The Path to Open-Angle Glaucoma Gene Discovery: Endophenotypic Status of Intraocular Pressure, Cup-to-Disc Ratio, and Central Corneal Thickness

Abstract

PURPOSE. Primary open-angle glaucoma (POAG) is a complex disease with a genetic architecture that can be simplified through the investigation of individual traits underlying disease risk. It has been well studied in twin models, and this study was undertaken to investigate the heritability of some of these key endophenotypes in extended pedigrees. METHODS. These data are derived from a large, multicenter study of extended, Caucasian POAG families from Australia and the United States. The study included 1181 people from 22 extended pedigrees. Variance components modeling was used to determine the heritabilities of maximum intraocular pressure (IOP), maximum vertical cup-to-disc ratio (VCDR), and mean central corneal thickness (CCT). Bivariate quantitative genetic analysis between these eye-related phenotypes and POAG itself was performed to determine whether any of these traits represent true endophenotypes. RESULTS. Heritability estimates for IOP, VCDR, and CCT (0.42, 0.66, and 0.72, respectively) were significant and show strong concordance with data in previous studies. Bivariate analysis revealed that both IOP (RhoG = 0.80; P = 9.6 x 10(-6)) and VCDR (RhoG = 0.76; P = 4.8 x 10(-10)) showed strong evidence of genetic correlation with POAG susceptibility. These two traits also correlated genetically with each other (RhoG = 0.45; P = 0.0012). Alternatively, CCT did not correlate genetically with risk of POAG. CONCLUSIONS. All the proposed POAG-related traits have genetic components. However, the significant genetic correlations observed between IOP, VCDR, and POAG itself suggest that they most likely represent true endophenotypes that could aid in the identification of genes underlying POAG susceptibility. CCT did not correlate genetically with disease and is unlikely to be a useful surrogate endophenotype for POAG.

New insights into the genetics of primary open-angle glaucoma based on meta-analyses of intraocular pressure and optic disc characteristics.

“…Charlesworth et al previously found a genetic correlation between VCDR and IOP (RhoG ¼ 0.45, P ¼ 0.0012), however, genes underlying this relationship have not yet been identified (17).…”

Gene-Finding Strategy in Glaucoma

“…There are many established risk factors for glaucoma, including high IOP, optic nerve cupping, age, race, family history, thin central corneal thickness (CCT), myopia, smoking, hypothyroidism, systolic hypertension, and diabetes (6)(7)(8)(9)(10). The role of ethnicity and family history in the development of glaucoma and its inheritability suggests the contribution of genetic factors to this disease (5).…”

Common and Rare Genetic Risk Factors for Glaucoma

“…For traits with complex inheritance, it can be useful to identify genetic factors influencing the underlying individual traits, such as IOP, optic nerve parameters, and CCT, that contribute to the overall disease (Charlesworth et al 2010). These traits are quantitative, highly heritable, and show substantial variation in human populations.…”