Identification of genes and translocations involved in human leukemia, as well as classification and clustering by gene arrays, have greatly evolved in the past years. However, the mechanisms of human leukemogenesis remain to be elucidated and the failure to develop an in vivo model where primary human hematopoietic cells are transformed into leukemic cells represents a significant limitation.

Using a retrovirus encoding the oncogene MLL-ENL resulting from the t(11;19)(q23;p13.3) translocation found in acute myeloid leukemias (AML) as well as in acute lymphoblastic leukemias (ALL) of B or T cell origin, we infected lineage-negative cord blood cells and injected those cells into sub-lethally irradiated NOD/SCID mice. 15 to 20 weeks after injection, all the mice developed an aggressive pro-B acute lymphoblastic leukemia characterized by immature B cells (CD10+, CD19+, CD20−, IgD−, IgM−) involving more than 90% of bone marrow. Spleen and thymus were increased in size and infiltrated with >90% leukemic cells. Furthermore, analysis of the lungs and liver showed significant infiltration of these organs. Transplantation of leukemic cells from primary mice to secondary recipients was able to recapitulate the disease with the same phenotype and the same organ involvement in a shorter period of time.

If MLL-ENL transduced cells are grown in suspension culture with IL-3 and SCF, there is massive proliferation of cells blocked in differentiation along the monocytic lineage. In contrast to untransduced cells, colony-forming progenitors were maintained long term in these cultures and could be serially replated, suggestive of an enhanced capacity for self-renewal. After 50 to 70 days in culture, these cells were injected in NOD/SCID mice and mice were analyzed after 12 to 15 weeks. Monoblastic cells were engrafted in the bone marrow and spleen with the same phenotype of the cultured cells (CD33+, CD11b+, CD15+, HLA DR+). These cells were able to engraft secondary and tertiary recipients formally demostrating increased self-renewal capacity of the transformed stem cell. In a limited number of primary mice, transplanted with high cell doses, AML developed at 15 weeks post-transplant.

To our knowledge, these results provide the first in vivo model where human hematopietic stem/progenitor cells are transformed into leukemia. Remarkably, depending on the cellular environment, MLL-ENL can induce ALL or AML in primary cells as a sole genetic event, although we cannot rule out the spontaneous acquistion of additional co-operating genetic or epigenetic abnormalities. This model provides a significant step forward to understand the mechanisms involved in human leukemogenesis.

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