Hyperactive STAT5 Hijacks T-Cell Receptor Signaling and Drives Immature T-Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive immature T-cell cancer. Mutations in IL7RA were analyzed genetically, but downstream effector functions such as STAT5A/B hyperactivation are poorly understood. Here, we studied the most frequent and clinically challenging STAT5BN642H driver in T-cell development and immature T-cell cancer onset and compared it to STAT5A hyperactive variants in transgenic mice. Enhanced STAT5 activity caused disrupted T-cell development and promoted an early T-cell progenitor (ETP)- ALL phenotype with upregulation of genes involved in T-cell receptor (TCR) signaling, even in absence of surface TCR. Importantly, TCR pathway genes were overexpressed in human T- ALL and mature T-cell cancers and activation of TCR pathway kinases was STAT5-dependent. We confirmed STAT5-binding to these genes using ChIP-seq analysis in human T-ALL cells, which were sensitive to pharmacologic inhibition by dual STAT3/5 degraders or ZAP70 tyrosine kinase blockers in vitro and in vivo. We provide first genetic and biochemical proof that STAT5A and STAT5B hyperactivation can initiate T-ALL through TCR pathway hijacking and suggest similar mechanisms for other T-cell cancers. Thus, STAT5 or TCR component blockade are targeted therapy options, particularly in patients with chemo-resistant clones carrying STAT5BN642H.