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* Human Genetics Center, University of Texas Health Science Center, Houston, TX
Department of Human Genetics, University of Michigan, Ann Arbor, MI
Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY
** Department of Preventive Medicine, Northwestern University, Chicago, IL
Published, JLR Papers in Press, December 16, 2004. DOI 10.1194/jlr.M400437-JLR200
1 To whom correspondence should be addressed. e-mail: kklos{at}sph.uth.tmc.edu
| ABSTRACT |
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The results of our study suggest a small but replicable context-dependent influence of the APOA5 gene region on plasma TG levels in young, healthy individuals.
Supplementary key words apolipoprotein AV haplotypes SNP association coronary artery disease genetics human
| INTRODUCTION |
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Recently, Pennacchio et al. (5) identified the apolipoprotein A-V gene (APOA5) as a new member of the APOA1/C3/A4 gene cluster by comparative sequencing. Overexpression in transgenic mice leads to reduced plasma TG level, whereas increased TG levels are observed in knockout mice (5). In humans, associations have been identified between plasma TG level and several APOA5 polymorphisms, including 1131T/C, 3A/G, S19W, and 1259T/C (519). However, there has been some inconsistency in the reported race- and gender-specific associations of this limited set of APOA5 polymorphisms with plasma TG levels [e.g., Pennacchio et al. (5, 7) and Evans, Buchwald, and Beil (10)].
The majority of studies have evaluated the influence of APOA5 variation on interindividual variation in TG levels in older individuals (5, 6, 8, 10, 12, 16) and/or in individuals at increased risk of coronary artery disease based on increased lipid levels or family history (5, 9, 1012, 14, 15). The purpose of our study was to evaluate 16 single-nucleotide polymorphisms (SNPs) in the APOA5 gene for association with variation among individuals for plasma lnTG levels in healthy young (1830 year old) African American and white females and males from the Coronary Artery Risk Development in Young Adults (CARDIA) Study.
| MATERIALS AND METHODS |
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For the present study, genotypes were obtained for 3,831 individuals (1,075 African American females, 783 African American males, 1,041 white females, and 932 white males). SNPs were identified by resequencing the entire APOA1/C3/A4/A5 gene cluster, including
1,000 bp upstream of each gene, in 24 unrelated individuals from each of three populations: African Americans from Jackson, MS; Europeans from North Karelia, Finland; and European Americans from Rochester, MN (22). Sixteen SNPs in the APOA5 gene region were polymorphic in the CARDIA samples and have been identified for the present study by their local APOA5 sequence position (Fig. 1)
. Genotyping was performed using PCR amplification of genomic DNA, a short extension reaction across the polymorphic site, and mass spectrometry to detect allele-specific mass differences of the extension product. Allele detection and genotype calling were performed using the MassARRAY System from Sequenom® (San Diego, CA).
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![]() | (Eq.1) |
where
AB = 2pAABB + pAaBB + 1/2pAaBb. (23). P values for the test of
AB = 0 were obtained using the Chi-square test statistic
![]() | (Eq.2) |
Haplotype pairs were assigned to individuals based on SNP genotype data using the Bayesian algorithm of Stephens, Smith, and Donnelly (24) as implemented in PHASE 2.0.2. (25). The homogeneity of quantitative trait variances was evaluated using Levene's test (26). Differences in means were evaluated by the one-way ANOVA (27). Where variances were unequal, Welch's modified F statistic (28) was used to assess significance. Equivalent conclusions were obtained using the Welch technique and the standard ANOVA. The Boerwinkle and Sing (29) bias-corrected estimator of genetic variance was used to measure the proportion of plasma lnTG variance attributable to deviations of APOA5 genotype means from the population mean. The bias-corrected estimates of variance attributable to APOA5 haplotypes were obtained from one-way ANOVA in which individuals were identified as having zero, one, or two copies of a given three-SNP haplotype. Instead of applying a correction for multiple testing at
= 0.05, statistical significance was defined as P < 0.01.
| RESULTS |
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A summary of the estimates of a composite measure of pairwise LD is presented in Fig. 2 . Patterns of LD were similar in both races, apart from those involving SNP 30730, which was not tested in African Americans. Strong LD was observed between 27450 and 27565, between 27709 (1131T/C) and 28837 (3A/G), between 29009 (S19W) and 29085, and among 29928, 30648, and 30966.
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0.01 in tests of association with plasma lnTG in at least one race/gender are summarized in Table 2. Significant associations were observed between plasma lnTG level and single-site genotype variation for 27376, 27709, 28837, 29009, and 29085 in white males. P values for 27376 and 28837 were also marginally significant (0.01
P < 0.05) in white females. In African Americans, significant association between plasma lnTG and genotypes defined by 29009 and by 30966 were observed in females. No significant associations were observed in the African American male sample. In all cases of significant association, carriers of the rare allele had higher plasma TG than did noncarriers.
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To compare the combined effects of SNPs evaluated in this sample with those reported for other samples, haplotypes were constructed with SNPs 27709 (1131T/C), 28837 (3A/G), and 29009 (S19W), which were each significantly associated with plasma lnTG in one or more race/gender in our study and which were also reported in haplotype analyses elsewhere. Four of six assigned haplotypes (111, 221, 121, and 112) accounted for 99.65% of the chromosomes in African Americans, and three of six assigned haplotypes (111, 221, and 112) accounted for 99.57% of the chromosomes in whites. The frequency of individuals with haplotype pairs containing two, one, or zero copies of each of these four common haplotypes is reported in Table 3. Haplotype 111 represents the common allele at all three SNPs. Haplotype 112 represents the rare 19W allele with the common alleles of the two other SNPs and accounted for 5.9% of chromosomes in African Americans and 5.6% of chromosomes in whites. The 221 haplotype accounted for 8.8% of chromosomes in African Americans and 6.9% in whites. The 121 haplotype accounted for 4.0% in African Americans but only 0.2% in whites and so was evaluated for association in African Americans only. Haplotype pairs within individuals were associated with plasma lnTG level in white males (P < 0.0001). Table 4 summarizes the association of APOA5 variation, defined by the number of 111, 112, 121, and 221 haplotypes in an individual, with variation in plasma lnTG level. In this sample, the 111 haplotype explained 3.99% of variation in plasma lnTG, the 221 haplotype explained 2.53% of plasma lnTG variation, and the 112 haplotype explained 1.45% of variation. Copy number of the 121 haplotype was not tested in whites, because of rarity, and did not account for significant plasma TG variation in African Americans. The estimates of plasma lnTG variation explained in these three analyses of haplotype effect are not independent as a result of sample overlap. Trends in mean plasma TG level for haplotype pairs (Table 4) were remarkably consistent across race and gender given the small number of individuals homozygous for the rare haplotypes. Where haplotype had a significant, or marginally significant, effect on plasma lnTG, mean plasma TG levels decreased with increasing copies of the common 111 haplotype and increased with copy number of the rare haplotypes, except for 221 homozygotes in African American females.
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| DISCUSSION |
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In other studies, plasma TG levels have been associated with one or more of the SNPs 1131T/C, 3A/G, 1259T/C, 12238T/C, and IVS3+476G/A in white females (10, 16, 19) and males (5, 6, 7, 10, 14, 16, 19), in Chinese men (18) and with genders combined (12, 13), in Japanese (17), and in Hispanics of both genders (7). S19W has been associated with TG variation in African American females and males (7) and white females (7, 16, 19) and males (6, 7, 14, 16, 19). Statistical analyses cannot predict function, and the presence of as-yet unidentified functional polymorphisms in LD with these cannot be ruled out; however, the potential function(s) of these SNP markers have been discussed elsewhere (6, 7, 9).
Evaluation of measured genotype effects in large population-based studies is important for determining the impact of genetic variation on cardiovascular health in the population at large. Five other large population-based samples of African Americans and/or whites have been evaluated for associations between plasma TG levels and APOA5 markers (Table 5). SNPs 1131T/C and S19W are the only markers in the current study that can be directly compared with results from several other population-based studies. For this reason, the remainder of this discussion will focus on these two markers.
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Significant association of the 1131T/C variation with lnTG variation in white males of the CARDIA Study was consistent with the results from four of five other population-based studies (Table 5). Interestingly, association was not identified in the European Atherosclerosis Study II (EARSII), the sample most similar in age (415 healthy white males aged 1828 years) to our sample (14). A significant association (P = 0.026), however, was observed in a sample of 407 males from the EARSII population whose fathers had suffered a myocardial infarction before age 55. In the CARDIA sample, 148 white males reported a known incidence (but not age) of heart attack in one or both parents. No statistically significant association between lnTG level and the 1131T/C marker in white males with a self-reported family history of heart attack was detected, nor did 1131T/C genotype predict family history in a logistic regression analysis (data not shown).
No statistically significant association between plasma lnTG and either 1131T/C or S19W was detected in white females in our study, in contrast with two of the three other studies (16, 19). Both of these studies were of individuals older than those in our study. The study by Pennacchio et al. (7) of 359 females who ranged in age from 18 to 65 years found association with S19W but not 1131T/C. Sample size differences and sampling error, or differences in population structure, may be responsible for the variable results among these studies. There may also be additional genetic or environmental factors that play a role in modifying the gene effect identified by these SNPs.
An association of lnTG level with S19W variation was observed in African American females and white males. The lack of significant association in African American males is in contrast to findings by Pennacchio et al. (7) of significant associations for S19W with plasma TG levels in both genders of African Americans from the DHDPP. The age at recruitment was broader (1865 years) in the DHDPP than in CARDIA, suggesting a role for gender-specific age and age-related factors. The association of lnTG level with variation in S19W in white males is consistent across population-based studies (Table 5).
In the Framingham Heart Study, variation in 1131T/C explained 0.51% of lnTG variance in females and 0.78% in males, and S19W explained 0.45% of lnTG variance in females and 0.78% in males (19). This was similar to the amount of lnTG variation accounted for by these SNPs in females of our study (<0.40%) but less than that accounted for in males (<2.50%). Given the relatively low proportion of variance explained by measured APOA5 genotype effects, the level of consistency among population-based studies is surprising. Replicated observations of the influence of APOA5 gene variation across samples suggests that some common variations in or near this gene play an important role in plasma TG regulation. Yet, the variable results among samples of African American and white females and males indicate that there may be additional unidentified genetic and/or environmental factors that are important modifiers of gene effect. Additionally, larger proportions of TG variation (714%) explained by S19W in individuals diagnosed with familial combined hyperlipidemia compared with their healthy spouses (4%) suggest that variation at or near the APOA5 gene may play a greater role in TG metabolism within certain strata of the population (9). It should be kept in mind, however, that the validity of comparisons among samples depends on uniformity of population structure, especially with regard to the percentage of variability explained, and caution should be exercised when drawing conclusions.
The proportion of plasma TG variance explained by measured variation in the APOA5 gene appears to be in agreement with that reported for other apolipoproteins. Kaprio et al. (30) examined associations between plasma TG and measured genotypic variation in APOE, APOA4, and APOH in white males and females of the Rochester Family Heart Study. APOE variation explained 2.5% of plasma TG variation in males, a similar proportion of variation to that explained by APOA5 in the white males of this study (1.453.99%). They also found gender specificity in the proportion of TG explained by APOE, but in that case the proportion explained in females was much larger (22.76%). They reported a proportion of plasma TG variance explained by the APOA4 and APOH genetic variations of less than 1% in both genders. This was not significant in their sample of 226 females and 227 males.
Pennacchio et al. (7), evaluating LD and association in African Americans, Hispanics, and whites, identified a pattern consistent with two functional sites in the APOA5 gene. The rare alleles of SNPs 1131T/C (27709), 3A/G (28837), and 1259T/C (30730), as well as the APOA5 markers 12238T/C and IVS3+476G/A not measured in our study, were shown to represent a single haplotype, whereas S19W (29009) had an independent effect on TG levels (57). Similar haplotype effects have been reported in analyses of other samples (13, 17, 19). The haplotype effects observed in our study were consistent with this pattern, but they were only significant in white males. Interestingly, greater lnTG variation was explained by copy number of the common haplotype (3.99% in white males) than by any single SNP. Mean plasma TG level decreased with increasing copy number of that haplotype in all samples.
The purpose of our study was to evaluate the influence of APOA5 polymorphisms on TG variation in a young, healthy population-based sample. Single-site and three-SNP haplotype effects on plasma lnTG were identified without attempting to identify a functional site(s). Comparisons with other population-based studies suggest that the influence of the APOA5 gene on lipid metabolism may be dependent on both race and gender and that such effects are present in young adults before the onset of clinical manifestations of cardiovascular disease. Finally, the consistency of findings among samples for a gene that accounts for less than 5% of risk factor variation demonstrates the potential for genetic association studies to identify genes that are involved in the pathogenesis of a complex multifactorial disease.
| ACKNOWLEDGMENTS |
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Manuscript received November 3, 2004 and in revised form December 7, 2004.
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