In juvenile macular degeneration (JMD) (e.g Stargardt's, Best's) and common dry age-related macular degeneration (dAMD), cellular and biochemical debris accumulates within and beneath the retinal pigment epithelium (RPE). These diseases reflect, in part, naturally occurring circadian shedding of rod and cone photoreceptor (PR) outer segment tips, and phagocytosis and lysosomal digestion by RPE cells. In the human parafovea, where dAMD starts, a single RPE cell underlies about 30-35 rods and a few cones. Due to RPE digestive limitations excess age-related materials called lipofuscin (LF) accumulate in RPE phagolysosomes. LF contains protein, lipid and carbohydrate components and contributes to sub-RPE deposits (flecks, drusen) seen in JMD and dAMD. LF has a brilliant autofluorescence under blue light excitation due to the dominant presence of toxic bis-retinoid pyridinium salts (A2E) and retinaldehyde dimers (RetDi). These derive from covalent reaction, in PR outer segments, of two molecules of all-trans-retinal (ATR), resulting from visual pigment bleaching, with a single molecule of the membrane aminolipid phosphatidyl-ethanolamine (PE). Autofluorescent LF pigments accumulate with age in normals, and by age 30 are readily quantitated by fundus autofluorescence (FAF). Accumulation of A2E and RetDi in RPE cells reflects the normal daily accumulation of ATR from the visual cycle, with bleaching and regeneration of rod and cone opsins, integrated over many years. In JMD and dAMD A2E and RetDi accumulation rates are accelerated. A2E and the more potent RetDi exert many toxic effects on the RPE cells and directly promote apoptosis. Accumulation of A2E/Ret-Di precedes spatial geographic loss of RPE cells and overlying PRs in dAMD/JMD. A2E and RetDi are well validated molecular targets for therapy of dAMD/JMD. Our hypothesis is that dAMD/JMD can be treated by reducing time-dependent accumulation of A2E and Ret-Di in RPE cells. The rationale is that steady- state reductions in A2E/RetDi would decrease its time-integrated toxicity and maintain viable RPE cells longer into life. This effect would act to preserve overlying PRs, maintain central vision, and slow or halt emergence of geographic macular atrophy. The long term objective is to develop a safe and effective gene therapy for dAMD/JMD. The objective of the proposed experiments is to use hammerhead ribozymes (hhRz) or RNA interference (shRNA) as genetic tools to knockdown (KD) expression of key proteins in rod PRs or RPE cells that quantitatively contribute to daily accumulation of A2E/RetDi in RPE. This novel strategy is tested in a new mouse model of dAMD/JMD (ABCR-/-//RDH8-/- double knockout), which has central inferior RPE and PR loss due to A2E/RetDi accumulation. The strategy is: 1) reduce rhodopsin (RHO) to constrain ATR formation that results mostly from rod pigment bleaching, and 2) constrain retinoid cycle regeneration rates by reduction of 11-cis-retinol dehydrogenases (RDH5/RDH11). By reducing the amount of RHO that forms and bleaches in rod PRs, daily ATR production will be reduced under normal lighting. As ATR is a substrate of A2E and RetDi formation, reduction in the rate of toxic retinoid accumulation is expected. After determining safe KD levels of targets (RHO, RDH5/RDH11) by hhRzs/shRNAs, the expected outcome is that reduction of these targets will rescue A2E/RetDi-mediated retinal degeneration in the mouse model, at the expense of slight scotopic sensitivity loss (<-0.3 log) and preserved photopic sensitivity (cones use a retinoid visual cycle involving M|ller cells).
Specific Aims are:
Aim 1. Identify and optimize lead candidate hhRz and shRNA expression constructs to target mouse RHO and RDH5 and RDH11 for specific knockdown.
Aim 2. Determine maximum tolerable (nontoxic) knockdown levels of RHO and RHD5/RDH11 by hhRzs/shRNAs transduced to photoreceptors or RPE by rAAV vectors after subretinal delivery.
Aim 3. Test for rescue of retinal degeneration in the ABCR-/-//RDH8-/- mouse model of JMD and dAMD by knockdown of RHO and RHD5/RDH11 targets following subretinal delivery of rAAV hhRz/shRNA expression constructs.
Potential Impact on Veterans Health: AMD is the most common form of visual disability in the elderly and is a health care crisis. There are 4 million people in the US with severe visual disability due to AMD, and this number is expected to triple by the year 2020. There are 10-15 million others who have early forms of the disease. AMD prevalence increases exponentially over the age of 55 and, with our aging society, there are many millions of American Veterans who will suffer from visual loss due to AMD. I see Veterans in my Retinal and Macular Degeneration clinic and can attest to the suffering that AMD causes. Nonspecific vitamin support is the only approved therapy for dry AMD, the most common form (80-85%). As yet, there is no specific therapy for dry AMD, which promotes profound visual disability in about 20% of people. This proposed study is a proof-of-concept test of a novel gene therapy strategy for dry AMD. Success in a mouse model is a first essential step toward human clinical trials, which could be conducted in the VA system.