This image depicts a conventional CRISPR-Cas9 system. The Cas9 enzyme acts like a wrench, and specific RNA guides act as different socket heads. Conventional CRISPR-Cas9 systems act continuously, raising the risk of off-target effects. But CRISPR-Cas9 systems that incorporate specially engineered RNAs could act transiently, potentially reducing unwanted changes. [Ernesto del Aguila III, NHGRI]
This image depicts a conventional CRISPR-Cas9 system. The Cas9 enzyme acts like a wrench, and specific RNA guides act as different socket heads. Conventional CRISPR-Cas9 systems act continuously, raising the risk of off-target effects. But CRISPR-Cas9 systems that incorporate specially engineered RNAs could act transiently, potentially reducing unwanted changes. [Ernesto del Aguila III, NHGRI]

By removing parts of the CRISPR/Cas9 gene-editing system, and replacing them with specially engineered molecules, researchers at the University of California, San Diego (UCSD) and Isis Pharmaceutical hope to limit the CRISPR/Cas9 system’s propensity for off-target effects. The researchers say that CRISPR/Cas9 needn’t remain continuously active. Instead, it could be transiently activated and deactivated. Such on/off control could prevent residual gene-editing activity that might go awry. Also, such control could be exploited for therapeutic purposes.

The key, report the scientists, is the introduction of RNA-based drugs that can replace the guide RNA that usually serves to guide the Cas9 enzyme to a particular DNA sequence. When Cas9 is guided by a synthetic RNA-based drug, its cutting action can be suspended whenever the RNA-based drug is cleared. The Cas9’s cutting action can be stopped even more quickly if a second, chemically modified RNA drug is added, provided that it is engineered to direct inactivation of the gene encoding the Cas9 enzyme.

Details about temporarily activated CRISPR/Cas9 systems appeared November 16 in the Proceedings of the National Academy of Sciences, in a paper entitled, “Synthetic CRISPR RNA-Cas9–guided genome editing in human cells.” The paper’s senior author, the USCD’s Don Cleveland, Ph.D., noted that the RNA-based drugs described in the study “provide many advantages over the current CRISPR/Cas9 system,” such as increased editing efficiency and potential selectivity.

“Here we develop a chemically modified, 29-nucleotide synthetic CRISPR RNA (scrRNA), which in combination with unmodified transactivating crRNA (tracrRNA) is shown to functionally replace the natural guide RNA in the CRISPR-Cas9 nuclease system and to mediate efficient genome editing in human cells,” wrote the authors of the PNAS paper. “Incorporation of rational chemical modifications known to protect against nuclease digestion and stabilize RNA–RNA interactions in the tracrRNA hybridization region of CRISPR RNA (crRNA) yields a scrRNA with enhanced activity compared with the unmodified crRNA and comparable gene disruption activity to the previously published single guide RNA.”

Not only did the synthetic RNA functionally replace the natural crRNA, it produced enhanced cleavage activity at a target DNA site with apparently reduced off-target cleavage. These findings, Dr. Cleveland explained, could provide a platform for multiple therapeutic applications, especially for nervous system diseases, using successive application of cell-permeable, synthetic CRISPR RNAs to activate and then silence Cas9 activity. “In addition,” he said, “[these designer RNAs] can be synthesized efficiently, on an industrial scale and in a commercially feasible manner today.”








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