Gene therapy for the hemophilias.

Significant progress has recently been made in the development of gene therapy for the treatment of hemophilia A and B. These advances parallel the development of improved gene delivery systems. Long-term therapeutic levels of factor (F) VIII and FIX can be achieved in adult FVIII- and FIX-deficient mice and in adult hemophiliac dogs using adeno-associated viral (AAV) vectors, high-capacity adenoviral vectors (HC-Ad) and lentiviral vectors. In mouse models, some of the highest FVIII or FIX expression levels were achieved using HC-Ad vectors with no or only limited adverse effects. Encouraging preclinical data have been obtained using AAV vectors, yielding long-term FIX levels above 10% in primates and in hemophilia B dogs, which prevented spontaneous bleeding. Non-viral ex vivo gene therapy approaches have also led to long-term therapeutic levels of coagulation factors in animal models. Nevertheless, the induction of neutralizing antibodies (inhibitors) to FVIII or FIX sometimes precludes stable phenotypic correction following gene therapy. The risk of inhibitor formation varies depending on the type of vector, vector serotype, vector dose, expression levels and promoter used, route of administration, transduced cell type and the underlying mutation in the hemophilia model. Some studies suggest that continuous expression of clotting factors may induce immune tolerance, particularly when expressed by the liver. Several gene therapy phase I clinical trials have been initiated in patients suffering from severe hemophilia A or B. Some subjects report fewer bleeding episodes and occasionally have low levels of clotting factor activity detected. Further improvement of the various gene delivery systems is warranted to bring a permanent cure for hemophilia one step closer to reality.