Our Scientists

Glaucoma, a group of diseases in which retinal ganglion cell death and optic nerve degeneration lead to blindness, affects 70 million people worldwide. Harmfully high intraocular pressure (IOP) is a strong contributing factor. The genetic factors determining IOP and susceptibility to pressure-induced damage are largely unknown, as are the specific processes that kill the retinal ganglion cells. We use mouse models to identify both genes and pathophysiologic processes that contribute to glaucoma.

Mechanisms of IOP Elevation
High IOP often causes glaucoma. To understand the initial processes leading to glaucoma, we must identify the genes that cause elevated IOP. Over the years, we have used both gene mapping and candidate gene approaches to identify genes that affect IOP. A major factor limiting the use of mice in the study of glaucoma is the scarcity of high-IOP mutants. We are identifying and characterizing new mouse mutants with altered IOP and glaucoma. Our characterization of new mutants will likely provide valuable insights into mechanisms of IOP elevation and glaucoma. We are currently investigating several new mutants that appear to involve different molecular processes. (The National Eye Institute funds our characterization of new mutants.)

Ocular Development and Glaucoma
Dysgenesis of the ocular drainage structures in the angle of the eye is one underlying cause of IOP elevation in developmental glaucomas. We have characterized genes that cause angle dysgenesis, including the genes for forkhead transcription factor (Foxc2), bone morphogenetic protein (Bmp4), and collagen type IV α1 (Col4a1). We have also studied mice with mutations in the genes for cytochrome P450 1b1 (Cyp1b1) and forkhead box transcription factor C1 (Foxc1). Mutations in these genes cause human developmental glaucoma. Because of extensive variability in the human phenotype, we tested mouse strains for modifier genes that enhance or suppress ocular abnormalities in Cyp1b1 and Foxc1 mutant mice. We identified the tyrosinase gene as a modifier whose deficiency exacerbates defects in both Cyp1b1 and Foxc1 mutant mice. Tyrosinase provides L-DOPA that protects against the developmental abnormalities. Future work will be aimed at increasing understanding of the role of L-DOPA in ocular development and glaucoma, evaluating how dietary DOPA modulates the severity of human glaucoma, and identifying other genes that modify the phenotypic effects of Cyp1b1 mutations.

Pigmentary Glaucoma
In human pigmentary glaucoma, abnormally dispersed iris pigment and cell debris enter the ocular drainage structures, leading to increased IOP. DBA/2J mice develop a form of pigmentary glaucoma involving iris pigment dispersion. The glaucoma results from mutations in related genes (glycoprotein nmb gene [Gpnmb] and tyrosinase-related protein 1 gene [Tyrp1]) encoding melanosomal proteins. We hypothesized that the mutant genes alter melanosomes, allowing toxic intermediates of pigment production to leak from melanosomes, causing iris disease and subsequent pigmentary glaucoma. This is supported by alleviation of pigmentary glaucoma in DBA/2J eyes with substantially decreased pigment production. These results suggest that pigment production and mutant melanosomal protein genes may contribute to human pigmentary glaucoma.

Gpnmb is expressed not only in melanocytes but also in immune cells that present antigens and modulate immune responses. This suggested that immune cell abnormalities may contribute to DBA/2J glaucoma. In support of this, we have characterized a mild inflammatory infiltrate and striking ocular immune abnormalities in DBA/2J eyes. Replacing the bone marrow (which gives rise to immune cells) of DBA/2J mice with normal donor bone marrow restored important aspects of ocular immunity and prevented the disease components associated with the Gpnmb mutation. These findings are the first evidence that an abnormal immune response may contribute to development of pigmentary glaucoma and that bone marrow genotype is an important determinant of disease severity. Ongoing experiments focus on the nature of the immune system's participation in this glaucoma and the immune cell types that are involved.

Not all mouse strains with pigment dispersion are equally susceptible to developing high IOP. We have transferred the DBA/2J genes (Tyrp1 and Gpnmb) that cause iris disease and pigment dispersion into the C57BL/6J strain. This mutant C57BL/6J strain develops prominent pigment dispersion, but IOP elevation is delayed and occurs in many fewer mice compared to DBA/2J mice. We are using genetics, genomics, and cell biology to study the strain differences that determine whether pigment dispersion leads to IOP elevation. This information may lead to new treatments aimed at preventing IOP elevation in individuals with pigment dispersion.

Neurodegeneration in Glaucoma
The neurobiology of pressure-induced cell death in glaucoma is poorly understood. DBA/2J mice provide a tractable model for dissecting pathways of cell death in inherited glaucoma and for investigating neuroprotective strategies.

We use the DBA/2J model to address how and why retinal ganglion cells (RGCs) die in glaucoma, since the damaging pathways are not clearly defined. We determined that distinct degeneration pathways kill the RGC soma and axons in glaucoma. We found that the proapoptotic molecule BAX is necessary for RGC death. BAX deficiency completely stopped RGC somal degeneration in the DBA/2J model. Although BAX deficiency protected the RGC soma, BAX was not necessary for axon degeneration. Thus, somal and axonal degeneration use different pathways that can be uncoupled. Although BAX deficiency did not stop axon degeneration, it did delay optic nerve damage. Optic nerve axon degeneration was delayed in both heterozygous and homozygous mutant mice. These data suggest that BAX inhibitors may be able to delay axon degeneration and that both somal and axonal degeneration pathways will have to be considered when therapeutic strategies for human glaucoma are designed.

We recently demonstrated that the RGC axon is locally insulted in the optic nerve in glaucoma. The insult almost certainly occurs within a small and specialized region of the optic nerve that is rich in glial cells known as astrocytes. We named this region the glial lamina, as it is in the same location as a similarly astrocyte-rich and specialized region of the human optic nerve known as the lamina cribrosa. The human lamina cribrosa has collagenous plates that have been suggested to be essential for developing glaucoma but are absent in the mouse glial lamina. The lack of collagenous plates and location of insult in the mouse optic nerve suggest that abnormal neuroglial interactions may be critical in damaging RGCs in glaucoma.

We are also investigating other molecules that contribute to the neurodegeneration in this inherited glaucoma. Although nitric oxide synthase–dependent damaging events are commonly thought to damage RGCs in glaucoma, our experiments do not support their involvement in DBA/2J glaucoma. Thus, their damaging effects are at least not essential for glaucomatous neurodegeneration. We are completing a comprehensive genomic study of gene expression changes at different stages of glaucoma and are identifying early changes. These studies are providing new insights into RGC death in glaucoma and will identify potential therapeutic targets. For example, complement molecules are up-regulated early in the glaucoma and may contribute to RGC synapse damage.

Neuroprotection
We discovered that whole-body irradiation accompanied by syngeneic bone marrow profoundly protects DBA/2J mice from glaucoma. The treatment has no effect on the iris disease or IOP, but glaucomatous neurodegeneration is undetectable in the vast majority of treated animals. We are not aware of any other neuroprotective effects this substantial. Since bone marrow genotype is not changed, it is not the causative agent. In a recent epidemiological study of atomic bomb survivors, radiation appeared to protect against human glaucoma. Experiments are under way to understand this important effect. We are also testing the ability of focal ocular irradiation to prevent glaucoma.

As of May 30, 2012