MicroRNA-126 Regulates HOXA9 by Binding to the Homeobox

Department of Medicine, Department of Veterans Affairs and University of California, San Francisco; Program of Development and Stem Cell Biology, Department of Medicine, University of California, San Francisco

^* To whom correspondence should be addressed. Email: wfshen{at}itsa.ucsf.edu. largman{at}cgl.ucsf.edu.

	Abstract

The PicTar program predicted that miR-126, miR-145, and let-7s target highly conserved sites within the Hoxa9 homeobox. There is increased nucleotide constraint 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 down-regulated HOXA9 protein and caused loss of biological activity. The microRNAs were shown to target their predicted sites within the homeobox. miR-126 and Hoxa9 mRNA are co-expressed in hematopoietic stem cells, and down regulated in parallel during progenitor cell differentiation, but miR-145 is barely detectable in hematopoietic cells and let-7s are highly expressed in bone marrow progenitors, suggesting that miR-126 may function in normal hematopoietic cells to modulate HOXA9 protein. In support of this hypothesis, expression of miR-126 alone in MLL-ENL-immortalized bone marrow cells decreased endogenous HOXA9 protein while, inhibition of endogenous miR-126 increased expression of HOXA9 in F9 cells.