Growth hormone and noonan syndrome: update in dysfunctional signaling aspects and in therapy for short stature

Short stature is a frequent feature of Noonan syndrome (NS), a disease caused by mutations of genes encoding components of the Ras/mitogen-activated protein kinases (MAPK) signalling pathway. To date numerous patients have been treated with growth hormone (GH) in various countries. However this treatment is still controversial, as its efficacy is a matter of debate. The final height gain of GH therapy represents 5 to 10 cm, at best, which is disappointing considering the length and burden of the treatment. The reasons explaining this lack of efficiency are poorly understood and they seemed to involve a waning effect after the first year of GH treatment or acceleration of bone maturation in patients on GH. Moreover, GH therapy raises questions about its safety, especially in subjects with NS who are at risk for cardiac defects and malignancies. Cases of severe adverse effects were described, yet too sporadically to conclude that they were certainly caused by GH therapy. Novels insights in understanding the dysfunction of GH-dependent processes in NS were recently reported and this manuscript aims at reviewing the latest advances. Clinical data suggested that growth retardation could be caused by a partial postreceptor resistance to GH, an impaired sensitivity to GH which could also explain the modest efficiency of GH therapy in NS patients. Similar observations were also obtained in a NS mouse model harboring a mutation in the gene encoding the tyrosine phosphatase Shp2. In the mouse, the mechanism of GH resistance is thought to involve upregulation by mutated Shp2 of GH-induced Ras/MAPK, a signaling pathway which seemed to play a negative regulatory role in the production of GH mediators involved in bone growth. Despite these recent finding, the underlying mechanisms of growth retardation and GH therapy in NS remain to be further explored in order to improve treatment for short stature.