AAV vectors as gene delivery vehicles in the central nervous system

Recombinant gene delivery vehicles based on the replication-defective AAV have gained a preeminent position in the field of gene delivery to the brain. Efficient global gene delivery to the CNS is beneficial for the study of gene products is the entire CNS as well as for introducing and expressing genes encoding potential therapeutic or protective proteins in neuronal tissue. Although efficient global gene delivery to the CNS is an ambitious goal, recent scientific developments, as for instance the expansion of the repertoire of AAV vectors by the isolation of new serotypes and the creation of AAV vectors with chimeric capsids, might open up new avenues to achieve this goal. The overall aim of this thesis was to achieve efficient, scientifically and therapeutically relevant AAV- mediated gene delivery to the mouse brain. First, experiments were performed to assess the ability of different serotype AAV vectors to achieve efficient gene delivery to the CNS upon neonatal ICV injection. AAV8 was able to achieve most efficient, widespread delivery of transgenes. Second, the transduction properties of novel mosaic vectors incorporating capsid proteins from AAV1, AAV2, and AAV8 capsids are described. The transduction profiles of these vectors were compared to those of parental serotypes in mouse primary cortical cultures and in vivo after injection into the cerebral lateral ventricles of neonatal mice. Both in culture and in vivo, these mosaic vectors displayed additional properties. Third, the properties of both parental serotype vectors and mosaic vectors were assessed upon injection into the striatum of adult mice. The lysosomal storage disease GM1-gangliosidosis, an autosomal recessive deficiency of lysosomal acid β-galactosidase is biochemically characterized by accumulation of GM1-ganglioside in the central nervous system (CNS), and partially degraded glycoproteins, keratan sulfate and oligosaccharides in visceral organs. GM1-gangliosidosis is a proper candidate for gene therapy, as not all cells in the central nervous system have to be genetically modified to achieve therapeutic efficacy, because mechanisms for efficient secretion and uptake of lysosomal enzymes exist. Next, experiments were conducted to characterize the distribution of lysosomal β-galactosidase in the adult GM1-gangliosidosis mouse brain after AAV-mediated gene delivery. Several mechanisms of distribution were observed including: diffusion, axonal transport of the enzyme and axonal transport of mRNA. To evaluate the therapeutic potential of AAV- mediated gene delivery in neonatal GM1-gangliosidosis mice, an AAV vector encoding for β-galactosidase was injected into the cerebroventricles of neonatal GM1-gangliosidosis mice. which resulted in complete correction of enzymatic deficiency and lysosomal storage throughout the GM1-gangliosidosis mouse forebrain. Based on the results described in this thesis, AAV vectors can be considered highly promising vehicles for the delivery of therapeutically relevant transgenes to the CNS.