Genetic determinants of plasma lipoproteins

Association of hepatic lipase gene polymorphisms with hypertriglyceridemia and low high-density lipoprotein-cholesterol levels among South Indian subjects without diabetes

AIM

The aim of this study was to investigate the association of four variants of the hepatic lipase (HL [or LIPC]) gene with various lipid parameters among South Indian subjects with normal glucose tolerance (NGT).

SUBJECTS AND METHODS

In total, 747 NGT subjects were randomly selected from the Chennai Urban Rural Epidemiological Study (CURES). Serum triglycerides, serum cholesterol, and high-density lipoprotein cholesterol (HDL-C) were measured using a Hitachi-912 autoanalyzer (Roche Diagnostics GmbH, Mannheim, Germany). Genotyping of HL gene variants was done by the polymerase chain reaction-restriction fragment length polymorphism method, and 20% of samples were sequenced to validate the genotypes obtained. Haplotype analysis was also carried out.

RESULTS

The TT genotype of the rs1800588 C/T (C-480T) polymorphism was significantly associated with hypertriglyceridemia, with an adjusted odds ratio of 2.58 (95% confidence interval 1.38-4.85, P=0.003), whereas those with the CC genotype of the rs6074 A/C (Thr479Thr) had significantly lower HDL-C levels (41.3±9.8 mg/dL) compared with the AA genotype (43.6±10.2 mg/dL, P=0.02). Haplotype analysis showed the TGC haplotype was significantly associated with low HDL-C levels.

CONCLUSIONS

Among South Indian subjects without diabetes, the rs1800588 C/T (C-480T) and rs6074 C/A (Thr479Thr) variants of the HL gene are associated with hypertriglyceridemia and low HDL-C, respectively. The TGC haplotype was significantly associated with low HDL-C.

Pleiotropic effects on subclasses of HDL, adiposity, and glucose metabolism in adult Alaskan Eskimos

The aim of this study was to analyze the heritability and the presence of pleiotropic effects on subfractions of high-density lipoproteins (HDLs) as measured by nuclear magnetic resonance (NMR), parameters for adiposity, and glucose metabolism in adult Alaskan Eskimos. The present family study included 1,214 adult Alaskan Eskimos (537 male/677 female). Body weight, height, circumferences, selected skinfolds, and blood pressure were measured in all participants. Blood samples were collected under fasting conditions for the isolation of plasma. Glucose, insulin, subclasses and size of lipoproteins, triglycerides, total, and HDL cholesterol and lipoprotein (a) were measured in plasma. HbA1c was measured in total blood. Univariate and bivariate quantitative genetic analyses were conducted between HDL subclasses and size and the anthropometric and biochemical measures using the variance decomposition approach. Variation in all the analyzed traits exhibits a significant genetic component. Heritabilities ranged between 0.18 +/- 0.11 for LDL(2) (intermediate) and 0.89 +/- 0.07 for small HDL. No common genetic effects were found on the HDL subclasses (small, intermediate, and large). Small HDL particles were genetically correlated with LDL particles and HbA1c. Negative genetic correlations were observed between intermediate and large HDL subfractions, HDL size and measures of adiposity, and LDL and parameters for glucose metabolism (HbA1, insulin). These observations confirm the presence of possible pleiotropic effects on HDL, adiposity, and cardiovascular risk factors and provide novel insight on the relationship between HDL subclasses, adiposity, and glucose regulation.

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.

Concordance of two multiple analytical approaches demonstrate that interaction between BMI and ADIPOQ haplotypes is a determinant of LDL cholesterol in a general French population

Genetic and environmental factors are involved in insulin resistance (IR). IR and dyslipidemia associate with increased risk of cardiovascular diseases. Plasma low-density lipoprotein cholesterol (LDL-C) level is a marker of cardiovascular risk. In a Caucasian general population we aimed at determining the multifactorial components of LDL-C levels using 10 genes and 3 phenotypes. In the PPARG, UCP3, ADIPOQ, TNF, LIPC, CARTPT, PCSK9, SCAP, SCARB1 and ENPP1 genes known to be associated with IR or dyslipidemia we genotyped 19 single nucleotide polymorphisms (SNPs) in 846 subjects. When several SNPs were genotyped for a given gene we constructed haplotypes. Including genetic and environmental variables (gender, body mass index (BMI) and adiponectin level) we used (1) the multifactor dimensionality reduction method to explain clusters of high and low LDL-C, and (2) the restricted partition method to explain LDL-C levels. Both methods showed that BMI and haplotypes at the ADIPOQ adiponectin encoding gene but not adiponectin level itself, were discriminant regarding to LDL-C. Subjects bearing an at-risk combination of BMI and ADIPOQ genotypes were prone to have a higher LDL-C (OR=3.13, 95% CI=2.20-4.46, P<0.0001). Our results suggest that in interaction with BMI, ADIPOQ haplotypes capture genetic variation(s) from neighboring gene(s) that would modulate LDL-C level.

Carnitine palmitoyltransferase IA polymorphism P479L is common in Greenland Inuit and is associated with elevated plasma apolipoprotein A-I

Carnitine palmitoyltransferase IA, encoded by CPT1A, is a key regulator of fatty acid metabolism. Previously, a loss-of-function mutation, namely, c.1436 C-->T (p.P479L), was reported in CPT1A in the homozygous state in Canadian aboriginal male with presumed CPT1A deficiency. To determine the population frequency of this variant, we determined CPT1A p.P479L genotypes in 1111 Greenland Inuit. Associations between genotype and variation in plasma total cholesterol, triglycerides, LDL, HDL, apolipoprotein (apo) B, and apoA-I was also investigated. We found the L479 allele occurs at a high frequency in this sample (0.73), while it was completely absent in 285 nonaboriginal samples. This suggests that the original proband's symptoms were not likely due to the CPT1A p.P479L mutation because it is very common in Inuit and because symptoms suggesting CPT1A deficiency have not been reported in any carrier subsequently studied. However, CPT1A p.P479L was associated with elevated plasma HDL and apoA-I levels. The association with increased levels of HDL and apoA-I suggest that the polymorphism might protect against atherosclerosis.

Plasma lipoproteins: genetic influences and clinical implications

Susceptibility to the growing global public health problem of cardiovascular disease is associated with levels of plasma lipids and lipoproteins. Several experimental strategies have helped us to clarify the genetic architecture of these complex traits, including classical studies of monogenic dyslipidaemias, resequencing, phenomic analysis and, more recently, genome-wide association studies and analysis of metabolic networks. The genetic basis of plasma lipoprotein levels can now be modelled as a mosaic of contributions from multiple DNA sequence variants, both rare and common, with varying effect sizes. In addition to filling gaps in our understanding of plasma lipoprotein metabolism, the recent genetic advances will improve our ability to classify, diagnose and treat dyslipidaemias.

Hypertriglyceridemia: phenomics and genomics

Hypertriglyceridemia is a common complex metabolic trait that is associated with increased atherosclerosis risk, presence of the metabolic syndrome and, with extreme elevation, increased risk of pancreatitis. Hierarchical cluster analysis using clinical and biochemical features of the Frederickson hyperlipoproteinemia types can generate hypotheses for molecular genetic studies. High throughput resequencing of individuals at the extremes of plasma triglyceride concentration has shown that both rare genetic variants with large effects and common genetic variants with moderate effects explain a relatively large proportion of variation. Very recent progress using high-density sets of genome-wide markers have identified additional genetic determinants of plasma triglyceride concentrations, albeit within largely normolipidemic subjects and with small effect sizes. Phenomic evaluation of patients with hypertriglyceridemia might help to clarify genotype-phenotype correlations and responses to interventions.

Association between glucokinase regulatory protein (GCKR) and apolipoprotein A5 (APOA5) gene polymorphisms and triacylglycerol concentrations in fasting, postprandial, and fenofibrate-treated states

BACKGROUND

Hypertriglyceridemia is a risk factor for cardiovascular disease. Variation in the apolipoprotein A5 (APOA5) and glucokinase regulatory protein (GCKR) genes has been associated with fasting plasma triacylglycerol.

OBJECTIVE

We investigated the combined effects of the GCKR rs780094C-->T, APOA5 -1131T-->C, and APOA5 56C-->G single nucleotide polymorphisms (SNPs) on fasting triacylglycerol in several independent populations and the response to a high-fat meal and fenofibrate interventions.

DESIGN

We used a cross-sectional design to investigate the association with fasting triacylglycerol in 8 populations from America, Asia, and Europe (n = 7,730 men and women) and 2 intervention studies in US whites (n = 1,061) to examine postprandial triacylglycerol after a high-fat meal and the response to fenofibrate. We defined 3 combined genotype groups: 1) protective (homozygous for the wild-type allele for all 3 SNPs); 2) intermediate (any mixed genotype not included in groups 1 and 3); and 3) risk (carriers of the variant alleles at both genes).

RESULTS

Subjects within the risk group had significantly higher fasting triacylglycerol and a higher prevalence of hypertriglyceridemia than did subjects in the protective group across all populations. Moreover, subjects in the risk group had a greater postprandial triacylglycerol response to a high-fat meal and greater fenofibrate-induced reduction of fasting triacylglycerol than did the other groups, especially among persons with hypertriglyceridemia. Subjects with the intermediate genotype had intermediate values (P for trend <0.001).

CONCLUSIONS

SNPs in GCKR and APOA5 have an additive effect on both fasting and postprandial triacylglycerol and contribute to the interindividual variability in response to fenofibrate treatment.

Polygenic determinants of severe hypertriglyceridemia

Recent genome-wide association (GWA) studies have identified new genetic determinants of complex quantitative traits, including plasma triglyceride (TG). We hypothesized that common variants associated with mild TG variation identified in GWA studies would also be associated with severe hypertriglyceridemia (HTG). We studied 132 patients of European ancestry with severe HTG (fasting plasma TG > 10 mmol/l), who had no mutations found by resequencing of candidate genes, and 351 matched normolipidemic controls. We determined genotypes for: GALNT2 rs4846914, TBL2/MLXIPL rs17145738, TRIB1 rs17321515, ANGPTL3 rs12130333, GCKR rs780094, APOA5 rs3135506 (S19W), APOA5 rs662799 (-1131T > C), APOE (isoforms) and LPL rs328 (S447X). We found that: (i) genotypes, including those of APOA5 S19W, APOA5 -1131T > C, APOE, GCKR, TRIB1 and TBL2/MLXIPL, were significantly associated with severe HTG; (ii) odds ratios for these genetic variables were significant in both univariate and multivariate regression analyses, irrespective of the presence or absence of diabetes or obesity; (iii) a significant fraction-about one-quarter-of the explained variation in disease status was associated with these genotypes. Therefore, common SNPs (single nucleotide polymorphisms) that are associated with mild TG variation in GWA studies of normolipidemic subjects are also associated with severe HTG. Our findings are consistent with the emerging model of a complex genetic trait. At the extremes of a quantitative trait, such as severe HTG, are found the cumulative contributions of both multiple rare alleles with large genetic effects and common alleles with small effects.

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.