The t(6;9) Associated DEK/CAN Fusion Protein Does Not Block Differentiation of Early Hematopoietic Stem Cells but Increases Their Stem Cell Capacity.

Leukemia-specific translocations such as t(15;17), t(11;17), or t(8;21) lead to the expression of aberrant fusion proteins (FP) such as PML/RAR, PLZF/RAR and AML-1/ETO. These FP induce and maintain the leukemic phenotype by blocking terminal differentiation of early hematopoietic progenitors and by interfering with the biology of the leukemic counterpart of the hematopoietic stem cells. The t(6;9) with its DEK/CAN FP is of particular interest because it is more frequent in young patients and associated with a poor prognosis. The t(6;9)-DEK/CAN fusion occurs with an incidence of 1–5% in adult patients with AML and in most of the cases t(6;9)-positive AML is classified as FAB-M2 or M4. In contrast to other FP the overexpression of DEK/CAN does not block VitD3-induced differentiation in U937 cells. These findings called in question the leukemogenic potential of DEK/CAN. To further disclose the role of DEK/CAN in leukemogenesis we investigated its effect on the biology of early hematopoietic stem cells (HSC) in comparison to PML/RAR. Therefore we retrovirally transduced Sca1+/linmurine HSC (SL cells) with DEK/CAN and PML/RAR and performed both differentiation and stem cells assays (morphology, surface marker expression, replating efficiency, colony forming unit - spleen - CFU-S). Here we show that i.) DEK/CAN in contrast to PML/RAR was apparently unable to block terminal differentiation of SL cells as revealed by morphology and the expression of differentiation-specific surface markers; ii.) DEK/CAN slightly increased the replating efficiency of SL cells, but did not reach the level of PML/RAR with a reduced number of either colony forming units as plating rounds with respect to PML/RAR; iii.) the increased replating efficiency was related to an increased stem cell capacity as revealed by the fact that in contrast to mock infected control cells DEK/CAN-positive cells gave origin to a positive CFU-S assay even after one or two plating rounds in semi-solid medium similar to PML/RAR. Apparently DEK/CAN seems to share with other FP the capacity to increase the self renewal of HSC, but not that to block terminal differentiation. These findings strongly suggest that the effect of DEK/CAN is limited to a very small subset of cells within the stem cell compartment which might represent the origin of a DEK/CAN-positive leukemia. Actually the capacity of DEK/CAN to give origin to leukemia in vivo is under investigation.