A cohort of beagles with inherited glaucoma is providing new clues for researchers about neurodegeneration in retinal ganglion cells (RGC) that contributes to glaucoma in humans.
Twelve years ago, researchers at Vanderbilt University Medical Center and the University of Florida, Gainesville, identified a variant of the gene ADAMTS10 as the glaucoma-causative mutation in the beagle cohort, which has been studied since the late 1970s. Those findings have since been independently verified in other canine models and ADAMTS genes have been associated with human glaucoma.
The Vanderbilt team led by John Kuchtey, Ph.D., and Rachel Kuchtey, M.D., Ph.D., carried out recent studies that went on to establish ADAMTS10 as a glaucoma-causative factor in both mouse and zebrafish models. The Kuchteys led the original ADAMTS10 discovery team.
“We’re still following through with our original work with the beagles,” said John Kuchtey, a research associate professor at Vanderbilt. “Finding a disease gene is just scratching the surface.”
Rachel Kuchtey, a clinician-scientist and glaucoma specialist at Vanderbilt Eye Institute, said the findings have relevance specific to human glaucoma.
“It’s really the pathways that these molecules take that may be more relevant than one single player,” she added. “Understanding how they contribute to disease pathogenesis will enable us to identify novel treatment strategies.”
The Critical Role of ADAMTS10
The core of elastic fibers found in the optic nerve is surrounded by a sheath of fibrillin microfibrils. The ADAMTS10 protein contributes to the normal formation of these microfibrils, which act as storage vehicles for latent transforming growth factor beta (TGF-β), a family of signaling proteins that play a role in glaucoma.
TGF-β promotes the development of RGCs and is also known to play other roles, John Kuchtey explained. In glaucoma, abnormal microfibrils can lead to dysregulation of TGF-β signaling, which could be a genesis of the disease.
Mutations in ADAMTS10 have been shown to cause the autosomal recessive form of Weill-Marchesani syndrome in humans, with lens thickening and angle closure glaucoma as common complications. The dominant form of Weill-Marchesani syndrome is caused by mutations in fibrillin-1. Marfan syndrome is also caused by fibrillin-1 disruptions.
“The question we’re asking is: What is the effect of ADAMTS10 on these microfibrils and how does it impact TGF-β signaling?” John Kuchtey said.
Preserving RGCs
Microfibrils contribute to the mechanical function of the lamina cribrosa, where the RGCs exit the back of the eye. Researchers believe that changes in the biomechanical properties of the lamina cribrosa and surrounding tissue cause abnormal remodeling of the extracellular matrix, resulting in impingement of the RGC axons and reduced transport of needed growth factors. This, in turn, leads to the eventual degradation and death of the RGCs.
In their latest study, the investigators introduced into mice the ADAMTS10 mutation found in beagles. Abundant ADAMTS10 was found in the RGC bodies in the retina and in optic nerve axons, but in somewhat of a surprise result, it was not associated with microfibrils.
“This was an unexpected finding, since previous studies of ADAMTS10 focused on interactions with microfibrils. Our results showed that in these tissues that essentially define glaucoma, ADAMTS10 has a previously unknown function independent of microfibrils,” John Kuchtey said.
The researchers found that developing mice with the glaucoma-causing mutation had a marked reduction of TGF-β activity, suggesting a novel role for ADAMTS10 in the regulation of this cytokine that itself is an important regulator of extracellular matrix structure.
“There are other ways to regulate TGF-β; we’re looking at other proteins,” John Kuchtey explained. “One of these is thrombspondin-1 (TSP1), which displaces fibrillin and allows active TGF-β to bind to its peptide and activate signaling.”
“Our hypothesis is that ADAMTS10 does a similar thing; we found that the two proteins do directly interact. Do they interact like TGF-β does with TSP1? We think this is likely the case because two other ADAMTS family members interact with TGF-β this way. The interactions would occur in very specific locations; you can dampen or enhance this interaction using corresponding peptides.”
Future Implications
Not all patients with elevated intraocular pressure (IOP) develop glaucoma while some patients with apparently normal IOP do and continue to progress, John Kuchtey noted. Therefore, a major unmet need is to identify targets for glaucoma treatment in addition to lowering IOP.
“Modulation of TGF-β signaling could offer a new approach to glaucoma pathogenesis that directly addresses the changes in the properties of the extracellular matrix in the optic nerve tissue,” he said.
