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Chloride Channel Activity of Bestrophin Mutants Associated with Mild or Late-Onset Macular Degeneration

From the Department of Cell Biology and The Center for Neurodegenerative Disease, Emory University School of Medicine, Atlanta, Georgia.

PURPOSE. Mutations in the hBest1 (VMD2) gene are linked to various kinds of macular degeneration, including Best vitelliform macular dystrophy (BVMD) and adult-onset vitelliform macular dystrophy (AVMD). The age at onset and severity of disease are quite variable. This study was conducted to examine Cl^– currents generated by six hBest1 mutations (E119Q, A146K, T216I, I295, D312N, and L567F) found in patients having adult-onset macular dystrophies or in BVMD patients having normal electro-oculograms (EOGs), to examine the hypothesis that the severity of disease is related to the effect of the hBest1 mutation on hBest1 Cl^– channel function.

METHODS. Wild-type (WT) hBest1 was mutated by PCR-based mutagenesis. WT and mutant channels were expressed in HEK293 cells and Cl^– currents analyzed by whole-cell patch clamp. The trafficking of proteins to the plasma membrane was tested by cell-surface biotinylation.

RESULTS. All the mutations except L567F and T216I produced a defect in Cl^– channel function. The D312N and I295 mutants do not generate functional Cl^– currents. Furthermore, they inhibit WT hBest1 function. The amplitudes of currents produced by the A146K mutant were smaller than WT and had altered anionic selectivity. The E119Q mutant produced currents similar in amplitude to those of WT, but had altered relative permeability to large anions.

CONCLUSIONS. These findings support the idea that hBest1 mutations produce variable forms of macular dystrophy via dysfunction of hBest1 Cl^– channels. However, because the light peak of the EOG of some patients with the I295, D312N, E119Q, and A243V mutations does not correlate with the Cl^– channel function, the results also support the suggestion that the light peak of the EOG may not be generated solely by hBest1.

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