Aernoud Fiolet

23 General introduction and thesis outline biological relationships viewed from a different perspective. In this regard one should realise that vascular threats come from many behavioural and biological pathophysiological concepts. In most cases, one should therefore see a biomarker primarily as just yet one of many links, rather than as the missing link. 61 In animal experiments, the roles of such cytokines are investigated using transgenic or gene-deficient mouse models, in which certain receptors, cytokines, or cell lines are absent (the “knockout model”) or over-expressed. The effects of under- or over-expression of specific components that are thought to be of immunological importance in atherosclerosis in humans are investigated in population-wide gene- association studies. In this sort of research, known genetic variations causing loss of function or over-expression caused by single-nucleotide polymorphisms are used as “nature’s own experiment.” When investigating the effects of certain types of (immunological) exposure in a conventional observation study design, causality as origin of the association is often hard to prove since the exposure might merely be the derivate of a different, yet unknown, causal factor. However, exactly those studies that use these genetic polymorphisms (or: “Mendelian randomisation” studies) remove confounding by any acquired disease-influencing factors and supply the strongest evidence of a possible causal relationship between the factor and disease outcomes. 62 The acute phase protein C – Reactive protein is an excellent example of such a marker of inflammation with an unmistakably strong association with the risk of cardiovascular disease. Findings from retrospective and prospective cohort studies all confirmed an increase in levels of circulating C – Reactive protein as risk factor for cardiovascular disease. 63,64 Those with the genetic polymorphism known to lead to increased levels of circulating C – Reactive protein do not however have an increased risk of cardiovascular disease. 65,66 Ergo, C – reactive protein is a powerful marker for disease, but lacks a causal role, making it not a suitable target for treatment. As discussed above, interleukin – 6 plays an important role in the increased release of C – reactive protein. Population based studies on nucleotide polymorphisms leading to anomalous interleukin – 6 levels show that an increased expression of interleukin – 6 due to genetic polymorphisms is directly associated with the lifetime risk of cardiovascular disease. This proves the case for a causal role of interleukin – 6 in atherothrombosis. 67