Genome engineering using the CRISPR-Cas9 system.

Schematic of the RNA-guided Cas9 nuclease. The Cas9 nuclease from S. pyogenes (in yellow) is targeted to genomic DNA (shown for example is the human EMX1 locus) by an sgRNA consisting of a 20-nt guide sequence (blue) and a scaffold (red). The guide sequence pairs with the DNA target (blue bar on top strand), directly upstream of a requisite 5′-NGG adjacent motif (PAM; pink). Cas9 mediates a DSB ~3 bp upstream of the PAM (red triangle).

DSB repair promotes gene editing. DSBs induced by Cas9 (yellow) can be repaired in one of two ways. In the error-prone NHEJ pathway, the ends of a DSB are processed by endogenous DNA repair machinery and rejoined, which can result in random indel mutations at the site of junction. Indel mutations occurring within the coding region of a gene can result in frameshifts and the creation of a premature stop codon, resulting in gene knockout. Alternatively, a repair template in the form of a plasmid or ssODN can be supplied to leverage the HDR pathway, which allows high fidelity and precise editing. Single-stranded nicks to the DNA can also induce HDR.

Timeline and overview of experiments. Steps for reagent design, construction, validation and cell line expansion are depicted. Custom sgRNAs (light blue bars) for each target, as well as genotyping primers, are designed in silico via the CRISPR Design Tool (http://tools.genome-engineering.org). sgRNA guide sequences can be cloned into an expression plasmid bearing both sgRNA scaffold backbone (BB) and Cas9, pSpCas9(BB). The resulting plasmid is annotated as pSpCas9(sgRNA). Completed and sequence-verified pSpCas9(sgRNA) plasmids and optional repair templates for facilitating HDR are then transfected into cells and assayed for their ability to mediate targeted cleavage. Finally, transfected cells can be clonally expanded to derive isogenic cell lines with defined mutations.

Target selection and reagent preparation. (a) For S. pyogenes Cas9, 20-bp targets (highlighted in blue) must be followed at their 3′ends by 5′-NGG, which can occur in either the top or the bottom strand of genomic DNA, as in the example from the human EMX1 gene. We recommend using the CRISPR Design Tool (http://tools.genome-engineering.org) to facilitate target selection. (b) Schematic for co-transfection of the Cas9 expression plasmid (pSpCas9) and a PCR-amplified U6-driven sgRNA expression cassette. By using a U6 promoter-containing PCR template and a fixed forward primer (U6-Fwd), sgRNA-encoding DNA can be appended onto the U6 reverse primer (U6-Rev) and synthesized as an extended DNA oligo (Ultramer oligos from IDT). Note that the guide sequence in the U6-Rev primer, designed against an example target from the top strand (blue), is the reverse complement of the 20-bp target sequence preceding the 5′-NGG PAM. An additional cytosine (‘C’ in gray rectangle) is appended in the reverse primer directly 3′ to the target sequence to allow guanine as the first base of the U6 transcript. (c) Schematic for scarless cloning of the guide sequence oligos into a plasmid containing Cas9 and the sgRNA scaffold (pSpCas9(BB)). The guide oligos for the top strand example (blue) contain overhangs for ligation into the pair of BbsI sites in pSpCas9(BB), with the top and bottom strand orientations matching those of the genomic target (i.e., the top oligo is the 20-bp sequence preceding 5′-NGG in genomic DNA). Digestion of pSpCas9(BB) with BbsI allows the replacement of the Type II restriction sites (blue outline) with direct insertion of annealed oligos. Likewise, a G-C base pair (gray rectangle) is added at the 5′ end of the guide sequence for U6 transcription, which does not adversely affect targeting efficiency. Alternate versions of pSpCas9(BB) also contain markers such as GFP or a puromycin resistance gene to aid the selection of transfected cells.

Anticipated results for multiplex-sgRNA-targeted NHEJ. (a) Schematic of the SURVEYOR assay used to determine the indel percentage. First, genomic DNA from the heterogeneous population of Cas9-targeted cells is amplified by PCR. Amplicons are then reannealed slowly to generate heteroduplexes. The reannealed heteroduplexes are cleaved by SURVEYOR nuclease, whereas homoduplexes are left intact. Cas9-mediated cleavage efficiency (percentage indel) is calculated on the basis of the fraction of cleaved DNA, as determined by integrated intensity of gel bands. (b) Two sgRNAs (orange and dark blue bars) are designed to target the human GRIN2B and DYRK1A loci. SURVEYOR gel shows modification at both loci in transfected cells. Colored arrowheads indicate expected fragment sizes for each locus. (c) Paired sgRNAs (light blue and green bars) are designed to excise an exon (dark blue) in the human EMX1 locus. Target sequences and PAMs (pink) are shown in respective colors, and sites of cleavage by Cas9 are indicated by red triangles. A predicted junction is shown below. Individual clones isolated from cell populations transfected with sgRNA 3, 4 or both are assayed by PCR (using the Out-Fwd and Out-Rev primers), reflecting a deletion of ~270 bp long. Representative clones with no modification (12/23), mono-allelic modification (10/23) and bi-allelic (1/23) modification are shown. (d) Quantification of clonal lines with EMX1 exon deletions. Two pairs of sgRNAs (3.1 and 3.2, left-flanking sgRNAs; 4.1 and 4.2, right flanking sgRNAs) are used to mediate deletions of various sizes around one EMX1 exon. Transfected cells are clonally isolated and expanded for genotyping analysis of deletions and inversion events. Of the 105 clones screened, 51 (49%) and 12 (11%) are carrying heterozygous and homozygous deletions, respectively. Only approximate deletion sizes are given, as deletion junctions may be variable.

Anticipated results for HDR in HEK and HUES9 cells. (a) Either a targeting plasmid or an ssODN (sense or antisense) with homology arms can be used to edit the sequence at a target genomic locus cleaved by Cas9 (red triangle). To assay the efficiency of HDR, we introduced a HindIII site (red bar) into the target locus, which was PCR-amplified with primers that anneal outside of the region of homology. Digestion of the PCR product with HindIII reveals the occurrence of HDR events. (b) ssODNs, oriented in either the sense or the antisense (s or a) direction relative to the locus of interest, can be used in combination with Cas9 to achieve efficient HDR-mediated editing at the target locus. A minimal homology region of 40 bp, and preferably 90 bp, is recommended on either side of the modification (red bar). (c) Example of the effect of ssODNs on HDR in the EMX1 locus is shown using both wild-type Cas9 and Cas9 nickase (D10A). Each ssODN contains homology arms of 90 bp flanking a 12-bp insertion of two restriction sites.