies that remain, in their majority, without effective treatment.

Exploiting Sphingo- and Glycerophospholipid Impairment to Select Effective Drugs and Biomarkers for CMT1A

In Charcot-Marie-Tooth type 1A (CMT1A), Schwann cells exhibit a preponderant transcriptional deficiency of genes involved in lipid biosynthesis. This perturbed lipid metabolism affects the peripheral nerve physiology and the structure of peripheral myelin. Nevertheless, the identification and functional characterization of the lipid species mainly responsible for CMT1A myelin impairment currently lack. This is critical in the pathogenesis of the neuropathy since lipids are many and complex molecules which play essential roles in the cell, including the structural components of cellular membranes, cell signaling, and membrane trafficking. Moreover, lipids themselves are able to modify gene transcription, thereby affecting the genotype-phenotype correlation of well-defined inherited diseases, including CMT1A. Here we report for the first time a comprehensive lipid profiling in experimental and human CMT1A, demonstrating a previously unknown specific alteration of sphingolipid (SP) and glycerophospholipid (GP) metabolism. Notably, SP, and GP changes even emerge in biological fluids of CMT1A rat and human patients, implying a systemic metabolic dysfunction for these specific lipid classes. Actually, SP and GP are not merely reduced; their expression is instead aberrant, contributing to the ultrastructural abnormalities that we detailed by X-ray diffraction in rat and human internode myelin. The modulation of SP and GP pathways in myelinating dorsal root ganglia cultures clearly sustains this issue. In fact, just selected molecules interacting with these pathways are able to modify the altered geometric parameters of CMT1A myelinated fibers. Overall, we propose to exploit the present SP and GP metabolism impairment to select effective drugs and validate a set of reliable biomarkers, which remain a challenge in CMT1A neuropathy.

Intraepidermal nerve fibre density as biomarker in Charcot-Marie-Tooth disease type 1A

Charcot-Marie-Tooth disease type 1A, caused by a duplication of the gene peripheral myelin protein 22 kDa, is the most frequent subtype of hereditary peripheral neuropathy with an estimated prevalence of 1:5000. Patients suffer from sensory deficits, muscle weakness and foot deformities. There is no treatment approved for this disease. Outcome measures in clinical trials were based mainly on clinical features but did not evaluate the actual nerve damage. In our case-control study, we aimed to provide objective and reproducible outcome measures for future clinical trials. We collected skin samples from 48 patients with Charcot-Marie-Tooth type 1A, 7 patients with chronic inflammatory demyelinating polyneuropathy, 16 patients with small fibre neuropathy and 45 healthy controls. To analyse skin innervation, 40-µm cryosections of glabrous skin taken from the lateral index finger were double-labelled by immunofluorescence. The disease severity of patients with Charcot-Marie-Tooth type 1A was assessed by the Charcot-Marie-Tooth neuropathy version 2 score, which ranged from 3 (mild) to 27 (severe) and correlated with age (P < 0.01, R = 0.4). Intraepidermal nerve fibre density was reduced in patients with Charcot-Marie-Tooth type 1A compared with the healthy control group (P < 0.01) and negatively correlated with disease severity (P < 0.05, R = -0.293). Meissner corpuscle (MC) density correlated negatively with age in patients with Charcot-Marie-Tooth type 1A (P < 0.01, R = -0.45) but not in healthy controls (P = 0.07, R = 0.28). The density of Merkel cells was reduced in patients with Charcot-Marie-Tooth type 1A compared with healthy controls (P < 0.05). Furthermore, in patients with Charcot-Marie-Tooth type 1A, the fraction of denervated Merkel cells was highly increased and correlated with age (P < 0.05, R = 0.37). Analysis of nodes of Ranvier revealed shortened paranodes and a reduced fraction of long nodes in patients compared with healthy controls (both P < 0.001). Langerhans cell density was increased in chronic inflammatory demyelinating polyneuropathy, but not different in Charcot-Marie-Tooth type 1A compared with healthy controls. Our data suggest that intraepidermal nerve fibre density might be used as an outcome measure in Charcot-Marie-Tooth type 1A disease, as it correlates with disease severity. The densities of Meissner corpuscles and Merkel cells might be an additional tool for the evaluation of the disease progression. Analysis of follow-up biopsies will clarify the effects of Charcot-Marie-Tooth type 1A disease progression on cutaneous innervation.

Targeted PMP22 TATA-box editing by CRISPR/Cas9 reduces demyelinating neuropathy of Charcot-Marie-Tooth disease type 1A in mice

Charcot-Marie-Tooth 1A (CMT1A) is the most common inherited neuropathy without a known therapy, which is caused by a 1.4 Mb duplication on human chromosome 17, which includes the gene encoding the peripheral myelin protein of 22 kDa (PMP22). Overexpressed PMP22 protein from its gene duplication is thought to cause demyelination and subsequently axonal degeneration in the peripheral nervous system (PNS). Here, we targeted TATA-box of human PMP22 promoter to normalize overexpressed PMP22 level in C22 mice, a mouse model of CMT1A harboring multiple copies of human PMP22. Direct local intraneural delivery of CRISPR/Cas9 designed to target TATA-box of PMP22 before the onset of disease, downregulates gene expression of PMP22 and preserves both myelin and axons. Notably, the same approach was effective in partial rescue of demyelination even after the onset of disease. Collectively, our data present a proof-of-concept that CRISPR/Cas9-mediated targeting of TATA-box can be utilized to treat CMT1A.

Regulating PMP22 expression as a dosage sensitive neuropathy gene

Structural variation in the human genome has emerged as a major cause of disease as genomic data have accumulated. One of the most common structural variants associated with human disease causes the heritable neuropathy known as Charcot-Marie-Tooth (CMT) disease type 1A. This 1.4 Mb duplication causes nearly half of the CMT cases that are genetically diagnosed. The PMP22 gene is highly induced in Schwann cells during development, although its precise role in myelin formation and homeostasis is still under active investigation. The PMP22 gene can be considered as a nucleoprotein complex with enzymatic activity to produce the PMP22 transcript, and the complex is allosterically regulated by transcription factors that respond to intracellular signals and epigenomic modifications. The control of PMP22 transcript levels has been one of the major therapeutic targets of therapy development, and this review summarizes those approaches as well as efforts to characterize the regulation of the PMP22 gene.

Schwann cell transcript biomarkers for hereditary neuropathy skin biopsies

OBJECTIVE

Charcot-Marie-Tooth (CMT) disease is most commonly caused by duplication of a chromosomal segment surrounding Peripheral Myelin Protein 22, or PMP22 gene, which is classified as CMT1A. Several candidate therapies reduce Pmp22 mRNA levels in CMT1A rodent models, but development of biomarkers for clinical trials in CMT1A is a challenge given its slow progression and difficulty in obtaining nerve samples. Quantitative PCR measurements of PMP22 mRNA in dermal nerves were performed using skin biopsies in human clinical trials for CMT1A, but this approach did not show increased PMP22 mRNA in CMT1A patients compared to controls. One complicating factor is the variable amounts of Schwann cells (SCs) in skin. The objective of the study was to develop a novel method for precise evaluation of PMP22 levels in skin biopsies that can discriminate CMT1A patients from controls.

METHODS

We have developed methods to normalize PMP22 transcript levels to SC-specific genes that are not altered by CMT1A status. Several CMT1A-associated genes were assembled into a custom Nanostring panel to enable precise transcript measurements that can be normalized to variable SC content.

RESULTS

The digital expression data from Nanostring analysis showed reproducible elevation of PMP22 levels in CMT1A versus control skin biopsies, particularly after normalization to SC-specific genes.

INTERPRETATION

This platform should be useful in clinical trials for CMT1A as a biomarker of target engagement that can be used to optimize dosing, and the same normalization framework is applicable to other types of CMT. ANN NEUROL 2019;85:887-898.

Challenges in modelling the Charcot-Marie-Tooth neuropathies for therapy development

Much has been achieved in terms of understanding the complex clinical and genetic heterogeneity of Charcot-Marie-Tooth neuropathy (CMT). Since the identification of mutations in the first CMT associated gene, PMP22, the technological advancement in molecular genetics and gene technology has allowed scientists to generate diverse animal models expressing monogenetic mutations that closely resemble the CMT phenotype. Additionally, one can now culture patient-derived neurons in a dish using cellular reprogramming and differentiation techniques. Nevertheless, despite the fact that finding a disease-causing mutation offers a precise diagnosis, there is no cure for CMT at present. This review will shed light on the exciting advancement in CMT disease modelling, the breakthroughs, pitfalls, current challenges for scientists and key considerations to move the field forward towards successful therapies.

Elevated Peripheral Myelin Protein 22, Reduced Mitotic Potential, and Proteasome Impairment in Dermal Fibroblasts from Charcot-Marie-Tooth Disease Type 1A Patients

A common form of hereditary autosomal dominant demyelinating neuropathy known as Charcot-Marie-Tooth disease type 1A (CMT1A) is linked with duplication of the peripheral myelin protein 22 (PMP22) gene. Although studies from animal models have led to better understanding of the pathobiology of these neuropathies, there continues to be a gap in the translation of findings from rodents to humans. Because PMP22 was originally identified in fibroblasts as growth arrest specific gene 3 (gas3) and is expressed broadly in the body, it was tested whether skin cells from neuropathic patients would display the cellular pathology observed in Schwann cells from rodent models. Dermal fibroblasts from two CMT1A pedigrees with confirmed PMP22 gene duplication were studied. Samples from age-matched non-neuropathic individuals were used as controls. CMT1A patient–derived cultures contain approximately 1.5-fold elevated levels of PMP22 mRNA, exhibit reduced mitotic potential, and display intracellular protein aggregates as compared to cells from unaffected individuals. The presence of cytosolic PMP22 coincides with a decrease in proteasome activity and an increase in autophagy-lysosomal proteins, including LC3-II and LAMP1. These results indicate that the abnormalities in the subcellular processing of excess PMP22 elicit a detectable response in human CMT1A fibroblasts, a phenotype that resembles Schwann cells from neuropathic mice. These findings support the use of human CMT1A fibroblasts as a platform for therapy testing.

PMP22 antisense oligonucleotides reverse Charcot-Marie-Tooth disease type 1A features in rodent models

Charcot-Marie-Tooth disease type 1A (CMT1A) is caused by duplication of peripheral myelin protein 22 (PMP22) and is the most common hereditary peripheral neuropathy. CMT1A is characterized by demyelination and axonal loss, which underlie slowed motor nerve conduction velocity (MNCV) and reduced compound muscle action potentials (CMAP) in patients. There is currently no known treatment for this disease. Here, we show that antisense oligonucleotides (ASOs) effectively suppress PMP22 mRNA in affected nerves in 2 murine CMT1A models. Notably, initiation of ASO treatment after disease onset restored myelination, MNCV, and CMAP almost to levels seen in WT animals. In addition to disease-associated gene expression networks that were restored with ASO treatment, we also identified potential disease biomarkers through transcriptomic profiling. Furthermore, we demonstrated that reduction of PMP22 mRNA in skin biopsies from ASO-treated rats is a suitable biomarker for evaluating target engagement in response to ASO therapy. These results support the use of ASOs as a potential treatment for CMT1A and elucidate potential disease and target engagement biomarkers for use in future clinical trials.

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.

Tolerability and efficacy study of P2X7 inhibition in experimental Charcot-Marie-Tooth type 1A (CMT1A) neuropathy

Charcot-Marie-Tooth 1A (CMT1A) is a demyelinating hereditary neuropathy for which pharmacological treatments are not yet available. An abnormally high intracellular Ca(2+) concentration was observed in Schwann cells (SC) from CMT1A rats, caused by the PMP22-mediated overexpression of the P2X7 purinoceptor. The purpose of this study was to investigate the tolerability and therapeutic potential of a pharmacological antagonist of the P2X7 receptor (A438079) in CMT1A. A438079 ameliorated in vitro myelination of organotypic DRG cultures from CMT1A rats. Furthermore, we performed an experimental therapeutic trial in PMP22 transgenic and in wild-type rats. A preliminary dose-escalation trial showed that 3mg/kg A438079 administered via intraperitoneal injection every 24h for four weeks was well tolerated by wild type and CMT1A rats. Affected rats treated with 3mg/kg A438079 revealed a significant improvement of the muscle strength, when compared to placebo controls. Importantly, histologic analysis revealed a significant increase of the total number of myelinated axons in tibial nerves. Moreover, a significant decrease of the hypermyelination of small caliber axons and a significant increase of the frequency and diameter of large caliber myelinated axons was highlighted. An improved distal motor latencies was recorded, whereas compound muscle action potentials (CMAP) remained unaltered. A438079 reduced the SC differentiation defect in CMT1A rats. These results show that pharmacological inhibition of the P2X7 receptor is well tolerated in CMT1A rats and represents a proof-of-principle that antagonizing this pathway may correct the molecular derangements and improve the clinical phenotype in the CMT1A neuropathy.

Ascorbic acid for the treatment of Charcot-Marie-Tooth disease

BACKGROUND

Charcot-Marie-Tooth disease (CMT) comprises a large group of different forms of hereditary motor and sensory neuropathy. The molecular basis of several CMT subtypes has been clarified during the last 20 years. Since slowly progressive muscle weakness and sensory disturbances are the main features of these syndromes, treatments aim to improve motor impairment and sensory disturbances to improve abilities. Pharmacological treatment trials in CMT are rare. This review was derived from a Cochrane review, Treatment for Charcot Marie Tooth disease, which will be updated via this review and a forthcoming title, Treatments other than ascorbic acid for Charcot-Marie-Tooth disease.

OBJECTIVES

To assess the effects of ascorbic acid (vitamin C) treatment for CMT.

SEARCH METHODS

On 21 September 2015, we searched the Cochrane Neuromuscular Specialised Register, Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE and LILACS for randomised controlled trials (RCTs) of treatment for CMT. We also checked clinical trials registries for ongoing studies.

SELECTION CRITERIA

We included RCTs and quasi-RCTs of any ascorbic acid treatment for people with CMT. Where a study aimed to evaluate the treatment of general neuromuscular symptoms of people with peripheral neuropathy including CMT, we included the study if we were able to identify the effect of treatment in the CMT group. We did not include observational studies or case reports of ascorbic acid treatment in people with CMT.

DATA COLLECTION AND ANALYSIS

Two review authors (BG and JB) independently extracted the data and assessed study quality.

MAIN RESULTS

Six RCTs compared the effect of oral ascorbic acid (1 to 4 grams) and placebo treatment in CMT1A. In five trials involving adults with CMT1A, a total of 622 participants received ascorbic acid or placebo. Trials were largely at low risk of bias. There is high-quality evidence that ascorbic acid does not improve the course of CMT1A in adults as measured by the CMT neuropathy score (0 to 36 scale) at 12 months (mean difference (MD) -0.37; 95% confidence intervals (CI) -0.83 to 0.09; five studies; N = 533), or at 24 months (MD -0.21; 95% CI -0.81 to 0.39; three studies; N = 388). Ascorbic acid treatment showed a positive effect on the nine-hole peg test versus placebo (MD -1.16 seconds; 95% CI -1.96 to -0.37), but the clinical significance of this result is probably small. Meta-analyses of other secondary outcome parameters showed no relevant benefit of ascorbic acid. In one trial, 80 children with CMT1A received ascorbic acid or placebo. The trial showed no clinical benefit of ascorbic acid treatment. Adverse effects did not differ in their nature or abundance between ascorbic acid and placebo.

AUTHORS' CONCLUSIONS

High-quality evidence indicates that ascorbic acid does not improve the course of CMT1A in adults in terms of the outcome parameters used. According to low-quality evidence, ascorbic acid does not improve the course of CMT1A in children. However, CMT1A is slowly progressive and the outcome parameters show only small change over time. Longer study durations should be considered, and outcome parameters more sensitive to change over time should be designed and validated for future studies.

Polytherapy with a combination of three repurposed drugs (PXT3003) down-regulates Pmp22 over-expression and improves myelination, axonal and functional parameters in models of CMT1A neuropathy

Charcot-Marie-Tooth disease type 1A (CMT1A) is the most common inherited sensory and motor peripheral neuropathy. It is caused by PMP22 overexpression which leads to defects of peripheral myelination, loss of long axons, and progressive impairment then disability. There is no treatment available despite observations that monotherapeutic interventions slow progression in rodent models. We thus hypothesized that a polytherapeutic approach using several drugs, previously approved for other diseases, could be beneficial by simultaneously targeting PMP22 and pathways important for myelination and axonal integrity. A combination of drugs for CMT1A polytherapy was chosen from a group of authorised drugs for unrelated diseases using a systems biology approach, followed by pharmacological safety considerations. Testing and proof of synergism of these drugs were performed in a co-culture model of DRG neurons and Schwann cells derived from a Pmp22 transgenic rat model of CMT1A. Their ability to lower Pmp22 mRNA in Schwann cells relative to house-keeping genes or to a second myelin transcript (Mpz) was assessed in a clonal cell line expressing these genes. Finally in vivo efficacy of the combination was tested in two models: CMT1A transgenic rats, and mice that recover from a nerve crush injury, a model to assess neuroprotection and regeneration. Combination of (RS)-baclofen, naltrexone hydrochloride and D-sorbitol, termed PXT3003, improved myelination in the Pmp22 transgenic co-culture cellular model, and moderately down-regulated Pmp22 mRNA expression in Schwannoma cells. In both in vitro systems, the combination of drugs was revealed to possess synergistic effects, which provided the rationale for in vivo clinical testing of rodent models. In Pmp22 transgenic CMT1A rats, PXT3003 down-regulated the Pmp22 to Mpz mRNA ratio, improved myelination of small fibres, increased nerve conduction and ameliorated the clinical phenotype. PXT3003 also improved axonal regeneration and remyelination in the murine nerve crush model. Based on these observations in preclinical models, a clinical trial of PTX3003 in CMT1A, a neglected orphan disease, is warranted. If the efficacy of PTX3003 is confirmed, rational polytherapy based on novel combinations of existing non-toxic drugs with pleiotropic effects may represent a promising approach for rapid drug development.