BEST1: the Best Target for Gene and Cell Therapies
Authors: Tingting Yang, Sally Justus, Yao Li, Stephen H Tsang
Journal: Molecular Therapy (2015)

A retinal pigmented epithelial (RPE) disorder, bestrophinopathy has recently been proven to be amenable to gene and cell-based therapies in preclinical models. RPE disorders and allied retinal degenerations exhibit significant genetic heterogeneity, and diverse mutations can result in similar disease phenotypes. Several RPE disorders have recently become targets for gene therapies in humans. The year 2011 brought a new advance in cell-based therapies, with the Food and Drug Administration approving clinical trials using embryonic stem cells for an RPE disorder known as age-related macular degeneration. Recent studies on induced pluripotent stem (iPS)-RPE generation indicate strong potential for developing patient-specific disease models in vitro, which could eventually enable personalized treatment. This mini-review will briefly highlight the suitability of the retina for gene and cell therapies, the pathophysiology of bestrophinopathy, and the research and treatment opportunities afforded by stem cell and genetic therapies. A retinal pigmented epithelial (RPE) disorder, bestrophinopathy has recently been proven to be amenable to gene and cell-based therapies in preclinical models. RPE disorders and allied retinal degenerations exhibit significant genetic heterogeneity, and diverse mutations can result in similar disease phenotypes. Several RPE disorders have recently become targets for gene therapies in humans. The year 2011 brought a new advance in cell-based therapies, with the Food and Drug Administration approving clinical trials using embryonic stem cells for an RPE disorder known as age-related macular degeneration. Recent studies on induced pluripotent stem (iPS)-RPE generation indicate strong potential for developing patient-specific disease models in vitro, which could eventually enable personalized treatment. This mini-review will briefly highlight the suitability of the retina for gene and cell therapies, the pathophysiology of bestrophinopathy, and the research and treatment opportunities afforded by stem cell and genetic therapies. RPE disorders, including bestrophinopathy, some forms of retinitis pigmentosa, and age-related macular degeneration, affect more than 10 million Americans, an incidence that is expected to double by 2020 and will place a significant burden on public health.1Friedman DS O'Colmain BJ Muñoz B Tomany SC McCarty C de Jong PT Eye Diseases Prevalence Research Group et al.Prevalence of age-related macular degeneration in the United States.Arch Ophthalmol. 2004; 122: 564-572Crossref PubMed Scopus (77) Google Scholar Aside from cancer, blindness is one of the most feared medical conditions among Americans,2Rosenberg EA Sperazza LC The visually impaired patient.Am Fam Physician. 2008; 77: 1431-1436PubMed Google Scholar and for many patients, the diagnosis of an eye disease implies reduced independence and limited ability to conduct normal, day-to-day activities. Several of these debilitating blinding disorders have recently become targets for gene3Bainbridge JW Smith AJ Barker SS Robbie S Henderson R Balaggan K et al.Effect of gene therapy on visual function in Leber's congenital amaurosis.N Engl J Med. 2008; 358: 2231-2239Crossref PubMed Scopus (1610) Google Scholar,4Barnard AR Groppe M MacLaren RE Gene therapy for choroideremia using an adeno-associated viral (AAV) vector.Cold Spring Harb Perspect Med. 2015; 5: a017293Crossref Scopus (48) Google Scholar,5Jacobson SG Cideciyan AV Roman AJ Sumaroka A Schwartz SB Heon E et al.Improvement and decline in vision with gene therapy in childhood blindness.N Engl J Med. 2015; 372: 1920-1926Crossref PubMed Scopus (267) Google Scholar,6Cideciyan AV Hauswirth WW Aleman TS Kaushal S Schwartz SB Boye SL et al.Human RPE65 gene therapy for Leber congenital amaurosis: persistence of early visual improvements and safety at 1 year.Hum Gene Ther. 2009; 20: 999-1004Crossref PubMed Scopus (292) Google Scholar,7Bainbridge JW Mehat MS Sundaram V Robbie SJ Barker SE Ripamonti C et al.Long-term effect of gene therapy on Leber's congenital amaurosis.N Engl J Med. 2015; 372: 1887-1897Crossref PubMed Scopus (508) Google Scholar and cell8Schwartz SD Hubschman JP Heilwell G Franco-Cardenas V Pan CK Ostrick RM et al.Embryonic stem cell trials for macular degeneration: a preliminary report.Lancet. 2012; 379: 713-720Abstract Full Text Full Text PDF PubMed Scopus (1101) Google Scholar therapies. There are currently 26 active and completed gene therapy trials (including RPE65 (MIM#613794), REP1 (MIM#300390), RS1 (MIM#300839), ABCA4 (MIM#248200), MYO7A (MIM#276900), MERTK (MIM#613862), RPGR (MIM#312610), CNGB3 (MIM#262300), and sFLT1 (MIM#165070)) and two embryonic stem cell trials8Schwartz SD Hubschman JP Heilwell G Franco-Cardenas V Pan CK Ostrick RM et al.Embryonic stem cell trials for macular degeneration: a preliminary report.Lancet. 2012; 379: 713-720Abstract Full Text Full Text PDF PubMed Scopus (1101) Google Scholar,9Schwartz SD Regillo CD Lam BL Eliott D Rosenfeld PJ Gregori NZ et al.Human embryonic stem cell-derived retinal pigment epithelium in patients with age-related macular degeneration and Stargardt's macular dystrophy: follow-up of two open-label phase ½ studies.Lancet. 2015; 385: 509-516Abstract Full Text Full Text PDF PubMed Scopus (878) Google Scholar for retinal degenerations. The human retina is the ideal testing ground for assessing experimental gene and cell therapies because of its relative immune privilege and accessibility for monitoring, imaging, and surgical manipulation. Numerous noninvasive techniques such as fundus autofluorescence, optical coherence tomography (OCT), and adaptive optics imaging enable micron-level examination of the inside of the retina in live animals and patients. The ability to quantify efficacy in subjects in a noninvasive manner at multiple time points is a major advantage of conducting research on the eye, one shared by few if any other organ systems. In fact, the eye is one of the only organs that permits remarkably targeted interventions that are not highly invasive or systemic. The blood–retina barrier provides relative immune privilege, and in the event of teratoma formation, the eye can be removed without risk of damaging other organ systems and is itself not a life-threatening event. Experimentally, eyes provide the ideal treatment-control groups, as the contralateral, untreated eye can be compared with the intervention eye. Bestrophinopathies are caused by mutations in the gene encoding BESTROPHIN1 (BEST1 (MIM#607854)), with over 120 amino acid substitution mutations associated with the disease.10Hartzell HC Qu Z Yu K Xiao Q Chien LT Molecular physiology of bestrophins: multifunctional membrane proteins linked to best disease and other retinopathies.Physiol Rev. 2008; 88: 639-672Crossref PubMed Scopus (265) Google Scholar,11Krawczak M Cooper DN The human gene mutation database.Trends Genet. 1997; 13: 121-122Abstract Full Text PDF PubMed Scopus (220) Google Scholar Diagnosis is typically made based on fundus appearance, EOG measurements, and family history.12North V Gelman R Tsang SH Juvenile-onset macular degeneration and allied disorders.Dev Ophthalmol. 2014; 53: 44-52Crossref PubMed Scopus (2) Google Scholar,13MacDonald IM Lee T. Pagon RA Bird TD Dolan CR Stephens K Adam MP Best Vitelliform Macular Dystrophy..GeneReviews. Seattle, WA1993Google Scholar A decreased EOG Arden ratio (light peak divided by dark trough) is a hallmark of all bestrophinopathies and is caused by malfunctions in the protein bestrophin-1, believed to be a Ca2+-regulated chloride channel (CaCC) in the basolateral membrane of the RPE.14Xiao Q Hartzell HC Yu K Bestrophins and retinopathies.Pflugers Arch. 2010; 460: 559-569Crossref PubMed Scopus (87) Google Scholar Figures 1, 2, 3, and 4 provide fundus and/or OCT examples ranging from healthy individuals (Figure 1) to those with severe bestrophinopathy (Figure 4).Figure 2Early bestrophinopathy vitelliform macular degeneration (VMD (MIM#153700)). Mild phenotypic manifestation in a 5-year-old VMD patient with a BEST1, p. Ala10Thr mutation. Color fundus photograph of the left eye shows the vitelliform lesion in the central macula (a). Subretinal fluid* and outer segment debris are visible in the spectral-domain optical coherence tomography images (b). The central opaque region* corresponds to an area of serous detachment. Photoreceptor outer segments are hyper-autofluorescent in FA imaging. *= subretinal fluid; black arrows = photoreceptor mitochondrial band.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 3Moderate vitelliform macular degeneration (VMD). Moderate phenotypic manifestation in a 58-year-old VMD patient with a novel p. Asp302Ala mutation. The vitelliform is pronounced in the color fundus imaging (a), the photoreceptor layer is thin compared to that of a healthy individual, and extensive fluid has developed subretinally in optical coherence tomography imaging (b). *= subretinal fluid; black arrows = photoreceptor mitochondrial band.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 4Severe vitelliform macular degeneration (VMD). Severe phenotypic manifestation in a 42-year-old VMD patient with p. Asp302Ala (novel) and p. Arg218His mutations. Photoreceptor mitochondrial band has disintegrated at various points, indicated by black arrows. FA is absent in the central area, which exhibits dystrophic RPE and loss of photoreceptors.View Large Image Figure ViewerDownload Hi-res image Download (PPT) While bestrophinopathies progressively worsen with age, the rate of decline is typically slow and provides a long therapeutic window, as central photoreceptors remain viable for decades despite the persistence of subretinal fluid and serous detachment (Figures 2 and 3). Most bestrophinopathy patients will maintain robust central vision well into the fifth decade of life,15Duncker T Greenberg JP Ramachandran R Hood DC Smith RT Hirose T et al.Quantitative fundus autofluorescence and optical coherence tomography in best vitelliform macular dystrophy.Invest Ophthalmol Vis Sci. 2014; 55: 1471-1482Crossref PubMed Scopus (73) Google Scholar as cones can tolerate subretinal fluid for many years.16Hurley JB Lindsay KJ Du J Glucose, lactate, and shuttling of metabolites in vertebrate retinas.J Neurosci Res. 2015; 93: 1079-1092Crossref PubMed Scopus (143) Google Scholar Eventually rod cell death (Figure 4) due to subretinal fluid will cause cones to degenerate soon after,16Hurley JB Lindsay KJ Du J Glucose, lactate, and shuttling of metabolites in vertebrate retinas.J Neurosci Res. 2015; 93: 1079-1092Crossref PubMed Scopus (143) Google Scholar,17Aït-Ali N Fridlich R Millet-Puel G Clérin E Delalande F Jaillard C et al.Rod-derived cone viability factor promotes cone survival by stimulating aerobic glycolysis.Cell. 2015; 161: 817-832Abstract Full Text Full Text PDF PubMed Scopus (248) Google Scholar,18Lin M Kim SH Zhang L Tsai Y Tsang SH Rod metabolic demand drives progression in retinopathies.Taiwan J Ophthalmol. 2015; 5: 105-108Abstract Full Text Full Text PDF Scopus (12) Google Scholar which results in blindness and cannot be treated with gene supplementation therapy. Thus, unless rod transplantation is applied, the ideal time for intervention is as soon as possible after diagnosis or at least before fluid-induced rod cell death. Quantifiable therapeutic endpoints are readily obtainable for bestrophinopathies, which feature subretinal fluid accumulation. Fluid resolution on OCT thickness measurements (in microns) provides sensitive therapeutic endpoints and can reduce the required sample size in a gene therapy trial. Contrastingly, cell preservation is the standard metric in many ongoing human gene therapy trials for retinal degeneration, even though this method is a much more challenging outcome measurement to achieve than quantifying fluid resolution. The relative ease of appraising disease development or treatment progress is more closely comparable to that of exudative age-related macular degeneration trials using OCT anatomical outcome measurements, where subretinal fluid can be resolved within a week after treatment. An autosomal dominant bestrophinopathy known as Best Vitelliform Macular Degeneration (VMD (MIM#153700)) is a debilitating, early-onset form of central macular degeneration. While rod and cone photoreceptor function remains unaffected, the disease leads to the formation of serous retinal detachment and lesions that resemble egg yolk, or vitelliform, due to abnormalities in the fluid and/or electrolyte homeostasis between the RPE and photoreceptor outer segments (Figures 2, 3, and 4). Normally, the RPE phagocytoses photoreceptor outer segments, which are composed of lipid and oxidized protein wastes, to allow cell renewal.14Xiao Q Hartzell HC Yu K Bestrophins and retinopathies.Pflugers Arch. 2010; 460: 559-569Crossref PubMed Scopus (87) Google Scholar Phagocytosis is affected by ion and fluid flux, which is believed to be under the control of bestrophin-1.19Singh R Shen W Kuai D Martin JM Guo X Smith MA et al.iPS cell modeling of Best disease: insights into the pathophysiology of an inherited macular degeneration.Hum Mol Genet. 2013; 22: 593-607Crossref PubMed Scopus (172) Google Scholar Although the mechanism is still unclear, mutations in BEST1 alter the CaCC conductance of the bestrophin-1 protein, which hinders the RPE from digesting the photoreceptor outer segments waste products.10Hartzell HC Qu Z Yu K Xiao Q Chien LT Molecular physiology of bestrophins: multifunctional membrane proteins linked to best disease and other retinopathies.Physiol Rev. 2008; 88: 639-672Crossref PubMed Scopus (265) Google Scholar The accumulation of this bisretinoid waste is termed lipofuscin, which has toxic effects on the RPE and is directly correlated with the severity of macular degeneration.10Hartzell HC Qu Z Yu K Xiao Q Chien LT Molecular physiology of bestrophins: multifunctional membrane proteins linked to best disease and other retinopathies.Physiol Rev. 2008; 88: 639-672Crossref PubMed Scopus (265) Google Scholar,15Duncker T Greenberg JP Ramachandran R Hood DC Smith RT Hirose T et al.Quantitative fundus autofluorescence and optical coherence tomography in best vitelliform macular dystrophy.Invest Ophthalmol Vis Sci. 2014; 55: 1471-1482Crossref PubMed Scopus (73) Google Scholar Consequently, the abnormal appearance of the macula in bestrophinopathy patients is caused by the autofluorescent properties of lipofuscin (Figure 2b). The ability to model this mechanism in the dish could provide unique opportunities for testing drug-like compounds that improve the rate of phagocytosis in induced pluripotent stem (iPS)-derived, diseased RPE. Autosomal recessive bestrophinopathy (ARB) is a similar disorder to VMD but differs in its inheritance pattern and some phenotypic manifestations, and it results in a nearly nonfunctional bestrophin-1 protein.20Boon CJ van den Born LI Visser L Keunen JE Bergen AA Booij JC et al.Autosomal recessive bestrophinopathy: differential diagnosis and treatment options.Ophthalmology. 2013; 120: 809-820Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar ARB is caused by loss-of-function alleles21Burgess R Millar ID Leroy BP Urquhart JE Fearon IM De Baere E et al.Biallelic mutation of BEST1 causes a distinct retinopathy in humans.Am J Hum Genet. 2008; 82: 19-31Abstract Full Text Full Text PDF PubMed Scopus (227) Google Scholar,22Johnson AA Bachman LA Gilles BJ Cross SD Stelzig KE Resch ZT et al.Autosomal Recessive Bestrophinopathy Is Not Associated With the Loss of Bestrophin-1 Anion Channel Function in a Patient With a Novel BEST1 Mutation.Invest Ophthalmol Vis Sci. 2015; 56: 4619-4630Crossref PubMed Scopus (33) Google Scholar and is characterized by multifocal vitelliform dystrophy with subretinal fluid (Figure 5). One of patients' first symptoms is central vision loss with variable onset, and they are typically hyperopic, exhibiting bilateral multifocal hyper-autofluorescent vitelliform lesions and shallow serous retinal detachment.20Boon CJ van den Born LI Visser L Keunen JE Bergen AA Booij JC et al.Autosomal recessive bestrophinopathy: differential diagnosis and treatment options.Ophthalmology. 2013; 120: 809-820Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar Scotopic and photopic full-field ERGs usually diminish with age and the EOG shows a reduced or absent light rise. Thus, one distinguishing diagnostic feature between VMD and ARB is that individuals with VMD typically have normal full-field ERGs while ARB patients do not.20Boon CJ van den Born LI Visser L Keunen JE Bergen AA Booij JC et al.Autosomal recessive bestrophinopathy: differential diagnosis and treatment options.Ophthalmology. 2013; 120: 809-820Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar,23Fung AT Yzer S Goldberg N Wang H Nissen M Giovannini A et al.New best1 mutations in autosomal recessive bestrophinopathy.Retina. 2015; 35: 773-782Crossref PubMed Google Scholar Despite bestrophin-1's clear pathological relevance to bestrophinopathy diseases,24Mullins RF Kuehn MH Faidley EA Syed NA Stone EM Differential macular and peripheral expression of bestrophin in human eyes and its implication for best disease.Invest Ophthalmol Vis Sci. 2007; 48: 3372-3380Crossref PubMed Scopus (94) Google Scholar the physiological role and disease-causing mechanisms of bestrophin-1 remain elusive. Therefore, dissecting the structural and functional properties of this protein would be a significant step towards understanding bestrophinopathies and developing treatment options. Recently, the crystal structures of a bacterial (Klebsiella pneumoniae, KpBest) and a chicken bestrophin-1 homolog were solved independently.25Yang T Liu Q Kloss B Bruni R Kalathur RC Guo Y et al.Structure and selectivity in bestrophin ion channels.Science. 2014; 346: 355-359Crossref PubMed Scopus (92) Google Scholar,26Kane Dickson V Pedi L Long SB Structure and insights into the function of a Ca(2+)-activated Cl(-) channel.Nature. 2014; 516: 213-218Crossref PubMed Scopus (149) Google Scholar The structure models reveal that bestrophin-1 forms a stable pentamer, with each protomer containing four transmembrane helices and cytosolic N- and C-termini. The ion permeation pathway has a flower-vase shape with two permeation restrictions. Electrophysiological analysis of structure-inspired mutations demonstrated that the first restriction is a sensitive control of ion selectivity, while the second restriction is an activation gate in both KpBest and human bestrophin-1, showcasing the power of crystallography in inspiring functional studies. Importantly, the crystallography of bestrophin-1 potentiates targeted drug design for bestrophinopathies by predicting the disease-causing mechanism of a given BEST1 mutation in a patient and providing a structural platform for small molecule-based drug screens. To understand characteristics of human bestrophin-1 and the disposition of disease-causing mutations, a homology model for its transmembrane portion was generated based on the KpBest structure. When plotting disease-causing mutations in human BEST1 onto the three-dimensional (3D) model, the distribution of mutations suggests that there may be multiple disease-causing mechanisms.25Yang T Liu Q Kloss B Bruni R Kalathur RC Guo Y et al.Structure and selectivity in bestrophin ion channels.Science. 2014; 346: 355-359Crossref PubMed Scopus (92) Google Scholar For example, mutations along the ion-conducting pathway (e.g., F80 and I205) may affect Cl− conduction, while mutations around a carboxylate loop, which is a possible Ca2+ binding domain (e.g., E300 and D302), may disrupt channel activation, as the channel is Ca2+-activated. The 3D structure of wild-type bestrophin-1 or patient-derived mutants will guide drug discovery and personalized medicine in the near future. Compounds can be screened and designed on the computer based on the predicted interacting interface of a desired structural/functional domain of bestrophin-1. Then the binding mode of synthesized compounds to bestrophin-1 will be examined by X-ray crystallography for confirmation, followed by further validation and optimization. In short, developing precision medicine-based approaches for bestrophinopathy patients is contingent on understanding the structure and function of bestrophin-1. VMD is likely due to a dominant-gene effect wherein the expression of the mutated protein interferes with the CaCC activity of wild-type bestrophin-1 (refs. 21Burgess R Millar ID Leroy BP Urquhart JE Fearon IM De Baere E et al.Biallelic mutation of BEST1 causes a distinct retinopathy in humans.Am J Hum Genet. 2008; 82: 19-31Abstract Full Text Full Text PDF PubMed Scopus (227) Google Scholar,27Davidson AE Sergouniotis PI Burgess-Mullan R Hart-Holden N Low S Foster PJ et al.A synonymous codon variant in two patients with autosomal recessive bestrophinopathy alters in vitro splicing of BEST1.Mol Vis. 2010; 16: 2916-2922PubMed Google Scholar) and/or the regulation of other ion channels.14Xiao Q Hartzell HC Yu K Bestrophins and retinopathies.Pflugers Arch. 2010; 460: 559-569Crossref PubMed Scopus (87) Google Scholar If VMD mutations simply antagonize wild-type bestrophin-1 function, AAV-mediated ribozyme and gene-editing delivery will be needed for correction, and subretinal fluid can be resolved by restoration of the CaCC conductance of bestrophin-1 through gene or cell supplementation. On the other hand, recessive, loss-of-function alleles, as in ARB, can be corrected by gene supplementation, in which a single wild type copy is transduced into the affected cells. An alternative expression hypothesis, haploinsufficieny was not supported by data presented on the parents of five ARB patients from different families.28Fung AT Yzer S Goldberg N Wang H Nissen M Giovannini A et al.New best1 mutations in autosomal recessive bestrophinopathy.Retina. 2015; 35: 773-782Crossref PubMed Scopus (23) Google Scholar Of note, one of the five ARB patients28Fung AT Yzer S Goldberg N Wang H Nissen M Giovannini A et al.New best1 mutations in autosomal recessive bestrophinopathy.Retina. 2015; 35: 773-782Crossref PubMed Scopus (23) Google Scholar had homozygous null alleles, yet her parents had no phenotypic manifestations, which further supports the notion of a dominant-negative effect of VMD. Until the development of patient-specific stem cell lines, progress to therapy was stalled because Best1 knockout mice and knockin mice do not recapitulate diseased human phenotypes.29Marmorstein AD Marmorstein LY The challenge of modeling macular degeneration in mice.Trends Genet. 2007; 23: 225-231Abstract Full Text Full Text PDF PubMed Scopus (34) Google Scholar In contrast to expectation, mice homozygous for a Best1 knockout mutation do not exhibit abnormal EOG responses or an eye phenotype that resembles human VMD.30Marmorstein AD Cross HE Peachey NS Functional roles of bestrophins in ocular epithelia.Prog Retin Eye Res. 2009; 28: 206-226Crossref PubMed Scopus (107) Google Scholar Furthermore, these knockout mice show normal RPE CaCC activity, indicating that mouse bestrophin-1 is not a major contributor to CaCC function in mouse RPE. Overexpression and knockdown of Best1 in rat RPE also does not have the expected effects. Contrastingly, a dog model of ARB called canine multifocal retinopathy (cmr) accurately replicated human ARB and was also treatable through genetic intervention: fundus lesions were resolved for at least 23 months post-rAAV2/2 injection,31Guziewicz KE Zangerl B Komáromy AM Iwabe S Chiodo VA Boye SL et al.Recombinant AAV-mediated BEST1 transfer to the retinal pigment epithelium: analysis of serotype-dependent retinal effects.PLoS One. 2013; 8: e75666Crossref PubMed Scopus (36) Google Scholar although improvement in ERG has yet to be reported. Thus, while dominant forms of the disorder may be more difficult to model in animals, canine models of the recessive forms are already successfully being treated by overriding the mutations responsible for them. An alternative to animal models, stem cells can readily be obtained from patient fibroblasts and differentiated into RPE, which may provide a useful experimental system to study bestrophin-1 function. In a recent study, patient-specific RPE derived from two VMD patients showed delayed rhodopsin degradation and decreased Ca2+-stimulated responses in phagocytosis assays, as compared to unaffected sibling controls.19Singh R Shen W Kuai D Martin JM Guo X Smith MA et al.iPS cell modeling of Best disease: insights into the pathophysiology of an inherited macular degeneration.Hum Mol Genet. 2013; 22: 593-607Crossref PubMed Scopus (172) Google Scholar With the employment of VMD patient-derived RPE, techniques to correct the genetic defect may be developed to treat the disease on a patient-specific basis.32Vasireddy V Mills JA Gaddameedi R Basner-Tschakarjan E Kohnke M Black AD et al.AAV-mediated gene therapy for choroideremia: preclinical studies in personalized models.PLoS One. 2013; 8: e61396Crossref PubMed Scopus (64) Google Scholar,33Li Y Wu WH Hsu CW Nguyen HV Tsai YT Chan L et al.Gene therapy in patient-specific stem cell lines and a preclinical model of retinitis pigmentosa with membrane frizzled-related protein defects.Mol Ther. 2014; 22: 1688-1697Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar While there are also clinical trials currently underway to determine whether embryonic stem cells can replace diseased RPE, the advantage of iPS therapies is that genetically-repaired iPS cells allow for autologous transplantation, which does not require immunosuppression therapy. Successful restoration of CaCC conductance in patient-specific iPS-RPE can be an indicator of the efficacy of this therapeutic approach in subsequent clinical trials. Until recently, retinal degenerations including bestrophinopathy faced overwhelming hurdles to treatment, as their pathophysiology and crystallographic structure were unavailable. Now, it is feasible to identity each patient's genotype and corresponding mutation, create autologous iPS-RPE, replace the mutated allele with the wild type form, and implant the healthy cells into the retina. Significant advances in our understanding of BEST1 and its involvement in bestrophinopathy must be made in order to refine and optimize this treatment approach, but the future of personalized medicine is quickly approaching and soon to be within reach. The Barbara & Donald Jonas Stem Cell & Regenerative Medicine Laboratory is supported by NIH R01EY018213, R01EY024698, and R21AG050437; the Research to Prevent Blindness Physician-Scientist Award; New York State (C029572); and the Foundation Fighting Blindness New York Regional Research Center Grant (C-NY05-0705-0312). Additionally, the authors would like to extend their gratitude and appreciation to our patients and especially to Lawrence Chan for reviewing this manuscript as well as serving as our leading research adviser for our macular degeneration program. The authors declare no conflict of interest.