Preclinical Assessment of the scAAV9-HEXM Gene Therapy Vector via a Novel Combined Delivery Method in the Treatment of Sandhoff Disease in a Murine Model

Abstract

Sandhoff disease (SD) is a genetic neurodegenerative disorder caused by an excessive accumulation of GM2 gangliosides in neuronal lysosomes. SD arises from a mutation in the HEXB gene which encodes the β-subunit of the enzyme, β-hexosaminidase A, a heterodimer responsible for the hydrolysis of GM2. Lack of functional enzymes and subsequent accumulation of GM2 results in death by age 4 in the most common, infantile form. There is no cure nor any highly effective treatments currently available. Therefore, the aim of this dissertation was to assess the efficacy of the scAAV9-HEXM gene therapy utilizing a novel method of delivery, via intravenous (IV) and intra-cisterna magna (ICM) routes, combined with osmotic induced flow, as a treatment for SD. The early effects and efficacy of the scAAV9-HEXM vector administered as a single dose IV or IV&ICM was compared. The results indicated that at 16 weeks, treated Hexb(-/-) mice had significant phenotypic and biochemical improvements compared to vehicle-only controls, but only minimal differences were seen between the two routes of administration. In a long-term study, the effects of dose and delivery route, with 2 doses of ICM-only, and 3 doses administered both IV-only and IV&ICM were assessed. Over a 2-year period a dose-dependent survival curve was established. Most notable was the restoration of a full life span in both high dose cohorts, although the IV&ICM cohort significantly outlived the IV-only cohort (margin of 16.6 weeks). Moreover, neither of the high dose cohorts developed any characteristic symptoms of murine SD. Biochemical analyses showed supraphysiological levels of Hex activity in serum and CNS tissues, and up to a 12.9-fold reduction in accumulated GM2 in the IV&ICM high dose cohort compared to the untreated SD mice. All treated cohorts showed a dose-dependent reduction of GM2, Hex activity level, and biodistribution of the scAAV9-HEXM vector. The results of this dissertation demonstrate that the scAAV9-HEXM vector with the combined method of delivery can reduce GM2 to nearly WT levels, exceed the level of Hex activity required for mice to be asymptomatic, and restore a full two-year life span to Hexb(-/-) mice. The implications of this work suggest potentially promising techniques that can improve the survival and quality of life for individuals with SD.