HOXA9 plays important roles in both development and hematopoiesis and is overexpressed in more than 50 percent of acute myeloid leukemias (AML). Nearly all cases of AML with mixed lineage leukemia (MLL) translocations show increased HOXA9 expression, as well as cases with mutation of the nucleophosmin gene NPM1, overexpression of CDX2, and fusions of NUP98. In most cases, upregulation of HOXA9 is accompanied by upregulation of its homeodomain-containing cofactor MEIS1, which directly interacts with HOXA9. While HOXA9 alone is sufficient for transformation of hematopoietic stem cells in culture, the addition of MEIS1 increases the transformation efficiency and results in rapidly fatal leukemias in transplanted animals. Despite the crucial role that HOXA9 plays in development, hematopoiesis, and leukemia, its transcriptional targets and mechanisms of action are poorly understood. We have used ChIP-seq to identify Hoxa9 and Meis1 binding sites on a genome-wide level in myeloblastic cells, profiled their associated epigenetic modifications, identified the target genes regulated by HOXA9 and identified HOXA9 interacting proteins. HOXA9 and MEIS1 cobind at hundreds of promoter distal, highly evolutionarily conserved sites showing high levels of histone H3K4 monomethylation and CBP/P300 binding. These include many proleukemogenic gene loci, such as Erg, Flt3, Myb, Lmo2, and Sox4. In addition, HOXA9 binding sites overlap a subset of enhancers previously implicated in myeloid differentiation and inflammation. HOXA9 binding at enhancers stabilizes association of MEIS1 and lineage-restricted transcription factors, including C/EBPα, PU.1, and STAT5A/B thereby promoting CBP/p300 recruitment, histone acetylation, and transcriptional activation. Current efforts are focused on using both biochemical and genetic approaches to assess the role of HOXA9 “enhanceosome” components C/EBPα, PU.1, and STAT5A/B in transcriptional regulation and leukemogenesis. Studies to date suggest that C/EBPα and PU.1 binding can occur in the absence of HOXA9/MEIS1, supporting a model in which these proteins act as pioneer transcription factors for establishment of poised, but not activated, HOXA9-regulated enhancers. Work is under way to assess the impact of high-level HOXA9 and MEIS1 on enhanceosome assembly and the role of recruitment of transcriptional coactivators involved in target gene up- or downregulation, including histone acetyltransferases and chromatin remodeling complexes. Collectively, our findings suggest that HOXA9-regulated enhancers are a fundamental mechanism of HOX-mediated transcription in normal development that is deregulated in leukemia.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.