Clinical and Biological Aspects of Cardiovascular microRNA

Abstract: Ischemic heart disease is the leading cause of death in high-income parts of the world and is caused mainly by atherosclerosis in the coronary arteries. The rupture of an atherosclerotic plaque with subsequent platelet activation and clot formation can lead to myocardial infarction (MI). Atherosclerosis is a complex process, which involves the accumulation and oxidation of low-density lipoprotein in the vessel wall followed by endothelial inflammation, infiltration of monocytes and proliferation and migration of vascular smooth muscle cells towards the vessel lumen. microRNA (miRNA) is a class of short non-coding RNA which regulate gene expression through part-complimentary binding to target sites preferably within the 3’-UTR of specific mRNAs. miRNAs have pervasive roles in animal biology and aberrant expression of miRNAs have been linked with a wide spectrum of human disease. Additionally, the tissue specific manner of miRNA expression together with remarkable stability in plasma and evidence of release of miRNA from stressed or apoptotic cells have made miRNAs interesting as biomarkers for various diseases. The aim of this thesis was (1) to assess the diagnostic and prognostic value of cardiac-enriched miRNAs in the context of coronary artery disease, (2) to screen for differences in miRNA content in platelets from myocardial infarction patients and elucidate a potential paracrine function for platelet miRNA and (3) to investigate the role of miRNAs in regulating endothelial inflammation in response to extracellular ATP/UTP. In Study I and II, we found that cardiac-enriched miRNAs are increased 100-3000 fold within 12 h following onset of symptoms in patients with myocardial infarction and that the levels of two specific miRNAs, miR-208b and-499-5p, could be used to discriminate MI-patients from non-MI patients and were associated with increased risk of death and development of heart failure. In Study III, we found differentially expressed platelet miRNAs in MI patients compared to healthy controls using RNA-sequencing. Release of miRNAs upon platelet activation, as well as microparticle-dependent transfer of functional platelet-derived miRNA to endothelial cells was also shown in vitro. In Study IV, we showed that the effects of ATP and UTP on cell surface expression of intercellular adhesion molecule 1 (ICAM-1) and leukocyte adhesion is mediated in part by miR-22 in endothelial cells. In conclusion, the levels of circulating cardiac-enriched miRNA have diagnostic and prognostic value in the context of coronary artery disease, platelet-derived miRNA can act as paracrine mediators in endothelial cells and miR-22 plays an anti-inflammatory role in the endothelium.