Genetic studies in rheumatoid arthritis : familial studies and analysis of relationships to atherothrombotic comorbidity

Background. Rheumatoid arthritis (RA) is an autoimmune disease mainly affecting the joints but has also extra articular manifestations and an increased cardiovascular (CV) co-morbidity. Rheumatoid factor (RF) and antibodies against citrullinated proteins/peptides (ACPA) are diagnostically important and are related to a more severe disease. The aetiology is unknown but RA is considered a complex disease caused by both genetic and environmental factors. The heritability is estimated to be 60% with the main contribution from the HLA region. The relative homogeneity of the population in northern Sweden due to low immigration and founder effects has shown to be suitable for genetic studies. Objectives. The aim of this thesis has been to identify genes contributing to the susceptibility of RA and the CV co-morbidity in particular. To achieve this, multi-case families from the four northern most counties of Sweden were collected for linkage studies to identify susceptibility genes. Association studies with genetic polymorphisms in genes, involved in inflammation or being of importance for atherothrombotic manifestations (ATM) in the general population, were performed in RA-patients concerning ATM e.g. myocardial infarction, angina pectoris with intervention, stroke/TIA, deep vein thrombosis/pulmonary embolism (DVT/PE) at follow-up. Methods & Results. 47 families with 134 affected and 216 unaffected relatives were included in a genome-wide linkage study (GWL) performed with microsatellite markers at an average of 10cM resolution analysed using ABI PRISM 3730 DNA sequencer and non-parametric multipoint linkage in the Merlin program. Eight linked loci were identified with HLA as the most significant and a novel region on chromosome 14. In a follow-up analysis on a custom Illumina chip, with 13 additional families, yielding a total of 198 affected and 197 unaffected relatives. The majority of the 1536 single nucleotide polymorphisms (SNPs) used in the Illumina follow-up analyses was focused on chromosome 14. Statistical analyses with linkage and transmission disequilibrium test narrowed the region to 4 cM, a region containing multiple plausible RA candidate genes (Paper I). In Paper II serum samples from 163 affected and 157 first degree relatives were analysed with EliA ACPA assay on ImmunoCAP250 for ACPA (IgA, IgG, IgM) and RF (IgA, IgM) isotypes. Both concentrations and frequencies were increased among the relatives compared with controls but lower compared with RA-patients and with a different relative distribution of the isotypes. The genetic contribution to ATM was studied in Paper III and IV using selected SNPs analysed using ABI PRISM 7900HT sequence detector system. In Paper III, RA-patients (n=467) were compared with age and sex matched controls (n=696) with respect to SNPs in tumor necrosis factor receptor II (TNFRII)(M196R), ß-fibrinogen -455 (G-455A), plasminogen activator inhibitor type-1 (PAI-1) (4G/5G) and Factor XIIIA (Val34Leu). Hypertension was predicted by TNFRII R allele and to a higher extent in combination with the A-allele in ß-fibrinogen. The 4G allele in PAI-1 was more frequent in patients with ischemic heart disease (IHD) and the FXIIIA Leu34 variant in patients with DVT/PE. In Paper IV, the minor allele of the polymorphism in growth differentiation factor 15 (GDF15) was found to be associated with RA (n=696) per se but also to ATM, a SNP in the 9p21.3 locus was also associated with ATM. A significant association to stroke was found in female patients homozygote for the minor allele of GDF15. Stoke among male patients was significantly associated with carrying the major allele of two SNPs in the CD40 gene. DVT/PE was associated with the minor allele of GDF15. Conclusion. A novel locus on chromosome 14 of importance for RA susceptibility in northern Sweden was found. The minor allele of TNFRII separately and together with the minor allele of ß-fibrinogen -455 was associated with hypertension and the 4G allele in PAI-1 was associated with IHD and the Leu34 variant was associated with DVT/PE in RA patients. The GDF15 minor allele was associated with RA per se, ATM and DVT/PE in RA patients and a genotype in the SNP on 9p21.3 was associated with ATM. Stroke among females was associated with GDF15 and stroke among males with two SNPs in CD40