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Lipoprotein subfractions help discriminate cardiometabolic disease risk. Genetic loci validated as associating with lipoprotein measures do not account for a large proportion of the individual variation in lipoprotein measures. We hypothesized that DNA methylation levels across the genome contribute to interindividual variation in lipoprotein measures. Using data from participants of the Genetics of Lipid Lowering Drugs and Diet Network (n = 663 for discovery and n = 331 for replication stages, respectively), we conducted the first systematic screen of the genome to determine associations between methylation status at ∼470,000 cytosine-guanine dinucleotide (CpG) sites in CD4+ T cells and 14 lipoprotein subfraction measures. We modeled associations between methylation at each CpG site and each lipoprotein measure separately using linear mixed models, adjusted for age, sex, study site, cell purity, and family structure. We identified two CpGs, both in the carnitine palmitoyltransferase-1A (CPT1A) gene, which reached significant levels of association with VLDL and LDL subfraction parameters in both discovery and replication phases (P < 1.1 × 10−7 in the discovery phase, P < .004 in the replication phase, and P < 1.1 × 10−12 in the full sample). CPT1A is regulated by PPARα, a ligand for drugs used to reduce CVD. Our associations between methylation in CPT1A and lipoprotein measures highlight the epigenetic role of this gene in metabolic dysfunction.

Modified LDL in human plasma including small, dense LDL (sdLDL) and oxidized LDL carries a more negative charge than unmodified LDL and is atherogenic. We examined the effects of apolipoprotein A-I (apoA-I)/POPC discs on charge-based LDL subfractions as determined by capillary isotachophoresis (cITP). Three normal healthy subjects and seven patients with metabolic disorders were included in the study. LDL in human plasma was separated into two major subfractions, fast- and slow-migrating LDL (fLDL and sLDL), by cITP. Normal LDL was characterized by low fLDL, and mildly oxidized LDL in vitro and mildly modified LDL in human plasma were characterized by increased fLDL. Moderately oxidized LDL in vitro and moderately modified LDL in a patient with hypertriglyceridemia and HDL deficiency were characterized by both increased fLDL and a new LDL subfraction with a faster mobility than fLDL [very-fast-migrating LDL as determined by cITP (vfLDL)]. cITP LDL subfractions with faster electrophoretic mobility (fLDL vs. sLDL, vfLDL vs. fLDL) were associated with an increased content of sdLDL. Incubation of a plasma fraction with d > 1.019 g/ml (depleted of triglyceride-rich lipoproteins) in the presence of apoA-I/POPC discs at 37°C greatly decreased vfLDL and fLDL but increased sLDL. Incubation of whole plasma from patients with an altered distribution of cITP LDL subfractions in the presence of apoA-I/POPC discs also greatly decreased fLDL but increased sLDL. ApoA-I/POPC discs decreased the cITP fLDL level, the free cholesterol concentration, and platelet-activating factor acetylhydrolase activity in the sdLDL subclasses (d = 1.040–1.063 g/ml) and increased the size of LDL. ApoA-I/POPC discs reduced charge-modified LDL in human plasma by remodeling cITP fLDL into sLDL subfractions.

The proportion of the electronegative low density lipoprotein [LDL(−)] subfraction, which is atherogenic, is increased in type 2 diabetes but is not reduced by glycemic control. Therefore, we evaluated the ability of a new technique, capillary isotachophoresis (cITP), to quantify charge-based LDL subfractions and examined the relation between insulin resistance and the cITP fast-migrating (f) LDL levels. Seventy-five 10-year-old boys were included. The two cITP LDL subfractions, fLDL and major LDL subfractions, were proportional to the LDL protein content within the range of 0.1–0.8 mg/ml LDL protein. Levels of cITP fLDL were positively correlated with triglyceride (TG) levels and negatively correlated with LDL size. Insulin resistance as assessed by the homeostasis model assessment (HOMA-IR) was positively correlated (P < 0.01) with cITP fLDL levels (r = 0.41). The relation between HOMA-IR and cITP fLDL levels depended on TG levels but was independent of body mass index and LDL size.cITP lipoprotein analysis is an accurate and sensitive method for quantifying charge-based LDL subfractions in human plasma, and insulin resistance is related to cITP fLDL independent of LDL size.

Small, dense LDL particles have been associated with an increased risk of coronary artery disease, and cholesteryl ester transfer protein (CETP) has been suggested to play a role in LDL particle remodeling. We examined the relationship between LDL heterogeneity and plasma CETP mass in familial hypercholesterolemia (FH). LDL particles were characterized by polyacrylamide gradient gel electrophoresis in a total of 259 FH heterozygotes and 208 nonFH controls. CETP mass was measured by enzyme-linked immunosorbent assay in a subgroup of 240 participants, which included 120 FH patients matched with 120 controls. As compared with controls, FH subjects had an 11% higher CETP mass. Moreover, LDL-peak particle diameter (LDL-PPD) was significantly smaller in FH heterozygotes than in controls (258.1 ± 4.8 vs. 259.2 ± 4.1 Å; P = 0.01) after adjustment for covariates. There was also an inverse relationship between LDL-PPD and CETP mass (R = −0.15; P = 0.02), and this relationship was abolished by adjustment for the FH/control status, indicating that LDL-PPD changes in FH are mediated, at least in part, by an increase in plasma CETP mass concentrations.These results suggest that increased plasma CETP mass concentrations could lead to significant LDL particle remodeling in FH heterozygotes and could contribute to the pathogenesis of atherosclerosis.

Substantial evidence exists suggesting that small, dense LDL particles are associated with an increased risk of coronary heart disease. This disease-related risk factor is recognized to be under both genetic and environmental influences. Several studies have been conducted to elucidate the genetic architecture underlying this trait, and a review of this literature seems timely. The methods and strategies used to determine its genetic component and to identify the genes have greatly changed throughout the years owing to the progress made in genetic epidemiology and the influence of the Human Genome Project. Heritability studies, complex segregation analyses, candidate gene linkage and association studies, genome-wide linkage scans, and animal models are all part of the arsenal to determine the susceptibility genes. The compilation of these studies clearly revealed the complex genetic nature of LDL particles.This work is an attempt to summarize the growing evidence of genetic control on LDL particle heterogeneity with the aim of providing a concise overview in one read.

The size of LDL is usually reported as particle diameter, with the implicit assumption that it is a spherical particle. On the other hand, data obtained by cryoelectron microscopy and crystallographic analysis suggest that LDL shape may be discoid. We have investigated LDL particle geometry by combining data on LDL lipid composition with size measurement. The mean LDL diameter of 160 samples was measured by high-performance gel-filtration chromatography (HPGC), and particle volume was calculated from its lipid composition. Assuming a spherical shape, diameters calculated from volume correlated poorly with values obtained by HPGC (R2 = 0.36). Assuming a discoid shape, particle height was calculated from volume and HPGC diameter. Diameter (20.9 ± 0.5 nm) and height (12.1 ± 0.8 nm) were not significantly related to each other (r = 0.14, P = 0.09) and accounted for 23% and 77%, respectively, of the variation in particle volume. In multivariate regression models, LDL core lipids were the main determinants of height (R2 = 0.83), whereas free cholesterol in the shell, which contributes only 5–9% to LDL mass, was the main determinant of diameter (R2 = 0.54).We conclude that combined data from composition and size measurements are compatible with a discoid particle shape and propose a structural model for LDL in which free cholesterol plays a major role in determining particle shape and diameter.

Mice lacking hepatic lipase have been reported to express mild hyperlipidemia characterized by increased concentrations of large high density lipoproteins, but normal concentrations of lipoproteins containing apolipoprotein B. Whereas hepatic lipase has been implicated in the clearance and processing of chylomicron remnants in rats, no such defect was found in these mice. We have further characterized the abnormal lipoprotein phenotype in young hepatic lipase-deficient mice and have found more pronounced elevations of high density lipoproteins associated in particular with a 5-fold increase in plasma concentrations of apolipoprotein E. In addition, there was a reduction in the concentration of low density lipoproteins containing apolipoprotein B-100 and B-48 relative to precursor lipoproteins of lower density and a pronounced deficiency of apolipoprotein B-containing low density lipoproteins with density exceeding 1.029 g/mL. Conversion of radiolabeled rabbit intermediate density lipoproteins to low density lipoproteins was reduced by 6-fold as compared with wild-type mice. Although clearance of cholesteryl ester-labeled chylomicrons from the blood was unimpaired in the deficient mice, that of chylomicron remnants was reduced. Furthermore, endocytosis of chylomicron cholesteryl esters into liver cells occurred more rapidly than in wild-type mice. The unimpaired hepatic clearance of injected chylomicron particles in hepatic lipase-deficient mice may be the result of greater acquisition of apoE from high density lipoproteins during remnant formation. These studies thus demonstrate a critical role for mouse hepatic lipase in the formation of small, dense low density lipoproteins, as well as participation in the normal clearance and processing of chylomicron remnants.—Qiu, S., N. Bergeron, L. Kotite, R. M. Krauss, A. Bensadoun, and R. J. Havel. Metabolism of lipoproteins containing apolipoprotein B in hepatic lipase-deficient mice.

Recently, the presence of small dense low density lipoprotein (LDL) has been postulated to be a stronger risk factor for coronary heart disease than large LDL. While small dense LDL has been associated with individual components of the insulin resistance syndrome such as hypertension, high triglyceride level, low high density (HDL) cholesterol, and diabetess mellitus, there has been little work exploring whether LDL size is decreased in subjects with multiple metabolic disorders. We examined the association of LDL size and pattern to specific insulin (which does not cross-react with proinsulin), proinsulin, increased triglyceride, decreased HDL, hypertension and impaired glucose tolerance in 488 non-diabetic subjects from the San Antonio Heart Study. LDL size was significantly related to specific insulin, proinsulin and the fasting proinsulin/insulin ratio. Small dense LDL was significantly associated with high triglyceride level, decreased HDL cholesterol, hypertension and impaired glucose tolerance. LDL size (å) decreased in a stepwise fashion with increasing number of the metabolic disorders described above (zero 262.6±9.4; one 257.0±9.3; two 256.4±9.4; three 249.0±9.1; and four 244.9±9.0). These results were similar in men and women and in non-Hispanic whites and Mexican Americans. The association between LDL size and the number of metabolic disorders remained statistically significant even after adjustment for obesity, body fat distribution, gender, ethnicity, proinsulin and insulin concentrations. Furthermore, decreases in LDL size are also significantly associated with both a selective beta-cell defect (as estimated by the fasting proinsulin/insulin ratio) and insulin resistance (as estimated by the fasting insulin concentrations) although the association was some-what stronger for the latter. We conclude that small dense LDL may form part of the insulin resistance syndrome in non-diabetic subjects. [ABSTRACT FROM AUTHOR]