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Genome Editing: Cas9 Stable Cell Lines for CRISPR sgRNA Validation, Library Screening, and More

  Ed Davis, Ph.D.

Introduction

The CRISPR-Cas9 system has become greatly popular for genome editing in recent years, due to its ease-of-design, efficiency, specificity, and relatively low cost (Wang, et al., 2016). In mammalian cell culture systems, most genome editing is achieved using transient transfection or lentiviral transduction, which works well for routine, low-throughput applications. However, for other applications, it would be beneficial to have a system in which one component, namely the CRISPR-Cas9 nuclease, was stably integrated into the genome. In this Technical Note, we introduce GeneCopoeia’s suite of Cas9 stable cell lines, and discuss the great utility that these cell lines provide for genome editing applications.

The CRISPR-Cas9 system for genome editing has been adapted from anti-viral immunity mechanisms present in many bacterial species. The most commonly used CRISPR-Cas9 system to date is from Streptococcus pyogenes (SpCas9). SpCas9 is a DNA endonuclease that, in the presence of a chimeric single guide RNA (sgRNA), makes a double strand break (DSB) in chromosomal DNA. Creation of a DSB is the necessary initiation event for most CRISPR-mediated genome editing applications

Why use a Cas9-expressing stable cell line?

In cell line-based genome editing, Cas9 and sgRNA are typically introduced into cells lacking stably integrated Cas9 using transient transfection of either DNA, RNA, or (for Cas9) protein. Frequencies of DSB formation, as measured in insertion/deletion (indel) transfection assays, can exceed 60% (Fu, et al., 2013). Therefore, transfection of cell lines that do not have Cas9 previously integrated is usually sufficient for gene knockouts resulting from nonhomologous end joining (NHEJ), and for applications requiring donor-mediated, homology-directed repair (HDR), including gene knockout, gene mutagenesis, gene tagging, and Safe Harbor transgene knock-in.

There are additional applications that, while it is not required, are enhanced by having Cas9 stably integrated into the genome: 1) sgRNA functional validation; 2) gene knockout using lentiviral CRISPR; 3) sgRNA library screening; and 4) inducible genome editing. Each of these applications is discussed below.

Application 1: sgRNA functional validation

Cell line-based CRISPR genome editing projects typically require several (>3) months of work, including transfection, clone isolation, and screening. While CRISPR typically provides a highly efficient method for genome editing, the individual sgRNAs themselves do exhibit variations in efficiency due to the nature of individual target sites. Therefore, it would be helpful to determine which CRISPR sgRNAs have the highest potential for successful genome editing before doing lots of screening work.

We recommend the mismatch cleavage assay (Qiu, et al., 2004) for sgRNA functional validation in a Cas9 stable cell line. GeneCopoeia’s IndelCheck™ mismatch cleavage assay (http://www.genecopoeia.com/product/indelcheck-detection-system/) is a cell culture-based assay designed to efficiently detect indels resulting from DSB-mediated NHEJ. The basic workflow for the IndelCheck™ mismatch cleavage assay for sgRNA functional validation is illustrated in Figure 1.

 

Figure 1. Workflow of the IndelCheck™ mismatch cleavage assay. A population of cells that has been exposed to CRISPR components is subjected to genomic DNA PCR using primers flanking the target site. Denaturation and reannealing of the PCR products creates a mixed population of homoduplex and heteroduplex molecules. The fragments are treated with T7 Endonuclease I, which cuts only the heteroduplex molecules. Cleavage products are detected by standard agarose gel electrophoresis.

 

Functional validation of CRISPR sgRNAs can be performed either in a commonly used, “model” cell line (e.g. HEK293), or in your own cell line. GeneCopoeia has several pre-made human and mouse Cas9-expressing stable cell lines in widely-used backgrounds (Table 1). Alternatively, you can purchase CRISPR reagents from us that allow you to readily build your own Cas9-expressing stable cell line.

Application 2: Gene knockout using lentiviral CRISPR

While transfection is a commonly used method for cell line-based genome editing, some cell lines do not transfect well. For these cell lines, researchers often turn to lentiviral-mediated CRISPR delivery, because lentiviruses efficiently infect a very large number of mammalian cell types. It is possible to express both Cas9 nuclease and an sgRNA from the same lentivirus, as has been shown previously (Sanjana, et al), or to co-transduce two different lentiviruses, one expressing Cas9 and the other expressing sgRNA. However, the titres of such “all in one” or Cas9-only lentiviral CRISPR viruses tend to be significantly lower than that achieved from sgRNA-only lentiviral vectors due to the large (>4.4 kb) size of the Cas9 ORF. It is for this reason that GeneCopoeia recommends, for genome editing using lentiviral CRISPR applications, that scientists transduce a Cas9-expressing stable cell line with sgRNA-only lentiviruses.

Application 3: sgRNA library screening

Another application for Cas9-expressing stable cell lines is sgRNA library screening. CRISPR sgRNA libraries are collections of hundreds-to-thousands of plasmids, each expressing a unique sgRNA, and are highly useful for forward mutagenesis screens (Sanjana, et al, 2014). GeneCopoeia carries pre-made CRISPR sgRNA libraries that cover several well-studied pathways and gene groups, and also offers custom sgRNA library construction (http://www.genecopoeia.com/product/sgrna_libraries/).

CRISPR sgRNA libraries enable researchers to knock out many genes simultaneously in mammalian cells, opening the door to drug target identification and validation, phenotypic changes, and reporter assays. Previously, these applications were achieved using RNA interference (RNAi). However, while shRNA libraries are useful for high-throughput loss-of-function screens, RNAi has a number of disadvantages compared with CRISPR: 1) RNAi causes knockdown of gene expression, not knockout, so false negatives resulting from residual gene expression can be missed; 2) RNAi acts only on cytoplasmic RNA, and so cannot silence nuclear RNAs like long non-coding RNAs. Conversely, CRISPR makes permanent changes to the genetic code. Thus, CRISPR can make a complete knockout of all alleles of a gene, whether their transcription products are localized to the nucleus or cytoplasm. So, CRISPR sgRNA library screening can be straightforward, provided that you have a good assay or phenotype for screening (Figure 2).

 

 

Figure 2. Workflow for CRISPR sgRNA libraries. A. Pooled screen. Cells infected with each sgRNA library pool are screened for the desired readout. Pooled cells are subjected to Sanger sequencing for individual sgRNAs, or deep sequencing to look for over- or under-representation of individual sgRNAs. B. GeneCopoeia sgRNA libraries are available either as pools or individual sgRNAs. Cells are infected with individual sgRNA lentiviruses. Wells are screened for the readout of interest. Individual sgRNAs corresponding to the phenotype of interest are already known without sequencing.

 

sgRNA libraries are best suited for lentiviral delivery rather than plasmid transfection, because it provides higher delivery efficiency. However, because the titer of Cas9-expressing lentiviruses is lower than sgRNA-expressing lentiviruses, GeneCopoeia sgRNA libraries are built without Cas9. We recommend using our sgRNA libraries in Cas9-expressing stable cell lines.

Application 4: Inducible genome editing

CRISPR is commonly used to generate genome modifications in a constitutive fashion, without regard to a specific time or stage in cell growth. Sometimes, though, it is useful to introduce the modification after a particular event has occurred, such as treatment of cells with a compound, or at a specific time during differentiation. Inducible CRISPR (iCRISPR) was first demonstrated by Gonzalez, et al. (2014), who built an induced pluripotent stem cell (iPSC) model cell line carrying Cas9 that can be expressed after addition of the tetracycline analog doxycycline.

GeneCopoeia Cas9 stable cell lines

GeneCopoeia carries a suite of human and mouse cell lines stably expressing the Cas9 nuclease (Table 1; http://www.genecopoeia.com/product/cas9-cell-line/).

Buy Catalog # Cell line Description Cell type Selection marker Price, Academic Price, Industry
Human Cas9-expressing stable cell lines
SL502 HEK293T Human cell line HEK293T stably expressing Cas9 from AAVS1 site Embryonic kidney Puro $945 $1995
SL501 NCI-H1299 Human cell line NCI-H1299 stably expressing Cas9 from AAVS1 site Lung cancer Puro $1395 $2695
SL533 NCI-H1299 Human cell line NCI-H1299 stably expressing Cas9 from random site Lung cancer Hygro $1395 $2695
SL504 A549 Human cell line A549 stably expressing Cas9 from AAVS1 site Lung cancer Hygro $1395 $2695
SL529 H1975 Human cell line H1975 stably expressing Cas9 from random site Lung cancer Hygro $1395 $2695
SL536 NCI-H1437 Human cell line NCI-H1437 stably expressing Cas9 from random site Lung cancer Hygro $1395 $2695
SL503 HeLa Human cell line HeLa stably expressing Cas9 from AAVS1 site Cervical cancer Hygro $1395 $2695
SL514 MCF-7 Human cell line MCF-7 stably expressing Cas9 from AAVS1 site Breast cancer Hygro $1395 $2695
SL524 MCF-7 Human cell line MCF-7 stably expressing Cas9 from random site Breast cancer Hygro $1395 $2695
SL515 MDA-MB-231 Human cell line MDA-MB-231 stably expressing Cas9 from random site Breast cancer Hygro $1395 $2695
SL516 MDA-MB-468 Human cell line MDA-MB-468 stably expressing Cas9 from random site Breast cancer Hygro $1395 $2695
SL517 T47D Human cell line T47D stably expressing Cas9 from random site Breast cancer Hygro $1395 $2695
SL526 SNU-1 Human cell line SNU-1 stably expressing Cas9 from random site Breast cancer Hygro $1395 $2695
SL535 SK-BR-3 Human cell line SK-BR-3 stably expressing Cas9 from random site Breast cancer Hygro $1395 $2695
SL544 HCC70 Human cell line HCC70 stably expressing Cas9 from random site Breast cancer Hygro $1395 $2695
SL547 DU4475 Human cell line DU4475 stably expressing Cas9 from random site Breast cancer Hygro $1395 $2695
SL518 HepG2 Human cell line HepG2 stably expressing Cas9 from AAVS1 site Liver cancer Hygro $1395 $2695
SL522 SNU-475 Human cell line SNU-475 stably expressing Cas9 from random site Liver cancer Hygro $1395 $2695
SL530 SNU-449 Human cell line SNU-449 stably expressing Cas9 from random site Liver cancer Hygro $1395 $2695
SL531 PLC/PRF/5 Human cell line PLC/PRF/5 stably expressing Cas9 from random site Liver cancer Hygro $1395 $2695
SL537 SNU-387 Human cell line SNU-387 stably expressing Cas9 from random site Liver cancer Hygro $1395 $2695
SL538 SNU-423 Human cell line SNU-423 stably expressing Cas9 from random site Liver cancer Hygro $1395 $2695
SL543 C3A Human cell line C3A stably expressing Cas9 from random site Liver cancer Hygro $1395 $2695
SL520 AGS Human cell line AGS stably expressing Cas9 from random site Stomach/gastric cancer Hygro $1395 $2695
SL527 KATO111 Human cell line KATO111 stably expressing Cas9 from random site Stomach/gastric cancer Hygro $1395 $2695
SL545 SNU-16 Human cell line SNU-16 stably expressing Cas9 from random site Stomach/gastric cancer Hygro $1395 $2695
SL521 BXPC-3 Human cell line BXPC-3 stably expressing Cas9 from random site Pancreatic cancer Hygro $1395 $2695
SL540 CFPAC-1 Human cell line CFPAC-1 stably expressing Cas9 from random site Pancreatic cancer (derived from metastatic liver) Hygro $1395 $2695
SL523 HT-29 Human cell line HT-29 stably expressing Cas9 from random site Colon cancer Hygro $1395 $2695
SL525 LoVo Human cell line LoVo stably expressing Cas9 from random site Colon cancer Hygro $1395 $2695
SL539 RKO Human cell line RKO stably expressing Cas9 from random site Colon cancer Hygro $1395 $2695
SL541 T84 Human cell line T84 stably expressing Cas9 from random site Colon cancer Hygro $1395 $2695
SL542 COLO 205 Human cell line COLO 205 stably expressing Cas9 from random site Colon cancer (derived frm metastatic site) Hygro $1395 $2695
SL532 LS411N Human cell line LS411N stably expressing Cas9 from random site Caecum cancer Hygro $1395 $2695
SL546 U-2 OS Human cell line U-2 OS stably expressing Cas9 from random site Bone cancer Hygro $1395 $2695
SL552 K562 Human cell line K562 stably expressing Cas9 from random site Bone marrow cancer Hygro $1395 $2695
SL555 Jurkat Human cell line Jurkat stably expressing Cas9 from random site Peripheral blood cancer Hygro $1395 $2695
SL550 SH-SY5Y Human cell line SH-SY5Y stably expressing Cas9 from random site Neuroblastoma
 
Hygro $1395 $2695
Human stable cell lines expressing high-fidelity Cas9 (SpCas9-HF1)
  SL553 HEK293 Human cell line HEK293 stably expressing SpCas9-HF1 from AAVS1 site Embryonic kidney Hygro Coming soon!
Mouse Cas9-expressing stable cell lines
SL509 Neuro2a Mouse cell line Neuro2a stably expressing Cas9 from ROSA26 site Neuroblastoma Hygro $1395 $2695
SL510 Neuro2a Mouse cell line Neuro2a stably expressing Cas9 from ROSA26 site Neuroblastoma Puro $1395 $2695
SL511 Neuro2a Mouse cell line Neuro2a stably expressing Cas9 from ROSA26 site Neuroblastoma Neo $1395 $2695
Rat Cas9-expressing stable cell lines
SL534 C6 Rat cell line C6 stably expressing Cas9 from random site Glioma Hygro $1395 $2695

  MSDS

 

Note: GeneCopoeia also offers custom-built Cas9-expressing stable cell line services. To inquire, please contact inquiry@genecopoeia.com.

Table 1. List of GeneCopoeia pre-made human and mouse Cas9 stable cell lines.

These cell lines provide several benefits and advantages for genome editing work, including:

  • Built-in, stable Cas9 expression. Minimizes need for co-transfection/transduction of sgRNAs. Ideal for sgRNA library screening, lentiviral CRISPR, sgRNA validation, and inducible CRISPR.
  • Single clone isolation.  GeneCopoeia’s Cas9 stable cell lines are derived from single cell clones, providing a uniform genetic background, unlike stable pools, which can vary in transgene expression and carry unwanted background mutations.
  • Functionally validated. The function of Cs9 is validated for cleavage ability using the IndelCheck™ T7 Endonuclease I assay, assuring high quality.
  • Compatibility. Compatible with GeneCopoeia Genome-CRISP™ sgRNA clones, sgRNA libraries, and donor clones.

 

Figure 3. Structure of a stably integrated CRISPR Cas9 nuclease construct. Hygromycin (Hygro) selection is used for isolation of single clones. CBh: Hybrid CBA promoter. 3X FLAG: N-terminal 3X FLAG tag. NLS: nuclear localization signal. bGH pA: Bovine growth hormone polyadenylation signal. EF1a: Elongation factor 1a promoter. copGFP: Copepod green fluorescent protein. T2A: Element to allow translation of copGFP and Puro independently from the same transcript. SV40 pA: Simian virus 40 polyadenylation signal.

 

If you don’t see your cell line of interest in Table 1, you can use either GeneCopoeia’s Cas9 lentiviral clones or GeneCopoeia’s Safe Harbor knock-in system to integrate the Cas9 gene yourself. GeneCopoeia’s Safe Harbor knock-in system consists of kits containing CRISPR reagents targeting the human AAVS1 and mouse ROSA26 Safe Harbor sites, and can be ordered with many donors for HDR, such as those expressing Cas9 used for the cell lines in Table 1, or any of more than 45,000 human and mouse ORFs. For more information about GeneCopoeia’s Safe Harbor kits and knock-in ORF clones, visit http://www.genecopoeia.com/product/safe-harbor/.

At GeneCopoeia, our Genome Editing team has a wealth of expertise with CRISPR applications in mammalian systems. We start at CRISPR design and deliver sequence-verified plasmid DNA. We also offer functional validation kits and services for your CRISPR constructs, construct stable cell lines or transgenic mice containing your CRISPR-mediated modification of interest, and provide scientific consulting services to help you plan your projects. For more information, visit our website: http://www.genecopoeia.com, call 1-866-360-9531, or email inquiry@genecopoeia.com

 

References

Fu, et al. (2013). High-frequency off-target mutagenesis induced by CRISPR-Cas nucleases in human cells. Nature Biotechnology 31, 822.

Gonzalez, et al. (2014). An iCRISPR Platform for Rapid, Multiplexable, and Inducible Genome Editing in Human Pluripotent Stem Cells. Cell Stem Cell 15, 215.Qiu, et al. (2004). Mutation detection using Surveyor™ nuclease. Biotechniques 36, 702.

Sanjana, et al. (2014). Improved vectors and genome-wide libraries for CRISPR screening. Nature Methods 11, 783.

Wang, et al. (2016). CRISPR/Cas9 in Genome Editing and Beyond. Ann. Rev. Biochemistry 85, 227.

 

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