An ancient RNA-guided system could simplify delivery of gene editing therapies

Amazing stuff!

"A vast search of natural diversity has led scientists ... to uncover ancient systems with the potential to expand the genome editing toolbox. These systems, which the researchers call TIGR (Tandem Interspaced Guide RNA) systems, use RNA to guide them to specific sites on DNA. TIGR systems can be reprogrammed to target any DNA sequence of interest, and they have distinct functional modules that can act on the targeted DNA. In addition to its modularity, TIGR is very compact compared to other RNA-guided systems, like CRISPR, which is a major advantage for delivering it in a therapeutic context.  ...

The TIGR-associated (Tas) proteins that Zhang’s team found share a characteristic RNA-binding component that interacts with an RNA guide that directs it to a specific site in the genome.

They note that CRISPR systems can only be directed to segments of DNA that are flanked by short motifs known as PAMs (protospacer adjacent motifs). TIGR Tas proteins, in contrast, have no such requirement. “This means theoretically that any site in the genome should be targetable,” ... The team’s experiments also show that TIGR systems have ...  a “dual-guide system,” interacting with both strands of the DNA double helix to home in on their target sequences, which should ensure they act only where they are directed by their RNA guide. What’s more, Tas proteins are compact—a quarter of the size Cas9 on average—making them easier to deliver, which could overcome a major obstacle to therapeutic deployment of gene editing tools.  "

From the abstract:

"RNA-guided systems provide remarkable versatility, enabling diverse biological functions. Through iterative structural and sequence homology-based mining starting with a guide RNA-interaction domain of Cas9, we identified a family of RNA-guided DNA-targeting proteins in phage and parasitic bacteria.

Each system consists of a Tandem Interspaced Guide RNA (TIGR) array and a TIGR-associated (Tas) protein containing a Nop domain, sometimes fused to HNH (TasH) or RuvC (TasR) nuclease domains.

We show that TIGR arrays are processed into 36-nt RNAs (tigRNAs) that direct sequence-specific DNA binding through a tandem-spacer targeting mechanism.

TasR can be reprogrammed for precise DNA cleavage, including in human cells. The structure of TasR reveals striking similarities to box C/D snoRNPs and IS110 RNA-guided transposases, providing insights into the evolution of diverse RNA-guided systems."

An ancient RNA-guided system could simplify delivery of gene editing therapies | Broad Institute "The programmable proteins are compact, modular, and can be directed to modify DNA in human cells."

An ancient RNA-guided system could simplify delivery of gene editing therapies "The programmable proteins are compact, modular, and can be directed to modify DNA in human cells."

TIGR-Tas: A family of modular RNA-guided DNA-targeting systems in prokaryotes and their viruses (no public access)

The Tas protein uses an RNA guide to recognize a specific target DNA sequence.