15. Development of Intrathecal scAAV9 Gene Therapy for Giant Axonal Neuropathy
Authors: Rachel M. Bailey, Diane Armao, Sahana Nagabhushan Kalburgi, Steven J. Gray
Journal: Molecular Therapy (2015)

Giant axonal neuropathy (GAN) is a rare pediatric neurodegenerative disorder characterized by progressive sensory and motor neuropathy that presents as early as 3 years of age and with ultimate mortality during the second or third decade of life. GAN is caused by autosomal recessive loss-of-function mutations in the GAN gene that encodes the gigaxonin protein. Gigaxonin plays a role in the organization/degradation of intermediate filaments (IFs) and a pathological hallmark of GAN is large axonal swellings filled with disorganized aggregates of IFs. While GAN is primarily described as a peripheral neuropathy, diffuse pathology from disorganized IFs is found throughout the nervous system and other organ systems. An NIH-sponsored Phase I study is underway to test the safety of intrathecal (IT) administration of scAAV9/JeT-GAN to treat the most severe aspects of GAN, namely the motor and sensory neuropathy. Gigaxonin gene transfer is the first proposed therapy for GAN. Our group developed the vector to be used in the Phase I clinical trial, which is a self-complementary AAV serotype 9 vector carrying a codon-optimized human GAN transgene controlled by the minimal synthetic JeT promoter (scAAV9/JeT-GAN). Preclinical studies show that scAAV/JeT-GAN can restore the normal arrangement of IFs in patient fibroblasts within days in cell culture and by 3 weeks in GAN KO mice. The safety and biodistribution of scAAV9/JeT-GAN was investigated in mice and non-human primates that received a single IT overdose of scAAV9/JeT-GAN. No safety concerns were apparent from these animal studies, with the longest endpoint at 1 year post-injection. To further support the translation of this approach to human subjects, IT delivery of the scAAV9/JeT-GAN vector in GAN KO mice showed sustained levels of human gigaxonin expression in therapeutically-relevant areas for at least 48 weeks without evidence of toxicity. Furthermore, treated GAN KO mice have improved motor function and preservation of peripheral nerve ultrastructure. In all, the results of our preclinical studies attest to the safety of IT scAAV9/JeT-GAN delivery and the potential benefit to treated patients. During the review and public discussion of the clinical trial protocol, the NIH Recombinant DNA Advisory Committee (RAC) raised the concern of re-dosing patients with AAV vectors should the dose used prove to be safe but ineffective. Specifically, the RAC made the recommendation for researchers to evaluate the effect of re-administration in preclinical models in advance of this trial or preceding a subsequent trial. Pilot studies in wild-type mice show that repeat injection of scAAV9 vectors of the same dose and via the same IT route results in 70% less vector delivery of the second transgene to the brain, 70-95% less delivery to regions of the spinal cord and 55% less delivery to the sciatic nerve. This data suggests that alternative AAV capsids and/or routes of AAV delivery will need to be explored to enable effective re-dosing following an initial dose of scAAV9/JeT-GAN.