Biomedical subjects

Trang Hoang

Publications and source records attributed to Trang Hoang.

14 recordsLinked to original sources

PubMed2006

ETO2 coordinates cellular proliferation and differentiation during erythropoiesis.

The passage from proliferation to terminal differentiation is critical for normal development and is often perturbed in malignancies. To define the molecular mechanisms that govern this process during erythropoiesis, we have used tagging/proteomics approaches and characterized protein complexes nucleated by TAL-1/SCL, a basic helix-loop-helix transcription factor that specifies the erythrocytic lineage. In addition to known TAL-1 partners, GATA-1, E2A, HEB, LMO2 and Ldb1, we identify the ETO2 repressor as a novel component recruited to TAL-1 complexes through interaction with E2A/HEB. Ectopic expression and siRNA knockdown experiments in hematopoietic progenitor cells show that ETO2 actively represses erythroid TAL-1 target genes and governs the expansion of erythroid progenitors. At the onset of erythroid differentiation, a change in the stoichiometry of ETO2 within the TAL-1 complex activates the expression of known erythroid-specific TAL-1 target genes and of Gfi-1b and p21(Cip), encoding two essential regulators of erythroid cell proliferation. These results suggest that the dynamics of ETO2 recruitment within nuclear complexes couple cell proliferation to cell differentiation and determine the onset of terminal erythroid maturation.

Animals↗

PubMed2006

Novel insights into the pathogenesis of the Graffi murine leukemia retrovirus.

The Graffi murine leukemia virus (MuLV) was isolated in 1954 by Arnold Graffi, who characterized it as a myeloid leukemia-inducing retrovirus. He and his team, however, soon observed the intriguing phenomenon of hematological diversification, which corresponded to a decrease of myeloid leukemias and an increase of other types of leukemias. Recently, we derived two different molecular clones corresponding to ecotropic nondefective genomes that were named GV-1.2 and GV-1.4. The induced leukemias were classified as myeloid based on morphological analysis of blood smears. In this study, we further characterized the two variants of the Graffi murine retrovirus, GV-1.2 and GV-1.4, in three different strains of mice. We show that the Graffi MuLV is a multipotent retrovirus capable of inducing both lymphoid (T- and B-cell) and nonlymphoid (myeloid, erythroid, megakaryocytic) leukemia. Many of these are very complex with concomitant expression of different hematopoietic lineages. Interestingly, a high percentage of megakaryocytic leukemias, a type of leukemia rarely observed with MuLVs, arise in the FVB/n strain of mice. The genetic backgrounds of the different strains of mice influence greatly the results. Furthermore, the enhancer region, different for GV-1.2 and GV-1.4, plays a pivotal role in the disease specificity: GV-1.2 induces more lymphoid leukemias, and GV-1.4 induces more nonlymphoid ones.

Animals↗

PubMed2004

The origin of hematopoietic cell type diversity.

The hematopoietic system remains robust with regards to extrinsic perturbations, in sharp contrast with the stochastic behavior of hematopoeitic stem cells (HSCs) at the single cell level, suggesting that stability may be achieved within a stem cell system that undergoes constant self-renewal, commitment to differentiation and generates cell type diversification. Converging evidence at the interface of cellular, molecular and numerical studies suggests that diversity is generated by the chaotic dynamics of transcription factor networks within a cell and of the combination of growth factors and cytokines in the environment, both involving cooperation and competition. Current evidence indicates that HSCs are primed for multilineage gene expression. A subtle shift in transcription factor dosage is sufficient to perturb this equilibrium and to drive lineage commitment that involves a resolution of complexity at the molecular level and a transition towards less chaotic behavior. This dynamical instability establishes a state of responsiveness to extrinsic signals. Evolutionary conserved environmental cues that drive pattern formation or migratory behavior during embryonic development operate in the adult to influence the decision between self-renewal and differentiation in HSCs, as exemplified by the role of Notch1, Wnt proteins, BMPs and VEGF. In contrast, a network of cytokines uniquely present in mammalians influences later developmental stages, from progenitors with more restricted potentials (tri-, bi- or unipotent) to mature functional cells. These cytokines have co-opted the ancient Jak-STAT pathway but also appear to trigger lineage-affiliated transcription factors, thus linking environmental signaling to cell fate decisions.

Acute Disease↗

PubMed2004

SCL: from the origin of hematopoiesis to stem cells and leukemia.

In the hematopoietic system, lineage commitment and differentiation is controlled by the combinatorial action of transcription factors from diverse families. SCL is a basic helix-loop-helix transcription factor that is an essential regulator at several levels in the hematopoietic hierarchy and whose inappropriate regulation frequently contributes to the development of pediatric T-cell acute lymphoblastic leukemia. This review discusses advances that have shed important light on the functions played by SCL during normal hematopoiesis and leukemogenesis and have revealed an unexpected robustness of hematopoietic stem cell function. Molecular studies have unraveled a mechanism through which gene expression is tightly controlled, as SCL functions within multifactorial complexes that exhibit an all-or-none switch-like behavior in transcription activation, arguing for a quantal process that depends on the concurrent occupation of target loci by all members of the complex. Finally, variations in composition of SCL-containing complexes may ensure flexibility and specificity in the regulation of lineage-specific programs of gene expression, thus providing the molecular basis through which SCL exerts its essential functions at several branch points of the hematopoietic hierarchy.

Animals↗

PubMed2004

SCL assembles a multifactorial complex that determines glycophorin A expression.

SCL/TAL1 is a hematopoietic-specific transcription factor of the basic helix-loop-helix (bHLH) family that is essential for erythropoiesis. Here we identify the erythroid cell-specific glycophorin A gene (GPA) as a target of SCL in primary hematopoietic cells and show that SCL occupies the GPA locus in vivo. GPA promoter activation is dependent on the assembly of a multifactorial complex containing SCL as well as ubiquitous (E47, Sp1, and Ldb1) and tissue-specific (LMO2 and GATA-1) transcription factors. In addition, our observations suggest functional specialization within this complex, as SCL provides its HLH protein interaction motif, GATA-1 exerts a DNA-tethering function through its binding to a critical GATA element in the GPA promoter, and E47 requires its N-terminal moiety (most likely entailing a transactivation function). Finally, endogenous GPA expression is disrupted in hematopoietic cells through the dominant-inhibitory effect of a truncated form of E47 (E47-bHLH) on E-protein activity or of FOG (Friend of GATA) on GATA activity or when LMO2 or Ldb-1 protein levels are decreased. Together, these observations reveal the functional complementarities of transcription factors within the SCL complex and the essential role of SCL as a nucleation factor within a higher-order complex required to activate gene GPA expression.

Adaptor Proteins, Signal Transducing↗

PubMed2004

SCL interacts with VEGF to suppress apoptosis at the onset of hematopoiesis.

During development, hematopoiesis initiates in the yolk sac through a process that depends on VEGF/Flk1 signaling and on the function of the SCL/Tal1 transcription factor. Here we show that VEGF modifies the developmental potential of primitive erythroid progenitors and prolongs their life span. Furthermore, the survival of yolk sac erythrocytes in vivo depends on the dose of VEGF. Interestingly, in Vegf(lo/lo) embryos carrying a hypomorph allele, Flk1-positive cells reach the yolk sac at E8.5, but are severely compromised in their ability to generate primitive erythroid precursors. These observations indicate that during embryonic development, different thresholds of VEGF are required for the migration and clonal expansion of hematopoietic precursors. The near absence of primitive erythroid precursors in Vegf(lo/lo) embryos correlates with low levels of Scl in the yolk sac. Strikingly, gain-of-function of SCL partially complements the hematopoietic defect caused by the hypomorph Vegf(lo) allele, and re-establishes the survival of erythroid cells and the expression of erythroid genes (Gata1 and betaH1). This indicates that SCL functions downstream of VEGF to ensure an expansion of the hematopoietic compartment.

Animals↗

PubMed2003

Biallelic transcriptional activation of oncogenic transcription factors in T-cell acute lymphoblastic leukemia.

Aberrant expression of transcription factor oncogenes such as HOX11, HOX11L2, TAL1/SCL, LYL1, LMO1, and LMO2 can be detected in lymphoblasts from up to 80% of patients with acute T-cell lymphoblastic leukemia (T-ALL). Transcriptional activation of these oncogenes in leukemic cells typically results from chromosomal rearrangements that place them next to highly active cis-acting transcriptional regulatory elements. However, biallelic activation of TAL1 in some T-ALL cases has been previously proposed. We have used allele-specific mRNA analysis to show that trans-acting mechanisms leading to biallelic overexpression of TAL1 are involved in 10 (42%) of 24 TAL1+ informative T-ALL cases, 2 (17%) of 12 HOX11+ informative cases, and 7 (64%) of 11 LMO2+ informative cases. We propose that aberrant expression of oncogenic transcription factors in a significant fraction of T-ALLs may result from loss of the upstream transcriptional mechanisms that normally down-regulate the expression of these oncogenes during T-cell development.

Adaptor Proteins, Signal Transducing↗

PubMed2003

Unsuspected role of the brain morphogenetic gene Otx1 in hematopoiesis.

Otx1 belongs to the paired class of homeobox genes and plays a pivotal role in brain development. Here, we show that Otx1 is expressed in hematopoietic pluripotent and erythroid progenitor cells. Moreover, bone marrow cells from mice lacking Otx1 exhibit a cell-autonomous impairment of the erythroid compartment. In agreement with these results, molecular analysis revealed decreased levels of erythroid genes that include the SCL and GATA-1 transcription factors. Accordingly, a gain of function of SCL rescues the erythroid deficiency in Otx1-/- mice. Taken together, our findings indicate a function for Otx1 in the regulation of blood cell production.

Animals↗

PubMed2003

Translational control of SCL-isoform expression in hematopoietic lineage choice.

We investigated the translational regulation of SCL protein expression and its role in hematopoietic lineage choice. We show that the expression of different SCL protein isoforms is regulated by signal transduction pathways that modulate translation initiation factor (eIF) function. A conserved small upstream open reading frame (uORF) in SCL transcripts acts as a cis-regulatory element for isoform expression. At the onset of erythroid differentiation, truncated SCL protein isoforms arise by alternative translation initiation and favor the erythroid lineage. In comparison, full-length SCL proteins are more efficient at enhancing the megakaryocyte lineage. Together, our studies unravel translational control as a novel mechanism regulating hematopoietic outcome.

Animals↗

PubMed2003

Growth factor independence-1B expression leads to defects in T cell activation, IL-7 receptor alpha expression, and T cell lineage commitment.

T cell differentiation in the thymus is dependent upon signaling through the TCR and is characterized by the resulting changes in expression patterns of CD4 and CD8 surface coreceptor molecules. Although recent studies have characterized the effects of proximal TCR signaling on T cell differentiation, the downstream integration of these signals remains largely unknown. The growth factor independence-1 (GFI1) and GFI1B transcriptional repressors may regulate cytokine signaling pathways to affect lymphocyte growth and survival. In this study, we show that Gfi1 expression is induced upon induction of the T cell program. Gfi1B expression is low and dynamic during T cell development, but is terminated in mature thymocytes. Transgenic expression of GFI1 and GFI1B in T cells allowed us to determine the functional consequences of constitutive expression. GFI1 potentiates response to TCR stimulation and IL-2, whereas GFI1B-transgenic T cells are defective in T cell activation. Moreover, GFI1B-transgenic thymocytes display reduced expression of the late-activation marker IL-7R alpha, and a decrease in CD4(-)8(+) single-positive T cells that can be mitigated by transgenic expression of BCL2 or GFI1. These data show that GFI1 and GFI1B are functionally unique, and implicate a role for GFI1 in the integration of activation and survival signals.

Animals↗

PubMed2003

Regulation of pT alpha gene expression by a dosage of E2A, HEB, and SCL.

The expression of the pT alpha gene is required for effective selection, proliferation, and survival of beta T-cell receptor (beta TCR)-expressing immature thymocytes. Here, we have identified two phylogenetically conserved E-boxes within the pT alpha enhancer sequence that are required for optimal enhancer activity and for its stage-specific activity in immature T cells. We have shown that the transcription factors E2A and HEB associate with high affinity to these E-boxes. Moreover, we have identified pT alpha as a direct target of E2A-HEB heterodimers in immature thymocytes because they specifically occupy the enhancer in vivo. In these cells, pT alpha mRNA levels are determined by the presence of one or two functional E2A or HEB alleles. Furthermore, E2A/HEB transcriptional activity is repressed by heterodimerization with SCL, a transcription factor that is turned off in differentiating thymocytes exactly at a stage when pT alpha is up-regulated. Taken together, our observations suggest that the dosage of E2A, HEB, and SCL determines pT alpha gene expression in immature T cells.

Amino Acid Sequence↗

PubMed2002

The SCL complex regulates c-kit expression in hematopoietic cells through functional interaction with Sp1.

The combinatorial interaction among transcription factors is believed to determine hematopoietic cell fate. Stem cell leukemia (SCL, also known as TAL1 [T-cell acute lymphoblastic leukemia 1]) is a tissue-specific basic helix-loop-helix (bHLH) factor that plays a central function in hematopoietic development; however, its target genes and molecular mode of action remain to be elucidated. Here we show that SCL and the c-Kit receptor are coexpressed in hematopoietic progenitors at the single-cell level and that SCL induces c-kit in chromatin, as ectopic SCL expression in transgenic mice sustains c-kit transcription in developing B lymphocytes, in which both genes are normally down-regulated. Through transient transfection assays and coimmunoprecipitation of endogenous proteins, we define the role of SCL as a nucleation factor for a multifactorial complex (SCL complex) that specifically enhances c-kit promoter activity without affecting the activity of myelomonocytic promoters. This complex, containing hematopoietic-specific (SCL, Lim-only 2 (LMO2), GATA-1/GATA-2) and ubiquitous (E2A, LIM- domain binding protein 1 [Ldb-1]) factors, is tethered to DNA via a specificity protein 1 (Sp1) motif, through direct interactions between elements of the SCL complex and the Sp1 zinc finger protein. Furthermore, we demonstrate by chromatin immunoprecipitation that SCL, E2A, and Sp1 specifically co-occupy the c-kit promoter in vivo. We therefore conclude that c-kit is a direct target of the SCL complex. Proper activation of the c-kit promoter depends on the combinatorial interaction of all members of the complex. Since SCL is down-regulated in maturing cells while its partners remain expressed, our observations suggest that loss of SCL inactivates the SCL complex, which may be an important event in the differentiation of pluripotent hematopoietic cells.

3T3 Cells↗

PubMed2002

Gradient of E2A activity in B-cell development.

The E2A locus is a frequent target of chromosomal translocations in B-cell acute lymphoblastic leukemia (B-ALL). E2A encodes two products, E12 and E47, that are part of the basic helix-loop-helix (bHLH) family of transcription factors and are central in B lineage differentiation. E2A haplo-insufficiency hinders progression through three major checkpoints in B-cell development: commitment into the B lineage, at the pro-B to pre-B transition, and in the induction of immunoglobulin M (IgM) expression required for a functional BCR. These observations underscore the importance of E2A gene dosage in B-cell development. Here we show that a higher proportion of pro-B cells in E2A(+/-) mice is in the cell cycle compared to that in wild-type littermates. This increase correlates with lower p21(waf/cip1) levels, indicating that E2A has an antiproliferative function in B-cell progenitors. Ectopic expression in the B lineage of SCL/Tal1, a tissue-specific bHLH factor that inhibits E2A function, blocks commitment into the B lineage without affecting progression through later stages of differentiation. Furthermore, ectopic SCL expression exacerbates E2A haplo-insufficiency in B-cell differentiation, indicating that SCL genetically interacts with E2A. Taken together, these observations provide evidence for a gradient of E2A activity that increases from the pre-pro-B to the pre-B stage and suggest a model in which low levels of E2A (as in pro-B cells) are sufficient to control cell growth, while high levels (in pre-B cells) are required for cell differentiation. The antiproliferative function of E2A further suggests that in B-ALL associated with t(1;19) and t(17;19), the disruption of one E2A allele contributes to leukemogenesis, in addition to other anomalies induced by E2A fusion proteins.

Animals↗

PubMed1995

Analysis of gene expression in a complex differentiation hierarchy by global amplification of cDNA from single cells.

BACKGROUND: Many differentiating tissues contain progenitor cells that differ in their commitment states but cannot be readily distinguished or segregated. Molecular analysis is therefore restricted to mixed populations or cell lines which may also be heterogeneous, and the critical differences in gene expression that might determine divergent development are obscured. In this study, we combined global amplification of mRNA transcripts in single cells with identification of the developmental potential of processed cells on the basis of the fates of their sibling cells from clonal starts. RESULTS: We analyzed clones of from four to eight hemopoietic precursor cells which had a variety of differentiative potentials; sibling cells generally each formed clones of identical composition in secondary culture. Globally amplified cDNA was prepared from individual precursors whose developmental potential was identified by tracking sibling fates. Further cDNA samples were prepared from terminally maturing, homogeneous hemopoietic cell populations. Together, the samples represented 16 positions in the hemopoietic developmental hierarchy. Expression patterns in the sample set were determined for 29 genes known to be involved in hemopoietic cell growth, differentiation or function. The cDNAs from a bipotent erythroid/megakaryocyte precursor and a bipotent neutrophil/macrophage precursor were subtractively hybridized, yielding numerous differentially expressed cDNA clones. Hybridization of such clones to the entire precursor sample set identified transcripts with consistent patterns of differential expression in the precursor hierarchy. CONCLUSIONS: Tracking of sibling fates reliably identifies the differentiative potential of a single cell taken for PCR analysis, and demonstrates the existence of a variety of distinct and stable states of differentiative commitment. Global amplification of cDNA from single precursor cells, identified by sibling fates, yields a true representation of lineage- and stage-specific gene expression, as confirmed by hybridization to a broad panel of probes. The results provide the first expression mapping of these genes that distinguishes between progenitors in different commitment states, generate new insights and predictions relevant to mechanism, and introduce a powerful set of tools for unravelling the genetic basis of lineage divergence.

Animals↗