EphA3 Is Expressed on Leukemia Stem Cells, and Eph/Ephrin Signalling Features in the Remodelling of the Leukemia Stem Cell Niche

EphA3 is a member of the Eph receptor tyrosine kinase family, the largest such family in the human genome. The bi-directional signalling of Eph receptors and their ligands (the ephrins) is responsible for correct migration and positioning of cells during development, as well as adult processes such as angiogenesis and neurogenesis. EphA3 was first discovered as a receptor expressed on the surface of LK63, a B-cell leukemia cell line. It is not expressed on normal human hematopoietic cells, but expression has been noted on the cell surface of leukemic blasts from a high percentage of patients with hematopoietic neoplasia of all lineages. It is particularly commonly expressed in myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML), where it is present in approximately 50% of patients. A therapeutic antibody against EphA3 has been developed and is in Phase II clinical trials for MDS and AML.

Consistent with data from human MDS patients, the NUP98-HOXD13 (NHD13) mouse model of MDS spontaneously expresses EphA3 on a percentage of cells from all populations throughout the hematopoietic compartment. The proportion of positive cells varies from mouse to mouse, but the proportion positively correlates with disease severity, indicating a possible causative relationship. Studies crossing the NHD13 transgene onto an EphA3 knockout background are ongoing and will definitively answer this question.

Recent studies have demonstrated the alteration (or “remodelling”) of the hematopoietic stem cell (HSC) micro-environment (or “niche”) by malignant hematopoietic cells, and the role that this plays in maintaining the leukemia stem cell (LSC) population. This self-reinforcing niche remodelling is critical to the maintenance of hematopoietic disease, particularly during a course of chemotherapy. These niche changes have also been shown to be sufficient to drive malignancy, and are therefore an attractive target for the design of disruptive therapeutics. EphA3 and other Eph/ephrin family members are expressed on normal mesenchymal stem/stromal cells (MSCs) and other stromal component cells of the HSC niche. EfnA5 (EphrinA5) is the high-affinity ligand for EphA3, and we observed an increased expression of EfnA5 on stromal cells in the NHD13 mice, indicating that the EphA3-EfnA5 interaction in these mice may facilitate LSC maintenance in the LSC niche. More broadly, it appears that Eph/Ephrin signalling may be a component of the remodelling of the niche to favour LSCs over normal HSCs. Possible indirect means by which this remodelling may occur include production of the cytokines CCL3 (decreased in NHD13 mice) or CXCL12 (variably decreased in NHD13 mice, and its receptor CXCR4 is increased on the cell surface of NHD13 HSCs). Imaging studies confirm preferential homing of EphA3-positive LSCs to niche locations, including co-localisation with CXCL12. It is therefore likely that the remodelled stem cell niche maintains stemness and leukemogenicity of the LSCs via interaction of the Eph/ephrin family members abnormally expressed on the LSCs and stromal cells, respectively.