Abstract
Background:
The Long Life Family Study (LLFS) is a family based, prospective study of healthy aging and familial longevity. The study includes two assessments of cognitive function that were administered approximately 8 years apart.
Objective:
To test whether APOE genotype is associated with change of cognitive function in older adults.
Methods:
We used Bayesian hierarchical models to test the association between APOE alleles and change of cognitive function. Six longitudinally collected neuropsychological test scores were modelled as a function of age at enrollment, follow-up time, gender, education, field center, birth cohort indicator (≤ 1935, or > 1935), and the number of copies of ε2 or ε4 alleles.
Results:
Out of 4,587 eligible participants, 2,064 were male (45.0%), and age at enrollment ranged from 25 to 110 years, with mean of 70.85 years (SD: 15.75). We detected a significant cross-sectional effect of the APOE ε4 allele on Logical Memory. Participants carrying at least one copy of the ε4 allele had lower scores in both immediate (−0.31 points, 95%CI: −0.57, −0.05) and delayed (−0.37 points, 95%CI: −0.64, −0.10) recall comparing to non-ε4 allele carriers. We did not detect any significant longitudinal effect of the ε4 allele. There was no cross-sectional or longitudinal effect of the ε2 allele.
Conclusion:
The APOE ε4 allele was identified as a risk factor for poorer episodic memory in older adults, while the APOE ε2 allele was not significantly associated with any of the cognitive test scores.
Keywords: ApoE, longevity, longitudinal studies, cognition
1. Introduction
Cognitive decline, both normal and pathologic, is one of the most common complications of reaching older age. Preservation of good cognitive function or delaying the onset of cognitive decline is essential to maintaining quality of life in older adults and it is important to identify risk factors of onset and rate of cognitive decline that can suggest therapeutic interventions. Several known factors including cardiovascular risk factors, alcohol use, smoking, high systemic levels of inflammatory markers, as well as socioeconomic status contribute to the cognitive decline process[1, 2]. Cognitive decline patterns vary among older adults, and are genetically regulated[3]. The apolipoprotein E (APOE) gene is one of the most important genes related to cognition. The gene has 3 alleles, namely ε2, ε3, and ε4 that result from the combination of the variations of two single nucleotide polymorphisms rs7412 and rs429358[4]. Several studies have shown that carriers of the ε4 allele are at increased risk of dementia and Alzheimer’s disease, while the ε2 allele might have a protective effect against age-related neurodegenerative diseases,[5, 6] and is associated with extreme human longevity. [7, 8] A relatively small number of studies have investigated the effect of both alleles on the rate of cognitive decline using longitudinally collected data.[9–11] The review by O’Donoghue[12] lists 40 studies of the association between APOE and cognition in longitudinal studies and only one study showed a protective effect of APOE ε2 on verbal episodic memory, while other studies showed a negative effect of APOE ε4 on various measures of cognition. Most of these studies were small (median sample size = 550), with largest sample size of 5,544 and length of follow-up ranging between 2 and 30 years (median = 5.6). Several factors may contribute to inconsistent findings, including sample size, short follow-up time, neuropsychological tests used, neurobiological mechanisms, and population ancestry.
The Long Life Family Study (LLFS) recruited over 5,000 individuals from longevous families. Participants underwent two in-person assessments, approximately 8 years apart, and attention, memory, and executive function were assessed through a battery of neuropsychological tests. The study included a relatively large sample of carriers of the APOE ε2 allele and provides a unique opportunity to assess whether APOE is associated with cross-sectional or longitudinal cognitive decline in this healthy aging cohort. In line with previous findings, we hypothesize that carriers of the APOE ε4 allele have increased risk for poorer cognitive function, while carriers of the ε2 allele are protected against cognitive decline.
2. Method
2.1. Study Population
The LLFS is a multicenter longitudinal study for healthy aging and familial longevity that recruited 5,086 participants from three sites in the United States (Boston, New York, Pittsburgh) and one site in Denmark. The recruitment process and inclusion criteria for this study have been described[13] and were based on a metric of familial longevity that was calculated from the aggregated survival probabilities of family members.[14] The study recruited spouses of members of long-lived families as referents. The participants completed two in-person visits, where their physical and cognitive functions were assessed through questionnaires, performance measures, and neuropsychological tests. Approximately 4,700 participants provided blood samples for genotyping, and APOE alleles were determined from the SNPs rs7412 and rs429358 that were genotyped using real time PCR. APOE alleles were defined as ε2: rs7412=T; rs429358=T, ε3: rs7412=C; rs429358=T, ε4: rs7412=C; rs429358=C. All subjects provided informed consent and data are available via dbGaP (dbGaP Study Accession: phs000397.v1.p1).
2.2. Cognitive Tests
Six neuropsychological tests were the main outcomes in our analyses. These tests include Verbal Fluency (category fluency for animals) to assess semantic memory and generativity; Digit Symbol Substitution Test (DSST) from the Wechsler Adult Intelligence Test (WAIS-R, 5) for processing speed; Digit Span forward and backward to measure working memory and attention; and Logical Memory (immediate and delayed recall) from the Wechsler Memory Scale – Revised (WMS-R, 4) to assess attention and episodic memory. The Mini-Mental State Examination (MMSE) was also administered but was not included in the analysis because of low variability.
2.3. Statistical Analysis
Participants were divided into three genotype groups defined as:
“APOE2 group”: carriers of the APOE genotypes ε2ε2 or ε2ε3;
“APOE3 group”: carriers of the genotype ε3ε3;
“APOE4 group”: carriers of the genotypes ε3ε4 or ε4ε4.
We used APOE3 as reference group. We summarized participants’ characteristics using mean and standard deviation. We compared participants’ characteristics of the APOE2 and APOE4 groups to the APOE3 group using t-tests or χ2 tests. We analyzed the effect of ε2 and ε4 in two separate analyses using additive genetic models. To test for association between APOE alleles and each of the neuropsychological tests, we used Bayesian hierarchical modelling of the longitudinal values of each test score as a function of age at enrollment, follow-up time, gender, education, field center, birth cohort indicator (≤ 1935, or > 1935), and the number of copies of ε2 or ε4 alleles. We started with a model with all main effects and interactions between the APOE variable, gender, education and age at enrollment and follow-up time, and used a backward model selection algorithm to identify the significant interactions and main effects. All analyses were conducted in R3.5 and the Bayesian analysis was conducted using the rjags package. Full details of the model specification, the algorithm for model selection and an example of the full model in rjags are in the supplemental material. The LLFS data used in this analysis was frozen by June 2018.
3. Results
Out of 5,086 LLFS participants we excluded 22 participants with missing sociodemographic data, 387 participants with missing APOE genotype, and 90 participants with APOE genotype ε2ε4. Tables 1 and 2 summarize demographic characteristics and cognitive test scores of the remaining 4,587 participants (1,785 in the older generation and 2,802 in the younger generation, Supplementary Tables 1 and 2 show similar information with breakdown of each APOE genotype). The APOE3 group (n=3,038) was the most prevalent and was used as the referent group. Age at enrollment ranged from 71 to 110 years among the older generation (mean=88.3 years; SD:7.71), and from 25 to 73 years among the younger generation (mean=59.7 years; SD:7.10). In the older generation, the APOE2 group was older (89.4 years vs. 88.3 years, p=0.02), while the APOE4 group was younger (86.8 years vs. 88.3 years, p<0.004) than the APOE3 group at enrollment. At visit 2, there were no other significant differences in age, sex, education, percent deceased among genotype groups. In the younger generation, there were no significant differences in the demographic characteristics among the genotype groups. At enrollment, the APOE4 group had significantly lower DSST score (50.4 vs. 51.7, p=0.03) and lower Digit Span – Backward score (6.6 vs. 6.9, p=0.02) than the APOE3 group. At visit 2, the APOE4 group had lower DSST score than the APOE3 group (47.4 vs. 48.9, p=0.02). There were no significant differences in the cognitive test scores comparing the APOE2 group to the APOE3 in both generations at either visit.
Table 1.
Demographic characteristics and test scores of 1,785 older generation (born in or before 1935) LLFS participants.
| APOE2(ε2ε2, ε2ε3) | APOE3(ε3ε3) | APOE4(ε3ε4, ε4ε4) | p-value (t-test, comparing APOE2 and APOE3) | p-value (t-test, comparing APOE4 and APOE3) | |
|---|---|---|---|---|---|
| N | 314 | 1239 | 232 | ||
| Age at Enrollment, mean(SD), years | 89.4(7.9) | 88.3(7.7) | 86.8(7.4) | 0.02 | 0.00 |
| Age at visit 2, mean(SD), years | 90.6(7) | 91.1(6.8) | 90.1(6.8) | 0.54 | 0.25 |
| Gender, male(%) | 152(48.4%) | 555(44.8%) | 118(50.9%) | 0.25 | 0.09 |
| Education, college and above(%) | 83(26.4%) | 353(28.5%) | 70(30.2%) | 0.46 | 0.61 |
| Deceased at follow up (%)a | 220(70.1%) | 816(65.9%) | 158(68.1%) | 0.15 | 0.50 |
| Test Scores at baseline (SD) | |||||
| MMSE | 25.7(4.3) | 25.8(4.2) | 25.9(3.9) | 0.58 | 0.75 |
| Animal Fluency | 14.4(5.5) | 14.8(5.4) | 15.3(5.7) | 0.27 | 0.22 |
| DSST | 30.1(13.8) | 30.8(12.6) | 30.5(12.2) | 0.47 | 0.70 |
| Digits Forward | 7.4(2.2) | 7.6(2.2) | 7.6(2.2) | 0.18 | 0.98 |
| Digits Backward | 5.3(2.2) | 5.5(2.1) | 5.4(2.1) | 0.15 | 0.30 |
| Logical Memory-Immediate | 8.7(4.3) | 8.5(4.7) | 8.8(4.8) | 0.57 | 0.38 |
| Logical Memory-Delayed | 6.4(4.5) | 6.5(4.8) | 6.8(4.8) | 0.66 | 0.46 |
| Test Scores at follow-up (SD) | |||||
| MMSE | 26(5.2) | 26.2(4) | 25.1(6) | 0.66 | 0.15 |
| Animal Fluency | 15.2(6.2) | 15(5.8) | 15.6(6) | 0.82 | 0.53 |
| DSST | 32.5(14.4) | 30.3(12.3) | 30.9(14.7) | 0.20 | 0.79 |
| Digits Forward | 6.7(2.1) | 6.8(2.1) | 6.7(2.4) | 0.65 | 0.73 |
| Digits Backward | 5.5(1.9) | 5.3(2) | 5.4(2.1) | 0.49 | 0.76 |
| Logical Memory-Immediate | 10.1(4.4) | 9.8(4.9) | 10(5) | 0.53 | 0.73 |
| Logical Memory-Delayed | 8(5.1) | 7.6(5.3) | 7.6(5.1) | 0.52 | 1.00 |
Notes: MMSE = Mini-Mental State Examination, DSST = Digit Span Substitution Test
Follow up from Visit 1 through April 2018
Table 2.
Demographic characteristics and test scores of 2,802 younger generation (born after 1935) LLFS participants.
| APOE2(ε2ε2, ε2ε3) | APOE3(ε3ε3) | APOE4(ε3ε4, ε4ε4) | p-value (t-test, comparing APOE2 and APOE3) | p-value (t-test, comparing APOE4 and APOE3) | |
|---|---|---|---|---|---|
| N | 419 | 1799 | 584 | ||
| Age at Enrollment, mean(SD), years | 59.1(7.3) | 59.8(7.2) | 60.1(6.7) | 0.08 | 0.30 |
| Age at visit 2, mean(SD), years | 67.1(7) | 67.8(6.9) | 68.2(6.6) | 0.13 | 0.29 |
| Gender, male(%) | 199(47.5%) | 774(43%) | 266(45.5%) | 0.10 | 0.29 |
| Education, college and above(%) | 229(54.7%) | 1027(57.1%) | 310(53.1%) | 0.37 | 0.09 |
| Deceased at follow up (%)a | 18(4.3%) | 95(5.3%) | 31(5.3%) | 0.38 | 0.98 |
| Test Scores at baseline (SD) | |||||
| MMSE | 28.9(2.5) | 29(1.8) | 28.8(2.6) | 0.52 | 0.18 |
| Animal Fluency | 22.3(5.9) | 22.5(5.9) | 22.4(5.4) | 0.43 | 0.70 |
| DSST | 52(12.4) | 51.7(12.2) | 50.4(11.8) | 0.59 | 0.03 |
| Digits Forward | 8.5(2.2) | 8.6(2.2) | 8.5(2.2) | 0.29 | 0.08 |
| Digits Backward | 6.8(2.3) | 6.9(2.4) | 6.6(2.2) | 0.81 | 0.02 |
| Logical Memory-Immediate | 13.3(3.9) | 13.4(3.9) | 13.2(4.1) | 0.57 | 0.29 |
| Logical Memory-Delayed | 11.9(4.2) | 12.1(4.2) | 11.8(4.5) | 0.49 | 0.24 |
| Test Scores at follow-up (SD) | |||||
| MMSE | 29.1(2) | 29.1(1.6) | 29.2(1.4) | 0.92 | 0.58 |
| Animal Fluency | 22.9(5.7) | 22.8(6.1) | 22.8(5.9) | 0.74 | 0.94 |
| DSST | 48.9(11.7) | 48.9(12) | 47.4(11.4) | 0.96 | 0.02 |
| Digits Forward | 7.7(2.7) | 7.7(2.5) | 7.6(2.5) | 0.85 | 0.33 |
| Digits Backward | 6.6(2.1) | 6.6(2.2) | 6.4(2.1) | 0.88 | 0.08 |
| Logical Memory-Immediate | 14.1(3.6) | 14.3(3.8) | 14.1(4.2) | 0.49 | 0.63 |
| Logical Memory-Delayed | 12.7(4.2) | 12.9(4.2) | 12.7(4.3) | 0.52 | 0.64 |
Notes: MMSE = Mini-Mental State Examination, DSST = Digit Span Substitution Test
Follow up from visit 1 through April 2018
Tables 3 & 4 and Supplementary Tables 3, 4, 5 & 6 show parameter estimates generated using the model selected with the credible interval algorithm. The overall conclusion is that there was no significant effect of the ε2 allele on either the baseline assessment or the rate of change over follow-up time on any of the neuropsychological tests, while the ε4 allele had a negative effect on the two logical memory tests at baseline but had no effect on their rate of decline. We describe below the details of the analysis of each test.
Table 3.
Parameter estimates of Animal Fluency, DSST, Digit Span tests by generation, without APOE genotype stratification.
| Animal Fluency | DSST | Digits Forward | Digits Backward | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Older Generation | Younger Generation | Older Generation | Younger Generation | Older Generation | Younger Generation | Older Generation | Younger Generation | ||
| Main Effects | age |
−0.17 (−0.18,−0.15) |
0.01 (−0.03,0.06) |
−0.67 (−0.71,−0.64) |
−0.4 (−0.48,−0.31) |
−0.03 (−0.03,−0.02) |
0.02 (0.004,0.04) |
−0.03 (−0.04,−0.02) |
0.01 (−0.003,0.03) |
| dage |
−0.06 (−0.09,−0.03) |
0.18 (0.11,0.25) |
−0.48 (−0.54,−0.43) |
−0.06 (−0.2,0.07) |
−0.12 (−0.13,−0.11) |
−0.07 (−0.1,−0.05) |
−0.04 (−0.05,−0.03) |
0.01 (−0.02,0.04) |
|
| sex, male | −0.06 (−0.33,0.21) |
−4.52 (−5.05,−4) |
0.13 (0.04,0.23) |
−0.08 (−0.18,0.02) |
|||||
| educ |
0.45 (0.4,0.49) |
0.92 (0.82,1.01) |
0.12 (0.1,0.14) |
0.14 (0.13,0.16) |
|||||
| ind1935 | 0.2 (−0.1,0.53) |
−0.41 (−1.03,0.2) |
−0.04 (−0.16,0.07) |
0 (−0.13,0.12) |
|||||
| Interactions | educ*age |
−0.01 (−0.01,−0.005) |
−0.01 (−0.01,−0.001) |
−0.001 (−0.002,−0.001) |
|||||
| educ*dage |
0.003 (0.0003,0.01) |
||||||||
| sex*age |
0.04 (0.02,0.05) |
0.12 (0.08,0.15) |
|||||||
| sex*dage |
0.11 (0.01,0.22) |
||||||||
Notes: Parameter estimates in this table are generated using data from all subjects, and the generation specific effects were estimated using the interaction effects in Supplement Tables 1 and 2. For example, the effect of age on the animal fluency score in the younger generation was estimated as the sum of the effects of age and 1935*age. Parameter estimates for ind1935 was only under the younger generation column because ind1935 was defined to have value 1 for younger generation and 0 for older generation.
Table 4.
Parameter estimates of Logical Memory tests – ε4 allele carriers vs. non-ε4 allele carriers.
| Logical Memory- Immediate | Logical Memory-Delayed | ||||
|---|---|---|---|---|---|
| Older Generation | Younger Generation | Older Generation | Younger Generation | ||
| Main Effects | age | −0.1(−0.12,−0.09) | 0.09(0.06,0.12) | −0.12(−0.13,−0.1) | 0.07(0.04,0.11) |
| dage | 0.06(0.03,0.08) | 0.2(0.14,0.25) | 0.04(0.01,0.06) | 0.19(0.13,0.25) | |
| sex, male | −0.87(−1.07,−0.67) | −1.08(−1.29,−0.87) | |||
| educ | 0.35(0.31,0.38) | 0.35(0.31,0.39) | |||
| APOE4 | −0.31(−0.57,−0.05) | −0.37(−0.64,−0.1) | |||
| ind1935 | −0.4(−0.63,−0.17) | −0.2(−0.45,0.05) | |||
| Interactions | educ*age | −0.003(−0.01,−0.001) | −0.003(−0.005,−0.001) | ||
| educ*dage | −0.01(−0.02,−0.002) | ||||
| sex*age | 0.02(0.002,0.03) | 0.03(0.02,0.04) | |||
| sex*dage | |||||
Notes: Parameter estimates in this table are generated using data from all subjects, comparing ε4 carriers to non-ε4 carriers.
Animal Fluency.
Neither ε2 nor ε4 allele of APOE was associated with performance on animal fluency (Supplementary Table 3 & 4). Age at enrollment, follow-up time, and some of the interactions with generation, sex and education were significant (Supplementary Table 3), suggesting that the cross-sectional and longitudinal effects of age were different in the younger and older generations and were modified by sex and education. Table 3 describes the estimated age and follow-up effects by generation. Older age at enrollment was associated with a lower score (age effect = −0.17, 95%CI: −0.18, −0.15) and, for every year of follow-up, the score decreased by −0.06 points (95%CI: −0.09, −0.03) in the older generation while, in the younger generation, the effect of age at enrollment was not significant (age effect =0.01, 95%CI: −0.03, 0.06). The analyses also predicted a significant increase in score for each year of follow-up time in the younger generation (0.18, 95%CI: 0.11, 0.25) that could be caused by a practice effect among the younger participants. Higher education had a positive effect on the score but slightly diminished with older age at enrollment (educ*age interaction effect = −0.01, 95%CI: −0.01, −0.005). The age effect was smaller in males (sex*age interaction effect = 0.04, 95%CI: 0.02, 0.05).
DSST.
Only age at enrollment, gender and education were significantly associated with DSST score, while the effects of ε2 and ε4 alleles of APOE were not significant (Supplementary Tables 3 & 4). In the older generation, an older year of age at enrollment was associated with a decrease of 0.67 points (95%CI: −0.71, −0.64) on the DSST (Table 3). Follow-up time also had a negative effect on DSST (dage effect = −0.48, 95%CI: −0.54, −0.43). The negative effects of age at enrollment in the younger generation was smaller (−0.4, 95%CI: −0.48, −0.31), while the effect of follow-up time was not significant (−0.06, 95%CI: −0.20, 0.07). Higher education and female sex were associated with higher scores but the effect was reduced with older age at enrollment and longer follow up (educ*age interaction effect = −0.01, 95%CI: −0.01, −0.001; sex*age interaction effect = 0.12, 95%CI: 0.08, 0.15; sex*dage interaction 0.11, 96%CI 0.01,0.22).
Digit Span – Forward.
Neither ε2 nor ε4 allele of APOE was associated with this test (Supplementary Tables 3 & 4). Age at enrollment, follow-up time, gender and education were associated with the digit span forward score, in both the older and younger generations (Table 3). In the older generation, the score was expected to decrease by 0.03 points (95%CI: −0.03, −0.02) for every year of age at enrollment, and decrease by 0.12 points (95CI: −0.13, −0.11) for every year of follow-up time. In the younger generation, there was an estimated increase in forward span score as baseline age increased (0.02, 95%CI: 0.004, 0.04) and for each additional year in the follow-up time, the score decreased by −0.07 points (95%CI: −0.10, −0.05), thus suggesting a smaller rate of decline in the younger generation. Education was positively associated with the score (0.12 points, 95%CI: 0.10, 0.14) and the effect increased as follow-up time increased (educ*dage interaction effect 0.003, 95%CI: 0.0003, 0.01). Males tended to score higher by 0.13 points (95%CI: 0.04, 0.23) than females.
Digit Span – Backward.
Similar to the Digit Span forward test, APOE was not associated with the backward span test score (Supplementary Tables 3 & 4). Older age at enrollment and longer follow-up time were negatively associated with the score only in the older generation (age effect = −0.03, 95%CI: −0.04, −0.02; dage effect = −0.04, 95%CI: −0.05, −0.03, Table 3), and had no significant effect in the younger generation. Higher education was positively correlated with the score but the effect decreased with older age at enrollment (educ*age interaction effect = −0.001, 95%CI: −0.002, −0.001). We did not detect any gender difference in this test.
Logical Memory Recall Tests.
As shown in Table 4, the ε4 allele had a negative effect on the logical memory tests (immediate recall ε4 allele effect = −0.31, 95%CI: −0.57, −0.05; delayed recall ε4 allele effect = −0.37, 95%CI: −0.64, −0.10) compared to carriers of ε3 or ε2. These effects were not modified by any of the other variables. In the older generation, older age at enrollment was associated with lower scores of both tests (immediate recall age effect = −0.10, 95%CI: −0.12, −0.09; delayed recall age effect = −0.12, 95%CI: −0.13, −0.10). However, consistent with a possible practice effect, follow-up time had positive effects on both tests (immediate recall dage effect = 0.06, 95%CI: 0.03, 0.08; delayed recall dage effect = 0.04, 95%CI: 0.01, 0.06).
The effects of age at baseline and follow-up time were different in the younger generation as indicated by the significant interactions of ind1935*age and ind1935*dage (Supplementary Table 6). In the younger generation, both older baseline age and follow-up time were associated with higher scores of both tests. In the immediate recall test, the analysis estimated an increase of 0.09 points (95%CI: 0.06, 0.12, Table 4) with every additional year increase in baseline age, and an increase of 0.20 points (95%CI: 0.14, 0.25) with every year of follow-up time. Similarly, in the delayed recall test, for every one-year increase in baseline age the score increased by 0.07 points (95%CI: 0.04, 0.11), and by 0.19 points (95%CI: 0.13, 0.25) for every year of follow-up time. The age affects were modified by sex and education. Male sex reduced the effect of age at enrollment (immediate recall sex*age interaction effect = 0.02, 95%CI: 0.002, 0.03; delayed recall sex*age interaction effect = 0.03, 95%CI: 0.02, 0.04, Table 4). The advantage of higher education diminished slightly as baseline age increased (immediate recall educ*age interaction effect = −0.003, 95%CI: −0.01, −0.001; delayed recall educ*age interaction effect = −0.003, 95%CI: −0.005, −0.001), and also diminished as follow-up time increased in immediate recall (educ*dage interaction effect = −0.01, 95%CI: −0.02, −0.002).
4. Discussion
We conducted a comprehensive analysis of the effect of APOE alleles on age-related change in various different cognitive domains. The analyses confirm the negative effect of ε4 allele on episodic memory assessed by immediate and delayed recall on logical memory: compared to the ε2 and ε3 alleles, carriers of one or more ε4 alleles scored lower in both tests although they did not exhibit a faster rate of decline. We did not detect any significantly protective effect of the ε2 allele compared to the ε3 allele.
There is substantial literature linking the ε4 allele of APOE to poorer cognition at older age, faster rate of cognitive decline and higher risk for Alzheimer’s disease. The review by O’Donoghue and colleagues[12] identified 12 cross-sectional and 15 longitudinal studies that reported a significant negative association between ε4 and episodic memory. Our findings show poorer memory in carriers of ε4 compared to other genotypes, but did not detect a significantly faster rate of decline. A study of centenarians (Xiang el al. 2020 preprint) also reported similar adverse effect of the ε4 allele on a memory cognitive test score using Beta regression modeling. Rawle et al. showed that ε4 homozygous carriers have a faster rate of cognitive decline compared to other genotype carriers[15] in a study of comparable sample size. LLFS is a study of healthy aging and longevity with approximately 50% fewer ε4 carriers compared to the study in Rawle et al. and the smaller number of ε4 carriers may have reduced the power of our study. Alternatively, the lack of a difference in the rate of decline in the older generation may be due to a survivor bias. A review paper by Smith et al. [16] had suggested that young ε4 carriers presented better mental performance compared to ε3ε3 carriers. A study by Caselli et al. [17] aiming to address the transition from cognitive advantage to cognitive deficit of the ε4 carriers showed the longitudinal decline began before age 60 and had faster acceleration compared to ε3ε3 carriers. In LLFS with an average age of 87 years, ε4 carriers may be survivors with increased resilience to the risk conferred by the ε4 allele and therefore are not showing the accelerated declines seen in other samples. In addition, Tao et al. [18] suggested that the adverse effect of the allele might be activated by chronic low-grade inflammation. We conducted an additional analysis adjusting for baseline C-Reactive Protein (CRP) level in the two Logical Memory Recall tests and the results (Supplementary Table 7) showed that the adverse effect of the allele is mildly affected in the Logical Memory Immediate Recall test while having no effect on the Logical Memory Delayed Recall test.
Our findings on the effect of ε4 and cognition are consistent with other analyses conducted in the LLFS but expand the set of results to longitudinal assessments of cognitive function. For example, Kulminski and colleagues showed that the ε4 allele increases the lifetime risk of neurological disorders including dementia and Alzheimer’s disease by 98% in both LLFS men and women.[19]. Barral et al.[20] defined exceptional cognitive performance using predominantly immediate and delayed memory, and showed that being in an exceptional cognitive performance family was significantly associated with being a non-carrier of the APOE ε4 allele.
The fact that Logical Memory was the only cognitive test affected by ε4 is consistent with early episodic memory changes in Alzheimer’s disease. Reduced verbal fluency has also been posited as a marker of early Alzheimer’s disease that affects semantic memory.[21] In our sample there was no relationship between animal fluency and ε4. It is possible that the difference between phonemic and semantic fluency, that is poorer semantic fluency relative to phonemic fluency, is more indicative of early Alzheimer’s disease[22] than semantic fluency in isolation.
Our analyses did not detect any significant protective effect of ε2 on cognition although the data set included 314 carriers of one or more ε2 alleles. The results regarding the effect of the ε2 allele on cognition have been mixed. The Religious Orders Study found that ε2 carriers had an annual increase in episodic memory score while the ε4 subgroup decreased more rapidly compared to ε3 carriers.[23] The study also found that ε4 carriers declined faster than ε3 in semantic memory and processing speed, but not in working memory. Four additional studies suggested the ε2 allele has a protective effect and is associated with reduced odds for developing cognitive impairment.[11, 24–26]. In contrast, the ε2 allele was not significantly associated with cognitive decline in[5]. A study of 18,000 people by Marioni et al.[27] did not detect any relationship between the ε2 allele and learning and episodic memory (Logical Memory), processing speed (DSST) and a fluency test. A clustering analysis of the LLFS cohort based on pattern of cognitive change by Sebastiani et al.[28] detected a cluster of slowest changers of DSST that was enriched for ε2 carriers, and a cluster of fastest changers that was enriched for ε4, suggesting that the effect of ε2 may be modified by other factors.
Our study has some limitations. Only 55% of LLLFS participants completed a second visit, though the dropout rates are comparable in each group (E2=46%, E3=45%, E4=40%). Supplementary Figure 2 shows a forest plot of the standardized mean differences of the neuropsychological tests among the three genotype groups at both visits and suggests that differences between participants who completed both visits and those that completed one visit should not effect the results. Secondly, having at most two time points restricts the analysis to a linear model rather than any nonlinear models.
In conclusion, APOE ε4 allele was confirmed as a risk factor for episodic memory in older adults, while APOE ε2 allele was not significantly associated with any of the cognitive tests, and neither allele appear to modify the rate of cognitive decline.
Supplementary Material
Acknowledgement
All subjects provided informed consent and data are available via dbGaP (dbGaP Study Accession: phs000397.v1.p1).
Fundings: This work was supported by the National Institute on Aging (NIA cooperative agreements U19-AG023122, 5U01AG023744, 5U01AG023755, 5U01AG023749, 5U01AG023746, 5U01AG023712, R21AG056630, R01AG061844, and K01AG057798 to SA). The funders had no role in drafting this manuscript.
Footnotes
Conflict of Interest: The authors have no conflicts of interest or financial disclosures.
References
- [1].Gottesman RF, Albert MS, Alonso A, Coker LH, Coresh J, Davis SM, Deal JA, McKhann GM, Mosley TH, Sharrett AR, Schneider ALC, Windham BG, Wruck LM, Knopman DS (2017) Associations Between Midlife Vascular Risk Factors and 25-Year Incident Dementia in the Atherosclerosis Risk in Communities (ARIC) Cohort. JAMA Neurol 74, 1246–1254. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [2].Krell-Roesch J, Vemuri P, Pink A, Roberts RO, Stokin GB, Mielke MM, Christianson TJ, Knopman DS, Petersen RC, Kremers WK, Geda YE (2017) Association Between Mentally Stimulating Activities in Late Life and the Outcome of Incident Mild Cognitive Impairment, With an Analysis of the APOE epsilon4 Genotype. JAMA Neurol 74, 332–338. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [3].Fan J, Tao W, Li X, Li H, Zhang J, Wei D, Chen Y, Zhang Z (2019) The Contribution of Genetic Factors to Cognitive Impairment and Dementia: Apolipoprotein E Gene, Gene Interactions, and Polygenic Risk. Int J Mol Sci 20. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [4].Liu CC, Liu CC, Kanekiyo T, Xu H, Bu G (2013) Apolipoprotein E and Alzheimer disease: risk, mechanisms and therapy. Nat Rev Neurol 9, 106–118. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [5].Henderson AS, Easteal S, Jorm AF, Mackinnon AJ, Korten AE, Christensen H, Croft L, Jacomb PA (1995) Apolipoprotein E allele epsilon 4, dementia, and cognitive decline in a population sample. Lancet 346, 1387–1390. [DOI] [PubMed] [Google Scholar]
- [6].Raber J, Huang Y, Ashford JW (2004) ApoE genotype accounts for the vast majority of AD risk and AD pathology. Neurobiol Aging 25, 641–650. [DOI] [PubMed] [Google Scholar]
- [7].Sebastiani P, Gurinovich A, Nygaard M, Sasaki T, Sweigart B, Bae H, Andersen SL, Villa F, Atzmon G, Christensen K, Arai Y, Barzilai N, Puca A, Christiansen L, Hirose N, Perls TT (2019) APOE Alleles and Extreme Human Longevity. J Gerontol A Biol Sci Med Sci 74, 44–51. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [8].Wolters FJ, Yang Q, Biggs ML, Jakobsdottir J, Li S, Evans DS, Bis JC, Harris TB, Vasan RS, Zilhao NR, Ghanbari M, Ikram MA, Launer L, Psaty BM, Tranah GJ, Kulminski AM, Gudnason V, Seshadri S, investigators EC (2019) The impact of APOE genotype on survival: Results of 38,537 participants from six population-based cohorts (E2-CHARGE). PLoS One 14, e0219668. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [9].Blair CK, Folsom AR, Knopman DS, Bray MS, Mosley TH, Boerwinkle E, Atherosclerosis Risk in Communities Study I (2005) APOE genotype and cognitive decline in a middle-aged cohort. Neurology 64, 268–276. [DOI] [PubMed] [Google Scholar]
- [10].Caselli RJ, Reiman EM, Osborne D, Hentz JG, Baxter LC, Hernandez JL, Alexander GG (2004) Longitudinal changes in cognition and behavior in asymptomatic carriers of the APOE e4 allele. Neurology 62, 1990–1995. [DOI] [PubMed] [Google Scholar]
- [11].Kim YJ, Seo SW, Park SB, Yang JJ, Lee JS, Lee J, Jang YK, Kim ST, Lee KH, Lee JM, Lee JH, Kim JS, Na DL, Kim HJ (2017) Protective effects of APOE e2 against disease progression in subcortical vascular mild cognitive impairment patients: A three-year longitudinal study. Sci Rep 7, 1910. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [12].O’Donoghue MC, Murphy SE, Zamboni G, Nobre AC, Mackay CE (2018) APOE genotype and cognition in healthy individuals at risk of Alzheimer’s disease: A review. Cortex 104, 103–123. [DOI] [PubMed] [Google Scholar]
- [13].Newman AB, Glynn NW, Taylor CA, Sebastiani P, Perls TT, Mayeux R, Christensen K, Zmuda JM, Barral S, Lee JH, Simonsick EM, Walston JD, Yashin AI, Hadley E (2011) Health and function of participants in the Long Life Family Study: A comparison with other cohorts. Aging (Albany NY) 3, 63–76. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [14].Sebastiani P, Hadley EC, Province M, Christensen K, Rossi W, Perls TT, Ash AS (2009) A family longevity selection score: ranking sibships by their longevity, size, and availability for study. Am J Epidemiol 170, 1555–1562. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [15].Rawle MJ, Davis D, Bendayan R, Wong A, Kuh D, Richards M (2018) Apolipoprotein-E (Apoe) epsilon4 and cognitive decline over the adult life course. Transl Psychiatry 8, 18. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [16].Smith CJ, Ashford JW, Perfetti TA (2019) Putative Survival Advantages in Young Apolipoprotein ɛ4 Carriers are Associated with Increased Neural Stress. Journal of Alzheimer’s disease : JAD 68, 885–923. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [17].Caselli RJ, Dueck AC, Osborne D, Sabbagh MN, Connor DJ, Ahern GL, Baxter LC, Rapcsak SZ, Shi J, Woodruff BK, Locke DE, Snyder CH, Alexander GE, Rademakers R, Reiman EM (2009) Longitudinal modeling of age-related memory decline and the APOE epsilon4 effect. N Engl J Med 361, 255–263. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [18].Tao Q, Ang TFA, DeCarli C, Auerbach SH, Devine S, Stein TD, Zhang X, Massaro J, Au R, Qiu WQ (2018) Association of Chronic Low-grade Inflammation With Risk of Alzheimer Disease in ApoE4 Carriers. JAMA Netw Open 1, e183597. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [19].Kulminski AM, Arbeev KG, Culminskaya I, Ukraintseva SV, Stallard E, Province MA, Yashin AI (2015) Trade-offs in the effects of the apolipoprotein E polymorphism on risks of diseases of the heart, cancer, and neurodegenerative disorders: insights on mechanisms from the Long Life Family Study. Rejuvenation Res 18, 128–135. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [20].Barral S, Singh J, Fagan E, Cosentino S, Andersen-Toomey SL, Wojczynski MK, Feitosa M, Kammerer CM, Schupf N, Long Life Family S(2017) Age-Related Biomarkers in LLFS Families With Exceptional Cognitive Abilities. J Gerontol A Biol Sci Med Sci 72, 1683–1688. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [21].Gomez RG, White DA (2006) Using verbal fluency to detect very mild dementia of the Alzheimer type. Arch Clin Neuropsychol 21, 771–775. [DOI] [PubMed] [Google Scholar]
- [22].Henry JD, Crawford JR, Phillips LH (2004) Verbal fluency performance in dementia of the Alzheimer’s type: a meta-analysis. Neuropsychologia 42, 1212–1222. [DOI] [PubMed] [Google Scholar]
- [23].Wilson RS, Bienias JL, Berry-Kravis E, Evans DA, Bennett DA (2002) The apolipoprotein E epsilon 2 allele and decline in episodic memory. J Neurol Neurosurg Psychiatry 73, 672–677. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [24].Shinohara M, Kanekiyo T, Yang L, Linthicum D, Shinohara M, Fu Y, Price L, Frisch-Daiello JL, Han X, Fryer JD, Bu G (2016) APOE2 eases cognitive decline during Aging: Clinical and preclinical evaluations. Ann Neurol 79, 758–774. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [25].Hyman BT, Gomez-Isla T, Briggs M, Chung H, Nichols S, Kohout F, Wallace R (1996) Apolipoprotein E and cognitive change in an elderly population. Ann Neurol 40, 55–66. [DOI] [PubMed] [Google Scholar]
- [26].Helkala EL, Koivisto K, Hanninen T, Vanhanen M, Kervinen K, Kuusisto J, Mykkanen L, Kesaniemi YA, Laakso M, Riekkinen P Sr. (1996) Memory functions in human subjects with different apolipoprotein E phenotypes during a 3-year population-based follow-up study. Neurosci Lett 204, 177–180. [DOI] [PubMed] [Google Scholar]
- [27].Marioni RE, Campbell A, Scotland G, Hayward C, Porteous DJ, Deary IJ (2016) Differential effects of the APOE e4 allele on different domains of cognitive ability across the life-course. Eur J Hum Genet 24, 919–923. [DOI] [PMC free article] [PubMed] [Google Scholar]
- [28].Sebastiani P AS, Sweigart B, Du M, Cosentino S, …, Perls TT, et al. (Manuscript submitted for publication.) Analysis of Longitudinal Changes of Neuropsychological Tests in Participants of the long Life Family Study. Journal of Gerontology. [Google Scholar]
- [29].Sun F, Sebastiani P, Schupf N, Bae H, Andersen SL, McIntosh A, Abel H, Elo IT, Perls TT (2015) Extended maternal age at birth of last child and women’s longevity in the Long Life Family Study. Menopause 22, 26–31. [DOI] [PMC free article] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.