Bestrophin Gene Research Articles

Autosomal recessive bestrophinopathy combined with neurofibromatosis type 1 in a patient

BackgroundNeurofibromatosis type 1 (NF1) is a multisystem genetic disorder that may affect multiple systems of the body. Autosomal recessive bestrophinopathy (ARB) is a rare retinal dystrophy caused by autosomal recessively mutations in bestrophin 1 (BEST1) gene. So far, we have not retrieved any case report of the same patient with both NF1 and BEST1 gene mutations.Case presentationAn 8-year-old female patient with café-au-lait spots, freckling on skin presented to our ophthalmology clinic for routine ophthalmological examination. Her best corrected visual acuity (BCVA) was 20/20 in both eyes. Slit-lamp examination of both eyes revealed few yellowish-brown dome-shaped Lisch nodules over the iris surface. Fundus examination was notable for bilateral confluent yellowish subretinal deposits at macula, few yellow flecks at temporal retina, and cup-to-disc ratio of 0.2. Optical coherence tomography (OCT) revealed subretinal fluid (SRF) involving the fovea, elongated photoreceptor outer segments and mild intraretinal fluid (IRF) at bilateral macula. Fundus autofluorescence demonstrated hyperautofluorescence in the area corresponding to the subretinal deposits. Whole-exome sequencing and Sanger sequencing were used to investigate genetic mutation in the patient and her parents. A BEST1 gene heterozygous missense c.604 C > T (p.Arg202Trp) was identified in the patient and her mother. Also, the patient carries an NF1 nonsense mutation c.6637 C > T (p.Gln2213*) with the mosaic generalized phenotype. There were no visual impairments or obvious neurological, musculoskeletal, behavioral or other symptoms in this patient, so she was managed conservatively and advised to follow up regularly for a long time.ConclusionsARB and NF1, which are caused by two different pathogenic gene mutations, have rarely coexisted in the same patient. The discovery of pathogenic gene mutations may play a crucial role in more accurate diagnostics and genetic consultations for individuals and their families.

Glial regulators of ions and solutes required for specific chemosensory functions in Caenorhabditis elegans.

Glia and accessory cells regulate the microenvironment around neurons and primary sensory cells. However, the impact of specific glial regulators of ions and solutes on functionally diverse primary cells is poorly understood. Here, we systemically investigate the requirement of ion channels and transporters enriched in Caenorhabditis elegans Amsh glia for the function of chemosensory neurons. Although Amsh glia ablated worms show reduced function of ASH, AWC, AWA, and ASE neurons, we show that the loss of glial enriched ion channels and transporters impacts these neurons differently, with nociceptor ASH being the most affected. Furthermore, our analysis underscores the importance of K+, Cl-, and nucleoside homeostasis in the Amphid sensory organ and uncovers the contribution of glial genes implicated in neurological disorders. Our findings build a unique fingerprint of each glial enriched ion channel and transporter and may provide insights into the function of supporting cells of mammalian sensory organs.

Autosomal Recessive Bestrophinopathy: Clinical and Genetic Characteristics of Twenty-Four Cases.

Background To describe ocular manifestations, imaging characteristics, and genetic test results of autosomal recessive bestrophinopathy (ARB). The study design is an observational case series. Methods Forty-eight eyes of 24 patients diagnosed with ARB underwent complete ophthalmic examinations including refraction, anterior and posterior segment examination, enhanced depth imaging optical coherence tomography (EDI-OCT), fluorescein angiography (FA), electroretinography (ERG), and electrooculography (EOG). Optical coherence tomography angiography (OCTA) and BEST1 gene sequencing were performed in selected patients. Results The age at onset was 4–35 years (mean: 18.6 years). The male-to-female ratio was 0.45. All patients were hyperopic, except one with less than one diopter myopia. EOG was abnormal in 18 cases with near-normal ERGs. Six patients did not undergo EOG due to their young age. Eighteen patients (75%) had a thick choroid on EDI-OCT, of which three had advanced angle-closure glaucoma, 15 patients were hyperopic, and eight of them had more than four diopters hyperopia in both eyes. Macular retinoschisis was observed in 46 eyes of 23 patients (95%) with cysts mostly located in the inner nuclear layer (INL) to the outer nuclear layer (ONL). Of the 18 patients who underwent FA, mild peripheral leakage was seen in eight eyes of four patients (22%). Subfoveal choroidal neovascularization (CNV) was seen in three eyes of two patients (6%) that responded well to intravitreal bevacizumab (IVB). Seven mutations of the bestrophin-1 (BEST1) gene were found in this study; however, only two of them (p.Gly34 = and p.Leu319Pro) had been previously reported as the cause of ARB based on ClinVar and other literature studies. Conclusions ARB can be presented with a wide spectrum of ocular abnormalities that may not be easily diagnosed. Pachychoroid can occur alongside retinal schisis and may be the underlying cause of angle-closure glaucoma in ARB. Our study also expands the pathogenic mutation spectrum of the BEST1 gene associated with ARB.

Bestrophinopathies: perspectives on clinical disease, Bestrophin-1 function and developing therapies.

Bestrophinopathies are a group of clinically distinct inherited retinal dystrophies that typically affect the macular region, an area synonymous with central high acuity vision. This spectrum of disorders is caused by mutations in bestrophin1 (BEST1), a protein thought to act as a Ca2+-activated Cl- channel in the retinal pigment epithelium (RPE) of the eye. Although bestrophinopathies are rare, over 250 individual pathological mutations have been identified in the BEST1 gene, with many reported to have various clinical expressivity and incomplete penetrance. With no current clinical treatments available for patients with bestrophinopathies, understanding the role of BEST1 in cells and the pathological pathways underlying disease has become a priority. Induced pluripotent stem cell (iPSC) technology is helping to uncover disease mechanisms and develop treatments for RPE diseases, like bestrophinopathies. Here, we provide a comprehensive review of the pathophysiology of bestrophinopathies and highlight how patient-derived iPSC-RPE are being used to test new genomic therapies in vitro.

Evaluating BEST1 mutations in pluripotent stem cell-derived retinal pigment epithelial cells.

Bestrophin-1 (BEST1) is a calcium-activated chloride channel (CaCC) predominantly expressed at the basolateral membrane of the retinal pigment epithelium (RPE). Over 250 mutations in the BEST1 gene have been documented to cause at least five retinal degenerative disorders, commonly termed bestrophinopathies, to which no treatment is currently available. Therefore, understanding the influences of BEST1 disease-causing mutations on the physiological function of BEST1 in RPE is critical for deciphering the pathology of bestrophinopathies and developing therapeutic strategies for patients. However, this task has been impeded by the rarity of BEST1 mutations and limited accessibility to native human RPE cells. Here, we describe a pluripotent stem cell (PSC)-based pipeline for reproducibly generating RPE cells expressing endogenous or exogenous mutant BEST1, which provides us with a powerful "disease-in-a-dish" approach for studying BEST1 mutations in physiological environments.

Generation of a human induced pluripotent stem cell line, BRCi005-A, derived from a Best disease patient with BEST1 mutations.

Best Disease is an inherited retinal dystrophy that results in progressive and irreversible central vision loss caused by mutations of BESTROPHIN1 (BEST1). We established human induced pluripotent stem cells (iPSCs) from a Best disease patient with mutations R218H and A357V in the BEST1 gene. The generated iPSCs showed pluripotency markers and three-germ layer differentiation ability in vitro. A genetic analysis revealed mutations of R218H and A357V in the iPSCs. This iPSC line will be useful for elucidating the pathomechanisms of and drug discovery for Best disease.

Novel therapies for Bestrophinopathies

AbstractBestrophinopathies are a group of clinically distinct diseases caused by mutations in Bestrophin1 (BEST1). BEST1 is expressed exclusively in the retinal pigment epithelial (RPE) cells of the eye, where it functions as an ion channel. Mutations in the BEST1 gene affect the function of the RPE leading to the death of overlying retinal cells and subsequent vision loss.The dominant and recessive nature of Bestrophinopathies may necessitate different approaches to prevent or ameliorate these diseases. Potential therapeutic options for Bestrophinopathies focus on restoring correct gene expression, though viral gene therapy and gene editing, or by targeting nonsense mutations through translational readthrough‐inducing drugs. We are using induced pluripotent stem‐derived RPE created from patients with Bestrophinopathies to investigate these potential therapies in vitro.

Mutation spectrum of the bestrophin-1 gene in a large Chinese cohort with bestrophinopathy

BackgroundBestrophin-1 (BEST1) gene is associated with a wide range of ocular phenotypes, collectively termed as bestrophinopathy. The aim of the current study was to identify the mutation spectrum of BEST1...

Bestrophin1: A Gene that Causes Many Diseases.

Bestrophinopathies are a group of clinically distinct inherited retinal dystrophies that lead to the gradual loss of vision in and around the macular area. There are no treatments for patients suffering from bestrophinopathies, and no measures can be taken to prevent visual deterioration in those who have inherited disease-causing mutations. Bestrophinopathies are caused by mutations in the Bestrophin1 gene (BEST1), a protein found exclusively in the retinal pigment epithelial (RPE) cells of the eye. Mutations in BEST1 affect the function of the RPE leading to the death of overlying retinal cells and subsequent vision loss. The pathogenic mechanisms arising from BEST1 mutations are still not fully understood, and it is not clear how mutations in BEST1 lead to diseases with distinct clinical features. This chapter discusses BEST1, the use of model systems to investigate the effects of mutations and the potential to investigate individual bestrophinopathies using induced pluripotent stem cells.

Differentiation, Maintenance, and Analysis of Human Retinal Pigment Epithelium Cells: A Disease-in-a-dish Model for BEST1 Mutations.

Although over 200 genetic mutations in the human BEST1 gene have been identified and linked to retinal degenerative diseases, the pathological mechanisms remain elusive mainly due to the lack of a good in vivo model for studying BEST1 and its mutations under physiological conditions. BEST1 encodes an ion channel, namely BESTROPHIN1 (BEST1), which functions in retinal pigment epithelium (RPE); however, the extremely limited accessibility to native human RPE cells represents a major challenge for scientific research. This protocol describes how to generate human RPEs bearing BEST1 disease-causing mutations by induced differentiation from human pluripotent stem cells (hPSCs). As hPSCs are self-renewable, this approach allows researchers to have a steady source of hPSC-RPEs for various experimental analyses, such as immunoblotting, immunofluorescence, and patch clamp, and thus provides a very powerful disease-in-a-dish model for BEST1-associated retinal conditions. Notably, this strategy can be applied to study RPE (patho)physiology and other genes of interest natively expressed in RPE.

BESTROPHIN1 mutations cause defective chloride conductance in patient stem cell-derived RPE.

Bestrophin1 (BEST1) is expressed in human retinal pigment epithelium (RPE) and mutations in the BEST1 gene commonly cause retinal dysfunction and macular degeneration. BEST1 is presumed to assemble into a calcium-activated chloride channel and be involved in chloride transport but there is no direct evidence in live human RPE cells to support this idea. To test whether BEST1 functions as a chloride channel in living tissue, BEST1-mutant RPE (R218H, L234P, A243T) were generated from patient-derived induced pluripotent stem cells and compared with wild-type RPE in a retinal environment, using a biosensor that visualizes calcium-induced chloride ion flux in the cell. Calcium stimulation elicited chloride ion export in normal RPE but not in RPE derived from three patients with BEST1 mutations. These data, along with three-dimensional modeling, provide evidence that BEST1 assembles into a key calcium-sensing chloride channel in human RPE.

Multifocal retinopathy in Dachshunds with CLN2 neuronal ceroid lipofuscinosis

The CLN2 form of neuronal ceroid lipofuscinosis is an autosomal recessively inherited lysosomal storage disease that is characterized by progressive vision loss culminating in blindness, cognitive and motor decline, neurodegeneration, and premature death. CLN2 disease results from mutations in the gene that encodes the soluble lysosomal enzyme tripeptidyl peptidase-1. A null mutation in the TPP1 gene encoding this enzyme causes a CLN2-like disease in Dachshunds. Dachshunds that are homozygous for this mutation serve as a model for human CLN2 disease, exhibiting clinical signs and neuropathology similar to those of children with this disorder. Affected dogs reach end-stage terminal disease status at 10–11 months of age. In addition to retinal changes typical of CLN2 disease, a retinopathy consisting of multifocal, bullous retinal detachment lesions was identified in 65% of (TPP1−/−) dogs in an established research colony. These lesions did not occur in littermates that were heterozygous or homozygous for the normal TPP1 allele. Retinal changes and the functional effects of this multifocal retinopathy were examined objectively over time using ophthalmic examinations, fundus photography, electroretinography (ERG), quantitative pupillary light response (PLR) recording, fluorescein angiography, optical coherence tomography (OCT) and histopathology. The retinopathy consisted of progressive multifocal serous retinal detachments. The severity of the disease-related retinal thinning was no more serious in most detached areas than in adjacent areas of the retina that remained in close apposition to the retinal pigment epithelium. The retinopathy observed in these dogs was somewhat similar to canine multifocal retinopathy (CMR), a disease caused by a mutation of the bestrophin gene BEST1. ERG a-wave amplitudes were relatively preserved in the Dachshunds with CLN2 disease, whether or not they developed the multifocal retinopathy. The retinopathy also had minimal effects on the PLR. Histological evaluation indicated that the CLN2 disease-related retinal degeneration was not exacerbated in areas where the retina was detached except where the detached areas were very large. DNA sequence analysis ruled out a mutation in the BEST1 exons or splice junctions as a cause for the retinopathy. Perfect concordance between the TPP1 mutation and the retinopathy in the large number of dogs examined indicates that the retinopathy most likely occurs as a direct result of the TPP1 mutation. Therefore, inhibition of the development and progression of these lesions can be used as an indicator of the efficacy of therapeutic interventions currently under investigation for the treatment of CLN2 disease in the Dachshund model. In addition, these findings suggest that TPP1 mutations may underlie multifocal retinopathies of unknown cause in animals and humans.

A V235L Mutation of the Human BEST1 Gene Associated with Best Macular Dystrophy with Intra-familial Clinical Variations

Introduction: Best vitelliform macular dystrophy (BVMD) accounts for 1% of all cases of macular degeneration resulting in progressive loss of central vision. In this work we sought to evaluate the clinical and genetic background in a two generation pedigree of autosomal dominant BVMD for clinical management and follow up. To our knowledge this is the first report on association of Bestrophin 1 (BEST1) mutation with BVMD in an Indian family. Case report: Complete ophthalmic examination done in a family with a complaint of impaired vision revealed the presence of yellow-orange yolk like lesions in the macula upon fundus examination. Further investigations through EOG revealed decreased Arden ratio. Besides the proband, two other members namely, mother and siblings were also affected in the family. Follow up clinical examination was done after three years to clinically document the progression of the disease, OCT examination was done in addition during follow up studies. Genomic DNA samples of the affected family members showed a sequence variation, c.703 G>T transversion in exon # 6 which results in substitution of valine by polar leucine as V235L. This variant was not observed in the unaffected father and 65 ethnically matched controls. Follow up clinical examination of the proband and his sib showed rapid progression of the disease. Conclusions: We report for the first time an attempt on genetic diagnosis in a case of Best disease from Indian ethnicity. The disease condition revealed a rapid progression in the proband as compared to his mother, depicting severity of the disease in successive generation. The study prompts the need for genetic prognosis to identify predisposed/ susceptible individuals to enable proper counseling to the family members about the disease.

Acute exudative polymorphous vitelliform maculopathy in a young woman: presymptomatic findings and 21-month follow-up.

To describe ocular findings before and after the diagnosis of acute exudative polymorphous vitelliform maculopathy, in an otherwise healthy 28-year-old woman. Case report with 21-months of follow-up. Fundus photography, optical coherence tomography, fluorescein angiography, indocyanine green angiography, and autofluorescence were used for imaging the retina. To examine retinal function, full-field electroretinogram, multifocal electroretinogram, electrooculography, and dark adaptometry were performed. Genetic analysis for mutations associated with Best disease was done. In the asymptomatic patient before diagnosis, white-yellow, drusen-like, subretinal depositions were found in both eyes. A few months later, the patient developed bilateral visual disturbances. Retinal examination at the acute phase revealed a characteristic pattern of multifocal white-yellow subretinal lesions in both posterior poles, imaged by ophthalmoscopy, fluorescein angiography, indocyanine green angiography, and optical coherence tomography. Additionally, electrooculography and dark adaptometry were abnormal. Full-field electroretinogram was normal, but multifocal electroretinogram revealed central depression of peak amplitudes. During the 21-month follow-up without any treatment, visual acuity recovered, electrooculography and dark adaptometry normalized, and the patient experienced one episode of relapse. Genetic studies excluded mutations in the bestrophin gene (BEST1). Acute exudative polymorphous vitelliform maculopathy is still a condition of unknown origin, primarily affecting the pigment epithelium. Earlier reports have discussed whether the condition is inherited or acquired. In this report, the presymptomatic retinal findings in acute exudative polymorphous vitelliform maculopathy are described for the first time, indicating that a condition may be associated with primarily affected retinal pigment epithelium.

Molecular Consequences ofBEST1Gene Mutations in Canine Multifocal Retinopathy Predict Functional Implications for Human Bestrophinopathies

Bestrophin-1 gene (BEST1) mutations are responsible for a broad spectrum of human retinal phenotypes, jointly called bestrophinopathies. Canine multifocal retinopathy (cmr), caused by mutations in the dog gene ortholog, shares numerous phenotypic features with human BEST1-associated disorders. The purpose of this study was the assessment of molecular consequences and pathogenic outcomes of the cmr1 (C(73)T/R(25)X) premature termination and the cmr2 (G(482)A/G(161)D) missense mutation of the canine model compared with the C(87)G/Y(29)X mutation observed in human patients. Dogs carrying the BEST1 mutation were introduced into a breeding colony and used to produce either carrier or affected offspring. Eyes were collected immediately after euthanatization at the disease-relevant ages and were harvested for expression studies. In parallel, an in vitro cell culture model system was developed and compared with in vivo The results demonstrate that cmr1 and human C(87)G-mutated transcripts bypass the nonsense-mediated mRNA decay machinery, suggesting the AUG proximity effect as an underlying transcriptional mechanism. The truncated protein, however, is not detectable in either species. The in vitro model accurately recapitulates transcriptional and translational expression events observed in vivo and, thus, implies loss of bestrophin-1 function in cmr1-dogs and Y(29)X-affected patients. Immunofluorescence microscopy of cmr2 mutant showed mislocalization of the protein. Molecular evaluation of cmr mutations in vivo and in vitro constitutes the next step toward elucidating genotype-phenotype interactions concerning human bestrophinopathies and emphasizes the importance of the canine models for studying the complexity of the BEST1 disease mechanism.

Characterisation of AnBEST1, a functional anion channel in the plasma membrane of the filamentous fungus, Aspergillus nidulans

Two distant homologues of the bestrophin gene family have been identified in the filamentous fungus, Aspergillus nidulans ( anbest1 and anbest2). AnBEST1 was functionally characterised using the patch clamp technique and was shown to be an anion selective channel permeable to citrate. Furthermore, AnBEST1 restored the growth of the pdr12Δ yeast mutant on inhibitory concentrations of extracellular propionate, benzoate and sorbate, also consistent with carboxylated organic anion permeation of AnBEST1. Similar to its animal counterparts, AnBEST1 currents were activated by elevated cytosolic Ca 2+ with a K d of 1.60 μM. Single channel currents showed long (>10 s) open and closed times with a unitary conductance of 16.3 pS. Transformation of A. nidulans with GFP-tagged AnBEST1 revealed that AnBEST1 localised to the plasma membrane. An anbest1 null mutant was generated to investigate the possibility that AnBEST1 mediated organic anion efflux across the plasma membrane. Although organic anion efflux was reduced from anbest1 null mutants, this phenotype was linked to the restoration of uracil/uridine-requiring A. nidulans strains to uracil/uridine prototrophy. In conclusion, this study identifies a new family of organic anion-permeable channels in filamentous fungi. We also reveal that uracil/uridine-requiring Aspergillus strains exhibit altered organic anion metabolism which could have implications for the interpretation of physiological studies using auxotrophic Aspergillus strains.

Anterior Segment Abnormalities and Angle-Closure Glaucoma in a Family with a Mutation in the BEST1 Gene and Best Vitelliform Macular Dystrophy

Purpose: To present the clinical and electrophysiological findings in four members of a family with Best vitelliform macular dystrophy (BVMD) and angle-closure glaucoma (ACG).Methods: Four members of a family with BVMD were examined clinically, including visual acuity, slit-lamp examination, biomicroscopy, Goldmann applanation tonometry and gonioscopy. Measurements of the anterior chamber depth and axial length, visual field, optical coherence tomography, full-field electroretinography, multifocal electroretinography and electrooculography were performed. In addition molecular genetic analysis of the bestrophin-1 gene (BEST1), the microphthalmia-associated transcription factor gene (MITF) and the cone-rod homeobox gene (CRX) were performed.Results: Four family members with the c.253T>C p.Y85H mutation in the BEST1 gene and BVMD in different stages also exhibited anterior segment abnormalities such as shallow anterior chambers (two cases), and reduced axial lengths in all cases. Microphthalmos (axial length ≤ 20mm) was found in the index patient and in her son. Hyperopia was found in all four examined patients. Closed angles/narrow angles were observed in patients with microphthalmos. The index patient developed ACG at the age of 12 years. Her son inherited microphthalmos, severe hyperopia, and narrow angles. He is at risk of developing ACG. No pathogenic mutation of the MITF or the CRX genes was detected in the index patient.Conclusions: BVMD could be associated with anterior segment abnormalities such as shallow anterior chambers, closed/narrow anterior chamber angles and ACG. Ophthalmologists should be aware of the association between ACG and BVMD. Examination of the anterior segment, gonioscopy and intraocular pressure control are recommended in patients with BVMD.

Water cotransport in pigmented epithelial cells

Water cotransport in pigmented epithelial cells

Rescue of Volume-regulated Anion Current by Bestrophin Mutants with Altered Charge Selectivity

Mutations in human bestrophin-1 are linked to various kinds of retinal degeneration. Although it has been proposed that bestrophins are Ca2+-activated Cl− channels, definitive proof is lacking partly because mice with the bestrophin-1 gene deleted have normal Ca2+-activated Cl− currents. Here, we provide compelling evidence to support the idea that bestrophin-1 is the pore-forming subunit of a cell volume-regulated anion channel (VRAC) in Drosophila S2 cells. VRAC was abolished by treatment with RNAi to Drosophila bestrophin-1. VRAC was rescued by overexpressing bestrophin-1 mutants with altered biophysical properties and responsiveness to sulfhydryl reagents. In particular, the ionic selectivity of the F81C mutant changed from anionic to cationic when the channel was treated with the sulfhydryl reagent, sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES−) (PCs/PCl = 0.25 for native and 2.38 for F81C). The F81E mutant was 1.3 times more permeable to Cs+ than Cl−. The finding that VRAC was rescued by F81C and F81E mutants with different biophysical properties shows that bestrophin-1 is a VRAC in S2 cells and not simply a regulator or an auxiliary subunit. F81C overexpressed in HEK293 cells also exhibits a shift of ionic selectivity after MTSES− treatment, although the effect is quantitatively smaller than in S2 cells. To test whether bestrophins are VRACs in mammalian cells, we compared VRACs in peritoneal macrophages from wild-type mice and mice with both bestrophin-1 and bestrophin-2 disrupted (best1−/−/best2−/−). VRACs were identical in wild-type and best1−/−/best2−/− mice, showing that bestrophins are unlikely to be the classical VRAC in mammalian cells.

Bestrophin Gene Mutations Cause Canine Multifocal Retinopathy: A Novel Animal Model for Best Disease

Canine multifocal retinopathy (cmr) is an autosomal recessive disorder of multiple dog breeds. The disease shares a number of clinical and pathologic similarities with Best macular dystrophy (BMD), and cmr is proposed as a new large animal model for Best disease. cmr was characterized by ophthalmoscopy and histopathology and compared with BMD-affected patients. BEST1 (alias VMD2), the bestrophin gene causally associated with BMD, was evaluated in the dog. Canine ortholog cDNA sequence was cloned and verified using RPE/choroid 5'- and 3'-RACE. Expression of the canine gene transcripts and protein was analyzed by Northern and Western blotting and immunocytochemistry. All exons and the flanking splice junctions were screened by direct sequencing. The clinical phenotype and pathology of cmr closely resemble lesions of BMD. Canine VMD2 spans 13.7 kb of genomic DNA on CFA18 and shows a high level of conservation among eukaryotes. The transcript is predominantly expressed in RPE/choroid and encodes bestrophin, a 580-amino acid protein of 66 kDa. Immunocytochemistry of normal canine retina demonstrated specific localization of protein to the RPE basolateral plasma membranes. Two disease-specific sequence alterations were identified in the canine VMD2 gene: a C(73)T stop mutation in cmr1 and a G(482)A missense mutation in cmr2. The authors propose these two spontaneous mutations in the canine VMD2 gene, which cause cmr, as the first naturally occurring animal model of BMD. Further development of the cmr models will permit elucidation of the complex molecular mechanism of these retinopathies and the development of potential therapies.