Functional and structural analysis of cytokine selective IL6ST defects that cause recessive hyper-IgE syndrome.

BACKGROUND: Biallelic variants in IL6ST cause a recessive form of hyper-IgE syndrome (HIES) characterized by high IgE, eosinophilia, defective acute phase response, susceptibility to bacterial infections and skeletal abnormalities due to cytokine selective loss-of-function in GP130 with defective IL-6 and IL-11, variable OSM and IL-27 but sparing LIF signaling. OBJECTIVE: To understand the functional and structural impact of recessive HIES-associated IL6ST variants. METHODS: We investigated a patient with HIES using exome, genome and RNA sequencing. Functional assays assessed IL-6, IL-11, IL-27, OSM, LIF, CT-1, CLC, and CNTF signaling. Molecular dynamic simulations and structural modeling of GP130 cytokine receptor complexes were performed. RESULTS: We identify a patient with compound heterozygous novel missense variants in IL6ST (p.Ala517Pro, and exon-skipping null variant p.Gly484_Pro518delinsArg). The p.Ala517Pro variant results in a more profound IL-6 and IL-11 dominated signaling defect compared to the previously identified recessive IL6ST variants p.Asn404Tyr, and p.Pro498Leu. Molecular dynamics simulations suggest that the p.Ala517Pro and p.Asn404Tyr variants result in increased flexibility of the extracellular membrane-proximal domains of GP130. We propose a structural model that explains the cytokine selectivity of pathogenic IL6ST variants that result in recessive HIES. The variants destabilize the hexameric cytokine receptor complexes whereas the trimeric LIF-GP130-LIFR complex remains stable by an additional membrane-proximal interaction. Deletion of this membrane-proximal interaction site in GP130 consequently causes additional defective LIF signaling and Stüve-Wiedemann syndrome. CONCLUSION: Our data provide a structural basis to understand clinical phenotypes in patients with IL6ST variants.