HEMATOPOIESIS AND STEM CELLS Hmga2 is a direct target gene of RUNX1 and regulates expansion of myeloid progenitors in mice

RUNX1, also known as AML1 and CBFa2, is a member of the Runt family of proteins. The major function of RUNX1 is to operate as a DNA-binding transcription factor. Studies over the past 20 years have established RUNX1 as a critical player in hematopoiesis and specification of hematopoietic stem and progenitor cells (HSPCs), becauseneither process canoccurwithoutRUNX1. The importance of RUNX1 is further validated by its prevalence in a variety of hematologic diseases and malignancies including myelodysplastic syndrome, myeloproliferative neoplasms, and multiple forms for acutemyeloid leukemia (AML).As a transcription factor, RUNX1 binds to DNA regulatory regions to guide the expression of its target genes. Most confirmed RUNX1 target genes are mainly expressed in differentiated blood cells. Direct targets of RUNX1 in HSPCs, however, have largely remained unexplored. The identification of these genes offers an insightful view into how this master regulator influences HSPC biology. To elucidate RUNX1 target genes in HSPCs, we performed genome-occupancy analysis with chromatin immunoprecipitation coupled with high-throughput sequencing (ChIP-seq) using RUNX1 antibodies and a murine HSPC cell line. These data were combined with gene expression signatures from wild-type and RUNX1-deficient HSPCs (Lineage/cKit/Sca1). The combination of both sets of data allowed elucidation of Hmga2 as a RUNX1 target gene. High mobility group AT-hook 2 (HMGA2) is a nonhistone chromatin-binding protein typically associated with enhancers but lacks its own transcriptional activity. Its expression is generally highest in stem cells and during embryogenesis. AberrantHMGA2 expression has been associated with a variety of mesenchymal tumors including several examples in the hematopoietic system. Hence, HMGA2 is considered to be an oncogene. More recently, ectopic expression of Hmga2 in transgenic mouse models or bone marrow transplantation with cells expressing Hmga2 have been demonstrated to result in myeloproliferative disease. In these models, the HSPC populations are expanded as well as the myeloid progenitors. All of these studies suggest that HMGA2 serves as a critical regulator of proliferation and survival. Conditional deletion of Runx1 in mice has allowed for the study of its role in HSPCs becausemice that are null forRunx1 display embryonic lethality. Interestingly, one of the most striking phenotypes in Runx1 conditional knockout mice is a marked expansion of their HSPCs and myeloid progenitors. In this study, we identified Hmga2 as a target gene using RUNX1 ChIP-seq and analyzed the role of