Abstracts

Rationale: Succinic semialdehyde dehydrogenase deficiency (SSADHD) is a rare inborn metabolic disorder caused by aldehyde dehydrogenase 5 family member A1 (aldh5a1) mutations. Aldh5a1 encodes SSADH which is essential for degrading the inhibitory neurotransmitter g-aminobutyric acid (GABA). Paradoxically, despite abundant ambient GABA, patients with SSADHD are susceptible to intractable seizures and sudden unexpected death in epilepsy (SUDEP). This distinct pathophysiology is reflected by global compensatory GABA receptor down-regulation in SSADH knock-out mice, aldh5a1^-/-, which exhibit obligatory epilepsy and pre-mature death. Proof-of-concept systemic SSADH enzyme injection and liver-directed adenoviral aldh5a1 gene transfer rescued aldh5a1^-/- mice, suggesting that enzyme replacement therapy (ERT) and gene therapy are realistic treatment options for SSADHD. However, it is unclear whether rescued aldh5a1^-/- mice remain life-long seizure-free. We develop an inducible SSADH mouse model, aldh5a1^{lox-rtTA-STOP}, which has no baseline SSADH activities but allows conditional Cre-mediated aldh5a1 reactivation. We use aldh5a1^{lox-rtTA-STOP} mice to investigate how SSADH restoration leads to seizure phenotype reversal.

Methods: We used an adeno-associated virus which has high blood-brain barrier penetrant to express Cre under a constitutive promoter (AAV-PHP.B-Cre). AAV was administered into homozygous aldh5a1^{lox-rtTA-STOP} mice via intraperitoneal injection (5x10¹¹ genome copies/100ml saline) to induce brain-wide aldh5a1 restoration at a symptomatic stage. Somatic (body weight) developmental trajectory and survivability of aldh5a1^{lox-rtTA-STOP} mice before and after injecting AAV were recorded. Motor seizures were video-recorded and analyzed. AAV-injected wild-type/heterozygous (asymptomatic) and vehicle-injected homozygous littermates were control.

Results: Sudden death occurred in all untreated aldh5a1^{lox-rtTA-STOP} mice at ~postnatal day P21 (20.88±0.72, n=8), phenocopying aldh5a1^-/-. Upon AAV-PHP.B-Cre injection (n=7), 3 mice survived beyond P30 (survivability at P30: untreated=0%, treated=43%, p=0.0401, t-test). Rescued aldh5a1^{lox-rtTA-STOP} mice gained weight similar to control (rescued=2.57±0.61g/wk, control=2.43±0.29g/wk, p=0.85, t-test), but they exhibited brief motor seizures (< 2s) not detected in control.