CRISPR/Cas9-Mediated Highly Efficient Gene Targeting in Embryonic Stem Cells for Developing Gene-Manipulated Mouse Models

Summary

Here we present a protocol for developing genetically modified mouse models using embryonic stem cells, especially for large DNA knock-in (KI). This protocol is tuned up using CRISPR/Cas9 genome editing, resulting in significantly improved KI efficiency compared with the conventional homologous recombination-mediated linearized DNA targeting method.

Abstract

The CRISPR/Cas9 system has made it possible to develop genetically modified mice by direct genome editing using fertilized zygotes. However, although the efficiency in developing gene-knockout mice by inducing small indel mutation would be sufficient enough, the efficiency of embryo genome editing for making large-size DNA knock-in (KI) is still low. Therefore, in contrast to the direct KI method in embryos, gene targeting using embryonic stem cells (ESCs) followed by embryo injection to develop chimera mice still has several advantages (e.g., high throughput targeting in vitro, multi-allele manipulation, and Cre and flox gene manipulation can be carried out in a short period). In addition, strains with difficult-to-handle embryos in vitro, such as BALB/c, can also be used for ESC targeting. This protocol describes the optimized method for large-size DNA (several kb) KI in ESCs by applying CRISPR/Cas9-mediated genome editing followed by chimera mice production to develop gene-manipulated mouse models.

Introduction

Producing genetically modified mice and analyzing their phenotype enables us to understand specific gene functions in detail, in vivo. Numerous important findings have been uncovered using gene-modified animal models in the life science field. Furthermore, since the report of genome editing technology using CRISPR/Cas9¹, research using genetically modified mice has quickly spread to many laboratories^2,3. Genome editing of mouse zygotes by CRISPR/Cas9 has achieved acceptable efficiency for developing short DNA modification, such as indel mutation-oriented gene knockout⁴, single nucleotide replacement, or short peptide-tag insertion using single-stranded oligonucleotides (ssODNs) as knock-in (KI) donors⁵. On the other hand, the KI of large DNA fragments into zygotes by genome editing remains at a low efficiency compared to the short-size DNA modification^6,7. In addition, it is difficult to use mouse strains such as BALB/c, which is an important strain for specific research areas like immunology, for zygote-based genome editing because their preimplantation embryos are susceptible to in vitro manipulation.

Another way to develop genetically modified mouse models is to use the embryonic stem cell (ESC) targeting technique followed by ESC injection into the preimplantation embryo to produce chimeras^8,9,10, which is still routinely used as a conventional method. Although the acquisition rate for obtaining accurate KI-ESC clones is not very high in conventional ESC targeting methods, ESC targeting offers some advantages compared to zygote genome editing, especially for long DNA KI. For example, the KI efficiency of long DNA fragments (> several kb) into the zygote genome is less evident^6,7, and many zygotes are needed to develop even one line of KI mouse, which is undesirable in the current perspective of animal experiments. In contrast to zygote genome editing, long DNA targeting to ESCs followed by chimera production needs significantly fewer embryos than zygote genome editing. Furthermore, even though the preimplantation embryos from BALB/c are susceptible to in vitro manipulation, their ESCs can be maintained and handled in vitro¹¹ as other competent 129 or F1 background ESCs, therefore, applicable for chimera productions. However, even though a targeting vector contains 5' and 3' homologous arms and drug resistance gene cassettes for positive or negative selection, the conventional KI efficiency of ESCs is generally insufficient, because of the high frequency of random genomic integration^8,10, Thus, an improved method with precise ESC targeting efficiency is required. Recently, we reported a tuned-up ESC KI method using CRISPR/Cas9-based genome editing to achieve higher KI efficiency than conventional targeting methods¹¹. The method we describe here is based on this procedure which enables long DNA (> several to 10 kb) KI to ESCs with acceptable efficiency for routine works without drug selection; thus, the vector construction procedure would be much easier and need a shorter period, or the cell culture period would also become significantly shorter.

Protocol

All mouse experiments were approved by the Institutional Animal Care and Use Committee of the University of Tokyo (approval number PA17-63) and Osaka University (approval number Biken-AP-H30-01) and performed according to their guidelines as well as the ARRIVE guidelines (https://arriveguidelines.org).

1. Targeting vector construction

1. Amplify a KI cassette (CreERT here, template DNA for the PCR is originally from a commercial gene synthesis company) and an approximate 1 kb fragment of 5'- or 3'-homology arms by PCR. For DNA cloning (step 1.3), add 15-mer overlap sequences at the 5' end of each PCR primer. Purify the PCR DNA fragments by agarose gel electrophoresis, followed by extraction of the DNA fragment using a DNA purification kit.
2. Linearize the backbone plasmid (e.g., pUC19 or pBS) using an appropriate restriction enzyme(s) that uniquely digests somewhere in the multi-cloning site for cloning. Then, purify the digested plasmid using a DNA purification kit.
3. Clone each PCR fragment (e.g., 5'-homology arm, 3'-homology arm) and KI sequence simultaneously into the digested plasmid using a DNA cloning kit according to the manufacturer's instructions.
4. Transform the competent cells using the constructed plasmid (step 1.3) by heating at 42 °C for 1 min in a water bath, then seed them on an LB plate containing the appropriate concentration of antibiotics such as ampicillin or kanamycin. Culture them overnight for resistant clone selections.
5. Pick up several (four to eight) individual colonies using 200 µL pipette tips and transfer the tips into 3 mL of liquid LB containing appropriate antibiotics (100 µg/mL ampicillin or 20 µL/mL kanamycin) and culture overnight at 37 °C with shaking.
6. Purify the plasmid the following day using an endotoxin-free grade commercial plasmid purification kit according to the manufacturer's instructions. Confirm the cloned sequence in the plasmid by Sanger sequencing. Adjust the plasmid concentration at 1 µg DNA/µL using nuclease-free water. Use the plasmid as the gene targeting vector.

2. Preparation of the mouse embryonic fibroblast (MEF) as feeder cells for ESC

1. Sacrifice 8-10-week-old pregnant ICR female mice (14.5 days postcoitum) by cervical dislocation. Wipe the abdomen well by using 70% (v/v) ethanol for sterilization, cut the abdomen using orbital scissors and forceps, and recover the fetus-containing uterus.
2. Place the uterus into a 100 mm dish containing 10 mL of phosphate bovine serum (PBS) containing 100 U/mL penicillin and 100 µg/mL streptomycin. Remove the placentas and extraembryonic tissues from the fetuses using orbital scissors and forceps. Mince the fetuses using orbital scissors and transfer the PBS solution containing the minced fetal cells to a 50 mL conical tube.
3. Centrifuge at 280 x g for 5 min at 4 °C and discard the supernatant by aspiration. Add 10 mL of 0.25% (w/v) trypsin-EDTA solution, mix well by pipetting, and then incubate for 10 min at 37 °C using a water bath for trypsin digestion.
4. Add 20 mL of MEF medium (DMEM containing 10% (v/v) fetal bovine serum, 100 U/mL penicillin, and 100 µg/mL streptomycin) to stop the enzymatic reaction, mix well by gentle inversion, and centrifuge at 280 x g for 5 min at 4 °C. Discard the supernatant by aspiration, add 10 mL of MEF medium, and mix well by pipetting.
5. Seed the cell suspension in several new 100 mm dishes (prepare the same number of dishes as the number of fetuses with 10 mL of MEF medium for one 100 mm dish). Change the medium 4 to 5 h post-seeding to remove unattached cells and let the attached MEF grow. Passage the cells when they reach ~80% confluency using trypsin.
6. Place the culture dishes containing the proliferating MEF in MEF medium, approximately 12-15 days post-seeding, in an X-ray irradiating device. Expose the MEF with a 50 Gy X-ray (total) to halt the cell cycle according to the manufacturer's instruction.
7. Trypsinize the irradiated MEF by adding 2 mL of 0.25% trypsin-EDTA for one 100 mm dish for 5 min at 37 °C, then add 4 mL of MEF medium to stop trypsin digestion and collect the cell suspension in a new 50 mL tube.
8. Wash the MEF with a fresh MEF medium by centrifugation at 280 x g for 5 min at 4 °C, count the number of cells using a cell counter with trypan blue staining, and freeze them at 1.6 x 10⁶ cells/tube in a cell-freezing medium. Store until use; cells can be stably stored in liquid nitrogen for several years.

3. Cas9-RNP-DNA mixture preparation

1. Dissolve tracrRNA and crisprRNA in dilution buffer (each RNA concentration at 200 µM) by pipetting. Mix the tracrRNA solution, crisprRNA solution, and dilution buffer (volume ratio at 2:2:5) by gentle tapping and anneal each RNA using a thermal cycler at 95 °C for 10 min followed by 1 °C/min stepdown cycles until the temperature reaches 4 °C.
   NOTE: The gRNA sequence was designed using CRISPOR, http://crispor.tefor.net.
2. Incubate the annealed RNAs, 10 µg/µL Cas9 nuclease, and the electroporation buffer mixed in a volume ratio of 5:3:2 at 37 °C for 20 min to form the Cas9-RNP complex. In routine work, mix, and incubate 1.25 µL of annealed RNA, 0.75 µL of Cas9 nuclease, and 0.5 µL of the electroporation buffer for one locus genome editing.
3. Mix the following materials in a 1.5 mL tube: 10 µL of electroporation buffer, 1 µL of Cas9-RNP complex (step 3.2), and 1 µL of circular targeting vector (1 µg/µL, step 1.6). The total amount of Cas9-RNP-DNA mixture is 12 µL. Keep the Cas9-RNP-DNA mixture on ice until electroporation.
   ​NOTE: To prevent re-cleavage of the gRNA after KI, design the gRNA in a position where the gRNA recognition sequence is split by the integration of the KI donor. If the gRNA cannot be designed in such a location, several nucleotide silent mutations, by which the amino acid sequence does not change, are included in the plasmid of the KI donor.

4. Gene targeting of ESCs

1. To prepare the gelatin-coated cell culture dishes, add 0.1% (w/v) gelatin solution at 2 mL for a 60 mm dish, 500 µL for a 24-well plate, 100 µL for a 96-well plate, and incubate for 2 h in a humid incubator. Remove the gelatin solution, wash twice with the same amount of PBS, and store at room temperature until use.
2. Thaw mitotically inactivated frozen MEF stock (step 2.2) using a 37 °C water bath for 1 min, and place the MEF on a gelatin-coated 60 mm dish (one frozen stock tube of MEF containing 1.6 x 10⁶ cells for two 60 mm dishes, step 2.2) 1 day before ESC seeding.
3. Thaw a frozen ESC stock tube (stored at 2 x 10⁵ ESCs/tube; cell counting was carried out using a cell counter) using a 37 °C water bath for 1 min, and place 1 x 10⁵ ESCs onto a 60 mm dish containing pre-seeded MEF (step 4.1).
4. Culture in 4 mL ESC culture medium (ESCM, DMEM-based modified medium with 15% (v/v) fetal bovine serum, 2 mM L-glutamine substrate, 100 U/mL penicillin, 100 µg/mL streptomycin, 0.1 mM 2-mercaptoethanol, leukemia inhibitory factor, and t2i (0.2 µM PD0325901 and 3 µM CHIR99021) which maintains pluripotency of ESC¹¹) at 37 °C with 5% CO₂ until the ESC reaches 50% to 70% confluency.
   NOTE: We use three different lines of ESC: JM8.A3 from B6N, V6.5 from B6-129 F1, and in-house developed BALB/c ESC. The same KI protocols in this manuscript are used for all ESC lines.
5. Wash the culturing ESC with 4 mL of PBS, then treat with 800 µL of 0.25% trypsin-EDTA solution for 5 min at 37 °C for digestion. Add 1 mL of ESCM to inactivate the trypsin and dissociate the ESCs to single cells by pipetting.
6. Centrifuge the ESC-containing solution for 5 min at 280 x g, discard the supernatant, resuspend ESCs in fresh 1 mL of ESCM, and count the number of cells using a cell counter with trypan blue staining. Transfer 1 x 10⁵ ESCs to a new 1.5 mL tube and wash them three times with PBS by centrifugation at 500 x g, 4 °C.
7. Resuspend the ESC pellet in 12 µL of Cas9-RNP-DNA mixture (step 3.3) and mix well by gentle pipetting to avoid bubbling. Serve the resuspended ESC for electroporation. The electroporation system uses a single pulse at 1,400 V and 30 ms for Cas9-RNP-DNA transduction (Figure 1).
8. Culture the electroporated ESCs in a 60 mm dish containing 4 mL of ESCM with mitotically inactivated MEF (section 4.2). Change the medium every day.
9. Wash the ESCs with 4 mL of PBS 3 to 5 days after the electroporation, then treat with 800 µL of 0.25% trypsin-EDTA and culture at 37 °C for 5 min in a humid incubator for digestion. Add 2 mL of ESCM to stop the trypsin digestion and centrifuge the cell mixture at 280 x g for 5 min.
10. Discard the supernatant, resuspend the ESCs in 1 mL of fresh ESCM, and count the cell concentration using a cell counter with trypan blue staining. Passage the ESCs at 1 x 10³ ESCs per 60 mm dish containing ESCM and feeder MEF. Change the medium every day until the colony picks-up.
    ​NOTE: The reason why the ESC is passed once before pick-up was that genome editing may continue to occur after ESC division, so the first colony is not always the clone in many cases. Therefore, the first passage of ESC before the colony pick-up is an essential step to pick-up single clones.

5. PCR genotyping of targeted ESCs

1. Aspirate ESCM from the ESC culture dish 5 to 7 days after the first passage (step 4.9) and add 4 mL of PBS. Pick up single ESC colonies with 5 µL of PBS using a 20 µL pipette under a stereomicroscope (30x to 40x magnification). Place the individual colonies in a well of a round-bottom 96-well plate containing 15 µL of 0.25% trypsin-EDTA solution.
2. Keep the 96-well plate on ice until 48 individual colonies are picked-up. Incubate the 96-well plate for 5 min in a 37 °C humid 5% CO₂ incubator, then add 80 µL of ESCM to stop trypsin digestion and dissociate ESC colonies into single cells by pipetting.
   NOTE: Since the KI efficiency of this method is usually 10%-50%, we routinely pick up 48 clones and first perform genotyping using 24 of them. If multiple KI clones cannot be obtained at this point, we further screen for KI using the remaining 24 clones.
3. Transfer 40 µL of ESC suspension (step 5) to a gelatin-coated feeder-free 96-well plate containing 50 µL of ESCM per well for PCR genotyping. Transfer the remaining 60 µL of ESC suspension into a well in a gelatin-coated 24-well plate, which contains 500 µL of ESCM and feeder MEF, to make the frozen ESC stock.
4. Stock individual ESC clones cultured in the 24-well plate (step 5.3) until they will reach 60% to 80% confluency. Recover individual ESC clones by trypsinization as mentioned above, collect cells in a new 1.5 mL tube, and centrifuge at 280 x g for 5 min. Discard the supernatant, resuspend the cells in 500 µL of cell freezing medium, and freeze them using a -80 °C deep freezer.
5. For PCR genotyping, culture ESC clones in the 96-well feeder-free plate (step 5.3) until the ESC reaches more than 90% confluency. Remove ESCM from each well by aspiration and wash twice with 100 µL of PBS.
6. Aspirate the PBS and add 100 µL of lysis buffer containing proteinase K, mix well, and transfer the ESC lysate to a new 1.5 mL tube. Heat the ESC lysate at 65 °C for at least 1 h using a heat chamber. Extract and purify the genomic DNA using a conventional phenol-chloroform DNA purification method followed by ethanol precipitation.
7. Dissolve the precipitated DNA in 20 µL of DNase-free water and determine the purity and concentration of DNA using a spectrophotometer. Perform genomic PCR using locus-specific primer sets to amplify the target region. Then, check the sequence and choose the ESC clones with the desired KI sequences.

6. Preparation of eight-cell stage embryo and microinjection of ESCs

1. Inject 5 IU of pregnant mare serum gonadotrophin (PMSG) intraperitoneally into adult ICR females. After 48 h, inject 5 IU of human chorionic gonadotrophin (hCG) into the same females, then mate the hormone-treated females with the ICR males. Check the copulatory plug in the vagina the following day (embryonic day 0.5, ED0.5).
   NOTE: To evaluate chimerism by coat color ratios, use blastocyst from the ICR strain as a recipient for a ESC that has black or agouti coat color background, or blastocyst from the C57BL/6 strain as a recipient for a ESC that has the albino background.
2. Collect the oviduct at ED1.5 and place it in HEPES buffered medium (FHM) drops. Recover the two-cell or four-cell stage embryos by flushing the oviduct with FHM using a flushing needle inserted into the infundibulum.
3. Wash the collected embryos by moving them into several fresh FHM drops using a mouth pipette. Culture the embryos in 50 µL of KSOM drop on a 35 mm cell culture dish covered with mineral oil at 37 °C, 5% CO₂ incubator for 1 day until the eight-cell or morula stage (ED2.5) is reached.
4. If the embryos are not used on the next collection day, freeze them by vitrification according to the CARD protocol (http://card.medic.kumamoto-u.ac.jp/card/english/sigen/manual/ebvitri.html), and store them in liquid nitrogen until use.
5. Prepare glass pipettes for holding the embryos (80-100 µm outer diameter) and ESC injection (about 13 µm diameter) using a puller and microforge.
6. Integrate a holding pipette that contains FHM into a capillary holder connected to a microinjector and set up on the left side of the micromanipulator. Connect an injection pipette to another microinjector and set it up on the right side. Align the two pipettes straight in the field of the view of microscope.
7. Dispense 5 µL drops of 12% (w/v) polyvinylpyrrolidone (PVP) in FHM medium and 5 µL drops of FHM onto the same lid of a 60 mm dish side-by-side and cover the drops with mineral oil. Transfer embryos in the 5 µL FHM drop and add 1-5 µL of the ESC suspension to the FHM drop that contains embryos.
   NOTE: Leave the ESC and MEF suspension for 30 min in the 4 mL ESM on 60mm culture dish before the drop preparation. Since MEFs stick to the bottom faster than ESCs, this 30-minute incubation allows for the concentration of unattached ES cells in the supernatant.
8. Wash inside the injection pipette with PVP, then move to the drop containing the embryos and ESCs.
9. Pick up three individual ECS doublets that have just finished their cell division (six cells) in the injection pipette. Hold an eight-cell or morula stage embryo, which has completed compaction, with the holding pipette by aspiration. Make a hole in the zona pellucida with a piezoelectric pulse (intensity: 3; speed: 3). Expel the six-celled ESC inside the zona pellucida and pull the pipette out of the embryos.
10. Wash the ESC-injected embryos with several KSOM drops gently using a mouth pipette and incubate them in a new 50 µL KSOM drop covered with mineral oil at 37 °C, 5% CO₂ until the embryos develop to the blastocyst stage.
11. Transfer the 10 ESC-injected blastocysts into each uterine horn of pseudopregnant female mice (2.5 days postcoitum, 20 blastocysts per head).
12. After the surrogate mother has given birth, select at least two male chimeras with the highest chimerism ratio (70% or higher as confirmed by coat color), and then mate them with appropriate strain females to obtain F1 heterozygous KI.
13. Breed individual KI mice with appropriate partners to obtain mice with desirable genotype(s) or genetic backgrounds suitable for research.

Representative Results

We have targeted specific gene(s) in the ESC followed by chimera production to develop gene-manipulated mouse production according to our previous manuscript¹¹. ESC genotyping (described in section 4) is routinely carried out by PCR using primers. The primers are designed on genomic sequences outside the homology arms and the specific sequences in the KI DNA fragment (Figure 2A). In that case, no wild-type allele is amplified, whereas a PCR amplicon of a specific size is detected only when the targeted exogenous DNA is KI at the target locus. Representative genotyping PCR results are shown in Figure 2B. Nine out of 22 clones (40.9%) showed a KI-specific band in this case. Three representative targeting results, including the result shown in Figure 2, are shown in Table 1. These results indicate that the method shown here is efficient and reproducible for gene KI without any drug selections.

ESC is picked up in an injection pipette, then a hole in the zona pellucida is made using a piezoelectric plus, and the ESC is released between the embryonic cells (Figure 3). Technically, this protocol to inject an ESC into an eight-cell or morula stage embryo is similar to the protocol for ESC injection into a blastocyst commonly used in many mouse facilities. Representative chimeric mice are shown in Figure 4. For coat color evaluation of chimerism, the embryo from the ICR strain (albino, white hair) was used as a recipient of B6 or B6-129 F1 ESC, and B6 embryos were used as a recipient of BALB/c ESCs (Figure 4).

Figure 1: A schematic of CRISPR/Cas9 ribonucleoprotein (RNP)-mediated circular plasmid integration into the specific locus of an ESC genome. Electroporation introduces a circular plasmid as a targeting vector into ESCs with Cas9-RNP. Abbreviations: GOI = gene of interest, HA = homology arm. Please click here to view a larger version of this figure.

Figure 2: Genomic PCR analyses for KI screening of targeted ESC clones. (A) KI-specific PCR primers are designed on genomic sequences outside the homology arms (forward) and in the specific sequences in the KI DNA fragment (reverse). (B) Representative genotyping PCR results. A wild-type genome was used as a negative control. Abbreviations: GOI = gene of interest, HA = homology arm. Please click here to view a larger version of this figure.

Figure 3: Representative images of eight-cell stage embryo injection. (A) For microinjection, three doublets of ESCs (six cells, arrowheads) are picked up. (B) A hole in the zona pellucida is made with a piezoelectric pulse, and ESCs are expelled in-between each blastomere. Scale bar shown is 50 µm. Please click here to view a larger version of this figure.

Figure 4: Representative images of chimera mice. (A,B) The ESC derived from C57BL/6N (JM8.A3, Agouti hair; A) or B6-129 F1 (Agouti hair; B) are injected into ICR (albino, white hair) embryos, followed by transferring the embryos to the mother surrogates. (C) The ESC derived from BALB/c (albino, white hair) is injected into C57BL/6J (black hair) embryos, followed by transferring the embryos to mother surrogates. Please click here to view a larger version of this figure.

Project ID	Choromosome	5'HA length (bp)	3'HA length (bp)	Insert size (bp)	Number of analyzed clones	Number of KI clones	Efficiency (%)	Remarks
R26-CC*	Chr6	965	1006	5321	23	2	8.7%	-
R4-03*	Chr8	1000	997	3070	22	6	27.3%	-
P4-01*	Chr15	1000	1000	2569	22	9	40.9%	Shown in Figure 2

Table 1: KI efficiencies in three independent genomic loci in the ESC. *These projects have not been published so far. The name of these genes will be disclosed in independent manuscripts in the future.

Discussion

Gene targeting of ESCs followed by chimera production has been conventionally used for developing gene-manipulated mice. Nevertheless, gene knock-in efficiency remains low even though the targeting vector contains long (> several kb in usual) homology arms with positive or negative drug selection gene cassettes. Our protocol introduced a tuned-up ESC KI method of long exogenous DNA using a circular plasmid without any drug selection cassettes as a targeting vector accompanied by Cas9-RNP-mediated genome editing at an acceptable efficiency for routine works. Thus, this protocol could help to substantially reduce the amount of time taken in producing genetically modified chimeric mice compared with the conventional ESC targeting.

In this protocol, we used CRISPR/Cas9-RNP to induce site-specific double-strand breaks in the genome, and a circular plasmid was used as the targeting vector instead of a linearized one. Linearized plasmids are conventionally used as a targeting vector for gene KI because of their increased efficiency of genomic integration^8,9,10. However, many genomic integrations are nonspecific even though the vector contains homologous arms⁹. On the other hand, circular plasmids are rarely used as targeting vectors because of their hard-to-integrate feature into the genome of ESCs¹² or fibroblasts¹³. Thus, it would be possible that applying a circular plasmid as a targeting vector accompanied by CRISPR/Cas9-mediated genome editing minimizes nonspecific random integration but maximizes the site-specific integration into the ESC genome. It would be notable that the induction efficiency of the double-strand break by CRISPR/Cas9 is quite important¹⁴, thus it would be essential to use gRNA which induces a double-strand break at high efficiency. It should be considered to re-design gRNA when the KI efficiency is too low. In the case shown in Figure 2, 40.9% of ESC clones showed a KI-specific band. In fact, as the core lab of the institute, we routinely perform gene targeting using ESCs by the method described here and have achieved KI efficiencies of 10%-50% for 1-2 kb sequences such as Cre, CreERT, or fluorescent reporters, although there are some variations depending on the gene locus. The longest DNA sequences we have successfully KI using this method is about 11.2 kb into the Rosa26 locus, and the efficiency was 12.2% (five KI colonies out of 41 analyzed colonies).

It would also be of note that this protocol uses eight-cell or morula stage embryos as recipients for ESC microinjection but not blastocyst stage embryos. Although we have not conducted comparing experiments in this paper, several reports have shown that injection of ESCs into eight-cell or morula stage embryos significantly improves the efficiency of ESC contribution to chimeric offspring compared to the ESC injection into blastocysts^15,16,17,18. Indeed, ESC injections of various ESC lines, including C57BL/6, B6-129 F1, and BALB/c, commonly resulted in high coat color chimera development in some, but not all, offspring (see Figure 4). The limitation of the method is that the ESCs injected into the preimplantation embryo could not always contribute to germ cells in a chimera^19,20. Germline transmission of ESCs would depend on the quality of each ESC clone¹⁹. Therefore, it would be advantageous to develop multiple ESC clone lines as a backup for stable chimeric mouse production. In conclusion, the protocols presented here use simple targeting vectors which include only each homology arm and gene of interest for KI without any drug-resistant cassette. Therefore, the vector construction would be much easier, and the culture period could be shortened compared to the conventional ESC targeting technique. This would help in quickly and facilitatively producing various types of genetically modified mice for future analysis in life science.

Materials

Name	Company	Catalog Number	Comments
BALB/c ESC	-	-	ESC developed from BALB/c strain
Bambanker	Nippon Genetics	CS-02-001	Cell-freezeing medium. Section 2.6 and elsewhere
Cas9 Nuclease V3	IDT	1081059	Section 3.2 and elsewhere.
CHIR99021	FUJIFILM Wako	038-23101	Section 4.3
CreERT gene fragment	GeneWiz		Section 1.1.
CRISPR-Cas9 crisprRNA	IDT		crisprRNA. Section 3.1 and elsewhere.
CRISPR-Cas9 tracrRNA	IDT	1072534	tracrRNA. Section 3.1 and elsewhere.
DMEM	Nacalai	08458-45	MEF medium. Section 2.3 and elsewhere
Duplex buffer	IDT	1072534	RNA dilution buffer. Section 3.1 and elsewhere.
FastGene Gel/PCR Extraction Kit	Nippon Genetics	FG-91302	Section 1.1 and 1.2.
GlutaMax	Thermo Fisher	35-050-061	L-glutatime substrate
hCG	ASKA Animal Health		Section 6.1.
In-Fusion HD Cloning Kit	Clontech	639648	DNA cloning kit. Section 1.3 and elsewhere
JM8.A3 ESC	EuMMCR	-	ESC developed from C57BL/6N strain
Knock-out DMEM	Thermo Fisher	10829018	Section 4.3 and elsewhere, DMEM-based modified commercial medium.
KSOM	Merck	MR-121-D	Section 6.3 and 6.9.
Leukemia inhibitory factor	FUJIFILM Wako	125-05603	Section 4.3. No unit concentration data is supplied by the provider. Used 1,000-fold dilution in this protocol.
Neon Electroporation system	Thermo Fisher	MPK5000	Section 3.2, 4.5 and elsewhere. The system containes electroporation buffer as well used in section 3.2.
NucleoSpin Plasmid Transfection-grade	Takara	U0490B	Section 1.6.
PD0325901	FUJIFILM Wako	162-25291	Section 4.3
PMSG	ASKA Animal Health		Section 6.1.
Tail lysis buffer	Nacalai	06169-95	Section 5.5.
Trypsin-EDTA	Nacalai	32777-15	Section 2.2 and elsewhere
V6.5 ESC	-	-	ESC developed from B6J-129 F1 strain
X-ray irradiation device	Hitachi	MBR-1618R-BE	Section 2.6.