Original Article
Objective
s:The aim of this study is to examine the impact of the apolipoprotein E (APOE) polymorphism on the risk of Alzheimer's disease (AD) in Koreans.
Materials and Methods:We determined the APOE genotype of 303 probable AD patients and 1,052 non-demented normal controls, and compared their genotypic and allelic frequencies. Then, we determined the odds ratios for AD of the APOE polymorphism in the subjects, stratified by age and gender.
Results
:The APOE ε4 allele was more prevalent in the AD patients than in the controls (P<0.0001). The APOE ε4 allele increased the risk of AD in a dose-dependent manner; the odds ratios for AD of the APOE ε4-heterozygous and the APOE ε4-homozygous subjects were 3.9 (95%CI=2.8-5.3) and 15.6 (95% CI=6.6-36.7), respectively. The APOE ε4-conferred AD risk was also dependent on the age and gender of the subjects; the odds ratios for AD were highest in the subjects aged 75-79 years and higher in the women than in the men.
Conclusions
:The APOE ε4 allele is a significant genetic risk factor for AD and confers the AD risk in a dose-, age-and gender-dependent manner in Koreans.
Correspondence: Ki Woong Kim, MD, PhD, Department of Neuropsychiatry, Seoul National University Bundang Hospital, 300 Gumi-dong, Bundang-gu, Seongnam, Gyeonggi-do 463-707, Korea
Tel: +82-31-787-7432, Fax: +82-31-787-4058, E-mail: psykim@kornet.net
The apolipoprotein E (APOE) ε4 allele has been established as an important genetic susceptibility factor for Alzheimer's disease (AD) in numerous ethnic populations1. In Koreans, we previously reported that the APOE ε4 allele increased the risk of AD in a dose-dependent manner; with the odds ratios for AD being 2.7 (95%CI=1. 6-4.5) and 17.4 (95%CI=2. 0-147.3) in the APOE ε4-heterozygous subjects and the APOE ε4-homozygous subjects, respectively. Thereafter, the APOE ε4-AD association was consistently replicated in a series of follow-up studies in Koreans2,3. The APOE ε4-conferred AD risk is also dependent on the gender and age of the subjects. According to a meta-analysis by Farrer et al1., the APOE ε4-conferred AD risk was higher in women than in men at most ages and across all genotypes, and increased steadily between the ages of 40 and 60 years and thereafter declined with increasing age. In our previous study of the Korean population, this gender-and age-dependency of the APOE ε4-conferred AD risk was also observed4. The odds ratios for AD in the women (OR=4.4, 95% CI=2.4-8.0) were much higher than those in the men (OR=0.6, 95%CI=0.1-2.2). In fact, the odds ratios for AD in the men did not reach statistical significance. The odds ratios for ADincreased steadily between ages 50 and 69 years and declined with age thereafter to a statistically insignificant level.
However, the gender-and age-dependency of the APOE ε4-conferred AD risk in Koreans has not been replicated yet and required further investigation in several respects. First, since the number of male AD patients was less than 30 in our previous study, we cannot conclude that the APOE ε4 allele does not confer a risk for AD in Korean men. In other ethnic groups, the APOE ε4 allele was found to be a significant genetic risk factor for AD in men, although its odds ratios for AD in men were lower than those in women1. Second, the difference in the gender-dimorphic AD risk of the APOE ε4 allele across the APOE genotypes has not been examined yet in Koreans. In the meta-analysis by Farrer et al1., the gender-dimorphic character of the AD risk was reported to be highest in those subjects with the APOE ε3/ε4 carriers. Third, in our previous study, the number of AD patients was not sufficient to set the age stratum finely, and each age strata had a cell containing 30 subjects or less4. The APOE ε4-conferred AD risk in the subjects aged 70 to 79 years is particularly questionable. Although the APOE ε4 allele was not found to confer a risk of AD in the subjects aged 70 or over in our previous study, the APOE ε4-AD association became insignificant at the age of 80 years or over in most previous studies5,6,7 and remained significant until the age of 95 years in the meta-analysis conducted by Farrer et al1.
Therefore, in the present study, we investigated again the impact of the APOE ε4 allele on the risk of AD in an extended Korean sample and the influence of age and gender on the APOE ε4-conferred AD risk.
Materials and Methods
The AD patients were enrolled from three dementia clinics (Seoul National University Hospital, Seoul National University Bundang Hospital, and Gyeonggi Provincial Hospital for the Elderly) and three communities in Seoul (Nowon-gu, Seocho-gu, and Kwanak-gu). The non-demented control subjects were community-dwelling elderly individuals randomly selected from three districts in Seoul (Nowongu, Seochogu, and Kwanakgu) who were functioning independently in the community and did not have any cognitive impairment that altered their daily activities. All of the subjects were unrelated Koreans.
We administered the Korean version of the CERAD Clinical Assessment Battery8 to all of the subjects. Then, a consensus committee meeting was held to establish a diagnosis for each subject; diagnoses for dementia were made according to the DSM-IV criteria9 and diagnoses for AD were made according to the NINCDS-ADRDA criteria10. Those subjects who were diagnosed as cognitively normal at the consensus committee meeting and who were given a modified Hachinski Ischemic Score11 of less than ε4 were included in the normal control group. Genomic DNA was extracted from venous blood, and APOE genotyping was done according to the method described by Kimet al4. We estimated the allele and genotype frequencies of APOE by counting the alleles and genotypes and calculating the sample proportions. The Hardy-Weinberg equilibrium was tested by means of the chi square test. Comparisons of the allele frequencies and genotype frequencies of APOE were made using the
X2
analysis and Fisher's exact test when appropriate. Logistic regression models adjusting for age, sex and education were used to calculate the odds ratios for AD. Based on these odds ratios, the attributable risks of the APOE ε4 allele for AD were estimated. The confidence interval of the attributable risk was estimated from the variance of the odds ratios using first-order Taylor series approximations12. Then, we quantified the impact of the APOE ε4 allele on the risk of AD in five different age-strata (under 65 years, 65 to 69 years, 70 to 74 years, 75 to 79 years, and 80 years or over) and two gender-strata (women and men). The level of education was computed as a categorical variable; 0 years, 1 to 3 years, 4 to 6 years, 7 to 9 years, 10 to 12 years, and 13 years or over.
Results
Finally, 303 probable AD patients and 1052 normal controls were enrolled in the present study. The AD group (72.4±8.7 years old) was slightly older than the control group (69.4±6.5 years old; P<0.0001 by student t-test). However, the gender distributions of the AD group (%of female=70.0) and control group (%of female=74.1) were not significantly different (P>0.1 by Pearson
X2
test). The mean ages-at-onset were 58.3±4.5 years old and 73.9±5.6 years old in the early-onset AD patients and late-onset AD patients, respectively. The distributions of the APOE genotypes and alleles of the AD group and the control group are presented in Table 1. The distributions of the APOE polymorphisms were in Hardy-Weinberg equilibrium in the AD group, as well as in the control group (P>0.05). The AOPE ε4 allele frequency of the AD group was significantly higher than that of the control
group(X2=116.4, d.f.=1, P<0.0001 for AD;
X2=47.5, d.f.=1, P<0.0001 for EOAD;
X2=95.7, d.f.=1, P<0.0001 for LOAD) at the expense of the APOE ε2
(X2=8.6, d.f.=1, P<0.01) and 3
(X2=55.9, d.f.=1, P<0.0001) alleles. In contrast to our previous report, this was true in the men as well as in the women. The disequilibrium of the APOE polymorphisms between the AD group and control group became insignificant in the subjects aged 80 or over, which is consistent with the meta-analysis conducted by Farrer et al1.
As shown in Table 2, the APOE ε4 allele increased the risk of AD in a dose-dependent manner. The odds ratios for AD of the APOE ε4 homozygotes were about four times higher than those of the APOE ε4 heterozygotes. Among the APOE ε4 heterozygotes, only the APOE ε3/ε4 genotype conferred a significant risk for AD. Although the APOE ε2/ε4 genotype has a copy of the APOE ε4 allele, it was not associated with an increased risk of AD. The odds ratios for AD in relation to the presence of the APOE ε4 allele were comparable to those in Caucasians1. The impact of the APOE ε4 allele on the AD risk was significant in the men, as well as in the women. However, the odds ratios for AD in the men were still lower than those in the women, and this difference was most pronounced in the APOE ε3/ε4 carriers. As shown in Table 3, the odds ratios of the APOE ε4 allele for AD increased steadily between ages 45 and 79 years, before declining abruptly to an insignificant level with increasing age thereafter. The odds ratios for AD were highest at ages 75 to 79 years. At these ages, the risk of AD was increased about six times by the presence of the APOE ε4 allele, and about eight times by the presence of the APOE ε3/ε4 or the APOE ε4/ε4 genotype. In the oldest subjects, aged 80 years or over, the APOE ε4 allele did not confer a risk for AD. The age-dependency of the APOE ε4-conferred AD risk was most evident in the APOE ε3/ε4 carriers (Table 3). The age-dependent changes of the APOE ε4-conferred AD risk were evident in the women, whereas no such changes were observed in the men (Figure 1).
The estimated risk of AD attributable to the presence of the APOE ε4 allele in Koreans was 31%(95%CI=19%-43%). Even in the 75 to 79 years age group, at which the odds ratios for AD were at a peak, the attributable risk was only 48%(95%CI=4%-91%).
Discussion
The APOE ε4 allele is an established genetic risk factor for AD. It confers the risk for AD in almost all ethnic groups except African Americans1. In East Asians, i.e., Koreans, Chinese and Japanese, the APOE ε4 allele has been consistently replicated as a genetic risk factor for AD. However the impact of the APOE ε4 allele was not uniform across East Asians. The APOE ε4-conferred AD risk in the Japanese was found to be much higher than those in the Koreans and Chinese4,13,14,15,16. As mentioned in our previous report, this discrepancy in the impact of the APOE ε4 allele can be attributed to the differences in the characteristics of the subjects, especially their gender and age. Although the APOE ε4-conferred AD risk is known to be dependent on gender and age, regardless of the ethnicity of the subjects, most of the previous studies on the APOE ε4-AD association in East Asians, including Koreans, did not control the influence of the age and gender of the subjects when they were estimating the APOE ε4-conferred AD risk2,3,14,15,16.
We confirmed again that the APOE ε4-conferred AD risk is gender-and age-dependent in Koreans. The odds ratios for AD in the men were much lower than those in the women, indicating that the APOE ε4-AD risk conferred was gender-dimorphic in Koreans. However, in contrast to our previous report, in the present study the APOE ε4-conferred AD risk was significant in the men as well as in the women, a finding which might be attributed to the increased number of male AD patients in the present study (N=91) compared to our previous study (N=23)4.
The gender-dimorphic character of the AD risk was most pronounced in those Korean subjects with the APOE ε3/ε4 carriers, which is consistent with the findings for Caucasians4,19,20.
In the present study, the odds ratios for AD increased steadily between ages 45 and 79 years and declined abruptly to an insignificant level with increasing age thereafter.
Although the finding that the APOE ε4-conferred AD risk declines in the oldest AD patients has been consistently replicated in numerous studies, the age at which the APOE ε4-conferred AD risk begins to decline was not consistent across the studies, with the results being 80 years or over in the study by Bickeboller et al17., 70 years or over in the study by Frisoni et al18., and 60 years or over in the meta-analysis by Farrer et al.
hin
in Caucasians. Compared to Caucasians, the odds ratios for AD may decline later in Koreans.
The age-dependent change in the APOE ε4-conferred AD risk was not uniform across the APOE genotypes. According to Farrer et al1., the age-dependency of the APOE ε4-conferred AD risk was observed in both the APOE ε3/ε4 carriers and APOE ε4/ε4 carriers. However, in the present study, the age-dependent change in the APOE ε4-conferred AD risk was evident only in the APOE ε3/ε4 carriers. In the APOE ε4/ε4 carriers, although the odds ratios for AD were significant between ages 45 and 79 years, they did not show any age-dependent change. Since, in the present study, the number of APOE ε4-homozygous subjects in each age-stratum was insufficient, the age-dependency of the APOE ε4/ε4-conferred AD risk needs further investigation.
Although the APOE ε4 allele conferred a significant risk of AD in Koreans, neither the presence nor absence of the APOE ε4 allele provides diagnostic certainty, since the APOE ε4-attributable risk of AD was less than 50%at best. Therefore, the diagnostic use of APOE genotyping outside research settings is premature, until such testing is shown to be of practical value.
In conclusion, the APOE ε4 allele increased the risk of AD in a dose-dependent manner and the APOE ε4-conferred AD risk was age-and gender-dependent in Koreans. However, the use of APOE genotyping is not recommended as a routine diagnostic test for AD in clinical settings.
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