AAV Plasmid ITR Integrity & Symmetry

AAV Plasmid ITR Integrity & Symmetry

Dedicated ITR sequence-quality control for AAV cis (vector) plasmids: leveraging PacBio or Nanopore long-read single-molecule sequencing to traverse the T-shaped hairpin structure of the ITR, this service verifies the integrity, symmetry, and sequence correctness of the 5′ and 3′ ITRs, localizes deletions, recombination events, and inversions within the ITR, and intercepts—prior to virus production—plasmid batches that would otherwise propagate ITR defects into the AAV genome.

1. Background

Each end of the AAV genome carries an Inverted Terminal Repeat (ITR) of approximately 145 bp that folds into a T-shaped hairpin secondary structure. The ITR serves both as the recognition site for Rep-mediated replication initiation and as the boundary at which the genome is cleaved and packaged into the capsid—without an intact ITR, no qualified AAV genome can be produced. The highly palindromic, GC-rich, and structurally stable nature of the ITR is precisely what makes it a hotspot for recombination, deletion, and asymmetry during E. coli amplification and serial passaging; the more a plasmid is passaged, the greater the risk that the ITRs become truncated or that the two termini lose symmetry.

ITR defects at the plasmid stage are directly transmitted as truncations and packaging anomalies in the AAV genome, undermining titer and transduction efficiency and increasing product heterogeneity. However, routine analytical methods rarely deliver a definitive answer: Sanger sequencing often stalls or misreads through hairpin structures, and restriction digestion can only indirectly indicate the gross presence of the ITR. Neither approach can address whether the ITRs are fully intact, whether the two ends are symmetric, or whether the sequence is correct.

A healthy AAV cis plasmid should ensure that the 5′ and 3′ ITRs are mirror images of one another, matched in both length and sequence. Once one end of the ITR sustains deletion or partial recombination, the two termini are no longer symmetric, which affects replication and packaging efficiency as well as genome integrity. ITR quality therefore must be assessed not only in terms of presence but also in terms of whether the two ends are identical.

From a regulatory and industry perspective, the ITR is a recognized critical functional element and CQA of AAV products, and the requirement for ITR integrity and sequence correctness is supported by well-established guidance: ICH Q5B (Analysis of the Expression Construct and Genetic Stability), Q6B (Specifications), and Q2(R2) (Method Validation); the FDA’s Chemistry, Manufacturing, and Control (CMC) Information for Human Gene Therapy INDs (2020), which controls production plasmids as starting materials / critical reagents; and the Center for Drug Evaluation (CDE) of China’s NMPA Technical Guideline for Pharmaceutical Research and Evaluation of In Vivo Gene Therapy Products (Trial) (2022), which requires that the sequence correctness, critical quality attributes, and control strategy of starting materials (including plasmids) be clearly defined. This service provides CQA-level data support, aligned with these directives, dedicated to ITR verification.

Figure 1. AAV ITR secondary structure and schematic of how plasmid ITR defects propagate to the AAV genome.

2. Technical Principle

The service is built around long-read single-molecule sequencing (PacBio / Nanopore), using the customer-supplied ITR design / full plasmid sequence as a custom reference for dedicated alignment and analysis of both terminal ITR regions of the AAV cis plasmid. Single-molecule long reads traverse the T-shaped hairpin secondary structure in a single pass, avoiding the stalls and misreads that conventional sequencing suffers in palindromic / hairpin regions; this enables structural events such as ITR deletions, partial recombination, and inversions to be directly localized. The 5′ and 3′ ITRs are independently evaluated for integrity and sequence correctness, and a symmetry comparison is performed between the two ends. The core workflow is as follows:

(1) Sample Receipt and QC

AAV cis plasmid DNA is received and assessed for concentration, purity, and preliminary integrity (including supercoiled topology) to confirm suitability for library construction.

(2) Long-Read Library Construction

Long-read libraries are prepared on the PacBio or Nanopore platform using strategies that favor coverage of the highly structured ITR regions as well as the entire plasmid.

(3) Long-Read Sequencing

Sequencing is performed on the PacBio or Nanopore platform to the targeted depth; depth can be increased on demand to support detection of low-frequency ITR events.

(4) Bioinformatics Analysis and Sequence Interpretation

Alignment is performed against the customer ITR / plasmid sequence as a custom reference. The integrity and symmetry of the two terminal ITRs, the positions and prevalences of deletion / recombination / inversion events, and SNVs / indels relative to the design reference are resolved on a per-read basis. An ITR-focused verification report is then issued.

Figure 2. Workflow of AAV plasmid ITR integrity and symmetry analysis.

3. Technical Features and Advantages

(1) Long-read coverage through the hairpin

Single-molecule long reads read through the T-shaped hairpin of the ITR in a single pass, overcoming the blind spots and ambiguities of Sanger and short-read sequencing in palindromic / hairpin regions, and directly revealing ITR deletions, partial recombination, and inversions.

(2) Dedicated symmetry comparison between the two termini

The 5′ and 3′ ITRs are independently evaluated for integrity and sequence, followed by a length- and sequence-based symmetry comparison—reliably answering whether the two ends are symmetric and, if not, where the asymmetry lies.

(3) Complementary dual-platform strategy

PacBio HiFi provides high per-base accuracy and is well suited to detecting subtle sequence variants and low-frequency heterogeneity within the ITR; Nanopore provides ultra-long reads and rapid turnaround, well suited to structural and recombination screening and to plasmid-bank qualification. The two platforms can be used independently or in an orthogonal combination for mutual cross-validation.

(4) Customer-designed reference for tailored alignment

Alignment is performed against the customer-supplied ITR design / full plasmid sequence as a custom reference, with every deviation from the design reference reported—delivering quantifiable, position-resolved, and comparable conclusions.

(5) Source-to-product evidence chain

Results can be jointly interpreted with particle-level AAV genome integrity and ITR analyses, forming a “plasmid ITR → AAV genome ITR” source-to-product evidence chain that aids in localizing the origin of observed heterogeneity.

4. Application Scenarios

AAV cis plasmid bank qualification: Serves as the ITR sequence baseline for the master / working cis plasmid bank (MCB / WCB), supporting bank release and cross-batch comparison.

Verification after construct optimization or serotype switching: Confirms the integrity, symmetry, and sequence correctness of both ITRs following vector design changes, construct optimization, or serotype switching.

Pre-production cis plasmid release: Completes the ITR-focused verification of the plasmid prior to virus production, preventing upstream ITR defects from being propagated as truncation or packaging anomalies in the downstream AAV genome.

Process / supplier change comparison: Performs ITR consistency comparisons before and after process or plasmid-supplier changes, supporting change assessment.

IND / BLA starting-material characterization: Provides ITR integrity and symmetry evidence for AAV starting materials (cis plasmids) in regulatory submissions.

5. Report and Deliverables

The report provides quantifiable, position-resolved, and comparable ITR sequence evidence rather than a single pass/fail conclusion. Core content includes:

·Integrity assessment of both ITRs: whether the 5′ and 3′ ITRs are intact, including coverage and confidence assessment.

·ITR event spectrum: positions, types, and proportions of deletion / partial recombination / inversion events.

·Symmetry assessment: length- and sequence-based comparison of the 5′ versus 3′ ITRs, annotating regions of asymmetry.

·ITR variant list: SNVs / indels relative to the design reference (with heterogeneity fraction where applicable).

·Actionable recommendations: directional guidance such as re-cloning, switching to an alternative plasmid batch, or locking an ITR-intact plasmid bank, based on observed deviations.

·Data deliverables: complete analysis report (PDF), sequence-alignment / variant data files, and raw sequencing data.

·Optional: combined with particle-level AAV genome integrity and ITR analyses to form a source-to-product evidence chain.

6. Service Workflow

* Turnaround time: standard workflow 40–45 working days.

7. Sample Requirements

* The latest Sample Submission Guide takes precedence. This service is not applicable to severely degraded samples. Please contact us in advance to confirm the scheme prior to shipment.

8. Technical Parameters

9. References

[1] International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH). Q5B: Quality of Biotechnological Products: Analysis of the Expression Construct in Cells Used for Production of r-DNA Derived Protein Products. Current Step 4 version, 30 November 1995.

[2] ICH. Q6B: Specifications: Test Procedures and Acceptance Criteria for Biotechnological / Biological Products. Current Step 4 version, 10 March 1999.

[3] ICH. Q2(R2): Validation of Analytical Procedures. Step 4 version, adopted 1 November 2023.

[4] U.S. Food and Drug Administration (FDA), Center for Biologics Evaluation and Research (CBER). Chemistry, Manufacturing, and Control (CMC) Information for Human Gene Therapy Investigational New Drug Applications (INDs); Guidance for Industry. Final, January 2020. (Docket No. FDA-2008-D-0205)

[5] Center for Drug Evaluation (CDE), National Medical Products Administration of China. Technical Guideline for Pharmaceutical Research and Evaluation of In Vivo Gene Therapy Products (Trial). Notice No. 31 of 2022, issued 26 May 2022. [in Chinese]

[6] Wright JF. Product-related impurities in clinical-grade recombinant AAV vectors: characterization and risk assessment. Biomedicines, 2014, 2(1): 80–97.

[7] Tai PWL, Xie J, Fong K, et al. Adeno-associated virus genome population sequencing achieves full vector genome resolution and reveals human-vector chimeras. Molecular Therapy — Methods & Clinical Development, 2018, 9: 130–141.