Ron G. Rosenfeld

GH insensitivity (GHI) is a condition characterized by pronounced IGF-I deficiency and severe short stature. We previously identified a novel compound heterozygous GH receptor (GHR) mutation, GHR:p.R229H/c.899dupC, in a patient presenting with GHI. The heterozygous p.R229H (prepeptide) variant was previously associated with GHI despite a lack of adequate functional studies. The novel heterozygous GHR:c.899dupC variant affects the critical JAK2-binding Box 1 region of the GHR intracellular domain; the duplication predicted a frameshift and early protein termination. The individual and synergistic effect(s) of the p.R229H and c.899dupC mutations on GHR function(s) were evaluated in reconstitution studies. The recombinant human GHR (hGHR):p.R229H variant was readily expressed, and unexpectedly, GH-induced signal transducer and activator of transcription 5b (STAT5b) phosphorylation was comparable to that induced by wild-type hGHR. The truncated, immunodetected hGHR:c.899dupC variant, in contrast, was unresponsive to GH. To mimic a compound heterozygous state, the two variants were coexpressed, and strikingly, the presence of the hGHR:c.899dupC effectively abolished the GH-induced STAT5b activities that were observed with hGHR:p.R229H alone. Furthermore, hGHR:c.899dupC dose-dependently reduced the GH-induced STAT5b activities associated with hGHR:p.R229H. This dominant negative effect was also observed when hGHR:c.899dupC was coexpressed with wild-type hGHR. The p.R229H variant, contrary to an earlier report, appeared to function like wild-type GHR and, therefore, is unlikely to cause GHI. The c.899dupC variant is a novel dominant negative mutation that disrupted normal GHR signaling and is the cause for the GHI phenotype of the reported patient.

The human growth hormone receptor (GHR), upon binding GH, induces JAK2-mediated phosphorylation of GHR intracellular tyrosines, which then recruit components of multiple signaling pathways, including STAT5b, with subsequent regulation of the growth promoting insulin-like growth factor (IGF)-1. Mutations found in the GHR gene are the most common cause of growth hormone insensitivity (GHI), a condition characterized by pronounced IGF-I deficiency (IGFD) and severe short stature. We had previously identified a novel compound heterozygous GHR mutation in a patient presenting with GHI (1). One of the heterozygous mutations was R211H (mature GHR), a variant lacking adequate functional studies but associated with GHI (2). The second heterozygous mutation was a novel duplication of a nucleotide in exon 9 (c.899dupC), within the critical JAK2-binding Box 1 region of the GHR intracellular domain; the duplication predicted a frameshift and early protein termination.

Article published by the Endocrine Society Article published by the Endocrine Society

Article published by the Endocrine Society Article published by the Endocrine Society

The mouse Stat5b−/− knock-out model and rare human STAT5B gene mutations identified to date have demonstrated the critical involvement of STAT5b in both growth and immunity. Patients with STAT5b deficiency show GH insensitivity (GHI) and a variable degree of immune dysregulation which results in a broad spectrum of skin and respiratory infections.

The myriad actions of growth hormone (GH) are still incompletely understood, despite decades of research. Although it is a major regulator of post-natal growth in mammals, much of its effects on skeletal growth are recognized to be mediated indirectly, through the stimulation of production of insulin-like growth factor (IGF)-I, as well as some of the major serum carrier proteins for IGF-I and -II, such as IGF-binding protein-3 (IGFBP-3) and the acid-labile subunit (ALS). The regulation of IGF-I production by GH appears to be mediated entirely by signalling through the Janus kinase (JAK) 2 pathway, via the phosphorylation of the transcription factor, signal transducer and activator of transcription (STAT) 5b. GH also signals, however, through additional pathways that are likely to be critical to the metabolic actions of GH.

Decades of experience with growth hormone (GH) therapy have indicated considerable variability in responsiveness to therapy, even within single diagnostic categories, such as GH deficiency, Turner syndrome, intrauterine growth retardation and idiopathic short stature. It is likely that the major explanation for such variability lies in the genetic composition of the patient, including mutations of genes participating in the GH-insulin growth factor I cascade and genetic polymorphisms. Future studies of pharmacogenomic and pharmacoproteomic markers may allow us to better predict and categorize responsiveness to therapy.

It has long been recognized that growth failure encompasses a diverse spectrum of underlying pathophysiological processes, a characteristic that has significantly impacted both the diagnosis and management of growth disorders. This problem is exacerbated by inherent difficulty in distinguishing the borders between the 'normal range' for stature and defined abnormal growth. Evaluation of GH secretion has proven problematic, both diagnostically and prognostically, except in cases of unequivocal GH deficiency. Measurement of serum concentrations of IGF-I, IGFBP-3, and ALS have proven useful in the assessment of GH responsiveness and have contributed to the concept of primary and secondary 'IGF deficiency'. Nevertheless, there is great need for biochemical and/or molecular biomarkers that could: i) predict short- and long-term responsiveness to various therapeutic modalities, such as GH and IGF-I, and ii) predict potential risk for adverse effects of therapy. Candidate proteins and genes identified to date, and worthy of further evaluation, include IGF-I, IGF-I receptor, GH receptor and its variants (such as exon 3-deleted GHR), STAT5b and short stature homeobox. Proteomic analysis of serum samples pre- and post-treatment and correlation with clinical responsiveness should provide additional candidate biomarkers. Molecular studies to consider include: i) sequencing and mutation analysis of known genetic components of the GH-IGF axis; ii) evaluation of single nucleotide polymorphisms of candidate genes; and iii) identification of new candidate genes. It is proposed that the major target population to study is that of children currently labeled as idiopathic short stature (ISS). These children can be divided into those with: i) primary IGFD, where the focus should be on genes related to GHR, GHR signaling, and IGF-I gene expression, or ii) no IGFD (i.e. 'true ISS'), where the focus should be on genes related to IGFR, IGF signaling and epiphyseal growth.

Patients with GH insensitivity, typically resulting from mutations affecting the GH receptor (GHR), GHR signaling cascade, or the IGF-I gene, are, generally, unresponsive to GH therapy. Beginning in the 1990s, clinical trials of IGF-I administration in such patients demonstrated both short- and long-term efficacy, although not to the degree observed with GH treatment of naïve GH-deficient patients. Adverse effects, including hypoglycemia, lymphoid overgrowth, benign intracranial pressure, and coarsening of facial features, have been observed, but, in general, have proven to be transient. As interest in the potential efficacy of IGF-I treatment for children currently labeled as idiopathic short stature increases, it will be important to have controlled clinical trials of GH, versus IGF-I versus combination, GH + IGF-I.