Functional genetics of the interleukin-4 receptors in allergic asthma

Sammanfattning: Asthma is a complex disorder, with both genetic and environmental factors contributing to disease development. Our aim was to analyse possible associations between genetic variations of immunologically important genes and the susceptibility to and severity of allergic asthma in adults. For this purpose we used the candidate gene approach in a well-defined subgroup of allergic asthma. We investigated polymorphisms in the various genes, CD14, CLEC4E, DNAJA2, HARS, IL4R, IL10, TBX21, TGFB1, TNF and CIITA, by dynamic allele-specific hybridization (DASH), TaqMan allelic discrimination, LightCycler allelic discrimination or restriction endonuclease mapping. We found that polymorphisms in the IL4R gene associated with susceptibility to the disease and that soluble and membrane bound IL-4R forms were differently expressed in allergic asthma patients compared to non-asthmatic controls. To validate the impact of IL4R polymorphisms on the expression of the IL-4R splice forms, we used the minigene assay, a method frequently applied to investigate alternative splicing. The results demonstrated significant differences in the expression of the soluble form as an effect of four genetic variations flanking the alternative exon 8 of the IL4R gene. Polymorphisms in the specific regulator of MHC class II expression, the class II transactivator, CIITA gene, had previously been associated with inflammatory diseases and expression of both CIITA and HLA-DRA. We investigated CIITA and HLA-DRA together with the soluble and membrane bound IL-4R expression, in cultured peripheral blood mononuclear cells from allergic patients and non-allergic controls, as candidates for pathways responsive to IL-4 and IFN-gamma stimulation. Adult allergic patients showed increasing response to IL-4 stimulation and non-responsiveness towards IFN-gamma induction, which was manifested by an altered CIITA and IL-4R expression. The results also demonstrated a possible functional link between the CIITA and IL-4 pathways in the human immune system.In conclusion, variations in the IL4R gene associate with susceptibility to allergic asthma in Swedish Caucasians. Furthermore soluble IL-4R can be seen as a marker for allergic asthma, but even though genetic variations in the IL4R gene have a direct effect on expression levels of the two different splice forms, they do not explain the differences in IL4R levels seen between asthmatic and non-asthmatic individuals. Furthermore, allergic individuals show a CIITA independent upregulation of HLA-DRA, possibly through the IL-4 pathway.