Homocysteine elevation in glaucoma reveals dysfunctional one-carbon metabolism as a target for neuroprotection

Abstract

Purpose : Metabolic dysfunction has emerged as a clear pathophysiological mechanism in glaucoma, yet little is known about the ability to maintain anabolic metabolism. We recently identified early elevation of retinal homocysteine in a rat model of glaucoma. Homocysteine has a central role in one-carbon metabolism and an established pathogenic role in cardiovascular disease, diabetes, and Alzheimer’s disease. We hypothesized that elevated homocysteine may directly contribute to retinal ganglion cell (RGC) degeneration and indicate dysfunction of one-carbon metabolism in glaucoma.

Methods : We used a rat microbead model of ocular hypertension (OHT) with intravitreal injection of homocysteine and a Mendelian randomization experiment utilizing large-scale human genetic and ocular phenotyping data. We assessed transcriptional changes in one-carbon metabolism pathways from microarray and RNA-sequencing in the DBA/2J. Rats (bead model) and mice (circumlimbal suture) were supplemented with co-factors B₆, B₉, B₁₂, and choline as a prophylactic treatment 7 days prior to induction of OHT. RGC density, optic nerve degeneration, and electroretinogram function were assessed.

Results : Elevation of homocysteine did not worsen RGC death with OHT in rats, similarly, genetic association with elevated homocysteine is not associated with risk of POAG or retinal thinning in humans. We identified early- and sustained-dysregulation of genes involved in one-carbon metabolism, including the transport and utilization of one-carbon metabolism cofactors and precursors (B₆, B₉, B₁₂, choline), in whole retina and optic nerve head, and in RGCs, microglia, and infiltrating monocytes. Co-factor supplementation significantly reduced RGC loss relative to vehicle treated controls in acute and chronic OHT, reduced the severity of optic nerve degeneration, and protected the scotopic threshold response.

Conclusions : Elevation of homocysteine does not worsen RGC death in glaucoma but is an indicator of dysfunctional one-carbon metabolism in the retina/optic nerve. This disruption is exacerbated by a local deficiency in the utilization of B-vitamin co-factors. Supplementation of these can provide structural and functional neuroprotection across multiple species and models of glaucoma, which coupled with good clinical safety profiles supports translation to clinical trial.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.