Abstract 1821: MiR-9 is an oncogenic microRNA specifically overexpressed in MLL-rearrangement leukemia.

Acute Leukemia is a heterogeneous disease derived from different hematopoietic cell lineages; acute leukemia both lymphoid and myeloid contains chromosomal including translocation of MLL (Mixed Lineage Leukemia), resulting in intermediate to poor survival. Aberrant expression of microRNAs (miRs) is an emerging hallmark of cancer biology; in particular, expression of microRNA-9 (miR-9) has been connected to several malignancies including breast, colon, prostate, lung, and ovarian cancers. However, miR-9 abnormalities have not been described in leukemia. We report that miR-9 is specifically overexpressed in MLL-rearranged acute myeloid leukemia (AML) relative to normal controls or other subtypes of AMLs. We showed that MLL fusion proteins directly regulate miR-9 expression by binding to its promoter region. Forced expression of miR-9 alone does not transform normal hematopoietic progenitor cells but in the presence of MLL-AF9, one of the most frequent MLL-rearranged fusion gene resulting from t(9;11), results in a significant enhance of the colony forming/replating capacity of normal hematopoietic progenitor cells. The use of a competitor inhibitor (miR-9 sponge) resulted in a significant reduction of colony formation and an increase in differentiated hematopoietic cells, indicating miR-9 is required for MLL-AF9 colony-forming growth and maintenance of the immature cell population. Through further functional analysis we found that miR-9 is required for MLL-AF9-mediated cell transformation in vitro. Using mouse bone marrow transplant assays we discovered that forced expression of miR-9 could significantly accelerate leukemogenesis mediated by MLL-AF9. The addition of miR-9 to MLL-AF9 also resulted in an increase in immature hematopoietic progenitor cells indicating more potent long-term leukemia potential in vivo. To identify miR-9 targets we analyzed our in-house and publically available gene expression data from the Gene Expression Omnibus (GEO) at the National Center for Biotechnology Information (NCB I) in Acute Leukemia patients containing MLL-rearrangements. We determined that MLL-leukemias contained a unique deregulated gene signature. As microRNAs most commonly inhibit target genes through binding of the 3’ untranslated region (UTR) we identified genes that were significantly inhibited in MLL-leukemias and were predicted targets of miR-9. From the several genes we identified we selected six genes (based on best inverse correlation to miR-9 expression) to validate as targets of miR-9 in MLL-rearranged leukemias. We validated these genes as miR-9 targets and continue to investigate their contribution to MLL-rearranged leukemias. As these genes are potentially involved in several biological processes we believe that miR-9 in the presence of MLL fusions is an important contributor to leukemogenesis and could be a potential target for future antimir therapy. Citation Format: Colles Price, Ping Chen, Zejuan Li, Yuanyuan Li, Anissa Wiley, Donglin Cao, Chunjiang He, Rejani Kunjamma, Hao Huang, Xi Jiang, Stephen Arnovitz, Mary Beth Neilly, Janet Rowley, Jianjun Chen. MiR-9 is an oncogenic microRNA specifically overexpressed in MLL-rearrangement leukemia. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1821. doi:10.1158/1538-7445.AM2013-1821