Distinct c-Myb Regulation by HoxA9, Meis1 and Pbx1 in Haemopoietic and Leukaemic-Like Stem Cells.

Abstract 1431

Poster Board I-454

The transcription factor c-Myb is an essential regulator of haemopoiesis and its expression is deregulated in several types of leukaemia. Although some c-Myb functions have been defined, the mechanisms involved in the control of its expression have yet to be elucidated. Previous studies have suggested that transcription initiation at the c-myb gene is constitutive, and that the level of mRNA is regulated by an elongation-blocking mechanism operating in its first intron. Here, we define and compare mechanisms influencing c-myb expression in haemopoietic stem cells (HSCs) versus leukaemic stem cell (LSC)-like cells. Using a nuclease sensitivity assay we have defined several potential regulatory elements in both HSC and LSC-like model cell lines. These hypersensitive sites are in the proximal promoter and the first intron, the latter correlating with the position of the putative transcription elongation regulatory region. Moreover, the hypersensitive sites are located in regions of sequence conservation and encompass a number of potential binding sites for homeodomain (HD)-containing proteins. In this study, we were able to demonstrate that the HD-containing transcription factors HoxA9 and Meis1, which are highly expressed in HSCs and whose co-expression in mouse bone marrow leads to rapid development of acute myeloid leukaemia (AML), are necessary but not sufficient for c-myb expression. In addition, we show that the pre-B-cell leukaemia transcription factor-1 (Pbx1), known to be a key binding partner of HD-containing factors, is indispensable in the regulation of c-myb expression. Comparing the effects of altered levels of HoxA9, Meis1 and Pbx1 in HSCs versus LSCs suggests that distinct mechanisms involving dimeric or trimeric complexes operate to regulate c-myb expression in these two stem cell types.