CRISPR/Cas9 has revolutionized our ability to engineer genomes and conduct genome-wide screens in human cells. Whereas some cell types are amenable to genome engineering, genomes of human pluripotent stem cells (hPSCs) have been difficult to engineer, with reduced efficiencies relative to tumour cell lines or mouse embryonic stem cells. Here, using hPSC lines with stable integration of Cas9 or transient delivery of Cas9-ribonucleoproteins (RNPs), we achieved an average insertion or deletion (indel) efficiency greater than 80%. This high efficiency of indel generation revealed that double-strand breaks (DSBs) induced by Cas9 are toxic and kill most hPSCs. In previous studies, the toxicity of Cas9 in hPSCs was less apparent because of low transfection efficiency and subsequently low DSB induction. The toxic response to DSBs was P53/TP53-dependent, such that the efficiency of precise genome engineering in hPSCs with a wild-type P53 gene was severely reduced. Our results indicate that Cas9 toxicity creates an obstacle to the high-throughput use of CRISPR/Cas9 for genome engineering and screening in hPSCs. Moreover, as hPSCs can acquire P53 mutations, cell replacement therapies using CRISPR/Cas9-enginereed hPSCs should proceed with caution, and such engineered hPSCs should be monitored for P53 function.

The ability to engineer genomes in a specific, systematic, and costeffective way is critical for functional genomic studies. Recent advances using the CRISPR-associated single-guide RNA system (Cas9/sgRNA) illustrate the potential of this simple system for genome engineering in a number of organisms. Here we report an effective and inexpensive method for genome DNA editing in Drosophila melanogaster whereby plasmid DNAs encoding short sgRNAs under the control of the U6b promoter are injected into transgenic flies in which Cas9 is specifically expressed in the germ line via the nanos promoter. We evaluate the off-targets associated with the method and establish a Web-based resource, along with a searchable, genome-wide database of predicted sgRNAs appropriate for genome engineering in flies. Finally, we discuss the advantages of our method in comparison with other recently published approaches.nanos-Cas9 | HRMA M uch of our knowledge of the mechanisms underlying biological processes relies on genetic approaches, whereby gene activity is perturbed and the phenotypic consequences of perturbation are analyzed in detail. In recent years, several major advances have been made in the design of methods for specifically and efficiently perturbing genomes. Arguably, the most exciting advances rely on the ability to induce double-strand breaks (DSBs) by targeting a nuclease to a specific genomic sequence. Repair of DSBs by the error-prone nonhomologous endjoining (NHEJ) mechanism allows for the recovery of small deletions; moreover, repair of DSBs by homologous recombination (HR) in the presence of a donor template opens the door to a wide range of specifically engineered changes at the targeted site (1).Two nuclease-based systems, the zinc-finger nuclease (ZFN) and transcription activator-like effector nuclease (TALEN) systems, work effectively in a number of organisms (2-7). But because these approaches require the production of a construct encoding a unique DNA-binding protein fused to the nuclease domain, they can be both cumbersome and costly. In contrast, the recent approach based on the bacterial CRISPR-associated single-guide RNA (Cas9/sgRNA) system does not require production of specific fusion proteins for each targeted sequence (8-10).Cas9 was first identified in type II Streptococcus pyogenes as an RNA-guided defense system against invading viruses and plasmids (11-13). This adaptive immune-like system contains three components: CRISPR RNA (crRNA), trans-activating CRISPR RNA (tracrRNA), and Cas9. The tracrRNA triggers Cas9 nuclease activity and the crRNA guides Cas9 to cleave the specific foreign dsDNA sequence via base-pairing between the crRNA and the target DNA. Importantly, a single-guide RNA (sgRNA, also known as chiRNA), comprising the minimal crRNA and tracrRNA, can function similarly to the crRNA and tracrRNA, thereby providing a simplified method for genome editing (8)(9)(10)(14)(15)(16)(17)(18)(19)(20).Given the great promise of the Cas9/sgRNA method for genome engineering, we set out to test the sys...

Summary The CRISPR/Cas9 system has recently emerged as a powerful tool for functional genomic studies in Drosophila melanogaster. However, sgRNA parameters affecting the specificity and efficiency of the system in flies are still not clear. Here, we found that off-target effects did not occur in regions of genomic DNA with three or more nucleotide mismatches to sgRNAs. Importantly, we document for the first time a strong positive correlation between mutagenesis efficiency and sgRNA GC content of the six protospacer adjacent motif-proximal nucleotides (PAMPNs). Furthermore, by injecting well-designed sgRNA plasmids at the optimal concentration we determined, we could efficiently generate mutations in four genes in one step. Finally, we generated null alleles of HP1a using optimized parameters through homology-directed repair, and achieved an overall mutagenesis rate significantly higher than previously reported. Our work presents the most comprehensive optimization of sgRNA and promises to vastly simplify CRISPR/Cas9 experiments in Drosophila.

Biliary epithelial cells (BECs) form bile ducts in the liver and are facultative liver stem cells that establish a ductular reaction (DR) to support liver regeneration following injury. Liver damage induces periportal LGR5+ putative liver stem cells that can form BEClike organoids, suggesting that RSPO-LGR4/5-mediated WNT/b-catenin activity is important for a DR. We addressed the roles of this and other signaling pathways in a DR by performing a focused CRISPRbased loss-of-function screen in BEC-like organoids, followed by in vivo validation and single-cell RNA sequencing. We found that BECs lack and do not require LGR4/5-mediated WNT/b-catenin signaling during a DR, whereas YAP and mTORC1 signaling are required for this process. Upregulation of AXIN2 and LGR5 is required in hepatocytes to enable their regenerative capacity in response to injury. Together, these data highlight heterogeneity within the BEC pool, delineate signaling pathways involved in a DR, and clarify the identity and roles of injury-induced periportal LGR5+ cells.