Early genetic studies (1999–2005) indicated possible linkage and association signals on many human chromosomes and two major loci at 1q31 and 10q26.
16–20 In 2005, several independent research groups demonstrated the value of genome-wide association studies (GWAS) for complex disease with identification of AMD risk genes. The groups identified a common variant in the complement factor H (
CFH) gene located on chromosome 1q as a risk factor for developing AMD.
CFH plays a role in the intrinsic inflammatory cascade of the immune system. It may also regulate retinal development.
21 A study that included participants from the NEI-sponsored Age-Related Eye Diseases Study (AREDS) found individuals with the CFHY402H variant are 7.4 times more likely to develop AMD.
22 The study was based on whole genome analysis of participants from the NEI-sponsored AREDS, a major clinical study that closely followed nearly 5000 patients with varying stages of AMD.
18,19,22–24 The discovery of
CFH association highlighted a critical role of the alternative complement pathway and immune dysregulation, which was suggested previously by examining drusen, a hallmark of AMD pathology. Subsequent work over the next few years identified susceptibility loci
ARMS2,
C2/CFB,
C3,
CFI,
TIMP3 and
LIPC.
25–31 As a result, in 2010 NEI convened the International AMD Genomics Consortium (IAMDGC, also known as AMDGene) that included scientists from 18 research groups in 14 countries. The first study conducted by this consortium was a meta-analysis of multiple GWAS datasets representing more than 8000 individuals with AMD and 50,000 controls. Further analyses by this consortium included data from over 17,000 advanced AMD cases and more than 60,000 controls of primarily European ancestry, resulting in the next big leap in AMD genetics. The group identified 19 genetic loci linked to AMD susceptibility, with seven previously unreported loci near the genes
COL8A1/FILIP1L,
IER3/DDR1,
SLC16A8,
TGFBR1,
RAD51B,
MIR548A2, and
B3GALTL. The genetic risk score analysis of all variants (including the seven new ones) was able to distinguish the affected from unaffected individuals in all samples.
32 A major highlight of this study was the appreciation of distinct cellular pathways that contribute to pathology of advanced AMD. In addition to complement and immune dysregulation, the genetic studies provided direct evidence of the importance of cholesterol transport and lipoprotein metabolism, extracellular matrix, and angiogenesis signaling pathways, as recognized in previous investigations.
33 A key issue at this juncture was dissecting causality from genetic association studies. There was a concurrent search for causal variants at the GWAS loci by targeted re-sequencing that led to identification of rare and likely causal variants at complement genes Arg1210Cys in
CFH, Gly119Arg in
CFI, and Lys155Gln in
C3. To further explore the role of rare variants and expand the genomic investigations, the IAMDGC analyzed 16,144 patients and 17,832 controls using a custom-designed HumanCoreExome Chip for de novo genotyping at one central place. The primary goal was to bridge the gap between association studies of common variants and sequencing studies to identify rare variants.