Novel gene therapy for CMT4C shows proof of principle in treating mice
Strategy led to lasting benefits in nerve function, muscle health in study
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An experimental gene therapy for Charcot-Marie-Tooth disease type 4C (CMT4C) — a rare form of the genetic disease that has muscle weakness as its hallmark symptom — was shown to improve nerve signaling, motor function, and muscle health in a mouse model.
Moreover, according to the researchers, the treatment raised no obvious safety concerns.
“Our gene therapy strategy, aimed at restoring nerve function in muscle tissue, effectively targeted key pathways involved in nerve-muscle interaction and regeneration,” the scientists wrote, adding that the treatment’s “regulation of different muscle proteins … suggests a therapeutic effect in terms of reinnervation and subsequent muscle regeneration.”
These findings provide proof of principle for the gene therapy, the team said, though further study is necessary to determine its potential effectiveness — and safety — in humans.
The study, “A dose-escalation and safety gene therapy study in a model of CMT4C neuropathy,” was published in the journal Gene Therapy by researchers in Europe.
CMT type 4 is a rare type of Charcot-Marie-Tooth caused by damage to myelin, the fatty protective coating that covers nerve fibers. It is further divided into several subtypes, characterized by specific genetic mutations and symptoms.
The CMT4C subtype is caused by mutations in the SH3TC2 gene and is characterized by early-onset, severe spine deformities. For the disease to develop, a person must inherit two mutated copies of the gene, one from each biological parent.
The SH3TC2 gene provides instructions for making a protein of the same name in Schwann cells, which produce myelin in the peripheral nervous system. When the SH3TC2 protein does not work properly, Schwann cells may have trouble maintaining myelin and communicating with nerve fibers, leading to nerve damage.
Gene therapy for CMT4C targets the SH3TC2 gene
Gene therapies that aim to replace the faulty gene with a working copy of SH3TC2 may be a way to treat CMT4C.
As such, a team led by scientists in Cyprus has been developing adeno-associated viruses, or AAVs — viruses modified in the lab so they do not cause disease — to deliver a healthy SH3TC2 gene into Schwann cells.
Earlier proof-of-concept work in a mouse model of CMT4C showed that this approach improved nerve function and structure when given at both early and later disease stages.
Now, the researchers tested a tweaked version of the gene therapy, replacing a rat myelin-specific switch with a human one to essentially turn on SH3TC2 production in Schwann cells. The new version also included genetic changes meant to increase the amount of SH3TC2 protein produced.
First, the researchers tested whether their AAV9 viral vector would target Schwann cells after intrathecal injection, or injection directly into the spinal canal. For that, the team used a fluorescent marker in the vector. They found AAV9 in peripheral nerve tissues, especially in spinal nerve roots in the lower back. Its activity was restricted to Schwann cells actively forming the myelin layer, the data showed.
The new vector carrying the SH3TC2 gene was then injected into the CMT4C mice. Six weeks later, the vector was found again across peripheral nervous system tissues, predominantly in spinal roots, but also in the central nervous system, which comprises the brain and spinal cord.
In an experiment testing three increasing doses, the researchers then found that higher treatment doses led to higher percentages of SH3TC2-producing Schwann cells.
Benefits seen in mice 2 months after receiving treatment
Two months after treatment, mice given the gene therapy showed benefits in motor coordination and strength compared with untreated controls. Mice given the mid and high doses also showed improved grip strength, reaching levels similar to those of healthy mice.
Signaling in the sciatic nerve, which extends from the buttocks down each leg, also improved with gene therapy, the researchers noted. However, it was still worse than in healthy animals, as shown by nerve conduction velocity tests. In this test, higher values indicate faster nerve signal transmission and better nerve function.
Additionally, mice given the mid and high doses had greater weight in certain hindlimb muscles, suggesting less muscle wasting. Treated mice also showed improvement in an abnormal hindlimb clasping and toe-clenching posture seen in the CMT4C mouse model.
Tissue analyses showed better nerve structure with gene therapy. The treatment helped normalize the nodes of Ranvier, tiny gaps in the myelin sheath that help nerve signals travel efficiently. It also partially improved myelination, particularly in spinal roots.
Safety analyses eight weeks after injection found no evidence that the gene therapy damaged neural tissues or peripheral organs. In fact, the researchers found fewer immune cells called macrophages in the femoral nerve, one of the largest nerves in the leg, “suggesting an amelioration of nerve pathology [disease] and secondary inflammation,” the team wrote.
Finally, protein analyses of a leg muscle suggested that the gene therapy corrected biological processes associated with loss of nerve supply to muscles.
Overall, the researchers wrote, these findings provide “proof of principle for dose-dependent effectiveness and safety of intrathecal AAV9-mediated gene replacement paving the way for clinical translation.” However, further studies are “needed to ensure patient safety in future clinical applications for CMT4C,” the team wrote.
