Genetic analysis of IL7R and other immune-regulatory genes in multiple sclerosis

Abstract

Multiple sclerosis is a chronic neurological disease, where both genetic and environmental factors are influencing the susceptibility and pathogenesis. Epidemiological studies have clearly demonstrated the existence of a genetic component by comparing the degree of shared genetic material and the risk of MS, where the degree of shared genetic material clearly correlates with the risk of MS. Until recently only one confirmed genetic risk factor for MS has been identified, HLA-DR*1501. In this thesis we present evidence for a new genetic risk factor for MS, interleukin 7 receptor alpha chain, (IL7R ) (study I and IV).

The IL7R was initially identified in study I, where 66 genes were investigated in up to 672 MS patients and as many controls. The genes investigated were selected based on chromosomal location and biological functions presumed to be of importance in MS. Two genes, the IL7R and the lymphocyte activating gene (LAG3), were identified to be associated with MS. In addition, two haplotypes in IL7R presented significant differences between cases and controls. The IL7R, located on chromosome 5p13, is important in the maturation and survival of T-cells in humans. LAG3, located on chromosome 12p13, is important in inhibiting activated T-cells.

In study II we analysed LAG3 and CD4 in two independent populations; a Swedish case/control material used for the initial study and a Nordic case/control material used for confirmation. CD4 was included due to the location close to LAG3 and the LD patterns between the genes as well as a prior association of CD4 with MS. None of the SNPs associated with MS in LAG3 in study I were confirmed in the Nordic material. Initial analysis of nine SNPs in CD4 revealed three associated SNPs, but none of these survived the confirmation step. From this data we conclude that CD4 and LAG3 do not present evidence to influence the genetic susceptibility in these populations.

In study III we investigated two polymorphisms located in the promotor region of the myeloperoxidase (MPO) gene. A number of studies have been reported for one of the SNPs (-463) and MS, without any conclusive result. The other SNP (-129) has not previously been investigated in MS. Neither of the SNPs presented any evidence of influencing the susceptibility to MS in this study. In addition, we investigated if any of the two SNPs showed any association with disease severity by using Multiple Sclerosis Severity Score, but no association between disease severity and genotype could be detected. We therefore conclude that these two polymorphisms do not contribute to either disease susceptibility or severity in our material.

In study IV we confirmed the three associated SNPs and the two haplotype associations in IL7R from study I in a large independent Nordic case/control material. In addition we fine-mapped the LD block harbouring the IL7R in a Swedish case/control material using a tagSNP approach. At this stage, three additional SNPs showed significant associations with MS, where one non-synonymous SNP in exon 6 presented the most significant p-value, and the importance of this SNP was proved by logistic regression analysis. Haplotype analysis presented convincing evidence for a protective effect of the most common haplotype. Analysis of cerebrospinal fluid from MS patients and from patients with non-inflammatory neurological diseases revealed an increased expression of IL7R in MS patients adding to the hypothesis of this pathway in MS.

Due to the mounting evidence for an importance of IL7R in MS we investigated the ligand, interleukin 7 (IL7), in study V. Nine SNPs were genotyped and no significant association was identified for any of the markers, thus we conclude that IL7 does not contribute to the genetic susceptibility in MS. These negative findings strengthen the role of IL7R in MS, as the functional regulation of this complex has been suggested to be due to the receptor and not the ligand.