TYK2-STAT1 Pathway Positively Regulates BCL2 Gene Expression in T-Cell Acute Lymphoblastic Leukemia

Abstract 1470

To discover oncogenic pathways that are characteristically deregulated in T-cell acute lymphoblastic leukemia (T-ALL), we performed RNA interference screens both in T-ALL cell lines and primary specimens. We found that the JAK tyrosine kinase family member, TYK2, and its downstream effector, STAT1, are each required for the survival of T-ALL cells. To identify the effector molecules downstream of the TYK2-STAT1 pathway in T-ALL, we analyzed global gene expression profiles in TYK2-dependent T-ALL cell lines after silencing of TYK2 or STAT1. As expected, gene set enrichment analysis revealed that genes downregulated by TYK2 knockdown were generally also downregulated by knockdown of STAT1. Importantly, we found that expression of the anti-apoptotic gene BCL2 was significantly downregulated after silencing of both TYK2 and STAT1. Analysis by quantitative PCR of additional T-ALL cell lines revealed that silencing of TYK2 resulted in significant reductions of BCL2 mRNA expression in multiple TYK2-dependent cell lines. Expression of the wild-type but not the kinase-dead TYK2 protein was sufficient to rescue BCL2 protein expression and to prevent apoptosis after knockdown of endogenous TYK2, indicating that the tyrosine kinase activity of TYK2 is required for BCL2 upregulation. Similarly, expression of the shRNA-resistant wild-type STAT1A protein partially rescued BCL2 protein expression and prevented apoptosis, while a variant of STAT1A (Y701F) that is incapable of becoming phosphorylated on a requisite tyrosine residue did not rescue BCL2 levels. Taken together, our findings indicate that aberrant activation of a TYK2-STAT1 pathway upregulates BCL2 expression in T-ALL cells, and that the T-ALL cells develop pathway dependence, in that they require these sustained high levels BCL2 expression for survival.