Evaluation of biomarkers in chronic inflammatory diseases

Abstract: The overall aim of this thesis was to assess the role of biomarkers in chronic inflammatory disease. The goal of the first study was to investigate the association of sIL-6R and sgp130 levels and risk of first myocardial infarction (MI), and to explore the potential interaction between these two biomarkers in association with risk of MI. In the second study, the aim was to identify genetic loci associated with the circulation of sgp130, known as an inhibitor of the IL-6 trans-signaling, and also to test the association between the single nucleotide polymorphisms (SNPs) associated with sgp130 and carotid intima-media thickness (c-IMT) as a surrogate marker of atherosclerosis. The main objective of the third study was to investigate whether environmental risk factors of rheumatoid arthritis (RA) affect the occurrence of different reactivity patterns to citrullinated peptides and the presence of rheumatoid factor (RF) isotypes. Study I was carried out in the Stockholm Heart Epidemiology Program (SHEEP), a case-control study designed to determine new risk factors. sIL-6R and sgp130 levels were measured in serum of 682 and 664 MI cases and 1103 and 1062 controls, respectively. Increased concentrations of sIL6R were associated with a high occurrence of MI while very high levels of sgp130 had an inverse association with incidence of MI. The obtained results indicated presence of an interaction between the low level of sgp130 and the high level of sIL6R that might have a synergistic effect on increased risk of MI. Our results highlighted the necessity of focusing on molecular pathways instead of only one biomarker when estimating the risk of CVDs. Study II was performed in IMPROVE, a European multicenter prospective study recruited subjects with high risk profile for cardiovascular diseases. c-IMT was measured at baseline and after 30 months follow-up. Genomic DNA of 3703 participants was genotyped using the CardioMetaboChip and ImmunoChip. A linear regression model was used to assess the association of 360,842 SNPs and sgp130 levels. Model was adjusted for gender, age and population structure. rs10935473 and rs1929666, located at chromosome 3 and 10 respectively, had an association with sgp130 circulation. Besides, 24 SNPs showed the suggestive association with sgp130 levels. A positive association between rs17688225 and serum level of sgp130 was observed while this SNP showed a negative association with c-IMT. Our results indicated that sgp130 levels is regulated by multiple genetic loci, which is likely these loci partly also regulate c-IMT measures. In study III, analyses were based on data from two Swedish RA patients cohorts: Epidemiological Investigation of RA (EIRA, n=2859), and early RA from Umeå (eRA-Umeå, n=1011). Our results corroborate the wide-spread co-occurrence of different RA-specific antibodies, which is likely mediated by epitope spreading or cross-reactivity. When analyzing all antibodies jointly, we found that smoking is mainly associated with the presence of anti-Cit-Fibα36-50, anti-CEP-1 as well as with IgA-RF. No conclusive associations were found between low alcohol consumption or high BMI with the presence of any specific autoantibody. Our study indicated smoking might play a part in pathogenesis of RA through specific anticitrulline immunity by increasing the exposure of these antigens in affected tissues like lungs.