A twin study of heritability of plasma lipoprotein-associated phospholipase A2 (Lp-PLA2) mass and activity

Quantile-Specific Heritability of Inflammatory and Oxidative Stress Biomarkers Linked to Cardiovascular Disease

PURPOSE

Heritability (h2 , the proportion of the phenotypic variance attributable to additive genetic effects) is traditionally assumed to be constant throughout the distribution of the phenotype. However, the heritabilities of circulating C-reactive protein, interleukin-6, plasminogen activator inhibitor type-1 (PAI-1), and monocyte chemoattractant protein-1 (MCP-1) concentrations depend upon whether the phenotype is high or low relative to their distributions (quantile-dependent expressivity), which may account for apparent gene-environment interactions. Whether the heritabilities of other inflammatory biomarkers linked to cardiovascular disease are quantile-dependent remain to be determined.

PATIENTS AND METHODS

Quantile-specific offspring-parent (βOP) and full-sib regression slopes (βFS) were estimated by applying quantile regression to the age- and sex-adjusted phenotypes of families surveyed as part of the Framingham Heart Study. Quantile-specific heritabilities were calculated as: h2 =2βOP/(1+rspouse) and h2 ={(1+8rspouseβFS)0.5-1}/(2rspouse).

RESULTS

Heritability (h2 ± SE) of lipoprotein-associated phospholipase A2 (Lp-PLA2) mass concentrations increased from 0.11 ± 0.03 at the 10th percentile, 0.08 ± 0.03 at the 25th, 0.12 ± 0.03 at the 50th, 0.20 ± 0.04 at the 75th, and 0.26 ± 0.06 at the 90th percentile, or 0.0023 ± 0.0006 per each one-percent increase in the phenotype distribution (Plinear trend= 0.0004). Similarly, h2 increased 0.0029 ± 0.0011 (Plinear trend= 0.01) for sP-selectin, 0.0032 ± 0.0009 (Plinear trend= 0.0001) for soluble intercellular adhesion molecule 1 (sICAM-1), and 0.0026 ± 0.0006 for tumor necrosis factor receptor 2 (TNFR2) (Plinear trend= 5.0 × 10-6) per each one-percent increase in their distributions when estimated from βOP. Osteoprotegerin and soluble ST2 heritability also increased significantly with increasing percentiles of their distributions when estimated from βFS. Lp-PLA2 activity, CD40 ligand, TNFα, interleukin-18, and myeloperoxidase heritability showed no significant quantile-dependence.

CONCLUSION

The heritabilities of circulating Lp-PLA2-mass, sP-selectin, sICAM-1, TNFR2, osteoprotegerin and soluble ST2 concentrations are quantile-dependent, which may contribute to purported genetic modulations of: 1) sP-selectin's relationships to venous thrombosis, pulmonary hypertension, type 2 diabetes and atorvastatin treatment; 2) sICAM-I's relationships to brain abscess and atorvastatin treatment; and 3) Lp-PLA2's relationships to myocardial infarction and preeclampsia.

Lipoprotein-associated phospholipase A2: The story continues

Inflammation is thought to play an important role in the pathogenesis of vascular diseases. Lipoprotein-associated phospholipase A2 (Lp-PLA2) mediates vascular inflammation through the regulation of lipid metabolism in blood, thus, it has been extensively investigated to identify its role in vascular inflammation-related diseases, mainly atherosclerosis. Although darapladib, the most advanced Lp-PLA2 inhibitor, failed to meet the primary endpoints of two large phase III trials in atherosclerosis patients cotreated with standard medical care, the research on Lp-PLA2 has not been terminated. Novel pathogenic, epidemiologic, genetic, and crystallographic studies regarding Lp-PLA2 have been reported recently, while novel inhibitors were identified through a fragment-based lead discovery strategy. More strikingly, recent clinical and preclinical studies revealed that Lp-PLA2 inhibition showed promising therapeutic effects in diabetic macular edema and Alzheimer's disease. In this review, we not only summarized the knowledge of Lp-PLA2 established in the past decades but also emphasized new findings in recent years. We hope this review could be valuable for helping researchers acquire a much deeper insight into the nature of Lp-PLA2, identify more potent and selective Lp-PLA2 inhibitors, and discover the potential indications of Lp-PLA2 inhibitors.

Residual atherosclerotic cardiovascular disease risk in statin-treated adults: The Multi-Ethnic Study of Atherosclerosis

BACKGROUND

Residual atherosclerotic cardiovascular disease (ASCVD) risk in statin-treated US adults without known ASCVD is not well described.

OBJECTIVE

To quantitate residual ASCVD risk and its predictors in statin-treated adults.

METHODS

We studied 1014 statin-treated adults (53.3% female, mean 66.0 years) free of clinical ASCVD in the Multi-Ethnic Study of Atherosclerosis. We examined ASCVD event rates by National Lipid Association risk groups over 11-year follow-up and the relation of standard risk factors, biomarkers, and subclinical atherosclerosis measures with residual ASCVD event risk.

RESULTS

Overall, 5.3% of participants were at low, 12.2% at moderate, 60.3% at high, and 22.2% at very high baseline risk. Despite statin therapy, age- and race-standardized ASCVD rates per 1000 person-years for men and women were both 4.9 for low/moderate risk, 19.1 and 14.2 for high risk, and 35.6 and 26.7 for very high risk, respectively. Specific independent predictors of residual risk included current smoking, family history, diabetes, high-sensitivity C-reactive protein, low-density lipoprotein particle number, carotid intimal medial thickness, and especially coronary artery calcium score. Those on moderate- or high-intensity statins at baseline (compared with low intensity) had 39% lower risks and those who increased statin intensity 62% lower ASCVD event risks (P < .01).

CONCLUSION

Residual risk of ASCVD remains high despite statin treatment and is predicted by specific risk factors and subclinical atherosclerosis. These findings may be helpful for identifying those at highest risk needing more aggressive treatment.

A previously unreported impact of a PLA2G7 gene polymorphism on the plasma levels of lipoprotein-associated phospholipase A2 activity and mass

Lipoprotein-associated phospholipase A2 (Lp-PLA₂) levels are associated with the development of atherosclerosis. We aimed to assess the genetic determinants of Lp-PLA₂ activity and mass by genotyping multiple polymorphisms in PLA2G7, the gene encoding Lp-PLA₂, among 1258 participants from the Chinese Multi-provincial Cohort Study-Beijing Project. The Sequenom MassARRAY system, Taqman assay and direct sequencing were adopted. For the first time, the rs13218408 polymorphism was found to be significantly associated with reduced Lp-PLA₂ levels. We also confirmed the significant association of previously validated polymorphisms (rs1421378, rs1805018, rs16874954 and rs2216465), even after adjusting for traditional cardiovascular risk factors and for Bonferroni correction. Percentages of variance attributable to rs13218408 were 7.2% for activity and 13.3% for mass, and were secondary to those of rs16874954 (8.1% for activity and 16.9% for mass). A significant joint effect of rs13218408 and rs16874954 was observed on Lp-PLA₂ activity (P = 0.058) and mass (P = 0.003), with their minor alleles together linking to the largest reduction in Lp-PLA₂ levels (37.8% reduction in activity and 41.6% reduction in mass). Taken together, our findings show a significant association of a PLA2G7 polymorphism with Lp-PLA₂ levels, which was previously unreported in any population. The functionality of this genetic variation deserves further investigations.

Lipoprotein-associated phospholipase A2 and risk of incident peripheral arterial disease in a multi-ethnic cohort: The Multi-Ethnic Study of Atherosclerosis

Prospective studies supporting a relationship between elevated lipoprotein-associated phospholipase A₂ (Lp-PLA₂) and incident peripheral arterial disease (PAD) are limited. We evaluated the association of Lp-PLA₂ with incident PAD in a multi-ethnic cohort without clinical cardiovascular disease. A total of 4622 participants with measurement of Lp-PLA₂ mass and Lp-PLA₂ activity and an ankle-brachial index (ABI) between 0.9 and 1.4 were followed for the development of PAD (median follow-up = 9.3 years), defined as an ABI ⩽0.9 and decline from baseline ⩾0.15. There were 158 incident PAD events during follow-up. In adjusted logistic regression models, each higher standard deviation of both Lp-PLA₂ activity and mass did not confer an increased risk of developing PAD [odds ratios, (95% confidence intervals)]: 0.92 (0.66-1.27) for Lp-PLA₂ activity and 1.06 (0.85-1.34) for mass. Additionally, no significant interaction was found according to ethnicity: p=0.43 for Lp-PLA₂ activity and p=0.55 for Lp-PLA₂ mass. We found no evidence of an association between Lp-PLA₂ and incident PAD.

Lipoprotein-associated phospholipase A2 prognostic role in atherosclerotic complications

Atherosclerosis manifests itself clinically at advanced stages when plaques undergo hemorrhage and/or rupture with superimposed thrombosis, thus abruptly stopping blood supply. Identification of markers of plaque destabilization at a pre-clinical stage is, therefore, a major goal of cardiovascular research. Promising results along this line were provided by studies investigating the lipoprotein-associated phospholipase A2 (Lp-PLA2), a member of phospholipase A2 proteins family that plays a key role in the metabolism of pro-inflammatory phospholipids, as oxidized low-density lipoproteins, and in the generation of pro-atherogenic metabolites, including lysophosphatidylcholine and oxidized free fatty acids. We herein review the experimental and clinical studies supporting use of Lp-PLA2 activity for predicting cardiovascular events. To his end we considered not only Lp-PLA2 activity and mass, but also Lp-PLA2 gene variations and their association with incident coronary artery disease, stroke, and cardiovascular mortality. Based on these evidences the major scientific societies have included in their guidelines the measurement of Lp-PLA2 activity among the biomarkers that are useful in risk stratification of adult asymptomatic patients at intermediate cardiovascular risk. The results of two recently published major clinical trials with the Lp-PLA2 inhibitor darapladib, which seem to challenge the pathogenic role of Lp-PLA2, will also be discussed.

Lipoprotein-associated phospholipase A2 and risk of incident cardiovascular disease in a multi-ethnic cohort: The multi ethnic study of atherosclerosis

OBJECTIVE

Prospective studies reporting a positive association of lipoprotein-associated phospholipase A2 (Lp-PLA2) mass and activity with incident cardiovascular disease (CVD) have included primarily white individuals. We evaluated associations of Lp-PLA2 and first-time cardiovascular events in a healthy multi-ethnic cohort characterized by presence or absence of baseline subclinical atherosclerosis.

METHODS

Lp-PLA2 mass and activity were measured at baseline in 5456 participants in the Multi-Ethnic Study of Atherosclerosis. Individuals were characterized for presence of baseline subclinical disease (coronary artery calcium score > 0 or carotid intima-media thickness value > 80th percentile) and followed prospectively for development of CVD events (coronary heart disease, ischemic stroke, and cardiovascular death).

RESULTS

516 incident CVD events occurred over median follow-up of 10.2 years. In adjusted Cox proportional hazards models, each higher standard deviation of both Lp-PLA2 activity and mass was associated with an increased risk of cardiovascular events; hazard ratios (HR; 95% confidence intervals (CI)) 1.12 (1.01-1.26) for Lp-PLA2 activity and 1.10 (1.01-1.21) for mass. Associations did not differ by subclinical disease status (p-value for interaction 0.99 for Lp-PLA2 activity and 0.32 for Lp-PLA2 mass) and there was no confounding by subclinical atherosclerosis measures. Associations of Lp-PLA2 activity but not mass were weaker in Chinese participants but there were relatively few events among Chinese in race-stratified analysis.

CONCLUSION

In this multi-ethnic cohort, Lp-PLA2 was positively associated with CVD risk, regardless of the presence of coronary artery calcium or a thickened carotid-intimal media.

Genome-Wide Association Study Evaluating Lipoprotein-Associated Phospholipase A 2 Mass and Activity at Baseline and After Rosuvastatin Therapy

Background—

Lipoprotein-associated phospholipase A 2 (Lp-PLA 2 ) is a proinflammatory enzyme bound to low-density lipoprotein cholesterol and other circulating lipoproteins. Two measures of Lp-PLA 2 , mass and activity, are associated with increased cardiovascular risk. Data are sparse regarding genetic determinants of Lp-PLA 2 mass and activity, and no prior data are available addressing genetic determinants of statin-induced changes for this proinflammatory biomarker.

Methods and Results —

We performed a genome-wide association study of Lp-PLA 2 mass and activity at baseline and after 12 months of rosuvastatin therapy (20 mg/d) among 6851 participants of European ancestry from the Justification for Use of Statins in Prevention: an Intervention Trial Evaluating Rosuvastatin (JUPITER) and performed replication in a meta-analysis of 13 664 participants from the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Consortium. Novel associations were identified and replicated at MS4A4E and TMEM49 for baseline Lp-PLA 2 activity with genome-wide significant joint P values ( P =2.0×10 −11 and P =2.9×10 −9 , respectively). In addition, genome-wide associations ( P <5×10 −8 ) were identified and replicated for baseline Lp-PLA 2 mass at CETP and for Lp-PLA 2 activity at the APOC1-APOE and PLA2G7 loci. Among 2673 statin-allocated participants, both Lp-PLA 2 mass and activity were reduced by >30% and low-density lipoprotein cholesterol by 50% after 12 months of statin therapy ( P <0.001 for both). Variants in ABCG2 and LPA were associated with change in statin-induced Lp-PLA 2 activity at genome-wide significance but were substantially attenuated after adjustment for statin-induced changes in lipid levels.

Conclusions—

Genome-wide significant associations at MS4A4E and TMEM49 may reflect novel influences on circulating levels of Lp-PLA 2 activity. In addition, genome-wide significant associations with rosuvastatin-induced change in Lp-PLA 2 activity were observed in ABCG2 and LPA , likely because of their impact on statin-induced low-density lipoprotein cholesterol lowering.

Genetic and environmental regulation of inflammatory CVD biomarkers Lp-PLA2 and IgM anti-PC

OBJECTIVE

We set out to investigate the relative contribution of genetic and environmental effect on two inflammatory CVD biomarkers; lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) and anti-phosphorylcholine IgM (anti-PC). Their relationships and possible co-regulation with other established CVD biomarkers are also examined.

METHODS

Lp-PLA(2) activity (N=1600) and anti-PC (N=2036) levels were measured in elderly Swedish twins. Correlation analyses and heritability estimation were conducted by structural equation modeling.

RESULTS

We attribute 0.37 of the variance of Lp-PLA(2) and 0.40 of anti-PC variance to genetic variance. In addition, a bivariate heritability of 0.33, 0.35 and 0.36 could be detected for levels of Lp-PLA(2) together with ApoB, total cholesterol and LDL, respectively. Anti-PC was only weakly related to other biomarkers of CVD, which may suggest a more independent role of anti-PC as a biomarker.

CONCLUSIONS

In this large sample, Lp-PLA(2) activity has lower heritability and higher environmental regulation than previously reported. Anti-PC levels are partly influenced by dominance genetics and appear to be regulated independently of more established CVD biomarkers.

Genome-wide association study of Lp-PLA(2) activity and mass in the Framingham Heart Study

Lipoprotein-associated phospholipase A₂ (Lp-PLA₂) is an emerging risk factor and therapeutic target for cardiovascular disease. The activity and mass of this enzyme are heritable traits, but major genetic determinants have not been explored in a systematic, genome-wide fashion. We carried out a genome-wide association study of Lp-PLA₂ activity and mass in 6,668 Caucasian subjects from the population-based Framingham Heart Study. Clinical data and genotypes from the Affymetrix 550K SNP array were obtained from the open-access Framingham SHARe project. Each polymorphism that passed quality control was tested for associations with Lp-PLA₂ activity and mass using linear mixed models implemented in the R statistical package, accounting for familial correlations, and controlling for age, sex, smoking, lipid-lowering-medication use, and cohort. For Lp-PLA₂ activity, polymorphisms at four independent loci reached genome-wide significance, including the APOE/APOC1 region on chromosome 19 (p = 6×10⁻²⁴); CELSR2/PSRC1 on chromosome 1 (p = 3×10⁻¹⁵); SCARB1 on chromosome 12 (p = 1×10⁻⁸) and ZNF259/BUD13 in the APOA5/APOA1 gene region on chromosome 11 (p = 4×10⁻⁸). All of these remained significant after accounting for associations with LDL cholesterol, HDL cholesterol, or triglycerides. For Lp-PLA₂ mass, 12 SNPs achieved genome-wide significance, all clustering in a region on chromosome 6p12.3 near the PLA2G7 gene. Our analyses demonstrate that genetic polymorphisms may contribute to inter-individual variation in Lp-PLA₂ activity and mass.

Blood levels of lipoprotein-associated phospholipase A₂ (Lp-PLA₂) show a strong association with atherosclerosis in humans. This enzyme is made by certain cells of the immune system, associates with lipoproteins (HDL and LDL), and is thought to be involved in inflammation. Studies have shown that Lp-PLA₂ is a good predictor of cardiovascular disease, independent of HDL and LDL cholesterol levels. This has led to the development of drugs aimed at inhibiting Lp-PLA₂ as a way to treat or prevent cardiovascular disease. The activity and mass of Lp-PLA₂ are heritable traits, but major genetic determinants have not been explored in a systematic fashion. We examined genetic variants across the human genome to identify genes influencing Lp-PLA₂ activity and mass. We studied 6,668 Caucasian subjects from the population-based Framingham Heart Study. Clinical data and genetic data on 550,000 genetic variants were available for association analysis. There was no overlap in the most significantly associated SNPs for activity and mass. We identified four distinct gene regions showing highly significant associations with Lp-PLA₂ activity, all of which are known to include genes involved in cholesterol metabolism. The only locus associated with Lp-PLA₂ mass was a region harboring PLA2G7, the gene that encodes lipoprotein-associated phospholipase A2.

Lp-PLA2 Inhibition-The Atherosclerosis Panacea?

Based on the complex pathophysiology of atherosclerosis, a large number of biomarkers that relate to lipids, inflammation, immunity, thrombosis and hemostasis, have been investigated experimentally, in epidemiologic studies and in clinical trials. Interest focuses on their potential role to aid in risk stratification, as possible surrogate markers of atherosclerosis, and potential targets for therapy. More recently, one lipid associated biomarker, lipoprotein-associated phospholipase A2 (Lp-PLA₂), has gained considerable interest. In addition to a plausible pathophysiological role by generating pro-inflammatory and pro-atherogenic compounds from oxidized LDL in the vessel wall, there is a large, fairly consistent epidemiological database indicating that increased levels of Lp-PLA₂ mass or activity are associated with increased risk for cardiovascular outcomes; such data further suggest that it might improve risk stratification. In addition, clinical studies indicate that increased Lp-PLA₂ levels are associated with endothelial dysfunction. Moreover, it may also serve as an interesting therapeutic target, since a specific inhibitor of the enzyme is available with promising animal data and initial positive data in humans. Recent experimental data from a hyperlipidemic diabetic pig model strongly suggest that increased Lp-PLA₂ in the vessel wall is associated with a more vulnerable plaque phenotype which can be modulated by inhibiting Lp-PLA₂ activity. A biomarker study in more than 1,000 patients with CHD over three months has demonstrated a positive effect on various inflammatory molecules. In addition, an imaging study using IVUS based modalities (greyscale, virtual histology, and palpography) together with a panel of biomarkers (IBIS-2) has been done in more than 300 patients with CHD treated over 12 months and results indicate that the progression of the necrotic core of the plaque can be retarded. Inhibition of the pro-atherogenic and pro-inflammatory effects of Lp-PLA₂ may therefore contribute to decrease the residual risk in high risk patients already on polypharmacotherapy. This hypothesis is now being tested in two large phase 3 clinical trials. Thus, Lp-PLA₂ indeed may represent a biomarker and a promising target for intervention.