Gene therapy for the hemophilias.

This review discusses the progress of gene therapy for the hemophilias. The development of gene therapy for hemophilia A has been more problematic than that for hemophilia B. It is now well established that reduced expression of the human clotting factor VIII cDNA is caused by transcriptional repression. Multiple sequences within the factor VIII cDNA are involved. So far, attempts to improve the factor VIII cDNA expression have been unsuccessful. However, improved retroviral vectors and adenovirus-based vectors have been constructed that increase factor VIII expression. The use of these vectors has resulted in clinically relevant levels of human factor VIII in mice and hemophilic dogs. Thus, gene therapy for hemophilia A has reached the same developmental stage as that for hemophilia B. If further improvements can increase the persistence of expression and decrease the immunologic responses, phase I clinical trials in human individuals can be considered.