This study firmly established that the APOE-ε4 allele, or a nearby allele in linkage disequilibrium, was associated with a decreased susceptibility for AMD manifestations. The protective effect of the ε4 allele did not vary by family history status or by AMD subtype; ε4 allele frequency was reduced in both familial and sporadic AMD and in advanced forms of AMD, GA, and CNV. Although familial patients have 3.5 years earlier age at diagnosis of AMD than the sporadic patients, this difference in diagnosis age (though significant between the two groups) was not apparently due to APOE alleles. Age at diagnosis of AMD was similar between ε4-carriers (ε3/ε4 and ε4ε4), ε2-carriers (ε2ε2 and ε2ε3), and ε3ε3 patients. The data suggested that other as yet unidentified gene(s) may modulate age at diagnosis of AMD.
In this study, patients and controls were matched for ethnicity to avoid confounding due to population stratification. All subjects were white and a majority resided in the same geographical location (i.e., the state of Michigan). APOE allele frequencies observed in the controls in our study were similar to those reported by other studies on APOE and AMD
23 25 26 and reported for the general white population.
30 31 Moreover, APOE allele frequencies in our patients were similar to those reported by others.
23 25 In AMD, frequency of the ε4 allele ranges from 0.07 to 0.12 and ε2 allele from 0.09 to 0.125.
23 25 26 We did not detect a significant difference in ε2 allele frequency between patients and controls, possibly due to the insufficient statistical power. Interestingly, the association between APOE and AMD has also been replicated in a group of Italian AMD patients,
32 but not in Chinese AMD patients.
33 This indicates that the association between APOE and AMD may vary among different ethnic groups.
To date, there has been one published report
34 that failed to detect the association between ε4 and AMD in white people, when examined in families with three or more affected members, and in unrelated AMD cases. Though no association was apparent among 56 AMD families, the data presented in that report
34 demonstrate a trend for reduced ε4 frequency in unrelated AMD patients compared with controls (0.09 vs. 0.12). The lack of association could be explained by the small sample size and insufficient statistical power. It is possible that the effect of APOE was masked by the effect of other genes with a greater impact on AMD.
Although our control population was slightly younger than affected patients (74.6 ± 4.9 vs. 79.2 ± 7.9), all control subjects had an ophthalmic examination and were at least 68 years old at the time of enrollment. The average age at the most recent clinical evaluation for controls was 3 years more than the average age at diagnosis of AMD patients. The reduced ε4 allele frequency in patients compared with controls was observed in both age groups, those before 75 years and after. Our data strongly argue that the observed association is real and not due to a result of varying allele frequencies between different age groups.
APOE is associated with both risk of developing AD and age at onset of AD. Although it is now clear that APOE is associated with risk of developing AMD, the effect is in the opposite direction. The effect of APOE genotypes on age at diagnosis of AMD had not been examined before our study. An effect on age at diagnosis of AMD by APOE alleles was not detected, suggesting yet another difference in the association of APOE and these two aging-associated yet distinct neurodegenerative diseases.
Our understanding of the mechanism(s) of association between APOE alleles and AD remains limited even after 10 years. The association between APOE alleles and AMD was first reported approximately 5 years ago and was met with controversy. Our results based on a large patient cohort recruited from a single center, along with the results of the meta-analysis report,
27 clearly demonstrate the existence of the association and should encourage further research into the role of APOE and its variants in pathogenesis of AMD. Because of a high rate of turnover in photoreceptor outer segments, especially in the macular region,
35 APOE may play a role in cell-membrane remodeling, which is essential for normal functioning and maintenance of the retina.
23 It is also possible that the inability of ε4 to form dimers compared with ε2 or ε3 variants may allow easier transport of lipids through Bruch’s membrane because of smaller size lipid particles.
25 Accumulation of neutral lipids with age may then lead to the formation of a hydrophobic barrier within Bruch’s membrane. An alternative hypothesis is based on the difference in ability to clear debris through Bruch’s membrane, since unlike ε2 and ε3, ε4 contains positive charges.
25 APOE is also suggested to reduce oxidative damage to the RPE by regulating nitric oxide production.
27
In summary, in the largest patient cohort recruited from a single center to date, the APOE-ε4 allele was shown to be associated with a significantly decreased risk of AMD. The age at diagnosis of AMD may be modified by a familial risk factor(s). As the search for additional AMD susceptibility loci continues, it is important to explore the interactions among various genetic and environmental risk factors. A clear understanding of AMD pathogenesis will be most readily achieved through appreciation of these complex interactions.
The authors express gratitude to numerous individuals and families who donated their time and resources. Thanks are also due to Susan Elner, John Heckenlively, Samuel G. Jacobson, Mark W. Johnson, Ron Kurtz, Paul R. Lichter, Donald Puro, Stephen Saxe, Paul A. Sieving, Andrew K. Vine, and David Zacks for their clinical inputs and support during patient collection, Julia E. Richards and Goncalo Abecasis for constructive discussions, a large number of retina physicians in the state of Michigan and the Great Lakes region for their involvement, and the clinical staff at the Kellogg Eye Center Retina Clinic, whose everyday assistance made this work possible. The efforts of John Bahling, Suja Hiriyanna, and Elena Fillipova in patient collection and sample preparation are appreciated. Sharyn Ferrara is acknowledged for administrative assistance.