STRC B8 Enhancer Selection
Core claim
OHC-specific expression requires a strong, cell-type-specific enhancer. The ARBITER panel of synthetic enhancers identified B8 as the optimal candidate: high OHC expression with zero ectopic expression in other cell types.
Why this matters
AAV without OHC-specific enhancer risks expression in wrong cells (support cells, neurons, spiral ganglion). Ectopic STRC expression could be toxic or immunogenic. B8 solves this.
CMV/CBA constitutive promoters are always-on — no cell-type selectivity. Myo7a is OHC-favored but too large. Prestin (SLC26A5) enhancer (Zheng 2020) is the closest alternative; B8 from ARBITER outperforms it on the ectopic-expression metric.
B8 details
- Source: ARBITER synthetic enhancer screening panel
- Length: 587 bp (used in current mini-STRC vector)
- OHC expression: high
- Ectopic expression: zero (critical)
- Currently incorporated in mini-STRC construct: SP + B8 + CDS + polyA = 4,103 bp total — see STRC AAV Vector Design
Status
B8 is already incorporated into the mini-STRC construct design. Effectively resolved at the computational level — B8 is the selected enhancer for the therapeutic construct.
ARBITER workflow (Zhao et al. 2025, Neuron)
B8 was identified through the ARBITER pipeline — a systematic in vivo enhancer screening workflow:
- Screen conserved non-coding elements (CNEs) near hearing loss gene loci
- Package reporter constructs in AAV-ie for in vivo testing in mice
- Identify active CNEs, dissect into critical modules (E1, E2)
- Reassemble into synthetic enhancer
B8 is the result of combining E1 + E2 modules from CNEs near the Slc26a5 (prestin) locus.
Key results from Zhao 2025:
- High-dose AAV-ie-B8-Slc26a5 (5×10¹⁰ gc) restored prestin expression to WT-comparable levels in OHCs, particularly apical cochlea
- ABR thresholds and wave 1 amplitudes at 16 kHz: fully restored to WT
- Ectopic expression: zero — no prestin mislocalization in IHCs or vestibular hair cells (unlike CAG promoter, which expressed in both)
The CAG/CMV promoter comparison is important. Current STRC gene therapy research (Iranfar 2026) uses generic promoters. CAG failed to restore hearing in Slc26a5-/- mice — it expressed everywhere, not just OHCs. B8 is OHC-exclusive.
Ultra-Mini synergy (2026-04-21)
Ultra-Mini STRC (residues 1075-1775) is 2,106 bp CDS. With IgK signal peptide + B8 enhancer + WPRE3-compact + bGH polyA, total construct fits within AAV at 3,446 bp between ITRs — 1.25 kb headroom. This combination is not possible with clinical 700-1775 (which cannot fit both B8 and WPRE3-compact). Ultra-Mini unlocks the full OHC-specific + posttranscriptional-boost architecture. See STRC Ultra-Mini Promoter Shortlist.
Open questions
- B8 performance validated in mouse cochlea — but in human OHCs?
- Expression level sufficient for functional STRC replacement?
- Long-term stability of enhancer activity (epigenetic silencing risk)?
- Do CNEs near the STRC locus contain similar enhancer activity? (ARBITER applied to STRC locus = open experiment)
Connections
[applies]STRC AAV Vector Design — B8 is the chosen enhancer in the construct- STRC Mini-STRC Single-Vector Hypothesis — enhancer selection only matters if mini-STRC is the payload
- STRC Ultra-Mini Promoter Shortlist — architecture tradeoff analysis showing B8+WPRE3-compact as winner
[see-also]Sonogenetic STRC Computational Proof — alternative inducible promoter (6xNFAT) instead of constitutive enhancer[see-also]STRC Anti-AAV Immune Response Model — ectopic expression could trigger additional immune response; B8’s specificity reduces this risk[about]Misha