HOX-A10 regulates hematopoietic lineage commitment: evidence for a monocyte-specific transcription factor

Hox genes are crucial regulators of periosteal stem cell identity

Periosteal stem and progenitor cells (PSPCs) are major contributors to bone maintenance and repair. Deciphering the molecular mechanisms that regulate their function is crucial for the successful generation and application of future therapeutics. Here, we pinpoint Hox transcription factors as necessary and sufficient for periosteal stem cell function. Hox genes are transcriptionally enriched in periosteal stem cells and their overexpression in more committed progenitors drives reprogramming to a naïve, self-renewing stem cell-like state. Crucially, individual Hox family members are expressed in a location-specific manner and their stem cell-promoting activity is only observed when the Hox gene is matched to the anatomical origin of the PSPC, demonstrating a role for the embryonic Hox code in adult stem cells. Finally, we demonstrate that Hoxa10 overexpression partially restores the age-related decline in fracture repair. Together, our data highlight the importance of Hox genes as key regulators of PSPC identity in skeletal homeostasis and repair.

Intrathymic dendritic cell-biased precursors promote human T cell lineage specification through IRF8-driven transmembrane TNF

The cross-talk between thymocytes and thymic stromal cells is fundamental for T cell development. In humans, intrathymic development of dendritic cells (DCs) is evident but its physiological significance is unknown. Here we showed that DC-biased precursors depended on the expression of the transcription factor IRF8 to express the membrane-bound precursor form of the cytokine TNF (tmTNF) to promote differentiation of thymus seeding hematopoietic progenitors into T-lineage specified precursors through activation of the TNF receptor (TNFR)-2 instead of TNFR1. In vitro recapitulation of TNFR2 signaling by providing low-density tmTNF or a selective TNFR2 agonist enhanced the generation of human T cell precursors. Our study shows that, in addition to mediating thymocyte selection and maturation, DCs function as hematopoietic stromal support for the early stages of human T cell development and provide proof of concept that selective targeting of TNFR2 can enhance the in vitro generation of T cell precursors for clinical application.

Immune Biomarkers in Blood from Sarcoma Patients: A Pilot Study

The main role of the host immune system is to identify and eliminate cancer cells, which is a complex process, but it is not a fail-safe mechanism. Many sarcoma patients succumb to this disease despite treatments rendered. The aim of this pilot study was to compare the levels of CD4+ T-cells, T-regulatory (Treg) cells, and cytokines such as tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ), interleukin-17A (IL-17A), and transforming growth factor-beta-1 (TGF-β1) in peripheral blood leukocytes of sarcoma patients and healthy controls. For gene expression studies, total ribonucleic acid (RNA) was extracted from peripheral blood leukocytes and genes that were differentially regulated in peripheral blood leukocytes of sarcoma patients compared with healthy controls were determined using a commercial T-helper cell differentiation quantitative polymerase chain reaction (qPCR) array. Flow cytometer analysis was performed on blood samples from 26 sarcoma patients and 10 healthy controls to identify the levels of CD4+ T-cells and T-reg cells. The level of cytokines in plasma and culture supernatant were quantified using commercial enzyme-linked immunosorbent assay (ELISA) kits. A marked reduction in the percentage of CD4+ T-cells (p = 0.037) and levels of TNF-α (p = 0.004) and IFN-γ (0.010) was observed in sarcoma patients. Gene expression analysis showed five genes (homeobox A10 (HOXA10), GATA binding protein 3 (GATA3), prostaglandin D2 receptor 2 (PTGDR2), thymocyte selection associated high mobility group box (TOX), and C-C motif chemokine receptor 3 (CCR3)) were dysregulated (p < 0.05) in sarcoma patients. This study suggests that T-helper-1 immune responses are reduced in sarcoma patients.

Reynolds L, Enquobahrie DA, L. Fitzpatrick A, Kerr KF, J. Budoff M, Lee SI, Siscovick D, D. Kaufman J. Epigenome-wide analysis of long-term air pollution exposure and DNA methylation in monocytes: results from the Multi-Ethnic Study of Atherosclerosis

Air pollution might affect atherosclerosis through DNA methylation changes in cells crucial to atherosclerosis, such as monocytes. We conducted an epigenome-wide study of DNA methylation in CD14+ monocytes and long-term ambient air pollution exposure in adults participating in the Multi-Ethnic Study of Atherosclerosis (MESA). We also assessed the association between differentially methylated signals and cis-gene expression. Using spatiotemporal models, one-year average concentrations of outdoor fine particulate matter (PM_2.5) and oxides of nitrogen (NO_X) were estimated at participants' homes. We assessed DNA methylation and gene expression using Illumina 450k and HumanHT-12 v4 Expression BeadChips, respectively (n = 1,207). We used bump hunting and site-specific approaches to identify differentially methylated signals (false discovery rate of 0.05) and used linear models to assess associations between differentially methylated signals and cis-gene expression. Four differentially methylated regions (DMRs) located on chromosomes 5, 6, 7, and 16 (within or near SDHAP3, ZFP57, HOXA5, and PRM1, respectively) were associated with PM_2.5. The DMRs on chromosomes 5 and 6 also associated with NO_X. The DMR on chromosome 5 had the smallest p-value for both PM_2.5 (p = 1.4×10^-6) and NO_X (p = 7.7×10^-6). Three differentially methylated CpGs were identified for PM_2.5, and cg05926640 (near TOMM20) had the smallest p-value (p = 5.6×10^-8). NO_X significantly associated with cg11756214 within ZNF347 (p = 5.6×10^-8). Several differentially methylated signals were also associated with cis-gene expression. The DMR located on chromosome 7 was associated with the expression of HOXA5, HOXA9, and HOXA10. The DMRs located on chromosomes 5 and 16 were associated with expression of MRPL36 and DEXI, respectively. The CpG cg05926640 was associated with expression of ARID4B, IRF2BP2, and TOMM20. We identified differential DNA methylation in monocytes associated with long-term air pollution exposure. Methylation signals associated with gene expression might help explain how air pollution contributes to cardiovascular disease.

Recent Advancements in Regenerative Approaches for Thymus Rejuvenation

The thymus plays a key role in adaptive immunity by generating a diverse population of T cells that defend the body against pathogens. Various factors from disease and toxic insults contribute to the degeneration of the thymus resulting in a fewer output of T cells. Consequently, the body is prone to a wide host of diseases and infections. In this review, first, the relevance of the thymus is discussed, followed by thymic embryological organogenesis and anatomy as well as the development and functionality of T cells. Attempts to regenerate the thymus include in vitro methods, such as forming thymic organoids aided by biofabrication techniques that are transplantable. Ex vivo methods that have shown promise in enhancing thymic regeneration are also discussed. Current regenerative technologies have not yet matched the complexity and functionality of the thymus. Therefore, emerging techniques that have shown promise and the challenges that lie ahead are explored.

Computational Identification of Master Regulators Influencing Trypanotolerance in Cattle

African Animal Trypanosomiasis (AAT) is transmitted by the tsetse fly which carries pathogenic trypanosomes in its saliva, thus causing debilitating infection to livestock health. As the disease advances, a multistage progression process is observed based on the progressive clinical signs displayed in the host’s body. Investigation of genes expressed with regular monotonic patterns (known as Monotonically Expressed Genes (MEGs)) and of their master regulators can provide important clue for the understanding of the molecular mechanisms underlying the AAT disease. For this purpose, we analysed MEGs for three tissues (liver, spleen and lymph node) of two cattle breeds, namely trypanosusceptible Boran and trypanotolerant N’Dama. Our analysis revealed cattle breed-specific master regulators which are highly related to distinguish the genetic programs in both cattle breeds. Especially the master regulators MYC and DBP found in this study, seem to influence the immune responses strongly, thereby susceptibility and trypanotolerance of Boran and N’Dama respectively. Furthermore, our pathway analysis also bolsters the crucial roles of these master regulators. Taken together, our findings provide novel insights into breed-specific master regulators which orchestrate the regulatory cascades influencing the level of trypanotolerance in cattle breeds and thus could be promising drug targets for future therapeutic interventions.

Distinct and temporary-restricted epigenetic mechanisms regulate human αβ and γδ T cell development

The development of TCRαβ and TCRγδ T cells comprises a step-wise process in which regulatory events control differentiation and lineage outcome. To clarify these mechanisms, we employed RNA-sequencing, ATAC-sequencing and ChIPmentation on well-defined thymocyte subsets that represent the continuum of human T cell development. The chromatin accessibility dynamics show clear stage specificity and reveal that human T cell-lineage commitment is marked by GATA3- and BCL11B-dependent closing of PU.1 sites. A temporary increase in H3K27me3 without open chromatin modifications is unique for β-selection, whereas emerging γδ T cells, which originate from common precursors of β-selected cells, show large chromatin accessibility changes due to strong T cell receptor (TCR) signaling. Furthermore, we unravel distinct chromatin landscapes between CD4⁺ and CD8⁺ αβ-lineage cells that support their effector functions and reveal gene-specific mechanisms that define mature T cells. This resource provides a framework for studying gene regulatory mechanisms that drive normal and malignant human T cell development.

Distinct Notch1 and BCL11B requirements mediate human γδ/αβ T cell development

γδ and αβ T cells have unique roles in immunity and both originate in the thymus from T-lineage committed precursors through distinct but unclear mechanisms. Here, we show that Notch1 activation is more stringently required for human γδ development compared to αβ-lineage differentiation and performed paired mRNA and miRNA profiling across 11 discrete developmental stages of human T cell development in an effort to identify the potential Notch1 downstream mechanism. Our data suggest that the miR-17-92 cluster is a Notch1 target in immature thymocytes and that miR-17 can restrict BCL11B expression in these Notch-dependent T cell precursors. We show that enforced miR-17 expression promotes human γδ T cell development and, consistently, that BCL11B is absolutely required for αβ but less for γδ T cell development. This study suggests that human γδ T cell development is mediated by a stage-specific Notch-driven negative feedback loop through which miR-17 temporally restricts BCL11B expression and provides functional insights into the developmental role of the disease-associated genes BCL11B and the miR-17-92 cluster in a human context.

Integrated scRNA-Seq Identifies Human Postnatal Thymus Seeding Progenitors and Regulatory Dynamics of Differentiating Immature Thymocytes

During postnatal life, thymopoiesis depends on the continuous colonization of the thymus by bone-marrow-derived hematopoietic progenitors that migrate through the bloodstream. The current understanding of the nature of thymic immigrants is largely based on data from pre-clinical models. Here, we employed single-cell RNA sequencing (scRNA-seq) to examine the immature postnatal thymocyte population in humans. Integration of bone marrow and peripheral blood precursor datasets identified two putative thymus seeding progenitors that varied in expression of CD7; CD10; and the homing receptors CCR7, CCR9, and ITGB7. Whereas both precursors supported T cell development, only one contributed to intrathymic dendritic cell (DC) differentiation, predominantly of plasmacytoid dendritic cells. Trajectory inference delineated the transcriptional dynamics underlying early human T lineage development, enabling prediction of transcription factor (TF) modules that drive stage-specific steps of human T cell development. This comprehensive dataset defines the expression signature of immature human thymocytes and provides a resource for the further study of human thymopoiesis.

Clonal copy-number mosaicism in autoreactive T lymphocytes in diabetic NOD mice

Concordance for type 1 diabetes (T1D) is far from 100% in monozygotic twins and in inbred nonobese diabetic (NOD) mice, despite genetic identity and shared environment during incidence peak years. This points to stochastic determinants, such as postzygotic mutations (PZMs) in the expanding antigen-specific autoreactive T cell lineages, by analogy to their role in the expanding tumor lineage in cancer. Using comparative genomic hybridization of DNA from pancreatic lymph-node memory CD4⁺ T cells of 25 diabetic NOD mice, we found lymphocyte-exclusive mosaic somatic copy-number aberrations (CNAs) with highly nonrandom independent involvement of the same gene(s) across different mice, some with an autoimmunity association (e.g., Ilf3 and Dgka). We confirmed genes of interest using the gold standard approach for CNA quantification, multiplex ligation-dependent probe amplification (MLPA), as an independent method. As controls, we examined lymphocytes expanded during normal host defense (17 NOD and BALB/c mice infected with Leishmania major parasite). Here, CNAs found were fewer and significantly smaller compared to those in autoreactive cells (P = 0.0019). We determined a low T cell clonality for our samples suggesting a prethymic formation of these CNAs. In this study, we describe a novel, unexplored phenomenon of a potential causal contribution of PZMs in autoreactive T cells in T1D pathogenesis. We expect that exploration of point mutations and studies in human T cells will enable the further delineation of driver genes to target for functional studies. Our findings challenge the classical notions of autoimmunity and open conceptual avenues toward individualized prevention and therapeutics.

Evolving DNA methylation and gene expression markers of B-cell chronic lymphocytic leukemia are present in pre-diagnostic blood samples more than 10 years prior to diagnosis

BACKGROUND

B-cell chronic lymphocytic leukemia (CLL) is a common type of adult leukemia. It often follows an indolent course and is preceded by monoclonal B-cell lymphocytosis, an asymptomatic condition, however it is not known what causes subjects with this condition to progress to CLL. Hence the discovery of prediagnostic markers has the potential to improve the identification of subjects likely to develop CLL and may also provide insights into the pathogenesis of the disease of potential clinical relevance.

RESULTS

We employed peripheral blood buffy coats of 347 apparently healthy subjects, of whom 28 were diagnosed with CLL 2.0-15.7 years after enrollment, to derive for the first time genome-wide DNA methylation, as well as gene and miRNA expression, profiles associated with the risk of future disease. After adjustment for white blood cell composition, we identified 722 differentially methylated CpG sites and 15 differentially expressed genes (Bonferroni-corrected p < 0.05) as well as 2 miRNAs (FDR < 0.05) which were associated with the risk of future CLL. The majority of these signals have also been observed in clinical CLL, suggesting the presence in prediagnostic blood of CLL-like cells. Future CLL cases who, at enrollment, had a relatively low B-cell fraction (<10%), and were therefore less likely to have been suffering from undiagnosed CLL or a precursor condition, showed profiles involving smaller numbers of the same differential signals with intensities, after adjusting for B-cell content, generally smaller than those observed in the full set of cases. A similar picture was obtained when the differential profiles of cases with time-to-diagnosis above the overall median period of 7.4 years were compared with those with shorted time-to-disease. Differentially methylated genes of major functional significance include numerous genes that encode for transcription factors, especially members of the homeobox family, while differentially expressed genes include, among others, multiple genes related to WNT signaling as well as the miRNAs miR-150-5p and miR-155-5p.

CONCLUSIONS

Our findings demonstrate the presence in prediagnostic blood of future CLL patients, more than 10 years before diagnosis, of CLL-like cells which evolve as preclinical disease progresses, and point to early molecular alterations with a pathogenetic potential.

Notch3 activation is sufficient but not required for inducing human T-lineage specification

Although the role for the individual Notch receptors in early hematopoiesis have been thoroughly investigated in mouse, studies in human have been mostly limited to the use of pan-Notch inhibitors. However, such studies in human are important to predict potential side effects of specific Notch receptor blocking reagents because these are currently being considered as therapeutic tools to treat various Notch-dependent diseases. In this study, we studied the individual roles of Notch1 and Notch3 in early human hematopoietic lineage decisions, particularly during T-lineage specification. Although this process in mice is solely dependent on Notch1 activation, we recently reported Notch3 expression in human uncommitted thymocytes, raising the possibility that Notch3 mediates human T-lineage specification. Although expression of a constitutive activated form of Notch3 (ICN3) results in the induction of T-lineage specification in human CD34(+) hematopoietic progenitor cells, similar to ICN1 overexpression, loss-of-function studies using blocking Abs reveal that only Notch1, but not Notch3, is critical in this process. Blocking of Notch1 activation in OP9-DLL4 cocultures resulted in a complete block in T-lineage specification and induced monocytic and plasmacytoid dendritic cell differentiation instead. In fetal thymus organ cultures, impeded Notch1 activation resulted in B and dendritic cell development. In contrast, Notch3 blocking Abs only marginally affected T-lineage specification and hematopoietic differentiation with a slight increase in monocyte development. No induction of B or dendritic cell development was observed. Thus, our results unambiguously reveal a nonredundant role for Notch1 in human T-lineage specification, despite the expression of other Notch receptors.

Transcriptional repression of CDKN2D by PML/RARα contributes to the altered proliferation and differentiation block of acute promyelocytic leukemia cells

Cell proliferation and differentiation are highly coordinated processes. These two processes are disrupted during leukemogenesis, resulting in differentiation block and uncontrolled proliferation in leukemia. To understand the mechanisms disrupting the coordination between the two processes in acute promyelocytic leukemia (APL), we investigated the regulatory mechanism of the negative cell cycle regulator CDKN2D by the promyelocytic leukemia/retinoic acid receptor α (PML/RARα) fusion protein and the role of CDKN2D in cell differentiation and proliferation. We found that CDKN2D expression in APL cells was significantly lower than that in normal promyelocytes. By chromatin immunoprecipitation and luciferase reporter assays, we showed that PML/RARα directly bound to and inhibited the transactivation of the CDKN2D promoter. Further evidence by the truncated and mutated CDKN2D promoters revealed that the everted repeat 8 (ER8) motif on the promoter was the binding site of PML/RARα. Forced expression of CDKN2D induced G0/G1 phase arrest and partial granulocytic differentiation in APL-derived NB4 cells, suggesting the function of CDKN2D in regulating both cell proliferation and granulocytic differentiation. Furthermore, all-trans retinoic acid (ATRA) significantly induced CDKN2D expression in APL cells and knockdown of CDKN2D expression during ATRA treatment partially blocked the ATRA-induced differentiation and cell cycle arrest. Collectively, our data indicate that CDKN2D repression by PML/RARα disrupts both cell proliferation and differentiation in the pathogenesis of APL, and induced expression of CDKN2D by ATRA alleviates the disruption of both processes to ensure treatment efficiency. This study provides a mechanism for coupling proliferation and differentiation in leukemic cells through the action of CDKN2D.

Adaptive evolution of the Hox gene family for development in bats and dolphins

Bats and cetaceans (i.e., whales, dolphins, porpoises) are two kinds of mammals with unique locomotive styles and occupy novel niches. Bats are the only mammals capable of sustained flight in the sky, while cetaceans have returned to the aquatic environment and are specialized for swimming. Associated with these novel adaptations to their environment, various development changes have occurred to their body plans and associated structures. Given the importance of Hox genes in many aspects of embryonic development, we conducted an analysis of the coding regions of all Hox gene family members from bats (represented by Pteropus vampyrus, Pteropus alecto, Myotis lucifugus and Myotis davidii) and cetaceans (represented by Tursiops truncatus) for adaptive evolution using the available draft genome sequences. Differences in the selective pressures acting on many Hox genes in bats and cetaceans were found compared to other mammals. Positive selection, however, was not found to act on any of the Hox genes in the common ancestor of bats and only upon Hoxb9 in cetaceans. PCR amplification data from additional bat and cetacean species, and application of the branch-site test 2, showed that the Hoxb2 gene within bats had significant evidence of positive selection. Thus, our study, with genomic and newly sequenced Hox genes, identifies two candidate Hox genes that may be closely linked with developmental changes in bats and cetaceans, such as those associated with the pancreatic, neuronal, thymus shape and forelimb. In addition, the difference in our results from the genome-wide scan and newly sequenced data reveals that great care must be taken in interpreting results from draft genome data from a limited number of species, and deep genetic sampling of a particular clade is a powerful tool for generating complementary data to address this limitation.

Notch signaling during human T cell development

Notch signaling is critical during multiple stages of T cell development in both mouse and human. Evidence has emerged in recent years that this pathway might regulate T-lineage differentiation differently between both species. Here, we review our current understanding of how Notch signaling is activated and used during human T cell development. First, we set the stage by describing the developmental steps that make up human T cell development before describing the expression profiles of Notch receptors, ligands, and target genes during this process. To delineate stage-specific roles for Notch signaling during human T cell development, we subsequently try to interpret the functional Notch studies that have been performed in light of these expression profiles and compare this to its suggested role in the mouse.

Hox transcription factor regulation of adult bone-marrow-derived cell behaviour during tissue repair and regeneration

INTRODUCTION

Bone marrow offers a valuable source of stem/progenitor cells that contribute to the repair of injured tissues. Failure in the function of these cells results in delayed or reduced tissue repair. Identification of factors that can correct these defects is critical to treating the underlying dysfunction. Notably, homeobox (Hox) transcription factors have been identified as having significant effects on BMDC behaviour, including differentiation, migration and adhesion in injured tissue, and may provide a basis for future therapies.

AREAS COVERED

Hox protein regulation of bone-marrow-derived cell (BMDC) differentiation, factors that influence BMDC behaviour in response to injury, the effects of the diabetic environment on BMDCs, methods that can be used to reprogramme BMDCs, and the use of Hox transcription factors to correct BMDC behaviour.

EXPERT OPINION

Hox gene therapy has been successfully employed to change cell behaviour using ex vivo 'reprogramming' strategies overexpressing selected Hox genes in BMDCs to direct the fate of these cells to the desired cell type, promoting tissue repair.

Expression profiling of cytogenetically normal acute myeloid leukemia identifies microRNAs that target genes involved in monocytic differentiation

MicroRNAs are short ribonucleic acids (RNAs) that play an important role in many aspects of cellular biology such as differentiation and apoptosis, due to their role in the regulation of gene expression. Using microRNA microarrays, we characterized the microRNA gene expression of 27 patients with acute myeloid leukemia (AML) with normal cytogenetics, focusing on the microRNAs differentially expressed between the M1 and M5 French-American-British (FAB) subtypes. An accurate delineation of these two AML entities was observed based on the expression of 12 microRNAs. We hypothesized that these microRNAs may potentially be involved in the differentiation block of M1 blasts and consequently monocytic differentiation. Using publically available mRNA data and microRNA target prediction software, we identified several key myeloid factors that may be targeted by our candidate microRNAs. The expression changes of the candidate microRNAs during monocytic differentiation of AML cell lines treated with Vitamin D and phorbol 12-myristate 13-acetate were examined. All six candidate microRNAs were significantly down-regulated over the time course by quantitative reverse transcriptase polymerase chain reaction suggesting a link between these microRNAs and monocytic differentiation. To further characterize these microRNAs, we confirmed by luciferase assays that these microRNA target several key myeloid factors such as MAFB, IRF8, and KLF4 identifying a possible mechanism for the control of differentiation by these microRNAs.

Targeting iron homeostasis induces cellular differentiation and synergizes with differentiating agents in acute myeloid leukemia

Differentiating agents have been proposed to overcome the impaired cellular differentiation in acute myeloid leukemia (AML). However, only the combinations of all-trans retinoic acid or arsenic trioxide with chemotherapy have been successful, and only in treating acute promyelocytic leukemia (also called AML3). We show that iron homeostasis is an effective target in the treatment of AML. Iron chelating therapy induces the differentiation of leukemia blasts and normal bone marrow precursors into monocytes/macrophages in a manner involving modulation of reactive oxygen species expression and the activation of mitogen-activated protein kinases (MAPKs). 30% of the genes most strongly induced by iron deprivation are also targeted by vitamin D3 (VD), a well known differentiating agent. Iron chelating agents induce expression and phosphorylation of the VD receptor (VDR), and iron deprivation and VD act synergistically. VD magnifies activation of MAPK JNK and the induction of VDR target genes. When used to treat one AML patient refractory to chemotherapy, the combination of iron-chelating agents and VD resulted in reversal of pancytopenia and in blast differentiation. We propose that iron availability modulates myeloid cell commitment and that targeting this cellular differentiation pathway together with conventional differentiating agents provides new therapeutic modalities for AML.

Rho GTPase Cdc42 is essential for human T-cell development

BACKGROUND

Rho GTPases are involved in the regulation of many cell functions, including some related to the actin cytoskeleton. Different Rho GTPases have been shown to be important for T-cell development in mice. However, their role in human T-cell development has not yet been explored.

DESIGN AND METHODS

We examined the expression and activation of Rho GTPases along different stages of T-cell development in the human thymus. Early stage human thymocytes were transduced with constitutively active and dominant negative mutants of different Rho GTPases to explore their role in human T-cell development, as analyzed in fetal thymus organ cultures. The use of these mutants as well as Rho GTPase-specific inhibitors allowed us to explore the role of GTPases in thymocyte migration.

RESULTS

We found that the expression of several Rho GTPases is differently regulated during successive stages of T-cell development in man, suggesting a specific role in human thymopoiesis. In chimeric fetal thymus organ culture, T-cell development was not or only mildly affected by expression of dominant negative Rac1 and Rac2, but was severely impaired in the presence of dominant negative Cdc42, associated with enhanced apoptosis and reduced proliferation. Kinetic analysis revealed that Cdc42 is necessary in human T-cell development both before and after expression of the pre-T-cell receptor. Using inhibitors and retrovirally transferred mutants of the aforementioned Rho GTPases, we showed that only Rac1 is necessary for migration of different thymocyte subsets, including the early CD34(+) fraction, towards stromal cell-derived factor-1 alpha. Constitutively active mutants of Rac1, Rac2 and Cdc42 all impaired migration towards stromal cell-derived factor-1 alpha and T-cell development to different degrees.

CONCLUSIONS

This is the first report on Rho GTPases in human T-cell development, showing the essential role of Cdc42. Our data suggest that enhanced apoptotic death and reduced proliferation rather than disturbed migration explains the decreased thymopoiesis induced by dominant negative Cdc42.

A NUP98-HOXD13 fusion gene impairs differentiation of B and T lymphocytes and leads to expansion of thymocytes with partial TCRB gene rearrangement

Expression of a NUP98-HOXD13 (NHD13) fusion gene leads to myelodysplastic syndrome in mice. In addition to ineffective hematopoiesis, we observed that NHD13 mice were lymphopenic; the lymphopenia was due to a decrease in both T and B lymphocytes. Although the pro-B cell (B220(+)/CD43(+)) populations from the NHD13 and wild-type mice were similar, the NHD13 mice showed decreased pre-B cells (B220(+)/CD43(-)), indicating impaired differentiation at the pro-B to pre-B stage. Thymi from NHD13 mice were smaller and overexpressed Hoxa cluster genes, including Hoxa7, Hoxa9, and Hoxa10. In addition, the NHD13 thymi contained fewer thymocytes, with an increased percentage of CD4(-)/CD8(-) (double-negative (DN)) cells and a decreased percentage of CD4(+)/CD8(+) (double-positive) cells; the DN1/DN2 population was increased and the DN3/DN4 population was decreased, suggesting a partial block at the DN2 to DN3 transition. To determine clonality of the thymocytes, we used degenerate RT-PCR to identify clonal Tcrb gene rearrangements. Five of six NHD13 thymi showed an unusual Tcrb gene rearrangement pattern with common, clonal DJ rearrangements, but distinct V-D junctions, suggesting a marked clonal expansion of thymocytes that had undergone a DJ rearrangement, but not completed a VDJ rearrangement. Taken together, these findings demonstrate that expression of the NHD13 transgene inhibits lymphoid as well as myeloid and erythroid differentiation, results in overexpression of Hoxa cluster genes, and leads to a precursor T cell lymphoblastic leukemia/lymphoma.

Quantitative analysis of mRNA transcripts of Hox, SHH, PTCH, Wnt, and Fzd genes in canine hematopoietic progenitor cells and various in vitro colonies differentiated from the cells

Homeobox (Hox), Sonic hedgehog (SHH), and Wingless-type MMTV integration site family (Wnt) are known to modulate the self-renewal and expansion of hematopoietic progenitor/stem cells in humans and mice. Frizzled (Fzd) and Patched1 (PTCH1) represent the receptors of Wnt and SHH, respectively. In this study, the amounts of mRNA transcripts of the genes associated with the self-renewal of hematopoietic stem cells, HoxB3, HoxB4, HoxA10, Wnt5a, Wnt2b, Fzd1, Fzd6, SHH, and PTCH1, were measured in canine unfractionated bone marrow cells, CD34-enriched cells, and various colony-forming units in culture (CFU-C). Partial cDNA sequences of these 9 canine genes were determined in this study. Quantitative real-time polymerase chain reaction was employed to indicate their relative amounts of mRNA transcripts. Amounts of mRNA transcripts of HoxB3, HoxA10, PTCH1, and Wnt5a genes in canine CD34-enriched cell fraction were significantly larger than those in the CD34-depleted cell fraction. Amounts of mRNA transcripts of HoxB3, HoxA10, PTCH1, Wnt5a, and Wnt2b genes in various CFU-C cells were significantly smaller than those in the seeded CD34-enriched cell fraction. These results suggested important roles of the products of these genes in self-renewal, expansion, and survival of hematopoietic progenitor cells in dogs as shown in humans and rodents.

Notch signaling is required for proliferation but not for differentiation at a well-defined beta-selection checkpoint during human T-cell development

Notch signaling is absolutely required for beta-selection during mouse T-cell development, both for differentiation and proliferation. In this report, we investigated whether Notch has an equally important role during human T-cell development. We show that human CD34(+) thymocytes can differentiate into CD4(+)CD8beta(+) double positive (DP) thymocytes in the absence of Notch signaling. While these DP cells phenotypically resemble human beta-selected cells, they lack a T-cell receptor (TCR)-beta chain. Therefore, we characterized the beta-selection checkpoint in human T-cell development, using CD28 as a differential marker at the immature single positive CD4(+)CD3(-)CD8alpha(-) stage. Through intracellular TCR-beta staining and gene expression analysis, we show that CD4(+)CD3(-)CD8alpha(-)CD28(+) thymocytes have passed the beta-selection checkpoint, in contrast to CD4(+)CD3(-)CD8alpha(-)CD28(-) cells. These CD4(+)CD3(-)CD8alpha(-)CD28(+) thymocytes can efficiently differentiate into CD3(+)TCRalphabeta(+) human T cells in the absence of Notch signaling. Importantly, preselection CD4(+)CD3(-)CD8alpha(-)CD28(-) thymocytes can also differentiate into CD3(+)TCRalphabeta(+) human T cells without Notch activation when provided with a rearranged TCR-beta chain. Proliferation of human thymocytes, however, is clearly Notch-dependent. Thus, we have characterized the beta-selection checkpoint during human T-cell development and show that human thymocytes require Notch signaling for proliferation but not for differentiation at this stage of development.

The vitamin D3/Hox-A10 pathway supports MafB function during the monocyte differentiation of human CD34+ hemopoietic progenitors

Although a considerable number of reports indicate an involvement of the Hox-A10 gene in the molecular control of hemopoiesis, the conclusions of such studies are quite controversial given that they support, in some cases, a role in the stimulation of stem cell self-renewal and myeloid progenitor expansion, whereas in others they implicate this transcription factor in the induction of monocyte-macrophage differentiation. To clarify this issue, we analyzed the biological effects and the transcriptome changes determined in human primary CD34(+) hemopoietic progenitors by retroviral transduction of a full-length Hox-A10 cDNA. The results obtained clearly indicated that this homeogene is an inducer of monocyte differentiation, at least partly acting through the up-regulation of the MafB gene, recently identified as the master regulator of such a maturation pathway. By using a combined approach based on computational analysis, EMSA experiments, and luciferase assays, we were able to demonstrate the presence of a Hox-A10-binding site in the promoter region of the MafB gene, which suggested the likely molecular mechanism underlying the observed effect. Stimulation of the same cells with the vitamin D(3) monocyte differentiation inducer resulted in a clear increase of Hox-A10 and MafB transcripts, indicating the existence of a precise transactivation cascade involving vitamin D(3) receptor, Hox-A10, and MafB transcription factors. Altogether, these data allow one to conclude that the vitamin D(3)/Hox-A10 pathway supports MafB function during the induction of monocyte differentiation.

Differential expression of HOX genes upon activation of leukocyte sub-populations

The HOX genes are key determinants of cellular identity both in early development and in the renewal and differentiation of adult blood cells. Although a number of studies have examined the expression of individual HOX genes in defined blood cell lineages, we have undertaken a comprehensive analysis of HOX gene expression in resting and activated lymphocytic and monocytic subpopulations. This has revealed distinct patterns of expression between different cell types and resting and activated states. (Main category A: Erythrocytes, Leukocytes and Hematopoiesis, subcategory: 8: Lymphocytes).

The recurrent SET-NUP214 fusion as a new HOXA activation mechanism in pediatric T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is mostly characterized by specific chromosomal abnormalities, some occurring in a mutually exclusive manner that possibly delineate specific T-ALL subgroups. One subgroup, including MLL-rearranged, CALM-AF10 or inv (7)(p15q34) patients, is characterized by elevated expression of HOXA genes. Using a gene expression-based clustering analysis of 67 T-ALL cases with recurrent molecular genetic abnormalities and 25 samples lacking apparent aberrations, we identified 5 new patients with elevated HOXA levels. Using microarray-based comparative genomic hybridization (array-CGH), a cryptic and recurrent deletion, del (9)(q34.11q34.13), was exclusively identified in 3 of these 5 patients. This deletion results in a conserved SET-NUP214 fusion product, which was also identified in the T-ALL cell line LOUCY. SET-NUP214 binds in the promoter regions of specific HOXA genes, where it interacts with CRM1 and DOT1L, which may transcriptionally activate specific members of the HOXA cluster. Targeted inhibition of SET-NUP214 by siRNA abolished expression of HOXA genes, inhibited proliferation, and induced differentiation in LOUCY but not in other T-ALL lines. We conclude that SET-NUP214 may contribute to the pathogenesis of T-ALL by enforcing T-cell differentiation arrest.

HOXA10 expression induced by Abl kinase inhibitors enhanced apoptosis through PI3K pathway in CML cells

Chronic myelogenous leukemia is characterized by the reciprocal chromosomal translocation (9;22), which generates a novel fusion gene, BCR-ABL. Bcr-Abl-expressing leukemia cells are highly resistant to apoptosis. Imatinib an Abl kinase inhibitor, is a highly effective agent for patients with CML. However, a small percentage of these patients and most advanced-phase patients relapse on imatinib therapy. It is poorly understood whether the Abl kinase inhibitors are able to eradicate CML progenitor or stem cells. In this study, we investigated the role of HOXA10 in CML cell lines and the hematopoietic progenitor cells derived from CML patients, and whether the regulation of HOXA10 eradicates Bcr-Abl(+) hematopoietic stem/progenitor cells. The Abl kinase inhibitors and PI3K inhibitor, LY294002, induced the expression of HOXA10, and it enhanced apoptosis in CML cells. Moreover, the reduction of HOXA10 expression by siRNA in CML cells inhibited apoptosis by treatment with the Abl kinase inhibitors and LY294002. These results revealed that HOXA10 had an important role in induction of apoptosis by the Abl kinase inhibitors in CML cells. Finally, we showed that the inhibition of HOXA10 expression by siRNA increased the numbers of CFU-GEMM, BFU-E, and CFU-GM when the cells were treated with the combination of BMS354825 and LY294002 compared to control cells, and HOXA10 played a critical role in the committed colony-formation in CML. This study shows for the first time that the Abl kinase inhibitor and LY294002 induced HOXA10, and HOXA10 had an important role in apoptosis or cell growth inhibition in CML cells in vitro.

Efficient gene transfer in CLL by mRNA electroporation

Chronic lymphocytic leukemia (CLL) consists of at least two major prognostic subgroups, characterized by different cellular and molecular markers. This observation sparked studies on the function and clinical importance of these markers. In order to address their function adequately, an efficient and reliable method for gene transfer is needed. In this study, we compared efficiency and utility of different gene transfer techniques in CLL. Lenti-, retro- and adenoviral transduction did not yield appreciable numbers of marker gene enhanced green fluorescent protein (EGFP) positive CLL cells, despite various prestimulation protocols. Efficient transgene expression was observed after nucleofection of CLL cells with plasmid DNA, at the expense of low survival rates. After optimization, electroporation of in vitro transcribed mRNA yielded up to 90% EGFP+CLL cells without affecting survival. Transgene expression remained detectable for at least 2 weeks after electroporation. Furthermore, we could demonstrate overexpression of ZAP70 and of a ZAP70-EGFP fusion protein after electroporation with ZAP70 or ZAP70-EGFP mRNA. We conclude that mRNA electroporation is a novel and straightforward method for highly efficient gene transfer in CLL. The application of this technique should facilitate functional studies on CLL cells, as well as clinical research.

Characterization of monocyte-macrophage-lineage cells induced from CD34+ bone marrow cells in vitro

We characterized the expression of cell surface antigens and cytokine-secreting ability of monocyte-macrophage-lineage cells induced in vitro from CD34+ bone marrow cells. After cultivation for 3 weeks, we observed 2 distinct cell fractions: a floating small, round cell fraction and an adherent large, protruding cell fraction. Both cell fractions expressed myelocyte-monocyte-lineage antigens, but mature-macrophage markers such as CD206 were expressed only by the adherent cells. An assessment of cells cultured for 5 weeks revealed spontaneous secretion of interleukin 8 (IL-8) and IL-6, and lipopolysaccharide (LPS)-induced tumor necrosis factor alpha (TNF-alpha) secretion in both fractions, but only the adherent cell fraction secreted IL-10 after LPS stimulation. In contrast, both fractions of cells cultured for 3 weeks spontaneously secreted low levels of IL-8, but none of the other cytokines. Upon LPS stimulation, the cells secreted IL-6 and TNF-alpha, but not IL-10. We also assessed the effect of granulocyte colony-stimulating factor (G-CSF) pretreatment on TNF-alpha secretion by each cell fraction and found that G-CSF reduced TNF-alpha secretion only in the adherent fraction of cells cultured for 3 weeks. Monocyte-macrophage-lineage cells induced in vitro should provide an ideal model for functional analysis of monocyte-macrophage cells.

HOXA10 is a critical regulator for hematopoietic stem cells and erythroid/megakaryocyte development

The Homeobox (Hox) transcription factors are important regulators of normal and malignant hematopoiesis because they control proliferation, differentiation, and self-renewal of hematopoietic cells at different levels of the hematopoietic hierarchy. In transgenic mice we show that the expression of HOXA10 is tightly regulated by doxycycline. Intermediate concentrations of HOXA10 induced a 15-fold increase in the repopulating capacity of hematopoietic stem cells (HSCs) after 13 days of in vitro culture. Notably, the proliferation induction of HSC by HOXA10 was dependent on the HOXA10 concentration, because high levels of HOXA10 had no effect on HSC proliferation. Furthermore, high levels of HOXA10 blocked erythroid and megakaryocyte development, demonstrating that tight regulation of HOXA10 is critical for normal development of the erythroid and megakaryocytic lineages. The HOXA10-mediated effects on hematopoietic cells were associated with altered expression of genes that govern stem-cell self-renewal and lineage commitment (eg, hepatic leukemia factor [HlF], Dickkopf-1 [Dkk-1], growth factor independent-1 [Gfi-1], and Gata-1). Interestingly, binding sites for HOXA10 were found in HLF, Dkk-1, and Gata-1, and Dkk-1 and Gfi-1 were transcriptionally activated by HOXA10. These findings reveal novel molecular pathways that act downstream of HOXA10 and identify HOXA10 as a master regulator of postnatal hematopoietic development.

Tfe3 expression is closely associated to macrophage terminal differentiation of human hematopoietic myeloid precursors

The MItf-Tfe family of basic helix-loop-helix leucine zipper (bHLH-Zip) transcription factors encodes four family members: MItf, Tfe3, TfeB and TfeC. In vitro, each protein of the family binds DNA in a homo- or heterodimeric form with other family members. Tfe3 is involved in chromosomal translocations recurrent in different tumors and it has been demonstrated, by in vivo studies, that it plays, redundantly with MItf, an important role in the process of osteoclast formation, in particular during the transition from mono-nucleated to multi-nucleated osteoclasts. Since mono-nucleated osteoclasts derive from macrophages we investigated whether Tfe3 might play a role upstream during hematopoietic differentiation. Here we show that Tfe3 is able to induce mono-macrophagic differentiation of U937 cells, in association with a decrease of cell proliferation and an increase of apoptosis. We also show that Tfe3 does not act physiologically during commitment of CD34+ hematopoietic stem cells (HSCs), since it is not able to direct HSCs toward a specific lineage as observed by clonogenic assay, but is a strong actor of terminal differentiation since it allows human primary myeloblasts' maturation toward the macrophage lineage.

Identification of Notch target genes in uncommitted T-cell progenitors: No direct induction of a T-cell specific gene program

Deregulated Notch signaling occurs in the majority of human T-ALL. During normal lymphoid development, activation of the Notch signaling pathway poses a T-cell fate on hematopoietic progenitors. However, the transcriptional targets of the Notch pathway are largely unknown. We sought to identify Notch target genes by inducing Notch signaling in human hematopoietic progenitors using two different methods: an intracellular signal through transfection of activated Notch and a Notch-receptor dependent signal by interaction with its ligand Delta1. Gene expression profiles were generated and evaluated with respect to expression profiles of immature thymic subpopulations. We confirmed HES1, NOTCH1 and NRARP as Notch target genes, but other reported Notch targets, including the genes for Deltex1, pre-T-cell receptor alpha and E2A, were not found to be differentially expressed. Remarkably, no induction of T-cell receptor gene rearrangements or transcription of known T-cell specific genes was found after activation of the Notch pathway. A number of novel Notch target genes, including the transcription factor TCFL5 and the HOXA cluster, were identified and functionally tested. Apparently, Notch signaling is essential to open the T-cell pathway, but does not initiate the T-cell program itself.

Notch 1 activation in the molecular pathogenesis of T-cell acute lymphoblastic leukaemia

The chromosomal translocation t(7;9) in human T-cell acute lymphoblastic leukaemia (T-ALL) results in deregulated expression of a truncated, activated form of Notch 1 (TAN1) under the control of the T-cell receptor-beta (TCRB) locus. Although TAN1 efficiently induces T-ALL in mouse models, t(7;9) is present in less than 1% of human T-ALL cases. The recent discovery of novel activating mutations in NOTCH1 in more than 50% of human T-ALL samples has made it clear that Notch 1 is far more important in human T-ALL pathogenesis than previously suspected.

Virally mediated MafB transduction induces the monocyte commitment of human CD34+ hematopoietic stem/progenitor cells

Upregulation of specific transcription factors is a generally accepted mechanism to explain the commitment of hematopoietic stem cells along precise maturation lineages. Based on this premise, transduction of primary hematopoietic stem/progenitor cells with viral vectors containing the investigated transcription factors appears as a suitable experimental model to identify such regulators. Although MafB transcription factor is believed to play a role in the regulation of monocytic commitment, no demonstration is, to date, available supporting this function in normal human hematopoiesis. To address this issue, we retrovirally transduced cord blood CD34+ hematopoietic progenitors with a MafB cDNA. Immunophenotypic and morphological analysis of transduced cells demonstrated the induction of a remarkable monomacrophage differentiation. Microarray analysis confirmed these findings and disclosed the upregulation of macrophage-related transcription factors belonging to the AP-1, MAF, PPAR and MiT families. Altogether our data allow to conclude that MafB is a key regulator of human monocytopoiesis.

Dissociation of expression patterns of homeodomain transcription factors in the evolution of developmental mode in the sea urchins Heliocidaris tuberculata and H. erythrogramma

The direct-developing sea urchin species Heliocidaris erythrogramma has a radically modified ontogeny. Along with gains of novel features, its entire ectoderm has been reorganized, resulting in the apparent absence of a differentiated oral ectoderm, a major module present in the pluteus of indirect-developing species, such as H. tuberculata. The restoration of an obvious oral ectoderm in H. erythrogrammaxH. tuberculata hybrids, indicates the action of dominant regulatory factors from the H. tuberculata genome. We sought candidate regulatory genes based on the prediction that they should include genes that govern development of the oral ectoderm in the pluteus, but play different roles in H. erythrogramma. Such genes may have a large effect in the evolution of development. Goosecoid (Gsc), Msx, and the sea urchin Abd-B-like gene (Hox11/13b) are present and expressed in both species and the hybrid embryos. Both Gsc and Msx are oral ectoderm specific in H. tuberculata, and show novel and distinct expression patterns in H. erythrogramma. Gsc assumes a novel ectodermal pattern and Msx shifts to a novel and largely mesodermal pattern. Both Gsc and Msx show a restoration of oral ectoderm expression in hybrids. Hox11/13b is not expressed in oral ectoderm in H. tuberculata, but is conserved in posterior spatial expression among H. tuberculata, H. erythrogramma and hybrids, serving as a control. Competitive RT-PCR shows that Gsc, Msx, and Hox11/13b are under different quantitative and temporal controls in the Heliocidaris species and the hybrids. The implications for the involvement of these genes in the rapid evolution of a direct developing larva are discussed.

Different thresholds of Notch signaling bias human precursor cells toward B-, NK-, monocytic/dendritic-, or T-cell lineage in thymus microenvironment

Notch receptors are involved in lineage decisions in multiple developmental scenarios, including hematopoiesis. Here, we treated hybrid human-mouse fetal thymus organ culture with the gamma-secretase inhibitor 7 (N-[N-(3,5-difluorophenyl)-l-alanyl]-S-phenyl-glycine t-butyl ester) (DAPT) to establish the role of Notch signaling in human hematopoietic lineage decisions. The effect of inhibition of Notch signaling was studied starting from cord blood CD34(+) or thymic CD34(+)CD1(-), CD34(+)CD1(+), or CD4ISP progenitors. Treatment of cord blood CD34(+) cells with low DAPT concentrations results in aberrant CD4ISP and CD4/CD8 double-positive (DP) thymocytes, which are negative for intracellular T-cell receptor beta (TCRbeta). On culture with intermediate and high DAPT concentrations, thymic CD34(+)CD1(-) cells still generate aberrant intracellular TCRbeta(-) DP cells that have undergone DJ but not VDJ recombination. Inhibition of Notch signaling shifts differentiation into non-T cells in a thymic microenvironment, depending on the starting progenitor cells: thymic CD34(+)CD1(+) cells do not generate non-T cells, thymic CD34(+)CD1(-) cells generate NK cells and monocytic/dendritic cells, and cord blood CD34(+)Lin(-) cells generate B, NK, and monocytic/dendritic cells in the presence of DAPT. Our data indicate that Notch signaling is crucial to direct human progenitor cells into the T-cell lineage, whereas it has a negative impact on B, NK, and monocytic/dendritic cell generation in a dose-dependent fashion.

The Kinetic Status of Hematopoietic Stem Cell Subpopulations Underlies a Differential Expression of Genes Involved in Self-Renewal, Commitment, and Engraftment

The gene expression profile of CD34(-) hematopoietic stem cells (HSCs) and the correlations with their biological properties are still poorly understood. To address this issue, we used the DNA microarray technology to compare the expression profiles of different peripheral blood hemopoietic stem/progenitor cell subsets, lineage-negative (Lin(-)) CD34(-), Lin(-)CD34(+), and Lin(+)CD34(+) cells. The analysis of gene categories differentially expressed shows that the expression of CD34 is associated with cell cycle entry and metabolic activation, such as DNA, RNA, and protein synthesis. Moreover, the significant upregulation in CD34(-) cells of pathways inhibiting HSC proliferation induces a strong differential expression of cyclins, cyclin-dependent kinases (CDKs), CDK inhibitors, and growth-arrest genes. According to the expression of their receptors and transducers, interleukin (IL)-10 and IL-17 showed an inhibitory effect on the clonogenic activity of CD34(-) cells. Conversely, CD34(+) cells were sensitive to the mitogenic stimulus of thrombopoietin. Furthermore, CD34(-) cells express preferentially genes related to neural, epithelial, and muscle differentiation. The analysis of transcription factor expression shows that the CD34 induction results in the upregulation of genes related to self-renewal and lineage commitment. The preferential expression in CD34(+) cells of genes supporting the HSC mobilization and homing to the bone marrow, such as chemokine receptors and integrins, gives the molecular basis for the higher engraftment capacity of CD34(+) cells. Thus, the different kinetic status of CD34(-) and CD34(+) cells, detailed by molecular and functional analysis, significantly influences their biological behavior.

A new recurrent inversion, inv(7)(p15q34), leads to transcriptional activation of HOXA10 and HOXA11 in a subset of T-cell acute lymphoblastic leukemias

Chromosomal translocations with breakpoints in T-cell receptor (TCR) genes are recurrent in T-cell malignancies. These translocations involve the TCRalphadelta gene (14q11), the TCRbeta gene (7q34) and to a lesser extent the TCRgamma gene at chromosomal band 7p14 and juxtapose T-cell oncogenes next to TCR regulatory sequences leading to deregulated expression of those oncogenes. Here, we describe a new recurrent chromosomal inversion of chromosome 7, inv(7)(p15q34), in a subset of patients with T-cell acute lymphoblastic leukemia characterized by CD2 negative and CD4 positive, CD8 negative blasts. This rearrangement juxtaposes the distal part of the HOXA gene cluster on 7p15 to the TCRbeta locus on 7q34. Real time quantitative PCR analysis for all HOXA genes revealed high levels of HOXA10 and HOXA11 expression in all inv(7) positive cases. This is the first report of a recurrent chromosome rearrangement targeting the HOXA gene cluster in T-cell malignancies resulting in deregulated HOXA gene expression (particularly HOXA10 and HOXA11) and is in keeping with a previous report suggesting HOXA deregulation in MLL-rearranged T- and B cell lymphoblastic leukemia as the key factor in leukaemic transformation. Finally, our observation also supports the previous suggested role of HOXA10 and HOXA11 in normal thymocyte development.

Polycomb group gene mel-18 modulates the self-renewal activity and cell cycle status of hematopoietic stem cells

OBJECTIVE

Mel-18 is a member of the mammalian Polycomb group (PcG) genes. This family of genes regulates global gene expression in many biologic processes, including hematopoiesis and anterior-posterior axis formation by manipulating specific target genes, including members of the Hox family. Here, we demonstrate that mel-18 negatively regulates the self-renewal activity of hematopoietic stem cells (HSCs).

MATERIALS AND METHODS

Long-term reconstitution activity was evaluated by competitive repopulating unit (CRU) and mean activity of the stem cells (MAS) assays in vivo in bone marrow cells (BMCs) derived from mel-18(-/-) and mel-18 tg mice. The expression levels of mel-18 and Hoxb4 were measured by quantitative real-time reverse transcription polymerase chain reaction.

RESULTS

The Hoxb4 gene was highly expressed in HSCs derived from mel-18(-/-) mice. The observed CRUs were 3.21, 4.77, 3.32, and 1.64 CRU per 10(5) BMCs in mel-18(+/+), mel-18(-/-), C57BL/6, and mel-18 tg, respectively. MAS was 0.58, 0.18, 0.41, and 5.89 in mel-18(+/+), mel-18(-/-), C57BL/6, and mel-18 tg, respectively. The percentage in G0 phase HSCs (lin(-)flk2(-)c-Kit(+)Sca1+ cells) was increased in mel-18(-/-) mice and decreased in mel-18 tg mice.

CONCLUSION

Loss or knockdown of mel-18 leads to the expression of Hoxb4, an increase in the proportion of HSCs in G0 phase, and the subsequent promotion of HSC self-renewal. These findings will enable us to develop new approaches for controlling HSC activity for hematopoietic transplantations based on ex vivo expansion of HSCs.

Thrombopoietin, flt3-ligand and c-kit-ligand modulate HOX gene expression in expanding cord blood CD133 cells

Haemopoietic stem/progenitor cell (HSPC) development is regulated by extrinsic and intrinsic stimuli. Extrinsic modulators include growth factors and cell adhesion molecules, whereas intrinsic regulation is achieved with many transcription factor families, of which the HOX gene products are known to be important in haemopoiesis. Umbilical cord blood CD133+ HSPC proliferation potential was tested in liquid culture with 'TPOFLK' (thrombopoietin, flt-3 ligand and c-kit ligand, promoting HSPC survival and self-renewal), in comparison to 'K36EG' (c-kit-ligand, interleukins-3 and -6, erythropoietin and granulocyte colony-stimulating factor, inducing haemopoietic differentiation). TPOFLK induced a higher CD133+ HSPC proliferation (up to 60-fold more, at week 8) and maintained a higher frequency of the primitive colony-forming cells than K36EG. Quantitative polymerase chain reaction analysis revealed opposite expression patterns for specific HOX genes in expanding cord blood CD133+ HSPC. After 8 weeks in liquid culture, TPOFLK increased the expression of HOX B3, B4 and A9 (associated with uncommitted HSPC) and reduced the expression of HOX B8 and A10 (expressed in committed myeloid cells) when compared to K36EG. These results suggest that TPOFLK induces CD133+ HSPC proliferation, self-renewal and maintenance, up-regulation of HOX B3, B4 and A9 and down-regulation of HOX B8 and A10 gene expression.

Identification of a Hoxd10-regulated transcriptional network and combinatorial interactions with Hoxa10 during spinal cord development

Hoxd10 is expressed in the posterior spinal cord and hindlimbs of the mouse. Hoxd10, along with other Hox transcription factors, is thought to regulate the activity of genes involved in nervous system patterning and motor neuron development, but little is known about the downstream targets regulated by this gene. cDNA microarrays were used to investigate the transcriptional network regulated by Hoxd10 in homozygous knockout animals. Sixty-nine genes were identified with altered expression levels in mutant spinal cords. Among these were genes involved in such diverse cellular events as cellular communication, cell cycle control, development and differentiation, and neuronal survival. The expression of some of these genes was investigated using reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization. Nine genes showed changes in expression of the same sign and similar magnitude using RT-PCR in Hoxd10 single mutant animals, with additional changes in expression seen in Hoxa10/Hoxd10 double mutant animals. In situ hybridization studies also demonstrated changes in expression consistent with microarray results. Analysis of putative promoter regions for Hox protein binding sites suggested that some genes may be direct Hoxd10 targets, whereas others likely are regulated through intermediate steps. Using cDNA microarrays to study a single gene knockout during critical developmental stages has identified a large number of genes regulated by Hoxd10, many of which would not have been approached as candidates for Hox gene regulation based on function or expression.

MLL-AF9 oncogene expression affects cell growth but not terminal differentiation and is downregulated during monocyte–macrophage maturation in AML-M5 THP-1 cells

The MLL-AF9 oncogene - one of the most frequent MLL/HRX/ALL-1 rearrangements found in infantile and therapy-related leukaemias - originates from t(9;11)(p22;q23) and is mainly associated with monocytic acute myeloid leukaemia (AML-M5; FAB-classification). Here, we investigated the MLL-AF9 function by means of an antisense phosphorothioate-oligodeoxyribonucleotide (MLL-AF9-PS-ODNas) using the THP-1 AML-M5 cell line carrying t(9;11). Having confirmed that MLL-AF9-PS-ODNas induces strong inhibition of THP-1 cell growth, but only a moderate increase in apoptosis, we found that MLL-AF9-PS-ODNas did not induce morpho-functional terminal differentiation or restore M-CSF-, G-CSF- or GM-CSF-induced differentiation. Moreover, THP-1 cells showed the same phenotype with/without MLL-AF9-PS-ODNas. In THP-1 cells differentiated to mature macrophage-like cells by PMA/TPA or ATRA, MLL-AF9 expression was downregulated. Thus, in the monocytic lineage, MLL-AF9 may be expressed only in early phases and can induce deregulated amplification in both nonmalignant and malignant cells, maintaining the monocytic phenotype without blocking final maturation. Our findings suggest that: (1) as well as directly promoting cell growth, MLL-AF9 may also indirectly determine phenotype; (2) other leukaemogenic mutations associated with MLL-AF9-related leukaemias should be searched for mainly in processes of resistance to apoptosis (where MLL-AF9 may play only a limited role) and differentiation blockage (where MLL-AF9 may play no role).

Signaling but not trafficking function of HIV-1 protein Nef is essential for Nef-induced defects in human intrathymic T-cell development

The HIV-1 gene nef is important for progression toward AIDS and cellular depletion of the infected thymus. Expression of the Nef protein alone impairs human thymopoiesis. Here, we performed a structure-function analysis of the Nef protein by comparing the effect on T-cell development of different nef alleles, either wild type or defective for selected functions, expressed by human thymocytes. We show that Nef-mediated impaired thymopoiesis is not due to altered surface marker trafficking, nor dependent on oligomerization of Nef. By contrast, mutations in the myristoylation site and in signaling sites of Nef, ie, sites important for interaction with phosphofurin acidic cluster sorting protein-1 (PACS-1), Src homology domain 3 (SH3) domains, and p21-activated kinase 2 (PAK2), were found to be critical for its effect on T-cell development. These results point to sites in Nef to target therapeutically for restoration of thymopoiesis in HIV infection.

Molecular biology of hematopoietic stem cells

Human CD34+ hematopoietic stem and progenitor cells are capable of maintaining a life-long supply of the entire spectrum of blood cells dependent on systemic needs. Recent studies suggest that hematopoietic stem cells are, beyond their hematopoietic potential, able to differentiate into nonhematopoietic cell types, which could open novel avenues in the field of cellular therapy. Here, we concentrate on the molecular biology underlying basic features of hematopoietic stem cells. Immunofluorescence analyses, culture assays, and transplantation models permit an extensive immunological as well as functional characterization of human hematopoietic stem and progenitor cells. New methods such as cDNA array technology have demonstrated that distinct gene expression patterns of transcription factors and cell cycle genes molecularly control self-renewal, differentiation, and proliferation. Furthermore, several adhesion molecules have been shown to play an important role in the regulation of hematopoiesis and stem cell trafficking. Progress has also been made in elucidating molecular mechanisms of stem cell aging that limit replicative potential. Finally, more recent data provide the first molecular basis for a better understanding of transdifferentiation and developmental plasticity of hematopoietic stem cells. These findings could be helpful for non-hematopoietic cell therapeutic approaches.

Homeobox gene expression profile in human hematopoietic multipotent stem cells and T-cell progenitors: implications for human T-cell development

Class I homeobox (HOX) genes comprise a large family of transcription factors that have been implicated in normal and malignant hematopoiesis. However, data on their expression or function during T-cell development is limited. Using degenerated RT-PCR and Affymetrix microarray analysis, we analyzed the expression pattern of this gene family in human multipotent stem cells from fetal liver (FL) and adult bone marrow (ABM), and in T-cell progenitors from child thymus. We show that FL and ABM stem cells are similar in terms of HOX gene expression, but significant differences were observed between these two cell types and child thymocytes. As the most immature thymocytes are derived from immigrated FL and ABM stem cells, this indicates a drastic change in HOX gene expression upon entry into the thymus. Further analysis of HOX-A7, HOX-A9, HOX-A10, and HOX-A11 expression with specific RT-PCR in all thymocyte differentiation stages showed a sequential loss of 3' region HOX-A cluster genes during intrathymic T-cell development and an unexpected expression of HOX-A11, previously not recognized to play a role in hematopoiesis. Also HOX-B3 and HOX-C4 were expressed throughout thymocyte development. Overall, these data provide novel evidence for an important role of certain HOX genes in human T-cell development.

Global down-regulation of HOX gene expression in PML-RARalpha + acute promyelocytic leukemia identified by small-array real-time PCR

Acute promyelocytic leukemia (APL) is associated with a reciprocal and balanced translocation involving the retinoic acid receptor-alpha (RARalpha). All-trans retinoic acid (ATRA) is used to treat APL and is a potent morphogen that regulates HOX gene expression in embryogenesis and organogenesis. HOX genes are also involved in hematopoiesis and leukemogenesis. Thirty-nine mammalian HOX genes have been identified and classified into 13 paralogous groups clustered on 4 chromosomes. They encode a complex network of transcription regulatory proteins whose precise targets remain poorly understood. The overall function of the network appears to be dictated by gene dosage. To investigate the mechanisms involved in HOX gene regulation in hematopoiesis and leukemogenesis by precise measurement of individual HOX genes, a small-array real-time HOX (SMART-HOX) quantitative polymerase chain reaction (PCR) platform was designed and validated. Application of SMART-HOX to 16 APL bone marrow samples revealed a global down-regulation of 26 HOX genes compared with normal controls. HOX gene expression was also altered during differentiation induced by ATRA in the PML-RARalpha(+) NB4 cell line. PML-RARalpha fusion proteins have been reported to act as part of a repressor complex during myeloid cell differentiation, and a model linking HOX gene expression to this PML-RARalpha repressor complex is now proposed.

Human hematopoietic lineage commitment

The ultimate goal of developmental immunology is to understand the normal processes that give rise to the immune system in order to diagnose and develop effective treatments for diseases that occur as a consequence of immune system defects. Central to achieving this goal is understanding the complex interplay between microenvironmental signals and transcription factors that direct human hematopoietic differentiation and lineage commitment. The ability to isolate highly purified populations of human hematopoietic cells at critical points in differentiation make it possible to answer very specific questions about the hematopoietic process and lineage restriction. This review describes the use of surface immunophenotypes to identify human hematopoietic cells at particular points in differentiation or with particular patterns of lineage restriction. Culture models are discussed in the context of the ability to detect, characterize and determine the lineage potential of human hematopoietic stem cells and progenitors. Variations in hematopoeises that correspond to ontogeny will be examined. Potential roles for the HOX and Ikaros proteins in human lineage commitment will be considered. Also included will be discussion of a number of factors that provide challenges to experimental design, to experimental interpretation, and to the development of a comprehensive model of human hematopoiesis.