GA Binding Protein (GABP) Is Required for Myeloid Cell Development and Differentiation.

GABPα is an ets transcription factor that regulates genes that are required for innate immunity, including CD18 (β2 leukocyte integrin), lysozyme, and neutrophil elastase. GABP consists of two distinct and unrelated proteins that, together, form a functional transcription factor complex. GABPα binds to DNA through its ets domain and forms a multimeric complex by recruiting its partner, GABPβ, which contains the transactivation domain. GABPα is a single copy gene in both the human and murine genomes and it is the only protein that can recruit GABPβ to DNA. We cloned GABPα from a murine genomic BAC library and prepared a targeting vector in which the GABPα ets domain is flanked by loxP recombination sites (floxed allele, designated fl). Mice that bear one intact (and one disrupted copy) of GABPα, i.e. hemizygous mice, are phenotypically normal. Intercrossing of hemizygous mice yielded no nullizygous mice, indicating that homozygous loss of GABPα causes an embryonic lethal defect. To determine the effect of GABPα deletion on myeloid cell development, we bred heterozygous and homozygous floxed mice to mice that bear the interferon-responsive Mx1-Cre transgene, which express Cre in response to injection of the synthetic polynucleotide, poly I-C. Bone marrow cells underwent efficient deletion of GABPα following poly I-C injection; in contrast, other somatic tissues did not efficiently delete the floxed allele. Bone marrow, peripheral blood, and other tissues were examined for cellular morphology and flow cytometry. We compared mice that lack GABPα in bone marrow (i.e. fl/fl Mx1-Cre mice injected with poly I-C) to littermate controls (i.e. fl/fl mice injected with poly I-C). Mice that lack GABPα exhibited a striking and statistically significant decrease in granulocytes and monocytes in bone marrow and peripheral blood, compared with controls; in contrast, there was an increase in erythroid cells in GABPα null bone marrow. This indicates that the loss of GABPα has lineage-specific effects on myeloid cell development. Morphologic analysis indicates that mice which lack GABPα possess more immature granulocytes compared to control mice. Thus, GABP disruption causes a striking loss of myeloid cells in the bone marrow and peripheral blood of mice in a lineage-specific manner. Furthermore, the maturation block of murine granulocytes that is caused by GABPα disruption demonstrates the crucial role of GABP in myeloid differentiation.