Mutations responsible for the blinding human retinal degenerative diseases categorized as retinitis pigmentosa (RP) have been identified in a least nine separate genes, most of which are expressed primarily in photoreceptor cells of the neural retina. These include genes encoding rhodopsin (RHO), betacGMP phosphodiesterase (betaPDE), ROM-1, peripherin/RDS, the cGMP-gated channel, ABCR, RPGR, TULP1 and CRALBP (1-12). Two of these genes, betaPDE and RHO, are particularly interesting with respect to gene therapy as their protein products have been well characterized biochemically and physiologically, animal models with the same mutations as humans exist and there is a large number of human patients with mutations in these genes. Two findings in our laboratory suggest that it may now be possible to treat hereditary retinal degenerations, such as those resulting from loss-of-function mutations in betaPDE or rhodopsin on a long-term if not permanent basis with genetic therapy: 1) Recombinant replication-defective adeno- associated viruses (rAAVs) can be used to transduce terminally differentiated photoreceptors efficiently in vivo with no apparent toxicity (13). Although onset of transgene expression is slow, high levels of expression persist for months and years. For therapy, rAAV- mediated transduction is expected to be permanent if the photoreceptors survive the 3-4 week period after delivery (the time period necessary to obtain high levels of transgene expression with this vector); 2) Adenovirus-mediated delivery of betaPDE to photoreceptors of animal models with betaPDE null mutations results in histological and functional evidence of rescue lasting at least 6 weeks (14). These findings suggest that rAAV-mediated delivery of therapeutic genes to photoreceptors degenerating due to loss-of-function mutations may result in stable transgene expression and long-term if not permanent rescue. The research proposed here aims to determine whether rAAV-mediated introduction of the appropriate wild-type transgene (betaPDE or rhodopsin) can reverse or slow the degeneration in animal models with betaPDE null mutations and rhodopsin haploinsufficiency. Therapeutic and other effects of wild-type betaPDE and rhodopsin will be assessed qualitatively and quantitatively after rAAV-mediated gene transfer. The ability to rescue photoreceptors in the betaPDE or rhodopsin-based animal models could ultimately pave the way for genetic therapy as a treatment for blinding and currently incurable inherited retinal degenerations such as RP.
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