Distinct expression requirements and rescue strategies for BEST1 loss- and gain-of-function mutations

Genetic mutation of the human BEST1 gene, which encodes a Ca²⁺-activated Cl^- channel (BEST1) predominantly expressed in retinal pigment epithelium (RPE), causes a spectrum of retinal degenerative disorders commonly known as bestrophinopathies. Previously, we showed that BEST1 plays an indispensable role in generating Ca²⁺-dependent Cl^- currents in human RPE cells, and the deficiency of BEST1 function in patient-derived RPE is rescuable by gene augmentation (Li et al., 2017). Here, we report that BEST1 patient-derived loss-of-function and gain-of-function mutations require different mutant to wild-type (WT) molecule ratios for phenotypic manifestation, underlying their distinct epigenetic requirements in bestrophinopathy development, and suggesting that some of the previously classified autosomal dominant mutations actually behave in a dominant-negative manner. Importantly, the strong dominant effect of BEST1 gain-of-function mutations prohibits the restoration of BEST1-dependent Cl^- currents in RPE cells by gene augmentation, in contrast to the efficient rescue of loss-of-function mutations via the same approach. Moreover, we demonstrate that gain-of-function mutations are rescuable by a combination of gene augmentation with CRISPR/Cas9-mediated knockdown of endogenous BEST1 expression, providing a universal treatment strategy for all bestrophinopathy patients regardless of their mutation types.