Cell-Extrinsic Differentiation Block Mediated by EphA3 in Pre-Leukaemic Thymus Contributes to Disease Progression.

We recently characterised the NUP98-HOXD13 (NHD13) mouse as a model of T-cell pre-leukaemia, featuring thymocytes that can engraft in recipient animals and progress to T-cell acute lymphoblastic leukaemia (T-ALL). However, loss of this engraftment ability by deletion of Lyl1 did not result in any loss of leukemogenesis activity. In the present study, we observe that NHD13 thymocytes overexpress EPHA3, and we characterise thymocyte behaviour in NHD13 mice with deletion of EphA3, which show a markedly reduced incidence of T-ALL. Deletion of EphA3 from the NHD13 mice does not prevent the abnormal accumulation or transplantation ability of these thymocytes. However, upon transplantation, these cells are unable to block the normal progression of recipient wild type (WT) progenitor cells through the normal developmental pathway. This is in contrast to the EphA3+/+ NHD13 thymocytes, which block the progression of incoming WT progenitors past the DN1 stage. Therefore, EphA3 is not critical for classical self-renewal, but is essential for mediating an interaction between the abnormally self-renewing cells and healthy progenitors-an interaction that results in a failure of the healthy cells to differentiate normally. We speculate that this may orchestrate a loss of healthy cell competition, which in itself has been demonstrated to be oncogenic, and that this may explain the decrease in T-ALL incidence in the absence of EphA3. We suggest that pre-leukaemic self-renewal in this model is a complex interplay of cell-intrinsic and -extrinsic factors, and that multiple redundant pathways to leukaemogenesis are active.

The NUP98-HOXD13 fusion oncogene induces thymocyte self-renewal via Lmo2/Lyl1.

T cell acute lymphoblastic leukaemia (T-ALL) cases include subfamilies that overexpress the TAL1/LMO, TLX1/3 and HOXA transcription factor oncogenes. While it has been shown that TAL1/LMO transcription factors induce self-renewal of thymocytes, whether this is true for other transcription factor oncogenes is unknown. To address this, we have studied NUP98-HOXD13-transgenic (NHD13-Tg) mice, which overexpress HOXA transcription factors throughout haematopoiesis and develop both myelodysplastic syndrome (MDS) progressing to acute myeloid leukaemia (AML) as well as T-ALL. We find that thymocytes from preleukaemic NHD13-Tg mice can serially transplant, demonstrating that they have self-renewal capacity. Transcriptome analysis shows that NHD13-Tg thymocytes exhibit a stem cell-like transcriptional programme closely resembling that induced by Lmo2, including Lmo2 itself and its critical cofactor Lyl1. To determine whether Lmo2/Lyl1 are required for NHD13-induced thymocyte self-renewal, NHD13-Tg mice were crossed with Lyl1 knockout mice. This showed that Lyl1 is essential for expression of the stem cell-like gene expression programme in thymocytes and self-renewal. Surprisingly however, NHD13 transgenic mice lacking Lyl1 showed accelerated T-ALL and absence of transformation to AML, associated with a loss of multipotent progenitors in the bone marrow. Thus multiple T cell oncogenes induce thymocyte self-renewal via Lmo2/Lyl1; however, NHD13 can also promote T-ALL via an alternative pathway.