Loss-of-function variants in endothelial lipase are a cause of elevated HDL cholesterol in humans

Elevated plasma concentrations of HDL cholesterol (HDL-C) are associated with protection from atherosclerotic cardiovascular disease. Animal models indicate that decreased expression of endothelial lipase (LIPG) is inversely associated with HDL-C levels, and genome-wide association studies have identified LIPG variants as being associated with HDL-C levels in humans. We hypothesized that loss-of-function mutations in LIPG may result in elevated HDL-C and therefore performed deep resequencing of LIPG exons in cases with elevated HDL-C levels and controls with decreased HDL-C levels. We identified a significant excess of nonsynonymous LIPG variants unique to cases with elevated HDL-C. In vitro lipase activity assays demonstrated that these variants significantly decreased endothelial lipase activity. In addition, a meta-analysis across 5 cohorts demonstrated that the low-frequency Asn396Ser variant is significantly associated with increased HDL-C, while the common Thr111Ile variant is not. Functional analysis confirmed that the Asn396Ser variant has significantly decreased lipase activity both in vitro and in vivo, while the Thr111Ile variant has normal lipase activity. Our results establish that loss-of-function mutations in LIPG lead to increased HDL-C levels and support the idea that inhibition of endothelial lipase may be an effective mechanism to raise HDL-C.

Elevated levels of HDL cholesterol (HDL-C) are inversely associated with atherosclerotic cardiovascular risk, independent of LDL (1). The heritability of HDL-C is approximately 50% (2), with significant influence from environmental factors such as physical activity, alcohol consumption, and smoking (3). Based on the study of monogenic conditions, genetic causes of low HDL-C levels include mutations in ABCA1 (4–7), APOA1 (7), and lecithin-cholesterol acyltransferase (LCAT) (7, 8), whereas genetic causes of high HDL-C levels include mutations in cholesterol ester transfer protein (CETP) (9). Homozygous CETP deficiency is found predominantly in Japanese individuals, and mutations in other genes are likely to contribute to the phenotype of elevated HDL.

Endothelial lipase (EL; LIPG) is an HDL candidate gene, in which loss-of-function mutations are hypothesized to result in elevated HDL-C concentration. EL is a member of the triglyceride lipase family of proteins that includes lipoprotein lipase and hepatic lipase, and it exhibits a conserved catalytic triad, heparin-binding properties, lipid-binding domains, and cysteine residues (10). EL primarily hydrolyzes phospholipids with little triglyceride lipase activity (10, 11) and hydrolyzes HDL the most efficiently of all the lipoprotein fractions (12). Overexpression of EL in mice decreases HDL-C levels (13), whereas the inhibition or deletion of EL in mice increases HDL-C levels (14). In baboons, HDL-C levels have also been associated with variation in LIPG expression (15).

We previously sequenced the LIPG gene in a limited number of individuals with high HDL-C levels and reported 4 nonsynonymous variants: the common Thr111Ile (rs2000813) variant, the low-frequency Asn396Ser variant, and the rare Gly26Ser and Thr298Ser variants (16). The Gly26Ser and Thr298Ser variants were primarily found in African Americans. Early association studies of SNPs in LIPG with HDL-C produced conflicting results with regard to Thr111Ile as well as other common noncoding LIPG SNPs (3, 17–23). Recently, SNPs near the LIPG gene were identified in several human genome-wide association studies (GWASs) as being associated with HDL-C levels (24–29). We hypothesized that rare loss-of-function EL variants are a cause of high HDL-C and that deep medical resequencing efforts would enable us to identify rare mutations that exhibit a substantial phenotypic effect. Thus, we sequenced the 10 exons of LIPG in participants of mixed European ancestry with extremely high HDL-C (≥95th percentile) and low HDL-C (≤25th percentile) and assessed the functionality of the newly identified nonsynonymous variants. We also assessed the association of the common Thr111Ile variant and of the low-frequency Asn396Ser variant with HDL-C in several population-based and case-control studies, as well as their lipolytic function.

Identification and functional analysis of rare nonsynonymous LIPG variants in participants with high HDL-C. We resequenced all 10 LIPG exons in 585 participants of mixed European ancestry from the extreme tails of the HDL-C phenotype distribution (Table 1), drawn from the University of Pennsylvania High HDL Cholesterol Study (HHDL) cohort; participants attending a lipid outpatient clinic, Universitätsklinikum Hamburg-Eppendorf (UKE Hamburg); and a cross-sectional community screening of healthy Canadian volunteers, including 372 individuals with high HDL-C (≥95th percentile for age and gender) and 213 individuals of mixed European descent with low HDL-C (≤25th percentile for age and gender), for a total of 1,170 chromosomes sequenced. Unique nonsynonymous variants identified through LIPG exon resequencing were exclusively found in the group with high HDL-C. In total, 10 rare nonsynonymous variants (7 unique) were identified in participants with high HDL-C compared with none in the participants with low HDL-C (Table 2). This represents a significant excess of nonsynonymous variants in the group with high HDL-C compared with the group with low HDL-C (P = 0.02; Fisher exact test). Individual fasting lipid and lipoprotein measurements for participants with rare LIPG variants are available in Supplemental Table 1 (supplemental material available online with this article; doi:10.1172/JCI37176DS1), along with summary information from sequenced participants not carrying a LIPG variant. Additional lipoprotein measurements and NMR lipoprotein profile measurements, when available for a subset of these individuals, are available in Supplemental Table 2.

Table 1

Baseline characteristics

Table 2

Nonsynonymous sequence variants in LIPG exons found exclusively in participants with high HDL over the 95th percentile

For variants affecting a single amino acid, computational prediction performed by PolyPhen (30) suggested that most of these variants are possibly or probably damaging to normal EL function (Table 2). PolyPhen was unable to perform predictions for the Fs114DelA or X501Arg variants, which produce multiple amino acid changes. The Fs114DelA variant is a single nucleotide deletion, which results in a frameshift that changes amino acids 115–117 (Asp, Ala, Asn) to Thr, Pro, Met and then truncates the protein with a premature stop codon, which results in a protein with a predicted molecular mass of 13.4 kDa (Figure 1A). This mutation results in termination of the protein well before the catalytic triad, yielding a predictably nonfunctional protein. The X501Arg variant is a mutation in the LIPG stop codon, which allows translation to continue for an additional 49 amino acids before reaching the next in-frame stop codon, resulting in a protein with a predicated molecular mass of 62 kDa (the predicted molecular mass of WT EL is 56.9 kDa). Nonsynonymous variants were also assessed by aligning the human EL amino acid sequence (AAD30434) with that from various other species, including mouse (NP_034850), rat (AAX11354), chimpanzee (XP_512126), and rhesus monkey (XP_001090086) using ClustalW (31). Four of the nonsynonymous variants were in residues conserved across all of the species and resulted in nonconservative changes (Ala116Thr, Gly176Arg, Ile239Thr, and Met342Val). The remaining variant (Arg476Trp) was in a nonconserved residue, but resulted in a nonconservative change from the amino acid found in any of the species.

Figure 1

Functional analysis of unique nonsynonymous variants identified in participants with high HDL-C. (A) Schematic of EL protein structure. Diamonds represent potential N-linked glycosylation sites. Filled diamonds indicate validated N-linked glycosylation sites (32), and diamonds with a black circle indicate N-linked glycosylation sites conserved across lipoprotein lipase and hepatic lipase. Numbering indicates amino acid residues and differs from previous publications due to the inclusion of the 20–amino acid signal peptide (SP) in the numbering. RNKR indicates the site of proteolytic cleavage of the 68-kDa full-length EL into a 40-kDa N-terminal fragment and a 28-kDa C-terminal fragment. The “‘lid” domain and conserved catalytic triad are indicated. Nonsynonymous EL variants identified in participants with high HDL-C were created by site-directed mutagenesis and transiently expressed using 293 cells in the presence of heparin. (B) Expressed EL was visualized by immunoblotting of the conditioned media and (C) cell lysate. (D) Media from cells transiently expressing WT EL or variant EL in the presence of heparin were collected and assayed for the hydrolysis of dipalmitoylphosphatidyl choline and (E) isolated human HDL₃. Activity data for the EL variants were normalized to a percentage of WT EL activity. Assays were performed in triplicate and separate transfections were repeated at least 3 times. Presented results are from a representative experiment. 1, Fs114DelA; 2, Ala116Thr; 3, Gly176Arg; 4, Ile239Thr; 5, Met342Val; 6, Arg476Trp; 7, X501Arg. Error bars indicate ± SD. *P ≤ 0.001 compared with WT.

To test the activities of the EL variants identified through resequencing, an EL expression plasmid (32) was modified through site-directed mutagenesis to contain each of the EL nonsynonymous variants. Each of the variants was expressed in 293 cells (Figure 1, B and C), except the X501Arg variant, which had only a faint N-terminal 40-kDa band and no detectable full-length band. The activity of each EL variant was tested against the synthetic phospholipid substrate dipalmitoylphosphatidyl choline as previously described (32) and against isolated human HDL₃ as previously described (33). Each assay was performed in triplicate and transfections were repeated at least 3 separate times. Each EL variant identified exclusively in the high HDL group exhibited a significant decrease in lipolytic activity compared with WT EL, with the majority of the variants having nearly undetectable activity in both assays of lipase function (Figure 1, D and E).

Association of common nonsynonymous LIPG variants with HDL-C and functional analysis. Not surprisingly, the common Thr111Ile variant and the low-frequency Asn396Ser variant, both of which we had previously reported in people of mixed European descent (16), were identified in both sequencing groups. Thr111Ile occurred at a similar frequency in both groups, whereas Asn396Ser was found with much greater frequency in the group with high HDL-C (P = 0.007; Table 3). Summary fasting lipid and lipoprotein measurements for sequenced participants with high HDL levels with the Asn396Ser LIPG variant are available in Supplemental Table 1. PolyPhen (30) predicted the Asn396Ser variant to be possibly damaging with regard to normal EL function but predicted the Thr111Ile variant to have a benign effect on normal EL function. Alignment analysis using ClustalW (31) revealed that Asn396 is highly conserved in all species and adjacent to an N-linked glycosylation site that is conserved across the triglyceride lipase family (Figure 2A). In contrast, Thr111 is not conserved across species, with the Ile111 variant found in rodent LIPG genes.

Figure 2

Functional analysis of the common Thr111Ile variant and the low-frequency Asn396Ser variant. (A) Schematic of EL protein structure as described in Figure 1. Thr111Ile and Asn396Ser were created by site-directed mutagenesis and transiently expressed using 293 cells in the presence of heparin. (B) Expressed EL was visualized by immunoblotting of the conditioned media and cell lysate. (C) Media from cells transiently expressing WT EL or variant EL in the presence of heparin were collected and assayed for the hydrolysis of dipalmitoylphosphatidyl choline and (D) isolated human HDL₃. Activity data for the EL variants were normalized to a percentage of WT EL activity. Assays were performed in triplicate, and separate transfections were repeated at least 3 times. Presented results are from a representative experiment. 1, Thr111Ile; 2, Asn396Ser. *P ≤ 0.001 compared with WT. (E) Somatic gene transfer of WT and Asn396Ser EL into male Lipg^–/– mice was performed by administering AAV2/8, carrying the designated transgene via i.p. injection. HDL-C was measured at indicated intervals. Triangles, LacZ (n = 4); circles, Asn396Ser (n = 3); squares, WT (n = 4). Error bars indicate ± SD.

Table 3

Sequence variants in LIPG exons occurring in participants from both extremes of the HDL phenotypic distribution

We formally tested for an association of these 2 LIPG variants with HDL-C in the Framingham Heart Study Offspring cohort (FHS; n = 1,796) (Table 4). The Thr111Ile variant was not associated with HDL-C concentration. In contrast, the Asn396Ser variant was found to be highly significantly associated with HDL-C, with the Asn396Ser minor allele associated with an approximately 8 mg/dl increase in HDL-C (P = 4 × 10^–5). Asn396Ser was also significantly associated with increased HDL₂ and HDL₃ subfractions, increased HDL particle size, increased large HDL particles, and increased apoA-I levels (Table 4). The Asn396Ser variant was not associated with LDL, triglyceride, apoB, or sizes of other lipoprotein particles, nor was the Thr111Ile variant associated with any of these additional lipid and lipoprotein measures (Supplemental Table 3).

Table 4

FHS association analysis

We attempted to replicate these findings in several other cohorts, including a subset of the Nurses’ Health Study II (NHS II), a subset of the Health Professionals Follow-Up Study (HPFS), the University of Pennsylvania Catheterization Study (PennCATH) cohort, and case-control cohorts from Universitätsklinikum Hamburg-Eppendorf (UKE Hamburg CC) and the University of Pennsylvania HDL Case-Control Study (Penn HDL CC; Tables 1 and 5). Statistical evidence from these cohorts was summarized using a weighted z-statistic meta-analysis. In none of these cohorts was Thr111Ile associated with HDL-C levels. In contrast, Asn396Ser was strongly associated with elevated HDL-C across all cohorts studied (combined P value = 1.7 × 10^–8; Table 6). Mean lipid values listed by genotype are available in Supplemental Table 3.

Table 5

Baseline characteristics

Table 6

Results of association analyses

Finally, to test for association of the variants with HDL-C levels within families, multigenerational families from our HHDL cohort, with vertical transmission of the minor alleles of the Thr111Ile and Asn396Ser variants, provided DNA for genotyping. The Thr111Ile variant was genotyped in 136 available individuals from 32 separate families, yielding 38 informative transmissions, and the Asn396Ser variant was genotyped in 100 available individuals from 20 separate families, yielding 39 informative transmissions. The quantitative transmission disequilibrium test (QTDT) was used to test for an association of both variants with HDL-C within the families. No population stratification was detected in our samples (P > 0.4). There was no evidence that Thr111Ile was associated with HDL-C levels within families (P = 0.365). In contrast, Asn396Ser was significantly associated with an increase in HDL-C in the within-family association (P = 0.008). Family members with the Asn396Ser variant had an increase of approximately 11 mg/dl in HDL-C above family members without the variant (WT = 63.7 ± 17.2 mg/dl, Asn396Ser = 74.9 ± 31.0 mg/dl).

In order to test their functionality, we created the Thr111Ile and Asn396Ser variants in the EL expression plasmid using site-directed mutagenesis and analyzed their lipolytic activity in vitro (Figure 2, B–D). Consistent with the epidemiologic data, Thr111Ile had lipolytic activity similar to WT EL, whereas Asn396Ser had significantly (P < 0.001) reduced lipolytic activity with less than 40% of WT activity. We also created adeno-associated virus (AAV) vectors encoding WT EL and the Asn396Ser variant and injected them into Lipg^–/– mice to compare their in vivo biological activity against HDL. The Asn396Ser variant was substantially less effective than WT EL at reducing HDL-C levels, consistent with reduced activity against HDL in vivo (Figure 2E).

Evaluation of linkage disequilibrium. Recently, several human GWASs have reported that correlated SNPs 40–65 kb downstream of the LIPG gene are associated with HDL-C levels (the most robustly associated being rs2156552 and rs4939883, which are in high linkage disequilibrium [LD] with each other). However, the causative variant is, as of yet, unidentified, and none of these correlated SNPs are in LD with known functional variants in LIPG (24–29). HapMap data (release 21) suggests that the Thr111Ile variant is not significantly correlated with the downstream variants (squared correlation coefficient [r²] = 0.078 – 0.083). Genotyping of these downstream LIPG variants performed in the HHDL (rs4939883), Penn HDL CC (rs4939883), and PennCATH (rs2156552) cohorts confirmed that the downstream LIPG SNPs from GWAS reports are not significantly correlated with either the Thr111Ile or the Asn396Ser variants (r² < 0.03 and r² < 0.01, respectively) in any of these cohorts (Supplemental Figure 1).

While biochemical and animal studies have suggested that EL is an important modulator of HDL-C concentration, definitive evidence that it plays an important role in human HDL metabolism has been notably lacking. Recent GWAS results have suggested that common variation near the LIPG locus is associated with HDL-C in humans, although the causal variant or variants are still unidentified. Resequencing of candidate genes in participants at the extremes of a quantitative trait has been shown to efficiently identify both common and functional rare variants of a gene (34) and has been particularly successful at identifying genetic causes of lipid phenotypes (7, 34–36). Here, we report the discovery of rare loss-of-function mutations in LIPG in persons with very high HDL-C levels and that a low-frequency variant with reduced activity is significantly (P = 1.7 × 10^–8) associated with higher HDL-C.

The first LIPG resequencing efforts were performed by our group in a small number (n = 20) of participants with high HDL, identifying 4 nonsynonymous variants of EL (16). This study lacked statistical power and did not characterize the activities of the variants. Only 1 nonsynonymous variant, Thr111Ile, was found to be common (minor allele frequency > 0.05), and thus has been the subject of several additional studies of limited power, with equivocal and inconsistent results. Several studies have attempted to determine if there is any effect of the Thr111Ile variant on a variety of cardiovascular measures. Overall, these studies have been underpowered and lack functional evidence to support their conclusions: (a) Ma et al. reported that Thr111Ile was associated with an increase in HDL-C among 372 participants (17); (b) Yamakawa-Kobayashi et al. failed to detect an association of Thr111Ile with HDL-C levels in 340 Japanese children (22); (c) Paradis and colleagues found an association of Thr111Ile with increased levels of the HDL₃ subfraction in 281 females (18); (d) Halverstadt et al. found an association of Thr111Ile with NMR measurements of HDL size in 83 healthy elderly participants but not with overall HDL-C levels (19); (e) Mank-Seymour et al. showed a weak association of Thr111Ile with increased HDL-C among 594 participants (3); (f) Hutter et al. found a weak association of Thr111Ile with HDL-C levels in 541 Japanese Americans (20); (g) Shimizu et al. failed to find an association of Thr111Ile with HDL-C in 107 Japanese acute myocardial infarction cases and controls (23); and (h) Tang et al. found a weak association of Thr111Ile with increased HDL-C in 265 Chinese coronary artery disease (CAD) cases and controls (21). Our data from a combined sample of 3,845 participants, and functional studies of the variant definitively establish that Thr111Ile is not associated with HDL-C and in vitro studies show that it has normal lipolytic activity. Furthermore, the Thr111Ile variant is not correlated with the downstream LIPG SNPs identified in GWASs, r² < 0.1, according to HapMap data (37) and our own genotyping (Supplemental Figure 1), further confirming that it is not the etiological variant under the HDL-C signal detected near LIPG in recent GWASs.

Here, we report an expanded LIPG resequencing effort, resequencing the LIPG exons on 1,170 chromosomes in participants from both extremes of the HDL concentration distribution. Our resequencing efforts did not identify any nonsynonymous variants strongly correlated with the downstream LIPG SNPs identified in the HDL GWASs, and it is unlikely that further resequencing of the LIPG locus will reveal any coding variants that can explain this signal. This suggests that the HDL-C peak near LIPG in GWASs is not due to a coding variant but rather due to some type of regulatory variant. Interestingly, studies in baboons have suggested that LIPG promoter variants are associated with variation in HDL-C levels (15). Ongoing studies in our lab are addressing the role of promoter variants in the human LIPG gene and their association with plasma HDL-C levels.

Our results validate deep medical resequencing in extremes of a quantitative phenotype to identify rare mutations that exert a substantial phenotypic effect. Notably, we show that nonsynonymous LIPG variants are significantly more common in participants with elevated HDL-C and that these variants are true loss-of-function variants, a finding which mirrors the accumulation of rare variants in ABCA1, LCAT, and APOA1 in participants with low HDL-C (7). Furthermore, we establish that while the common Thr111Ile variant is definitively not associated with variation in HDL-C concentration and has normal lipolytic activity, the much less common Asn396Ser is significantly associated with elevated HDL-C concentration, with a P value on meta-analysis considered to be significant genome wide (P = 1.7 × 10^–8), and that it has substantially reduced lipolytic activity in vitro and in vivo. We further replicated these findings in families with elevated HDL-C, an approach that is robust to population stratification, a problem that theoretically can affect population association studies.

Our results highlight the utility of a functional assay for experimentally testing mutations found upon resequencing. Computational programs such as PolyPhen (30) or SNPs3D (38) primarily predict changes in protein structure stability. However, they cannot predict specific functional impairment (such as ligand binding, disruption of the catalytic site, disruption of posttranslational modification, or the introduction of some allosteric effects) and may introduce a bias that can obscure the true relationship between the variants and the phenotype of interest when used exclusively. While the majority of the LIPG variants we found were correctly predicted by PolyPhen to have a deleterious effect on EL function, likely through decreased protein structure stability, the Ala116Thr variant was incorrectly predicted to be a benign change. It may prove particularly insightful to further characterize this variant, as its significant (P < 0.001) reduction in lipolytic activity may result from a specific functional impairment of a novel functional element in EL.

It will be important to use loss-of-function variants in LIPG as an approach to assess the effects of lifetime exposure to elevated HDL-C due to reduced EL activity, much as loss-of-function proprotein convertase subtilisin/kexin type 9 (PCSK9) variants that reduce LDL cholesterol (LDL-C) concentrations were found to greatly decrease cardiovascular risk (39). However, the low allele frequency of the Asn396Ser variant and its comparatively modest HDL-C effect require a very large number of participants in order to have adequate power to achieve this goal. Specifically, the low-frequency Asn396Ser variant is present in roughly 2.2% of people of mixed European descent, which is lower than the 2.6% of African American participants or the 3.2% of participants of mixed European descent harboring sequence variants of PCSK9 shown to protect against coronary heart disease. The Asn396Ser variant also appears to have an approximate 10%–16% increase in HDL-C, lower than the 15%–28% decrease in LDL-C from PCSK9 variants (39). As the minor allele frequency and effect size decrease, increasingly large cohorts are needed to detect a phenotypic effect on cardiovascular disease. We attempted to quantify an effect of the Asn396Ser variant on cardiovascular disease in FHS using a surrogate, namely carotid intimal medial thickness measured by carotid ultrasonography. These analyses showed no significant difference (Supplemental Table 4) but were clearly underpowered. A definitive answer will hopefully come with the widespread use of a cardiovascular candidate gene SNP array (40), being used to genotype multiple cohorts, including the National Heart, Lung and Blood Institute’s Candidate-gene Association Resource.

Pharmacologic options to raise HDL-C concentration are currently limited. The recent failure of torcetrapib (41), and the ongoing debate as to whether the increased mortality was an off-target or mechanism-based effect (42), highlights the need for the development of new pharmacological approaches to raise HDL-C. The increasing costs of developing and testing new drugs, particularly when so many fail human trials even after extensive testing in animal models, necessitates a prioritization of potential targets that are most likely to produce safe and effective drugs. Human genetic data linked to both intermediate phenotypes, such as elevated HDL-C, and outcomes, such as reduced CAD events, offers a powerful validation of a clinical target (43), particularly when individuals with loss-of-function variants appear healthy. Our results provide important human genetic evidence that inhibition of EL would be likely to raise HDL-C concentrations in humans. Whether or not the resulting increase in HDL-C due to EL inhibition will have an impact on cardiovascular outcome remains an open question.

View Supplemental data

We would like to acknowledge expert technical support from the London Regional Genomics Centre. The Broad Institute Center for Genotyping and Analysis is supported by grant U54 RR020278 from the National Center for Research Resources. This work was supported by an National Heart, Lung and Blood Institute Ruth L. Kirschstein National Research Service Award for Individual Predoctoral MD/PhD Fellows (F30, to A.C. Edmondson), a Research Fellowship of the Heart and Stroke Foundation of Canada (to R.J. Brown), a Doris Duke Charitable Foundation Distinguished Clinical Scientist Award (to D.J. Rader), and support from Genome Canada through the Ontario Genomics Institute (to R.A. Hegele). The FHS of the National Heart, Lung and Blood Institute of the NIH and Boston University School of Medicine is supported by the National Heart, Lung and Blood Institute’s FHS (contract no. N01-HC-25195). A portion of this research was conducted using the Linux Cluster for Genetic Analysis (LinGA-II) funded by the Robert Dawson Evans Endowment of the Department of Medicine at Boston University School of Medicine and Boston Medical Center. We would also like to thank the individuals and families from all of these studies for their participation as well as the referring physicians both in and outside the University of Pennsylvania Health System, and in particular, John Hoekstra and James Underberg for their help with recruitment of the HHDL study.

Address correspondence to: Daniel J. Rader, University of Pennsylvania School of Medicine, 654 BRBII/III Labs, 421 Curie Blvd., Philadelphia, Pennsylvania 19104-6160, USA. Phone: (215) 573-4176; Fax: (215) 573-8606; E-mail: rader@mail.med.upenn.edu.

Conflict of interest: The authors have declared that no conflict of interest exists.

Nonstandard abbreviations used: AAV, adeno-associated virus; CAD, coronary artery disease; EL, endothelial lipase; FHS, Framingham Heart Study Offspring cohort; GWAS, genome-wide association study; HDL-C, HDL cholesterol; HHDL, University of Pennsylvania High HDL Cholesterol Study; HPFS, Health Professionals Follow-Up Study; LD, linkage disequilibrium; LDL-C, LDL cholesterol; NHS II, Nurses’ Health Study II; PennCATH, University of Pennsylvania Catheterization Study; Penn HDL CC, University of Pennsylvania HDL Case-Control Study; QTDT, quantitative transmission disequilibrium test; SIRCA, Study of Inherited Risk of Coronary Atherosclerosis; UKE Hamburg CC, Universitätsklinikum Hamburg-Eppendorf Case-Control Study.

Reference information: J. Clin. Invest.119:1042–1050 (2009). doi:10.1172/JCI37176

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