An sgRNA-scoring algorithm (CRISPRscan) based on molecular features that enhance activity allows users to predict the most efficient sgRNA for in vivo targets. CRISPR-Cas9 technology provides a powerful system for genome engineering. However, variable activity across different single guide RNAs (sgRNAs) remains a significant limitation. We analyzed the molecular features that influence sgRNA stability, activity and loading into Cas9 in vivo. We observed that guanine enrichment and adenine depletion increased sgRNA stability and activity, whereas differential sgRNA loading, nucleosome positioning and Cas9 off-target binding were not major determinants. We also identified sgRNAs truncated by one or two nucleotides and containing 5′ mismatches as efficient alternatives to canonical sgRNAs. On the basis of these results, we created a predictive sgRNA-scoring algorithm, CRISPRscan, that effectively captures the sequence features affecting the activity of CRISPR-Cas9 in vivo. Finally, we show that targeting Cas9 to the germ line using a Cas9-nanos 3′ UTR led to the generation of maternal-zygotic mutants, as well as increased viability and decreased somatic mutations. These results identify determinants that influence Cas9 activity and provide a framework for the design of highly efficient sgRNAs for genome targeting in vivo.

Cpf1 is a novel class of CRISPR-Cas DNA endonucleases, with a wide range of activity across different eukaryotic systems. Yet, the underlying determinants of this variability are poorly understood. Here, we demonstrate that LbCpf1, but not AsCpf1, ribonucleoprotein complexes allow efficient mutagenesis in zebrafish and Xenopus. We show that temperature modulates Cpf1 activity by controlling its ability to access genomic DNA. This effect is stronger on AsCpf1, explaining its lower efficiency in ectothermic organisms. We capitalize on this property to show that temporal control of the temperature allows post-translational modulation of Cpf1-mediated genome editing. Finally, we determine that LbCpf1 significantly increases homology-directed repair in zebrafish, improving current approaches for targeted DNA integration in the genome. Together, we provide a molecular understanding of Cpf1 activity in vivo and establish Cpf1 as an efficient and inducible genome engineering tool across ectothermic species.

This protocol describes how to generate and genotype mutants using an optimized CRISPR–Cas9 genome-editing system in zebrafish (CRISPRscan). Because single guide RNAs (sgRNAs) have variable efficiency when targeting specific loci, our protocol starts by explaining how to use the web tool CRISPRscan to design highly efficient sgRNAs. The CRISPRscan algorithm is based on the results of an integrated analysis of more than 1000 sgRNAs in zebrafish, which uncovered highly predictive factors that influence Cas9 activity. Next, we describe how to easily generate sgRNAs in vitro, which can then be injected in vivo to target specific loci. The use of highly efficient sgRNAs can lead to biallelic mutations in the injected embryos, causing lethality. We explain how targeting Cas9 to the germline increases viability by reducing somatic mutations. Finally, we combine two methods to identify F₁ heterozygous fish carrying the desired mutations: (i) Mut-Seq, a method based on high-throughput sequencing to detect F₀ founder fish; and (ii) a polymerase chain reaction–based fragment analysis method that identifies F₁ heterozygous fish characterized by Mut-Seq. In summary, this protocol includes the steps to generate and characterize mutant zebrafish lines using the CRISPR–Cas9 genome engineering system.