EPIGENETIC DYSREGULATION OF CEBPE ENHANCER FUNCTION IN MLL-REARRANGED LEUKEMIA

Abstract

Mixed lineage leukemia (MLL)-rearranged leukemia is characterized by the presence of MLL fusion proteins that mediate differentiation blockage through aberrant epigenetic regulation. CCAAT enhancer binding proteins (CEBPs) are key transcription factors involved in myeloid lineage commitment and maturation, among which CEBPE is crucial in terminal differentiation of granulocytes/macrophages. Here we report that CEBPE expression is significantly lower in human MLL acute myeloid leukemia (AML) cell lines (HL60, NB4, U937 and Kasumi-1), when compared to the non-MLL AML cell lines (ML-2, MV4:11, Monomac-6 and THP-1). Importantly, TCGA dataset shows that low CEBPE expression is significantly associated with poor survival of AML patients. We further identified the putative CEBPE enhancer and validated its interaction with promoter by chromatin conformation capture (3C) assay. Strikingly, DNA hypermethylation is observed specially at the CEBPE enhancer in MLL AML cells. We further demonstrated an induction of CEBPE and myeloid marker CD11b in MLL AML cells treated with DNA methylation inhibitor 5’-aza-cytidine (5’-aza-C), suggesting the crucial epigenetic regulatory role of CEBPE enhancer. While all-trans retinoid acid failed to trigger cell differentiation in MLL AML cells, co-treatment with 5’-aza-C resulted in strong induction of CEBPE with terminal differentiated cell morphology, compared to 5’-aza-C treatment per se. It therefore suggests that de-repression of CEBPE by removal of enhancer DNA methylation may prone MLL AML cells to undergo differentiation upon chemical/pharmaceutical induction. Collectively, our work showed that aberrant DNA methylation mediated by MLL fusion proteins represses CEBPE expression through altering its enhancer activity, which subsequently leads to cell differentiation blockage in MLL AML cells.