Metabolome-Wide Association Study of Primary Open Angle Glaucoma

Glaucoma is an insidiously progressive optic neuropathy that affects nearly 90 million people worldwide, making it a leading cause of irreversible blindness.1 The high variation of incidence among races2 and the variation in clinical manifestations of glaucoma3 emphasize the need to understand how specific genetic and/or metabolic differences among individuals significantly influence disease status. The risk of glaucoma is increased by multiple genetic factors susceptible to the influence of numerous environmental exposures.4 Mutations in MYOC5 and OPTN6 have been associated with juvenile open angle glaucoma and normal tension glaucoma, respectively, and have also been found in a small percentage of primary open angle glaucoma (POAG) patients. Genome-wide association studies have found links between common genetic variants and POAG, such as CAV1 and CAV2,7 as well as TMCO1 and CDKN2BAS.8 However, in total, these genetic variants linked to glaucoma are thought to account for approximately 5% of all POAG and normal tension glaucoma cases.9 More recent studies have revealed three new loci, ABCA1, AFAP1, and GMDS, to be associated with POAG,10 but the extent to which these associations involve the general population has yet to be characterized. The presence of glaucoma has also been associated with metabolic factors, including an increase in plasma markers of oxidative stress, such as malondialdehyde (MDA) and conjugated diene,11 as well as reduced total antioxidant status (TAS).12 However, solitary genetic polymorphisms and single protein or metabolic biomarkers have been limited in their predictive capacity. These biomarker studies hint that future predictive measures are possible, but greater discovery of biomarkers associated with glaucoma is necessary to advance knowledge of the disease mechanism at a personalized level. Ultrahigh resolution metabolic profiling using liquid chromatography-mass spectrometry (LC-MS) provides a practical approach to personalized medicine and has great potential to advance the evaluation of glaucoma risk. The ultrahigh mass resolution and mass accuracy of current Fourier transform mass spectrometers, coupled with advanced adaptive processing algorithms for data extraction, allow the detection and separation of over 10,000 chemicals in plasma. This includes metabolites from 146 of the 154 pathways in the Kyoto Encyclopedia of Genes and Genomes (KEGG) human metabolic pathways,13 providing an effective coverage of central metabolic pathways. Importantly, greater than 90% of the KEGG metabolites have distinct elemental composition13 so that they can be measured based upon the accurate mass and mass resolution without requiring ion dissociation.14–16 Metabolomic analysis of serum and plasma has revealed panels of metabolites that distinguish patients with cardiovascular disease,17 breast cancer,18,19 Parkinson disease,20 and diabetes21 from control patients. This technique is able to differentiate between individuals despite intraindividual variation due to factors such as dietary intake.14 We have established this metabolomic method as a powerful tool for assessing risk of ocular disease through identifying metabolites that discriminate neovascular AMD patients from controls.22 The purpose of this study was 2-fold: first, to determine whether plasma metabolic profiling can detect systemic metabolic differences between POAG patients and controls; and second, to identify metabolites associated with POAG. Metabolic profiles incorporate both individual biomarkers and entire pathways associated with disease, providing a better understanding of POAG pathophysiology.