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Plasmid identity and sequencing are core release testing items for plasmid products, used to confirm the genetic identity, sequence fidelity, and structural integrity of the production plasmid. This testing is critical in gene therapy and biopharmaceutical manufacturing – as a key starting material for AAV/LV production or an IVT template for mRNA vaccines, the sequence accuracy of the plasmid directly determines the identity, purity, and potency of downstream products.
Creative Biogene's plasmid identity and sequencing service covers a complete method matrix from Sanger full plasmid sequencing to NGS deep verification. The service supports the entire spectrum from preliminary plasmid verification at the R&D stage to GMP-grade MCB/WCB release testing for IND submissions.
The core challenge of plasmid identity and sequencing lies in the fact that plasmids are circular DNA molecules. Although traditional Sanger sequencing is the gold standard, its read length (~900 bp per reaction) limits its ability to accurately sequence regions containing repeats (ITR, LTR), high GC content, or poly(A) tracts. Additionally, low-frequency sequence variants may be masked in traditional sequencing, yet these variants need thorough characterization for IND submissions. Creative Biogene offers three complementary sequencing technology routes, flexibly selectable based on plasmid complexity and project stage.
1. Sanger Full Plasmid Sequencing
Based on the Sanger dideoxy chain termination method, a primer set covering the entire plasmid is designed, and each fragment is independently sequenced. The fragment sequences are then assembled into the complete plasmid sequence. Sanger sequencing is the long-standing gold standard for plasmid verification, offering single-base resolution and high accuracy (>99.99%), and is widely used to confirm the correctness of the insert sequence and the integrity of key functional elements.
2. NGS Short Read Sequencing (Illumina)
Plasmid DNA is fragmented to 200-600 bp, a library is constructed, and paired-end sequencing is performed on a high-throughput sequencer. The full plasmid sequence and variant information are obtained through bioinformatics alignment. Compared to Sanger sequencing, NGS offers significant advantages in throughput and depth, enabling the detection of low-abundance variants (down to 1% variant allele frequency) and exogenous DNA contamination.
3. Long Read Sequencing (Nanopore/PacBio)
For plasmids containing complex secondary structures (e.g., AAV ITR regions, ~145 bp inverted repeats), high GC content (>70%), or long repetitive sequences, short read sequencing may fail due to fragmentation destroying structural information or inability to span repetitive regions. Long read sequencing (Nanopore up to 10-50 kb) enables single-molecule full-length sequencing, completely spanning repeat regions. OnRamp studies have shown that Nanopore long read sequencing achieves full-length plasmid sequencing at less than half the cost of Sanger and can detect sequence variations in regions with high secondary structure.
| Method | Resolution | Turnaround | Suitable Scenarios | Advantages | Limitations |
| Sanger full plasmid | Single base | Days–1 week | Routine plasmid full-length verification; confirmation of key functional elements | Extremely high accuracy (>99.99%); gold standard; easy interpretation | Cannot sequence through ITR/high GC/repeats; primer design is cumbersome; no low-frequency variant detection |
| NGS short read (Illumina) | Single base + variant frequency | 1-2 weeks | GMP release; IND submission; low-frequency variant monitoring | High depth; detects 1% variants; exogenous contamination screening; multiplexing | Short reads cannot span long repeats; fragmentation may destroy structural information |
| NGS long read (Nanopore/PacBio) | Single base + structural variants | 1-2 weeks | ITR plasmids; AAV/LV plasmids; high GC regions; poly(A) plasmids | Full-length single-molecule sequencing; spans repeats; no primers/PCR; low-cost multiplexing | Raw accuracy lower than Sanger (requires consensus correction); high-quality long fragment preparation |
| Regulation/Guideline | Relevant Section | Creative Biogene's Alignment |
| FDA Guidance: CMC for Human Gene Therapy INDs (2020) | Identity Testing section: Identity confirmation is a core element of the IND CMC section; plasmids as gene therapy starting materials require sequence verification | Plasmid identity testing strategy designed per FDA guidance, supports IND submission |
| ICH Q6B – Specifications: Test Procedures and Acceptance Criteria | Section 2.1: Identity tests should be highly specific; full plasmid sequencing (Sanger or NGS) is recommended for plasmid identity | Sanger and NGS methods designed per ICH Q6B identity testing requirements |
| ICH Q2(R2) – Validation of Analytical Procedures | Validation parameters: specificity, detection limit, precision, robustness | GMP-level NGS plasmid identity methods validated per ICH Q2(R2) |
| USP <1047> – Gene Therapy Products | Identity confirmation section: vector/cargo DNA/RNA sequence confirmation, restriction analysis, PCR, vector serotype/type identification | Plasmid identity method combination follows the USP <1047> framework. |
| BioPhorum Plasmid Release Specifications (2020/2023) | Full plasmid sequencing is recommended for identity; for plasmids containing ITR/LTR/poly(A), NGS is recommended over Sanger | Differentiated sequencing strategy based on plasmid structure: Sanger for routine plasmids, NGS recommended for complex structures |
Creative Biogene's plasmid identity and sequencing services can be flexibly customized based on the client's project stage and plasmid structural characteristics. For a tailored identity strategy integrating Sanger and NGS methods, please contact the Creative Biogene technical team for a consultation.
Q1: How to sequence AAV plasmids containing ITR (Inverted Terminal Repeat) regions?
A: ITR regions consist of ~145 bp highly repetitive sequences that form hairpin secondary structures, posing severe challenges to Sanger sequencing – primers cannot bind effectively, and sequencing signals are typically completely lost in the ITR region. Creative Biogene recommends the Nanopore long-read sequencing solution: long reads (10-50 kb) can completely span ITR regions without special primers, and single-molecule full-length sequencing ensures accurate characterization of ITR structures.
Q2: How does host gDNA contamination in plasmid samples affect sequencing results?
A: Host gDNA (e.g., E. coli genomic DNA) contamination affects NGS analysis in two ways: it consumes effective sequencing reads, reducing plasmid coverage depth; and it may lead to gDNA sequences being misinterpreted as plasmid sequence variants or exogenous contamination. Creative Biogene's mitigation strategies include: removal of gDNA by gel purification or restriction digestion before library construction; and setting up the host genome as a filtering database in bioinformatics analysis to remove host-derived reads, retaining only plasmid sequences for variant detection.
Q3: What sequencing depth is sufficient for plasmid sequencing to support an IND submission?
A: For routine full-length sequence confirmation, ≥50× coverage is sufficient. For low-frequency variant detection, ≥500× coverage is recommended to ensure statistical confidence for 1% VAF. Creative Biogene can recommend coverage targets based on the client's risk tolerance level and confirm the LOD during method validation.
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