The Vascular Bone Marrow Niche Influences Outcome in Chronic Myeloid Leukemia

The endosteal bone marrow niche is known to protect leukemic stem cells (LSC) from chemotherapy, but the role of the vascular niche in the bone marrow microenvironment in leukemia is largely unknown. E-selectin, which is expressed on bone marrow endothelial cells, has been described to regulate the dormancy of normal hematopoietic stem cells (HSC) by increasing HSC proliferation (Winkler IG, 2012). In chronic myeloid leukemia (CML), one of the myeloproliferative neoplasias caused by the oncogene BCR-ABL1, E-selectin (Krause DS, 2014) and the adhesion molecule CD44, which is overexpressed on CML-initiating cells (Krause DS, 2006) and to which E-selectin binds (Videira PA, 2018), was shown to be essential for homing and engraftment of LSC. However, exact mechanisms of the interactions between BCR-ABL1, CD44 expression and interactions with the vascular niche are unclear.

Hypothesizing that E-selectin influences the cell cycle of leukemic stem cells (LSC) in CML leading to improved eradication of LSC in case of treatment with the tyrosine kinase inhibitor imatinib, we treated murine recipients of CML-initiating cells in the retroviral transduction/transplantation model with the E -selectin inhibitor GMI-1271 (uproleselan, GlycoMimetics, Inc.) and imatinib, considered standard of care in CML. This increased the survival of mice compared to animals treated with imatinib alone and decreased the engraftment of CML-initiating cells in this model, as well as in the majority of mice in a xenotransplantation model of NOD SCID IL2 receptor gamma knockout (NSG) mice injected with human CML cells and treated with uproleselan. Treatment of NSG mice with uproleselan also decreased the contact time of injected human CML cells with the bone marrow endothelium. As shown in in vitro and in vivo experiments treatment with GMI-1271 and non-adhesion to E-selectin increased the cell cycle of BCR-ABL1⁺ LSC with a concomitant increase in expression of the transcriptional regulator and protooncogene Scl/Tal1. SCL/TAL1 was shown to negatively regulate CD44 expression by binding to the CD44 regulatory element, which was increased in presence of imatinib. In addition, we demonstrated SCL/TAL1 to be an indirect phosphorylation target of BCR-ABL1 and a negative transcriptional regulator of CD44 expression.

In confirmation of our findings in mice, we also showed a negative correlation between SCL/TAL1 and CD44 expression in leukocytes of human patients with CML and that increased SCL/TAL1 expression is associated with decreased probability of relapse and increased relapse-free survival in human CML patients after allogeneic hematopoietic stem cell transplantation.

In summary, inhibition of E-selectin in murine CML may lead to 'non-adherence' of LSC to the vascular niche and improved eradication by imatinib. This is likely due to an increase in cell cycle and an increase of Scl/Tal1 expression, which is shown to be regulated by BCR-ABL1. These data connect the adhesion of LSC in CML to the vascular niche via CD44 with the regulation of CD44 expression by SCL/TAL1 and BCR-ABL1, offering a potential new avenue for treatment.