HOXA9 Modulates Its Oncogenic Partner Meis1 To Influence Normal Hematopoiesis

Abstract

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. Hoxa9 and Meis1 expression was correlated in hematopoietic progenitors and acute leukemias. Meis1 ؉/؊ Hoxa9 ؊/؊ deficient mice, generated to test HOXA9 regulation of endogenous Meis1, were small and had reduced bone marrow Meis1 mRNA and significant defects in fluorescence-activated cell sorting-enumerated monocytes, mature and pre/pro-B cells, and functional B-cell progenitors. These data indicate that HOXA9 modulates Meis1 during normal murine hematopoiesis. Chromatin immunoprecipitation analysis did not reveal direct binding of HOXA9 to Meis1 promoter/enhancer regions. However, Creb1 and Pknox1, whose protein products have previously been reported to induce Meis1, were shown to be direct targets of HOXA9. Loss of Hoxa9 resulted in a decrease in Creb1 and Pknox1 mRNA, and forced expression of CREB1 in Hoxa9 ؊/؊ bone marrow cells increased Meis1 mRNA almost as well as HOXA9, suggesting that CREB1 may mediate HOXA9 modulation of Meis1 expression.While the Hox homeobox genes are widely recognized as important developmental genes (26), we and others have shown that several Hox genes, and Hoxa9 in particular, are important for both normal hematopoiesis (27, 28) and leukemic transformation (25,29). While the Hoxa9 gene plays a role in embryonic development, much of the research on this gene has focused on its role as an oncogene that is often upregulated in acute myeloid leukemias (12,29). In an analysis of 6,817 genes, Hoxa9 was the most highly positively correlated with treatment failure in acute myeloid leukemia patients (18). Meis1 is a member of the TALE family of non-Hox homeobox genes, which was initially identified as a frequent viral integration site in myeloid leukemias arising in BXH2 mice (32). The Hoxa9 gene is also upregulated in many of the leukemias arising in the BXH2 animals (33). Hoxa9 is expressed in numerous tissues during development, including rib (8), limb (17), motor neuron progenitors (10), reproductive tract (9), and mammary gland (7). Hoxa9 is also expressed in normal adult BM (24, 43), and loss of Hoxa9 leads to multiple relatively mild defects in normal hematopoiesis (23,27,28). Retroviral expression studies have also shown that HOXA9 and MEIS1 are important for myeloid blood cell differentiation (3, 4). Despite the broad expression of Hoxa9 and other Hox genes, relatively little is known about how the HOX proteins function. An important advance was the discovery that many HOX proteins gain DNA binding specificity by forming complexes with the PBX (6, 31), MEIS1 (41), and PREP1 (2) proteins. Although HOXA9 is capable of bind...

Down-regulation of homeobox genes MEIS1 and HOXA in MLL -rearranged acute leukemia impairs engraftment and reduces proliferation

“…As controls we used RS4;11 cells infected with the lentiviral vector, and cells expressing HOXA9 shRNA found to be nonpotent in elimination of the protein. Two recent studies indicated down-regulation of MEIS1 RNA (29,30), as well as of RNAs of multiple HOXA genes (30), in MLL-rearranged cells knocked down for HOXA9. To examine whether similar cross-modulation is associated with MEIS1, HOXA7, and HOXA10 knockdowns, we analyzed the abundance of HOXA/MEIS1 RNAs by applying the technology of NanoString nCounter gene expression system, which captures and counts individual mRNA transcripts by their hybridization to a multicomplex probe library (31).…”

Identification of E74-like factor 1 (ELF1) as a transcriptional regulator of the Hox cofactor MEIS1

“…Pan-chromatin modifiers, such as the trithorax group member, MLL, or the polycomb group member, EZH2 appear to be important regulators of MEIS1 (19,20). Transcription factors such as CREB, GFI1 and recently HOXA9 also have been reported to be involved in MEIS1 regulation, but the molecular mechanisms of how they affect MEIS1 transcription are unclear (21)(22)(23). Detailed comparative sequence analysis of the 140 kb human MEIS1 locus reveals a high degree of evolutionary conservation not only in the exons, but also in the non-coding regions, namely introns and adjacent upstream and downstream regions.…”

MEIS1 regulates early erythroid and megakaryocytic cell fate

“…MEIS1 is expressed in hematopoietic stem and progenitor cells 9,19,20 and increases during human megakaryocytic differentiation. 21 Meis1 null-mutant mice die at embryonic day 14.5 due to a poorly developed hematopoietic compartment, lack of megakaryocytes and platelets and defective vascularization.…”