Gene Therapy Seen to Rescue Motor, Nerve Defects in CMTX1 Mouse Model

Delivering a healthy neurotrophin-3 (NT-3) gene to muscle cells in a mouse model of Charcot-Marie-Tooth disease type X1 (CMTX1) prevented motor function decline and eased defects in nerve and muscle structure, a study found.

These results support an NT-3 gene therapy as a potential treatment approach for this X-linked form of CMT and for other subtypes marked by problems with myelination.

The study, “AAV1.NT-3 gene therapy for X-linked Charcot-Marie-Tooth neuropathy type 1,” was published in the journal Gene Therapy.

CMTX1 accounts for about 10% of all cases of CMT. It is caused by mutations in the GJB1 gene that ultimately result in demyelination (loss of myelin) of nerves in the peripheral nervous system, affecting nerve cell communication and causing muscle weakness and atrophy. 

Myelin is a fatty substance that surrounds nerve cells like a sheath, facilitating the proper transmission of signals throughout the nervous system. In the peripheral nervous system — nerves outside the brain and spinal cord — this myelin sheath is produced by supporting cells called Schwann cells. Connexin-32 (Cx32), the protein encoded by GJB1, is required for proper Schwann cell function.

NT-3 is a nervous system protein that supports Schwann cell survival and myelination. Previously, researchers at Nationwide Children’s Hospital found that NT-3 gene therapy could rescue Schwann cell function and neuromuscular defects in a mouse model of CMT type 1A — a CMT subtype also characterized by loss of myelin.

This same group of researchers now investigated the use of NT-3 gene therapy in a mouse model of CMTX1, which lacks Cx32 expression. NT-3 was delivered to cells using an attenuated viral vector, ensuring that only muscle cells would be able to produce this protein. 

This vector was injected into the calf muscle of 3-month-old CMTX1 mice, at which point progressive loss of myelin is already evident. Treated and untreated mice were then evaluated for motor function and neuromuscular defects, and also compared with normal mice.

While untreated CMTX1 mice showed a significant loss of motor abilities between the ages of 3 months (baseline study measure) and 9 months (six months after therapy’s use), animals given the NT-3 gene therapy showed no such loss in motor function, with performance comparable to wild-type (normal) animals.

Treatment also rescued muscle function, as measured by compound muscle action potential (CMAP) amplitude, to levels similar to healthy animals. Untreated CMTX1 mice, in comparison, had a 13% reduction in CMAP. Untreated mice also showed a 30.5% reduction in the speed at which nerve signals travel down axons, compared with normal mice. Treated mice showed a 16% reduction.

Signs of demyelination and axon degeneration were evident in nerve samples from 9-month-old untreated CMTX1 mice, but not in treated mice, and more so in treated females than males.

NT-3 gene therapy also increased the number of myelin incisures — clefts in myelin that allow ions to reach nerve cells — to levels that were comparable to normal mice, the researchers wrote. Notably, a normalization of myelin incisure density was observed in both male and female mice, although the increase was only significant in females.

The NT-3 treated CMTX1 mice also had thicker myelin and a higher percentage of highly myelinated axons, or nerve fibers, at six months post-treatment than did untreated mice. No difference was seen between males and females. In contrast, untreated male CMTX1 mice had a lower percentage of highly myelinated axons (12.3%) than did females (24%).

CMTX1 mice given the NT-3 gene therapy also showed significantly lesser wasting of calf muscle fibers and greater muscle fiber size than did untreated male and female mice. Again, these muscle structures in animals given the therapy six months earlier were comparable to these structures in wild-type mice.

“AAV1.NT-3 gene therapy, administered after the onset of neuropathy [nerve damage] leads to meaningful improvements” in muscle and nerve cell function and structure in an animal model of CMTX1, the researchers wrote.

“Our proof of principle studies in the mouse model for CMTX along with previously demonstrated safety and efficacy of our method provide a potential path for clinical translation,” they concluded.