Regulation of the T-cell receptor delta enhancer by functional cooperation between c-Myb and core-binding factors

Regulation of T-cell Receptor Gene Expression by Three-Dimensional Locus Conformation and Enhancer Function

The adaptive immune response in vertebrates depends on the expression of antigen-specific receptors in lymphocytes. T-cell receptor (TCR) gene expression is exquisitely regulated during thymocyte development to drive the generation of αβ and γδ T lymphocytes. The TCRα, TCRβ, TCRγ, and TCRδ genes exist in two different configurations, unrearranged and rearranged. A correctly rearranged configuration is required for expression of a functional TCR chain. TCRs can take the form of one of three possible heterodimers, pre-TCR, TCRαβ, or TCRγδ which drive thymocyte maturation into αβ or γδ T lymphocytes. To pass from an unrearranged to a rearranged configuration, global and local three dimensional (3D) chromatin changes must occur during thymocyte development to regulate gene segment accessibility for V(D)J recombination. During this process, enhancers play a critical role by modifying the chromatin conformation and triggering noncoding germline transcription that promotes the recruitment of the recombination machinery. The different signaling that thymocytes receive during their development controls enhancer activity. Here, we summarize the dynamics of long-distance interactions established through chromatin regulatory elements that drive transcription and V(D)J recombination and how different signaling pathways are orchestrated to regulate the activity of enhancers to precisely control TCR gene expression during T-cell maturation.

Mechanism of ETV6-RUNX1 Leukemia

The t(12;21)(p13;q22) translocation is the most frequently occurring single genetic abnormality in pediatric leukemia. This translocation results in the fusion of the ETV6 and RUNX1 genes. Since its discovery in the 1990s, the function of the ETV6-RUNX1 fusion gene has attracted intense interest. In this chapter, we will summarize current knowledge on the clinical significance of ETV6-RUNX1, the experimental models used to unravel its function in leukemogenesis, the identification of co-operating mutations and the mechanisms responsible for their acquisition, the function of the encoded transcription factor and finally, the future therapeutic approaches available to mitigate the associated disease.

Association of single-nucleotide polymorphisms of high-mobility group box 1 with susceptibility and clinicopathological characteristics of uterine cervical neoplasia in Taiwanese women

To date, no study associated the genetic polymorphisms of high-mobility group box 1 protein (HMGB1) with the development of uterine cervical cancer. We therefore conducted this study to investigate the associations of HMGB1 single-nucleotide polymorphisms (SNPs) with cervical carcinogenesis and clinicopathological characteristics of cancer patients. Five hundred two women, including 112 with invasive cancer, 85 with precancerous lesions of the uterine cervix, and 305 normal controls, were consecutively enrolled into this study. Analysis of HMGB1 SNPs was done by real-time polymerase chain reaction and genotyping. Our results found that the risk of susceptibility to cervical invasive cancer was 1.85 (95 % CI 1.12-3.04; p = 0.016) in women with TC and 1.99 (95 % CI 1.24-3.23; p = 0.005) in women with TC/CC after adjusting for age, using TT as a comparison reference in HMGB1 SNP rs1412125. In rs2249825, the increased risk was also seen for the development of cervical invasive cancer in women with CG [adjusted odds ratio (AOR) 2.04, 95 % CI 1.22-3.40; p = 0.006] or CG/GG (AOR 2.02, 95 % CI 1.22-3.32; p = 0.006) using CC as a comparison reference. An additional integrated in silico analysis confirmed that rs2249825 creates a binding site for v-Myb, which may affect HMGB1 expression. In conclusion, Taiwanese women with TC or TC/CC in HMGB1 SNP rs1412125 as well as CG or CG/GG in rs2249825 were susceptible to the development of cervical invasive cancer.

The RUNX complex: reaching beyond haematopoiesis into immunity

Among their diverse roles as transcriptional regulators during development and cell fate specification, the RUNX transcription factors are best known for the parts they play in haematopoiesis. RUNX proteins are expressed throughout all haematopoietic lineages, being necessary for the emergence of the first haematopoietic stem cells to their terminal differentiation. Although much progress has been made since their discoveries almost two decades ago, current appreciation of RUNX in haematopoiesis is largely grounded in their lineage-specifying roles. In contrast, the importance of RUNX to immunity has been mostly obscured for historic, technical and conceptual reasons. However, this paradigm is likely to shift over time, as a primary purpose of haematopoiesis is to resource the immune system. Furthermore, recent evidence suggests a role for RUNX in the innate immunity of non-haematopoietic cells. This review takes a haematopoiesis-centric approach to collate what is known of RUNX's contribution to the overall mammalian immune system and discuss their growing prominence in areas such as autoimmunity, inflammatory diseases and mucosal immunity.

Recent insights into the transcriptional control of the Tcra/Tcrd locus by distant enhancers during the development of T-lymphocytes

Tcra/Tcrd includes 2 genes with distinct developmental programs controlled by 2 distant enhancers, Eα and Eδ. These enhancers work as a developmental switch during thymocyte development and they are essential for generation of αβ and γδ T-lymphocytes. Tcra and Tcrd transit from an unrearranged configuration to a rearranged configuration during T-cell development. Eα and Eδ are responsible for transcription of their respective unrearranged genes in thymocytes but are dispensable for such functions in the context of the rearranged genes in mature T-cells. Interestingly, Eα activates transcription of the rearranged Tcrd in γδ T-lymphocytes but it is inactive in αβ T-lymphocytes.

Association of HMGB1 and HMGB2 genetic polymorphisms with lung cancer chemotherapy response

The aim of the present study was to investigate the association of genetic polymorphisms in high mobility group box 1 and 2 (HMGB1 and HMGB2, respectively) with platinum-based chemotherapy responses in Chinese lung cancer patients. In total, 338 Chinese lung cancer patients (154 responders and 184 non-responders) were recruited to the study. All patients received at least two cycles of first-line platinum-based chemotherapy. Three tagging single nucleotide polymorphisms (SNPs) of HMGB1 and two tagging SNPs of HMGB2 were detected in patients. We found that rs1412125 and rs2249825 of HMGB1 were significantly associated with the platinum-based chemotherapy response in both recessive and genotypic models. In addition, rs1412125 showed significant association with platinum-based chemotherapy response for the subgroup of patients aged >55 years in additive, recessive and genotypic models. No significant associations were detected between other SNPs and the platinum-based chemotherapy response. The HMGB1 SNPs (rs1412125 and rs2249825) were associated with platinum-based chemotherapy responses in Chinese lung cancer patients. In conclusion, HMGB1 SNPs may serve as potential biomarkers for predicting the efficacy of platinum-based chemotherapy.

Endothelio-mesenchymal interaction controls runx1 expression and modulates the notch pathway to initiate aortic hematopoiesis

Hematopoietic stem cells (HSCs) are produced by a small cohort of hemogenic endothelial cells (ECs) during development through the formation of intra-aortic hematopoietic cell (HC) clusters. The Runx1 transcription factor plays a key role in the EC-to-HC and -HSC transition. We show that Runx1 expression in hemogenic ECs and the subsequent initiation of HC formation are tightly controlled by the subaortic mesenchyme, although the mesenchyme is not a source of HCs. Runx1 and Notch signaling are involved in this process, with Notch signaling decreasing with time in HCs. Inhibiting Notch signaling readily increases HC production in mouse and chicken embryos. In the mouse, however, this increase is transient. Collectively, we show complementary roles of hemogenic ECs and mesenchymal compartments in triggering aortic hematopoiesis. The subaortic mesenchyme induces Runx1 expression in hemogenic-primed ECs and collaborates with Notch dynamics to control aortic hematopoiesis.

Association of HMGB1 polymorphisms with outcome after allogeneic hematopoietic cell transplantation

Several studies have demonstrated that genetic variation in cytokine genes can modulate the immune reactions after allogeneic hematopoietic cell transplantation (HCT). High mobility group box 1 protein (HMBG1) is a pleiotropic cytokine that functions as a pro-inflammatory signal, important for the activation of antigen presenting cells (APCs) and propagation of inflammation. HMGB1 is implicated in the pathophysiology of a variety of inflammatory diseases, and we have recently found the variation in the HMGB1 gene to be associated with mortality in patients with systemic inflammatory response syndrome. To assess the impact of the genetic variation in HMGB1 on outcome after allogeneic HCT, we genotyped 276 and 146 patient/donor pairs treated with allogeneic HCT for hematologic malignancies following myeloablative (MA) or nonmyeloablative (NMA) conditioning. Associations between genotypes and outcome were only observed in the cohort treated with MA conditioning. Patient homozygosity or heterozygosity for the-1377delA minor allele was associated with increased risk of relapse (hazard ratio [HR] 2.11, P = .02) and increased relapse related mortality (RRM) (P = .03). Furthermore, patient homozygosity for the 3814C > G minor allele was associated with increased overall survival (OS; HR 0.13, P = .04), progression free survival (PFS; HR 0.30, P = .05) and decreased probability of RRM (P = .03). Patient carriage of the 2351insT minor allele reduced the risk of grade II to IV acute graft-versus-host disease (aGVHD) (HR 0.60, P = .01), whereas donor homozygosity was associated with chronic GVHD (cGVHD) (HR 1.54, P = .01). Our findings suggest that the inherited variation in HMGB1 is associated with outcome after allogeneic HCT following MA conditioning. None of the polymorphisms were associated with treatment-related mortality (TRM).

Runx-CBFbeta complexes control expression of the transcription factor Foxp3 in regulatory T cells

Foxp3 plays an indispensable role in establishing stable transcriptional and functional programs of regulatory T (T_reg) cells. Loss of Foxp3 expression in mature T_reg cells results in a failure of suppressor function, yet the molecular mechanisms ensuring steady heritable Foxp3 expression in the Treg cell lineage remain unknown. Using T_reg cell-specific gene targeting we found that Runx-CBFβ complexes were required for maintenance of Foxp3 mRNA and protein expression in Treg cells. Consequently, mice lacking CBFβb exclusively in the Treg cell lineage exhibited a moderate lymphproliferative syndrome. Thus, Runx-CBFβ complexes maintain stable expression of high amounts of Foxp3 and serve as an essential determinant of Treg cell lineage stability.

Development of macrophages of cyprinid fish

The innate immune responses of early vertebrates, such as bony fishes, play a central role in host defence against infectious diseases and one of the most important effector cells of innate immunity are macrophages. In order for macrophages to be effective in host defence they must be present at all times in the tissues of their host and importantly, the host must be capable of rapidly increasing macrophage numbers during times of need. Hematopoiesis is a process of formation and development of mature blood cells, including macrophages. Hematopoiesis is controlled by soluble factors known as cytokines, that influence changes in transcription factors within the target cells, resulting in cell fate changes and the final development of specific effector cells. The processes involved in macrophage development have been largely derived from mammalian model organisms. However, recent advancements have been made in the understanding of macrophage development in bony fish, a group of organisms that rely heavily on their innate immune defences. Our understanding of the growth factors involved in teleost macrophage development, as well as the receptors and regulatory mechanisms in place to control them has increased substantially. Furthermore, model organisms such as the zebrafish have emerged as important instruments in furthering our understanding of the transcriptional control of cell development in fish as well as in mammals. This review highlights the recent advancements in our understanding of teleost macrophage development. We focused on the growth factors identified to be important in the regulation of macrophage development from a progenitor cell into a functional macrophage and discuss the important transcription factors that have been identified to function in teleost hematopoiesis. We also describe the findings of in vivo studies that have reinforced observations made in vitro and have greatly improved the relevance and importance of using teleost fish as model organisms for studying developmental processes.

Proleukemic RUNX1 and CBFbeta mutations in the pathogenesis of acute leukemia

The existence of non-random mutations in critical regulators of cell growth and differentiation is a recurring theme in cancer pathogenesis and provides the basis for our modern, molecular approach to the study and treatment of malignant diseases. Nowhere is this more true than in the study of leukemogenesis, where research has converged upon a critical group of genes involved in hematopoietic stem and progenitor cell self-renewal and fate specification. Prominent among these is the heterodimeric transcriptional regulator, RUNX1/CBFbeta. RUNX1 is a site-specific DNA-binding protein whose consensus response element is found in the promoters of many hematopoietically relevant genes. CBFbeta interacts with RUNX1, stabilizing its interaction with DNA to promote the actions of RUNX1/CBFbeta in transcriptional control. Both the RUNX1 and the CBFbeta genes participate in proleukemic chromosomal alterations. Together they contribute to approximately one-third of acute myelogenous leukemia (AML) and one-quarter of acute lymphoblastic leukemia (ALL) cases, making RUNX1 and CBFbeta the most frequently affected genes known in the pathogenesis of acute leukemia. Investigating the mechanisms by which RUNX1, CBFbeta, and their proleukemic fusion proteins influence leukemogenesis has contributed greatly to our understanding of both normal and malignant hematopoiesis. Here we present an overview of the structural features of RUNX1/CBFbeta and their derivatives, their roles in transcriptional control, and their contributions to normal and malignant hematopoiesis.

Critical roles for c-Myb in hematopoietic progenitor cells

While it has long been known that the transcription factor c-Myb is an essential regulator of hematopoiesis, its precise molecular targets have remained elusive. Cell line studies suggest that c-Myb promotes proliferation and at the same time inhibits differentiation, however the early lethality of c-Myb deficient embryos precluded analysis of its role in adult hematopoiesis. Here we review insights derived from recently developed mouse models of c-Myb deficiency that are viable as adults. These studies reveal a complex array of functions for c-Myb in multiple hematopoietic cell types that will redefine our understanding of this crucial transcription factor.

The genetic variation of the human HMGB1 gene

High-mobility group box 1 protein (HMGB1) is a nuclear DNA-binding protein, which also functions as a pleiotropic cytokine, implicated in the pathology of several different immune-mediated diseases. The purpose of this study was to examine the HMGB1 gene for putative polymorphisms in 103 healthy Caucasian Danish blood donors. A total of six polymorphisms and four mutations were identified in the HMGB1 gene. Subsequent MatInspector estimation revealed that several polymorphisms might have a potential regulatory impact on HMGB1 transcription. This study has characterized genetic variations in the HMGB1 gene locus, which may have a regulating role in the expression of HMGB1, providing the basis for molecular investigations of the HMGB1 gene in different disease settings.

RUNX1-RUNX1 Homodimerization Modulates RUNX1 Activity and Function

RUNX1 (AML1, CBFalpha2, PEBP2alphaB) is a transcription factor essential for the establishment of the hematopoietic stem cell. It is generally thought that RUNX1 exists as a monomer that regulates hematopoietic differentiation by interacting with tissue-specific factors and its DNA consensus through its N terminus. RUNX1 is frequently altered in human leukemia by gene fusions or point mutations. In general, these alterations do not affect the N terminus of the protein, and it is unclear how they consistently lead to hematopoietic transformation and leukemia. Here we report that RUNX1 homodimerizes through a mechanism involving C terminus-C terminus interaction. This RUNX1-RUNX1 interaction regulates the activity of the protein in reporter gene assays and modulates its ability to induce hematopoietic differentiation of hematopoietic cell lines. The promoters of genes regulated by RUNX1 often contain multiple RUNX1 binding sites. This arrangement suggests that RUNX1 could homodimerize to bring and hold together distant chromatin sites and factors and that if the dimerization region is removed by gene fusions or is altered by point mutations, as observed in leukemia, the ability of RUNX1 to regulate differentiation could be impaired.

Normal and transforming functions of RUNX1: a perspective

Converging studies from many investigators indicate that RUNX1 has a critical role in the correct maintenance of essential cellular functions during embryonic development and after birth. The discovery that this gene is also frequently mutated in human leukemia has increased the interest in the role that RUNX1 plays in both normal and transforming pathways. Here, we provide an overview of the many roles of RUNX1 in hematopoietic self-renewal and differentiation and summarize the information that is currently available on the many mechanisms of RUNX1 deregulation in human leukemia.

Regulation of the murine Ddelta2 promoter by upstream stimulatory factor 1, Runx1, and c-Myb

Accessibility control of V(D)J recombination at Ag receptor loci depends on the coordinate activities of transcriptional enhancers and germline promoters. Recombination of murine Tcrd gene segments is known to be regulated, at least in part, by the Tcrd enhancer (Edelta) situated in the Jdelta2-Cdelta intron. However, there has been little characterization of promoters and other cis-acting elements that are activated by or collaborate with Edelta and that might function to regulate Tcrd gene recombination events. We now describe a strong promoter that is tightly associated with the murine Ddelta2 gene segment. EMSAs reveal that upstream stimulatory factor 1, Runx1, c-Myb, lymphoid enhancer binding factor 1, NF1, and E47 all interact with this promoter in vitro. Of these, upstream stimulatory factor 1, Runx1, and c-Myb appear necessary for full promoter activity in transiently transfected cells. Moreover, the same three factors were found to interact with the promoter in vivo by chromatin immunoprecipitation. We suggest that these factors play important roles as Edelta-dependent regulators of Ddelta2 accessibility in vivo. Consistent with the established roles of c-Myb and Runx factors in Edelta function, we detected low level, enhancer-independent activity of the Ddelta2 promoter in transient transfection experiments. We speculate that the Ddelta2 promoter may play a role as a weak, enhancer-independent regulator in vivo, and might contribute to residual Tcrd rearrangement in Edelta(-/-) mice.

Functional domains of Runx1 are differentially required for CD4 repression, TCRbeta expression, and CD4/8 double-negative to CD4/8 double-positive transition in thymocyte development

Runx1 (AML1) has multiple functions in thymocyte development, including CD4 repression in immature thymocytes, expression of TCRbeta, and efficient beta-selection. To determine the functional domains of Runx1 important for thymocyte development, we cultured Runx1-deficient murine fetal liver (FL) cells on OP9-Delta-like 1 murine stromal cells, which express Delta-like 1 and support thymocyte development in vitro, and introduced Runx1 or C-terminal-deletion mutants of Runx1 into the FL cells by retrovirus infection. In this system, Runx1-deficient FL cells failed to follow normal thymocyte development, whereas the introduction of Runx1 into the cells was sufficient to produce thymocyte development that was indistinguishable from that in wild-type FL cells. In contrast, Runx1 mutants that lacked the activation domain necessary for initiating gene transcription did not fully restore thymocyte differentiation, in that it neither repressed CD4 expression nor promoted the CD4/8 double-negative to CD4/8 double-positive transition. Although the C-terminal VWRPY motif-deficient mutant of Runx1, which cannot interact with the transcriptional corepressor Transducin-like enhancer of split (TLE), promoted the double-negative to double-positive transition, it did not efficiently repress CD4 expression. These results suggest that the activation domain is essential for Runx1 to establish thymocyte development and that Runx1 has both TLE-dependent and TLE-independent functions in thymocyte development.

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.

Developmental Activation of the TCR α Enhancer Requires Functional Collaboration among Proteins Bound Inside and Outside the Core Enhancer

The TCR delta enhancer (Edelta) and TCR alpha enhancer (Ealpha) play critical roles in the temporal and lineage-specific control of V(D)J recombination and transcription at the TCR alphadelta locus, working as a developmental switch controlling a transition from TCR delta to TCR alpha activity during thymocyte development. Previous experiments using a transgenic reporter substrate revealed that substitution of the 116-bp minimal Ealpha, denoted Talpha1-Talpha2, for the entire 1.4-kb Ealpha led to a premature activation of V(D)J recombination. This suggested that binding sites outside of Talpha1-Talpha2 are critical for the strict developmental regulation of TCR alpha rearrangement. We have further analyzed Ealpha to better understand the mechanisms responsible for appropriate developmental regulation in vivo. We found that a 275-bp Ealpha fragment, denoted Talpha1-Talpha4, contains all binding sites required for proper developmental regulation in vivo. This suggests that developmentally appropriate enhancer activation results from a functional interaction between factors bound to Talpha1-Talpha2 and Talpha3-Talpha4. In support of this, EMSAs reveal the formation of a large enhanceosome complex that reflects the cooperative assembly of proteins bound to both Talpha1-Talpha2 and Talpha3-Talpha4. Our data suggest that enhanceosome assembly is critical for developmentally appropriate activation of Ealpha in vivo, and that transcription factors, Sp1 and pCREB, may play unique roles in this process.

Role of the TEL-AML1 fusion gene in the molecular pathogenesis of childhood acute lymphoblastic leukaemia

Balanced chromosomal translocations are frequently associated with haematopoietic neoplasms and often involve genes that encode transcription factors, which play critical roles in normal haematopoiesis. Fusion oncoproteins that arise from chimeric genes generated by such translocations are usually stable and consistent molecular markers for a given disease subtype and contribute to the leukaemogenic processes. The t(12;21)(p13;q22) chromosomal translocation is the most frequent illegitimate gene recombination in paediatric cancer, occurring in approximately 25% of common (c) B-cell precursor acute lymphoblastic leukaemia (cALL) cases. The rearrangement results in the in-frame fusion of the 5' region of the ETS-related gene, TEL (ETV6), to almost the entire AML1 (RUNX1) locus and is associated with favourable prognosis following conventional therapeutic strategies. We discuss here the prenatal origins of the TEL/AML1 translocation as an initiating mutation, the role of TEL-AML1 in cellular transformation and the molecular mechanisms by which the chimeric protein imposes altered patterns of gene expression.

Identification of a Myb-responsive enhancer of the chicken C/EBPβ gene

The retroviral oncogene v-myb encodes a transcription factor (v-Myb) that disrupts myelomonocytic differentiation and transforms myelomonocytic cells. It is thought that the biological effects of v-Myb are caused by deregulation of specific target genes. The CCAAT box/enhancer binding protein beta (C/EBPbeta), a member of the basic region-leucine zipper (bzip) class of transcription factors, which itself plays an important role during myelomonocytic differentiation, has previously been shown to be regulated by Myb. Here we have addressed the mechanism by which v-Myb affects C/EBPbeta expression. We have employed the mapping of DNase I hypersensitive sites (DHSs) in chromatin as a tool to detect in vivo target sites of v-Myb. Our data identify a DHS downstream of the C/EBPbeta gene that appears to be specific for v-myb-transformed myeloblasts. We have confirmed by chromatin immunoprecipitation that v-Myb is bound to this region in vivo. Furthermore, we have found that ectopic expression of v-Myb in a myelomonocytic cell line is able to induce a DHS downstream of the C/EBPbeta gene, showing for the first time that v-Myb can affect chromatin structure. Reporter gene experiments demonstrate that the downstream DHS acts as a Myb-dependent enhancing element in transiently as well as in stably transfected myelomonocytic cells. Previous work has shown that v-Myb acts on the C/EBPbeta promoter; it now appears that Myb stimulates C/EBPbeta expression by acting on the promoter as well as on an enhancer of the C/EBPbeta gene. Interestingly, the mechanisms by which Myb acts on both elements differ; while Myb activation of the promoter requires the cooperation with C/EBPbeta, activation of the enhancer by Myb is independent of C/EBPbeta. Apart from the identification of a novel Myb-dependent enhancer, our work demonstrates the potential of chromatin structure analysis for the identification of Myb target sites.

Isoforms of the Ets transcription factor NERF/ELF-2 physically interact with AML1 and mediate opposing effects on AML1-mediated transcription of the B cell-specific blk gene

We previously isolated different isoforms of a new Ets transcription factor family member, NERF/ELF-2, NERF-2, NERF-1a, and NERF-1b. In contrast to the inhibitory isoforms NERF-1a and NERF-1b, NERF-2 acts as a transactivator of the B cell-specific blk promoter. We now report that NERF-2 and NERF-1 physically interact with AML1 (RUNX1), a frequent target for chromosomal translocations in leukemia. NERF-2 bound to AML1 via an interaction site located in a basic region upstream of the Ets domain. This is in contrast to most other Ets factors such as Ets-1 that bind to AML1 via the Ets domain, suggesting that different Ets factors utilize different domains for interaction with AML1. The interaction between AML1 and NERF-2 led to cooperative transactivation of the blk promoter, whereas the interaction between AML1 and NERF-1a led to repression of AML1-mediated transactivation. To delineate the differences in function of the different NERF isoforms, we determined that the transactivation domain of NERF-2 is encoded by the N-terminal 100 amino acids, which have been replaced in NERF-1a by a 19-amino acid transcriptionally inactive sequence. Furthermore, acidic domains A and B, which are conserved in NERF-2 and the related proteins ELF-1 and MEF/ELF-4, but not in NERF-1a, are largely responsible for NERF-2-mediated transactivation. Because translocation of the Ets factor Tel to AML1 is a frequent event in childhood pre-B leukemia, understanding the interaction of Ets factors with AML1 in the context of a B cell-specific promoter might help to determine the function of Ets factors and AML1 in leukemia.

The phosphorylation state of an autoregulatory domain controls PACS-1-directed protein traffic

PACS-1 is a cytosolic sorting protein that directs the localization of membrane proteins in the trans-Golgi network (TGN)/endosomal system. PACS-1 connects the clathrin adaptor AP-1 to acidic cluster sorting motifs contained in the cytoplasmic domain of cargo proteins such as furin, the cation-independent mannose-6-phosphate receptor and in viral proteins such as human immunodeficiency virus type 1 Nef. Here we show that an acidic cluster on PACS-1, which is highly similar to acidic cluster sorting motifs on cargo molecules, acts as an autoregulatory domain that controls PACS-1-directed sorting. Biochemical studies show that Ser278 adjacent to the acidic cluster is phosphorylated by CK2 and dephosphorylated by PP2A. Phosphorylation of Ser278 by CK2 or a Ser278-->Asp mutation increased the interaction between PACS-1 and cargo, whereas a Ser278-->Ala substitution decreased this interaction. Moreover, the Ser278-->Ala mutation yields a dominant-negative PACS-1 molecule that selectively blocks retrieval of PACS-1-regulated cargo molecules to the TGN. These results suggest that coordinated signaling events regulate transport within the TGN/endosomal system through the phosphorylation state of both cargo and the sorting machinery.

AML1 interconnected pathways of leukemogenesis

The AML1 transcription factor, identified by the cloning of the translocation t(8;21) breakpoint, is one of the most frequent targets for chromosomal translocations in leukemia. Furthermore, polysomies and point mutations can also alter AML1 function. AML1, also called CBF alpha 2, PEBP alpha 2 or RUNX1, is thus implicated in a great number of acute leukemias via a variety of pathogenic mechanisms and seems to act either as an oncogene or a tumor suppressor gene. Characterization of AML1 knockout mice has shown that AML1 is necessary for normal development of all hematopoietic lineages and alterations in the overal functional level of AML1 can have a profound effect on hematopoiesis. Numerous studies have shown that AML1 plays a vital role in the regulation of expression of many genes involved in hematopoietic cell development, and the impairment of AML1 function disregulates the pathways leading to cellular proliferation and differentiation. However, heterozygous AML1 mutations alone may not be sufficient for the development of leukemia. A cumulative process of mutagenesis involving additional genetic events in functionally related molecules, may be necessary for the development of leukemia and may determine the leukemic phenotype. We review the known AML1 target genes, AML1 interacting proteins, AML1 gene alterations and their effects on AML1 function, and mutations in AML1-related genes associated with leukemia. We discuss the interconnections between all these genes in cell signaling pathways and their importance for future therapeutic developments.

A DNA-binding-independent pathway of repression by the Drosophila Runt protein

DNA-binding proteins are important for regulating gene expression during development. It is widely assumed that this regulation involves sequence-specific DNA binding by these transcription factors to cognate cis-regulatory sequences of their downstream target genes. However, studies in both the Drosophila and the mouse model systems have provided examples in which the DNA-binding activity of a transcription factor is not essential for in vivo function. Using a system that allows for quantitative analysis of gene function in the Drosophila embryo, we have discovered a DNA-binding-independent activity of Runt, the founding member of the RUNX family of transcriptional regulators. Examination of the in vivo potency of a DNA-binding-defective form of Runt reveals differential requirements for DNA binding in the regulation of different downstream target genes. DNA binding is not required for establishing repression of the odd-numbered stripes of the segment polarity gene engrailed, but does contribute to Runt's role as a regulator of sloppy-paired, another downstream target gene in the pathway of segmentation. We investigate this DNA-binding-independent pathway using a genetic screen for dose-dependent modifiers of runt activity. These studies reveal that DNA-binding proteins encoded by the tramtrack locus cooperate with Runt to repress engrailed. These results provide new insights into the context-dependent regulatory functions of Runt domain proteins and provide a paradigm for understanding DNA-binding-independent regulation by developmentally important transcription factors.

The t(8;21) fusion protein contacts co-repressors and histone deacetylases to repress the transcription of the p14ARF tumor suppressor

The t(8;21) is one of the most frequent chromosomal translocations associated with acute leukemia. The translocation fuses the DNA binding domain of AML1 to nearly all of the ETO co-repressor. ETO associates with the mSin3 and N-CoR co-repressors as well as histone deacetylases 1, 2, and 3. Although this is one of the most frequent chromosomal translocations in acute leukemia, accounting for 10-15% of the cases of acute myeloid leukemia (AML), the direct targets for transcriptional regulation that stimulate leukemogenesis are unknown. We found that AML1-ETO repressed the promoter of p14(ARF) tumor suppressor in transient transfection assays and reduced endogenous levels of p14(ARF) expression in multiple cell types. Chromatin immunoprecipitation assays demonstrated that AML1-ETO bound to the p14(ARF) promoter. In acute myeloid leukemia samples containing the t(8;21), levels of p14(ARF) mRNA were markedly lower when compared to other acute myeloid leukemias. Therefore, p14(ARF) is a direct transcriptional target of AML1-ETO.

Runx1, c-Myb, and C/EBPalpha couple differentiation to proliferation or growth arrest during hematopoiesis

Immature hematopoietic precursors proliferate as they differentiate, whereas terminal differentiation is associated with cell cycle arrest. Stem cell lineage commitment and subseqent maturation is regulated predominantly by transcription factors. Runx1 and c-Myb act in early stage hematopoietic cells to both stimulate proliferation and differentiation, whereas C/EBPalpha, and perhaps other C/EBP family members, block progression from G1 to S and induce terminal maturation. Coupling of differentiation to either proliferation or growth arrest by transcription factors is likely an important regulatory mechanism in multiple developmental systems.

Cooperativity in transcription factor binding to the coactivator CREB-binding protein (CBP). The mixed lineage leukemia protein (MLL) activation domain binds to an allosteric site on the KIX domain

The interactions between cAMP-response element-binding protein (CREB)-binding protein (CBP) and gene-specific transcription factors play an important role in activation of transcription from numerous genes. Cooperative interactions between CBP and multiple transcriptional activators may provide a mechanism for synergistic increases in transcriptional activation. Here we report the characterization of ternary complexes formed by the KIX domain of CBP and the transactivation domain of the trithorax group protein mixed lineage leukemia protein (MLL), together with either the phosphorylated kinase-inducible domain (pKID) of CREB or the activation domain from c-Myb. Isothermal titration calorimetry experiments show that KIX in complex with the MLL activation domain binds the c-Myb activation domain and pKID domain with 2-fold higher affinity than does the KIX domain alone. Thus, the activation domains of Myb and MLL or of pKID and MLL bind cooperatively to KIX. The thermodynamics of these interactions imply different mechanisms of binding cooperativity for the two ternary complexes; the KIX.MLL.pKID complex is stabilized by entropy increases, whereas the enhancement of Myb binding in the presence of the MLL activation domain is due to more favorable enthalpy. NMR experiments show that the MLL-binding site on KIX is distinct from the surface that binds the pKID and c-Myb activation domains. Our data indicate that KIX can directly mediate cooperative interactions between pairs of transcriptional regulatory proteins. In the case of MLL and c-Myb, both proteins are involved in proliferation of hematopoietic cells and leukemogenesis, and synergistic interactions mediated by CBP may play a functional role.

Distinct roles for c-Myb and core binding factor/polyoma enhancer-binding protein 2 in the assembly and function of a multiprotein complex on the TCR delta enhancer in vivo

Enhancers and promoters within TCR loci functionally collaborate to modify chromatin structure and to confer accessibility to the transcription and V(D)J recombination machineries during T cell development in the thymus. Two enhancers at the TCRalphadelta locus, the TCR alpha enhancer and the TCR delta enhancer (Edelta), are responsible for orchestrating the distinct developmental programs for V(D)J recombination and transcription of the TCR alpha and delta genes, respectively. Edelta function depends critically on transcription factors core binding factor (CBF)/polyoma enhancer-binding protein 2 (PEBP2) and c-Myb as measured by transcriptional activation of transiently transfected substrates in Jurkat cells, and by activation of V(D)J recombination within chromatin-integrated substrates in transgenic mice. To understand the molecular mechanisms for synergy between these transcription factors in the context of chromatin, we used in vivo footprinting to study the requirements for protein binding to Edelta within wild-type and mutant versions of a human TCR delta minilocus in stably transfected Jurkat cells. Our data indicate that CBF/PEBP2 plays primarily a structural role as it induces a conformational change in the enhanceosome that is associated with augmented binding of c-Myb. In contrast, c-Myb has no apparent affect on CBF/PEBP2 binding, but is critical for transcriptional activation. Thus, our data reveal distinct functions for c-Myb and CBF/PEBP2 in the assembly and function of an Edelta enhanceosome in the context of chromatin in vivo.

Runx1/AML1 in leukemia: disrupted association with diverse protein partners

Runx1/AML1, a chromosome 21q22 hematopoietic regulator, is frequently translocated in leukemia. Its protein product, a relatively weak transcriptional activator, becomes an effective transcriptional enhancer or repressor, when co-operating with transcriptional co-activators or co-repressors. Runx1/AML1 association with its partners is disrupted in leukemia. For example, Runx1/AML1 mutations and translocations (e.g. t(8;21), t(12;21) and t(3;21)) impair binding of Runx1/AML1-CBFbeta complexes to Runt motifs in myelopoietically active promoters, preventing normal hematopoiesis. However, Runx1/AML1-associated translocations are not leukemogenic in animal models, suggesting the involvement of yet unidentified regulatory proteins. New candidates are cholinesterases, inhibition of which increases leukemic risk in a manner potentially associated with Runx1/AML1.

Mechanism of c-Myb-C/EBP beta cooperation from separated sites on a promoter

c-Myb, but not avian myeloblastosis virus (AMV) v-Myb, cooperates with C/EBP beta to regulate transcription of myeloid-specific genes. To assess the structural basis for that difference, we determined the crystal structures of complexes comprised of the c-Myb or AMV v-Myb DNA-binding domain (DBD), the C/EBP beta DBD, and a promoter DNA fragment. Within the c-Myb complex, a DNA-bound C/EBP beta interacts with R2 of c-Myb bound to a different DNA fragment; point mutations in v-Myb R2 eliminate such interaction within the v-Myb complex. GST pull-down assays, luciferase trans-activation assays, and atomic force microscopy confirmed that the interaction of c-Myb and C/EBP beta observed in crystal mimics their long range interaction on the promoter, which is accompanied by intervening DNA looping.

A DAP12-mediated pathway regulates expression of CC chemokine receptor 7 and maturation of human dendritic cells

Gene targeting of the adaptor molecule DAP12 in mice caused abnormal distribution and impaired antigen presentation capacity of dendritic cells (DCs). However, the DAP12-associated receptors expressed on DCs and their functions have not been identified yet. Here we show that the triggering receptor expressed on myeloid cells-2 (TREM-2) is a cell surface receptor on human monocyte-derived DCs, which is associated with DAP12. TREM-2/DAP12 promotes upregulation of CC chemokine receptor 7, partial DC maturation, and DC survival through activation of protein tyrosine kinases and extracellular signal-regulated kinase. In contrast to Toll-like receptor-mediated signaling, TREM2/DAP12 stimulation is independent of nuclear factor-kappaB and p38 stress-activated protein kinase. This novel DC activation pathway may regulate DC homeostasis and amplify DC responses to pathogens, explaining the phenotype observed in DAP12-deficient mice.

The upstream enhancer is necessary and sufficient for the expression of the pre-T cell receptor alpha gene in immature T lymphocytes

The expression of the pre-T cell receptor alpha (pTa) gene occurs exclusively in immature T lymphocytes and is regulated by poorly defined mechanisms. We have analyzed the role of the upstream enhancer in pTa expression using conventional and bacterial artificial chromosome (BAC) reporter transgenes. The deletion of the enhancer completely abolished the expression of pTa BAC reporter in transgenic mice. Conversely, the combination of pTa enhancer and promoter targeted transgenes specifically to immature thymocytes, recapitulating the expression pattern of pTa. The core enhancer is conserved between mice and humans and contains a critical binding site for the transcription factor c-Myb. We also show that pTa promoter contains a conserved tandem E box site activated by E protein, HEB. These data establish the enhancer as a critical element regulating pTa gene expression and identify additional targets for c-Myb and E proteins in T cell development.

Transcriptional regulation of hemopoiesis

The regulation of blood cell formation, or hemopoiesis, is central to the replenishment of mature effector cells of innate and acquired immune responses. These cells fulfil specific roles in the host defense against invading pathogens, and in the maintenance of homeostasis. The development of hemopoietic cells is under stringent control from extracellular and intracellular stimuli that result in the activation of specific downstream signaling cascades. Ultimately, all signal transduction pathways converge at the level of gene expression where positive and negative modulators of transcription interact to delineate the pattern of gene expression and the overall cellular hemopoietic response. Transcription factors, therefore, represent a nodal point of hemopoietic control through the integration of the various signaling pathways and subsequent modulation of the transcriptional machinery. Transcription factors can act both positively and negatively to regulate the expression of a wide range of hemopoiesis-relevant genes including growth factors and their receptors, other transcription factors, as well as various molecules important for the function of developing cells. The expression of these genes is dependent on the complex interactions between transcription factors, co-regulatory molecules, and specific binding sequences on the DNA. Recent advances in various vertebrate and invertebrate systems emphasize the importance of transcription factors for hemopoiesis control and the evolutionary conservation of several of such mechanisms. In this review we outline some of the key issues frequently identified in studies of the transcriptional regulation of hemopoietic gene expression. In teleosts, we expect that the characterization of several of these transcription factors and their regulatory mechanisms will complement recent advances in a number of fish systems where identification of cytokine and other hemopoiesis-relevant factors are currently under investigation.

Transcription factors and translocations in lymphoid and myeloid leukemia

Chromosomal translocations involving transcription factors and aberrant expression of transcription factors are frequently associated with leukemogenesis. Transcription factors are essential in maintaining the regulation of cell growth, development, and differentiation in the hematopoietic system. Alterations in the mechanisms that normally control these functions can lead to hematological malignancies. Further characterization of the molecular biology of leukemia will enhance our ability to develop disease-specific treatment strategies, and to develop effective methods of diagnosis and prognosis.

Expression and function of a stem cell promoter for the murine CBFalpha2 gene: distinct roles and regulation in natural killer and T cell development

The Runt family transcription factor CBFalpha2 (AML1, PEBP2alphaB, or Runx1) is required by hematopoietic stem cells and expressed at high levels in T-lineage cells. In human T cells CBFalpha2 is usually transcribed from a different promoter (distal promoter) than in myeloid cells (proximal promoter), but the developmental and functional significance of this promoter switch has not been known. Here, we report that both coding and noncoding sequences of the distal 5' end are highly conserved between the human and the murine genes, and the distal promoter is responsible for the overwhelming majority of CBFalpha2 expression in murine hematopoietic stem cells as well as in T cells. Distal promoter activity is maintained throughout T cell development and at lower levels in B cell development, but downregulated in natural killer cell development. The distal N-terminal isoform binds to functionally important regulatory sites from known target genes with two- to threefold higher affinity than the proximal N-terminal isoform. Neither full-length isoform alters growth of a myeloid cell line under nondifferentiating conditions, but the proximal isoform selectively delays mitotic arrest of the cell line under differentiating conditions, resulting in the generation of greater numbers of neutrophils.

Chemokine production by G protein-coupled receptor activation in a human mast cell line: roles of extracellular signal-regulated kinase and NFAT

Chemoattractants are thought to be the first mediators generated at sites of bacterial infection. We hypothesized that signaling through G protein-coupled chemoattractant receptors may stimulate cytokine production. To test this hypothesis, a human mast cell line (HMC-1) that normally expresses receptors for complement components C3a and C5a at low levels was stably transfected to express physiologic levels of fMLP receptors. We found that fMLP, but not C3a or C5a, induced macrophage inflammatory protein (MIP)-1ss (CCL4) and monocyte chemoattractant protein-1 (CCL2) mRNA and protein. Although fMLP stimulated both sustained Ca(2+) mobilization and phosphorylation of extracellular signal-regulated kinase (ERK), these responses to C3a or C5a were transient. However, transient expression of C3a receptors in HMC-1 cells rendered the cells responsive to C3a for sustained Ca(2+) mobilization and MIP-1ss production. The fMLP-induced chemokine production was blocked by pertussis toxin, PD98059, and cyclosporin A, which respectively inhibit G(i)alpha activation, mitgen-activated protein kinase kinase-mediated ERK phosphorylation, and calcineurin-mediated activation of NFAT. Furthermore, fMLP, but not C5a, stimulated NFAT activation in HMC-1 cells. These data indicate that chemoattractant receptors induce chemokine production in HMC-1 cells with a selectivity that depends on the level of receptor expression, the length of their signaling time, and the synergistic interaction of multiple signaling pathways, including extracellular signal-regulated kinase phosphorylation, sustained Ca(2+) mobilization and NFAT activation.

Mechanisms of transcriptional regulation by Runt domain proteins

Runt domain proteins have vital roles in regulating transcription in developmental pathways extending from sex determination and segmentation in fruit fly embryos to the development of blood and bone in mammals. Many of the insights into the mechanisms by which these proteins act to regulate transcription originate either from studies on the Drosophila runt gene, the founding member of this family, or from work on the mammalian PEBP2/CBF transcription factor. Genetic experiments in the Drosophila system reveal that runt functions both to activate and to repress transcription of different downstream target genes and indicate that different mechanisms are used in the regulation of different specific downstream target genes. These studies have also identified other nuclear factors that work with Runt in some of these pathways. Studies in mammalian systems have provided additional evidence for the complexity of transcriptional regulation by Runt domain proteins and have identified other transcription factors that cooperate with Runt domain proteins to regulate the activity of different specific cis-regulatory enhancers. The emerging view from studies in both systems is that these proteins act as context-dependent regulators of transcription, activating or repressing gene expression dependent upon the constititution of a particular promoter/enhancer in a particular cell type. These results have yielded new insights into the molecular mechanisms that control animal development and provide a framework for investigating fundamental issues in eukaryotic transcriptional regulation.

Regulation of granulopoiesis by transcription factors and cytokine signals

The development of mature granulocytes from hematopoietic precursor cells is controlled by a myriad of transcription factors which regulate the expression of essential genes, including those encoding growth factors and their receptors, enzymes, adhesion molecules, and transcription factors themselves. In particular, C/EBPalpha, PU.1, CBF, and c-Myb have emerged as critical players during early granulopoiesis. These transcription factors interact with one another as well as other factors to regulate the expression of a variety of genes important in granulocytic lineage commitment. An important goal remains to understand in greater detail how these various factors act in concert with signals emanating from cytokine receptors to influence the various steps of maturation, from the pluripotent hematopoietic stem cell, to a committed myeloid progenitor, to myeloid precursors, and ultimately to mature granulocytes.

Role of the transcription factor AML-1 in acute leukemia and hematopoietic differentiation

Chromosomal translocations affecting the AML-1 gene are among the most frequent aberrations found in acute leukemia. Because the AML-1 transcription factor is a critical regulator of hematopoeitic cell development, normal homeostasis is disrupted in cells containing these translocations. In this review we describe the mechanisms of transcriptional activation and repression by AML-1 and how this transcriptional control is disrupted by the chromosomal translocations that affect AML-1. Finally, we discuss how the mechanism of transcriptional repression by these chromosomal translocation fusion proteins is a possible target of therapeutic intervention in acute leukemia.

A mechanism of repression by acute myeloid leukemia-1, the target of multiple chromosomal translocations in acute leukemia

AML1 is one of the most frequently translocated genes in human leukemia. Here we demonstrate that acute myeloid leukemia-1 (AML-1) (Runx-1) represses transcription from a native promoter, p21(Waf1/Cip1). Unexpectedly, this repression did not require interactions with the Groucho co-repressor. To define the mechanism of repression, we asked whether other co-repressors could interact with AML-1. We demonstrate that AML-1 interacts with the mSin3 co-repressors. Moreover, endogenous AML-1 associated with endogenous mSin3A in mammalian cells. A deletion mutant of AML-1 that did not interact with mSin3A failed to repress transcription. The AML-1/mSin3 association suggests a mechanism of repression for the chromosomal translocation fusion proteins that disrupt AML-1.

Transcriptional activation by the myb proteins requires a specific local promoter structure

The biological effects of the cellular c-Myb and the viral v-Myb proteins are strikingly different. While c-Myb is indispensable for normal hematopoiesis, v-Myb induces acute leukemia. The v-Myb DNA-binding domain (DBD) differs from that of c-Myb mainly by deletion of the first of three repeats which correlates with efficient oncogenic transformation and a decrease in DNA-binding activity. To investigate the difference in DNA-binding and transcriptional activation, oligonucleotide selection and electrophoretic mobility shift assays were employed. The v-Myb DBD (R2R3) shows an intrinsic DNA-binding specificity for an AT-rich downstream extension of the Myb recognition element (MRE) PyAAC(T)/(G)G for efficient binding to this site, whereas R1 within the c-Myb DBD allows for more flexibility for this downstream extension. Therefore, due to the presence of repeat R1, c-Myb can bind to a greater number of target sites. The intrinsic DNA-binding specificity of R2R3 is further supported with the results from in vivo transcriptional activation experiments which demonstrated that both the v-Myb and c-Myb DBDs require an extension of the MRE (motif #1) by a downstream T-stretch (motif #2) for full activity. Surprisingly, the T-stretch improves binding when present on either strand, but is required on a specific strand for transcriptional activation.