Project summary/abstract (30 lines) Succinic Semialdehyde Dehydrogenase Deficiency (SSADHD) is a rare inborn metabolic disorder caused by aldh5a1 mutations. Aldh5a1 encodes SSADH which is essential for the catabolism of the inhibitory neurotransmitter γ-aminobutyric acid (GABA). In SSADHD, pathologic accumulation of GABA and its metabolite γ-hydroxybutyrate (GHB) leads to broad spectrum encephalopathy. Paradoxically, despite ambient GABA is heightened, SSADHD patients are susceptible to seizures and sudden unexpected death in epilepsy (SUDEP), highlighting the significance of compensatory GABA receptor reduction over pathologic GABA build- up. A major unmet medical need for SSADHD is treatment directly addressing the underlying enzyme deficiency such as enzyme replacement therapy (ERT) and gene therapy. Proof-of-concept systemic ERT and liver-directed aldh5a1 over-expression increased aldh5a1-/- mice survival. However, enzyme or viral vector injections in aldh5a1-/- mice lead to uncontrollable, non-specific SSADH restoration difficult to evaluate therapeutic efficacy and dose-response relationship. It is unclear restoring SSADH at what time, rate or in what cell types would suffice for phenotype reversal. We thus propose to develop a novel mouse model which allows conditional aldh5a1 reactivation under independent Cre or doxycycline regulation. In this novel mouse strain, aldh5a1 gene activity is disrupted at basal level, but is reconstituted upon Cre-mediated recombination or is reversibly regulated by the level of doxycycline. Our specific aims in this two-year exploratory grant are to: 1) Develop the aldh5a1lox-rtTA-STOP mouse model and characterize its baseline phenotype trajectory across development. 2) Test for an age-dependent therapeutic window when SSADH restoration is safe and effective, by injecting AAV-Cre into this mouse at contrasting developmental time points. 3) Test whether abrupt SSADH restoration leads to epileptic seizures, and whether gradual SSADH restoration has a therapeutic advantage, by injecting single dose versus multiple lower doses of AAV-Cre across days in early postnatal development. 4) Test whether inhibitory cell-directed partial SSADH restoration might be sufficient for phenotype reversal, by breeding aldh5a1lox-rtTA-STOP mice with Gad2-IRES-Cre mice. This novel mouse model enables testing of preclinical readiness of SSADH-restoring strategies such as gene therapy and ERT in a controlled, quantifiable and cell-specific manner. This project's long-term objectives are two-fold: 1) Provision of mechanistic insights into SSADH pathophysiology and how SSADH restoration might rescue symptoms. The proposed mouse model allows conditional SSADH depletion and restoration, so that pathological mechanisms of SSADHD and the impacts of SSADH restoration can be studied in great details. 2) Establishment of key parameters for therapeutic SSADH restoration. The proposed model provides necessary insights into the dose, pace an...