Independent effects of Apo E phenotype and plasma triglyceride on lipoprotein particle sizes in the fasting and postprandial states

Quantile-dependent expressivity of postprandial lipemia

Purpose

“Quantile-dependent expressivity” describes an effect of the genotype that depends upon the level of the phenotype (e.g., whether a subject’s triglycerides are high or low relative to its population distribution). Prior analyses suggest that the effect of a genetic risk score (GRS) on fasting plasma triglyceride levels increases with the percentile of the triglyceride distribution. Postprandial lipemia is well suited for testing quantile-dependent expressivity because it exposes each individual’s genotype to substantial increases in their plasma triglyceride concentrations. Ninety-seven published papers were identified that plotted mean triglyceride response vs. time and genotype, which were converted into quantitative data. Separately, for each published graph, standard least-squares regression analysis was used to compare the genotype differences at time t (dependent variable) to average triglyceride concentrations at time t (independent variable) to assess whether the genetic effect size increased in association with higher triglyceride concentrations and whether the phenomenon could explain purported genetic interactions with sex, diet, disease, BMI, and drugs.

Results

Consistent with the phenomenon, genetic effect sizes increased (P≤0.05) with increasing triglyceride concentrations for polymorphisms associated with ABCA1, ANGPTL4, APOA1, APOA2, APOA4, APOA5, APOB, APOC3, APOE, CETP, FABP2, FATP6, GALNT2, GCKR, HL, IL1b, LEPR, LOX-1, LPL, MC4R, MTTP, NPY, SORT1, SULF2, TNFA, TCF7L2, and TM6SF2. The effect size for these polymorphisms showed a progressively increasing dose-response, with intermediate effect sizes at intermediate triglyceride concentrations. Quantile-dependent expressivity provided an alternative interpretation to their interactions with sex, drugs, disease, diet, and age, which have been traditionally ascribed to gene-environment interactions and genetic predictors of drug efficacy (i.e., personalized medicine).

Conclusion

Quantile-dependent expressivity applies to the majority of genetic variants affecting postprandial triglycerides, which may arise because the impaired functionalities of these variants increase at higher triglyceride concentrations. Purported gene-drug interactions may be the manifestations of quantile-dependent expressivity, rather than genetic predictors of drug efficacy.

Apolipoprotein E in lipoprotein metabolism, health and cardiovascular disease

Apolipoprotein E (apoE), a 34 kDa circulating glycoprotein of 299 amino acids, predominantly synthesised in the liver, associates with triglyceride-rich lipoproteins to mediate the clearance of their remnants after enzymatic lipolysis in the circulation. Its synthesis in macrophages initiates the formation of high density-like lipoproteins to effect reverse cholesterol transport to the liver. In the nervous system apoE forms similar lipoproteins which perform the function of distributing lipids amongst cells. ApoE accounts for much of the variation in plasma lipoproteins by three common variants (isoforms) that influence low-density lipoprotein concentration and the risk of atherosclerosis. ApoE2 generally is most favourable and apoE4 least favourable for cardiovascular and neurological health. The apoE variants relate to different amino acids at positions 112 and 158: cysteine in both for apoE2, arginine at both sites for apoE4, and respectively cysteine and arginine for apoE3 that is viewed as the wild type. Paradoxically, under metabolic stress, homozygosity for apoE2 may result in dysbetalipoproteinaemia in adults owing to impaired binding of remnant lipoproteins to the LDL receptor and related proteins as well as heparan sulphate proteoglycans. This highly atherogenic condition is also seen with other mutations in apoE, but with autosomal dominant inheritance. Mutations in apoE may also cause lipoprotein glomerulopathy. In the central nervous system apoE binds amyloid β-protein and tau protein and fragments may incur cellular damage. ApoE4 is a strong risk factor for the development of Alzheimer's disease. ApoE has several other physiological effects that may influence health and disease, including supply of docosahexaenoic acid for the brain and modulating immune and inflammatory responses. Genotyping of apoE may have application in disorders of lipoprotein metabolism as well as glomerulopathy and may be relevant to personalised medicine in understanding cardiovascular risk, and the outcome of nutritional and therapeutic interventions. Quantitation of apoE will probably not be clinically useful. ApoE is also of interest as it may generate peptides with biological function and could be employed in nanoparticles that may allow crossing of the blood-brain barrier. Therapeutic options may emerge from these newer insights.

Interactions between age and apoE genotype on fasting and postprandial triglycerides levels

OBJECTIVE

The influences of genetic determinants on the magnitude of postprandial lipaemia are presently unclear. Here the impact of the common apolipoprotein (apo)E epsilon mutation on the postprandial triglyceride (TG) response is determined, along with an assessment of genotype penetrance according to age, body mass index and gender.

METHODS AND RESULTS

Healthy adults (n=251) underwent a postprandial investigation, in which blood samples were taken at regular intervals after a test breakfast (0 min, 49 g fat) and lunch (330 min, 29 g fat) until 480 min after the test breakfast. There was a significant impact of apoE genotype on fasting total cholesterol (TC), (P=0.027), LDL-cholesterol (LDL-C), (P=0.008), and %LDL(3) (P=0.001), with higher and lower levels in the E4 and E2 carriers respectively relative to the E3/E3 genotype. Reflective of a higher fasting TG (P=0.001), a significantly higher area under the curve for the postprandial TG response (TG AUC) was evident in the E4 carriers relative to the E3/E3 group (P=0.038). In the group as a whole, a significant age×genotype interaction was observed for fasting TC (P=0.021). In the participants>50 years there was a significant impact of genotype on TC (P=0.005), LDL-C (P=0.001) and TAG AUC (P=0.028).

CONCLUSIONS

It is possible that an exaggerated postprandial lipaemia contributes to the increased coronary heart disease risk associated with carriers of the E4 allele; an effect which is more evident in older adults.

Atherogenic dyslipidemia: cardiovascular risk and dietary intervention

Atherogenic dyslipidemia comprises a triad of increased blood concentrations of small, dense low-density lipoprotein (LDL) particles, decreased high-density lipoprotein (HDL) particles, and increased triglycerides. A typical feature of obesity, the metabolic syndrome, insulin resistance, and type 2 diabetes mellitus, atherogenic dyslipidemia has emerged as an important risk factor for myocardial infarction and cardiovascular disease. A number of genes have now been linked to this pattern of lipoprotein changes. Low-carbohydrate diets appear to have beneficial lipoprotein effects in individuals with atherogenic dyslipidemia, compared to high-carbohydrate diets, whereas the content of total fat or saturated fat in the diet appears to have little effect. Achieving a better understanding of the genetic and dietary influences underlying atherogenic dyslipidemia may provide clues to improved interventions to reduce the risk of cardiovascular disease in high-risk individuals.

Gender-related effect of apo E polymorphism on lipoprotein particle sizes in the middle-aged subjects

OBJECTIVES

We determined the frequencies of apolipoprotein E (apo E) alleles and examined the effect of apo E polymorphism on lipoprotein particle sizes in Serbian healthy, middle-aged individuals.

DESIGN AND METHODS

We performed apo E phenotype by immunobloting method in 183 men and 143 women (mean years: 56.3+/-10.60 and 54.9+/-10.31, respectively). Plasma lipid and apolipoprotein levels were measured by routine laboratory methods. LDL and HDL particle sizes were determined by nondenaturing polyacrylamide gradient (3-31%) gel electrophoresis.

RESULTS

The apo E allele frequencies were epsilon2--4.9%, epsilon3--86.5%, and epsilon4--8.6%. Men with epsilon4 allele had lower HDL-C and Apo AI concentrations than epsilon3 men. The epsilon2 allele men had the smallest LDL particles, highest percent of subjects with LDL phenotype B and highest TG/HDL-C ratio. Women with epsilon2 allele had lowest concentration of apo B. The epsilon4 allele women had smallest HDL particles and highest percent of the subjects with small-sized HDL phenotype.

CONCLUSIONS

This study showed gender-related effect of apo E polymorphism on lipoprotein particle size. In men, possession of the epsilon2 allele is associated with small LDL particles, whereas in women, epsilon4 allele is associated with small HDL particles. Differences in gender-related influence of apo E polymorphism on LDL and HDL particle sizes could be clinically useful in strategy for reduction of coronary disease risk in middle-aged men and women.

The composition, structural properties and binding of very-low-density and low-density lipoproteins to the LDL receptor in normo- and hypertriglyceridemia: relation to the apolipoprotein E phenotype

The composition, apolipoprotein structure and lipoprotein binding to the LDL receptor were studied for very-low-density (VLDL) and low-density lipoprotein (LDL) particles isolated from subjects with apoE phenotype E3/3 (E3), E2/2 or E2/3 (E2+) and E3/4 or E4/4 (E4+) and a wide range of plasma triglyceride (TG) contents. The data combined for all three phenotype groups can be summarized as follows. (i) A decrease in accessibility of VLDL tryptophan residues to I- anions with a decrease in tryptophan surface density, concomitant with an increase in VLDL dimensions, reflects the increased efficiency of protein-protein interactions. (ii) A gradual increase in the quenching constant for LDL apoB fluorescence with an increase in TG/cholesterol (Chol) ratio reflects the 'freezing' effect of Chol molecules on apoB dynamics. (iii) Different mechanisms specific for a particular lipoprotein from E3/3 or E2/3 subjects are responsible for apoE-mediated VLDL binding and apoB-mediated LDL binding to the LDL receptor in a solid-phase binding assay. (iv) The 'spacing' effect of apoC-III molecules on apoE-mediated VLDL binding results in a decrease in the number of binding sites. (v) The maximum of the dependence of the LDL binding affinity constant on relative tryptophan density corresponds to LDL intermediate size. VLDL particles from hypertriglyceridemic E2/3 heterozygotic individuals had remnant-like properties (increased cholesterol, apoE and decreased apoC-III content) while their binding efficiency was unchanged. Based on the affinity constant value and LDL-Chol content, increased competition between VLDL and LDL for the binding to the LDL receptor upon increase in plasma TG is suggested, and LDL from hypertriglyceridemic E3/3 homozygotic individuals is the most efficient competitor.

The effect of dietary fat on LDL size is influenced by apolipoprotein E genotype in healthy subjects

LDL particle size is dependent on both genetic factors and environmental factors such as dietary fat composition. The apolipoprotein E (apoE) genotype is a major genetic determinant of LDL size. Thus, the aim of this work was to study whether the apoE genotype interacts with the quantity and quality of dietary fat, modifying LDL size in young healthy subjects. Healthy subjects (n = 84; 66 apoE 3/3, 8 apoE 4/3, 10 apoE 3/2) were subjected to 3 dietary periods, each lasting 4 wk. The first was an SFA-enriched diet (38% fat, 20% SFA), which was followed by a carbohydrate (CHO)-rich diet (30% fat, < 10% SFA, 55% carbohydrate) or a monounsaturated fatty acid (MUFA) olive oil-rich diet (38% fat, 22% MUFA) following a randomized crossover design. At the end of each diet period, LDL particle size and plasma levels of total cholesterol, LDL cholesterol (LDL-C), HDL-C, apoB, apoA-I, and triacylglycerols were determined. LDL particle size was significantly higher (P < 0.04) in subjects with the apoE 4/3 genotype compared with those with apoE 3/3 and apoE 3/2 in the basal state. LDL size was smaller (P < 0.02) after the CHO diet than after the MUFA or SFA diets. After the CHO diet, a significant increase in LDL particle size (P < 0.035) was noted with respect to the MUFA diet in apoE 4/3 subjects, whereas a significant decrease was observed in the apoE 3/3 individuals (P < 0.043). In conclusion, a Mediterranean diet, high in MUFA-fat increases LDL particle size compared with a CHO diet, and this effect is dependent of apoE genotypes.

Genetics of LDL particle heterogeneity

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.

Oxidized LDL autoantibodies are related to apolipoprotein E phenotype, independently of postprandial change in plasma triglycerides and LDL size, among patients with angiographically verified coronary artery disease and healthy controls

OBJECTIVE

Oxidized low-density lipoprotein (LDL) autoantibodies (oxLDLab), apolipoprotein E (apoE) phenotype, postprandial triglyceride changes and LDL size are suggested to be risk factors for coronary artery disease (CAD). Our aim was to study the interaction between these new risk factors among patients with CAD and healthy controls.

METHODS

oxLDLab from 31 men with angiographically verified CAD and 31 healthy men were analyzed by enzyme-linked immunosorbent assay. Isoelectric focusing and immunoblotting were used for apoE phenotyping. Triglyceride level was measured after 12 h of fasting and 3, 5 and 7 h after a high-fat meal. Nondenaturing gradient gel electrophoresis was used to separate LDL particles according to size.

RESULTS

oxLD- Lab levels increased according to apoE phenotype in the following order: E2 < E3 < E4 (p = 0.004, ANOVA). The postprandial response of triglycerides, the size of LDL particles and the concentration of LDL and high-density lipoprotein (HDL) cholesterol did not differ between apoE phenotypes, and the use of these variables as covariates did not change the statistically significant difference in oxLDLab levels between apoE phenotypes (p = 0.01, ANCOVA). oxLDLab levels did not differ between the patient and control groups.

CONCLUSION

We found an association between apoE allele epsilon2 and decreased levels of oxLDLab, which was independent of the postprandial response of triglycerides, the size of LDL particles and plasma LDL and HDL cholesterol levels. The mechanism by which apoE affects oxidation of LDL remains unknown.

ApoE polymorphism in a small Mediterranean island: relationships with plasma lipids, lipoproteins and LDL particle size

Polymorphisms of apoE gene are able to modulate lipoprotein metabolism at different steps and to influence LDL-cholesterol (LDL-C) levels and also other lipoproteins features. Population studies documented large differences in the frequency of apoE alleles which could be even related to the prevalence of cardiovascular disease. In this study we evaluated the apoE genotypes and allele frequency in 576 subjects living in a small island in the Tyrrhenian Sea and the relative contribution of apoE polymorphism on plasma lipid and lipoprotein profile, including LDL particle size. We found a cumulative frequency of 0.073, 0.866 and 0.061 for epsilon2, epsilon3 and epsilon4 alleles respectively. Moreover epsilon3 subjects had only triglyceride levels significantly lower and LDL-C and lipoprotein (a) (Lp(a)) levels higher than epsilon2 carriers. LDL-particle size was significant smaller in epsilon2 subjects than both epsilon3 and epsilon4 carriers, but the difference disappeared when data were adjusted for triglycerides. In conclusion we have provided further evidence of a low prevalence of epsilon4 allele in a Mediterranean population which may represent a genetic protective factor of these populations. Environmental factors, such as diet, occurring in this area may have attenuated the influence of this gene on plasma lipoproteins.

Lipid profile in the severely obese: changes with weight loss after lap-band surgery

OBJECTIVE

To characterize dyslipidemia before and after weight loss in the severely obese.

RESEARCH METHODS AND PROCEDURES

Five hundred fifteen subjects who had Lap-Band surgery were followed with yearly conventional lipid profiles for up to 4 years. Preoperative data were collected from the most recent 381 subjects, and predictors of dyslipidemia were sought. One hundred seventy-one subjects completed a 1-year review, providing data to assess predictors of change in lipids.

RESULTS

Favorable changes in fasting triglycerides (TG), high-density lipoprotein-cholesterol (HDL-C), and total cholesterol (TC):HDL-C ratio occurred within 1 year. All improvements were maintained up to 4 years. Male gender, central obesity, elevated fasting glucose, and insulin resistance were associated with less favorable lipid levels. Fasting plasma glucose best predicted TG (r = 0.46, p < 0.001), whereas insulin sensitivity using the homeostatic model assessment (HOMA %S) correlated best with the HDL-C (r =0.34, p < 0.001). Higher preoperative fasting glucose best predicted the decrease in TG; improved HOMA %S with weight loss correlated best with HDL-C. The extent of weight loss had limited influence on lipid changes. However, low preoperative HOMA %S was associated with lower weight loss. Greater weight loss was associated with more favorable lipid measures after controlling for preoperative HOMA %S.

DISCUSSION

Dyslipidemia of obesity is related to weight distribution, insulin sensitivity, and impaired glucose tolerance. Improvement with weight loss is related to the decrease in fasting glucose, improvement in insulin sensitivity, and the extent of weight lost. Improvement in dyslipidemia is sustained with long-term weight loss.