Genetic Variation and Expression of the IRF5 Gene in Autoimmune Diseases

Abstract

The interferon regulatory factor 5 (IRF5) gene encodes a transcription factor that plays an important role in the innate as well as in the cell-mediated immune response. The IRF5 gene has received considerable attention since it was shown to be associated with two autoimmune diseases; systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). The aim of this thesis was to examine if IRF5 is associated with other autoimmune diseases and to investigate the role of the genetic variation of IRF5. In the first study a set of common polymorphisms in IRF5 were analyzed for their association with two subgroup of inflammatory bowel disease (IBD); Crohn´s diseases (CD) and ulcerative colitis (UC). A strong signal of association of IRF5 with IBD was found. The most strongly associated polymorphism is a 5 base pair (bp) insertion-deletion (indel) in the promoter region of the IRF5 gene. The association was detected within both UC and CD, and appeared to be stronger in UC. In the second study we investigated the association of IRF5 with multiple sclerosis (MS). A similar set of polymorphisms as in the IBD study were genotyped in a cohort of MS patients and controls. The same polymorphisms that were associated with IBD were also found to be associated with MS. In the third study, we performed a comprehensive investigation of the IRF5 gene to detect most of the polymorphisms in the gene, and to determine to what extent they account for the association signals obtained from the gene. IRF5 was sequenced and 34 new polymorphisms were identified. Twenty seven of these, and 20 previously known SNPs in IRF5 were genotyped in an SLE case-control cohort. We found that only two polymorphisms, the 5bp indel and a SNP downstream of IRF5, account for the association signal from all the remaining markers in the IRF5 gene, and that these two polymorphisms are independently associated with SLE. Interestingly, in our studies on IBD and MS, we only observed the signal from the 5bp indel polymorphism as a risk factor for IBDs. In the fourth study the two independent risk alleles in IRF5, were tested for their association with primary Sjögren´s syndrome (pSS). In this study we also included one SNP in the STAT4 gene, since STAT4 had recently been shown to be associated with SLE. Both risk factors in IRF5 and STAT4 were found to be associated with pSS.

The regulation of expression of IRF5 was also investigated in the first three studies. We observed allele-specific differences in protein binding as well as increased binding of the transcription factor SP1 to the 5bp risk allele. We also detected increased expression of the IRF5 mRNA from a promoter containing the risk allele.

Taken together, the results of our studies suggest a general function for IRF5 as a regulator of the autoimmune response, where the 5bp indel is associated with IBD, MS, SLE and pSS. The additionally polymorphisms, which account for the remaining association signal obtained with SLE and pSS, may contribute to the disease manifestations that are specific for rheumatic diseases. Our studies add to the evidence that there are genes or pathways that are common in multiple autoimmune diseases, and that the type I interferon system is likely to be involved in the development of these diseases.