Disease modeling of myocilin mutation-dependent normal tension glaucoma: human retinal ganglion cell susceptibility to unfolded protein response and mTOR signaling

Glaucoma represents a group of diseases where the unifying theme is the progressive degeneration of retinal ganglion cells (RGCs), causing irreversible vision loss. Mutations in the myocilin (MYOC) gene represent one of the most common genetic factors associated with primary open-angle glaucoma (POAG). However, the mechanism underlying MYOC mutation-associated POAG is poorly understood. Here, using human disease modeling of MYOC mutation (A445V)-dependent POAG, which is usually without ocular hypertension, we have tested a hypothesis that human RGCs (hRGCs) are the target of the mutant protein, making them vulnerable to degenerative changes. Examination of hRGCs generated from MYOCA445V POAG patient-specific induced pluripotent stem cells (iPSCs) revealed that their differentiation is adversely affected, compared to those generated from isogenic control iPSCs. Retinal ganglion cells regulatory and axon growth and guidance gene expression is decreased in patient-specific hRGCs vs isogenic controls. Consequently, the former display immature neurites and their ability to form synapses with the target cells and regenerate are compromised. Furthermore, they display immature networking physiology compared to isogenic controls. The pathological burden of the mutant protein is reflected in their preferential retention in the endoplasmic reticulum (ER) of patient-specific hRGCs, activating the unfolded protein response (UPR) toward mutation-associated developmental phenotype. Furthermore, we demonstrate that REDD1, a stress-induced factor, is a mechanistic link between the MYOCA445V-activated UPR axis and inhibited mTOR signaling, a critical regulator of RGC development and function. Ours is the first demonstration of MYOC mutation-dependent hRGC phenotype and posits a mechanism for hRGC susceptibility toward degeneration independent of ocular hypertension.