STAT3 SH2 Domain Aspartic Acid 661 Mutations Activate Immune Gene Programs

The conserved aspartic acid residue D661 within the STAT3 SH2 domain is a recurrent mutational hotspot in hematologic malignancies, including T-cell large granular lymphocytic leukaemia, myelodysplastic syndromes and acute lymphoblastic leukaemia. To define the functional consequences of distinct STAT3^D661 variants, we integrated computational, structural and in vitro and in vivo genetic approaches. AlphaMissense and PolyPhen-2 classified all four STAT3^D661 variants (D661Y, D661V, D661N and D661H) as pathogenic. ClinVar classified D661Y and D661V as variants of uncertain significance. AlphaFold 3-based modelling predicted that D661Y and D661V strongly promoted SH2-TAD-mediated dimerization, while D661N and D661H exerted weaker structural effects. Functional in vitro assays in Stat3-deficient T cells demonstrated a gain-of-function (GOF) hierarchy of the STAT3 variants (D661Y ≈ V > H > N) resulting in activation of canonical STAT3 target genes and immune transcriptional programs. In vivo, only STAT3^D661H mice were viable, displaying reduced CD4⁺ T cells, expansion of memory CD8⁺ T cells and enhanced immune gene expression. Collectively, our findings define a gradient of STAT3 D661 GOF variants, consistent with in vitro and in vivo experiments. D661Y and D661V mutants exhibited stronger transcriptional activity in T cells with impaired viability of mice carrying these variants.