Patients with age-related macular degeneration typically have two avenues of therapeutic intervention—nutritional supplements to reduce the risk of vision loss from intermediate AMD or anti-vascular endothelial growth factor treatment for neovascular AMD. However, the response to either intervention can vary substantially among patients,1 so a working familiarity of pharmacogenetics—the study of how individual genetic traits may influence the clinical response to medications2—can help the retina specialist better understand patients’ responses to treatments.3,4 Genetics may influence some of these responses.
 


AMD remains the leading cause of irreversible visual loss among the elderly in the United States.5 The Age-Related Eye Disease Study (AREDS) reported a significant risk reduction among patients with intermediate or advanced AMD who were treated with antioxidants plus zinc,6 and the potential benefits of anti-VEGF treatments in neovascular AMD have been well documented.7 This review discusses what we understand about the pharmacogenetics of these approaches to managing AMD.

Complex vs. Monogenic Disease

Diseases with genetic risk factors may be divided broadly into monogenic (single-gene or Mendelian) diseases and complex genetic diseases. These two categories differ in etiology and in general suitability for genetic screening.

Monogenic diseases are typically uncommon. They are, by definition, caused by a single gene defect, or mutation, and follow a recognizable inheritance pattern. In general, individuals with the mutation are likely to develop the disease. Therefore, genetic screening of individuals at risk for monogenic disease is valuable. Examples of monogenic retinal diseases include retinitis pigmentosa, Best vitelliform macular dystrophy and Leber congenital amaurosis. Other articles in this issue discuss the genetics of monogenic disease and the role of screening and testing in managing these diseases.

In contrast, complex genetic diseases may be common. While mutations cause monogenic diseases, one or more gene variants (risk alleles) plus one or more environmental risk factors have been associated with complex genetic diseases. Risk alleles are not necessarily “abnormal” or deleterious, but are associated with increased risk of disease. This is why they are referred to as risk alleles rather than mutations. AMD is a complex genetic disease.

Complex genetic diseases do not follow a recognizable inheritance pattern, although patients with these diseases may report a positive family history. The presence of one or more risk alleles does not necessarily imply that the patient will develop the disease. Patients with multiple risk alleles may remain disease-free, while other patients with few or no risk alleles may develop disease. For this reason, genetic screening of individuals at risk for complex genetic disease is less valuable and may provide misleading information compared to screening for monogenic retinal disease.

Complex Profile of AMD

In AMD, 52 risk alleles within 34 loci have been reported to date.8 The two major associated loci are complement factor H (CFH) (Figure 1) and age-related maculopathy susceptibility 2 (ARMS2) (Figure 2);9 the latter is in very strong linkage disequilibrium with high temperature requirement A serine peptidase 1 (HTRA1)—two loci that cannot be statistically differentiated.10 Investigators have not determined which of the two is more clinically meaningful. The CFH and ARMS2/HTRA1 variants have been reported to contribute to more than 50 percent of the genetic risk for AMD.11

In addition, AMD is significantly associated with multiple non-genetic risk factors, including advancing age, body mass index (BMI) and cigarette smoking.12 Gene-environment interactions are poorly understood but may also further modulate risk of disease.
 
Figure 1. A rendering of the complement factor H (CFH) locus.


When reviewing studies on genotype-phenotype correlations in AMD (or any complex genetic disease), one should remember that a positive finding must be validated in a separate population. Highly significant correlations may be reported that are not clinically “true,” especially with large numbers of individual comparisons. This may occur for a variety of reasons, including differences in clinical endpoints, baseline demographics in patient populations and inadvertent selection bias. Therefore, a report of a genotype-phenotype correlation in a single population does not “prove” the association.13

Genetic Screening for AMD

At least three genetic tests for AMD are currently available: Macula Risk PGx (ArcticDx, Toronto), Genetic Predisposition Text for Macular Degeneration (EasyDNA, Kent, U.K.) and Asper Ophthalmics (Asper Biotech, Tartu, Estonia). The Easy- DNA test is not available to U.S. residents, and a fourth test that had been available, RetnaGene (Sequenom, San Diego) is not currently available. A doctor or other provider orders the Macula Risk test, whereas EasyDNA and Asper Biotech are direct-to-consumer genetic tests.

The ACCE model, which the Centers for Disease Control and Prevention and the National Institutes for Health developed, can evaluate the utility of genetic tests.14 The ACCE model considers four factors:

Analytic validity—sensitivity and specificity of genetic information.

Clinical validity—how well the genetic test predicts the clinical phenotype.

Clinical utility—how well the test improves clinical outcomes.

Ethical—along with legal and social implications of the test.

A statistic called the area under the curve (AUC), in which an AUC ≥ 0.75 indicates a valid test, has been used to evaluate the accuracy of a test.15
 

The Macula Risk test analyzes 15 variants in 12 loci, plus age, BMI, smoking history and educational level. It has a reported AUC of 0.883 for five-year progression,15 which indicates excellent analytic and clinical validity. However, the clinical utility of Macula Risk is uncertain, because no interventions currently available have been reported to improve outcomes in patients at high risk for advanced AMD. More frequent examination of high-risk patients may seem logical, but to our knowledge there is little or no peer-reviewed evidence that has demonstrated an actual benefit from this approach.

Further, the ethical, legal and social implications of Macula Risk appear substantial. An incorrect false-negative result may give patients a false sense of security, while an incorrect false-positive result could cause anxiety and possibly unnecessary examinations and clinical interventions.

The American Academy of Ophthalmology Task Force on Genetic Testing published recommendations in 2012,16 which it then updated in 2014,17 that advise clinicians to offer genetic testing to patients suspected of having a monogenic (Mendelian) disease, and to provide patients with genetic counseling or referral to a genetic counselor. These guidelines also recommended against direct-to-consumer genetic testing and against routine testing for complex genetic diseases such as AMD.

AREDS and Intermediate AMD

With regard to anti-VEGF therapy, many relatively small series and meta-analyses have reported significant associations between various risk alleles and outcomes.18 However, two large, prospective randomized clinical trials did not validate these findings: Comparison of AMD Treatments Trials (CATT) and Inhibit VEGF in Patients with Age-Related Choroidal Neovascularization (IVAN). The CATT investigators reported no significant associations between variants in CFH, ARMS2, HTRA1 and complement factor 3 (C3) with treatment outcomes.19 Similarly, the IVAN investigators reported no significant associations between variants in CFH, ARMS2, HTRA1 and others with treatment outcomes.20

Some risk alleles may be associated with the response to nutritional supplementation in AMD. AREDS was a randomized clinical trial that studied the effect of nutritional supplementation on AMD progression. It randomized a total of 3,640 patients to receive antioxidants (beta-carotene, vitamin C and vitamin E), zinc (defined as zinc plus copper), both, or neither (placebo).6 The AREDS investigators graded disease severity with fundus photography on a 1-4 scale.
 
Figure 2. A gene map of the location of the age-related maculopathy susceptibility 2 (ARMS2) loci.


They reported that in patients with category 3 or 4 disease, treatment with antioxidants plus zinc was associated with a significant reduction in disease progression rates by about 25 percent at five years. AREDS defined category 3 disease as at least one large druse (>125 µm), extensive intermediate drusen and/or non-central geographic atrophy (GA). Category 4 disease was defined as central GA, neovascular AMD or visual loss resulting from AMD in one eye.

The AREDS2 study subsequently reported that substituting lutein and zeaxanthin for beta-carotene resulted in similar outcomes,21 and many clinicians currently recommend the AREDS2 formulation rather than the original AREDS formulation.

Retrospective AREDS Analyses

AREDS obtained and stored genetic data from some participants. Six subsequent retrospective analyses of the original AREDS study data have investigated genetic associations with treatment outcomes (progression rates). These studies have reported conflicting results (Table).
 


Michael Klein, MD, and colleagues investigated 867 patients with category 3 and 4 disease and reported that all patients benefited from antioxidants plus zinc (the AREDS formula). However, significantly more favorable outcomes were associated with patients with no risk alleles at CFH compared to patients with two risk alleles at CFH. There were no associations with ARMS2.22 The investigators noted this association but did not recommend a change in clinical management.  

Carl Awh, MD, and colleagues analyzed 995 patients with category 3 disease and reported a complex relationship in which certain combinations of risk alleles at CFH and ARMS2 were associated with more favorable clinical outcomes with certain nutritional supplements. They concluded that 49 percent of patients in their analysis had risk allele combinations for which nutritional supplements other than the AREDS formula were most beneficial. They suggested that individualized nutritional supplementation based on CFH and ARMS2 variants might improve clinical outcomes.23

The AREDS investigators responded to this publication by performing an “unplanned retrospective analysis” of 1,237 patients with category 3 or 4 disease. This analysis reported that all evaluated patients benefited from antioxidants plus zinc, and that CFH and ARMS2 risk alleles were not significantly associated with treatment outcomes.24

Dr. Awh and colleagues published a second study of 989 patients with category 3 and 4 disease. They defined four groups of patients based on CFH and ARMS2 and reported that, for each group, different nutritional supplements were most beneficial.

They concluded that “most” patients with category 3 or 4 disease would have more favorable outcomes if treated with a combination other than antioxidants plus zinc. They noted the lack of an available second population with which to validate their findings, but did recommend individualized nutritional supplementation based on genotype.25

The AREDS investigators then attempted to validate these findings by identifying a “residual cohort” of 526 patients from the original AREDS study not included in the subgroup that Dr. Awh and colleagues analyzed. In these 526 patients, the authors found no significant associations between CFH, ARMS2 and treatment outcomes.

Further, to illustrate the importance of a validation sample, the AREDS investigators also analyzed all patients in both cohorts according to astrological sign. They reported that, in the subgroup of patients that Dr. Awh and colleagues analyzed, the signs of Aries and Cancer were associated with significantly worse outcomes when treated with zinc. However, these associations were not found in the “residual cohort,” and were thus not validated.26

Most recently, Johanna Seddon, MD, MSc, and colleagues retrospectively studied the AREDS data, analyzing individual eyes (4,124 in total) rather than patients.27 They reported that antioxidants plus zinc conferred no treatment benefits in patients with two risk alleles at CFH or no risk alleles at ARMS2. The authors did not make any recommendations for clinical management, but did call for additional studies to investigate these outcome differences.

Conclusion

AMD is a complex genetic disease, with both genetic and environmental influences, so analysis of only two risk alleles (CFH and ARMS2) may yield misleading information.28 No randomized clinical trials have been published regarding genotype-phenotype relationships with AREDS supplementation.

Six retrospective subgroup analyses of the original AREDS data (but none of the AREDS2 data) have been published. The significant associations that some investigators have identified from the AREDS study have not been replicated in a second study population.12

Clearly, a prospective randomized clinical trial that was designed to investigate a genotype-phenotype association is preferable to a retrospective subgroup analysis of a randomized trial that was not designed to investigate the association.29 Unfortunately, no such randomized clinical trial has been published, and it seems unlikely one will occur in the near future. Because of this, clinicians must make treatment decisions based on the available data, which has yielded conflicting results.  

In summary, AMD pharmacogenetics is an intriguing area of research, but at this time there is insufficient evidence to use genetic information to guide treatment decisions in individual patients.  RS

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