Efficacy of gene therapy for a prototypical lysosomal storage disease (GSD-II) is critically dependent on vector dose, transgene promoter, and the tissues targeted for vector transduction

Lysosomal storage diseases are an intriguing target for gene therapy approaches, as transduction of a "depot" organ with a transgene encoding a lysosomal enzyme can be followed by secretion, systemic distribution, downstream uptake, and lysosomal targeting of the enzyme into non-transduced tissues. These benefits are of utmost importance when considering gene therapy approaches for glycogen storage disease type-II (GSD-II). GSD-II is a prototypical lysosomal storage disorder caused by lack of intralysosomal acid alpha-glucosidase (GAA) activity. Lack of GAA can result in a proximal limb myopathy and respiratory and cardiac failure, each due to abnormal glycogen accumulation in the skeletal muscles or cardiac tissues, respectively. After converting the liver into a "depot" organ, we found that intravenous injection of the [E1-,polymerase-]AdGAA vector allowed for hepatic secretion of GAA over an at least 20-fold dosage range. We noted that very low plasma GAA levels (derived from hepatic secretion of GAA) can allow for GAA uptake by muscle tissues (skeletal or cardiac), but significantly higher plasma GAA levels are required before glycogen "cross-correction" can occur in these same tissues. We also demonstrated that liver-specific enhancer/promoters prolonged GAA transgene expression from persistent [E1-,polymerase-] adenovirus based vector genomes for at least 180 days, and significantly diminished the amounts of neutralizing anti-GAA antibodies elicited in this animal model. Finally, we demonstrated that skeletal muscles can also serve as a "depot" organ for GAA secretion, allowing for secretion of GAA and its uptake by noninfected distal tissues, although glycogen reductions in non-injected muscles were not achieved by the latter approach.