Glaucoma; The Silent Thief

Diagnosis and Treatment

Damage to the optic nerve usually happens when the pressure of the liquid inside the eye - the intraocular fluid - becomes too high. This can be a result of health conditions, such as diabetes, or particular medications like steroids. Other times, however, high intraocular pressure occurs without a clear cause. Furthermore, in some people, the optic nerve can become damaged without any high intraocular pressure at all. This means that testing eye pressure alone isn’t enough to check if a person has glaucoma - a more complex eye examination is required, including careful interpretation of the patient’s symptoms.

Once the optic nerve is damaged by glaucoma, it is impossible to repair. However, there are medications available to stop glaucoma in its tracks and prevent any further loss of sight - for many people, regular use of medicinal eyedrops is enough to lower eye pressure and halt the progression of glaucoma. In other cases, laser surgery or conventional operating room surgery can be used to improve drainage in the eye and stop the disease. In order for these treatments to be effective, however, it is important that glaucoma can be caught early - ideally before patients have suffered severe sight loss - so that people with glaucoma can enjoy healthy vision and a better quality of life.

Glaucoma Genes

One way of detecting glaucoma is to search for a gene, or multiple genes, that are responsible. Since glaucoma is a group of related diseases rather than a single condition, different genes are thought to be responsible for each type of glaucoma. Early onset types of glaucoma – those that develop before age 40 - often involve rare mutations in one or two genes that result in significant biological effects. For example, junior open angle glaucoma (JOAG) – a severe form of glaucoma that develops in young people – is associated with inherited mutations in the MYOC gene, which encodes the protein myocilin. Research has found that mutations in the MYOC gene cause misfolding and aggregation of myocilin, which is thought to contribute to increased intraocular pressure.

Familial normal tension glaucoma (NTG) is also associated with rare mutations - in this case, two genes are affected: OPTN, which encodes optineurin, and TBK1, which encodes tank-binding protein 1. These two proteins are known to interact, and are both associated with important cellular processes, particularly autophagy, which is the controlled degradation and recycling of cells.

The genetic basis of adult-onset glaucoma is more complex, involving a greater number of genetic mutations which each contribute in small amounts to the overall disease. For instance, primary open angle glaucoma (POAG) – the most common type of glaucoma – is associated with mutations in up to 16 different gene loci, making it highly heterogeneous. The cellular processes affected by these mutations are diverse, ranging from lipid metabolism to cell division. Primary angle-closure glaucoma (PACG), a major cause of blindness, is associated with 8 different genetic loci. Therefore, in these cases, it is much harder to pinpoint an exact genetic cause – meaning it is unlikely that we will soon access a genetic test for these types of glaucoma.

Molecular Biomarkers

An alternative to genetic testing is the use of biomarkers. The term biomarker refers to any characteristic that can be objectively measured and evaluated to provide some indication of an internal biological or pathological process, and can be anything from large proteins to small molecule metabolites. Modern technology and the development of advanced analytical platforms such as mass spectrometry coupled with gas chromatography (GC-MS) and liquid chromatography (LC-MS) has enabled biomarker research to skyrocket in recent years – in fact, for glaucoma alone, over 450 potential biomarkers have been identified as of 2021 (Cueto et al., 2021).

Many biomarker studies for glaucoma have examined the tissues and fluids of the eye, as this is where the disease occurs. Numerous proteins of interest have been identified in the intraocular fluid, such as the cytokine transforming growth factor-beta 2 (TGF-β2), which has been repeatedly implicated in biomarker studies and is reportedly up-regulated in multiple types of glaucoma (Tripathi et al., 1994)(Min et al., 2006). Some other notable biomarkers from the eye include matricellular proteins – a group of extracellular proteins which are known to be expressed in the eyes of glaucoma patients. In particular, CTGF is a matricellular protein that has been highlighted in several studies and is reportedly elevated in cases of glaucoma (Cueto et al., 2021). Smaller molecules from inside the eye have also been explored as potential biomarkers - for example, the amino acid homocysteine has been reported as upregulated in glaucoma eyes, and these high levels are thought to contribute to the development of the disease. However, sampling the intraocular fluid is highly invasive, and is really only possible when patients are undergoing surgery.

Accessible and useful clinical biomarkers might be found in the tear fluid, which covers the surface of the eye in a fine film. This fluid can be sampled much more easily than the fluid inside the eye, by using a glass capillary or paper strips. Several studies investigating the tear film in glaucoma patients have found that the inflammatory response pathway is affected, with inflammatory-associated proteins such as immunoglobins and proinflammatory cytokines showing altered expression. This sounds promising, but these proposed biomarkers – particularly the cytokines – could be expressed as a result of topical glaucoma treatment, rather than the disease itself (Cueto et al., 2021).

More conventional biomarker research has looked into the blood as a source of molecules of interest. The blood, as the body’s transport system, is a bountiful source of signaling molecules and small metabolites from cells all over the body. Indeed, inflammatory biomarkers associated with glaucoma have been found in the blood, including the hormone thymulin (Noureddin et al., 2006) and several inflammatory cytokines (Huang et al., 2010). More recently, a comparison of the composition of blood plasma between healthy people and individuals with POAG found differences in a range of metabolic processes, which could be indicative of mitochondrial dysfunction and a significant change in energy metabolism (Burgess et al., 2015). Another study focusing on pseudoexfoliation glaucoma found that levels of malondialdehyde (MDA) were higher in the glaucoma group than controls, while levels of catalase (CAT) and superoxide dismutase (SOD) enzyme activity were higher in the controls (Yaz et al., 2019); a later study examining serum found consistent results (Li et al., 2020). These biomarkers as associated with oxidative stress, which supports the idea that oxidative stress and the alteration of antioxidant defense mechanisms play a role in the pathogenesis of glaucoma.

Outlook

Given the number of biomarker studies performed to date, one might wonder why we aren’t already using biomarkers to diagnose glaucoma in the clinic. Unfortunately, although many molecules have been put forward as potential biomarkers, many studies show conflicting results, and the suggested biomarkers are yet to be successfully validated. Many of the studies performed so far have used small numbers of participants, so we can’t be sure that their results are applicable to the population as a whole. Also, many glaucoma patients included in these studies are likely to have used glaucoma medications, which are themselves responsible for molecular changes to the body.

Overall, despite research into glaucoma genes and biomarkers, glaucoma remains a silent thief of sight across the world. However, there is hope. Every piece of research has helped us understand more about glaucoma – whether it is inherited, how it develops, and what treatments might help us in the future. While more research is needed before we have access to a suitable genetic or biomarker test, we are still getting closer every day to a world where no one needs to suffer through preventable sight loss from glaucoma.

References:

Burgess, L. G., Uppal, K., Walker, D. I., Roberson, R. M., Tran, V. L., Parks, M. B., Wade, E. A., May, A. T., Umfress, A. C., Jarrell, K. L., Stanley, B. O. C., Kuchtey, J., Kuchtey, R. W., Jones, D. P., & Brantley, M. A. (2015). Metabolome-wide association study of primary open angle glaucoma. Investigative Ophthalmology and Visual Science, 56(8). https://doi.org/10.1167/iovs.15-16702

Cueto, A. F. V., Álvarez, L., García, M., Álvarez-barrios, A., Artime, E., Cueto, L. F. V., Coca-Prados, M., & González-iglesias, H. (2021). Candidate glaucoma biomarkers: From proteins to metabolites, and the pitfalls to clinical applications. In Biology (Vol. 10, Issue 8). https://doi.org/10.3390/biology10080763

Huang, P., Qi, Y., Xu, Y. S., Liu, J., Liao, D., Zhang, S. S. M., & Zhang, C. (2010). Serum cytokine alteration is associated with optic neuropathy in human primary open angle glaucoma. Journal of Glaucoma, 19(5). https://doi.org/10.1097/IJG.0b013e3181b4cac7

Li, S., Shao, M., Li, Y., Li, X., Wan, Y., Sun, X., & Cao, W. (2020). Relationship between Oxidative Stress Biomarkers and Visual Field Progression in Patients with Primary Angle Closure Glaucoma. Oxidative Medicine and Cellular Longevity, 2020. https://doi.org/10.1155/2020/2701539

Min, S. H., Lee, T. Il, Chung, Y. S., & Kim, H. K. (2006). Transforming growth factor-beta levels in human aqueous humor of glaucomatous, diabetic and uveitic eyes. Korean Journal of Ophthalmology : KJO, 20(3). https://doi.org/10.3341/kjo.2006.20.3.162

Noureddin, B. N., Al-Haddad, C. E., Bashshur, Z., & Safieh-Garabedian, B. (2006). Plasma thymulin and nerve growth factor levels in patients with primary open angle glaucoma and elevated intraocular pressure. Graefe’s Archive for Clinical and Experimental Ophthalmology, 244(6). https://doi.org/10.1007/s00417-005-0143-z

Tripathi, R. C., Li, J., Chan, W. F. A., & Tripathi, B. J. (1994). Aqueous humor in glaucomatous eyes contains an increased level of TGF-β2. Experimental Eye Research, 59(6). https://doi.org/10.1006/exer.1994.1158

Yaz, Y. A., Yıldırım, N., Yaz, Y., Tekin, N., İnal, M., & Şahin, F. M. (2019). Role of oxidative stress in pseudoexfoliation syndrome and pseudoexfoliation glaucoma. Turkish Journal of Ophthalmology, 49(2). https://doi.org/10.4274/tjo.galenos.2018.10734