Endocrine Abstracts

Background: Defects in growth hormone (GH) action account for a substantial percentage of endocrine causes of growth restriction but are frequently unrecognised due to wide clinical and biochemical variability. We report five probands from three families who present with short stature, features of atypical growth hormone insensitivity (GHI), immune dysfunction, atopic eczema, and gastrointestinal pathology associated with recessive variants in QSOX2.

Methods: QSOX2 domain boundary locations were modelled using the IntFOLD7 server and interaction partners identified using the PINOT server. Mutagenesis of an N-terminal FLAG tagged-QSOX2 cDNA generated variant constructs. All experiments were performed using a HEK 293-hGHR cell line. QSOX2 and STAT5B cellular localisation were assessed by western blotting and immunofluorescence. Nano-luciferase complementation assays evaluated QSOX2-STAT5B interactions. Mitochondrial morphology/membrane potential of patient fibroblasts were examined by confocal microscopy and TMRE assays.

Results: We describe the first pathological QSOX2 variants, discovered by next generation sequencing of five individuals with growth restriction. We demonstrated a direct interaction between QSOX2 and STAT5B using NanoBit complementation assays. Bioinformatic analyses suggested that this interaction occurred via the QSOX2 sulfhydryl oxidase structural domain, which was disrupted for all variants. All variants led to robust GH-stimulated tyrosine phosphorylation of STAT5B. STAT5B nuclear translocation was attenuated with resultant reduced STAT5B downstream transcriptional activities. Intriguingly, robust GH-induced STAT5B phosphorylation correlated with reorganisation of oxidative phosphorylation complexes and diminished mitochondrial membrane potential in patient-derived dermal fibroblasts. Confocal microscopy showed markedly fragmented mitochondria in patient derived fibroblasts only following GH, but not when untreated or following IGF-1 stimulation. A concomitant increase in phospho-Ser616-DRP1 (Dynamin-related protein 1), a pro-fission marker of mitochondrial fragmentation, was observed. Increased cytoplasmic p-STAT5 in fibroblasts co-localised to the mitochondrial outer membrane suggesting that in the absence of functional QSOX2, p-STAT5 may impact mitochondrial fragmentation via enhanced DRP1-S616 phosphorylation.

Conclusion: QSOX2 deficiency should be suspected in individuals with features of growth restriction, atypical GHI, low IGF-1, atopic eczema, feeding difficulties, gastrointestinal dysmotility and recurrent infections. QSOX2 deficiency modulates human growth by impairing GH-STAT5B downstream activities and mitochondrial dynamics, contributing to multi-system dysfunction. Therapeutic rhIGF-1 may circumvent the GH-mediated STAT5B molecular defect and potentially alleviate organ specific disease.