731. High-Fidelity CRISPR-Cas9 Nucleases with No Detectable Genome-Wide Off-Target Effects

Abstract

CRISPR-Cas9 nucleases are widely used for genome editing but can induce unwanted off-target mutations at genomic locations that resemble the intended target. These so-called off-target effects can confound research applications and are important considerations for potential therapeutic use. Existing strategies for reducing genome-wide off-targets of the broadly used Streptococcus pyogenes Cas9 (SpCas9) have thus far proven to be imperfect by possessing only partial efficacy and/or other limitations that constrain their use. Here we describe a high-fidelity variant of SpCas9, called SpCas9-HF1, that contains alterations in the amino acid sequence designed to reduce non-specific contacts to the target strand DNA. SpCas9-HF1 retains on-target activities comparable to wild-type SpCas9 with >85% of the 37 single-guide RNAs (sgRNAs) tested in human cells. Strikingly, with eight different sgRNAs targeted to standard non-repetitive sequences in human cells, SpCas9-HF1 rendered all or nearly all off-target events imperceptible by genome-wide break capture and targeted sequencing methods. Even for atypical, repetitive target sites, the vast majority of off-targets induced by SpCas9-HF1 were not detected. With its exceptional precision, SpCas9-HF1 provides an important and easily employed alternative to wild-type SpCas9 that can eliminate off-target effects when using CRISPR-Cas9 for research and therapeutic applications. Our findings also suggest a general strategy for improving or optimizing the genome-wide specificities of other Cas9 orthologues and engineered variants.