STRC is too big for one AAV. The coding sequence is 5,325 base pairs. AAV packaging limit is 4,700. That is why Iranfar et al. (2026) use a dual-vector approach: split the gene into two halves, package in two separate AAV particles, inject both, hope they recombine inside the same cell.
Hope is the key word. For dual-vector to work, both viruses must enter the same cell. Then the two halves must find each other and recombine. Each step has its own probability. The losses multiply.
Our mini-STRC (residues 594-1775, 3,546 bp) fits in a single AAV. One particle. One transduction event. No recombination needed. This is not an incremental improvement. The math shows it is a different category.
Viral particle uptake follows a Poisson distribution. If the average number of particles reaching a cell (MOI) is λ, then probability of getting at least one is P = 1 - e-λ.
For dual-vector: both AAV-A and AAV-B must enter the same cell. If they share the same receptors (AAVR/HSPG), they compete. Each gets half the effective MOI. Co-transduction probability = P(A≥1) × P(B≥1).
Even after co-transduction, the two halves must recombine intracellularly. Published efficiency: 10-30% (overlap or split-intein designs).
We calibrate delivery efficiency from Omichi et al. (2020): single AAV2 via RWM achieved 83.9% OHC transduction in mice at 3.75×1012 GC/mL. This implies an effective delivery rate of 0.12% of injected particles actually reaching OHC. Most particles stay in perilymph, bind non-target cells, or degrade.
Omichi et al. (2020). Cochlear gene therapy with dual-AAV. PMC7270144
At standard clinical titer (3.75×1012 GC/mL), single-vector mini-STRC delivers functional protein to 56.5 times more hair cells than dual-vector full STRC. The dual-vector approach is further penalized by 10-30% intracellular recombination efficiency, which is already included in the 1.2% figure above.
The gap between single and dual widens at clinically realistic titers. Only at extreme titers (>1013 GC/mL, difficult to manufacture) does dual approach single efficiency.
| Viral titer | Single-vector | Dual-vector | Gap |
|---|---|---|---|
| 10¹⁰ | 0.3% | 0% | +0.3% |
| 3×10¹⁰ | 0.9% | 0% | +0.9% |
| 10¹¹ | 2.9% | 0% | +2.9% |
| 3×10¹¹ | 9% | 0.1% | +9% |
| 10¹² | 25.8% | 0.5% | +25.3% |
| 3.75×10¹² ← clinical | 67.4% | 1.2% | +66.2% |
| 10¹³ | 94.9% | 27.7% | +67.3% |
| 3×10¹³ | 100% | 82% | +18% |