Gene identification leads to genetic testing

Genetic testing offers clues regarding the nature and course of ophthalmic diseases.

PORTLAND, Ore. — The commercialization of glaucoma genetic testing, now a reality, is proof that identifying genes involved in glaucoma and retinal disease is not just of interest to academics, but will become a lucrative business.

A test called OcuGene, marketed by InSite Vision, provides information on the role of the trabecular meshwork inducible glucocorticoid response (TIGR) gene in primary open-angle glaucoma (POAG). The test is designed to detect the genetic marker mt-1 in the promoter region of the glaucoma-related TIGR gene for patients with POAG. It screens for the presence of a number of mutations in the coding region of the TIGR gene in high-risk people such as relatives of glaucoma patients, ocular hypertensives and glaucoma suspects.

Although there is debate in the genetics community regarding the test’s usefulness and cost effectiveness, John R. Samples, MD, says InSite Vision deserves acclaim for developing the test, regardless of its current limitations.

“The reason testing is important is not only because it will eventually predict who is going to get the disease, but because each one of these underlying proteins behaves a little differently, so the real diagnostic importance lies in predicting how that glaucoma is going to behave.

“This is important in terms of the disease’s response to medication, in terms of its aggressiveness and in response to how aggressively it needs to be treated,” said Dr. Samples, professor of ophthalmology at Oregon Health Sciences University and director of glaucoma service at Casey Eye Institute.

S. Kumar Chandrasekaran, PhD, president and CEO of InSite Vision, said, “InSite’s OcuGene tests are demonstrating clinical usefulness in a number of clinical centers and trials. The availability of our first test, and subsequent tests being developed for other glaucoma indications, can provide eye care specialists with useful diagnostic and management tools – as they design their management and treatment plans for each patient.”

Research funding

While some might be wary of the commercialization of glaucoma genetic testing, Dr. Samples is more optimistic.

“It’s probably a good thing in the long haul, because it’s only through commercialization that these tests will become readily available and perfected. It is only through commercialization that the research and development money necessary to perfect genetic therapy of glaucoma will be made available. At this stage, I don’t see the government devoting the necessary product development money for glaucoma gene therapies,” he said.

While the government may not yet be willing to make an adequate investment in genetic testing or therapies for glaucoma or retina disease, it appears to stand behind its worth. A report by the National Eye Institute’s Glaucoma Panel said, “The availability of genetics as a diagnostic tool may allow clinicians to customize an intervention strategy based on the risk of blindness for an individual patient and balance treatment with quality-of-life considerations.”

At the time of the Glaucoma Panel’s report last June, a number of loci had been mapped for glaucomas or ocular diseases associated with secondary glaucomas (see accompanying chart).

“This work and the mapping of other glaucoma-related loci have substantiated the concept of a genetic component to glaucoma,” the panel reported.

In addition to mapping of glaucoma loci by genetic linkage, there have been significant advances in the discovery of glaucoma-causing genes. The gene for juvenile primary open-angle glaucoma (GLC1A) was identified, as were the gene codes for trabecular meshwork glucocorticoid response protein (TIGR), which was first identified as a protein made by trabecular meshwork cells exposed to glucocorticoid hormones.

Subsequently, the panel reported, this gene was found to be the same as genes identified from cDNA libraries made from ciliary body and retina. This gene cloned from the retina was named myocilin because of the myosin-like domain within the gene. Mutations in this gene have been associated with juvenile-onset primary open-angle glaucoma and, in a small percentage of cases, adult-onset primary open-angle glaucoma.

Meanwhile, in a recent report in Science magazine, Mansoor Sarfarazi, PhD, and colleagues reported another major gene linked with POAG, with a particular relevance to normal tension glaucoma (NTG). The gene, OPTN, leads to the expression of Optineurin, or optic-neuropathy-inducing protein. Mutations in this gene are present in 16.7% of the primary open-angle glaucoma families evaluated with this disorder (see Ocular Surgery News April 1, 2002, page 1).

Dr. Sarfarazi reported, “Our data suggest that mutations in OPTN may be responsible for 16.7% of hereditary forms of normal-tension glaucoma with an additional attributable risk factor of 13.6% in both familial and sporadic cases.

“However, because we sequenced the entire OPTN gene in only one family with over 60 members, 19 of whom are affected, and thereafter used single-strand conformational polymorphism (SSCP) analysis to screen the remaining glaucoma cases, it is possible that additional mutations have been missed. We also identified additional non-disease-causing sequence alterations in the OPTN gene.”

Testing for the NTG mutation will assist with identification of patients who may develop this form of glaucoma that may otherwise go undetected. Such detection could mitigate visual loss by allowing time for implementation of treatment before substantial visual loss has occurred. Dr. Sarfarazi and his team also discovered a major gene, CYP1B1, linked with primary congenital glaucoma (PCG). Testing for the congenital glaucoma gene may help identify children at risk, permitting earlier detection and less complicated surgery.

Dr. Sarfarazi concluded that identification of OPTN as an adult-onset glaucoma gene provides an opportunity to study the biochemical pathways that may be involved in the pathogenesis of this group of optic neuropathies. In addition, because OPTN mutations are a contributing factor in patients with NPG, the gene may be a useful tool for presymptomatic screening of the general population.

Retinal genetics

Much progress has been made in the area of the molecular genetics of various types of retinal degeneration as well, according to the NEI’s Retinal Diseases Panel. Since 1992, the genes for numerous forms of retinal disease have been mapped to specific chromosomes.

One of the most recent genetic findings occurred at the University of Texas School of Public Health in Houston, where researchers discovered a surprising link between a well-studied gene involved in DNA synthesis and retinitis pigmentosa. According to a research study published in the March issue of Human Molecular Genetics, mutations in inosine monophosphate dehydrogenase 1 (IMPDH1) are expected to cause from 5% to 10% of one form of retinitis pigmentosa.

“Several classes of drugs are known to affect IMPDH proteins, and this suggests that treatments may someday be available for people with this form of retinitis pigmentosa,” Sara Bowne, PhD, said. Dr. Bowne is lead author of the study and a research associate in the Human Genetics Center at the School of Public Health.

The body produces two forms of this same molecule, IMPDH1 and IMPDH2, and both play critical roles in nucleotide synthesis. Interestingly, IMPDH1 alone seems to be critical for normal vision, and mutations in this gene result in the RP10 form of autosomal dominant RP (adRP).

Stephen Daiger, PhD, is the director of the Laboratory for Molecular Diagnosis of Inherited Eye Diseases at the university, where Bowne and Lori S. Sullivan, PhD, assistant professor in the Human Genetics Center, work at unraveling the role of IMPDH1 in human vision.

“It is interesting from a scientific point of view [that] this enzyme is found throughout the body, yet we find that individuals with mutations in the IMPDH1 gene have no other physical problems except retinal degeneration,” Dr. Daiger said.

Research from the University of North Carolina at Chapel Hill shows that when the mouse IMPDH1 gene is removed, the mice appear to be normal. However, according to Dr. Bowne, no one thought to find out if the mice could actually see.

“Our collaborators at UNC may have never known about the effects in the retina if IMPDH1 hadn’t been found to be associated with adRP,” she said.

Mutations in IMPDH1 have been identified in six adRP families to date, but Dr. Bowne said that number is expected to increase as other families with histories of RP are tested.

Genetic testing survey

A 2001 ARVO poster reported that only a very small percentage of glaucoma patients surveyed were uncomfortable with or not interested in genetic testing for glaucoma. However, a significant number of participants were unsure. The higher the level of education, the more comfortable the participants were with genetic testing. Patients who were comfortable with or interested in testing were more likely to have their children tested. The Glaucoma Research Foundation and Research to Prevent Blindness supported the survey.

Researchers at the department of ophthalmology at Duke University Medical Center in Durham, N.C., reported on glaucoma patients seen at the eye center there. Patients were questioned about their family history of glaucoma, knowledge about genetics, sources of knowledge, level of education and comfort with genetic testing, as well as age and gender. Information on the level of interest in genetic testing, use of the results, and willingness to have children tested were also collected. One hundred and thirty-three consecutive patients with a diagnosis of glaucoma participated in and completed the survey.

Out of these patients, 43.6% said they were comfortable and 6.8% said they were uncomfortable with the idea of genetic testing. The rest (49.6%) were unsure. Of the variables, the source of knowledge, level of education and willingness to have children tested were significantly associated with the comfort level.

For this same group of patients, 61.8% were interested in the testing, 8.4% were not interested and 29.8% were unsure. Among all variables, willingness to have children tested was significantly associated with level of interest. The level of comfort was significantly associated with level of interest.

For Your Information:

• John R. Samples, MD, can be reached at Casey Eye Institute, 3375 SW Terwilliger Blvd., Portland, OR 97201; (503) 494-7667; fax: (503) 494-3017.
• Mansoor Sarfarazi, PhD, can be reached at the University of Connecticut Health Center, 263 Farmington Ave., Farmington, CT 06030: (860) 679-4306; fax: (860) 679-1282.
• Sara Bowne, PhD, Stephen Daiger, PhD, and Lori S. Sullivan, PhD, can be reached at The Human Genetics Center, 1200 Herman Pressler St., Houston, TX 77225; (713) 500-9829; fax: (713) 500-0900.
• InSite Vision, manufacturer of the OcuGene glaucoma genetic test, can be reached at 965 Atlantic Ave., Alameda, CA 94501; (510) 865-8800; fax: (510) 865-5700; Web site: www.insitevision.com.