Genes influencing HDL metabolism: new perspectives and implications for atherosclerosis prevention

The Question of a Role for Statins in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is the leading cause of irreversible central vision loss in patients over the age of 65 years in industrialized countries. Epidemiologic studies suggest that high dietary fat intake is a risk factor for the development and progression of both vascular and retinal disease. These, and other associations, suggest a hypothesis linking elevated cholesterol and AMD progression. It follows, therefore, that cholesterol-lowering medications, such as statins, may influence the onset and progression of AMD. However, the findings have been inconclusive as to whether statins play a role in AMD. Due to the significant public health implications of a potential inhibitory effect of statins on the onset and progression of AMD, it is important to continually evaluate emerging findings germane to this question.

A graphical weighted power improving multiplicity correction approach for SNP selections

Controlling for the multiplicity effect is an essential part of determining statistical significance in large-scale single-locus association genome scans on Single Nucleotide Polymorphisms (SNPs). Bonferroni adjustment is a commonly used approach due to its simplicity, but is conservative and has low power for large-scale tests. The permutation test, which is a powerful and popular tool, is computationally expensive and may mislead in the presence of family structure. We propose a computationally efficient and powerful multiple testing correction approach for Linkage Disequilibrium (LD) based Quantitative Trait Loci (QTL) mapping on the basis of graphical weighted-Bonferroni methods. The proposed multiplicity adjustment method synthesizes weighted Bonferroni-based closed testing procedures into a powerful and versatile graphical approach. By tailoring different priorities for the two hypothesis tests involved in LD based QTL mapping, we are able to increase power and maintain computational efficiency and conceptual simplicity. The proposed approach enables strong control of the familywise error rate (FWER). The performance of the proposed approach as compared to the standard Bonferroni correction is illustrated by simulation and real data. We observe a consistent and moderate increase in power under all simulated circumstances, among different sample sizes, heritabilities, and number of SNPs. We also applied the proposed method to a real outbred mouse HDL cholesterol QTL mapping project where we detected the significant QTLs that were highlighted in the literature, while still ensuring strong control of the FWER.

A genetic basis for coronary artery disease

CAD and cancer account for over one-half of all deaths in the world. It is claimed that the 21st century is the last century for CAD. This is, in part, because CAD is preventable based on randomized, placebo-controlled clinical trials, which show modifying known risk factors such as cholesterol is associated consistently with 40-60% reduction in morbidity and mortality from CAD. Comprehensive prevention will require modifying genetic risk factors that are claimed to account for 40-60% of predisposition to CAD. The 21st century is meeting this challenge with over 50 genetic risk variants discovered and replicated in large genome-wide association studies involving over 200,000 cases and controls. Similarly, 157 genetic variants have been discovered that regulate plasma lipids including, LDL-C, HDL-C, triglycerides, and total cholesterol. A major finding from these studies is that only 15 of the 50 genetic variants for CAD act through known risk factors. Hence, the pathogenesis of CAD in addition to cholesterol and other known risk factors is due to various other factors, many of which remain unknown. Secondly, genes regulating the plasma triglyceride levels are strongly associated with the pathogenesis of CAD. Thirdly, Mendelian randomization studies show no protection from genes that increase plasma HDL cholesterol. This is contrary to current opinion. These genetic risk variants have provided new targets for the development of novel therapies to prevent CAD. Already a new and potent drug has been developed targeting PCSK9, which is in phase 3 clinical trials and shows great efficacy and safety for prevention of CAD. The 21st century is looking very bright for the prevention of CAD.

Genetics of premature myocardial infarction

Common multigene disorders account for 80% of deaths in the world and all have significant genetic predisposition. Coronary artery disease and myocardial infarction (MI) account for more than 40% of these deaths. The genetic component is due to multiple genes, each contributing only minimally to the phenotype. Linkage analysis, which has been successful in identifying rare disorders that cause MI, is not sensitive for multigene disorders. The recent candidate case-control approach has been equally unsuccessful. Multigene disorders require genome-wide association studies involving genotyping hundreds of thousands of DNA markers in thousands of individuals with replication in independent populations. Platforms with 500,000 and 1 million single nucleotide polymorphisms provide the necessary high-throughput genotyping for genome-wide association. The first confirmed common locus, 9p21, is independent of conventional risk factors. Identifying the 9p21 gene will elucidate novel mechanisms responsible for MI. Comprehensive prevention of MI based on individual genetic variants (personalized medicine) is expected in the next decade.

Dietary myristic acid modifies the HDL-cholesterol concentration and liver scavenger receptor BI expression in the hamster

The influence of myristic acid in a narrow physiological range (0·5 to 2·4 % of total dietary energy) on the plasma and hepatic cholesterol metabolism was investigated in the hamster. The hamsters were fed on a diet containing 12·5 g fat/100 g and 0·05 g cholesterol/100 g with 0·5 % myristic acid (LA diet) for 3 weeks (pre-period). During the following 3 weeks (test period), they were divided into four dietary groups with 0·5 % (LA), 1·2 % (LM), 1·8 % (ML) or 2·4 % (M) myristic acid. Finally, half the hamsters in each group were again fed the LA diet for another 3 weeks (post-period). At the end of the test period, the hepatic expression of the scavenger receptor BI (SR-BI) was lower in the LM, ML and M groups than in the LA group whereas the hepatic cholesteryl ester concentration was higher. Cholesterol 7α hydroxylase activity was lower in the ML and M groups than in the LA and LM groups while the sterol 27 hydroxylase and 3-hydroxy-3-methyl glutaryl coenzyme A reductase activities were not modulated by dietary myristic acid. This is the first time a negative correlation has been observed between the HDL-cholesterol concentration and the hepatic mass of SR-BI (r-0·69;P<0·0001) under physiological conditions. An inverse linear regression was also shown between SR-BI and the percentage of myristic acid in the diet (r-0·75;P<0·0001). The hepatic mass of SR-BI in the M group had increased at the end of the post-period compared with the test-period values. The present investigation shows that myristic acid modulates HDL-cholesterol via a regulation of the SR-BI expression.

Are plasma lipid levels related to ABCG5/ABCG8 polymorphisms? A preliminary study in siblings with gallstones

BACKGROUND

The role of ABCG5 and ABCG8 genes in the determination of plasma lipid levels is currently under intensive investigation. The aim of this study was to evaluate plasma lipid levels in sibling pairs with gallstones and to assess their correlation with common gene polymorphisms in the ABCG5/ABCG8 genes.

METHODS

Plasma levels of cholesterol, HDL-cholesterol, and triglycerides were measured in 68 patients belonging to 34 sibling pairs with gallstones (affected sibling pairs, mean age 56.3 years) and in 68 gallstone carriers with stone-free siblings (age/gender-matched controls in a ratio of 2:1 with the index patients of the study group). Four and one non-synonymous sequence variants in ABCG8 and ABCG5 genes, respectively, were determined in the affected sibling pairs, employing allelic discrimination with 5' nuclease assays.

RESULTS

Plasma triglyceride levels were higher and HDL-cholesterol levels lower in the index patients than in controls. Plasma lipid levels were correlated in the members of the affected sibling pairs. Triglyceride levels were higher in carriers of the common alleles for ABCG5 Q604E and ABCG8 D19H sequence variants, and HDL-cholesterol was lower in carriers of the common alleles for ABCG5 Q604E than in carriers of the rare alleles.

CONCLUSIONS

The significantly different plasma lipid levels in siblings with gallstones versus controls, as well as the correlation of plasma lipids in affected sibling pairs, confirm the genetic influence in gallstone disease. Polymorphisms in ABCG5/ABCG8 genes might contribute to the genetic variation in plasma lipid levels and in cholesterol saturation of the bile.

Increased sphingomyelin content impairs HDL biogenesis and maturation in human Niemann-Pick disease type B

We previously reported that human Niemann-Pick Disease type B (NPD-B) is associated with low HDL. In this study, we investigated the pathophysiology of this HDL deficiency by examining both HDL samples from NPD-B patients and nascent high density lipoprotein (LpA-I) generated by incubation of lipid-free apolipoprotein A-I (apoA-I) with NPD-B fibroblasts. Interestingly, both LpA-I and HDL isolated from patient plasma had a significant increase in sphingomyelin (SM) mass ( approximately 50-100%). Analysis of LCAT kinetics parameters (V(max) and K(m)) revealed that either LpA-I or plasma HDL from NPD-B, as well as reconstituted HDL enriched with SM, exhibited severely decreased LCAT-mediated cholesterol esterification. Importantly, we documented that SM enrichment of NPD-B LpA-I was not attributable to increased cellular mass transfer of SM or unesterified cholesterol to lipid-free apoA-I. Finally, we obtained evidence that the conditioned medium from HUVEC, THP-1, and normal fibroblasts, but not NPD-B fibroblasts, contained active secretory sphingomyelinase (S-SMase) that mediated the hydrolysis of [(3)H]SM-labeled LpA-I and HDL(3). Furthermore, expression of mutant SMase (DeltaR608) in CHO cells revealed that DeltaR608 was synthesized normally but had defective secretion and activity. Our data suggest that defective S-SMase in NPD leads to SM enrichment of HDL that impairs LCAT-mediated nascent HDL maturation and contributes to HDL deficiency. Thus, S-SMase and LCAT may act in concert and play a crucial role in the biogenesis and maturation of nascent HDL particles.

Investigating the Genetic Determinants of Cardiovascular Disease Using Candidate Genes and Meta-analysis of Association Studies

Coronary artery disease (CAD) has a polygenic basis, and identification of CAD susceptibility genes has the potential to aid the development of new treatments and enhance prediction of disease risk. Thus far, the strategy has firstly been to choose "candidate" genes coding for important "rate-limiting" proteins in the homeostatic systems involved in maintaining cardiovascular health; secondly to identify common variants in these candidate genes; thirdly to carry out genotyping and statistical analysis using genetic association studies; and finally to test the functional effects of the identified variants in vitro and in vivo. However, lack of reproducibility of genetic association studies has led to uncertainty about the nature and number of genes involved. In part this is because many of the studies conducted have not been adequately powered to detect small risk effects, or to permit adequate exploration of gene-gene or gene-environment interactions in a robust manner. Spurious positive and negative associations due to type I and type II statistical errors are likely to co-exist with real associations in the published literature. By utilising all available data to increase statistical power, meta-analysis of genetic association studies is increasingly being used to identify genotypic risk with a greater degree of precision. Though potentially powerful, this approach may be prone to publication bias. Therefore, very large genetic association studies will also be required to identify risk genes for CAD. This review lays out the framework for the candidate gene approach for CAD and illustrates this with published results from a UK prospective study of 3000 middle-aged men.

Contribution of regulatory and structural variations in APOE to predicting dyslipidemia

The objective of this study was to evaluate 1) whether non single nucleotide polymorphisms-coding (non-cSNP) in the apolipoprotein E gene (APOE) identified by resequencing studies contribute to statistically explaining dyslipidemia if variations in the two cSNPs in exon 4 that define the 2, 3, and 4 alleles are ignored, and 2) whether the contribution of these additional SNPs persists when variations in the cSNPs are considered. We used an ecological, multiple-population, data-mining strategy to identify single-SNP and two-SNP genotypes that distinguish between high and low levels of plasma lipids in three training samples, European-Americans from Rochester, MN, African-Americans from Jackson, MS, and Europeans from North Karelia, Finland. We found that a pair of SNPs located in the 5' region define genotypes A560T832/A560T832, A560T832/A560G832, and A560T832/T560T832, which distinguish between high and low levels of HDL-cholesterol (HDL-C), triglycerides (TG), and/or total cholesterol (T-C). The A560T832/- genotypes predicted high TG and high T-C in both genders in a large independent test sample from Copenhagen, Denmark. Prediction of high T-C in the Danish females was dependent on genotypes defined by the cSNPs. Our study suggests that both regulatory and structural variations should be considered when evaluating the utility of APOE for predicting dyslipidemia in the population at large.

Compromised LCAT Function Is Associated With Increased Atherosclerosis

BACKGROUND

Prospective epidemiological studies have shown that low plasma levels of HDL cholesterol (HDL-C) are associated with an increased risk for cardiovascular disease (CVD). Despite nearly 40 years of research, however, it is unclear whether this also holds true for individuals with severely reduced levels of HDL-C due to mutations in the lecithin:cholesterol acyltransferase (LCAT) gene. Better insight into CVD risk in these individuals may provide clues toward the potential of LCAT as a pharmaceutical target to raise HDL-C levels.

METHODS AND RESULTS

Lipids, lipoproteins, high-sensitivity C-reactive protein (CRP), and carotid artery intima-media thickness (IMT) were assessed in 47 heterozygotes for LCAT gene mutations and 58 family controls. Compared with controls, heterozygotes presented with a mean 36% decrease in HDL-C levels (P<0.0001), a 23% increase in triglyceride levels (P<0.0001), and a 2.1-fold increase in CRP levels (P<0.0001). Mean carotid IMT was significantly increased in heterozygotes compared with family controls (0.623+/-0.13 versus 0.591+/-0.08 mm). After adjustment for age, gender, and alcohol use, this difference proved statistically significant (P<0.0015).

CONCLUSIONS

The data show that heterozygosity for LCAT gene defects is associated with low HDL-C levels and elevated concentration of triglycerides and CRP in plasma. This phenotype underlies increased IMT in carriers versus controls, which suggests that LCAT protects against atherosclerosis. This in turn indicates that targeting LCAT to raise HDL-C may reduce CVD risk.

HDL: the 'new' target of cardiovascular medicine

Clinical, experimental and epidemiological research has shown the undeniable causal relationship between low HDL plasma concentrations and cardiovascular disease. Low HDL levels are present in about 10% of the general population and represent the most frequent form of dyslipidemia in patients with coronary disease. Reduced HDL concentrations seem to be unable to eliminate efficiently the cholesterol excess at vascular wall level, contributing to the onset of the inflammatory response that typically occurs in the pathogenesis of atherosclerosis right from its earliest stages. The results of numerous studies quite convincingly suggest that HDL is capable of exerting anti-inflammatory activity either directly or by modulating the expression of a number of acute phase proteins. Although the therapeutic options currently available for raising HDL levels still show modest efficacy, both in experimental and pre-clinical fields, genetic investigation and specifically aimed pharmacological treatment have produced more encouraging results, shedding some light on the concrete possibility of being able to treat this disease in the very near future.

HMG CoA reductase inhibitors (statins): do they have a role in age-related macular degeneration?

Age-related macular degeneration is a progressive late onset disease affecting central vision. It is the leading cause of irreversible blindness in developed countries, and with the aging population the problem is increasing. Current treatment options are limited to the late stage of the disease when central vision is already under great threat, and even new treatments make little impact on the rate of blindness. Intervention earlier in the disease may prove more rewarding, but to date little progress has been made with this approach. Epidemiologic, genetic, and pathological evidence continues to accumulate, suggesting a possible link between risk factors for cardiovascular diseases and age-related macular degeneration. This article reviews the evidence and discusses the rationale behind the recent suggestions that cholesterol-lowering agents may be useful in the treatment of early age-related macular degeneration. The cholesterol-lowering family of drugs called statins are 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) inhibitors with pleiotropic actions. Their therapeutic effects in cardiovascular disease and dyslipidaemia have been well proven. In this review we will outline the known actions of statins and discuss possible ways that they may impact on age-related macular degeneration.

Triglyceride-rich lipoproteins inhibit cholesterol efflux to apolipoprotein (apo) A1 from human macrophage foam cells

High circulating levels of triglyceride-rich lipoproteins (TGRL) represent an independent risk factor for coronary artery disease. Here, we show that TGRL inhibit the efflux of cholesterol from 'foam cell' macrophages to lipid-poor apolipoprotein (apo) A1, and may thereby inhibit arterial reverse cholesterol transport and promote the formation of atherosclerotic lesions. Human (THP-1) monocyte-derived macrophages were pre-incubated (48 h) with acetylated low-density lipoprotein (AcLDL) to provide a foam cell model of cholesterol efflux to apoA1. Pre-incubation of macrophage 'foam cells' with TGRL (0-200 microg/ml, 0-24 h) inhibited the efflux of exogenously radiolabelled ([3H]), endogenously synthesised ([14C]) and cellular cholesterol mass to lipid-poor apoA1, but not control medium, during a (subsequent) efflux period. This inhibition is dependent upon the length of prior exposure to, and concentration of, TGRL employed, but is independent of changes in intracellular triglyceride accumulation or turnover of the cholesteryl ester pool. Despite the negative impact of TGRL on cholesterol efflux, major proteins involved in this process--namely apoE, ABCA1, SR-B1 and caveolin-1--were unaffected by TGRL pre-incubation, suggesting that exposure to these lipoproteins inhibits an alternate, and possibly novel, anti-atherogenic pathway.

Association between single-nucleotide polymorphisms in the endothelial lipase (LIPG) gene and high-density lipoprotein cholesterol levels

Endothelial lipase (LIPG) is the latest addition to the triglyceride lipase family of genes that includes pancreatic lipase (PL), hepatic lipase (HL), and lipoprotein lipase (LPL). These lipolytic enzymes demonstrate both triglyceride lipase as well as phospholipase activities and are integrally involved in lipid absorption, transport, and metabolism. Several studies have demonstrated that LIPG is important for affecting lipid levels in mice but the data in humans is less complete. To more thoroughly characterize the LIPG gene, we resequenced it from an ethnically diverse population. Thirteen novel single-nucleotide polymorphisms (SNPs) were identified and seven others confirmed. High linkage disequilibrium was found among these SNPs spanning the length of the transcript, allowing interrogation of the entire gene for functional variation. Subjects with either high or low HDL cholesterol were used to investigate its association with LIPG gene variation. Associations were found with the most significant being the intronic variants C+42T/In5 and T+2864C/In8 (P=0.007 and 0.004, respectively). A trend for an association of the same SNPs with fewer myocardial infarctions (P=0.03) was also observed but was not significant after correction for multiple testing. The results of this study provide data linking variation in the human LIPG gene with HDL cholesterol levels as well as further evidence in support of LIPG as a potential target for therapeutic intervention.

Design, Synthesis, and Biological Evaluation of Thio-Containing Compounds with Serum HDL-Cholesterol-Elevating Properties

A novel series of substituted sulfanyldihydroimidazolones (1) that modulates high-density lipoprotein cholesterol (HDL-C) has been reported to have HDL-elevating properties in several animal models. Concerns about the chemical and metabolic stability of 1 directed us to explore the structure-activity relationship (SAR) of a related series of substituted thiohydantoins (2). Expansion of the scope of the thiohydantoin series led to exploration of compounds in related thio-containing ring systems 3-7 and the N-cyanoguanidine derivative 8. Compounds were tested sequentially in three animal models to assess their HDL-C elevating efficacy and safety profiles. Further evaluation of selected compounds in a dose-response paradigm culminated in the identification of compound 2.39 as a candidate compound for advanced preclinical studies.

Action of ciprofibrate in type IIb hyperlipoproteinemia: modulation of the atherogenic lipoprotein phenotype and stimulation of high-density lipoprotein-mediated cellular cholesterol efflux

The effects of ciprofibrate (100 mg/d) on apolipoprotein (apo)B- and apoAI-containing lipoprotein subclasses, cholesteryl ester (CE) transfer protein activity, and plasma high-density lipoprotein (HDL)-mediated cellular cholesterol efflux were evaluated in 10 patients displaying type IIB hyperlipidemia. Plasma concentrations of large very low-density lipoprotein (VLDL)-1 (Sf 60-400) and of small VLDL-2 (Sf 20-60) were markedly diminished after fibrate treatment (-40%, P = 0.001; and -25%, P = 0.003, respectively). We observed a reduction (-17%; P = 0.005) in plasma low-density lipoprotein (LDL) levels resulting from significant reductions in concentrations of dense LDL particles (-46%; P < 0.0001). Ciprofibrate induced elevation in plasma total HDL (+13%; P = 0.005) levels; such elevation occurred preferentially in HDL-3 (+22%; P = 0.009). Marked reduction in numbers of atherogenic apoB100-containing particle acceptors was associated with a 25% decrease (P < 0.02) in CE transfer protein-mediated CE transfer from HDL. Finally, a significant fibrate-mediated elevation (+13%; P = 0.01 compared with baseline) in the capacity of plasma from type IIB subjects to mediate free cholesterol efflux from scavenger receptor class B, type I-expressing Fu5AH hepatoma cells was observed. In conclusion, the action of ciprofibrate in type IIB dyslipidemia leads to preferential reduction in particle numbers of atherogenic VLDL-1, VLDL-2, and dense LDL and, concomitantly, to elevation in HDL-3 levels that are associated with stimulation of HDL-mediated cellular free cholesterol efflux through the scavenger receptor class B, type I receptor pathway.

Using advanced intercross lines for high-resolution mapping of HDL cholesterol quantitative trait loci

Mapping quantitative trait loci (QTLs) with high resolution facilitates identification and positional cloning of the underlying genes. The novel approach of advanced intercross lines (AILs) generates many more recombination events and thus can potentially narrow QTLs significantly more than do conventional backcrosses and F2 intercrosses. In this study, we carried out QTL analyses in (C57BL/6J x NZB/BlNJ) x C57BL/6J backcross progeny fed either chow or an atherogenic diet to detect QTLs that regulate high-density lipoprotein cholesterol (HDL)concentrations, and in (C57BL/6J x NZB/BlNJ) F11 AIL progeny to confirm and narrow those QTLs. QTLs for HDL concentrations were found on chromosomes 1, 5, and 16. AIL not only narrowed the QTLs significantly more than did a conventional backcross but also resolved a chromosome 5 QTL identified in the backcross into two QTLs, the peaks of both being outside the backcross QTL region. We tested 27 candidate genes and found significant mRNA expression differences for 12 (Nr1i3, Apoa2, Sap, Tgfb2, Fgfbp1, Prom, Ppargc1, Tcf1, Ncor2, Srb1, App, and Ifnar). Some of these underlay the same QTL, indicating that expression differences are common and not sufficient to identify QTL genes. All the major HDL QTLs in our study had homologous counterparts in humans, implying that their underlying genes regulate HDL in humans.

Quantitative trait loci and candidate genes regulating HDL cholesterol: a murine chromosome map

OBJECTIVE

Summarizing the many discovered mouse and human quantitative trait loci (QTL) for high density lipoprotein (HDL) cholesterol (HDL-C) levels is important for guiding future research on the genetic regulation of HDL concentrations and for finding gene targets for upregulating HDL levels in mice and humans.

METHODS AND RESULTS

We summarized the 27 QTL and candidate genes associated with HDL-C concentrations in mice and plotted them on a mouse chromosome map. We also summarized the 22 human QTL for HDL-C levels and compared them with those of the mouse by comparative genomics. At least part of the mouse homologies for 18 of the 22 human HDL-C QTL were within the murine HDL-C QTL.

CONCLUSIONS

Murine QTL for HDL-C levels may predict their homologous location in humans, and their underlying genes may be appropriate genes to test in humans.

Pharmacodynamics and pharmacogenomics of diverse receptor-mediated effects of methylprednisolone in rats using microarray analysis

Corticosteroids such as methylprednisolone (MPL) produce many of their anti-inflammatory, immunosuppressive, and exaggerated physiological effects by receptor and gene-mediated mechanisms. The temporal pattern of change in four genes in rat tissues was measured by quantitative Northern hybridization and rtPCR after a single dose of MPL. Two profiles were observed: two genes with enhanced expression showed a slow onset and moderate rate of decline within a 24 hr time frame while two genes with reduced expression exhibited a rapid onset and prolonged suppression over a > or = 72 hr time span. These patterns are consistent with and rationalized by pharmacodynamic expectations based on earlier models. cDNA microarrays used to assess the expression levels of 5200 genes at one optimal time-point showed marked variation in baseline values. Of these, 20 genes showed statistically significant enhanced expression with increases ranging from 130 to 1690%, 31 genes exhibited reduced expression ranging from 31 to 72% of control. Many genes could be categorized as affecting acute phase/immune response, energy metabolism, microsomal metabolism, and hepatic function. These studies provide the first simultaneous assessment of the diversity in pharmacogenomic effects of corticosteroids. They also provide some insight into the advantages and limitations of microarray measurements in regard to the pharmacodynamics of drugs having complex, multi-faceted, and integrated mechanisms of action.

Sequencing and chromosomal assignment of the rat endothelial-derived lipase gene (Lipg)

Part of the nucleotide sequence of the Lipg gene in the rat was established using primers based on the mRNA sequence described in the mouse. The rat intron sequence served as a template for designing primers for the specific amplification of rat Lipg. A rat-hamster radiation hybrid (RH) panel was used for chromosomal assignment of the rat Lipg gene. The Lipg gene was found to be located on rat chromosome 18 in the vicinity of the marker D18Mit11; a region reported to be homologous with both human and mouse chromosome 18.

Genetic susceptibility to macrovascular complications of type 2 diabetes mellitus

Patients with type 2 diabetes mellitus have a threefold increased risk of developing macrovascular disease such that 75% of such patients will die of cardiovascular complications. This increased risk is, however, not completely explained by traditional risk factors such as smoking, hypercholesterolaemia, hypertension and glycaemic control. Moreover, the fact that not all patients with type 2 diabetes develop these complications, together with evidence of family clustering (a heritability of 50%), suggests that a proportion of the susceptibility to ischaemic heart disease in type 2 diabetes may be genetic. Unravelling the polygenic susceptibility factors for the complications of a disease that itself has multifactorial inheritance has proved difficult and has focused largely on the candidate gene approach. A review of some of the studies testing candidate genes specifically in patients with both type 2 diabetes and ischaemic heart disease is presented. These studies focus largely on four main areas: lipoprotein metabolism, glycation and oxidation pathways, haemostatic cascade, and other candidate genes.

The farnesoid X-activated receptor mediates bile acid activation of phospholipid transfer protein gene expression

Bile acids facilitate the absorption of dietary lipids and fat-soluble vitamins and are physiological ligands for the farnesoid X-activated receptor (FXR), a member of the nuclear hormone receptor superfamily. FXR functions as a heterodimer with the retinoid X receptor and in the presence of ligand, the heterodimer binds to specific DNA sequences in the promoters of target genes to regulate gene transcription. Phospholipid transfer protein (PLTP) has been identified as a possible target gene for FXR because the human promoter contains a potential FXR response element, an inverted repeat in which consensus receptor-binding hexamers are separated by one nucleotide (inverted repeat-1). PLTP is essential in the transfer of very low density lipoprotein phospholipids into high density lipoprotein (Jiang, X. C., Bruce, C., Mar, J., Lin, M., Ji, Y., Francone, O. L., and Tall, A. R. (1999) J. Clin. Invest. 103, 907-914). Here we report the regulation of PLTP gene expression by FXR and bile acids. In CV-1 cells, cotransfection of FXR and the retinoid X receptor resulted in bile acid-dependent transactivation of a luciferase reporter construct containing the human PLTP promoter. Mutation analysis demonstrated that the inverted repeat-1 (IR-1) in the PLTP promoter is required for this transactivation. Finally, we demonstrate that bile acids are able to regulate PLTP gene expression in vivo. Mice fed a chow diet supplemented with bile acid showed increased hepatic PLTP mRNA levels. These results suggest that FXR may play a role in high density lipoprotein metabolism via the regulation of PLTP gene expression.

Prevention of coronary heart disease by raising high-density lipoprotein cholesterol?

Consistent with several potentially anti-atherogenic activities of high-density lipoproteins in vitro, low plasma levels of high-density lipoprotein cholesterol are associated with an increased risk of coronary heart disease. In addition to genes, lifestyle factors (e.g. smoking, being overweight and physical inactivity) strongly affect plasma high-density lipoprotein cholesterol levels. Moreover, a low level of high-density lipoprotein cholesterol interacts with other risk factors. Raising of high-density lipoprotein cholesterol by either adjustments of lifestyle or drug intervention as well as elimination of additional risk factors are thus thought to affect coronary risk. Here, we summarize the outcomes of observational and interventional studies as well as genetic and experimental research which have recently much advanced our understanding of the function and regulation of high-density lipoprotein metabolism. We conclude from the data that changes in the kinetics and functionality of high-density lipoprotein rather than changes in plasma high-density lipoprotein cholesterol levels per se will affect the anti-atherogenicity of therapeutic interference with high-density lipoprotein metabolism.

High-density lipoprotein: gene-based approaches to the prevention of atherosclerosis

Although the atheroprotective role of high-density lipoprotein (HDL) has been well documented in epidemiological and animal studies, highly effective therapeutic approaches for the selective increase of plasma HDL levels or function are not yet available. Several mechanisms by which HDL exerts an atheroprotective effect have been proposed on the basis of experiments in vitro and in vivo. These mechanisms include directing excess cellular cholesterol from the peripheral tissues to the liver in 'reverse cholesterol transport', inhibiting oxidative modification or aggregation of LDL, and modulating inflammatory responses to favour vasoprotection. This review gives an overview of the genes regulating these mechanisms, such as those encoding apolipoprotein AI, lecithin:cholesterol acyltransferase (LCAT), scavenger receptor B1 (SR-BI), and the ATP-binding cassette transporter 1 (ABC1), and the potential to exploit them to develop gene-based therapeutic approaches to increase the level or function of HDL.

Acceleration of reverse cholesterol transport

A low level of high-density lipoprotein (HDL) cholesterol is an important risk factor for coronary heart disease. Levels of HDL cholesterol and composition of HDL subclasses in plasma are regulated by many factors, including apolipoproteins, lipolytic enzymes, lipid transfer proteins, receptors, and cellular transporters. Reverse transport of cholesterol from cells of the arterial wall to the liver is an important mechanism by which HDL exerts its anti-atherogenic properties. Enhancement of reverse cholesterol transport is considered as a potential target for anti-atherosclerotic drug therapy. It is suggested, however, that the serum level of HDL cholesterol does not necessarily reflect the efficacy of reverse cholesterol transport.