Project description:CRISPR-Cas is an RNA-based defense system that enables prokaryotes to recognize invading foreign DNA by cognate crRNA guides and destroy it by CRISPR-associated Cas nucleases 1,2 . Elucidation of the interference mechanism of the Streptococcus pyogenes Type II CRISPR- Cas9 system has allowed for the successful repurposing of SpCas9 as a generic genome editing tool, with great promise for human gene therapy 3 . However, especially for therapeutic applications, some caution seems appropriate, because Cas9 systems from some human pathogens may induce a cytotoxic response via an unknown mechanism 4 . Here we show that when released in human cells, Cas9 nucleases from the pathogenic bacteria Campylobacter jejuni and S. pyogenes have the potential to cause severe DNA damage. In the absence of a CRISPR RNA guide, native Cas9 nucleases from both pathogens enter the host nucleus, where their presence leads to promiscuous double stranded DNA breaks (DSBs) and induction of cell death. DSB induction can be reduced to background levels either by saturation of CjCas9 and SpCas9 with crRNA guides or by inactivating their nuclease activity. Our results demonstrate that guide-free Cas9 of bacterial pathogens might play an important role in pathogenicity. Furthermore, we propose that saturating Cas9 with appropriate guide RNAs is crucial for efficient and safe therapeutic applications.
Project description:West Nile virus (WNV) causes an acute neurological infection attended by massive neuronal cell death. However, the mechanism(s) behind the virus-induced cell death is poorly understood. Using a library containing 77,406 sgRNAs targeting 20,121 genes, we performed a genome-wide screen using CRISPR/Cas9. HEK 293FT cells were infected with lentivirus expressing sgRNAs and then transfected with a Cas9 expressing construct. WNV infection killed most cells during a 12d selection. Survivor cells were harvested, from which DNA was isolated. The sgRNAs integrated in genome of survivor cells were amplified with PCR. The PCR product was sequenced with Illumina MiSeq to profile the sgRNA population in the survivor cells. Three replicates were conducted. Similarly, a second round of screen was conducted. Among the genes identified, seven genes, EMC2, EMC3, SEL1L, DERL2, UBE2G2, UBE2J2, and HRD1, stood out as having the strongest phenotype, whose knockout conferred strong protection against WNV-induced cell death with two different WNV strains and in three cell lines. Interestingly, knockout of these genes did not block WNV replication. Thus, these appear to be essential genes that link WNV replication to downstream cell death pathway(s). In addition, the fact that all of these genes belong to the endoplasmic reticulum-associated protein degradation (ERAD) pathway suggests that this might be the primary driver of WNV-induced cell death. Examination of sgRNA populations in survival 293FT cells
Project description:The CRISPR-Cas9 system enables efficient sequence-specific mutagenesis for creating germline mutants of model organisms. Key constraints in vivo remain the expression and delivery of active Cas9-guideRNA ribonucleoprotein complexes (RNPs) with minimal toxicity, variable mutagenesis efficiencies depending on targeting sequence, and high mutation mosaicism. Here, we established in vitro-assembled, fluorescent Cas9-sgRNA RNPs in stabilizing salt solution to achieve maximal mutagenesis efficiency in zebrafish embryos. Sequence analysis of targeted loci in individual embryos reveals highly efficient bi-allelic mutagenesis that reaches saturation at several tested gene loci. Such virtually complete mutagenesis reveals preliminary loss-of-function phenotypes for candidate genes in somatic mutant embryos for subsequent generation of stable germline mutants. We further show efficient targeting of functional non-coding elements in gene-regulatory regions using saturating mutagenesis towards uncovering functional control elements in transgenic reporters and endogenous genes. Our results suggest that in vitro assembled, fluorescent Cas9-sgRNA RNPs provide a rapid reverse-genetics tool for direct and scalable loss-of-function studies beyond zebrafish applications.
Project description:To study target sequence specificity, selectivity, and reaction kinetics of Streptococcus pyogenes Cas9 activity, we challenged libraries of random variant targets with purified Cas9::guide RNA complexes in vitro. Cleavage kinetics were nonlinear, with a burst of initial activity followed by slower sustained cleavage. Consistent with other recent analyses of Cas9 sequence specificity, we observe considerable (albeit incomplete) impairment of cleavage for targets mutated in the PAM sequence or in "seed" sequences matching the proximal 8 bp of the guide. A second target region requiring close homology was located at the other end of the guide::target duplex (positions 13-18 relative to the PAM). Strikingly, a subset of variants which broke homology in the intervening region consistently increased the capacity of Cas9 to cleave in extended reactions. Sequences flanking the guide+PAM region had measurable (albeit modest) effects on cleavage. Taken together, these studies provide both a basis for predicting effective cleavage targets and a basis for potential optimization of guide RNAs to yield efficiency beyond that of the simple perfect-match guides.