ABSTRACT
The translocation t(11;19) is a recurrent feature of a subgroup of acute leukemias occurring in infants. This event fuses the genes MLL and ENL and creates the leukemogenic oncoprotein MLL-ENL. We studied the effect of retroviral MLL-ENL expression in primary mouse hematopoietic cells and show here that MLL-ENL requires the oncoprotein Myc to establish a reversible differentiation arrest of a myelomonocytic precursor population. MLL-ENL-transduced cells proliferated as immature myeloid cells in the presence of interleukin 3. The addition of granulocyte colony-stimulating factor reversed the maturation block set by MLL-ENL and induced the development of mature granulocytes and macrophages accompanied by growth arrest. Gene expression analysis indicated a down-regulation of the proto-oncogene c-myc and of several c-myc target genes during granulocyte colony-stimulating factor-mediated differentiation. The role of c-myc in the MLL-ENL transformation pathway was tested by modulating the effective Myc protein concentrations in MLL-ENL transduced cells. Cotransduction of dominant-negative Myc neutralized the MLL-ENL effect and precluded transformation. In contrast, constitutive expression of Myc cooperated with MLL-ENL and caused the transformation of a cell population with an irreversible maturation arrest.
Subject(s)
Cell Transformation, Neoplastic/genetics , Genes, myc/physiology , Hematopoietic Stem Cells/cytology , Oncogene Proteins, Fusion/physiology , Animals , Apoptosis/drug effects , Apoptosis/physiology , Cell Differentiation/drug effects , Cell Differentiation/genetics , Cell Differentiation/physiology , Cell Transformation, Neoplastic/pathology , Gene Expression Regulation , Granulocyte Colony-Stimulating Factor/pharmacology , Interleukin-3/pharmacology , Leukemia/genetics , Leukemia/pathology , Mice , Mice, Inbred BALB C , Myeloid-Lymphoid Leukemia Protein , Oncogene Proteins, Fusion/biosynthesis , Oncogene Proteins, Fusion/genetics , Proto-Oncogene Mas , Proto-Oncogene Proteins c-myc/biosynthesis , Proto-Oncogene Proteins c-myc/physiology , Retroviridae/genetics , Transduction, GeneticABSTRACT
The translocation t(11;19) is frequently found in acute leukemia in infants. This event truncates the proto-oncogene MLL and fuses the 5' end of MLL in frame with the ENL gene. ENL contributes a crucial protein-protein interaction domain to the resulting oncoprotein MLL-ENL. Here we show by yeast two-hybrid assays, GST-pull-down experiments and in a far western blot analysis that this domain is necessary and sufficient to recruit a novel member of the human Polycomb protein family (hPc3). hPc3 RNA was detected throughout the human hematopoietic system. Similar to other Polycomb proteins hPc3 acts as a transcriptional repressor. The ENL-hPc3 interaction was verified by mutual co-precipitation of the proteins from cell extracts. ENL and hPc3 tagged with fluorescent proteins co-localized in living cells in a nuclear dot pattern. An internal region of hPc3 was responsible for binding to ENL. Finally, hPc3 binds to the C-terminus of AF9, another common MLL fusion partner. The recruitment of a repressive function by ENL opens up a new insight into a possible mechanism of leukemogenesis by the fusion protein MLL-ENL.
