The Ets Transcription Factor, GABP, Is Required by Myeloid Progenitors for Normal Myeloid Differentiation.

Abstract 256

GABP transcription factor regulates genes that are required for innate immunity. GABP is an obligate tetrameric protein complex that contains two molecules of GABPa, which binds to DNA through its ets domain, and two molecules of GABPb, which contains a transcription activation domain. GABP is an essential component of a multiprotein enhanceosome that is required for retinoic acid-dependent myeloid gene transcription. Disruption in mouse embryo fibroblasts of Gabpa, the mouse gene that encodes mouse Gabpa, causes profound cell cycle arrest at the G1-S boundary, due to reduced expression of DNA Polymerase a and Thymidylate Synthase, which are required for DNA synthesis, and of Skp2, a ubiquitin ligase that controls degradation of the cyclin-dependent kinase inhibitors (CDKIs), p21 and p27. Thus, GABP is a key regulator of the cell cycle. In order to define the role of GABP in myeloid differentiation, we generated mice in which exons that encode the Gabpa ets domain are flanked by loxP recombination sites, and bred these floxed mice to mice that bear the Mx1-Cre transgene. Their progeny were treated with pI-C and Gabpa was efficiently deleted in hematopoietic cells of these Gabpa^−/− mice. As controls for all experiments, mice that bear Mx1-Cre but which lack the floxed Gabpa allele were also injected with pI-C. Within days, the peripheral blood white blood cell count fell in the Gabpa^−/− mice compared to the controls; half of the Gabpa^−/− mice died within two weeks. Gabpa^−/− mice exhibited a striking loss of Gr1⁺, CD11b⁺ cells in the peripheral blood, spleen, and bone marrow. Myeloid cells of Gabpa^−/− mice were immature, morphologically dysplastic, and demonstrated aberrant patterns of myeloid gene expression. Bone marrow from Gabpa^−/− mice formed reduced numbers of in vitro myeloid colonies in the presence of G-CSF, M-CSF, or GM-CSF; cells isolated from in vitro colonies from Gabpa^−/− mice exhibited a strong bias toward macrophage-like morphology. Multicolor flow cytometry revealed a loss of granulocyte-monocyte committed progenitor cells (GMPs) in the bone marrow of Gabpa^−/− mice, and these progenitors expressed aberrant patterns of key transcription factors. Especially notable in Gabpa^−/− GMPs was reduced expression of Gfi-1, a transcriptional repressor that is mutated in some congenital neutropenic syndromes, and whose genetic disruption causes abnormalities in granulocyte development. We used chromatin immunoprecipitation (ChIP) to identify ets sites in the Gfi-1 promoter that are bound by GABP in vivo. We conclude that GABP is required for proliferation or survival of committed myeloid progenitor cells and for normal maturation of granulocytes. We hypothesize that defects in myeloid cell proliferation and differentiation associated with Gabpa disruption are caused, at least in part, by its regulation of the Gfi-1 transcriptional repressor. Furthermore, we propose that the regulation of Gfi-1 by GABP constitutes a key regulatory pathway in myeloid cell development.