Mimicking Behçet’s disease: GM‐CSF gain of function mutation in a family suffering from a Behçet’s disease‐like disorder marked by extreme pathergy

Summary Behçet’s disease (BD) is an inflammatory disease mainly affecting men along the ancient Silk Route. In the present study we describe a Dutch family suffering from BD‐like disease with extreme pathergic responses, but without systemic inflammation. Genetic assessment revealed a combination of the human leukocyte antigen (HLA)‐B*51 risk‐allele together with a rare heterozygous variant in the CSF2 gene (c.130A>C, p.N44H) encoding for granulocyte–macrophage colony‐stimulating factor (GM‐CSF) found by whole exome sequencing. We utilized an over‐expression vector system in a human hepatocyte cell line to produce the aberrant variant of GM‐CSF. Biological activity of the protein was measured by signal transducer and activator of transcription 5 (STAT‐5) phosphorylation, a downstream molecule of the GM‐CSF receptor, in wild‐type peripheral mononuclear cells (PBMCs) using flow cytometry. Increased STAT‐5 phosphorylation was observed in response to mutated GM‐CSF when compared to the wild‐type or recombinant protein. CSF2 p.N44H results in disruption of one of the protein’s two N‐glycosylation sites. Enzymatically deglycosylated wild‐type GM‐CSF also enhanced STAT‐5 phosphorylation. The patient responded well to anti‐tumor necrosis factor (TNF)‐α treatment, which may be linked to the capacity of TNF‐α to induce GM‐CSF in phorbol 12‐myristate 13‐acetate (PMA)‐treated PBMCs, while GM‐CSF itself only induced dose‐dependent interleukin (IL)‐1Ra production. The identified CSF2 pathway could provide novel insights into the pathergic response of BD‐like disease and offer new opportunities for personalized treatment.