Frontiers of lipid research: cholesterol variability, HDL biogenesis, genetics of myalgia, and lipoprotein(a)

Over the last two centuries, the cholesterol hypothesis has been developed and supported by an overwhelming body of evidence of accumulated pathological, experimental, epidemiological, genetic, and eventually also pharmacological data. Already in the mid 19th century, the pathologist Rudolf Virchow visionarily stated that atherosclerosis is an inflammation induced by cholesterol.1,2 A few decades later, Nicolay Anitschkov performed seminal experiments in rabbits demonstrating that a high-fat diet indeed induces lipid-rich aortic plaques.3–5 The Framingham study, initiated by president Harry Truman after the Second World War, confirmed that cholesterol plasma levels are indeed associated with myocardial infarction, stroke, and premature death.6 To prove that this association is causal in nature, it required a tool to lower plasma cholesterol levels chronically. This was provided by Akiro Endo, the discoverer of statins.7 Eventually, the 4S trial convingingly demonstrated that lowering plasma cholesterol levels does reduce major cardiovascular events.8 While the predictive value of plasma cholesterol has been confirmed in numerous studies,9 the impact of the variability of it remains less clear. In their Fast Track entitled ‘Cholesterol variability and the risk of mortality, myocardial infarction, and stroke: a nationwide population-based study’ by Seung-Hwan Lee and colleagues from the Catholic University of Korea School of Medicine in Seoul, Korea evaluated high visit to visit variability in cholesterol and its possible role as a predictor of major adverse cardiovascular events in patients with coronary artery disease.10 In 3 656 648 subjects without a history of myocardial infarction and stroke, 2.3% deaths, 1.1% myocardial infarctions, and 1.2% strokes occurred during 8.3 years. There was a linear association between higher total cholesterol variability and outcomes. In the multivariable adjusted model, the hazard ratios comparing the highest vs. lowest quartiles of the coefficient of variation of total cholesterol were 1.26 for all-cause mortality, 1.08 for myocardial infarction, and 1.11 for stroke, which was independent of mean total cholesterol and the use of lipid-lowering agents (Figure1). Thus, high variability in lipid levels is associated with adverse health-related outcomes, suggesting that this represents a novel risk factor in the general population. These provocative findings are put into context in an interesting Editorial by Sripal Bangalore from the New York University School of Medicine.11 Incidence rates, hazard ratios, and 95% confidence intervals of all-cause mortality, myocardial infarction, and stroke by deciles of total cholesterol variability. Adjusted for age, sex, body mass index, alcohol consumption, smoking, regular exercise, income, diabetes mellitus, hypertension, mean total cholesterol level, and use of lipid lowering-agent (from Kim MK, Han K, Kim H-S, Park Y-M, Kwon H-S, Yoon K-H, Lee S-H. Cholesterol variability and the risk of mortality, myocardial infarction, and stroke: a nationwide population-based study. See pages 3560–3566). Although statins are usually well tolerated, myalgia is a common side effect.12 A genetic variant in the leucocyte immunoglobulin-like receptor subfamily-B or LILRB5, i.e rs12975366:T>C:Asp247Gly, appears to be associated with lower creatine phosphokinase and lactate dehydrogenase levels. Both biomarkers are released from injured muscle, making this variant a candidate for susceptibility to muscle-related symptoms. In a research manuscript entitled ‘A common missense variant of LILRB5 is associated with statin intolerance and myalgia’, Colin Palmer and colleagues from the University of Dundee School of Medicine in the UK examined this association.13 In their GoDARTS cohort, the LILRB5 Asp247 variant was associated with statin intolerance, as defined as raised levels of creatine phosphokinase and being non-adherent to therapy with an odds ratio of 1.8. Furthermore, it increased the probability of being intolerant to the lowest approved dose of a statin before being switched to two or more other statins with an odds ratio of 1.36. Those homozygous for Asp247 had increased odds of developing both forms of intolerance. Thus, Asp247Gly is a novel immunogenetic factor associated with statin intolerance and cardiovascular outcomes, suggesting that true statin-induced myalgia and non-specific myalgia are distinct, with a potential role for the immune system. This novel concept is put into further perspective in an Editorial by Winfried Maerz from the Synlab Center of Laboratory Diagnostics Heidelberg in Eppelheim, Germany.14 The recent failures of HDL-raising therapies15–17 have underscored our incomplete understanding of HDL biology.18 Indeed, HDL particles may become dysfunctional as atherosclerotic disease develops.19 Furthermore, extremely high HDL levels may even be harmful.20,21 Therefore, there is an urgent need to investigate HDL metabolism to enable the development of effective HDL therapies. In a Basic Science article entitled ‘TRAK2, a novel regulator of ABCA1 expression, cholesterol efflux, and HDL biogenesis’, Kiymet Bozaoglu and colleagues from the Baker Heart and Diabetes Institute in Melbourne, Australia analysed human genetic and transcriptomic data and plasma HDL-cholesterol concentration, and identified a novel association between the trafficking protein, kinesin binding 2 or TRAK2, and HDL-cholesterol, in order to identify novel regulators of HDL metabolism.22 Analysis of lymphocyte transcriptomics together with plasma HDL of 1240 subjects revealed a negative correlation between TRAK2 mRNA levels and HDL-cholesterol, HDL particle diameter, and HDL subspecies heterogeneity. TRAK2 small interfering RNA (siRNA)-mediated knockdown increased cholesterol efflux to apolipoprotein A-I and isolated HDL from human macrophage and liver cells by increasing the mRNA and protein expression of the cholesterol transporter ABCA1. The effect of TRAK2 knockdown on cholesterol efflux was abolished in the absence of ABCA1, indicating that TRAK2 regulates ABCA1-dependent efflux. TRAK2 knockdown significantly increased liver X receptor binding at the ABCA1 promoter, establishing TRAK2 as a regulator of liver X receptor-mediated transcription of ABCA1. Thus, TRAK2 is a novel regulator of liver X-receptor mediated ABCA1 expression, cholesterol efflux, and HDL biogenesis. TRAK2 may therefore be a novel target for anti-atherosclerotic therapies (Figure2). These findings are commented on in a timely Editorial by Alan Fogelman from the University of California Los Angeles in California, USA.23 Summary of key findings. TRAK2 siRNA-mediated knockdown significantly increases LXR-α binding at LXR response elements (LXR RE) in the ABCA1 promoter, which increases ABCA1 mRNA and protein expression. This leads to an increase in cholesterol efflux to both apoA-I and isolated HDL particles. Therefore low-TRAK2 levels can promote both HDL biogenesis and RCT (from Lake NJ, Taylor RL, Trahair H, Harikrishnan KN, Curran JE, Almeida M, Kulkarni H, Mukhamedova N, Hoang A, Low H, Murphy AJ, Johnson MP, Dyer TD, Mahaney MC, Göring HHH, Moses EK, Sviridov D, Blangero J, Jowett JBM, Bozaoglu K. TRAK2, a novel regulator of ABCA1 expression, cholesterol efflux and HDL biogenesis. See pages 3579–3587). The co-stimulatory receptor CD27 modulates responses of T cells, B cells, and natural killer cells. Various T-cell subsets participate in atherogenesis. However, the role of CD27 in atherosclerosis remains unexplored. In another Basic Science article entitled ‘CD27 co-stimulation increases the abundance of regulatory T cells and reduces atherosclerosis in hyperlipidaemic mice’, Norbert Gerdes and colleagues from the Universitätsklinikum Düsseldorf in Germany investigated the effect of bone marrow-derived and systemic CD27 deficiency in apolipoprotein E-deficient or ApoE–/– mice.24 Lethally irradiated ApoE–/– mice reconstituted with CD27–/–/ApoE–/– bone marrow on an atherogenic diet had increased plaque size and lesional inflammation compared with mice receiving CD27+/+/ApoE–/– bone marrow. Accordingly, chow diet-fed CD27–/–/ApoE–/– mice showed exacerbated lesion development and increased inflammation at the age of 18 weeks. At 28 weeks, lesion size and phenotype did not differ between the two groups. Systemic and bone marrow-derived CD27 deficiency reduced the abundance of regulatory T cells (Tregs) in blood, lymphoid organs, and the aorta. Numbers of other immune cells were not affected, while expression of inflammatory cytokine genes [e.g. interleukin-1 β (IL-1β) and IL-6] was increased in the aorta when haematopoietic CD27 was lacking. In vitro, Tregs of CD27-deficient mice showed similar suppressive capacity to their wild-type controls and migrated equally towards CCL19 and CCL21. However, thymic CD27–/– Tregs underwent increased apoptosis and expressed fewer markers of proliferation in vivo. Reconstitution of CD27–/–/ApoE–/– mice with CD27+/+/ApoE–/– Tregs reversed the increase in atherosclerosis. Thus, during early stages, CD27 co-stimulation increases the number of Tregs and limits lesion development and inflammation in atherosclerosis. As such, promotion of CD27 function may mitigate atherosclerosis. This manuscript is accompanied by a comprehensive Editorial authored by Christian M. Matter from the University of Zurich, Switzerland.25 Besides LDL-cholesterol, lipoprotein(a) is an important risk factor for atherosclerosis.26 In a Current Opinion entitled ‘Depicting new pharmacological strategies for familial hypercholesterolaemia involving lipoprotein(a)’, Alpo Vuorio and colleagues from Mehiläinen Airport Health Centre in Vantaa, Finland note that ∼35 million people worldwide suffer from heterozygous familial hypercholesterolaemia, a condition characterized by genetically determined life-long elevation of plasma LDL-cholesterol.27 One in three of these patients also inherit an elevated plasma concentration of lipoprotein(a), a particle with atherogenic, inflammatory, and pro-thrombotic properties. Accordingly, the combination of high plasma LDL-cholesterol and lipoprotein(a) can accelerate premature atherosclerosis. Neither statin nor ezetimibe lowers lipoprotein(a), so that familial hypercholesterolaemia patients with high lipoprotein(a) remain at increased residual risk. Proprotein convertase subtilisin/kexin type 9 (PCSK9) monoclonal antibodies are indicated for heterozygous familial hypercholesterolaemia patients,9,28,29 but only lower lipoprotein(a) by 15–30%.30 Recent trials employing apolipoprotein(a) antisense therapy show up to 90% reductions in plasma lipoprotein(a).31 The combination of a PCSK9 inhibitor and apolipoprotein(a) antisense therapy appears to be the optimal strategy for mitigating residual risk of atherosclerotic cardiovascular disease in heterozygous familial hypercholesterolaemia patients with high lipoprotein(a). The editors hope that this issue of the European Heart Journal will be of interest to its readers.