For the first time, researchers have found anti-CRISPR proteins that shut off the genome editor and also demonstrated it for use to control the cutting of DNA in human cells.
Molecular biochemist Erik Sontheimer of the University of Massachusetts Medical School in Worcester and colleagues describe three proteins in Neisseria meningitidis that inhibit its version of Cas9, an enzyme that some CRISPR systems employ to cut the DNA.
"Any time you have greater control over a useful technology, the utility improves," says Sontheimer.
The potential revolutionary implications of using CRISPR's Cas9 molecular scissors, it has been suggested, can cure genetic defects, eradicate diseases, and even end the organ transplant shortage.
But along with the various benefits, Sontheimer and his co-authors warn of "practical difficulties and safety concerns" because there are no "off switches" to prevent Cas9 from making cuts in the wrong places or to stop gene drives using the enzyme from running amok.
Though biochemists Karen Maxwell and Alan Davidson at the University of Toronto in Canada reported in Nature magazine that they had found anti-CRISPR genes in a bacterial system that uses a different DNA-cutting enzyme than Cas9, Rodolphe Barrangou, a functional genomics specialist, said that "a nature-driven approach" to controlling Cas9 is "very powerful."
As explained in the Cell paper, "These proteins likely emerged first in phages as a way for those viruses to counter the Cas9-based immune defense of bacteria. But the essence of the CRISPR defense mechanism is that bacteria incorporate the phage genetic material into their own DNA and use it against the parasite if it reinfects. So N. meningitidis ended up with anti-CRISPR genes in its own genome, which may limit both the bacteria's ability to stop subsequent invasions from a familiar phage and to build an effective CRISPR defense against novel phages."
Sontheimer also cautions that "there are a lot of things to work out."
The CRISPR system that most research teams now use to modify human cells and those of other complex organisms was adapted from the bacterium Streptococcus pyogenes, not N. meningitidis, and the three newfound proteins can't shut off its Cas9. But Sontheimer and others predict they will find ones that do. "This paper is a proof of principle," he says. "There definitely are going to be other Cas9 inhibitors. Now, the search is on."
