Rapid, efficient generation of knock-in mice with targeted large insertions remains a major hurdle in mouse genetics. Here, we describe two-cell homologous recombination (2C-HR)-CRISPR, a highly efficient gene-editing method based on introducing CRISPR reagents into embryos at the two-cell stage, which takes advantage of the open chromatin structure and the likely increase in homologous-recombination efficiency during the long G2 phase. Combining 2C-HR-CRISPR with a modified biotin–streptavidin approach to localize repair templates to target sites, we achieved a more-than-tenfold increase (up to 95%) in knock-in efficiency over standard methods. We targeted 20 endogenous genes expressed in blastocysts with fluorescent reporters and generated reporter mouse lines. We also generated triple-color blastocysts with all three lineages differentially labeled, as well as embryos carrying the two-component auxin-inducible degradation system for probing protein function. We suggest that 2C-HR-CRISPR is superior to random transgenesis or standard genome-editing protocols, because it ensures highly efficient insertions at endogenous loci and defined 'safe harbor' sites.
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The authors acknowledge technical support from the Model Production Core staff led by M. Gertsenstein at the Centre for Phenogenomics; L. Lavis (HHMI Janelia Research Campus) for synthetic dyes against the Halo-tag; D. Durocher (Lunenfeld-Tanenbaum Research Institute, Mount Sinai Hospital) for discussion of the parameters affecting HR; Y. Yamanaka (McGill Cancer Research Center) for previously setting up the two-cell microinjection system in the Rossant laboratory; and S. Park (University at Buffalo), R. Kuehn (Berlin Institute of Health) and F. Zhang (Broad Institute of Massachusetts Institute of Technology (MIT) and Harvard) for providing materials. This work was funded by CIHR (FDN-143334) to J.R. and Genome Canada and Ontario Genomics (OGI-099).
The authors declare no competing financial interests.
Integrated supplementary information
(a) by immunofluorescence staining of 2C-HR-CRISPR microinjected embryos cultured to the blastocyst stage. Embryos were stained for the inserted tag and targeted endogenous gene product (Sox2, Nanog, Oct4, Esrrb, Klf4, Gata6, Gata4, Sox17, Cdx2, Eomes) or where antibodies for the endogenous protein were not available (Fgfr2, Pdgfra, Id2, Dppa1, Gata3) correct lineage localization of the tag was confirmed using established lineage markers. Extended focus images are shown. Scale bars: 40 μm. These experiments were repeated independently 3 times with similar results. (b) by PCR genotyping of Halo-Erk2 and Ctcf-Halo targeting. Single targeted and non-targeted (negative control) embryos were pre-selected by checking for the presence of fluorescently labeled (JF549) Halo-tag at the blastocyst stage and were individually genotyped. These experiments were repeated independently 3 times with similar results.
Table showing all loci targeted, design of targeting construct (C- or N-terminal fusion or C-terminal 2A-fusion), sgRNAs used and targeting efficiencies achieved at the blastocyst stage using 2C-HR-CRISPR (n indicates the number of microinjected embryos, which developed to the blastocyst stage). sgRNA sequence is indicated in the genomic sequence surrounding the insertion site: the N20 bases of the sgRNA are underlined, the PAM sequence is in blue. Start or stop codons in the genomic DNA are highlighted in red or green, respectively. Black arrows show site where the reporter construct was inserted (always after the start codon or before the stop codon). If a synonymous mutation was introduced into the targeting donor to avoid cleavage by the sgRNA/Cas9, this is indicated above the DNA sequence.
Supplementary Figure 3 Analysis of mosaicism in targeted embryos on the basis of immunofluorescence staining data.
(a) Efficiency of correct targeting of knock-in (KI) allele within individual embryos estimated from counting number of reporter positive cells/total number of cells positive for endogenous protein or appropriate lineage marker (where antibody for endogenous protein was not available). Targeting was done using the Cas9/plasmid method. n = number of targeted embryos. Error bars represent s.d. of mean. Results show various levels of KI-mosaicism in individual targeted embryos. (b) Unique mosaicism observed in embryos microinjected with CRISPR reagents to generate Gata6-GFP using the Cas9/plasmid method. We noted that targeting the C-terminus of Gata6 with CRISPR often resulted in a lack of endogenous Gata6 protein in some cells – a unique knock-out phenotype we did not observe with other targeted genes. We suggest this may be due to an indel generated before or at the STOP codon that gives rise to an aberrant transcript or protein that is degraded. Thus in the case of Gata6 targeting we noted mosaicism in microinjected embryos consisting of correct reporter targeting, knock-out and wild-type cells. Examples of different alleles are shown: left embryo likely has wild-type/wild-type (+/+) cells, due to the uniform expression of endogenous Gata6 in PE (stronger) and TE (weaker) cells and no detectable GFP signal. Middle embryo likely has a mix of Gata6-GFP/+ and Gata6-GFP/deleted (-) cells, as GFP is detected in seemingly all Gata6 positive cells, but endogenous Gata6 expression levels vary. Right embryo likely has a mix of +/+, +/- and -/- cells, corresponding to strong, intermediate and completely absent endogenous Gata6 signals. All embryos are from the same microinjection session. Embryos were cultured to the blastocyst stage and immunostained with indicated antibodies. Scale bars: 40 μm. Extended focus images are shown. These experiments were repeated independently 3 times with similar results. (c) Comparison of correct targeting efficiencies of reporter tags in individual embryos using the Cas9/plasmid or Cas9-SA/BIO-PCR methods. Targeting efficiencies were estimated by counting number of reporter positive cells/total number of cells positive for endogenous protein (Nanog-mAID-mCherry and Gata6-Halo) or total cell number marked by DAPI staining (Ctcf-Halo-mAID) (n=number of targeted embryos analyzed). Error bars represent s.d. of mean. p-values were calculated using the Mann-Whitney U-test (2-sided). Results show increased targeting using the Cas9-mSA/BIO-PCR method.
Targeting efficiencies were scored in microinjected embryos cultured to the blastocyst stage. n represents number of embryos microinjected. (a) 3 different insert sizes between ~700 to ~1400 bps were tested using both Cas9/plasmid and Cas9-mSA/BIO-PCR methods. Homology arm length was kept constant (3kb:2kb) No obvious decrease in targeting efficiency was observed at this insert size range. (b) 4 different homology arm lengths were compared ranging from 3 kb to 100 bp. Insert size was kept constant (Halo tag, 921 bp). We observed a gradual decrease in targeting efficiencies if the homology arm was shorter than 1 kb.
Supplementary Figure 5 2C-HR-CRISPR targeting efficiencies compared between CD1 and C57BL/6J backgrounds.
Microinjected embryos were cultured to the blastocyst stage where targeting efficiency was evaluated by scoring correct localization of reporter expression (n indicates number of microinjected embryos that developed to the blastocyst stage).
Supplementary Figure 6 Analysis of mosaicism in 2C-HR-CRISPR-targeted blastocysts and mice by genotyping and sequencing.
(a) Schematic overview of strategy to analyze mosaicism in 2C-HR-CRISPR-targeted blastocysts and results of analysis (table). ES cells were derived from Gata6-Halo Cas9/plasmid and Cas9mSA/BIO-PCR microinjected embryos (from 5 knock-in-positive embryos for each injection method). (1) 16 individual ES cell colonies were picked from each derivation and zygosity of the correct knock-in (KI) allele was determined using genotyping PCR. (2) To analyze CRISPR-editing byproduct alleles the targeting region was PCR-amplified, cloned and sequenced (15 bacterial clones from each derivation) from the mixed ES cell population. Wild-type (wt), various deletion (del) and point mutation (point mut) alleles were detected. Last column of table indicates allele types present in each Gata6-Halo-targeted embryo. (b) Schematic overview of strategy to analyze mosaicism in 2C-HR-CRISPR-generated founder mice and results of analysis (table). (3) Presence of KI allele in the germ line of founder animals was assessed by genotyping 2 consecutive N1 litters from each founder (% positive N1 pups (positive pups/total pups)). (4) To analyze CRISPR-editing byproduct alleles the targeting region was PCR-amplified, cloned and sequenced from founder animal tail samples (at least 8 bacterial clones from each founder). Wild-type (wt), various deletion (del) and point mutation (point mut) alleles were detected. Last column of table indicates allele types present in each founder animal.
(a) Immunofluorescence staining of N1 blastocysts (all cells are heterozygous, non-mosaic for the reporter) from Nanog-mAID-mCherry and Gata6-Halo mouse lines. These lines were crossed to create the triple-color lineage reporter line. All images show extended focus. Scale bars: 40 μm. These experiments were repeated independently 3 times with similar results. (b) Non-mosaic live triple-color blastocysts with all three lineages labeled with different reporter alleles. Extended focus and single plane image of an early (embryonic day 3.5) and late (embryonic day 4.5) blastocyst are shown. Embryos were produced by breeding 2C-HR-CRISPR-generated mouse lines (Nanog-mAID-mCherry and Gata6-Halo) with an existing Cdx2-eGFP reporter mouse line. The Halo-tag was visualized using the live JF646 dye. Scale bar: 40 μm. These experiments were repeated independently 3 times with similar results.
Snapshots from time-lapse imaging of non-mosaic Cdx2-eGFP/Gata6-Halo embryos from the 16-cell stage to the blastocyst stage. The Halo-tag was visualized using the live JF549 dye. 1 Z-stack image was acquired every hour for 30 hours with a spinning disk confocal. All images show extended focus. Scale bars: 40 μm. These experiments were repeated independently 3 times with similar results.
Supplementary Figure 9 Additional examples of auxin-inducible degradation of endogenously mAID-tagged protein in early embryos.
Examples of auxin-inducible degradation of Gata6-Halo-mAID and Sox2-mAID-mCherry in preimplantation embryos. Simultaneous targeting of Halo-mAID to the Gata6 locus or mAID-mCherry to the Sox2 locus and TIR1-GFP to the Actb locus was achieved with 2C-HR-CRISPR. Z-stack images were acquired every 10 minutes following addition of IAA. Arrows point to double positive cells with both Gata6-Halo-mAID or Sox2-mAID-mCherry, and TIR1-GFP expression. Gata6-Halo-mAID or Sox2-mAID-mCherry are only degraded in TIR1-GFP positive cells, but not in TIR1-GFP negative cells in the same embryos. Extended focus images are shown. Scale bars: 40 μm. These experiments were repeated independently 3 times with similar results.
Supplementary Figure 10 Schematic overviews of genotyping strategy, donor vectors and BIO-PCR donors.
(a) Schematic overview of strategy to genotype and sequence knock-in reporter alleles, used in mice and ES cells. (b) Schematic overview of strategy to PCR-amplify and sequence editing byproduct alleles, used in mice and ES cells. (c) Schematic overview of EasyFusion targeting vectors. All plasmids are available from Addgene. MCS – multiple cloning site; SA – splice acceptor. (d) Schematic overview of pan-BIO primers used to produce BIO-PCR products.
Supplementary Figures 1–10 and Supplementary Table 1 (PDF 2662 kb)
Oligo sequences (XLSX 184 kb)
Live imaging of mouse embryos carrying primitive endoderm reporter Gata6-Halo and Trophectoderm reporter Cdx2-eGFP for 30 hours 16-cell stage to blastocyst (MOV 5697 kb)
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Gu, B., Posfai, E. & Rossant, J. Efficient generation of targeted large insertions by microinjection into two-cell-stage mouse embryos. Nat Biotechnol 36, 632–637 (2018). https://doi.org/10.1038/nbt.4166
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