Objective: The gene (myocilin: MYOC) has been attributed to be involved in over 6% of inherited types of human glaucoma, the highest correlation for any gene to date. This study determines myocilin protein levels in the aqueous humor (AH) of normal laboratory quality, genetic carrier (offspring of normal laboratory quality and POAG Beagles), and primary open angle glaucoma (POAG) Beagles.
Materials and methods: Eighteen dogs were used and classified as either normal, carrier or having mild, moderate or advanced POAG. A 0.1 mL sample of AH was drawn from the anterior chamber of each dog in the study and frozen on dry ice. A modified Coomassie stain and Western blot, using a polyclonal rabbit antihuman myocilin antibody (Santa Cruz Biotechnologies, Santa Cruz, CA), was run on each sample to compare the myocilin levels. A purified human trabecular meshwork excreted myocilin protein sample was used as a control (Alcon Research Laboratories, Fort Worth, TX) and its band/densitometry measurement was defined as one unit of myocilin for comparisons.
Results: Comparisons of AH myocilin levels differed among normal laboratory quality, genetic carrier, and POAG Beagles at different stages of the disease. In the normal laboratory, Beagles the AH myocilin measured 0.817 +/- 0.075 units (mean +/- SEM); in the carrier Beagles the AH myocilin was 3.117 +/- 0.290 units. As POAG progressed, myocilin protein levels also increased to 6.097 +/- 0.810, 8.844 +/- 1.079, and 17.228 +/- 1.198 units in the early, moderate, and advanced forms, respectively. Overall comparisons between normal, carrier and all POAG Beagles combined showed significant differences (P < 0.0010). Individual comparisons between normal and carrier eyes showed significant differences (P < 0.0193). Comparisons between normal and all POAG eyes also showed significant differences (P < 0.0426).
Conclusion: This study shows myocilin protein is present in normal Beagles, markedly increased in POAG Beagles, and mildly increased in genetic carrier Beagles. There is a strong correlation between amounts of AH myocilin protein and the presence and severity of POAG. The exact role of AH myocilin levels in the genesis of ocular hypertension remains unresolved, but myocilin may adversely affect AH outflow.