Scientists extend the capabilities of the CRISPR-Cas system to include precise manipulations of RNA sequences in human cells.

Fusing an RNA-editing enzyme to an RNA-targeting Cas protein has enabled researchers to edit specific nucleotides within RNA molecules in human cells. The approach, called RNA Editing for Programmable A-to-I replacement (REPAIR), is described today (October 25) in Science, and has the potential to serve not only as a research tool, but as a temporary correctional therapy for disease-causing mutations, the researchers propose.

“This work is an impressive study from a highly productive research group that suggests the possibility of editing RNA transcripts to alter their coding potential in a programmable manner,” David Liu, a chemical biologist at Harvard University who was not involved in the project, writes in an email to The Scientist. “For applications that are best addressed through a transient change in a target RNA's sequence, this approach has strong potential,” he adds. Liu himself has a report out today in Nature describing specific nucleotide editing of DNA by a similar method.

The CRISPR-Cas9 system—a bacterial antiviral immune mechanism first discovered in Streptococcus thermophilus—is now widely used as a DNA editing technique, wherein the DNA nuclease Cas9 is directed to cut any DNA sequence of choice. By searching for similar immune systems in other microorganisms, researchers went on to discover a Cas9-related enzyme called Cas13 (originally termed C2c2), which, in 2016, the Broad Institute’s Feng Zhang and colleagues revealed targeted RNA not DNA.

Further developments with Cas13 led to a Nature paper earlier this month in which Zhang’s team showed that one family member, Cas13a, could be used in mammalian cells to knockdown particular messenger RNAs with similar efficiency to RNA interference. The team also engineered a catalytically inactive version of Cas13a that, when fused to a fluorescent protein, could be used to track RNAs of interest.

[link to www.the-scientist.com]