Statistical analysis

Both SNPs were assessed for departure from Hardy- Weinberg equilibrium by study, using a v2 goodness-of-fit test. Data were categorized on the basis of age into 8 groups ((cid:1)60, 61–65, 66–70, 71–75, 76–80, 81–85, 86–90, and >90 years). Separate analyses were performed for each of the 3 APOE alleles in the data set. Logistic regression was used to assess the variation in APOE allele frequencies across age groups, with center, gender, and smoking included in the regression to adjust for possible confounding. A likelihood ratio v2 test was used to compare models that included and excluded a linear term for age group, thus providing a test for trend in allele frequency across age groups. This analysis also provided an odds ratio summarizing the change in odds for each allele per 5-year increase in age. Interactions be- tween gender and age and between center and age were also tested in the logistic regression using likelihood ratio tests which compared models that included and excluded the interaction terms.

Life expectancy in Western countries has risen by 3 months per year over the past 160 years, with a net increase of approximately 40 years within this time frame, largely as a consequence of reductions in malnutrition and childhood infection (9). A number of reports on human longevity have shown that the frequency of the e4 allele is lower in older age groups, such as octogenarians, nonagenarians, and cen- tenarians, than in younger or middle-aged persons (10) and that absence of an e4 allele appears to be a favorable sur- vival factor. In populations of European origin, an elevated mortality risk has been reported for the e3/e4 genotype rel- ative to the e3/e3 genotype, with a slightly decreased risk being associated with the e2/e3 genotype (11, 12). Further- more, a gender-specific survival effect associated with e2 has been reported, although whether this is specific to males or females is as yet unclear, with opposing associations with both genders being reported (13, 14).

The analyses undertaken in this study were subsidiary to a pooled data analysis assessing APOE variation in the con- text of AMD. The authors examined the association of APOE with age as a marker for longevity and assessed the potential for a gender-specific effect.

RESULTS

APOE allele frequency with age

MATERIALS AND METHODS

tected for either SNP by center or within the entire data set. APOE allele frequencies varied between studies (data not shown), with ranges of 6.7%–10.0% (APOE e2), 75.3%– 82.8% (APOE e3), and 7.5%–15.6% (APOE e4). APOE genotype frequencies for the 10,623 controls are shown in Table 2. The frequency of the e4 allele decreased from 17.6% to 8.3% ((cid:2)9.3%) with increasing age, while the fre- quency of the e3 allele increased from 73.3% to 83.3% (þ10.0%) (Table 3, Figure 1). Following adjustment for center and smoking status to limit potential confounding, we observed a significant decrease in the frequency of e4 with increasing age (v2 ¼ 14.9 (1 df); P ¼ 0.0001), repre- senting a 5% decrease in odds per 5-year increase in age (odds ratio ¼ 0.95, 95% confidence interval: 0.92, 0.97), as well as a significantly increased frequency of e3 (v2 ¼ 11.3 (1 df); P ¼ 0.001), representing a 4% increase in odds per 5- year increase in age (odds ratio ¼ 1.04, 95% confidence interval: 1.02, 1.07). The frequency of the e2 allele showed little variation with age. The relations between allele fre- quency and age in males and females were compared by including an age 3 gender interaction in the logistic regres- sion analysis, but none of the interactions were signifi- cant (likelihood ratio v2 test: e2, P ¼ 0.13; e3, P ¼ 0.25; e4,

Study population

The data originated from 15 studies carried out at 40 study centers in 11 countries: 9 in Europe (United Kingdom, Germany, Netherlands, Norway, Estonia, Italy, France, Greece, and Spain), the United States, and Australia, which had previously examined the association of APOE with AMD (15–28). The dates of the studies ranged from 1990 to 2009. Analysis was restricted to samples derived from participants of European descent (n ¼ 24,774). Individual participant data, including age, gender, smoking status (ever smoker vs. never smoker), and APOE genotype, were avail- able for 23,686 persons, enabling us to conduct a pooled data analysis. The Alzheimer’s disease status of these participants was not available. We restricted the analysis to 10,623 par- ticipants with no evidence of any AMD, classified by means of retinal photography or clinical examination, in order to avoid confounding by disease status due to a prior association of variation within the APOE gene and AMD (4). Investiga- tors from 3 studies—the Rotterdam Study, the Women’s Health Initiative Sight Exam Study, and the European Eye Study—provided data derived from samples acquired

1. Sample Size, Age Distribution, and Gender Composition Among Control Subjects From Contributing in a Pooled Analysis of Variations in APOE Allele Frequencies With Age (n ¼ 10,623), 1990–2009