each variant and its consequence on AMD pathology. Among the 15 variants mainly three SNPs are of particular interest, namely

•rs10490924, an nscSNP within exon 1 of ARMS2 (A69S),

•the c.*del443ins54 variant in the 3¢-untranslated region of ARMS2, and

•rs11200638, a possible regulatory SNP approximately 600 nucleotides upstream of the HTRA1 start codon. Each of the three variants was also associated with

AMD in Asian populations, even with higher frequencies than in Caucasian ethnicities. Therefore, the AMDassociated ARMS2/HTRA1 variants appear to be more global than the CFH risk variant rs1061170 (Table 2.1).

2.5.1Functional Implications

The 15 risk variants in the 23-kb region of 10q26 center over the ARMS2 and HTRA1 locus and thus point to two equally probable candidates for the sought-after AMD susceptibility gene. Although putative functional consequences for several associated SNPs have been suggested, so far evidence is elusive, and the role of ARMS2 and/or HTRA1 in AMD pathology needs further clarification.

HTRA1 is a member of the HTRA family of serine proteases and was initially described as a secretory protein involved in the degradation and maintenance of the ECM [53], in the modification of the complement pathway [54, 55], and in amyloid deposition [53, 54], all processes playing an important role in AMD pathogenesis. More recent studies focused on an intracellular form of HTRA1 influencing cell migration [56] and apoptosis [57]. Chan et al. [58] demonstrated an upregulation of HTRA1 in macular lesions of AMD eyes. In addition, the protein was detected in drusen of advanced AMD retinae [50, 59]. Despite these arguments in favor of HTRA1 as the AMD gene, the risk haplotype tagging polymorphisms do not reveal any obvious functional consequence on HTRA1 protein sequence or structure, but rather may influence expression levels. Thus far, however, contradictory results have been reported on HTRA1 expression levels with regard to risk [51, 54, 60–62].

Few but also highly controversial functional data are available on ARMS2, a phylogenetically young gene existent only in primates [63]. It is specifically

expressed in the human retina and placenta, but not in a number of other tissues tested [52, 60, 63]. The putative amino acid sequence has no similarity to any known protein or protein domain. While the localization of the putative ARMS2 protein is still unclear [60, 64, 65], it cannot be excluded that ARMS2 operates on the RNA level or might even be a spurious transcript without cellular function. A reasonable argument in favor of ARMS2 as the AMD susceptibility gene is a recently identified indel polymorphism (c.*del443ins54) in the 3’ untranslated region of the gene, which leads to the deletion of the polyadenylation signal of the ARMS2 mRNA and an insertion of an AU-rich element. As a consequence, the ARMS2 mRNA risk isoform is highly instable compared to the mRNA isoform of the non-risk haplotype [52]. If ARMS2 can be ascribed a cellular function, consequently the riskassociated indel variant would result in partial or complete insufficiency of such a function.

2.6Latest Findings from Genome-Wide Association Studies (GWAS)

In the GWAS by Klein et al. [11], which led to the identification of CFH as an AMD susceptibility gene, a relatively small number of AMD patients (n=96) and controls (n=50) was analyzed. Nevertheless, the statistical power in this study was sufficient to detect the frequent and strong risk effects of the CFH gene. Subsequent GWAS have been based on several hundred to thousands of individuals and thus are suited to detect somewhat weaker association signals at genome-wide significance levels, like those at the C3, CFB, and CFI loci [66, 67]. Additional AMD susceptibility genes have been identified, namely the gene for the tissue inhibitor of metalloproteinases-3 (TIMP3), the gene for the hepatic lipase (LIPC), and the gene for the plasma cholesteryl ester transfer protein (CETP), all of which were not a focus of AMD research before (Table 2.1). Nevertheless, their functions and the associated pathways fit well into actual concepts of AMD pathogenesis, implicating diffusion disturbances at the level of the extracellular matrix of Bruch’s membrane [3]. Of interest is the fact that TIMP3 mutations were previously associated with Sorsby fundus dsytrophy, a rare autosomal dominant form of macular dystrophy with striking phenotypic overlaps with late-stage AMD [68].

Variations in both LIPC and CETP were associated with alterations in HDL cholesterol concentrations, an important regulator of lipid accumulation in Bruch’s membrane.

2.7Prospects of Genetics in AMD Therapy and Prevention

To date, AMD may be one of the best characterized complex diseases with extensive information on the genetic and environmental risk factors and implicated biological pathways [69]. Nevertheless, an effective treatment is not yet available for the majority of AMD patients. However, preventive measures or successful therapies will be indispensable, especially when considering the predicted demographic shift toward an older population within the next few decades. Due to the complex nature of the disease, this will require a comprehensive understanding of the genetic, demographic, and environmental factors, and their mutual interplay in the development of AMD, strongly encouraging the need for further major research efforts in this disorder.

Available data estimate that up to 70% of the AMD risk might be due to genetic influences [4]. This should focus our priorities on the functional impact of gene variants associated with AMD, especially their impact on early and late stages as well as the progression of the disease. A deeper insight into AMD genetics promises to discover so far unknown cellular pathways or markers that might provide novel targets for therapeutic approaches. A prominent illustration of the power of the new genetics is the identification of CFH, the first major AMD susceptibility gene, whose discovery was a breakthrough greatly boosting intense research into the association of AMD with the immune system [8–11]. So far, the known AMD susceptibility genes have not been associated exclusively with one or the other of the two late stages of the disease, namely GA or CNV. It is hoped, however, that a profound understanding of AMD genetics may provide clues as to the central switches controlling prognosis and disease progression.

A recent multilocus analysis of known genetic risk factors estimated that about 80% of individuals within the highest of ten risk groups will develop AMD at the age of 75 years [66]. It follows that despite the complex nature of the disease, genetic factors are highly accurate predictors of disease, bringing the era of personalized medicine closer to reality and the prospects to enhance quality of life at older age within reach.

Summary for the Clinician

›Estimates assume that up to 71% of AMD susceptibility can be ascribed to genetic factors.

›First studies analyzing the genetic contribution to AMD were published in the late 1990s and suggested two AMD susceptibility genes, ABCA4 and APOE. Both genes, however, make only minor contributions to overall disease load.

›It was not until 2005 that two major AMD susceptibility loci, CFH and ARMS2/HTRA1, were identified. Together, the risk variants at these two loci likely account for over 50% of AMD cases.

›The findings in CFH strongly suggest an involvement of the alternative complement pathway in AMD pathogenesis. Subsequent studies revealed three additional AMDassociated genes within this pathway, namely

CFB, C3, and CFI.

›The functional roles of ARMS2 or HTRA1 in the disease process are still controversial and objects of intense research. This second AMD locus may uncover another important pathway in AMD etiology distinct from the complement system.

›Recent improvements in high-throughput technologies promise to identify additional AMD susceptibility genes, specifically those with a minor contribution to the overall disease load, as shown for example for TIMP3,

CETP, and LIPC.

›Establishing a comprehensive profile of the genetic susceptibility to AMD will pave the way for novel and innovative options in prevention and personalized treatment.

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