Background/Purpose: Most autoimmune diseases are polygenic, frequently with more than 100 gene variants contributing to genetic predisposition. Given this complexity, conclusions on disease mechanisms are generally difficult to draw. Monogenic autoimmune diseases, while rare, offer a unique opportunity to develop models that enable a more mechanistic understanding of the disease process. Here, we have identified a family with IgG4-related disease (IgG4-RD) and dominant atopic features. All three affected members shared mutations in the IKZF1 (c. 548G >A, p. Arg183His) and UBR4 (c. 12537T >A, p. Cys4179Ter) genes, suggesting that either one of them or the combination of both conferred disease with high penetrance. UBR4 encodes a ubiquitin ligase involved in protein degradation, IKZF1 the transcription factor IKAROS.

Methods: PBMCs from patients and healthy controls were phenotyped by mass cytometry and flow cytometry. The influence of the variants on the transcriptome and proteome was determined and confirmed by gene silencing. In functional studies, T cell receptor signaling, T cell activation and polarization were examined in patient-derived T cell lines. Gene silencing and forced overexpression of candidate molecules was used to confirm their role in disease-relevant pathways and construct a model of distorted T cell activation and T cell differentiation due to the variants’ functions.

Results: Consistent with previous findings in non-hereditary IgG4-RD, all three patients had a prominent Th2 bias as determined by cytokine production, transcription factor expression and chemokine receptor profiles. The UBR4 mutation caused truncation resulting in reduced function. The IKZF1 mutation was mapped to the DNA-binding domain, predicting increased transcriptional regulation. Flow studies showed increased CD45 levels in all patients’ T and B cells. Consistent with increased CD45 phosphatase activity, LCK phosphorylation at inhibitory and stimulatory sites were reduced. Unexpectedly, TCR signaling and T cell activation were upregulated in patients. Despite reduced LCK activity, the enhanced TCR signaling was SRC family kinase (SFKs) dependent. After further examination of SFKs, FYN abundance was identified to be upregulated in the patients. In contrast to its inhibitory effect on LCK, CD45 abundance activated FYN, which resulted in augmented T cell activation. Mechanistically, the IKZF1 mutation resulted in increased binding to the FYN promoter and elevated transcription of FYN. The UBR4 mutation affected CD45 levels post-transcriptionally by preventing its lysosomal degradation. The hyperactivity of FYN not only reduced the threshold for T cell activation but also accounted for the increased Th2 polarization by phosphorylating JunB.

Conclusion: We identified variants of the IKZF1 and UBR4 genes in familial IgG4-RD. The two mutations synergize to promote the expression and activity of the kinase FYN in T and B cells. Increased FYN activity induces unopposed T cell activation and enhances Th2 differentiation. Based on this disease model, we propose several molecular targets that can be explored for therapeutic interventions in IgG4-RD and possibly other atopic diseases.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-pathogenetic-model-of-igg4-related-disease-developed-from-familial-ikzf1-and-ubr4-gene-variants/