BEST1 — bestrophin 1

The BEST1 gene provides the body with instructions for making a protein called bestrophin-1. This protein is found in the retinal pigment epithelium (RPE), a crucial layer of support cells located at the very back of the eye, just underneath the light-sensitive retina. Bestrophin-1 acts like a microscopic gate or channel that allows specific salts (like chloride) and fluid to flow in and out of these support cells. This flow is essential for keeping the retina healthy, nourished, and clear of waste products, allowing the eye to process light and maintain sharp vision. When there is a mutation (a harmful change) in the BEST1 gene, the bestrophin-1 gates do not work correctly. Because the flow of fluid and salts is disrupted, waste material—specifically a yellowish, fatty substance called lipofuscin—begins to build up underneath the retina. Over time, this buildup forms blister-like deposits that can damage the support cells and the overlying retina. For patients, this damage typically leads to a slow, gradual loss of central vision, making it difficult to read, recognize faces, or drive, though peripheral (side) vision usually remains intact. Diseases caused by BEST1 mutations are collectively called bestrophinopathies, with the most common being Best disease (also known as Best vitelliform macular dystrophy). Best disease is usually inherited in an autosomal dominant pattern, meaning a person only needs to inherit one copy of the mutated gene from one parent to develop the condition. Interestingly, the severity of the disease can vary greatly; some family members with the mutation may experience significant vision loss early in life, while others may have very mild symptoms or remain unaware of the condition until later in adulthood.

Gene description: BEST1 encodes a transmembrane protein forming a calcium-activated chloride channel primarily expressed in the retinal pigment epithelium (RPE).

Patient and family guide: The BEST1 gene provides the body with instructions for making a protein called bestrophin-1. This protein is found in the retinal pigment epithelium (RPE), a crucial layer of support cells located at the very back of the eye, just underneath the light-sensitive retina. Bestrophin-1 acts like a microscopic gate or channel that allows specific salts (like chloride) and fluid to flow in and out of these support cells. This flow is essential for keeping the retina healthy, nourished, and clear of waste products, allowing the eye to process light and maintain sharp vision. When there is a mutation (a harmful change) in the BEST1 gene, the bestrophin-1 gates do not work correctly. Because the flow of fluid and salts is disrupted, waste material—specifically a yellowish, fatty substance called lipofuscin—begins to build up underneath the retina. Over time, this buildup forms blister-like deposits that can damage the support cells and the overlying retina. For patients, this damage typically leads to a slow, gradual loss of central vision, making it difficult to read, recognize faces, or drive, though peripheral (side) vision usually remains intact. Diseases caused by BEST1 mutations are collectively called bestrophinopathies, with the most common being Best disease (also known as Best vitelliform macular dystrophy). Best disease is usually inherited in an autosomal dominant pattern, meaning a person only needs to inherit one copy of the mutated gene from one parent to develop the condition. Interestingly, the severity of the disease can vary greatly; some family members with the mutation may experience significant vision loss early in life, while others may have very mild symptoms or remain unaware of the condition until later in adulthood.

Gene function: BEST1 functions as a calcium-activated chloride channel located in the basolateral membrane of the RPE. It is crucial for maintaining ion homeostasis, fluid transport, and adhesion between the RPE and photoreceptors. BEST1 also contributes to the transepithelial potential. Mutations disrupt these functions, leading to RPE dysfunction and secondary photoreceptor degeneration, characteristic of Best vitelliform macular dystrophy.

Protein structure: The BEST1 gene encodes bestrophin-1, a 585-amino acid integral membrane protein. Structurally, bestrophin-1 is characterized by a highly conserved N-terminal region that contains four to five transmembrane domains, which anchor the protein into the basolateral membrane of the retinal pigment epithelium. The C-terminal half of the protein is largely intracellular and is less conserved, containing domains involved in protein-protein interactions and regulatory functions. To form a functional ion channel, bestrophin-1 must assemble into a homopentamer—a complex made of five identical bestrophin-1 protein subunits arranged in a ring. This pentameric assembly creates a central pore through which chloride and bicarbonate ions can pass. A critical structural feature is the "calcium bowl," a highly conserved intracellular domain that binds calcium ions. The binding of calcium to this region induces a conformational change in the pentameric structure, opening the central pore and allowing anion transport, which is the fundamental mechanism of its calcium-activated channel activity.

Molecular function: The BEST1 gene encodes bestrophin-1, an integral membrane protein that functions primarily as a calcium-activated anion channel. Localized to the basolateral membrane of the retinal pigment epithelium (RPE), bestrophin-1 forms a pentameric pore that allows the passive transport of chloride (Cl-) and bicarbonate (HCO3-) ions across the cell membrane in response to increases in intracellular calcium (Ca2+) levels. This ion transport is a critical component of the RPE's physiological role in regulating the volume and composition of the subretinal space, facilitating the removal of fluid and metabolic waste products from the photoreceptors to the choroidal blood circulation. In addition to its role as an anion channel, bestrophin-1 is implicated in the regulation of intracellular calcium signaling within the RPE. It has been shown to interact with voltage-dependent calcium channels (VDCCs) and modulate their activity, thereby influencing calcium homeostasis. Proper bestrophin-1 function is essential for the generation of the light peak observed in the electrooculogram (EOG), a clinical measure of the standing potential across the RPE. Mutations in BEST1 disrupt these functions, leading to impaired fluid and ion transport across the RPE. This dysfunction is thought to cause an abnormal microenvironment in the subretinal space, leading to the accumulation of unphagocytosed photoreceptor outer segments and lipofuscin (a lipid-rich waste product). The buildup of this toxic material forms the characteristic vitelliform lesions seen in Best disease, which eventually causes RPE cell death and secondary degeneration of the overlying photoreceptors, resulting in vision loss.

Expression pattern: The BEST1 gene is predominantly expressed in the retinal pigment epithelium (RPE), a monolayer of cells situated between the neural retina and the choroid. Within the RPE, the bestrophin-1 protein is specifically localized to the basolateral plasma membrane. This strategic positioning is essential for its role in regulating ion and fluid transport between the subretinal space and the choroidal blood supply, which is critical for maintaining the health and function of the overlying photoreceptors. While the highest levels of BEST1 expression are found in the eye, lower levels of expression have been detected in other tissues, including the brain, spinal cord, and testis. However, the physiological significance of bestrophin-1 in these non-ocular tissues remains less well understood, and the clinical manifestations of BEST1 mutations are almost exclusively confined to the visual system.

Mutation spectrum: Over 250 pathogenic variants have been identified in the BEST1 gene, encompassing a wide spectrum of mutation types. The vast majority of these are missense mutations, which account for most cases of autosomal dominant Best vitelliform macular dystrophy (BVMD) and adult-onset vitelliform macular dystrophy (AVMD). These missense mutations are not evenly distributed but tend to cluster in four distinct "hotspot" regions within the highly conserved N-terminal half of the protein, which includes the transmembrane domains and the calcium-binding bowl essential for channel function. In addition to missense variants, the mutation spectrum includes nonsense mutations, small deletions, insertions, and splice-site alterations. These types of mutations, which often result in a truncated or absent protein, are more frequently associated with autosomal recessive bestrophinopathy (ARB). Specific splice-site mutations that lead to in-frame deletions or insertions are uniquely linked to the rare phenotype of autosomal dominant vitreoretinochoroidopathy (ADVIRC). The high degree of allelic heterogeneity in BEST1 contributes to the diverse clinical presentations observed in bestrophinopathies.

Pathogenic variants: 1. p.Thr6Arg (c.17C>G): A well-characterized missense mutation frequently associated with classic autosomal dominant Best vitelliform macular dystrophy (BVMD). It affects a highly conserved residue near the N-terminus. 2. p.Arg141His (c.422G>A): One of the most common missense mutations found in BVMD patients. It is located in a mutational hotspot within the second transmembrane domain, severely disrupting the channel's anion permeability. 3. p.Ala243Val (c.728C>T): A frequent missense variant associated with BVMD, located in the intracellular loop containing the calcium-binding domain, impairing the channel's response to intracellular calcium. 4. p.Tyr227Asn (c.679T>A): A missense mutation often associated with adult-onset vitelliform macular dystrophy (AVMD), typically presenting with a later onset and milder phenotype compared to classic Best disease. 5. c.238-240del (p.Val80del): An in-frame deletion uniquely associated with autosomal dominant vitreoretinochoroidopathy (ADVIRC), leading to altered splicing and a distinct panophthalmic phenotype.

Clinical significance: Mutations in the BEST1 gene cause a spectrum of inherited retinal diseases collectively known as bestrophinopathies. The most common is Best vitelliform macular dystrophy (BVMD), an autosomal dominant condition characterized by the bilateral accumulation of yellowish, egg-yolk-like (vitelliform) lipofuscin deposits in the macula. Onset typically occurs in childhood or adolescence. As the disease progresses, the vitelliform lesions can rupture and lead to macular atrophy or scarring, resulting in a slow, progressive decline in central vision. Another dominant condition is adult-onset vitelliform macular dystrophy (AVMD), which presents similarly to BVMD but typically appears later in life (between ages 30 and 50) and often has a milder visual prognosis. Autosomal dominant vitreoretinochoroidopathy (ADVIRC) is a rarer phenotype involving peripheral retinal hyperpigmentation, breakdown of the blood-retinal barrier, and panophthalmic features such as nanophthalmos (small eyes) and angle-closure glaucoma. Autosomal recessive bestrophinopathy (ARB) is caused by biallelic BEST1 mutations. It is characterized by more widespread retinal changes, including multifocal subretinal deposits, macular edema, and a higher risk of choroidal neovascularization. Unlike the dominant forms, ARB often involves more severe and progressive visual impairment affecting both central and peripheral vision. Across all bestrophinopathies, a hallmark clinical finding is an abnormal electrooculogram (EOG) with a reduced Arden ratio, reflecting generalized RPE dysfunction.

Inheritance: Autosomal Dominant

Chromosomal location: 11q12.3

Genotype-phenotype correlations: Genotype-phenotype correlations in BEST1 mutations are complex due to significant variable expressivity and incomplete penetrance, even among individuals carrying the same mutation within a single family. Generally, missense mutations clustered in the highly conserved N-terminal half of the protein (which forms the channel pore and calcium-binding domains) are frequently associated with the classic autosomal dominant Best vitelliform macular dystrophy (BVMD). These mutations typically exert a dominant-negative effect, where the mutant protein incorporates into the pentameric channel and disrupts its overall function. In contrast, mutations causing autosomal recessive bestrophinopathy (ARB) are often null mutations (such as nonsense or frameshift variants) or missense mutations that lead to rapid degradation of the protein, resulting in a complete loss of bestrophin-1 function when both alleles are affected. Some specific mutations are strongly linked to distinct phenotypes; for example, certain splice-site altering mutations are uniquely associated with autosomal dominant vitreoretinochoroidopathy (ADVIRC), leading to altered protein isoforms that may have distinct toxic effects on the RPE and developing eye.

Research and therapeutic approaches: Currently, there are no FDA-approved treatments or cures for BEST1-related bestrophinopathies. Clinical management is primarily supportive, focusing on regular monitoring with imaging (OCT) and treating complications such as choroidal neovascularization (CNV) with off-label intravitreal injections of anti-VEGF agents (e.g., bevacizumab, ranibizumab). Patients are also provided with low-vision aids and counseling to help manage the impact of progressive central vision loss. However, significant progress is being made in the development of targeted therapies, particularly gene therapy. Because the RPE is highly accessible to subretinal injections and does not divide rapidly, it is an excellent target for viral vector-mediated gene delivery. A leading approach is AAV-mediated gene augmentation therapy, which aims to deliver a healthy copy of the BEST1 gene to the RPE. Opus Genetics has recently launched a Phase 1/2 clinical trial (OPGx-BEST1) to evaluate the safety and preliminary efficacy of an AAV-based gene therapy for BEST1-related inherited retinal diseases. For autosomal dominant forms of the disease where the mutant protein exerts a dominant-negative effect, simple gene augmentation may not be sufficient. In these cases, researchers are exploring advanced strategies such as CRISPR/Cas9 gene editing or RNA interference (RNAi) to specifically knock down the mutant allele while preserving or supplementing the wild-type allele. These approaches have shown promise in preclinical cellular and animal models, paving the way for future clinical applications.

Diagnostic testing: Diagnosis of BEST1-related bestrophinopathies relies on a combination of clinical examination, electrophysiological testing, and molecular genetic testing. A hallmark diagnostic test is the electrooculogram (EOG), which typically shows a severely reduced light peak to dark trough ratio (Arden ratio < 1.5) in affected individuals, even in asymptomatic carriers of dominant mutations. Full-field electroretinogram (ERG) is usually normal in early BVMD but may be abnormal in ARB or advanced disease. High-resolution imaging, including spectral-domain optical coherence tomography (SD-OCT) and fundus autofluorescence (FAF), is essential for visualizing the characteristic subretinal vitelliform deposits and monitoring disease progression. Genetic testing confirms the diagnosis and is typically performed using targeted multi-gene panels for inherited retinal diseases or macular dystrophies, which include the BEST1 gene. Whole exome sequencing (WES) may also be used. Identifying the specific mutation is crucial for genetic counseling, as it distinguishes between autosomal dominant (BVMD, AVMD, ADVIRC) and autosomal recessive (ARB) inheritance patterns. Genetic counseling should inform patients about the variable penetrance and expressivity of dominant BEST1 mutations, meaning that family members with the same mutation may experience vastly different disease severities.

Animal models: The most prominent animal model for BEST1-related diseases is the canine model of canine multifocal retinopathy (cmr), which naturally occurs in several dog breeds due to BEST1 mutations. This model closely mimics the clinical and pathological features of human Best vitelliform macular dystrophy (BVMD), including the characteristic subretinal lesions and electrooculogram (EOG) abnormalities, making it an excellent large animal model for translational research and gene therapy testing. Mouse models, including Best1 knockout (KO) mice and knock-in models carrying specific human mutations (e.g., W93C), have also been developed. While Best1 KO mice exhibit reduced light peak in EOG and altered calcium signaling in the retinal pigment epithelium (RPE), they generally lack the prominent macular lesions seen in humans, partly because mice lack a macula. Despite this limitation, mouse models have been crucial for understanding the basic physiological role of bestrophin-1 as a calcium-activated chloride channel and its impact on RPE function and fluid transport.

Population genetics: Bestrophinopathies are rare genetic disorders with a global distribution, though precise prevalence data is limited. The estimated prevalence of Best disease (BVMD) is approximately 1 in 10,000 to 1 in 100,000 individuals, depending on the population studied. Some studies suggest higher carrier frequencies in specific isolated populations due to founder effects. For instance, a higher prevalence has been noted in certain Swedish populations where the disease was first extensively described, and specific founder mutations have been identified in other distinct ethnic groups. However, BEST1 mutations are found worldwide across diverse ethnic backgrounds, with a high rate of novel mutations continually being discovered in different populations.

Selected references: 1. Petrukhin K, et al. Identification of the gene responsible for Best macular dystrophy. Nat Genet. 1998;19(3):241-7. PMID: 9662395 2. Sun H, et al. The vitelliform macular dystrophy protein defines a new family of chloride channels. Proc Natl Acad Sci U S A. 2002;99(6):4008-13. PMID: 11904445 3. Marmorstein AD, et al. Bestrophin, the product of the Best vitelliform macular dystrophy gene (VMD2), localizes to the basolateral plasma membrane of the retinal pigment epithelium. Proc Natl Acad Sci U S A. 2000;97(23):12758-63. PMID: 11050159 4. Boon CJ, et al. The spectrum of ocular phenotypes caused by mutations in the BEST1 gene. Prog Retin Eye Res. 2009;28(3):187-205. PMID: 19375515 5. Guziewicz KE, et al. Bestrophin gene mutations cause canine multifocal retinopathy: a novel animal model for best disease. Invest Ophthalmol Vis Sci. 2007;48(5):1959-67. PMID: 17460247 6. Johnson AA, et al. Bestrophin 1 and retinal disease. Prog Retin Eye Res. 2017;58:45-69. PMID: 28153808 7. MacDonald IM, et al. Bestrophinopathies. 2003 Sep 30 [Updated 2020 Jul 16]. In: Adam MP, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2024. PMID: 20301344