Preclinical gene therapy in a mouse model of Charcot-Marie-Tooth disease type 4J

Abstract

Charcot-Marie-Tooth Disease Type 4J (CMT4J) is an ultra-rare, autosomal recessively inherited peripheral neuropathy, first described in 2007 and caused by mutations in the Fig4 Gene. FIG4 is a PIP2 phosphatase, and its loss leads to lysosomal abnormalities and “spongiform-like” vacuolization in neurons. Clinically, CMT4J is characterized by motor developmental delay, slow nerve conduction velocities, rapidly progressive paralysis, quadriplegia, resulting respiratory complications and premature death. Later-onset patients have a more variable disease course, with some experiencing mild symptoms. There are currently no approved therapies for CMT4J patients. Gene therapy represents a reasonable and promising approach to provide a meaningful and long term therapeutic benefit for this patient population. To explore this, we conducted preclinical studies in the pale tremor mouse model, which lacks Fig4 expression. These preclinical studies used a codon optimized human FIG4 gene, delivered by AAV9. Administering this vector directly to the nervous system by either intracerebroventricular (ICV) in early neonates or later, after the onset of symptoms by intrathecal (IT) injection produced benefit, as indicated by improved survival, improved gross motor function including grip strength, improved peripheral motor nerve conduction, and improved histopathological outcomes in spinal cord and dorsal root ganglia, as well as other parts of the central nervous system. Importantly, these results indicate that the benefit is greatest the earlier the FIG4-AAV9 is delivered. Preclinical study results also indicate a clear dose-response of the treatment up to the maximum feasible dose in mice, justifying the maximum feasible dose to provide the most effective treatment. Importantly, there were no observable adverse effects, even when Fig4 was overexpressed in WT mice. Overall, our results predict a benefit of this treatment to pediatric subjects, with no obvious toxicity expected, and support the further exploration of clinical trials for CMT4J patients.