Background Zebrafish have practical features that make them a useful model for higher-throughput tests of gene function using CRISPR/Cas9 editing to create ‘knockout’ models. In particular, the use of G0 mosaic mutants has potential to increase throughput of functional studies significantly but may suffer from transient effects of introducing Cas9 via microinjection. Further, a large number of computational and empirical tools exist to design CRISPR assays but often produce varied predictions across methods leaving uncertainty in choosing an optimal approach for zebrafish studies. Methods To systematically assess accuracy of tool predictions of on- and off-target gene editing, we subjected zebrafish embryos to CRISPR/Cas9 with 50 different guide RNAs (gRNAs) targeting 14 genes. We also investigate potential confounders of G0-based CRISPR screens by assaying control embryos for spurious mutations and altered gene expression. Results We compared our experimental in vivo editing efficiencies in mosaic G0 embryos with those predicted by eight commonly used gRNA design tools and found large discrepancies between methods. Assessing off-target mutations (predicted in silico and in vitro) found that the majority of tested loci had low in vivo frequencies (< 1%). To characterize if commonly used ‘mock’ CRISPR controls (larvae injected with Cas9 enzyme or mRNA with no gRNA) exhibited spurious molecular features that might exacerbate studies of G0 mosaic CRISPR knockout fish, we generated an RNA-seq dataset of various control larvae at 5 days post fertilization. While we found no evidence of spontaneous somatic mutations of injected larvae, we did identify several hundred differentially-expressed genes with high variability between injection types. Network analyses of shared differentially-expressed genes in the ‘mock’ injected larvae implicated a number of key regulators of common metabolic pathways, and gene-ontology analysis revealed connections with response to wounding and cytoskeleton organization, highlighting a potentially lasting effect from the microinjection process that requires further investigation. Conclusion Overall, our results provide a valuable resource for the zebrafish community for the design and execution of CRISPR/Cas9 experiments.

Zebrafish have practical features that make them a useful model for higher-throughput tests of gene function using CRISPR/Cas9 editing to create ‘knockout’ models. Due to the large number of available tools to design CRISPR assays and diversity of theories/model systems they were originally built on, we sought to systematically compare computational and empirical approaches for predicting gene-editing efficacy in zebrafish. We subjected zebrafish embryos to CRISPR/Cas9 with 50 different guide RNAs (gRNAs) targeting 14 genes and assayed individual editing efficiencies. We compared our experimental in vivo efficiencies in mosaic G0 embryos with those predicted by seven commonly used gRNA design tools and found large discrepancies between methods. Assessing off-target mutations (predicted in silico and in vitro) found that the majority of tested loci had low in vivo frequencies (<1%). Moreover, understanding that recent segmental duplications in the zebrafish genome could exacerbate CRISPR targeting of individual genes, we cataloged these loci and have made them available as a resource. Lastly, we assessed the transcriptome of negative ‘mock’ control CRISPR larvae injected with Cas9 enzyme or mRNA with no gRNA using RNA-seq and identified differentially expressed genes with high variability between injections. Using these same data, we discovered on average ~60 putative somatic mosaic frameshift mutations impacting genes per pool of injected larvae, potentially due to background cutting of DNA with Cas9 in the absence of gRNA. To verify this previously unreported phenomenon in zebrafish, we validated seven of twelve genes tested carrying low frequency mosaic somatic mutations in the genomes of a separate batch of injected larvae. These results suggest that negative control embryos may carry mutations within genes leading to spurious phenotypes. Overall, our results provide a valuable resource for the zebrafish community for the design and execution of CRISPR/Cas9 experiments.

In recent years zebrafish have become commonly used as a model for studying human traits and disorders. Their small size, high fecundity, and rapid development allow for more high-throughput experiments compared to other vertebrate models. Given that zebrafish share >70% gene homologs with humans and their genomes can be readily edited using highly efficient CRISPR methods, we are now able to rapidly generate mutations impacting practically any gene of interest. Unfortunately, our ability to phenotype mutant larvae has not kept pace. To address this challenge, we have developed a protocol that obtains multiple phenotypic measurements from individual zebrafish larvae in an automated and parallel fashion, including morphological features (i.e., body length, eye area, and head size) and movement/behavior. By assaying wild-type zebrafish in a variety of conditions, we determined optimal parameters that avoid significant developmental defects or physical damage; these include morphological imaging of larvae at two time points (3 days post fertilization (dpf) and 5 dpf) coupled with motion tracking of behavior at 5 dpf. As a proof-of-principle, we tested our approach on two novel CRISPR-generated mutant zebrafish lines carrying predicted null-alleles of syngap1b and slc7a5, orthologs to two human genes implicated in autism-spectrum disorder, intellectual disability, and epilepsy. Using our optimized high-throughput phenotyping protocol, we recapitulated previously published results from mouse and zebrafish models of these candidate genes. In summary, we describe a rapid parallel pipeline to characterize morphological and behavioral features of individual larvae in a robust and consistent fashion, thereby improving our ability to better identify genes important in human traits and disorders.AUTHOR SUMMARYZebrafish (Danio rerio) are a well-established model organism for the study of neurodevelopmental disorders. Due to their small size, fast reproduction, and genetic homology with humans, zebrafish have been widely used for characterizing and screening candidate genes for many disorders, including autism-spectrum disorder, intellectual disability, and epilepsy. Although several studies have described the use of high-throughput morphological and behavioral assays, few combine multiple assays in a single zebrafish larva. Here, we optimized a platform to characterize morphometric features at two developmental time points in addition to behavioral traits of zebrafish larvae. We then used this approach to characterize two autism candidate genes (SYNGAP1 and SLC7A5) in two CRISPR-generated zebrafish null mutant models we developed in house. These data recapitulate previously published results related to enhanced seizure activity, while identifying additional defects not previously reported. We propose that our phenotyping platform represents a feasible method for maximizing the use of single zebrafish larvae in the characterization of additional mutants relevant to neurodevelopmental disorders.