A gain-of-function mutation in STAT6.

Interleukin-4 (IL-4) is a cytokine that plays a crucial role in the pathophysiology of asthma and allergic diseases. IL-4-induced gene expression is largely mediated through the activation of the latent transcription factor STAT6. We identified a STAT6 mutant (STAT6VT)) that is activated independently of IL-4 stimulation. STAT6VT carries two amino acid changes in the SH2 domain that affect the overall structure and stability of the monomeric and dimeric protein. When overexpressed in mammalian cells, STAT6VT undergoes tyrosine phosphorylation, binds DNA, and activates transcription in the absence of IL-4 stimulation. Using the Jak1- and Jak3-deficient fibroblast line U4A, we demonstrate that phosphorylation is mediated by an IL-4-independent tyrosine kinase that is not able to activate the wild-type STAT6 protein. These results suggest that small changes in STAT6 could result in hyperactivation of the protein and constitutive expression of STAT6-dependent genes. Such a mutation, if found in vivo, could cause genetic predisposition for atopic diseases.