CRISPR-Cas9 is know for its ability to edit eukaryotic genomes, but from now it could be famous for its multifunctionality. MIT and Harvard laboratories have led a research project that transforms CRISPR-Cas9 into a gene activator/repressor. The researchers altered the length of the guide RNA (gRNA) that directs the nuclease to its target, which resulted in a new ability: the control of transcriptional regulation. The results have been published in the journal Nature Methods.
CRISPR/Cas is an immune system found in prokaryotes that confers resistance to foreign DNA like plasmids and phages. This DNA is incorporated in the prokaryote genome, transcribed and processed into gRNA, which binds Cas endonucleases and targets them to cleave alien, complementary DNA sequences. Researchers saw the potential of this acquired immunity mechanism as a gene editing tool, engineering its gRNA to target selected sequences. Since its first applied use as a genome editor in 2012, CRISPR/Cas has been extensively studied and/or modified to enhance its function or to grant it new roles: photoactivatable CRISPR/Cas9, tools to facilitate gRNA design, inhibition of the NHEJ pathway to boost homology directed repair, software to improve targeting, etc.
The CRISPR/Cas system can be used for different functions depending of the Cas protein attached to the gRNA. Cas9 is a nuclease used for cleaving the target DNA. Cas9 variants have been engineered to render them unable to cut the DNA, but still bind to it, effectively working as gene expression regulators in combination with attached proteins. The last improvement to this technique comes from Harvard University and the MIT, where Dr. Church and Dr. Weiss respectively led their teams to develop a new technique that allows gene editing and regulation with the same Cas molecule. The researchers found out that very short gRNAs did not allow the attached Cas9 to cut the DNA, but Cas9 still could bind the targeted sequence. They subsequently tested how different gRNA lengths affected Cas9 activity. With the right gRNA length, a single Cas molecule can control both genome edition and transcription. By binding gene regulation proteins to Cas9, any genomic sequence can be converted into a regulatory sequence.
The possibilities of this innovation on CRISPR/Cas9 are immense. Scientists have proposed its use to decipher genetic interactions, molecular bases of diseases or mechanisms of drug action.