Retinitis Pigmentosa (RP) is one of the most frequent causes of hereditary blindness in the United States, affecting approximately 1 in every 3000 individuals. There is currently no treatment available for this disease. The broad, long term objectives of this study are to develop a therapy for RP. Some forms of RP in humans, dogs and mice (rd) are caused by mutations in the beta subunit of cGMP phosphodiesterase (bPDE). In order to deliver a normal cDNA encoding bPDE to the rd retina, we have developed a novel class of safer and less-toxic adenovirus vectors termed Encapsidated Adenovirus Minichromosomes - (EAMs or 'gutted' vectors). EAMs have a 36Kb cloning capacity, allowing insertion of large upstream regulatory elements for regulated transgene expression and/or inclusion of multiple therapeutic transgene cassettes. We have used EAMs to temporarily rescue retinal degeneration in rd1 mice by approximately 10 weeks, which would otherwise be complete by 3 weeks postnatal. Our objectives are now focused on extending this short period of rescue to one that might be more therapeutically relevant to humans. EAMs bind to their target cells by attachment of the antenna-like fiber protein to the Coxsackievirus B-adenovirus Receptor (CAR). We have determined that CAR is not significantly expressed on murine and human photoreceptors which could explain the low levels of infection by EAMs and adenovirus vectors. In order to overcome this problem, we propose to modify the structure of EAMs such that they now bind cell surface sialic acid instead of CAR. Sialic acid is abundantly present on the rod cell membrane. We also propose to test adenovirus fibers recently shown to have significantly enhanced tropism for neurons. We will use these improved EAMs to deliver bPDE to the rd10 retina. We will measure therapeutic effects on photoreceptors by PDE assays, histology, electroretinograms and western analysis. Upon completion of this study, we will have constructed and tested a vector which will have potential use in a Phase 1 clinical trial for treatment of RP in humans.
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