The association of inflammatory biomarkers with cardiovascular events : a long and winding path

Aims: The overall aim of this thesis was to investigate the association of inflammatory biomarkers with cardiovascular disease (CVD) with the final scope to identify potential novel predictors of incident cardiovascular events (CVE). The specific objectives were: to study the association between levels of IL8 and the genetic variants at the IL8 and IL8 receptor genes with the occurrence of myocardial infarction (MI) and to explore if these genetic variants regulate IL8 levels (Study 1); to identify novel genetic variants associated with IL8 levels (Study II); to study the association of IL8 levels with the risk of CVE (Study III) and, finally, to investigate the association between the levels of soluble IL6 receptors (sIL6R and sgp130) with the occurrence of MI (Study IV). Materials and Methods: Study I, II and IV were based on the Stockholm Heart Epidemiology Program (SHEEP), a population-based case-control study of incident MI performed between 1992 and 1994 at the ten emergency hospitals within the county of Stockholm. Controls were randomly selected from the study base, using density sampling and matching for age, sex and hospital catchment area. The analyses performed in this thesis were restricted to non-fatal cases (n=1213) and matched controls (n=1561), with available biomarkers data. Exposure information was available from questionnaires, anthropometric measurements, blood samples and medical records. Study III was based on a cohort of 60year-old men and women from Stockholm (60YO), a population-based prospective study performed between 1997 and 1999 in Stockholm County. A total of 4232 individuals (78%) agreed to be enrolled in the study. Participants were followed for an average of 14.5 years for the assessment of CVE and cardiovascular mortality. Up to December 31, 2012, n= 491 cases were recorded. A nested-case control was conducted based on the full enumerated cohort. Controls (n= 981) were randomly selected from the study base having the same sex as the case, being alive, and not been classified as a case at the date of the CVE (+/60 days). Results and Conclusions: Circulating levels of IL8 were associated with a reduced occurrence of non-fatal MI in the SHEEP population aged 45-70 years (Study I), whereas no association was observed with the risk of CVEdefined as fatal and non-fatal MI, fatal and non-fatal stroke and hospitalization due to angina pectoris in the 60YO cohort (Study III). Median levels of IL8 did not vary according to IL8 and IL8 receptor genetic variants (Study I). A SNP rs12075 A/G, Asp42Gly, mapping at the Duffy antigen receptor for chemokines (DARC) gene on chromosome 1 was associated with circulating IL8 in serum, but not in plasma. Homozygous carriers of the G allele had lower levels of IL8 (Study II). These findings were replicated in independent populations. SIL6R and sgp130 had opposing associations with MI. Sgp130 was an effect modifier of the association of sIL6R with MI and there was an indication of a possible interaction between high sIL6R and low sgp130 with the risk of MI (Study IV). Our results suggest that differences in baseline characteristics of a study population, genetic predisposition and genetic regulation of biomarkers should be taken into account when interpreting the results of associations of biomarkers with CVD risk. In a broader sense, the role of molecular pathways should be evaluated in studies aimed at analysing the risk associated with complex outcomes rather than applying single biomarker approaches. LIST OF SCIENTIFIC PAPERS This thesis is based on the following original articles/manuscript, which will be referred to in the text by their Roman numerals