Ascorbic acid for treatment in CMT1A: what's next?

A translatable RNAi-driven gene therapy silences PMP22/Pmp22 genes and improves neuropathy in CMT1A mice

Charcot-Marie-Tooth disease type 1A (CMT1A), the most common inherited demyelinating peripheral neuropathy, is caused by PMP22 gene duplication. Overexpression of WT PMP22 in Schwann cells destabilizes the myelin sheath, leading to demyelination and ultimately to secondary axonal loss and disability. No treatments currently exist that modify the disease course. The most direct route to CMT1A therapy will involve reducing PMP22 to normal levels. To accomplish this, we developed a gene therapy strategy to reduce PMP22 using artificial miRNAs targeting human PMP22 and mouse Pmp22 mRNAs. Our lead therapeutic miRNA, miR871, was packaged into an adeno-associated virus 9 (AAV9) vector and delivered by lumbar intrathecal injection into C61-het mice, a model of CMT1A. AAV9-miR871 efficiently transduced Schwann cells in C61-het peripheral nerves and reduced human and mouse PMP22 mRNA and protein levels. Treatment at early and late stages of the disease significantly improved multiple functional outcome measures and nerve conduction velocities. Furthermore, myelin pathology in lumbar roots and femoral motor nerves was ameliorated. The treated mice also showed reductions in circulating biomarkers of CMT1A. Taken together, our data demonstrate that AAV9-miR871–driven silencing of PMP22 rescues a CMT1A model and provides proof of principle for treating CMT1A using a translatable gene therapy approach.

AAV2/9-mediated silencing of PMP22 prevents the development of pathological features in a rat model of Charcot-Marie-Tooth disease 1 A

Charcot-Marie-Tooth disease 1 A (CMT1A) results from a duplication of the PMP22 gene in Schwann cells and a deficit of myelination in peripheral nerves. Patients with CMT1A have reduced nerve conduction velocity, muscle wasting, hand and foot deformations and foot drop walking. Here, we evaluate the safety and efficacy of recombinant adeno-associated viral vector serotype 9 (AAV2/9) expressing GFP and shRNAs targeting Pmp22 mRNA in animal models of Charcot-Marie-Tooth disease 1 A. Intra-nerve delivery of AAV2/9 in the sciatic nerve allowed widespread transgene expression in resident myelinating Schwann cells in mice, rats and non-human primates. A bilateral treatment restore expression levels of PMP22 comparable to wild-type conditions, resulting in increased myelination and prevention of motor and sensory impairments over a twelve-months period in a rat model of CMT1A. We observed limited off-target transduction and immune response using the intra-nerve delivery route. A combination of previously characterized human skin biomarkers is able to discriminate between treated and untreated animals, indicating their potential use as part of outcome measures.

Biomarkers predict outcome in Charcot-Marie-Tooth disease 1A

BACKGROUND

Charcot-Marie-Tooth disease type 1A (CMT1A) is the most common inherited neuropathy, a debilitating disease without known cure. Among patients with CMT1A, disease manifestation, progression and severity are strikingly variable, which poses major challenges for the development of new therapies. Hence, there is a strong need for sensitive outcome measures such as disease and progression biomarkers, which would add powerful tools to monitor therapeutic effects in CMT1A.

METHODS

We established a pan-European and American consortium comprising nine clinical centres including 311 patients with CMT1A in total. From all patients, the CMT neuropathy score and secondary outcome measures were obtained and a skin biopsy collected. In order to assess and validate disease severity and progression biomarkers, we performed qPCR on a set of 16 animal model-derived potential biomarkers in skin biopsy mRNA extracts.

RESULTS

In 266 patients with CMT1A, a cluster of eight cutaneous transcripts differentiates disease severity with a sensitivity and specificity of 90% and 76.1%, respectively. In an additional cohort of 45 patients with CMT1A, from whom a second skin biopsy was taken after 2-3 years, the cutaneous mRNA expression of GSTT2, CTSA, PPARG, CDA, ENPP1 and NRG1-Iis changing over time and correlates with disease progression.

CONCLUSIONS

In summary, we provide evidence that cutaneous transcripts in patients with CMT1A serve as disease severity and progression biomarkers and, if implemented into clinical trials, they could markedly accelerate the development of a therapy for CMT1A.

Rehabilitation in Charcot-Marie-Tooth disease type 1

Charcot-Marie-Tooth disease is the most common inherited peripheral neuropathy with a prevalence of approximately 1 in 2,500 [1]. The most common subtype is the autosomal dominant type 1A, which is caused by an intrachromosomal duplication on chromosome 17p11.2 [2,3]. A consecutive primary loss of the myelin sheath leads to secondary axonal degeneration. Characteristic clinical findings include distally pronounced muscle wasting, secondary skeletal deformities, sensory loss, and reduced deep tendon reflexes [4,5]. The individual clinical phenotypes vary, even among monozygotic twins [6]. They range from sub-clinical manifestations to rare cases of wheelchair-bound patients. Overall, the quality of life is significantly impaired [7]. Despite ongoing research, no curative treatments are currently available [8]. A recently published ascorbic acid trial showed no significant effect on the clinical phenotype of CMT1A patients [9,32,36]. Nevertheless, physical therapy and moderate exercises are proven to be positively disease-modifying. While a cure lies beyond the scope of physical therapy, it may prevent the rapid aggravation of the clinical phenotype [10,11,17]. Recent studies suggest that CMT patients experience physical as well as mental benefits from rehabilitation programmes, but they also perceive that the performed exercises were not specifically designed to their needs [11]. In fact, there is little evidence-based data and no common consensus on rehabilitation in patients suffering from Charcot-Marie-Tooth disease.

PMP22 related neuropathies: Charcot-Marie-Tooth disease type 1A and Hereditary Neuropathy with liability to Pressure Palsies

PMP22 related neuropathies comprise (1) PMP22 duplications leading to Charcot-Marie-Tooth disease type 1A (CMT1A), (2) PMP22 deletions, leading to Hereditary Neuropathy with liability to Pressure Palsies (HNPP), and (3) PMP22 point mutations, causing both phenotypes. Overall prevalence of CMT is usually reported as 1:2,500, epidemiological studies show that 20-64% of CMT patients carry the PMP22 duplication. The prevalence of HNPP is not well known. CMT1A usually presents in the first two decades with difficulty walking or running. Distal symmetrical muscle weakness and wasting and sensory loss is present, legs more frequently and more severely affected than arms. HNPP typically leads to episodic, painless, recurrent, focal motor and sensory peripheral neuropathy, preceded by minor compression on the affected nerve. Electrophysiological evaluation is needed to determine whether the polyneuropathy is demyelinating. Sonography of the nerves can be useful. Diagnosis is confirmed by finding respectively a PMP22 duplication, deletion or point mutation. Differential diagnosis includes other inherited neuropathies, and acquired polyneuropathies. The mode of inheritance is autosomal dominant and de novo mutations occur. Offspring of patients have a chance of 50% to inherit the mutation from their affected parent. Prenatal testing is possible; requests for prenatal testing are not common. Treatment is currently symptomatic and may include management by a rehabilitation physician, physiotherapist, occupational therapist and orthopaedic surgeon. Adult CMT1A patients show slow clinical progression of disease, which seems to reflect a process of normal ageing. Life expectancy is normal.

Murine therapeutic models for Charcot-Marie-Tooth (CMT) disease

INTRODUCTION OR BACKGROUND

Charcot-Marie-Tooth (CMT) disease represents a broad group of inherited motor and sensory neuropathies which can originate from various genetic aberrations, e.g. mutations, deletions and duplications.

SOURCES OF DATA

We performed a literature review on murine animal models of CMT disease with regard to experimental therapeutic approaches. Hereby, we focussed on the demyelinating subforms of CMT (CMT1). PubMed items were CMT, animal model, demyelination and therapy.

AREAS OF AGREEMENT

Patients affected by CMT suffer from slowly progressive, distally pronounced muscle atrophy caused by an axonal loss. The disease severity is highly variable and impairments may result in wheelchair boundness. No therapy is available yet.

AREAS OF CONTROVERSY

Numerous rodent models for the various CMT subtypes are available today. The selection of the correct animal model for the specific CMT subtype provides an important prerequisite for the successful translation of experimental findings in patients.

GROWING POINTS

Despite more than 20 years of remarkable progress in CMT research, the disease is still left untreatable. There is a growing number of experimental therapeutic strategies that may be translated into future clinical trials in patients with CMT.

AREAS TIMELY FOR DEVELOPING RESEARCH

The slow disease progression and insensitive outcome measures hamper clinical therapy trials in CMT. Biomarkers may provide powerful tools to monitor therapeutic efficacy. Recently, we have shown that transcriptional profiling can be utilized to assess and predict the disease severity in a transgenic rat model and in affected humans.

A rat model of Charcot-Marie-Tooth disease 1A recapitulates disease variability and supplies biomarkers of axonal loss in patients

Charcot-Marie-Tooth disease is the most common inherited neuropathy and a duplication of the peripheral myelin protein 22 gene causes the most frequent subform Charcot-Marie-Tooth 1A. Patients develop a slowly progressive dysmyelinating and demyelinating peripheral neuropathy and distally pronounced muscle atrophy. The amount of axonal loss determines disease severity. Although patients share an identical monogenetic defect, the disease progression is strikingly variable and the impending disease course can not be predicted in individual patients. Despite promising experimental data, recent therapy trials have failed. Established clinical outcome measures are thought to be too insensitive to detect amelioration within trials. Surrogate biomarkers of disease severity in Charcot-Marie-Tooth 1A are thus urgently needed. Peripheral myelin protein 22 transgenic rats harbouring additional copies of the peripheral myelin protein 22 gene ('Charcot-Marie-Tooth rats'), which were kept on an outbred background mimic disease hallmarks and phenocopy the variable disease severity of patients with Charcot-Marie-Tooth 1A. Hence, we used the Charcot-Marie-Tooth rat to dissect prospective and surrogate markers of disease severity derived from sciatic nerve and skin tissue messenger RNA extracts. Gene set enrichment analysis of sciatic nerve transcriptomes revealed that dysregulation of lipid metabolism associated genes such as peroxisome proliferator-activated receptor gamma constitutes a modifier of present and future disease severity. Importantly, we directly validated disease severity markers from the Charcot-Marie-Tooth rats in 46 patients with Charcot-Marie-Tooth 1A. Our data suggest that the combination of age and cutaneous messenger RNA levels of glutathione S-transferase theta 2 and cathepsin A composes a strong indicator of disease severity in patients with Charcot-Marie-Tooth 1A, as quantified by the Charcot-Marie-Tooth Neuropathy Score. This translational approach, utilizing a transgenic animal model, demonstrates that transcriptional analysis of skin biopsy is suitable to identify biomarkers of Charcot-Marie-Tooth 1A.