Histochemical Analysis of Glaucoma Caused by a Myocilin Mutation in a Human Donor Eye

Abstract

Purpose: Mutations in myocilin (MYOC) may cause either juvenile-onset open-angle glaucoma (JOAG) or adult-onset primary open-angle glaucoma. Myocilin encodes a glycoprotein that normally is secreted from trabecular meshwork cells that regulate intraocular pressure. Prior in vitro, transgenic rodent, and organ culture experiments have suggested that abnormal accumulation of MYOC protein within trabecular meshwork cells is a key step in glaucoma pathophysiology. We investigated the pathogenesis of MYOC glaucoma by examining a donor eye from a patient with JOAG caused by a Tyr437His MYOC mutation. Design: Case-control immunohistochemical study of a donor eye from a patient with JOAG caused by a Tyr437His MYOC mutation and age-matched control donor eyes. Participants: An eye from a 59-year-old man with JOAG caused by a Tyr437His MYOC mutation and eyes from 5 donors (51–66 years of age) with no known ocular disease were examined. Methods: Frozen fixed sections of the iridocorneal angle were prepared from the donor eyes of the MYOC glaucoma patient and control eyes. We used antibodies directed against MYOC, collagen IV, and GRP78 (BiP) as well as wheat germ agglutinin and concanavalin A lectins to localize MYOC protein in the trabecular meshwork. Main Outcome Measures: Qualitative comparison of MYOC protein labeling and localization in the trabecular meshwork of donor eyes from a glaucoma patient with a MYOC mutation and from control participants. Results: Using immunohistochemistry, we detected more abundant MYOC protein within the trabecular meshwork of the MYOC glaucoma patient's eye than in control eyes. We further localized MYOC protein within the trabecular meshwork cells of the MYOC glaucoma patient's eye by colabeling with the endoplasmic reticulum (ER) marker GRP78 (BiP). Little to no MYOC was identified within the trabecular meshwork cells of control eyes. Minimal extracellular MYOC was detected in both MYOC glaucoma eyes and control eyes. Conclusions: This histopathologic analysis of a rare donor eye from a glaucoma patient with a MYOC mutation supports our model of MYOC-associated glaucoma, in which MYOC mutations cause abnormal intracellular retention of MYOC within the ER of trabecular meshwork cells as a key step toward development of glaucoma.