Improved Helper-Dependent Adenoviral Vectors for High-Level, Neuron-Restricted Gene Transfer into the CNS

Adenoviral vectors (Ad) possess a number of attributes which favour their use for CNS gene delivery, e.g., their ability to infect post-mitotic tissues. However, the widespread distribution of the cellular receptor for Ad, the Coxsackie/adenovirus receptor (CAR), allows adenoviral vectors to infect a broad range of cells in the host, thus limiting cell-type specificity for gene transfer in the CNS. Also, the safety issue of these vectors is still a concern. Thus, there is a rationale for the development of Ad vectors capable of enhanced gene transfer specificity and efficacy, yet without evoking an immune response that may lead to reduced transgene expression with time. The use of neuron-specific promoters is a successful means to achieve neuron-restricted expression although gene expression driven by tissue specific promoters is often weak. To boost gene expression, we added the woodchuck hepatitis virus posttranscriptional regulatory element (WPRE) to the expression cassette, which is required for the cytoplasmic accumulation of viral RNAs. High-capacity adenoviral (HC-Ad) vectors are devoid of all viral genes. These vectors feature reduced toxicity, immunogenicity, and increased capacity for foreign DNA. Injection of an adenoviral vector with an EGFP-WPRE cassette under the control of the neuron specific synapsin-promoter (Ad-Syn-EGFP-WPRE) into the brain of NMRI mice results in an increase of EGFP expression in the neurons. In addition, no signs of inflammation were seen using an HC-Ad vector in the brain, hence allowing for the administration of high vector doses. The combination of these strategies leads to significant improvement in the safety and efficacy of Ad vectors and may have the potential to yield increased therapeutic benefit for therapeutic applications.