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Mice with a defective gene for factor IX have virtually no hepatic production of this clotting protein (the cause of hemophilia B). Viral vectors can carry healthy genes into an animal born with a defective gene, but two problems have dogged this therapy: delivering the genes to the organ that needs them (the liver, in hemophilia) and random insertion of the therapeutic gene into an oncogene (which can cause a cell to become cancerous).
A team from the University of Pennsylvania created a viral vector that included the gene for normal factor IX as well as chemical signals designed to direct the gene to insert specifically into hepatocytes at the chromosomal location of the defective factor IX gene. This vector was injected intraperitoneally …