Genome Editing Cures Hemophilia in Mouse Model

Genome editing improves on simple gene-replacement strategies by correcting the mutant gene in situ, thus, restoring normal gene function under the control of endogenous regulatory elements, and reducing risks associated with random insertion into the genome.

The ability to edit selectively a gene’s DNA sequence springs from the development of zinc finger nucleases (ZFNs). ZFNs are artificial restriction enzymes generated by fusing a zinc finger DNA-binding domain to a DNA-cleavage domain. Zinc finger domains can be engineered to target desired DNA sequences, which enable zinc-finger nucleases to target a unique sequence within a complex genome. By taking advantage of endogenous DNA repair machinery, these reagents can be used to precisely alter the genomes of higher organisms.

In the current study, investigators at The Children’s Hospital of Philadelphia (PA, USA) worked with a line of mice that had been genetically engineered to lack the gene for blood clotting factor IX. These mice demonstrated a blood clotting disorder similar to human hemophilia B.

To treat the disorder two versions of a viral vector based on adenovirus-associated virus (AAV) were used. One AAV vector carried ZFNs into the livers of the mice to selectively introduce a double strand cut into the DNA, while the other vector delivered a correctly functioning version of the F9 (factor IX) gene.

Results published in the June 26, 2011 online edition of the journal Nature revealed that mice having received the ZFN/factor IX gene combination incorporated the gene into their genomes and became able to generate enough clotting factor to reduce blood-clotting times to nearly normal levels. Control mice receiving vectors lacking the ZFNs or the factor IX gene had no significant improvements in circulating factor or in clotting times. The positive results of genome editing persisted over the eight months of the study, and showed no toxic effects on growth, weight gain, or liver function.

“We established a proof of concept that we can perform genome editing in vivo, to produce stable and clinically meaningful results,” said senior author Dr. Katherine High, professor of hematology at The Children’s Hospital of Philadelphia. “We need to perform further studies to translate this finding into safe, effective treatments for hemophilia and other single-gene diseases in humans, but this is a promising strategy for gene therapy. The clinical translation of genetic therapies from mouse models to humans has been a lengthy process, nearly two decades, but we are now seeing positive results in a range of diseases from inherited retinal disorders to hemophilia. In vivo genome editing will require time to mature as a therapeutic, but it represents the next goal in the development of genetic therapies. 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.”

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