PU.1 immortalizes hematopoietic progenitors in a GM-CSF-dependent manner

ETV6-RUNX1 interacts with a region in SPIB intron 1 to regulate gene expression in pre-B-cell acute lymphoblastic leukemia

The most frequently occurring genetic abnormality in pediatric B-lymphocyte-lineage acute lymphoblastic leukemia is the t(12;21) chromosomal translocation that results in a ETV6-RUNX1 (also known as TEL-AML1) fusion gene. Expression of ETV6-RUNX1 induces a preleukemic condition leading to acquisition of secondary driver mutations, but the mechanism is poorly understood. SPI-B (encoded by SPIB) is an important transcriptional activator of B-cell development and differentiation. We hypothesized that SPIB is directly transcriptionally repressed by ETV6-RUNX1. Using chromatin immunoprecipitation, we identified a regulatory region in the first intron of SPIB that interacts with ETV6-RUNX1. Mutation of the RUNX1 binding site in SPIB intron 1 prevented transcriptional repression in transient transfection assays. Next, we sought to determine to what extent gene expression in REH cells can be altered by ectopic SPI-B expression. SPI-B expression was forced using CRISPR-mediated gene activation and also using a retroviral vector. Forced expression of SPI-B resulted in altered gene expression and, at high levels, impaired cell proliferation and induced apoptosis. Finally, we identified CARD11 and CDKN1A (encoding p21) as transcriptional targets of SPI-B involved in regulation of proliferation and apoptosis. Taken together, this study identifies SPIB as an important target of ETV6-RUNX1 in regulation of B-cell gene expression in t(12;21) leukemia.

Proliferation and Differentiation of Murine Myeloid Precursor 32D/G-CSF-R Cells

Understanding of the hematopoietic stem and progenitor cell biology has important implications for regenerative medicine and the treatment of hematological pathologies. Despite the most relevant data that can be acquired using in vivo models or primary cultures, the low abundance of hematopoietic stem and progenitor cells considerably restricts the pool of suitable techniques for their investigation. Therefore, the use of cell lines allows sufficient production of biological material for the performance of screenings or assays that require large cell numbers. Here we present a detailed description, readout, and interpretation of proliferation and differentiation assays which are used for the investigation of processes involved in myelopoiesis and neutrophilic differentiation. These experiments employ the 32D/G-CSF-R cytokine dependent murine myeloid cell line, which possesses the ability to proliferate in the presence of IL-3 and differentiate in G-CSF. We provide optimized protocols for handling 32D/G-CSF-R cells and discuss major pitfalls and drawbacks that might compromise the described assays and expected results. Additionally, this article contains protocols for lentiviral and retroviral production, titration, and transduction of 32D/G-CSF-R cells. We demonstrate that genetic manipulation of these cells can be employed to successfully perform functional and molecular studies, which can complement results obtained with primary hematopoietic stem and progenitor cells or in vivo models.

PU.1 affects proliferation of the human acute myeloid leukemia U937 cell line by directly regulating MEIS1

The transcription factor PU.1 is a member of the ETS family, which is expressed in a wide variety of hematopoietic lineages. Accumulating evidence has indicated that PU.1 plays a key role in hematopoiesis, and reduced expression of PU.1 leads to the pathogenesis of human myeloid leukemia. As a multi-functional factor, PU.1 is also required for mixed lineage leukemia (MLL) stem cell potential and the development of MLL. However, the function of PU.1 in human non-MLL leukemia and its molecular mechanism remains poorly understood. In the present study, PU.1 siRNA was demonstrated to efficiently inhibit the transcription level of oncogene MEIS1 in the human acute myeloid non-MLL leukemia U937 cell line. In addition, PU.1, as a positive regulator of MEIS1, performed a crucial role in maintaining cell proliferation. Using electrophoretic mobility shift assay, chromatin immunoprecipitation analysis and luciferase reporter assay, previously unexplored evidence that PU.1 activated the MEIS1 promoter through a conserved binding motif in vitro and in vivo was further defined. Overall, the present study provides insight into the molecular mechanism of the contribution of PU.1 to the pathogenesis of non-MLL U937 cells, which is mediated by direct regulation of MEIS1 transcription. The present data reveal the possibility of developing an alternative therapy for non-MLL leukemia by targeting PU.1-mediated MEIS1 gene activation.

Hypermethylation of IGSF4 gene for noninvasive prenatal diagnosis of thalassemia

Background

For patients with pregnancy-induced thalassemia, fetal cord blood or amniotic fluid is invasively collected in the traditional diagnosis and prediction of thalassemia. However, there is no specific molecular target in the diagnosis of thalassemia using fetal DNA from the plasma of pregnant women.

Material/Methods

The promoter of cell surface adhesion molecule (IGSF4) gene was found to be down-regulated in patients with homozygous thalassemia, and the expression of IGSF4 was closely associated with the methylation of its promoter. In the present study, mass spectrometric sequencing of methylation was performed using MassARRAY to detect the 12 CpG sites in the promoter of IGSF4 gene.

Results

The methylation degree of these 12 CpG sites was significantly higher than that in healthy subjects (P<0.05). Hierarchical clustering was done in 23 patients with thalassemia and 5 healthy individuals. Results revealed the promoter of IGSF4 gene was highly methylated in thalassemia patients, which was dramatically different from that in healthy subjects (P<0.05). Methylation-specific PCR (MSP) was employed to confirm the methylation of the promoter of IGSF4 gene and results were consistence with those obtained in sequencing with MassARRAY. Real-time PCR showed, when compared with heterozygous subjects, the expression of IGSF4 was significantly down-regulated in thalassemia patients (ratio=0.18).

Conclusions

The expression of IGSF4 was closely related to the methylation of its promoter, suggesting the methylation of IGSF4 gene is tissue-specific for thalassemia. These findings provide evidence for the non-invasive prenatal diagnosis of thalassemia in terms of epigenetics.

Functional PU.1 in macrophages has a pivotal role in NF-κB activation and neutrophilic lung inflammation during endotoxemia

Although the role of ETS family transcriptional factor PU.1 is well established in macrophage maturation, its role in mature macrophages with reference to sepsis- related animal model has not been elucidated. Here, we report the in vivo function of PU.1 in mediating mature macrophage inflammatory phenotype by using bone marrow chimera mice with conditional PU.1 knockout. We observed that the expression of monocyte/macrophage-specific markers CD 11b, F4/80 in fetal liver cells, and bone marrow-derived macrophages were dependent on functional PU.1. Systemic inflammation as measured in terms of NF-κB reporter activity in lung, liver, and spleen tissues was significantly decreased in PU.1-deficient chimera mice compared with wild-type chimeras on lipopolysaccharide (LPS) challenge. Unlike wild-type chimera mice, LPS challenge in PU.1-deficient chimera mice resulted in decreased lung neu-trophilic inflammation and myeloperoxidase activity. Similarly, we found attenuated inflammatory gene expression (cyclooxygenase-2, inducible nitric-oxide synthase, and TLR4) and inflammatory cytokine secretion (IL-6, MCP-1, IL-1β, TNF-α, and neutrophilic chemokine keratinocyte-derived chemokine) in PU.1-deficient mice. Most importantly, this attenuated lung and systemic inflammatory phenotype was associated with survival benefit in LPS-challenged heterozygotic PU.1-deficient mice, establishing a novel protective mechanistic role for the lineage-specific transcription factor PU.1.

Dose-dependent repression of T-cell and natural killer cell genes by PU.1 enforces myeloid and B-cell identity

The Ets transcription factor PU.1, encoded by the gene Sfpi1, functions in a concentration-dependent manner to promote myeloid and B-cell development and has been implicated in myeloid and lymphoid leukemias. To determine the consequences of reducing PU.1 concentration during hematopoiesis, we analyzed mice with two distinct hypomorphic alleles of Sfpi1 that produce PU.1 at approximately 20% (BN) or approximately 2% (Blac) of wild-type levels. Myeloid development was impaired in these mice, but less severely than in Sfpi1 null mice. To identify the downstream target genes that respond to changes in PU.1 concentration, we analyzed ex vivo interleukin-3 dependent myeloid cell lines established from Sfpi1(BN/BN), Sfpi1(Blac/Blac) and Sfpi1(-/-) fetal liver cells. Unexpectedly, many T-cell and natural killer cell genes were expressed in Sfpi1(-/-) cells and repressed in a dose-dependent manner in Sfpi1(Blac/Blac) and Sfpi1(BN/BN) cells. This pattern of dose-dependent T/NK-cell gene repression also occurred in ex vivo interleukin-7 dependent progenitor B cell lines. These results suggest that PU.1 functions in a concentration-dependent manner to repress T-cell and natural killer cell fates while promoting myeloid and B-cell fates.

PU.1 binding to the p53 family of tumor suppressors impairs their transcriptional activity

The transcription factor PU.1 is essential for terminal myeloid differentiation, B- and T-cell development, erythropoiesis and hematopoietic stem cell maintenance. PU.1 functions as oncogene in Friend virus-induced erythroleukemia and as tumor suppressor in acute myeloid leukemias. Moreover, Friend virus-induced erythroleukemia requires maintenance of PU.1 expression and the disruption of p53 function greatly accelerates disease progression. It has been hypothesized that p53-mediated expression of the p21(Cip1) cell cycle inhibitor during differentiation of pre-erythroleukemia cells promotes selection against p53 function. In addition to the blockage of erythroblast differentiation provided by increased levels of PU.1, we propose that PU.1 alters p53 function. We demonstrate that PU.1 reduces the transcriptional activity of the p53 tumor suppressor family and thus inhibits activation of genes important for cell cycle regulation and apoptosis. Inhibition is mediated through binding of PU.1 to the DNA-binding and/or oligomerization domains of p53/p73 proteins. Lastly, knocking down endogenous PU.1 in p53 wild-type REH B-cell precursor leukemia cells leads to increased expression of the p53 target p21(Cip1).

Reduction in PU.1 activity results in a block to B-cell development, abnormal myeloid proliferation, and neonatal lethality

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.

MATERIALS AND 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.

CONCLUSION

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.

Analysis of concentration-dependent functions of PU.1 in hematopoiesis using mouse models

The Ets family transcription factor PU.1, encoded by the gene Sfpi1, is essential for normal hematopoiesis. A number of studies have suggested that changes in PU.1 concentration play a role in directing cell fate decisions during hematopoiesis. However, the stages of hematopoietic development at which changes in PU.1 concentration are important have not been defined until recently. Experiments using conditional null alleles, reporter alleles, and hypomorphic alleles of the Sfpi1 gene in mice demonstrate that PU.1 concentration is uniformly high during early stages of hematopoietic development. However, reduction of PU.1 concentration is required for normal development of megakaryocyte-erythroid progenitors, B cell progenitors, and T cell progenitors. PU.1 concentration increases in granulocyte-macrophage progenitors. Furthermore, experimental reduction of PU.1 concentration in the myeloid lineages leads to failed differentiation, abnormal proliferation, and leukemia. In this review, we summarize recent studies to develop a new model of PU.1 function in hematopoiesis.

Cancer stem cells in leukemia, recent advances

The history of stem cell research was started in the early 1900s in Europe where the researcher realized that various types of blood cells came from a particular "stem cells." However, it was not until 1963 that the first quantitative description of the self-renewal activities of transplanted mouse bone marrow cells were documented by Canadian scientist Ernest A McCulloch and James E Till in Toronto. The concept of cancer stem cells has been used over 50 years ago; whereas the strong evidence for the existence of a Cancer Stem Cells was obtained recently. Consequently, there is increasing attention in recent year about cancer stem cells. The findings from recent studies support the concept that stem cells are integral to the development of several forms of human cancer. Changes in stem cell behavior can contribute to tumor formation. Leukemia is a cancer of blood-forming tissue, including the bone marrow and lymphatic system. Leukemic stem cells represent the cancer stem cells in the leukemia. In this review, we summarize the recent advance in the study of leukemic stem cells.