CRISPR ‘kill switch’ wards off unwanted side effects

Anti-CRISPR proteins could reduce the occurrence of unintended editing and its side effects.

CRISPR-Cas9 can neatly snip out disease-causing mutations from DNA but comes with a question of safety: What if the system edits untargeted genes, bringing about harmful side effects? An anti-CRISPR protein that reduces off-target editing might be the answer.

CRISPR is based on a system that bacteria use to protect themselves against viruses. Researchers from UC Berkeley and UC San Francisco found that viruses counter this bacterial defense using proteins that disable CRISPR.

The team applied an anti-CRISPR protein to a CRISPR-Cas9 molecule that treats sickle cells disease by replacing the mutated gene that causes it. The study, published in Science Advances, showed the protein AcrIIA4 reduced off-target effects by a factor of four without significantly diminishing the desired gene editing.

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CRISPR-blocking proteins work by targeting a spot on the CRISPR-Cas9 molecule and binding with it, rendering the molecule unable to cut DNA, the team said. Scientists from the University of Toronto and the University of Massachusetts discovered a trio of anti-CRISPR proteins last year.

"The ability to turn Cas9 gene editing off is just as important as the ability to turn it on," said Corn, scientific director for biomedicine of the IGI and a UC Berkeley assistant adjunct professor of molecular and cell biology. "For eventual therapeutic applications, it is critical to be able to precisely control when and where gene editing is active. The anti-CRISPR proteins offer opportunities to completely turn off Cas9 as well as fine-tune its activity."

Administering an anti-CRISPR protein after delivering CRISPR was the most effective way to cut down on off-target effects, found first author Jiyung Jenny Shin, a postdoctoral fellow in Jacob Corn’s lab at the Innovative Genomics Institute in Berkeley.

"Even after six hours of effective CRISPR, inserting anti-CRISPR decreases off-target effects by more than two-fold compared to on-target effects," Shin said. "Therapeutically, you could treat a patient with CRISPR first, and then treat with anti-CRISPR at a later time and decrease off-target effects."