Human promoter mutations unveil Oct-1 and GATA-1 opposite action on Gfi1b regulation

Growth Factor Independence 1B-Mediated Transcriptional Repression and Lineage Allocation Require Lysine-Specific Demethylase 1-Dependent Recruitment of the BHC Complex

Growth factor independence 1B (GFI1B) coordinates assembly of transcriptional repressor complexes comprised of corepressors and histone-modifying enzymes to control gene expression programs governing lineage allocation in hematopoiesis. Enforced expression of GFI1B in K562 erythroleukemia cells favors erythroid over megakaryocytic differentiation, providing a platform to define molecular determinants of binary fate decisions triggered by GFI1B. We deployed proteome-wide proximity labeling to identify factors whose inclusion in GFI1B complexes depends upon GFI1B's obligate effector, lysine-specific demethylase 1 (LSD1). We show that GFI1B preferentially recruits core and putative elements of the BRAF-histone deacetylase (HDAC) (BHC) chromatin-remodeling complex (LSD1, RCOR1, HMG20A, HMG20B, HDAC1, HDAC2, PHF21A, GSE1, ZMYM2, and ZNF217) in an LSD1-dependent manner to control acquisition of erythroid traits by K562 cells. Among these elements, depletion of both HMG20A and HMG20B or of GSE1 blocks GFI1B-mediated erythroid differentiation, phenocopying impaired differentiation brought on by LSD1 depletion or disruption of GFI1B-LSD1 binding. These findings demonstrate the central role of the GFI1B-LSD1 interaction as a determinant of BHC complex recruitment to enable cell fate decisions driven by GFI1B.

POU2F1 over-expression correlates with poor prognoses and promotes cell growth and epithelial-to-mesenchymal transition in hepatocellular carcinoma

Despite recent efforts to understand activities of POU domain class 2 transcription factor 1 (POU2F1), little is known about the roles of POU2F1 in hepatocellular carcinoma (HCC) tumorigenesis and its correlation with any clinicopathological feature of HCC. In this study, we found that POU2F1 was significantly up-regulated in HCC specimens compared with adjacent non-cancerous liver specimens. The high POU2F1 protein expression level positively correlated with large tumor size, high histological grade, tumor metastasis and advanced clinical stage, and HCC patients with high POU2F1 levels exhibited poor prognoses. We further demonstrated that POU2F1 over-expression promoted HCC cell proliferation, colony formation, epithelial-to-mesenchymal transition (EMT), migration and invasion, while silencing of POU2F1 inhibited these malignant phenotypes. POU2F1 induced the expression of Twist1, Snai1, Snai2 and ZEB1 genes which are involved in the regulation of EMT. Furthermore, POU2F1 was up-regulated by AKT pathway in HCC, and POU2F1 over-expression reversed the inhibition of malignant phenotypes induced by AKT knock-down, indicating POU2F1 is a key down-stream effector of AKT pathway. Collectively, our results indicate that POU2F1 over-expression is positively associated with aggressive phenotypes and poor survival in patients with HCC, and POU2F1 regulated by AKT pathway promotes HCC aggressive phenotypes by regulating the transcription of EMT genes. POU2F1 may be employed as a new prognostic factor and therapeutic target for HCC.

Transcription Factor GFI1B in Health and Disease

Many human diseases arise through dysregulation of genes that control key cell fate pathways. Transcription factors (TFs) are major cell fate regulators frequently involved in cancer, particularly in leukemia. The GFI1B gene, coding a TF, was identified by sequence homology with the oncogene growth factor independence 1 (GFI1). Both GFI1 and GFI1B have six C-terminal C2H2 zinc fingers and an N-terminal SNAG (SNAIL/GFI1) transcriptional repression domain. Gfi1 is essential for neutrophil differentiation in mice. In humans, GFI1 mutations are associated with severe congenital neutropenia. Gfi1 is also required for B and T lymphopoiesis. However, knockout mice have demonstrated that Gfi1b is required for development of both erythroid and megakaryocytic lineages. Consistent with this, human mutations of GFI1B produce bleeding disorders with low platelet count and abnormal function. Loss of Gfi1b in adult mice increases the absolute numbers of hematopoietic stem cells (HSCs) that are less quiescent than wild-type HSCs. In keeping with this key role in cell fate, GFI1B is emerging as a gene involved in cancer, which also includes solid tumors. In fact, abnormal activation of GFI1B and GFI1 has been related to human medulloblastoma and is also likely to be relevant in blood malignancies. Several pieces of evidence supporting this statement will be detailed in this mini review.

A somatic mutation of GFI1B identified in leukemia alters cell fate via a SPI1 (PU.1) centered genetic regulatory network

We identify a mutation (D262N) in the erythroid-affiliated transcriptional repressor GFI1B, in an acute myeloid leukemia (AML) patient with antecedent myelodysplastic syndrome (MDS). The GFI1B-D262N mutant functionally antagonizes the transcriptional activity of wild-type GFI1B. GFI1B-D262N promoted myelomonocytic versus erythroid output from primary human hematopoietic precursors and enhanced cell survival of both normal and MDS derived precursors. Re-analysis of AML transcriptome data identifies a distinct group of patients in whom expression of wild-type GFI1B and SPI1 (PU.1) have an inverse pattern. In delineating this GFI1B-SPI1 relationship we show that (i) SPI1 is a direct target of GFI1B, (ii) expression of GFI1B-D262N produces elevated expression of SPI1, and (iii) SPI1-knockdown restores balanced lineage output from GFI1B-D262N-expressing precursors. These results table the SPI1-GFI1B transcriptional network as an important regulatory axis in AML as well as in the development of erythroid versus myelomonocytic cell fate.

Growth factor independence 1b (gfi1b) is important for the maturation of erythroid cells and the regulation of embryonic globin expression

Growth factor independence 1b (GFI1B) is a DNA binding repressor of transcription with vital functions in hematopoiesis. Gfi1b-null embryos die at midgestation very likely due to defects in erythro- and megakaryopoiesis. To analyze the full functionality of Gfi1b, we used conditionally deficient mice that harbor floxed Gfi1b alleles and inducible (Mx-Cre, Cre-ERT) or erythroid specific (EpoR-Cre) Cre expressing transgenes. In contrast to the germline knockout, EpoR-Cre mediated erythroid specific ablation of Gfi1b allows full gestation, but causes perinatal lethality with very few mice surviving to adulthood. Both the embryonic deletion of Gfi1b by EpoR-Cre and the deletion in adult mice by Mx-Cre or Cre-ERT leads to reduced numbers of erythroid precursors, perturbed and delayed erythroid maturation, anemia and extramedullary erythropoiesis. Global expression analyses showed that the Hba-x, Hbb-bh1 and Hbb-y embryonic globin genes were upregulated in Gfi1b deficient TER119⁺ fetal liver cells over the gestation period from day 12.5–17.5 p.c. and an increased level of Hbb-bh1 and Hbb-y embryonic globin gene expression was even maintained in adult Gfi1b deficient mice. While the expression of Bcl11a, a regulator of embryonic globin expression was not affected by Gfi1b deficiency, the expression of Gata1 was reduced and the expression of Sox6, also involved in globin switch, was almost entirely lost when Gfi1b was absent. These findings establish Gfi1b as a regulator of embryonic globin expression and embryonic and adult erythroid maturation.

Origin of the brush cell lineage in the mouse intestinal epithelium

Mix progenitors are short-lived multipotential cells formed as intestinal epithelial stem cells initiate a differentiation program. Clone dynamics indicates that various epithelial cell lineages arise from Mix via a sequence of progressively restricted progenitor states. Lateral inhibitory Notch signaling between the daughters of Mix (DOM) is thought to break their initial symmetry, thereby determining whether a DOM invokes a columnar (absorptive) or granulocytic (secretory) cell lineage program. This is supported by the absence of granulocytes following enforced Notch signaling or Atoh1 deletion. Conversely, granulocytes increase in frequency following inhibition of Notch signaling or Hes1 deletion. Thus reciprocal repression between Hes1 and Atoh1 is thought to implement a Notch signaling-driven cell-fate-determining binary switch in DOM. The brush (tuft) cells, a poorly understood chemosensory cell type, are not incorporated into this model. We report that brush cell numbers increase dramatically following conditional Atoh1-deletion, demonstrating that brush cell production, determination, differentiation and survival are Atoh1-independent. We also report that brush cells are derived from Gfi1b-expressing progenitors. These and related results suggest a model in which initially equivalent DOM progenitors have three metastable states defined by the transcription factors Hes1, Atoh1, and Gfi1b. Lateral inhibitory Notch signaling normally ensures that Hes1 dominates in one of the two DOMs, invoking a columnar lineage program, while either Atoh1 or Gfi1b dominates in the other DOM, invoking a granulocytic or brush cell lineage program, respectively, and thus implementing a cell fate-determining ternary switch.

Evidence that growth factor independence 1b regulates dormancy and peripheral blood mobilization of hematopoietic stem cells

Donor-matched transplantation of hematopoietic stem cells (HSCs) is widely used to treat hematologic malignancies but is associated with high mortality. The expansion of HSC numbers and their mobilization into the bloodstream could significantly improve therapy. We report here that adult mice conditionally deficient for the transcription Growth factor independence 1b (Gfi1b) show a significant expansion of functional HSCs in the bone marrow and blood. Despite this expansion, Gfi1b(ko/ko) HSCs retain their ability to self-renew and to initiate multilineage differentiation but are no longer quiescent and contain elevated levels of reactive oxygen species. Treatment of Gfi1b(ko/ko) mice with N-acetyl-cystein significantly reduced HSC numbers indicating that increased reactive oxygen species levels are at least partially responsible for the expansion of Gfi1b-deficient HSCs. Moreover, Gfi1b(-/-) HSCs show decreased expression of CXCR4 and Vascular cell adhesion protein-1, which are required to retain dormant HSCs in the endosteal niche, suggesting that Gfi1b regulates HSC dormancy and pool size without affecting their function. Finally, the additional deletion of the related Gfi1 gene in Gfi1b(ko/ko) HSCs is incompatible with the maintenance of HSCs, suggesting that Gfi1b and Gfi1 have partially overlapping functions but that at least one Gfi gene is essential for the generation of HSCs.