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Summary Table: Retina / Eye-Specific Promoters
| Promoter | Target Cell Type | Size (bp) | Origin | Key Characteristics |
| RHO (Rhodopsin) | Rods | ~0.8 – 2.2 kb | Human / Mouse | The primary promoter for rod-specific targeting. The 2.2 kb version is most specific, but 0.8 kb is common in AAV. |
| GRK1 | Rods & Cones | ~300 bp | Human | G protein-coupled receptor kinase 1. Very popular in AAV because it is tiny and targets both types of photoreceptors. |
| Cone Arrestin (CAR) | Cones | ~0.5 – 1.0 kb | Human | Specific for cone photoreceptors. Excellent for treating conditions like Achromatopsia. |
| IRBP | Photoreceptors | ~250 bp – 1.3 kb | Human / Mouse | Interphotoreceptor Retinoid-Binding Protein. Drives expression in both rods and cones. |
| BEST1 (VMD2) | RPE | ~600 bp | Human | Highly specific for the Retinal Pigment Epithelium (RPE). Used for Best Disease. |
| RPE65 | RPE | ~0.8 – 1.6 kb | Human | Specific to the RPE layer. Famously used in the first FDA-approved AAV therapy (Luxturna). |
| Opsin (L/M/S) | Cone Subtypes | ~0.6 – 1.0 kb | Human | Used to target specific cone populations (Long, Medium, or Short-wavelength sensitive). |
Key Insights for Ocular Vector Design
1. Targeting the RPE vs. Photoreceptors For diseases like Leber Congenital Amaurosis (LCA), the RPE65 or BEST1 promoters are used because the defect lies in the pigmented layer. However, for Retinitis Pigmentosa, researchers focus on RHO or GRK1 to reach the photoreceptor cells themselves.
2. The Versatility of hGRK1 The human GRK1 promoter is a staple in retinal gene therapy. Because it is so small (~300 bp), it allows for the packaging of large genes or multiple regulatory elements. It provides robust expression in both rod and cone cells, making it a “pan-photoreceptor” tool.
3. Enhancer-Promoter Pairs To increase the specificity of cone targeting, researchers often combine the IRBP enhancer with the CAR promoter. This dual-element approach helps ensure that expression is strictly limited to cones, even if the AAV serotype (like AAV2 or AAV5) has a broader tropism.
4. Sub-retinal vs. Intravitreal Injection Promoter specificity is often paired with the delivery method. Sub-retinal injections physically place the virus near the RPE and photoreceptors, while intravitreal injections target the inner retina (like Ganglion cells).
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