Update on the role of genetics in AMD

Extensive epidemiologic and genetic analyses have lead to the conclusion that AMD, like many other chronic age-related diseases, results from the interplay of multiple environmental and genetic factors, which in combination account for development of the phenotype. The condition is strongly age-related, and tobacco smoking is the most consistent and modifiable significant risk factor. Other environmental risk factors that have been reported include cardiovascular disease, hypertension, high body mass index, and low education level.

GENETIC SUSCEPTIBILITY AND AMD

It is now beyond question that genes play a significant etiological role in AMD. Studies to identify genetic AMD-susceptibility variants have utilized all available techniques such as genome-wide linkage approaches (twins, sib pairs, and families) and case-control association studies. For the most part, studies have been limited to the study of the phenotypic extremes; that is, advanced cases or those with no signs of the condition, and case-control populations of extreme phenotypes. This is because it is reasonably achievable to ascertain these phenotypes.

Early studies

Early studies concentrated on genome-wide linkage and familial association analyses (twins, sib pairs, and families). The first genetic locus for AMD was localized in a single large pedigree to chromosome 1q. Later, a co-segregating variant in HMCN-1 (hemicentin-1) was identified. HMCN-1 lies in close proximity to the CFH (complement factor H) gene, discussed below. Meta-analysis of a number of linkage studies consistently identified this same locus and several other genomic regions which were later shown to harbor specific genetic variants. Others remain the subject of further investigation.

Complement genes

The first well established specific genetic variant to be associated with advanced AMD was the single nucleotide polymorphism (SNP) rs1061170 (T1277C; Y402H) in the CFH gene. This finding has been replicated by numerous studies. Additional analyses of the RCA locus on chromosome 1q in which the gene resides have concluded that haplotypes encompassing both CFH and neighboring genes, acting independently or in concert with the Y402H change, confer increased risk of drusen formation and advanced AMD. Subsequent analyses of the complement pathway identified SNPs in other complement components: complement factors C2, CFB, C3, and CFI. CFH is a regulator of complement activation, dysfunction of which has been linked to retinal pathology.

The challenges of 10q26

Early genome-wide linkage studies consistently identified an AMD susceptibility locus on chromosome 10q26. A combination of genotyping and direct sequencing of this region initially identified two SNPs, 6 kb apart, in high linkage disequilibrium in many Caucasian populations, that are strongly associated with advanced AMD, as follows.

The rs10490924 (A69S) variant lies within the putative gene, LOC387715, now named ARMS2 (age-related maculopathy susceptibility 2). ARMS2 has no known function, and the predicted protein shows little homology with other proteins. ARMS2 is only present in higher primates, and mRNA transcripts can be detected in the retina. Whether the protein is translated is still debated. Immunohistochemical analyses have provided conflicting evidence localizing protein within the mitochondrion in the inner segment of the photoreceptor; the cytoplasm, among other locations. Most recently, an indel in ARMS2 has been reported that appears to affect translation of the protein and has been postulated to be the functional variant at the 10q26 locus.

The rs11200638 SNP resides in the promoter of the gene, HTRA1, a serine protease found in the retina (among other tissues). Preliminary, functional analyses suggest that the polymorphism at this position alters expression levels of the gene.

The era of genome-wide SNP-association studies (GWAS)

Modern advances in genotyping technology have facilitated the high-throughput analysis of hundreds of thousands of single-nucleotide polymorphisms on a single chip. In 2010, a consortium of researchers published the results of two independent GWASs with subsequent replication of positive findings. These studies identified several new genes associated with advanced AMD status. Of interest, this study implicated genes associated with lipid metabolism, specifically the HDL pathway, ABCA1, LIPC, CETP, and LPL. Other replicated findings included significantly associated SNPs near the gene encoding TIMP3 (tissue inhibitor of metalloproteinase 3), which is involved in remodeling of the extracellular matrix in the retina.

Other genes

Associations in the genes APOE (apolipoprotein E), ABCA4 (ATP-binding cassette A4), CX3CR1 (chemokine 3 receptor 1), PON1, TLR4 (toll-like receptor 4), ERCC6, ELOVL4, VLDLR (very low density lipoprotein receptor), fibulin-5, hemicentin-1, TLR3 (toll-like receptor 3), C1q (complement factor C1q), VEGF (vascular endothelial growth factor), SERPING1, and LRP6 have been reported in single populations.

PHARMACOGENETICS IN AMD

Pharmacogenetics attempts to define the genetic variants that determine variable response to medication. The ultimate goal is to identify those who respond best and avoid adverse reactions. Garrod first recognized a familial or genetic tendency to variability in drug response and hypothesized that drugs were metabolized by specific pathways of genes in which defects would result in differences in drug concentrations and therefore drug effect. A large number of studies have now defined pharmacogenetic interactions in many biomedical fields. These include therapies for neurological and psychiatric disorders, asthma, cardiovascular disease, and cancer.

Initial studies in AMD have focused on three different treatments: Age-Related Eye Disease Study (AREDS) supplementation, photodynamic therapy (PDT), and anti-VEGF therapy. In all instances, studies to date have been limited to retrospective analyses.

Anti-VEGF agents

In one retrospective study, 86 patients being treated with bevacizumab (Avastin™) alone were evaluated for associations between treatment response and common polymorphisms in the genes CFH and ARMS2. Patients homozygous for both CFH risk alleles (CC) had worse visual outcomes than those with the CFH TC and TT genotypes. In a similar retrospective analysis, but involving 156 patients who were receiving ranibizumab, the same authors were able to replicate this finding. These studies were well conducted; however, the associations do not necessarily imply causality and there may have been additional confounders.

AREDS supplements

The AREDS was an 11-center National Institutes of Health-funded study initiated in 1992 with 4757 participants. It included an 8-year randomized control trial which established that a combination of zinc and antioxidants (beta-carotene, vitamin C, and vitamin E) produced a 25% reduction in development of advanced AMD and a 19% reduction in severe vision loss in individuals determined to be at high risk of developing the advanced forms of the disease. Conversely, 22% of participants receiving antioxidants and zinc had a 15-letter decrease in visual acuity despite treatment. Use of these oral supplements is now current standard of practice in the United States. Indeed, they remain the only therapy for early, intermediate, and dry AMD.

A recent evaluation of the AREDS cohort found evidence of an interaction between the CFH genotype and treatment with antioxidants plus zinc when compared with placebo. This interaction appears to have arisen because supplementation was associated with a greater reduction in AMD progression (68%) in those with the low risk TT genotype compared with those with the high risk CC genotype (11%).

These results may imply that the strong genetic predisposition to AMD conferred by the CC genotype limits the benefits available from zinc and antioxidants (beta-carotene, vitamin C, and vitamin E). In this pharmacogenetics study, the authors evaluated whether known AMD-susceptibility genotypes in those who at entry into the study had early to intermediate AMD and progressed to advanced disease were associated with treatment assignment. Previously, these same genes had been reported to be independently associated with progression to advanced AMD. There is good biological plausibility to support a possible role for CFH. Evidence supports the assertion that CFH protein dysfunction results in excessive inflammation and tissue damage of the type involved in the pathogenesis of AMD. Inflammation is known to intensify oxidative stress, and since AREDS supplements are thought to have an antioxidant effect, it seems reasonable to assume that CFH polymorphisms could play a role in treatment response.

PDT

PDT was until recently the most widely used therapy for neovascular AMD and still retains a role for individuals in whom anti-VEGF agents are contraindicated. PDT to the macula induces thrombosis of neovascular vessels (choroidal neovascularization) which have been photosensitized by the administration of verteporfin. Efficacy was originally established in a series of randomized control trials including the TAP (Treatment of Age-Related Macular Degeneration with Photodynamic Therapy), VIP (Visudyne in Photodynamic Therapy), and Visudyne in Minimally Classic Choroidal Neovascularization studies. Considerable variability in response is observed with PDT and may vary by ethnicity. In an attempt to identify whether genetic influences are involved, a set of variants in genes associated with thrombosis were retrospectively evaluated in two studies (84 and 90 subjects). Patients were divided into those that were PDT “responders” and those that were “nonresponders” (3-month follow-up). Patients were genotyped for factor V G1691A, prothrombin G20210A, factor XIII-A G185T, methylenetetrahydrofolate reductase C677T, methionine synthase A2756G, and methionine synthase reductase A66G. “Nonresponse” was more frequent in those with the hyperfibrinolytic G185T gene polymorphism of factor XIII-A, and response was associated with those with the thrombophilic factor V 1691A and prothrombin 20210A alleles.

PREDICTING THE RISK OF DEVELOPING ADVANCED AMD

The idea of employing a risk assessment algorithm to identify individuals at risk of developing AMD is attractive. The fact that drusen, the hallmark of the condition, appear prior to the development of vision loss offers an unusually useful clinical feature that might be combined with genetic and environmental risk factors to give an accurate risk assessment. Several such models have been proposed. Seddon et al described a model derived from the AREDS study population that included all these factors using the AREDS clinical AMD grading scale. In the model, points are assigned for the risk factors in their model to determine an individual’s risk score. Zanke et al described a model that gives a lifetime risk estimate based on genetics and environmental factors, and recently Chen et al proposed a model that examined risk of bilateral involvement. There is no conclusive evidence that genetic variants assist in predicting progression of disease once advanced AMD is established. One study found no association of progression of geographic atrophy with variants in the CFH, C3, and ARMS2 genes. A second study found no association of progression with variants in CFH, C2, C3, and CFI, but did note a nominal association with ARMS2.

CONCLUDING REMARKS

AMD is a major health burden and one that is rapidly growing as the population of the Western world ages, en masse. Although the introduction of anti-VEGF agents has revolutionized outcomes for those with the less common neovascular form of AMD, there is limitation to the effectiveness of these regimens. There is currently neither effective treatment for geographic atrophy nor for earlier stages of disease. Dissecting the genetic etiology of the condition holds substantial promise for the identification of new avenues for therapeutic development. It is likely that conventional genome-wide and candidate gene approaches may have reached their limit to resolve new variants. Genome-wide strategies are not themselves redundant but will be superseded by next-generation technology such as whole Exmore and full genome sequencing. Furthermore, the analysis of individuals with intermediate AMD phenotypes and the use of extended pedigrees with carefully quantified endophenotypes offer the opportunity to investigate less common, rarer, and private mutations, otherwise largely unidentifiable using case-control populations.


Source:
Clin Ophthalmol. 2011;5:1127-33
http://www.ncbi.nlm.nih.gov/pubmed/21887094