Gene therapy for hemophilia

Summary Hemophilia  A and B are X ‐linked monogenic disorders resulting from deficiencies of factor  VIII and FIX , respectively. Purified clotting factor concentrates are currently intravenously administered to treat hemophilia, but this treatment is non‐curative. Therefore, gene‐based therapies for hemophilia have been developed to achieve sustained high levels of clotting factor expression to correct the clinical phenotype. Over the past two decades, different types of viral and non‐viral gene delivery systems have been explored for hemophilia gene therapy research with a variety of target cells, particularly hepatocytes, hematopoietic stem cells, skeletal muscle cells, and endothelial cells. Lentiviral and adeno‐associated virus ( AAV )‐based vectors are among the most promising vectors for hemophilia gene therapy. In preclinical hemophilia  A and B animal models, the bleeding phenotype was corrected with these vectors. Some of these promising preclinical results prompted clinical translation to patients suffering from a severe hemophilic phenotype. These patients receiving gene therapy with AAV vectors showed long‐term expression of therapeutic FIX levels, which is a major step forwards in this field. Nevertheless, the levels were insufficient to prevent trauma or injury‐induced bleeding episodes. Another challenge that remains is the possible immune destruction of gene‐modified cells by effector T  cells, which are directed against the AAV vector antigens. It is therefore important to continuously improve the current gene therapy approaches to ultimately establish a real cure for hemophilia.