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

Enhancers in T Cell development and malignant lesions

Enhancers constitute a vital category of cis-regulatory elements with a Mediator complex within DNA sequences, orchestrating gene expression by activating promoters. In the development of T cells, some enhancers regulate the critical genes, which might also regulate T cell malignant lesions. This review is to comprehensively elucidate the contributions of enhancers in both normal T cell development and its malignant pathogenesis, proposing the idea that the precise subunits of the Mediator complex are the potential drug target for disrupting the specific gene enhancer for T cell malignant diseases.

Multi-modular structure of the gene regulatory network for specification and commitment of murine T cells

T cells develop from multipotent progenitors by a gradual process dependent on intrathymic Notch signaling and coupled with extensive proliferation. The stages leading them to T-cell lineage commitment are well characterized by single-cell and bulk RNA analyses of sorted populations and by direct measurements of precursor-product relationships. This process depends not only on Notch signaling but also on multiple transcription factors, some associated with stemness and multipotency, some with alternative lineages, and others associated with T-cell fate. These factors interact in opposing or semi-independent T cell gene regulatory network (GRN) subcircuits that are increasingly well defined. A newly comprehensive picture of this network has emerged. Importantly, because key factors in the GRN can bind to markedly different genomic sites at one stage than they do at other stages, the genes they significantly regulate are also stage-specific. Global transcriptome analyses of perturbations have revealed an underlying modular structure to the T-cell commitment GRN, separating decisions to lose "stem-ness" from decisions to block alternative fates. Finally, the updated network sheds light on the intimate relationship between the T-cell program, which depends on the thymus, and the innate lymphoid cell (ILC) program, which does not.

Direct regulation of TCR rearrangement and expression by E proteins during early T cell development

γδ T cells are widely distributed throughout mucosal and epithelial cell-rich tissues and are an important early source of IL-17 in response to several pathogens. Like αβ T cells, γδ T cells undergo a stepwise process of development in the thymus that requires recombination of genome-encoded segments to assemble mature T cell receptor (TCR) genes. This process is tightly controlled on multiple levels to enable TCR segment assembly while preventing the genomic instability inherent in the double-stranded DNA breaks that occur during this process. Each TCR locus has unique aspects in its structure and requirements, with different types of regulation before and after the αβ/γδ T cell fate choice. It has been known that Runx and Myb are critical transcriptional regulators of TCRγ and TCRδ expression, but the roles of E proteins in TCRγ and TCRδ regulation have been less well explored. Multiple lines of evidence show that E proteins are involved in TCR expression at many different levels, including the regulation of Rag recombinase gene expression and protein stability, induction of germline V segment expression, chromatin remodeling, and restriction of the fetal and adult γδTCR repertoires. Importantly, E proteins interact directly with the cis-regulatory elements of the TCRγ and TCRδ loci, controlling the predisposition of a cell to become an αβ T cell or a γδ T cell, even before the lineage-dictating TCR signaling events. This article is categorized under: Immune System Diseases > Stem Cells and Development Immune System Diseases > Genetics/Genomics/Epigenetics.

The divergence between T cell and innate lymphoid cell fates controlled by E and Id proteins

T cells develop in the thymus from lymphoid primed multipotent progenitors or common lymphoid progenitors into αβ and γδ subsets. The basic helix-loop-helix transcription factors, E proteins, play pivotal roles at multiple stages from T cell commitment to maturation. Inhibitors of E proteins, Id2 and Id3, also regulate T cell development while promoting ILC differentiation. Recent findings suggest that the thymus can also produce innate lymphoid cells (ILCs). In this review, we present current findings that suggest the balance between E and Id proteins is likely to be critical for controlling the bifurcation of T cell and ILC fates at early stages of T cell development.

Transcriptional dynamics and epigenetic regulation of E and ID protein encoding genes during human T cell development

T cells are generated from hematopoietic stem cells through a highly organized developmental process, in which stage-specific molecular events drive maturation towards αβ and γδ T cells. Although many of the mechanisms that control αβ- and γδ-lineage differentiation are shared between human and mouse, important differences have also been observed. Here, we studied the regulatory dynamics of the E and ID protein encoding genes during pediatric human T cell development by evaluating changes in chromatin accessibility, histone modifications and bulk and single cell gene expression. We profiled patterns of ID/E protein activity and identified up- and downstream regulators and targets, respectively. In addition, we compared transcription of E and ID protein encoding genes in human versus mouse to predict both shared and unique activities in these species, and in prenatal versus pediatric human T cell differentiation to identify regulatory changes during development. This analysis showed a putative involvement of TCF3/E2A in the development of γδ T cells. In contrast, in αβ T cell precursors a pivotal pre-TCR-driven population with high ID gene expression and low predicted E protein activity was identified. Finally, in prenatal but not postnatal thymocytes, high HEB/TCF12 levels were found to counteract high ID levels to sustain thymic development. In summary, we uncovered novel insights in the regulation of E and ID proteins on a cross-species and cross-developmental level.

Monoallelic Heb/Tcf12 Deletion Reduces the Requirement for NOTCH1 Hyperactivation in T-Cell Acute Lymphoblastic Leukemia

Early T-cell development is precisely controlled by E proteins, that indistinguishably include HEB/TCF12 and E2A/TCF3 transcription factors, together with NOTCH1 and pre-T cell receptor (TCR) signalling. Importantly, perturbations of early T-cell regulatory networks are implicated in leukemogenesis. NOTCH1 gain of function mutations invariably lead to T-cell acute lymphoblastic leukemia (T-ALL), whereas inhibition of E proteins accelerates leukemogenesis. Thus, NOTCH1, pre-TCR, E2A and HEB functions are intertwined, but how these pathways contribute individually or synergistically to leukemogenesis remain to be documented. To directly address these questions, we leveraged Cd3e-deficient mice in which pre-TCR signaling and progression through β-selection is abrogated to dissect and decouple the roles of pre-TCR, NOTCH1, E2A and HEB in SCL/TAL1-induced T-ALL, via the use of Notch1 gain of function transgenic (Notch1ICtg) and Tcf12+/- or Tcf3+/- heterozygote mice. As a result, we now provide evidence that both HEB and E2A restrain cell proliferation at the β-selection checkpoint while the clonal expansion of SCL-LMO1-induced pre-leukemic stem cells in T-ALL is uniquely dependent on Tcf12 gene dosage. At the molecular level, HEB protein levels are decreased via proteasomal degradation at the leukemic stage, pointing to a reversible loss of function mechanism. Moreover, in SCL-LMO1-induced T-ALL, loss of one Tcf12 allele is sufficient to bypass pre-TCR signaling which is required for Notch1 gain of function mutations and for progression to T-ALL. In contrast, Tcf12 monoallelic deletion does not accelerate Notch1IC-induced T-ALL, indicating that Tcf12 and Notch1 operate in the same pathway. Finally, we identify a tumor suppressor gene set downstream of HEB, exhibiting significantly lower expression levels in pediatric T-ALL compared to B-ALL and brain cancer samples, the three most frequent pediatric cancers. In summary, our results indicate a tumor suppressor function of HEB/TCF12 in T-ALL to mitigate cell proliferation controlled by NOTCH1 in pre-leukemic stem cells and prevent NOTCH1-driven progression to T-ALL.

The transcription factor TAL1 and miR-17-92 create a regulatory loop in hematopoiesis

A network of gene regulatory factors such as transcription factors and microRNAs establish and maintain gene expression patterns during hematopoiesis. In this network, transcription factors regulate each other and are involved in regulatory loops with microRNAs. The microRNA cluster miR-17-92 is located within the MIR17HG gene and encodes six mature microRNAs. It is important for hematopoietic differentiation and plays a central role in malignant disease. However, the transcription factors downstream of miR-17-92 are largely elusive and the transcriptional regulation of miR-17-92 is not fully understood. Here we show that miR-17-92 forms a regulatory loop with the transcription factor TAL1. The miR-17-92 cluster inhibits expression of TAL1 and indirectly leads to decreased stability of the TAL1 transcriptional complex. We found that TAL1 and its heterodimerization partner E47 regulate miR-17-92 transcriptionally. Furthermore, miR-17-92 negatively influences erythroid differentiation, a process that depends on gene activation by the TAL1 complex. Our data give example of how transcription factor activity is fine-tuned during normal hematopoiesis. We postulate that disturbance of the regulatory loop between TAL1 and the miR-17-92 cluster could be an important step in cancer development and progression.

Secretome Analysis of Inductive Signals for BM-MSC Transdifferentiation into Salivary Gland Progenitors

Severe dry mouth in patients with Sjögren's Syndrome, or radiation therapy for patients with head and neck cancer, significantly compromises their oral health and quality of life. The current clinical management of xerostomia is limited to palliative care as there are no clinically-proven treatments available. Previously, our studies demonstrated that mouse bone marrow-derived mesenchymal stem cells (mMSCs) can differentiate into salivary progenitors when co-cultured with primary salivary epithelial cells. Transcription factors that were upregulated in co-cultured mMSCs were identified concomitantly with morphological changes and the expression of acinar cell markers, such as α-amylase (AMY1), muscarinic-type-3-receptor(M3R), aquaporin-5(AQP5), and a ductal cell marker known as cytokeratin 19(CK19). In the present study, we further explored inductive molecules in the conditioned media that led to mMSC reprogramming by high-throughput liquid chromatography with tandem mass spectrometry and systems biology. Our approach identified ten differentially expressed proteins based on their putative roles in salivary gland embryogenesis and development. Additionally, systems biology analysis revealed six candidate proteins, namely insulin-like growth factor binding protein-7 (IGFBP7), cysteine-rich, angiogenetic inducer, 61(CYR61), agrin(AGRN), laminin, beta 2 (LAMB2), follistatin-like 1(FSTL1), and fibronectin 1(FN1), for their potential contribution to mMSC transdifferentiation during co-culture. To our knowledge, our study is the first in the field to identify soluble inductive molecules that drive mMSC into salivary progenitors, which crosses lineage boundaries.

Oncogenic transcriptional program driven by TAL1 in T-cell acute lymphoblastic leukemia

TAL1/SCL is a prime example of an oncogenic transcription factor that is abnormally expressed in acute leukemia due to the replacement of regulator elements. This gene has also been recognized as an essential regulator of hematopoiesis. TAL1 expression is strictly regulated in a lineage- and stage-specific manner. Such precise control is crucial for the switching of the transcriptional program. The misexpression of TAL1 in immature thymocytes leads to a widespread series of orchestrated downstream events that affect several different cellular machineries, resulting in a lethal consequence, namely T-cell acute lymphoblastic leukemia (T-ALL). In this article, we will discuss the transcriptional regulatory network and downstream target genes, including protein-coding genes and non-coding RNAs, controlled by TAL1 in normal hematopoiesis and T-cell leukemogenesis.

HEB is required for the specification of fetal IL-17-producing γδ T cells

IL-17-producing γδ T (γδT17) cells are critical components of the innate immune system. However, the gene networks that control their development are unclear. Here we show that HEB (HeLa E-box binding protein, encoded by Tcf12) is required for the generation of a newly defined subset of fetal-derived CD73⁻ γδT17 cells. HEB is required in immature CD24⁺CD73⁻ γδ T cells for the expression of Sox4, Sox13, and Rorc, and these genes are repressed by acute expression of the HEB antagonist Id3. HEB-deficiency also affects mature CD73⁺ γδ T cells, which are defective in RORγt expression and IL-17 production. Additionally, the fetal TCRγ chain repertoire is altered, and peripheral Vγ4 γδ T cells are mostly restricted to the IFNγ-producing phenotype in HEB-deficient mice. Therefore, our work identifies HEB-dependent pathways for the development of CD73⁺ and CD73⁻ γδT17 cells, and provides mechanistic evidence for control of the γδT17 gene network by HEB.

The γδ T cell pool includes abundant IL-17-producing cells that protect mucosal surfaces, but the signals that control γδ T cell specification are unclear. Here the authors identify a role for the transcription factor HEB, and antagonistic activity of Id3, in the development of these cells.

Transcription factors and target genes of pre-TCR signaling

Almost 30 years ago pioneering work by the laboratories of Harald von Boehmer and Susumo Tonegawa provided the first indications that developing thymocytes could assemble a functional TCRβ chain-containing receptor complex, the pre-TCR, before TCRα expression. The discovery and study of the pre-TCR complex revealed paradigms of signaling pathways in control of cell survival and proliferation, and culminated in the recognition of the multifunctional nature of this receptor. As a receptor integrated in a dynamic developmental process, the pre-TCR must be viewed not only in the light of the biological outcomes it promotes, but also in context with those molecular processes that drive its expression in thymocytes. This review article focuses on transcription factors and target genes activated by the pre-TCR to drive its different outcomes.

SCL, LMO1 and Notch1 reprogram thymocytes into self-renewing cells

The molecular determinants that render specific populations of normal cells susceptible to oncogenic reprogramming into self-renewing cancer stem cells are poorly understood. Here, we exploit T-cell acute lymphoblastic leukemia (T-ALL) as a model to define the critical initiating events in this disease. First, thymocytes that are reprogrammed by the SCL and LMO1 oncogenic transcription factors into self-renewing pre-leukemic stem cells (pre-LSCs) remain non-malignant, as evidenced by their capacities to generate functional T cells. Second, we provide strong genetic evidence that SCL directly interacts with LMO1 to activate the transcription of a self-renewal program coordinated by LYL1. Moreover, LYL1 can substitute for SCL to reprogram thymocytes in concert with LMO1. In contrast, inhibition of E2A was not sufficient to substitute for SCL, indicating that thymocyte reprogramming requires transcription activation by SCL-LMO1. Third, only a specific subset of normal thymic cells, known as DN3 thymocytes, is susceptible to reprogramming. This is because physiological NOTCH1 signals are highest in DN3 cells compared to other thymocyte subsets. Consistent with this, overexpression of a ligand-independent hyperactive NOTCH1 allele in all immature thymocytes is sufficient to sensitize them to SCL-LMO1, thereby increasing the pool of self-renewing cells. Surprisingly, hyperactive NOTCH1 cannot reprogram thymocytes on its own, despite the fact that NOTCH1 is activated by gain of function mutations in more than 55% of T-ALL cases. Rather, elevating NOTCH1 triggers a parallel pathway involving Hes1 and Myc that dramatically enhances the activity of SCL-LMO1 We conclude that the acquisition of self-renewal and the genesis of pre-LSCs from thymocytes with a finite lifespan represent a critical first event in T-ALL. Finally, LYL1 and LMO1 or LMO2 are co-expressed in most human T-ALL samples, except the cortical T subtype. We therefore anticipate that the self-renewal network described here may be relevant to a majority of human T-ALL.

Deciphering the initiating events in lymphoid leukemia is important for the development of new therapeutic strategies. In this manuscript, we define oncogenic reprogramming as the process through which non-self-renewing progenitors are converted into pre-leukemic stem cells with sustained self-renewal capacities. We provide strong genetic evidence that this step is rate-limiting in leukemogenesis and requires the activation of a self-renewal program by oncogenic transcription factors, as exemplified by SCL and LMO1. Furthermore, NOTCH1 is a pathway that drives cell fate in the thymus. We demonstrate that homeostatic NOTCH1 levels that are highest in specific thymocyte subsets determine their susceptibilities to oncogenic reprogramming by SCL and LMO1. Our data provide novel insight into the acquisition of self-renewal as a critical first step in lymphoid cell transformation, requiring the synergistic interaction of oncogenic transcription factors with a cellular context controlled by high physiological NOTCH1.

Regulatory mechanisms of thymus and T cell development

The thymus is a central hematopoietic organ which produces mature T lymphocytes with diverse antigen specificity. During development, the thymus primordium is derived from the third pharyngeal endodermal pouch, and then differentiates into cortical and medullary thymic epithelial cells (TECs). TECs represent the primary functional cell type that forms the unique thymic epithelial microenvironment which is essential for intrathymic T-cell development, including positive selection, negative selection and emigration out of the thymus. Our understanding of thymopoiesis has been greatly advanced by using several important animal models. This review will describe progress on the molecular mechanisms involved in thymus and T cell development with particular focus on the signaling and transcription factors involved in this process in mouse and zebrafish.

Dynamic interaction between TAL1 oncoprotein and LSD1 regulates TAL1 function in hematopoiesis and leukemogenesis

TAL1/SCL is a hematopoietic-specific oncogene and its activity is regulated by associated transcriptional co-activators and corepressors. Dysregulation of TAL1 activity has been associated with T-cell leukemogenesis. However, it remains unclear how the interactions between TAL1 and corepressors versus co-activators are properly regulated. Here, we reported that protein kinase A (PKA)-mediated phosphorylation regulates TAL1 interaction with the lysine-specific demethylase (LSD1) that removes methyl group from methylated Lys 4 on histone H3 tails. Phosphorylation of serine 172 in TAL1 specifically destabilizes the TAL1-LSD1 interaction leading to promoter H3K4 hypermethylation and activation of target genes that have been suppressed in normal and malignant hematopoiesis. Knockdown of TAL1 or LSD1 led to a derepression of the TAL1 target genes in T-cell acute lymphoblast leukemia (T-ALL) Jurkat cells, which is accompanied by elevating promoter H3K4 methylation. Similarly, treatment of PKA activator forskolin resulted in derepression of target genes by reducing its interaction with LSD1 while PKA inhibitor H89 represses them by suppressing H3K4 methylation levels. Consistent with the dual roles of TAL1 in transcription, TAL1-associated LSD1 is decreased while recruitment of hSET1 is increased at the TAL1 targets during erythroid differentiation. This process is accompanied by a dramatic increase in H3K4 methylation. Thus, our data revealed a novel interplay between PKA phosphorylation and TAL1-mediated epigenetic regulation that regulates hematopoietic transcription and differentiation programs during hematopoiesis and leukemogenesis.

HEB in the spotlight: Transcriptional regulation of T-cell specification, commitment, and developmental plasticity

The development of T cells from multipotent progenitors in the thymus occurs by cascades of interactions between signaling molecules and transcription factors, resulting in the loss of alternative lineage potential and the acquisition of the T-cell functional identity. These processes require Notch signaling and the activity of GATA3, TCF1, Bcl11b, and the E-proteins HEB and E2A. We have shown that HEB factors are required to inhibit the thymic NK cell fate and that HEBAlt allows the passage of T-cell precursors from the DN to DP stage but is insufficient for suppression of the NK cell lineage choice. HEB factors are also required to enforce the death of cells that have not rearranged their TCR genes. The synergistic interactions between Notch1, HEBAlt, HEBCan, GATA3, and TCF1 are presented in a gene network model, and the influence of thymic stromal architecture on lineage choice in the thymus is discussed.

Stage-specific functions of E-proteins at the β-selection and T-cell receptor checkpoints during thymocyte development

The E-protein transcription factors E2A and HEB function in a lineage- and stage-specific manner to orchestrate many critical events throughout lymphocyte development. The function of E-proteins in both B- and T-lymphocyte development has been extensively studied through the use of single-gene knockout animals. Unlike B cells, which rely primarily on E2A alone, T cells are regulated by the combinatorial expression of both E2A and HEB. Therefore, many of the roles of E-proteins during T-cell development may be masked in single-gene knockout studies due to the compensatory function of E2A and HEB. More recently, our laboratory has established double-conditional knockout models to eliminate both E2A and HEB in a stage-specific manner throughout T-cell development. These models, in combination with other complimentary genetic approaches, have identified new E-protein functions at each of the two major T-cell developmental checkpoints. Here, we will discuss how E-proteins function to regulate the expression of T-cell receptor components and cell cycle at the β-selection checkpoint, and how they control positive selection, survival, and lineage-specific gene expression at the subsequent T-cell receptor checkpoint.

Transcriptomic analysis supports similar functional roles for the two thymuses of the tammar wallaby

Background

The thymus plays a critical role in the development and maturation of T-cells. Humans have a single thoracic thymus and presence of a second thymus is considered an anomaly. However, many vertebrates have multiple thymuses. The tammar wallaby has two thymuses: a thoracic thymus (typically found in all mammals) and a dominant cervical thymus. Researchers have known about the presence of the two wallaby thymuses since the 1800s, but no genome-wide research has been carried out into possible functional differences between the two thymic tissues. Here, we used pyrosequencing to compare the transcriptomes of a cervical and thoracic thymus from a single 178 day old tammar wallaby.

Results

We show that both the tammar thoracic and the cervical thymuses displayed gene expression profiles consistent with roles in T-cell development. Both thymuses expressed genes that mediate distinct phases of T-cells differentiation, including the initial commitment of blood stem cells to the T-lineage, the generation of T-cell receptor diversity and development of thymic epithelial cells. Crucial immune genes, such as chemokines were also present. Comparable patterns of expression of non-coding RNAs were seen. 67 genes differentially expressed between the two thymuses were detected, and the possible significance of these results are discussed.

Conclusion

This is the first study comparing the transcriptomes of two thymuses from a single individual. Our finding supports that both thymuses are functionally equivalent and drive T-cell development. These results are an important first step in the understanding of the genetic processes that govern marsupial immunity, and also allow us to begin to trace the evolution of the mammalian immune system.

Identification of DeltaEF1 as a novel target that is negatively regulated by LMO2 in T-cell leukemia

The lmo2 gene is a specific oncogene in T-cell leukemia, for its ectopic expression causes both increased pro-T-cell proliferation and differentiation arrest, leading to the onset of leukemia. Notably, DeltaEF1 (also known as ZEB1), a member of zinc finger-homeodomain family transcription factor, also exhibits crucial function in promoting T-cell differentiation. In this study, we found that DeltaEF1 was positively regulated by T-lineage-specific transcriptional regulator GATA3, while ectopically expressed LMO2 targeted to DeltaEF1 promoter by interaction with GATA3 and inhibited DeltaEF1 expression in transcriptional level. Moreover, LMO2 interacted with the N-terminal zinc finger domain of DeltaEF1 protein and inhibited its positive transcriptional regulatory function by this interaction. Taken together, our findings revealed that ectopically expressed LMO2 impaired the function of DeltaEF1 in both transcriptional and protein levels and identified DeltaEF1 as a novel pathogenic target of LMO2 in T-cell leukemia.

Context-dependent regulation of hematopoietic lineage choice by HEBAlt

Hematopoietic development is controlled by combinatorial interactions between E-protein transcription factors and other lineage regulators that operate in the context of gene-regulatory networks. The E-proteins HEB and E2A are critical for T cell and B cell development, but the mechanisms by which their activities are directed to different genes in each lineage are unclear. We found that a short form of HEB, HEBAlt, acts downstream of Delta-like (DL)-Notch signaling to promote T cell development. In this paper, we show that forced expression of HEBAlt in mouse hematopoietic progenitors inhibited B cell development, but it allowed them to adopt a myeloid fate. HEBAlt interfered with the activity of E2A homodimers and with the expression of the transcription factor Pax5, both of which are critical for B cell development. However, when combined with DL-Notch signaling, HEBAlt enhanced the generation of T cell progenitors at the expense of myeloid cells. The longer form of HEB, HEBCan, also inhibited E47 activity and Pax5 expression, but it did not collaborate with DL-Notch signaling to suppress myeloid potential. Therefore, HEBAlt can suppress B cell or myeloid potential in a context-specific manner, which suggests a role for this factor in maintaining T lineage priming prior to commitment.

SSBP2 is an in vivo tumor suppressor and regulator of LDB1 stability

SSBP proteins bind and stabilize transcriptional cofactor Lim Domain Binding protein1 (LDB1) from proteosomal degradation to promote tissue specific transcription through an evolutionarily conserved pathway. The human SSBP2 gene was isolated as a candidate tumor suppressor from a critical region of loss in chromosome 5q14.1. By gene targeting, we demonstrate increased predisposition to B cell lymphomas and carcinomas in Ssbp2^−/− mice. Remarkably, loss of Ssbp2 causes increased LDB1 turnover in the thymus, a pathway exploited in Trp53^−/−Ssbp2^−/− mice to develop highly aggressive, immature thymic lymphomas. Using T cell differentiation as a model, we report a stage specific up regulation of Ssbp2 expression which in turn regulates LDB1 turnover under physiological conditions. Furthermore, transcript levels of pTα, a target of LDB1 containing complex, and a critical regulator T cell differentiation is reduced in Ssbp2^−/− immature thymocytes. Our findings suggest disruption of the SSBP2 regulated pathways may be an infrequent but critical step in malignant transformation of multiple tissues.

Homo-binding character of LMO2 isoforms and their both synergic and antagonistic functions in regulating hematopoietic-related target genes

Background

The human lmo2 gene plays important roles in hematopoiesis and is associated with acute T lymphocyte leukemia. The gene encodes two protein isoforms, a longer form LMO2-L and a shorter form LMO2-S. Both isoforms function as bridge molecules to assemble their partners together to regulate their target genes. A typical LMO2 binding site consists of two elements, a GATA site and an E-box, with an interval of 9~12 bp.

Methods

In this study, the combination of MBP pulldown assay and mammalian two hybrid assay were used to confirm the homo-binding character of LMO2-L/-S isoforms. Luciferase reporter assay and Real-time PCR assay were used to detect expression levels and relative promoter activities of LMO2-L/-S isoforms. Co-transfection and Luciferase reporter assay were used to reveal the detailed regulatory pattern of LMO2-L/-S isoforms on their targets.

Results

Herein we report the homo-interaction character of LMO2-L and LMO2-S and their major difference in manner of regulating their target genes. Our results showed that LMO2-L and LMO2-S could only bind to themselves but not each other. It was also demonstrated that LMO2-L could either positively or negatively regulate the transcription of its different target genes, depending on the arrangement and strand location of the two elements GATA site and E-box, LMO2-S, however, performed constitutively transcriptional inhibiting function on all target genes.

Conclusion

These results suggest that LMO2 isoforms have independent functions while there is no interaction between each other and they could play synergetic or antagonistic roles precisely in regulating their different genes involved in normal and aberrant hematopoiesis.

Early T cell differentiation lessons from T-cell acute lymphoblastic leukemia

T cells develop from bone marrow-derived self-renewing hematopoietic stem cells (HSC). Upon entering the thymus, these cells undergo progressive commitment and differentiation driven by the thymic stroma and the pre-T cell receptor (pre-TCR). These processes are disrupted in T-cell acute lymphoblastic leukemia (T-ALL). More than 70% of recurring chromosomal rearrangements in T-ALL activate the expression of oncogenic transcription factors, belonging mostly to three families, basic helix-loop-helix (bHLH), homeobox (HOX), and c-MYB. This prevalence is indicative of their importance in the T lineage, and their dominant mechanisms of transformation. For example, bHLH oncoproteins inhibit E2A and HEB, revealing their tumor suppressor function in the thymus. The induction of T-ALL, nonetheless, requires collaboration with constitutive NOTCH1 signaling and the pre-TCR, as well as loss-of-function mutations for CDKN2A and PTEN. Significantly, NOTCH1, the pre-TCR pathway, and E2A/HEB proteins control critical checkpoints and branchpoints in early thymocyte development whereas several oncogenic transcription factors, HOXA9, c-MYB, SCL, and LYL-1 control HSC self-renewal. Together, these genetic lesions alter key regulatory processes in the cell, favoring self-renewal and subvert the normal control of thymocyte homeostasis.

A role for E2-2 at the DN3 stage of early thymopoiesis

Roles for the E-proteins E2A and HEB during T lymphocyte development have been well established. Based on our previous observations of counter selection against T cells lacking E2-2, it seemed reasonable to assume that there would be a function also for E2-2 in thymocyte development. Aiming at assigning such a role for E2-2, we analyzed the expression of E2-2, E2A, HEB as well as Id mRNA during T cell development. Interestingly, whereas all three E-proteins were expressed during early thymocyte development, significant expression beyond the DP stage was detected only for E2A. Among the Id proteins, Id2 displayed a prominent expression exclusively in DN1, whereas Id3 showed some expression in DN1, followed by a down regulation and then a prominent induction, peaking in the DP stage. E2-2 was expressed during the DN stages, as well as in the DP stage, suggesting that E2-2 operates in concert with the other E-proteins during early thymocyte development. We found that E2-2 null thymocytes displayed a partial block at the DN3 stage of development, as well as a reduced expression of pre-T alpha, known to be regulated also by E2A and HEB. The fact that E2-2 deficient thymocytes develop without gross abnormalities is likely to stem from redundancy due to the co-expression of E2A and HEB.

Protein Stability and Transcription Factor Complex Assembly Determined by the SCL-LMO2 Interaction

Gene expression programs are established by networks of interacting transcription factors. The basic helix-loop-helix factor SCL and the LIM-only protein LMO2 are components of transcription factor complexes that are essential for hematopoiesis. Here we show that LMO2 and SCL are predominant interaction partners in hematopoietic cells and that this interaction occurs through a conserved interface residing in the loop and helix 2 of SCL. This interaction nucleates the assembly of SCL complexes on DNA and is required for target gene induction and for the stimulation of erythroid and megakaryocytic differentiation. We also demonstrate that SCL determines LMO2 protein levels in hematopoietic cells and reveal that interaction with SCL prevents LMO2 degradation by the proteasome. We propose that the SCL-LMO2 interaction couples protein stabilization with higher order protein complex assembly, thus providing a powerful means of modulating the stoichiometry and spatiotemporal activity of SCL complexes. This interaction likely provides a rate-limiting step in the transcriptional control of hematopoiesis and leukemia, and similar mechanisms may operate to control the assembly of diverse protein modules.

NOTCH1 pathway activation is an early hallmark of SCL T leukemogenesis

The acquired activation of stem cell leukemia (SCL) during T lymphopoiesis is a common event in T-cell acute lymphoblastic leukemia (T-ALL). Here, we generated tamoxifen (TAM)-inducible transgenic mice (lck-ER(T2)-SCL) to study the consequences of acquired SCL activation during T-cell development. Aberrant activation of SCL in thymocytes resulted in the accumulation of immature CD4(+)CD8(+) (double-positive, DP) cells by preventing normal surface expression of the T-cell receptor alphabeta (TCRalphabeta) complex. SCL-induced immature DP cells were further characterized by up-regulated NOTCH1 and generated noncycling polyclonal CD8(+)TCRbeta(low) cells. The prevalence of these cells was SCL dependent because TAM withdrawal resulted in their disappearance. Furthermore, we observed that SCL activation led to a dramatic up-regulation of NOTCH1 target genes (Hes-1, Deltex1, and CD25) in thymocytes. Strikingly, NOTCH1 target gene up-regulation was already observed after short-term SCL induction, implying that enhanced NOTCH signaling is mediated by SCL and is not dependent on secondary genetic events. These data represent the basis for a novel pathway of SCL-induced leukemogenesis and provide a functional link between SCL and NOTCH1 during this process.

Molecular dissection of prethymic progenitor entry into the T lymphocyte developmental pathway

Notch signaling activates T lineage differentiation from hemopoietic progenitors, but relatively few regulators that initiate this program have been identified, e.g., GATA3 and T cell factor-1 (TCF-1) (gene name Tcf7). To identify additional regulators of T cell specification, a cDNA library from mouse Pro-T cells was screened for genes that are specifically up-regulated in intrathymic T cell precursors as compared with myeloid progenitors. Over 90 genes of interest were identified, and 35 of 44 tested were confirmed to be more highly expressed in T lineage precursors relative to precursors of B and/or myeloid lineage. To a remarkable extent, however, expression of these T lineage-enriched genes, including zinc finger transcription factor, helicase, and signaling adaptor genes, was also shared by stem cells (Lin(-)Sca-1(+)Kit(+)CD27(-)) and multipotent progenitors (Lin(-)Sca-1(+)Kit(+)CD27(+)), although down-regulated in other lineages. Thus, a major fraction of these early T lineage genes are a regulatory legacy from stem cells. The few genes sharply up-regulated between multipotent progenitors and Pro-T cell stages included those encoding transcription factors Bcl11b, TCF-1 (Tcf7), and HEBalt, Notch target Deltex1, Deltex3L, Fkbp5, Eva1, and Tmem131. Like GATA3 and Deltex1, Bcl11b, Fkbp5, and Eva1 were dependent on Notch/Delta signaling for induction in fetal liver precursors, but only Bcl11b and HEBalt were up-regulated between the first two stages of intrathymic T cell development (double negative 1 and double negative 2) corresponding to T lineage specification. Bcl11b was uniquely T lineage restricted and induced by Notch/Delta signaling specifically upon entry into the T lineage differentiation pathway.

E2A and HEB are required to block thymocyte proliferation prior to pre-TCR expression

Thymocytes undergoing TCRbeta gene rearrangements are maintained in a low or nonproliferating state during early T cell development. This block in cell cycle progression is not released until the expression of a functional pre-TCR, which is composed of a successfully rearranged TCRbeta-chain and the Pre-Talpha-chain. The regulatory molecules responsible for the coordination of these differentiation and proliferation events are currently unknown. E2A and HEB are structurally and functionally related basic helix-loop-helix transcription factors involved in T cell development. To reveal the function of E2A and HEB through the stage of pre-TCR expression and alleviate functional compensation between E2A and HEB, we use a double-conditional knockout model. The simultaneous deletion of E2A and HEB in developing thymocytes leads to a severe developmental block before pre-TCR expression and a dramatic reduction of Pre-Talpha expression. These developmentally arrested thymocytes exhibit increased proliferation in vivo and dramatic expansion ex vivo in response to IL-7 signaling. These results suggest that E2A and HEB are not only critical for T cell differentiation but also necessary to retain developing thymocytes in cell cycle arrest before pre-TCR expression.

NFKB1 is a direct target of the TAL1 oncoprotein in human T leukemia cells

We recently showed that a subset of human T acute lymphoblastic leukemia (T-ALL) cell lines expresses low basal levels of p50, a nuclear factor-kappaB (NF-kappaB)/Rel family member, resulting in their capacity to activate the atypical p65:cRel complex rather than the classic p50:p65 dimer. Here, we show that the transcription factor TAL1 (also known as SCL) binds to the promoter of the NFKB1 gene that encodes p50 and represses its transcription to set up this unique response in T-ALL cells. When TAL1 expression is reduced in CEM T leukemia cells, basal NFKB1 expression is increased, and the levels of p65:cRel complex and transcription of its target gene, such as intercellular adhesion molecule-1 (ICAM-1), are reduced in response to etoposide treatment. Moreover, a significant negative correlation between NFKB1 and TAL1 or LMO1 was found in primary human TAL1/LMO1 double-positive T-ALL samples previously described by Ferrando et al. Thus, TAL1 modulates NFKB1 expression and an NF-kappaB-dependent transcriptional program in a subset of human T-cell leukemia cells.

Cytogenetics and molecular genetics of T-cell acute lymphoblastic leukemia: from thymocyte to lymphoblast

For long, T-cell acute lymphoblastic leukemia (T-ALL) remained in the shadow of precursor B-ALL because it was more seldom, and showed a normal karyotype in more than 50% of cases. The last decennia, intense research has been carried out on different fronts. On one side, development of normal thymocyte and its regulation mechanisms have been studied in multiple mouse models and subsequently validated. On the other side, molecular cytogenetics (fluorescence in situ hybridization) and mutation analysis revealed cytogenetically cryptic aberrations in almost all cases of T-ALL. Also, expression microarray analysis disclosed gene expression signatures that recapitulate specific stages of thymocyte development. Investigations are still very much actual, fed by the discovery of new genetic aberrations. In this review, we present a summary of the current cytogenetic changes associated with T-ALL. The genes deregulated by translocations or mutations appear to encode proteins that are also implicated in T-cell development, which prompted us to review the 'normal' and 'leukemogenic' functions of these transcription regulators. To conclude, we show that the paradigm of multistep leukemogenesis is very much applicable to T-ALL and that the different genetic insults collaborate to maintain self-renewal capacity, and induce proliferation and differentiation arrest of T-lymphoblasts. They also open perspectives for targeted therapies.

Determinants of lymphoid-myeloid lineage diversification

In recent years, investigators have made great progress in delineating developmental pathways of several lymphoid and myeloid lineages and in identifying transcription factors that establish and maintain their fate. However, the developmental branching points between these two large cell compartments are still controversial, and little is known about how their diversification is induced. Here, we give an overview of determinants that play a role at lymphoid-myeloid junctures, in particular transcription factors and cytokine receptors. Experiments showing that myeloid lineages can be reversibly reprogrammed into one another by transcription factor network perturbations are used to highlight key principles of lineage commitment. We also discuss experiments showing that lymphoid-to-myeloid but not myeloid-to-lymphoid conversions can be induced by the enforced expression of a single transcription factor. We close by proposing that this asymmetry is related to a higher complexity of transcription factor networks in lymphoid cells compared with myeloid cells, and we suggest that this feature must be considered when searching for mechanisms by which hematopoietic stem cells become committed to lymphoid lineages.

E proteins and Notch signaling cooperate to promote T cell lineage specification and commitment

The helix-loop-helix protein, E47, is essential for both B- and T-lineage development. Here we demonstrate that in vitro E47 and Notch signaling act in concert to promote T cell development from fetal hematopoietic progenitors and to restrain development into the natural killer and myeloid cell lineages. The expression of an ensemble of genes associated with Notch signaling is activated by E47, and additionally, Notch signaling and E47 act in parallel pathways to induce a T lineage-specific program of gene expression. Enforced expression of the intracellular domain of Notch rescues the developmental arrest at the T cell commitment stage in E2A-deficient fetal thymocytes. Finally, we demonstrate that regulation of Hes1 expression by Notch signaling and E47 is strikingly similar to that observed during Drosophila melanogaster sensory development. Based on these observations, we propose that in developing fetal thymocytes E47 acts to induce the expression of an ensemble of genes involved in Notch signaling, and that subsequently E47 acts in parallel with Notch signaling to promote T-lineage maturation.

Transcriptional regulatory networks downstream of TAL1/SCL in T-cell acute lymphoblastic leukemia

Aberrant expression of 1 or more transcription factor oncogenes is a critical component of the molecular pathogenesis of human T-cell acute lymphoblastic leukemia (T-ALL); however, oncogenic transcriptional programs downstream of T-ALL oncogenes are mostly unknown. TAL1/SCL is a basic helix-loop-helix (bHLH) transcription factor oncogene aberrantly expressed in 60% of human T-ALLs. We used chromatin immunoprecipitation (ChIP) on chip to identify 71 direct transcriptional targets of TAL1/SCL. Promoters occupied by TAL1 were also frequently bound by the class I bHLH proteins E2A and HEB, suggesting that TAL1/E2A as well as TAL1/HEB heterodimers play a role in transformation of T-cell precursors. Using RNA interference, we demonstrated that TAL1 is required for the maintenance of the leukemic phenotype in Jurkat cells and showed that TAL1 binding can be associated with either repression or activation of genes whose promoters occupied by TAL1, E2A, and HEB. In addition, oligonucleotide microarray analysis of RNA from 47 primary T-ALL samples showed specific expression signatures involving TAL1 targets in TAL1-expressing compared with -nonexpressing human T-ALLs. Our results indicate that TAL1 may act as a bifunctional transcriptional regulator (activator and repressor) at the top of a complex regulatory network that disrupts normal T-cell homeostasis and contributes to leukemogenesis.

At the crossroads: diverse roles of early thymocyte transcriptional regulators

Transcriptional regulation of T-cell development involves successive interactions between complexes of transcriptional regulators and their binding sites within the regulatory regions of each gene. The regulatory modules that control expression of T-lineage genes frequently include binding sites for a core set of regulators that set the T-cell-specific background for signal-dependent control, including GATA-3, Notch/CSL, c-myb, TCF-1, Ikaros, HEB/E2A, Ets, and Runx factors. Additional regulators in early thymocytes include PU.1, Id-2, SCL, Spi-B, Erg, Gfi-1, and Gli. Many of these factors are involved in simultaneous regulation of non-T-lineage genes, T-lineage genes, and genes involved in cell cycle control, apoptosis, or survival. Potential and known interactions between early thymic transcription factors such as GATA-3, SCL, PU.1, Erg, and Spi-B are explored. Regulatory modules involved in the expression of several critical T-lineage genes are described, and models are presented for shifting occupancy of the DNA-binding sites in the regulatory modules of pre-Talpha, T-cell receptor beta (TCRbeta), recombinase activating genes 1 and 2 (Rag-1/2), and CD4 during T-cell development. Finally, evidence is presented that c-kit, Erg, Hes-1, and HEBAlt are expressed differently in Rag-2(-/-) thymocytes versus normal early thymocytes, which provide insight into potential regulatory interactions that occur during normal T-cell development.

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.

The role of LMO2 in development and in T cell leukemia after chromosomal translocation or retroviral insertion

Chromosomal translocations are primary events in the development of leukemias, representing at least one genetic feature of the putative cancer stem cell. Studies of genes influenced by chromosomal translocations have yielded a vast amount of information about how cancer is initiated and maintained. In particular, acute leukemias have demonstrated that chromosomal translocations often involve transcription regulators that function by interacting with proteins and by controlling cell fate in the aberrant setting of the developing cancer cell. As a quintessential chromosomal translocation gene product, LMO2 has many properties that typify this class of molecule. In addition to its involvement in chromosomal translocations, the LMO2 gene was inadvertently activated in an X-SCID gene therapy trial by retroviral insertion. New molecular therapies targeted directly at the LMO2 protein could have major impact as adjuncts to existing therapies or as therapeutics in their own right. In this review, we outline the current knowledge about LMO2 and some possible routes to develop reagents that might be possible macromolecular drugs in the future.

Molecular genetics of T cell development

T cell development is guided by a complex set of transcription factors that act recursively, in different combinations, at each of the developmental choice points from T-lineage specification to peripheral T cell specialization. This review describes the modes of action of the major T-lineage-defining transcription factors and the signal pathways that activate them during intrathymic differentiation from pluripotent precursors. Roles of Notch and its effector RBPSuh (CSL), GATA-3, E2A/HEB and Id proteins, c-Myb, TCF-1, and members of the Runx, Ets, and Ikaros families are critical. Less known transcription factors that are newly recognized as being required for T cell development at particular checkpoints are also described. The transcriptional regulation of T cell development is contrasted with that of B cell development, in terms of their different degrees of overlap with the stem-cell program and the different roles of key transcription factors in gene regulatory networks leading to lineage commitment.

Identification of cyclin D3 as a direct target of E2A using DamID

The transcription factor E2A can promote precursor B cell expansion, promote G(1) cell cycle progression, and induce the expressions of multiple G(1)-phase cyclins. To better understand the mechanism by which E2A induces these cyclins, we characterized the relationship between E2A and the cyclin D3 gene promoter. E2A transactivated the 1-kb promoter of cyclin D3, which contains two E boxes. However, deletion of the E boxes did not disrupt the transactivation by E2A, raising the possibility of indirect activation via another transcription factor or binding of E2A to non-E-box DNA elements. To distinguish between these two possibilities, promoter occupancy was examined using the DamID approach. A fusion construct composed of E2A and the Escherichia coli DNA adenosine methyltransferase (E47Dam) was subcloned in lentivirus vectors and used to transduce precursor B-cell and myeloid progenitor cell lines. In both cell types, specific adenosine methylation was identified at the cyclin D3 promoter. Chromatin immunoprecipitation analysis confirmed the DamID findings and localized the binding to within 1 kb of the two E boxes. The methylation by E47Dam was not disrupted by mutations in the E2A portion that block DNA binding. We conclude that E2A can be recruited to the cyclin D3 promoter independently of E boxes or E2A DNA binding activity.

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.

TAL1/SCL induces leukemia by inhibiting the transcriptional activity of E47/HEB

Activation of the basic-helix-loop-helix (bHLH) gene TAL1 (or SCL) is a frequent gain-of-function mutation in T cell acute lymphoblastic leukemia (T-ALL). To provide genetic evidence that tal1/scl induces leukemia by interfering with E47 and HEB, we expressed tal1/scl in an E2A or HEB heterozygous background. These mice exhibit disease acceleration and perturbed thymocyte development due to repression of E47/HEB target genes. In tal1/scl thymocytes, we find the corepressor mSin3A bound to the CD4 enhancer, whereas an E47/HEB/p300 complex is detected in wild-type thymocytes. Furthermore, tal1/scl tumors are sensitive to pharmacologic inhibition of HDAC and undergo apoptosis. These data demonstrate that tal1/scl induces leukemia by repressing E47/HEB and suggest that HDAC inhibitors may prove efficacious in T-ALL patients who express TAL1/SCL.

Leukaemia - a developmental perspective

Leukaemia is characterized by the accumulation of malignant haematopoietic precursors. Recent studies have revealed that acquired alterations in genes that regulate normal haematopoiesis are frequently detected in leukaemia. The progression to leukaemia depends on additional mutations that promote the survival of developmentally arrested cells. This review describes three examples of this general paradigm of leukaemogenesis: RUNX1 abnormalities in acute leukaemias, GATA1 mutations in the leukaemias of Down syndrome, and SCL and LMO2 ectopic expression in T cell acute lymphoblastic leukaemia.

Basic helix-loop-helix proteins bind to TrkB and p21(Cip1) promoters linking differentiation and cell cycle arrest in neuroblastoma cells

Differentiation of precursor into specialized cells involves an increasing restriction in proliferative capacity, culminating in cell cycle exit. In this report we used a human neuroblastoma cell line to study the molecular mechanisms that coordinate cell cycle arrest and neuronal differentiation. Exposure to retinoic acid (RA), a differentiation agent in many cell types, causes an upregulation of neurotrophin receptor TrkB and the cyclin kinase inhibitor p21(Cip1) at a transcriptional level. Full transcriptional activation of these two genes requires canonical E-box sequences found in the TrkB and p21(Cip1) promoters. As reported for other E-box-regulated promoters, ectopic expression of E47 and E12 basic helix-loop-helix (bHLH) proteins enhances RA-dependent expression of TrkB and p21(Cip1), whereas the inhibitory HLH Id2 exerts opposite effects. In addition, ectopic expression of E47 and NeuroD, a neuronal bHLH protein, is able to activate TrkB transcription in the absence of RA. More importantly, we show that E47 and NeuroD proteins bind the TrkB and p21(Cip1) promoter sequences in vivo. Since they establish a direct transcriptional link between a cell cycle inhibitor, p21(Cip1), and a neurotrophic receptor, TrkB, bHLH proteins would play an important role in coordinating key events of cell cycle arrest and neuronal differentiation.

The basic helix-loop-helix transcription factor TAL1/SCL inhibits the expression of the p16INK4A and pTalpha genes

The Tal1 gene (also called Scl or TCL5) encodes a basic helix-loop-helix transcription factor required for hematopoiesis and vasculogenesis. Additionally, aberrant transcriptional activation of the Tal1 gene is a frequent event in human T cell acute lymphoblastic leukemia (T-ALL). T cell specific expression of TAL1 in mice induces aggressive T cell malignancies, demonstrating the oncogenic potential of TAL1. Yet, the underlying mechanisms of TAL1 induced tumorigenesis are poorly understood. By inhibiting E protein mediated transcription of the pTalpha gene, TAL1 can interfere with the T cell differentiation program. In addition, several studies suggest that TAL1 expression might also enhance proliferation rate. We report here that TAL1 can bind the E boxes in both the p16 and the pTalpha promoters, and functionally suppress the activity of both promoters. These results indicate that TAL1 can affect both T cell proliferation and differentiation. Moreover, we show that overexpression of TAL1 in hematopoietic progenitor cells promotes cell cycle division.

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.

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.

The LIM-only protein LMO4 modulates the transcriptional activity of HEN1

The basic helix-loop-helix protein HEN1 and the LIM-only proteins LMO2 and LMO4 are expressed in neuronal cells. HEN1 was cloned by virtue of its homology to TAL1, a bHLH protein important for early hematopoiesis. Since it has been shown that TAL1 forms complex with LMO proteins in erythroid and leukemic cells we investigated the capacity of HEN1 to form complex with LMO2 and LMO4. By mammalian two-hybrid analysis, we show that HEN1 interacts with both LMO2 and LMO4. To characterize the transcriptional capacity of HEN1 alone or together with LMO2 and LMO4, we performed reporter gene assays. In comparison with the ubiquitously expressed bHLH protein E47, HEN1 is a very modest transcriptional activator and titration experiments indicate that HEN1, like TAL1, represses E47 mediated transcriptional activation. Furthermore, LMO4 but not LMO2 was able to augment this effect. Overexpression of HEN1 in hippocampal precursor cells resulted in neurite extension, which could be prevented by LMO4. Taken together, these results indicate that LMO proteins can modulate the transcriptional activity of HEN1.