Genetic factors affecting HDL levels, structure, metabolism and function

A decreased level of high-density lipoprotein is a possible risk factor for type 2 diabetes mellitus: A review

Introduction

Type 2 diabetes mellitus (T2DM) is characterized primarily by dyslipidemia and hyperglycemia due to insulin resistance. High-density lipoprotein (HDL) play a significant role in preventing the incidence of dyslipidemia and its complications. HDL has different protective functions, such as reducing oxidation, vascular inflammation, and thrombosis; additionally, its anti-diabetic role is one of the most significant recent discoveries about HDL and some of its constituent lipoproteins.

Methods

This research reviews ongoing studies and preliminary investigations into the assessment of relation between decreased level of HDL and T2DM.

Results

The levels of HDL and its functions contribute to glucose hemostasis and the development of T2DM through four possible mechanisms, including insulin secretion by beta cells, peripheral insulin sensitivity, non-insulin-dependent glucose uptake, and adipose tissue metabolic activity. Additionally, the anti-oxidant properties of HDL protect beta cells from apoptosis caused by oxidative stress and inflammation induced by low-density lipoprotein, which facilitate insulin secretion.

Conclusion

Therefore, HDL and its compositions, especially Apo A-I, play an important role in regulating glucose metabolism, and decreased levels of HDL can be considered a risk factor for DM. Different factors, such as hypoalphalipoproteinemia that manifests as a consequence of genetic factors, such as Apo A-I deficiency, as well as secondary causes arising from lifestyle choices and underlying medical conditions that decrease the level of HDL, could be associated with DM. Moreover, intricate connections between HDL and diabetic complications extend beyond glucose metabolism to encompass complications like cardiovascular disease and kidney disease. Therefore, the exact interactions between HDL level and DM should be evaluated in future studies.

Genetic factors exist behind the high prevalence of reduced high-density lipoprotein cholesterol levels in the Roma population

BACKGROUND AND AIMS

Previous findings showed that reduced plasma high-density lipoprotein cholesterol (HDL-C) levels are more frequent in all age groups of the Hungarian Roma compared to the general population. It suggests that genetic factors may exist behind this phenomenon. Our present study was designed to test this hypothesis, i.e., to define whether genetic factors contribute to the higher prevalence of reduced HDL-C among Roma. Single nucleotide polymorphisms (N = 21) contributing to the variation in plasma HDL-C concentrations were analysed in the Hungarian Roma (N = 646) and general (N = 1542) populations.

METHODS

Genetic risk scores, unweighted (GRS) and weighted (wGRS), were computed and compared. Associations between the GRSs and the prevalence of reduced HDL-C levels were analysed.

RESULTS

The GRS and wGRS were significantly higher in the Roma compared to the general population (GRS: 22.2 ± 3.2 vs. 21.5 ± 3.3; wGRS: 0.57 ± 0.1 vs. 0.53 ± 0.1; p<0.001). One half per cent of Roma subjects were in the bottom fifth of the wGRS (wGRS≤ 0.3) compared with 1.8% of those in the general population (p=0.025), while 5% of the Roma subjects were in the top fifth of the wGRS (wGRS≥ 0.75) compared with 2.6% of those in the general population (p=0.004). The GRS showed similar correlation with reduced plasma HDL-C levels in the two populations, whilst the wGRS showed stronger correlation with the trait among Roma after controlling for confounders.

CONCLUSIONS

These results strongly suggest that genetic factors contribute to the higher prevalence of reduced HDL-C levels among Roma, so interventions aiming to improve Roma health status need to consider their increased genetic susceptibility.

Taq1B Polymorphism of Cholesteryl Ester Transfer Protein (CETP) and Its Effects on the Serum Lipid Levels in Metabolic Syndrome Patients

The metabolic syndrome (MetS) is one of the most important risk factors for type 2 diabetes and cardiovascular disease. This syndrome is characterized by abdominal obesity, hypertension, insulin resistance, and dyslipidemia. The plasma origin of Cholesteryl ester transfer protein (CETP) is responsible for transferring cholesterol esters from high-density lipoprotein particles to apolipoprotein B containing lipoproteins compartment. We conducted this study to investigate the association between CETP gene Taq1B (rs708272) polymorphism in the metabolic syndrome among Iranian subjects. A sample size of 200 patients diagnosed with MetS together with 200 healthy donors as control were enrolled in this study. The investigation of polymorphism was performed by the use of polymerase chain reaction and restriction fragment length polymorphism analysis. To determine the relationship between polymorphism and lipid profile, we measured lipids and CETP concentration in metabolic syndrome and control subjects. Genotype distribution and allelic frequencies of polymorphism were determined and compared in both groups. Our findings showed that all clinical and biochemical characteristics in patients differed from the control group. The results showed that genotype and allele frequency of the Taq1B polymorphism was not significantly different between two groups. Instinctively, CETP was significantly higher in metabolic syndrome (1.64 ± 0.32 µg/ml) than in control (1.53 ± 0.34 µg/ml). A low level of CETP was found in blood of B2B2 typified genotype. In spite of Taq1B polymorphism on ester transfer protein concentration, no direct correlation was found between this polymorphism and metabolic syndrome.

Single-nucleotide polymorphisms: a perspective of cardiovascular prevention

INTRODUCTION

several studies have evaluated the utilization of lipid biomarkers in an attempt to correlate them with clinical cardiovascular events. Nevertheless, the investigation of clinical conditions under specific plasmatic levels of lipoproteins for long periods presents limitations due to inherent difficulties that are related to the follow-up of individuals throughout their lives. Better understanding of the clinical response and occasional resistance to the action of hypolipidemic drugs in several clinic scenarios is also necessary.

OBJECTIVES

to determine the role of evaluation of single-nucleotide polymorphisms (SNPs) related to the metabolism of lipids, and its implications in different clinical scenarios.

METHODS

a search of the literature in English and Spanish languages was performed in Medline, Lilacs via Bireme, IBECS via Bireme, and Cochrane databases. The expected results included information regarding plasmatic lipid profile and SNPs, cardiovascular clinical outcomes and polymorphisms related to the effectiveness of statins in the treatment of hypercholesterolemia.

RESULTS

in order to perform this analysis, 19 studies were included from a total of 89 identified citations. The evaluation of the results suggests that low plasmatic levels of LDL-c are associated with a reduction in the risk of heart attacks, although this was not observed for the rise of plasmatic levels of HDL-c.

CONCLUSION

polymorphisms in different populations and clinical perspectives may bring important contributions for a better understanding and adequacy of plasmatic lipoproteins aiming at reducing cardiovascular risk.

Changes in high-density-lipoprotein cholesterol upon statin treatment in type 2 diabetic patients: a meta-analysis

BACKGROUND AND AIM

To investigate the diversity of change in high-density-lipoprote in cholesterol (HDL-C) after statin treatment in patients with type 2 diabetes mellitus (T2DM).

METHODS AND RESULTS

A systemic review searched for trials that reported a serum change in HDL-C in patients with T2DM after statin treatment, and extracted data for meta-analysis. Of 6709 articles surveyed, 160 articles were identified as eligible articles. In the analysis of simvastatin, serum HDL-C was increased in Non-Asian and Asian patients with T2DM by 2.17 mg/dl (95% CI 1.43 ∼ 2.90 mg/dl, p < 0.001) and 2.31 mg/dl (95% CI 1.37 ∼ 3.25 mg/dl, p < 0.001), respectively. In the analysis of atorvastatin, although significant, serum HDL-C was subtly increased in Non-Asian patients with T2DM by 1.14 mg/dl (95% CI 0.28 ∼ 2.01 mg/dl, p = 0.010) mg/dl; however, atorvastatin treatment did not significantly change the serum HDL-C by 0.12 mg/dl (95% CI -1.04 ∼ 1.27 mg/dl, p = 0.839) mg/dl in Asian patients with T2DM. According to meta-regression analysis, the baseline HDL-C did not affect the change in serum HDL-C in Asian patients with T2DM after either simvastatin or atorvastatin treatment. However, contrary to simvastatin, the coefficient of regression (r) showed a significant negative association (r = -0.18; 95% CI -0.32 to -0.04; p = 0.01) between baseline HDL-C and the change of HDL-C in non-Asian patients with T2DM after atorvastatin treatment.

CONCLUSION

We have demonstrated for the first time that there may be a discrepancy in the change of serum HDL-C in Asian patients with T2DM after atorvastatin treatment.

Expression and clinical significance of ATP-binding cassette transporter 1 in serum and placental tissue in Chinese patients with preeclampsia

AIM

To investigate the expression and clinical significance of ATP-binding cassette transporter 1 (ABCA1) in pregnant women with preeclampsia (PE).

METHODS

52 pregnant women with PE who were admitted for delivery were enrolled in the study, while 30 normal pregnant inpatients were chosen as controls. Blood lipid and serum ABCA1 concentrations were assayed by enzymatic analysis and ELISA, respectively, and the expression of the ABCA1 gene and its encoded protein were detected and quantified by RT-PCR and Western blotting.

RESULTS

In the study group, blood lipid levels were significantly higher than those in the control group (p < 0.01), while the ABCA1 gene and its encoded protein expression in both serum and placental tissue were lower than that of controls. These differences were highly correlated with disease severity (p < 0.05). In PE patients, serum ABCA1 concentration was positively correlated with ABCA1 protein expression in placental tissue (r = 0.384, p < 0.01) and high-density lipoprotein level (r = 0.318, p < 0.05), but negatively correlated with low-density lipoprotein level (r = -0.279, p < 0.05).

CONCLUSION

In PE women, expression of ABCA1 was decreased, suggesting that ABCA1 may play an important role in onset of PE by altering blood lipid metabolism.

The Effect of Residing Altitude on Levels of High-Density Lipoprotein Cholesterol: A Pilot Study From the Omani Arab Population

Lower mortality rates from coronary heart disease and higher levels of serum high-density lipoprotein cholesterol (HDL-C) have been observed in populations residing at high altitude. However, this effect has not been investigated in Arab populations, which exhibit considerable genetic homogeneity. We assessed the relationship between residing altitude and HDL-C in 2 genetically similar Omani Arab populations residing at different altitudes. The association between the levels of HDL-C and other metabolic parameters was also investigated. The levels of HDL-C were significantly higher in the high-altitude group compared with the low-altitude group. Stepwise regression analysis showed that altitude was the most significant factor affecting HDL-C, followed by gender, serum triglycerides, and finally the 2-hour postprandial plasma glucose. This finding is consistent with previously published studies from other populations and should be taken into consideration when comparing cardiovascular risk factors in populations residing at different altitudes.

Niacin: another look at an underutilized lipid-lowering medication

Niacin, or water-soluble vitamin B(3), when given at pharmacologic doses, is a powerful lipid-altering agent. This drug, which lowers the levels of atherogenic, apolipoprotein-B-containing lipoproteins, is one of few medications that can raise the levels of atheroprotective HDL cholesterol. Niacin also has beneficial effects on other cardiovascular risk factors, including lipoprotein(a), C-reactive protein, platelet-activating factor acetylhydrolase, plasminogen activator inhibitor 1 and fibrinogen. Many clinical trials have confirmed the lipid effects of niacin treatment; however, its effects on cardiovascular outcomes have been called into question owing to the AIM-HIGH trial, which showed no benefit of niacin therapy on cardiovascular endpoints. Furthermore, use of niacin has historically been limited by tolerability issues. In addition to flushing, worsened hyperglycaemia among patients with diabetes mellitus has also been a concern with niacin therapy. This article reviews the utility of niacin including its mechanism of action, clinical trial data regarding cardiovascular outcomes, adverse effect profile and strategies to address these effects and improve compliance.

Deletion of scavenger receptor A gene in mice resulted in protection from septic shock and modulation of TLR4 signaling in isolated peritoneal macrophages

Scavenger receptor A (Sra), also known as macrophage scavenger receptor 1 (Msr1), is a surface glycoprotein preferentially present in macrophages that plays a primary role in innate immunity. Previous studies have shown that Sra is a modifier gene for the response to bacterial LPS in mice at the level of IL-10 production, in particular. In the present study, we found that Sra(-/-) mice are more resistant to septic shock induced by cecal ligation and puncture than wild-type C57BL/6 J (B6) mice. In addition, Sra(-/-) mice displayed initial elevated high density lipoprotein (HDL) circulating levels. Naïve peritoneal macrophages (PMs) were isolated from Sra(-/-) mice to understand the possible protective mechanism. Incubation of these cells with LPS was found to modulate TLR4 signaling, leading to a reduction in IL-10 and IL-6 mRNA levels, but not TNF-α expression, at low concentrations of LPS in comparison with PMs isolated from B6 mice. No differences were found in LPS binding between PMs derived from Sra(-/-) or B6 mice. The lack of Sra binding to LPS was confirmed after transfection of Chinese hamster ovary (CHO) cells with the Sra gene. The contribution of Sra to the outcome of sepsis may be a combination of changes in TLR4 signaling pathway and elevated levels of HDL in circulation, but also LPS toxicity.

The Association Between the Uncoupling Protein-1 Gene A-3826G Polymorphism and High-density Lipoprotein Cholesterol in A General Japanese Population: A Consideration of the Obesity Status

BACKGROUND

Limited studies have shown inconsistent data about the association between the uncoupling protein 1 (UCP1) gene A-3826G polymorphism and high-density lipoprotein (HDL) cholesterol levels. The present study investigated the association between the A-3826G polymorphism and low HDL-cholesterolemia in non-obese and obese subjects.

METHODS

Anthropometric and biochemical factors, in addition to genotyping by an allele-specific DNA assay, were measured in 294 community-dwelling Japanese subjects (male/female: 127/167, mean age: 65 years). Obesity was defined as a body mass index (BMI) ≥ 25 kg/m(2), and low HDL-cholesterolemia was defined as < 1.04 mmol/L of HDL-cholesterol.

RESULTS

The subjects with the G/G genotype (n = 27) showed a significantly higher prevalence of low HDL-cholesterolemia (37%) than those with the A/A + A/G genotype (13%) in the obese group (n = 102). There was a non-significant difference in the prevalence of low HDL-cholesterolemia between subjects with the G/G genotype (n = 45, 13%) and with the A/A + A/G genotype (15%) in the non-obese group (n = 192). A multivariate-adjusted logistic regression analysis of the presence of low HDL-cholesterolemia revealed that carrying the G/G genotype was an independent and significant factor positively associated with low HDL-cholesterolemia [odds ratio (OR): 6.85, 95% confidence interval (CI): 1.65-28.49] in the obese group, while carrying the G/G genotype exhibited a non-significant but reduced OR, by one-half, for low HDL-cholesterolemia (OR: 0.51, 95% CI: 0.13-1.96) in the non-obese group.

CONCLUSIONS

The obesity status could have opposing impacts on the relationship between the G/G genotype and low HDL-cholesterolemia, providing insight into the need to consider the obesity levels when studying the association between the UCP-1 gene A-3826G polymorphism and HDL-cholesterol.

KEYWORDS

Obesity; Body mass index; HDL-C; Atherosclerotic risk.

Effects of diet on genetic regulation of lipoprotein metabolism in baboons

Several measures of lipoprotein phenotype are significant predictors of cardiovascular risk. Although such lipoprotein phenotypes are under strong genetic control, it is not clear to what extent they are controlled by the same - and by different - genes and whether these relationships may be altered in different dietary environments. Therefore, we measured six lipoprotein traits (three LDL traits - LDLC and apoB concentrations and LDL size - and three HDL traits - HDLC and apoA1 concentrations and HDL size) on each of three diets differing in level of fat and cholesterol. In bivariate analyses, all but two metabolically related trait pairs were genetically correlated, though none were completely correlated, implying additive genetic effects by both pleiotropic and unique genes. In comparing genetic correlations for the same pair of traits across diet, we detected evidence of diet effects on genetic control of these metabolically related traits; specifically, increasing level of dietary cholesterol was associated with a significant decrease in the genetic correlation of apoA1 with HDL size, and a significant increase in the genetic correlations of LDL size with LDLC and apoB. The results suggest a complex network of genes affecting lipoprotein metabolism: the genes may exert both unique and pleiotropic effects; the genes may exert detectable effects in many or only in specific dietary environments.

Gene therapy for dyslipidemia: a review of gene replacement and gene inhibition strategies

Despite numerous technological and pharmacological advances and more detailed knowledge of molecular etiologies, cardiovascular diseases remain the leading cause of morbidity and mortality worldwide claiming over 17 million lives a year. Abnormalities in the synthesis, processing and catabolism of lipoprotein particles can result in severe hypercholesterolemia, hypertriglyceridemia or low HDL-C. Although a plethora of antidyslipidemic pharmacological agents are available, these drugs are relatively ineffective in many patients with Mendelian lipid disorders, indicating the need for new and more effective interventions. In vivo somatic gene therapy is one such intervention. This article summarizes current strategies being pursued for the development of clinical gene therapy for dyslipidemias that cannot effectively be treated with existing drugs.

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.

No interaction between alcohol consumption and HDL-related genes on HDL cholesterol levels

OBJECTIVE

To assess the relationships and possible interactions between polymorphisms related to HDL levels and alcohol consumption.

METHODS

Cross-sectional population-based study including 2863 women and 2546 men aged 35-75 years (CoLaus study). Alcohol intake was assessed by the reported alcohol consumption of the last 7 days. Nineteen candidate genes known to influence HDL levels were studied.

RESULTS

Alcohol consumption increased HDL cholesterol levels in both genders. After multivariate adjustment for gender, age, body mass index, smoking, hypolipidaemic drug treatment, physical activity and alcohol consumption, APOA5, CETP, LIPC and LPL gene polymorphisms were significantly (10(-5) threshold) related with HDL cholesterol levels, while no genexalcohol intake interaction was found for all SNPs studied. ABCA1 polymorphisms were related to HDL cholesterol levels on bivariate analysis but the relationship was no longer significant after multivariate analysis.

CONCLUSION

Our data confirm the association of alcohol consumption and of APOA5, CETP, LIPC and LPL gene polymorphisms with HDL cholesterol levels. Conversely, no genexalcohol consumption interactions were found, suggesting that the effect of alcohol consumption on HDL cholesterol levels is not mediated via a modulation of HDL related genes.

Hypolipidemic effects of proanthocyanidins and their underlying biochemical and molecular mechanisms

Proanthocyanidins are the most abundant polyphenols in human diets. Epidemiological studies strongly suggest that proanthocyanidins protect against cardiovascular diseases. Despite the antioxidant and anti-inflammatory properties of these flavonoids, one of the mechanisms by which proanthocyanidins exert their cardiovascular protection is improving lipid homeostasis. Animal studies demonstrate that proanthocyanidins reduce the plasma levels of atherogenic apolipoprotein B-triglyceride-rich lipoproteins and LDL-cholesterol but increase antiatherogenic HDL-cholesterol. The results in humans, however, are less clear. This review summarizes the results that have been published on plasma triglyceride, apolipoprotein B, HDL-cholesterol and LDL-cholesterol levels in humans and animal models in response to proanthocyanidin extracts and proanthocyanidin-rich foods. The physiological processes and biochemical pathways that are related to lipid homeostasis and affected by proanthocyanidin consumption are also discussed. Intestinal lipid absorption, chylomicron secretion by the intestine and VLDL secretion by the liver are the processes that are most repressed by proanthocyanidins, which, therefore, induce hypolipidemic effects.

Metabolic syndrome-related composite factors over 5 years in the STANISLAS family study: genetic heritability and common environmental influences

BACKGROUND

We estimated genetic heritability and common environmental influences for various traits related to metabolic syndrome in young families from France.

METHODS

At entrance and after 5 years, nineteen traits related to metabolic syndrome were measured in a sample of families drawn from the STANISLAS study. In addition, 5 aggregates of these traits were identified using factor analysis.

RESULTS

At entrance, genetic heritability was high (20 to 44%) for plasma lipids and lipoproteins, uric acid, fasting glucose, and the related clusters "risk lipids" and "protective lipids". Intermediate or low genetic heritability (less than 20%) was shown for triglycerides, adiposity indices, blood pressure, hepatic enzyme activity, inflammatory makers and the related clusters: "liver enzymes", "adiposity/blood pressure" and "inflammation". Moreover, common environmental influences were significant for all the parameters. With regard to 5-year changes, polygenic variance was low and not statistically significant for any of the individual variables or clusters whereas shared environment influence was significant.

CONCLUSIONS

In these young families, genetic heritability of metabolic syndrome-related traits was generally lower than previously reported while the common environmental influences were greater. In addition, only shared environment contributed to short-term changes of these traits.

High-density lipoprotein metabolism and the human embryo

BACKGROUND

High-density lipoprotein (HDL) appears to be the dominant lipoprotein particle in human follicular fluid (FF). The reported anti-atherogenic properties of HDL have been attributed in part to reverse cholesterol transport. The discoveries of the scavenger receptor class B type I (SR-BI) and the ATP-binding cassette A1 lipid (ABCA1) transporter have generated studies aimed at unraveling the pathways of HDL biogenesis, remodeling and catabolism. The production of SR-BI and ABCA1 knockout mice as well as other lipoprotein metabolism-associated mutants has resulted in reduced or absent fertility, leading us to postulate the existence of a human hepatic-ovarian HDL-associated axis of fertility. Here, we review an evolving literature on the role of HDL metabolism on mammalian fertility and oocyte development.

METHODS

An extensive online search was conducted of published articles relevant to the section topics discussed. All relevant English language articles contained in Pubmed/Medline, with no specific time frame for publication, were considered for this narrative review. Cardiovascular literature was highly cited due to the wealth of relevant knowledge on HDL metabolism, and the dearth thereof in the reproductive field.

RESULTS

Various vertebrate models demonstrate a role for HDL in embryo development and fertility. In our clinical studies, FF levels of HDL cholesterol and apolipoprotein AI levels were negatively associated with embryo fragmentation, but not with embryo cell cleavage rate. However, the HDL component, paraoxonase 1 arylesterase activity, was positively associated with embryo cell cleavage rate.

CONCLUSIONS

HDL contributes to intra-follicular cholesterol homeostasis which appears to be important for successful oocyte and embryo development.

Association of polymorphisms in genes involved in lipoprotein metabolism with plasma concentrations of remnant lipoproteins and HDL subpopulations before and after hormone therapy in postmenopausal women

OBJECTIVE

A high degree of inter-individual variability in plasma lipid level response to hormone therapy (HT) has been reported. Variations in the oestrogen receptor alpha gene (ESR1) and in genes involved in lipid metabolism may explain some of the variability in response to HT. Subjects Postmenopausal Caucasian women (n = 208) participating in a placebo-controlled randomized trial of 3.2 years of hormone therapy (HT).

METHODS

Plasma triglyceride (TG), remnant lipoprotein cholesterol (RLP-C), and high-density lipoprotein cholesterol (HDL-C) levels and HDL subpopulations were assessed at baseline and at follow up. Single nucleotide polymorphisms (SNPs) in ESR1 and in the ATP binding cassette A1 (ABCA1), cholesteryl ester transfer protein (CETP), hepatic lipase (LIPC), lipoprotein lipase (LPL), and scavenger receptor class B type I (SRB1) genes were assessed for their association with baseline plasma levels and HT-related changes in levels of RLP-C and HDL subpopulations.

RESULTS

Carriers of the ESR1 PvuII or IVS1-1505 variants had lower plasma TG concentrations and higher plasma HDL-C and alpha-1 and prealpha-1 HDL particle levels at baseline and showed greater increases in HDL-C, apo A-I and alpha-1 particle levels after HT than wild-type carriers. Carriers of the N291S and D9N variants in the LPL gene had significantly higher remnant lipoproteins and lower alpha-2 HDL particle levels at baseline. The CETP TaqIB SNP was a significant determinant of baseline plasma HDL-C and HDL subpopulation profile.

CONCLUSIONS

Single nucleotide polymorphisms in ESR1, CETP and LPL had significant effects on baseline plasma levels of TG-rich and HDL subpopulations. With the exception of ESR1 SNPs, variation in genes involved in lipid metabolism has a very modest effect on lipoprotein response to HT.

The genetics of high-density lipoprotein metabolism: clinical relevance for therapeutic approaches

The risk for coronary artery disease (CAD) is inversely correlated with high-density lipoprotein (HDL) cholesterol plasma levels. These plasma HDL cholesterol levels are influenced by the activity of a number of enzymes and receptors, and therefore, variations in the genes encoding for these proteins may consequently result in an altered CAD risk. Identification of such pivotal players in HDL cholesterol metabolism that are also strongly associated with CAD risk is crucial for the materialization of novel therapeutic modalities. A large amount of knowledge has been obtained by studies involving families with extreme HDL phenotypes specific to molecular defects. In fact, thus far, monogenetic defects have been described in the genes coding for apolipoprotein A-I, adenosine triphosphate-binding cassette transporter A1, cholesterol ester transfer protein, the lack of endothelial lipase (LIPG), phospholipid transfer protein, and lecithin-cholesterol acyltransferase. Despite the fact that the total number of carriers of such mutations is rather small, much can be gained by extensively studying the metabolic and vascular consequences of these mutations. Surrogate markers for atherosclerosis have proved to be useful to overcome this sample size limitation and have been widely exploited to study families with decreased or increased HDL cholesterol levels in order to correlate HDL cholesterol phenotypes to atherosclerotic burden in cases and controls. Apart from such extreme phenotype approaches, novel population-based genome-wide association studies have been used to decipher the link between genetic loci and HDL cholesterol levels, and the identification of novel HDL cholesterol-related genes is eagerly awaited. These might be instrumental in the ongoing fight against atherosclerosis.

HDL subspecies in young adult twins: heritability and impact of overweight

The association between abdominal obesity and atherogenic lipid profile emerges from complex interactions of genes and environment. We aimed to explore the heritability and effects of overweight on serum lipid profile (high-density lipoprotein-cholesterol (HDL-C), HDL mean particle size, percentages of HDL(2b, 2a, 3a, 3b, and 3c,) low-density lipoprotein-cholesterol (LDL-C), LDL peak particle size and triglycerides (TGs)) in healthy, young adults. HDL-C, LDL-C, and TG were measured in 52 monozygotic (MZ) and 89 dizygotic (DZ) twin pairs, aged 23-32 years, chosen to represent a wide range of BMIs (17.6-42.9 kg/m2). Of them, 24 MZ and 26 DZ pairs were chosen at random for measurements of HDL mean and LDL peak particle sizes and percentages of HDL subspecies. The heritabilities of the lipid parameters adjusted for BMI were HDL-C 73%, HDL mean particle size 56%, HDL subspecies 46-63%, LDL-C 79%, LDL peak particle size 49%, and TG 64%. Genetic and environmental correlations between BMI and HDL-C, LDL-C, and TG were modest (0.3-0.4). Abdominal overweight (waist circumference>or=94 cm for males and >or=80 cm for females) associated with decreased HDL-C, increased LDL-C, and TG concentrations, smaller HDL mean particle size, lower HDL2b, and higher HDL3c percentages in both genders. Within MZ twins, controlling for genetic influences, within-pair differences in HDL3c percentage were associated with those in waist (r=0.46, P=0.032) and BMI (r=0.51, P=0.013). In conclusion, serum lipid parameters, including LDL peak and HDL mean particle sizes and HDL subspecies distribution are under strong genetic control. Overweight associated with significant lipid profile changes, particularly, small HDL3c increased in overweight independent of genetic influences.

Genetic-epidemiological evidence on genes associated with HDL cholesterol levels: a systematic in-depth review

High-density lipoprotein (HDL) particles exhibit multiple antiatherogenic effects. They are key players in the reverse cholesterol transport which shuttles cholesterol from peripheral cells (e.g. macrophages) to the liver or other tissues. This complex process is thought to represent the basis for the antiatherogenic properties of HDL particles. The amount of cholesterol transported in HDL particles is measured as HDL cholesterol (HDLC) and is inversely correlated with the risk for coronary artery disease: an increase of 1mg/dL of HDLC levels is associated with a 2% and 3% decrease of the risk for coronary artery disease in men and women, respectively. Genetically determined conditions with high HDLC levels (e.g. familial hyperalphalipoproteinemia) often coexist with longevity, and higher HDLC levels were found among healthy elderly individuals. HDLC levels are under considerable genetic control with heritability estimates of up to 80%. The identification and characterization of genetic variants associated with HDLC concentrations can provide new insights into the background of longevity. This review provides an extended overview on the current genetic-epidemiological evidence from association studies on genes involved in HDLC metabolism. It provides a path through the jungle of association studies which are sometimes confusing due to the varying and sometimes erroneous names of genetic variants, positions and directions of associations. Furthermore, it reviews the recent findings from genome-wide association studies which have identified new genes influencing HDLC levels. The yet identified genes together explain only a small amount of less than 10% of the HDLC variance, which leaves an enormous room for further yet to be identified genetic variants. This might be accomplished by large population-based genome-wide meta-analyses and by deep-sequencing approaches on the identified genes. The resulting findings will probably result in a re-drawing and extension of the involved metabolic pathways of HDLC metabolism.

A mouse model of harlequin ichthyosis delineates a key role for Abca12 in lipid homeostasis

Harlequin Ichthyosis (HI) is a severe and often lethal hyperkeratotic skin disease caused by mutations in the ABCA12 transport protein. In keratinocytes, ABCA12 is thought to regulate the transfer of lipids into small intracellular trafficking vesicles known as lamellar bodies. However, the nature and scope of this regulation remains unclear. As part of an original recessive mouse ENU mutagenesis screen, we have identified and characterised an animal model of HI and showed that it displays many of the hallmarks of the disease including hyperkeratosis, loss of barrier function, and defects in lipid homeostasis. We have used this model to follow disease progression in utero and present evidence that loss of Abca12 function leads to premature differentiation of basal keratinocytes. A comprehensive analysis of lipid levels in mutant epidermis demonstrated profound defects in lipid homeostasis, illustrating for the first time the extent to which Abca12 plays a pivotal role in maintaining lipid balance in the skin. To further investigate the scope of Abca12's activity, we have utilised cells from the mutant mouse to ascribe direct transport functions to the protein and, in doing so, we demonstrate activities independent of its role in lamellar body function. These cells have severely impaired lipid efflux leading to intracellular accumulation of neutral lipids. Furthermore, we identify Abca12 as a mediator of Abca1-regulated cellular cholesterol efflux, a finding that may have significant implications for other diseases of lipid metabolism and homeostasis, including atherosclerosis.

Genome scan of a nonword repetition phenotype in families with dyslexia: evidence for multiple loci

To understand the genetic architecture of dyslexia and identify the locations of genes involved, we performed linkage analyses in multigenerational families using a phonological memory phenotype--Nonword Repetition (NWR). A genome scan was first performed on 438 people from 51 families (DS-1) and linkage was assessed using variance components (VC), Bayesian oligogenic (BO), and parametric analyses. For replication, the genome scan and analyses were repeated on 693 people from 93 families (DS-2). For the combined set (DS-C), analyses were performed with all three methods in the regions that were identified in both samples. In DS-1, regions on chromosomes 4p, 6q, 12p, 17q, and 22q exceeded our initial threshold for linkage, with 17q providing a parametric LOD score of 3.2. Analysis with DS-2 confirmed the locations on chromosomes 4p and 12p. The strongest VC and BO signals in both samples were on chromosome 4p in DS-C, with a parametric multipoint LOD(max) of 2.36 for the 4p locus. Our linkage analyses of NWR in dyslexia provide suggestive and reproducible evidence for linkage to 4p12 and 12p in both samples, and significant evidence for linkage to 17q in one of the samples. These results warrant further studies of phonological memory and chromosomal regions identified here in other datasets.

Altered lipoprotein metabolism in chronic inflammatory states: proinflammatory high-density lipoprotein and accelerated atherosclerosis in systemic lupus erythematosus and rheumatoid arthritis

In this review, the authors discuss the formation and structure of high-density lipoproteins (HDLs) and how those particles are altered in inflammatory or stress states to lose their capacity for reverse cholesterol transport and for antioxidant activity. In addition, abnormal HDLs can become proinflammatory (piHDLs) and actually contribute to oxidative damage. The assay by which piHDLs are identified involves studying the ability of test HDLs to prevent oxidation of low-density lipoproteins. Finally, the authors discuss the potential role of piHDLs (found in some 45% of patients with systemic lupus erythematosus and 20% of patients with rheumatoid arthritis) in the accelerated atherosclerosis associated with some chronic rheumatic diseases.

Effects of statins on high-density lipoproteins: a potential contribution to cardiovascular benefit

PURPOSE

The objective was to systematically review clinical trial data on the effects of statins on high-density lipoproteins (HDL) and to examine the possibility that this provides cardiovascular benefits in addition to those derived from reductions in low-density lipoproteins (LDL).

METHODS

The PubMed database was searched for publications describing clinical trials of atorvastatin, pravastatin, rosuvastatin, and simvastatin. On the basis of predefined criteria, 103 were selected for review.

RESULTS

Compared with placebo, statins raise HDL, measured as HDL-cholesterol (HDL-C) and apolipoprotein A-I (apo A-I); these elevations are maintained in the long-term. In hypercholesterolemia, HDL-C is raised by approximately 4% to 10%. The percentage changes are greater in patients with low baseline levels, including those with the common combination of high triglycerides (TG) and low HDL-C. These effects do not appear to be dose-related although there is evidence that, with the exception of atorvastatin, the changes in HDL-C are proportional to reductions in apo B-containing lipoproteins. The most likely explanation is a reduced rate of cholesteryl ester transfer protein (CETP)-mediated flow of cholesterol from HDL. There is some evidence that the statin effects on HDL reduce progression of atherosclerosis and risk of cardiovascular disease independently of reductions in LDL.

CONCLUSION

Statins cause modest increases in HDL-C and apo A-I probably mediated by reductions in CETP activity. It is plausible that such changes independently contribute to the cardiovascular benefits of the statin class but more studies are needed to further explore this possibility.

The value of HDL genetics

PURPOSE OF REVIEW

To review studies on hereditary disorders of high-density lipoprotein (HDL) metabolism and studies on HDL genetics in mice, which have both provided valuable insight into the pathways of this intriguing lipoprotein and moreover revealed targets to raise HDLc to reduce atherosclerosis.

RECENT FINDINGS

To date, as many as 11 genes are considered key players in the synthesis, maturation, conversion and/or catabolism of HDL. Five of these genes have been identified in humans, APOA1, LCAT, ABCA1, LIPC, and CETP, whereas the other six genes have been identified in mice, SCARB1, ABCG1, ATPB5, PLTP, LIPG and APOM. Genetic association studies are as yet the best line of evidence of the roles of the 'murine genes' in human HDL pathways. In addition to recent genetic association studies, a third section describes exciting news on six newly proposed HDL genes VNN1, GALNT2, MMAB/MVK, CTalpha, BMP-1 and SIRT1.

SUMMARY

This review provides a summary of the current literature on the genetics of HDL. New information from this research area may assist us in obtaining a better understanding of HDL biology and identifying novel pharmacological targets.

HDL metabolism and CETP inhibition

High density lipoprotein-cholesterol (HDL-C) concentration in the blood is independently and inversely associated with an increased risk of coronary heart disease. Some of the cholesterol-lowering drugs (niacin, fibrates, and statins) incidentally raise HDL-C. These drugs are not effective in causing major changes in HDL-C. Since the discovery of human genetic cholesteryl ester transfer protein (CETP) deficiency in a Japanese population with high levels of HDL-C and apolipoprotein A-I, CETP inhibition has become a novel strategy for raising HDL-C in humans. Mice, a species naturally lacking CETP, were transduced with the human CETP gene, which resulted in dose-related reductions in HDL-C. Rabbits, a species with naturally high levels of CETP, were fed a synthetic CETP inhibitor, JTT-705, leading to both a 90% increase in HDL-C and a 70% reduction in aortic atherosclerotic lesion area. Human intervention trials with a new potent and selective CETP inhibitor, torcetrapib, have taken place. In a phase I multidose trial, HDL-C increased by 91% with torcetrapib 120 mg twice daily. A phase II trial conducted with multiple combinations of torcetrapib and atorvastatin showed that the combination was well tolerated and doses 30 mg and higher of torcetrapib caused 8.3-40.2% changes from baseline HDL-C across the dose range of atorvastatin at 12 weeks. Recently the phase III clinical trial ILLUMINATE (Investigation of Lipid Level Management to Understand its Impact in Atherosclerotic Events) was prematurely terminated because of an increase in mortality in the torcetrapib/atorvastatin treatment arm compared with atorvastatin used alone. In companion studies no improvement in carotid or coronary atherosclerosis could be detected in patients treated with the torcetrapib/atorvastatin combination despite favorable changes in both low density lipoprotein (LDL)- and HDL-cholesterol levels. The future for CETP inhibition with drug therapy is now unclear, and must include a closer look at CETP inhibitor's effects on blood pressure and HDL itself. Accordingly, it was recently shown in 2 double-blind, placebo-controlled, randomized, phase I studies with the CETP inhibitor anacetrapib in healthy individuals and in patients with dyslipidemias that the drug increased HDL and reduced LDL, while having no effect on blood pressure.

Is raising HDL a futile strategy for atheroprotection?

The dramatic failure of clinical trials evaluating the cholesterol ester transfer protein inhibitor torcetrapib has led to considerable doubt about the value of raising high-density lipoprotein cholesterol (HDL-C) as a treatment for cardiovascular disease. These results have underscored the intricacy of HDL metabolism, with functional quality perhaps being a more important consideration than the circulating quantity of HDL. As a result, HDL-based therapeutics that maintain or enhance HDL functionality warrant closer investigation. In this article, we review the complexity of HDL metabolism, discuss clinical-trial data for HDL-raising agents, including possible reasons for the failure of torcetrapib, and consider the potential for future HDL-based therapies.

Prediction of cardio- and cerebro-vascular events in patients with subclinical carotid atherosclerosis and low HDL-cholesterol

Low HDL-cholesterol concentrations are associated with increased cardiovascular risk and recent evidences suggest that HDL may aggravate the atherosclerotic process promoting inflammation: HDL are anti-inflammatory in the absence of inflammation but can become proinflammatory in the presence of atherosclerosis. Yet, no data is available on the cardiovascular outcome in subjects with low HDL-cholesterol and early stages of atherosclerosis. Therefore, we included in a prospective 5-year follow-up study 150 subjects with low HDL-cholesterol concentrations and subclinical carotid atherosclerosis, as assessed by carotid colour doppler, evaluating at baseline all the established traditional cardiovascular risk factors (e.g. male gender, older age, obesity, hypertension, diabetes, smoking, family history of coronary artery disease, hypercholesterolemia), as well as levels of two markers of inflammation (C-reactive protein and fibrinogen). At the end of the follow-up we registered vascular events in the 21% of patients and we found that lower HDL-cholesterol concentrations were associated with ischemic stroke (p=.0164), peripheral arterial disease (p=.0248) and the presence of any clinical event (p=.0105). By multivariate analysis we searched, among all baseline parameters, for independent variables associated with the events and we found a predictive role for elevated fibrinogen concentrations (OR 6.3, 95% CI 2.0-19.6, p=.0016), family history of coronary artery disease (OR 4.5, 95% CI 1.7-12.8, p=.0045) and lower HDL-cholesterol levels (OR 1.4, 95% CI 1.1-1.9, p=.0278). These findings further suggest a synergistic role of low-HDL and inflammation on the atherosclerotic disease progression from subclinical lesions to clinical events. Yet, their therapeutical implications remain to be established in future studies.

The paradox of dysfunctional high-density lipoprotein

PURPOSE OF REVIEW

This review addresses how, in atherosclerosis or systemic inflammation, HDL can lose its usual atheroprotective characteristics and even paradoxically assume proinflammatory properties.

RECENT FINDINGS

Specific chemical and structural changes within HDL particles can impede reverse cholesterol transport, enhance oxidation of LDL, and increase vascular inflammation. HDL may be viewed as a shuttle that can be either anti-inflammatory or proinflammatory, depending on its cargo of proteins, enzymes, and lipids. Some therapeutic approaches that reduce coronary risk, such as statins and therapeutic lifestyle changes, can favorably moderate the characteristics of proinflammatory HDL. In addition, apolipoprotein A-I mimetic peptides and other compounds that target functional aspects of HDL may offer novel approaches to reduction in cardiovascular risk.

SUMMARY

Current data suggest that under some conditions HDL can become dysfunctional and even proinflammatory, but this characterization can change with resolution of systemic inflammation or use of certain treatments.