Scientists from Salk Institute have made a revolutionary breakthrough: they could now, for the first time, insert DNA at a target location. Once the DNA was inserted into the non-dividing cells that make up the majority of adult organs and tissues, the team of researchers was able to partially restore visual responses in blind rodents. This discovery is bound to open new channels for basic research and a variety of treatments, primarily targeting retinal, heart, and neurological diseases.

"We are very excited by the technology we discovered because it's something that could not be done before," said Juan Carlos Izpisua Belmonte, a professor in Salk's Gene Expression Laboratory and senior author of the paper published on Nov. 16, 2016 in Nature. "For the first time, we can enter into cells that do not divide and modify the DNA at will. The possible applications of this discovery are vast."

Earlier DNA modifying techniques were effective in dividing cells, but the new Salk technology presents the first time scientists have managed to insert a new gene into a precise DNA location in adult cells that no longer divide. Prime examples would be the cells of the eye, brain, pancreas, or heart, opening new possibilities for therapeutic applications in these cells.

To achieve this, the Salk researchers targeted a DNA-repair cellular pathway called NHEJ or "non-homologous end-joining." This pathway repairs routine DNA breaks by rejoining the original strand ends. They paired this process with existing gene-editing technology to successfully place new DNA into a precise location in non-dividing cells.

"Using this NHEJ pathway to insert entirely new DNA is revolutionary for editing the genome in live adult organisms," said Keiichiro Suzuki, one of the paper's lead authors and a senior research associate in the Izpisua Belmonte lab. "No one has done this before."

The Salk team worked on optimizing the NHEJ machinery for use with the CRISPR-Cas9 system to insert DNA at very precise locations within the genome. Then the researchers created a custom insertion package made up of a nucleic acid cocktail, which they call HITI, or homology-independent targeted integration and delivered it to neurons derived from human embryonic stem cells through an inert virus.

The team then successfully delivered the construct to the brains of adult mice and tested the technique on a rat model of retinitis pigmentosa, an inherited retinal degeneration condition that causes blindness in humans. This time, the team used HITI to deliver to the eyes of three-week-old rats a functional copy of Mertk, one of the genes that is damaged in retinitis pigmentosa. Analysis performed when the rats were eight weeks old showed that the animals were able to respond to light and passed several tests indicating healing in their retinal cells.

"We were able to improve the vision of these blind rats," shared co-lead author Reyna Hernandez-Benitez, a Salk research associate. "This early success suggests that this technology is very promising."