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 ¿-aminobutyric acid (GABA). In SSADHD, pathologic accumulation of GABA and its
metabolite ¿-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 and cell-specificity of SSADH restoration, advancing future
translational and clinical studies including ERT and gene therapy for SSADHD patients.
