Summary Expression of microRNAs (miRNAs) is under stringent regulation at both transcriptional and post-transcriptional levels. Disturbance at either level could cause dysregulation of miRNAs. Here we show that MLL fusion proteins negatively regulate production of miR-150, an miRNA widely repressed in acute leukemia, by blocking miR-150 precursors from being processed to mature miRNAs through MYC/LIN28 functional axis. Forced expression of miR-150 dramatically inhibited leukemic cell growth and delayed MLL-fusion-mediated leukemogenesis, likely through targeting FLT3 and MYB and thereby interfering with the HOXA9/MEIS1/FLT3/MYB signaling network, which in turn caused downregulation of MYC/LIN28. Collectively, we revealed a MLL-fusion/MYC/LIN28⊣miR-150⊣FLT3/MYB/HOXA9/MEIS1 signaling circuit underlying the pathogenesis of leukemia, where miR-150 functions as a pivotal gatekeeper and its repression is required for leukemogenesis.

Increased expression levels of miR-181 family members have been shown to be associated with favorable outcome in patients with cytogenetically normal acute myeloid leukemia. Here we show that increased expression of miR-181a and miR-181b is also significantly (P < .

Acute myeloid leukemia (AML) is a heterogeneous group of hematopoietic malignancies with variable response to treatment. AMLs bearing MLL (mixed lineage leukemia) rearrangements are associated with intermediate or poor survival. MicroRNAs (miRNAs), a class of small noncoding RNAs, have been postulated to be important gene expression regulators virtually in all biological processes, including leukemogenesis. Through a large-scale, genomewide miRNA expression profiling assay of 85 human AML and 15 normal control samples, we show that among 48 miRNAs that are significantly differentially expressed between MLL-and non-MLLrearranged AML samples, only one (miR-495) is expressed at a lower level in MLL-rearranged AML than in non-MLL-rearranged AML; meanwhile, miR-495 is also significantly down-regulated in MLLrearranged AML samples compared with normal control samples. Through in vitro colony-forming/replating assays and in vivo bone marrow transplantation studies, we show that forced expression of miR-495 significantly inhibits MLL-fusion-mediated cell transformation in vitro and leukemogenesis in vivo. In human leukemic cells carrying MLL rearrangements, ectopic expression of miR-495 greatly inhibits cell viability and increases cell apoptosis. Furthermore, our studies demonstrate that PBX3 and MEIS1 are two direct target genes of miR-495, and forced expression of either of them can reverse the effects of miR-495 overexpression on inhibiting cell viability and promoting apoptosis of human MLL-rearranged leukemic cells. Thus, our data indicate that miR-495 likely functions as a tumor suppressor in AML with MLL rearrangements by targeting essential leukemia-related genes.L eukemia arises as a result of genetic lesions that cause uncontrolled proliferation in cells of the hematopoietic lineage (1, 2). Chromosome translocations are frequently observed in both acute myeloid leukemia (AML) (2-4) and other hematologic malignancies. The MLL (mixed lineage leukemia; HRX, ALL-1, Htrx) gene, located at 11q23, is frequently involved in chromosome translocations with more than 60 different partner genes (5-8). The critical feature of these chromosomal rearrangements is the generation of an in-frame fusion transcript consisting of 5′ MLL and 3′ sequences of the gene on the partner chromosome (7,8). MLL-rearranged leukemia occurs in approximately 10% of patients with de novo or treatment-related acute leukemia (9-11). MLL-rearranged leukemia is classified as a disease with an intermediate or poor risk of prognosis (1, 12).The most well-studied downstream target genes of MLL fusion proteins are HOXA9 and MEIS1 (13-15), and their aberrant overexpression has been shown to be required for the induction and maintenance of MLL-rearranged leukemia (16-21). HOX proteins can form heterodimers or heterotrimers with members of the three-amino-acid loop extension family of cofactors, including PBX and MEIS proteins, to regulate the transcription of multiple downstream targets directly (22)(23)(24). Similar to HOXA9 and MEIS1, overexpression of...

The study of a class of small non-coding RNA molecules, named microRNAs (miRNAs), has advanced our understanding of many of the fundamental processes of cancer biology and the molecular mechanisms underlying tumor initiation and progression. MiRNA research has become more and more attractive as evidence is emerging that miRNAs likely play important regulatory roles virtually in all essential bioprocesses. Looking at this field over the past decade it becomes evident that our understanding of miRNAs remains rather incomplete. As research continues to reveal the mechanisms underlying cancer therapy efficacy, it is clear that miRNAs contribute to responses to drug therapy and are themselves modified by drug therapy. One important area for miRNA research is to understand the functions of miRNAs and the relevant signaling pathways in the initiation, progression and drug-resistance of tumors to be able to design novel, effective targeted therapeutics that directly target pathologically essential miRNAs and/or their target genes. Another area of increasing importance is the use of miRNA signatures in the diagnosis and prognosis of various types of cancers. As the study of non-coding RNAs is increasingly more popular and important, it is without doubt that the next several years of miRNA research will provide more fascinating results.

MicroRNAs (miRNAs), small noncoding RNAs that regulate target gene mRNAs, are known to contribute to pathogenesis of cancers. Acute myeloid leukemia (AML) is a group of heterogeneous hematopoietic malignancies with various chromosomal and/or molecular abnormalities. AML with chromosomal translocations involving the mixed lineage leukemia ( MLL ) gene are usually associated with poor survival. In the present study, through a large-scale, genomewide miRNA expression assay, we show that microRNA-9 (miR-9) is the most specifically up-regulated miRNA in MLL- rearranged AML compared with both normal control and non– MLL -rearranged AML. We demonstrate that miR-9 is a direct target of MLL fusion proteins and can be significantly up-regulated in expression by the latter in human and mouse hematopoietic stem/progenitor cells. Depletion of endogenous miR-9 expression by an appropriate antagomiR can significantly inhibit cell growth/viability and promote apoptosis in human MLL -rearranged AML cells, and the opposite is true when expression of miR-9 is forced. Blocking endogenous miR-9 function by anti-miRNA sponge can significantly inhibit, whereas forced expression of miR-9 can significantly promote, MLL fusion–induced immortalization/transformation of normal mouse bone marrow progenitor cells in vitro. Furthermore, forced expression of miR-9 can significantly promote MLL fusion–mediated leukemogenesis in vivo. In addition, a group of putative target genes of miR-9 exhibited a significant inverse correlation of expression with miR-9 in a series of leukemia sample sets, suggesting that they are potential targets of miR-9 in MLL -rearranged AML. Collectively, our data demonstrate that miR-9 is a critical oncomiR in MLL -rearranged AML and can serve as a potential therapeutic target to treat this dismal disease.