Leukemic Transformation of Primary Human CD34+ Progenitor Cells by MLL-AF9 Recapitulates the Phenotype and Gene Expression Profile of MLL Rearranged AML.

The t(9;11)(p22;q23) results in the creation of an MLL-AF9 fusion gene and is observed in 27% of 11q23 leukemias. t(9;11) is commonly associated with an M5 monocytic leukemia and patients harboring this translocation are included in the intermediate or poor cytogenetic risk groups, with an average 42% overall survival at five years. Using primary human hematopoietic progenitor cells, we have developed a system that closely mimics the MLL-AF9 associated disease observed in human acute myeloid leukemia (AML). Injection of MLL-AF9 expressing cells into sublethally-irradiated NOD/SCID mice produced an aggressive FAB-M5-like leukemia that infiltrated the spleen and liver and induced death with an average latency of 8 weeks. Serial transplantation of leukemic cells recapitulated the disease with a similar latency. MLL-AF9 expression conferred the ability to grow indefinitely in culture, promoted an accumulation of cells resembling monoblasts and was associated with an increase in telomerase activity. Mono- or oligoclonal outgrowth arose in vitro as well as in vivo. Numeric karyotypic changes or normal karyotypes, in the absence of structural rearrangements, additions or deletions were detected in cultured cells as well as samples from leukemic mice. Principal Component Analysis of genome-wide gene expression data from MLL-AF9 expressing cultured cells revealed a gene expression signature that closely mirrors that observed in MLL-rearranged leukemic cells from patient samples. Furthermore, training of a (linear) Support Vector Machine with gene expression data from patient samples resulted in the correct classification of all MLL-AF9-expressing cell cultures as MLL-rearranged leukemia. We conclude that expression of MLL-AF9 in a normal human hematopoietic progenitor is sufficient to recapitulate many key aspects of the clinical disease. This model will yield valuable insights into the molecular pathogenesis of MLL fusion driven leukemia and will serve as a powerful in vivo model for testing much needed novel therapeutic targets.