In common dry age-related macular degeneration (dAMD) and orphan juvenile macular degeneration (JMD) (e.g., Stargardt) cellular and biochemical debris accumulates within and beneath the retinal pigment epithelium (RPE). These diseases reflect 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 and JMD starts, a single RPE cell underlies and supports about 30-35 rods and fewer cones. RPE enzymatic limitations cause age-related lipofuscin to accumulate in phagolysosomes. Lipofuscin has protein, lipid and carbohydrate components and contributes to intra- and sub-RPE deposits (flecks, drusen) seen in JMD/dAMD. Lipofuscin has a brilliant autofluorescence and photoreactivity under blue light excitation due to the dominant presence of toxic bis-retinoids (TBR) such as pyridinium salts (A2E) and retinaldehyde dimers (RetDi). These derive from covalent reaction, in PR outer segments, of two molecules of retinaldehydes, resulting from visual pigment bleaching and regeneration, with lipids and proteins. TBR pigments accumulate in RPE cells normally with age (> 30 years) integrated over many years. Any stressors (e.g., macular degenerative processes) that promote accelerated TBRs buildup will exert toxic apoptotic effects on RPE cells, loss of support of overlying PRs and cell death (e.g., dAMD/JMD). TBRs are validated molecular targets for therapy. Daily turnover of massive amounts of rod visual pigment in the parafovea is a huge burden for the RPE (most metabolically active cells in human body) and contributes to accumulation of intracellular and subretinal debris; 50% reduction of normal human rod rhodopsin (RHO) has no degenerative effect and people still see stars. 90% of rod photoreceptor mass is RHO. RHO is a validatable target. Our hypothesis is that dAMD/JMD can be treated by reducing RHO and related time-dependent accumulation of debris and TBRs in PR and RPE cells. The rationale or strategy is that steady-state reductions in RHO and related TBRs would slow age- related toxic buildup of byproducts, decrease metabolic burden, and maintain viable RPE cells longer into life. This would preserve overlying rods and cones, maintain central cone vision, and slow or halt emergence of geographic 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) RHO expression to relieve daily outer retinal stresses (TBRs, metabolic stresses). This novel strategy is tested in multiple mouse models of dAMD/JMD (ABCR-/-//RDH8-/-; ELOVL4mut, SOD2ko), which all have outer retinal PR/RPE degenerations. The expected outcome is that RHO reduction will rescue outer retinal degenerations, with preserved photopic sensitivity (cones use a Mller cell retinoid visual cycle) at the expense of slight scotopic sensitivity loss (< -0.3 log) but preserved scotopic function.
Specific Aims are:
Aim 1. Optimize the novel lead candidate anti-RHO hhRzs, and shRNAs and develop stable, non-toxic cellular expression constructs for these agents.
Aim 2. Phase I model study for toxicity of candidate therapeutic agents (hhRz, shRNA) after subretinal or intravitreal rAAV delivery in a mouse humanized for normal RHO.
Aim 3. Phase II model study for rescue of retinal degeneration after subretinal or intravitreal rAAV delivery of anti-RHO therapeutic candidates in multiple mouse models of dAMD/JMD, with diverse stresses, measured as changes in the intrinsic rate of retinal degeneration or protection against photoreceptor loss in a retinal light damage paradigm. dAMD is highly prevalent in our aging Veteran population and is a visually devastating disease. Currently, vitamins are the only (limited) therapy. Successful rescue in mouse models sets the stage for preclinical- Investigational New Drug (IND) testing in nonhuman primate dAMD models, as a step toward clinical trials of a novel dAMD gene therapy in the VA Healthcare System.
Potential Impact on Veterans Health. AMD is the most common form of visual disability in the elderly and is a health care crisis. Four million people in the US have severe visual disability due to AMD, and this burden is expected to triple by 2020. There are 10-15 million others who have early forms of the disease. Prevalence increases exponentially over the age of 55 and, with an aging society, there are millions of American Veterans added to these ranks each year. I see Veterans in my unique Retinal and Macular Degeneration clinic and attest to the suffering AMD causes. Nonspecific vitamin support is the only approved therapy for dry AMD (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. The proposed study continues development and preclinical testing of a novel gene therapy strategy for dry AMD. Success in diverse mouse models is a first essential step toward human clinical trials, which could be conducted in the VA Healthcare System.
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