Massachusetts General Hospital Develops Improved CRISPR

A new engineered version of the gene-editing CRISPR-Cas9 nuclease appears to robustly abolish the unwanted, off-target DNA breaks that are a significant current limitation of the technology, reducing them to undetectable levels. In their report receiving advance online publication in Nature, Massachusetts General Hospital (MGH) researchers describe how altering the Cas9 enzyme to reduce non-specific interactions with the target DNA may greatly expand applications of the gene-editing technology.

"Our creation of a Cas9 variant that brings off-target effects to levels where we can no longer detect them, even with the most sensitive methods, provides a substantial advance for therapeutic applications in which you want to accurately hit your target without causing damage anywhere else in the genome," says J. Keith Joung, MD, PhD, associate chief for Research and the Jim and Ann Orr MGH Research Scholar in the MGH Department of Pathology, senior author of the Nature paper. "But its impact will also be incredibly important for research applications because off-target effects can potentially confound the results of any experiment. As a result, we envision that our high-fidelity variant will supplant the use of standard Cas9 for many research and therapeutic applications."

Used to create targeted DNA breaks at which genetic changes can be introduced, CRISPR-Cas9 nucleases combine a bacterial DNA-cutting enzyme called Cas9 with a short guide RNA sequence that can bind to the target DNA sequence. While easier to use than previous gene-editing tools, CRISPR-Cas9 nucleases have a well-characterized and significant limitation. As described in 2013 studies led by Joung and others, CRISPR-Cas9 nucleases can induce off-target DNA breaks at sites that resemble the on-target sequence. Subsequent investigations by Joung's team and others have reduced but never completely and consistently eliminated these off-target effects.

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