Directed Evolution of New Adeno-Associated Viral Vectors for Clinical Gene Therapy

Gene therapy – the delivery of genetic material to the cells of a patient for therapeutic benefit – has been increasingly successful in human clinical trials over the past decade, and there are numerous FDA-approved gene therapies.  The most successful gene delivery vehicles, or vectors, are based on adeno-associated viruses (AAV); however, vectors based on natural versions of AAV face a number of delivery barriers that limit their efficacy and will thus preclude the extension of these successes to the majority of human diseases.  These delivery limitations arise since the parent viruses upon which these vectors are based were not evolved by nature for our convenience to use as human medicines.  Unfortunately, due to the highly complex mechanisms of virus-host interactions, there is currently insufficient mechanistic knowledge to enable rational design to be sufficiently successful in creating new vectors.  As an alternative, however, we developed the concept of using directed evolution to engineer highly optimized variants of AAV for a broad range of cell and tissue targets.  Directed evolution involves the iterative genetic diversification of a biomolecule to create a gene pool and functional selection to isolate variants with optimal properties.  Using this approach, we have engineered AAV variants with greatly improved delivery efficiency to multiple organs including the retina; lung, and muscle; targeted delivery to specific cell types; and the capacity to evade immune responses.  Our novel AAV variants are currently used in 5 human clinical trials involving delivery to the retina and heart, and two additional clinical trials will be initiated this year.

In parallel, the advent of genome editing technologies such as the CRISPR/Cas9 system raises the possibility of using gene delivery not only for gene replacement but for repair or knockout of endogenous genes.  We have thus been combining engineered AAVs with CRISPR/Cas9 for a range of applications.  The integration of these new technologies – AAV delivery or genome editing machinery – can enable a broad range of basic and therapeutic applications.