Supplementary Material for: Short Stature in a Boy with Multiple Early-Onset Autoimmune Conditions due to a STAT3 Activating Mutation: Could Intracellular Growth Hormone Signalling Be Compromised?

<i>Background:</i> Germline <i>STAT3</i> gain-of-function (GOF) mutations cause multiple endocrine and haematologic autoimmune disorders, lymphoproliferation, and growth impairment. As the JAK-STAT pathway is known to transduce the growth hormone (GH) signalling, and STAT3 interacts with STAT5 in growth regulation, we hypothesised that short stature in <i>STAT3</i> GOF mutations results mostly from GH insensitivity via involving activation of STAT5. <i>Case Report:</i> A boy with a novel <i>STAT3</i> c.2144C>T (p.Pro715Leu) mutation presented with short stature (–2.60 SD at 5.5 years). He developed diabetes mellitus at 11 months, generalised lympho­proliferation, autoimmune thyroid disease, and immune bicytopenia in the subsequent years. At 5.5 years, his insulin-like growth factor-1 (IGF-I) was 37 µg/L (–2.22 SD) but stimulated GH was 27.7 µg/L. Both a standard IGF-I generation test (GH 0.033 mg/kg/day sc; 4 days) and a high-dose prolonged IGF-I generation test (GH 0.067 mg/kg/day sc; 14 days) failed to significantly increase IGF-I levels (37–46 and 72–87 µg/L, respectively). The boy underwent haematopoietic stem cell transplantation at 6 years due to severe neutropenia and massive lymphoproliferation, but unfortunately deceased 42 days after transplantation from reactivated generalised adenoviral infection. <i>Conclusions:</i> Our findings confirm the effect of <i>STAT3</i> GOF mutation on the downstream activation of STAT5 resulting in partial GH insensitivity.