Massively parallel Cas13 screens reveal principles for guide RNA design

Type VI CRISPR enzymes are RNA-targeting proteins with nuclease activity that enable specific and robust target gene knock-down without altering the genome. To define rules for the design of Cas13d guide RNAs, we conducted massively-parallel screens targeting mRNAs of a green fluorescent protein (GFP) transgene and CD46, CD55 and CD71 cell surface proteins in human cells. In total, we measured the activity of 24,460 guide RNAs with and without mismatches relative to the target sequences. Knock-down efficacy is driven by guide RNA-specific features and target site context. Single mismatches generally reduce knock-down to a modest degree, but spacer nucleotides 15 – 21 are largely intolerant to target site mismatches. We developed a computational model to identify optimal guide RNAs and confirm its generalizability testing 3,979 guides targeting mRNAs of 48 endogenous genes. We show that Cas13 can be used in forward transcriptomic pooled screens and, using our model, predict optimized Cas13 guide RNAs for all protein-coding transcripts in the human genome.