Gene Therapy Extends Life, Nerve Defects in CMT4J Mice

In a mouse model of Charcot-Marie-Tooth disease type 4J (CMT4J), gene therapy administered in the first few days of life significantly prolonged survival and eased motor and nerve defects. 

The study with that finding, “AAV9-mediated FIG4 delivery prolongs life span in Charcot Marie Tooth disease type 4J mouse model, was published in The Journal of Clinical Investigation. 

CMT4J develops when an individual inherits a mutant copy of the FIG4 gene from both parents. The disorder is characterized by dysfunction of motor and sensory nerves, and the formation of small compartments called vacuoles inside the cell — a sign of defects in the endosome/lysosomal pathway that transports and digests substances in the cell. 

In CMT4J patients, symptoms typically emerge in childhood and can reduce lifespan significantly. The introduction of a healthy FIG4 gene has been shown to rescue characteristic symptoms of the disease in mice, and FIG4 expression (activity) specifically in neurons restored the formation of myelin — a fatty coating insulating nerve fibers and enabling rapid transmission of nerve signals. Taken together, this suggests that gene therapy may be effective in CMT4J.

Researchers in the U.S. tested the use of an engineered viral vector, called AAV9, to deliver copies of healthy human FIG4 gene in a mouse model of CMT4J. These mice have features of the human disorder, including motor neuron and myelin loss, as well as vacuole formation in the nervous system.

While untreated animals had no FIG4 protein in the brain and spinal cord, mice treated at day 1 or day 4 after birth (by injection into the cerebral ventricles) showed a dose-dependent restoration in FIG4 activity levels.

Survival benefits also were found. In contrast, to the median survival of 23 days in untreated mice, animals surviving more than one year were 90% of the group treated on postnatal day 1 and 65% of the group treated on postnatal day 4.

Notably, mice treated at these early timepoints showed growth and weight gain improvements.

Treatment at day 7 after birth — via injection into the spinal canal — significantly increased survival, but not to the same degree as earlier treatment, suggesting that the degree of survival benefits are age-dependent. Improvements were further seen in growth and weight gain, as also found in animals treated on post-natal day 11 that did not survive longer. Notably, these two later timepoints were meant to assess treatment effects when animals were already showing symptoms.

Untreated mice with CMT4J-like-features did not survive long enough to perform motor tests. However, animals treated at days 1 or 4 showed a similar motor performance at six months compared to healthy controls.

Later treatment at day 7 resulted in a significant, but incomplete, rescue of the motor defects.

Reduced nerve conduction velocity (NCV) — the speed at which electrical signals are transmitted in the nerve — is a sign of myelin loss. At six months, mice treated on postnatal day 1 exhibited NCV equivalent to that observed in controls, while animals treated at day 4 exhibited diminished NCV. 

By one year post-treatment, both early treatment groups were similar to control mice. Mice treated on postnatal day 7 showed only partial rescue of nerve defects. 

Six months into treatment on day 1 of life, mice had reduced vacuole formation in the nervous system, nearly to the level of controls. Treatment on day 7 induced a similar but less-pronounced effect, particularly in the brain.

“Our best rescue of [CMT4J-like features] was obtained by treating as early as possible,” the investigators wrote. 

Mice treated at the symptomatic stage (later treatment) exhibited signs of less endosome/lysosome pathway defects observed in non-treated mice, specifically a rescue of the accumulation of autophagosomes that deliver substances to be digested in the lysosomes.

In addition, tissue analysis revealed no unanticipated adverse effects, suggesting the treatment was well-tolerated.

“Considering the often severe and debilitating symptoms of CMT4J patients and the lack of other treatment options, our results indicate that this gene therapy approach warrants further investigation,” the researchers concluded.