BCR-ABL-mediated recruitment of STAT5 revealed by ChIPseq

Even in the era of ABL tyrosine kinase inhibitors, eradication of chronic myeloid leukemia (CML) stem cells still remains to be a prerequisite for the complete cure of the disease. Interferon-α (IFNα), which has been used long for the treatment for CML-chronic phase, are now being re-evaluated. However, the molecular mechanism involved in the action of IFNα on CML stem cells have not been elucidated yet. In this study we have found that IFNα upregulates CCAAT/Enhancer Binding Protein β (C/EBPβ), a transcription factor required for demand-driven granulopoiesis, through activation of STAT1 and STAT5 in BCR-ABL-expressing cells. Activated STAT1 and STAT5 were recruited to the newly identified 3’ distal enhancer region of Cebpb, which contains tandemly aligned interferon-g activated sequences (GAS). Genome editing-mediated repression or deletion of the GAS elements significantly abrogated the IFNα-dependent upregulation of C/EBPβ. IFNα induces differentiation and exhaustion of CML stem cells both in vitro and in vivo in a C/EBPβ-dependent manner. In addition, IFNα upregulates C/EBPβ and induces exhaustion of CD34+ CML stem cells obtained from CML patients. Collectively, these data clearly show that C/EBPβ is a critical factor in IFNα signaling that induces differentiation and exhaustion of CML stem cells. Examination of STAT5 binding in 3 conditions in a cell line