Gene therapy usually involves removing cells from a patient, repairing their damaged genes in a Petri dish, reinserting them, and hoping that they’ll take, a fraught, expensive process. These researchers performed the whole procedure within their mouse subjects, sending in enzymes to snip out the defective gene and a virus carrying a normal gene to replace it.
The mice had been engineered to carry a human gene for hemophilia, and the enzymes and virus had been specifically engineered as well: the enzymes would cut only certain sequences of DNA, patterns that were known to appear on either side of the defective gene, and the virus, which naturally infects the liver cells where the clotting factor is made, would swap in an unmutated gene, instead of the viral genes it would carry in nature.
The treated mice bled for a significantly shorter time than untreated mice and made 3-7% of the normal level of the clotting factor, a level that would result in only mild bleeding in humans. What’s more, even after part of the liver had been removed and allowed to regenerate, the mice continued to produce clotting factor, a sign that the modified cells were passing the normal gene down to their daughter cells.
