Repression of Mir-495, a Microrna Associated with Favorable Outcome of Acute Myeloid Leukemia Patients, Is Required for the MLL-Associated Leukemogenesis,

Abstract 3462

Acute myeloid leukemia (AML) bearing MLL (mixed lineage leukemia) translocations are associated with poor survival, and only fewer than 50% of the patients survive longer than 5 years. Thus, an improved strategy leading to a higher cure rate is urgently needed to treat MLL-associated AML. MicroRNAs (miRNAs), a class of small non-coding RNAs, have been postulated to be important gene expression regulators in all biology including human leukemia. Through large-scale, genome-wide miRNA expression profiling assays, we determined that miR-495 is significantly down-regulated in the majority of human AML samples, particularly, in those with MLL rearrangements. More interestingly, through correlating the expression signature of miR-495 with clinical outcome of AML patients, we revealed that a low expression level of miR-495 is a predictor of poor prognosis in most AML patients. Our further qPCR assays confirmed that the expression of miR-495 is even more significantly downregulated in MLL-rearranged AML primary patient samples and cell lines. Through in vitro colony-forming/replating assays and in vivo bone marrow transplantation studies, we found that forced expression of miR-495 significantly inhibits the capacity of the MLL-AF9 fusion gene to support colony formation in mouse bone marrow progenitor cells in vitro and to induce leukemia in vivo. In leukemia cell lines, overexpression of miR-495 greatly inhibits the viability of the cells, while increasing apoptosis. Furthermore, by using 3 algorithms for miR-495 3'UTR binding sites, we identified several well-known MLL leukemia-related genes, e. g. BMI1, MEF2C, BID and MEIS1, as potential targets of miR-495. Results of qPCR revealed that forced expression of miR-495 significantly inhibits the expression levels of these genes in leukemia cell lines, mouse bone marrow progenitor cells, as well as mouse peripheral blood cells with MLL fusion genes. Therefore we hypothesize that miR-495 may function as a tumor suppressor in AML with MLL rearrangements by targeting essential tumor-related genes. Further studies will focus on: 1) effects of miR-495 on the functions of target genes studied in vitro and in vivo; 2) the epigenetic mechanisms and the signaling pathways involved in regulating the expression level of miR-495 in human leukemia.