CD70/CD27 Signaling Mediates Resistance of Chronic Myeloid Leukemia Stem Cells to Tyrosine Kinase Inhibitors By Compensatory Activation of the Wnt Pathway

The introduction of BCR/ABL-specific tyrosine kinase inhibitors (TKIs) a decade ago revolutionized chronic myelogenous leukemia (CML) therapy. However, disease-initiating leukemia stem cells (LSCs) in CML are resistant to TKIs despite BCR/ABL inhibition. Therefore, CML will ultimately relapse upon drug discontinuation. We have previously shown that blocking CD70/CD27 signaling targets LSCs by inhibiting the activation of the Wnt pathway. Here, we investigated a combination therapy of TKIs and CD70/CD27 blocking monoclonal antibodies in human and murine CML. We demonstrate that TKI-mediated BCR/ABL inhibition down-regulates miR-29, leading to increased expression of specificity protein 1 (SP1), a transcription factor with binding site in the CD70 promoter. In addition, TKI treatment reduced the expression of DNA methyltransferases resulting in de-methylation of the CD70 promoter. These combined effects resulted in CD70 up-regulation on LSCs, enhanced CD70/CD27 signaling and compensatory Wnt pathway activation. Combined BCR/ABL and CD70/CD27 inhibition synergistically reduced Wnt signaling and eradicated leukemia cells in vitro. More importantly, combination therapy effectively eliminated CD34⁺ CML stem/progenitor cells in murine xenografts and LSCs in a murine CML model. Therefore, TKI-induced CD70 up-regulation triggers CD70/CD27 signaling leading to compensatory Wnt activation. These findings identify an important targetable TKI resistance mechanism of CML LSCs and may lead to new therapeutic strategies to directly target LSCs to overcome treatment resistance.