The booming field of gene therapy hinges on the idea that genetic diseases can be corrected at their source. While techniques are still considered risky, huge advancements have been made in recent years with genetic engineering tools like CRISPR, which has been used on animal models to cure muscular dystrophy and edit dangerous mutations in human embryos. Now, for the first time, scientists have employed genetic engineering tools inside a human adult.

According to AP News, the scientists used another gene editing tool called a zinc finger nuclease to edit a gene that’s inside a living human body. The experiment was conducted on Monday in California on Brian Madeux, a 44-year old who has a metabolic disease called Hunter syndrome, a condition with prominent effects on a person’s physical appearance as well as on the inner organs.

This attempt to provide therapy for this disease won’t cure Madeux but will hopefully end the need for the costly weekly enzyme treatments that are the current default form of care for people with Hunter syndrome. In a month, the team of physicians, led by Dr. Paul Harmatz at the University of California-San Francisco Benioff Children’s Hospital Oakland, expect to see signs of whether it worked. In three months, tests will confirm the effects of the therapy.

Scientists have edited genes while inside a body for the first time.

Zinc finger nuclease is a tool that experts say has the same effect of sending a tiny surgeon into the body to place a new gene in the right location. AP reports that the therapy relies on three parts: the new gene and two zinc proteins that are placed in the body:

“DNA instructions for each part are placed in a virus that’s been altered to not cause infection but to ferry them into cells. Billions of copies are given through a vein. They travel to the liver, where cells use the instructions to make the zinc fingers and prepare the corrective gene. The fingers cut the DNA, allowing the new gene to slip in. The new gene then directs the cell to make the enzyme the patient lacked.”

While there are some risks to the therapy — it’s still relatively untested, and there’s a chance the new gene will have accidental effects on other genes — the safety tests on animals leading up to this procedure have suggested that the chances that something will go wrong are slim. For better or worse, the process is also irreversible — there’s no way to correct mistakes that may occur.

“We cut your DNA, open it up, insert a gene, stitch it back up. Invisible mending,” Dr. Sandy Macrae, president of Sangamo Therapeutics, tells AP. “It becomes part of your DNA and is there for the rest of your life.”

While Madeux is the first human to have this in-body procedure done, there are plans for initial studies on 30 adults to test the safety of the procedure. Ultimately, scientists would like to use this technique to treat children, before metabolic diseases begin to take effect. Approximately 10,000 people worldwide are living with metabolic diseases like Hunter syndrome — a low number because many diagnosed die young.

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