Knock-in/Point-Mutation Animal Model

• CRISPR/Cas9 components (Cas9 mRNA or protein, single-guide RNA, and donor DNA templates such as ssODN or plasmid DNA) are used to induce targeted DSBs and provide repair templates for HDR-mediated point mutation insertion in embryos or ES cells.

• Small molecules such as Scr7 (NHEJ inhibition) and RS-1 (HDR enhancement) have been reported to improve knock-in efficiency by modulating DNA repair pathway choice during genome editing in mammalian embryos or somatic cells.

• PCR genotyping reagents and restriction enzyme assays (e.g., site-specific digestion assays) are commonly used to identify successful knock-in events and distinguish edited from wild-type alleles.

• Microinjection systems or electroporation devices are used to deliver CRISPR reagents into zygotes or embryos, while embryonic stem cell culture systems support in vitro genome editing workflows for generating genetically modified animal models.

• Guide RNA is designed to target a genomic region adjacent to the intended mutation site, and donor templates are synthesized with homology arms flanking the desired point mutation sequence.

• Reported studies indicate that homology arm length and structure can influence HDR efficiency, with optimization being locus- and species-dependent.

• Embryos (including zygotes or vitrified/warmed fertilized oocytes) or embryonic stem cells are prepared for microinjection or electroporation-based delivery of CRISPR components, depending on the experimental model system.

• CRISPR/Cas9 components and donor DNA are introduced into zygotes or embryonic stem cells via microinjection or electroporation to induce site-specific DSBs and promote HDR-mediated repair incorporating the desired point mutation.

• In embryo-based systems, timing of manipulation (e.g., post-fertilization culture duration prior to microinjection) can affect knock-in efficiency, indicating that early developmental stage and cell cycle status are important determinants of HDR success.

• In some systems, chemical modulation of DNA repair pathways (such as inhibition of NHEJ or enhancement of HDR) has been used to increase precise editing efficiency, although effects may vary depending on species and experimental context.

• Successful knock-in events are typically identified using PCR amplification of the target locus followed by sequencing to confirm the presence of the intended nucleotide substitution.

• In some designs, restriction enzyme digestion assays are used when mutations introduce or remove restriction sites, enabling rapid screening of edited embryos or cells.

• Editing outcomes are commonly categorized into precise HDR-mediated knock-ins, NHEJ-derived indels, and mosaic alleles in founder animals, with genotyping performed across multiple biological replicates to assess editing efficiency and fidelity.

Low efficiency of point mutation knock-in in embryos or ES cells

May be due to the dominance of NHEJ repair over HDR and suboptimal donor design or delivery timing.
This can be addressed by increasing HDR favorability through chemical enhancers such as RS-1 or optimizing CRISPR delivery conditions.

• High frequency of indels instead of precise point mutations

May result from double-strand break repair predominantly proceeding through NHEJ rather than HDR, which can be mitigated by modulating DNA repair pathways using NHEJ inhibitors (e.g., Scr7) or improving HDR conditions.

• Mosaicism in founder animals after zygote editing

May occur if editing happens after the first cell division or persists across multiple embryonic stages, which can be reduced by early-stage delivery of CRISPR components in zygotes and optimized embryo handling timing.

• Off-target or unintended mutations at target locus

May arise from Cas9 activity and error-prone repair processes during DSB repair, which can be minimized by using modified Cas9 variants or improved editing systems.