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Clinical Research |
From the Sections of Cardiology and Atherosclerosis, Department of Medicine, Baylor College of Medicine, Houston, Tex, and Weill Medical College of Cornell University (A.M.G.), New York, NY.
Correspondence to A.J. Marian, MD, Associate Professor of Medicine, Section of Cardiology, One Baylor Plaza, 543E, Houston, TX 77030. E-mail amarian{at}bcm.tmc.edu
| Abstract |
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1 coronary
lesion (P=0.001). No
association between the genotypes and progression of
coronary atherosclerosis or clinical events was
detected. We conclude that
ABCA1 genotypes are
potential risk factors for coronary
atherosclerosis in the general population.
Key Words: atherosclerosis genetics HDL cholestero l ATP binding cassette transporter apolipoprotein A1
| Introduction |
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| Materials and Methods |
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1 coronary lesion causing 30% to 75% diameter
stenosis and LDL cholesterol (LDL-C) of 115 to 190
mg/dL despite diet were randomized to fluvastatin (40 mg
daily) or placebo. Total choles- terol, LDL-C, HDL-C,
triglyceride, lipoprotein(a), and apolipoprotein levels
were measured in all subjects, and quantitative coronary
angiography was performed in 340 subjects at baseline and 2.5 years
after randomization. Baseline and final plasma level of lipids and
indices of quantitative coronary angiograms were determined and
used to assess the association of the genotypes with plasma
lipids, with the severity, progression, and regression of
coronary atherosclerosis, and with the response
to treatment (pharmacogenetics). Clinical events monitored were
definite or probable myocardial infarction, unstable angina requiring
hospitalization, percutaneous coronary
interventions, coronary artery bypass grafting, and death of
any cause.
Identification of Novel Polymorphisms in
the Promoter Region
A 660-bp 5' fragment of the
ABCA1
gene10 was amplified by
polymerase chain reaction (PCR) (forward primer
5'-AGCAGTAAGATGTTCCTCTGGG-3', reverse
5'-CCGAGCGCAGAGGTTACTATCG-3') in 12 subjects with HDL <35 mg/dL and 12
subjects with HDL >75 mg/dL selected from the LCAS population and was
sequenced by using the Big Dye Terminator Cycle Sequencing Ready
Reaction Kit on an ABI Genetic Analyzer 310 (PE Biosystem).
Sequences were analyzed and compared with the published
sequence of the ABCA1 promoter
sequence10 to detect
polymorphisms.
Genotpying
We identified 3 novel polymorphisms, -477C/T,
-419A/C, and -320G/C. We designed PCRrestriction fragment length
polymorphism assays, and an investigator who had no knowledge of
the angiographic and clinical data performed the genotyping in 372
subjects in the LCAS population. The -477C/T polymorphism creates
a novel site for the AciI
restriction enzyme (C/CGC) in the presence of the C allele. A set
of primers was designed to amplify a 351-bp fragment of the
ABCA1 promoter by PCR (forward
5'-CTCGGGTCCTCTGAGGGACCT-3', reverse
5'-CCGCAGA-CTCTCTAGTCCAC-3') encompassing the -477C/T
polymorphic site and an additional site for
AciI as a control, at an
annealing temperature of 56°C. The resulting amplicon was digested
overnight with the AciI
enzyme.
The -320G/C polymorphism did not affect a restriction enzyme site; therefore, with use of a mismatch primer, an artificial site was generated for the MflI enzyme (R/GATCY) in the presence of the G allele (forward 5'-TGCTTGGCGTTCCTGAGGGAGATTC-3', reverse 5'-GGGCACCAGTGGAATTTGCTTCCTCTAGATC-3'; mismatch nucleotide is underlined). The size of the expected PCR product was 133 bp, and the annealing temperature was 66°C.
Similarly, to identify the -419A/C genotypes, a mismatch primer (forward 5'-GGAAACAAAAGACAAGACAGAA-3', reverse 5'-GGCAGCCAAGGGCACCAGTGG-3') was used to generate a restriction site for the BsmI (GAATGCN/) enzyme in the presence of the C allele. The expected size of the PCR product was 164 bp, and the annealing temperature was 45°C.
Statistical Analysis
Continuous variables are expressed as mean±SD.
Differences among the genotypes were compared by ANOVA for
phenotypes with equal variance and by the Kruskal-Wallis test
for those with unequal variance. Distribution of the categorical
variables among genotypes was compared by the Pearson
2 or Fisher exact test. To detect
interactions between the genotypes and the response to
treatment, mean changes in plasma lipid levels and minimal lumen
diameter (MLD) among the genotypes were compared by ANOVA.
Statistical analysis was performed with the use of STATA,
version 5.0 (Stata Corp).
| Results |
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The PCR amplicon encompassing the -320G/C polymorphic
site was 133 bp long. Subjects with GG (n=131), GC (n=166), and CC
(n=75) genotypes were identified by gel electrophoretic
patterns (GG, 102 and 31 bp; GC, 133, 102, and 31 bp; and CC, 133 bp).
The frequency of the C allele was 0.42. The -477C/T and -320G/C
variants were in complete linkage disequilibrium, and the -320G
allele cosegregated with the -477C allele (127 GG/CC, 163
GC/CT, 74 CC/TT, 5 GG/CT, 3 CC/CT, and 0 GG/TT, GC/CC, GC/TT, and
CC/CC;
2=699,
P<0.0001). The results of
association studies with phenotypes were similar for the 2
polymorphisms, and data for the -477C/T polymorphism is
presented.
The size of the PCR amplicon encompassing the -419A/C polymorphic site was 164 bp. Only 1 of 63 subjects carried the A allele, identified by the presence of 140- and 24-bp digestion products (allele frequency of 0.01).
-477C/T Variants and Demographic and
Clinical Phenotypes
Demographic and clinical characteristics, such as age,
sex, ethnic background, height, weight, body mass index,
systolic and diastolic blood pressure, waist/hip
ratio, history of smoking, and history of myocardial infarction, were
not significantly different among the genotypes (data not
shown). Diabetes mellitus was present in 7.1%, 2.3%, and 1.4% of
the subjects with CC, CT, and TT genotypes, respectively
(P=0.074).
-477C/T Variants and Plasma Lipids at
Baseline and Response to Fluvastatin
Plasma lipids at baseline and response to
fluvastatin in -477C/T variants are shown in
Table 1
. There was a trend toward lower mean plasma levels
of HDL-C (P=0.094) and apoA1
(P=0.054) in heterozygous
subjects. There were no significant interactions between the
genotypes and in the response of plasma lipids to treatment
with fluvastatin, with the exception of apoA1, which showed
a significant genotype-treatment interaction
(P=0.0385), as shown in
Table 2
.
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-477C/T Variants and Severity of
Coronary Atherosclerosis
There were strong associations between the
genotypes and 2 indices of severity of coronary
atherosclerosis, as shown in
Table 1
. The mean number of qualifying lesions (30% to
75% diameter stenosis) was greater in subjects with the TT
genotype (3.1±2.1) or CT genotype (2.9±1.9) than in
those with the CC genotype (2.2±1.8)
(P=0.002). Similarly, greater
numbers of subjects with the TT genotype (88%) or the CT
genotype (90%) had
1 qualifying lesion on quantitative
coronary angiograms than did those with the CC genotype
(75%) (P=0.002). When study
subjects were stratified according to the mean plasma levels of HDL-C
(43.9±11.5 mg/dL), the -477C/T genotypes were strongly
associated with the number of qualifying coronary lesions, in a
gene-dosedependent manner, only in subjects with the lower HDL-C
levels (2.0±1.6 [CC] versus 2.8±2.0 [CT] versus 3.4±2.0 [TT],
F=7.2, P=0.001).
Analysis in subjects with the plasma levels of HDL-C of <35
mg/dL also showed a strong association between the genotypes
and the number of coronary lesions (2.1±1.5 [CC]
versus 2.8±2.0 [CT] versus 4.4±1.2 [TT], P=0.003).
Analysis of the data after exclusion of 14 subjects with
diabetes mellitus did not change the observed association between the
genotypes and the number of qualifying coronary lesions
(2.2±1.9 [CC] versus 2.8±1.9 [CT] versus 3.1±2.1 [TT]; F=6.9,
P=0.001). The number of total
occlusions, the number of subjects with
1 total occlusion, and the
mean MLD did not differ significantly among the genotypes
(Table 1
).
-477C/T Variants and Progression/Regression
of Coronary Atherosclerosis
The mean number of new coronary lesions or
total occlusions, the number of subjects who had progression of
coronary atherosclerosis, and the changes in
mean MLD during the course of 2.5 years of follow-up were not
significantly different among the genotypes
(Table 2
). There were no significant interactions between
the genotypes and response to treatment with
fluvastatin with regard to angiographic indices of the
progression/regression of atherosclerosis. No
significant association between the -477C/T genotypes and the
progression or regression of coronary
atherosclerosis and response to therapy with
fluvastatin was detected in subjects stratified according
to the mean plasma levels of HDL-C and in subjects with plasma levels
of HDL-C <35 mg/dL.
Clinical Events
Morbid or fatal events occurred in 54 patients
(14%). Distribution of clinical events among the genotypes in
the placebo and fluvastatin groups was not significantly
different
(Table 2
).
| Discussion |
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The hypothesis that common variants of the
ABCA1 gene could modulate
plasma levels of HDL-C and apoA1 stems largely from the discovery of
mutations in ABCA1 in patients
with Tangier disease and HA, who exhibit significantly reduced plasma
levels of HDL-C and apoA1 and a high incidence of premature
atherosclerosis.1 2 3 11
Members of families with Tangier disease or HA who are heterozygous for
mutations in the ABCA1 gene
exhibit a wide variation in HDL-C levels, an age-dependent decline in
HDL-C levels, and an increased risk of coronary
atherosclerosis.4
In the LCAS population, which is a representative of a
general population with mildly to moderately elevated LDL-C
levels,8 we observed only a
modest association between HDL-C or apoA1 levels and heterozygosity for
polymorphisms in ABCA1,
which remained unchanged when analyzed in subjects aged >50
years (n=313). Thus, unlike familial disorders, such as Tangier disease
or HA, in which common variants in
ABCA1 reduce HDL-C levels
significantly,4 our data
suggest that common variants in the promoter region of the
ABCA1 gene exert, at most, a
modest effect on plasma levels of HDL-C and apoA1. Low HDL-C levels in
the general population, unlike familial disorders, could result from a
variety of disorders, of which only a portion operate through the
reverse cholesterol transport, a function that has been
attributed to ABCA1 protein.7
The lack of a major effect on HDL-C levels in this general
population is consistent with ABCA1 acting as a flippase
at the plasma membrane that stimulates cholesterol and
phospholipid efflux to apoA1 and HDL-C; the efflux to HDL-C is
relatively minor.7 Multiple
complex pathways are involved in the determination of plasma HDL-C
levels, which include not only HDL-C synthesis and catabolism but also
the metabolism of triglyceride-rich
lipoproteins. Although a complete absence of ABCA1 leads to marked
reductions in HDL-C levels, subjects heterozygous for mutations
in the ABCA1 gene in families
with Tangier disease and HA exhibit wide variations in HDL-C levels and
apoA1,4 illustrating
the limitations in assessing the function of both total reverse
cholesterol transport and the initial step mediated by
ABCA1 by examining only plasma HDL-C levels. In families with Tangier
disease and HA, reduction in the relative efflux was greater in
heterozygous subjects who had low plasma levels of
HDL-C.4 Although the primary
determinant of low HDL-C levels in LCAS was clearly not related to the
477T variant, we postulate that in patients with low levels of HDL-C,
the 477T variant in the ABCA1
promoter leads to a reduction in the expression of ABCA1 and reverse
cholesterol transport, resulting in increased
cholesterol accumulation in the vessel wall and increased
atherosclerotic burden as assessed by an increased number of lesions at
baseline angiography. Angiographic progression may be driven more by
plaque destabilization and thrombosis instead of a gradual accumulation
of cholesterol in macrophages and thus showed no
relation to the 477T variant. In accordance with the results of studies
in families with Tangier disease and HA, our results demonstrated a
strong association between the severity of coronary
atherosclerosis and the
ABCA1 genotypes in the
LCAS population. The latter finding suggests dissociation between the
proatherosclerotic effect of mutations and their impact on plasma
levels of HDL-C and apoA1 in the general population, which has also
been reported in subjects with mutations in cholesteryl ester transfer
protein, resulting in high HDL-C levels but an increased risk of
coronary atherosclerosis. Inversely, subjects
with the apoA1-Milano mutation have very low levels of HDL-C and apoA1
but a low risk of coronary
atherosclerosis.12
In the absence of a significant association between the -477C/T
genotypes and the plasma levels of HDL-C and apoA1, the
progression or regression of coronary
atherosclerosis, and the response to therapy, the
observed association of the T allele with the severity of
coronary atherosclerosis requires confirmation
in additional data sets. The disparity between the effect of
ABCA1 variants on HDL-C levels
and the severity of atherosclerosis in the present
study also raises the possibility of statistical errors. Regarding ß
error, the sample size of the present study provided >90% power
to detect a 15% difference in mean plasma levels of HDL-C (6.6 mg/dL)
and apoA1 (20 mg/dL) among the genotypes. The possibility of an
error, leading to a spurious association between the
ABCA1 variants and the severity
of the coronary atherosclerosis, is unlikely
given the strength of the association
(P=0.001). We made no
adjustment for multiple testing, and if it is assumed that the
hypotheses being tested are not fully independent of one another, a
significant probability value
(P<0.05) should be regarded as
a potential association. We also note that the choice of end points,
the duration of the study, and the inclusion criteria were determined
before genetic analysis. In addition, the duration of LCAS (2.5
years) was relatively short, and the number of new clinical events was
relatively low (54 events), which may not be sufficient to detect
possible genotype-treatment interactions or association of the
genotypes with clinical events or the progression/regression of
coronary atherosclerosis. Overall, there were
no genetic or biological gradients or a trend toward an association
with multiple dependent phenotypes to suggest the possibility
of type II (ß) error. Furthermore, 108 subjects in LCAS were also
treated with cholestyramine. Distributions of the genotypes
among those who were or were not treated with adjunctive cholestyramine
were not significantly different, and there were also no significant
cholestyramine therapy-genotype interactions regarding plasma
lipids or angiographic phenotypes (data not shown). The main
results of the LCAS showed that treatment with fluvastatin
reduced mean LDL-C by 24% and slowed the progression of
coronary lesions
significantly.8
In summary, we identified 2 common and 1 uncommon polymorphism in the promoter region of the ABCA1 gene and showed a strong association between the -477C/T genotypes and the severity of coronary atherosclerosis, defined by quantitative coronary angiography in the LCAS population. There was a modest association between the genotypes and plasma levels of HDL-C and apoA1 and the response of apoA1 to treatment with fluvastatin. We conclude that ABCA1 -477C/T genotypes are potential risk factors for coronary atherosclerosis in the LCAS population, representative of the general population with mildly to moderately elevated LDL-C.8
| Acknowledgments |
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| Footnotes |
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| References |
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