Gene therapy for hemophilia

PURPOSE OF REVIEW: This review will highlight the progress achieved in the past 2 years on using gene therapy to treat hemophilia in animals and humans. RECENT FINDINGS: There has been substantial progress in using gene therapy to treat animals with hemophilia. Novel approaches for hemophilia A in mice include expression of Factor VIII in blood cells or platelets derived from ex-vivo transduced hematopoietic stem cells, or in-vivo transfer of transposons expressing Factor VIII into endothelial cells or hepatocytes. Advances in large-animal models include the demonstration that neonatal administration of a retroviral vector expressing canine Factor VIII completely corrected hemophilia A in dogs, and that double-stranded adeno-associated virus vectors resulted in expression of Factor IX that is 28-fold that obtained using single-stranded adeno-associated virus vectors. In humans, one hemophilia B patient achieved 10% of normal activity after liver-directed gene therapy with a single-stranded adeno-associated virus vector expressing human Factor IX. Expression fell at 1 month, however, which was likely due to an immune response to the modified cells. SUMMARY: Gene therapy has been successful in a patient with hemophilia B, but expression was unstable due to an immune response. Abrogating immune responses is the next major hurdle for achieving long-lasting gene therapy.