Enhanced accumulation of A2E in individuals homozygous or heterozygous for mutations in BEST1 (VMD2)

Disease-associated missense mutations in bestrophin-1 affect cellular trafficking and anion conductance

Bestrophin-1, an integral membrane protein encoded by the BEST1 gene, is localized predominantly to the basolateral membrane of the retinal pigment epithelium. Mutations in the BEST1 gene have been associated with Best vitelliforme macular dystrophy (BMD), a central retinopathy with autosomal dominant inheritance and variable penetrance. Over 120 disease-causing mutations are known, the majority of which result in amino acid substitutions within four mutational hot-spot regions in the highly conserved N-terminal half of the protein. Although initially thought to impair Cl⁻ channel function, the molecular pathology of BEST1 mutations is still controversial. We have analyzed the subcellular localization of 13 disease-associated BEST1 mutant proteins in polarized MDCK II cells, an established model of apical to basolateral protein sorting. Immunostaining demonstrated that nine of the 13 mutant proteins failed to integrate into the cell membrane. The defective proteins were predominantly retained in the cytoplasm, whereas wild-type bestrophin-1 revealed cell membrane localization. Functional analysis of I⁻ fluxes in HEK-293 cells showed that all mutants exhibited a significant reduction in anion conductance. Our data indicate that defective intracellular trafficking could be a common cause of BMD accompanied by impaired anion conductance, representing a loss of anion channel function that is probably due to mistargeting of mutant protein.

WITHDRAWN: Ultrastructure of the human retina in aging and various pathological states

This article has been withdrawn at the request of the author(s) and/or editor. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.

Phenotype and genotype of patients with autosomal recessive bestrophinopathy

PURPOSE

To describe the phenotype and genotype of patients with autosomal recessive bestrophinopathy.

METHODOLOGY

The phenotype of the subjects was described after a complete ophthalmological examination, and in various cases, ancillary testing of the visual field, optical coherent tomography, full field electroretinography and electrophysiology. Genetic analysis was carried out by screening the Bestrophin-1 (BEST1) gene for mutations by dideoxy sequencing and segregation analysis.

RESULTS

We identified three previously described mutations (Ala195Val, Leu191Pro and Arg141His) and two potentially pathogenic changes (Trp93Pro and Trp287Ter) in the Best-1 gene. Two patients carried compound heterozygous mutations, Trp93Pro/Ala195Val, and Leu191Pro/Trp287Ter. Two sisters were homozygous for an Arg141His mutation. All individuals with Best1 gene mutations had signs of maculopathy.

CONCLUSIONS

Our observations expand the limited number of phenotypes associated with mutations in the Best1 gene. Patients with compound heteroyzygous Best1 mutations developed atypical forms of Best disease. Two siblings with homozygous Arg141His mutation developed symptoms of typical Best vitelliform dystrophy while their parents had clinical features of mild maculopathy.

Interaction of bestrophin-1 and Ca2+ channel β-subunits: identification of new binding domains on the bestrophin-1 C-terminus

Bestrophin-1 modulates currents through voltage-dependent L-type Ca²⁺ channels by physically interacting with the β-subunits of Ca²⁺ channels. The main function of β-subunits is to regulate the number of pore-forming Ca_V-subunits in the cell membrane and modulate Ca²⁺ channel currents. To understand the influence of full-length bestrophin-1 on β-subunit function, we studied binding and localization of bestrophin-1 and Ca²⁺ channel subunits, together with modulation of Ca_V1.3 Ca²⁺ channels currents. In heterologeous expression, bestrophin-1 showed co-immunoprecipitation with either, β3-, or β4-subunits. We identified a new highly conserved cluster of proline-rich motifs on the bestrophin-1 C-terminus between amino acid position 468 and 486, which enables possible binding to SH3-domains of β-subunits. A bestrophin-1 that lacks these proline-rich motifs (ΔCT-PxxP bestrophin-1) showed reduced efficiency to co-immunoprecipitate with β3 and β4-subunits. In the presence of ΔCT-PxxP bestrophin-1, β4-subunits and Ca_V1.3 subunits partly lost membrane localization. Currents from Ca_V1.3 subunits were modified in the presence of β4-subunit and wild-type bestrophin-1: accelerated time-dependent activation and reduced current density. With ΔCTPxxP bestrophin-1, currents showed the same time-dependent activation as with wild-type bestrophin-1, but the current density was further reduced due to decreased number of Ca²⁺ channels proteins in the cell membrane. In summary, we described new proline-rich motifs on bestrophin-1 C-terminus, which help to maintain the ability of β-subunits to regulate surface expression of pore-forming Ca_V Ca²⁺-channel subunits.

Morphological and functional changes in multifocal vitelliform retinopathy and biallelic mutations in BEST1

PURPOSE

To describe morphological and functional changes in a single patient with multifocal Best vitelliform macular dystrophy (BVMD) and to perform a genotype/phenotype correlation.

METHODS

The proband with multifocal BVMD and three of her family members were examined with electrooculography (EOG), full-field electroretinography (full-field ERG), multifocal electroretinography (mfERG) and optical coherence tomography (OCT). Genomic DNA was screened for mutation in the BEST1 gene by DNA sequencing analysis.

RESULTS

The proband was observed regularly during a follow-up period of 4 years. Full-field ERG demonstrated reduced and delayed responses of both rods and cones. OCT demonstrated intra- and subretinal fluid which seemed to fluctuate with periods of stress, similar to that seen in chronic central serous chorioretinopathy. Two distinct heterozygous BEST1 mutations were identified in the proband, the recurrent p.R141H mutation and the p.P233A mutation. Heterozygous p.R141H mutations were also identified in two family members, while p.P233A was a de novo mutation. Abnormal EOG findings were observed in both the proband and in the carriers of p.R141H. Heterozygous carriers showed delayed implicit times in a- and b-waves of combined total rod and cone full-field ERG responses.

CONCLUSIONS

The p.R141H mutation is frequently seen together with multifocal vitelliform retinopathy and biallelic mutations in BEST1. Our results show that carriers of the p.R141H mutation are clinically unaffected but present with abnormal EOG and full-field ERG findings. A patient with biallelic mutations of the BEST1 gene, causing multifocal BVMD with progressive, widespread functional disturbance of the retina, confirmed by full-field and mfERG is described.

C20-D3-vitamin A slows lipofuscin accumulation and electrophysiological retinal degeneration in a mouse model of Stargardt disease

Stargardt disease, also known as juvenile macular degeneration, occurs in approximately one in 10,000 people and results from genetic defects in the ABCA4 gene. The disease is characterized by premature accumulation of lipofuscin in the retinal pigment epithelium (RPE) of the eye and by vision loss. No cure or treatment is available. Although lipofuscin is considered a hallmark of Stargardt disease, its mechanism of formation and its role in disease pathogenesis are poorly understood. In this work we investigated the effects of long-term administration of deuterium-enriched vitamin A, C20-D(3)-vitamin A, on RPE lipofuscin deposition and eye function in a mouse model of Stargardt's disease. Results support the notion that lipofuscin forms partly as a result of the aberrant reactivity of vitamin A through the formation of vitamin A dimers, provide evidence that preventing vitamin A dimerization may slow disease related, retinal physiological changes and perhaps vision loss and suggest that administration of C20-D(3)-vitamin A may be a potential clinical strategy to ameliorate clinical symptoms resulting from ABCA4 genetic defects.

Deuterium enrichment of vitamin A at the C20 position slows the formation of detrimental vitamin A dimers in wild-type rodents

Degenerative eye diseases are the most common causes of untreatable blindness. Accumulation of lipofuscin (granular deposits) in the retinal pigment epithelium (RPE) is a hallmark of major degenerative eye diseases such as Stargardt disease, Best disease, and age-related macular degeneration. The intrinsic reactivity of vitamin A leads to its dimerization and to the formation of pigments such as A2E, and is believed to play a key role in the formation of ocular lipofuscin. We sought a clinically pragmatic method to slow vitamin A dimerization as a means to elucidate the pathogenesis of macular degenerations and to develop a therapeutic intervention. We prepared vitamin A enriched with the stable isotope deuterium at carbon twenty (C20-D(3)-vitamin A). Results showed that dimerization of deuterium-enriched vitamin A was considerably slower than that of vitamin A at natural abundance as measured in vitro. Administration of C20-D(3)-vitamin A to wild-type rodents with no obvious genetic defects in vitamin A processing, slowed A2E biosynthesis. This study elucidates the mechanism of A2E biosynthesis and suggests that administration of C20-D(3)-vitamin A may be a viable, long-term approach to retard vitamin A dimerization and by extension, may slow lipofuscin deposition and the progression of common degenerative eye diseases.

Multimodal fundus imaging in Best vitelliform macular dystrophy

BACKGROUND

Best vitelliform macular dystrophy (BVMD) is a rare autosomal dominant retinal disease of highly variable phenotypic expression. Interpretations of disease mechanisms based on histopathology, electrophysiology, genetic analysis, and retinal imaging are somewhat discordant in fundamental issues such as the location and extension of primary retinal changes. Herein we describe the morphological macular features in patients with BVMD undergoing simultaneous multimodal fundus imaging and compare to those of normal age-matched subjects.

METHODS

Comparative study including seven patients with BVMD (14 eyes) and seven age-matched healthy subjects (14 eyes). All participants were submitted to complete ophthalmological examination, fundus photography, and standardized multimodal fundus imaging protocol including Fourier-domain optical coherence tomography (Fd-OCT) combined with near-infrared reflectance and blue-light fundus autofluorescence (FAF).

RESULTS

In two eyes in the "subclinical" stage, Fd-OCT revealed thickening of the middle highly reflective layer (HRL) localized between the photoreceptors' inner/outer segments junction (inner-HRL) and RPE/Bruch's membrane reflective complex (outer-HRL) throughout the macula. In one eye in the "vitelliform" stage, a homogeneous hyper-reflective material on Fd-OCT was observed between the middle-HRL and outer-HRL; this material presented increased fluorescence on FAF. The outer nuclear layer (ONL) was thinned in the central macula and subretinal fluid was not identified in these earlier disease stages. In patients of "pseudohypopyon" (two eyes), "vitelliruptive" (eight eyes) and "atrophic" (one eye) stages, Fd-OCT revealed a variety of changes in the middle- and inner-HRLs and thinning of ONL. These changes were found to be associated with the level of visual acuity observed. Thickening of the middle-HRL was observed beyond the limits of the clinically evident macular lesion in all eyes.

CONCLUSIONS

Multimodal fundus imaging demonstrated thickening of the reflective layer corresponding to the photoreceptors' outer segments throughout the macula with no subretinal fluid accumulation as the earliest detectable feature in BVMD. Changes detected in the photoreceptors' reflective layers (middle- and inner- HRLs) and ONL thinning seemed to be progressive with direct implications for the level of visual acuity impairment observed among the different stages of the disease.

Bestrophins and retinopathies

Best vitelliform macular dystrophy (BVMD, also called Best's disease) is a dominantly inherited, juvenile-onset form of macular degeneration, which is characterized by abnormal accumulation of yellow pigment in the outer retina and a depressed electro-oculogram light peak (LP). Over 100 disease-causing mutations in human bestrophin-1 (hBest1) are closely linked to BVMD and several other retinopathies. However, the physiological role of hBest1 and the mechanisms of retinal pathology remain obscure partly because hBest1 has been described as a protein with multiple functions including a Ca2+-activated Cl- channel, a Ca2+ channel regulator, a volume-regulated Cl- channel, and a HCO3- channel. This review focuses on how dysfunction of hBest1 is related to the accumulation of yellow pigment and a decreased LP. The dysfunction of hBest1 as a HCO3- channel or a volume-regulated Cl- channel may be associated with defective regulation of the subretinal fluid or phagocytosis of photoreceptor outer segments by retinal pigment epithelium cells, which may lead to fluid and pigment accumulation.

Suppression of Ca2+ signaling in a mouse model of Best disease

Mutations in BEST1, encoding bestrophin-1 (Best1), cause Best vitelliform macular dystrophy (BVMD), a dominantly inherited macular degeneration characterized by a diminished electrooculogram light peak (LP), lipofuscin in retinal pigment epithelial cells (RPE), and fluid- and debris-filled retinal detachments. To understand the pathogenesis of BVMD we generated knock-in mice carrying the BVMD-causing mutation W93C in Best1. Both Best1(+/W93C)and Best1(W93C/W93C) mice had normal ERG a- and b-waves, but exhibited an altered LP luminance response reminiscent of that observed in BVMD patients. Morphological analysis identified fluid- and debris-filled retinal detachments in mice as young as 6 months of age. By 18-24 months of age Best1(+/W93C)and Best1(W93C/W93C) mice exhibited enhanced accumulation of lipofuscin in the RPE, and a significant deposition of debris composed of unphagocytosed photoreceptor outer segments and lipofuscin granules in the subretinal space. Although Best1 is thought to function as a Ca(2+)-activated Cl(-) channel, RPE cells from Best1(W93C) mice exhibited normal Cl(-) conductances. We have previously shown that Best1(-/-) mice exhibit increased [Ca(2+)](i) in response to ATP stimulation. However, ATP-stimulated changes in [Ca(2+)](i) in RPE cells from Best1(+/W93C) and Best1(W93C/W93C) mice were suppressed relative to Best1(+/+) littermates. Based on these data we conclude that mice carrying the Best1(W93C) mutation are a valid model for BVMD. Furthermore, these data suggest that BVMD is not because of Best1 deficiency, as the phenotypes of Best1(+/W93C) and Best1(W93C/W93C) mice are distinct from that of Best1(-/-) mice with regard to lipofuscin accumulation, and changes in the LP and ATP Ca(2+) responses.

Phenotypic variability due to a novel Glu292Lys variation in exon 8 of the BEST1 gene causing best macular dystrophy

OBJECTIVE

To study the phenotypic characteristics of patients with a novel p.E292K mutation in BEST1.

METHODS

Affected individuals underwent ophthalmic examination and testing that included photography, autofluorescence, optical coherence tomography, and electrophysiological testing. Their DNA was analyzed for BEST1 mutations.

RESULTS

Five patients aged 5 to 59 years who expressed the p.E292K mutation in BEST1 were identified in 3 families. Electro-oculographic light-rise was subnormal in all probands and carriers. Carriers had normal findings from fundus examination, multifocal electroretinography, and visual acuity, and were emmetropic or myopic. Only probands had hyperopia and fundus findings typical of Best macular dystrophy. Optical coherence tomography of vitelliform lesions demonstrated retinal pigment epithelium elevation without subretinal fluid; atrophic lesions exhibited disruption of the hyperreflective outer retina-retinal pigment epithelium complex. Intense hyperautofluorescence correlated with the vitelliform lesion.

CONCLUSIONS

Patients with the Glu292Lys variation in BEST1 exhibit intrafamilial and interfamilial phenotypic variability. A disproportionate fraction (26%) of Best disease-causing mutations occurs in exon 8, suggesting that the portion of protein encoded by this exon (amino acids 290-316) may be especially important to bestrophin's function. Relatively good visual acuity with vitelliform lesions can be explained by preservation of the outer retina, demonstrated by optical coherence tomography. Clinical Relevance A novel mutation in this region of BEST1 carries implications for disease pathogenesis.

A2E, a component of lipofuscin, is pro-angiogenic in vivo

A recent study in vitro demonstrated that a major lipofuscin component, A2E, serves as a retinoic acid receptor ligand. The current study investigated the effects of A2E on retinal pigment epithelial (RPE) cells in vivo and was performed to extend the understanding of the effects of A2E. Firstly, subretinal injection of A2E was performed and 3 weeks after the injection, and it was demonstrated that subretinal injection of A2E induced RPE cell death, and concomitant upregulation of vascular endothelial growth factor (VEGF) in the RPE and choroid. The upregulation of VEGF was attenuated by an RARalpha antagonist. Next we performed laser photocoagulation in mice that accumulated A2E either after subretinal injection, by Ccl2 gene knockout or by aging demonstrated that mice that accumulated A2E in the RPE, which showed higher rates of choroidal neobascularization (CNV) formation after weak laser injury than the controls and the formation of CNV was inhibited by an RARalpha antagonist in all models tested. The data suggest that A2E accumulation induces RPE cell death, and concomitant increase of VEGF. Accumulation of A2E alone is not sufficient to induce CNV in vivo, but induces the expression of VEGF in RPE and choroid. The mice that accumulated A2E in RPE cells are vulnerable to CNV development via RAR activation, at least in part.

Functional roles of bestrophins in ocular epithelia

There are four members of the bestrophin family of proteins in the human genome, of which two are known to be expressed in the eye. The gene BEST1 (formerly VMD2) which encodes the protein bestrophin-1 (Best1) was first identified in 1998. Mutations in this gene have now been associated with four clinically distinguishable human eye diseases, collectively referred to as "bestrophinopathies". Over the last decade, laboratories have sought to understand how Best1 mutations could result in eye diseases that range in presentation from macular degeneration to nanophthalmos. The majority of our knowledge comes from studies that have sought to understand how Best1 mutations or dysfunction could induce the classical symptoms of the most common of these diseases: Best vitelliform macular dystrophy (BVMD). BVMD is a dominant trait that is characterized electrophysiologically by a diminished electrooculogram light peak with a normal clinical electroretinogram. This together with the localization of Best1 to the retinal pigment epithelium (RPE) basolateral plasma membrane and data from heterologous expression studies, have led to the proposal that Best1 generates the light peak, and that bestrophins are a family of Ca(2+) activated Cl(-) channels (CaCCs). However, data from Best1 knock-out and knock-in mice, coupled with the recent discovery of a recessive bestrophinopathy suggest that Best1 does not generate the light peak. Recently Best2 was found to be expressed in non-pigmented epithelia in the ciliary body. However, aqueous dynamics in Best2 knock-out mice do not support a role for Best2 as a Cl(-) channel. Thus, the purported CaCC function of the bestrophins and how loss of this function relates to clinical disease needs to be reassessed. In this article, we examine data obtained from tissue-type and animal models and discuss the current state of bestrophin research, what roles Best1 and Best2 may play in ocular epithelia and ocular electrophysiology, and how perturbation of these functions may result in disease.

Detailed analysis of retinal function and morphology in a patient with autosomal recessive bestrophinopathy (ARB)

The objective of the paper is to study the retinal microstructure and function in a patient with autosomal recessive bestrophinopathy (ARB). Retinal function and morphology assessment in a patient diagnosed with a biallelic mutation in the BEST1 gene (heterozygote mutations: Leu88del17 and A195V) included: full-field electroretinogram (ffERG) and multifocal electroretinogram (mfERG), electro-oculogram (EOG) testing, and imaging with a high-resolution Fourier-domain optical coherence tomography (Fd-OCT) system (UC Davis Medical Center; axial resolution: 4.5 microm, acquisition speed: 9 frames/s, 1,000 A-scans/frame) combined with a flexible scanning head (Bioptigen Inc.). The 11-year old asymptomatic boy showed a well-demarcated retinopathy with deposits. Functional assessment revealed normal visual acuity, reduced central mfERG responses, delayed rod and rod-cone b-wave ffERG responses, and reduced light rise in the EOG. Fd-OCT demonstrated RPE deposits, photoreceptor detachment, elongated and thickened photoreceptor outer segments, but preserved inner retinal layers. In conclusion, ARB associated retinal dystrophy shows functional and morphological changes that overlap with classic Best disease. For the first time, high-resolution imaging provided in vivo evidence of RPE and photoreceptor involvement in ARB.

Retinal carotenoids can attenuate formation of A2E in the retinal pigment epithelium

A2E, an important constituent of lipofuscin in human retinal pigment epithelium (RPE), is thought to mediate light-induced oxidative damage associated with aging and other ocular disorders. Ocular carotenoids in overlying retinal tissues were measured by HPLC and mass spectrometry and were correlated with levels of RPE A2E. We observed a statistically significant increase in total A2E levels in human RPE/choroid with age, and A2E levels in macular regions were approximately 1/3 lower than in peripheral retinal regions of the same size. There was a statistically significant inverse correlation between peripheral retina carotenoids and peripheral RPE/choroid A2E. Prospective carotenoid supplementation studies in Japanese quail demonstrated nearly complete inhibition of A2E formation and oxidation. These findings support current recommendations to increase dietary intake of xanthophyll carotenoids in individuals at risk for macular degeneration and highlight a new potential mechanism for their protective effects-inhibition of A2E formation and oxidation in the eye.

Molecular physiology of bestrophins: multifunctional membrane proteins linked to best disease and other retinopathies

This article reviews the current state of knowledge about the bestrophins, a newly identified family of proteins that can function both as Cl(-) channels and as regulators of voltage-gated Ca(2+) channels. The founding member, human bestrophin-1 (hBest1), was identified as the gene responsible for a dominantly inherited, juvenile-onset form of macular degeneration called Best vitelliform macular dystrophy. Mutations in hBest1 have also been associated with a small fraction of adult-onset macular dystrophies. It is proposed that dysfunction of bestrophin results in abnormal fluid and ion transport by the retinal pigment epithelium, resulting in a weakened interface between the retinal pigment epithelium and photoreceptors. There is compelling evidence that bestrophins are Cl(-) channels, but bestrophins remain enigmatic because it is not clear that the Cl(-) channel function can explain Best disease. In addition to functioning as a Cl(-) channel, hBest1 also is able to regulate voltage-gated Ca(2+) channels. Some bestrophins are activated by increases in intracellular Ca(2+) concentration, but whether bestrophins are the molecular counterpart of Ca(2+)-activated Cl(-) channels remains in doubt. Bestrophins are also regulated by cell volume and may be a member of the volume-regulated anion channel family.

Fundus autofluorescence imaging of retinal dystrophies

Fundus autofluorescence (FAF) is a non-invasive imaging technique that enables the visualization of lipofuscin changes in the retinal pigment epithelium. This study aims to illustrate the spectrum of FAF changes in a variety of retinal dystrophies. For this purpose, we examined patients with retinal dystrophies such as Stargardt disease, Best vitelliform macular dystrophy, and retinal dystrophies associated with mutations in the peripherin/RDS gene. All retinal dystrophies were confirmed by molecular genetic analysis. A broad range of characteristic FAF patterns was observed. Our results indicate that FAF imaging constitutes a useful additive tool in the diagnosis and follow-up of various retinal dystrophies.