ESPE Abstracts

Background: Growth Hormone Insensitivity (GHI) is characterised by short stature and functional IGF-I deficiency associated with normal/elevated GH levels. Marked genetic and phenotypic heterogeneity exist, and heritable defects in GH-IGF-I axis genes/associated pathways account for mild-moderate to severe GHI cases. We report twin brothers from a non-consanguineous kindred who present with short stature and bi-allelic mutations in QSOX2 encoding the nuclear membrane protein, quiescin sulfhydryl oxidase 2. Genome-wide association studies have identified the LHX3-QSOX2 locus as a significant adult height quantitative trait locus. We hypothesise QSOX2 is a novel regulator of STAT5B nuclear translocation, a key step in GH-induced IGF-I production.

Methods: Variant constructs generated by mutagenesis of an N-terminal FLAG tagged-QSOX2 cDNA were expressed in HEK 293-hGHR cells. QSOX2 and STAT5B cellular localisation were assessed by immunoblotting and immunofluorescence. Nano-luciferase complementation and dual luciferase reporter assays evaluated QSOX2-STAT5B interactions and GH-induced transcriptional activity, respectively. Mitochondrial morphology and membrane potential of patient fibroblasts were examined by confocal microscopy and TMRE assays.

Results: Monozygotic twin brothers presented with severe postnatal growth restriction, immunodeficiency, relative macrocephaly, mild dysmorphism, recurrent respiratory tract infections, oral feeding aversion and reduced gastrointestinal motility. Peripheral blood immune profiling revealed low IgM levels and elevated basal levels of tyrosine phosphorylated STAT5 compared to controls. Next generation sequencing revealed compound heterozygous variants in QSOX2; a novel paternally inherited single base deletion, predicted to result in a frameshift truncation and a maternally inherited missense variant, predicted deleterious in silico. We demonstrated a direct interaction between QSOX2 and STAT5B. Nano-luciferase complementation assays revealed attenuation of the interaction of both mutants with STAT5B when compared to wild type. Both mutations led to robust tyrosine phosphorylation of STAT5 following GH agonist stimulation with concomitant cytosolic accumulation and attenuation of nuclear translocation of STAT5B. This effect was phenocopied in patient-derived dermal fibroblasts and similar to the dominant negative STAT5B p.Gln177Pro mutant which disrupts the CCD, suggesting that this domain is integral for interaction with QSOX2. Furthermore, both mutants exhibit reduced STAT5B downstream transcriptional activity. A distinct mitochondrial phenotype was also observed in patient fibroblasts, with reorganisation of oxidative phosphorylation complexes and diminished mitochondrial membrane potential compared to WT.

Conclusion: We describe a definitive role of QSOX2 in modulating human growth, broadening the GHI spectrum. Deficiency of QSOX2 impairs STAT5B downstream activity and mitochondrial dynamics leading to a unique syndrome of postnatal growth failure and mild immunodeficiency similar to germline dominant-negative STAT5B deficiency.