Date: 22nd April 2021
Charcot-Marie-Tooth disease (CMT) is a group of inherited neurological disorders, also known as hereditary motor and sensory neuropathy. They affect the peripheral nerves, and are one of the most common inherited neurological disorders, affecting ~2.6 million people worldwide. To date there are no available cures for CMT, and they are currently managed with supportive therapy. Now, researchers evaluate the safety and efficacy of gene therapy carrying therapeutic shRNAs that restore disease-associated protein levels to normal, preventing motor and sensory impairments for over a year, in a model for the most common form of CMT.
The most common of myelin-related CMT diseases is CMT1A, which results from a duplication of the PMP22 gene. This encodes a transmembrane glycoprotein located in the protective myelin sheath, which wraps around axons and is formed by Schwann cells. PMP22 protein overexpression in the Schwann cells leads to defects in the myelin, causing myelin degeneration (demyelination) and eventually leading to axon loss. Gene therapy may represent a long-term way to treat CMT1A however, whilst preclinical experiments using lentiviral methods have shown some success for CMTs, the more popular and perhaps safer adeno-associated virus (AAV)-based strategy of gene therapy has yet to be tested, and transduction pattern of AAVs is currently unknown.
Now, researchers at the University of Montpellier, France, led by Nicolas Tricaud, and Robert Fledrich at Leipzig University, Germany, have shown that intra-nerve delivery of AAV2/9 (two AAV serotypes) in the sciatic nerve allowed widespread transgene expression in resident myelinating Schwann cells in mice, rats and non-human primates. In a rat model of CMT1A, this gene therapy carried shRNAs targeting Pmp22 mRNA, restoring expression levels of PMP22 to near wild-type levels. This resulted in increased myelination and prevention of motor and sensory impairments for up to a year.
To start the team wanted to assess transduction potential of myelinating Schwann cells (mSC) using AAVs carrying a reporter gene. They found a high transduction rate of mSC in rats and mice via intra-nerve injection, with diffusion of reporter expression found away from the injection site and down the nerve.
Next, they added shRNAs targeting human PMP22 mRNA into the AAV in order to decrease PMP22 expression in CMT1A mSC. Using a rat model of CMT1A, which closely resembled the clinical aspects of the human disease, the team administered the gene therapy bilaterally via intra-nerve injection. This restored PMP22 levels back to that found in the wild type animals, and reduced myelinated fiber defects in the sciatic nerve such as demyelination and focal hypermyelination, and increased myelinated fibers density.
Having established that the therapy prevented myelin loss and the occurrence of myelinated fiber defects, the team wanted to assess whether this translated into alleviating CMT symptoms. Symptoms in humans include motor and sensory defects, as is seen in CMT1A rats. Upon administration of the gene therapy, both motor and sensory deficits were reduced, and even up until 12 months after treatment no significant evidence of a reduction in the effectiveness of the treatment was observed.
Lastly, assessing gene therapy effectiveness can be a difficult process, and clinical trials greatly benefit from sensitive, easily measured outcomes. With this in mind, the team tested whether recently discovered human skin mRNA biomarkers could be used as an outcome measure for their gene therapy efficiency. Using a set of 9 validated human biomarkers, the team showed that 3 biomarkers from skin biopsies of the animals, could reliably and robustly measure CMT1A gene therapy outcomes (functional phenotypes) in CMT1A rats.
The team here have demonstrated that AAV-based gene therapy using shRNAs to decrease levels of PMP22 are effective, and alleviate myelinated fibers defects and the resulting motor and sensory defects associated with CMT1A. The work suggests intra-nerve delivery of the gene therapy is an highly efficient system and supports further work required for translation into clinical trials.
Importantly for any gene therapy, initial safety assessments were positive. The intra-nerve delivery was highly reliable (91% success rate), and resulted in very little off-target transduction. Furthermore, there was only a weak humoral immune response towards the vector. The data indicated that there was no correlation between the presence of low-titer neutralising factors and a reduce therapy effectiveness, which opens up the possibility for successive treatments in several nerves of CMT1A patients in the future.
The development of CMT biomarkers in order to assess outcome measures will likely be a valuable tool in the clinic. We are starting to see a rapid development of diagnostic biosensors, such as nanosensors that can diagnose lung disease from exhaled biomarkers in the breath, ultra-sensitive biosensors which detect cancer markers in patient blood or serum, or those that can detect early signs of infection and rejection following organ transplants or detection of SARS-CoV-2. Scientists have even engineered immunological niches to monitor disease dynamics and effectiveness of treatments for multiple sclerosis, another disease that affects myelin. The advantage with the biomarkers assessed here, is they can be sampled relatively easily from skin.
Regarding potential future clinical studies, the results presented here indicate that this gene therapy to treat CMT1A would be most effective if administered as early as possible. This would represent a major shift in all current pharmacological strategies as they target adults and not children. Whether this approach would be efficient in adults, after the onset of disease progression, has yet to be tested, and will be an important question looking forward to clinical trials.
Gautier, B., Hajjar, H., Soares, S., Berthelot, J., Deck, M., Abbou, S., Campbell, G., Ceprian, M., Gonzalez, S., Fovet, C.-M., et al. (2021). AAV2/9-mediated silencing of PMP22 prevents the development of pathological features in a rat model of Charcot-Marie-Tooth disease 1 A. Nature Communications 12, 2356.