Expansion of BCR-ABL Positive Leukemic Stem Cells Is Dependent on Hedgehog Pathway Activation.

Chronic Myeloid Leukemia is characterized by the expansion of a leukemic stem cell clone carrying a Philadelphia translocation, which outgrows the non-malignant haematopoietic stem cells. Here we show that BCR-ABL directly enhances self renewal of haematopoietic stem and progenitor cells by activating the hedgehog signaling pathway through upregulation of SMO. Induction of hedgehog signaling in regular bone marrow by overexpression of SMO or loss of PTCH induces expansion of short term repopulating cells, but also enhances the expansion of long term repopulating haematopoietic stem cells. Pharmacological inhibition of SMO activity in BCR-ABL positive bone marrow cultures induced apoptosis in BCR-ABL positive differentiated cells and inhibited colony forming capacity of BCR-ABL positive self renewing cells in vitro. While loss of SMO in regular haematopoiesis only impacts the reconstitution of CD8 positive T-cells and short term repopulating cells (transplantation of SMO−/− fetal liver cells), the development of BCR-ABL positive leukemias was impaired and retransplantability abolished in the absence of SMO expression. Combined treatment of leukemic mice with AMN107 (Abl inhibitor) and Cyclopamine (SMO inhibitor) lead to a reduction of Bcr-Abl positive self-renewing cells in vivo and enhanced the time to relapse more than 3-fold compared to mice treated with AMN107 alone. Thus we conclude that BCR-ABL enhances the self renewal of leukemic stem cells through intrinsic activation of hedgehog signaling by upregulation of SMO. Therefore Hh pathway inhibition alone or in combination with Abl inhibitors could serve as an effective therapeutic strategy to reduce the malignant stem cell pool in BCR-ABL positive leukemias.