Autosomal dominant hyper IgE syndrome (AD-HIES) is a primary immunodeficiency caused by heterozygous dominant negative mutations in the gene encoding Signalling transducer and activator of protein 3 (STAT3). These mutations lead to a multisystemic disorder, which is characterized immunologically by susceptibility to infection with mucocutaneous pathogens such as Candida and Staphylococcus and Streptococcus, a failure to generate a normal humoral immune response and an increased incidence of B-cell lymphoma.
In response to cytokines such as IL-6, IL-10 and IL-21, STAT3 normally becomes phosphorylated, undergoes multimerisation and then translocates to the nucleus where it binds to consensus sites in DNA to regulate gene expression. In AD-HIES patients, mutations have been found in the DNA binding domain, SH2 domain and transactivation domain of STAT3. However there is limited knowledge regarding how these mutations impact on the biochemistry and function of STAT3. We have addressed this by using B-cell lines derived from AD-HIES patients or cell lines transfected with mutant forms of STAT3, and determining the abilities of the mutant STAT3 proteins to undergo phosphorylation, dimerisation, nuclear translocation, and bind to DNA.
We found that STAT3 phosphorylation, was unaffected by mutations in the DNA binding domain, however this was reduced by mutations in the transactivation domain and some, but not all, SH2 domain mutations. However, the mutations did affect translocation of phospho-STAT3 from the cytoplasm into the nucleus, and also diminished DNA binding. This study reveals the biochemical mechanism for impaired STAT3 function in AD-HIES and provides insights into the functional requirements for STAT3 in normal immune responses.