The Streptococcus pyogenes Cas9 (SpCas9) nuclease can be efficiently targeted to genomic loci by means of single-guide RNAs (sgRNAs) to enable genome editing1,2,3,4,5,6,7,8,9,10. Here, we characterize SpCas9 targeting specificity in human cells to inform the selection of target sites and avoid off-target effects. Our study evaluates >700 guide RNA variants and SpCas9-induced indel mutation levels at >100 predicted genomic off-target loci in 293T and 293FT cells. We find that SpCas9 tolerates mismatches between guide RNA and target DNA at different positions in a sequence-dependent manner, sensitive to the number, position and distribution of mismatches. We also show that SpCas9-mediated cleavage is unaffected by DNA methylation and that the dosage of SpCas9 and sgRNA can be titrated to minimize off-target modification. To facilitate mammalian genome engineering applications, we provide a web-based software tool to guide the selection and validation of target sequences as well as off-target analyses.
We thank A. Shalek, E. Stamenova and D. Gray for expert help with DNA sequencing, R. Barretto for genome-wide PAM analysis, as well as D. Altshuler, P.A. Sharp, and the entire Zhang Lab for their support and advice. P.D.H. is a James Mills Pierce Fellow. D.A.S. is an National Science Foundation pre-doctoral fellow and J.A.W. is supported by a Life Science Research Foundation Fellowship. X.W. is a Howard Hughes Medical Institute International Student Research Fellow and is supported by National Institutes of Health (NIH) grants R01-GM34277 and R01-CA133404 to P.A. Sharp, X.W.'s thesis advisor. G.B. is supported by an NIH Nanomedicine Development Center Award (PN2EY018244).This work is supported by an NIH Director's Pioneer Award (DP1-MH100706), an NIH Transformative R01 grant (R01-DK097768) to D. Altshuler, the Keck, McKnight, Damon Runyon, Searle Scholars, Klingenstein and Simons Foundations, and Bob Metcalfe and Jane Pauley. The authors wish to dedicate this paper to the memory of Officer Sean Collier, for his caring service to the MIT community and for his sacrifice. Reagents are available to the academic community through Addgene, and associated protocols, support forums and computational tools are available through the Zhang lab website (http://www.genome-engineering.org/).