MicroRNA-708 is a novel regulator of the Hoxa9 program in myeloid cells.

MicroRNAs (miRNAs) are commonly deregulated in acute myeloid leukemia (AML), affecting critical genes not only through direct targeting, but also through modulation of downstream effectors. Homeobox (Hox) genes balance self-renewal, proliferation, cell death, and differentiation in many tissues and aberrant Hox gene expression can create a predisposition to leukemogenesis in hematopoietic cells. However, possible linkages between the regulatory pathways of Hox genes and miRNAs are not yet fully resolved. We identified miR-708 to be upregulated in Hoxa9/Meis1 AML inducing cell lines as well as in AML patients. We further showed Meis1 directly targeting miR-708 and modulating its expression through epigenetic transcriptional regulation. CRISPR/Cas9 mediated knockout of miR-708 in Hoxa9/Meis1 cells delayed disease onset in vivo, demonstrating for the first time a pro-leukemic contribution of miR-708 in this context. Overexpression of miR-708 however strongly impeded Hoxa9 mediated transformation and homing capacity in vivo through modulation of adhesion factors and induction of myeloid differentiation. Taken together, we reveal miR-708, a putative tumor suppressor miRNA and direct target of Meis1, as a potent antagonist of the Hoxa9 phenotype but an effector of transformation in Hoxa9/Meis1. This unexpected finding highlights the yet unexplored role of miRNAs as indirect regulators of the Hox program during normal and aberrant hematopoiesis.

Genetic modification of murine hematopoietic stem cells by retroviruses.

Among the currently available methods for gene transfer, recombinant murine retroviruses remain the best established method for achieving stable integration of a transgene with high efficiency. Pioneering work by a number of groups has demonstrated the feasibility of using this method for gene transfer to primitive, multipotential long-term repopulating hematopoietic stem cells (HSC) (1-4). In the case of the hematopoietic system, it is required that the introduced gene integrates into the genome of HSC in order to be expressed in multiple lineages over an extended period of time. However, HSC are found at low frequency, and are normally in a quiescent or slow cycling state. Both factors represent challenges to successful retroviral gene transfer. The former places a premium on high titer, and the latter dictates methods to trigger HSC cycling during the infection, since stable integration of murine retroviruses requires cell division of the target cell and breakdown of the nuclear membrane (5,6). In general, titers greater than 1 × 10(5) U/mL allow some degree of gene transfer for HSC, but 1 × 10(6) or higher are a reasonable goal for achieving useful efficiencies of at least 20%. For activation of HSC, most protocols invoke a combination of in vivo and in vitro stimulation. The former is most easily and routinely achieved by administration of cytotoxic agents like 5-fluorouracil (5-FU) 4 d prior to bone-marrow harvest. This procedure removes a large proportion of actively cycling, more differentiated cells, thus achieving a.