CRISPR Gene Editing Efficiency Detection

CRISPR Gene Editing Efficiency Detection

1. Background

The CRISPR-Cas system employs guide RNA (gRNA) to direct Cas nucleases—including Cas9, Cas12, and related variants—to induce DNA breaks at specified genomic loci, harnessing endogenous cellular repair mechanisms to achieve gene editing. The non-homologous end joining (NHEJ) pathway predominantly generates small insertion-deletion (indel) mutations applicable to gene disruption, while the homology-directed repair (HDR) pathway facilitates precise sequence replacement or gene insertion. Additionally, the editing process may generate complex genomic rearrangements, including large-fragment deletions, chromosomal inversions, or translocations. Although distinct Cas proteins differ in their cleavage modality, PAM recognition specificity, and editing product profiles, all are capable of producing a diverse array of on-target and unintended editing products that necessitate comprehensive, accurate detection and quantification.

CRISPR technology has achieved significant milestones in clinical translation. As of late 2025, the first CRISPR-based gene therapy, Casgevy (exa-cel), developed jointly by Vertex Pharmaceuticals and CRISPR Therapeutics for the treatment of sickle cell disease and transfusion-dependent β-thalassemia, has received regulatory approval in multiple jurisdictions including the United States, United Kingdom, and European Union. Furthermore, more than 150 CRISPR-related clinical trials are actively ongoing worldwide, spanning hematological disorders, hereditary blindness, cancer immunotherapy, and numerous other indications—collectively signaling the accelerating commercialization of CRISPR technology.

As gene editing technologies continue to evolve, regulatory agencies worldwide—including the U.S. Food and Drug Administration (FDA) and the Center for Drug Evaluation (CDE) under China's National Medical Products Administration (NMPA)—have established comprehensive and stringent safety assessment requirements for gene editing products. These requirements specifically encompass off-target effects, chromosomal structural variants, vector integration risks, and residual editing components. The FDA guidance document Human Gene Therapy Products Incorporating Human Genome Editing, published in January 2024, explicitly mandates comprehensive off-target risk assessments, emphasizing the synergistic application of bioinformatics, biochemical, and cellular approaches for genome-wide evaluation, along with systematic assessment of chromosomal integrity, clonal expansion risks, and the biological consequences of editing products (Figure 1).

Figure 1. Core safety assessment requirements for gene editing products as stipulated in the FDA guidance: Human Gene Therapy Products Incorporating Human Genome Editing

In response to the practical requirements of CRISPR-based gene editing product development and regulatory evaluation, conventional PCR-based Sanger sequencing methods are limited by low sensitivity (limit of detection: approximately 10%–20%) and insufficient throughput, and are incapable of comprehensively capturing the diverse array of editing products generated by CRISPR editing. Generulor has developed a specialized CRISPR editing efficiency detection platform based on high-throughput next-generation sequencing (NGS). By designing target-specific primers flanking the editing locus for high-depth amplicon sequencing, this platform enables highly sensitive and accurate detection and quantification of all editing products at on-target sites, providing a robust solution for quality control and safety assessment of CRISPR-based therapeutics.

2. Principle of CRISPR Editing Efficiency Detection

The on-target CRISPR amplicon sequencing technology leverages the high-throughput capability, exceptional sensitivity, and single-nucleotide resolution of next-generation sequencing (NGS), combined with an experimental workflow and a specialized bioinformatics analysis pipeline optimized for the CRISPR editing mechanism. The technology involves specific amplification of the genomic region encompassing the CRISPR cleavage site (3 bp upstream of the PAM sequence) and its flanking sequences (typically 150–300 bp), followed by ultra-deep sequencing (≥1,000,000×) and advanced sequence analysis algorithms for precise identification and quantification of editing products at on-target loci.

The analytical workflow comprises the following steps: (i) design of highly specific amplification primers based on the sgRNA target sequence and Cas9 cleavage site, ensuring complete coverage of the cleavage site and flanking regions; (ii) high-fidelity PCR amplification to minimize the introduction of artifactual mutations; (iii) library construction and ultra-deep paired-end sequencing (PE150), ensuring acquisition of ≥1,000,000 effective reads per locus to enable detection of editing events at frequencies as low as 0.1%; and (iv) application of a purpose-built CRISPR editing analysis algorithm for precise detection and quantification of all editing product classes, including insertions, deletions, and substitutions.

Relative to conventional detection methods, this platform offers five core advantages: (1) Ultra-high sensitivity—with a detection limit of 0.1%, enabling identification of extremely low-frequency editing events and rare indel variants; (2) Comprehensive editing product profiling—systematic detection and quantification of all editing product types, including indels of diverse lengths and configurations, and complex mutations; (3) High-precision quantification—accurate determination of the relative abundance of each editing product class via ultra-deep sequencing; (4) Editing efficiency assessment—precise calculation of critical metrics including gene knockout efficiency and the proportion of precise insertions or substitutions; (5) High-throughput capacity—supporting simultaneous analysis of hundreds of samples and multiple target loci within a single experimental run, suited for large-scale screening and quality control applications.

Figure 2. Schematic diagram of the on-target amplicon sequencing workflow for CRISPR gene editing

3. Technical Innovations and Advantages in CRISPR Editing Efficiency Detection

3.1 Core Technical Innovations

3.1.1 CRISPR-Specific Primer Design Strategy

A specialized primer design strategy has been developed to address the unique features of CRISPR-Cas9 cleavage:

(1) Amplification regions provide complete coverage of the Cas9 cleavage site (3 bp upstream of the PAM) and its flanking sequences extending 100–250 bp in each direction, ensuring comprehensive capture of all indel variants generated at the cleavage site.

(2) Primer positions are placed outside potential editing-affected regions to prevent amplification failure or PCR bias resulting from target site mutations.

(3) Amplicon length is optimized (150–300 bp) to ensure complete coverage of the cleavage site by overlapping paired-end reads while retaining the ability to detect intermediate-length deletion events.

3.1.2 Ultra-Deep Sequencing

Stringent standards for sequencing depth and deduplication-based quality control have been established:

(1) Effective sequencing depth ≥1,000,000×, ensuring statistically reliable detection of editing events at frequencies as low as 0.1%.

(2) Multiple negative controls (unedited samples and sgRNA-free controls) to accurately characterize background mutation levels.

3.1.3 CRISPR-Specific Bioinformatics Analysis

Analytical algorithms and assessment metrics specifically designed for CRISPR editing patterns have been developed:

(1) Precise identification of the Cas9 cleavage site: accurate localization of the expected cleavage position (3 bp upstream of the PAM) based on sgRNA sequence and PAM coordinates.

(2) Comprehensive indel analysis: systematic detection and classification of all insertion and deletion mutations, including mutation type (insertion/deletion), mutation length (1 bp to hundreds of bp), position relative to the cleavage site, and sequence characteristics.

(3) Calculation of editing efficiency metrics: determination of total editing efficiency (proportion of non-wild-type sequences), and proportional contributions of precise insertions, deletions, and substitutions.

(4) Complex mutation detection: identification of complex editing events, such as sequences simultaneously harboring insertions and deletions, or multi-site mutations.

(5) Visualization output: provision of editing pattern heatmaps providing an intuitive representation of the full spectrum of editing products.

3.2 Methodological Validation and Performance Metrics

Generulor has completed a comprehensive and systematic methodological validation. The technical performance metrics are as follows:

4. Applications and Service Advantages

4.1 Application Scenarios

The on-target CRISPR amplicon sequencing technology encompasses broad applications across the entire continuum of gene editing therapeutic product development and regulatory evaluation:

(1) sgRNA screening and optimization: Systematic evaluation of on-target editing efficiency and indel distribution patterns for candidate sgRNAs, enabling identification of optimal sgRNAs with high editing efficiency and elevated frameshift mutation proportions.

(2) Delivery system evaluation: Comparative assessment of the impact of different delivery modalities (AAV, LNP, RNP electroporation, etc.) on editing efficiency, optimizing delivery conditions.

(3) Cell product quality control: Assessment of editing efficiency at key genetic loci (e.g., TRAC, B2M, PD-1) during the manufacture of CAR-T, TCR-T, and other cell therapy products to ensure product quality consistency and batch release compliance.

(4) IND submission support: Provision of on-target editing efficiency data and comprehensive methodological validation reports meeting regulatory requirements for clinical trial applications.

(5) Preclinical safety evaluation: Assessment of editing efficiency, editing product distribution, and durability in animal models, organoids, or primary cells to provide data for clinical risk assessment.

(6) Clinical sample monitoring: Detection of on-target editing efficiency and editing product stability in patient samples to support evaluation of clinical efficacy and safety.

(7) Multi-target gene editing assessment: Simultaneous evaluation of editing efficiency across multiple loci for complex editing strategies involving concurrent modification of several targets (e.g., TRAC + B2M dual knockout in CAR-T applications).

4.2 Service Advantages

(1) Technical leadership: An assay platform optimized specifically for CRISPR editing mechanisms, enabling comprehensive and accurate detection and quantification of all on-target editing products.

(2) Certified quality management system: The laboratory simultaneously operates under the ISO 9001 quality management system and ISO/CNAS accreditation standards, ensuring data reliability and regulatory traceability.

(3) Comprehensive methodological validation: Full validation covering sensitivity, specificity, accuracy, linearity, and precision; validation reports are directly applicable to IND submissions.

(4) Standardized reporting: Analytical reports fully compliant with the latest NMPA-CDE and FDA guidance documents, comprehensively supporting regulatory submissions and inspections.

(5) Expert technical support: A technical team with extensive experience in gene editing product development and regulatory submissions, providing end-to-end consulting from experimental design through data interpretation.

(6) Proven track record: Established CRISPR detection services for dozens of leading gene therapy companies and research institutions, with successful support for multiple IND submissions.

5. Representative Report for CRISPR Editing Efficiency Detection

Generulor provides comprehensive CRISPR on-target analysis reports compliant with regulatory requirements, encompassing detailed sequencing data quality assessment and alignment rate analysis. In addition, reports include the following core components:

(1) Sequencing data quality and alignment statistics: The report provides a detailed assessment of sequencing data quality, including the number of clean reads and clean ratio following raw data filtration, paired-end read merging efficiency (merge ratio), and amplicon reference sequence alignment efficiency (align ratio). These metrics comprehensively reflect library quality and sequencing data reliability, ensuring that subsequent editing efficiency analyses are grounded in high-quality data.

Figure 3. Summary table of read merging and alignment statistics (representative example)

(2) Quantitative analysis of gene editing efficiency: Employing the CRISPResso2 software framework, sequencing reads are aligned to reference sequences to enumerate the number of modified reads (Modified read counts) and calculate editing efficiency (Modified rate) for each sample. Background noise is removed by computing the differential between experimental and control groups, enabling precise quantification of the actual on-target editing efficiency at CRISPR loci, thereby providing a reliable basis for editing tool activity assessment and normalized comparisons across samples.

Figure 4. Gene editing efficiency summary table (representative example)

(3) Classification of editing events: All detected editing events are subjected to fine-grained classification by mutation type, with separate enumeration of the frequencies of sole insertions (Only Ins), sole deletions (Only Del), sole substitutions (Only Sub), and various complex editing combinations, along with aggregated InDel rates. In the context of the DSB repair characteristics of the CRISPR-Cas system, this analysis provides comprehensive characterization of the indel mutation spectrum generated by the NHEJ pathway and the distribution of unintended editing products, providing granular data for specificity assessment and quality control of editing tools.

Figure 5. Editing classification summary table (representative example)

(4) Visualization of editing product sequences: All high-frequency editing events within the editing locus region (constituting >0.2% of total reads) are displayed at single-nucleotide resolution. The first row depicts the unedited reference amplicon sequence; subsequent rows represent the spectrum of detected editing sequences. The four nucleotides are distinguished by color; substitutions are marked in bold, insertions are highlighted with rectangular boxes, deletions are indicated with dashed lines, and the sgRNA-predicted cleavage site is denoted by a vertical dashed line. The right-hand panel displays the read proportion and read count for each editing sequence, providing an intuitive visualization of the indel distribution pattern surrounding the cleavage site and the diversity of editing products—serving as direct sequence-level evidence for cleavage activity interpretation and off-target risk assessment.

Figure 6. Editing product visualization (representative example)

6. Service Workflow for CRISPR Editing Efficiency Detection

*Standard turnaround time: 20–30 business days.

7. Sample Requirements

*Notes: ① All samples must conform to the quality standards described above. ② Clients may also submit tissue or cell pellet samples for DNA extraction; tissue samples must weigh >50 mg, and cell pellets must contain >2×10⁷ cells per locus. ③ For non-standard sample types, please consult with the Generulor technical team in advance (Tel: 400-6309596; service@generulor.com).

8. References

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[2] 国家药品监督管理局药品审评中心. 体外基因修饰系统药学研究与评价技术指导原则(试行)[EB/OL]. 北京: 国家药品监督管理局药品审评中心, 2022-05.

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[4] U.S. Food and Drug Administration. Human Gene Therapy Products Incorporating Human Genome Editing: Guidance for Industry [EB/OL]. Silver Spring, MD: FDA, January 2024.

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