MicroRNA Maestros of Hematopoiesis

Abstract SCI-32

MicroRNAs (miRs) inhibit stability and/or translation of mRNAs, usually by binding to specific sites in the 32′UTRs of their target mRNAs. Due to imperfect (i.e. partially complementary) miR:mRNA base-pairing, miRs can block translation of many mRNAs and serve as powerful master switches to regulate cell functions. Therefore, we profiled miR expression in human CD34+ hematopoietic stem-progenitor cells (HSPCs) and combined human HSPC miR expression, mRNA expression, and miR-mRNA target predictions to hypothesize that certain HSPC-expressed miRs (HE-miRs) target several mRNAs critical to hematopoiesis. On this informatic basis, we formulated a model of hematopoietic differentation in which many genes specifying hematopoietic differentiation are expressed by early HSPCs, but held in check by miRs [1]. In addition, we noted that the miR-23a cluster (i.e. adjacent, co-transcribed miR-23a, miR-27a, and miR-24-2) is not expressed or is expressed at levels >2-fold lower in 50% of acute myeloid leukemias and 80% of acute lymphoid leukemias tested compared to normal human HSPCs. ‘Re-expressing’ 1 or more of these miR-23a cluster members in leukemia cells promotes their apoptosis and reduces their proliferation, thus suggesting that these miRs have a tumor suppressive role. We have identified YWHAQ (14-3-3q) and several other 14-3-3 isoforms, which are anti-apoptotic and have established roles as oncogenes, as miR-23a cluster target molecules. Artificial manipulation of these HE-miRs and their target genes may lead to novel strategies for leukemia treatment and/or for expansion of normal HSPCs. Since the CD34⁺ HSPCs that we studied initially include rare stem cells and various stages of progenitors, we have expanded our miR profiling to more highly purified subsets of mouse HSPCs. Several previously described (e.g. miR-155 [1], miR-451 [2], miR-146 [3]) and novel HE-miRs are expressed differentially in lineages/stages of HSPCs, and their selective expression has been confirmed in human HSPC subsets. We are using cellular gain- and loss-of-function approaches with hematopoietic functional assays to determine whether these HE-miRs control human hematopoiesis. Understanding the effects of HE-miRs in hematopoiesis may elucidate hematopoietic and general stem cell biologic mechanisms.

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