Base editing
Materials Required
Principle
Base editing introduces targeted single-base substitutions without intentionally generating double-stranded DNA breaks or requiring donor DNA templates; cytosine base editors mainly mediate C•G-to-T•A changes, while adenine base editors mediate A•T-to-G•C changes[1][2][3][4]. The readout is the fraction of sequencing reads carrying the intended base conversion at the target site, together with bystander edits, indels, and off-target edits when measured[1][6][7].
MCE has not independently verified the accuracy of these methods. They are for reference only.
Experimental Materials
• Use cultured mammalian cells, base-editor expression plasmid or mRNA/protein format, guide RNA expression plasmid or synthetic guide RNA, cell-culture reagents, transfection or electroporation reagents, genomic DNA extraction reagents, PCR reagents, and sequencing-library reagents for amplification and measurement of edited loci[1][5][10].
• Use amplicon sequencing or Sanger/NGS analysis reagents to detect target-site editing; fluorescent reporters or selectable markers may be used only when encoded in the selected editor or delivery construct and analyzed by flow cytometry or FACS as reported in mammalian-cell protocols[1][6][7].
• Use standard mammalian cell-culture equipment, a transfection or nucleofection/electroporation platform, PCR thermocycler, gel-electrophoresis equipment if PCR confirmation is performed, flow cytometer or cell sorter when reporter enrichment is used, and Sanger or next-generation sequencing instrumentation for editing quantification[1][6][7].
• Use amplicon sequencing or Sanger/NGS analysis reagents to detect target-site editing; fluorescent reporters or selectable markers may be used only when encoded in the selected editor or delivery construct and analyzed by flow cytometry or FACS as reported in mammalian-cell protocols[1][6][7].
• Use standard mammalian cell-culture equipment, a transfection or nucleofection/electroporation platform, PCR thermocycler, gel-electrophoresis equipment if PCR confirmation is performed, flow cytometer or cell sorter when reporter enrichment is used, and Sanger or next-generation sequencing instrumentation for editing quantification[1][6][7].
Experimental Procedure
• Select a genomic target containing the desired editable base within the activity window of the chosen editor and near a compatible PAM; use a base-editing design tool or equivalent sequence analysis to list candidate guides and potential off-target sites[1][6].
• Choose CBE for C•G-to-T•A editing or ABE for A•T-to-G•C editing, and prefer optimized editor architectures when higher expression is required because codon optimization and nuclear-localization-signal optimization improved CBE and ABE activity in mammalian cells[1][5].
• Seed mammalian cells so they are healthy and suitable for transfection at the time of delivery; HEK293T cells are a commonly described model for standard base-editing experiments in cultured mammalian cells[1].
• Prepare the editor and guide-delivery format consistently across experimental and control groups, including an editor-plus-targeting-guide condition, a no-guide or non-targeting-guide negative control, and a known active guide or reporter-positive condition when available[1][6][7].
• Deliver the base editor and guide RNA into cells by the selected published delivery method, such as plasmid transfection, mRNA/guide delivery, protein or RNP delivery, nucleofection, electroporation, or embryo microinjection/electroporation when working in zygotes[1][10].
• Maintain cells after delivery and collect genomic DNA at the endpoint used for the selected protocol; cultured mammalian-cell base-editing workflows from target selection through analysis are typically completed within 1-3 weeks[1].
• Amplify the target locus from genomic DNA using locus-specific PCR, prepare sequencing libraries or Sanger sequencing templates, and sequence the amplicon to quantify intended edits, bystander substitutions, indels, and unedited alleles[1][6][7].
• When off-target assessment is part of the experiment, analyze predicted DNA off-target sites and consider RNA-seq or transcriptome-wide assays for RNA off-target editing because DNA base editors have been reported to induce RNA edits and genome-wide off-target SNVs in some contexts[8][9][11].
• Calculate editing efficiency as the percentage of reads with the intended base conversion at the target nucleotide; report bystander edits within the editing window, indel frequency, and allelic outcome distributions rather than reporting only the desired edit[1][6][7].
• Use CRISPResso2 or BE-Analyzer for amplicon-sequencing analysis because both were developed for genome/base-editing outcome analysis[6][7].
• Compare edited samples with non-targeting or no-guide controls to identify background sequencing noise and delivery-related effects, and compare candidate guides or editor variants using the same sequencing and analysis pipeline[1][6][7].
• Interpret high intended-edit frequency cautiously if bystander edits, indels, RNA off-target edits, or DNA off-target edits are also detected[8][9][11].
• Choose CBE for C•G-to-T•A editing or ABE for A•T-to-G•C editing, and prefer optimized editor architectures when higher expression is required because codon optimization and nuclear-localization-signal optimization improved CBE and ABE activity in mammalian cells[1][5].
• Seed mammalian cells so they are healthy and suitable for transfection at the time of delivery; HEK293T cells are a commonly described model for standard base-editing experiments in cultured mammalian cells[1].
• Prepare the editor and guide-delivery format consistently across experimental and control groups, including an editor-plus-targeting-guide condition, a no-guide or non-targeting-guide negative control, and a known active guide or reporter-positive condition when available[1][6][7].
• Deliver the base editor and guide RNA into cells by the selected published delivery method, such as plasmid transfection, mRNA/guide delivery, protein or RNP delivery, nucleofection, electroporation, or embryo microinjection/electroporation when working in zygotes[1][10].
• Maintain cells after delivery and collect genomic DNA at the endpoint used for the selected protocol; cultured mammalian-cell base-editing workflows from target selection through analysis are typically completed within 1-3 weeks[1].
• Amplify the target locus from genomic DNA using locus-specific PCR, prepare sequencing libraries or Sanger sequencing templates, and sequence the amplicon to quantify intended edits, bystander substitutions, indels, and unedited alleles[1][6][7].
• When off-target assessment is part of the experiment, analyze predicted DNA off-target sites and consider RNA-seq or transcriptome-wide assays for RNA off-target editing because DNA base editors have been reported to induce RNA edits and genome-wide off-target SNVs in some contexts[8][9][11].
• Calculate editing efficiency as the percentage of reads with the intended base conversion at the target nucleotide; report bystander edits within the editing window, indel frequency, and allelic outcome distributions rather than reporting only the desired edit[1][6][7].
• Use CRISPResso2 or BE-Analyzer for amplicon-sequencing analysis because both were developed for genome/base-editing outcome analysis[6][7].
• Compare edited samples with non-targeting or no-guide controls to identify background sequencing noise and delivery-related effects, and compare candidate guides or editor variants using the same sequencing and analysis pipeline[1][6][7].
• Interpret high intended-edit frequency cautiously if bystander edits, indels, RNA off-target edits, or DNA off-target edits are also detected[8][9][11].
Troubleshooting
Problem: Low intended editing.
• Possible cause: The target base may not fall in an efficient editing window, the PAM may be incompatible, or editor expression may be limiting.• Literature-supported solution: Redesign guides using base-editor design tools and consider optimized editor expression architectures when supported by the selected cell system[1][5][6].
Problem: High bystander editing.
• Possible cause: Additional editable bases within the editor activity window can be deaminated.• Literature-supported solution: Test alternative guides or editor variants with different editing windows or activity profiles, and quantify all editable bases in the amplicon rather than only the intended nucleotide[1][6][7].
Problem: Detectable RNA or DNA off-target editing.
• Possible cause: Deaminase domains can edit RNA or DNA sites outside the intended target in some systems.• Literature-supported solution: Include predicted off-target-site sequencing and RNA-level assessment when needed, and select editor variants or experimental designs with documented fidelity in the relevant model[8][9][11].
References:
- [1]. Huang TP, et al. Precision genome editing using cytosine and adenine base editors in mammalian cells. Nat Protoc. 2021;16(2):1089-1128. [Content Brief]
- [2]. Komor AC, et al. Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage. Nature. 2016;533(7603):420-424. [Content Brief]
- [3]. Gaudelli NM, Komor AC, Rees HA, Packer MS, Badran AH, Bryson DI, et al. Programmable base editing of A•T to G•C in genomic DNA without DNA cleavage. Nature. 2017;551(7681):464-471. [Content Brief]
- [4]. Nishida K, Arazoe T, Yachie N, Banno S, Kakimoto M, Tabata M, et al. Targeted nucleotide editing using hybrid prokaryotic and vertebrate adaptive immune systems. Science. 2016;353(6305):aaf8729. [Content Brief]
- [5]. Koblan LW, Doman JL, Wilson C, Levy JM, Tay T, Newby GA, et al. Improving cytidine and adenine base editors by expression optimization and ancestral reconstruction. Nat Biotechnol. 2018;36(9):843-846. [Content Brief]
- [6]. Hwang GH, Park J, Lim K, Kim S, Yu J, Yu E, et al. Web-based design and analysis tools for CRISPR base editing. BMC Bioinformatics. 2018;19(1):542. [Content Brief]
- [7]. Clement K, Rees H, Canver MC, Gehrke JM, Farouni R, Hsu JY, et al. CRISPResso2 provides accurate and rapid genome editing sequence analysis. Nat Biotechnol. 2019;37(3):224-226. [Content Brief]
- [8]. Zhou C, Sun Y, Yan R, Liu Y, Zuo E, Gu C, et al. Off-target RNA mutation induced by DNA base editing and its elimination by mutagenesis. Nature. 2019;571(7764):275-278. [Content Brief]
- [9]. Zuo E, Sun Y, Wei W, Yuan T, Ying W, Sun H, et al. Cytosine base editor generates substantial off-target single-nucleotide variants in mouse embryos. Science. 2019;364(6437):289-292. [Content Brief]
- [10]. Kim K, Ryu SM, Kim ST, Baek G, Kim D, Lim K, et al. Highly efficient RNA-guided base editing in mouse embryos. Nat Biotechnol. 2017;35(5):435-437. [Content Brief]
- [11]. Lee HK, Willi M, Miller SM, Kim S, Liu C, Liu DR, et al. Targeting fidelity of adenine and cytosine base editors in mouse embryos. Nat Commun. 2018;9(1):4804. [Content Brief]
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