© 1999 American Heart Association, Inc.
Atherosclerosis and Lipoproteins |
Association of Polymorphisms at the SR-BI Gene Locus With Plasma Lipid Levels and Body Mass Index in a White Population
From Millennium Pharmaceuticals, Inc, Cambridge, Mass (S.A., M.D., J.K., S.S., E.A.W.); the Jean Mayer–US Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, Mass (D.O., D.C., J.M.O.); and the Department of Biochemistry and Molecular and Cellular Biology, University of Zaragoza, Zaragoza, Spain (M.P., A.C., P.M.).
Correspondence to Jose M. Ordovas, Jean Mayer–US Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, 711 Washington St, Boston, MA 02111. E-mail ordovas_li{at}hnrc.tufts.edu
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Abstract—The scavenger receptor class B type I (SR-BI) is a lipoprotein receptor that has been shown to be important in high density lipoprotein cholesterol (HDL-C) metabolism in mice. To determine its role in humans, we have characterized the human SR-BI gene and investigated its genetic variation in 489 white men and women. Five variants were demonstrated: 2 in introns (3 and 5) and 3 in exons (1, 8, and 11). Three variants at exons 1 and 8 and intron 5 with allele frequencies >0.1 were used to examine associations with lipid or anthropometric variables. The exon 1 variant was significantly (P<0.05) associated with increased HDL-C and lower low density lipoprotein cholesterol (LDL-C) values in men, but no associations were observed in women. The exon 8 variant was associated in women with lower LDL-C concentrations (3.05±0.98 mmol/L and 3.00±0.93 mmol/L for heterozygotes and homozygotes, respectively) compared with women homozygous for the common allele (3.39±1.09 mmol/L, P=0.043). No associations for this variant were observed in men. Women carriers of the intron 5 variant showed a higher body mass index (23.8±3.8 kg/m2, P=0.031) than those women homozygous for the common allele (22.4±3.4 kg/m2). Similar results were observed after haplotype analysis. Multiple regression analysis using HDL-C, LDL-C, and body mass index as dependent variables and age, sex, and each of the genetic variants as predictors also provided similar results. The associations found with both LDL-C and HDL-C suggest that SR-BI may play a role in the metabolism of both lipoprotein classes in humans.
Key Words: scavenger receptor class B type I • single-nucleotide polymorphism • body mass index • LDL cholesterol • HDL cholesterol
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The scavenger receptor class B type I (SR-BI) is a multilipoprotein receptor found in the liver and steroidogenic glands of both mice1 and humans2 3 (for a review, see Reference 44 ). The cDNA for human SR-BI (also known as CLA-1) was originally cloned by homology to human CD36 and rat LIMPII, which are members of a family of transmembrane proteins.5 An independent expression cloning study identified the hamster homologue by its ability to mediate the binding of modified LDL, and it was also shown to bind native LDL.6 Subsequently, murine SR-BI was shown to mediate the uptake of lipid, but not apoprotein, from HDL into cells,1 a process described as selective uptake.7 8 9 This finding established SR-BI as the first HDL transmembrane receptor to be identified and cloned. Further studies of the human homologue demonstrated that it also is a multilipoprotein receptor that binds HDL, LDL, and VLDL.2 10
Further analysis in vivo in mice and rats has supported a role for SR-BI in cholesterol metabolism. Targeted disruption of apoAI, the major protein component of HDL, leads to an increase in SR-BI expression in the adrenal glands of mice,11 where HDL-C is used for steroid hormone synthesis. In addition, SR-BI expression levels in the adrenal glands are increased in response to adrenocorticotropic hormone and decreased in response to dexamethasone.12 Estrogen treatment at high doses in rats greatly reduces SR-BI expression in the liver while it increases SR-BI expression in the adrenal gland and ovarian corpus luteal cells.13 Transient overexpression of SR-BI in the livers of mice by adenoviral infection leads to a marked reduction in plasma HDL levels and a concomitant increase in plasma LDL/IDL cholesterol levels.14 Finally, targeted disruption of the SR-BI gene in mice leads to a significant increase in plasma HDL15 16 and reduced selective uptake of cholesterol from HDL into the liver.16 Thus, SR-BI has clearly been shown to be a very important player in HDL metabolism in mice. However, although mice have HDL as the major cholesterol-carrying lipoprotein in plasma, adult humans carry the bulk of plasma cholesterol in LDL and VLDL particles.
As a first step to determine the physiological role of SR-BI in humans, we have isolated the gene and determined the intron-exon boundaries. This information was used to search for genetic variability at this gene locus in a random population of unrelated white individuals. Moreover, in this well-characterized population, we investigated associations between common polymorphisms at this gene locus and plasma lipid levels as well as anthropometric characteristics.
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Subjects
Blood samples for DNA isolation and biochemical measurements were collected from 489 unrelated subjects (201 men and 288 women) living in Zaragoza, Spain. All subjects were apparently healthy factory and hospital workers and students at the local university. They were recruited during their regular physical examinations, and they gave informed consent to their participation in the study. Height and weight for each participant were measured with the individual dressed in an examining gown and wearing no shoes. Body mass index (BMI) was expressed as weight in kilograms divided by the square of height in meters. Cigarette use was based on smoking status in the year before the study and included assessment of the number of cigarettes smoked per day. Alcohol intake was based on average weekly intake during the same period. All subjects were of white descent. The study was approved by the institutional review committee at the University of Zaragoza, Zaragoza, Spain.
Plasma Lipid Measurements
Plasma lipids were measured after a 12- to 14-hour overnight
fast by using blood collected in tubes containing 0.1% EDTA. Plasma
HDL-C was determined after precipitation of plasma apoB-containing
lipoproteins with dextran sulfate–Mg2+. Plasma
total cholesterol, HDL-C, and triglyceride
levels were measured using standard enzymatic procedures. LDL-C was
calculated by the Friedewald equation when triglyceride
levels were <400 mg/dL (4.52 mmol/L). CVs between runs for all
lipid assays were <5%.
Isolation of the Human SR-BI Gene
A probe consisting of a 474-bp fragment of the human SR-BI cDNA
was given to Research Genetics (Huntsville, Ala) to isolate bacterial
artificial chromosomes (BACs) containing genomic DNA encoding the human
SR-BI protein from a human BAC library (catalog No. 96041). Two BACs
were isolated by hybridizing the probe to this library. These BACs were
then sized by pulse field electrophoresis, and the inserts were found
to be 
80 and 70 kb for BAC 179 m10 and BAC 256i19, respectively. All
further work was done using BAC 179 m10. BAC 179 m10 was digested with
restriction enzymes, analyzed by Southern blot hybridization
with portions of human SR-BI cDNA, and shown to contain a large portion
of the SR-BI sequence. This BAC was then sheared by nebulizing the DNA
into fragments that were inserted into a vector for sequencing
(pminisk), and the resulting insert sizes were shown to range from 1 to
3 kb. Initially, clones that hybridized to the coding sequence of the
full-length human SR-BI cDNA were sequenced, leading to the
identification of most of the exons of the gene. Further random
sequencing of the BAC sheared library led to the identification of the
remaining coding exons and the adjacent intron flanking sequences.
Sequences were not polished unless they were considered important for
selection of polymerase chain reaction (PCR) primers.
Amplification of Genomic DNA Fragments
Multiple pairs of primers were synthesized to amplify each of
the exonic regions with intronic borders. Genomic DNA from a human
subject was subjected to PCR in 25-µL reactions (1x PCR Amplitaq
polymerase buffer, 0.1 mmol/L dNTPs, 0.8 mmol/L 5' primer,
0.8 mmol/L 3' primer, 0.75 U of Amplitaq polymerase, and 50 ng
genomic DNA) by using each of the described pairs of primers under the
following cycle conditions: 94°C for 2 minutes, 35 times (94°C for
40 seconds, annealing temperature for 30 seconds, and 72°C for 1
minute), 72°C for 5 minutes, and a 4°C hold. The resulting PCR
products were analyzed on a 2% agarose gel. The identity
of the PCR product was confirmed by digestion with a restriction
enzyme and subsequent agarose electrophoresis. Thirteen pairs of
oligomers were chosen to serve as PCR primers to amplify regions
containing each of the 12 coding exons of the human SR-BI gene. The
nucleotide sequence of these primers is indicated in Table 2
. The optimum PCR annealing temperatures for each primer pair,
the expected sizes of the PCR products, as well as
diagnostic restriction sites, are indicated in Table 2.
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Single-Strand Conformation Polymorphism (SSCP)
Analysis
The amplified genomic DNA fragments were analyzed by
SSCP17 18 19 from 96 subjects. From each 25-µL reaction, 3
µL was taken and added to 7 µL of loading buffer. The mixture was
heated to 94°C for 5 minutes and then immediately cooled in a slurry
of ice water. Three to 4 µL was then loaded onto a 10%
polyacrylamide gel containing 10% glycerol and subjected to
electrophoresis, typically overnight at 4 W at room temperature or for
6 hours at 20 W at 4°C. The secondary structure of single-stranded
nucleic acids varies according to sequence, thus allowing the detection
of small differences in nucleic acid sequence between similar nucleic
acids. At the end of the electrophoretic period, the DNA was detected
by gently overlaying a mixture of dyes onto the gel (1x the
manufacturers' recommended concentration of SYBR green I and SYBR
green II in 0.5x Tris-borate-EDTA buffer; Molecular Probes) onto the
gel for 5 minutes, followed by rinsing in distilled water and detection
in a Fluorimager 575 (Molecular Dynamics).
Identification of Mutations by Direct Sequencing of PCR
Products
On detection of a polymorphism in an amplified SR-BI genomic
region by SSCP, this region was reamplified using the aforementioned
primers that were modified to contain additional sequences that could
be used to directly sequence the PCR product (M13 forward sequence
[5'-TGT AAA ACG ACG GCC AGT-3'] on the 5' end of the 5' primer and
M13 reverse sequence [5'-CAG GAA ACA GCT ATG ACC-3'] on the 5' end of
the 3' primer). The genomic DNA from 3 to 6 subjects per
polymorphism was subjected to PCR in 50-µL reactions (1x PCR
Amplitaq polymerase buffer, 0.1 mmol/L dNTPs, 0.8 mmol/L 5'
primer, 0.8 mmol/L 3' primer, 0.75 U of Amplitaq polymerase, and
50 ng genomic DNA) by using each of the above-described pairs of
primers under the following cycle conditions: 94°C for 2 minutes, 35
times (94°C for 40 seconds, annealing temperature for 30 seconds,
72°C for 1 minute), 72°C for 5 minutes, and a 4°C hold. The
optimum PCR annealing temperatures are given in Table 2
. The
newly amplified products were then purified by agarose gel
electrophoresis and subjected to sequencing by using M13 forward and
reverse primers.
Genotyping of Single-Nucleotide Polymorphisms
(SNPs) in the Population
After characterization of the SNPs by direct sequencing,
subjects were typed by digestion of PCR products for exon 1, intron
5, and exon 8 by using the primers and enzymes listed in Table 5. Introns 3 and 11 were typed by SSCP.
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Statistical Analysis
The Statistical Package for the Social Sciences
(SPSS), version 8.0 for Windows, was used for the
statistical analysis. Power analyses for the different
patterns of association by sex were carried out using PC-Size
Consultant, version 1.01, from Statools. Sample size was
calculated a priori considering an 
error <5% and a ß error
<0.2. Owing to missing genetic information for some of the subjects,
the statistical power was recalculated a posteriori for each tested
hypothesis. The statistical power for the different patterns of
association by sex ranged from 0.745 to 0.847, and the average was
0.8024, suggesting that our sample size was large enough to minimize
type I errors and to assume standard type II errors.
A normal distribution for all continuous variables was checked by
graphical methods and by hypotheses tests. Only
triglyceride concentrations were markedly skewed, and this
variable was logarithmically transformed to improve normality for
statistical testing. To assess mean differences of lipid and
anthropometric variables between sexes and between
genotypes, Student's t test for independent samples
was used after determining the homogeneity of variances by the Levene
statistic. All probability values were calculated using the assumption
of potentially 2-sided differences. For multiple comparisons of means,
1-way ANOVAs were performed. Once established that differences existed
among means, to determine which mean differed with correction for
multiple comparisons, Tukey's test was applied. The correction is
needed because when several t tests are made, each at the
=0.05 level, then the probability of incorrectly rejecting at least
1 null hypothesis will be much larger than and will increase with
the number of tests made. Unadjusted probability values (obtained by
Student's t test) and adjusted probability values (obtained
by Tukey's test) are reported in the tables to show the extent of the
correction. To test the null hypotheses of no association between
genotypes and lipid or anthropometric variables,
controlling for 1 or more potential confounders (such as age, sex, BMI,
alcohol intake, and tobacco use), multiple linear regression models
with dummy variables were fitted. To improve statistical power,
data from men and women were analyzed together, including a
dummy variable for sex (1=male, 0=female) into the model. Several
regression models were fitted for each outcome variable: BMI, total
cholesterol, LDL-C, HDL-C, and triglycerides
(after logarithmic transformation). Genotypes or haplotypes
were included as k-1 dummy variables (k is
the number of categories) to avoid collinearity, because the regression
model contains an intercept and the 1/1 genotype was considered
for each intron or exon as the reference category. Age was considered a
control variable, and alcohol intake, tobacco use, or BMI was
retained in the model when the increase in
R2 (F of change) was statistically
significant (P<0.05). Taking into account the differences
observed in mean values of lipids for men and women, additional
interactions terms (with dummies) between sex and each genotype
were included into the models. Because the power to detect associations
is generally lower when testing interaction terms, a probability value
<0.08 was used as the criterion to retain these terms. Regression
coefficients, SEs, 95% CIs, as well as probability values were
estimated for each independent variable. When the interaction terms
between sex and genotype were statistically significant
(indicative that the relationship of interest was different at
different categories of the variable sex), separate linear
regression models, by applying the criterion mentioned above, were
computed for men and women. Regression diagnostics were
employed to check the assumptions and to assess the accuracy of
computations. Finally, to calculate adjusted means for each outcome
variable, controlling for the effects of the variables retained
in the corresponding fitted linear regression models,
covariance analyses with the general linear model
procedure were carried out. To test the statistical significance and to
estimate the 95% CIs for multiple comparisons of adjusted means,
Tukey's method was applied. The allele and haplotype frequencies
were estimated using the EH linkage utility
program.20 Significance of linkage disequilibrium
between SNPs within SR-BI was assessed by
2.
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Gene Structure and SNP Characterization
To begin to understand the physiological role of SR-BI in humans, we have defined the structure of the human gene by complete sequencing of the coding region as well as the majority of the noncoding region of the gene (see Methods and Reference 33 ). Thirteen exons were identified, ranging in size from 52 to 204 bp (Table 1
). The first exon contained all 5'
untranslated sequences found in SR-BI cDNAs as well as a portion of the
coding region. The last exon comprised the entire 3' untranslated
sequence. The exonic structure was remarkably similar to that of human
CD36 (data not shown and References 3 and 213 21 ), another member of the
CD36 family of proteins. To amplify the exonic regions as well as the
splice junction sites, PCR primers were chosen in the intronic regions
25 to 60 bp from the intron-exon boundaries (Table 2). The resulting PCR products were
then analyzed by SSCP or restriction endonuclease
analysis on a sample of 489 unrelated white men and women who
were born and living in Spain (Table 3).
The age range was between 18 and 75 years with a mean value of 37
years. These subjects were not selected on the basis of any
preestablished disease. Polymorphisms were found in regions in or
near exons 1, 3, 5, 8, and 11 (Table 4)
and further identified by direct sequencing (see Methods). All
polymorphisms were SNPs. SNPs near exons 5 and 11 were found in
adjacent introns but were not in canonical splice-site sequences. The
SNP in the region of exon 8 found by SSCP was determined to constitute
a change in base position 1050 (cDNA position 1 is the first base of
the initiator methionine) encoding amino acid 350, but there was no
resultant change in the amino acid. The SNP in exon 3 at bp position
403 encoded a change from valine to isoleucine at amino acid
135. The SNP in exon 1 at bp 4 encoded a change from glycine to serine
at the second amino acid position.
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Association of Common SNPs at the SR-BI Gene With Plasma Lipids and
Anthropometric Parameters
The frequencies of the less-common allele (allele 2) for
each of the SNPs described at the SR-BI gene locus were as follows:
exon 1, 0.117; exon 3, 0.018; intron 5, 0.105; exon 8, 0.438; and
intron 11, 0.043. Because the frequencies of the SNPs in exon 3 and
intron 11 were low (<0.1), no further analyses were done on
these. The associations between the common SNPs (exon 1, intron 5, and
exon 8) and plasma lipid concentrations and BMI are presented
in Table 6 for men and Table 7 for women (1/1
represents homozygosity for the most common allele, 1/2
represents heterozygosity, and 2/2 represents
homozygosity for the less-common allele). Linkage disequilibrium
analysis suggested that there was significant linkage
disequilibrium between the intron 5 and exon 8 SNPs
(2 P=0.003). The intron 5
allele 2 was found more often with the exon 8 allele 1,
suggesting that they form a haplotype. No evidence of linkage
disequilibrium was found between exon 1 and either intron 5 or exon
8.
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For men, the allele 2 defined by the exon 1 SNP was associated with significantly lower mean LDL-C concentrations (125±38 mg/dL [3.23±0.98 mmol/L] and 82 mg/dL [2.12 mmol/L] for heterozygotes and the homozygote, respectively) than those observed in subjects homozygous for allele 1 (145±44 mg/dL [3.75±1.14 mmol/L]; P=0.029). Conversely, the presence of allele 2 at this SNP was associated with increased HDL-C concentrations (48±10 mg/dL [1.24±0.26 mmol/L] and 56 mg/dL [1.45 mmol/L] for heterozygotes and the homozygote, respectively) relative to the homozygotes for allele 1 (42±13 mg/dL [1.19±0.44 mmol/L]; P=0.035). Moreover, allele 2 was also associated with lower triglyceride concentrations (P=0.038). Allele 2 at the intron 5 SNP was associated in men only with reduced triglyceride concentrations (102±60 mg/dL [1.15±0.68 mmol/L] versus 79±30 mg/dL [0.89±0.34 mmol/L]; P=0.017). No significant associations were observed between any of the variables examined and the exon 8 SNP in men. In women, no significant associations were noted for the exon 1 SNP; however, allele 2 at the exon 8 SNP was associated with significantly lower mean plasma LDL-C concentrations (118±38 mg/dL [3.05±0.98 mmol/L] and 116±36 mg/dL [3.00±0.93 mmol/L] for heterozygotes and homozygotes, respectively) than those observed in subjects homozygous for allele 1 (131±42 mg/dL [3.39±1.09 mmol/L]; P=0.043). No other significant associations were observed between these SNPs and other lipid variables.
A significant association was observed between the intron 5 SNP and BMI. Women carriers of allele 2 showed a mean BMI value (23.8±3.8 kg/m2) that was significantly greater (P=0.031) than that in those women homozygous for the most common allele (22.4±3.4 kg/m2).
The association between these SNPs and plasma lipid levels and BMI was
further explored using multiple regression analysis (Table 8). Regression models were
examined for BMI, LDL-C, and HDL-C as dependent variables and sex,
age, and specific SNPs (exon 1, intron 5, or exon 8) as independent
variables. For BMI, the model including both men and women showed
that sex and age were the most significant determinants of BMI
(P<0.001), with the intron 5 SNP contributing significantly
to this model (P=0.011). Age, sex, and the intron 5 SNP
accounted for 28% of the BMI variance. Cigarettes and alcohol were
consumed by most subjects, and for this reason, they were not
significant determinants of the variability in this specific population
and thus were excluded from the model (see Statistical Methods). When
men and women were analyzed separately, the model predicted
that the presence of 1 allele of the intron 5 SNP contributed
positively to BMI in women (+1.09 kg/m2) and in
men (+0.45 kg/m2), but this effect was
statistically significant only in women (P=0.036 versus
P=0.336 in men). For LDL-C levels, exon 8 was a significant
determinant for women (-0.426 mmol/L, P=0.006 for 1/2
versus 1/1 subjects and -0.422 mmol/L, P=0.045 for 2/2
versus 1/1 subjects). The exon 1 SNP (P=0.053) was close to
reaching the predetermined significance level in men. Regarding HDL-C
levels, exon 1 was a significant predictor for men (+0.144 mmol/L,
P=0.025) but not for women (-0.025 mmol/L,
P=0.633). A significant genotypexsex interaction
was noted for this SNP (P=0.044).
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We performed haplotype analyses using the 3 most common SNPs
identified at this locus (exon 1, intron 5, and exon 8). Six of the 8
possible haplotypes were identified according to the absence (1) or
presence (2) of the variant allele at each of the 3 polymorphic
sites. Four of the haplotypes were common: 111 (wild type), 112 (exon 8
variant), 121 (intron 5 variant), and 211 (exon 1 variant), whereas 2
of the estimated haplotypes were rare: 221 (variants at exon 1 and
intron 5) and 212 (variants at exons 1 and 8). Each subject was
assigned to the most plausible haplotype; however, because of the
uncertainty associated with genotype assignments in double
heterozygotes when studying unrelated subjects for whom the phase of
the polymorphisms cannot be directly ascertained, in further
analysis we used only those subjects with unequivocal
haplotypes. For simplicity, 112/112 and 112/111 subjects were pooled
together in these analyses (represented as
112/11X). Panel A of the Figure displays
mean HDL-C differences for each genotype versus
genotype 111/111 by sex. No significant
genotype-related differences were noted for women; however, men
with the 112/11X genotypes had lower HDL-C levels than those
with the 211/111 (P=0.002) genotype, whereas those
with the 211/111 genotype had significantly elevated HDL-C
concentrations compared with 111/111 (P=0.009) and 112/11X
(P=0.002) subjects. Panel B of the Figure
presents mean LDL-C differences for each genotype
versus genotype 111/111 by sex. Women with the 112/11X
genotypes had significantly lower LDL-C concentrations compared
with 111/111 women (P=0.008). No other differences were
found to be statistically significant. For BMI, no differences were
observed for men, but in women, the 121/111 genotype was
associated with higher BMIs (25.0±3.3 kg/m2)
compared with 111/111 (22.8±3.2 kg/m2,
P=0.010), 112/11X (22.8±3.9 kg/m2,
P=0.004), and 121/111 (21.5±2.1
kg/m2, P<0.01) subjects (panel C of
the Figure).
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The goals of this study were to identify genetic variants in the human SR-BI gene locus and to determine possible roles for SR-BI in lipid metabolism in humans by evaluating whether associations exist between these variants and lipid parameters in a healthy, white population. Although this receptor has been shown to play a significant role in the metabolism of HDL in mice,15 16 lipoprotein metabolism in mice is significantly different from that in humans. Most notably, mice have HDL as their principal circulating lipoprotein, whereas humans carry a more significant proportion of their lipids within LDL and VLDL. It is precisely those high levels of circulating LDL that are thought to contribute to the higher atherosclerosis susceptibility observed in humans than in other animal species. Moreover, mice lack cholesterol ester transfer protein, a key element of reverse cholesterol transport in humans. Because SR-BI has been shown to bind LDL and VLDL in vitro, it is plausible that it may also recognize apoB-containing lipoproteins in vivo and may be a significant contributor to LDL and VLDL catabolism in humans.
Our analysis of the SR-BI gene, located in 12q24,3 reveals that this locus is polymorphic in whites. We found evidence for significant associations between several SNPs and plasma lipids and anthropometric measures, and these associations were sex-specific. In men, we found significant associations between the exon 1 SNP and LDL-C and HDL-C levels, with allele 2 being associated with a less atherogenic lipid profile. This SNP did change the amino acid sequence, and therefore further work is warranted to examine the functionality of this mutation. Allele 2 at the intron 5 SNP was found to be associated with lower triglyceride levels, whereas no significant associations were noted for the exon 8 SNP. In women, the associations were different than those observed in men. Allele 2 at the exon 8 SNP was associated with lower LDL-C levels. This SNP, despite being within an exon, did not change the amino acid sequence and therefore, does not appear to be a functional mutation. These observations suggest that the SR-BI or a linked gene may play a role in LDL-C metabolism. Though less efficient than the LDL receptor, SR-BI is able to mediate the degradation of LDL in vitro.22 SR-BI may, however, play an indirect role in LDL-C metabolism in vivo by altering cholesterol homeostasis through its interaction with HDL and VLDL. Moreover, it has been shown that liver overexpression of SR-BI results in stimulation of excretion of cholesterol into the bile and suppresses the percentage of dietary cholesterol absorption.23 All of these data suggest that the SR-BI is an important candidate gene in terms of cholesterol metabolism. However, we did not find any linked functional mutation in the SR-BI gene in carriers of the exon 8 SNP. This SNP could be in linkage disequilibrium with a functional mutation at a neighboring relevant locus. Several other candidate genes involved in lipid metabolism are localized in the 12q24 chromosomal region (ie, ACACB, PLA2, CLTA, MVK, ACADS, and TCF1). Moreover, the SR-BI gene has been assigned to mouse chromosome 5, in a region homologous with human chromosome 12 harboring the SR-BI locus. Several of the previously indicated candidate genes related to lipids have been found in the mouse homologous region (MVK, ACADS, and TCF1),24 and more careful analysis of these loci appears to be warranted.
Additional analysis with the common SNPs revealed an
association between the intron 5 polymorphism and BMI in women
only. The finding was most significant in premenopausal women (not
shown). None of the subjects had morbid obesity (BMI >40), and thus,
this effect was observed in individuals within the common weight range.
Some estimates suggest that 40% to 70% of the variation in
obesity-related phenotypes in humans is heritable and probably
the result of the interaction of multiple genes. Consequently, the
effect of each single gene will be rather limited, making the search
for obesity genes in humans especially challenging. Several common
polymorphisms have been associated with BMI values in
humans.25 26 27 Moreover, evidence for linkage has been
demonstrated for the following chromosomal regions: 1p, 6p (TNF-),
7q (OB), 11q, and 20q (ADA and MC3R). An increased BMI has been
associated with higher mortality from all causes and from
cardiovascular disease in particular. For mortality
from cardiovascular disease, the relative risk
associated with an increment of 1 in BMI in women in the age range of
30 to 44 years has been reported to be 1.08 (95% CI, 1.05 to
1.11).28 29 Our data show that for an average woman, the
presence of the 121 haplotype raises the BMI by 2.2
kg/m2, which corresponds to 6 kg in body
weight. This increase in BMI could result in an increase in
coronary heart disease mortality of 17.6%, primarily due to
a greater risk of developing non–insulin-dependent diabetes
mellitus,28 29 a major risk factor for coronary
artery atherosclerosis.
It is not clear whether the SR-BI gene itself plays a role in weight control or whether a neighboring gene could be the cause of these observations. Sequencing of the entire coding portion in 3 individuals with the intron 5 polymorphism and 3 control subjects did not reveal a possible functional mutation in linkage with the SNP (not shown). Other genes in the chromosome 12q24 region are good candidates to be involved in the regulation of BMI and lipids, some of them acting through diabetic phenotypes. These are MODY3, NIDDM2, and ACACB. The lattermost is especially interesting, given the fact that ACACB may be involved in the regulation of fatty acid oxidation, thus affecting both lipid and energy metabolism.30 It should be noted that the BMI, HDL-C, and LDL-C associations were found within different haplotypes. This result would suggest that there may be functional mutations linked separately to the SNPs in exon 1, intron 5, and exon 8. Linkage disequilibrium analysis would support this concept. The exon 1 SNP is in linkage equilibrium with the other 2 SNPs; therefore, one would not expect to find similar associations for this SNP and those in intron 5 and exon 8. Linkage disequilibrium was detected between allele 2 of intron 5 and allele 1 of exon 8. Consequently, we would not expect to find similar patterns of associations for these SNPs.
SR-BI mRNA is expressed in adipocytes and adipose tissue, where SR-BI might mediate the uptake of lipids into those cells; however, steady-state levels of SR-BI protein are not high in adipose tissue.1 6 In addition, SR-BI expression in adrenal cells is thought to be important for the uptake of cholesterol for glucocorticoid synthesis,11 12 13 and excess production of glucocorticoids can lead to glucose intolerance, lipid alterations, and increased BMI.31 Because the association with BMI was most evident in premenopausal women, there may be hormonal regulation of the SR-BI gene in humans. Indeed, estrogen (albeit at nonphysiological doses) causes SR-BI expression to be significantly reduced in the liver and dramatically increased in the adrenal glands of male rats.13
This study is the first to demonstrate genetic variation at this gene locus and a possible role for SR-BI in humans. Our data suggest that future studies should focus on LDL metabolism and weight regulation, in addition to HDL lipid metabolism. Moreover, these associations are sex-specific, suggesting that the role of SR-BI in steroidogenesis and its hormonal regulation, as demonstrated in animal models, could also be relevant in humans. However, with this sample size, we cannot reject the possibility that some of the sex differences observed could be due to type II errors. We do not know whether the polymorphisms and frequencies reported will be similar in other geographic areas and ethnic backgrounds. Moreover, the associations observed in this study were derived from a normal population in southern Europe. It remains to be determined whether these associations can be replicated in other populations with different environmental factors.
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Acknowledgments |
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This work was supported in part by National Heart, Lung, and Blood Institute grant HL54776 (to J.M.O.) and by Eli Lilly & Co. We would like to thank Lisa Holmgren, John Lee, Tenzing Nyima, and Tenzin Lhasang for expert technical assistance. We would also like to thank Wendy Chin for technical advice, Jeanette Just for helpful discussions and assistance in DNA ascertainment, and Attilio Rigotti and Roger Breitbart for their critical review of this manuscript.
Received April 27, 1998; accepted December 16, 1998.
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