Clinical Heterogeneity in Autosomal Recessive Bestrophinopathy with Biallelic Mutations in the BEST1 Gene

Non-vasogenic cystoid maculopathy in autosomal recessive bestrophinopathy: novel insights from NIR-FAF and OCTA imaging

BACKGROUND

Autosomal Recessive Bestrophinopathy (ARB) is an inherited retinal disease caused by biallelic mutations in the BEST1 gene. Herein, we report the multimodal imaging findings of ARB presenting with cystoid maculopathy and investigate the short-term response to combined systemic and topical carbonic anhydrase inhibitors (CAIs).

MATERIAL AND METHODS

An observational, prospective, case series on two siblings affected by ARB is presented. Patients underwent genetic testing and optical coherence tomography (OCT), blue-light fundus autofluorescence (BL-FAF), near-infrared fundus autofluorescence (NIR-FAF), fluorescein angiography (FA), MultiColor imaging, and OCT angiography (OCTA).

RESULTS

Two male siblings, aged 22 and 16, affected by ARB resulting from c.598C>T, p.(Arg200*) and c.728C>A, p.(Ala243Glu) BEST1 compound heterozygous variants, presented with bilateral multifocal yellowish pigment deposits scattered through the posterior pole that corresponded to hyperautofluorescent deposits on BL-FAF. Vice versa, NIR-FAF mainly disclosed wide hypoautofluorescent areas in the macula. A cystoid maculopathy and shallow subretinal fluid were evident on structural OCT, albeit without evidence of dye leakage or pooling on FA. OCTA demonstrated disruption of the choriocapillaris throughout the posterior pole and sparing of intraretinal capillary plexuses. Six months of combined therapy with oral acetazolamide and topical brinzolamide resulted in limited clinical benefit.

CONCLUSIONS

We reported two siblings affected by ARB, presenting as non-vasogenic cystoid maculopathy. Prominent alteration of NIR-FAF signal and concomitant choriocapillaris rarefaction on OCTA were noted in the macula. The limited short-term response to combined systemic and topical CAIs might be explained by the impairment of the RPE-CC complex.

Genetic and clinical features of BEST1-associated retinopathy based on 59 Chinese families and database comparisons

Variants in BEST1 are one of the most common cause of retinopathy mainly involving the retinal pigment epithelium with both dominant and recessive traits. This study aimed to describe the characteristics of potential pathogenic variants (PPVs) in BEST1 and their associated clinical features. Variants in BEST1 were collected from our in-house exome sequencing data and systematically evaluated by in silico prediction tools as well as genotype-phenotype analysis. The pathogenicity features of the BEST1 variants were further assessed through database comparison among the in-house data, Genome Aggregation Database from the general population, and all previously published literature. The clinical information of the in-house patients was summarized. The PPVs in BEST1 were identified in 66 patients from 59 families, including 32 families with Best vitelliform macular dystrophy (BVMD) and 27 families with autosomal recessive bestrophinopathy (ARB). These PPVs included 31 missense variants, seven truncation variants, one in-frame deletion, and a known 3-untranslated region variant. All the truncations detected in our study were exclusively involved in ARB but not BVMD. Among the 31 missense variants, 18 missenses associated with BVMD in the dominant trait were clustered in four hotspot regions with statistically significant differences from the recessive missenses. Except for distinct macular changes, there were no statistically significant differences among the other associated clinical features between BVMD and ARB, including peripheral retinopathy, high hyperopia, and angle-closure glaucoma. In conclusion, BEST1-associated dominant retinopathy was preferentially caused by missense variants located in important functional regions. Truncations were most likely benign in heterozygous status. Future studies are expected to elucidate the mystery of the same missense variants contributing to both BVMD and ARB.

Impaired Bestrophin Channel Activity in an iPSC-RPE Model of Best Vitelliform Macular Dystrophy (BVMD) from an Early Onset Patient Carrying the P77S Dominant Mutation

Best Vitelliform Macular dystrophy (BVMD) is the most prevalent of the distinctive retinal dystrophies caused by mutations in the BEST1 gene. This gene, which encodes for a homopentameric calcium-activated ion channel, is crucial for the homeostasis and function of the retinal pigment epithelia (RPE), the cell type responsible for recycling the visual pigments generated by photoreceptor cells. In BVMD patients, mutations in this gene induce functional problems in the RPE cell layer with an accumulation of lipofucsin that evolves into cell death and loss of sight. In this work, we employ iPSC-RPE cells derived from a patient with the p.Pro77Ser dominant mutation to determine the correlation between this variant and the ocular phenotype. To this purpose, gene and protein expression and localization are evaluated in iPSC-RPE cells along with functional assays like phagocytosis and anion channel activity. Our cell model shows no differences in gene expression, protein expression/localization, or phagocytosis capacity, but presents an increased chloride entrance, indicating that the p.Pro77Ser variant might be a gain-of-function mutation. We hypothesize that this variant disturbs the neck region of the BEST1 channel, affecting channel function but maintaining cell homeostasis in the short term. This data shed new light on the different phenotypes of dominant mutations in BEST1, and emphasize the importance of understanding its molecular mechanisms. Furthermore, the data widen the knowledge of this pathology and open the door for a better diagnosis and prognosis of the disease.

Best Vitelliform Macular Dystrophy (BVMD) is a phenocopy of North Carolina Macular Dystrophy (NCMD/MCDR1)

PURPOSE

North Carolina Macular Dystrophy (NCMD) and Best Vitelliform Macular Dystrophy (BVMD) are rare autosomal dominant macular dystrophies. Both BVMD and NCMD have markedly variable expressivity. In some individuals, it can be difficult to differentiate between the two disease entities.

METHODS

Clinical findings including fundus photography, fundus autofluorescence (FAF), and spectral domain optical coherence tomography (SD-OCT) were evaluated in 5 individuals with NCMD and 3 with BMD. Electrooculography (EOG) was performed in 2 NCMD subjects. Molecular diagnosis was performed using Sanger DNA sequencing. IRB approval was obtained.

RESULTS

Five NCMD subjects had clinical findings indistinguishable from three of our BVMD subjects. Molecular diagnosis was confirmed in all but one BVMD subject who had an abnormal EOG prior to discovery of the BEST1 gene. Two NCMD subjects had an abnormal EOG with a normal ERG, which has been considered a unique feature of BVMD. SD-OCT in one BVMD subject demonstrated a small lucency/excavation into the choroid similar to that in grade 3 lesions of NCMD. Two NCMD subjects had elevated sub-macular lesions giving a pseudo-vitelliform appearance on OCT similar to BVMD.

CONCLUSION

Best Vitelliform Macular Dystrophy can be a phenocopy of NCMD. There is considerable clinical overlap between NCMD and BVMD, which can cause diagnostic inaccuracies. Our new findings demonstrate that like BVMD, NCMD can also have an abnormal EOG with a normal ERG. The overlapping phenotypes of BVMD with NCMD may provide insights into the mechanisms of the macular changes.

Clinical reassessments and whole-exome sequencing uncover novel BEST1 mutation associated with bestrophinopathy phenotype

BACKGROUND

The diagnosis of retinal dystrophies can be challenging due to the spectrum of protean phenotypic manifestations. This study employed trio-whole-exome sequencing (trio-WES) to unveil the genetic cause of an inherited retinal disorder in a south Indian family.

MATERIALS AND METHODS

Proband's initial ophthalmic examinations was performed in the year 2016. WES was performed on a proband-parent trio to identify causative mutation followed by Sanger validation, segregation analysis, sequence and structure-based computational analysis to assess its pathogenicity. Based on the genetic findings, detailed clinical reassessments were performed in year 2020 for the proband and available family members.

RESULTS

WES revealed a novel homozygous BEST1 mutation c.G310A (p.D104N) in the proband and heterozygous for the parents, indicating autosomal recessive inheritance. Segregation analysis showed heterozygous mutation in maternal grandfather and normal genotype for younger brother and maternal grandmother. Moreover, the structure-based analysis revealed the mutation p.D104N in the cytoplasmic domain, causing structural hindrance by altering hydrogen bonds and destabilizing the BEST1 protein structure. Proband's clinical assessments were consistent with autosomal recessive bestrophinopathy (ARB) phenotype. Additionally, characteristic absent light rise and decreased light peak-to-dark trough ratio (LP:DT) was observed bilaterally in EOG.

CONCLUSIONS

Our study demonstrates the utility of WES and clinical re-evaluations in establishing the precise diagnosis of autosomal recessive bestrophinopathy associated with a novel mutation, thus expanding the BEST1-related mutation spectrum.

Novel disease-causing variant in RDH12 presenting with autosomal dominant retinitis pigmentosa

Aim

To describe the clinical and molecular features of a novel, autosomal dominant RDH12-retinopathy.

Methods

Retrospective cross-sectional study. Twelve individuals from a four-generation British pedigree underwent ophthalmic examination, genotyping using next generation sequencing, including whole genome sequencing and multimodal retinal imaging including fundus photography, optical coherence tomography (OCT), autofluorescence imaging and adaptive optics (AO) scanning light ophthalmoscopy were performed. Visual electrophysiology was performed in a subset.

Results

Eight family members were confirmed as affected by genotyping heterozygous for RDH12 c.763delG. Visual acuity ranged from −0.1 to 0.2 logMAR. Affected individuals had constricted visual fields. A parafoveal and peripapillary ring of hyper-autofluorescence was seen initially, and with progression the area of perifoveal hypo-autofluorescence increased to involve the parafoveal area. Mild retinal thinning was identified on OCT imaging with reduction in both foveal total retinal and outer nuclear layer thickness. Cone densities along the temporal meridian were reduced in affected individuals compared with normative values at all temporal eccentricities studied. One individual with incomplete penetrance, was identified as clinically affected primarily on the basis of AO imaging. Full-field electroretinography demonstrated a rod-cone pattern of dysfunction and large-field pattern electroretinography identified peripheral macular dysfunction.

Conclusions

This novel heterozygous variant RDH12 c.763delG is associated with a rod-cone dystrophy with variable expression. Determination of the degree of penetrance may depend on the modality employed to phenotypically characterise an individual. This rare and specific heterozygous (dominant) variant is predicted to result in a gain of function, that causes disease in a gene typically associated with biallelic (recessive) variants.