GENE ANALYSIS
Myocilin
The first gene associated with the development of juvenile and primary open-angle glaucoma (JOAG, POAG) was TIGR, now known as myocilin (MYOC). Initially localized2 to the GLC1A locus in 1993 and subsequently linked3 to the MYOC gene in 1997, researchers have identified more than 30 different point mutations in multiple ethnic groups worldwide. Mutations in the coding regions of MYOC are associated with 2% to 4% of POAG cases4-15 and account for a larger proportion of JOAG cases (from 6% to 36%).16,17
MYOC is expressed in multiple ocular and non-ocular tissues, but, despite intense investigation, the function of this protein is largely unknown. Some studies have shown that cells do not secrete the mutated form of MYOC and that this protein accumulates within the endoplasmic reticulum.18 Some scientists have hypothesized that MYOC accumulation may overload the normal proteosome degradation pathway and lead to cellular dysfunction and death.19 Studies are currently underway to analyze this mechanism.
Optineurin
The second identified gene associated with open-angle glaucoma is optineurin (OPTN). This gene was identified by the same group that reported CYP1B1's association with congenital glaucoma.20 These investigators found that OPTN gene polymorphisms occur in approximately 17% of normal tension glaucoma patients, and its expression has been localized to several ocular and non-ocular tissues, including the trabecular meshwork, retina, and nonpigmented ciliary epithelium. This gene's function is unknown, but it has a putative role in the TNF-alpha signaling pathway and therefore may be involved in cellular apoptosis.21 Research to verify OPTN's role in the pathogenesis of glaucoma is underway, and several investigators are confirming polymorphisms identified in this gene.
GENETIC TESTING
One of the potential benefits of discovering the genes associated with glaucoma is the development of glaucoma screening tests. Currently, the only commercially available test is the OcuGene test (InSite Vision Incorporated, Alameda, CA). This blood test screens patients for three mutations in the promoter region of the MYOC gene. Although it tests for only a small number of potential MYOC mutations, some investigators have suggested that these mutations are associated with a more aggressive form of POAG. One group reports an increased rate of progression over 15 years for individuals harboring the mt.1 promoter mutation.22,23
Other investigators evaluated the utility of the OcuGene test by screening 393 patients with POAG and 92 healthy patients. They found that 3.3% of the former group had disease-associated mutations in the actual coding region of the gene. When they looked for the mt.1 promoter mutations, they found that 15.5% of POAG and 23.9% of healthy controls exhibited a polymorphism. Furthermore, these investigators did not find any significant clinical differences between patients with POAG who did and did not exhibit the mt.1 polymorphism.16
The primary differences between the two studies just outlined are that the first was a 15-year retrospective analysis, whereas the second was a cross-sectional analysis at a single time point. Both studies raise important questions. First, why is the mt.1 change present in such a large percentage of healthy, normal-appearing patients? Second, what is the sensitivity of the OcuGene test given the low prevalence of MYOC promoter mutations among POAG patients and its similar prevalence among healthy individuals? These unresolved issues make it unclear how the currently available test for MYOC should be used.
FUTURE DIRECTIONS
Gene Identification
Currently, several large, ongoing research projects are directed at identifying the genes associated with glaucoma. Thanks to the Human Genome project, advances in bioinformatics, and advances in gene identification technologies, researchers should eventually identify most of the glaucoma genes. Like most diseases, however, glaucoma appears to be polygenic with multiple different genes producing similar disease manifestations. A comparable situation exists with regard to retinitis pigmentosa, in which multiple gene defects have been implicated to result in a loss of photoreceptors.24 As a confounding factor, environmental influences appear to play a large role in many late onset disorders such as Alzheimer's disease25 and glaucoma.26 For that reason, it may be that dozens of genes are involved in the pathogenesis of glaucoma. If such is the case, several advancements will help improve patient care, including better diagnostic tests, drug therapies, and gene therapy.
Screening Examinations
The identification of multiple glaucoma genes will enable the creation of a battery of tests to identify patients at risk for developing glaucoma. Using modern gene array technologies, thousands of gene mutations can be screened on a single microarray chip using a very small amount of blood or tissue. By combining this information and technology with advances in bioinformatics, scientists will be able to use genotype/phenotype correlations to develop an overall risk profile for an individual. If such a profile determines that a given patient has a high likelihood of developing glaucoma, his physician may decide to start therapy before any signs or symptoms of the disease develop. Some investigators advocate this sort of risk assessment for patients at high risk for breast cancer.27
Drug Choice and Development
Scientists may use genetic information in order to develop pharmacogenomics for glaucoma. In other words, they would be able to identify the genetic characteristics of individuals who respond to specific medications. As a result, physicians could select appropriate medications at specific dosages for an individual based on his genetic profile. This approach would lead to the more efficient treatment of patients compared with the current trial-and-error method of prescribing medications.
Identifying glaucoma-associated genes will also help elucidate the biochemical pathways that produce glaucoma. Knowing such pathways will facilitate the development of novel drug therapies that can be tailored to individual forms of glaucoma. New therapies could include agents based on the protein, enzymes, or RNA transcripts associated with glaucoma. Scientists could create safer and more disease-specific therapies because the biochemical pathways involved in glaucoma could be specifically targeted.
Furthermore, identifying glaucoma genes will allow the development of animal models of glaucoma. Currently, no good models exist, an absence making it difficult to study disease progression and modifying factors. Moreover, the existence of adequate models would accelerate drug development and testing. Investigators are already using primate and rodent models with mutant variants of MYOC to elucidate the function of this protein.
Because glaucoma appears to be a polygenic condition, specific gene therapies aimed at individual mutations may be difficult to develop. Instead, scientists may develop therapies aimed at common pathways that lead to glaucoma development. As mentioned earlier, the proteosome overload phenomenon may be a unifying principle for glaucoma, so its modulation may lead to novel glaucoma therapies.28
CONCLUSION
In short, genetic investigations are elucidating many of the basic mechanisms of glaucoma. Although there is only one genetic test for glaucoma presently available, it is worth remembering that small steps are the precursors to giant leaps.
Pratap Challa, MD, is Assistant Professor of Ophthalmology at the Glaucoma Service of Duke University in Durham, North Carolina. He holds no financial interest in the product or company mentioned herein. Dr. Challa may be reached at (919) 684-3283; chall001@mc.duke.edu.
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