Something closer to the clinic is the zinc-finger nuclease, an enzyme created by the fusion of zinc finger DNA-binding domains with an endonuclease's cleavage domain. The modularity of zinc fingers allows for predictable customization of the DNA that the nuclease recognizes and cuts, and there are kits for making the process easier.
Such nucleases form the basis for Sangamo's HIV treatment, SB-728-T. It consists of removing T cells and treating them with the nucleases to introduce a deletion in CCR5, creating the same sort of HIV resistance as seen in the patient who was cured through marrow transplants. Sangamo presented positive Phase I results last month and is working on a similar method to treat stem cells.
Zinc-finger nucleases (ZFN) have room for improvement, though; use of other modular DNA-binding proteins called TAL effectors suggests that TAL effector nucleases (TALEN) might be even easier to predict and engineer.
In any case, modular nucleases offer more precision in gene therapy than has previously been possible, reducing the risk of side effects (such as leukemia) and may lead to further applications and treatments. Gene therapy in situ is still considerably more difficult, so don't expect a ZFN or TALEN to repair BRCA genes—yet.
One project that showcases all the principles of synthetic biology is the tumor-killing bacterium. It requires modifications to the base genome, removing the immunogenic capsule and antigens from the bacterium; a sophisticated sensing, guidance, and activation logic system to direct the bacteria to tumors and activate them appropriately; and finally redundant safeguards to ensure the bacteria behave as predicted and dispose of the bacteria after their purpose is served. It is an engineering project at heart and demonstrates both design principles and new genetic parts.
Synthetic biology has the ability to enhance regulation and genetic failsafes. Every measure to prevent these activities makes for a more predictable (and safer) biological therapy. Eventually, it might be bacteria delivering gene repair packages, and the gene repairs might be as varied as treatments for phenylketonuria to cystic fibrosis. Considering all that's been done lately, a world where one can opt-out of genetic diseases doesn't seem so far away.