and across Bruch’s membrane which would otherwise accumulate leading to drusen formation and AMD [173, 178, 179]. APOE e4 carriers have a lower cellular accumulation than e3 carriers, whereas e2 carriers have more lipid accumulation [117, 202]. An alternative explanation for the protective effect of the e4 allele and the seemingly predisposing effect of e2 is that APOE variants modify inflammatory response. Several studies reported that e4 carriers have significantly lower CRP levels than noncarriers, especially compared to e2 carriers. CRP level reportedly decreases in a dose-depen- dent order of e2/ e2, e2/e3, e3/e3, e2/e4, e3/e4, and e4/e4 [203–208].

In addition, APOE e2 appears to enhance expression by RPE cells of the vascular endothelial growth factor and fibroblast growth factor [202], whereas their expression is reportedly suppressed by APOE e4 [186, 209]. This indicates that APOE e2 induces neovascularization by altering angiogenic cytokines, whereas APOE e4 limits this process. And in contrast to e2, APOE e4 has positive charges which diminish hydrophobicity of Bruch’s membrane, and result in better clearance of debris compared to APOE e2. Moreover, e4 carriers reportedly have a 36% lower risk of hypertension than noncarriers [210]. Another interesting finding is that APOE e4 levels seem to decrease with advancing age [208], which may reduce transport of lipids and cell debris, culminating in a higher rate of AMD in older age. APOE also plays an important role in the maintenance of retinal cell membranes: Lipids are released from the degenerating cell membrane and astrocytes react by synthesizing APOE to bind the free cholesterol and lipids in order to distribute them for reuse in cell membrane biosynthesis [211–213]. Based on the cumulative empirical evidence and pooled data outlined above, it may be proposed that the APOE e4 offers a reduced risk for onset, severity, and progression rate of AMD, in contrast to APOE e2.

1.5.3.2Tissue Inhibitor of Metalloproteinases-3 (TIMP3), Lipase C (LIPC), Cholesterylester Transfer Protein (CETP), Lipoprotein Lipase (LPL), ATP-Binding Cassette Subfamily A

Member 1 (ABCA1)

Candidate gene analyses initially found no evidence of linkage or association between AMD and TIMP3 on chromosome 22q12.1–13.2 [214, 215] Recently, a genome-wide association study (GWAS) found the region near TIMP3 to be a susceptibility locus [135],

which had previously been reported by one linkage study [50]. TIMP3 is a metalloproteinase involved in the degradation of the extracellular matrix. It is a RPE signature gene [216], which is mutated in Sorby’s fundus dystrophy [217]. Very common alleles at rs9621532 and nearby variants at TIMP3 were associated with increased risk of AMD (OR, 1.41; 95% CI, 1.27–1.57; frequency in controls ~0.94; P = 1.1 × 10−11).

In addition, they found causative associations with alleles at loci that were associated with high-density lipoprotein cholesterol (HDL-c) levels in blood [218, 219], namely, the common allele of rs493258 at the LIPC gene on chromosome 15q22 (OR, 1.14; 95% CI, 1.09–1.20; frequency in controls ~0.53; P = 1.3 × 10−7), the rare allele of rs3764261 at the CETP gene on chromosome 16q21 (OR, 1.19; 95% CI, 1.12–1.27; frequency in controls ~0.36; P = 7.4 × 10−7), rs12678919 at LPL on chromosome 8p22 (OR, 1.38; 95% CI, 1.17–1.63; P = 1.8 × 10−3), and rs1883025 near ABCA1 on chromosome 9q22 (OR, 1.25; 95% CI, 1.12–1.40; P = 2.6 × 10−3) [135]. It should be noted that only variants near TIMP3 reached genome-wide significance (P < 5 × 10−8).

In a parallel GWAS, only associations between variants at LIPC and AMD reached genome-wide significance [220]. They found rs10468017 to be most associated with AMD (OR, 0.82; 95% CI, 0.77–0.88; frequency in controls ~0.30; P = 1.34 × 10−8). However, confirmation was achieved after targeted examination of the suggestive markers of the other GWAS. CETP and LPL play important roles in the production and degradation of HDL-c, whereas LIPC and ABCA1 are involved in mediating the uptake of HDL-c at the cell surface [221]. HDL is an important transporter of lutein/zeaxanthin. Modification of HDL-related efficiency of carotenoid delivery to the retina could influence the risk of AMD [222]. LIPC, CETP, and ABCA1 have been shown to be expressed in the retina [220, 223]. With increasing age, lipids and cholesterol accumulate underneath the RPE and are constituents of drusen [118, 224]. These HDL-c associated variants might affect the formation of drusen and subsequently the development of AMD.

1.5.4Candidate Gene Association Studies

Several variations in other genes have been tentatively identified through candidate gene approaches and are not consistently linked to AMD. Toll-like receptors