E Pluribus Unum: 50 Years of Research, Millions of Viruses, and One Goal--Tailored Acceleration of AAV Evolution.

Fifty years ago, a Science paper by Atchison et al. reported a newly discovered virus that would soon become known as adeno-associated virus (AAV) and that would subsequently emerge as one of the most versatile and most auspicious vectors for human gene therapy. A large part of its attraction stems from the ease with which the viral capsid can be engineered for particle retargeting to cell types of choice, evasion from neutralizing antibodies or other desirable properties. Particularly powerful and in the focus of the current review are high-throughput methods aimed at expanding the repertoire of AAV vectors by means of directed molecular evolution, such as random mutagenesis, DNA family shuffling, in silico reconstruction of ancestral capsids, or peptide display. Here, unlike the wealth of prior reviews on this topic, we especially emphasize and critically discuss the practical aspects of the different procedures that affect the ultimate outcome, including diversification protocols, combinatorial library complexity, and selection strategies. Our overall aim is to provide general guidance that should help users at any level, from novice to expert, to safely navigate through the rugged space of directed AAV evolution while avoiding the pitfalls that are associated with these challenging but promising technologies.