The high-density lipoprotein puzzle: Why classic epidemiology, genetic epidemiology, and clinical trials conflict?

Classical epidemiology has established the incremental contribution of the high-density lipoprotein (HDL) cholesterol measure in the assessment of atherosclerotic cardiovascular disease risk; yet, genetic epidemiology does not support a causal relationship between HDL cholesterol and the future risk of myocardial infarction. Therapeutic interventions directed toward cholesterol loading of the HDL particle have been based on epidemiological studies that have established HDL cholesterol as a biomarker of atherosclerotic cardiovascular risk. However, therapeutic interventions such as niacin, cholesteryl ester transfer protein inhibitors increase HDL cholesterol in patients treated with statins, but have repeatedly failed to reduce cardiovascular events. Statin therapy interferes with ATP-binding cassette transporter-mediated macrophage cholesterol efflux via miR33 and thus may diminish certain HDL functional properties. Unraveling the HDL puzzle will require continued technical advances in the characterization and quantification of multiple HDL subclasses and their functional properties. Key mechanistic criteria for clinical outcomes trials with HDL-based therapies include formation of HDL subclasses that improve the efficiency of macrophage cholesterol efflux and compositional changes in the proteome and lipidome of the HDL particle that are associated with improved antioxidant and anti-inflammatory properties. These measures require validation in genetic studies and clinical trials of HDL-based therapies on the background of statins.