MiR-29a Maintains Hematopoietic Stem Cell Self-Renewal and Is Required For Myeloid Leukemogenesis

microRNAs (miRNAs) are important regulators of both embryonic and adult tissue stem cell self-renewal. We previously showed that ectopic expression of miR-29a, a miRNA highly expressed in HSCs as well as in human acute myeloid leukemia (AML) stem cells, in immature mouse hematopoietic cells is sufficient to induce a myeloproliferative disorder that progresses to AML. During the early phase of this disease, miR-29a induces aberrant self-renewal of committed myeloid progenitors, strongly suggesting a role for miR-29a in regulating HSC self-renewal. In order to determine the role of miR-29a in HSC function, we have evaluated our recently described miR-29a/b1 null mouse. Homozygous deletion of miR-29a/b1 resulted in reduced bone marrow cellularity and reduced colony forming capacity of hematopoietic stem and progenitor cells (HSPCs). The phenotype was mediated specifically by miR-29a since miR-29b expression was not significantly altered in HSCs and reconstitution of miR-29a/b1 null HSPCs with miR-29a, but not miR-29b, rescued in vitro colony formation defects. Self-renewal defects were observed in miR-29a deficient HSCs in both competitive and non-competitive transplantation assays, and these deficits were associated with increased HSC cell cycling and apoptosis. Gene expression studies of miR-29a deficient HSCs demonstrated widespread gene dysregulation including a number of up-regulated miR-29a target genes including DNA methylation enzymes (Dnmt3a, -3b) and cell cycle regulators (e.g. Cdk6, Tcl1, Hbp1, Pten). Knockdown of one of these targets, Dnmt3a, in miR-29a deficient HSCs resulted in partial restoration of colony formation, providing functional validation that Dnmt3a mediates part of miR-29a null HSPCs functional defects. miR-29a loss also abrogated leukemogenesis in the MLL-AF9 retroviral AML model. Together, our results demonstrate that miR-29a positively regulates HSC self-renewal and is required for myeloid leukemogenesis.