Harnessing a virus to cure hemophilia

Nursing, May 1999

The Food and Drug Administration is considering human trials on a genetically engineered drug that's cured hemophilia in mice and controlled it in dogs. The treatment is especially promising because it was developed at two different research centers by scientists working independently of each other.

At both centers, researchers modified a virus called Adeno-Associated Viral Vector (AAV), which causes no known disease in humans, to carry a gene that people with hemophilia B lack. Injected into the liver in animal trials, AAV seems capable of integrating itself into liver tissue. Some evidence suggests that the new genes may then pass from one generation of cells to the next, raising hopes of a cure.

The modified AAV virus has also been effective when injected into muscle, although whether it integrates into muscle tissue isn't clear. If not, a patient treated this way might require retreatment every few years as muscle tissue dies.

Until recently, most gene therapy research has focused on modifying the common cold virus. The fatal flaw in this line of investigation is that the body's immune system quickly identifies and destroys the cold virus. The body then becomes immune to it, making retreatments unsuccessful. For unknown reasons, AAV eludes the body's immune system, making it a promising vector for gene therapy.

This research targets only hemophilia B, which affects about 20% of American hemophiliacs. Hemophilia A, the more common type, is caused by a different genetic defect.

Sources: "Long-Term Correction of Canine Hemophilia B by Gene Transfer of Blood Coagulation Factor IX Mediated by Adeno-Associated Viral Vector," Nature Medicine. R. Herzog, et al., January, 1999; "Correction of Hemophilia B in Canine and Murine Models Using Recombinant Adeno-Associated Viral Vectors," Nature Medicine, R. Snyder, et al., January 1999.