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Polyhistidine tags (His-tags) are commonly employed in protein purification strategies due to the high affinity and specificity for metal-NTA columns, the relative simplicity of such protocols, and the assumption that His-tags do not affect the native activities of proteins. However, there is a growing body of evidence that such tags can modulate protein structure and function. In this study, we demonstrate that a His-tag impacts DNA complex formation by the C-terminal domain of the α-subunit (αCTD) of Helicobacter pylori RNA polymerase in a metal-dependent fashion. The αCTD was purified with a cleavable His-tag, and complex formation between αCTD, the nickel-responsive metalloregulator HpNikR, and DNA was investigated using electrophoretic mobility shift assays. An interaction between His-tagged αCTD (HisαCTD) and the HpNikR-DNA complex was observed; however, this interaction was not observed upon removal of the His-tag. Further analysis revealed that complex formation between HisαCTD and DNA is non-specific and dependent on the type of metal ions present. Overall, the results indicate that a histidine tag is able to modulate DNA-binding activity and suggests that the impact of metal affinity tags should be considered when analyzing the in vitro biomolecular interactions of metalloproteins., (Copyright © 2019 Elsevier Inc. All rights reserved.)

FUR (Ferric uptake regulator) proteins are among the most important families of transcriptional regulators in prokaryotes, often behaving as global regulators. In the cyanobacterium Anabaena PCC 7120, FurB (Zur, Zinc uptake regulator) controls zinc and redox homeostasis through the repression of target genes in a zinc-dependent manner. In vitro , non-specific binding of FurB to DNA elicits protection against oxidative damage and avoids cleavage by deoxyribonuclease I. The present study provides, for the first time, evidence of the influence of redox environment in the interaction of FurB with regulatory zinc and its consequences in FurB-DNA-binding affinity. Calorimetry studies showed that, in addition to one structural Zn(II), FurB is able to bind two additional Zn(II) per monomer and demonstrated the implication of cysteine C93 in regulatory Zn(II) coordination. The interaction of FurB with the second regulatory zinc occurred only under reducing conditions. While non-specific FurB-DNA interaction is Zn(II)-independent, the optimal binding of FurB to target promoters required loading of two regulatory zinc ions. Those results combined with site-directed mutagenesis and gel-shift assays evidenced that the redox state of cysteine C93 conditions the binding of the second regulatory Zn(II) and, in turn, modulates the affinity for a specific DNA target. Furthermore, differential spectroscopy studies showed that cysteine C93 could also be involved in heme coordination by FurB, either as a direct ligand or being located near the binding site. The results indicate that besides controlling zinc homeostasis, FurB could work as a redox-sensing protein probably modifying its zinc and DNA-binding abilities depending upon environmental conditions., (© 2018 The Author(s). Published by Portland Press Limited on behalf of the Biochemical Society.)

Given the similar chemical properties of zinc and cadmium, zinc finger domains have been often proposed as mediators of the toxic and carcinogenic effects exerted by this xenobiotic metal. The effects of zinc replacement by cadmium in different eukaryotic zinc fingers have been reported. In the present work, to evaluate the effects of such substitution in the prokaryotic zinc finger, we report a detailed study of its functional and structural consequences on the Ros DNA binding domain (Ros87). We show that this protein, which bears important structural differences with respect to the eukaryotic domains, appears to structurally tolerate the zinc to cadmium substitution and the presence of cadmium does not affect the DNA binding activity of the protein. Moreover, we show for the first time how zinc to cadmium replacement can also take place in a cellular context. Our findings both complement and extend previous results obtained for different eukaryotic zinc fingers, suggesting that metal substitution in zinc fingers may be of relevance to the toxicity and/or carcinogenicity mechanisms of this metal.

Zur regulators control zinc homeostasis by repressing target genes under zinc-sufficient conditions in a wide variety of bacteria. This paper describes how part of a survey of duplicated genes led to the identification of the open reading frame all2473 as the gene encoding the Zur regulator of the cyanobacterium Anabaena sp. strain PCC 7120. All2473 binds to DNA in a zinc-dependent manner, and its DNA-binding sequence was characterized, which allowed us to determine the relative contribution of particular nucleotides to Zur binding. A zur mutant was found to be impaired in the regulation of zinc homeostasis, showing sensitivity to elevated concentrations of zinc but not other metals. In an effort to characterize the Zur regulon in Anabaena, 23 genes containing upstream putative Zur-binding sequences were identified and found to be regulated by Zur. These genes are organized in six single transcriptional units and six operons, some of them containing multiple Zur-regulated promoters. The identities of genes of the Zur regulon indicate that Anabaena adapts to conditions of zinc deficiency by replacing zinc metalloproteins with paralogues that fulfill the same function but presumably with a lower zinc demand, and with inducing putative metallochaperones and membrane transport systems likely being involved in the scavenging of extracellular zinc, including plasma membrane ABC transport systems and outer membrane TonB-dependent receptors. Among the Zur-regulated genes, the ones showing the highest induction level encode proteins of the outer membrane, suggesting a primary role for components of this cell compartment in the capture of zinc cations from the extracellular medium.

Nature has illustrated through numerous examples that protein dimerization has structural and functional advantages. We previously reported the design and characterization of an engineered "metallohomeodomain" protein (C2) based on a chimera of the EF-hand Ca-binding motif and the helix-turn-helix motif of homeodomains (Lim and Franklin in Protein Sci. 15:2159-2165, 2004). This small metalloprotein binds the hard metal ions Ca(II) and Ln(III) and interacts with DNA with modest sequence preference and affinity, yet exhibits only residual DNA cleavage activity. Here we have achieved substantial improvement in function by constructing a covalent dimer of this C2 module (F2) to create a larger multidomain protein. As assayed via fluorescence spectroscopy, this F2 protein binds Ca(II) more avidly (25-fold) than C2 on a per-domain basis; in gel shift selection experiments, metallated F2 exhibits a specificity toward 5'-TAATTA-3' sequences. Finally, Ca(2)F2 cleaves plasmid DNA and generates a linear product in a Ca(II)-dependent way, unlike the CaC2 monomer. To the best of our knowledge this activation of Ca(II) in the context of an EF-hand binding motif is unique and represents a significant step forward in the design of artificial metallonucleases by utilizing biologically significant metal ions.

Human germinal center-associated lymphoma (HGAL) and LIM domain only-2 (LMO2) are proteins highly expressed in germinal center (GC) B lymphocytes. HGAL and LMO2 are also expressed in GC-derived lymphomas and distinguish biologically distinct subgroups of diffuse large B-cell lymphomas (DLBCL) associated with improved survival. However, little is known about their regulation. PRDM1/Blimp1 is a master regulator of terminal B cell differentiation and may also function as a tumor suppressor in the pathogenesis of DLBCL, where it is frequently inactivated by mutations and deletions. We now demonstrate that both HGAL and LMO2 are directly regulated by the transcription repressor PRDM1. In vivo studies demonstrate that PRDM1 directly binds to the recognition sites within the upstream promoters of both HGAL and LMO2. PRDM1 binding suppresses endogenous protein and mRNA levels of HGAL and LMO2. In addition, promoter analysis reveals that site-specific binding of PRDM1 to the promoters is capable of repressing transcriptional activity. This inhibitory effect of PRDM1 suggests that it has a key role in the loss of HGAL and LMO2 expression upon differentiation of GC B cells to plasma cells and may also contribute to absence of HGAL and LMO2 expression in post-GC lymphoid tumors., (© 2011 The Authors Journal compilation © 2011 FEBS.)

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy largely caused by aberrant activation of the TAL1/SCL, LMO1/2, and NOTCH1 oncogenes. Approximately 30% of T-ALL patients relapse, and evidence is emerging that relapse may result from a failure to eliminate leukemia-initiating cells (LICs). Thymic expression of the Tal1 and Lmo2 oncogenes in mice results in rapid development of T-ALL; and similar to T-ALL patients, more than half the leukemic mice develop spontaneous mutations in Notch1. Using this mouse model, we demonstrate that mouse T-ALLs are immunophenotypically and functionally heterogeneous with approximately 1 of 10,000 leukemic cells capable of initiating disease on transplantation. Our preleukemic studies reveal expansion of Notch-active double-negative thymic progenitors, and we find the leukemic DN3 population enriched in disease potential. To examine the role of Notch1 in LIC function, we measured LIC activity in leukemic mice treated with vehicle or with a γ-secretase inhibitor. In 4 of 5 leukemias examined, Notch inhibition significantly reduced or eliminated LICs and extended survival. Remarkably, in 2 mice, γ-secretase inhibitor treatment reduced LIC frequency below the limits of detection of this assay, and all transplanted mice failed to develop disease. These data support the continued development of Notch1 therapeutics as antileukemia agents.

Background: LMO2 is highly expressed at the most immature stages of lymphopoiesis. In T-lymphocytes, aberrant LMO2 expression beyond those stages leads to T-cell acute lymphoblastic leukemia, while in B cells LMO2 is also expressed in germinal center lymphocytes and diffuse large B-cell lymphomas, where it predicts better clinical outcome. The implication of LMO2 in B-cell acute lymphoblastic leukemia must still be explored., Design and Methods: We measured LMO2 expression by real time RT-PCR in 247 acute lymphoblastic leukemia patient samples with cytogenetic data (144 of them also with survival and immunophenotypical data) and in normal hematopoietic and lymphoid cells., Results: B-cell acute lymphoblastic leukemia cases expressed variable levels of LMO2 depending on immunophenotypical and cytogenetic features. Thus, the most immature subtype, pro-B cells, displayed three-fold higher LMO2 expression than pre-B cells, common-CD10+ or mature subtypes. Additionally, cases with TEL-AML1 or MLL rearrangements exhibited two-fold higher LMO2 expression compared to cases with BCR-ABL rearrangements or hyperdyploid karyotype. Clinically, high LMO2 expression correlated with better overall survival in adult patients (5-year survival rate 64.8% (42.5%-87.1%) vs. 25.8% (10.9%-40.7%), P= 0.001) and constituted a favorable independent prognostic factor in B-ALL with normal karyotype: 5-year survival rate 80.3% (66.4%-94.2%) vs. 63.0% (46.1%-79.9%) (P= 0.043)., Conclusions: Our data indicate that LMO2 expression depends on the molecular features and the differentiation stage of B-cell acute lymphoblastic leukemia cells. Furthermore, assessment of LMO2 expression in adult patients with a normal karyotype, a group which lacks molecular prognostic factors, could be of clinical relevance.

LMO2 is a target of chromosomal translocations in T-cell tumors and was activated by retroviral vector insertions in T-cell tumors from X-SCID patients in gene therapy trials. To better understand the cooperating genetic events in LMO2-associated T-cell acute lymphoblastic leukemia (T-ALL), we investigated the roles of Arf tumor suppressor loss and Notch activation in murine models of transplantation. Lmo2 overexpression enhanced the expansion of primitive DN2 thymocytes, eventually facilitating the stochastic induction of clonal CD4(+)/CD8(+) malignancies. Inactivation of the Arf tumor suppressor further increased the self-renewal capacity of the primitive, preleukemic thymocyte pool and accelerated the development of aggressive, Lmo2-induced T-cell lympholeukemias. Notch mutations were frequently detected in these Lmo2-induced tumors. The Arf promoter was not directly engaged by Lmo2 or mutant Notch, and use of a mouse model in which activation of a mutant Notch allele depends on previous engagement of the Arf promoter revealed that Notch activation could occur as a subsequent event in T-cell tumorigenesis. Therefore, Lmo2 cooperates with Arf loss to enhance self-renewal in primitive thymocytes. Notch mutation and Arf inactivation appear to independently cooperate in no requisite order with Lmo2 overexpression in inducing T-ALL, and all 3 events remained insufficient to guarantee immediate tumor development.

We studied the efficacy of 2 germinal center B-cell markers, HGAL and LMO2, in the separation of lymphomas derived from small B cells, particularly follicular lymphoma (FL) and marginal zone lymphoma occurring in nodal, extranodal, splenic, and bone marrow sites using immunohistochemical analysis for CD10, BCL6, BCL2, HGAL, and LMO2. Our results showed that HGAL and LMO2 are sensitive and specific markers for detecting FL in nodal and extranodal sites. In contrast, all markers were down-regulated in FL infiltrates in the bone marrow. CD10 and HGAL were expressed in a subset of FLs in the bone marrow and were highly correlated with each other and with CD21, a marker of follicular dendritic cells. We conclude that HGAL and LMO2 should be considered in immunohistochemical panels used for the routine workup of lymphomas derived from small B cells. In the bone marrow, staining for HGAL or CD10 can be helpful in making a diagnosis of FL, although they are absent in a subset of cases.

The most common translocation in childhood T-cell acute lymphoblastic leukemia (T-ALL) involves the LMO2 locus, resulting in ectopic expression of the LMO2 gene in human thymocytes. The LMO2 gene was also activated in patients with X-linked Severe Combined Immune Deficiency treated with gene therapy because of retroviral insertion in the LMO2 locus. The LMO2 insertions predisposed these children to T-ALL, yet how LMO2 contributes to T cell transformation remains unclear. The LIM (Lin 11, Isl-1, Mec-3) domain containing LMO2 protein regulates erythropoiesis as part of a large transcriptional complex consisting of LMO2, TAL1, E47, GATA1 and LDB1 that recognizes bipartite E-box-GATA1 sites on target genes. Similarly, a TAL1/E47/LMO2/LDB1 complex is observed in human T-ALL and Tal1 and Lmo2 expression in mice results in disease acceleration. To address the mechanism(s) of Tal1/Lmo2 synergy in leukemia, we generated Lmo2 transgenic mice and mated them with mice that express wild-type Tal1 or a DNA-binding mutant of TAL1. Tal1/Lmo2 and MutTAL1/Lmo2 bitransgenic mice exhibit perturbations in thymocyte development due to reduced E47/HEB transcriptional activity and develop leukemia with identical kinetics. These data demonstrate that the DNA-binding activity of Tal1 is not required to cooperate with Lmo2 to cause leukemia in mice and suggest that Lmo2 may cooperate with Tal1 to interfere with E47/HEB function(s)., (© 2011 Macmillan Publishers Limited)

The LIM only protein 2 (LMO2) is a key regulator of hematopoietic stem cell development whose ectopic expression in T cells leads to the onset of acute lymphoblastic leukemia. Through its LIM domains, LMO2 is thought to function as the scaffold for a DNA-binding transcription regulator complex, including the basic helix-loop-helix proteins SCL/TAL1 and E47, the zinc finger protein GATA-1, and LIM-domain interacting protein LDB1. To understand the role of LMO2 in the formation of this complex and ultimately to dissect its function in normal and aberrant hematopoiesis, we solved the crystal structure of LMO2 in complex with the LID domain of LDB1 at 2.4 Å resolution. We observe a largely unstructured LMO2 kept in register by the LID binding both LIM domains. Comparison of independently determined crystal structures of LMO2 reveals large movements around a conserved hinge between the LIM domains. We demonstrate that such conformational flexibility is necessary for binding of LMO2 to its partner protein SCL/TAL1 in vitro and for the function of this complex in vivo. These results, together with molecular docking and analysis of evolutionarily conserved residues, yield the first structural model of the DNA-binding complex containing LMO2, LDB1, SCL/TAL1, and GATA-1.

The oncogenic fusion protein E2A-HLF is a chimeric transcription factor that arises from the t(17;19) translocation in childhood B-cell acute lymphoblastic leukemias (B-precursor ALL) and is associated with very poor outcome. We show that retroviral-mediated expression of E2A-HLF alone is sufficient to immortalize primary lymphoid progenitors. We identify Lmo2 and Bcl-2 as direct target genes downstream of E2A-HLF. We use real-time PCR analysis to show that LMO2 and BCL-2 expression is preferentially upregulated both in biopsy material from t(17;19) B-precursor ALL patients and lymphoid cell lines derived from t(17;19) leukemias. Co-expression of Lmo2 and Bcl-2 was sufficient to immortalize lymphoid progenitor cells resulting in a similar phenotype to that induced by E2A-HLF alone. Both shRNA-mediated knockdown of Lmo2 expression and pharmacological inhibition of BCL-2 function in E2A-HLF immortalized cells severely compromised their viability. These data suggest that both Lmo2 and Bcl-2 are required for the action of E2A-HLF in leukemogenesis.

T-cell acute lymphoblastic leukemia (T-ALL) is commonly caused by the overexpression of oncogenic transcription factors in developing T cells. In a mouse model of one such oncogene, LMO2, the cellular effect is to induce self-renewal of committed T cells in the thymus, which persist long-term while acquiring additional mutations and eventually giving rise to leukemia. These precancerous stem cells (pre-CSC) are intrinsically resistant to radiotherapy, implying that they may be refractory to conventional cancer therapies. However, they depend on an aberrantly expressed stem cell-like self-renewal program for their maintenance, in addition to a specialized thymic microenvironmental niche. Here, we discuss potential approaches for targeting pre-CSCs in T-ALL by using therapies directed at oncogenic transcription factors themselves, downstream self-renewal pathways, and the supportive cell niche., (©2010 AACR.)

The lmo2 gene is a specific oncogene in T-cell leukemia, for its ectopic expression causes both increased pro-T-cell proliferation and differentiation arrest, leading to the onset of leukemia. Notably, DeltaEF1 (also known as ZEB1), a member of zinc finger-homeodomain family transcription factor, also exhibits crucial function in promoting T-cell differentiation. In this study, we found that DeltaEF1 was positively regulated by T-lineage-specific transcriptional regulator GATA3, while ectopically expressed LMO2 targeted to DeltaEF1 promoter by interaction with GATA3 and inhibited DeltaEF1 expression in transcriptional level. Moreover, LMO2 interacted with the N-terminal zinc finger domain of DeltaEF1 protein and inhibited its positive transcriptional regulatory function by this interaction. Taken together, our findings revealed that ectopically expressed LMO2 impaired the function of DeltaEF1 in both transcriptional and protein levels and identified DeltaEF1 as a novel pathogenic target of LMO2 in T-cell leukemia., (© 2010 John Wiley & Sons A/S.)

The c-myb transcription factor is highly expressed in immature hematopoietic cells and down-regulated during differentiation. To define its role during the hematopoietic lineage commitment, we silenced c-myb in human CD34(+) hematopoietic stem/progenitor cells. Noteworthy, c-myb silencing increased the commitment capacity toward the macrophage and megakaryocyte lineages, whereas erythroid differentiation was impaired, as demonstrated by clonogenic assay, morphologic and immunophenotypic data. Gene expression profiling and computational analysis of promoter regions of genes modulated in c-myb-silenced CD34(+) cells identified the transcription factors Kruppel-Like Factor 1 (KLF1) and LIM Domain Only 2 (LMO2) as putative targets, which can account for c-myb knockdown effects. Indeed, chromatin immunoprecipitation and luciferase reporter assay demonstrated that c-myb binds to KLF1 and LMO2 promoters and transactivates their expression. Consistently, the retroviral vector-mediated overexpression of either KLF1 or LMO2 partially rescued the defect in erythropoiesis caused by c-myb silencing, whereas only KLF1 was also able to repress the megakaryocyte differentiation enhanced in Myb-silenced CD34(+) cells. Our data collectively demonstrate that c-myb plays a pivotal role in human primary hematopoietic stem/progenitor cells lineage commitment, by enhancing erythropoiesis at the expense of megakaryocyte diffentiation. Indeed, we identified KLF1 and LMO2 transactivation as the molecular mechanism underlying Myb-driven erythroid versus megakaryocyte cell fate decision.

The T-cell oncogene Lim-only 2 (LMO2) critically influences both normal and malignant haematopoiesis. LMO2 is not normally expressed in T cells, yet ectopic expression is seen in the majority of T-acute lymphoblastic leukaemia (T-ALL) patients with specific translocations involving LMO2 in only a subset of these patients. Ectopic lmo2 expression in thymocytes of transgenic mice causes T-ALL, and retroviral vector integration into the LMO2 locus was implicated in the development of clonal T-cell disease in patients undergoing gene therapy. Using array-based chromatin immunoprecipitation, we now demonstrate that in contrast to B-acute lymphoblastic leukaemia, human T-ALL samples largely use promoter elements with little influence from distal enhancers. Active LMO2 promoter elements in T-ALL included a previously unrecognized third promoter, which we demonstrate to be active in cell lines, primary T-ALL patients and transgenic mice. The ETS factors ERG and FLI1 previously implicated in lmo2-dependent mouse models of T-ALL bind to the novel LMO2 promoter in human T-ALL samples, while in return LMO2 binds to blood stem/progenitor enhancers in the FLI1 and ERG gene loci. Moreover, LMO2, ERG and FLI1 all regulate the +1 enhancer of HHEX/PRH, which was recently implicated as a key mediator of early progenitor expansion in LMO2-driven T-ALL. Our data therefore suggest that a self-sustaining triad of LMO2/ERG/FLI1 stabilizes the expression of important mediators of the leukaemic phenotype such as HHEX/PRH.

miR-142 and miR-223 have been identified as hematopoietic specific microRNAs. miR-223 has crucial functions in myeloid lineage development. However, the function of miR-142 remains unclear. In this study, we found that both miR-142 and miR-223 attenuated the proliferation of hematopoietic cells, and that miR-223 up-regulated miR-142 expression through the LMO2-L/-S isoforms and CEBP-β. miR-223 negatively regulated both LMO2-L/-S isoforms and CEBP-β post-transcriptionally, while CEBP-β positively regulated the LMO2-L/-S isoforms and both of the LMO2-L/-S isoforms negatively regulated miR-142. These results reveal a novel miR-223--CEBP-β--LMO2--miR-142 regulatory pathway, which has pivotal functions in hematopoiesis.

LMO2, a critical transcription regulator of hematopoiesis, is involved in human T-cell leukemia. The binding site of proline and acidic amino acid-rich protein (PAR) transcription factors in the promoter of the LMO2 gene plays a central role in hematopoietic-specific expression. E2A-HLF fusion derived from t(17;19) in B-precursor acute lymphoblastic leukemia (ALL) has the transactivation domain of E2A and the basic region/leucine zipper domain of HLF, which is a PAR transcription factor, raising the possibility that E2A-HLF aberrantly induces LMO2 expression. We here demonstrate that cell lines and a primary sample of t(17;19)-ALL expressed LMO2 at significantly higher levels than other B-precursor ALLs did. Transfection of E2A-HLF into a non-t(17;19) B-precursor ALL cell line induced LMO2 gene expression that was dependent on the DNA-binding and transactivation activities of E2A-HLF. The PAR site in the LMO2 gene promoter was critical for E2A-HLF-induced LMO2 expression. Gene silencing of LMO2 in a t(17;19)-ALL cell line by short hairpin RNA induced apoptotic cell death. These observations indicated that E2A-HLF promotes cell survival of t(17;19)-ALL cells by aberrantly up-regulating LMO2 expression. LMO2 could be a target for a new therapeutic modality for extremely chemo-resistant t(17;19)-ALL.

LIM domain only-2 (LMO2) is an important regulator of hematopoietic stem cell development. LMO2 is also expressed in blast cells of different types of acute leukemia. Here, we analyzed the LMO2 protein expression in acute myeloid leukemia (AML) and B-cell acute lymphoblastic leukemia (B-ALL) and examined whether the LMO2 protein expression can predict outcomes of patients with acute leukemia. Patients with acute B-ALL (22 cases) and AML (57 cases) were examined using immunohistochemistry for LMO2 on paraffin-embedded bone marrow biopsies. We report that LMO2 protein is expressed in a significant proportion of B-ALL and AML and the staining of LMO2 protein does not predict survival in acute leukemia.

Background: The human lmo2 gene plays important roles in hematopoiesis and is associated with acute T lymphocyte leukemia. The gene encodes two protein isoforms, a longer form LMO2-L and a shorter form LMO2-S. Both isoforms function as bridge molecules to assemble their partners together to regulate their target genes. A typical LMO2 binding site consists of two elements, a GATA site and an E-box, with an interval of 9 approximately 12 bp., Methods: In this study, the combination of MBP pulldown assay and mammalian two hybrid assay were used to confirm the homo-binding character of LMO2-L/-S isoforms. Luciferase reporter assay and Real-time PCR assay were used to detect expression levels and relative promoter activities of LMO2-L/-S isoforms. Co-transfection and Luciferase reporter assay were used to reveal the detailed regulatory pattern of LMO2-L/-S isoforms on their targets., Results: Herein we report the homo-interaction character of LMO2-L and LMO2-S and their major difference in manner of regulating their target genes. Our results showed that LMO2-L and LMO2-S could only bind to themselves but not each other. It was also demonstrated that LMO2-L could either positively or negatively regulate the transcription of its different target genes, depending on the arrangement and strand location of the two elements GATA site and E-box, LMO2-S, however, performed constitutively transcriptional inhibiting function on all target genes., Conclusion: These results suggest that LMO2 isoforms have independent functions while there is no interaction between each other and they could play synergetic or antagonistic roles precisely in regulating their different genes involved in normal and aberrant hematopoiesis.

The gene encoding LIM-only 2 (LMO2), an oncogenic transcription factor, is frequently activated in T cell acute lymphoblastic leukemia (T-ALL), but how LMO2 transforms primary hematopoietic cells to induce T-ALL remains an open question. McCormack et al. now show that, in mice, Lmo2 confers self-renewal potential on normally nonrenewing thymocyte progenitor cells, and this property is maintained over four serial transplantations when the cells are transplanted into irradiated mice that lack thymocytes. These leukemia-initiating cells are resistant to irradiation, indicating the need to develop new therapeutic drugs that specifically target the oncogene itself.

The LMO2 oncogene causes a subset of human T cell acute lymphoblastic leukemias (T-ALL), including four cases that arose as adverse events in gene therapy trials. To investigate the cellular origin of LMO2-induced leukemia, we used cell fate mapping to study mice in which the Lmo2 gene was constitutively expressed in the thymus. Lmo2 induced self-renewal of committed T cells in the mice more than 8 months before the development of overt T-ALL. These self-renewing cells retained the capacity for T cell differentiation but expressed several genes typical of hematopoietic stem cells (HSCs), suggesting that Lmo2 might reactivate an HSC-specific transcriptional program. Forced expression of one such gene, Hhex, was sufficient to initiate self-renewal of thymocytes in vivo. Thus, Lmo2 promotes the self-renewal of preleukemic thymocytes, providing a mechanism by which committed T cells can then accumulate additional genetic mutations required for leukemic transformation.

Therapeutic retroviral vector integration near the oncogene LMO2 is thought to be a cause of leukemia in X-SCID gene therapy trials. However, no published studies have evaluated the frequency of vector integrations near exon 1 of the LMO2 locus. We identified a high incidence region (HIR) of vector integration using PCR techniques in the upstream region close to the LMO2 transcription start site in the TPA-Mat T cell line. The integration frequency of the HIR was one per 4.46 x 10(4) cells. This HIR was also found in Jurkat T cells but was absent from HeLa cells. Furthermore, using human cord blood-derived CD34+ cells we identified a HIR in a similar region as the TPA-Mat T cell line. One of the X-linked severe combined immunodeficiency (X-SCID) patients that developed leukemia after gene therapy had a vector integration site in this HIR. Therefore, the descriptions of the location and the integration frequency of the HIR presented here may help us to better understand vector-induced leukemogenesis.

Purpose: LIM domain only 2 (LMO2) has been identified as a novel oncogene associated with carcinogenesis and better prognosis in several malignant tumors. We investigate the involvement of LMO2 in pancreatic cancer., Experimental Design: We evaluated LMO2 expression in cultured cells, bulk tissues, and microdissected cells from pancreatic cancers by quantitative reverse transcription-polymerase chain reaction and immunohistochemistry., Results: Of 164 pancreatic cancers, 98 (60%) were positive for LMO2 expression. LMO2 was more frequently detected in high-grade pancreatic intraepithelial neoplasia (PanIN) lesions (PanIN-2 and -3) than in low-grade PanIN lesions (PanIN-1A and -1B; P < .001) and was not detected in normal pancreatic ductal epithelium. The LMO2 messenger RNA levels were significantly higher in invasive ductal carcinoma cells than in normal pancreatic cells as evaluated by quantitative reverse transcription-polymerase chain reaction analyses of microdissected cells (P = .036). We also found higher incidence of LMO2 expression in histologic grade G1/G2 cancers than in grade G3 cancers (P < .001). The median survival time of LMO2-positive patients was significantly longer than that of LMO2-negative patients (P < .001), and multivariate analyses revealed that high LMO2 expression was an independent predictor of longer survival (risk ratio, 0.432, P < .001). Even among patients with a positive operative margin, LMO2-positive patients had a significant survival benefit compared with LMO2-negative patients. We further performed a large cohort study (n = 113) to examine the LMO2 messenger RNA levels in formalin-fixed paraffin-embedded samples and found similar results., Conclusions: LMO2 is a promising marker for predicting a better prognosis in pancreatic cancer.

The Lmo2 gene encodes a transcriptional cofactor critical for the development of hematopoietic stem cells. Ectopic LMO2 expression causes leukemia in T-cell acute lymphoblastic leukemia (T-ALL) patients and severe combined immunodeficiency patients undergoing retroviral gene therapy. Tightly controlled Lmo2 expression is therefore essential, yet no comprehensive analysis of Lmo2 regulation has been published so far. By comparative genomics, we identified 17 highly conserved noncoding elements, 9 of which revealed specific acetylation marks in chromatin-immunoprecipitation and microarray (ChIP-chip) assays performed across 250 kb of the Lmo2 locus in 11 cell types covering different stages of hematopoietic differentiation. All candidate regulatory regions were tested in transgenic mice. An extended LMO2 proximal promoter fragment displayed strong endothelial activity, while the distal promoter showed weak forebrain activity. Eight of the 15 distal candidate elements functioned as enhancers, which together recapitulated the full expression pattern of Lmo2, directing expression to endothelium, hematopoietic cells, tail, and forebrain. Interestingly, distinct combinations of specific distal regulatory elements were required to extend endothelial activity of the LMO2 promoter to yolk sac or fetal liver hematopoietic cells. Finally, Sfpi1/Pu.1, Fli1, Gata2, Tal1/Scl, and Lmo2 were shown to bind to and transactivate Lmo2 hematopoietic enhancers, thus identifying key upstream regulators and positioning Lmo2 within hematopoietic regulatory networks.

LMO2 is a transcription regulator involved in human T-cell leukemia, including some occurring in X-SCID gene therapy trials, and in B-cell lymphomas and prostate cancer. LMO2 functions in transcription complexes via protein-protein interactions involving two LIM domains and causes a preleukemic T-cell development blockade followed by clonal tumors. Therefore, LMO2 is necessary but not sufficient for overt neoplasias, which must undergo additional mutations before frank malignancy. An open question is the importance of LMO2 in tumor development as opposed to sustaining cancer. We have addressed this using a peptide aptamer that binds to the second LIM domain of the LMO2 protein and disrupts its function. This specificity is mediated by a conserved Cys-Cys motif, which is similar to the zinc-binding LIM domains. The peptide inhibits Lmo2 function in a mouse T-cell tumor transplantation assay by preventing Lmo2-dependent T-cell neoplasia. Lmo2 is, therefore, required for sustained T-cell tumor growth, in addition to its preleukemic effect. Interference with LMO2 complexes is a strategy for controlling LMO2-mediated cancers, and the finger structure of LMO2 is an explicit focus for drug development.

Five X-linked severe combined immunodeficiency patients (SCID-X1) successfully treated with autologous bone marrow stem cells infected ex vivo with an IL2RG-containing retrovirus subsequently developed T-cell leukemia and four contained insertional mutations at LMO2. Genetic evidence also suggests a role for IL2RG in tumor formation, although this remains controversial. Here, we show that the genes and signaling pathways deregulated in murine leukemias with retroviral insertions at Lmo2 are similar to those deregulated in human leukemias with high LMO2 expression and are highly predictive of the leukemias induced in SCID-X1 patients. We also provide additional evidence supporting the notion that IL2RG and LMO2 cooperate in leukemia induction but are not sufficient and require additional cooperating mutations. The highly concordant nature of the genetic events giving rise to mouse and human leukemias with mutations at Lmo2 are an encouraging sign to those wanting to use mice to model human cancer and may help in designing safer methods for retroviral gene therapy., Competing Interests: The authors have declared that no competing interests exist.

Objective: To explore the expression of the transcription factors LMO2 and LYL1 and the interaction between these 2 factors in myeloid leukemia cells and to analyze the significance thereof in leukemogenesis., Methods: Samples of peripheral blood and bone marrow were collected form 51 AML patties, and 5 normal bone marrow donors to isolate mononuclear cells (MNCs) with high percentage of CD34(+) cells. Western blotting (WB) was used to detect the protein expression of LMO2 and LYL1 in the cells. Human myeloid leukemia cells of the line K562 were cultured and transfected with pcDNA3-LMO2, plasmid containing LMO2, pcDNA3-LYL1, plasmid containing LYL1, or pcDNA-GFP, blank plasmid containing green fluorescent protein. RT-PCR was used to detect the mRNA expression of LMO2 and LYL1. Co-immunoprecipitation (co-IP) and WB were used to detect the binding protein of LMO2 and LYL1., Results: The MNCs of 51.1% of the patients with acute myeloblastic leukemia (AML) without remission expressed higher levels of LMO2, the MNCs of 62.2% of the AML patients expressed higher levels of LYL1, and the MNCs of 31.1% of those expressed both. The K562 cells transfected with pcDNA3-LMO2 showed higher mRNA and protein expression levels of both LMO2 and LYL1, and the K562 cells transfected with pcDNA3-LYL1 showed higher mRNA and protein expression levels of both LYL1 and LMO2 too, as indicated by RT-PCR and WB, which suggested that the expression of LMO2 and the expression of LYL1 stimulated each other in the myeloid leukemia cells. Co-IP assay detected the presence of LMO2-LYL1 complex in those cells., Conclusion: The abnormal expression and protein interaction of LMO2 and LYL1 may play a role in the abnormal proliferation and differentiation of myeloid hematopoietic cells.

Background: MicroRNAs are small non-coding RNAs that regulate gene expression through mRNA degradation or translational inhibition. MicroRNAs are emerging as key regulators of normal hematopoiesis and hematologic malignancies. Several miRNAs are differentially expressed during hematopoiesis and their specific expression regulates key functional proteins involved in hematopoietic lineage differentiation. This study focused on the functional role of microRNA-223 (miR-223) on erythroid differentiation., Design and Methods: Purified cord blood CD34+ hematopoietic progenitor cells were grown in strictly controlled conditions in the presence of saturating dosage of erythropoietin to selectively induce erythroid differentiation. The effects of enforced expression of miR-223 in unilin-eage erythroid cultures were evaluated in liquid phase culture experiments and clonogenic studies., Results: In unilineage erythroid culture of cord blood CD34+ hematopoietic progenitor cells miR-223 is down-regulated, whereas LMO2, an essential protein for erythroid differentiation, is up-regulated. Functional studies showed that enforced expression of miR-223 reduces the mRNA and protein levels of LMO2, by binding to LMO2 3' UTR, and impairs differentiation of erythroid cells. Accordingly, knockdown of LMO2 by short interfering RNA mimics the action of miR-223. Furthermore, hematopoietic progenitor cells transduced with miR-223 showed a significant reduction of their erythroid clonogenic capacity, suggesting that downmodulation of this miRNA is required for erythroid progenitor recruitment and commitment., Conclusions: These results show that the decline of miR-223 is an important event for erythroid differentiation that leads to the expansion of erythroblast cells at least partially mediated by unblocking LMO2 protein expression.

The LIM-domain protein LMO2 is a T-cell oncogenic protein first recognized by gene activation through chromosomal translocations, but it is also responsible for leukaemias arising as secondary, adverse effects in an X-SCID gene therapy trial. There are no specific reagents currently available to analyse the LMO2 multiprotein complex or to combat LMO2-dependent leukaemias. Accordingly, we have isolated an anti-LMO2 single chain Fv antibody fragment to determine if intracellular interference with LMO2-protein complexes can avert LMO2-dependent functions in normal and cancer settings. The anti-LMO2 single chain Fv, obtained using Intracellular Antibody Capture (IAC) technology, is specific for LMO2 among the LIM-only protein family and binds LMO2 through the third and fourth LIM fingers. Using vector-mediated expression of anti-LMO2 scFv, we show inhibition of Lmo2-dependent erythropoiesis but not endothelial development. We also demonstrate inhibition of Lmo2-dependent leukaemia in a mouse T-cell tumourigenesis transplantation assay with retroviral-mediated expression of anti-LMO2 scFv. Our studies establish that interference with the LMO2 multiprotein complex inhibits both normal and tumourigenic roles. The antibody fragment is a tool for dissecting LMO2 function in haematopoiesis and leukaemia and is a lead for development of therapeutics against LMO2-dependent T-ALL.

Cre/loxP system is a powerful tool to manipulate the genome. Transgenic animals expressing Cre recombinase in specific tissues or cells have been widely used for conditional gene targeting, lineage tracing, and other genetic analyses. In zebrafish, the transgenic line with stable expression of Cre in specific tissues and cell subtypes has not been generated and its functional activity remains to be defined. Here we report the establishment of a stable transgenic fish Tg(zlmo2:Cre), which specifically expresses Cre in the primitive hematopoietic progenitors and vascular endothelial cells, under the control of lmo2 promoter. Our result shows that the Cre expression pattern recapitulates the endogenous lmo2 expression pattern during embryogenesis. Crossing of the Tg(zlmo2:Cre) line with another established transgenic reporter line Tg(zlmo2:loxP-DsRed-loxP-EGFP), induces a robust recombination activity in hematopoietic progenitors and vascular endothelial cells. Thus, the Tg(zlmo2:Cre) transgenic line provides an invaluable tool to dissect genetic pathways in hematopoietic development and diseases., (Copyright (c) 2008 Wiley-Liss, Inc.)

Objective: To identify the interaction partners of a new splicing product of LMO2 gene (LMO2-C), and study its function in K562 cells., Methods: Maltose binding protein (MBP) pull down and mammalian two-hybrid assay (MTHA) were used to identify the interaction partners of LMO2-C in K562 cells. Semiquantitative RT-PCR was used to detect the expression of hematopoietic specific gene glycoprotein (GPA) in K562 cells., Results: MBP-LMO2-C fusion protein was expressed and purified in soluble form successfully. Endogenous GATA1 and LDB1 proteins were confirmed to bind to LMO2-C by MBP pull down analysis. The MTHA also showed that LMO2-C had comparable binding affinities to LDB1 with LMO2-L, and over expression of LMO2-C prevented LMO2-L from binding to LDB1, the inhibition rate being (81.13 +/- 0.68)%. RT-PCR results showed that the expression level of GPA was reduced [(51.00 +/- 1.58)%] in K562 cells while LMO2-C overexpressed., Conclusion: LMO2-C can bind endogenous GATA1 and LDB1 protein in K562 cells and down regulates the expression of GPA.

Pathogenic activation of the LMO2 proto-oncogene by an oncoretroviral vector insertion in a clinical trial for X-linked severe combined immunodeficiency (X-SCID) has prompted safety concerns. We used an adeno-associated virus vector to achieve targeted insertion of a gamma-retroviral long terminal repeat (LTR) driving a GFP expression cassette with flanking loxP sites in a human T-cell line at the precise location of vector integration in one of the patients with X-SCID. The LTR-GFP cassette was inserted into the first intron of the LMO2 gene, resulting in strong activation of LMO2. Cre-mediated cassette exchange was used to replace the original LTR-GFP cassette with one flanked by insulator elements leading to a several fold reduction in LMO2 expression. The LTR-GFP cassette was also replaced with a globin gene regulatory cassette that failed to activate the LMO2 gene in lymphoid cells. A gamma-retroviral vector with 2 intact LTRs resulted in activation of the LMO2 gene when inserted into the first intron, but a self-inactivating lentiviral vector with an internal cellular promoter and flanking insulator elements did not activate the LMO2 gene. Thus, this system is useful for comparing the safety profiles of vector cassettes with various regulatory elements for their potential for proto-oncogene activation.

To program pluripotent cells into blood, a knowledge of the locations of precursors during their journey through the embryo and the signals they experience would be informative. The anterior (a) and posterior (p) ventral blood islands (VBIs) in Xenopus are derived from opposite sides of the pregastrula embryo. The aVBI goes through a "hemangioblast" state, characterized by coexpression of blood and endothelial genes at neurula stages, whereas the pVBI expresses these genes in a nonoverlapping fashion several hours later, after commitment to either a blood or an endothelial fate. We describe a novel role for fibroblast growth factor (FGF) in controlling the timing of Scl, Lmo2, and Runx1 expression in the 2 VBI compartments. Blocking FGF signaling during gastrulation expands expression at neurula stages into posterior regions. We show, by lineage labeling, explant analysis, and targeted blocking of FGF signaling, that this is due to the pVBI prematurely expressing these genes with the timing of the aVBI. In contrast, overexpression of FGF in aVBI precursors eliminates the anterior hemangioblast program. Using this information, we have recapitulated the anterior hemangioblast program in pluripotent cells in vitro by inhibiting FGF signaling in anterior mesoderm induced by activin and exposed to bone morphogenetic protein (BMP) signaling.

Long-range interactions between distant regulatory elements, such as enhancers, and their target genes underlie the specificity of gene expression in many developmentally regulated gene families. NLI/Ldb1, a widely expressed nuclear factor, is a potential mediator of long-range interactions. Here, we show that NLI/Ldb1 and erythroid-binding partners GATA-1/SCL/LMO2 bind in vivo to the beta-globin locus control region (LCR). The C-terminal LIM interaction domain of NLI is required for formation of the complex on chromatin. Loss of the LIM domain converts NLI into a dominant-negative inhibitor of globin gene expression, and knockdown of NLI by using shRNA results in failure to activate beta-globin expression. Kinetic studies reveal that the NLI/GATA-1/SCL/LMO2 complex is detected at the beta-globin promoter coincident with RNA Pol II recruitment, beta-globin transcription, and chromatin loop formation during erythroid differentiation, providing evidence that NLI facilitates long-range gene activation.

The t(11;14)(p13;q11) is presumed to arise from an erroneous T-cell receptor delta TCRD V(D)J recombination and to result in LMO2 activation. However, the mechanisms underlying this translocation and the resulting LMO2 activation are poorly defined. We performed combined in vivo, ex vivo, and in silico analyses on 9 new t(11;14)(p13;q11)-positive T-cell acute lymphoblastic leukemia (T-ALL) as well as normal thymocytes. Our data support the involvement of 2 distinct t(11;14)(p13;q11) V(D)J-related translocation mechanisms. We provide compelling evidence that removal of a negative regulatory element from the LMO2 locus, rather than juxtaposition to the TCRD enhancer, is the main determinant for LMO2 activation in the majority of t(11;14)(p13;q11) translocations. Furthermore, the position of the LMO2 breakpoints in T-ALL in the light of the occurrence of TCRD-LMO2 translocations in normal thymocytes points to a critical role for the exact breakpoint location in determining LMO2 activation levels and the consequent pressure for T-ALL development.

Ectopic expression of LIM-only protein 2 (LMO2) in T-cells, as a result of chromosomal translocations or retroviral insertion, plays an important role in the onset of T-cell leukemias. Two transcripts of LMO2 gene (LMO2-a and LMO2-b) have been reported to encode a same 158-amino-acid protein. We have previously reported a novel transcript of human LMO2 gene (LMO2-c) encoding a 151-amino-acid protein, and defined its promoter region. In the present study, we investigated the regulation of the LMO2-c expression and the functions of LMO2-c. We found that LMO2-c expression is regulated by the cooperation of two essential hematopoietic transcription factors GATA-1 and PU.1 in various hematopoietic cell lines, suggesting an important functional role for LMO2-c in the hematopoietic system. More importantly, we demonstrated that LMO2-c acts as an antagonist of LMO2-a/b binding to its partners, therefore blocking the transactivation of LMO2-a/b on its target genes. These findings provide novel evidence to the functions of LMO2 gene in the hematopoietic system and leukemia.

The occurrence of leukemia in a gene therapy trial for SCID-X1 has highlighted insertional mutagenesis as an adverse effect. Although retroviral integration near the T-cell acute lymphoblastic leukemia (T-ALL) oncogene LIM-only protein 2 (LMO2) appears to be a common event, it is unclear why LMO2 was preferentially targeted. We show that of classical T-ALL oncogenes, LMO2 is most highly transcribed in CD34+ progenitor cells. Upon stimulation with growth factors typically used in gene therapy protocols transcription of LMO2, LYL1, TAL1 and TAN1 is most prominent. Therefore, these oncogenes may be susceptible to viral integration. The interleukin-2 receptor gamma chain (IL2Rgamma), which is mutated in SCID-X1, has been proposed as a cooperating oncogene to LMO2. However, we found that overexpressing IL2Rgamma had no effect on T-cell development. In contrast, retroviral overexpression of LMO2 in CD34+ cells caused severe abnormalities in T-cell development, but B-cell and myeloid development remained unaffected. Our data help explain why LMO2 was preferentially targeted over many of the other known T-ALL oncogenes. Furthermore, during T-cell development retrovirus-mediated expression of IL2Rgamma may not be directly oncogenic. Instead, restoration of normal IL7-receptor signaling may allow progression of T-cell development to stages where ectopic LMO2 expression causes aberrant thymocyte growth.

The transcription factors Scl and Lmo2 are crucial for development of all blood. An important early requirement for Scl in endothelial development has also been revealed recently in zebrafish embryos, supporting previous findings in scl(-/-) embryoid bodies. Scl depletion culminates most notably in failure of dorsal aorta formation, potentially revealing a role in the formation of hemogenic endothelium. We now present evidence that the requirements for Lmo2 in zebrafish embryos are essentially the same as for Scl. The expression of important hematopoietic regulators is lost, reduced, or delayed, panendothelial gene expression is down-regulated, and aorta-specific marker expression is lost. The close similarity of the phenotypes for Scl and Lmo2 suggest that they perform these early functions in hemangioblast development within a multiprotein complex, as shown for erythropoiesis. Consistent with this, we find that scl morphants cannot be rescued by a non-Lmo2-binding form of Scl but can be rescued by non-DNA-binding forms, suggesting tethering to target genes through DNA-binding partners linked via Lmo2. Interestingly, unlike other hematopoietic regulators, the Scl/Lmo2 complex does not appear to autoregulate, as neither gene's expression is affected by depletion of the other. Thus, expression of these critical regulators is dependent on continued expression of upstream regulators, which may include cell-extrinsic signals.

LIM domain only 2 (LMO2) proteins are important regulators in determining cell fate and controlling cell growth and differentiation. This study has investigated LMO2 expression in human prostatic tissue specimens, prostate cancer cell lines, and xenografts; and has assessed the possible role and mechanism of LMO2 in prostate carcinogenesis. Immunohistochemical analysis on a tissue microarray consisting of 91 human prostate specimens, including normal, prostatic hyperplasia, high-grade prostatic intraepithelial neoplasia, and invasive carcinoma, revealed that overexpression of LMO2 was significantly associated with advanced tumour stage, as measured by Gleason score (p = 0.012), as well as with the development of distant metastasis (p = 0.018). These data were supported by quantitative real-time PCR experiments, where LMO2 mRNA levels were found to be significantly higher in prostate tumour specimen than in normal epithelium (p = 0.037). The expression of LMO2 in cell lines and xenografts representing androgen-dependent (AD) and androgen-independent (AI) prostate cancer stages was further studied. Consistent with the in vivo data, LMO2 mRNA and protein were found to be overexpressed in the more aggressive AI cells (PC3, DU145, and AI CWR22 xenografts) compared with less aggressive AD cells (LNCaP and AD CWR22 xenografts). Furthermore, stable introduction of LMO2 into LNCaP cells conferred enhanced cell motility and invasiveness in vitro, accompanied by down-regulation of E-cadherin expression. Taken together, these findings provide the first evidence to support the hypothesis that LMO2 may play an important role in prostate cancer progression, possibly via repression of E-cadherin expression., (Copyright 2006 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.)

To identify new cytogenetic abnormalities associated with leukemogenesis or disease outcome, T-cell acute lymphoblastic leukemia (T-ALL) patient samples were analyzed by means of the array-comparative genome hybridization technique (array-CGH). Here, we report the identification of a new recurrent and cryptic deletion on chromosome 11 (del(11)(p12p13)) in about 4% (6/138) of pediatric T-ALL patients. Detailed molecular-cytogenetic analysis revealed that this deletion activates the LMO2 oncogene in 4 of 6 del(11)(p12p13)-positive T-ALL patients, in the same manner as in patients with an LMO2 translocation (9/138). The LMO2 activation mechanism of this deletion is loss of a negative regulatory region upstream of LMO2, causing activation of the proximal LMO2 promoter. LMO2 rearrangements, including this del(11)(p12p13) and t(11;14) (p13;q11) or t(7;11)(q35;p13), were found in the absence of other recurrent cytogenetic abnormalities involving HOX11L2, HOX11, CALM-AF10, TAL1, MLL, or MYC. LMO2 abnormalities represent about 9% (13/138) of pediatric T-ALL cases and are more frequent in pediatric T-ALL than appreciated until now.

The overexpression of LIM-only protein 2 (LMO2) in T-cells, as a result of chromosomal translocations, retroviral insertion during gene therapy, or in transgenic mice models, leads to the onset of T-cell leukemias. LMO2 comprises two protein-binding LIM domains that allow LMO2 to interact with multiple protein partners, including LIM domain-binding protein 1 (Ldb1, also known as CLIM2 and NLI), an essential cofactor for LMO proteins. Sequestration of Ldb1 by LMO2 in T-cells may prevent it binding other key partners, such as LMO4. Here, we show using protein engineering and enzyme-linked immunosorbent assay (ELISA) methodologies that LMO2 binds Ldb1 with a twofold lower affinity than does LMO4. Thus, excess LMO2 rather than an intrinsically higher binding affinity would lead to sequestration of Ldb1. Both LIM domains of LMO2 are required for high-affinity binding to Ldb1 (K(D) = 2.0 x 10(-8) M). However, the first LIM domain of LMO2 is primarily responsible for binding to Ldb1 (K(D) = 2.3 x 10(-7) M), whereas the second LIM domain increases binding by an order of magnitude. We used mutagenesis in combination with yeast two-hybrid analysis, and phage display selection to identify LMO2-binding "hot spots" within Ldb1 that locate to the LIM1-binding region. The delineation of this region reveals some specific differences when compared to the equivalent LMO4:Ldb1 interaction that hold promise for the development of reagents to specifically bind LMO2 in the treatment of leukemia.

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

Transcriptional control has been identified as a key mechanism regulating the formation and subsequent behavior of hematopoietic stem cells. We have used a comparative genomics approach to identify transcriptional regulatory elements of the LMO2 gene, a transcriptional cofactor originally identified through its involvement in T-cell leukemia and subsequently shown to be critical for normal hematopoietic and endothelial development. Of the 2 previously characterized LMO2 promoters, the second (proximal) promoter was highly conserved in vertebrates ranging from mammals to fish. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) expression analysis identified this promoter as the predominant source of transcription in hematopoietic tissue. Transient and stable transfections indicated that the proximal promoter was active in hematopoietic progenitor and endothelial cell lines and this activity was shown to depend on 3 conserved Ets sites that were bound in vivo by E74-like factor 1 (Elf1), Friend leukemia integration 1 (Fli1), and erythroblastosis virus oncogene homolog E twenty-six-1 (Ets1). Finally, transgenic analysis demonstrated that the LMO2 proximal promoter is sufficient for expression in endothelial cells in vivo. No hematopoietic expression was observed, indicating that additional enhancers are required to mediate transcription from the proximal promoter in hematopoietic cells. Together, these results suggest that the conserved proximal promoter is central to LMO2 transcription in hematopoietic and endothelial cells, where it is regulated by Ets factors.

Objective: The LIM-finger protein LMO2 forms a transcription factor complex with other hematopoietic regulator proteins, such as TAL1 (SCL), LDB1, GATA1, 2, and 3, in the promoters of several erythroid genes. To elucidate the functional role of two LIM domains in LMO2, we introduced deletion or mutation in each of the LIM domains and analyzed their phenotypic effects on the hematopoietic system when overexpressed in vivo or in vitro., Materials and Methods: Protein interactions of LIM-modified LMO2 constructs with TAL1, LDB1, and GATAs were examined in an immunoprecipitation assay. In vivo hematopoiesis in transgenic mice with wild-type and LIM-modified Lmo2 was studied morphologically and by measuring the progenitor cells in fetal liver. Their effects on the erythroid differentiation of the dimethylsulfoxide (DMSO)-induced murine erythroleukemia (MEL) cells were evaluated., Results: Deletion of the LIM2 domain, but not of the LIM1 domain, abolished its binding of GATA proteins. Overexpression of wild-type LMO2 is known to have dominant negative inhibitory effects on erythropoietic development. Enforced expression of LMO2 constructs with mutant or absent LIM2 but with an intact LIM1 domain resulted in fetal death, small livers and hearts, and decreased hematopoiesis, as well as a hypoplastic thymus. DMSO-induced erythroid differentiation of the MEL cells was inhibited by the overexpressed LMO2 with mutant LIM2 but not by the LMO2 with modified LIM1., Conclusion: Overexpression of the LMO2 with modified LIM2 inhibited hematopoiesis probably by interfering with the formation of the physiological complex or by replacing the functional LMO2 with mutants with reduced affinity to GATA proteins. In this experiment, no evident effect of the LMO2 with modified LIM1 could be observed.

The Lmo2 transcription factor, a T-cell oncoprotein, is required for both hematopoiesis and angiogenesis. To investigate the fate of lmo2-expressing cells and the transcriptional regulation of lmo2 in vivo, we generated stable transgenic zebrafish that express green fluorescent protein (EGFP) or DsRed under the control of an lmo2 promoter. A 2.5-kb fragment contains the cis-regulatory elements required to recapitulate endogenous lmo2 expression in embryonic hematopoietic and vascular tissues. We further characterized embryonic Lmo2+ cells through transplantation into vlad tepes (vlt), an erythropoietic mutant. These Lmo2+ primitive wave donor cells differentiated into circulating hematopoietic cells and extended the life span of vlt recipients, but did not demonstrate long-term repopulation of the erythroid lineage. Promoter analysis identified a 174-bp proximal promoter that was sufficient to recapitulate lmo2 expression. This element contains critical ETS-binding sites conserved between zebrafish and pufferfish. Furthermore, we show that ets1 is coexpressed with lmo2, and overexpression experiments indicate that ets1 can activate the lmo2 promoter through this element. Our studies elucidate the transcriptional regulation of this key transcription factor, and provide a transgenic system for the functional analysis of blood and blood vessels in zebrafish.

Ectopic expression of LMO2 occurs in approximately 45% of T-lineage acute lymphoblastic leukemias (T-ALL), sometimes in association with chromosomal translocations. Recently, a lymphoproliferative disorder developed in two participants in a gene therapy trial due to LMO2 activation via integration of the retroviral vector. To investigate these regulatory disruptions, we analyzed the promoter region and identified a tissue-specific repressor. The fragment containing this element could also produce tissue-specific suppression of transcription from the SV40 promoter. This suppression involves histone acetylation which can be relieved with Trichostatin A (TSA). The negative element is in a region consistently removed from LMO2 in the known chromosomal translocations.