Context: A natural missense mutation in the signal transducer and activator of transcription 5b (STAT5b) gene was recently identified in association with a female patient presenting with severe growth failure and immune dysfunction. The mutation results in an alanine to proline substitution at residue 630 (A630P) in the src-homology-2 (SH2) domain, a region essential for docking of STATs to phospho-tyrosines on activated receptors, for STAT dimerization, and for stabilization of phospho-STAT-DNA interactions.

Objective: The purpose of this study is to explore the molecular mechanisms underlying the Growth Hormone Insensitivity and IGF-I deficiency caused by the A630P mutated STAT5b.

Results: In reconstitution experiments employing HEK293 cells, both growth hormone (GH) and interferon- (IFN) were unable to activate mutant STAT5b(A630P), as demonstrated by lack of immuno-detectable phospho-tyrosyl-STAT5b(A630P) and inability to drive luciferase-reporter activity. However, the Src family of non-receptor kinases (constitutively active v-src and EGF-induced c-src) tyrosine-phosphorylated STAT5b(A630P). The v-src-induced phospho-STAT5b(A630P) translocated to the nucleus, but, unlike wild-type Stat5b, was unable to bind DNA.

Conclusions: The A630P mutation disrupts the SH2 architecture such that: (1) mutant STAT5b most likely cannot dock to phospho-tyrosines on ligand-activated receptors; and (2) stable interactions with DNA are prevented. Since STAT5b(A630P) is an inefficient signal transducer and transcription factor, the detrimental impact on signaling pathways important for normal growth and immunity explain, in part, the complex clinical phenotype of GH insensitivity and immune dysfunction.