Project 4: MicroRNA targeting of normal and leukemia stem-progenitor cells (Civin)MicroRNAs bind to 3' untranslated region sites in target mRNAs to down-regulate translation to protein.Since microRNAs do not base-pair exactly with their target mRNAs, they can block protein translation ofmany mRNAs and serve as powerful switches to regulate cell functions. To extend our long-term studies onregulation of hematopoiesis and stem cell biology, we profiled microRNA expression in hematopoietic stemprogenitorcells (HSPCs). We combined this data with human HSPC mRNA expression results andmicroRNA-mRNA target predictions into a novel database which predicted that certain of the HSPCexpressedmicroRNAs (HE-microRNAs) targeted several mRNAs critical to hematopoiesis. On this informaticbasis, we formulated a model for microRNA control of hematopoiesis in which many genes specifyinghematopoietic differentiation are expressed by early HSPCs, but held in check by HE-microRNAs. Forseveral target mRNAs, we then demonstrated experimentally that translation is actually decreased bymicroRNAs. Mir-155 potently reduced myeloid and erythroid colony formation of normal human HSPCs, andmir-16 selectively inhibited erythropoiesis.Since the cells that we studied include rare stem cells and various stages of progenitors, we propose inAim 1, to expand our microRNA profiles of HSPCs to more highly purified subsets of primary human andmouse HSPCs and primary human acute myeloid leukemia (AMI) cells.
In Aim 2, we will determine ifselected individual microRNAs experimentally inhibit development of primary HSPCs, as our model predicts,and if these microRNAs affect AMI stem cells and leukemogenesis. We will determine the proteins whosesynthesis is inhibited by each functionally-active microRNA and thereby the molecular mechanisms of thehematopoietic effects. At this point in our studies, at least 2 of these microRNAs, mir-16 and mir-155 appearto be new regulators of normal hematopoietic and cancer stem cells.Relevance: We suggest that a major barrier impeding cure of the many cancer patients whom we cannotcure today is our failure to effectively attack and eliminate cancer stem cells. Specifically, we believe thatdevelopment of more effective therapies in leukemia hinges on fuller understanding of the regulation of rarenormal and leukemic hematopoietic stem cells. The studies in this project investigate the effects of newregulatory molecules called microRNAs, which appear to play previously unexpected, potent roles in controlof blood formation and leukemia. Understanding the actions of these hematopoietic-regulatory microRNAsmay provide new targets, for both expansion of normal stem cells and leukemia therapy.
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