CRISPR Blocking

Day by day new tools are being found to manage CRISPRs. In Cell today they announce:

Bacterial CRISPR-Cas systems utilize sequence-specific RNA-guided nucleases to defend against bacteriophage infection. As a countermeasure, numerous phages are known that produce proteins to block the function of class 1 CRISPR-Cas systems. However, currently no proteins are known to inhibit the widely used class 2 CRISPR-Cas9 system. To find these inhibitors, we searched cas9-containing bacterial genomes for the co-existence of a CRISPR spacer and its target, a potential indicator for CRISPR inhibition. This analysis led to the discovery of four unique type II-A CRISPR-Cas9 inhibitor proteins encoded by Listeria monocytogenes prophages. More than half of L. monocytogenes strains with cas9 contain at least one prophage-encoded inhibitor, suggesting widespread CRISPR-Cas9 inactivation. Two of these inhibitors also blocked the widely used Streptococcus pyogenes Cas9 when assayed in Escherichia coli and human cells. These natural Cas9-specific “anti-CRISPRs” present tools that can be used to regulate the genome engineering activities of CRISPR-Cas9.

Likewise in The Scientist they note:

The researchers found these Cas9 blockers by searching bacterial genomes for both a CRISPR sequence and its target, under the assumption that the genome likely contained an inhibitor to prevent CRISPR from cutting that target in the bacterium’s own genome. Indeed, Rauch and colleagues uncovered several anti-CRISPRs in Listeria whose sequences had been left behind in the bacterial genome by prior phage infection. “Just as CRISPR technology was developed from the natural anti-viral defense systems in bacteria, we can also take advantage of the anti-CRISPR proteins that viruses have sculpted to get around those bacterial defenses,” Rauch said in the statement. Two of the inhibitors blocked Cas9 from Streptococcus pyogenes, the form of the DNA-cutting enzyme frequently used in genome editing. In a study published earlier this month in Cell, a different team of researchers reported the discovery of several anti-CRISPRs that block Cas9, but none of them acted against the activity of the Cas9 from S. pyogenes.

It seems we are building up a powerful tool box for cell manipulation. Hopefully the patent war can be settled in 2017.