The T cell leukemia LIM protein Lmo2 is necessary for adult mouse hematopoiesis

Analysis of the effect of LMO2 on indicating CD8+ T-lymphocyte infiltration in pan-cancers

LMO2 is a critical factor in hematopoiesis but widely expressed in kinds of epithelia and solid tumors as well, and it has been demonstrated that increased LMO2 level in some tumor cells exhibits an effect on promoting CD8+ T-lymphocyte infiltration in tumor microenvironment. Herein we expanded the investigation of such effect in pan-cancer spectrum, and the transcriptome data analysis revealed extensively positive associations between LMO2 and CD8+ T-lymphocyte infiltration in pan-cancers. Meanwhile, tissue staining revealed variable expression level and consistently cytosolic localization of LMO2 in different tissues and their tumor counterparts, and further indicated that high LMO2 expression in malignant cells was strongly associated with positive CD8+ T-lymphocyte infiltration selectively in digestive tract, breast and kidney tumors. Taken together, this study depicted an overall view of LMO2 functional linkage with tumor immunology in pan-cancers and provided novel insight of LMO2 significance in the field of tumor immune therapy.

The LIM-domain-only protein LMO2 and its binding partner LDB1 are differentially required for class switch recombination

The LIM-domain-only protein LMO2 interacts with LDB1 in context-dependent multiprotein complexes and plays key roles in erythropoiesis and T cell leukemogenesis, but whether they have any roles in B cells is unclear. Through a CRISPR/Cas9-based loss-of-function screening, we identified LMO2 and LDB1 as factors for class switch recombination (CSR) in murine B cells. LMO2 contributes to CSR at least in part by promoting end joining of DNA double-strand breaks (DSBs) and inhibiting end resection. Although LDB1 stabilizes LMO2 proteins, it is not required for end joining but functions as a positive regulator of AID transcription independent of LMO2, and this function of LDB1 requires its dimerization domain. Moreover, LDB1 directly binds to and promotes the looping of the AID promoter to upstream enhancers through dimerization. Our study revealed the mechanistically separated roles of LMO2 and LDB1 in different steps of CSR for antibody diversification.

In vivo CRISPR/Cas9-mediated screen reveals a critical function of TFDP1 and E2F4 transcription factors in hematopoiesis

Hematopoiesis is a continuous process of blood cell production driven by hematopoietic stem and progenitor cells (HSPCs) in the bone marrow. Proliferation and differentiation of HSPCs are regulated by complex transcriptional networks. In order to identify transcription factors with key roles in HSPC-mediated hematopoietic reconstitution, we developed an efficient and robust CRISPR/Cas9-based in vivo genetic screen. Using this experimental system, we identified the TFDP1 transcription factor to be essential for HSPC proliferation and post-transplant hematopoiesis. We further discovered that E2F4, an E2F transcription factor, serves as a binding partner of TFDP1 and is required for HSPC proliferation. Deletion of TFDP1 caused downregulation of genes associated with the cell cycle, with around 50% of these genes being identified as direct targets of TFDP1 and E2F4. Thus, our study expands the transcriptional network governing hematopoietic development through an in vivo CRISPR/Cas9-based genetic screen and identifies TFDP1/E2F4 as positive regulators of cell cycle genes in HSPCs.

Aberrant stem cell and developmental programs in pediatric leukemia

Hematopoiesis is a finely orchestrated process, whereby hematopoietic stem cells give rise to all mature blood cells. Crucially, they maintain the ability to self-renew and/or differentiate to replenish downstream progeny. This process starts at an embryonic stage and continues throughout the human lifespan. Blood cancers such as leukemia occur when normal hematopoiesis is disrupted, leading to uncontrolled proliferation and a block in differentiation of progenitors of a particular lineage (myeloid or lymphoid). Although normal stem cell programs are crucial for tissue homeostasis, these can be co-opted in many cancers, including leukemia. Myeloid or lymphoid leukemias often display stem cell-like properties that not only allow proliferation and survival of leukemic blasts but also enable them to escape treatments currently employed to treat patients. In addition, some leukemias, especially in children, have a fetal stem cell profile, which may reflect the developmental origins of the disease. Aberrant fetal stem cell programs necessary for leukemia maintenance are particularly attractive therapeutic targets. Understanding how hijacked stem cell programs lead to aberrant gene expression in place and time, and drive the biology of leukemia, will help us develop the best treatment strategies for patients.

Lung Cancer Gene Regulatory Network of Transcription Factors Related to the Hallmarks of Cancer

The transcriptomic analysis of microarray and RNA-Seq datasets followed our own bioinformatic pipeline to identify a transcriptional regulatory network of lung cancer. Twenty-six transcription factors are dysregulated and co-expressed in most of the lung cancer and pulmonary arterial hypertension datasets, which makes them the most frequently dysregulated transcription factors. Co-expression, gene regulatory, coregulatory, and transcriptional regulatory networks, along with fibration symmetries, were constructed to identify common connection patterns, alignments, main regulators, and target genes in order to analyze transcription factor complex formation, as well as its synchronized co-expression patterns in every type of lung cancer. The regulatory function of the most frequently dysregulated transcription factors over lung cancer deregulated genes was validated with ChEA3 enrichment analysis. A Kaplan-Meier plotter analysis linked the dysregulation of the top transcription factors with lung cancer patients' survival. Our results indicate that lung cancer has unique and common deregulated genes and transcription factors with pulmonary arterial hypertension, co-expressed and regulated in a coordinated and cooperative manner by the transcriptional regulatory network that might be associated with critical biological processes and signaling pathways related to the acquisition of the hallmarks of cancer, making them potentially relevant tumor biomarkers for lung cancer early diagnosis and targets for the development of personalized therapies against lung cancer.

In vivo clonal tracking reveals evidence of haemangioblast and haematomesoblast contribution to yolk sac haematopoiesis

During embryogenesis, haematopoietic and endothelial lineages emerge closely in time and space. It is thought that the first blood and endothelium derive from a common clonal ancestor, the haemangioblast. However, investigation of candidate haemangioblasts in vitro revealed the capacity for mesenchymal differentiation, a feature more compatible with an earlier mesodermal precursor. To date, no evidence for an in vivo haemangioblast has been discovered. Using single cell RNA-Sequencing and in vivo cellular barcoding, we have unravelled the ancestral relationships that give rise to the haematopoietic lineages of the yolk sac, the endothelium, and the mesenchyme. We show that the mesodermal derivatives of the yolk sac are produced by three distinct precursors with dual-lineage outcomes: the haemangioblast, the mesenchymoangioblast, and a previously undescribed cell type: the haematomesoblast. Between E5.5 and E7.5, this trio of precursors seeds haematopoietic, endothelial, and mesenchymal trajectories.

A dissected LMO2 functional analysis and clinical relevance in brain gliomas

Brain glioma is one of the cancer types with worst prognosis, and LMO2 has been reported to play oncogenic functions in brain gliomas. Herein, analysis of datasets from The Cancer Genome Atlas (TCGA) indicated that higher LMO2 level in patient samples indicated worse prognosis in lower grade gliomas (LGG) but not glioblastoma multiforme (GBM). Further, in tumor tissues consisting of a variety of cell types, LMO2 level indicated intratumoral endothelium and pattern recognition receptor (PRR) response in both LGGs and GBMs, and additionally indicated cytotoxic T-lymphocyte, M2 macrophage infiltration and fibroblast specifically in LGGs. Moreover, only in LGGs these aspects were significantly associated with patient survival, in either risky or protective manner, and these dissected associations can give a better prediction on patient prognosis than LMO2 alone. This study not only provided more detailed understandings of LMO2 functional representatives in brain gliomas but also demonstrated that dealing with certain gene (LMO2 in this study) in transcriptome data with the Weighted Gene Co-Expression Network Analysis (WGCNA) method was a robust strategy for dissecting exact and reasonable gene functions/associations in a complicated tumor environment.

Identification of a minority population of LMO2+ breast cancer cells that integrate into the vasculature and initiate metastasis

Metastasis is responsible for most breast cancer-related deaths; however, identifying the cellular determinants of metastasis has remained challenging. Here, we identified a minority population of immature THY1+/VEGFA+ tumor epithelial cells in human breast tumor biopsies that display angiogenic features and are marked by the expression of the oncogene, LMO2. Higher abundance of LMO2+ basal cells correlated with tumor endothelial content and predicted poor distant recurrence-free survival in patients. Using MMTV-PyMT/Lmo2CreERT2 mice, we demonstrated that Lmo2 lineage-traced cells integrate into the vasculature and have a higher propensity to metastasize. LMO2 knockdown in human breast tumors reduced lung metastasis by impairing intravasation, leading to a reduced frequency of circulating tumor cells. Mechanistically, we find that LMO2 binds to STAT3 and is required for STAT3 activation by tumor necrosis factor-α and interleukin-6. Collectively, our study identifies a population of metastasis-initiating cells with angiogenic features and establishes the LMO2-STAT3 signaling axis as a therapeutic target in breast cancer metastasis.

A comprehensive analysis of LMO2 pathogenic regulatory profile during T-lineage development and leukemic transformation

LMO2 is a well-known leukemic proto-oncogene, its ectopic expression in T-lineage specifically initiates malignant transformation of immature T cells and ultimately causes the onset of acute T-lymphocytic leukemia (T-ALL) in both mouse models and human patients. In this study, we systematically explored the LMO2 performance on the profiles of transcriptome, DNA-binding and protein interactions during T-lineage development in the pre-leukemic stage. Our data indicated that large-scale transcriptional dysregulation caused by LMO2 primarily occurred in DN3 thymocytes, characterized by enriched upregulation of the target genes of typical LMO2 complex, RUNX, ETS and STATs, and ectopic LMO2 primarily targeted to RUNX motifs along with intensive interaction with RUNX1 and H3K4 methyltransferase component ASH2L in this stage. However, binding of LMO2 on specific motifs was largely reduced in the following DP and SP stages, along with gradually disappeared LMO2-RUNX1 and LMO2-ASH2L interactions and less alteration of certain transcriptional factor profiles. Moreover, LMO2 showed relatively less influence on cellular behavior of DN3 thymocyte whereas displayed more prominent effects in DP and SP stages, including promoting Notch signaling and cell cycles. These findings provide a high-resolution landscape of the pathogenic role of LMO2 during T-lineage development in molecular level, and may benefit further clinical investigations for LMO2-associated T-lineage malignancies.

Cytoplasmic LMO2-LDB1 Complex Activates STAT3 Signaling through Interaction with gp130-JAK in Glioma Stem Cells

The oncogenic role of nuclear LIM domain only 2 (LMO2) as a transcriptional regulator is well established, but its function in the cytoplasm is largely unknown. Here, we identified LMO2 as a cytoplasmic activator for signal transducer and activator of transcription 3 (STAT3) signaling in glioma stem cells (GSCs) through biochemical and bioinformatics analyses. LMO2 increases STAT3 phosphorylation by interacting with glycoprotein 130 (gp130) and Janus kinases (JAKs). LMO2-driven activation of STAT3 signaling requires the LDB1 protein and leads to increased expression of an inhibitor of differentiation 1 (ID1), a master regulator of cancer stemness. Our findings indicate that the cytoplasmic LMO2-LDB1 complex plays a crucial role in the activation of the GSC signaling cascade via interaction with gp130 and JAK1/2. Thus, LMO2-LDB1 is a bona fide oncogenic protein complex that activates either the JAK-STAT signaling cascade in the cytoplasm or direct transcriptional regulation in the nucleus.

Understanding Aberrant Signaling to Elude Therapy Escape Mechanisms in Myeloproliferative Neoplasms

Aberrant signaling in myeloproliferative neoplasms may arise from alterations in genes coding for signal transduction proteins or epigenetic regulators. Both mutated and normal cells cooperate, altering fragile balances in bone marrow niches and fueling persistent inflammation through paracrine or systemic signals. Despite the hopes placed in targeted therapies, myeloid proliferative neoplasms remain incurable diseases in patients not eligible for stem cell transplantation. Due to the emergence of drug resistance, patient management is often very difficult in the long term. Unexpected connections among signal transduction pathways highlighted in neoplastic cells suggest new strategies to overcome neoplastic cell adaptation.

LMO2 is essential to maintain the ability of progenitors to differentiate into T-cell lineage in mice

Notch signaling primarily determines T-cell fate. However, the molecular mechanisms underlying the maintenance of T-lineage potential in pre-thymic progenitors remain unclear. Here, we established two murine Ebf1-deficient pro-B cell lines, with and without T-lineage potential. The latter expressed lower levels of Lmo2; their potential was restored via ectopic expression of Lmo2. Conversely, the CRISPR/Cas9-mediated deletion of Lmo2 resulted in the loss of the T-lineage potential. Introduction of Bcl2 rescued massive cell death of Notch-stimulated pro-B cells without efficient LMO2-driven Bcl11a expression but was not sufficient to retain their T-lineage potential. Pro-B cells without T-lineage potential failed to activate Tcf7 due to DNA methylation; Tcf7 transduction restored this capacity. Moreover, direct binding of LMO2 to the Bcl11a and Tcf7 loci was observed. Altogether, our results highlight LMO2 as a crucial player in the survival and maintenance of T-lineage potential in T-cell progenitors via the regulation of the expression of Bcl11a and Tcf7.

Fli1+ cells transcriptional analysis reveals an Lmo2-Prdm16 axis in angiogenesis

A network of molecular factors drives the development, differentiation, and maintenance of endothelial cells. Friend leukemia integration 1 transcription factor (FLI1) is a bona fide marker of endothelial cells during early development. In zebrafish Tg( f li1:EGFP) ^y1 , we identified two endothelial cell populations, high-fli1 ⁺ and low-fli1 ⁺, by the intensity of green fluorescent protein signal. By comparing RNA-sequencing analysis of non-fli1 expressing cells (fli1 ^-) with these two (fli1 ⁺) cell populations, we identified several up-regulated genes, not previously recognized as important, during endothelial development. Compared with fli1 ^- and low-fli1 ⁺ cells, high-fli1 ⁺ cells showed up-regulated expression of the zinc finger transcription factor PRDI-BF1 and RIZ homology domain containing 16 (prdm16). Prdm16 knockdown (KD) by morpholino in the zebrafish larva was associated with impaired angiogenesis and increased number of low-fli1 ⁺ cells at the expense of high-fli1 ⁺ cells. In addition, PRDM16 KD in endothelial cells derived from human-induced pluripotent stem cells impaired their differentiation and migration in vitro. Moreover, zebrafish mutants (mut) with loss of function for the oncogene LIM domain only 2 (lmo2) also showed reduced prdm16 gene expression combined with impaired angiogenesis. Prdm16 expression was reduced further in endothelial (CD31⁺) cells compared with CD31^- cells isolated from l mo2-mutants (l mo2-mut) embryos. Chromatin immunoprecipitation-PCR demonstrated that Lmo2 binds to the promoter and directly regulates the transcription of prdm16 This work unveils a mechanism by which prdm16 expression is activated in endothelial cells by Lmo2 and highlights a possible therapeutic pathway by which to modulate endothelial cell growth and repair.

LIM domain only 1: an oncogenic transcription cofactor contributing to the tumorigenesis of multiple cancer types

ABSTRACT

The LIM domain only 1 (LMO1) gene belongs to the LMO family of genes that encodes a group of transcriptional cofactors. This group of transcriptional cofactors regulates gene transcription by acting as a key "connector" or "scaffold" in transcription complexes. All LMOs, including LMO1, are important players in the process of tumorigenesis. Unique biological features of LMO1 distinct from other LMO members, such as its tissue-specific expression patterns, interacting proteins, and transcriptional targets, have been increasingly recognized. Studies indicated that LMO1 plays a critical oncogenic role in various types of cancers, including T-cell acute lymphoblastic leukemia, neuroblastoma, gastric cancer, lung cancer, and prostate cancer. The molecular mechanisms underlying such functions of LMO1 have also been investigated, but they are currently far from being fully elucidated. Here, we focus on reviewing the current findings on the role of LMO1 in tumorigenesis, the mechanisms of its oncogenic action, and the mechanisms that drive its aberrant activation in cancers. We also briefly review its roles in the development process and non-cancer diseases. Finally, we discuss the remaining questions and future investigations required for promoting the translation of laboratory findings to clinical applications, including cancer diagnosis and treatment.

Small molecule inhibits T-cell acute lymphoblastic leukaemia oncogenic interaction through conformational modulation of LMO2

Ectopic expression in T-cell precursors of LIM only protein 2 (LMO2), a key factor in hematopoietic development, has been linked to the onset of T-cell acute lymphoblastic leukaemia (T-ALL). In the T-ALL context, LMO2 drives oncogenic progression through binding to erythroid-specific transcription factor SCL/TAL1 and sequestration of E-protein transcription factors, normally required for T-cell differentiation. A key requirement for the formation of this oncogenic protein-protein interaction (PPI) is the conformational flexibility of LMO2. Here we identify a small molecule inhibitor of the SCL-LMO2 PPI, which hinders the interaction in vitro through direct binding to LMO2. Biophysical analysis demonstrates that this inhibitor acts through a mechanism of conformational modulation of LMO2. Importantly, this work has led to the identification of a small molecule inhibitor of the SCL-LMO2 PPI, which can provide a starting point for the development of new agents for the treatment of T-ALL. These results suggest that similar approaches, based on the modulation of protein conformation by small molecules, might be used for therapeutic targeting of other oncogenic PPIs.

Lineage Decision-Making within Normal Haematopoietic and Leukemic Stem Cells

To produce the wide range of blood and immune cell types, haematopoietic stem cells can “choose” directly from the entire spectrum of blood cell fate-options. Affiliation to a single cell lineage can occur at the level of the haematopoietic stem cell and these cells are therefore a mixture of some pluripotent cells and many cells with lineage signatures. Even so, haematopoietic stem cells and their progeny that have chosen a particular fate can still “change their mind” and adopt a different developmental pathway. Many of the leukaemias arise in haematopoietic stem cells with the bulk of the often partially differentiated leukaemia cells belonging to just one cell type. We argue that the reason for this is that an oncogenic insult to the genome “hard wires” leukaemia stem cells, either through development or at some stage, to one cell lineage. Unlike normal haematopoietic stem cells, oncogene-transformed leukaemia stem cells and their progeny are unable to adopt an alternative pathway.

Beadex, a homologue of the vertebrate LIM domain only protein, is a novel regulator of crystal cell development in Drosophila melanogaster

Haematopoiesis is a complex process in which the regulatory mechanisms of several implicated transcription factors remain uncertain. Drosophila melanogaster is an excellent model to resolve the unanswered questions about the blood cell development. This study describes the role of Beadex, a Drosophila homologue of LIM domain only 2 (LMO2), in haematopoiesis. Mutants of Beadex were analysed for blood cell abnormalities. Crystal cells, a subset of haemocytes, were significantly more in Beadex hypermorphic flies. Similarly, Beadex misexpression in prohemocytes altered the crystal cell numbers. Stage-specific misexpression analyses demonstrated that Beadex functions after the prohemocytes enter the crystal cell lineage. We also discovered that Pannier-U-shaped complex is a negative regulator of the crystal cell differentiation and is possibly negatively regulated by Beadex through its interaction with Pannier. We, therefore, suggest the mechanism of two novel regulators of crystal cell specification-Beadex and Pannier-during Drosophila haematopoiesis.

Conditional Immortalization of Lymphoid Progenitors via Tetracycline-Regulated LMO2 Expression

Conditional immortalization of hematopoietic progenitors through lentiviral expression of selected transcription factors in hematopoietic stem and progenitor cells provides a promising tool to study stem cell and leukemia biology. In this study, to generate conditionally immortalized lymphoid progenitor (ciLP) cell lines, murine hematopoietic progenitor cells were transduced with an inducible lentiviral "all-in-one" vector expressing LMO2 under doxycycline (DOX) stimulation and the reverse tetracycline-regulated transactivator (rtTA3). For selection of LMO2-expressing ciLPs (LMO2-ciLPs) and longitudinal manipulation in T cell differentiation lymphoid conditions, we developed a robust approach based on coculture with OP9-DL1 stromal cells and improved cytokine conditions allowing a controlled balance between cell proliferation and differentiation in vitro. LMO2-ciLP cell lines with the highest proliferation, vector copy number, and similar insertion pattern were selected for LMO2 "on/off" in vitro study. LMO2 expression under DOX induction resulted in a double negative (DN) 2 differentiation arrest and a propagation of CD44⁺CD25^- myeloid cell population characterized by lymphoid and myeloid phenotypes, respectively. Both DN2 and CD44⁺CD25^- myeloid cell subpopulations expressed c-KIT, suggesting that LMO2-ciLPs were similar to uncommitted progenitors under DOX supplementation. DOX removal resulted in cessation of ectopic LMO2 expression and LMO2-ciLPs continued T cell lymphoid differentiation accompanied by c-KIT downregulation and interleukin 7 receptor expression. Switching off LMO2 expression was accompanied by increased Notch signaling and significant reduction of the CD44⁺CD25^- myeloid cell population under T cell differentiation lymphoid conditions. Although vector insertions in cooperation with LMO2 expression could influence the fate of LMO2-ciLPs and additional experiments are required to evaluate it, our approach provides a promising tool to investigate mechanisms underlying stem cell, leukemia, and lymphocyte biology, leading to novel approaches for disease modeling and therapy evaluation.

Mapping Active Gene-Regulatory Regions in Human Repopulating Long-Term HSCs

Genes that regulate hematopoietic stem cell (HSC) self-renewal, proliferation, and differentiation are tightly controlled by regulatory regions. However, mapping such regions relies on surface markers and immunophenotypic definition of HSCs. Here, we use γ-retroviral integration sites (γRV ISs) from a gene therapy trial for 10 patients with Wiskott-Aldrich syndrome to mark active enhancers and promoters in functionally defined long-term repopulating HSCs. Integration site clusters showed the highest ATAC-seq signals at HSC-specific peaks and strongly correlated with hematopoietic risk variants. Tagged genes were significantly enriched for HSC gene sets. We were able to map over 3,000 HSC regulatory regions in late-contributing HSCs, and we used these data to identify miR-10a and miR-335 as two miRNAs regulating early hematopoiesis. In this study, we show that viral insertion sites can be used as molecular tags to assess chromatin conformation on functionally defined cell populations, thereby providing a genome-wide resource for regulatory regions in human repopulating long-term HSCs.

Insights into Endothelial Progenitor Cells: Origin, Classification, Potentials, and Prospects

With the discovery of endothelial progenitor cells (EPCs) in the late 1990s, a paradigm shift in the concept of neoangiogenesis occurred. The identification of circulating EPCs in peripheral blood marked the beginning of a new era with enormous potential in the rapidly transforming regenerative field. Overwhelmed with the revelation, researchers across the globe focused on isolating, defining, and interpreting the role of EPCs in various physiological and pathological conditions. Consequently, controversies emerged regarding the isolation techniques and classification of EPCs. Nevertheless, the potential of using EPCs in tissue engineering as an angiogenic source has been extensively explored. Concomitantly, the impact of EPCs on various diseases, such as diabetes, cancer, and cardiovascular diseases, has been studied. Within the limitations of the current knowledge, this review attempts to delineate the concept of EPCs in a sequential manner from the speculative history to a definitive presence (origin, sources of EPCs, isolation, and identification) and significance of these EPCs. Additionally, this review is aimed at serving as a guide for investigators, identifying potential research gaps, and summarizing our current and future prospects regarding EPCs.

EVI1 overexpression reprograms hematopoiesis via upregulation of Spi1 transcription

Inv(3q26) and t(3:3)(q21;q26) are specific to poor-prognosis myeloid malignancies, and result in marked overexpression of EVI1, a zinc-finger transcription factor and myeloid-specific oncoprotein. Despite extensive study, the mechanism by which EVI1 contributes to myeloid malignancy remains unclear. Here we describe a new mouse model that mimics the transcriptional effects of 3q26 rearrangement. We show that EVI1 overexpression causes global distortion of hematopoiesis, with suppression of erythropoiesis and lymphopoiesis, and marked premalignant expansion of myelopoiesis that eventually results in leukemic transformation. We show that myeloid skewing is dependent on DNA binding by EVI1, which upregulates Spi1, encoding master myeloid regulator PU.1. We show that EVI1 binds to the -14 kb upstream regulatory element (-14kbURE) at Spi1; knockdown of Spi1 dampens the myeloid skewing. Furthermore, deletion of the -14kbURE at Spi1 abrogates the effects of EVI1 on hematopoietic stem cells. These findings support a novel mechanism of leukemogenesis through EVI1 overexpression.

Hepatic Leukemia Factor Maintains Quiescence of Hematopoietic Stem Cells and Protects the Stem Cell Pool during Regeneration

The transcription factor hepatic leukemia factor (HLF) is strongly expressed in hematopoietic stem cells (HSCs) and is thought to influence both HSC self-renewal and leukemogenesis. However, the physiological role of HLF in hematopoiesis and HSC function is unclear. Here, we report that mice lacking Hlf are viable with essentially normal hematopoietic parameters, including an intact HSC pool during steady-state hematopoiesis. In contrast, when challenged through transplantation, Hlf-deficient HSCs showed an impaired ability to reconstitute hematopoiesis and became gradually exhausted upon serial transplantation. Transcriptional profiling of Hlf-deficient HSCs revealed changes associated with enhanced cellular activation, and cell-cycle analysis demonstrated a significant reduction of quiescent HSCs. Accordingly, toxic insults targeting dividing cells completely eradicated the HSC pool in Hlf-deficient mice. In summary, our findings point to HLF as a critical regulator of HSC quiescence and as an essential factor for maintaining the HSC pool during regeneration.

Beadex, a Drosophila LIM domain only protein, function in follicle cells is essential for egg development and fertility

LIM domain, constituted by two tandem C2H2 zinc finger motif, proteins regulate several biological processes. They are usually found associated with various functional domains like Homeodomain, kinase domain and other protein binding domains. LIM proteins that are devoid of other domains are called LIM only proteins (LMO). LMO proteins were first identified in humans and are implicated in development and oncogenesis. They regulate various cell specifications by regulating the activity of respective transcriptional complexes. The Drosophila LMO protein (dLMO), Beadex (Bx), regulates various developmental processes like wing margin development and bristle development. It also regulates Drosophila behavior in response to cocaine and ethanol. We have previously generated Bx null flies and shown its essential function in neurons for multiple aspects of female reproduction. However, it was not known whether Bx affects reproduction through its independent function in ovaries. In this paper we show that female flies null for Bx lay eggs with multiple defects. Further, through knock down studies we demonstrate that function of Bx in follicle cells is required for normal egg development. We also show that function of Bx is particularly required in border cells for Drosophila fertility.

Epigenetic dysregulation of ZEB1 is involved in LMO2-promoted T-cell acute lymphoblastic leukaemia leukaemogenesis

T-cell acute lymphoblastic leukaemia (T-ALL) is a hematological malignancy caused by the accumulation of genomic lesions that affect the development of T-cells. ZEB1, a member of zinc finger-homeodomain family transcription factor, exhibits crucial function in promoting T-cell differentiation and potentially acts as a tumor suppressor in T-ALL. However, the molecular mechanism by which ZEB1 regulates T-ALL leukaemogenesis remains obscure. Here, we showed that oncogenic LIM only 2 (LMO2) could recruit Sap18 and HDAC1 to assemble an epigenetic regulatory complex, thus inducing histone deacetylation in ZEB1 promoter and chromatin remodeling to achieve transcriptional repression. Furthermore, downregulation of ZEB1 by LMO2 complex results in an increased leukaemia stem cell (LSC) phenotype as well as unsensitivity in response to methotrexate (MTX) chemotherapy in T-ALL cells. Importantly, we demonstrated that Trichostatin A (TSA, a HDAC inhibitor) addition significantly attenuates MTX unsensitivity caused by dysfunction of LMO2/ZEB1 signaling. In conclusion, these findings have identified a molecular mechanism underlying LMO2/ZEB1-mediated leukaemogenesis, paving a way for treating T-ALL with a new strategy of epigenetic inhibitors.

Reciprocal Negative Regulation Between Lmx1a and Lmo4 Is Required for Inner Ear Formation

LIM-domain containing transcription factors (LIM-TFs) are conserved factors important for embryogenesis. The specificity of these factors in transcriptional regulation is conferred by the complexes that they form with other proteins such as LIM-domain-binding (Ldb) proteins and LIM-domain only (LMO) proteins. Unlike LIM-TFs, these proteins do not bind DNA directly. LMO proteins are negative regulators of LIM-TFs and function by competing with LIM-TFs for binding to Ldb's. Although the LIM-TF Lmx1a is expressed in the developing mouse hindbrain, which provides many of the extrinsic signals for inner ear formation, conditional knock-out embryos of both sexes show that the inner ear source of Lmx1a is the major contributor of ear patterning. In addition, we have found that the reciprocal interaction between Lmx1a and Lmo4 (a LMO protein within the inner ear) mediates the formation of both vestibular and auditory structures. Lmo4 negatively regulates Lmx1a to form the three sensory cristae, the anterior semicircular canal, and the shape of the utricle in the vestibule. Furthermore, this negative regulation blocks ectopic sensory formation in the cochlea. In contrast, Lmx1a negatively regulates Lmo4 in mediating epithelial resorption of the canal pouch, which gives rise to the anterior and posterior semicircular canals. We also found that Lmx1a is independently required for the formation of the endolymphatic duct and hair cells in the basal cochlear region.

SIGNIFICANCE STATEMENT The mammalian inner ear is a structurally complex organ responsible for detecting sound and maintaining balance. Failure to form the intricate 3D structure of this organ properly during development most likely will result in sensory deficits on some level. Here, we provide genetic evidence that a transcription factor, Lmx1a, interacts with its negative regulator, Lmo4, to pattern various vestibular and auditory components of the mammalian inner ear. Identifying these key molecules that mediate formation of this important sensory organ will be helpful for designing strategies and therapeutics to alleviate hearing loss and balance disorders.

LMO2 promotes tumor cell invasion and metastasis in basal-type breast cancer by altering actin cytoskeleton remodeling

LMO2 is traditionally recognized as a pivotal transcriptional regulator during embryonic hematopoiesis and angionenesis, and its ectopic expression in T lymphocyte progenitors is closely correlated to the onset of acute T lymphocytic leukemia. However, recently studies revealed complicated expression features and dual functions of LMO2 on tumor behaviors in a variety of cancer types, including breast cancers. Basal-type breast cancer is one of the breast cancer subtypes and a prognostically unfavorable subtype among all breast cancers. Herein we found that in basal-type breast cancer specifically, high LMO2 expression was positively correlated with lymph node metastases in patients, promoted tumor cell migration and invasion and increased distant metastasis in SCID mice. Moreover, the novel function of LMO2 was achieved by its predominantly cytoplasmic location and interaction with cofilin1, which is a critical regulator in actin cytoskeleton dynamics. These findings suggest a subtype-dependent role of LMO2 in breast cancers and the potential of LMO2 as a subtype-specific biomarker for clinical practice.

A comprehensive function analysis of LMO2 in different breast cancer subtypes

Breast cancer is the most common invasive cancer in women worldwide, and can be subdivided into Luminal A, Luminal B, Her2, and Basal subtype (the PAM50 subtyping system). The lmo2 gene was traditionally recognized as a proto-oncogene in hematopoietic system but its functions in breast cancers remained largely unclear. Based on the Cancer Genome Atlas (TCGA) breast cancer dataset, herein we found that the significantly LMO2-correlated genes in normal or malignant samples were enriched in rather divergent cellular pathways, suggesting the function complexity of LMO2 in breast tissues. Moreover, high LMO2 expression level was found to predict a shorter patient survival in Luminal A type whereas a better outcome in Her2 type. Correspondingly, LMO2 also revealed function diversities in different PAM50 subtypes. In Luminal A type, the LMO2 related genes were primarily enriched in cancer-promoting pathways, including VEGF production, stemness, PPAR signal pathways, MAPK cascade and cell cycle regulation. In Her2 type however, the LMO2 related genes lacked the enrichment on most of the generally cancer-related pathways and were particularly enriched in negative regulation of ErbB pathway as well as MAPK cascade, suggesting a potentially anti-oncogenic role of LMO2 on this subtype. Taken together, this study drew a comprehensive overview of divergent functions of LMO2 on breast cancers, provided additional evidence for the function complexity of LMO2 in solid tumors and suggested the potential usage of LMO2 as a PAM50 subtype dependent biomarker for breast cancer clinic in the precision medicine era.

Identification of Novel Hemangioblast Genes in the Early Chick Embryo

During early vertebrate embryogenesis, both hematopoietic and endothelial lineages derive from a common progenitor known as the hemangioblast. Hemangioblasts derive from mesodermal cells that migrate from the posterior primitive streak into the extraembryonic yolk sac. In addition to primitive hematopoietic cells, recent evidence revealed that yolk sac hemangioblasts also give rise to tissue-resident macrophages and to definitive hematopoietic stem/progenitor cells. In our previous work, we used a novel hemangioblast-specific reporter to isolate the population of chick yolk sac hemangioblasts and characterize its gene expression profile using microarrays. Here we report the microarray profile analysis and the identification of upregulated genes not yet described in hemangioblasts. These include the solute carrier transporters SLC15A1 and SCL32A1, the cytoskeletal protein RhoGap6, the serine protease CTSG, the transmembrane receptor MRC1, the transcription factors LHX8, CITED4 and PITX1, and the previously uncharacterized gene DIA1R. Expression analysis by in situ hybridization showed that chick DIA1R is expressed not only in yolk sac hemangioblasts but also in particular intraembryonic populations of hemogenic endothelial cells, suggesting a potential role in the hemangioblast-derived hemogenic lineage. Future research into the function of these newly identified genes may reveal novel important regulators of hemangioblast development.

BRACHYURY directs histone acetylation to target loci during mesoderm development

T-box transcription factors play essential roles in multiple aspects of vertebrate development. Here, we show that cooperative function of BRACHYURY (T) with histone-modifying enzymes is essential for mouse embryogenesis. A single point mutation (T^Y88A) results in decreased histone 3 lysine 27 acetylation (H3K27ac) at T target sites, including the T locus, suggesting that T autoregulates the maintenance of its expression and functions by recruiting permissive chromatin modifications to putative enhancers during mesoderm specification. Our data indicate that T mediates H3K27ac recruitment through a physical interaction with p300. In addition, we determine that T plays a prominent role in the specification of hematopoietic and endothelial cell types. Hematopoietic and endothelial gene expression programs are disrupted in T^Y88A mutant embryos, leading to a defect in the differentiation of hematopoietic progenitors. We show that this role of T is mediated, at least in part, through activation of a distal Lmo2 enhancer.

LMO2 is required for TAL1 DNA binding activity and initiation of definitive haematopoiesis at the haemangioblast stage

LMO2 is a bridging factor within a DNA binding complex and is required for definitive haematopoiesis to occur. The developmental stage of the block in haematopoietic specification is not known. We show that Lmo2^−/− mouse embryonic stem cells differentiated to Flk-1⁺ haemangioblasts, but less efficiently to haemogenic endothelium, which only produced primitive haematopoietic progenitors. Genome-wide approaches indicated that LMO2 is required at the haemangioblast stage to position the TAL1/LMO2/LDB1 complex to regulatory elements that are important for the establishment of the haematopoietic developmental program. In the absence of LMO2, the target site recognition of TAL1 is impaired. The lack of LMO2 resulted in altered gene expression levels already at the haemangioblast stage, with transcription factor genes accounting for ∼15% of affected genes. Comparison of Lmo2^−/− with Tal1^−/− Flk-1⁺ cells further showed that TAL1 was required to initiate or sustain Lmo2 expression.

Efforts to enhance blood stem cell engraftment: Recent insights from zebrafish hematopoiesis

Hematopoietic stem cell transplantation (HSCT) is an important therapy for patients with a variety of hematological malignancies. HSCT would be greatly improved if patient-specific hematopoietic stem cells (HSCs) could be generated from induced pluripotent stem cells in vitro. There is an incomplete understanding of the genes and signals involved in HSC induction, migration, maintenance, and niche engraftment. Recent studies in zebrafish have revealed novel genes that are required for HSC induction and niche regulation of HSC homeostasis. Manipulation of these signaling pathways and cell types may improve HSC bioengineering, which could significantly advance critical, lifesaving HSCT therapies.

Lmo2 (LIM-Domain-Only 2) Modulates Sphk1 (Sphingosine Kinase) and Promotes Endothelial Cell Migration

OBJECTIVE

Lmo (LIM-domain-only)2 transcription factor is involved in hematopoiesis and vascular remodeling. Sphk (sphingosine kinase)1 phosphorylates sphingosine to S1P (sphingosine-1-phosphate). We hypothesized that Lmo2 regulates Sphk1 to promote endothelial cell (EC) migration and vascular development. APPROACH AND RESULTS: Lmo2 and Sphk1 knockdown (KD) were performed in Tg(fli1:EGFP) ^y1 zebrafish and in human umbilical vein EC. Rescue of phenotypes or overexpression of these factors were achieved using mRNA encoding Lmo2 or Sphk1. EC proliferation in vivo was assessed by BrdU (bromodeoxyuridine) immunostaining and fluorescence-activated cell sorter analysis of dissociated Tg(fli1:EGFP) ^y1 embryos. Cell migration was assessed by scratch assay in human umbilical vein EC and mouse aortic rings. Lmo2 interactions with Sphk1 promoter were assessed by ChIP-PCR (chromatin immunoprecipitation-polymerase chain reaction). Lmo2 or Sphk1 KD reduced number and length of intersegmental vessels. There was no reduction in the numbers of GFP⁺ (green fluorescent protein) ECs after Lmo2 KD. However, reduced numbers of BrdU⁺GFP⁺ nuclei were observed along the dysmorphic intersegmental vessels, accumulating instead at the sprouting origin of the intersegmental vessels. This anomaly was likely because of impaired EC migration, which was confirmed in migration assays using Lmo2 KD human umbilical vein ECs and mouse aortic rings. Both in vivo and in vitro, Lmo2 KD reduced Sphk1 gene expression, associated with less Lmo2 binding to the Sphk1 promoter as assessed by ChIP-PCR. Sphk1 mRNA rescued the Lmo2 KD phenotype.

CONCLUSIONS

Our data showed that Lmo2 is necessary for Sphk1 gene expression in ECs. Lmo2 KD reduced Lmo2-Sphk1 gene interaction, impaired intersegmental vessels formation, and reduced cell migration. We identified for the first time Sphk1 as downstream effector of Lmo2.

LMO2 Enhances Lamellipodia/Filopodia Formation in Basal-Type Breast Cancer Cells by Mediating ARP3-Profilin1 Interaction

Background

The human LMO2 gene was first cloned from an acute T lymphocytic leukemia patient; it is primarily expressed in hematopoietic and vascular endothelial systems, and functions as a pivotal transcriptional regulator during embryonic hematopoiesis and angiogenesis. However, some recent reports indicated that LMO2 is widely expressed in many tissues and tumors, predominantly in cytoplasm, and revealed complicated functions on tumor behaviors in a variety of cancer types. As an adaptor molecule, binding partners and function details of LMO2 in these solid tumors need to be further investigated.

Material/Methods

In this study, we used yeast two-hybrid method to screen potential LMO2 interacting partners, MBP-pulldown, and co-immunoprecipitation assay to confirm protein-protein interactions, and confocal microscopy to reveal the subcellular localization of relevant proteins and actin cytoskeleton changes in relevant cells.

Results

We found that ARP3 and profilin1 were 2 binding partners of LMO2, primarily in cytoplasm. LMO2. Functionally, LMO2 mediated the assembly of a complex including ARP3, profilin1, and actin monomer, increased actin monomer binding to profilin1, and promoted lamellipodia/filopodia formation in basal-type breast cancer cells.

Conclusions

Our data indicate a novel functional mechanism of LMO2 in facilitating the delivery of actin monomers to the branched microfilament and increasing lamellipodia/filopodia formation in basal-type breast cancer cells, suggesting a cancer-promoting role of LMO2 in a subtype-dependent manner and its potential as a subtype-specific biomarker for clinical treatment of breast cancers.

LMO4 is a prognostic marker involved in cell migration and invasion in non-small-cell lung cancer

BACKGROUND

The aims of this study were to analyze the association of LMO4 with non-small-cell lung cancer (NSCLC) survival rate, and to determine its functional role and signaling pathway in lung cancer.

METHODS

Immunohistochemistry (IHC) was used to detect the expression of LMO4 in NSCLC cell lines and tumor tissues. Migration and invasion ability was detected respectively by wound healing test and transwell test. Immunofluorescence and western blot were detected of AKT/PI3K pathway related genes MAPK, PI3K, AKT.

RESULTS

LMO4 has high expression level of NSCLC cell lines and tumor tissues, and correlated with a lower survival rate. LMO4 can regulate the migration and invasion of NSCLC cells through the AKT/PI3K pathway.

CONCLUSIONS

LMO4 could serve as a promising biomarker and therapeutic target for NSCLC.

Reprogramming mouse fibroblasts into engraftable myeloerythroid and lymphoid progenitors

Recent efforts have attempted to convert non-blood cells into hematopoietic stem cells (HSCs) with the goal of generating blood lineages de novo. Here we show that hematopoietic transcription factors Scl, Lmo2, Runx1 and Bmi1 can convert a developmentally distant lineage (fibroblasts) into ‘induced hematopoietic progenitors' (iHPs). Functionally, iHPs generate acetylcholinesterase⁺ megakaryocytes and phagocytic myeloid cells in vitro and can also engraft immunodeficient mice, generating myeloerythoid and B-lymphoid cells for up to 4 months in vivo. Molecularly, iHPs transcriptionally resemble native Kit⁺ hematopoietic progenitors. Mechanistically, reprogramming factor Lmo2 implements a hematopoietic programme in fibroblasts by rapidly binding to and upregulating the Hhex and Gfi1 genes within days. Moreover the reprogramming transcription factors also require extracellular BMP and MEK signalling to cooperatively effectuate reprogramming. Thus, the transcription factors that orchestrate embryonic hematopoiesis can artificially reconstitute this programme in developmentally distant fibroblasts, converting them into engraftable blood progenitors.

Direct reprogramming of closely-related lineages can generate hematopoietic stem cells. Here, the authors show hematopoietic transcription factors Scl, Lmo2, Runx1 and Bmi1 can reprogram fibroblasts into induced hematopoietic progenitors (iHPs), which are engraftable blood progenitors.

LMO2 attenuates tumor growth by targeting the Wnt signaling pathway in breast and colorectal cancer

The proto-oncogene LIM-domain only 2 (lmo2) was traditionally considered to be a pivotal transcriptional regulator in hematopoiesis and leukemia. Recently, the cytosolic localization of LMO2 was revealed in multiple epithelial tissues and a variety of solid tumors. However, the function of LMO2 in these epithelia and solid tumors remains largely unclear. The Wnt signaling pathway is a crucial determinant of development, and abnormalities in several key segments of this pathway contribute to oncogenesis. The current study demonstrated that LMO2 participates in the regulation of canonical Wnt signaling in the cytoplasm by binding to Dishevelled-1/2 (DVL-1/2) proteins. These interactions occurred at the PDZ domain of Dishevelled, and LMO2 subsequently attenuated the activation of the key factor β-catenin in the canonical Wnt signaling pathway. Meanwhile, significantly decreased expression of LMO2 was detected in breast and colorectal cancers, and the downregulation of LMO2 in these cells increased cell proliferation and reduced apoptosis. Taken together, the data in this study revealed a novel crosstalk between LMO2 and the Wnt signaling pathway during tumorigenesis and suggested that LMO2 might be a tumor suppressor in certain solid tumors, in contrast to its traditional oncogenic role in the hematopoietic system.

LIM-Only Protein 4 (LMO4) and LIM Domain Binding Protein 1 (LDB1) Promote Growth and Metastasis of Human Head and Neck Cancer (LMO4 and LDB1 in Head and Neck Cancer)

Squamous cell carcinoma of the head and neck (HNSCC) accounts for more than 300,000 deaths worldwide per year as a consequence of tumor cell invasion of adjacent structures or metastasis. LIM-only protein 4 (LMO4) and LIM-domain binding protein 1 (LDB1), two directly interacting transcriptional adaptors that have important roles in normal epithelial cell differentiation, have been associated with increased metastasis, decreased differentiation, and shortened survival in carcinoma of the breast. Here, we implicate two LDB1-binding proteins, single-stranded binding protein 2 (SSBP2) and 3 (SSBP3), in controlling LMO4 and LDB1 protein abundance in HNSCC and in regulating specific tumor cell functions in this disease. First, we found that the relative abundance of LMO4, LDB1, and the two SSBPs correlated very significantly in a panel of human HNSCC cell lines. Second, expression of these proteins in tumor primaries and lymph nodes involved by metastasis were concordant in 3 of 3 sets of tissue. Third, using a Matrigel invasion and organotypic reconstruct assay, CRISPR/Cas9-mediated deletion of LDB1 in the VU-SCC-1729 cell line, which is highly invasive of basement membrane and cellular monolayers, reduced tumor cell invasiveness and migration, as well as proliferation on tissue culture plastic. Finally, inactivation of the LDB1 gene in these cells decreased growth and vascularization of xenografted human tumor cells in vivo. These data show that LMO4, LDB1, and SSBP2 and/or SSBP3 regulate metastasis, proliferation, and angiogenesis in HNSCC and provide the first evidence that SSBPs control LMO4 and LDB1 protein abundance in a cancer context.

Understanding the regulation of vertebrate hematopoiesis and blood disorders - big lessons from a small fish

Hematopoietic stem cells (HSCs) give rise to all differentiated blood cells. Understanding the mechanisms that regulate self-renewal and lineage specification of HSCs is key for developing treatments for many human diseases. Zebrafish have emerged as an excellent model for studying vertebrate hematopoiesis. This review will highlight the unique strengths of zebrafish and important findings that have emerged from studies of blood development and disorders using this system. We discuss recent advances in our understanding of hematopoiesis, including the origin of HSCs, molecular control of their development, and key signaling pathways involved in their regulation. We highlight significant findings from zebrafish models of blood disorders and discuss their application for investigating stem cell dysfunction in disease and for the development of new therapeutics.

LMO2 blocks the UBA6-USE1 interaction and downstream FAT10ylation by targeting the ubiquitin fold domain of UBA6

In eukaryotic cells, the post-translational modification of proteins by ubiquitin or ubiquitin-like proteins (UBLs) is the most common trigger for protein degradation and is involved in the regulation of a wide range of biological processes. FAT10 (HLA-F-adjacent transcript 10), which belongs to the UBL family, is activated specifically through the UBA6-USE1 cascade and targets substrates covalently for 26S proteasomal degradation. LMO2 is a well-recognized transcriptional regulator in hematopoietic and endothelial systems; however, it is predominantly located in the cytoplasm of epithelium-derived cells. The current study revealed that LMO2 protein interacted with the E1 ubiquitin-activating enzyme UBA6 at the C-terminal ubiquitin fold domain (UFD), which mediates the recognition and recruitment of the E2-conjugating enzyme USE1. Functionally, the LMO2-UBA6 interaction disturbed the interaction between UBA6 and USE1 and led to the decline of the overall cellular FAT10ylation level as well as the FAT10ylation and degradation of a known FAT10 substrate p62. Taken together, this study revealed a novel function of LMO2 involving in the regulatory hierarchy of UBA6-USE1-FAT10ylation pathway by targeting the E1 enzyme UBA6.

Beyond transcription factors: how oncogenic signalling reshapes the epigenetic landscape

Cancer, once thought to be caused largely by genetic alterations, is now considered to be a mixed genetic and epigenetic disease. The epigenetic landscape, which is dictated by covalent DNA and histone modifications, is profoundly altered in transformed cells. These abnormalities may arise from mutations in, or altered expression of, chromatin modifiers. Recent reports on the interplay between cellular signalling pathways and chromatin modifications add another layer of complexity to the already complex regulation of the epigenome. In this Review, we discuss these new studies and how the insights they provide can contribute to a better understanding of the molecular pathogenesis of neoplasia.

Specification and function of hemogenic endothelium during embryogenesis

Hemogenic endothelium is a specialized subset of developing vascular endothelium that acquires hematopoietic potential and can give rise to multilineage hematopoietic stem and progenitor cells during a narrow developmental window in tissues such as the extraembryonic yolk sac and embryonic aorta-gonad-mesonephros. Herein, we review current knowledge about the historical and developmental origins of hemogenic endothelium, the molecular events that govern hemogenic specification of vascular endothelial cells, the generation of multilineage hematopoietic stem and progenitor cells from hemogenic endothelium, and the potential for translational applications of knowledge gained from further study of these processes.

LMO2 at 25 years: a paradigm of chromosomal translocation proteins

LMO2 was first discovered through proximity to frequently occurring chromosomal translocations in T cell acute lymphoblastic leukaemia (T-ALL). Subsequent studies on its role in tumours and in normal settings have highlighted LMO2 as an archetypical chromosomal translocation oncogene, activated by association with antigen receptor gene loci and a paradigm for translocation gene activation in T-ALL. The normal function of LMO2 in haematopoietic cell fate and angiogenesis suggests it is a master gene regulator exerting a dysfunctional control on differentiation following chromosomal translocations. Its importance in T cell neoplasia has been further emphasized by the recurrent findings of interstitial deletions of chromosome 11 near LMO2 and of LMO2 as a target of retroviral insertion gene activation during gene therapy trials for X chromosome-linked severe combined immuno-deficiency syndrome, both types of event leading to similar T cell leukaemia. The discovery of LMO2 in some B cell neoplasias and in some epithelial cancers suggests a more ubiquitous function as an oncogenic protein, and that the current development of novel inhibitors will be of great value in future cancer treatment. Further, the role of LMO2 in angiogenesis and in haematopoietic stem cells (HSCs) bodes well for targeting LMO2 in angiogenic disorders and in generating autologous induced HSCs for application in various clinical indications.

High expression of LMO2 predicts a favorable outcome in adult patients with BCR/ABL negative B-cell acute lymphoblastic leukemia

The LIM domain only protein 2 (LMO2) is a key regulator of hematopoietic stem cell development. Expression of LMO2 has been evaluated in B-cell lymphoma, T-cell acute lymphoblastic leukemia and acute myeloid leukemia; however, information concerning its role in breakpoint cluster region/Abelson murine leukemia viral oncogene homolog 1 (BCR/ABL) negative B-cell acute lymphoblastic leukemia (B-ALL) remains limited. The present study investigated LMO2 expression using quantitative polymerase chain reaction in 85 adult patients with BCR/ABL negative B-ALL, and associated the expression of LMO2 with established prognostic factors. LMO2 expression levels in patients with BCR/ABL negative B-ALL was not significantly different compared with control individuals (P=0.25). However, LMO2 expression levels were associated with the immunophenotypical features of the patients; a high LMO2 expression was associated with a higher incidence of complete remission (P=0.03) and lower rate of relapse (P=0.01), and patients with a high LMO2 expression had a significantly improved overall survival rate (P=0.01) and disease-free survival (P=0.01). The present results suggest that LMO2 expression is a favorable prognostic marker in adult patients with BCR/ABL negative B-ALL and may be used as a diagnostic marker and therapeutic target. However, additional studies regarding its prognostic role in patients with BCR/ABL negative B-ALL are required.

LMO2 Oncoprotein Stability in T-Cell Leukemia Requires Direct LDB1 Binding

LMO2 is a component of multisubunit DNA-binding transcription factor complexes that regulate gene expression in hematopoietic stem and progenitor cell development. Enforced expression of LMO2 causes leukemia by inducing hematopoietic stem cell-like features in T-cell progenitor cells, but the biochemical mechanisms of LMO2 function have not been fully elucidated. In this study, we systematically dissected the LMO2/LDB1-binding interface to investigate the role of this interaction in T-cell leukemia. Alanine scanning mutagenesis of the LIM interaction domain of LDB1 revealed a discrete motif, R(320)LITR, required for LMO2 binding. Most strikingly, coexpression of full-length, wild-type LDB1 increased LMO2 steady-state abundance, whereas coexpression of mutant proteins deficient in LMO2 binding compromised LMO2 stability. These mutant LDB1 proteins also exerted dominant negative effects on growth and transcription in diverse leukemic cell lines. Mass spectrometric analysis of LDB1 binding partners in leukemic lines supports the notion that LMO2/LDB1 function in leukemia occurs in the context of multisubunit complexes, which also protect the LMO2 oncoprotein from degradation. Collectively, these data suggest that the assembly of LMO2 into complexes, via direct LDB1 interaction, is a potential molecular target that could be exploited in LMO2-driven leukemias resistant to existing chemotherapy regimens.

The LMO2 -25 Region Harbours GATA2-Dependent Myeloid Enhancer and RUNX-Dependent T-Lymphoid Repressor Activity

Lim domain only 2 (LMO2) is a transcriptional co-factor required for angiogenesis and the specification of haematopoietic cells during development. LMO2 is widely expressed within haematopoiesis with the exception of T-cells. Failure to downregulate LMO2 during T-cell maturation leads to leukaemia, thus underlining the critical nature of context-dependent regulation of LMO2 expression. We previously identified a distal regulatory element of LMO2 (element -25) that cooperates with the proximal promoter in directing haematopoietic expression. Here we dissected the functional activity of element -25 and showed it to consist of two modules that conferred independent and cell-type specific activities: a 3' myeloid enhancer and a 5' T-cell repressor. The myeloid enhancer was bound by GATA2 in progenitors and its activity depended on a highly conserved GATA motif, whereas the T-cell repressor moiety of element -25 was bound by the Core Binding Factor in T-cells and its repressive activity depended on a highly conserved RUNT motif. Since the myeloid enhancer and nearby downstream region is recurrently involved in oncogenic translocations, our data suggest that the -25 enhancer region provides an open chromatin environment prone to translocations, which in turn cause aberrant LMO2 expression in T-cells due to the removal of the adjacent T-cell repressor.