Prevalence and phenotypic spectrum of cholesteryl ester transfer protein gene mutations in Japanese hyperalphalipoproteinemia

Cholesteryl Ester Transfer Protein (CETP) Variations in Relation to Lipid Profiles and Cardiovascular Diseases: An Update

Lipid metabolism plays an essential role in the pathogenesis of cardiovascular and metabolic diseases. Cholesteryl ester transfer protein (CETP) is a crucial glycoprotein involved in lipid metabolism by transferring cholesteryl esters (CE) and triglycerides (TG) between plasma lipoproteins. CETP activity results in reduced HDL-C and increased VLDL- and LDL-C concentrations, thus increasing the risk of cardiovascular and metabolic diseases. In this review, we discuss the structure of CETP and its mechanism of action. Furthermore, we focus on recent experiments on animal CETP-expressing models, deciphering the regulation and functions of CETP in various genetic backgrounds and interaction with different external factors. Finally, we discuss recent publications revealing the association of CETP single nucleotide polymorphisms (SNPs) with the risk of cardiovascular and metabolic diseases, lifestyle factors, diet and therapeutic interventions. While CETP SNPs can be used as effective diagnostic markers, diet, lifestyle, gender and ethnic specificity should also be considered for effective treatment.

Variants in the CETP gene affect levels of HDL cholesterol by reducing the amount, and not the specific lipid transfer activity, of secreted CETP

BACKGROUND

Cholesteryl ester transfer protein (CETP) transfers cholesteryl esters in plasma from high density lipoprotein (HDL) to very low density lipoprotein and low density lipoprotein. Loss-of-function variants in the CETP gene cause elevated levels of HDL cholesterol. In this study, we have determined the functional consequences of 24 missense variants in the CETP gene. The 24 missense variants studied were the ones reported in the Human Gene Mutation Database and in the literature to affect HDL cholesterol levels, as well as two novel variants identified at the Unit for Cardiac and Cardiovascular Genetics, Oslo University Hospital in subjects with hyperalphalipoproteinemia.

METHODS

HEK293 cells were transiently transfected with mutant CETP plasmids. The amounts of CETP protein in lysates and media were determined by Western blot analysis, and the lipid transfer activities of the CETP variants were determined by a fluorescence-based assay.

RESULTS

Four of the CETP variants were not secreted. Five of the variants were secreted less than 15% compared to the WT-CETP, while the other 15 variants were secreted in varying amounts. There was a linear relationship between the levels of secreted protein and the lipid transfer activities (r = 0.96, p<0.001). Thus, the secreted variants had similar specific lipid transfer activities.

CONCLUSION

The effect of the 24 missense variants in the CETP gene on the lipid transfer activity was mediated predominantly by their impact on the secretion of the CETP protein. The four variants that prevented CETP secretion cause autosomal dominant hyperalphalipoproteinemia. The five variants that markedly reduced secretion of the respective variants cause mild hyperalphalipoproteinemia. The majority of the remaining 15 variants had minor effects on the secretion of CETP, and are considered neutral genetic variants.

Endothelial Function in Dyslipidemia: Roles of LDL-Cholesterol, HDL-Cholesterol and Triglycerides

Dyslipidemia is associated with endothelial dysfunction. Endothelial dysfunction is the initial step for atherosclerosis, resulting in cardiovascular complications. It is clinically important to break the process of endothelial dysfunction to cardiovascular complications in patients with dyslipidemia. Lipid-lowering therapy enables the improvement of endothelial function in patients with dyslipidemia. It is likely that the relationships of components of a lipid profile such as low-density lipoprotein cholesterol, high-density lipoprotein cholesterol and triglycerides with endothelial function are not simple. In this review, we focus on the roles of components of a lipid profile in endothelial function.

Hyperalphalipoproteinemia and Beyond: The Role of HDL in Cardiovascular Diseases

Hyperalphalipoproteinemia (HALP) is a lipid disorder characterized by elevated plasma high-density lipoprotein cholesterol (HDL-C) levels above the 90th percentile of the distribution of HDL-C values in the general population. Secondary non-genetic factors such as drugs, pregnancy, alcohol intake, and liver diseases might induce HDL increases. Primary forms of HALP are caused by mutations in the genes coding for cholesteryl ester transfer protein (CETP), hepatic lipase (HL), apolipoprotein C-III (apo C-III), scavenger receptor class B type I (SR-BI) and endothelial lipase (EL). However, in the last decades, genome-wide association studies (GWAS) have also suggested a polygenic inheritance of hyperalphalipoproteinemia. Epidemiological studies have suggested that HDL-C is inversely correlated with cardiovascular (CV) risk, but recent Mendelian randomization data have shown a lack of atheroprotective causal effects of HDL-C. This review will focus on primary forms of HALP, the role of polygenic inheritance on HDL-C, associated risk for cardiovascular diseases and possible treatment options.

Atherosclerotic cardiovascular disease in hyperalphalipoproteinemia due to LIPG variants

BACKGROUND

High density lipoprotein cholesterol (HDL-C) concentration correlates inversely with atherosclerotic cardiovascular disease (ASCVD) risk and is included in risk calculations. Endothelial lipase (EL) is a phospholipase that remodels HDL. Deficiency of EL due to mutations in its gene, LIPG, is associated with hyperalphalipoproteinemia. The effects of EL on HDL function and ASCVD risk remain poorly understood.

OBJECTIVES

To determine whether hyperalphalipoproteinemia due to EL deficiency is protective against ASCVD.

METHODS

We identified LIPG variants amongst patients with severe hyperalphalipoproteinemia (HDL-C >2.5 mmol/L) attending a referral lipid clinic in the Western Cape Province of South Africa. We analysed the clinical and biochemical phenotypes amongst primary hyperalphalipoproteinemia cases (males HDL-C >1.6 mmol/L; females HDL-C >1.8 mmol/L) due to LIPG variants, and the distribution of variants in normal and hyperalphalipoproteinemia ranges of HDL-C.

RESULTS

1007 patients with HDL-C concentration ranging from 1.2 to 4.5 mmol/L were included. Seventeen females had primary hyperalphalipoproteinemia. Vascular disease was prominent, but not associated with HDL-C concentration, LDL-C concentration or carotid artery intima media thickness. Two novel and three known LIPG variants were identified in severe hyperalphalipoproteinemia. Four additional variants were identified in the extended cohort. Two common variants appeared normally distributed across the HDL-C concentration range, while six less-common variants were found only at higher HDL-C concentrations. One rare variant had a moderate effect.

CONCLUSION

Hyperalphalipoproteinemia due to LIPG variants is commoner in females and may not protect against ASCVD. Use of current risk calculations may be inappropriate in patients with hyperalphalipoproteinemia due to EL deficiency. Our study cautions targeting EL to reduce risk.

Relationship between high-density lipoprotein cholesterol levels and endothelial function in women: a cross-sectional study

OBJECTIVES

The purpose of this study was to evaluate the relationship between high-density lipoprotein cholesterol (HDL-C) levels and endothelial function in women.

DESIGN

Cross-sectional study.

SETTING

22 university hospitals and affiliated clinics in Japan.

PARTICIPANTS

1719 Japanese women aged 17-90 years who were not receiving lipid-lowering therapy.

MEASURES

We evaluated flow-mediated vasodilation (FMD) and serum levels of HDL-C. All participants were divided into four groups by HDL-C level: low HDL-C (<40 mg/dL), moderate HDL-C (40-59 mg/dL), high HDL-C (60-79 md/dL) and extremely high HDL-C (≥80 mg/dL).

RESULTS

Univariate regression analysis revealed a significant relationship between FMD and HDL-C (r=0.12, p<0.001). FMD values were significantly smaller in the low HDL-C group (5.2%±3.8%) and moderate HDL-C group (5.2%±3.8%) than in the extremely high HDL-C group (6.7%±3.4%) (p=0.024 and p=0.003, respectively), while there was no significant difference in FMD between the high HDL-C group and the extremely high HDL-C group. Multiple logistic regression analysis did not show a significant association between HDL-C levels and FMD.

CONCLUSIONS

Endothelial function increased in relation to HDL-C levels. However, there was no association of HDL-C levels with endothelial function after adjustment of traditional cardiovascular risk factors in women.

TRIAL REGISTRATION NUMBER

UMIN000012950; Results.

Is Anacetrapib Better Than Its CETP Inhibitor Counterparts?

Cholesterol metabolism and transport has been a major focus in cardiovascular disease risk modification over the past several decades. Hydroxymethylglutaryl-CoA reductase inhibitors (statins) have been the most commonly used agents, with the greatest benefit in reducing both the primary and secondary risks of cardiovascular disease. However, heart disease remains the leading cause of death in both men and women in the United States. Further investigation and intervention are required to further reduce the risk for cardiovascular disease and cardiovascular-related deaths. This review will focus on high-density lipoprotein metabolism and transport, looking particularly at cholesteryl ester transfer protein (CETP) inhibitors. While studies of the other CETP inhibitors in its class have not shown a significant improvement in the prevention of primary or secondary cardiovascular risk, anacetrapib, the fourth and latest of the CETP inhibitors to be investigated, may be more promising.

Retinal Vein Occlusion is Associated with Low Blood High-Density Lipoprotein Cholesterol: A Nationwide Cohort Study

PURPOSE

To investigate association between the development of retinal vein occlusion (RVO) and blood high-density lipoprotein cholesterol (HDL-C).

DESIGN

A retrospective, nationwide, population-based cohort study.

METHODS

This study was set in the Republic of Korea and included 23,149,403 people ≥20 years of age who underwent the Korean National Health Screening Program examination between January 2009 and December 2012. Among them, the RVO group was composed of patients with an initial diagnosis of RVO made between 2009 and 2015 (n = 117,639). The earliest claim with an RVO diagnostic code was considered as the incident time. The predictive value of HDL-C level for RVO was analyzed using hazard ratios. The primary outcome measure was the incident cases of RVO.

RESULTS

Subjects with RVO were generally older; had high body mass index, waist circumference, fasting blood glucose, blood pressure, total and low-density lipoprotein cholesterol, and triglyceride values, and low glomerular filtration rate and HDL-C values; and were more likely to experience diabetes mellitus and hypertension compared with the non-RVO group. The fully adjusted hazard ratio of RVO was 1.12 (95% confidence interval 1.10-1.14) in the lowest quartile of HDL-C versus in the highest quartile. The association between the development of RVO and HDL-C was higher those with a younger age, male sex, current smoking habit, diabetes mellitus, and hypercholesterolemia. In addition, we observed a significant synergistic effect of low HDL-C level with obesity and hypertension.

CONCLUSION

This is the first nationwide population-based epidemiologic study evaluating the association between HDL-C level and the risk of RVO development. A significant association between low HDL-C and RVO development was found.

Inheritance of high and low HDL: mechanisms and management

PURPOSE OF REVIEW

The inverse association between plasma high-density lipoprotein cholesterol (HDL-C) concentration and the incidence of cardiovascular disease (CVD) has been unequivocally proven by many epidemiological studies. There are several genetic disorders affecting HDL-C plasma levels, either providing atheroprotection or predisposing to premature atherosclerosis. However, up to date, there has not been any pharmacological intervention modulating HDL-C levels, which has been clearly shown to prevent the progression of CVD. Thus, clarifying the exact underlying mechanisms of inheritance of these genetic disorders that affect HDL is a current goal of the research, as key roles of molecular components of HDL metabolism and function can be revealed and become targets for the discovery of novel medications for the prevention and treatment of CVD.

RECENT FINDINGS

Primary genetic disorders of HDL can be either associated with longevity or, in contrast, may lead to premature CVD, causing high morbidity and mortality to their carriers. A large body of recent research has closely examined the genetic disorders of HDL and new promising therapeutic strategies have been developed, which may be proven beneficial in patients predisposed to CVD in the near future.

SUMMARY

We have reviewed recent findings on the inheritance of genetic disorders associated with high and low HDL-C plasma levels and we have discussed their clinical features, as well as information about new promising HDL-C-targeted therapies that are under clinical trials.

Primary genetic disorders affecting high density lipoprotein (HDL)

There is extensive evidence demonstrating that there is a clear inverse correlation between plasma high density lipoprotein cholesterol (HDL-C) concentration and cardiovascular disease (CVD). On the other hand, there is also extensive evidence that HDL functionality plays a very important role in atheroprotection. Thus, genetic disorders altering certain enzymes, lipid transfer proteins, or specific receptors crucial for the metabolism and adequate function of HDL, may positively or negatively affect the HDL-C levels and/or HDL functionality and subsequently either provide protection or predispose to atherosclerotic disease. This review aims to describe certain genetic disorders associated with either low or high plasma HDL-C and discuss their clinical features, associated risk for cardiovascular events, and treatment options.

Re-evaluation of cholesteryl ester transfer protein function in atherosclerosis based upon genetics and pharmacological manipulation

PURPOSE OF REVIEW

To re-evaluate the functions of plasma cholesteryl ester transfer protein (CETP) in atherosclerosis based upon recent findings from human genetics and pharmacological CETP manipulation.

RECENT FINDINGS

CETP is involved in the transfer of cholesteryl ester from HDL to apolipoprotein B-containing lipoproteins, a key step of reverse cholesterol transport (RCT). CETP inhibitors have been developed to raise serum HDL-cholesterol (HDL-C) levels and reduce cardiovascular events. However, outcome studies of three CETP inhibitors (torcetrapib, dalcetrapib and evacetrapib) were prematurely terminated because of increased mortality or futility despite marked increases in HDL-cholesterol and decreases in LDL-cholesterol except for dalcetrapib. Patients with CETP deficiency show remarkable changes in HDL and LDL and are sometimes accompanied by atherosclerotic cardiovascular diseases. Recent prospective epidemiological studies demonstrated atheroprotective roles of CETP. CETP inhibition induces formation of small dense LDL and possibly dysfunctional HDL and downregulates hepatic scavenger receptor class B type I (SR-BI). Therefore, CETP inhibitors may interrupt LDL receptor and SR-BI-mediated cholesterol delivery back to the liver.

SUMMARY

For future drug development, the opposite strategy, namely enhancers of RCT via CETP and SR-BI activation as well as the inducers of apolipoprotein A-I or HDL production might be a better approach rather than delaying HDL metabolism by inhibiting a main stream of RCT in vivo.

A patient with a novel homozygous missense mutation in FTO and concomitant nonsense mutation in CETP

The fat mass and obesity associated gene (FTO) has previously been associated with a variety of diseases and conditions, notably obesity, acute coronary syndrome and metabolic syndrome. Reports describing mutations in FTO as well as FTO animal models have further demonstrated a role for FTO in the development of the brain and other organs. Here, we describe a patient born of consanguineous union who presented with microcephaly, developmental delay, behavioral abnormalities, dysmorphic facial features, hypotonia, and other various phenotypic abnormalities. Whole exome sequencing revealed a novel homozygous missense mutation in FTO and a nonsense mutation in the cholesteryl ester transfer protein (CETP). Exome CNV analysis revealed no disease causing large duplications or deletions within coding regions. Patient’s, her parents’ and non-related control’ fibroblasts were analyzed for morphologic defects, abnormal proliferation, apoptosis and transcriptome profile. We have shown that FTO is located in nucleus of cells from each tested samples. Western blot analysis demonstrated no changes in patient FTO. Q-PCR analysis revealed slightly decreased levels of FTO expression in patient cells compared to controls. No morphological or proliferation differences between the patient and control fibroblasts were observed. There is still much to be learned about the molecular mechanisms by which mutations in FTO contribute to such severe phenotypes.

Alcohol-independent beneficial cardiometabolic profile of individuals with hyper-HDL cholesterolemia in Japanese men and women

BACKGROUND

There is limited information on characterization of individuals with hyper-high-density lipoprotein (HDL) cholesterolemia.

OBJECTIVES

The purpose of this study was to investigate the cardiometabolic profile of individuals with hyper-HDL cholesterolemia in comparison with the profile of individuals with normo-HDL cholesterolemia.

METHODS

The subjects were Japanese men and women who had hyper-HDL cholesterolemia (≥100 mg/dL) and their control subjects who had normal HDL cholesterol levels (≥40 and <80 mg/dL) and were matched for age or age and alcohol consumption. The cardiometabolic profiles were compared between the hyper- and normo-HDL cholesterolemic groups.

RESULTS

Both in men and women, body mass index, waist-to-height ratio, triglycerides, low-density lipoprotein cholesterol, and hemoglobin A1c were significantly lower in subjects with hyper-HDL cholesterolemia than in subjects with normo-HDL cholesterolemia, whereas systolic and diastolic blood pressure levels were not significantly different between the 2 groups. In generalized estimating equation with adjustment for smoking and regular exercise, odds ratios of the hyper- vs normo-HDL cholesterolemic groups were significantly lower than the reference level of 1.00 for high body mass index, high waist-to-height ratio, hypertriglyceridemia, hyper-low-density lipoprotein cholesterolemia, high lipid accumulation product, and metabolic syndrome. The previously mentioned results were obtained both in age-matched analysis and in age- and alcohol intake-matched analysis, although the percentage of regular drinkers was significantly higher in the hyper-HDL cholesterolemic group than in the age-matched control group.

CONCLUSIONS

Hyper-HDL cholesterolemia was inversely associated with obesity, dyslipidemia, and metabolic syndrome in the analysis using alcohol intake-matched subject groups. Therefore, the association of hyper-HDL cholesterolemia with lower cardiometabolic risk is thought to be independent of habitual alcohol drinking.

A novel microdeletion affecting the CETP gene raises HDL-associated cholesterol levels

We describe a novel, inherited 16q13 microdeletion that removes cholesteryl ester transfer protein (CETP) and several nearby genes. The proband was originally referred for severe childhood-onset obesity and moderate developmental delay, but his fasting lipid profile revealed relatively high levels of high density lipoprotein cholesterol (HDL-C) and relatively low levels of low density lipoprotein cholesterol (LDL-C) for age, despite his obesity. Testing of first-degree relatives identified two other microdeletion carriers. Functional assays in affected individuals showed decreased CETP mRNA expression and enzymatic activity. This microdeletion may or may not be pathogenic for obesity and developmental delay, but based on the lipid profile, the functional studies, and the phenotype of other patients with loss-of-function mutations of CETP, we believe this microdeletion to be antipathogenic for cardiovascular disease.

HDL biogenesis, remodeling, and catabolism

In this chapter, we review how HDL is generated, remodeled, and catabolized in plasma. We describe key features of the proteins that participate in these processes, emphasizing how mutations in apolipoprotein A-I (apoA-I) and the other proteins affect HDL metabolism. The biogenesis of HDL initially requires functional interaction of apoA-I with the ATP-binding cassette transporter A1 (ABCA1) and subsequently interactions of the lipidated apoA-I forms with lecithin/cholesterol acyltransferase (LCAT). Mutations in these proteins either prevent or impair the formation and possibly the functionality of HDL. Remodeling and catabolism of HDL is the result of interactions of HDL with cell receptors and other membrane and plasma proteins including hepatic lipase (HL), endothelial lipase (EL), phospholipid transfer protein (PLTP), cholesteryl ester transfer protein (CETP), apolipoprotein M (apoM), scavenger receptor class B type I (SR-BI), ATP-binding cassette transporter G1 (ABCG1), the F1 subunit of ATPase (Ecto F1-ATPase), and the cubulin/megalin receptor. Similarly to apoA-I, apolipoprotein E and apolipoprotein A-IV were shown to form discrete HDL particles containing these apolipoproteins which may have important but still unexplored functions. Furthermore, several plasma proteins were found associated with HDL and may modulate its biological functions. The effect of these proteins on the functionality of HDL is the topic of ongoing research.

Cardiovascular and cancer events in hyper-high-density lipoprotein cholesterolemic patients: a post hoc analysis of the MEGA study

BACKGROUND

The prognosis for hyper-high-density lipoprotein (HDL) cholesterolemic patients has not been fully elucidated. We conducted a post hoc analysis of MEGA study data to investigate prospectively the incidence of cardiovascular events and cancer in hyper-HDL cholesterolemic patients.

METHODS

A total of 7832 patients with mild hypercholesterolemia were randomly allocated to either the National Cholesterol Education Program step 1 diet alone (n = 3966) or the diet plus pravastatin (n = 3866) and followed for 5 years. The incidences of coronary heart disease (CHD), CHD plus cerebral infarction (CI), cardiovascular disease (CVD), and cancer were calculated using the Cox proportional hazards model according to the level of HDL cholesterol (HDL-C).

RESULTS

CHD incidence was lower in patients with HDL-C >60-90 mg/dL (-52%, p = 0.0018) and HDL-C > 90 mg/dL (-46%, p = 0.4007) than in patients with HDL-C ≤ 60 mg/dL. The incidences of CHD, CHD plus CI, and CVD were significantly lower in patients with HDL-C >60-90 mg/dL than in those with HDL-C ≤ 60 mg/dL in both diet-alone and diet-plus-pravastatin groups. Cancer incidence was not increased in patients with HDL-C >60-90 mg/dL.

CONCLUSION

Patients not receiving statin therapy should aim for a target HDL-C of between 60 and 90 mg/dL to achieve a significant reduction in CHD without the occurrence of adverse events.

TRIAL REGISTRATION

Clinical trials.gov NCT00211705.

Disease-associated marked hyperalphalipoproteinemia

Marked hyperalphalipoproteinemia (HAL) is a heterogeneous syndrome. To clarify the pathophysiological significance of HAL, we compared clinical profiles between marked HAL subjects with and without cholesteryl ester transfer protein (CETP) deficiency. CETP deficiency was associated with cardiovascular diseases and strokes in the HAL population, particularly in female. HAL women without CETP deficiency tended to have higher prevalence with cancer history. HAL may not always be a longevity marker, but be sometimes accompanied with pathological conditions.

Cholesteryl ester transfer protein inhibition as a strategy to reduce cardiovascular risk

Human and rabbit plasma contain a cholesteryl ester transfer protein (CETP) that promotes net mass transfers of cholesteryl esters from high density lipoproteins (HDL) to other plasma lipoprotein fractions. As predicted, inhibition of CETP in both humans and rabbits increases the concentration of cholesterol in the potentially protective HDL fraction, while decreasing it in potentially proatherogenic non-HDL fractions. Inhibition of CETP in rabbits also inhibits the development of diet-induced atherosclerosis. However, use of the CETP inhibitor torcetrapib in humans did not reduce atheroma in three imaging trials and caused an excess of deaths and cardiovascular events in a large clinical outcome trial. The precise explanation for the harm caused by torcetrapib is unknown but may relate to documented, potentially harmful effects unrelated to inhibition of CETP. More recently, a trial using the weak CETP inhibitor dalcetrapib, which raises HDL levels less effectively than torcetrapib and does not lower non-HDL lipoprotein levels, was terminated early for reasons of futility. There was no evidence that dalcetrapib caused harm in that trial. Despite these setbacks, the hypothesis that CETP inhibitors will be antiatherogenic in humans is still being tested in studies with anacetrapib and evacetrapib, two CETP inhibitors that are much more potent than dalcetrapib and that do not share the off-target adverse effects of torcetrapib.

Background to discuss guidelines for control of plasma HDL-cholesterol in Japan

A decrease in high density lipoprotein-cholesterol (HDL-C) is a strong risk factor for atherosclerotic disorders in Japan, probably more important than an increase in low density lipoprotein-cholesterol (LDL-C). While there are rational grounds for the argument that elevation of HDL-C leads to decreased risk, there has as yet been no direct evidence of such an effect. If elevation of HDL-C decreases the risk, this effect is expected throughout the normal range of HDL-C or perhaps even higher than that. Simulation based on epidemiological data indicated that it may eventually reduce the incidence of ischemic heart disease by 60-70% in Japan. In the risk management guideline, "low" HDL-C is presently defined as 40 mg/dL or below. While there is no evidence that strongly urges a change in this definition, the results of epidemiological studies support "The higher the HDL-C level, the lower the risk,"even in the "normal range". Elevation of the HDL-C level may reduce the risk, probably at least up to 70 mg/dL; however, there are no supportive data for this effect still being obtained over 80 mg/dL. Patients with homozygous CETP deficiency should be followed-up while controlling other risk factors, so as not to dismiss the possibility of a risk increase with an extremely elevated HDL-C level.

CETP deficiency due to a novel mutation in the CETP gene promoter and its effect on cholesterol efflux and selective uptake into hepatocytes

OBJECTIVES

To identify the genetic variant in the CETP gene of the proband with high HDL-C and low CETP activity and to investigate whether HDL from the CETP-deficient subject was dysfunctional in the reverse cholesterol transport (RCT) pathway.

METHODS

We sequenced the CETP gene and assessed its promoter activity. Cholesterol efflux and hepatic cholesteryl ester delivery studies were also performed using the proband's HDL.

RESULTS

A proband was a compound heterozygote for a known D459G variant and a novel 18-bp deletion mutation in the CETP promoter. This promoter mutation markedly reduced the transcriptional activity in HepG2 cells. HDL2 from this subject increased SR-BI-mediated cholesterol efflux, whereas cholesteryl ester delivery into hepatocytes was maintained.

CONCLUSION

A novel deletion mutation in the CETP promoter is associated with high HDL-C and decreased promoter activity. HDL from this CETP-deficient subject was not dysfunctional in mediating two main steps of RCT assessed in vitro.

Significance of high HDL cholesterol levels in Japanese men with metabolic syndrome

OBJECTIVE

The objective was to clarify the significance of high high-density lipoprotein cholesterol (HDL-C) levels in the metabolic syndrome (MetS). The evaluation focused on insulin resistance as an indicator of early-stage MetS.

METHODS AND SUBJECTS

Of 2705 men who first underwent an annual health check-up at Tokai University Hachioji Hospital, 2129 men were included in this study, after exclusion of those on medication for hypertension, diabetes or dyslipidemia, and those with a prior history of ischemic heart disease, cerebrovascular disease or chronic renal failure. MetS risk factors include the following five parameters: waist circumference, blood pressure, plasma glucose, triglycerides and HDL-C. The correlations between HDL-C and number of MetS risks with homeostasis model assessment of insulin resistance (HOMA-IR) were analyzed. HOMA-IR, number of risks, habits of smoking, exercise and drinking alcohol, stratified by HDL-C levels, were compared in MetS subjects.

RESULTS

In cases with ≤2 risk factors, the higher the HDL-C, the lower the HOMA-IR. However, with ≥3 risk factors for MetS, the HOMA-IR increased when HDL-C was ≥90 mg/dL. In MetS subjects, the rate of alcohol intake ≥75 g/day was high when HDL-C was ≥90 mg/dL.

CONCLUSION

In MetS subjects with high HDL-C levels, insulin resistance was increased. Therefore, in persons with high HDL-C levels, it is important to monitor the amount of alcohol consumption and reduce alcohol consumption to <75 g/day.

Genetic causes of high and low serum HDL-cholesterol

Plasma levels of HDL cholesterol (HDL-C) have a strong inherited basis with heritability estimates of 40-60%. The well-established inverse relationship between plasma HDL-C levels and the risk of coronary artery disease (CAD) has led to an extensive search for genetic factors influencing HDL-C concentrations. Over the past 30 years, candidate gene, genome-wide linkage, and most recently genome-wide association (GWA) studies have identified several genetic variations for plasma HDL-C levels. However, the functional role of several of these variants remains unknown, and they do not always correlate with CAD. In this review, we will first summarize what is known about HDL metabolism, monogenic disorders associated with both low and high HDL-C levels, and candidate gene studies. Then we will focus this review on recent genetic findings from the GWA studies and future strategies to elucidate the remaining substantial proportion of HDL-C heritability. Comprehensive investigation of the genetic factors conferring to low and high HDL-C levels using integrative approaches is important to unravel novel pathways and their relations to CAD, so that more effective means of diagnosis, treatment, and prevention will be identified.

Two novel mutations and functional analyses of the CETP and LIPC genes underlying severe hyperalphalipoproteinemia

Previous studies have shown that CETP and LIPC mutations contribute to hyperalphalipoproteinemia (HALP) in some populations. We investigated whether activities in cholesteryl ester transfer protein (CETP) and hepatic lipase (HL) contribute to HALP in the Thai population and performed genetic analyses of the CETP and LIPC genes. We recruited 38 individuals with high-density lipoprotein cholesterol (HDL-C) levels of at least 2.59 mmol/L (100 mg/dL) (HALP group) and an equal number of individuals with normal serum HDL-C levels (control group). The CETP and HL activities were determined in both groups. Genetic analyses covering all the coding regions and exon-intron junctions of the CETP and LIPC genes were performed in subjects who had low CETP activity and HL activity, respectively. The mean CETP and HL activities were significantly lower in the HALP group than in the control group (34 +/- 4 vs 44 +/- 3 pmol/[microL h], P = .04 and 150 +/- 17 vs 227 +/- 16 nmol free fatty acid/[mL min] P = .002, respectively). Of the 38 individuals with HALP, 19 and 16 were found to have low CETP activity and HL activity, respectively. Of the 19 subjects with low CETP activity, 6 subjects were found to be heterozygous for a known functionally relevant c.1325A>G (D442G) mutation. The other subject was found to be heterozygous for a novel deletion mutation, c.734_737delTCCC mutation. Of the 16 subjects with low HL activity, 8 and 2 subjects were found to be heterozygous for known variants, c.283 G>A (V73M) and c.1068A>C (L334F), respectively. These variants have previously been shown not to be associated with HALP. Another subject was found to be heterozygous for a novel missense mutation, c.421G>A (G119S). Its amino acid change, absence in controls, evolutionary conservation, occurrence in functionally important domain, and predicted damaging function suggested that the G119S mutation is functionally relevant. Two novel mutations in the CETP and LIPC genes found in this study are likely to be the causes of low enzyme activities and elevated HDL-C levels.

A novel homozygous mutation in CETP gene as a cause of CETP deficiency in a Caucasian kindred

OBJECTIVE

To analyze the cholesteryl ester transfer protein (CETP) gene and the plasma HDL phenotype in a Caucasian subject with extremely elevated plasma high density lipoprotein-cholesterol (HDL-C).

METHODS AND RESULTS

The proband, a 63-year-old male of Swedish ancestry with elevated HDL-C (208mg/dl) and apoA-I (and 272mg/dl), was found to be homozygous for a point mutation in exon 2 of CETP gene (c.109 C>T) resulting in a premature termination codon (R37X). Plasma CETP mass and activity were undetectable. Plasma HDL were characterized by predominance of large HDL with enhanced prebeta-HDL content. The proband's sons, heterozygotes for the mutation, had reduced plasma CETP activity and moderately elevated HDL-C. Serum of CETP deficient subjects showed a normal or enhanced cholesterol efflux capacity via ABCG1/SR-BI; cholesterol efflux via ABCA1 and macrophage cholesterol removal were lower than normal. The proband was healthy and had no atherosclerotic plaques in carotid or femoral arteries.

CONCLUSION

Complete CETP deficiency caused by mutations in CETP gene is exceedingly rare in Caucasians; the description of this single case indicates that CETP deficiency does not predispose to atherosclerosis in the absence of major cardiovascular risk factors.

Distribution of dolichol in the serum and relationships between serum dolichol levels and various laboratory test values

We examined the correlations between serum dolichol levels and laboratory test parameters in patients affected by disease, as well as the distribution of dolichol in sera from patients with hyperbetalipoproteinemia and hyperalphalipoproteinemia. Serum dolichol was evaluated by a reverse-phase HPLC method. After centrifugation, the serum dolichol found in healthy controls was mainly associated with medium-sized particles of the high-density lipoprotein (HDL) fraction. For patients with hyperbetalipoproteinemia, serum dolichol was also associated with the medium HDL fractions. However, for hyperalphalipoproteinemia patients the levels of large HDL and serum dolichol were increased, and serum dolichol was mainly associated with the large HDL fraction. On laboratory tests of components, the dolichol level was not correlated with the values for markers of the liver and biliary system, with the values of renal function markers, with creatine kinase activity, amylase activity or uric acid concentration, but was correlated with total cholesterol, HDL-cholesterol and apoA-I concentrations, and with lactate dehydrogenase (LDH) activity. These results suggest that serum dolichol exclusively localized in HDL, and in subpopulation, that in normocholesterolemia or hyperbeta-cholesterolemia is associated with HDL(3), which is small sized and high density HDL, however, that in hyperalphacholesterolemia is associated with HDL(2), which is large sized and lower density HDL.

Genetic determinants of HDL: monogenic disorders and contributions to variation

PURPOSE OF REVIEW

This review focuses on recent progress towards the characterization of genetic variations that contribute to interindividual variation in plasma high-density lipoprotein cholesterol levels in the general population.

RECENT FINDINGS

Many of the genes that harbor rare mutations leading to extreme high-density lipoprotein cholesterol levels contain common variation that influences plasma high-density lipoprotein cholesterol in several study populations. Candidate gene association studies provide evidence that some of these variations have an effect on high-density lipoprotein cholesterol, dependent on epistatic interactions or environmental context. Both rare and common variations contribute to interindividual high-density lipoprotein cholesterol variation. Recent comparisons of candidate gene sequences between individuals in the tails of the high-density lipoprotein cholesterol distributions (the upper or lower 1-5%) of several study populations indicate that as many as 20% of individuals with low high-density lipoprotein cholesterol harbor a rare mutation in an investigated gene. For example, the ABCA1 gene region harbors rare mutations and common variants that contribute to interindividual high-density lipoprotein cholesterol variation in the general population.

SUMMARY

The genetic control of high-density lipoprotein cholesterol level is complex. Maximizing the utility of genetic knowledge for predicting an individual's high-density lipoprotein cholesterol level or response to intervention will require a better understanding of the action of combinations of genetic variants and environmental exposures.

Human genetics of variation in high-density lipoprotein cholesterol

Longitudinal population studies have confirmed plasma levels of high-density lipoprotein (HDL) cholesterol to be an important inverse coronary risk factor. Although environmental influences are known to regulate HDL cholesterol levels, genetic factors are also known to be important, and over 25 candidate genes have been proposed to be associated with variation in HDL cholesterol levels. A variety of monogenic conditions of extremely low or high HDL cholesterol has helped to delineate the physiology of HDL cholesterol metabolism in humans, which has led to the development of new therapeutic approaches to HDL cholesterol. However, most causes of genetic variation in HDL cholesterol in the general population are likely oligogenic or polygenic. We review the monogenic disorders associated with both high and low HDL cholesterol and the relevance of mutations and polymorphisms in these genes to variation in HDL cholesterol levels in the general population.

Role of apoA-I, ABCA1, LCAT, and SR-BI in the biogenesis of HDL

The concentration, composition, shape, and size of plasma high-density lipoprotein (HDL) are determined by numerous proteins that influence its biogenesis, remodeling, and catabolism. The discoveries of the HDL receptor (scavenger receptor class B type I, SR-BI) and the ABCA1 (ATP-binding cassette transporter A1) lipid transporter provided two missing links that were necessary to understand the biogenesis and some of the functions of HDL. Existing data indicate that functional interactions between apoA-I and ABCA1 are necessary for the initial lipidation of apoA-I. Through a series of intermediate steps, lipidated apoA-I proceeds to form discoidal HDL particles that can be converted to spherical particles by the action of lecithin:cholesterol acyltransferase (LCAT). Discoidal and spherical HDL can interact functionally with SR-BI and these interactions lead to selective lipid uptake and net efflux of cholesterol and thus remodel HDL. Defective apoA-I/ABCA1 interactions prevent lipidation of apoA-I that is necessary for the formation of HDL particles. In the same way, specific mutations in apoA-I or LCAT prevent the conversion of discoidal to spherical HDL particles. The interactions of lipid-bound apoA-I with SR-BI are affected in vitro by specific mutations in apoA-I or SR-BI. Furthermore, deficiency of SR-BI affects the lipid and apolipoprotein composition of HDL and is associated with increased susceptibility to atherosclerosis. Here we review the current status of the pathway of HDL biogenesis and mutations in apoA-I, ABCA1, and SR-BI that disrupt different steps of the pathway and may lead to dyslipidemia and atherosclerosis in mouse models. The phenotypes generated in experimental mouse models for apoA-I, ABCA1, LCAT, SR-BI, and other proteins of the HDL pathway may facilitate early diagnosis of similar phenotypes in the human population and provide guidance for proper treatment.

Targeting cholesteryl ester transfer protein for the prevention and management of cardiovascular disease

Epidemiologic studies have shown that the concentration of high-density lipoprotein cholesterol (HDL-C) is a strong, independent, inverse predictor of coronary heart disease risk. This identifies HDL-C as a potential therapeutic target. Compared with low-density lipoprotein cholesterol (LDL-C)-lowering agents, however, currently available HDL-raising drugs are relatively ineffective. Consequently, recent years have seen considerable efforts expended on identifying new drugs that can raise HDL-C. Cholesteryl ester transfer protein (CETP) plays an important role in cholesterol metabolism, being responsible for the transfer of cholesteryl esters from HDL to very low-density lipoproteins and LDLs. The observation that Japanese populations with CETP deficiency exhibited high levels of HDL-C has led to the concept that drugs targeting CETP activity may elevate HDL-C levels and potentially decrease cardiovascular risk. Support of this proposition has been obtained in rabbits where inhibition of CETP activity is markedly antiatherogenic. Two CETP inhibitors-torcetrapib and JTT-705-are currently in the preliminary stages of clinical development. Initial studies with these drugs in humans show that they substantially increase HDL-C levels and modestly decrease LDL-C levels. Larger, long-term, randomized, clinical end point trials are required to determine whether the beneficial effects of CETP inhibitors on lipoprotein metabolism can translate into reductions in cardiovascular events.

Inherited disorders of HDL metabolism and atherosclerosis

PURPOSE OF REVIEW

Genetic disorders of HDL metabolism are rare and, as a result, the assessment of atherosclerosis risk in individuals suffering from these disorders has been difficult. Ultrasound imaging of carotid arteries has provided a tool to assess the risk in hereditary hypo and hyperalphalipoproteinemia. This review gives a comprehensive summary.

RECENT FINDINGS

Epidemiological studies have unequivocally shown that HDL cholesterol levels are inversely related to coronary artery disease risk, but the literature concerning genetic disorders of HDL metabolism provides less convincing information. Fortuitously, we were able to directly compare carotid intima media thickness data of substantial numbers of individuals with mutations in either apolipoprotein A-I (apoA-I), ATP binding cassette AI (ABCA1), lecithin: cholesterol acyltransferase (LCAT) or cholesteryl ester transfer protein. These data show that carriers of an apoA-I mutation exhibit the most pronounced accelerated atherosclerosis compared with those carrying mutations in ABCA1 and LCAT. Heterozygosity for a non-sense mutation in cholesteryl ester transfer protein did, by contrast, not distinguish carriers from controls in terms of intima media thickness progression. We will discuss these results in the context of the current literature.

SUMMARY

Intima media thickness studies have provided evidence that hypoalphalipoproteinemia due to mutations in apoA-I, ABCA1, and LCAT is associated with increased progression of atherosclerosis. In contrast, hyperalphalipoproteinemia as a result of loss of cholesteryl ester transfer protein function is associated with unaltered atherosclerosis progression compared with family controls. This insight is of interest, since it can assist in the prioritizing of antiatherogenic therapy by increasing HDL cholesterol levels.

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.

Effort Angina in a Middle-Aged Woman With Abnormally High Levels of Serum High-Density Lipoprotein Cholesterol A Case of Cholesteryl-Ester Transfer Protein Deficiency

A 54-year-old female was admitted to hospital complaining of oppressive anterior chest pain during exercise. Treadmill exercise ECG testing showed significant ischemic ECG changes, and electron-beam computed tomography demonstrated patchy calcifications in the coronary artery. Coronary angiography revealed a significant stenotic lesion of the right coronary artery. On routine investigations, no classical coronary risk factors were found, although a very high concentration (209 mg/dl) of high-density lipoprotein cholesterol (HDL-C) was detected. The serum concentration of cholesteryl-ester transfer protein (CETP), which plays a central role in the reverse cholesterol transport system, was measured and found to be less than the measurable minimum. The patient showed one of the typical genetic CETP mutations (intone 14 splicing defect), and her lipid profile was improved by administration of probucol for 3 months. A very high concentration of HDL-C with a defect of CETP activity may be a specific biochemical indicator pointing to an increased risk of premature coronary artery disease, and the lipid profile can be improved by use of lipid-lowering drugs.

Mutations in Japanese subjects with primary hyperlipidemia--results from the Research Committee of the Ministry of Health and Welfare of Japan since 1996--

Primary hyperlipidemia is caused by various molecular defects in lipid metabolism. The Research Committee on Primary Hyperlipidemia organized by the Ministry of Health and Welfare of Japan (present: the Ministry of Health, Labour and Welfare) has investigated reported mutations in Japanese patients with primary hyperlipidemia and related disorders (including hypolipidemia), and has created a database based on the questionnaire sent to the members of council board of the Japan Atherosclerosis Society. Mutations in the following genes were investigated: low density lipoprotein receptor, lecithin: cholesteryl acyltransferase, lipoprotein lipase (LPL), hepatic lipase, apolipoproteins A-I, A-II, A-IV, B, C-II, C-III and E, microsomal triglyceride transfer protein, and cholesterol ester transfer protein (CETP). Until 1998, 922 patients with primary hyperlipidemia and related disorders has been registered with the Research Committee, and 190 mutations in 15 genes had been reported, showing a marked variation in Japanese patients with primary hyperlipidemia and related disorders. So-called "common mutations" have been described in Japanese patients with familial hypercholesterolemia, LPL deficiency and CETP deficiency. The genetic defect of familial combined hyperlipidemia (FCHL) is still unknown although FCHL is speculated to be the most prevalent genetic hyperlipidemia, and further investigations should be performed to elucidate the molecular mechanisms of FCHL

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.

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

Since the identification of cholesteryl ester transfer protein (CETP), its role in the modulation of HDL levels and cardiovascular disease has been debated. With the early detection of genetic variants followed by the finding of families deficient in CETP, genetic studies have played a large role in the attempts to understand the association of CETP with lipids and disease; however, results of these studies have often led to disparate conclusions. With the availability of a greater variety of genetic polymorphisms and larger studies in which disease has been examined, it is now possible to compare the breadth of CETP genetic studies and draw better conclusions. The most broadly studied polymorphism is TaqIB for which over 10,000 individuals have been genotyped and had HDL levels determined. When these studies are subjected to a meta-analysis, the B2B2 homozygotes are found to have higher HDL levels than B1B1 homozygotes (0.12 mmol/l, 95% CI = 0.11-0.13, P < 0.0001). A similar analysis of the I405V polymorphism yields 0.05 mmol/l higher HDL levels in 405VV homozygotes than in 405II homozygotes (95% CI = 0.03-0.07, P < 0.0001). The implications of these studies for cardiovascular disease will be addressed.