Modulation of cell cycle by graded expression of MLL-AF4 fusion oncoprotein

Acute lymphoblastic leukemia ( ALLs) expressing MLL-AF4, the fusion product of t(4; 11)(q21; q23), show marked leucocytosis and extramedullary disease in multiple organs, respond poorly to chemotherapy and have poor prognosis. In vitro, leukemic cells with the t( 4; 11) show resistance to serum deprivation-induced or interferon gamma-regulated CD95-mediated apoptosis. In addition, t( 4; 11) cells have prolonged doubling time and lower percentage of cells in cycle compared to non-t(4; 11) B lineage cell lines. In this study, we examine the time- and level-dependent effects of MLL-AF4 conditional expression on cell cycle and differentiation of myelomonocytic leukemia cell line U937. By varying the concentration of tetracycline in growth media, we found that increasing levels of MLL-AF4 expression result in a progressive decrease in growth rate and fraction of S phase cells, paralleled by an increase in percentage of cells expressing CD11b. Our results demonstrate a dosage-dependent effect of MLL-AF4 fusion oncoprotein on cell cycle progression, with increasing expression levels resulting in the accumulation in G1, prolonged doubling time, both findings that might be responsible for the increased resistance to etoposide-mediated cytotoxicity. We propose the cell cycle control exerted by MLL-AF4 may be responsible of resistance to cell-death promoting stimuli in leukemia carrying the t( 4; 11) translocation.