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Genome "Editing" Corrects Hemophilia B in Mice

Researchers at The Children’s Hospital of Philadelphia (CHOP) used genome editing, a gene therapy method, to treat hemophilia in mice. This is the first time genome editing has been successfully done on study animals with clinically relevant results. The study was led by Katherine A. High, MD, director of CHOP’s Center for Cellular and Molecular Therapeutics. She is also a Howard Hughes Medical Institute Investigator. High has been conducting hemophilia gene therapy clinical trials for more than a decade. Results of the study were published in the online version of Nature on June 26.


The novel therapy uses zinc-finger nucleases (ZFNs), genetically engineered enzymes that act as “molecular scissors” that edit DNA sequences. ZFNs replace targeted DNA sequences responsible for hemophilia by cutting through the double helix (the double-stranded DNA molecule that resembles a spiral staircase) to initiate the cell’s emergency repair mechanism. Once the repair begins, healthy genetic material is inserted to replace the defective gene, in this case for the factor IX (FIX) gene that causes hemophilia B. CHOP researchers joined with investigators from Sangamo BioSciences, a clinical stage biopharmaceutical company in Richmond, CA, to test this “cut-and-paste” method of gene therapy delivery in mice with hemophilia B.


High’s team used two versions of the adeno-associated virus, which does not cause disease, as delivery vehicles. One version carried the ZFNs; the other the healthy version of the FIX gene. The scientists injected the ZFNs into the livers of the mice. The liver is often the target organ for gene therapy because it is the manufacturing and storage site for clotting proteins, including factor IX. The enzymes sliced the FIX gene segment at the precise location, activating the cellular repair response, which then generated a healthy (hemophilia-free) copy of the FIX gene.


Prior to the study the mice had no detectable levels of FIX. After receiving the therapy, they experienced a rise in FIX production to approximately 5%, enough to reduce clotting time to near normal levels. This lasted throughout the duration of the eight-month trial. The treatment was well tolerated and there were no significant side effects.


These results, although seemingly modest, could downgrade a person’s diagnosis and symptoms. “If you have 5% of factor 9, you will have mild hemophilia instead of severe hemophilia and the difference is huge,” said High. “People with mild hemophilia usually only bleed if they are in surgery or suffer a trauma.”


The difference between the use of other therapies and ZFNs is their precision. Conventional gene therapy techniques may randomly deliver a replacement gene into unfavorable locations. In contrast, ZFNs can target a specific site on a chromosome.


“Our research raises the possibility that genome editing can correct a genetic defect at a clinically meaningful level after in vivo delivery of the zinc-finger nucleases,” High said.

However, this therapy is still in the early stages of development and could take years to finalize. Studies on larger animals, such as dogs, and clinical studies in humans are necessary before such a therapy might become available. 


The study, “In Vivo Genome Editing Restores Haemostasis in a Mouse Model of Haemophilia,” was published online in Nature on June 26, 2011.


Sources: The Guardian and Nature online, June 26, 2011; CHOP news release June 27, 2011


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