Gyrate atrophy (GA) of the choroid and retina is a rare, inherited, blinding chorioretinal degeneration caused by deficiency of the mitochondrial matrix enzyme, ornithine-delta-aminotransferase (OAT). Aside from their visual symptoms, most GA patients are asymptomatic; thus, GA is one of a small number of isolated chorioretinal de generations for which the biochemical defect is known. The human OAT cDNA and structural gene have been cloned, sequenced and mapped (10q26). A cluster of OAT processed pseudogenes map to Xp11-Xp21. More than 50 mutant OAT alleles have been identified in GA. Despite this progress, there is much that remains to be learned about the regulation of OAT, the pathophysiology of the chorioretinal degeneration and how we can prevent it in patients with GA. There are four long-term objectives of this proposal. The first is to extend our understanding of the molecular basis of GA by identifying the cis- and trans-acting regulators of OAT transcription responsible for its localized expression in liver; delineating the mutations causing OAT deficiency in GA; determining the distribution of these mutations in our large collection of GA families (92 pedigrees); determining the functional consequences of these mutations; and correlating the molecular defects with phenotypic variation. The second goal is to investigate potential pathophysiologic mechanisms in GA in order to determine why the retina is particularly sensitive to this systemic biochemical defect. Studies to this end will include an examination of the pattern of expression of enzymes and transport proteins metabolically related to OAT in human ocular tissues. These studies should establish the metabolic requirements met by OAT in the various cell types of the retina.
The third aim i s to continue our long-term evaluation of possible treatments of GA, focusing particularly on reduction of ornithine accumulation with an arginine- restricted diet, focusing particularly on the outcomes of younger patients.
The fourth aim i s to produce and characterize a mouse model of GA by homologous recombination mediated gene knockout. The resultant animals will be characterized biochemically by amino acid and OAT activity determinations and ophthalmologically by ophthalmoscopy, electroretinography and by histopathologic studies of the retinas of affected mice at various ages. The last aim is to utilize these mice for studies relevant to our understanding of the pathophysiology and to determine the requirements (location, amount of activity and optimal timing) for effective somatic gene therapy of GA.
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