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

Abstract

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.

Keywords: BEST1; CRISPR; bestrophinopathies; gene editing; gene therapy; induced pluripotent stem cells.

Figure 1.

Representative illustrations of Bestrophinopathy fundus appearance. (a) Best disease, with the egg yolk-like vitelliform lesion observed at the macula. (b) Autosomal recessive bestrophinopathy, characterised by multifocal deposits and lesions around and beyond the macula. (c) Autosomal dominant vitreoretinochoroidopathy typified by presence of a hyperpigmented circumferential band of pigmentation in the peripheral retina. (d) Best-related retinitis pigmentosa characterised by the presence of peripheral pigment changes, bone spicules and foveal deposits.

Figure 2.

Architecture of the BEST1 channel. (a) The structure of a BEST1 protein unit is divided into four segments, composed of alpha helices represented as S1a-c (red), S2a-h (yellow), S3a-b (blue) and S4a-b (magenta), the transmembrane regions (TM) are indicated. The calcium clasp is represented by a turquoise sphere and the start of the C-terminal tail is coloured green. (b) The BEST1 channel, viewed from the extracellular side, is formed from five BEST1 proteins arranged in a pentameric structure, forming a barrel shaped ion pore (c).

Figure 3.

BEST1 disease causing mutations. (a) Annotated BEST1 protein sequence with ClinVar benign/likely benign mutations indicated in green. (b) Potential effects of mutation on BEST1 channel.

Figure 4.

Crucial roles of retinal pigment epithelium (Adapted from Strauss, 2005).

Figure 5.

Generation of patient derived iPSC-RPE as a human disease model system for the study of bestrophinopathies and development novel forms of treatments for patients.

Figure 6.

Gene therapy approaches for bestrophinopathies can be tested in patient iPSC-RPE cells in a trial in a dish scenario. This approach could be used as a screen to identify mutations responsive to the treatment prior to clinical application in the patient.

Figure 7.

CRISPR genome editing can be used to treat patient mutations at the molecular level. (a) The CRISPR-Cas9 complex is used to create a double strand DNA break in close proximity to the patient. (b) A corrected DNA template can be incorporated into the sequence using homology directed repair, alternatively and (c) non-homologous end joining can be used to create insertions/deletions, resulting in frame-shifts that disrupt the target gene.