Liver-directed adenoviral gene transfer in murine succinate semialdehyde dehydrogenase deficiency

Murine succinate semialdehyde dehydrogenase (SSADH) deficiency (OMIM 271980; EC 1.2.1.24), a model of the corresponding human disorder, displays 100% mortality at weeks 3-4 of life, associated with lethal tonic-clonic seizures. The biochemical hallmark, γ-hydroxybutyrate (GHB), accumulates in both human and murine disorders. In the current study we evaluated rescue of the murine model with liver-directed gene therapy using the E1-deleted adenoviral vector AD:pAD-RSV-humanSSADH. Our working hypotheses were: (1) liver expresses considerable SSADH activity and therefore represents a major source of GHB output, (2) correction of liver enzyme deficiency will reduce GHB load both peripherally and in the central nervous system, and (3) SSADH expression will improve survival. SSADH^-/- and SSADH^+/+ mice were treated under two protocols: (A) intraperitoneal injection of 10⁸-10¹¹ viral particles by day 10 of life or (B) retro-orbital injection of 10¹¹ viral particles at day 13 of life. Intravenous administration was prohibited by the small size and fragility of the mice. Maximal survival (39%; P < 0.001) was achieved with intraperitoneal administration (10⁸ particles) at day 10; intraperitoneal (10¹⁰ and 10¹¹ particles) and retro-orbital administration (10¹¹ particles) yielded lower survival of 11-25% (P < 0.02). Under both protocols, the maximal hepatic SSADH enzyme activity was ∼20% of SSADH^+/+ liver activity (retro-orbital > ip). At various time points postinjection, ip-treated animals (10⁸ viral particles) demonstrated upward of 80% reduction in liver GHB concentrations, with little impact on brain or serum GHB levels except at 48-72 h posttreatment (∼50% reduction for both tissues). Accordingly, we harvested retro-orbitally treated animals at 72 h and observed significant reductions of 60-70% for GHB in liver, kidney, serum, and brain extracts. Histochemical analysis of liver from retro-orbitally treated mutants demonstrated substantial SSADH staining, but with variability both within tissues and between animals. Our studies provide proof-of-principle that liver-mediated gene therapy has efficacy in treating SSADH deficiency and that hepatic tissue contributes significantly to the pool of GHB within the CNS.