Short stature in two siblings heterozygous for a novel bioinactive GH mutant (GH-P59S) suggesting that the mutant also affects secretion of the wild-type GH

Abstract

ObjectiveShort stature caused by biologically inactive GH is clinically characterized by lack of GH action despite normal-high secretion of GH, pathologically low IGF1 concentrations and marked catch-up growth on GH replacement therapy.Design and methodsAdopted siblings (girl and a boy) of unknown family history were referred for assessment of short stature (−4.5 and −5.6 SDS) at the age of 10 and 8.1 years respectively. They had delayed bone ages (6.8 and 4.5 years), normal GH peaks at stimulation tests, and severely reduced IGF1 concentrations (−3.5 and −4.0 SDS). Genetic analysis of theGH1gene showed a heterozygous P59S mutation at position involved in binding to GH receptor (GHR).ResultsIsoelectric focusing analysis of secreted GH in patient serum revealed the presence of higher GH-P59S peak compared with that of wt-GH. Furthermore, computational simulation of GH-P59S binding to GHR suggested problems in correct binding of the mutant to the GHR.In vitroGHR binding studies revealed reduced binding affinity of GH-P59S for GHR (IC50, 30 ng/ml) when compared with the wt-GH (IC50, 11.8 ng/ml) while a significantly decreased ability of the mutant to activate the Jak2/Stat5 signaling pathway was observed at physiological concentrations of 25–100 ng/ml.ConclusionsThe clinical and biochemical data of our patients support the diagnosis of partial bioinactive GH syndrome. The higher amount of GH-P59S secreted in their circulation combined with its impact on the wt-GH function on GHR binding and signaling may alter GHR responsiveness to wt-GH and could ultimately explain severe short stature found in our patients.