HOXA9 regulates miR-155 in hematopoietic cells.

HOXA9-mediated up-regulation of miR-155 was noted during an array-based analysis of microRNA expression in Hoxa9(-/-)bone marrow (BM) cells. HOXA9 induction of miR-155 was confirmed in these samples, as well as in wild-type versus Hoxa9-deficient marrow, using northern analysis and qRT-PCR. Infection of wild-type BM with HOXA9 expressing or GFP(+) control virus further confirmed HOXA9-mediated regulation of miR-155.

HOXA9 modulates its oncogenic partner Meis1 to influence normal hematopoiesis.

While investigating the mechanism of action of the HOXA9 protein, we serendipitously identified Meis1 as a HOXA9 regulatory target. Since HOXA9 and MEIS1 play key developmental roles, are cooperating DNA binding proteins and leukemic oncoproteins, and are important for normal hematopoiesis, the regulation of Meis1 by its partner protein is of interest. Loss of Hoxa9 caused downregulation of the Meis1 mRNA and protein, while forced HOXA9 expression upregulated Meis1.

MicroRNA-126 regulates HOXA9 by binding to the homeobox.

The PicTar program predicted that microRNA-126 (miR-126), miR-145, and let-7s target highly conserved sites within the Hoxa9 homeobox. There are increased nucleotide constraints in the three microRNA seed sites among Hoxa9 genes beyond that required to maintain protein identity, suggesting additional functional conservation. In preliminary experiments, forced expression of these microRNAs in Hoxa9-immortalized bone marrow cells downregulated the HOXA9 protein and caused loss of biological activity.

Evidence that the Pim1 kinase gene is a direct target of HOXA9.

The HOXA9 homeoprotein exerts dramatic effects in hematopoiesis. Enforced expression of HOXA9 enhances proliferation of primitive blood cells, expands hematopoietic stem cells (HSCs), and leads to myeloid leukemia. Conversely, loss of HOXA9 inhibits proliferation and impairs HSC function.