Genome Editing in Mammalian Cell Lines using CRISPR-Cas

Summary

CRISPR-Cas is a powerful technology to engineer the complex genomes of plants and animals. Here, we detail a protocol to efficiently edit the human genome using different Cas endonucleases. We highlight important considerations and design parameters to optimize editing efficiency.

Abstract

The clustered regularly interspaced short palindromic repeats (CRISPR) system functions naturally in bacterial adaptive immunity, but has been successfully repurposed for genome engineering in many different living organisms. Most commonly, the wildtype CRISPR associated 9 (Cas9) or Cas12a endonuclease is used to cleave specific sites in the genome, after which the DNA double-stranded break is repaired via the non-homologous end joining (NHEJ) pathway or the homology-directed repair (HDR) pathway depending on whether a donor template is absent or present respectively. To date, CRISPR systems from different bacterial species have been shown to be capable of performing genome editing in mammalian cells. However, despite the apparent simplicity of the technology, multiple design parameters need to be considered, which often leave users perplexed about how best to carry out their genome editing experiments. Here, we describe a complete workflow from experimental design to identification of cell clones that carry desired DNA modifications, with the goal of facilitating successful execution of genome editing experiments in mammalian cell lines. We highlight key considerations for users to take note of, including the choice of CRISPR system, the spacer length, and the design of a single-stranded oligodeoxynucleotide (ssODN) donor template. We envision that this workflow will be useful for gene knockout studies, disease modeling efforts, or the generation of reporter cell lines.

Introduction

The ability to engineer the genome of any living organism has many biomedical and biotechnological applications, such as the correction of disease-causing mutations, construction of accurate cellular models for disease studies, or generation of agricultural crops with desirable traits. Since the turn of the century, various technologies have been developed for genome engineering in mammalian cells, including meganucleases^1,2,3, zinc finger nucleases^4,5, or transcription activator-like effector nucleases (TALENs)

Protocol

1. Design of sgRNAs

1. Select an appropriate CRISPR-Cas system.
   1. First, inspect the target region for the PAM sequences of all Cas9 and Cas12a nucleases that have been shown to be functional in mammalian cells^16,21-32. Five frequently used enzymes are given in Table 1 together with their respective PAMs.
      NOTE: Besides the endonucleases listed in Table 1, there are other less commonly used Cas enzymes that have been successfully deployed in mammalian cells as well, such as a Cas9 nuclease from Streptococcus thermophilus (St1Cas9) that recognizes the PAM NNAGAAW. If the desired target site d....

Results

To perform a genome editing experiment, a CRISPR plasmid expressing a sgRNA targeting the locus-of-interest needs to be cloned. First, the plasmid is digested with a restriction enzyme (typically a type IIs enzyme) to linearize it. It is recommended to resolve the digested product on a 1% agarose gel alongside an undigested plasmid to distinguish between a complete and partial digestion. As undigested plasmids are supercoiled, they tend to run faster than their linearized counterparts (see Figure 7

Discussion

The CRISPR-Cas system is a powerful, revolutionary technology to engineer the genomes and transcriptomes of plants and animals. Numerous bacterial species have been found to contain CRISPR-Cas systems, which may potentially be adapted for genome and transcriptome engineering purposes⁴⁴. Although the Cas9 endonuclease from Streptococcus pyogenes (SpCas9) was the first enzyme to be deployed successfully in human cells^21,22,

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
T4 Polynucleotide Kinase (PNK)	NEB	M0201
Shrimp Alkaline Phosphatase (rSAP)	NEB	M0371
Tris-Acetate-EDTA (TAE) Buffer, 50X	1st Base	BUF-3000-50X4L	Dilute to 1X before use. The 1X solution contains 40 mM Tris, 20 mM acetic acid, and 1 mM EDTA.
Tris-EDTA (TE) Buffer, 10X	1st Base	BUF-3020-10X4L	Dilute to 1X before use. The 1X solution contains 10 mM Tris (pH 8.0) and 1 mM EDTA.
BbsI	NEB	R0539
BsmBI	NEB	R0580
T4 DNA Ligase	NEB	M0202	400,000 units/ml
Quick Ligation Kit	NEB	M2200	An alternative to T4 DNA Ligase.
Rapid DNA Ligation Kit	Thermo Scientific	K1423	An alternative to T4 DNA Ligase.
Zero Blunt TOPO PCR Cloning Kit	Thermo Scientific	451245	The salt solution comes with the TOPO vector in the kit.
NEBuilder HiFi DNA Assembly Master Mix	NEB	E2621L	Kit for Gibson assembly.
One Shot Stbl3 Chemically Competent E.Coli	Thermo Scientific	C737303
LB Broth (Lennox), powder	Sigma Aldrich	L3022	Reconstitute in ddH20, and autoclave before use
LB Broth with Agar (Lennox), powder	Sigma Aldrich	L2897	Reconstitute in ddH20, and autoclave before use
SOC media	-	-	2.5 mM KCl, 10 mM MgCl2, 20 mM glucose in 1 L of LB Broth
Ampicillin (Sodium), USP Grade	Gold Biotechnology	A-301
REDiant 2X PCR Mastermix	1st Base	BIO-5185
Agarose	1st Base	BIO-1000
T7 Endonuclease I	NEB	M0302
Plasmid DNA Extraction Miniprep Kit	Favorgen	FAPDE 300
Dulbecco's Modified Eagle Medium (DMEM), High Glucose	Hyclone	SH30081.01	4.5 g/L Glucose, no L-glutamine, HEPES and Sodium Pyruvate
L-Glutamine, 200mM	Gibco	25030
Penicillin-Streptomycin, 10, 000U/mL	Gibco	15140
0.25% Trypsin-EDTA, 1X	Gibco	25200
Fetal Bovine Serum	Hyclone	SV30160	FBS is heat inactivated before use at 56 oC for 30 min
Phosphate Buffered Saline, 1X	Gibco	20012
jetPRIME transfection reagent	Polyplus Transfection	114-75
QuickExtract DNA Extraction Solution, 1.0	Epicentre	LUCG-QE09050
ISOLATE II Genomic DNA Kit	Bioline	BIO-52067	An alternative to QuickExtract
Q5 High-Fidelity DNA Polymerase	NEB	M0491
Deoxynucleotide (dNTP) Solution Mix	NEB	N0447
6X DNA Loading Dye	Thermo Scientific	R0611	10 mM Tris-HCl (pH 7.6) 0.03% bromophenol blue, 0.03% xylene cyanol FF, 60% glycerol, 60 mM EDTA
Protease Inhibitor Cocktail, Set3	Merck	539134
Nitrocellulose membrane, 0.2µm	Bio-Rad	1620112
Tris-glycine-SDS buffer, 10X	Bio-Rad	1610772	Dilute to 1X before use. The 1x solution contains 25 mM Tris, 192 mM glycine, and 0.1% SDS.
Tris-glycine buffer, 10X	1st base	BUF-2020	Dilute to 1X before use. The 1x solution contains 25 mM Tris and 192 mM glycine.
Ponceau S solution	Sigma Aldrich	P7170
TBS, 20X	1st base	BUF-3030	Dilute to 1X before use. The 1x solution contains 25 mM Tris-HCl (pH 7.5) and 150 mM NaCl.
Tween 20	Sigma Aldrich	P9416
Skim Milk for immunoassay	Nacalai Tesque	31149-75
WesternBright Sirius-femtogram HRP	Advansta	K12043
Antibody for β-actin (C4)	Santa Cruz Biotechnology	sc-47778	Lot number: C0916
MiSeq system	Illumina	SY-410-1003
NanoDrop spectrophotometer	Thermo Scientific	ND-2000
Qubit fluorometer	Thermo Scientific	Q33226
EVOS FL Cell Imaging System	Thermo Scientific	AMF4300
CRISPR plasmid: pSpCas9(BB)-2A-GFP (PX458)	Addgene	48138	Single vector system: The gRNA is expressed from the same plasmid.
CRISPR plasmid: pX601-AAV-CMV::NLS-SaCas9-NLS-3xHA-bGHpA	Addgene	61591	Single vector system: The gRNA is expressed from the same plasmid.
CRISPR plasmid: xCas9 3.7	Addgene	108379	Dual vector system: The gRNA is expressed from a different plasmid.
CRISPR plasmid: pX330-U6-Chimeric_BB-CBh-hSpCas9	Addgene	42230	Single vector system: The gRNA is expressed from the same plasmid.
CRISPR plasmid: hCas9	Addgene	41815	Dual vector system: The gRNA is expressed from a different plasmid.
CRISPR plasmid: eSpCas9(1.1)	Addgene	71814	Single vector system: The gRNA is expressed from the same plasmid.
CRISPR plasmid: VP12 (SpCas9-HF1)	Addgene	72247	Dual vector system: The gRNA is expressed from a different plasmid.