Researchers have used CRISPR — a revolutionary new genetic engineering technique —  to convert cells isolated from mouse connective tissue directly into neuronal cells.

In 2006, Shinya Yamanaka, a professor at the Institute for Frontier Medical Sciences at Kyoto University at the time, discovered how to revert adult connective tissue cells, called fibroblasts, back into immature stem cells that could differentiate into any cell type. These so-called induced pluripotent stem cells won Yamanaka the Nobel Prize in medicine just six years later for their promise in research and medicine.

Since then, researchers have discovered other ways to convert cells between different types. This is mostly done by introducing many extra copies of “master switch” genes that produce proteins that turn on entire genetic networks responsible for producing a particular cell type.

Now, researchers at Duke University have developed a strategy that avoids the need for the extra gene copies. Instead, a modification of the CRISPR genetic engineering technique is used to directly turn on the natural copies already present in the genome.

These early results indicate that the newly converted neuronal cells show a more complete and persistent conversion than the method where new genes are permanently added to the genome. These cells could be used for modeling neurological disorders, discovering new therapeutics, developing personalized medicines and, perhaps in the future, implementing cell therapy.

The study was published on August 11, 2016, in the journal Cell Stem Cell.

“This technique has many applications for science and medicine. For example, we might have a general idea of how most people’s neurons will respond to a drug, but we don’t know how your particular neurons with your particular genetics will respond,” said Charles Gersbach, the Rooney Family Associate Professor of Biomedical Engineering and director for the Center of Biomolecular and Tissue Engineering at Duke. “Taking biopsies of your brain to test your neurons is not an option. But if we could take a skin cell from your arm, turn it into a neuron, and then treat it with various drug combinations, we could determine an optimal personalized therapy.”

In the new study, Gersbach and colleagues used CRISPR to precisely activate the three genes that naturally produce the master transcription factors that control the neuronal gene network, rather than having a virus introduce extra copies of those genes.

CRISPR is a modified version of a bacterial defense system that targets and slices apart the DNA of familiar invading viruses. In this case, however, the system has been tweaked so that no slicing is involved. Instead, the machinery that identifies specific stretches of DNA has been left intact, and it has been hitched to a gene activator.

The CRISPR system was administered to mouse fibroblasts in the laboratory. The tests showed that, once activated by CRISPR, the three neuronal master transcription factor genes robustly activated neuronal genes. This caused the fibroblasts to conduct electrical signals, a hallmark of neuronal cells. And even after the CRISPR activators went away, the cells retained their neuronal properties.

“When blasting cells with master transcription factors made by viruses, it’s possible to make cells that behave like neurons,” Gersbach said. “But if they truly have become autonomously functioning neurons, then they shouldn’t require the continuous presence of that external stimulus.”

The experiments showed that the new CRISPR technique produced neuronal cells with an epigenetic program at the target genes matching the neuronal markings naturally found in mouse brain tissue.

The next steps are to extend the method to human cells, raise the efficiency of the technique and try to clear other epigenetic hurdles so that it could be applied to model particular diseases.

“In the future, you can imagine making neurons and implanting them in the brain to treat Parkinson’s disease or other neurodegenerative conditions,” Gersbach said. “But even if we don’t get that far, you can do a lot with these in the lab to help develop better therapies.”