Designing CRISPR Knockout Guides in Benchling | Tutorial

CRISPR/Cas9 plasmid construction can be simplified using oligo annealing cloning with the GoldenGate method, which achieves near 100% cloning efficiency without gel purification; this technique involves designing synthetic guide RNAs with 20-nucleotide protospacers followed by a PAM sequence (NGG), ordering complementary oligos with specific adapters (ATTG and AAAC) that match restriction enzyme overhangs, annealing them in ligase buffer, and performing thermocycling with restriction enzymes to insert the guide RNA into the vector.

CRISPR/Cas9 gene editing plasmids can be constructed with high efficiency (near 100%) using oligo annealing cloning, which involves designing 20-base pair guide RNA spacers with NGG PAM sequences, ordering complementary oligos with BBS1/BSA1 restriction site adapters, annealing them in ligase buffer, and performing a thermocycling reaction with the appropriate restriction enzyme to insert the gRNA into the vector without gel purification.

To design effective gRNA sequences for CRISPR-Cas9 knockout experiments, first identify NGG or CCN PAM sequences in the target gene's coding region, then select 20 bases upstream (for NGG) or downstream (for CCN) of the PAM while avoiding introns and UTRs; use online tools like CHOPCHOP to evaluate candidates for off-target sites, GC content (30-80%), and secondary structures, then clone the selected gRNA into a Cas9 expression plasmid using BBSI restriction sites by designing complementary oligos with CACC and AAC overhangs that anneal to form a duplex for ligation.

Guide RNA cloning involves designing sequences with specific adapter regions for vector integration. The spacer sequence (20 nucleotides) matches the target DNA, followed by a PAM sequence (NGG for Cas9). The guide RNA is cloned into vectors like pX330 using the BbsI restriction enzyme, which creates incompatible ends preventing self-ligation. A critical detail is that the first base of the spacer should be G for optimal transcription by RNA polymerase III. If the spacer does not start with G, researchers should add a G at the 5' end. The annealed guide RNA product is then ligated into the vector and transformed into bacteria for plasmid amplification.

CRISPR-based gene editing requires guide RNA synthesis and cloning into appropriate expression vectors. Guide RNA design services identify optimal gRNA sequences for target sites. Vectors from the Jinek lab at Broad Institute are available, along with services to synthesize and clone guide RNAs into expression-ready constructs. This enables targeted genomic integration for stable expression without random viral integration risks.

This section presents a breakthrough strategy for achieving reliable gene knockouts through multiplexed guide RNA approaches. Combining two or three guide RNAs targeting closely spaced sites in early exons generates larger fragment deletions (50+ bp) that reliably cause gene knockouts, dramatically increasing efficiency compared to single guides. Surprisingly, distant spacing (not overlapping) produces optimal cooperative effects. This approach eliminates the need to screen multiple guides per gene and rescues genes that fail with single guides. The section culminates in describing the development of an arrayed whole human genome knockout library targeting ~20,000 genes using this multiplexed approach, achieving 84-100% knockout efficiency validated by western blotting. This resource enables functional genomics screening across virtually any cell type and phenotype.

CRISPR technology has evolved beyond basic genome editing to enable advanced applications. Modified Cas9 variants retain targeting ability without nuclease activity, enabling fusion with fluorescent proteins for multiplexed imaging of multiple genomic loci simultaneously. Dual-guide systems create staggered cuts for enhanced specificity and directed repair. These innovations expand CRISPR utility from simple gene disruption to precise spatial and temporal control of genomic loci, enabling visualization of chromatin dynamics and measurement of distances between genomic regions in living cells.

The Surveyor assay detects CRISPR/Cas9 activity by identifying DNA mutations. Genomic DNA is amplified with fluorescent primers spanning the target region. Control samples with unmodified DNA show single bands. Edited samples show multiple bands due to Surveyor enzyme cuts at mismatch sites. For large deletions, two guide RNAs target sequences flanking the region to be deleted. Cas9 cuts at both sites, and the intervening DNA is lost when ends rejoin. Gel electrophoresis shows wild-type alleles as bands at expected sizes, while edited alleles show smaller bands for deletions. Heterozygous editing shows both wild-type and edited bands, while homozygous editing shows only edited bands. Plasmids can be designed with multiple U6 promoters to express multiple guide RNAs simultaneously.

Long oligonucleotides (up to 300 nucleotides) enable sophisticated CRISPR experiments beyond single-guide approaches. Dual/triple guide libraries encode multiple sgRNAs on single oligos, enabling simultaneous expression in single cells for precise deletion of genetic elements like long non-coding RNAs, increased loss-of-function rates for redundant genes, and enhanced activation/inhibition effects. Prime editing combines modified Cas9 with reverse transcriptase to perform precise genome modifications without double-strand breaks, using pegRNAs that encode both target sequences and desired edits. This technology requires high-quality long oligonucleotides to ensure accurate mutation incorporation. These advances enable systematic probing of gene pairs for synthetic lethality screening, identifying cancer-specific therapeutic targets by exploiting tumor-specific mutations.

Next Generation CRISPR Technologies, including base editing (CBE and ABE systems) and prime editing, offer safer and more precise alternatives to traditional CRISPR-Cas9 by avoiding double-stranded breaks; base editors use nuclease-defective Cas9 fused with deaminases to directly convert cytosines to uridines (CBE) or adenines to inosines (ABE), while prime editing employs a Cas9 nickase fused with reverse transcriptase to enable all 12 possible base substitutions, insertions up to 44 bp, and deletions up to 80 bp without requiring donor DNA.

This section covers advanced CRISPR technologies. Base editors are CRISPR-based systems that make precise point mutations without creating double-strand breaks. They consist of Cas9 nickase fused to cytidine or adenine deaminase enzymes. The system recognizes the target sequence and deaminates the target base (C to U or A to G), which is then converted to T or G during DNA replication. This allows correction of point mutations but has limitations: can only edit specific base pairs, and efficiency depends on local sequence context. Prime editors are next-generation systems that can make any type of point mutation, insertions, and deletions. They consist of Cas9 nickase fused to reverse transcriptase and a specially designed guide RNA with a primer binding site. The system creates a single-strand break and uses the guide RNA as a template to synthesize the desired edit. Prime editors can insert up to 44 base pairs and make any type of point mutation, overcoming many limitations of base editors and traditional CRISPR systems.

Base editing enables direct conversion of individual nucleotides without double-strand breaks by fusing Cas9 to deaminase enzymes (APOBEC or ADAR), converting C→U or A→G. Prime editing combines Cas9 with reverse transcriptase to enable precise insertions, deletions, and substitutions using prime editing guide RNAs that encode both target sequences and desired edits. CRISPR can also deliver epigenetic modifiers—dCas9 fused to VP64 activators, p300 histone acetyltransferases, or DNA methyltransferases—to regulate gene expression at specific promoters or enhancers without altering DNA sequence.

Prime editing is a revolutionary genome editing technology that overcomes the limitations of base editing and traditional CRISPR-Cas9 by using a three-component system consisting of nicking Cas9 (nCas9), pegRNA (Prime Editing guide RNA), and reverse transcriptase; unlike base editing which is limited to specific base transitions, prime editing can achieve any base change including purine-purine and pyrimidine-pyrimidine interchanges, as well as insertions and deletions, through a mechanism where nCas9 creates a single-strand nick, the pegRNA's primer binding site hybridizes with the exposed DNA strand, and reverse transcriptase synthesizes the new genetic information using the pegRNA template, followed by complex processing steps involving flap formation and mismatch repair to permanently incorporate the edits into the genome.

CRISPR base editors enable precise single-nucleotide modifications (C→T or A→G) without generating double-strand breaks by fusing Cas9 with cytosine or adenosine deaminases, while Prime editors extend this capability to insertions, deletions, and complex mutations up to 40 base pairs by combining Cas9 with reverse transcriptase; both systems offer advantages over traditional HDR methods by eliminating donor DNA requirements and reducing off-target effects, making them promising tools for treating rare genetic diseases affecting approximately 7,000 conditions.