mGem: AAV, from almost a virus to an awesome vector—or is it?

Adeno-associated virus (AAV) vectors have taken center stage for gene therapy and have shown clinical efficacy in 15 human diseases to date. The Food and Drug Administration has approved seven AAV "drugs" for one-time treatment respectively for Leber's congenital amaurosis, spinal muscular atrophy, hemophilia B, Duchenne muscular dystrophy, hemophilia A, and aromatic L-amino acid decarboxylase deficiency. Despite these remarkable developments, it has become increasingly clear that the first generation of AAV vectors is less than optimal since in most, if not all, cases, exceedingly high doses are needed to achieve clinical efficacy, and as a consequence, in some patients, serious adverse events have been observed, and to date, at least 21 patients have died. Thus, there is a need to reassess the limitations of the first generation of AAV vectors as well as an urgent need to develop the next generation of AAV vectors that are safe and effective.