Crystal Structure of the Minimal Cas9 from Campylobacter jejuni Reveals the Molecular Diversity in the CRISPR-Cas9 Systems

The RNA-guided endonuclease Cas9 generates a double-strand break at DNA target sites complementary to the guide RNA and has been harnessed for the development of a variety of new technologies, such as genome editing. Here, we report the crystal structures of Campylobacter jejuni Cas9 (CjCas9), one of the smallest Cas9 orthologs, in complex with an sgRNA and its target DNA. The structures provided insights into a minimal Cas9 scaffold and revealed the remarkable mechanistic diversity of the CRISPR-Cas9 systems. The CjCas9 guide RNA contains a triple-helix structure, which is distinct from known RNA triple helices, thereby expanding the natural repertoire of RNA triple helices. Furthermore, unlike the other Cas9 orthologs, CjCas9 contacts the nucleotide sequences in both the target and non-target DNA strands and recognizes the 5′-NNNVRYM-3′ as the protospacer-adjacent motif. Collectively, these findings improve our mechanistic understanding of the CRISPR-Cas9 systems and may facilitate Cas9 engineering. Keywords: CRISPR-Cas system; Cas9; protospacer adjacent motif; RNA triplex; crystal structureUnited States. Department of Energy (Grant DE-FG02-97ER25308)National Institute of Mental Health (U.S.) (Grant 5DP1-MH100706)National Institute of Mental Health (U.S.) (Grant 1R01-MH110049

Discovery and Functional Characterization of Diverse Class 2 CRISPR-Cas Systems

Microbial CRISPR-Cas systems are divided into Class 1, with multisubunit effector complexes, and Class 2, with single protein effectors. Currently, only two Class 2 effectors, Cas9 and Cpf1, are known. We describe here three distinct Class 2 CRISPR-Cas systems. The effectors of two of the identified systems, C2c1 and C2c3, contain RuvC-like endonuclease domains distantly related to Cpf1. The third system, C2c2, contains an effector with two predicted HEPN RNase domains. Whereas production of mature CRISPR RNA (crRNA) by C2c1 depends on tracrRNA, C2c2 crRNA maturation is tracrRNA independent. We found that C2c1 systems can mediate DNA interference in a 5'-PAM-dependent fashion analogous to Cpf1. However, unlike Cpf1, which is a single-RNA-guided nuclease, C2c1 depends on both crRNA and tracrRNA for DNA cleavage. Finally, comparative analysis indicates that Class 2 CRISPR-Cas systems evolved on multiple occasions through recombination of Class 1 adaptation modules with effector proteins acquired from distinct mobile elements.National Institute of Mental Health (U.S.) (Grant 5DP1-MH100706)National Institute of Mental Health (U.S.) (Grant 1R01-MH110049)National Institute of Diabetes and Digestive and Kidney Diseases (U.S.) (Grant 5R01DK097768-03)National Institutes of Health (U.S.) (Grant GM10407

Nucleic acid detection with CRISPR-Cas13a/C2c2

Rapid, inexpensive, and sensitive nucleic acid detection may aid point-of-care pathogen detection, genotyping, and disease monitoring. The RNA-guided, RNA-targeting clustered regularly interspaced short palindromic repeats (CRISPR) effector Cas13a (previously known as C2c2) exhibits a "collateral effect" of promiscuous ribonuclease activity upon target recognition. We combine the collateral effect of Cas13a with isothermal amplification to establish a CRISPR-based diagnostic (CRISPR-Dx), providing rapid DNA or RNA detection with attomolar sensitivity and single-base mismatch specificity. We use this Cas13a-based molecular detection platform, termed Specific High-Sensitivity Enzymatic Reporter UnLOCKing (SHERLOCK), to detect specific strains of Zika and Dengue virus, distinguish pathogenic bacteria, genotype human DNA, and identify mutations in cell-free tumor DNA. Furthermore, SHERLOCK reaction reagents can be lyophilized for cold-chain independence and long-term storage and be readily reconstituted on paper for field applications.United States. Air Force Office of Scientific Research (Grant FA9550-14-1-0060)Defense Threat Reduction Agency (DTRA) (Grant HDTRA1-14-1-0006)National Institute of Mental Health (U.S.) (Grant 5DP1-MH100706)National Institutes of Health (U.S.) (Grant 1R01-MH110049

RNA targeting with CRISPR–Cas13

RNA has important and diverse roles in biology, but molecular tools to manipulate and measure it are limited. For example, RNA interference1-3 can efficiently knockdown RNAs, but it is prone to off-target effects4, and visualizing RNAs typically relies on the introduction of exogenous tags5. Here we demonstrate that the class 2 type VI6,7 RNA-guided RNA-targeting CRISPR-Cas effector Cas13a8(previously known as C2c2) can be engineered for mammalian cell RNA knockdown and binding. After initial screening of 15 orthologues, we identified Cas13a from Leptotrichia wadei (LwaCas13a) as the most effective in an interference assay in Escherichia coli. LwaCas13a can be heterologously expressed in mammalian and plant cells for targeted knockdown of either reporter or endogenous transcripts with comparable levels of knockdown as RNA interference and improved specificity. Catalytically inactive LwaCas13a maintains targeted RNA binding activity, which we leveraged for programmable tracking of transcripts in live cells. Our results establish CRISPR-Cas13a as a flexible platform for studying RNA in mammalian cells and therapeutic development.National Institute of Mental Health (U.S.) (Grant 5DP1-MH100706)National Institute of Mental Health (U.S.) (Grant 1R01-MH110049

Structure and Engineering of Francisella novicida Cas9

Summary The RNA-guided endonuclease Cas9 cleaves double-stranded DNA targets complementary to the guide RNA and has been applied to programmable genome editing. Cas9-mediated cleavage requires a protospacer adjacent motif (PAM) juxtaposed with the DNA target sequence, thus constricting the range of targetable sites. Here, we report the 1.7 Å resolution crystal structures of Cas9 from Francisella novicida (FnCas9), one of the largest Cas9 orthologs, in complex with a guide RNA and its PAM-containing DNA targets. A structural comparison of FnCas9 with other Cas9 orthologs revealed striking conserved and divergent features among distantly related CRISPR-Cas9 systems. We found that FnCas9 recognizes the 5′-NGG-3′ PAM, and used the structural information to create a variant that can recognize the more relaxed 5′-YG-3′ PAM. Furthermore, we demonstrated that the FnCas9-ribonucleoprotein complex can be microinjected into mouse zygotes to edit endogenous sites with the 5′-YG-3′ PAM, thus expanding the target space of the CRISPR-Cas9 toolbox

In vivo genome editing using Staphylococcus aureus Cas9

The RNA-guided endonuclease Cas9 has emerged as a versatile genome-editing platform. However, the size of the commonly used Cas9 from Streptococcus pyogenes (SpCas9) limits its utility for basic research and therapeutic applications that employ the highly versatile adeno-associated virus (AAV) delivery vehicle. Here, we characterize six smaller Cas9 orthologs and show that Cas9 from Staphylococcus aureus (SaCas9) can edit the genome with efficiencies similar to those of SpCas9, while being >1kb shorter. We packaged SaCas9 and its sgRNA expression cassette into a single AAV vector and targeted the cholesterol regulatory gene Pcsk9 in the mouse liver. Within one week of injection, we observed >40% gene modification, accompanied by significant reductions in serum Pcsk9 and total cholesterol levels. We further demonstrate the power of using BLESS to assess the genome-wide targeting specificity of SaCas9 and SpCas9, and show that SaCas9 can mediate genome editing in vivo with high specificity

Nucleic Acid Detection of Plant Genes Using CRISPR-Cas13

Nucleic acid detection is vital for agricultural applications including trait detection during breeding, pest surveillance, and pathogen identification. Here, we use a modified version of the CRISPR-based nucleic acid detection platform SHERLOCK to quantify levels of a glyphosate resistance gene in a mixture of soybeans and to detect multiple plant genes in a single reaction. SHERLOCK is rapid (∼15 min), quantitative, and portable, and can process crude soybean extracts as input material for minimal nucleic acid sample preparation. This field-ready SHERLOCK platform with color-based lateral flow readout can be applied for detection and quantitation of genes in a range of agricultural applications

A Survey of Genome Editing Activity for 16 Cas12a Orthologs

© 2019 by The Keio Journal of Medicine. The class 2 CRISPR-Cas endonuclease Cas12a (previously known as Cpf1) offers several advantages over Cas9, including the ability to process its own array and the requirement for just a single RNA guide. These attributes make Cas12a promising for many genome engineering applications. To further expand the suite of Cas12a tools available, we tested 16 Cas12a orthologs for activity in eukaryotic cells. Four of these new enzymes demonstrated targeted activity, one of which, from Moraxella bovoculi AAX11_00205 (Mb3Cas12a), exhibited robust indel formation. We also showed that Mb3Cas12a displays some tolerance for a shortened PAM (TTN versus the canonical Cas12a PAM TTTV). The addition of these enzymes to the genome editing toolbox will further expand the utility of this powerful technology.National Institutes of Health (Grants 1R01-HG009761, 1R01-MH110049 and 1DP1-HL141201

Barcode extension for analysis and reconstruction of structures

Collections of DNA sequences can be rationally designed to self-assemble into predictable three-dimensional structures. The geometric and functional diversity of DNA nanostructures created to date has been enhanced by improvements in DNA synthesis and computational design. However, existing methods for structure characterization typically image the final product or laboriously determine the presence of individual, labelled strands using gel electrophoresis. Here we introduce a new method of structure characterization that uses barcode extension and next-generation DNA sequencing to quantitatively measure the incorporation of every strand into a DNA nanostructure. By quantifying the relative abundances of distinct DNA species in product and monomer bands, we can study the influence of geometry and sequence on assembly. We have tested our method using 2D and 3D DNA brick and DNA origami structures. Our method is general and should be extensible to a wide variety of DNA nanostructures