Common variants associated with plasma triglycerides and risk for coronary artery disease

The effect of plant sterols and different low doses of omega-3 fatty acids from fish oil on lipoprotein subclasses

SCOPE

Consumption of a low-fat spread enriched with plant sterols (PS) and different low doses (<2 g/day) of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from fish oil reduces serum triglycerides (TGs) and low-density lipoprotein-cholesterol (LDL-Chol) and thus beneficially affects two blood lipid risk factors. Yet, their combined effects on TG and Chol in various lipoprotein subclasses have been investigated to a limited extent.

METHODS AND RESULTS

In a randomized, double-blind, placebo-controlled, parallel study, we determined TG and Chol in 13 LP subclasses in fasting serum of 282 hypercholesterolemic subjects, who consumed either a placebo spread or one of the four spreads containing PS (2.5 g/day) and EPA+DHA (0.0, 0.9, 1.3, and 1.8 g/day) for 4 weeks. After PS treatment, total LDL-Chol was reduced, which was not further changed by EPA+DHA. No shift in the LDL-Chol particle distribution was observed. The addition of EPA+DHA to PS dose-dependently reduced VLDL-Chol and VLDL-TG mainly in larger particles. Furthermore, the two highest doses of EPA+DHA increased Chol and TG in the larger HDL particles, while these concentrations were decreased in the smallest HDL particles.

CONCLUSION

The consumption of a low-fat spread enriched with both PS and EPA+DHA induced shifts in the lipoprotein distribution that may provide additional cardiovascular benefits over PS consumption alone.

Identifying an Optimal Cutpoint for the Diagnosis of Hypertriglyceridemia in the Nonfasting State

BACKGROUND

Nonfasting triglycerides are similar or superior to fasting triglycerides at predicting cardiovascular events. However, diagnostic cutpoints are based on fasting triglycerides. We examined the optimal cutpoint for increased nonfasting triglycerides.

METHODS

We obtained baseline nonfasting (<8 h since last meal) samples from 6391 participants in the Women's Health Study who were followed prospectively for ≤17 years. The optimal diagnostic threshold for nonfasting triglycerides, determined by logistic regression models by use of c-statistics and the Youden index (sum of sensitivity and specificity minus 1), was used to calculate hazard ratios (HRs) for incident cardiovascular events. Performance was compared to thresholds recommended by the American Heart Association (AHA) and European guidelines.

RESULTS

The optimal threshold was 175 mg/dL (1.98 mmol/L), with a c-statistic of 0.656, statistically better than the AHA cutpoint of 200 mg/dL (c-statistic 0.628). For nonfasting triglycerides above and below 175 mg/dL, after adjusting for age, hypertension, smoking, hormone use, and menopausal status, the HR for cardiovascular events was 1.88 (95% CI 1.52-2.33, P < 0.001), and for triglycerides measured at 0-4 and 4-8 h since the last meal, 2.05 (1.54- 2.74) and 1.68 (1.21-2.32), respectively. We validated performance of this optimal cutpoint by use of 10-fold cross-validation and bootstrapping of multivariable models that included standard risk factors plus total and HDL cholesterol, diabetes, body mass index, and C-reactive protein.

CONCLUSIONS

In this study of middle-aged and older apparently healthy women, we identified a diagnostic threshold for nonfasting hypertriglyceridemia of 175 mg/dL (1.98 mmol/L), with the potential to more accurately identify cases than the currently recommended AHA cutpoint.

Plasma apolipoprotein C-III levels, triglycerides, and coronary artery calcification in type 2 diabetics

OBJECTIVE

Triglyceride-rich lipoproteins have emerged as causal risk factors for developing coronary heart disease independent of low-density lipoprotein cholesterol levels. Apolipoprotein C-III (ApoC-III) modulates triglyceride-rich lipoprotein metabolism through inhibition of lipoprotein lipase and hepatic uptake of triglyceride-rich lipoproteins. Mutations causing loss-of-function of ApoC-III lower triglycerides and reduce coronary heart disease risk, suggestive of a causal role for ApoC-III. Little data exist about the relationship of ApoC-III, triglycerides, and atherosclerosis in patients with type 2 diabetes mellitus (T2DM). Here, we examined the relationships between plasma ApoC-III, triglycerides, and coronary artery calcification in patients with T2DM.

APPROACH AND RESULTS

Plasma ApoC-III levels were measured in a cross-sectional study of 1422 subjects with T2DM but without clinically manifest coronary heart disease. ApoC-III levels were positively associated with total cholesterol (Spearman r=0.36), triglycerides (r=0.59), low-density lipoprotein cholesterol (r=0.16), fasting glucose (r=0.16), and glycosylated hemoglobin (r=0.12; P<0.0001 for all). In age, sex, and race-adjusted analysis, ApoC-III levels were positively associated with coronary artery calcification (Tobit regression ratio, 1.78; 95% confidence interval, 1.27-2.50 per SD increase in ApoC-III; P<0.001). As expected for an intermediate mediator, these findings were attenuated when adjusted for both triglycerides (Tobit regression ratio, 1.43; 95% confidence interval, 0.94-2.18; P=0.086) and separately for very low-density lipoprotein cholesterol (Tobit regression ratio, 1.14; 95% confidence interval, 0.75-1.71; P=0.53).

CONCLUSIONS

In persons with T2DM, increased plasma ApoC-III is associated with higher triglycerides, less favorable cardiometabolic phenotypes, and higher coronary artery calcification, a measure of subclinical atherosclerosis. Therapeutic inhibition of ApoC-III may thus be a novel strategy for reducing plasma triglyceride-rich lipoproteins and cardiovascular risk in T2DM.

Apoc2 loss-of-function zebrafish mutant as a genetic model of hyperlipidemia

Apolipoprotein C-II (APOC2) is an obligatory activator of lipoprotein lipase. Human patients with APOC2 deficiency display severe hypertriglyceridemia while consuming a normal diet, often manifesting xanthomas, lipemia retinalis and pancreatitis. Hypertriglyceridemia is also an important risk factor for development of cardiovascular disease. Animal models to study hypertriglyceridemia are limited, with no Apoc2-knockout mouse reported. To develop a genetic model of hypertriglyceridemia, we generated an apoc2 mutant zebrafish characterized by the loss of Apoc2 function. apoc2 mutants show decreased plasma lipase activity and display chylomicronemia and severe hypertriglyceridemia, which closely resemble the phenotype observed in human patients with APOC2 deficiency. The hypertriglyceridemia in apoc2 mutants is rescued by injection of plasma from wild-type zebrafish or by injection of a human APOC2 mimetic peptide. Consistent with a previous report of a transient apoc2 knockdown, apoc2 mutant larvae have a minor delay in yolk consumption and angiogenesis. Furthermore, apoc2 mutants fed a normal diet accumulate lipid and lipid-laden macrophages in the vasculature, which resemble early events in the development of human atherosclerotic lesions. In addition, apoc2 mutant embryos show ectopic overgrowth of pancreas. Taken together, our data suggest that the apoc2 mutant zebrafish is a robust and versatile animal model to study hypertriglyceridemia and the mechanisms involved in the pathogenesis of associated human diseases.

Highlighted Article: Apoc2 loss-of-function zebrafish display severe hypertriglyceridemia, which is characteristic of human patients with defective lipoprotein lipase activity.

A Mendelian randomization study of the effect of type-2 diabetes on coronary heart disease

In observational studies, type-2 diabetes (T2D) is associated with an increased risk of coronary heart disease (CHD), yet interventional trials have shown no clear effect of glucose-lowering on CHD. Confounding may have therefore influenced these observational estimates. Here we use Mendelian randomization to obtain unconfounded estimates of the influence of T2D and fasting glucose (FG) on CHD risk. Using multiple genetic variants associated with T2D and FG, we find that risk of T2D increases CHD risk (odds ratio (OR)=1.11 (1.05–1.17), per unit increase in odds of T2D, P=8.8 × 10⁻⁵; using data from 34,840/114,981 T2D cases/controls and 63,746/130,681 CHD cases/controls). FG in non-diabetic individuals tends to increase CHD risk (OR=1.15 (1.00–1.32), per mmol·per l, P=0.05; 133,010 non-diabetic individuals and 63,746/130,681 CHD cases/controls). These findings provide evidence supporting a causal relationship between T2D and CHD and suggest that long-term trials may be required to discern the effects of T2D therapies on CHD risk.

In order to effectively design interventions, it is useful to understand the complex interplay between multiple syndromes. Here, Ahmad et al. use genome-wide association study data and Mendelian randomisation to examine the influence of Type 2 diabetes and fasting glucose levels on coronary heart disease.

Extended-Release Niacin Versus Fenofibrate in HIV-Infected Participants With Low High-Density Lipoprotein Cholesterol: Effects on Endothelial Function, Lipoproteins, and Inflammation

BACKGROUND

Low levels of high-density lipoprotein cholesterol (HDL-C) are common in individuals with human immunodeficiency virus (HIV) infection, persist during antiretroviral therapy (ART), and are associated with increased cardiovascular disease (CVD) risk.

METHODS

Virologically controlled participants without CVD on stable ART with low HDL-C (men <40 mg/dL, women <50 mg/dL) and triglycerides >150 mg/dL were randomized to receive open-label extended-release niacin 1500 mg/day with aspirin 325 mg/day or fenofibrate 200 mg/day for 24 weeks. The primary endpoint was the week 24 within-arm change in brachial artery flow-mediated dilation (FMD) in participants with complete follow-up scans.

RESULTS

Of 99 participants, 74 had complete data (35 niacin, 39 fenofibrate). Median age was 45 years, 77% were male, median CD4(+) count was 561 cells/µL, and brachial FMD was 4.2%. Median HDL-C was 32 mg/dL for men and 38 mg/dL for women, low-density lipoprotein cholesterol was 103 mg/dL, and triglycerides were 232 mg/dL. In men, HDL-C increased a median of 3 mg/dL with niacin and 6.5 mg/dL with fenofibrate (P < .001 for both). In women, HDL-C increased a median of 16 mg/dL with niacin and 8 mg/dL with fenofibrate (P = .08 for both). After 24 weeks, there was no significant change in FMD in either arm; the median (interquartile range) change was +0.6% (-1.6 to 2.3) with niacin (P = .28) and +0.5% (-1.0 to 3.0) with fenofibrate (P = .19). Neither treatment significantly affected C-reactive protein, interleukin 6, or D-dimer levels.

CONCLUSIONS

Despite improvements in lipids, niacin or fenofibrate treatment for 24 weeks did not improve endothelial function or inflammatory markers in participants with well-controlled HIV infection and low HDL-C.

CLINICAL TRIALS REGISTRATION

NCT01426438.

New Insights into the Regulation of Chylomicron Production

Dietary lipids are efficiently absorbed by the small intestine, incorporated into triglyceride-rich lipoproteins (chylomicrons), and transported in the circulation to various tissues. Intestinal lipid absorption and mobilization and chylomicron synthesis and secretion are highly regulated processes. Elevated chylomicron production rate contributes to the dyslipidemia seen in common metabolic disorders such as insulin-resistant states and type 2 diabetes and likely increases the risk for atherosclerosis seen in these conditions. An in-depth understanding of the regulation of chylomicron production may provide leads for the development of drugs that could be of therapeutic utility in the prevention of dyslipidemia and atherosclerosis. Chylomicron secretion is subject to regulation by various factors, including diet, body weight, genetic variants, hormones, nutraceuticals, medications, and emerging interventions such as bariatric surgical procedures. In this review we discuss the regulation of chylomicron production, mechanisms that underlie chylomicron dysregulation, and potential avenues for future research.

Interactive effects of a common γ-glutamyltransferase 1 variant and low high-density lipoprotein-cholesterol on diabetic macro- and micro-angiopathy

BACKGROUND

We investigated the clinical relevance of a common variant, rs4820599, in the γ-glutamyltransferase (GGT)1 gene, associated with the serum GGT level, in Japanese type 2 diabetes mellitus (T2DM) subjects.

METHODS

We conducted a retrospective longitudinal study (4.9 ± 2.5 years) including 352 T2DM patients (T2DM subjects) and a cross-sectional study including 796 health screening program participants (general subjects). A real-time TaqMan allelic discrimination assay was used to identify the genotypes. Risk factors for a high brachial-ankle pulse wave velocity (baPWV) (≥1750 cm/sec) or diabetic retinopathy (DR) were determined using a generalized estimating equations approach, receiver operating characteristic (ROC) analysis or Cox proportional hazards model, etc.

RESULTS

The frequency of the GGT1 G allele was 20.8% in the T2DM subjects, and no associations were found between the GGT1 genotype and risk of T2DM. The mean log GGT values in the T2DM and general subjects were significantly higher among G allele carriers than non-carriers. The G allele and a low HDL-C level were identified to be risk factors for a high baPWV in the T2DM subjects [odds ratio (OR) 1.80, P = 0.008; OR 1.71, P = 0.03; respectively), and a significant interactive effect between these factors was found on the risk of a high baPWV and DR. The HDL-C level at baseline was a significant predictor of a high baPWV only in G allele carriers according to the ROC analysis. This result regarding baPWV in the T2DM subjects was replicated in the general population. Meanwhile, the GGT1 genotype was not associated with the risk of DR, although it affected the principal factors involved in the risk of DR, and a low HDL-C level was also found to be a risk factor for DR only in G allele carriers.

CONCLUSIONS

We herein describe for the first time the significant interactive effects of the GGT1 G allele and a low HDL-C level on a high baPWV and DR. These findings may encourage future clinical trials comparing the efficacy of agents increasing the HDL-C levels among the GGT1 genotypes. However, well-designed studies in larger cohorts are needed to confirm our results.

Genome-wide association studies of late-onset cardiovascular disease

Human genetics is a powerful tool for discovering causal mediators of human disease and physiology. Cardiovascular diseases with late onset in the lifecourse have historically not been considered genetic diseases, but in recent years the contribution of a heritable factor has been established. More importantly, over the last decade genome-wide association studies (GWASs) have identified many loci associated with late-onset cardiovascular diseases including coronary artery disease, carotid artery disease, ischemic stroke, aortic aneurysm, peripheral vascular disease, atrial fibrillation, valvular disease and correlates of vascular and myocardial function. Here we review findings from GWASs considered statistically robust with regard to multiple testing (p<5×10(-8)) for late-onset cardiovascular diseases and traits. Although for only a handful of the 92 genetic loci described here have the mechanisms underlying disease association been established, new and previously unsuspected pathways have been implicated for several conditions. Examples include a role for NO signaling in myocardial repolarization and sudden cardiac death and a role for the protein sortilin in lipid metabolism and coronary artery disease. Genetic loci with multiple trait associations have also provided novel biological insights. For example, of the 46 genetic loci associated with coronary artery disease, only 16 are also associated with conventional risk factors for cardiovascular disease whereas the remaining two thirds may reflect novel pathways. Much work remains to functionally characterize genetic loci and for clinical utility, but accruing insights into the biological basis of cardiovascular aging in human populations promise to point to novel therapeutic and preventive strategies. This article is part of a Special Issue entitled 'SI:CV Aging'.

Medical nutrition therapy is the essential cornerstone for effective treatment of "refractory" severe hypertriglyceridemia regardless of pharmaceutical treatment: Evidence from a Lipid Management Program

BACKGROUND

Patients with refractory severe hypertriglyceridemia are at risk of pancreatitis and cardiovascular disease. The role of individualized nutrition therapy in these patients independent of pharmaceutical treatment has not been documented.

OBJECTIVE

To document the effect of nutrition intervention on severe hypertriglyceridemia regardless of medication status or prior nutrition counseling.

METHODS

Outcomes of new patients with triglycerides ≥ 500 mg/dL presenting to a Lipid Management Program over a 6-year period were tracked. Patients received comprehensive laboratory assessment, nutrition assessment, and initiation of an individualized diet intervention before seeing the lipidologist. Clinical and behavioral outcomes were recorded.

RESULTS

In all, 168 patients (117 men; mean age, 49.03 ± 11.22 years; body mass index, 32.61 ± 5.85 kg/m(2); 110 (65.5%) on lipid-lowering medications) returned for assessment of nutrition intervention. Triglycerides were reduced from median (interquartile range) 961.5 (611.5-1785.3) to 493.0 (337-736.3) mg/dL (P < .0001 for log transformation of triglycerides). There was no difference in median percentage reduction in triglycerides after nutrition intervention between those not on lipid-lowering medication, on a fibric acid derivative, on other lipid-lowering medication, or on a combination of lipid-lowering medications (P = .376) in a median (interquartile range) of 5 (3-7) weeks. Effect was independent of prior nutrition counseling (P = .260). Reported percentage fat in the diet at second visit correlated with log-transformed triglycerides achieved, independent of initial triglycerides level (r = 0.290; P = .001).

CONCLUSIONS

Individualized nutrition therapy results in changes in eating behavior and reductions in triglyceride levels in patients with refractory severe hypertriglyceridemia independent of lipid-lowering medication(s) and prior nutrition counseling.

HDL re-examined

PURPOSE OF REVIEW

To summarize the current evidence concerning the role of HDL-C and HDL-associated parameters in the risk for cardiovascular disease (CVD).

RECENT FINDINGS

Numerous population studies have shown that plasma levels of HDL-C are inversely associated with CVD risk; in patient care HDL-C levels are therefore widely implemented in risk estimation models. A number of antiatherogenic properties have been ascribed to the HDL particle, but the hypothesis that HDL is causally related to CVD has been seriously challenged by recent data obtained from both human genetic studies and clinical trials. The final word on HDL-C as a therapeutic target is pending, as a number of clinical endpoint trials specifically focusing on the effect(s) of HDL-C increasing agents are underway. Moreover, recent data show that HDL efflux capacity could hold independent predictive value for CVD events, which clearly highlights the potential need to focus on HDL functionality, rather than on HDL-C levels.

SUMMARY

The dogmatic concept that HDL-C levels predict future CVD events is undisputed, but the role of HDL-C as a causal factor in atherosclerosis has been challenged by a number of different types of studies. In recent years, a paradigm shift toward 'HDL functionality' is apparent. Whether or not optimizing these markers of HDL functionality actually does reduce CVD risk requires formal testing in prospective controlled studies.

Pilot study on the effects of a 2-week hiking vacation at moderate versus low altitude on plasma parameters of carbohydrate and lipid metabolism in patients with metabolic syndrome

Background

Hypoxic and hypobaric conditions may augment the beneficial influence of training on cardiovascular risk factors. This pilot study aimed to explore for effects of a two-week hiking vacation at moderate versus low altitude on adipokines and parameters of carbohydrate and lipid metabolism in patients with metabolic syndrome.

Methods

Fourteen subjects (mean age: 55.8 years, range: 39 – 69) with metabolic syndrome participated in a 2-week structured training program (3 hours of guided daily hiking 4 times a week, training intensity at 55-65% of individual maximal heart rate; total training time, 24 hours). Participants were divided for residence and training into two groups, one at moderate altitude (1,900 m; n = 8), and the other at low altitude (300 m; n = 6). Anthropometric, cardiovascular and metabolic parameters were measured before and after the training period.

Results

In study participants, training overall reduced circulating levels of total cholesterol (p = 0.024), low-density lipoprotein cholesterol (p = 0.025) and adiponectin (p < 0.001). In the group training at moderate altitude (n = 8), lowering effects on circulating levels were significant not only for total cholesterol, low-density-lipoprotein cholesterol and adiponectin (all, p < 0.05) but also for triglycerides (p = 0.025) and leptin (p = 0.015), whereas in the low altitude group (n = 6), none of the lipid parameters was significantly changed (each p > 0.05). Hiking-induced relative changes of triglyceride levels were positively associated with reductions in leptin levels (p = 0.006). As compared to 300 m altitude, training at 1,900 m showed borderline significant differences in the pre-post mean reduction rates of triglyceride (p = 0.050) and leptin levels (p = 0.093).

Conclusions

Preliminary data on patients with metabolic syndrome suggest that a 2-week hiking vacation at moderate altitude may be more beneficial for adipokines and parameters of lipid metabolism than training at low altitude. In order to draw firm conclusions regarding better corrections of dyslipidemia and metabolic syndrome by physical exercise under mild hypobaric and hypoxic conditions, a sufficiently powered randomized clinical trial appears warranted.

Trial registration

ClinicalTrials.gov ID NCT02013947 (first received November 6, 2013).

Coronary heart disease mortality and hormone therapy before and after the Women's Health Initiative

OBJECTIVE

To assess whether coronary heart disease mortality in Finnish hormone therapy (HT) users differed before and after 2002 when the Women's Health Initiative study was published.

METHODS

The risks of coronary heart disease death in HT users in relation to the age-matched background population were compared between the pre- (1995-2001) and post- (2002-2009) Women's Health Initiative eras. We used a nationwide register on HT (ie, estradiol with or without progestin) reimbursement and linked them to causes of death in 290,272 women aged 40 years or older.

RESULTS

Exposure to HT for 1 year or less was accompanied by a 29% reduction (0.71; 0.63-0.80; three per 10,000 fewer deaths) and an exposure of 1-8 years with a 43% reduction (0.57; 0.48-0.66; three per 10,000 fewer deaths) in the risk of coronary heart disease death in the pre-Women's Health Initiative era. In the post-Women's Health Initiative era, HT use of 1 year or less was associated with an 18% reduction (0.82; 0.76-1.00; one per 10,000 fewer deaths) and an exposure of 1-8 years with a 54% reduction (0.46; 0.32-0.64; two per 10,000 fewer deaths) in coronary heart disease mortality. Discontinuation of HT was associated with an increased risk of cardiac death of 42% (1.42; 1.17-1.71; seven per 10,000 extra deaths) in the pre-Women's Health Initiative era and 31% (1.31; 0.92-1.82; two per 10,000 extra deaths) in the post-Women's Health Initiative era during the first posttreatment year. This risk increase vanished in further follow-up during both eras.

CONCLUSION

Changes in HT use after the Women's Health Initiative failed to affect coronary heart disease mortality of HT users in this nationwide study.

Modulators of hepatic lipoprotein metabolism identified in a search for small-molecule inducers of tribbles pseudokinase 1 expression

Recent genome wide association studies have linked tribbles pseudokinase 1 (TRIB1) to the risk of coronary artery disease (CAD). Based on the observations that increased expression of TRIB1 reduces secretion of VLDL and is associated with lower plasma levels of LDL cholesterol and triglycerides, higher plasma levels of HDL cholesterol and reduced risk for myocardial infarction, we carried out a high throughput phenotypic screen based on quantitative RT-PCR assay to identify compounds that induce TRIB1 expression in human HepG2 hepatoma cells. In a screen of a collection of diversity-oriented synthesis (DOS)-derived compounds, we identified a series of benzofuran-based compounds that upregulate TRIB1 expression and phenocopy the effects of TRIB1 cDNA overexpression, as they inhibit triglyceride synthesis and apoB secretion in cells. In addition, the compounds downregulate expression of MTTP and APOC3, key components of the lipoprotein assembly pathway. However, CRISPR-Cas9 induced chromosomal disruption of the TRIB1 locus in HepG2 cells, while confirming its regulatory role in lipoprotein metabolism, demonstrated that the effects of benzofurans persist in TRIB1-null cells indicating that TRIB1 is sufficient but not necessary to transmit the effects of the drug. Remarkably, active benzofurans, as well as natural products capable of TRIB1 upregulation, also modulate hepatic cell cholesterol metabolism by elevating the expression of LDLR transcript and LDL receptor protein, while reducing the levels of PCSK9 transcript and secreted PCSK9 protein and stimulating LDL uptake. The effects of benzofurans are not masked by cholesterol depletion and are independent of the SREBP-2 regulatory circuit, indicating that these compounds represent a novel class of chemically tractable small-molecule modulators that shift cellular lipoprotein metabolism in HepG2 cells from lipogenesis to scavenging.

Multivariable Mendelian randomization: the use of pleiotropic genetic variants to estimate causal effects

A conventional Mendelian randomization analysis assesses the causal effect of a risk factor on an outcome by using genetic variants that are solely associated with the risk factor of interest as instrumental variables. However, in some cases, such as the case of triglyceride level as a risk factor for cardiovascular disease, it may be difficult to find a relevant genetic variant that is not also associated with related risk factors, such as other lipid fractions. Such a variant is known as pleiotropic. In this paper, we propose an extension of Mendelian randomization that uses multiple genetic variants associated with several measured risk factors to simultaneously estimate the causal effect of each of the risk factors on the outcome. This “multivariable Mendelian randomization” approach is similar to the simultaneous assessment of several treatments in a factorial randomized trial. In this paper, methods for estimating the causal effects are presented and compared using real and simulated data, and the assumptions necessary for a valid multivariable Mendelian randomization analysis are discussed. Subject to these assumptions, we demonstrate that triglyceride-related pathways have a causal effect on the risk of coronary heart disease independent of the effects of low-density lipoprotein cholesterol and high-density lipoprotein cholesterol.

Genetics and causality of triglyceride-rich lipoproteins in atherosclerotic cardiovascular disease

Triglycerides represent 1 component of a heterogeneous pool of triglyceride-rich lipoproteins (TGRLs). The reliance on triglycerides or TGRLs as cardiovascular disease (CVD) risk biomarkers prompted investigations into therapies that lower plasma triglycerides as a means to reduce CVD events. Genetic studies identified TGRL components and pathways involved in their synthesis and metabolism. We advocate that only a subset of genetic mechanisms regulating TGRLs contribute to the risk of CVD events. This "omic" approach recently resulted in new targets for reducing CVD events.

New insights into the pathophysiology of dyslipidemia in type 2 diabetes

Cardiovascular disease (CVD) remains the leading cause of morbidity and mortality for patients with type 2 diabetes, despite recent significant advances in management strategies to lessen CVD risk factors. A major cause is the atherogenic dyslipidemia, which consists of elevated plasma concentrations of both fasting and postprandial triglyceride-rich lipoproteins (TRLs), small dense low-density lipoprotein (LDL) and low high-density lipoprotein (HDL) cholesterol. The different components of diabetic dyslipidemia are not isolated abnormalities but closely linked to each other metabolically. The underlying disturbances are hepatic overproduction and delayed clearance of TRLs. Recent results have unequivocally shown that triglyceride-rich lipoproteins and their remnants are atherogenic. To develop novel strategies for the prevention and treatment of dyslipidaemia, it is essential to understand the pathophysiology of dyslipoproteinaemia in humans. Here, we review recent advances in our understanding of the pathophysiology of diabetic dyslipidemia.

Hypertriglyceridemia in the genomic era: a new paradigm

Hypertriglyceridemia (HTG) is a highly prevalent condition that is associated with increased cardiovascular disease risk. HTG may arise as a result of defective metabolism of triglyceride-rich lipoproteins and their remnants, ie, impaired clearance, or increased production, or both. Current categorization of HTG segregates primary and secondary cases, implying genetic and nongenetic causes for each category. Many common and rare variants of the genes encoding factors involved in these pathways have been identified. Although monogenic forms of HTG do occur, most cases are polygenic and often coexist with nongenetic conditions. Cumulative, multiple genetic variants can increase the risks for HTG, whereas environmental and lifestyle factors can force expression of a dyslipidemic phenotype in a genetically susceptible person. HTG states are therefore best viewed as a complex phenotype resulting from the interaction of cumulated multiple susceptibility genes and environmental stressors. In view of the heterogeneity of the HTG states, the absence of a unifying metabolic or genetic abnormality, overlap with the metabolic syndrome and other features of insulin resistance, and evidence in some patients that accumulation of numerous small-effect genetic variants determines whether an individual is susceptible to HTG only or to HTG plus elevated low-density lipoprotein cholesterol, we propose that the diagnosis of primary HTG and further delineation of familial combined hyperlipidemia from familial HTG is neither feasible nor clinically relevant at the present time. The hope is that with greater understanding of genetic and environmental causes and their interaction, therapy can be intelligently targeted in the future.

New therapies for coronary artery disease: genetics provides a blueprint

The development of new therapies for coronary artery disease (CAD) poses a substantial challenge, and several recent approaches have failed for lack of efficacy. Human genetics has the potential to identify new targets for which the likelihood of therapeutic success is considerably greater. The intense focus on the genetics of CAD will revitalize the field and lead to future therapies for this common disease.

Extreme nonfasting remnant cholesterol vs extreme LDL cholesterol as contributors to cardiovascular disease and all-cause mortality in 90000 individuals from the general population

BACKGROUND

Increased nonfasting remnant cholesterol, like increased LDL cholesterol, is causally associated with increased risk for ischemic heart disease (IHD). We tested the hypothesis that extreme concentrations of nonfasting remnant and LDL cholesterol are equal contributors to the risk of IHD, myocardial infarction (MI), and all-cause mortality.

METHODS

We compared stepwise increasing concentrations of nonfasting remnant and LDL cholesterol for association with risk of IHD, MI, and all-cause mortality in approximately 90 000 individuals from the Danish general population. During up to 22 years of complete follow-up, 4435 participants developed IHD, 1722 developed MI, and 8121 died.

RESULTS

Compared with participants with nonfasting remnant cholesterol <0.5 mmol/L (19.3 mg/dL), hazard ratios for IHD ranged from 1.3 (95% CI 1.1-1.5) for remnant cholesterol of 0.5-0.99 mmol/L (19.3-38.2 mg/dL) to 2.4 (1.9-2.9) for remnant cholesterol of ≥1.5 mmol/L (58 mg/dL) (P for trend <0.001). Compared with participants with LDL cholesterol <3.0 mmol/L (115.8 mg/dL), hazard ratios for IHD ranged from 1.3 (1.1-1.5) for LDL cholesterol of 3-3.99 mmol/L (115.8-154 mg/dL) to 2.3 (1.9-2.8) for LDL cholesterol of ≥5 mmol/L (193 mg/dL) (P < 0.001). Corresponding hazard ratios for MI ranged from 1.8 (1.4-2.3) to 3.4 (2.5-4.8) for remnant cholesterol (P < 0.001), and from 1.7 (1.4-2.2) to 4.7 (3.5-6.3) for LDL cholesterol (P < 0.001). Nonfasting remnant cholesterol concentrations were associated stepwise with all-cause mortality ranging from hazard ratio 1.0 (0.9-1.1) to 1.6 (1.4-1.9) (P < 0.001), whereas LDL cholesterol concentrations were associated with decreased all-cause mortality risk in a U-shaped pattern, with hazard ratios from 0.8 (0.7-0.8) to 0.9 (0.8-1.0) (P = 0.002). After mutual adjustment, LDL cholesterol best predicted MI, and remnant cholesterol best predicted all-cause mortality.

CONCLUSIONS

Both lipoproteins were associated equally with risk of IHD and MI; however, only nonfasting remnant cholesterol concentrations were associated stepwise with increased all-cause mortality risk.

Serum lipid levels, body mass index, and their role in coronary artery calcification: a polygenic analysis

BACKGROUND

Coronary artery calcification (CAC) is widely regarded as a cumulative lifetime measure of atherosclerosis, but it remains unclear what is the relationship between calcification and traditional risk factors for coronary artery disease (CAD) and myocardial infarction (MI). This study characterizes the genetic architecture of CAC by evaluating the overall impact of common alleles associated with CAD/MI and its traditional risk factors.

METHODS AND RESULTS

On the basis of summary-association results from the CARDIoGRAMplusC4D study of CAD/MI, we calculated polygenic risk scores in 2599 participants of the Dutch and Belgian Lung Cancer Screening (NELSON) trial, in whom quantitative CAC levels (Agatston scores) were determined from chest computerized tomographic imaging data. The most significant polygenic model explained ≈14% of the observed CAC variance (P=1.6×10(-11)), which points to a residual effect because of many as yet unknown loci that overlap between CAD/MI and CAC. In addition, we constructed risk scores based on published single-nucleotide polymorphism associations for traditional cardiovascular risk factors and tested these scores for association with CAC. We found nominally significant associations for genetic risk scores of low-density lipoprotein-cholesterol, total cholesterol, and body mass index, which were successfully replicated in 2182 individuals of the Heinz Nixdorf Recall Study.

CONCLUSIONS

Pervasive polygenic sharing between CAC and CAD/MI suggests that a substantial fraction of the heritable risk for CAD/MI is mediated through arterial calcification. We also provide evidence that genetic variants associated with serum lipid levels and body mass index influence CAC levels.

Atherogenic dyslipidemia and cardiovascular risk factors in obese children

Childhood obesity when associated with serum lipoprotein changes triggers atherosclerosis. Evidences suggest that the atherosclerotic process begins in childhood and that the extent of early atherosclerosis of the aorta and coronary arteries can be associated with lipoprotein levels and obesity. Furthermore, many studies in childhood demonstrate an important relationship between parameters of insulin sensitivity, body fat distribution, and the development of lipid abnormalities. This review focuses on the most recent findings on the relationship between obesity, dyslipidemia, and cardiovascular risk in children.

Large-scale metabolomic profiling identifies novel biomarkers for incident coronary heart disease

Analyses of circulating metabolites in large prospective epidemiological studies could lead to improved prediction and better biological understanding of coronary heart disease (CHD). We performed a mass spectrometry-based non-targeted metabolomics study for association with incident CHD events in 1,028 individuals (131 events; 10 y. median follow-up) with validation in 1,670 individuals (282 events; 3.9 y. median follow-up). Four metabolites were replicated and independent of main cardiovascular risk factors [lysophosphatidylcholine 18∶1 (hazard ratio [HR] per standard deviation [SD] increment = 0.77, P-value<0.001), lysophosphatidylcholine 18∶2 (HR = 0.81, P-value<0.001), monoglyceride 18∶2 (MG 18∶2; HR = 1.18, P-value = 0.011) and sphingomyelin 28∶1 (HR = 0.85, P-value = 0.015)]. Together they contributed to moderate improvements in discrimination and re-classification in addition to traditional risk factors (C-statistic: 0.76 vs. 0.75; NRI: 9.2%). MG 18∶2 was associated with CHD independently of triglycerides. Lysophosphatidylcholines were negatively associated with body mass index, C-reactive protein and with less evidence of subclinical cardiovascular disease in additional 970 participants; a reverse pattern was observed for MG 18∶2. MG 18∶2 showed an enrichment (P-value = 0.002) of significant associations with CHD-associated SNPs (P-value = 1.2×10⁻⁷ for association with rs964184 in the ZNF259/APOA5 region) and a weak, but positive causal effect (odds ratio = 1.05 per SD increment in MG 18∶2, P-value = 0.05) on CHD, as suggested by Mendelian randomization analysis. In conclusion, we identified four lipid-related metabolites with evidence for clinical utility, as well as a causal role in CHD development.

Non-targeted metabolomic profiling of large population-based studies has become feasible only in the past 1–2 years and this hypothesis-free exploration of the metabolome holds a great potential to fuel the discovery of novel biomarkers for coronary heart disease (CHD). Such biomarkers are not only important for risk stratification and treatment decisions, but can also improve understanding of cardiovascular disease pathophysiology to identify new drug targets. In this study, we investigated the metabolic profiles of more than 3,600 individuals from three population-based studies, and discovered four metabolites that are consistently associated with incident CHD. We integrate genetic and metabolomic analysis to delineate the underlying biological mechanisms and evaluate potential causal effects of the novel biomarkers. Specifically, we found one metabolite to be strongly associated with single nucleotides polymorphisms previously reported for association with CHD, and consistent with a potential causal role in CHD development, as suggested by Mendelian randomization analysis.

Exome sequencing identifies rare LDLR and APOA5 alleles conferring risk for myocardial infarction

Myocardial infarction (MI), a leading cause of death around the world, displays a complex pattern of inheritance. When MI occurs early in life, genetic inheritance is a major component to risk. Previously, rare mutations in low-density lipoprotein (LDL) genes have been shown to contribute to MI risk in individual families, whereas common variants at more than 45 loci have been associated with MI risk in the population. Here we evaluate how rare mutations contribute to early-onset MI risk in the population. We sequenced the protein-coding regions of 9,793 genomes from patients with MI at an early age (≤50 years in males and ≤60 years in females) along with MI-free controls. We identified two genes in which rare coding-sequence mutations were more frequent in MI cases versus controls at exome-wide significance. At low-density lipoprotein receptor (LDLR), carriers of rare non-synonymous mutations were at 4.2-fold increased risk for MI; carriers of null alleles at LDLR were at even higher risk (13-fold difference). Approximately 2% of early MI cases harbour a rare, damaging mutation in LDLR; this estimate is similar to one made more than 40 years ago using an analysis of total cholesterol. Among controls, about 1 in 217 carried an LDLR coding-sequence mutation and had plasma LDL cholesterol > 190 mg dl(-1). At apolipoprotein A-V (APOA5), carriers of rare non-synonymous mutations were at 2.2-fold increased risk for MI. When compared with non-carriers, LDLR mutation carriers had higher plasma LDL cholesterol, whereas APOA5 mutation carriers had higher plasma triglycerides. Recent evidence has connected MI risk with coding-sequence mutations at two genes functionally related to APOA5, namely lipoprotein lipase and apolipoprotein C-III (refs 18, 19). Combined, these observations suggest that, as well as LDL cholesterol, disordered metabolism of triglyceride-rich lipoproteins contributes to MI risk.

Hypertriglyceridemia: a too long unfairly neglected major cardiovascular risk factor

The existence of an independent association between elevated triglyceride (TG) levels, cardiovascular (CV) risk and mortality has been largely controversial. The main difficulty in isolating the effect of hypertriglyceridemia on CV risk is the fact that elevated triglyceride levels are commonly associated with concomitant changes in high density lipoprotein (HDL), low density lipoprotein (LDL) and other lipoproteins. As a result of this problem and in disregard of the real biological role of TG, its significance as a plausible therapeutic target was unfoundedly underestimated for many years. However, taking epidemiological data together, both moderate and severe hypertriglyceridaemia are associated with a substantially increased long term total mortality and CV risk. Plasma TG levels partially reflect the concentration of the triglyceride-carrying lipoproteins (TRL): very low density lipoprotein (VLDL), chylomicrons and their remnants. Furthermore, hypertriglyceridemia commonly leads to reduction in HDL and increase in atherogenic small dense LDL levels. TG may also stimulate atherogenesis by mechanisms, such excessive free fatty acids (FFA) release, production of proinflammatory cytokines, fibrinogen, coagulation factors and impairment of fibrinolysis. Genetic studies strongly support hypertriglyceridemia and high concentrations of TRL as causal risk factors for CV disease. The most common forms of hypertriglyceridemia are related to overweight and sedentary life style, which in turn lead to insulin resistance, metabolic syndrome (MS) and type 2 diabetes mellitus (T2DM). Intensive lifestyle therapy is the main initial treatment of hypertriglyceridemia. Statins are a cornerstone of the modern lipids-modifying therapy. If the primary goal is to lower TG levels, fibrates (bezafibrate and fenofibrate for monotherapy, and in combination with statin; gemfibrozil only for monotherapy) could be the preferable drugs. Also ezetimibe has mild positive effects in lowering TG. Initial experience with en ezetimibe/fibrates combination seems promising. The recently released IMPROVE-IT Trial is the first to prove that adding a non-statin drug (ezetimibe) to a statin lowers the risk of future CV events. In conclusion, the classical clinical paradigm of lipids-modifying treatment should be changed and high TG should be recognized as an important target for therapy in their own right. Hypertriglyceridemia should be treated.

Diabetic dyslipidemia

Diabetic dyslipidemia is characterized by elevated fasting and postprandial triglycerides, low HDL-cholesterol, elevated LDL-cholesterol and the predominance of small dense LDL particles. These lipid changes represent the major link between diabetes and the increased cardiovascular risk of diabetic patients. The underlying pathophysiology is only partially understood. Alterations of insulin sensitive pathways, increased concentrations of free fatty acids and low grade inflammation all play a role and result in an overproduction and decreased catabolism of triglyceride rich lipoproteins of intestinal and hepatic origin. The observed changes in HDL and LDL are mostly sequence to this. Lifestyle modification and glucose control may improve the lipid profile but statin therapy mediates the biggest benefit with respect to cardiovascular risk reduction. Therefore most diabetic patients should receive statin therapy. The role of other lipid lowering drugs, such as ezetimibe, fibrates, omega-3 fatty acids, niacin and bile acid sequestrants is less well defined as they are characterized by largely negative outcome trials. This review examines the pathophysiology of diabetic dyslipidemia and its relationship to cardiovascular diseases. Management approaches will also be discussed.

Long-term risk of cardiovascular events across a spectrum of adverse major plasma lipid combinations in the Framingham Heart Study

BACKGROUND

Blood levels of high low-density lipoprotein cholesterol (LDL-C), high triglycerides (TG), and low high-density lipoprotein cholesterol (HDL-C) have been associated with increased risk of cardiovascular disease (CVD). The long-term comparative CVD risk associated with these 3 major lipid classes in various combinations is, however, unknown.

METHODS

A total of 3,501 participants of the Framingham Offspring Study (mean age 51 ± 10 years, 56% women) without CVD at baseline were followed up for incident CVD between 1987 and 2011. Participants were grouped according to baseline lipid values into 8 distinct categories to compare the prognostic significance of values within an optimal range to Third Report of the National Cholesterol Educational Program-defined high LDL-C (> 130 mg/dL), high TG (> 150 mg/dL), and/or low HDL-C (< 40 mg/dL) in various combinations using multivariable-adjusted Cox regression models.

RESULTS

On follow-up (median 20.2 years), 724 (21%) had new-onset CVD. Adjusted for confounders and compared with the group with optimal lipid values, hazards ratios and population-attributable risks (PARs) were as follows: isolated low HDL-C, 1.93 (95% CI 1.37-2.71), PAR = 3.1%; isolated high LDL-C, 1.28 (1.03-1.59), PAR 6.4%; isolated high TG, 1.35 (0.91-1.98), PAR = 1.1% (not significant); low HDL-C and high LDL-C, 1.82 (1.33-2.49), PAR = 3.9%; low HDL-C and high TG, 1.74 (1.28-2.37), PAR = 3.9%; high LDL-C and high TG, 1.52 (1.12-2.07), PAR = 6.4%; and high LDL-C, high TG and low HDL-C 2.28 (1.73-3.02), PAR = 7.5%.

CONCLUSIONS

Aside from isolated hypertriglyceridemia, low levels of HDL-C, high levels of LDL-C, and high levels of TG in any combination were associated with increased risk of CVD.

Inhibition of ApoCIII: the next PCSK9?

PURPOSE OF REVIEW

Recent large Mendelian randomization studies associate loss-of-function mutations in apolipoprotein CIII (APOCIII) with low levels of triglycerides and decreased incidence of cardiovascular disease. With ample in-vitro and in-vivo evidence for a role of apoCIII in lipoprotein lipase-mediated triglyceride clearance and remnant removal, it is, thus, an attractive target for the treatment of hypertriglyceridemia and the prevention of cardiovascular disease. This review evaluates the current position of apoCIII in clinical practice and provides a glimpse into the future in terms of treatment options.

RECENT FINDINGS

Two large Mendelian randomization studies have shown three identical loss-of-function mutations in APOCIII to be linked to favorable lipid profiles and lower incidence of coronary artery disease. A second-generation antisense oligonucleotide, which selectively inhibits apoCIII, was able to decrease serum apoCIII and triglyceride levels in rodents, nonhuman primates and humans.

SUMMARY

The central role of apoCIII in hypertriglyceridemia and cardiovascular disease was further cemented by recent findings and promising intervention data that showed the possibility of using antisense therapy to lower apoCIII and triglyceride levels. Currently, planned phase 3 trials should provide answers in regards to long-term efficacy and safety of this novel therapy.

An experimentally robust model of monomeric apolipoprotein A-I created from a chimera of two X-ray structures and molecular dynamics simulations

High-density lipoprotein (HDL) retards atherosclerosis by accepting cholesterol from the artery wall. However, the structure of the proposed acceptor, monomeric apolipoprotein A-I (apoA-I), the major protein of HDL, is poorly understood. Two published models for monomeric apoA-I used cross-linking distance constraints to derive best fit conformations. This approach has limitations. (i) Cross-linked peptides provide no information about secondary structure. (ii) A protein chain can be folded in multiple ways to create a best fit. (iii) Ad hoc folding of a secondary structure is unlikely to produce a stable orientation of hydrophobic and hydrophilic residues. To address these limitations, we used a different approach. We first noted that the dimeric apoA-I crystal structure, (Δ185-243)apoA-I, is topologically identical to a monomer in which helix 5 forms a helical hairpin, a monomer with a hydrophobic cleft running the length of the molecule. We then realized that a second crystal structure, (Δ1-43)apoA-I, contains a C-terminal structure that fits snuggly via aromatic and hydrophobic interactions into the hydrophobic cleft. Consequently, we combined these crystal structures into an initial model that was subjected to molecular dynamics simulations. We tested the initial and simulated models and the two previously published models in three ways: against two published data sets (domains predicted to be helical by H/D exchange and six spin-coupled residues) and against our own experimentally determined cross-linking distance constraints. We note that the best fit simulation model, superior by all tests to previously published models, has dynamic features of a molten globule with interesting implications for the functions of apoA-I.

Association of low-density lipoprotein cholesterol-related genetic variants with aortic valve calcium and incident aortic stenosis

IMPORTANCE

Plasma low-density lipoprotein cholesterol (LDL-C) has been associated with aortic stenosis in observational studies; however, randomized trials with cholesterol-lowering therapies in individuals with established valve disease have failed to demonstrate reduced disease progression.

OBJECTIVE

To evaluate whether genetic data are consistent with an association between LDL-C, high-density lipoprotein cholesterol (HDL-C), or triglycerides (TG) and aortic valve disease.

DESIGN, SETTING, AND PARTICIPANTS

Using a Mendelian randomization study design, we evaluated whether weighted genetic risk scores (GRSs), a measure of the genetic predisposition to elevations in plasma lipids, constructed using single-nucleotide polymorphisms identified in genome-wide association studies for plasma lipids, were associated with aortic valve disease. We included community-based cohorts participating in the CHARGE consortium (n = 6942), including the Framingham Heart Study (cohort inception to last follow-up: 1971-2013; n = 1295), Multi-Ethnic Study of Atherosclerosis (2000-2012; n = 2527), Age Gene/Environment Study-Reykjavik (2000-2012; n = 3120), and the Malmö Diet and Cancer Study (MDCS, 1991-2010; n = 28,461).

MAIN OUTCOMES AND MEASURES

Aortic valve calcium quantified by computed tomography in CHARGE and incident aortic stenosis in the MDCS.

RESULTS

The prevalence of aortic valve calcium across the 3 CHARGE cohorts was 32% (n = 2245). In the MDCS, over a median follow-up time of 16.1 years, aortic stenosis developed in 17 per 1000 participants (n = 473) and aortic valve replacement for aortic stenosis occurred in 7 per 1000 (n = 205). Plasma LDL-C, but not HDL-C or TG, was significantly associated with incident aortic stenosis (hazard ratio [HR] per mmol/L, 1.28; 95% CI, 1.04-1.57; P = .02; aortic stenosis incidence: 1.3% and 2.4% in lowest and highest LDL-C quartiles, respectively). The LDL-C GRS, but not HDL-C or TG GRS, was significantly associated with presence of aortic valve calcium in CHARGE (odds ratio [OR] per GRS increment, 1.38; 95% CI, 1.09-1.74; P = .007) and with incident aortic stenosis in MDCS (HR per GRS increment, 2.78; 95% CI, 1.22-6.37; P = .02; aortic stenosis incidence: 1.9% and 2.6% in lowest and highest GRS quartiles, respectively). In sensitivity analyses excluding variants weakly associated with HDL-C or TG, the LDL-C GRS remained associated with aortic valve calcium (P = .03) and aortic stenosis (P = .009). In instrumental variable analysis, LDL-C was associated with an increase in the risk of incident aortic stenosis (HR per mmol/L, 1.51; 95% CI, 1.07-2.14; P = .02).

CONCLUSIONS AND RELEVANCE

Genetic predisposition to elevated LDL-C was associated with presence of aortic valve calcium and incidence of aortic stenosis, providing evidence supportive of a causal association between LDL-C and aortic valve disease. Whether earlier intervention to reduce LDL-C could prevent aortic valve disease merits further investigation.

Aging and cardiovascular diseases: the role of gene-diet interactions

In the study of longevity, increasing importance is being placed on the concept of healthy aging rather than considering the total number of years lived. Although the concept of healthy lifespan needs to be defined better, we know that cardiovascular diseases (CVDs) are the main age-related diseases. Thus, controlling risk factors will contribute to reducing their incidence, leading to healthy lifespan. CVDs are complex diseases influenced by numerous genetic and environmental factors. Numerous gene variants that are associated with a greater or lesser risk of the different types of CVD and of intermediate phenotypes (i.e., hypercholesterolemia, hypertension, diabetes) have been successfully identified. However, despite the close link between aging and CVD, studies analyzing the genes related to human longevity have not obtained consistent results and there has been little coincidence in the genes identified in both fields. The APOE gene stands out as an exception, given that it has been identified as being relevant in CVD and longevity. This review analyzes the genomic and epigenomic factors that may contribute to this, ranging from identifying longevity genes in model organisms to the importance of gene-diet interactions (outstanding among which is the case of the TCF7L2 gene).

Using multivariable Mendelian randomization to disentangle the causal effects of lipid fractions

BACKGROUND

Previous Mendelian randomization studies have suggested that, while low-density lipoprotein cholesterol (LDL-c) and triglycerides are causally implicated in coronary artery disease (CAD) risk, high-density lipoprotein cholesterol (HDL-c) may not be, with causal effect estimates compatible with the null.

PRINCIPAL FINDINGS

The causal effects of these three lipid fractions can be better identified using the extended methods of 'multivariable Mendelian randomization'. We employ this approach using published data on 185 lipid-related genetic variants and their associations with lipid fractions in 188,578 participants, and with CAD risk in 22,233 cases and 64,762 controls. Our results suggest that HDL-c may be causally protective of CAD risk, independently of the effects of LDL-c and triglycerides. Estimated causal odds ratios per standard deviation increase, based on 162 variants not having pleiotropic associations with either blood pressure or body mass index, are 1.57 (95% credible interval 1.45 to 1.70) for LDL-c, 0.91 (0.83 to 0.99, p-value  = 0.028) for HDL-c, and 1.29 (1.16 to 1.43) for triglycerides.

SIGNIFICANCE

Some interventions on HDL-c concentrations may influence risk of CAD, but to a lesser extent than interventions on LDL-c. A causal interpretation of these estimates relies on the assumption that the genetic variants do not have pleiotropic associations with risk factors on other pathways to CAD. If they do, a weaker conclusion is that genetic predictors of LDL-c, HDL-c and triglycerides each have independent associations with CAD risk.

Genome-wide enrichment analysis between endometriosis and obesity-related traits reveals novel susceptibility loci

Endometriosis is a chronic inflammatory condition in women that results in pelvic pain and subfertility, and has been associated with decreased body mass index (BMI). Genetic variants contributing to the heritable component have started to emerge from genome-wide association studies (GWAS), although the majority remain unknown. Unexpectedly, we observed an intergenic locus on 7p15.2 that was genome-wide significantly associated with both endometriosis and fat distribution (waist-to-hip ratio adjusted for BMI; WHRadjBMI) in an independent meta-GWAS of European ancestry individuals. This led us to investigate the potential overlap in genetic variants underlying the aetiology of endometriosis, WHRadjBMI and BMI using GWAS data. Our analyses demonstrated significant enrichment of common variants between fat distribution and endometriosis (P = 3.7 × 10⁻³), which was stronger when we restricted the investigation to more severe (Stage B) cases (P = 4.5 × 10⁻⁴). However, no genetic enrichment was observed between endometriosis and BMI (P = 0.79). In addition to 7p15.2, we identify four more variants with statistically significant evidence of involvement in both endometriosis and WHRadjBMI (in/near KIFAP3, CAB39L, WNT4, GRB14); two of these, KIFAP3 and CAB39L, are novel associations for both traits. KIFAP3, WNT4 and 7p15.2 are associated with the WNT signalling pathway; formal pathway analysis confirmed a statistically significant (P = 6.41 × 10⁻⁴) overrepresentation of shared associations in developmental processes/WNT signalling between the two traits. Our results demonstrate an example of potential biological pleiotropy that was hitherto unknown, and represent an opportunity for functional follow-up of loci and further cross-phenotype comparisons to assess how fat distribution and endometriosis pathogenesis research fields can inform each other.

Low doses of eicosapentaenoic acid and docosahexaenoic acid from fish oil dose-dependently decrease serum triglyceride concentrations in the presence of plant sterols in hypercholesterolemic men and women

Plant sterols (PSs) lower LDL cholesterol (LDL-C) concentrations, whereas the n-3 (ω-3) fish fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) lower triglyceride (TG) concentrations. Incorporating both PSs and EPA+DHA from fish oil (FO) in a single food format was expected to beneficially affect 2 blood lipid risk factors. The aim of this study was to investigate the dose-response relation between low doses (<2 g/d) of EPA+DHA from FO, incorporated in a low-fat PS-enriched spread, and TG concentrations. In addition, effects on LDL-C were investigated. The study was designed as a randomized, double-blind, placebo-controlled parallel study. After a 4-wk run-in period, subjects were randomly assigned to consume either a control (C) spread (no PSs, no FO) or 1 of 4 intervention spreads containing a fixed amount of PSs (2.5 g/d) and varying amounts of FO (0.0, 0.9, 1.3, and 1.8 g/d of EPA+DHA) for 4 wk. Before and after the intervention, fasting blood samples were drawn for measuring serum lipids and EPA and DHA in erythrocyte membranes. In total, 85 hypercholesterolemic men and 247 women with a mean age of 57.9 y (range: 25-74 y) were included. Eighteen subjects dropped out during the study. At baseline, mean TG and LDL-C concentrations were 1.09 and 4.00 mmol/L, respectively. After the intervention, a significant dose-response relation for the TG-lowering effect of EPA+DHA [βln (TG) = -0.07 mmol/L per gram of EPA+DHA; P < 0.01] was found. Compared with the C group, TG concentrations were 9.3-16.2% lower in the different FO groups (P < 0.05 for all groups). LDL-C concentrations were 11.5-14.7% lower in the different PS groups than in the C group (P < 0.01 for all groups). EPA and DHA in erythrocyte membranes were dose-dependently higher after FO intake than after the C spread, indicating good compliance. Consumption of a low-fat spread enriched with PSs and different low doses of n-3 fatty acids from FO decreased TG concentrations in a dose-dependent manner and decreased LDL-C concentrations. This trial was registered at clinicaltrials.gov as NCT01313988.

Gene-Lifestyle Interactions in Complex Diseases: Design and Description of the GLACIER and VIKING Studies

Most complex diseases have well-established genetic and non-genetic risk factors. In some instances, these risk factors are likely to interact, whereby their joint effects convey a level of risk that is either significantly more or less than the sum of these risks. Characterizing these gene-environment interactions may help elucidate the biology of complex diseases, as well as to guide strategies for their targeted prevention. In most cases, the detection of gene-environment interactions will require sample sizes in excess of those needed to detect the marginal effects of the genetic and environmental risk factors. Although many consortia have been formed, comprising multiple diverse cohorts to detect gene-environment interactions, few robust examples of such interactions have been discovered. This may be because combining data across studies, usually through meta-analysis of summary data from the contributing cohorts, is often a statistically inefficient approach for the detection of gene-environment interactions. Ideally, single, very large and well-genotyped prospective cohorts, with validated measures of environmental risk factor and disease outcomes should be used to study interactions. The presence of strong founder effects within those cohorts might further strengthen the capacity to detect novel genetic effects and gene-environment interactions. Access to accurate genealogical data would also aid in studying the diploid nature of the human genome, such as genomic imprinting (parent-of-origin effects). Here we describe two studies from northern Sweden (the GLACIER and VIKING studies) that fulfill these characteristics.