Mutations in the gene BEST1 (formerly VMD2) encoding Bestrophin-1 (Best1) cause 5 clinically distinctinherited retinal degenerative diseases. Among these are Best vitelliform macular dystrophy (BVMD),autosomal dominant vitreoretinochoroidopathy (ADVIRC), and retinitis pigmentosa (RP). The'bestrophinopathies' are characterized by accumulation of lipofuscin in the retinal pigment epithelium(RPE), and fluid and debris filled retinal detachments. BVMD, the most common bestrophinopathy, ischaracterized by diminished central vision resulting from an 'egg-yolk' like vitelliform lesion in thefovea. These lesions eventually become disrupted leading to an atrophic form of macular degeneration.All individuals with BVMD exhibit a depressed electrooculogram (EOG) light peak (LP) with a normalclinical electroretinogram (ERG). The LP is generated by a Ca2+ dependent Cl- conductance across thebasolateral plasma membrane of RPE cells, where Best1 is localized. This led to the hypothesis thatBest1 is a Ca2+ activated Cl- channel (CaCC) that generates the LP, and that BVMD results from loss ofBest1 CaCC activity. We found that Best1 is not required to generate the LP, and that RPE Cl-conductances are normal in Best1 knock-out and knock-in mice carrying a BVMD causing mutation. Inour quest to understand how Best1 mutants cause these symptoms we found that Best1 and Best1mutants antagonize Ca2+ signaling and Ca2+ sensitive Cl- conductances, explaining the diminished LP inBVMD. Best1 mutants also affect cellular pH homeostasis. We have found that bestrophin-2 (Best2)carries a HCO3- conductance in colon cells. Thus, we hypothesize that Best1, like Best2, physiologicallycarries a HCO3- conductance and regulates Ca2+ signaling in RPE cells. Dysfunction of Best1, asoccurs in BVMD, alters pHi and Ca2+ homeostasis, perturbing RPE fluid transport and altering the ioniccomposition of the sub-retinal space, causing the symptoms of BVMD. The pathogenesis of ADVIRCand RP, on the other hand, has been suggested to involve defective Best1 mRNA splicing or defectiveprotein folding or trafficking. We will investigate the function of Best1 in the RPE and the differentialpathogenesis of the bestrophinopathies via 3 specific aims.
In aim 1 we will determine whether RPEcells use an anion channel for HCO3- transport and whether Best1 is that channel or a regulator of it.
In aim 2 we will examine the effects of Best1 on RPE Ca2+ signaling and fluid transport.
In aim 3 we willexplore the novel pathogenic mechanisms potentially underlying ADVIRC and RP in cultured cells andin new knock-in mouse models. Success in these aims will result in identification of Best1 function andthe pathogenic mechanisms of bestrophinopathies beyond our current, limited understanding. It willalso provide information and animal models essential for the development and testing of therapeuticstrategies for the treatment of these currently incurable eye diseases.
Mutations in the gene BEST1; encoding the protein bestrophin-1 (Best1); cause 5 clinicallydistinct retinal degenerative eye diseases in man. Understanding the function of Best1 anddetermining how mutations in Best1 cause disease; is a critical public health objective. Success inthis application will set the stage for identification of therapies to prevent or cure vision loss incarriers of BEST1 mutations; and potentially other blinding eye diseases.
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