title: Reduction in PU.1 activity results in a block to B-cell development, abnormal myeloid proliferation, and neonatal lethality.
abstract: OBJECTIVE
  It has been demonstrated that high concentration of the transcription factor PU.1 (encoded by Sfpi1) promotes macrophage development, whereas low concentration induces B-cell development in vitro. This has led to the hypothesis that lower levels of PU.1 activity are required for B cell than for macrophage development in vivo. We utilized an allele of Sfpi1 (termed BN) with a mutation in the first coding exon, which resulted in a reduction of PU.1 expression in order to test this hypothesis.
  
  
  METHODS
  Using gene targeting in embryonic stem cells, two ATG-start site codons of PU.1 were mutated, resulting in reduced PU.1 expression originating from a third start codon. Mice were assayed for phenotypic abnormalities using fluorescence-activated cell sorting, microscopy, and colony-forming ability. In addition, isolated cells were tested for their differentiation potential in vitro and in vivo.
  
  
  RESULTS
  Lymphoid and myeloid cells derived from cultured Sfpi1(BN/BN) fetal liver cells had reduced levels of PU.1 expression and activity. B-cell development was intrinsically blocked in cells isolated from Sfpi1(BN/BN) mice. In addition, myeloid development was impaired in Sfpi1(BN/BN) fetal liver. However, neonatal Sfpi1(BN/BN) mice had a dramatic expansion and infiltration of immature myeloid cells.
  
  
  CONCLUSIONS
  Contrary to our original hypothesis, high levels of PU.1 activity are required to induce both myeloid and B-cell development. In addition, neonatal mice homozygous for the hypomorphic allele acquire a myeloproliferative disorder and die within 1 month of age.
authors.affiliation: Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati College of Medicine, Cincinnati, Ohio, USA.
authors.forename: Isaac B
authors.initials: IB
authors.lastname: Houston
authors.forename: Meghana B
authors.initials: MB
authors.lastname: Kamath
authors.forename: Brock L
authors.initials: BL
authors.lastname: Schweitzer
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Chlon
authors.forename: Rodney P
authors.initials: RP
authors.lastname: DeKoter
mesh_terms: D000483:Alleles; D000818:Animals; D000831:Animals, Newborn; D001402:B-Lymphocytes / Q000502:physiology*; D049109:Cell Proliferation; D002478:Cells, Cultured; D006801:Humans; D051379:Mice; D008810:Mice, Inbred C57BL; D038042:Myelopoiesis; D009196:Myeloproliferative Disorders / Q000209:etiology*; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000502:physiology*; D015534:Trans-Activators / Q000235:genetics / Q000502:physiology*; D000099232:Proto-Oncogene Protein Spi-1
chemical_list: D011518:Proto-Oncogene Proteins; D015534:Trans-Activators; D000099232:Proto-Oncogene Protein Spi-1