Targeting Noncoding RNA Domains to Genomic Loci with CRISPR-Display: Guidelines for Designing, Building, and Testing sgRNA-ncRNA Expression Constructs

CRISPR-Display uses the S. pyogenes Cas9 protein to posttranscriptionally localize noncoding RNA (ncRNA) domains to any genomic site, by directly fusing these domains to the Cas9 sgRNA cofactor. This versatile technology enables numerous applications for interrogating natural chromatin-regulatory ncRNAs, or for utilizing artificial ncRNA and ribonucleoprotein (RNP) devices at individual chromatin loci. To achieve these, a successful CRISPR-Display experiment requires that chimeric sgRNA-ncRNA fusions are stably expressed and incorporated into Cas9 complexes, and that they retain their ncRNA "cargo" domains at the targeted genomic sites. Here, I describe a workflow for designing, building, and testing such chimeric sgRNA-ncRNA expression constructs. I detail strategies for choosing expression systems and sgRNA insertion topologies, for assaying the incorporation of sgRNA-ncRNA fusions into functional Cas9 complexes, and for surveying the activities of ncRNA domains at targeted genomic loci. This establishes an initial set of "best practices" for the design and implementation of CRISPR-Display experiments.

Crossing enhanced and high fidelity SpCas9 nucleases to optimize specificity and cleavage

Background

The propensity for off-target activity of Streptococcus pyogenes Cas9 (SpCas9) has been considerably decreased by rationally engineered variants with increased fidelity (eSpCas9; SpCas9-HF1). However, a subset of targets still generate considerable off-target effects. To deal specifically with these targets, we generated new “Highly enhanced Fidelity” nuclease variants (HeFSpCas9s) containing mutations from both eSpCas9 and SpCas9-HF1 and examined these improved nuclease variants side by side to decipher the factors that affect their specificities and to determine the optimal nuclease for applications sensitive to off-target effects.

Results

These three increased-fidelity nucleases can routinely be used only with perfectly matching 20-nucleotide-long spacers, a matching 5′ G extension being more detrimental to their activities than a mismatching one. HeFSpCas9 exhibit substantially improved specificity for those targets for which eSpCas9 and SpCas9-HF1 have higher off-target propensity. The targets can also be ranked by their cleavability and off-target effects manifested by the increased fidelity nucleases. Furthermore, we show that the mutations in these variants may diminish the cleavage, but not the DNA-binding, of SpCas9s.

Conclusions

No single nuclease variant shows generally superior fidelity; instead, for highest specificity cleavage, each target needs to be matched with an appropriate high-fidelity nuclease. We provide here a framework for generating new nuclease variants for targets that currently have no matching optimal nuclease, and offer a simple means for identifying the optimal nuclease for targets in the absence of accurate target-ranking prediction tools.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1318-8) contains supplementary material, which is available to authorized users.