Modification of Adenovirus Capsid with a Designed Protein Ligand Yields a Gene Vector Targeted to a Major Molecular Marker of Cancer

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Abstract
The future of genetic interventions in humans critically depends on the selectivity and efficiency of gene transfer to target tissues. The viral gene vectors explored to date cannot selectively transduce the desired targets. While substantial progress has been made in developing targeting strategies for adenovirus (Ad) vectors, future advances in this direction are severely limited by the shortage of naturally existing molecules available for use as targeting ligands. This shortage is due to fundamental and irresolvable differences at the level of both posttranslational modifications and intracellular trafficking between the Ad structural proteins and those natural proteins that are involved in interactions with the cell surface and could otherwise be considered as potential targeting ligands. We hypothesized that this problem could be resolved by altering the natural tropism of Ad vector through incorporation into its capsid of a rationally designed protein ligand, an affibody, whose structural, functional, and biosynthetic properties make it compatible with the Ad assembly process. We tested this hypothesis by redesigning the receptor-binding Ad protein, the fiber, using affibodies specific for human epidermal growth factor receptor type 2 (Her2), a major molecular marker of human tumors. The biosynthesis and folding of these fiber chimeras were fully compatible with Ad virion formation, and the resultant viral vectors were capable of selective delivery of a dual-function transgene to Her2-expressing cancer cells. By establishing the feasibility of this affibody-based approach to Ad vector targeting, the present study lays the foundation for further development of Ad vector technology toward its clinical use.