Authors :
Presenting Author: Amanda Liebhardt, BS – Boston Children's Hospital
Gabrielle McGinty, BS – Neurology – Boston Children's Hospital; Phillip Pearl, MD – William G. Lennox Chair and Professor of Neurology, Harvard Medical School, Neurology, Boston Children's Hospital; Guangping Gao, PhD – Professor of Microbiology and Physiological Systems, Penelope Booth Rockwell Professor in Biomedical Research, UMass Chan Medical School., UMass Medical School; Henry Lee, MPhil PhD – Senior Scientist, FM Kirby Neurobiology Center, Preclinical Science Program Manager, RSZ Translational Neuroscience Center, Neurology, Boston Children's Hospital; Alexander Rotenberg, MD PhD – Professor of Neurology, Joseph J. Volpe Chair in Neurology, Harvard Medical School, Neurology, Boston Children's Hospital
Rationale:
SSADHD is a rare autosomal recessive disorder caused by loss-of-function mutations of the ALDH5A1 gene, encoding SSADH critical for catabolizing the inhibitory neurotransmitter g-aminobutyric acid (GABA). In SSADHD, GABA and its metabolite g-hydroxybutyrate (GHB) accumulate to pathological levels resulting in ~50% of patients developing epilepsy and a 15% reported incidence of sudden unexpected death in epilepsy (SUDEP) in adolescent and adult patients. This paradoxical seizure risk in a hyper-GABAergic condition is possibly due to compensatory downregulation of GABA receptors and compromised inhibitory tone. Epilepsy in SSADHD is often drug resistant. Treatment options for SSADHD remain symptomatic. Gene replacement therapy is a potential cure for SSADHD but is unavailable. Our recent work using a Cre-dependent SSADH mouse model, aldh5a1lox-STOP, showed that 69 ± 4% SSADH protein restoration results in increased survival at postnatal day P90 (86.7%) compared to 0% survival (p< 0.00001, Fisher Exact Test) in untreated mutants (100% mortality due to status epilepticus by postnatal day P21, analogous to human early adolescence). As a step toward clinical translation of SSADH gene therapy, we tested the efficacy of a novel vector encompassing a native human ALDH5A1 promoter tethered with a human ALDH5A1 coding sequence, namely AAV-SSADH.
Methods:
We packaged AAV-SSADH into the blood-brain barrier (BBB)-penetrating capsid AAV-PHP.eB and injected into aldh5a1lox-STOP mice and wild-type (WT) littermates via intraperitoneal (IP) injection at high dose (~6-8x1013 gc/kg) at symptomatic stage P14. Body weight and survival was recorded and tracked until P100. Tissues and internal organs known to express high levels of SSADH were collected and their SSADH content was analyzed by western blot.
Results:
AAV-SSADH administered systemically into aldh5a1lox-STOP mice at P14 resulted in significant increase in survival at P40 (70%) compared to 0% (p=0.0002, Fisher Exact Test) in untreated mutants (Figure 1a). Both male and female mice were amenable to SSADH replacement therapy. Weight gain was seen within 2 weeks after AAV-SSADH treatment indicating symptom reversal. However, survival at P60 was reduced to 33% (p=0.0415, Fisher Exact Test) in these treated mutants suggesting residual life-threatening seizures. Western blot data indicated that SSADH was restored to 70-95% WT levels in the brain of treated mutants, and up to 50% in other vital organs (Figure 1b). Finally, there is no detrimental effect on growth or survival in AAV-SSADH-treated WT mice (Figure 2).
Conclusions:
Our data indicates that AAV-SSADH has therapeutic efficacy towards SUDEP prevention and seizure suppression in SSADH deficient mice. AAV-SSADH administered at high dosage is tolerable in WT and mutant mice, indicating safety in case of overexpression. Both male and female mice are rescuable via AAV-SSADH. We conclude that symptom reversal is possible via SSADH gene replacement therapy even after symptom manifestation. Systemic administration of a BBB-penetrating AAV encompassing a human SSADH promoter is likely a viable strategy for SSADH gene replacement therapy.
Funding:
SSADH Association, NINDS, Translational Research Program (BCH)