Genetic research opens doorways for testing, treatment for AMD

Michael J. Tolentino

The completion of the Human Genome Project has led to tremendous advances in the understanding of age-related macular degeneration.

The project allowed scientists to identify small variations in the genome called single nucleotide polymorphisms (SNPs), which could be used to screen samples of DNA from patients with and without AMD. The SNPs found to have higher frequency in patients with AMD were identified and could be used to localize a mutation in a particular gene.

The Y402H SNP was identified by multiple groups as a SNP strongly correlated with patients who had macular degeneration. This polymorphism is found in chromosome 1 in a region of the chromosome that contains the complement factor H gene. This exciting breakthrough has led to the identification of other SNPs correlated to patients with macular degeneration. While some of these polymorphisms were localized in the complement H genes, several SNPs were found in locations corresponding to other genes related to the complement pathway as well as other novel gene locations.

These discoveries expand our ever growing knowledge base and, more importantly, allow insight into the pathophysiology of macular degeneration. This new genetic understanding can be used to develop diagnostic tests to predict a patient’s risk of developing disease. It can also be used to design novel treatments and predict a patient’s probability of responding favorably.

The complement system

Before the discovery associating macular degeneration with mutations in the complement system, “complement” was an obscure branch of immunology for ophthalmic researchers. The complement system represents the innate portion of the immune system that complements the ability of antibodies to clear foreign pathogens. If our immune system is the armed forces protecting our bodies against foreign organisms, the complement system represents the munitions of the army.

The complement system consists of proteins produced in our body predominantly in the liver and found in the blood stream in an inactive form. Like gunpowder, the system requires a trigger to initiate an amplification cascade that results in a destructive force. The trigger initiates a chain reaction that activates the inert complement proteins and eventually produces the membrane attack complex (MAC), which, when localized next to a cell such as a bacterium, will produce holes in the cell membranes with subsequent destruction of the cell.

Triggering of this explosive cascade can be “classically” mediated by antibodies, but spontaneous triggers result in formation of this MAC through an alternative pathway. Because of the volatility of this alternative pathway, the complement system has factors that can defuse the formation of the MAC and prevent damage to the host’s healthy cells. Complement factor H (CFH) is a protein that defuses the formation of the MAC and protects the body from the destructive force of the complement system.

CFH mutation and AMD

The genetic association between CFH mutation and AMD served as the smoking gun implicating the complement system in the pathogenesis of AMD. Aside from genetic data, histopathologic evidence pointed to the involvement of complement in AMD. Analysis of drusen, the hallmark of AMD, revealed the presence of extensive byproducts of complement proteins, in particular, the presence of MAC in higher levels in patients with geographic atrophy and exudative AMD.

The high level of complement factor H in association with high levels of MAC also suggested a malfunctioning role of CFH. Combining all these clues has led researchers to conclude that malfunctioning of the complement systems serves as a causative role in the development of AMD.

Armed with this information, scientists and therapeutic developers are seeking to study and modulate the complement pathway to treat complications of AMD such as geographic atrophy and choroidal neovascularization. Scientists are also developing risk tables that will enable clinicians to use genetic testing to stratify patients’ probability of blindness and also probability of treatment response. This will allow clinicians to emphasize preventive medicine and monitoring.

Therapeutics in clinical trials

Currently, several therapeutics are being evaluated in phase 1 and 2 trials that involve inhibiting the complement cascade. Our group is involved in several of these trials.

Phase 1 studies involving two molecules, ARC-1905 and POT-4, have concluded. These molecules inhibit the formation of the MAC by stopping critical complement factors that are amplified once the complement system is triggered. In particular, POT-4 inhibits C3 while ARC-1905 inhibits C5, a factor downstream of C3.

Other molecules are just starting their phase 2 program or are still in preclinical stages. Because the complement cascade involves several steps, there are many points in which to intervene.

One strategy is to enhance the effects of CFH. A protective CFH that decreases the risk of developing AMD is being developed as a therapeutic. Other methods exist for enhancing the deactivation of complement activation. Suffice it to say that there is an explosion of strategies for stopping the complement system in the eye and treating dry AMD and enhancing our treatments for wet AMD.

Genetic testing a closer reality

While therapeutics developed using this genetic knowledge may be further in the future, genetic testing will demonstrate clinical utility much sooner. Discovery of SNPs associated with AMD provides genotype-phenotype correlation. Genetic testing is currently available; however, validation of these tests for commercial applicability is not yet completed and is being actively pursued by our research center.

Although it is not validated for clinical use, several studies have shown that attributable risk can be identified depending on the type and number of SNPs. For example, if a patient has one of three well characterized SNPs, the risk of developing blinding complications of AMD is several times greater than normal. If a patient has all three, the risk is more than 200-fold.

Use in clinical practice

This knowledge is useful to a clinician in several ways. If a patient is young, characterizing the risk can help devise preventive strategies such as smoking cessation, nutritional supplementation and UV protection. If a patient is older, high risk DNA analysis can identify populations who need more frequent examination, personalized nutritional supplementation and perhaps prescription of home monitoring devices.

In the not-too-distant future, this risk characterization can be used to manage our patients with AMD. Several groups are studying which SNPs are associated with better response to treatments such as anti-VEGF therapies and even response to photodynamic therapy. A recent phase 2 study of pazopanib eyedrops, an inhibitor of tyrosine kinase, has demonstrated a correlation with efficacy depending on the presence of a particular SNP. These types of correlations will allow some patients who develop exudative AMD an alternative to frequent intravitreal injections.

We are at the dawn of personalized medicine, where genetic knowledge can help design custom therapies for our patients. As clinicians we need to understand these advances so we can improve our care for our patients.

For more information:

• Michael J. Tolentino, MD, is a Primary Care Optometry News Editorial Board member and director of research at the Center for Retina and Macular Disease. He can be reached at 250 Avenue K, SW, Winter Haven, FL 33880; (863) 297-5400; e-mail: miket@crmd.net; website: www.crmd.net.