Natural genetic variation as a tool in understanding the role of CETP in lipid levels and disease

Cholesteryl ester transfer protein (CETP) polymorphism modifies the Alzheimer's disease risk associated with APOE ε4 allele

Cholesterol regulates the production of amyloid beta (Abeta), which is central to the pathogenesis of Alzheimer's disease (AD), with high cellular cholesterol promoting and low cellular cholesterol reducing Abeta in vitro and in vivo. High density lipoprotein (HDL) plays a central role in the removal of excess cholesterol from cells, and cholesteryl ester transfer protein (CETP) is a crucial protein involved in the regulation of HDL levels. Two common polymorphisms in the promoter region (C-629A) and exon 14 I405V of the CETP gene are associated with CETP activity and HDL levels. To investigate if these sequence variants in CETP might be of importance in mediating susceptibility to AD, independently or in concert with apolipoprotein E (APOE) epsilon4 allele, we studied a sample of 286 Spanish AD patients and 315 healthy controls. In APOE epsilon4 carriers, homozygous for the CETP (-629) A allele had approximately a three times lower risk of developing AD (odds ratio 2.33, 95% CI 1.01-5.37), than homozygous and heterozygous carriers of the CETP (-629) C allele (odds ratio 7.12, 95% CI 4.51-11.24, P for APOE epsilon4/CETP (-629) AA genotype interaction < 0.001). Our data suggest that CETP behaves as a modifier gene of the AD risk associated with the APOE epsilon4 allele, possibly through modulation of brain cholesterol metabolism.

High-Density Lipoprotein Deficiency and Dyslipoproteinemia Associated With Venous Thrombosis in Men

BACKGROUND

Although dyslipoproteinemia is associated with arterial atherothrombosis, little is known about plasma lipoproteins in venous thrombosis patients.

METHODS AND RESULTS

We determined plasma lipoprotein subclass concentrations using nuclear magnetic resonance spectroscopy and antigenic levels of apolipoproteins AI and B in blood samples from 49 male venous thrombosis patients and matched controls aged <55 years. Venous thrombosis patients had significantly lower levels of HDL particles, large HDL particles, HDL cholesterol, and apolipoprotein AI and significantly higher levels of LDL particles and small LDL particles. The quartile-based odds ratios for decreased HDL particle and apolipoprotein AI levels in patients compared with controls were 6.5 and 6.0 (95% CI, 2.3 to 19 and 2.1 to 17), respectively. Odds ratios for apolipoprotein B/apolipoprotein AI ratio and LDL cholesterol/HDL cholesterol ratio were 6.3 and 2.7 (95% CI, 1.9 to 21 and 1.1 to 6.5), respectively. When polymorphisms in genes for hepatic lipase, endothelial lipase, and cholesteryl ester transfer protein were analyzed, patients differed significantly from controls in the allelic frequency for the TaqI B1/B2 polymorphism in cholesteryl ester transfer protein, consistent with the observed pattern of lower HDL and higher LDL.

CONCLUSIONS

Venous thrombosis in men aged <55 years old is associated with dyslipoproteinemia involving lower levels of HDL particles, elevated levels of small LDL particles, and an elevated ratio of apolipoprotein B/apolipoprotein AI. This dyslipoproteinemia seems associated with a related cholesteryl ester transfer protein genotype difference.

Cholesteryl Ester Transfer Protein Variants Have Differential Stability but Uniform Inhibition by Torcetrapib

Cholesteryl ester transfer protein (CETP) is an important modulator of high density lipoprotein cholesterol in humans and thus considered to be a therapeutic target for preventing cardiovascular disease. The gene encoding CETP has been shown to be highly variable, with multiple single nucleotide polymorphisms responsible for altering both its transcription and sequence. Examining nine missense variants of CETP, we found some had significant associations with CETP mass and high density lipoprotein cholesterol levels. Two variants, Pro-373 and Gln-451, appear to be more stable in vivo, an observation mirrored by partial proteolysis studies performed in vitro. Because these naturally occurring variant proteins are potentially present in clinical populations that will be treated with CETP inhibitors, all commonly occurring haplotypes were tested to determine whether the proteins they encode could be inhibited by torcetrapib, a compound currently in clinical trials in combination with atorvastatin. Torcetrapib behaved similarly with all variants, with no significant differences in inhibition.

Cholesteryl ester transfer protein promoter single-nucleotide polymorphisms in Sp1-binding sites affect transcription and are associated with high-density lipoprotein cholesterol

Genetic variation in the human cholesteryl ester transfer protein (CETP) promoter has been shown to be associated with high-density lipoprotein cholesterol (HDL-C) levels and cardiovascular disease. Some of this variation occurs in Sp1/Sp3 binding sites in the proximal promoter. We find that both the known promoter polymorphism at -629 and the previously uncharacterized polymorphism at -38 are associated with HDL-C levels in vivo and affect transcription in vitro. While the -629 polymorphism is common in all ethnic groups, the -38 polymorphism is found at significant levels (6.4%) only among African Americans. Those homozygous for the less common -38A allele have higher HDL-C levels than those with the more frequent -38G allele. This association was found in a population of African Americans at risk of cardiovascular disease and then replicated in a different population chosen from among patients with extremes of HDL-C. When studied in vitro, the most transcriptionally active allele (-629C/-38G) yields 51% more reporter protein than the least active allele (-629A/-38A) in HepG2 cells. These transcriptional effects reflect the projected impact of increased CETP expression on HDL-C phenotypes seen in vivo.

Inhibition of cholesteryl ester transfer protein activity: a new therapeutic approach to raising high-density lipoprotein

High-density lipoprotein (HDL) cholesterol levels are inversely associated with risk of atherosclerotic cardiovascular disease (ASCVD), leading to the concept that pharmacologic therapy to raise HDL cholesterol levels may reduce ASCVD risk. There is substantial interest in the concept of inhibition of the cholesteryl ester transfer protein (CETP) as a novel strategy for raising HDL cholesterol levels, as well as reducing levels of atherogenic lipoproteins. This article reviews the physiology of CETP in lipoprotein metabolism and the data in animals and humans that are relevant to the question of whether CETP inhibition may some day be part of the clinical armamentarium for treating dyslipidemia and atherosclerotic vascular disease.

CETP polymorphisms associated with HDL cholesterol may differ from those associated with cardiovascular disease

To better understand the role of cholesteryl ester transfer protein (CETP) in cardiovascular disease, nine polymorphisms spanning the gene from the upstream promoter region to beyond the 3'UTR were genotyped in 2553 individuals from multiple ethnic groups and with different cardiovascular disease profiles. The frequency of four of these SNPs varied by 40-300% between Caucasians and African Americans. SNPs in each ethnic group fell into two haploblocks with significant linkage disequilibrium within each block. SNPs in the 5' haploblock were significantly associated with HDL cholesterol while SNPs in the 3' haploblock were, at best, only weakly associated with HDL-C. One SNP in the 3' haploblock (rs1800774 in intron 12) was highly associated with history of myocardial infarction even though it was not associated with HDL-C. This association was driven by the effect in Caucasian women where 11.9% of the women with no history of MI are homozygous for the less common allele while 23.7% of those with a history of MI share this genotype. In addition, this SNP was highly associated with BMI among Caucasians (p < 0.0001). The association of HDL-C with CETP genotype was found to be independent of smoking or alcohol consumption. These results replicate some earlier findings and also help to explain some of the apparent contradictions in the literature surrounding the role of CETP in modulating HDL-C and cardiovascular disease.

Lipid transfer proteins (LTP) and atherosclerosis

This review deals with four lipid transfer proteins (LTP): three are involved in cholesteryl ester (CE) synthesis or transport, the fourth deals with plasma phospholipid (PL) transfer. Experimental models of atherosclerosis, clinical and epidemiological studies provided information as to the relationship of these LTP(s) to atherosclerosis, which is the main focus of this review. Thus, inhibition of acyl-CoA:cholesterol acyltransferase (ACAT) 1 and 2 decreases cholesterol absorption, plasma cholesterol and aortic cholesterol esterification in the aorta. The discovery that tamoxifen is a potent ACAT inhibitor explained the plasma cholesterol lowering of the drug. The use of ACAT inhibition in humans is under current investigation. As low cholesteryl ester transfer protein (CETP) activity is connected with high HDL-C, several CETP inhibitors were tried in rabbits, with variable results. A new CETP inhibitor, Torcetrapib, was tested in humans and there was a 50-100% increase in HDL-C. Lecithin cholesterol acyl-transferase (LCAT) influences oxidative stress, which can be lowered by transient LCAT gene transfer in LCAT-/- mice. Phospholipid transfer protein (PLTP) deficiency reduced apo B production in apo E-/- mice, as well as oxidative stress in four models of mouse atherosclerosis. In conclusion, the ability to increase HDL-C so markedly by inhibitors of CETP introduces us into a new era in prevention and treatment of coronary heart disease (CHD).

A genomewide search using an original pairwise sampling approach for large genealogies identifies a new locus for total and low-density lipoprotein cholesterol in two genetically differentiated isolates of Sardinia

A powerful approach to mapping the genes for complex traits is to study isolated founder populations, in which genetic heterogeneity and environmental noise are likely to be reduced and in which extended genealogical data are often available. Using graph theory, we applied an approach that involved sampling from the large number of pairwise relationships present in an extended genealogy to reconstruct sets of subpedigrees that maximize the useful information for linkage mapping while minimizing calculation burden. We investigated, through simulation, the properties of the different sets in terms of bias in identity-by-descent (IBD) estimation and power decrease under various genetic models. We applied this approach to a small isolated population from Sardinia, the village of Talana, consisting of a unique large and complex pedigree, and performed a genomewide search through variance-components linkage analysis for serum lipid levels. We identified a region of significant linkage on chromosome 2 for total serum cholesterol and low-density lipoprotein (LDL) cholesterol. Through higher-density mapping, we obtained an increased linkage for both traits on 2q21.2-q24.1, with a LOD score of 4.3 for total serum cholesterol and of 3.9 for LDL cholesterol. A replication study was performed in an independent and larger set from a genetically differentiated isolated population of the same region of Sardinia, the village of Perdasdefogu. We obtained consistent linkage to the region for total serum cholesterol (LOD score 1.4) and LDL cholesterol (LOD score 2.2), with a level of concordance uncommon for complex traits, and refined the location of the quantitative-trait locus. Interestingly, the 2q21.1-22 region has also been linked to premature coronary heart disease in Finns, and, in the adjacent 2q14 region, significant linkage with triglycerides has been reported in Hutterites.

Plasma Levels of Cholesteryl Ester Transfer Protein and the Risk of Future Coronary Artery Disease in Apparently Healthy Men and Women

Background— Low plasma levels of cholesteryl ester transfer protein (CETP) are associated with elevated levels of HDL cholesterol (HDL-C), but it remains unclear whether this translates into a concomitant reduction in the risk of coronary artery disease (CAD). Evidence exists that the effect of CETP depends on metabolic context, in particular on triglyceride levels.

Methods and Results— A nested case-control study was performed in the prospective EPIC-Norfolk cohort study. Cases were apparently healthy men and women aged 45 to 79 years who developed fatal or nonfatal CAD during follow-up. Control subjects were matched by age, sex, and enrollment time. CETP levels were not significantly different between cases and controls (4.0±2.2 versus 3.8±2.1 mg/L, P =0.07). CETP levels were significantly related to plasma levels of total cholesterol, LDL cholesterol, and HDL-C. The risk of CAD increased with increasing CETP quintiles ( P for linearity=0.02), such that subjects in the highest quintile had an adjusted OR of 1.43 (95% CI 1.03 to 1.99, P =0.03) versus those in the lowest. Among individuals with triglyceride levels below the median (1.7 mmol/L), no relationship between CETP levels and CAD risk was observed ( P for linearity=0.5), but this relationship was strong among those with high triglyceride levels ( P for linearity=0.02), such that those in the highest CETP quintile had an OR of 1.87 (95% CI 1.06 to 3.30, P =0.02).

Conclusions— Elevated CETP levels are associated with an increasing risk of future CAD in apparently healthy individuals, but only in those with high triglyceride levels.

A review of CETP and its relation to atherosclerosis

Although the atheroprotective role of HDL cholesterol (HDL-c) is well documented, effective therapeutics to selectively increase plasma HDL-c levels are not yet available. Recent progress in unraveling human HDL metabolism has fuelled the development of strategies to decrease the incidence and progression of coronary artery disease (CAD) by raising HDL-c. In this quest for novel drugs, cholesteryl ester transfer protein (CETP) represents a pivotal target. The role of this plasma protein in HDL metabolism is highlighted by the discovery that genetic CETP deficiency is the main cause of high HDL-c levels in Asian populations. The use of CETP inhibitors to effectively increase HDL-c concentration in humans was recently published and data with regard to the effect on human atherosclerosis are expected shortly. This review discusses the potential of CETP inhibitors to protect against atherosclerosis in the context of the current knowledge of CETP function in both rodents and humans.

CETP gene variation: relation to lipid parameters and cardiovascular risk

PURPOSE OF REVIEW

Over the past decade lowering of low-density lipoprotein-cholesterol levels has been established as the foundation for preventing coronary artery disease, but substantial additional risk reduction remains to be gained by modifying risk factors other than low-density lipoprotein-cholesterol. Raising high-density lipoprotein-cholesterol levels by inhibiting activity of the cholesteryl ester transfer protein (CETP) is a prime target. Research on naturally occurring variants in the CETP gene has yielded numerous insights that have been relevant for understanding lipoprotein metabolism, and crucial to the development of pharmacological CETP inhibition.

RECENT FINDINGS

This review discusses a number of recently published studies, including a haplotype analysis of the CETP promoter region confirming that the -629 C-->A variant, not the TaqIB variant, is instrumental in determining CETP activity, as previously suggested. In addition, we discuss a recent meta-analysis which confirms that the I405V and TaqIB variants are indeed associated with lower CETP activity and higher high-density lipoprotein-cholesterol levels. Also, we review two subanalyses of large randomized controlled pravastatin trials which found no evidence for a proposed pharmacogenetic interaction between the CETP TaqIB variant and pravastatin treatment.

SUMMARY

The currently available evidence suggests that several genetic variants in the CETP gene are associated with altered CETP plasma levels and activity, high-density lipoprotein-cholesterol plasma levels, low-density lipoprotein and high-density lipoprotein particle size, and perhaps the risk of coronary artery disease. No evidence exists for a pharmacogenetic interaction between the CETP TaqIB variant and pravastatin efficacy.

Highly polymorphic repeat region in the CETP promoter induces unusual DNA structure

Genetic variation in the human cholesteryl ester transfer protein (CETP) promoter is associated with HDL cholesterol levels and cardiovascular disease with much of the genetic variation in CETP attributed to the promoter region. In this region, there are several single nucleotide polymorphisms as well as a variable length tandem repeat located 1946 base pairs upstream of the CETP transcription start that is highly polymorphic with respect to both length and sequence. There are more than 10 different long alleles and these vary in their repeat structure. We find that the short allele of this repeat is associated with high HDL cholesterol levels in vivo (P<0.0001). In males, this association is independent of the nearby -629 polymorphism. In addition, the variable length GAAA repeat can stimulate an adjacent GGGGA repeat to form a structure that hinders DNA amplification and sequencing. This structure also has an effect in vivo as shown by orientation effects and cloning efficiency in Escherichia coli.

5th Annual International Conference on HDL Cholesterol: metabolic pathways and drug developments

Cardiovascular diseases (CVD) are the number one killer in the USA and are likely to be a significant cause of mortality in developing countries in the near future. High levels of high-density lipoprotein (HDL) are known to be inversely related to CVD incidence. However, whether or not HDL plays a direct preventative role or is simply a coincident marker for other protective processes has been controversial. Fortunately, recent discoveries have dramatically increased understanding of HDL function and recent clinical studies suggest that raising HDL levels may indeed be an effective strategy for preventing CVD. This meeting was organised to highlight state-of-the art studies focusing on the mechanisms of HDL function. Important work concerning small-molecule and immunological approaches for raising HDL were presented, including the targeting of regulatory nuclear receptors, plasma enzymes/transfer proteins that remodel HDL, and cell surface proteins that influence HDL formation and function. A detailed understanding of the molecular basis for the protective effect of HDL will hopefully lead to the development of new therapeutics that exploit this pathway.

Cholesteryl ester transfer protein concentration is associated with progression of atherosclerosis and response to pravastatin in men with coronary artery disease (REGRESS)

BACKGROUND

The TaqIB polymorphism in the cholesteryl ester transfer protein (CETP) gene is associated with HDL-C, progression of coronary artery disease (CAD) and response to pravastatin treatment in men with angiographically proven CAD (REGRESS). We hypothesized that differences in CETP concentration could explain these associations and now investigated whether CETP concentration is an independent determinant of these parameters.

MATERIALS AND METHODS

Plasma CETP concentrations at baseline and after 2 years' treatment with pravastatin or placebo were measured (n=674), and correlations with lipid and angiographic parameters (mean segment- and obstruction-diameter; MSD and MOD), and TaqIB genotype were studied.

RESULTS

After segregation into three groups (baseline CETP<1.58, 1.58-2.21, >2.21 mg L(-1)), subjects with the highest CETP had significantly higher baseline total cholesterol, LDL-C and triglycerides (P<0.01), while HDL-C, MSD and MOD were not different among these groups. After 2 years of placebo, the MSD decreased threefold (P<0.001) and the MOD decreased 2.4-fold (P=0.042) more in the highest compared with the lowest CETP quartile. Pravastatin treatment reduced total cholesterol LDL-C and triglycerides significantly more in the highest CETP quartile. Moreover, only in the highest CETP quartile, pravastatin significantly reduced the MSD- (P=0.003) and MOD-decrease (P=0.014) compared with placebo, and, notably, this was independent of baseline lipids and differential lipid changes in these quartiles. Strikingly, baseline associations and treatment responses according to baseline CETP were independent of TaqIB genotype.

CONCLUSIONS

High CETP concentration is associated with faster progression of coronary atherosclerosis in men with proven CAD. Second, pravastatin yielded the highest improvement of lipid and angiographic parameters in patients with high baseline CETP independent of baseline lipids, lipid changes and TaqIB genotype, indicating that the plasma CETP level itself is an important determinant of the response to statins.

Alterations in high-density lipoprotein metabolism and reverse cholesterol transport in insulin resistance and type 2 diabetes mellitus: role of lipolytic enzymes, lecithin:cholesterol acyltransferase and lipid transfer proteins

Insulin resistance and type 2 diabetes mellitus are generally accompanied by low HDL cholesterol and high plasma triglycerides, which are major cardiovascular risk factors. This review describes abnormalities in HDL metabolism and reverse cholesterol transport, i.e. the transport of cholesterol from peripheral cells back to the liver for metabolism and biliary excretion, in insulin resistance and type 2 diabetes mellitus. Several enzymes including lipoprotein lipase (LPL), hepatic lipase (HL) and lecithin: cholesterol acyltransferase (LCAT), as well as cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP), participate in HDL metabolism and remodelling. Lipoprotein lipase hydrolyses lipoprotein triglycerides, thus providing lipids for HDL formation. Hepatic lipase reduces HDL particle size by hydrolysing its triglycerides and phospholipids. A decreased postheparin plasma LPL/HL ratio is a determinant of low HDL2 cholesterol in insulin resistance. The esterification of free cholesterol by LCAT increases HDL particle size. Plasma cholesterol esterification is unaltered or increased in type 2 diabetes mellitus, probably depending on the extent of triglyceride elevation. Subsequent CETP action results in transfer of cholesteryl esters from HDL towards triglyceride-rich lipoproteins, and is involved in decreasing HDL size. An increased plasma cholesteryl ester transfer is frequently observed in insulin-resistant conditions, and is considered to be a determinant of low HDL cholesterol. Phospholipid transfer protein generates small pre beta-HDL particles that are initial acceptors of cell-derived cholesterol. Its activity in plasma is elevated in insulin resistance and type 2 diabetes mellitus in association with high plasma triglycerides and obesity. In insulin resistance, the ability of plasma to promote cellular cholesterol efflux may be maintained consequent to increases in PLTP activity and pre beta-HDL. However, cellular cholesterol efflux to diabetic plasma is probably impaired. Besides, cellular abnormalities that are in part related to impaired actions of ATP binding cassette transporter 1 and scavenger receptor class B type I are likely to result in diminished cellular cholesterol efflux in the diabetic state. Whether hepatic metabolism of HDL-derived cholesterol and subsequent hepatobiliary transport is altered in insulin resistance and type 2 diabetes mellitus is unknown. Specific CETP inhibitors have been developed that exert major HDL cholesterol-raising effects in humans and retard atherosclerosis in animals. As an increased CETP-mediated cholesteryl ester transfer represents a plausible metabolic intermediate between high triglycerides and low HDL cholesterol, studies are warranted to evaluate the effects of these agents in insulin resistance- and diabetes-associated dyslipidaemia.