RNA Targeting and Editing with CRISPR-Cas13: Dr. David Cox, Stanford University
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The ability to modify nucleic acids is critical for establishing the role of genetic elements in mediating biological phenotypes. Manipulating endogenous DNA sequences in eukaryotic genomes has been greatly aided by the advent of genome editing technologies that utilize programmable nucleases. RNA-guided DNA nucleases derived from class 2 CRISPR systems, which provide adaptive immunity in prokaryotes, have been particularly useful in this regard because they enable targeting of new sites through simple Watson-Crick base pairing rules. Following the initial demonstration of DNA editing with CRISPR-Cas9, computational studies predicted the existence of RNA-targeting class 2 CRISPR systems, raising the possibility that the flexibility of CRISPR-based genome editing might be extended to the level of transcripts.
In this talk, I will present my work describing the discovery and characterization of the RNA-targeting class 2 CRISPR system: type VI-B. Using a combination of biochemistry and bacterial genetics, we demonstrate that the predicted nuclease of the VI-B system, Cas13b, is an RNA-guided RNase, whose activity can be modulated by the csx genes that often appear in genetic proximity to cas13b.
Additionally, we characterized the behavior of Cas13b and the related enzymes Cas13a and Cas13c in mammalian cells, identifying orthologs of Cas13a and Cas13b with specific RNA interference activity. We also show that catalytically inactive versions of a Cas13b ortholog can direct adenosine to inosine deaminase activity to transcripts in human cells when fused to the catalytic domain of ADAR2. Using structure-guided mutagenesis, we created a high specificity version of this system that can be utilized in research or potentially therapeutic contexts. The description of a Cas13b ortholog that can be used to knockdown or recruit RNA modifying domains to transcripts in mammalian cells suggests the utility of this protein to interrogate and modify transcript function in diverse contexts.
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