Growth Factor Independence 1b (Gfi1b) Is Required for the Regulation of Fetal Globin Genes in Both Fetal and Adult Erythroid Cells

Abstract 350

Gfi1b is hematopoietic transcription factor most highly expressed in hematopoietic stem cells, megakaryocyte-erythroid precursors, megakaryocytes and throughout erythroid development. Gfi1b deficiency is lethal in mice around 13.5 dpc caused by a failure to produce functional erythrocytes, megakaryocytes and platelets, which causes severe hemorrhaging. Since this lethality has hampered further analysis of the function of Gfi1b, we used Cre-recombinase inducible conditional Gfi1b knock-out mice (Gfi1b^fl/fl). The pIpC induced knock-out of Gfi1b in Gfi1b^fl/flMxCre mice leads to a pronounced drop in peripheral blood platelet numbers and induces a strong extramedullary erythropoiesis in the spleen. We sorted Ter119+ bone marrow cells from wt and pIpC induced Gfi1b^fl/flMxCre mice for a genome wide expression array analysis and found a significant increase in the expression levels of platelet/coagulation related genes such as PF4, vWF, F2r or Ppbp as well as of the fetal globin genes Hba-x, Hbb-ey and Hbb-ßh1, suggesting that Gfi1 regulates globin gene expression or globin switching.

It remained unclear whether the disturbed erythropoiesis in Gfi1b deficient mice was caused by a bone marrow failure, or was a reaction to the anemia caused by internal bleedings as a result of low platelet counts. To clarify this and to avoid deletion of Gfi1b in megakaryocytes, we crossed Gfi1b^fl/fl mice with EpoR-EGFP-Cre mice allowing a Gfi1b deletion specifically in erythroid cells at the pro-erythroblast stage. Gfi1b^fl/flEpoR-EGFP-Cre embryos were paler than wt littermates, but in contrary to complete knock-outs showed no internal bleedings and had normal platelet counts. In addition, EpoR-EGFP-Cre embryos showed a mild block in terminal erythroid differentiation and a pronounced hyper-proliferation at the Ter119-,CD71+, cKit+ proerythroblast stage where Cre expression is activated. Gfi1b^fl/flEpoR-Cre cells showed a strong increase of fetal Hbb-ßH1 globin gene expression and a pronounced decrease of the expression of the adult globin genes Hba, Hbb, as well as of Gata1, Foxo3a and Nfe2l2 but not Gata2. Gene expression-array analysis of fetal liver cells from wt and Gfi1b^fl/flEpoR-Cre embryos from day 14.5 dpc showed that besides fetal globin genes, many genes where up-regulated that normally decrease in expression during the development between the embryonic stages 11.5 dpc to 14.5 dpc. These findings confirm that Gfi1b is required for the regulation of globin gene expression during or at the transition from embryonic/fetal to adult stages.

Interestingly, Gfi1b^fl/flEpoR-Cre mice were viable very likely because these animals have normal platelet counts and do not suffer from hemorrhaging like constitutive Gfi1b deficient mice. However, this also suggested that the block in erythroid development is tolerable, or that it can be overcome during maturation of the embryo. Q-PCR analysis on mRNA from sorted erythroid cells from wt and adult Gfi1b^fl/flEpoR-Cre mice showed a highly increased expression of the fetal globin genes Hbb-ßh1, Hbb-ey and Hba-x but only a slight decrease of Gata1 expression, a mild increase in Nfe2l2 expression and no significant expression of Gata2 compared to age matched wild type controls. A recently published study of genome wide in vivo DNA binding of ten major hematopoietic transcription factors (Wilson et al., Cell Stem Cell, 2010) showed Gfi1b binding to hypersensitive site 2 in the globin locus control region (LCR) where also the Gfi1b interaction partner Gata1 and Nfe2 bind. From these data we conclude, that Gfi1b is required to regulate the expression of fetal globin genes during the switch from embryonic/fetal to adult stages and thereafter during adult globin expression and exerts this function by directly binding to regulatory sites in the globin locus. Since the re-expression of fetal globin genes in adult stages is a therapeutic approach for ß-thalassemia, the function of Gfi1b and its regulatory mechanisms could point to new therapeutic strategies for this disease.