Two mutations associated with leukemia that change a single tyrosine residue in the STAT5B SH2 domain have opposite effects on the transcriptional programs controlling hematopoiesis [RNA-seq]

Normal hematopoiesis is regulated by various cytokines via the JAK-STAT signaling pathway. Gene deletion studies in mice have demonstrated the crucial role of STAT5B in the proliferation, differentiation, and survival of immune cells. However, the physiological effects of human STAT5B mutations remain poorly understood. Mutations within the SH2 domain of STAT5B have been identified in leukemia patients. In this study, we investigate two specific variants: one that substitutes tyrosine 665 with phenylalanine (STAT5BY665F) and another that substitutes tyrosine 665 with histidine (STAT5BY665H). We introduced these two mutations into the mouse genome and observed distinct and divergent impacts on immune cell function. The STAT5BY665F mutation enhanced STAT5B activity and the establishment of transcriptional enhancers and genetic programs. Conversely, the STAT5BY665H mutation was largely inactive, failing to induce the interleukin-regulated enhancer landscape and gene expression. Both mutations modified immune cell profiles, inducing features characteristic of autoimmune disease. In STAT5BY665F mice, there was an expansion of CD8+ and regulatory CD4+ T cells, whereas STAT5BY665H mice didn’t. Both mutant strains exhibited skin abnormalities; however, only the STAT5BY665F mice developed progressive dermatitis. Introduction of equivalent mutations into STAT5A did not significantly impact hematopoiesis. However, mice carrying mutations in both STAT5A and STAT5B displayed additive effects with highly elevated CD4+ and CD8+ T cells. Our findings demonstrate that different naturally occurring missense mutations identified in leukemia patients, changing a single amino acid within the SH2 domain, can either activate or inactivate STAT5B and thereby impact T cell populations.