Biomarker research centers on early glaucoma diagnosis

By Fred Gebhart

The search for glaucoma biomarkers is kicking into high gear. A multiyear grant, funded by Catalyst for a Cure, has already produced new biomarker candidates and novel techniques that promise to advance research in glaucoma, as well as in retinitis pigmentosa, gene therapy of inherited photoreceptor conditions, and other conditions.

“The way this program is structured pushes us to be very creative,” said Alfredo Dubra, PhD, assistant professor and co-director of the Advanced Ocular Imaging Program at the Medical College of Wisconsin, Milwaukee. “Three of the four of us were not even working in glaucoma before this project. We are encouraged to bring in out-of-the-field ideas. And it is paying off.”

The GRF research program, entitled “Biomarker Initiative Program,” has four principal investigators. Dr. Dubra presented current results during “GRF Update: Adaptive Optics and Glaucoma Biomarkers.” The co-investigators are: Jeffrey Goldberg, MD, PhD, professor of ophthalmology and director of research at the Shiley Eye Center, University of California, San Diego (UCSD); Andrew Huberman, PhD, assistant professor of neurosciences/neurobiology/ophthalmology at Shiley Eye Center and UCSD; and Vivek J. Srinivasan, PhD, assistant professor of biomedical engineering at the University of California, Davis.

New biomarkers sought

The goal of the program is to discover new biomarkers for glaucoma. It is not that glaucoma biomarkers don’t already exist. IOP, thinning of nerve fiber layers, visual field testing, and other changes are helpful and in use today, but they lack specificity and sensitivity, especially over shorter time scales.

“If we could discover a very sensitive, very specific biomarker for glaucoma, we could avoid much of the vision loss that is associated with the disease today,” Dr. Dubra said. “Our research focus is to be able to diagnose glaucoma very early and detect the specific, minute changes that could allow us to reduce vision loss.”

There is no shortage of biomarker candidates. Research teams are actively looking at metabolic changes, synapse loss, axon loss, glial activation, and vascular compromise. One of the most fundamental questions is whether there is a specific type of retinal ganglion cell (RGC) that shows changes first, at the earliest stages of glaucoma, and whether those changes can be detected. The answer appears to be yes.

A bead model of glaucoma produces chronic increases in IOP. Animal studies have found that RGCs in the “off” sublaminal layer are strongly impacted by rising IOP with dendritic structures that are clearly deformed. This same layer is rich in vasculature and fits with other evidence showing how different laminal layers are affected depending on their degree of vascularization.

Evidence to date suggests that early selective damage to connections in this “off” sublayer of the inner retina is a reliable biomarker for glaucoma. Human studies will begin later this year.

Depth-resolved OCT spectroscopy is also emerging as a more useful tool than it has been in the past. New techniques use motion to generate contrast without the use of exogenous dyes, extract extremely high-resolution data on the relative oxygenation within specific layers of the retina and measure blood flow velocity.

“For the first time, you can calculate how much oxygen the retina uses in specific locations,” Dr. Dubra said. “We may be able to see if regions affected by glaucoma show higher or lower oxygen consumption and whether there are changes over time.”

Advances in adaptive optics

The biggest news may be advances in adaptive optics. The latest techniques offer clear views of sides of the eye and high-resolution images of individual cells and layers within the retina, including the nerve fiber layer. The technique offers a non-invasive view of the retina and the photoreceptor mosaic that requires no dye or physical discomfort.

“We stumbled on this technique while trying to visualize the ganglion cells,” Dr. Dubra said. “It was the kind of fishing expedition that this grant was designed to encourage. And sometimes, when you go on a fishing expedition, you catch a very big fish. We have seen a whole new kind of epi-retinal membrane with structures that show visible changes as glaucoma progresses.”

Micrographs clearly show the nerve fiber layer with individual fibers in the 20-m range and even smaller structures in the 2-m range. The epi-retinal membrane exhibits changes over time that are consistent with advanced glaucoma, but the changes are not specific to glaucoma. More than two dozen different pathologies from macular holes and optic atrophy to multiple sclerosis, diabetic retinopathy, and age-related macular degeneration show similar changes.

This new retinal structure is not a useful biomarker for glaucoma, Dr. Dubra continued, but adaptive optics has evolved into a highly sensitive investigative tool that can be used in real time. The technique can image individual blood cells moving through the microvasculature and capillaries. Researchers have already identified previously unknown forms of microaneurysms and types of retinal damage specific to physical trauma, autosomal dominant optic atrophy, and other causes.       

“Even though it has only been two years, we have made remarkable progress,” Dr. Dubra said. “We are excited about the new tools we have developed and the new directions our findings have taken us.”