Transcriptional Regulation of Oncogenic MEF2C In T-Cell Acute Lymphoblastic Leukemia (T-ALL).

Abstract 3636

Myocyte enhancer factor 2C (MEF2C) is a transcription factor of the MADS-box family which is physiologically expressed in hematopoietic stem cells and during development of B-cells. Recently, we identified ectopic expression of MEF2C in T-cell acute lymphoblastic leukemia (T-ALL) cell lines activated either via chromosomally mediated ectopic expression of homeodomain protein NKX2-5 or via deletion of non-coding exon and promoter regions at 5q14, suggesting loss of negative regulatory elements. Our aim was to identify additional transcriptional regulators of MEF2C in T-ALL. Therefore, we analyzed the sequence of the MEF2C 5′-region, thus identifying potential regulatory binding sites for GFI1B, basic helix-loop-helix (bHLH) proteins, STAT5 and HOXA9/HOXA10. Overexpression studies demonstrated MEF2C activation by GFI1B (strong), LYL1 and TAL1 leukemic bHLH proteins (weak), and inhibition by STAT5 (strong) and HOXA9/HOXA10 (weak). Chromatin-Immuno-Precipitation analysis demonstrated direct binding of GFI1B, LYL1 and STAT5 but not of HOXA10 to the MEF2C 5′-region in T-ALL cells. However, HOXA9/HOXA10 activated expression of NMYC which in turn mediated MEF2C repression, indicating an indirect mode of MEF2C regulation. Chromosomal deletion of the 5′-MEF2C STAT5 binding site in LOUCY cells by del(5)(q14), reduced expression levels of STAT5 protein in some MEF2C-positve T-ALL cell lines, and the presence of inhibitory IL7-JAK-STAT5-signaling highlighted the repressive impact of this factor in MEF2C regulation. Taken together, our results indicate that ectopic expression of MEF2C in T-ALL cells is mainly regulated via activating leukemic transcription factors GFI1B or NKX2-5 and by escaping inhibitory STAT5-signaling.