Background/Purpose: Tyrosine kinase 2 (TYK2) is a member of the Janus kinase family that associates with the cytoplasmic domain of type I/II cytokine receptors and STATs to transmit signals that mediate both innate and adaptive immune responses. Genetic variants in TYK2 gene are associated with a number of inflammatory diseases including axial spondyloarthritis (AxSpA). Exome sequencing of a patient with severe inflammatory bowel disease and AxSpA revealed compound heterozygous TYK2 gene variants, p.A53T and p.R703W, with allele frequencies of 0.007301 and 0.006692, respectively, in gnomAD. The amino acid substitutions at position 53 and 703 are present in the FERM and pseudokinase domains, respectively, of the protein. In Silico predictors such as PolyPhen, SIFT and Mutation Taster revealed these variants to be damaging. Thus, we were interested in whether these variants were of functional significance

Methods: We used skin fibroblasts to produce induced pluripotent stem cells (iPSCs) and then differentiated the iPSCs into mesenchymal stem cells (MSCs) and osteoblasts. Cell lysates form cultured MSCs of patient and three controls were used in Western blotting analysis for measuring phosphorylated TYK2 and downstream STAT1 and phospho-STAT1 levels using specific antibodies.  Insilico protein modeling was done to predict the effect of these variants on protein function. Bulk RNA sequencing was performed and comparative profiling of IFN-a response gene set was done in patient and control cells

Results: Preliminary results suggest that patients cells have elevated STAT1 levels and stimulation with IFNα resulted in higher phospho-STAT1 levels and possibly higher phospho-TYK2 levels as compared to healthy controls. Protein modeling data suggests that the A53T variant could change the conformation of the FERM-SH2 domain, which might alter cytokine receptor binding. The R703W variant may affect dimerization, or it might affect the conformation of the pseudo-kinase domain, and thereby the activity of the kinase domain. As compared to controls, RNA sequencing data showed increased expression of IFN-a response gene set in patient cells

Conclusion: Preliminary evidence suggests that these TYK2 gene variants contribute to gain-of-function. IFN responses in patient cells which may partially be responsible for elevated proinflammatory response downstream of TYK2. Further experiments are being performed to determine downstream effects of increased signaling on gene expression and to better understand the possible role of these variants in AxSpA and IBD pathogenesis

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/alteration-of-jak-stat-signaling-in-an-axial-spondyloarthritispatient-with-tyk2-variants/