Modelling of leukemic fusion genes in human embryonics stem cells: integrated genomic and biological sudy of the effects of the fusion gene MLL-AF9

Resumen

Leukemias harboring the MLL-AF9 fusion gene are associated with a dismal prognosis and their clinical consequences are well characterized. Concerning the biological processes affected by this fusion gene a little is known and our understanding of its transformation capacities is limited. Human Embryonic Stem Cells (hESCs) are becoming a powerful tool for modeling human diseases. It is well known, that the process of leukemogenesis manifests as altered cell differentiation. Having this in mind, hematopoiesis-directed differentiation of hESCs is becoming a promising strategy to study the onset of hematopoiesis, especially the emergence of the earliest events leading to the specification of both normal and abnormal hematopoietic tissue (Lensch and Daley 2006). In our study, we created the first human ESC model for the MLL-AF9 fusion gene and we then explored the biological and developmental impact of the MLL-AF9 leukemic fusion gene on the onset of hematopoiesis from hES cells. In addition, we have studied for the first time the effects of the co-expression of the MLL-AF9 fusion oncogene and a known leukemic secondary hit FLT3-ITD on the process of hematopoiesis-directed differentiation of hES cells. Based on in vitro studies, we observed that the expression of the MLL-AF9 fusion oncogene in hES cells provoked a significant blockage in the process of hematopoietic differentiation of hES cells. Moreover, the MLL-AF9 fusion gene strongly compromised the clonogenic potential of hematopoietic precursors derived from hESCs. Since the process of in vitro hematopoietic differentiation from hESCs closely mirrors early events in embryonic hematopoietic development, the results of our study provide the first indication showing the capacity of the MLL-AF9 leukemic fusion gene to impair embryonic blood formation when experimentally overexpressed in hESCs. Additionally, we observed that the co-expression of the MLL-AF9 and the FLT3- WT/FLT3-ITD genes in hES cells completely abrogated their hematopoietic potential. These results suggest that the co-expression of the MLL-AF9 and the FLT3 genes may exert a synergistic effect on the process of hematopoiesis-directed differentiation of hES cells and lead to the blockage at a very early level of hematopoietic differentiation. Taken together, our results provide the first indication showing how the leukemic fusion gene MLL-AF9 when overexpressed in hESCs (separately or together with FLT3 genes) impairs embryonic blood formation, establishing a potential novel experimental system to further study the developmental impact of the MLL-AF9 fusion gene.