FAM167A-BLK is a susceptibility locus in autoimmune diseases : characterization of the FAM167 gene family

Abstract

Autoimmune diseases are complex multifactorial diseases, and their pathogenesis is only partially understood, but both genetic, as well as environmental factors, have been correlated to an increased risk for the diseases. Genome-wide association studies of Sjögren’s syndrome, systemic lupus erythematosus, and rheumatoid arthritis have identified many risk loci; most studies include associations to the FAM167A-BLK locus. A significantly increased expression of FAM167A in cells carrying disease-associated alleles was shown by expression quantitative trait loci analyses in peripheral blood cells. This finding makes FAM167A a candidate gene for disease susceptibility. However, the function of FAM167A and the only homologous protein, its gene family member, FAM167B, was unknown when this thesis was initiated.

This thesis aims to elucidate the function of the FAM167 genes and their role in the pathogenesis of systemic autoimmune diseases. Immunohistochemistry staining of the autoimmune target organ, salivary glands of patients with Sjögren’s syndrome, revealed expression of FAM167A in cells both in the inflammatory foci and interstitium. Most were confirmed as B cells, including plasma cells, by double staining. Further, the degree of the FAM167A staining correlated with the focus score, IgG levels, and autoantibodies present in the patients. Investigating the FAM167 genes and the encoded proteins with bioinformatic methods revealed that they are highly conserved in vertebrates, contain no known protein motifs but a high content of disordered secondary structures. Based on this observation the encoded proteins were denoted disordered autoimmunity (DIORA) -1 and -2. The proteins both localize to the cytosol but are found in distinct immune cell subsets and different organs. To investigate their function at the whole organism level, we established two knock-out mouse strains. Both Fam167a and Fam167b deficient mice were viable - but Fam167a deficient mice showed lower body weight, increased kidney weight, and proteinuria in older animals. Further, alterations in serum immunoglobulins and B cell populations were detected together with an expanded B1a cell population, which exhibited signs of metabolic activation at the transcriptomic level. Contrarily, Fam167b deficient mice demonstrated no gross malformations. However, their microglia displayed altered expression of genes belonging to pathways regulated by interferon as well as pathways of chemotaxis, cell adhesion, migration, and inflammatory responses. Based on proximity labeling experiments and annotated protein-protein interactions, we suggest that DIORA-1 and DIORA-2 share MRCKA and MRCKB, both regulators of the cytoskeleton, as interaction partners.

In summary, this thesis presents the initial characterization of two genes with unknown function. Based on the findings, we speculate that the FAM167 genes contribute to immune function including antibody isotype determination and immunoglobulin production. However, further studies are needed to explore the molecular activity of FAM167A and FAM167B in more detail. In the future, a detailed understanding of the pathways involved in autoimmune pathogenesis such as Sjögren’s syndrome, lupus, or rheumatoid arthritis might result in the development of novel therapeutics for the benefit of the patients.