Novel Gene Therapy for Giant Axonal Neuropathy

By Mary L. Vo, MD, PharmD

Assistant Professor of Neurology, Weill Cornell Medicine

SYNOPSIS: The authors conducted a Phase I trial of a gene editing approach delivered by adeno-associated virus vector carrying a codon-optimized human giant axonal neuropathy transgene. The transgene targets anterior horn motor neuron bodies and sensory neurons in the dorsal root ganglia with the goal of repairing peripheral nerves and their axons. Each patient received a single intrathecal injection of the investigational agent. At one year, patients had less progression of disease compared to pretreatment intervals.

SOURCE: Bharucha-Goebel DX, Todd JJ, Saade D, et al. Intrathecal gene therapy for giant axonal neuropathy. N Engl J Med 2024;390:1092-1104.

Giant axonal neuropathy (GAN) is an ultra-rare childhood neurodegenerative disease affecting both the peripheral and central nervous systems and ultimately resulting in early demise. GAN is an autosomal recessive condition resulting from biallelic mutation in the GAN gene on chromosome 16q23.2, resulting in loss of functional gigaxonin. Gigaxonin is a ubiquitin ligase adaptor protein required to regulate intermediate neurofilament turnover in the central nervous system and peripheral nervous system. Absence of functional gigaxonin results in toxic accumulation of intermediate filaments forming cellular inclusions and swollen axons densely packed with disordered microtubules and intermediate filaments.¹

Patients with GAN develop a length-dependent sensorimotor neuropathy in the first years of life that later progresses to sensory ataxia, cerebellar dysfunction, and loss of ambulation by 10 years of age. Other features of the condition include tightly coiled hair, vision loss, and respiratory complications. Most patients succumb to pulmonary complications in the third decade of life. No effective treatments exist. This study is the first to assess the safety of gene therapy for GAN.

Adeno-associated virus (AAV) vector gene therapy for monogenic disease has become increasingly popular and is bolstered by the successes of AAV therapies for Leber’s congenital amaurosis and spinal muscle atrophy. One of the greatest advantages of AAV therapy is the durability of treatment, often resulting in long-term transgene expression and therapeutic benefit following single-dose administration.

AAV9 is of particular interest because of its tropism for motor and sensory neurons. Intrathecal delivery is preferred because the vector bypasses the blood-brain barrier and evades host neutralizing antibodies, possibly leading to improved therapeutic response and lower post-treatment inflammatory reactions. Additionally, intrathecal administration permits treatment with lower doses, further reducing the risk of systemic side effects.^2,3

Although less common compared with intravenous AAV therapies, common side effects encountered with intrathecal therapy includes inflammatory responses and transaminitis. Of note, high-dose AAV administration has been associated with dorsal root ganglion toxicity. Host immunogenicity from prior AAV infection also may be a factor in reduced transgene expression following treatment.⁴ AAV-mediated GAN gene replacement therapy showed enhanced GAN expression and reduction of intermediate filament aggregates in patient fibroblasts as well as reduced neuronal intermediate filaments in the brain stem and spinal cord of GAN knockout mice.⁵

This is a Phase I dose-ranging study of AAV9/JeT-GAN therapy administered to 14 children with GAN. Fourteen subjects older than 6 years of age with confirmed GAN were recruited from an observational study of GAN from April 2015 to November 2020 with National Institutes of Health Institutional Review Board approval. Written informed consent and assent for all subjects was obtained prior to study procedures. The subjects received a single dose of intrathecal scAAV9/JeT-GAN and were monitored for safety and efficacy.

The cohort included participants who were predicted to have no residual gigaxonin expression. Patients with and without AAV9 serum neutralizing antibodies were included in this cohort.

Subjects were treated with a single intrathecal infusion of scAAV9/JeT-GAN at one of four doses followed by immunomodulation with steroids. A subgroup also received rapamycin. AAV9 seronegative patients received rapamycin and tacrolimus to mitigate T cell-mediated response to the transgene vector.

The primary safety endpoint was the incidence of serious adverse events assessed by serial examinations with magnetic resonance imaging (MRI) and laboratory testing. The secondary endpoint was disease progression at one year after gene transfers using Bayesian analysis to estimate rate of change of the motor score measured by 32-item Motor Function Measurement (MFM-32) score at three, six, eight, 12, and 18 months post-treatment. Secondary endpoints also included modified Friedreich Ataxia Rating scale and Neuropathy Impairment score. Exploratory endpoints included nerve conduction studies and superficial radial sensory histopathologic analyses before and following treatment.

Bayesian statistical analyses of pre- and post-treatment metrics were estimated using Markov chain Monte Carlo computational methods. Mean rates of change during the natural history study were used for comparison with post-treatment metrics. All analyses were performed with R software, version 4.2.0.

Of the 14 participants, nine were female and five were male, with a mean age of 9.1 years (range 6.3 to 14.5). Ten subjects had positive cross-reactive immunologic material and were predicted to have some residual GAN protein based on their genotype. Six subjects were AAV seropositive. All patients had significant disability, with only three subjects being ambulant and eight subjects able to ambulate with assistance at the time of the study.

There were two fatalities, including one death from respiratory complications 60 months following treatment. Another patient died postoperatively from aspiration, cardiac arrest, and multiorgan failure after spine surgery eight months after dose administration.

In total, 48 serious adverse events were reported, including scoliosis, urinary tract infection, and upper respiratory tract infection. Three patients required hospitalization for side effects associated with steroids, including pneumonia, skin infection, and bone infection. Of the 682 adverse events, 129 were deemed to be at least possibly related to scAAV9/JeT-GAN vector. Adverse events included cerebrospinal fluid (CSF) pleocytosis, elevated CSF immunoglobulin G index, leukocytosis, thrombocytosis, and mild transaminitis.

Except for the group receiving the lowest vector dose, motor scores in the remaining participants reflected slowing disease progression at one year after gene transfer. Additionally, five subjects had newly detectable median sensory nerve responses on nerve conduction studies and eight subjects had histological evidence of superficial radial sensory nerve regeneration one year after treatment.

COMMENTARY

This trial represents the first gene therapy trial for a peripheral neuropathy. Although rare, this trial establishes a new benchmark for a treatment approach of GAN. The study highlights the advantages of intrathecal AAV gene therapy in eluding circulating neutralizing antibodies and mitigating adverse effects associated with intravenous AAV therapy. The high morbidity and mortality of GAN underscores the urgency for disease-modifying therapy.

Safety findings were favorable overall. The vast majority of the 682 adverse events were mild, and 129 were at least possibly tied to the vector. The three cases of transaminitis were mild relative to those typically associated with intravenous AAV therapy. Further, electrodiagnostic and histological findings supporting peripheral nerve recovery suggest that gene therapy may meaningfully stabilize this progressive neurodegenerative condition.

The lack of differences in safety endpoints between AAV seropositive and seronegative groups is very informative and may allow seropositive patients to access gene therapy in the future. Although more research is needed, preliminary experience with scAAV9/JeT-GAN therapy is relatively well tolerated and offers some hope for this fatal neurodegenerative disorder.

REFERENCES

1. Echaniz-Laguna A, Cuisset JM, Guyant-Marechal L, et al. Giant axonal neuropathy: A multicenter retrospective study with genotypic spectrum expansion. Neurogenetics 2020;21:29-37.
2. Flotte TR. Intrathecal gene therapy for neurologic disease in humans. Mol Ther 2024;32:1185-1186.
3. Shirakaki S, Roshmi RR, Yokota T. Genetic approaches for the treatment of giant axonal neuropathy. J Pers Med 2022;13:91.
4. Benatti HR, Gray-Edwards HL. Adeno-associated virus delivery limitations for neurological indications. Hum Gene Ther 2022;33:1-7.
5. Mussche S, Devreese B, Nagabhushan Kalburgi S, et al. Restoration of cytoskeleton homeostasis after gigaxonin gene transfer for giant axonal neuropathy. Hum Gene Ther 2013;24:209-219.