Best1 Shot Through the Eye—Structure, Functions and Clinical Implications of Bestrophin-1 Protein

A novel compound heterozygous BEST1 gene mutation in two siblings causing autosomal recessive bestrophinopathy

PURPOSE

To describe the clinical features, imaging characteristics, and genetic test results associated with a novel compound heterozygous mutation of the BEST1 gene in two siblings with autosomal recessive bestrophinopathy.

METHODS

Two siblings underwent a complete ophthalmic examination, including dilated fundus examination, fundus photography, fundus autofluorescence imaging, spectral-domain optical coherence tomography, fluorescein angiography, electroretinography, and electrooculography. A clinical diagnosis of autosomal recessive bestrophinopathy was established based on ocular examination and multimodal retinal imaging. Subsequently, clinical exome sequencing consisting of a panel of 6670 genes was carried out to confirm the diagnosis and assess genetic alterations in the protein-coding region of the genome of the patients. The identified mutations were tested in the two affected siblings and one of their parents.

RESULTS

Two siblings (a 17-year-old female and a 15-year-old male) presented with reduced visual acuity and bilaterally symmetrical subretinal deposits of hyperautofluorescent materials in the posterior pole, which showed staining in the late phase of fluorescein angiogram. Spectral-domain optical coherence tomography demonstrated hyperreflective subretinal deposits and subretinal fluid accumulation. Both patients shared two mutations in the protein-coding region of the BEST1 gene, c.103G > A, p.(Glu35Lys) and c.313C > A, p.(Arg105Ser) (a novel disease-causing mutation). Sanger sequencing confirmed that the unaffected mother of the proband was carrying p.(Glu35Lys) variant in a heterozygous state.

CONCLUSIONS

We have identified and described the phenotype of a novel disease-causing mutation NM_004183.4:c.313C > A, p.(Arg105Ser) in a heterozygous state along with a previously reported mutation NM_004183.4:c.103G > A, p.(Glu35Lys) of the BEST1 gene in two related patients with autosomal recessive bestrophinopathy.

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.

Next generation sequencing identifies novel disease-associated BEST1 mutations in Bestrophinopathy patients

Bestinopathies are a spectrum of retinal disorders associated with mutations in BEST1 including autosomal recessive bestrophinopathy (ARB) and autosomal dominant Best vitelliform macular dystrophy (BVMD). We applied whole-exome sequencing on four unrelated Indian families comprising eight affected and twelve unaffected individuals. We identified five mutations in BEST1, including p.Tyr131Cys in family A, p.Arg150Pro in family B, p.Arg47His and p.Val216Ile in family C and p.Thr91Ile in family D. Among these, p.Tyr131Cys, p.Arg150Pro and p.Val216Ile have not been previously reported. Further, the inheritance pattern of BEST1 mutations in the families confirmed the diagnosis of ARB in probands in families A, B and C, while the inheritance of heterozygous BEST1 mutation in family D (p.Thr91Ile) was suggestive of BVMD. Interestingly, the ARB families A and B carry homozygous mutations while family C was a compound heterozygote with a mutation in an alternate BEST1 transcript isoform, highlighting a role for alternate BEST1 transcripts in bestrophinopathy. In the BVMD family D, the heterozygous BEST1 mutation found in the proband was also found in the asymptomatic parent, suggesting an incomplete penetrance and/or the presence of additional genetic modifiers. Our report expands the list of pathogenic BEST1 genotypes and the associated clinical diagnosis.

Transepithelial resistance in human bestrophin-1 stably transfected Madin-Darby canine kidney cells

Bestrophin-1 (Best1) is a transmembrane protein, found in the basolateral plasma membrane of retinal pigmented epithelial cells. The exact structure and functions of Best1 protein are still unclear. The protein is thought to be a regulator of ion channels, or an ion channel itself: it was shown to be permeable for chloride, thiocyanate, bicarbonate, glutamate and γ-aminobutyric acid (GABA). Mutations in the gene for Best1 are leading to best vitelliform macular dystrophy (BVMD) and are found in several other types of maculopathy. In order to obtain additional information about Best1 protein, we determined cell polarization of a stably transfected Madin–Darby canine kidney cell line II (MDCK II) cell line, expressing human Best1. We measured the transepithelial resistance of transfected and non-transfected MDCK cells by voltmeter EVOM, over 10 days at 24 hour intervals. The first few days (first–fourth day) both cell lines showed the same or similar values ​​of transmembrane resistance. As expected, on the fifth day the non-transfected cells showed maximum value of epithelial resistance, corresponding to the forming of monolayer. The transfected cells showed maximum value of transepithelial resistance on the ninth day of their cultivation. Phalloidin staining of actin demonstrated the difference in actin arrangements between transfected and non-transfected cells due to Best1. As a consequence of actin rearrangement, Best1 strongly affects the transepithelial resistance of polarizing stably transfected MDCK cells. Our results suggest that Best1 protein has an effect on transepithelial resistance and actin rearrangements of polarized stably transfected MDCK cells.

Interaction of Bestrophin-1 with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in surface films

Human bestrophin-1 (hBest1) is a transmembrane channel protein, predominantly expressed in the membrane of retinal pigment epithelium (RPE) cells. Although it is clear that hBest1's interactions with lipids are crucial for its function such studies were not performed as the protein was not purified. Here we describe an effective purification of hBest1 from Madin-Darby Canine Kidney (MDCK) cells via simple gel-filtration and affinity chromatographic steps, which makes possible to probe the protein interplay with lipids. The interaction of the purified hBest1 with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was studied in Langmuir monolayers. The surface pressure (π)-area (A) isotherms and compression/expansion isocycles of POPC monolayer were recorded in absence and presence of hBest1 in the subphase. The π(A) isotherms were analyzed in terms of surface compressional modulus and via two-dimensional virial equation of state. The dilatational rheological properties of the surface films and their surface potential were also measured. The morphology of the films was observed by Brewster angle microscopy. The inclusion of the protein in the film subphase does not lead to in-depth penetration of hBest1 but interaction takes place in the headgroup region of the monolayer. The hBest1/POPC interaction resulted in formation of more condensed films, which rheological properties and lateral structure differed significantly from the pure POPC monolayers. Our study sheds light on the still unclear question how hBest1 gets in touch with biomembrane phospholipids of eukaryotic cells that might be of key importance for the proper structure and function of RPE biomembranes.

Disease-causing mutations in BEST1 gene are associated with altered sorting of bestrophin-1 protein

Mutations in BEST1 gene, encoding the bestrophin-1 (Best1) protein are associated with macular dystrophies. Best1 is predominantly expressed in the retinal pigment epithelium (RPE), and is inserted in its basolateral membrane. We investigated the cellular localization in polarized MDCKII cells of disease-associated Best1 mutant proteins to study specific sorting motifs of Best1. Real-time PCR and western blots for endogenous expression of BEST1 in MDCK cells were performed. Best1 mutant constructs were generated using site-directed mutagenesis and transfected in MDCK cells. For protein sorting, confocal microscopy studies, biotinylation assays and statistical methods for quantification of mislocalization were used. Analysis of endogenous expression of BEST1 in MDCK cells revealed the presence of BEST1 transcript but no protein. Confocal microscopy and quantitative analyses indicate that transfected normal human Best1 displays a basolateral localization in MDCK cells, while cell sorting of several Best1 mutants (Y85H, Q96R, L100R, Y227N, Y227E) was altered. In contrast to constitutively active Y227E, constitutively inactive Y227F Best1 mutant localized basolaterally similar to the normal Best1 protein. Our data suggest that at least three basolateral sorting motifs might be implicated in proper Best1 basolateral localization. In addition, non-phosphorylated tyrosine 227 could play a role for basolateral delivery.