BE PCR-free WGS Off-Target Detection

BE PCR-free WGS Off-Target Detection

1. Background

1.1 Single Base Editing Technology

Base Editing (BE) technology, as a revolutionary member of the CRISPR gene editing toolbox, enables precise conversion of individual bases in genomic DNA without relying on DNA double-strand breaks (DSBs) and homologous recombination repair. This characteristic demonstrates tremendous potential in the treatment of monogenic hereditary diseases, development of cell therapy products, and construction of disease models [1, 2].

Base editing systems are mainly divided into two categories:

(1) Cytosine Base Editor (CBE): Converts cytosine (C) to thymine (T) through cytosine deaminase (such as APOBEC family proteins), achieving C·G to T·A base pair conversion.

(2) Adenine Base Editor (ABE): Converts adenine (A) to guanine (G) through engineered adenine deaminase (such as TadA-TadA*), achieving A·T to G·C base pair conversion.

However, the safety of base editing systems, particularly their off-target effects, is a critical aspect that must be carefully evaluated before clinical application. Unlike traditional CRISPR/Cas9 nucleases that mainly introduce insertions/deletions (InDels) at off-target sites, base editing off-target effects have their unique characteristics:

·BE-Specific Off-Target: Mainly manifested as specific types of single nucleotide variants (SNVs), with CBE primarily inducing C→T/G→A conversions, while ABE primarily induces A→G/T→C conversions.

·sgRNA-Dependent Off-Target: Occurs at genomic sites with certain homology to the sgRNA guide sequence, representing the main form of BE off-target.

·sgRNA-Independent Off-Target: Caused by random, non-specific deamination modifications of genomic DNA by deaminases without sgRNA guidance, this genome-wide “background noise” increases the complexity of assessment [3].

1.2 Whole Genome Sequencing (WGS) in BE Editing Safety Assessment

To comprehensively and unbiasedly identify off-target events generated by these two different mechanisms, high-depth whole genome sequencing (WGS) is recognized as the most comprehensive recommended method currently available. Compared to candidate site detection methods (such as GUIDE-seq, CHANG-seq BE, Tracking-seq, Selict-seq), whole genome sequencing (WGS) can unbiasedly detect variations across the entire genome, including off-target sites beyond predictions, and is the BE editing safety assessment recommendedmethod.

1.3 Advantages of PCR-free WGS Technology

Traditional WGS library preparation processes include PCR amplification steps, which may introduce amplification bias, GC bias, and PCR errors, affecting the accuracy of variant detection. PCR-free library construction technology has significant advantages by omitting the PCR amplification step and directly sequencing genomic DNA fragments:

·Reduced sequence bias: Uniform coverage of high and low GC regions, avoiding coverage unevenness caused by PCR amplification bias.

·Lower false positive rate: Eliminates errors and chimeric sequences introduced by PCR, improving the specificity of variant detection.

·Accurate detection of structural variants: Better identification of complex variant types such as large fragment insertions/deletions and chromosomal rearrangements.

·Accurate reflection of allele frequencies: Avoids the impact of PCR amplification bias on variant abundance and accurately quantifies mosaicism.

Figure 1. Schematic Comparison of PCR-free and Standard WGS Library Preparation

This figure demonstrates the workflow differences between the two library preparation methods. Standard WGS (left) includes PCR amplification steps, which may introduce sequence bias, GC bias, PCR errors, and uneven coverage. PCR-free WGS (right) omits the PCR amplification step and proceeds directly to sequencing, thereby achieving higher accuracy and more uniform coverage.

2. Technical Principles and Analysis Workflow

This protocol integrates high-depth PCR-free WGS with base editing-specific advanced bioinformatics analysis workflows, aiming to provide the most scientific and rigorous BE off-target effect assessment for research and clinical applications.

2.1 Principles of Base Editing

2.1.1 CBE (C→T) Editing Principle

The CBE system, guided by sgRNA, targets the nCas9-cytosine deaminase fusion protein to specific genomic sites. Within the editing window upstream of the PAM sequence, cytosine deaminase catalyzes the deamination of C on the target strand to form uracil (U). Cellular DNA repair mechanisms or DNA replication processes recognize U as T, thereby incorporating adenine (A) on the complementary strand, ultimately completing the C·G to T·A base pair conversion.

Figure2. Schematic of CBE Editing Principle

2.1.2 ABE (A→G) Editing Principle

The ABE system, guided by sgRNA, targets the nCas9-adenine deaminase fusion protein to specific genomic sites. Within the editing window upstream of the PAM sequence, adenine deaminase catalyzes the deamination of A on the target strand to form hypoxanthine (I). Cellular DNA repair mechanisms or DNA replication processes recognize I as G, thereby incorporating cytosine (C) on the complementary strand, ultimately completing the A·T to G·C base pair conversion.

Figure3. Schematic of ABE Editing Principle

2.2 WGS-BE PCR-free Off-Target Detection Workflow

Our detection workflow follows strict standardized procedures, starting from PCR-free library construction of samples, through high-depth sequencing and advanced bioinformatics analysis in three core stages, ensuring data accuracy and analysis depth.

Figure4. BE Off-Target Variant Classification and Filtering Workflow

This flowchart integrates the analysis pathways for CBE and ABE, in the “BE-specific Variant Identification” step clearly indicates the key differences between them with yellow annotation boxes: CBE screens for C→T/G→A variants, while ABE screens for A→G/T→C variants.

Figure5. Schematic of BE On-Target and Off-Target Types

This figure integrates the on-target and off-target situations of CBE and ABE, clearly marking the differences between them in specific base conversion types through yellow annotation boxes.

3. Technical Advantages

4. Application Scenarios

(1) BE Drug IND Application: Provides comprehensive and rigorous off-target safety assessment reports that meet regulatory requirements.

(2) BE System Optimization and Comparison: Quantitatively compares the specificity and off-target spectrum of different BE systems (e.g., different deaminases, Cas variants), guiding iterative optimization of editors.

(3) sgRNA Screening and Validation: Through parallel off-target detection, screens for optimal guide sequences with both high activity and high safety from multiple candidate sgRNAs.

(4) Gene/Cell Therapy Product QC Release: Serves as a critical quality control step in the production process of gene-edited cell products, ensuring the safety of each batch.

(5) Basic Scientific Research: In-depth exploration of the molecular mechanisms, sequence preferences, and distribution patterns of BE off-targets in different genomic environments.

5. Example Report

5.1 Data Quality Control and Alignment

Table 5.1-1 Quality Control Information

Table 5.1-2 Alignment Information

5.2 Main Results Display

Figure6. Variant Intersection Venn Diagram Display

Figure7. On-Target Editing Visualization

Figure8. Dependent Off-Target sgRNA Mismatch Diagram

Table 5.2-1 Dependent Off-Target Annotation

6. Service Content and Sample Requirements

6.1 Service Content

We provide one-stop service from experimental design consultation to final report delivery, ensuring smooth project progress.

6.2 Sample and Information Requirements

Accurate analysis relies on high-quality samples and complete information. Please prepare according to the following requirements:

7. References

[1] Komor, A. C., et al. (2016). Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage. Nature, 533(7603), pp.420–424.

[2] Gaudelli, N. M., et al. (2017). Programmable base editing of A•T to G•C in genomic DNA without DNA cleavage. Nature, 551(7681), pp.464–471.

[3] Zuo, E., et al. (2020). Cytosine base editor generates substantial off-target single-nucleotide variants in mouse embryos. Science, 367(6473), pp.114-114.