Molecular and functional studies of the BCR/ABL1 fusion gene

The BCR/ABL1 fusion gene is associated with chronic myeloid leukemia and a subgroup of acute lymphoblastic leukemia. The general aim of this thesis was to increase the understanding of BCR/ABL1-induced leukemogenesis by molecular and functional studies of this fusion gene. In Article I, an experimental U937 cell line model with inducible expression of BCR/ABL1 was established and characterized. Expression of BCR/ABL1 activated the JAK/STAT pathway, but showed no marked effects on proliferation, differentiation, and apoptosis. In addition, a reversible upregulation of the cell-surface marker CEACAM1, implicated in cell adhesion and signal transduction, was identified upon expression of BCR/ABL1. In Article II, the BCR/ABL1-inducible U937 cells were used to investigate the transcriptional effects of BCR/ABL1. Approximately 60 genes were induced by BCR/ABL1, including genes encoding transcription factors, kinases, and signal transduction molecules. About one third of the genes were IFN-responsive, which may indicate an overlap with IFN-induced signal transduction or activation of cellular defense and/or negative feedback mechanisms. In Article III, the tyrosine kinase activity of BCR/ABL1 was blocked with imatinib in five Ph-positive cell lines. Approximately 140 genes, mainly involved in signal transduction and metabolic pathways, were identified as affected by BCR/ABL1. Several genes implicated in negative feedback-regulation of well-known signaling pathways were positively regulated by BCR/ABL1, which may act to suppress tumor-promoting effects elicited by the fusion gene. In Article IV, the P190 or the P210 BCR/ABL1 fusion variant was retrovirally expressed in primitive human cord blood cells. In these cells, expression of both P190 and P210 BCR/ABL1 resulted in increased proliferation and differentiation towards the erythroid lineage. Approximately 220 genes were identified as targets of P190/P210 BCR/ABL1-mediated signaling. The similar biological and transcriptional effects induced by these two variants indirectly support the hypothesis of a separate cellular origin for these fusion genes to explain their association with different leukemias. A recurrent finding between the three experimental models was a deregulated expression of different SOCS family members, which regulate signaling mediated particularly via the JAK/STAT pathway. Apart from providing important pathogenetic insights into BCR/ABL1-induced leukemogenesis, the present study identified a number of pathways/individual genes that may provide attractive targets for the development of novel therapies against Ph-positive leukemias.