Mutant Myocilin Nonsecretion In Vivo Is Not Sufficient To Cause Glaucoma

Abstract

Glaucoma is a leading cause of blindness, affecting over 70 million people worldwide. Vision loss is the result of death of the retinal ganglion cells. The best-known risk factor for glaucoma is an elevated intraocular pressure (IOP); however, factors leading to IOP elevation are poorly understood. Mutations in the MYOC gene are an important cause of open-angle glaucoma. Over 70 MYOC mutations have been identified, and they lead to approximately 5% of all primary open-angle glaucoma cases. Nevertheless, the pathogenic mechanisms by which these mutations elevate IOP are presently unclear. Data suggest that a dominant interfering effect of misfolded mutant MYOC molecules may be pathogenic. To test this hypothesis, we have generated mice carrying a mutant allele of Myoc that is analogous to a human mutation that leads to aggressive glaucoma in patients. We show that mutant MYOC is not secreted into the aqueous humor. Instead of being secreted, mutant MYOC accumulates within the iridocorneal angle of the eye, consistent with the behavior of abnormally folded protein. Surprisingly, the accumulated mutant protein does not activate the unfolded protein response and lead to elevated intraocular pressure or glaucoma in aged mice of different strains. These data suggest that production, apparent misfolding, and nonsecretion of mutant MYOC are not, by themselves, sufficient to cause glaucoma in vivo.Glaucoma is a progressive neurodegenerative disease and a leading cause of blindness, especially in the elderly (27). There are approximately 70 million people with glaucoma worldwide, and this number is expected to increase as the population ages (39). Vision loss in patients with glaucoma results from the demise of the retinal ganglion cells (RGC). Development of an elevated intraocular pressure (IOP) is an important risk factor for glaucoma (14,26,45). IOP elevation most often appears to be caused by an increased resistance to outflow of aqueous humor through the drainage structures in the iridocorneal angle (the trabecular meshwork and Schlemm's canal). The molecular mechanisms that lead to IOP elevation remain poorly defined. Although current drug treatments that lower IOP are effective in decreasing visual loss in many patients, they are not effective in significant numbers of individuals.Understanding the pathological mechanisms that lead to IOP elevation will help to suggest new treatment targets, with the goal of more effective care for more patients. Deciphering the pathogenic mechanisms of mutations in specific glaucoma genes promises more targeted and tailored treatments based on genotype. A number of genes and loci have been associated with primary open-angle glaucoma (POAG), a common glaucoma subtype (27). Mutations in the myocilin gene (MYOC) are an important cause of POAG (46). Mutations in MYOC underlie glaucoma in up to 5% of patients with POAG and up to 30% of patients with juvenile open-angle glaucoma, an earlier-onset and more severe form of POAG (9). Of the approximately 70 MYOC mutations so far identified...

Myocilin Stimulates Osteogenic Differentiation of Mesenchymal Stem Cells through Mitogen-activated Protein Kinase Signaling

“…Although the exact mechanisms are still not clear, it has been suggested that accumulation of mutated myocilin induces the unfolded protein response, makes cells more sensitive to oxidative stress, and, consequently, leads to deterioration of trabecular meshwork function and/or trabecular meshwork cell death (40 -42). At the same time, no other phenotypes have been reported in humans carrying MYOC mutations or in mice expressing mutated myocilin (43)(44)(45). Myoc-null mice appeared to be normal and do not develop glaucoma, but careful analysis of non-ocular phenotypes have not been performed.…”

Primary open-angle glaucoma genes

“…Several mouse models of myocilin and optineurin-related POAG have been generated and will facilitate further studies of the biological mechanisms of vision loss in glaucoma. 43,44,59,89 In contrast, for complex genetic forms of disease, an individual genetic risk factor for POAG is not capable of causing glaucoma on its own, and most carriers of this type of risk factor will never develop glaucoma. In the future, when more of these risk factors have been identified, testing patients for the presence of several genetic factors may have more clinical utility for predicting which patients are at highest risk for developing disease and need closer surveillance.…”

Expression of Myocilin Mutants Sensitizes Cells to Oxidative Stress-Induced Apoptosis

“…52 Expression of mutated mouse myocilin Y423H corresponding to the Y437H human mutant in the mouse TM in vivo did not lead to the activation of UPR as judged by unchanged levels of several markers including GRP78. 34 Here we used stably transfected HEK293 cells expressing wild-type and mutated myocilins under an inducible promoter to demonstrate that both mutations that we used, Y437H and I477N, induced UPR, with the I477N mutant producing higher activation of UPR markers (GRP78 and PDI) than the Y437H mutant. We do not believe that higher activation of UPR markers by the I477N mutant was due to the presence of the C-terminal FLAG tag, which was also present in wild-type myocilin.…”

“…32,33 However, the expression of mutated myocilin allele (Tyr423His) specifically in the mouse iridocorneal angle did not lead to detectable TM cell death. 34,35 Environment may contribute to the effects of mutated myocilin. In particular, it is well established that oxidative stress may be a factor in the progression of many diseases, including glaucoma.…”