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B.D. Perkins, J. Bilotta; Mutation of the Zebrafish choroideremia Gene (Rab Escort Protein 1) Causes Photoreceptor Degeneration and Loss of Visual Function . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4557.
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The survival and maintenance of vertebrate photoreceptors and the retinal pigment epithelium (RPE) requires intracellular vesicular transport mediated by Rab GTP–binding proteins. Rab proteins require a geranyl–geranyl modification to bind target membranes and recognize cargo. The Rab escort protein 1 (REP1) binds newly synthesized Rab proteins and facilitates the addition of the geranyl–geranyl groups. Mutations in REP1 lead to choroideremia, an X–linked retinal degeneration characterized by photoreceptor death, atrophy of the RPE, and severe visual defects. To gain insight into the pathology of choroideremia, we analyzed retinas from zebrafish ru848 mutants, which harbor a stop codon in the second exon of the zebrafish REP1 gene. Zebrafish REP1 mutants were examined for normal rhodopsin localization, photoreceptor survival, visual function, and organization of the RPE.
Photoreceptor and RPE defects in the zebrafish choroideremia mutant were examined by light microscopy and transmission electron microscopy. Immunohistochemistry was used to determine the localization of several photoreceptor proteins and analyze photoreceptor structure. Electroretinograms (ERGs) were obtained to analyze retinal responses to 360, 460, and 560 nm stimuli.
Zebrafish REP1 mutants exhibited phenotypes similar to those of humans with choroideremia. Photoreceptor outer segments were shorter and more disorganized and photoreceptor degeneration was observed. The RPE was patchy and discontinuous with some RPE cells invading the photoreceptor layer. Melanosomes were disorganized and cellular debris was observed in the RPE layer. Rhodopsin staining was observed in the inner nuclear layer, indicating defects in photoreceptor transport. Finally, ERG analysis revealed a dramatic decrease in b– and d–wave amplitudes in the REP1 mutants compared to controls; the slope of the irradiance–response function based on the b–wave component of the REP1 mutant ERGs was very shallow compared to that of controls.
Loss of REP1 results in photoreceptor and RPE degeneration. As REP1 is required for the geranyl–geranylation of Rab proteins, photoreceptors and RPE cells must require normal Rab modification for survival. As the phenotypes we observed closely resemble that of human choroideremia patients, the zebrafish ru848 mutant represents a useful animal model for understanding the pathology of choroideremia.
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