The transcription factor CCAAT Enhancer Binding Protein alpha (C/EBPα) is crucial for normal granulopoiesis and frequently disrupted in acute myeloid leukemia (AML). Mutations in the CEBPA gene are reported for about 10% of all AML. Loss of function of C/EBPα leads to a block of myeloid differentiation. MicroRNAs (miR) inhibiting translation of mRNA into protein were identified as critical players in granulocytic differentiation. We and others have already shown that C/EBPα exerts its effects by regulating miRNAs such as miR-223 and miR-34a. In a global microRNA-array we found miR-30c as a novel most important target of C/EBPα during granulopoiesis. Wild-type C/EBPα-p42 up regulates miR-30c expression, whereas the C/EBPα-p30 mutant, found in AML, does not. Furthermore, miR-30c expression is up regulated by G-CSF during granulocytic differentiation of primary human CD34-positive progenitor cells and down regulated in various subtypes of AML. Among these, miR-30c is significantly down regulated in samples from AML with normal karyotype and CEBPA mutations compared to AML patients with CEBPA wildtype. In mice, miR-30c shows a high expression in GMP and granulocytes. An induced tissue specific knock-out of C/EBPα in mice leads to a significant suppression of miR-30c expression in bone marrow cells. A luciferase reporter assay identifies NOTCH1 as a direct target of miR-30c as evident by its binding to the 3'UTR. Recent studies show that Notch1 blocks protein expression of C/EBPα-p42 by activation of Tribbles homolog 2 (Trib2). On the other hand, Proteins of Notch1 and Trib2 are down regulated by C/EBPα-p42. A block of miR-30c by locked nucleic acid (LNA) oligonucleotides prevents C/EBPα–induced downregulation of Notch1 protein expression, thus, miR-30c is necessary for C/EBPα to block NOTCH1. In this direction, the network leads to a block of granulopoiesis and further to leukemogenesis. Our study indicates that C/EBPα-induced miR-30c inactivates Notch1 during granulopoiesis and is down regulated in AML. This data stress the important role of deregulated miRNA expression in leukemia and may provide novel biomarkers and therapeutic targets in AML.
Schnittger:MLL Munich Leukemia Laboratory: Equity Ownership.
Asterisk with author names denotes non-ASH members.