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Gene Therapy Helps Patients with Hemophilia B



Hemophilia B is less common than hemophilia A, a deficiency of clotting factor VIII. About 1 in 5 hemophilia patients has hemophilia B, while the other four have hemophilia A. Hemophilia is an inherited condition that affects men more frequently. Worldwide, about 1 in 5,000 men are born with hemophilia A and 1 in 25,000 men are born with hemophilia B each year. People with hemophilia bleed more following trauma than people without the disease, and those with severe disease may bleed spontaneously. Hemophilia B patients are unable to produce enough human clotting factor IX, also referred to as FIX, which is essential for normal blood clotting. Liver cells are the only cells in the body capable of producing a form of factor IX that is active in the clotting process.


According to an article published online in the New England Journal of Medicine (10 December 2011), an experimental gene therapy technique has been shown to boost the production of a vital blood clotting factor in six people with hemophilia B. As a result the therapy could give patients a long-term solution for preventing prolonged bleeding episodes and spontaneous bleeding.


“Hemophilia has long been one of the disorders thought most likely to be correctible with gene therapy, but previous approaches to deliver the gene have been disappointing. Results from this study represent a promising step toward making gene therapy a viable treatment option for hemophilia B,” said Susan B. Shurin, M.D., acting director of the NIH’s National Heart, Lung, and Blood Institute (NHLBI),. “If future studies support these findings, it would bring a significant improvement in the quality of life for those living with the disease.”


In preparation for the clinical trial, the FIX gene was packaged in a modified adeno-associated virus, AAV8, which targets liver cells. This potentially allowed the normal FIX gene to be delivered using intravenous infusion to the liver cells which should allow the patients to produce more FIX. The clinical trial then evaluated six people with severe hemophilia B who were producing clotting factor IX at less than 1% of normal levels before receiving the gene therapy. Prior to enrolling in the study, participants were receiving the standard treatment for severe hemophilia B of infusions of manufactured FIX several times a month.


Results showed that after receiving the FIX gene therapy, each study subject produced FIX at between 2 and 11% of normal levels. In the short-term follow-up period (six to 16 months), four of the six participants no longer required FIX infusions for routine bleeding. The other two participants required FIX infusions less frequently than before the study.


According to the authors, the unique feature of this approach is the use of AAV8 to deliver the gene to the appropriate cells, and if future studies show that immunologic or other factors do not limit its long-term effectiveness, people with hemophilia B could produce their own FIX and potentially avoid reliance upon FIX infusions. While more work is needed, these results are more promising than prior attempts at gene therapy for the hemophilias.


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