About 36,000 cases of simplex and familial retinitis pigmentosa (RP) worldwide are due to defects in rod-specific PDE6, which consists of catalytic (PDE6a and PDE6b) and regulatory (PDE6g) subunits. In mouse models, low PDE6 activity leads to RP-like features. The Pde6brd1 null mouse exhibits dramatic elevation of retinal cGMP and rapid rod degeneration, which is complete within 3 weeks after birth. There are likely multiple changes in intercellular signaling induced by excessive cGMP, but the exact mechanisms underlying these pathways are unknown. A recently discovered weak Pde6b allele, H620Q, can further our understanding of early degeneration mechanisms and enable us to test novel therapeutic hypotheses. Like Pde6brd1 mice, Pde6bH620Q mutants show RP-like features and dramatic elevation of retinal cGMP. However, the time course of degeneration of Pde6bH620Q is significantly slower, with complete rod loss occurring after 6 weeks. Moreover, for the first three weeks after birth Pde6bH620Q display relatively normal rod histology and quantifiable rod physiology, which is not the case in Pde6brd1 mice. Our long-term goal is to use Pde6bH620Q mice to find therapies that inhibit the intracellular effects of excessive cGMP and/or enhance PDE6 specific activity to prevent further rod and cone degeneration.
Aim 1. Establish if lower than normal PDE6 activity, combined with normal guanylate cyclase activity, results in elevated cGMP levels in Pde6bH620Qmutant mouse.
Aim 2. Identify kinomic (kinases in the genome) survival and apoptotic effectors of cGMP rise in Pde6bH620Q mutants before morphological signs of degeneration. Such effectors will provide novel pharmacological targets to retard degeneration.
Aim 3. Determine if progressive rod-cone degeneration can be genetically arrested by increasing PDE6 activity in Pde6b mutant rods. We intent to halt degeneration using Opsin::Pde6b rescue transgene and subretinal injections of Opsin::Pde6b rescue lentivirus. To test if the rod as well as gradual secondary cone loss can be arrested after the onset of rod death, we will employ a tamoxifen-inducible reverse Cre/loxP system to restore wild-type Pde6b in expression in mid-phase of the disease.
Inherited forms of retinal degeneration are incurable and affect about one in 2000 people;1.5 million people worldwide are affected by retinitis pigmentosa (RP). Can the remaining rod and cone photoreceptor death be halted once degeneration has begun? Our proposal addresses a clinically relevant question, as most retinal degeneration patients have significant night blindness (rod death) when they are first seen by an optometrist/ophthalmologist. If one can impede further rod and cone loss by correcting the primary cause of the pathogenesis at the mid-stage of disease, then there is hope for a drug- or gene-based therapy to restore activities of daily living for newly diagnosed patients.
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