Like all fields in medicine, ophthalmology stands to benefit tremendously from a thorough under-standing of the human genome. Of course, our knowledge of the human genome right now is anything but thorough; for example, we’re just beginning to identify the genes that
are involved in different ophthalmologic diseases such as glaucoma. For that reason, practical clinical uses for the knowledge we do have are still limited. However, that doesn’t mean they’re nonexistent.
Here, we’d like to review what we currently know; where we stand in terms of being able to use this knowledge in the clinic today; and what every ophthalmologist should be doing to be ready for the next wave of genetic information.
Reviewing the Basics
Although ophthalmologists who attended medical school in recent decades tend to be pretty familiar with the fundamentals of genetics, many of us received our schooling before this subject was the hot topic that it has become. So, a quick review.
As you know, all cells contain DNA, and DNA contains the genes that are responsible for the production of a number of proteins. Those proteins, in turn, are designed to manage one or more functions that are essential for the normal behavior of the cell. Genes that encode proteins are made up of a series of three-nucleotide sequences called codons; when there’s a change in the sequence of the molecules making up a codon, i.e., a mutation, then the protein that’s produced will be abnormal and the cell won’t function properly. That’s why a mutation in a gene may generate a malfunction in the cell, which may in turn cause what we would consider a disease.
The practical consequences of a mutation are not always manifest, thanks to a number of mitigating fac
tors—some environmental, some involving other genes that interact with the mutated gene. For that reason, we also consider the penetrance of a gene. Penetrance refers to how likely it is that a person harboring a particular mutation will actually express the resulting dysfunction or disease. Be-
cause penetrance can vary, not every-one with a given disease-related mutation will exhibit the disease.
For example, consider the myocilin gene, the first gene found to be associated with glaucoma, back in 1997. One hundred percent of patients with a proline to leucine mu-
tation in codon #370 of the myocilin gene develop juvenile open-angle glaucoma by age 27.1 But a mutation in a different codon of the same gene may produce a different effect; 55 percent of those harboring the Glutamine 368STOP mutation will develop glaucoma by age 50.2
In addition to our incomplete un
derstanding of the mitigating factors that can limit penetrance, we don’t fully understand how a given muta-tion produces a disease effect. In the case of the myocilin gene, the most accepted current theory is that the mutant myocilin accumulates in the endoplasmic reticulum of the affected cells, including trabecular meshwork cells. The accumulation leads to trabecular meshwork cell apoptosis, causing decreased aqueous outflow and increased IOP. Assuming this is correct, if we could avoid the
accumulation of that myocilin in-side the cells, we could prevent the development of glaucoma in these individuals. (Whether this mechanism-of-action theory is correct remains to be seen.)
Unfortunately, the complexity of genetic medicine is significant. When the myocilin gene was found, the entire scientific community was very excited; we thought this knowledge would allow us to use gene therapy very quickly to prevent the development of glaucoma in individuals with mutations in the myocilin gene. But this mutation only accounts for a small proportion of glaucoma cases. Moreover, we still don’t completely understand the mechanisms associated with the mutation.
Glaucoma Genes (So Far)
At least 14 regions of the human genome are associated with primary open-angle glaucoma. However, in those 14 areas, only four specific genes have been identified. They are:
• the myocilin gene (MYOC
-1q23-24]), associated with 3 to 5 percent of POAG cases and 30 to 35 percent of juvenile open-angle glaucoma cases;
• the optineurin gene (OPTN
-10p14-15]), associated with 16 percent of familial cases of normal-tension glaucoma;
• the WDR36
-5q22.1), believed to be a modifier gene that increases susceptibility to the development of glaucoma; and
• the recently discovered ASB10
-7q35-36), which has been associated with 6.6 percent of POAG patients.
Other forms of glaucoma have been associated with different genes:
• One particular variant in the LOXL1
gene (15q24)—which is
considered to be a modifier gene—is associated with a 27-fold increase in the risk of developing pseudoex-foliation syndrome.
• In primary congenital glaucoma, which is an autosomal recessive dis-ease seen frequently here in Brazil, there are mutations in what we call the CYP1B1
-2p21) that are found in 30 to 50 percent of these patients.
• Mutations in the PAX6
gene (11p13) are seen frequently in cases of aniridia.
• Mutations in the FOXC1
gene (6p25) and the PITX2
gene (4p25-27) are found frequently in Axenfeld-Rieger syndrome.
Genetics in Practice
There are three areas in which genetics should ultimately help us manage glaucoma: screening; genetic counseling and gene therapy.
• Genetic screening.
Ideally, we could screen large populations for the presence of mutations in genes that are known to be associated with glaucoma. Unfortunately, our current level of understanding makes this impractical; if an individual shows a negative result, that wouldn’t eliminate the possibility of him developing glaucoma because only a small proportion of glaucomas have been associated with genes so far. And if the result is positive, we can’t say for sure that the individual will develop glaucoma because of the issue of the penetrance of the mutation. Until we know more about the factors that activate or suppress the effect of a given mutation, we can’t make accurate predictions about disease development. So we’re not yet ready to attempt large-scale screening.
• Genetic counseling.
This is the second potential area in which genetics should be useful. It’s po-tentially important for helping parents and relatives understand their risk of developing glaucoma, which can have significant consequences, especially for a newborn family member. In contrast to screening, there are situ-ations in which our current knowledge is already sufficient to make this feasible.
The first step in genetic counseling is to establish the type of glaucoma that the family has and verify that we know of specific genes associated with this particular type of glaucoma. The second step is to create what we call a heredogram, which shows the pattern of disease inheritance, and whether it’s an autosomal-recessive, autosomal-dominant, or sex-linked chromosome mutation. (See example, p. 77)
The third step is to classify the heredogram according to the number of people in the family who have been affected by the disease. I favor the classification system invented by Gordon Gong, MD:
- When at least three first-degree relatives from two generations are considered affected by the disease, it’s referred to as hereditary.
- When at least two first- or second-degree relatives are examined and considered to be affected by the disease, it’s considered familial.
- If no first- or second-degree relative is affected, the heredogram would be referred to as sporadic.
|Knowing that the risk [of developing glaucoma] is high, a family can take concrete steps to ensure that the damage will be minimized if glaucoma does occur.|
The reason this is important is that we can attempt genetic counseling when the pattern is classified as hereditary. Consider the family shown in the sample heredogram on page 77. Several members of this family have been treated for juvenile open-angle glaucoma. We know that the myocilin gene is associated with this type of glaucoma. In this case, the family has three affected generations, and the heredogram shows an autosomal-dominant pattern.
Now suppose a young woman in the youngest generation is about to have a child. The family would be justified in being concerned about the possibility of the child also having the disease. The heredogram indicates that in this family the child has a 50 percent chance of having the same mutation, and we can share that information with the family. It’s true that harboring the mutation doesn’t necessarily mean that the child will develop glaucoma—but it does mean that this child should be checked for the mutation. If the child does, in fact, have the mutation, the family can then proactively make sure the child is examined frequently so that glaucoma—if it materializes—will be diagnosed and treated at the earliest possible moment.
Until we know more about the fac-tors that influence the penetrance of the mutation, we can’t predict for certain whether the child will or won’t develop the disease. However, knowing that the risk is high, the family can take concrete steps to en-sure that damage will be minimized if juvenile open-angle glaucoma does emerge.
Obviously, we cannot yet provide this kind of counseling for every type of glaucoma; we simply don’t know enough. But in some situations, like this one, genetic counseling is already feasible.
One very important caveat: When providing this type of information it’s crucial to be aware of the potential for misuse. As genetic information gives us more insight into the risk of disease development, that information could be used by insurance companies, for example, to justify refusing to provide coverage. So as the availability of this type of information increases in the next few years, we have to be careful about how the information is shared, remembering that such information is intended only for the benefit of the patient.
• Gene therapy.
This refers to modifying problematic genes in order to prevent the development of a disease, or to treat an existing disease by putting it into remission. The idea in gene therapy is to transfer genetic material to a target cell in order to correct the mutation in the gene. The two basic options are to cause gene suppression, thereby limiting the production of the faulty protein, or to increase gene expression if the problem is caused by the non-production of a protein.
There are currently several ways to transfer genetic material. Most researchers use viral vectors to get genetic material into cells, a process referred to as transduction. Viruses have evolved specific molecular mechanisms that allow them to enter cells; the viral vector approach modifies the contents of a virus so it delivers a payload of desired genetic material, but is prevented from causing harm to the infected cell.
Some research has already sup-ported the potential effectiveness of this approach; for example, studies in other areas of medicine are showing some success in treating conditions such as hemophilia; and in ophthalmology, one research group has reported improved visual acuity and visual fields in animals and humans after using this approach to treat Leber’s congenital amaurosis. (LCA is autosomal-recessive and has been associated with seven different genes, one of which is very important to the production of rhodopsin.) How-ever, so far their results in humans have not been replicated.
There are several ways that gene therapy could be applied for the benefit of glaucoma patients. For example, we could genetically alter conjunctival fibroblasts to prevent wound healing in a patient who is undergoing a filtering procedure; we could target the ciliary body to decrease aqueous production; we could target the trabecular meshwork to increase outflow; or we could target the ciliary muscle area to increase uveoscleral outflow, as prostaglandins do. We might even be able to target retinal ganglion cells to induce neuroprotection.
Of course, these approaches would only address symptoms. The ultimate goal of gene therapy in glaucoma would be to inhibit the disease-causing gene. For example, if we were able to inhibit the myocilin gene mutation in a patient with juvenile open-angle glaucoma, we could prevent the production of the abnormal protein associated with this mutation and thus prevent the development of the disease. Unfortunately, we’re not yet ready for clinical application of this technique.
What to Do Today
Despite the fact that current clinical use of genetics for managing glaucoma is limited, that’s going to change in upcoming years. With that in mind, I’d recommend that clinicians take several steps to make the most of this information now and as it unfolds:
• Become more familiar with genetics in medicine.
Advances in genetics have been impressive in the past 10 to 15 years, but some of us who attended medical school 25 or 30 years ago may not be well-informed in this area. I’d encourage general ophthalmologists, especially if they’re older, to take the time to learn more about genetics and the mechanisms by which genes influence the course of disease.
• Stay up-to-date regarding new discoveries of genes associated with ophthalmic diseases.
Specific genetic information relating to ophthalmology in general and glaucoma in particular is being uncovered every year. It’s important to stay on top of any new discoveries that may eventually affect your practice.
• Learn to build a heredogram and establish a pattern of inheritance.
This is a practical skill that can already be applied in the clinic and should be increasingly useful in the future. You can see examples and get helpful advice on doing this at
• Provide genetic counseling whenever possible and appropriate.
Used in the right circumstances, genetic counseling can be of sig-nificant value to your patients.
Only the Beginning
It’s easy to pay little attention to an area of medicine that seems to have minimal clinical relevance at
the moment. But genetics is so close
ly tied to health and disease that it will eventually become a central part of medicine, including ophthalmology. A few years ago, very few ophthalmologists paid attention to blood flow. The general feeling was that it didn’t matter. Today, however, the impact of factors such as perfusion pressure is a frequent topic.
The same shift is likely to happen with genetics. Right now its clinical usefulness is limited, but we’re certain to identify many more genes that are associated with glaucoma and other ocular diseases, and in the next few years we’ll learn a lot more about how those genes interact with the environment and with other genes. The inevitable result is that as the years go by we’ll be using the science of genetics more and more to help our patients.
It’s not something any of us should be overlooking.
Dr. Costa is director of the Glaucoma Service at the University of Campinas in Brazil and professor of ophthalmology at the University of Campinas and University of São Paulo. Dr. Vasconcellos is a member of the Glaucoma Service and assistant professor of ophthalmology at the University of Campinas; Dr. Melo is a researcher at the Center of Molecular Biology and Genetic Engineering at the University of Campinas.
1. Shimizu S, Lichter PR, Johnson AT, et al. Age-dependent prevalence of mutations at the GLC1A locus in primary open-angle glaucoma. Am J Ophthal 2000: 130;2;165-177.
2. Craig JE, Baird PN, Healey DL, et al. Evidence for genetic heterogeneity within eight glaucoma families, with the GLC1A Gln368STOP mutation being an important phenotypic modifier. Ophthalmology 2001;108:9:1607-20.