Scientists have edited genes in food, animals, and in human embryos not used for implantation. Now, they have taken an ambitious leap forward by carrying out the experiment inside the human body.
Researchers in the United States are trying to rid Brian Madeux of a rare disease called Hunter syndrome. The condition is caused by the lack of a gene that produces an enzyme that breaks down certain carbohydrates. In people with Hunter syndrome, the leftover carbohydrates build up in the body’s cells and cause permanent, progressive damage, affecting appearance, mental development, organ function, and physical abilities. 
Hunter syndrome has no cure, and one of the most common ways of treating the illness – giving patients regular doses of the missing enzyme – is enough to drive someone to the poor house.
Each treatment costs between $100,000 and $400,000 in the U.S. For that price, patients should expect a miracle, but in reality, it does nothing to reverse the damage already done by Hunter syndrome, and it doesn’t stop further deterioration in the brain.
Madeux, 44, has undergone 26 operations to treat everything from hernias to eye problems. He is the first individual to receive an experimental gene therapy as part of a clinical trial.
In mid-November 2017, Madeux was injected with viruses produced by Sangamo Therapeautics that contain a “package” of gene-editing material. If all goes according to plan, the viruses will enter Madeux’s cells – specifically in the liver – and inject the missing gene at the correct spot in his DNA. Only 1% of the cells in his liver need to be repaired. Theoretically speaking, this would allow his liver to start making the enzyme that Madeux has been missing all his life.  
Scientists have edited human DNA before, except that they extracted human immune cells, edited their genes outside of the human body, and then returned them to the body equipped with the ability to attack lung cancer. Madeux’s case is the first time gene therapy has been carried inside the human body.
(This video provides some information on CRISPR and gene-editing.)
Precise but Unproven
Sangamo is attempting to remedy Madeux’s Hunter syndrome using a technology called zinc-finger nucleases. It is similar to the CRISPR gene-editing tool in the sense that it has the ability to locate the precise spot in strands of DNA and make a “snip.”
In addition to the zinc-finger nucleases, the virus contains fragments of the missing gene, which the body itself inserts into its gene-repairing system into the precise spot where the cut is made.
Once the nucleases and gene snippets are in place, there is no going back. The procedure is irreversible.
In order for the clinical trial to move forward, the scientists at the helm of the ship had to put the therapy through rigorous tests to make sure the process was as safe as possible.
Experiments in animals suggest the treatment is safe; however, the possibility of inserting a mistake into the genome – a new mutation – remains. There is also a risk of editing the genes of cells that aren’t intended to be edited. 
The good news is that the edits cannot be passed down to future generations, but there is no guarantee the treatment will work.
It will take several months before the scientists know whether the treatment was successful.