Gene Therapy for Hemophilia

The X-linked bleeding disorder hemophilia causes frequent and exaggerated bleeding that can be life-threatening if untreated. Conventional therapy requires frequent intravenous infusions of the missing coagulation protein (factor VIII [FVIII] for hemophilia A and factor IX [FIX] for hemophilia B). However, a lasting cure through gene therapy has long been sought. After a series of successes in small and large animal models, this goal has finally been achieved in humans by in vivo gene transfer to the liver using adeno-associated viral (AAV) vectors. In fact, multiple recent clinical trials have shown therapeutic, and in some cases curative, expression. At the same time, cellular immune responses against the virus have emerged as an obstacle in humans, potentially resulting in loss of expression. Transient immune suppression protocols have been developed to blunt these responses. Here, we provide an overview of the clinical development of AAV gene transfer for hemophilia, as well as an outlook on future directions.