AAV Genome Integrity & ITR

AAV Genome Integrity & ITR

Genome-quality characterization for AAV vectors — including ssAAV/scAAV and complex designs such as genome-editing vectors carrying CRISPR components: Using PacBio single-molecule long-read sequencing, individual AAV genomes are read end-to-end from ITR to ITR, quantifying the full-length ratio, truncation and secondary-structure risks, chimeric host DNA, and ITR integrity and heterogeneity, to provide traceable molecular evidence for pre-production design screening, batch release, and IND/BLA characterization.

1. Background

The AAV genome is not a homogeneous population. During plasmid packaging, replication, and capsid loading, genomes can undergo truncation, recombination with heterologous sequences, tandem rearrangements, and changes in ITR orientation and sequence mutations. These events directly bear on vector potency and safety. The ITR is a hairpin structure at each end of the AAV genome; it drives replication, packaging, second-strand synthesis after nuclear entry, and expression regulation. It exists in two mirror-image orientations, flip and flop, and is particularly prone to mutation and deletion during plasmid amplification. Secondary structures and palindromic regions — such as long inverted repeats — can cause polymerase slippage and template switching during replication, leading to large-scale truncations. Particles may also contain chimeric genomes recombined with host DNA, often anchored at the 3′ ITR, that can form episomes and persist long-term; these fragments commonly span 1–2 kb and may include complete genes or promoters.

Risk is particularly pronounced for genome-editing AAVs. When packaging Cas9 with guide RNAs into AAV, the sgRNA backbone contains multiple stem-loops. If a dual-sgRNA design intended for large-fragment deletion is arranged in a tail-to-tail orientation, a long palindromic region results, and the proportion of truly full-length genomes can drop to roughly 0.21%, making it virtually impossible to obtain a functional vector; in contrast, single-sgRNA designs typically show a truncation rate of only about 2.48%. This means design-stage risks must be identified before production, or substantial time and cost losses will result.

Conventional short-read sequencing struggles to read through the strong ITR secondary structure and to distinguish chimeric from tandem integrations; agarose-gel profiles reflect only approximate structure. Answering the questions "is the genome complete from ITR to ITR, where are the truncation hotspots, has the ITR been repaired, and is there heterogeneity within the population?" requires single-molecule long reads spanning ITR to ITR.

From a regulatory standpoint, ICH Q5B (Analysis of the Expression Construct and Genetic Stability), Q6B (Specifications), and Q2(R2) (Validation of Analytical Procedures) provide the general basis for this test. The FDA guidance "CMC Information for Human Gene Therapy INDs" (2020) sets CMC characterization requirements for AAV vector identity, purity, and integrity; the FDA guidance "Human Gene Therapy Products Incorporating Human Genome Editing" (Final, 2024) and "Safety Assessment of Genome Editing Using NGS" (Draft, 2026) provide specific recommendations for product design, manufacturing testing, and NGS-based safety assessment of genome-editing products; and China NMPA/CDE's "Technical Guideline for Pharmaceutical Research and Evaluation of In Vivo Gene Therapy Products (Trial)" (2022) requires clear identification of critical quality attributes (CQAs) for AAV products. This service provides CQA-grade data support along these lines through ITR-to-ITR single-molecule characterization.

2. Technical Principle

This service is built on PacBio single-molecule (SMRT) long-read sequencing, using the customer-provided full vector sequence/map as a custom reference, to read individual AAV genomes end-to-end from ITR to ITR and resolve each one read by read. Its technical characteristics align precisely with the requirements of AAV integrity characterization — reading through the ITR, clearly resolving junctions, and identifying rare events. The core workflow is as follows:

(1) Sample receipt and QC

Receive AAV particles or extracted vector DNA; assess concentration, purity, and integrity; and confirm compliance with library construction requirements.

(2) Strand processing and library preparation

Through strand annealing of the single-stranded genome with adapter ligation strategies, both ssAAV and scAAV can be loaded onto the platform. A spike-in control is added in parallel for downstream artifact investigation.

(3) PacBio HiFi sequencing

Polymerases with strand-displacement activity are used to read through the strong ITR secondary structure, and circular consensus sequences (CCS) yield high per-base accuracy long reads; sequencing depth is set according to target sensitivity.

(4) Bioinformatic analysis and sequence interpretation

Read-by-read ITR-to-ITR alignment is performed against the customer-provided vector as a custom reference. The full-length ratio and read-length distribution are quantified; truncation hotspots are localized; chimeric events are identified; ITR flip/flop orientations, TRS termination positions, and mutation/repair status are resolved; and an integrity characterization report is issued.

Figure 1. Workflow of the AAV Genome Integrity and ITR Analysis service.

3. Technical Features and Advantages

(1) ITR-to-ITR single-molecule end-to-end reads

A single long read spans the entire genome and both ITRs, eliminating the need for short-read assembly and faithfully reflecting the junction structure of truncations, recombinations, and tandem rearrangements.

(2) Compatible with both ssAAV and scAAV

The strand-processing strategy accommodates both single-stranded and self-complementary AAV, addressing the difficulty of directly loading ssAAV.

(3) Fine-grained characterization of ITR quality

Quantitative readout of ITR flip/flop orientation distribution, TRS termination positions, ITR mutations, and the extent of in-process repair.

(4) Reliable identification of chimeric events

Spike-in controls are used to rule out library-construction–derived false positives, ensuring that detected chimeras are genuine biological events.

(5) Custom alignment to the client's vector

The customer-provided full vector sequence/map serves as a custom reference, yielding results that are locus-resolved, quantitative, and comparable across runs.

4. Applications

Genome-editing vector design assessment: Identify high-risk designs such as dual-sgRNA configurations and long palindromes before production, avoiding batches with low full-length ratios.

Batch release and cross-batch comparison: Use the full-length ratio, truncation profile, and ITR configuration as a CQA baseline for AAV product release and stability studies.

Process and plasmid optimization tracking: Verify the extent of ITR repair during production and inform plasmid ITR quality-control strategies.

Rare-event surveillance: Detect chimeric genomes recombined with host DNA to support long-term safety arguments.

IND/BLA characterization: Provide genome-integrity evidence for the regulatory submission of genome-editing and complex-design AAVs.

5. Report and Deliverables

The report provides quantitative, locus-resolved, comparable integrity evidence rather than a single "pass/fail" verdict. Core contents include:

·Full-length genome proportion and read-length distribution.

·Truncation hotspot localization and secondary-structure/palindrome risk assessment: with annotation of tail-to-tail, head-to-head, and long-palindrome structures.

·Chimeric genome detection: types, anchor sites, and abundance of recombinations with host DNA or plasmid backbone.

·ITR integrity and heterogeneity: proportions of the four flip/flop configurations, TRS termination statistics, and ITR mutation and repair rates.

·Actionable recommendations: directions for redesign, sequence correction, or plasmid bank locking based on truncation hotspots and high-risk designs.

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

Figure 2. Schematic of ITR flip/flop configuration distribution and detection of truncations and chimeras.

6. Service Workflow

* Standard turnaround: 40–50 business days.

7. Sample Requirements

* The latest Sample Submission Guide takes precedence. This service is not applicable to severely degraded samples. Please schedule and confirm the study plan before sample submission.

8. Technical Specifications

9. References

[1] International Council for Harmonisation (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). 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] FDA. Human Gene Therapy Products Incorporating Human Genome Editing; Guidance for Industry. Final, January 2024. (Docket No. FDA-2021-D-0398)

[6] FDA. Safety Assessment of Genome Editing Components Delivered in Gene Therapy Products Using Next-Generation Sequencing; Draft Guidance for Industry. 2026. (Docket No. FDA-2026-D-1255)

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

[8] Radukic MT, et al. Sequencing of recombinant adeno-associated virus genomes reveals high frequencies of recombinations and mutations. Nucleic Acids Research, 2020.

[9] Tai PWL, et al. Adeno-associated virus genome population sequencing achieves full vector genome resolution and reveals human-vector chimeras. Molecular Therapy — Methods & Clinical Development, 2018.