Post Receptor Growth Hormone Insensitivity (GHI) Associated with Severe Immune Dysfunction Caused by a Novel Bioinactive STAT5b.

HORACIO M. DOMENÉ

Buenos Aires

Simposio; Genomic and Pharmacogenomics of Growth Disorders; 2011

Fundation Serono Symposia International

Post Receptor Growth Hormone Insensitivity (GHI) Associated with Severe Immune Dysfunction Caused by a Novel Bioinactive STAT5b. Growth hormone insensitivity (GHI) syndrome is a condition characterized by severe postnatal growth retardation associated with low insulin-like growth factor-I (IGF-I) and normal to elevated growth hormone (GH) serum levels. The first cases described by Laron et al in 1966 were later demonstrated to be linked to molecular defects in the GH receptor gene (GHR). More recently, specific defects downstream of the GH receptor have been identified. To date, the only signaling molecule that has been directly implicated with severe GHI phenotype is the Signal Transducer and Activator of Transcription 5b (STAT5b). Since the first report in 2003 of a patient with a homozygous missense mutation, p.Ala630Pro (p.A630P), in the STAT5B gene by Kofoed el al, only eight other STAT5b deficient patients have been described world-wide, carrying nonsense or frame-shift mutations within the STAT5B gene, all of which caused premature truncation of the STAT5b protein. The STAT5b protein is a member of the STAT family of transcription factors (STATs 1, 2, 3, 4, 5a, 5b and 6) that are activated by hormones, such as GH and prolactin, and by cytokines (e.g. interferon-gamma, IL-2, IL-7, IL-15). Extracellular binding of cytokines or GH to their specific receptors triggers the activation of receptor-associated Janus Tyrosine Kinase (JAK) which, subsequently, phosphorylates intracellular tyrosines on the receptors. Signaling molecules possessing SH2 domains, including STAT5b, are capable of binding these phosphotyrosine residues and become themselves tyrosine-phosphorylated by the JAKs. In the GH-GHR system, JAK2-mediated phosphorylation of STAT5b at Tyr699 results in STAT5b dimer formation through reciprocal Tyr699 binding via their SH2 domains. The STAT5b dimers then translocate to the cell nucleus where they act as transcription factors by binding to GH response elements in the promoter region of responsive genes, and interact with co-activating proteins to stimulate transcription of genes such as IGF1, IGFALS and IGFBP3.  Reconstitution studies demonstrated that expression of the p.Phe646Ser variant was less robust than wild-type. In contrast to the previously described SH2 missense STAT5B mutation, p.Ala630Pro, the p.Phe646Ser variant can still be phosphorylated in response to GH, as well as to interferon-gamma (IFN-g). However, neither the p.Ala630Pro nor phosphorylated p.Phe646Ser, could drive transcription. Although, both p.A630P and p.F646S mutations are located within the critical SH2 domain, while the p.A630P substitution, mapping within the âC sheet, completely abrogated both the phosphopeptide binding and subsequent transcriptional functions, the p.F646S variant, located within the âD’ sheet, could still bind phosphopeptides but was unable to drive transcription. Results of our functional analysis have shed light on the importance of the conserved motifs in âD’ strand of the SH2 domain for full transcriptional activity. This differential functional effect of p.F646S, a bioinactive STAT5b mutant, is unique amongst the STAT5B mutations described to date and may be correlated to the relatively mild pulmonary symptoms displayed by the patient. It is likely that identification of additional STAT5b deficient patients will result in more thorough genotype-phenotype correlations.