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Laurie L Molday, Rando Allikmets, Robert S Molday; Localization and Functional Analysis of ABCA4 Variants Associated with Stargardt Disease. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3191.
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© ARVO (1962-2015); The Authors (2016-present)
ABCA4 is an ATP-binding cassette (ABC) protein that transports N-retinylidene-phosphatidylethanolamine (N-Ret-PE) from the lumen to cytoplasmic leaflet of photoreceptor outer segment disc membranes. Over 800 mutations in ABCA4 are known to cause Stargardt disease (STGD1), a macular degeneration characterized by the accumulation of bisretinoid compounds and a loss in vision. The purpose of this study is to determine the effect of relatively common missense mutations associated with STGD1 on the cellular localization and functional activity of ABCA4.
Wild-type (WT) ABCA4 and variants associated with STGD1 (G863A, N965S, T983A, T1019M, A1038V, R1108C, A1219P, G1961E, R2030Q) containing a 1D4 tag or green fluorescent protein (GFP) were generated by site-directed mutagenesis. ABCA4-1D4 variants were expressed in COS-7 cells for localization by immunofluorescence and in HEK293 cells for purification and analysis of their ATPase and N-Ret-PE tranport activities. Plasmids containing ABCA4-GFP variants were electroporated into the retina of P1-4 Abca4 knockout mice. At P21, the mice were sacrificed and examined for ABCA4-GFP localization by confocal microscopy.
WT ABCA4 localized primarily to the outer segments (OS) of transfected photoreceptors and intracellular vesicles in COS-7 cells. In contrast, STGD1-associated ABCA4 mutants were found in both the inner segment (IS) and outer segment (OS) to varying degrees. Variants such as T983A and A1219P were more abundantly localized to the IS of photoreceptors and retained in the endoplasmic reticulum (ER) of COS-7 cells whereas variants such as N965S and R2030Q localized evenly to the photoreceptor IS and OS and were distributed in both vesicles and the ER of COS-7 cells. WT ABCA4 exhibited basal and N-Ret-PE stimulated ATPase activity and N-Ret-PE transport activity as previously reported. All ABCA4 mutants associated with STGD1 showed significantly reduced basal ATPase activity and little if any N-Ret-PE stimulated ATPase activity or N-Ret-PE transport activity.
STGD1-associated ABCA4 variants showed altered localization in photoreceptors and COS-7 cells presumably due to abnormal protein folding. These mutations also caused a significant loss in the ATPase and N-Ret-PE transport function of ABCA4.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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