The oncogenic fusion protein RUNX1-CBFA2T1 supports proliferation and inhibits senescence in t(8;21)-positive leukaemic cells

The t(8;21)-positive cell line Kasumi-1 was electroporated with RUNX1-CBFA2T1 or control siRNAs followed by analysis of proliferation, colony formation, cell cycle distribution, apoptosis and senescence.Electroporation of Kasumi-1 cells with RUNX1-CBFA2T1 siRNAs, but not with control siRNAs, resulted in RUNX1-CBFA2T1 suppression which lasted for at least 5 days. A single electroporation with RUNX1-CBFA2T1 siRNA severely diminished the clonogenicity of Kasumi-1 cells. Prolonged RUNX1-CBFA2T1 depletion inhibited proliferation in suspension culture and G1-S transition during the cell cycle, diminished the number of apoptotic cells, but induced cellular senescence. The addition of haematopoetic growth factors could not rescue RUNX1-CBFA2T1-depleted cells from senescence, and could only partially restore their clonogenicity.RUNX1-CBFA2T1 supports the proliferation and expansion of t(8;21)-positive leukaemic cells by preventing cellular senescence. These findings suggest a central role of RUNX1-CBFA2T1 in the maintenance of the leukaemia. Therefore, RUNX1-CBFA2T1 is a promising and leukaemia-specific target for molecularly defined therapeutic approaches.The chromosomal translocation t(8;21) (q22;q22), which is associated with 10–15% of all cases of acute myeloid leukaemia, fuses the DNA binding domain of the transcription factor RUNX1 (also called AML1 or CBFα) to the almost complete open reading frame of CBFA2T1 (also named MTG8 or ETO) [1,2]. The resulting fusion protein RUNX1-CBFA2T1 (AML1/MTG8, AML1/ETO) interferes with haematopoetic gene expression by recruiting histone deacetylases via N-CoR and mSin3 to promoters, thereby inhibiting the transcription of the respective target gene [3-7]. Moreover, by directly binding to and sequestering transcription factors, such as SMAD3, C/EBPα or vitamin D receptor, RUNX1-CBFA2T1 interferes with signal transduction pathways controlling differentiation and proliferation [8-12]. Consequently, RUNX1-CBFA2T1 blocks myeloid different