A Model of Best Vitelliform Macular Dystrophy in Rats

The VMD2 gene is mutated in individuals with Best vitelliform macular dystrophy (BMD),1,2 an autosomal dominant disease characterized by early-onset degeneration of the macula, 3 a specialized region of the retina with the fovea at its center. VMD2 encodes bestrophin, a 68-kDa transmembrane protein localized to the basolateral plasma membrane of retinal pigment epithelial (RPE) cells.4 Bestrophin is a member of the RFP-TM family of proteins that is named for a highly conserved arginine, phenylalanine, proline (RFP) motif.1,2,5 In humans, four RFP-TM family members including bestrophin have been identified from genomic data.5 BMD is one of several diseases that present clinically with an egg-yolk–like vitelliform lesion in the ocular fundus.6,7 However, the only fully penetrant symptom of the disease is the finding of an abnormal ratio of the electro-oculogram (EOG) light peak (LP) to the dark trough, without aberrations in the a-or b-waves of the clinical electroretinogram (ERG).8,9 This is the defining characteristic of BMD and distinguishes it from other vitelliform dystrophies. The LP can be monitored more precisely using DC amplification of the ERG.10 In vitro studies on chick retina/RPE/choroid preparations have shown that the LP is generated by a depolarization of the basolateral plasma membrane of the RPE due to activation of Cl conductance11,12 and studies of RPE Cl conductances (reviewed in Ref. 13) indicate that a Ca2+ sensitive Cl channel probably underlies the LP. It has been proposed that bestrophin functions directly as a Ca2+-sensitive Cl channel, based on patch-clamp studies of bestrophin and other RFP-TM family members heterologously overexpressed in cell culture.14–16 Although these patch-clamp studies suggest a potential function for bestrophin, understanding the etiology of BMD requires animal models that recapitulate the major symptoms of the disease. Therefore, we set out to develop a model of BMD by transiently overexpressing mutant forms of bestrophin in the eyes of rats, by using adenovirus vectors. Our data indicate that the amplitude of the LP is indeed depressed by overexpression of BMD-associated bestrophin mutants without affecting the a- or b-waves of the ERG. These are exactly the symptoms observed in patients with BMD. Overexpression of wild-type (wt) besstrophin did not result in an increase in the LP, which suggests either that the limiting step in generating the LP occurs before activation of bestrophin channels or that bestrophin is not responsible directly for the LP conductance.