CRISPR/Cas systems in genome editing: methodologies and tools for sgRNA design, off-target evaluation and strategies to mitigate off-target effects

Life sciences have been revolutionized by genome editing (GE) tools, including zinc finger nucleases, transcription activator‐Like effector nucleases, and CRISPR (clustered regulatory interspaced short palindromic repeats)/Cas (CRISPR‐associated) systems, which make the targeted modification of genomic DNA of all organisms possible. CRISPR/Cas systems are being widely used because of their accuracy, efficiency, and cost‐effectiveness. Various classes of CRISPR/Cas systems have been developed, but their extensive use may be hindered by off‐target effects. Efforts are being made to reduce the off‐target effects of CRISPR/Cas9 by generating various CRISPR/Cas systems with high fidelity and accuracy. Several approaches have been applied to detect and evaluate the off‐target effects. Here, the current GE tools, the off‐target effects generated by GE technology, types of off‐target effects, mechanisms of off‐target effects, major concerns, and outcomes of off‐target effects in plants and animals are summarized. The methods to detect off‐target effects, tools for single‐guide RNA (sgRNA) design, evaluation and prediction of off‐target effects, and strategies to increase the on‐target efficiency and mitigate the off‐target impact on intended genome‐editing outcomes are summarized