Primary open-angle glaucoma, a heterogeneous spectrum of disorders, affects 1-2 percent of the population1 with varying severity and age of onset. With advances in molecular genetics and the discovery of new glaucoma genes occurring rapidly, it is evident that there will soon be new ways of classifying, diagnosing and treating the various forms of glaucoma.
Historically, the diagnosis of glaucoma has been based on a combination of intraocular pressure measurements, optic nerve head appearance and visual field testing. The current definition relies on a characteristic appearance of the optic nerve head. However, sometimes the diagnosis of glaucoma is quite difficult to make due to the variability of the "normal" disc appearance among any population. What one physician assesses as an early glaucomatous nerve another may classify as suspect, or even within physiological limits.
An objective diagnosis, via molecular genetic testing, would make classification easier for ophthalmologists and has the promise of ensuring early and effective treatment for patients. The therapeutic goal for patients with glaucoma remains early diagnosis and initiation of treatment, in order to retain vision and quality of life.
Predisposition Towards Disease
We have known for some time that in the majority of glaucoma patients there is a genetic predisposition towards the disease, but until recently there was little molecular evidence of this. It was realized that such a common disorder was likely complex in its etiology, i.e., not caused by a single gene but more likely by a number of genes, possibly with environmental influences, thus making detection of the responsible genes more difficult. As the molecular etiology of POAG becomes more fully understood, we are learning that in fact POAG is quite heterogeneous and that there are several subtypes, distinguishable largely based on molecular etiology (specific genetic mutations).
Molecular genetics has given us a new way of understanding disease. By identifying the genetic defect we can begin to classify glaucoma based on etiology, and then work out the pathophysiology of the disease. This provides a way of looking at glaucoma from the inside out, rather than using clinical observations and formulating hypotheses, which is the outside-in approach. So far, genetic linkage and positional cloning/candidate gene identification are the key methods that have been used to identify a number of glaucoma genes.
Successes in Molecular Genetics
In 1997, the first glaucoma gene, myocilin or MYOC, was discovered. It was initially named Trabecular Meshwork-Inducible Glucocorticoid Response Protein gene or TIGR. Genetic linkage was used to localize this gene to chromosome 1q24 and eventually it was cloned. Myocilin is expressed in the trabecular meshwork, the ciliary body and the retina, as well as in other tissues. Myocilin appears to be responsible for 3-5 percent of adult POAG by causing increased intraocular pressure due to obstruction of aqueous outflow through the trabecular meshwork.2
Since then two other genes have been identified, CYP1B1 for congenital glaucoma and Optineurin or OPTN for normal-tension/high tension glaucoma. CYP1B1 is thought to be important in the development and regulation of outflow facility of the trabecular meshwork as well as in the secretion of aqueous humour.3 Mutations may interfere with normal function and could contribute to increased intraocular pressure, which is seen in congenital glaucoma. Optineurin may play a role in normal-tension glaucoma,4 possibly by providing neuroprotection to the retinal ganglion cells. Defects in this gene may mean that retinal ganglion cells are more susceptible to damage or death, leading to glaucoma without pressure necessarily being high. Other genes have been discovered for systemic diseases where glaucoma is part of the phenotype. They are listed in the table below.
The Power of Molecular Genetics
Knowing the specific genetic mutation that a patient carries can be extremely helpful, both for diagnosis as well as treatment. Identifying the mutation will provide some clues as to the approximate age of onset, the severity of the disease, the prognosis, and possibly the effectiveness of different treatments.
For example, knowing that a person is likely to develop glaucoma at a relatively young age and that the disease will be fairly aggressive would be helpful in determining the type of treatment the individual might need to receive.
On the other hand, if we know that a patient will most likely develop a mild form of glaucoma late in life then our treatment would change considerably. An example of the former is an individual with the myocilin mutation Pro370Leu.5 These patients will generally have an onset of aggressive glaucoma at less than 10 years of age and they often require filtration surgery. In contrast, an individual with the myocilin mutation Gln368STOP6 will have an adult onset of the disease, usually after age 35, and a relatively less aggressive form of glaucoma. Therefore, knowing the specific mutation will guide follow-up frequency, the time at which treatment starts, and the type of treatment that would be most beneficial.
|Table 1. Molecular Etiology of Glaucoma Associated with Systemic Disease|
|GLAUCOMA TYPE||OMIM* NUMBER||LOCUS SYMBOL||LOCUS/ LOCI||GENE|
|Nail Patella syndrome||161200||NPS||9q34.1||LMX1B|
|Neurofibromatosis type I||162200||NF1||17q11.2||Neurofibromin|
|Mucopolysaccharidosis type VI||253200||MPS6||5q11-13||ARSB (arylsulfatase B)|
|Ehlers-Danlos syndrome type VI||225400||EDS6||1p36.3-36.2|
|Basal cell nevus syndrome||109400||BCNS||9q22.3, 9q31||PCTH (patched)|
|Lowe oculocerebrorenal syndrome||309000||OCRL1||Xq26.1||OCRL-1|
|*References can be obtained from On-line Mendelian Inheritance in Man: http://www3.ncbi.nlm.nih.gov/omim/ This is an up-to-date guide on genetics of various diseases.|
Depending on the mutation, and once the underlying cause is understood, more rational treatments can be developed that are based on the mechanism of the disease. For example, hypothetically, if it is determined that a specific mutation results in glaucoma due to restriction of blood flow, then treatment to increase blood flow to the optic nerve may be most helpful. On the other hand, if the optic nerve damage is due to high pressure caused by a decrease in trabecular meshwork function, then treatment could focus on enhancing outflow facility. In this way, treatment can be developed based on the underlying etiology for a specific mutation, and take advantage of a number of pathophysiological mechanisms
In the future, knowing the molecular basis for glaucoma may lead to the ability to prevent glaucoma, in some cases, by using gene therapy, i.e., the genetic defect could be modified before the onset of the disease process. Presently, glaucoma is usually detected once visual symptoms are present, or at best, once there is evidence of nerve fiber layer, optic nerve or visual field defects. It is well-known that the onset of the disease is actually many years before detectable signs are present. This is illustrated in the diagram below.
Armed with molecular information, physicians could initiate treatment for patients before any damage from glaucoma occurred, and patients could be counseled as to what to expect the course of the disease to be. For example, individuals found to be at risk for developing glaucoma could start treatment while still in the ocular hypertension phase, before any nerve damage occurred. This knowledge would be extremely helpful for patients and for any family members who might be at increased risk of developing glaucoma.
At this time, a test for the myocilin gene, specifically the mt-1 promotor region variation, is available. This test may be used to confirm the diagnosis and subtype of glaucoma in patients, as well as in at-risk relatives. Some have reported that it is useful as a prognostic indicator of future optic nerve damage.7 Others have questioned the utility of this test8 and further studies will have to be done to determine its benefit, in terms of wide-spread or selective screening.
Testing is also available at present to screen for mutations in the coding region
of myocilin, which may be a benefit for those with juvenile onset open-angle glaucoma or those at high risk for developing glaucoma, such as ocular hypertensives, glaucoma suspects or those with a positive family history. As more genes are discovered and the demand for genetic testing increases, such screening tests will likely become commonplace.
The progress that is being made in the area of molecular genetics of glaucoma has begun to change the way we classify, diagnose and treat our patients who are at risk for or who have glaucoma. Earlier detection and the ability to determine a predisposition to the disease means that fewer patients will suffer visual impairment if follow-up and treatment is based on a predictable natural history of the disease. This also means that patients can gain peace of mind since they can be counseled on what to expect in the disease process and what risk might be posed to future generations. The treatment of glaucoma will likely be revolutionized as a result of discovering more about the pathophysiology of this complex group of diseases. Primary prevention, through the advent of gene therapy, has the hope of making some forms of glaucoma diseases of the past.
Ms. Bovell is the study coordinator for the Ocular Genetics Lab at the University of Ottawa Eye Institute. Dr. Damji specializes in glaucoma and genetics at the University of Ottawa Eye Institute and is director of the Ocular Genetics Lab. Direct comments or questions to Ms. Bovell at (613) 737-8629 or abovell@ottawa hospital.on.ca.
1. Tielsch JM, Sommer A, Witt K, et al. Blindness and visual impairment in an American urban population. The Baltimore Eye Survey. Arch Ophthalmol 1990;108:286-290.
2. Stone EM, Fingert JH, Alward WL, et al. Identification of a gene that causes primary open angle glaucoma Science 1997;275:668-670.
3. Stoilov I, Akarsu AN, Alozie I, et al. Sequence analysis and homology modeling suggest that primary congenital glaucoma on 2p21 results from mutations disrupting either the hinge region or the conserved core structures of cytochrome P4501B1. Am J Hum Genet 1998; 62(3):573-584.
4. Rezaie T, Child A, Hitchings R, et al. Adult onset primary open angle glaucoma caused by mutations in Optineurin. Science 2002;295: 1077-1079.
5. Damji KF, Song X, Gupta SK, et al. Childhood-onset primary open angle glaucoma in a Canadian kindred: clinical and molecular genetic features. Ophthal Genet 1999;20: 211-218.
6. Allingham RR, Wiggs JL, De La Paz MA, et al. Gln368STOP myocilin mutation in families with late-onset primary open-angle glaucoma. Invest Ophthalmol Vis Sci 1998;39:2288-2295.
7. Colomb E, Nguyen TD, Bechetoille A, et al. Association of a single nucleotide polymorphism (SNP) in the TIGR/Myocilin gene promoter with the severity of Primary Open Angle Glaucoma. Clin Genet 2001; 60(3):220-225.
8. Alward WL, Kwon YH, Khanna CL, et al. Variations in the myocilin gene in patients with open-angle glaucoma. Arch Ophthalmol 2002; 120(9):1189-1197.