Hemophilia, a rare bleeding disorder, is an example of a monogenic disease. There are several types of hemophilia caused by mutations in genes that encode factors which help the blood clot and stop bleeding when blood vessels are injured. The most prevalent form of the disease, hemophilia A, is caused by a defect in clotting Factor VIII while defects in clotting Factor IX lead to hemophilia B. The most severe forms of hemophilia affect males. According to the National Hemophilia Foundation, hemophilia A occurs in about one in every 5,000 male births in the US, and hemophilia B in about 1 in every 25,000. The standard treatment for individuals with hemophilia is replacement of the defective clotting factor with regular infusion of concentrates or recombinant factors, which are expensive, carry the risk of transmission of blood-borne diseases and sometimes stimulate the body to produce antibodies against the factors that inhibit the benefits of treatment. In these situations, other clotting factors such as Factor VII and X may be used to treat patients.
Using a mouse model of hemophilia B, Sangamo scientists and its collaborators have already established proof of concept that ZFN-mediated genome editing can be accomplished in vivo and is curative in the animal. They have demonstrated the production of stable levels of corrected human clotting Factor IX that are clinically meaningful, restoring clotting times to normal, after a single, systemic administration of ZFNs specific for the Factor IX gene. The data were published in the scientific journal Nature in June 2011 ( Nature. 2011 Jun 26; 475(7355):217-21. doi: 10.1038/nature10177).
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