MLL/AF10(OM-LZ) Transformed Cells Induce Myeloproliferative Disease-Like Myeloid Leukemia in the Sublethally Irradiated Mice

Chromosomal translocation t(10;11)(p12;q23) resulting in the fusion of MLL gene with AF10 is seen in patients with acute leukemias, especially in acute monoblastic leukemia. The OM-LZ domains of AF10 have been shown to be sufficient for immortalization when fused with 5′MLL, however, the leukemogenesis of MLL/AF10(OM-LZ) is not clear. By retrovirally transducing MLL/AF10(OM-LZ) fusion gene into the murine bone marrow cells, we established six MLL/AF10(OM-LZ)-immortalized cell lines in medium containing IL3, IL6, SCF and GM-CSF. The cytological and immunophenotypic studies showed that all the cell lines were consisted of immature blast-like cells and monocyt/macrophage cells, except that line MA6 also contained about 2% mature granulocytes. The percentage of mature granulocytes in line MA6 raised to 20% upon treatment with G-CSF. Virus integration site (VIS) analysis demonstrated that line MA6 had a VIS at PD-L2 gene. Whether interruption of PD-L2 gene plays a role in releasing of the granulocytic differentiation blockage caused by MLL/AF10(OM-LZ) is under investigation. Nevertheless, line MA6 and the other immortalized cell lines all induced a myeloproliferative disease (MPD)-like myeloid leukemia when they were transplanted into the sublethally-irradiated syngeneic mice. Further studies revealed that the excessive mature granulocytes in the peripheral blood of leukemic mice were generated from the recipient hematopoietic stem/progenitor cells. On the other hand, the hyperleukocytosis found in line MA6 leukemic mice may be partly attributed to the expansion and maturation of the immortalized cells. RT-PCR analysis revealed that the MLL/AF10(OM-LZ)-immortalized cells expressed mRNAs encoding colony-stimulating factors and inflammatory cytokines. The latter were known to be able to stimulate or up-regulate endothelial cells, fibroblasts, T-lymphocytes, granulocytes and monocytes to produce GM-CSF, G-CSF or M-CSF, and, therefore, indirectly regulate myelopoiesis. Our results demonstrated that although the MLL/AF10(OM-LZ)-immortalized cells could not terminally differentiated into granulocytes, they could still induce a MPD by expression and secretion of the colony-stimulating factors and cytokines to over-stimulate the myelopoiesis of the recipient hematopoietic stem/progenitor cells.