ETS-core binding factor: a common composite motif in antigen receptor gene enhancers

Activation of Mouse Tcrb: Uncoupling RUNX1 Function from Its Cooperative Binding with ETS1

T lineage commitment requires the coordination of key transcription factors (TFs) in multipotent progenitors that transition them away from other lineages and cement T cell identity. Two important TFs for the multipotent progenitors to T lineage transition are RUNX1 and ETS1, which bind cooperatively to composite sites throughout the genome, especially in regulatory elements for genes involved in T lymphopoiesis. Activation of the TCR β (Tcrb) locus in committed thymocytes is a critical process for continued development of these cells, and is mediated by an enhancer, Eβ, which harbors two RUNX-ETS composite sites. An outstanding issue in understanding T cell gene expression programs is whether RUNX1 and ETS1 have independent functions in enhancer activation that can be dissected from cooperative binding. We now show that RUNX1 is sufficient to activate the endogenous mouse Eβ element and its neighboring 25 kb region by independently tethering this TF without coincidental ETS1 binding. Moreover, RUNX1 is sufficient for long-range promoter-Eβ looping, nucleosome clearance, and robust transcription throughout the Tcrb recombination center, spanning both DβJβ clusters. We also find that a RUNX1 domain, termed the negative regulatory domain for DNA binding, can compensate for the loss of ETS1 binding at adjacent sites. Thus, we have defined independent roles for RUNX1 in the activation of a T cell developmental enhancer, as well as its ability to mediate specific changes in chromatin landscapes that accompany long-range induction of recombination center promoters.

Targeting of somatic hypermutation by immunoglobulin enhancer and enhancer-like sequences

Immunoglobulin gene enhancers have a conserved function in targeting somatic hypermutation to immunoglobulin genes, thereby supporting the production of high affinity antibodies.

Somatic hypermutation (SH) generates point mutations within rearranged immunoglobulin (Ig) genes of activated B cells, providing genetic diversity for the affinity maturation of antibodies. SH requires the activation-induced cytidine deaminase (AID) protein and transcription of the mutation target sequence, but how the Ig gene specificity of mutations is achieved has remained elusive. We show here using a sensitive and carefully controlled assay that the Ig enhancers strongly activate SH in neighboring genes even though their stimulation of transcription is negligible. Mutations in certain E-box, NFκB, MEF2, or Ets family binding sites—known to be important for the transcriptional role of Ig enhancers—impair or abolish the activity. Full activation of SH typically requires a combination of multiple Ig enhancer and enhancer-like elements. The mechanism is evolutionarily conserved, as mammalian Ig lambda and Ig heavy chain intron enhancers efficiently stimulate hypermutation in chicken cells. Our results demonstrate a novel regulatory function for Ig enhancers, indicating that they either recruit AID or alter the accessibility of the nearby transcription units.

During the B cell immune response, immunoglobulin (Ig) genes are subject to a unique mutation process known as somatic hypermutation that allows the immune system to generate high-affinity antibodies. Somatic hypermutation preferentially affects Ig genes, relative to other genes, and this is important in preventing catastrophic levels of general genomic mutations that could lead to B cell cancers. We hypothesized that this preferential targeting of somatic hypermutation is assisted by specific DNA sequences in or near Ig genes that focus the action of the mutation machinery on those genes. In this study, we show that Ig genes across species—from human, mouse, and chicken—do indeed contain such mutation targeting sequences and that they coincide with transcriptional regulatory regions known as enhancers. We show that combinations of Ig enhancers cooperate to achieve strong mutation targeting and that this action depends on well-known transcription factor binding sites in these enhancer elements. Our findings establish an evolutionarily conserved function for enhancers in somatic hypermutation targeting, which operates by a mechanism distinct from the conventional enhancer function of increasing levels of transcription. We propose that combinations of Ig enhancers target somatic mutation to Ig genes by recruiting the mutation machinery and/or by making the Ig genes better substrates for mutation.

Using BAC transgenesis in zebrafish to identify regulatory sequences of the amyloid precursor protein gene in humans

Background

Non-coding DNA in and around the human Amyloid Precursor Protein (APP) gene that is central to Alzheimer’s disease (AD) shares little sequence similarity with that of appb in zebrafish. Identifying DNA domains regulating expression of the gene in such situations becomes a challenge. Taking advantage of the zebrafish system that allows rapid functional analyses of gene regulatory sequences, we previously showed that two discontinuous DNA domains in zebrafish appb are important for expression of the gene in neurons: an enhancer in intron 1 and sequences 28–31 kb upstream of the gene. Here we identify the putative transcription factor binding sites responsible for this distal cis-acting regulation, and use that information to identify a regulatory region of the human APP gene.

Results

Functional analyses of intron 1 enhancer mutations in enhancer-trap BACs expressed as transgenes in zebrafish identified putative binding sites of two known transcription factor proteins, E4BP4/ NFIL3 and Forkhead, to be required for expression of appb. A cluster of three E4BP4 sites at −31 kb is also shown to be essential for neuron-specific expression, suggesting that the dependence of expression on upstream sequences is mediated by these E4BP4 sites. E4BP4/ NFIL3 and XFD1 sites in the intron enhancer and E4BP4/ NFIL3 sites at −31 kb specifically and efficiently bind the corresponding zebrafish proteins in vitro. These sites are statistically over-represented in both the zebrafish appb and the human APP genes, although their locations are different. Remarkably, a cluster of four E4BP4 sites in intron 4 of human APP exists in actively transcribing chromatin in a human neuroblastoma cell-line, SHSY5Y, expressing APP as shown using chromatin immunoprecipitation (ChIP) experiments. Thus although the two genes share little sequence conservation, they appear to share the same regulatory logic and are regulated by a similar set of transcription factors.

Conclusion

The results suggest that the clock-regulated and immune system modulator transcription factor E4BP4/ NFIL3 likely regulates the expression of both appb in zebrafish and APP in humans. It suggests potential human APP gene regulatory pathways, not on the basis of comparing DNA primary sequences with zebrafish appb but on the model of conservation of transcription factors.

Identifying cis-acting DNA elements within a control region

Computational methods can be used to identify DNA sequence motifs that have been conserved through evolution, as well as motifs that correspond to recognition sites for known DNA-binding proteins. These computational methods, when combined with chromatin immunoprecipitation and other basic experiments, can provide preliminary insight into the elements and factors that regulate a gene of interest. When pursuing a more complete understanding of a control region of interest, a comprehensive mutant analysis should generally be performed as a critical step toward more advanced functional studies. This article describes strategies for such a comprehensive analysis. It also summarizes the insights provided by a comprehensive mutant analysis versus a phylogenetic analysis.

RUNX1 and RUNX1-ETO: roles in hematopoiesis and leukemogenesis

RUNX1 is a transcription factor that regulates critical processes in many aspects of hematopoiesis. RUNX1 is also integral in defining the definitive hematopoietic stem cell. In addition, many hematological diseases like myelodysplastic syndrome and myeloproliferative neoplasms have been associated with mutations in RUNX1. Located on chromosomal 21, the RUNX1 gene is involved in many forms of chromosomal translocations in leukemia. t(8;21) is one of the most common chromosomal translocations found in acute myeloid leukemia (AML), where it results in a fusion protein between RUNX1 and ETO. The RUNX1-ETO fusion protein is found in approximately 12% of all AML patients. In this review, we detail the structural features, functions, and models used to study both RUNX1 and RUNX1-ETO in hematopoiesis over the past two decades.

Next-generation insights into regulatory T cells: expression profiling and FoxP3 occupancy in Human

Regulatory T-cells (Treg) play an essential role in the negative regulation of immune answers by developing an attenuated cytokine response that allows suppressing proliferation and effector function of T-cells (CD4(+) Th). The transcription factor FoxP3 is responsible for the regulation of many genes involved in the Treg gene signature. Its ablation leads to severe immune deficiencies in human and mice. Recent developments in sequencing technologies have revolutionized the possibilities to gain insights into transcription factor binding by ChiP-seq and into transcriptome analysis by mRNA-seq. We combine FoxP3 ChiP-seq and mRNA-seq in order to understand the transcriptional differences between primary human CD4(+) T helper and regulatory T-cells, as well as to study the role of FoxP3 in generating those differences. We show, that mRNA-seq allows analyzing the transcriptomal landscape of T-cells including the expression of specific splice variants at much greater depth than previous approaches, whereas 50% of transcriptional regulation events have not been described before by using diverse array technologies. We discovered splicing patterns like the expression of a kinase-dead isoform of IRAK1 upon T-cell activation. The immunoproteasome is up-regulated in both Treg and CD4(+) Th cells upon activation, whereas the 'standard' proteasome is up-regulated in Tregs only upon activation.

The transcription factor Ets1 is important for CD4 repression and Runx3 up-regulation during CD8 T cell differentiation in the thymus

The transcription factor Ets1 contributes to the differentiation of CD8 lineage cells in the thymus, but how it does so is not understood. In this study, we demonstrate that Ets1 is required for the proper termination of CD4 expression during the differentiation of major histocompatability class 1 (MHC I)–restricted thymocytes, but not for other events associated with their positive selection, including the initiation of cytotoxic gene expression, corticomedullary migration, or thymus exit. We further show that Ets1 promotes expression of Runx3, a transcription factor important for CD8 T cell differentiation and the cessation of Cd4 gene expression. Enforced Runx3 expression in Ets1-deficient MHC I–restricted thymocytes largely rescued their impaired Cd4 silencing, indicating that Ets1 is not required for Runx3 function. Finally, we document that Ets1 binds at least two evolutionarily conserved regions within the Runx3 gene in vivo, supporting the possibility that Ets1 directly contributes to Runx3 transcription. These findings identify Ets1 as a key player during CD8 lineage differentiation and indicate that it acts, at least in part, by promoting Runx3 expression.

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.

The Runx3 distal transcript encodes an additional transcriptional activation domain

The runt family transcriptional regulator, Runx3, is upregulated during the differentiation of CD8 single-positive thymocytes and is expressed in peripheral CD8(+) T cells. Mice carrying targeted deletions in Runx3 have severe defects in the development and activation of CD8(+) T cells, resulting in decreased CD8(+) T-cell numbers, aberrant coexpression of CD4, and failure to expand CD8(+) effector cells after activation in vivo or in vitro. Expression of each of the three vertebrate runt family members, including Runx3, is controlled by two promoters that generate proteins with alternative N-terminal sequences. The longer N-terminal region of Runx3, expressed from the distal promoter, is highly conserved among family members and across species. We show that transcripts from the distal Runx3 promoter are selectively expressed in mature CD8(+) T cells and are upregulated upon activation. We show that the N-terminal region encoded by these transcripts carries an independent transcriptional activation domain. This domain can activate transcription in isolation, and contributes to the increased transcriptional activity observed with this isoform as compared to those expressed from the ancestral, proximal promoter. Together, these data suggest an important role for the additional N-terminal Runx3 activation domain in CD8(+) T-cell function.

RUNX1 (AML-1) and RUNX2 (AML-3) cooperate with prostate-derived Ets factor to activate transcription from the PSA upstream regulatory region

The RUNX transcription factors (RUNX1, RUNX2, and RUNX3) play essential roles in hematopoiesis and skeletal development. Consistent with these roles in differentiation and cell cycle, the activity of both RUNX1 and RUNX3 is perturbed in cancer. To determine a role for the RUNX factors in prostate biology, we investigated the expression of RUNX factors in prostate epithelial cell lines and normal prostate tissue. RUNX1, RUNX2, and RUNX3 were expressed in both normal prostate tissue and an immortalized, non-transformed cell line. We found that prostate cancer-derived cell lines expressed RUNX1 and RUNX2, but not RUNX3. Next, we sought to identify prostate-specific genes whose expression could be regulated by RUNX proteins. Four consensus RUNX sites are located within the prostate-specific antigen (PSA) regulatory region. Chromatin immunoprecipitation (ChIP) analysis showed that RUNX1 is specifically bound to the PSA regulatory region in LNCaP cells. RUNX1 and RUNX2 activated the PSA regulatory region alone or cooperatively with prostate-derived ETS factor (PDEF) and RUNX1 physically associated with PDEF. Taken together, our results suggest that RUNX factors participate in prostate epithelial cell function and cooperate with an Ets transcription factor to regulate PSA gene expression.

Conservation and divergence of the Eμ3′ enhancer in the IGH locus of teleosts

The core region of the Emicro3' transcriptional enhancer that drives the expression of the teleost IGH locus has been characterized functionally in two species, the catfish (Ictalurus punctatus) and the zebrafish (Danio rerio). These studies have suggested important differences: whereas the catfish enhancer acts through an E-box and two octamer motifs, the zebrafish enhancer exerts its major effects through two E-box motifs alone. In this study, the function of the catfish enhancer was reexamined in a broader comparative context within the teleosts. Electrophoretic mobility shift assays of motifs from catfish, zebrafish, and Fugu were conducted to determine their ability to bind catfish E-protein and Oct transcription factors. Transient expression assays were conducted using a region of the catfish core enhancer that includes a newly described hybrid octamer/E-box motif. Sequences homologous to the Emicro3' enhancer region from six teleosts were aligned to determine conserved regions ("phylogenetic footprinting"). These studies allowed the following conclusions to be drawn: (1) The important 3'E-box motif described in the zebrafish corresponds in the homologous region of the catfish enhancer to an Oct motif with a newly described negative regulatory function and (2) Comparison of the Emicro3' enhancer sequences of six teleosts indicates that while a variety of octamer and E-box motifs are found in this region, strict evolutionary conservation of the important functional elements of the teleost Emicro3' enhancer has not occurred.

Regulation of the human LAT gene by the Elf-1 transcription factor

Background

The LAT gene encodes an intracellular adaptor protein that links cell-surface receptor engagement to numerous downstream signalling events, and thereby plays an integral role in the function of cell types that express the gene, including T cells, mast cells, natural killer cells, and platelets. To date, the mechanisms responsible for the transcriptional regulation of this gene have not been investigated.

Results

In this study we have mapped the transcriptional start sites for the human LAT gene and localized the 5' and 3' boundaries of the proximal promoter. We find that the promoter contains both positive and negative regulatory regions, and that two binding sites for the Ets family of transcription factors have a strong, positive effect on gene expression. Each site binds the Ets family member Elf-1, and overexpression of Elf-1 augments LAT promoter activity. The promoter also contains a Runx binding site adjacent to one of the Ets sites. This site, which is shown to bind Runx-1, has an inhibitory effect on gene expression. Finally, data is also presented indicating that the identified promoter may regulate cell-type specific expression.

Conclusion

Collectively, these results provide the first insights into the transcriptional regulation of the LAT gene, including the discovery that the Ets transcription factor Elf-1 may play a central role in its expression.

Oct2 transcription factors in fish--a comparative genomic analysis

The Oct2 transcription factor is important in driving expression of the IgH locus of the channel catfish, Ictalurus punctatus. Two isoforms, catfish Oct2alpha and Oct2beta, have been characterized at the level of expression and function, but little is known of the structure of the Oct2 gene in catfish. To gain insight into the diversity of Oct2 gene structure and expression in the teleost fish, a comparative genomic analysis of Oct2 was undertaken in the pufferfish (Fugu rubripes) and the zebrafish (Danio rerio). The orthologues of zebrafish and Fugu Oct2 were identified, and share with catfish Oct2 the expression of a limited number (two in zebrafish, three in Fugu) of isotypes produced by alternative pathways of RNA processing. The alternatively spliced variants of catfish Oct2 showed a different pattern of exon use from those of Fugu and zebrafish. The analysis also identified a novel homologue of Oct2 in both zebrafish and Fugu. This homologue, termed Oct2x, shares similarities to both Oct1 and Oct2. A phylogenetic analysis of the relationships of Oct2x gave an unexpected result, with Oct2x occupying a position basal to the Oct gene families of both vertebrates and Drosophila.

Alternative splicing removes an Ets interaction domain from Lozenge during Drosophila eye development

Physical and functional characteristics of the RUNX family of transcription factors are conserved between vertebrates and the Drosophila protein Lozenge. The runt-homology domain responsible for DNA binding and also the C-terminus are both nearly identical between the two proteins. The mammalian and fly proteins heterodimerize with a non-DNA binding partner protein to form a core binding factor essential for gene regulation during cell differentiation. The mammalian protein RUNX1 (AML1/PEBP2alphaB) interacts with the transcription factor Ets-1 to increase DNA binding and transactivation potential. Alternative splicing of the mammalian RUNX1 removes a domain required for this cooperative transactivation. In this work we determine the structure of the lozenge transcription unit and map 21 mutations. We show that the lozenge transcript is alternatively spliced during eye development to remove an Ets interaction domain. Emphasis is placed on Pointed the Drosophila homolog of the vertebrate Ets-1 protein; both Lozenge and Pointed proteins are needed for the activation of prospero expression. We use site-directed mutagenesis and yeast two-hybrid analysis to show that conserved amino acids within the alternate Lozenge exon are important for interaction with Pointed. Furthermore, the ectopic expression of Lozenge is sufficient to rescue Prospero expression in the presence of the Pointed competitor, Yan(ACT). We show that both lozenge isoforms are expressed during eye development and that the relative ratio of the transcripts for the two isoforms is sensitive to changes in Ras activity. We suggest that during eye development, Lozenge isoforms function in divergent roles, either interacting with Pointed on downstream targets or by functioning independently to establish distinct cell fates.

Regulation of immunoglobulin gene transcription in a teleost fish: identification, expression and functional properties of E2A in the channel catfish

The function of the transcriptional enhancer, Emu3', of the IgH locus of the channel catfish, Ictalurus punctatus, involves the interaction of E-protein and Oct family transcription factors. The E-proteins [class I basic helix-loop-helix (bHLH) family] are encoded in mammals by three genes: E2A (of which E12/E47 are alternatively spliced products), HEB, and E2-2. An E2A homologue has been identified in a catfish B-cell cDNA library and contains regions homologous to the bHLH and activation domains of mammalian and other vertebrate E2A proteins. E2A message is widely expressed, being readily detected in catfish B cells, T cells, kidney, spleen, brain, and muscle. Its expression is lower than that previously observed for TF12/CFEB, the catfish homologue of HEB. E2A strongly activated transcription of a muE5 motif-dependent construct in catfish B cells, and also activated transcription from the core region of the catfish IgH enhancer (Emu3') in a manner dependent on the presence of the muE5 site. Catfish E2A, expressed in vitro, bound the muE5 motif present in the core region of Emu3'. These results document the conservation of structure and function in vertebrate E2A and suggest a potential role of E2A in driving expression of the IgH locus at the phylogenetic level of a teleost fish.

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.

Evolution of Transcriptional Control of theIgHLocus: Characterization, Expression, and Function of TF12/HEB Homologs of the Catfish

The transcriptional enhancer (Emu3') of the IgH locus of the channel catfish, Ictalurus punctatus, differs from enhancers of the mammalian IgH locus in terms of its position, structure, and function. Transcription factors binding to multiple octamer motifs and a single muE5 motif (an E-box site, consensus CANNTG) interact for its function. E-box binding transcription factors of the class I basic helix-loop-helix family were cloned from a catfish B cell cDNA library in this study, and homologs of TF12/HEB were identified as the most highly represented E-proteins. Two alternatively spliced forms of catfish TF12 (termed CFEB1 and -2) were identified and contained regions homologous to the basic helix-loop-helix and activation domains of other vertebrate E-proteins. CFEB message is widely expressed, with CFEB1 message predominating over that of CFEB2. Both CFEB1 and -2 strongly activated transcription from a muE5-dependent artificial promoter. In catfish B cells, CFEB1 and -2 also activated transcription from the core region of the catfish IgH enhancer (Emu3') in a manner dependent on the presence of the muE5 site. Both CFEB1 and -2 bound the muE5 motif, and formed both homo- and heterodimers. CFEB1 and -2 were weakly active or inactive (in a promoter-dependent fashion) in mammalian B-lineage cells. Although E-proteins have been highly conserved in vertebrate evolution, the present results indicate that, at the phylogenetic level of a teleost fish, the TF12/HEB homolog differs from that of mammals in terms of 1) its high level of expression and 2) the presence of isoforms generated by alternative RNA processing.

ESE-1 is a novel transcriptional mediator of angiopoietin-1 expression in the setting of inflammation

Angiogenesis is a critical component of the inflammatory response associated with a number of conditions. Angiopoietin-1 (Ang-1) is an angiogenic growth factor that promotes the chemotaxis of endothelial cells and facilitates the maturation of new blood vessels. Ang-1 expression is up-regulated in response to tumor necrosis factor-alpha (TNF-alpha). To begin to elucidate the underlying molecular mechanisms by which Ang-1 gene expression is regulated during inflammation, we isolated 3.2 kb of the Ang-1 promoter that contain regulatory elements sufficient to mediate induction of the promoter in response to TNF-alpha, interleukin-1beta, and endotoxin. Surprisingly, sequence analysis of this promoter failed to reveal binding sites for transcription factors that are frequently associated with mediating inflammatory responses, such as NF-kappaB, STAT, NFAT, or C/EBP. However, putative binding sites for ETS and AP-1 transcription factor family members were identified. Interestingly, among a panel of ETS factors tested in a transient transfection assay, only the ETS factor ESE-1 was capable of transactivating the Ang-1 promoter. ESE-1 binds to specific ETS sites within the Ang-1 promoter that are functionally important for transactivation by ESE-1. ESE-1 and Ang-1 are induced in synovial fibroblasts in response to inflammatory cytokines, with ESE-1 induction slightly preceding that of Ang-1. Mutation of a high-affinity ESE-1 binding site leads to a marked reduction in Ang-1 transactivation by ESE-1, inducibility by inflammatory cytokines, and DNA binding to the ESE-1 protein. Transcriptional profiling of cells transiently transfected with an ESE-1 expression vector demonstrates that the endogenous Ang-1 gene is directly inducible by ESE-1. Finally, Ang-1 and ESE-1 exhibit a similar and strong expression pattern in the synovium of patients with rheumatoid arthritis. Our results support a novel paradigm for the ETS factor ESE-1 as a transcriptional mediator of angiogenesis in the setting of inflammation.

HMGA1 co-activates transcription in B cells through indirect association with DNA

The immunoglobulin heavy chain enhancer, or mu enhancer, is required for B cell development. Only the appropriate combination of transcription factors results in B cell-specific enhancer activation. HMGA1 (formerly (HMG-I(Y)) is a proposed co-activator of the ETS transcription factors required for mu enhancer activity. HMGA1 associates with the ETS factor PU.1, resulting in changes in PU.1 structure, and enhanced transcriptional synergy with Ets-1 on the mu enhancer in nonlymphoid cells. New data show HMGA1 directly interacts with Ets-1 in addition to PU.1. In vitro HMGA1/Ets-1 interaction facilitates Ets-1/mu enhancer binding in the absence of an HMGA1.Ets-1.DNA complex. To address whether HMGA1 is present in the transcriptionally active mu nucleoprotein complex, we completed DNA pull-down assays to detect protein tethering in the context of protein/DNA interaction. Results show that HMGA1 is not tightly associated with mu enhancer DNA through PU.1 or Ets-1, despite strong associations between these proteins in solution. However, chromatin immunoprecipitation assays show HMGA1 associates with the endogenous enhancer in B cells. Furthermore, antisense HMGA1 substantially decreases mu enhancer activity in B cells. Taken together, these data suggest that HMGA1 functions as a transcriptional mu enhancer co-activator in B cells through indirect association with DNA.

The role of a Runt domain transcription factor AML1/RUNX1 in leukemogenesis and its clinical implications

A Runt domain transcription factor AML1/RUNX1 is essential for generation and differentiation of definitive hematopoietic stem cells. AML1 is the most frequent target of chromosomal translocations in acute leukemias. Several chimeric proteins such as AML1-MTG8 and TEL-AML1 have transdominant properties for wild-type AML1 and acts as transcriptional repressors. The transcriptional repression in AML1 fusion proteins is mediated by recruitment of nuclear corepressor complex that maintains local histone deacetylation. Inhibition of the expression of AML1-responsive genes leads to a block in hematopoietic cell differentiation and consequent leukemic transformation. On the other hand, mutations in the Runt domain of the AML1 are identified in both sporadic acute myeloblastic leukemia (AML) without AML1 translocation and familial platelet disorder with predisposition to AML. These observations indicate that a decrease in AML1 dosage resulting from chromosomal translocations or mutations contributes to leukemogenesis. Furthermore, dysregulated chromatin remodeling and transcriptional control appears to be a common pathway in AML1-associated leukemias that could be an important target for the development of new therapeutic agents.

AML1-ETO inhibits maturation of multiple lymphohematopoietic lineages and induces myeloblast transformation in synergy with ICSBP deficiency

The translocation (8;21), generating the AML1-ETO fusion protein, is one of the most frequent chromosomal abnormalities associated with acute myelogenous leukemia (AML). To elucidate its role in oncogenesis, bone marrow (BM) cells were infected with a retroviral vector carrying AML1-ETO and transplanted into mice. In contrast to previous transgenic mouse models, we show that AML1-ETO directly stimulates granulopoiesis, suppresses erythropoiesis, and impairs the maturation of myeloid, B, and T lymphoid cells in vivo. To determine the significance of earlier findings that expression of the tumor suppressor ICSBP is often downregulated in AML myeloblasts, AML1-ETO was introduced into BM cells derived from mice lacking the interferon regulatory factor ICSBP. Our findings demonstrate that AML1-ETO synergizes with an ICSBP deficiency to induce myeloblastic transformation in the BM, reminiscent of AML.

Expression of transcription factor AML-2 (RUNX3, CBF(alpha)-3) is induced by Epstein-Barr virus EBNA-2 and correlates with the B-cell activation phenotype

To identify cell proteins regulated by the Epstein-Barr virus (EBV) transcription factor EBNA-2, we analyzed a cell line with conditional EBNA-2 activity by using microarray expression profiling. This led to the identification of two novel target genes induced by EBNA-2. The first of these, interleukin-16, is an immunomodulatory cytokine involved in the regulation of CD4 T cells. The second, AML-2, is a member of the Runt domain family of transcription factors. Quiescent B cells initially expressed AML-1 but, 48 h after virus infection, the levels of AML-1 decreased dramatically, whereas the amount of AML-2 protein increased. Analysis of a panel of B-cell lines indicated that AML-2 expression is normally predominant in EBV latency III, whereas AML-1 is associated with EBV latency I or EBV-negative cells. The AML genes are the first example of cell transcription factors whose expression correlates with the latency I/III phenotype.

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.

The expression of ETV6/CBFA2 (TEL/AML1) is not sufficient for the transformation of hematopoietic cell lines in vitro or the induction of hematologic disease in vivo

ETV6/CBFA2 (TEL/AML1) is the most frequent genetic abnormality associated with acute lymphoblastic leukemias in children, and is associated with a favorable prognosis. To investigate the influence of ETV6/CBFA2 on cellular transformation, the fusion gene was cloned into a murine ecotropic retroviral vector and transduced into IL-3-dependent Ba/F3 and 32Dcl.3 and IL-7-dependent IxN/2b murine hematopoietic cell lines. Different variants of ETV6/CBFA2, corresponding to CBFA2 alternatively spliced variants, and the reciprocal product CBFA2/ETV6, were stably expressed in each of these cell lines. However, although Western blot analysis demonstrated expression of each variant, none of the stable cell lines expressing CBFA2/ETV6 or the variants conferred factor-independent growth. We further investigated the effect of ETV6/CBFA2 expression in vivo by generating transgenic mice in which expression of the fusion was directed to lymphoid cells using the immunoglobulin heavy chain enhancer/promoter. Four founder mice were identified showing transmission and expression of the chimeric product. The mice were bred for five generations and followed for more than 24 months. The mice did not develop a malignant hematologic disorder, nor did they display histopathologic, morphologic, or immunophenotypic abnormalities, although ETV6/CBFA2 expression was confirmed in each line. We conclude that the expression of ETV6/CBFA2 alone is not sufficient for induction of growth factor independence in hematopoietic cell lines or hematologic disease in transgenic mice.

Roles of Ets proteins, NF-kappa B and nocodazole in regulating induction of transcription of mouse germline Ig alpha RNA by transforming growth factor-beta 1

Antibody class switch recombination (CSR) occurs after antigen activation of B cells. CSR is directed to specific heavy chain isotypes by cytokines and B cell activators that induce transcription from the unrearranged, or germline (GL), C(H) region genes. Transforming growth factor (TGF)-beta1 is essential for switch recombination to IgA due to its ability to induce transcription from GL Ig alpha genes. It has been shown that the promoters which regulate transcription of mouse and human GL alpha RNAs contain a TGF-beta1-responsive element that binds Smad and core binding factor (CBFalpha)/AML/PEBPalpha/RUNX: They also contain other elements which bind the transcription factors CREB, BSAP and Ets family proteins. In this manuscript we demonstrate that two tandem Ets sites in the mouse GL alpha promoter bind the transcription factors Elf-1 and PU.1, and that the 3' site is essential for expression of a luciferase reporter gene driven by the GL alpha promoter. Binding of Elf-1 to the GL alpha promoter is inducible by lipopolysaccharide in nuclear extracts from splenic B cells. An NF-kappaB site is identified, although it does not contribute to expression of the promoter in reporter gene assays. Since CSR to IgA is greatly reduced in NF-kappaB/p50-deficient mice, these data support the hypothesis that NF-kappaB has roles in switching in addition to regulation of GL transcription. Finally, we demonstrate that nocodazole, which disrupts microtubules that sequester Smad proteins in the cytoplasm, stimulates transcription from the GL alpha promoter.

An IgH Enhancer That Drives Transcription through Basic Helix-Loop-Helix and Oct Transcription Factor Binding Motifs

The transcriptional enhancer (E(mu)3') of the IgH locus of the channel catfish, Ictalurus punctatus, shows strong B cell-specific activity and differs from the mammalian E(mu) enhancer in both location and structure. It occurs between the mu and delta genes and contains numerous transcription factor binding sites, predominantly octamer and muE5 motifs of consensus and variant sequences. It lacks the classical muA-muE3(CBF)-muB core array of binding motifs seen within mammalian IgH E(mu) enhancers. To determine the functionally important motifs, a series of mutant enhancers was created using sequence-targeted polymerase chain reaction. Whereas the mutation of consensus and variant octamer motifs (individually or in multiples) decreased enhancer function, mutation of a single consensus muE5 motif destroyed the function of this enhancer in mammalian plasmacytomas. Mutation of this consensus muE5 site, combined with mutations of certain octamer sites, destroyed function in catfish B cells. Experiments using artificial enhancers containing multimers of motifs or short regions of the native enhancer suggested that the minimal E(mu)3' enhancer (a) contains a consensus muE5 site and two octamer sites, (b) is B cell-specific, and (c) is active across species. The dependence of an Ig enhancer on sites that bind basic helix-loop-helix and Oct transcription factors has not previously been observed and confirms large differences in structure and function between fish and mammalian IgH enhancers.

Functional mutagenesis of AML1/RUNX1 and PEBP2 beta/CBF beta define distinct, non-overlapping sites for DNA recognition and heterodimerization by the Runt domain

The Runt domain family of transcription factors play key roles in transcriptional regulation of definitive hematopoiesis and osteogenesis. This transcription factor family is characterized by a DNA-binding alpha-subunit harboring the Runt domain and a secondary subunit, beta, which binds to the Runt domain and enhances its interaction with DNA. Missense mutations in the Runt domain from either the blood or bone-related gene product are associated with the onset of acute human leukemia as well as a disease of skeletal patterning known as cleidocranial dysplasia. NMR "footprinting" analysis of Runt domain/beta/DNA ternary complexes in solution previously identified the likely residues that form the heterodimerization and DNA-binding surfaces of the Runt domain. Functional mutagenesis at 37 positions in the Runt domain or beta confirms the original identification of these interaction surfaces and reveals that the heterodimerization and DNA-binding surfaces of the Runt domain occur at distinct, non-overlapping sites within the domain. The analysis of an additional 21 disease-related missense mutations identified from patients with either blood or bone disease demonstrates that the primary defect in these patients is a failure in DNA-recognition by the Runt domain. The molecular basis for the DNA-binding defect is analyzed in the context of the three-dimensional structure of the Runt domain in binary and ternary protein/DNA complexes.

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.

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.

Cell specificity of the transcription-factor repertoire used by a lentivirus: motifs important for expression of equine infectious anemia virus in nonmonocytic cells

The equine infectious anemia virus (EIAV) long-terminal repeat (LTR) has been identified as highly variable, both in infected horses and in cell culture. This nucleotide hypervariation is localized to the LTR enhancer region. The EIAV LTR has been implicated in controlling both the cell tropism and virulence of the virus and it is postulated that the enhancer-region hypervariation may be responsible for the LTR effects. Our previous studies have demonstrated that the presence of DNA motifs bound by the ets transcription-factor family member PU.1 are critically important for EIAV expression in equine macrophages. Here we identify and characterize the EIAV LTR enhancer motifs PEA-2, Lvb, Oct, and CRE, that bind to fibroblast nuclear extracts. Three of these four motifs, PEA-2, Oct, and CRE, were determined to be important for expression of the LTR in a fibroblast cell line that supports productive infection of EIAV. These motifs that are important for expression of the LTR in fibroblasts were found to be interdigitated between the PU.1 sites. We hypothesize that the combination of motif interdigitation and cell-specific usage of these motifs may be responsible for the observed EIAV LTR enhancer-region hypervariation.

Auto-inhibition of Ets-1 is counteracted by DNA binding cooperativity with core-binding factor alpha2

Auto-inhibition is a common transcriptional control mechanism that is well characterized in the regulatory transcription factor Ets-1. Autoinhibition of Ets-1 DNA binding works through an inhibitory module that exists in two conformations. DNA binding requires a change in the inhibitory module from the packed to disrupted conformation. This structural switch provides a mechanism to tightly regulate Ets-1 DNA binding. We report that the Ets-1 partner protein core-binding factor alpha2 (CBFalpha2; also known as AML1 or PEBP2) stimulates Ets-1 DNA binding and counteracts auto-inhibition. Support for this conclusion came from three observations. First, the level of cooperative DNA binding (10-fold) was similar to the level of repression by auto-inhibition (10- to 20-fold). Next, a region necessary for cooperative DNA binding mapped to the inhibitory module. Third, an Ets-1 mutant with a constitutively disrupted inhibitory module did not bind DNA cooperatively with CBFalpha2. Furthermore, two additional lines of evidence indicated that CBFalpha2 affects the structural switch by direct interactions with Ets-1. First, the retention of cooperative DNA binding on nicked duplexes eliminated a potential role of through-DNA effects. Second, cooperative DNA binding was observed on composite sites with altered spacing or reversed orientation. We suggest that only protein interactions can accommodate this observed flexibility. These findings provide a mechanism by which CBF relieves the auto-inhibition of Ets-1 and illustrates one strategy for the synergistic activity of regulatory transcription factors.

Auto-inhibition and partner proteins, core-binding factor beta (CBFbeta) and Ets-1, modulate DNA binding by CBFalpha2 (AML1)

Core-binding factor alpha2 (CBFalpha2; otherwise known as AML1 or PEBP2alphaB) is a DNA-binding subunit in the family of core-binding factors (CBFs), heterodimeric transcription factors that play pivotal roles in multiple developmental processes in mammals, including hematopoiesis and bone development. The Runt domain in CBFalpha2 (amino acids 51 to 178) mediates DNA binding and heterodimerization with the non-DNA-binding CBFbeta subunit. Both the CBFbeta subunit and the DNA-binding protein Ets-1 stimulate DNA binding by the CBFalpha2 protein. Here we quantify and compare the extent of cooperativity between CBFalpha2, CBFbeta, and Ets-1. We also identify auto-inhibitory sequences within CBFalpha2 and sequences that modulate its interactions with CBFbeta and Ets-1. We show that sequences in the CBFalpha2 Runt domain and sequences C terminal to amino acid 214 inhibit DNA binding. Sequences C terminal to amino acid 214 also inhibit heterodimerization with the non-DNA-binding CBFbeta subunit, particularly heterodimerization off DNA. CBFbeta rescinds the intramolecular inhibition of CBFalpha2, stimulating DNA binding approximately 40-fold. In comparison, Ets-1 stimulates CBFalpha2 DNA binding 7- to 10-fold. Although the Runt domain alone is sufficient for heterodimerization with CBFbeta, sequences N terminal to amino acid 41 and between amino acids 190 and 214 are required for cooperative DNA binding with Ets-1. Cooperative DNA binding with Ets-1 is less pronounced with the CBFalpha2-CBFbeta heterodimer than with CBFalpha2 alone. These analyses demonstrate that CBFalpha2 is subject to both negative regulation by intramolecular interactions, and positive regulation by two alternative partnerships.

Definition of a T-cell receptor beta gene core enhancer of V(D)J recombination by transgenic mapping

V(D)J recombination in differentiating lymphocytes is a highly regulated process in terms of both cell lineage and the stage of cell development. Transgenic and knockout mouse studies have demonstrated that transcriptional enhancers from antigen receptor genes play an important role in this regulation by activating cis-recombination events. A striking example is the T-cell receptor beta-chain (TCRbeta) gene enhancer (Ebeta), which in the mouse consists of at least seven nuclear factor binding motifs (betaE1 to betaE7). Here, using a well-characterized transgenic recombination substrate approach, we define the sequences within Ebeta required for recombination enhancer activity. The Ebeta core is comprised of a limited set of motifs (betaE3 and betaE4) and an additional previously uncharacterized 20-bp sequence 3' of the betaE4 motif. This core element confers cell lineage- and stage-specific recombination within the transgenic substrates, although it cannot bypass the suppressive effects resulting from transgene integration in heterochromatic centromeres. Strikingly, the core enhancer is heavily occupied by nuclear factors in immature thymocytes, as shown by in vivo footprinting analyses. A larger enhancer fragment including the betaE1 through betaE4 motifs but not the 3' sequences, although active in inducing germ line transcription within the transgenic array, did not retain the Ebeta recombinational activity. Our results emphasize the multifunctionality of the TCRbeta enhancer and shed some light on the molecular mechanisms by which transcriptional enhancers and associated nuclear factors may impact on cis recombination, gene expression, and lymphoid cell differentiation.

The Ets factors PU.1 and Spi-B regulate the transcription in vivo of P2Y10, a lymphoid restricted heptahelical receptor

To investigate the in vivo functions of PU.1 and Spi-B, two highly related Ets transcription factors, we previously generated PU. 1(+/+)Spi-B(-/-) and PU.1(+/-)Spi-B(-/-) mice and demonstrated a significant decrease in B-cell receptor (BCR) signaling in mutants. Major components of BCR signaling appear to be expressed at normal levels in these mice, implying that PU.1 and Spi-B cooperate in the transcription of additional target genes important for antigen receptor signaling. We used subtractive hybridization to identify novel in vivo PU.1/Spi-B target genes and determined that the expression of a heptahelical receptor, P2Y10, is dramatically reduced in PU.1(+/-)Spi-B(-/-) B-cells. Further analysis shows that P2Y10 expression is restricted to lymphoid cells and parallels that of Spi-B in B-lymphocytes. Lastly, the P2Y10 promoter contains a PU. 1/Spi-B binding site functionally required for efficient transcription in B-cells. Thus, P2Y10 is likely to be a direct in vivo transcriptional target for PU.1 and Spi-B and provides a unique model to explore transcriptional regulation by this Ets factor subfamily. Furthermore, P2Y10 suggests an intriguing connection between heterotrimeric G-proteins and BCR signaling.

AML1 (CBFalpha2) cooperates with B cell-specific activating protein (BSAP/PAX5) in activation of the B cell-specific BLK gene promoter

AML1 plays a critical role during hematopoiesis and chromosomal translocations involving AML1 are commonly associated with different forms of leukemia, including pre-B acute lymphoblastic leukemia. To understand the function of AML1 during B cell differentiation, we analyzed regulatory regions of B cell-specific genes for potential AML1-binding sites and have identified a putative AML1-binding site in the promoter of the B cell-specific tyrosine kinase gene, blk. Gel mobility shift assays and transient transfection assays demonstrate that AML1 binds specifically to this site in the blk promoter and this binding site is important for blk promoter activity. Furthermore, in vitro binding analysis revealed that the AML1 runt DNA-binding domain physically interacts with the paired DNA-binding domain of BSAP, a B cell-specific transcription factor. BSAP has been shown previously to be important for B cell-specific regulation of the blk gene. Physical interaction of AML1 with BSAP correlates with functional cooperativity in transfection studies where AML1 and BSAP synergistically activate blk promoter transcription by more than 50-fold. These results demonstrate physical and functional interactions between AML1 and BSAP and suggest that AML1 is an important factor for regulating a critical B cell-specific gene, blk.

Core-binding factor influences the disease specificity of Moloney murine leukemia virus

The core site in the Moloney murine leukemia virus (Moloney MLV) enhancer was previously shown to be an important determinant of the T-cell disease specificity of the virus. Mutation of the core site resulted in a significant shift in disease specificity of the Moloney virus from T-cell leukemia to erythroleukemia. We and others have since determined that a protein that binds the core site, one of the core-binding factors (CBF) is highly expressed in thymus and is essential for hematopoiesis. Here we test the hypothesis that CBF plays a critical role in mediating pathogenesis of Moloney MLV in vivo. We measured the affinity of CBF for most core sites found in MLV enhancers, introduced sites with different affinities for CBF into the Moloney MLV genome, and determined the effects of these sites on viral pathogenesis. We found a correlation between CBF affinity and the latent period of disease onset, in that Moloney MLVs with high-affinity CBF binding sites induced leukemia following a shorter latent period than viruses with lower-affinity sites. The T-cell disease specificity of Moloney MLV also appeared to correlate with the affinity of CBF for its binding site. The data support a role for CBF in determining the pathogenic properties of Moloney MLV.

SPI-B activates transcription via a unique proline, serine, and threonine domain and exhibits DNA binding affinity differences from PU.1

SPI-B is a B lymphocyte-specific Ets transcription factor that shares a high degree of similarity with PU.1/SPI-1. In direct contrast to PU.1(-/-) mice that die in utero and lack monocytes, neutrophils, B cells, and T cells, Spi-B-/- mice are viable and exhibit a severe B cell proliferation defect. Since PU.1 is expressed at wild type levels in Spi-B-/- B cells, the mutant mice provide genetic evidence that SPI-B and PU.1 have at least some non-redundant roles in B lymphocytes. To begin to understand the molecular basis for these defects, we delineated functional domains of SPI-B for comparison to those of PU.1. By using a heterologous co-transfection system, we identified two independent transactivation domains in the N terminus of SPI-B. Interestingly, only one of these domains (amino acids 31-61), a proline/serine/threonine-rich region, unique among Ets proteins, is necessary for transactivation of the immunoglobulin lambda light chain enhancer. This transactivation motif is in marked contrast to PU.1, which contains acidic and glutamine-rich domains. In addition, we describe a functional PU.1 site within the c-FES promoter which SPI-B fails to bind efficiently and transactivate. Finally, we show that SPI-B interacts with the PU.1 cofactors Pip, TBP, c-Jun and with lower affinity to nuclear factor interleukin-6beta and retinoblastoma. Taken together, these data suggest that SPI-B binds DNA with a different affinity for certain sites than PU.1 and harbors different transactivation domains. We conclude that SPI-B may activate unique target genes in B lymphocytes and interact with unique, although currently unidentified, cofactors.

CBFα3 (AML2) Is Induced by TGF-β1 to Bind and Activate the Mouse Germline Ig α Promoter

TGF-β1 directs class switching to IgA by splenic B cells and by the surface IgM+ B cell line, I.29μ, by inducing germline (GL) Ig α transcripts. The promoter segment between −130 and +46, relative to the first initiation site for mouse GL α transcripts, is sufficient for expression and TGF-β1 inducibility of a reporter gene in B cell lines. Within this segment resides a TGF-β1-responsive element (TβRE) that is required for induction of the promoter by TGF-β1 and, when multimerized, is sufficient to transfer TGF-β1 inducibility to another promoter. In this report we show that a TGF-β1-inducible complex binds the TβRE and contains the transcription factor core-binding factor (CBF; also known as acute myeloid leukemia, AML). Although all three CBFα family members activate the GL α promoter, only CBFα3 (AML-2) is induced by TGF-β1 in splenic B and I.29μ cells. The TβRE contains two CBF binding sites. Mutation of both sites reduces but does not eliminate induction of the GL α promoter by TGF-β1 or by overexpression of CBF, possibly due to the presence of an additional CBF site in the promoter. In addition, the TβRE contains two copies of another sequence motif. Mutation of these motifs eliminates TGF-β1 induction of the GL α promoter. Together the data indicate that TGF-β1 induction of the α promoter involves induction of CBFα3, which binds to the TβRE of the promoter along with one or more proteins.

Deletional and mutational analyses of the human CD4 gene promoter: characterization of a minimal tissue-specific promoter

In recent years, considerable interest has arisen in understanding the mechanisms of the CD4 gene transcriptional control which resulted in the identification of a promoter, enhancers and a silencer. While the murine CD4 gene promoter has been well studied, little is known about its human counterpart that we previously identified as a 1.1 kb region. Here, we show that the -170/+20 region represents a minimal tissue-specific promoter with a size compatible with its inclusion in viral vectors for gene therapy. In addition, mutational analyses provided evidence that this fragment contains three regions critical for transcriptional activity: an initiator-like sequence, an Ets consensus site and an ATF consensus site. The latter site is absent in the murine promoter, suggesting that some differences exist between the transcriptional control of the human and murine CD4 genes.

Exploring functional redundancy in the immunoglobulin mu heavy-chain gene enhancer

Immunoglobulin (Ig) mu heavy-chain gene enhancer activity is mediated by multiple DNA binding proteins. Mutations of several protein binding sites in the enhancer do not affect enhancer activity significantly. This feature, termed redundancy, is thought to be due to functional compensation of the mutated sites by other elements within the enhancer. In this study, we identified the elements that make the basic helix-loop-helix (bHLH) protein binding sites, muE2 and muE3, redundant. The major compensatory element is a binding site for interferon regulatory factors (IRFs) and not one of several other bHLH protein binding sites. These studies also provide the first evidence for a role of IRF proteins in Ig heavy-chain gene expression. In addition, we reconstituted the activity of a monomeric mu enhancer in nonlymphoid cells and defined the domains of the ETS gene required for function.

Transcriptional regulation during B cell development

Information is increasingly available concerning the molecular events that occur during primary and antigen-dependent stages of B cell development. In this review the roles of transcription factors and coactivators are discussed with respect to changes in expression patterns of various genes during B cell development. Transcriptional regulation is also discussed in the context of developmentally regulated immunoglobulin gene V(D)J recombination, somatic hypermutation, and isotype switch recombination.