Autoimmune diseases are clinical manifestations of a loss-of-tolerance of the immune system against the body’s own substances and healthy tissues. Primary Sjögren’s syndrome (pSS) and systemic lupus erythematosus (SLE) are two chronic inflammatory autoimmune diseases characterized by autoantibody production and an activated type I interferon system. Although the precise mechanisms leading to autoimmune processes are not well defined, recent studies suggest that aberrant DNA methylation and gene expression patterns may play a central role in the pathogenesis of these disorders. The aim of this thesis was to investigate DNA methylation and gene expression in pSS and SLE on a genome-wide scale to advance our understanding of how these factors contribute to the diseases and to identify potential biomarkers and novel treatment targets. In study I, differential DNA methylation was analyzed in multiple tissues from pSS patients and healthy controls. We identified thousands of CpG sites with perturbed methylation; the most prominent finding was a profound hypomethylation at regulatory regions of type I interferon induced genes in pSS. In study II, a cases-case study comparing DNA methylation in pSS patients with high fatigue to patients with low fatigue, we found methylation patterns associated to the degree of fatigue. In study III, RNA-sequencing was applied to investigate the transcriptome of B cells in pSS in comparison to controls. Increased expression of type I and type II interferon regulated genes in pSS was observed, indicating ongoing immune activation in B cells. In study IV, the impact of DNA methylation on disease susceptibility and phenotypic variability in SLE was investigated. We identified DNA methylation patterns associated to disease susceptibility, SLE manifestations and different treatments. In addition, we mapped methylation quantitative trait loci and observed evidence for genetic regulation of DNA methylation in SLE.   In conclusion, the results presented in this thesis provide new insights into the molecular mechanisms underlying autoimmunity in pSS and SLE. The studies confirm the central role of the interferon system in pSS and SLE and further suggest novel genes and mechanisms to be involved in the pathogenesis these autoimmune diseases.

Human autoimmune diseases are hallmarked by inappropriate loss-of-tolerance and self-attacking response of the immune system. Studies included in this thesis are focusing on the implication and functional impact of genetic factors in three autoimmune diseases rheumatoid arthritis (RA), asthma, and systemic lupus erythematosus (SLE). Using genetic association studies, we found in study I and II that sequence variants of the interferon regulatory factor 5 (IRF5) gene were associated with RA and asthma, and the associations were more pronounced in certain disease subtypes. Distinct association patterns or risk alleles of the IRF5 gene variants were revealed in different diseases, indicating that IRF5 contributes to disease manifestations in a dose-dependent manner. In study III, we found that seven out of eight genetic risk loci for SLE, which were originally identified in East Asian populations, also conferred disease risk with the same risk alleles and comparable magnitudes of effect sizes in Caucasians. Remarkable differences in risk allele frequencies were observed for all associated loci across ethnicities, which seems to be the major source of genetic heterogeneity for SLE. In study IV we explored an exhaustive spectrum of sequence variants in the genes inhibitor of kappa light polypeptide gene enhancer in B-cells kinase epsilon (IKBKE) and interferon induced with helicase C domain 1 (IFIH1) by gene resequencing, and identified nine variants in IKBKE and three variants in IFIH1 as genetic risk factors for SLE. One of the associated variants may influence splicing of IKBKE mRNA. In study V we provided genome-wide transcriptional regulatory profiles for IRF5 and signal transducer and activator of transcription 4 (STAT4) using chromatin immunoprecipitation-sequencing (ChIP-seq). The target genes of IRF5 and STAT4 were found to play active roles in pathways related with inflammatory response, and their expression patterns were characteristic for SLE patients. We also identified potential cooperative transcription factors for IRF5 and STAT4, and disease-associated sequence variants which may affect the regulatory function of IRF5 and STAT4. In conclusion, this thesis illuminates the contribution of several genetic risk factors to susceptibility of human autoimmune diseases, which facilitates our understanding of the genetic basis of their pathogenesis.

In autoimmune diseases an individual’s immune system becomes targeted at the body’s own healthy cells. The aim of this thesis was to identify genetic risk factors for the two autoimmune diseases multiple sclerosis (MS) and systemic lupus erythematosus (SLE). In Study I, we found that genetic variation in the interferon regulatory factor 5 gene (IRF5), previously shown to be associated with SLE, rheumatoid arthritis and inflammatory bowel diseases, was associated also with MS. An insertion/deletion polymorphism in the first intron of IRF5 is as a good functional candidate for this association. IRF5, together with the signal transducer and activator of transcription 4 gene (STAT4), are the most important genetic risk factors for SLE, outside the HLA region. In Study II we showed using a family-based study design that genetic variation in STAT4 is associated with SLE also in the Finnish population. In Study III, we investigated a STAT4 risk allele for SLE for its association with cardiovascular disease in SLE patients. The risk allele of STAT4 proved to be strongly associated with ischemic cerebrovascular disease and anti-phospholipid antibodies in SLE patients. A possible mechanism for this association is that the risk allele leads to increased production of pro-thrombotic anti-phospholipid antibodies, which in turn increases the risk for stroke. Both IRF5 and STAT4 are involved in signalling of the type I interferon system. In Study IV, we investigated 78 additional genes in this system for their association with SLE in a Swedish cohort. The most promising results were followed up in additional patients and controls from Sweden and the US. Two novel SLE genes were identified. In Study V a large follow-up of a genome-wide association study was performed. Five new SLE loci were identified: TNIP1, PRDM1, JAZF1, UHRF1BP1 and IL10. A number of genes previously shown to be associated with other autoimmune diseases were also tested for association with SLE. This analysis identified the type I interferon system gene IFIH1 as a novel SLE risk locus. These studies confirms the central role of the type I interferon system in SLE and further suggests common genetic risk factors in autoimmunity.

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