Key Points• Both overexpression and knockout of miR-126 result in enhanced leukemogenesis.• Overexpression and knockout of miR-126 activate distinct gene signaling and are associated with different biological consequences.It is generally assumed that gain-and loss-of-function manipulations of a functionally important gene should lead to the opposite phenotypes. We show in this study that both overexpression and knockout of microRNA (miR)-126 surprisingly result in enhanced leukemogenesis in cooperation with the t(8;21) fusion genes AML1-ETO/RUNX1-RUNX1T1 and AML1-ETO9a (a potent oncogenic isoform of AML1-ETO). In accordance with our observation that increased expression of miR-126 is associated with unfavorable survival in patients with t(8;21) acute myeloid leukemia (AML), we show that miR-126 overexpression exhibits a stronger effect on long-term survival and progression of AML1-ETO9a-mediated leukemia stem cells/leukemia initiating cells (LSCs/LICs) in mice than does miR-126 knockout. Furthermore, miR-126 knockout substantially enhances responsiveness of leukemia cells to standard chemotherapy. Mechanistically, miR-126 overexpression activates genes that are highly expressed in LSCs/LICs and/or primitive hematopoietic stem/progenitor cells, likely through targeting ERRFI1 and SPRED1, whereas miR-126 knockout activates genes that are highly expressed in committed, more differentiated hematopoietic progenitor cells, presumably through inducing FZD7 expression. Our data demonstrate that miR-126 plays a critical but 2-faceted role in leukemia and thereby uncover a new layer of miRNA regulation in cancer. Moreover, because miR-126 depletion can sensitize AML cells to standard chemotherapy, our data also suggest that miR-126 represents a promising therapeutic target. (Blood. 2015;126(17):2005-2015 Introduction MicroRNAs (miRNAs) have been implicated in the pathogenesis of various types of cancers. [1][2][3][4][5][6][7][8] Some miRNAs play distinct roles in different types of cancers. For example, miRNA (miR)-126, originally identified as an endothelial-specific miRNA playing an essential role in angiogenesis and vascular integrity, [9][10][11] has been shown to function as a critical tumor suppressor in various types of solid tumors. 5,[12][13][14][15][16][17][18][19] In contrast, we have shown that miR-126 is aberrantly overexpressed and likely plays an oncogenic role in core binding factor (CBF) leukemia. 20 CBF leukemia is characterized by the presence of a t(8;21)(q22;q22) or an inv(16)(p13.1q22) chromosomal rearrangement, which accounts for ;20% to 30% of primary acute myeloid leukemia (AML) cases. [21][22][23] The potential oncogenic role of miR-126 in AML was further confirmed by other groups. 24,25 However, it was reported that attenuation of miR-126 expression in normal hematopoietic stem/progenitor cells (HSPCs) resulted in expansion of long-term repopulating hematopoietic stem cells. 26 Thus, the definitive role of miR-126 in the hematopoietic system warrants further investigation.To precisely define ...