Induced pluripotent stem cell model of chronic myeloid leukemia revealed olfactomedin 4 as a novel therapeutic target in leukemia stem cells

The definitive cure of leukemia requires identification of novel therapeutic targets to eradicate leukemia stem cells (LSCs). However, rarity of LSCs within the pool of malignant cells remains major limiting factor for their study in humans. Reprogramming somatic cells to pluripotency allows generation of induced pluripotent stem cells (iPSCs) that are capable of self-renewal and differentiation toward derivatives of all three germ layers, including blood. Because iPSCs capture the entire genome of diseased cell, they have been already used successfully to model human genetic diseases. Recently, we generated transgene-free iPSCs from bone marrow mononuclear cells from the patient in chronic phase of chronic myeloid leukemia (CML). iPSCs generated from CML patient captured the entire genome of neoplastic myeloid cells in this patient, including the unique 4-way translocation between chromosomes 1, 9, 22, and 11. Following differentiation on OP9, CML-iPSCs generated lin-CD34+CD45+ primitive hematopoietic cells with HSC phenotype. These cells displayed many unique features of CML LSCs, including expression of Bcr/Abl, high ALDH and rhodamine efflux activity, LTC-IC potential, adhesion defect, rapid cytokine independent proliferation, and imatinib resistance. Following differentiation in vitro these cells loss CD34 expression and became sensitive to imatinib. Comparative analysis of gene expression in lin-CD34+CD45+ cells obtained from normal bone marrow- and CML-iPSCs treated and not treated with imatinib identified olfactomedin 4 (OLFM4) as the top-ranking gene induced by imatinib. OLFM4 knockdown using siRNA decreased CFCs and increased apoptosis in CML-iPSCs-derived lin-CD34+CD45+, as well as parental somatic LSCs and LSCs from other unrelated CML patients. Overall this study successfully demonstrates for the first time the validity of iPSC model for CML LSC studies and identification of novel targets for drugs in CML LSCs.