For the past 25 years, the field of gene therapy has all but remained in its experimental phase. But now, its unfortunately lethargic progress should experience a well-deserved boom. A team of scientists at the Children's Hospital of Philadelphia, led by Katherine High, have performed the first successful, in vivo genome editing procedure. Genome editing is a revolutionary gene therapy treatment that allows scientists to precisely target and repair a gene defect, effectively cutting out the mutated sequence and splicing in a healthy copy of the gene. Before genome editing, gene therapy techniques mostly consisted of randomly delivering replacement genes, sometimes to a wrong location, which could cause leukaemia and other cancers. This is the first time this treatment has been performed on live animals with clinically meaningful results.
High's team injected haemophiliac mice with enzymes called zinc finger nucleases (ZFNs). These enzymes were contained within artificial virus shells designed to "find their way to the liver, where blood clotting proteins are made." The ZFNs cut through the mutated gene (hence the 'genomic scissors' sobriquet), and delivered a DNA template of the healthy blood clotting gene in its place. This new gene was integrated, eliminating the mutation, and the mice's blood clotting times dramatically improved, a "a welcome breakthrough in a field that has struggled to deliver on the promise many scientists believe it holds."
High's team hopes to use a variation of this technique to "alter immune cells in HIV patients, which would make them resistant to the virus."
If you want to know more about how ZFN enzymes work, this easy-to-understand article has some very helpful diagrams. I have also embedded a YouTube video that nicely illustrates how ZFNs operate. The healthy gene integration procedure that High's team used in their experiments is shown around the 2:33 mark, although the entire video is worth a look.