Integrated characterisation of carotid atherosclerosis - from magnetic resonance imaging to plaque macrophage profiling

Thesis

Cardiovascular morbidity and mortality remain the most serious global public health challenge. With the evolution in global socio-economic activities and the associated lifestyle changes, there is an impending epidemic of obesity, insulin resistance, metabolic syndrome, and diabetes mellitus, all of which fuel premature coronary artery disease and stroke. In the post-statin era, despite considerable efforts in anti-atherosclerotic drug development, a significant number of these potential compounds with initial success in early phase clinical studies failed to translate to clinical outcome benefits. This shows a disparity of various biomarkers and molecular disease mechanisms, and highlighted the inadequacy of current methods of risk stratification and atherosclerosis characterisation.

Atherosclerosis imaging such as by MRI does not only serve as 'gate keepers' in the contemporary drug development process; but in combination with modern "omics", would allow precise disease stratification with resolution never achieved before. This integrated characterisation of atherosclerosis will generate novel insights about plaque biology, bridge the gap between molecular insights and clinical outcomes, and help achieve the goal of stratified, personalized, medicine.

This thesis hypothesized that plaque compositional heterogeneity such as lipid content is associated with specific gene expression profiles or signatures in human carotid plaque macrophages in a spatially distinct pattern; and that this macrophage functional heterogeneity may in turn play a role in altering response to anti-atherosclerotic treatment. To test these hypotheses, human subjects with carotid atherosclerosis were recruited before carotid surgery and scanned using novel MRI T₂ mapping technique. Explanted carotid plaques were then examined by immunohistochemistry-guided laser capture microdissection and microarray analysis.

T₂ mapping was shown to be able to accurately and reproducibly discriminate all major plaque components, and in particular, quantify plaque lipid content with excellent histological validation. Symptomatic plaques were shown to contain larger lipid-core by T₂ mapping compared to asymptomatic plaques, despite similar degree of luminal stenosis. Macrophages from the same cohorts of plaques were procured by immunohistochemistry-guided laser capture microdissection and they were shown to display distinct gene expression profiles based on their micro-anatomical locations and clinical presentations; importantly, macrophages from plaques with high lipid content detected by T₂ mapping were found to activate specific interferon / STAT1 pathways. Finally, using nicotinic acid as an example, the functional significance of plaque macrophage heterogeneity was shown in vitro to affect response to pharmacological intervention.

Authors: Chai, J

• DOI: 10.5287/ora-dp4eovjpr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: nicotinic acid; MRI; GPR109A; macrophage; atherosclerosis; laser capture microdissection; cholesterol efflux; lipid; carotid; transcriptomic; T2 mapping; microarray
• Subjects: Cardiovascular system--Diseases