Runx3 and Runx1 are required for CD8 T cell development during thymopoiesis

Transcription factors T-bet and Runx3 cooperate to activate Ifng and silence Il4 in T helper type 1 cells

Cell differentiation involves activation and silencing of lineage-specific genes. Here we show that the transcription factor Runx3 is induced in T helper type 1 (T(H)1) cells in a T-bet-dependent manner, and that both transcription factors T-bet and Runx3 are required for maximal production of interferon-gamma (IFN-gamma) and silencing of the gene encoding interleukin 4 (Il4) in T(H)1 cells. T-bet does not repress Il4 in Runx3-deficient T(H)2 cells, but coexpression of Runx3 and T-bet induces potent repression in those cells. Both T-bet and Runx3 bind to the Ifng promoter and the Il4 silencer, and deletion of the silencer decreases the sensitivity of Il4 to repression by either factor. Our data indicate that cytokine gene expression in T(H)1 cells may be controlled by a feed-forward regulatory circuit in which T-bet induces Runx3 and then 'partners' with Runx3 to direct lineage-specific gene activation and silencing.

CBFB-MYH11 hinders early T-cell development and induces massive cell death in the thymus

Recent studies suggest that the chromosome 16 inversion, associated with acute myeloid leukemia M4Eo, takes place in hematopoietic stem cells. If this is the case, it is of interest to know the effects of the resulting fusion gene, CBFB-MYH11, on other lineages. Here we studied T-cell development in mice expressing Cbfb-MYH11 and compared them with mice compound-heterozygous for a Cbfb null and a hypomorphic GFP knock-in allele (Cbfb(-/GFP)), which had severe Cbfb deficiency. We found a differentiation block at the DN1 stage of thymocyte development in Cbfb-MYH11 knock-in chimeras. In a conditional knock-in model in which Cbfb-MYH11 expression was activated by Lck-Cre, there was a 10-fold reduction in thymocyte numbers in adult thymus, resulting mainly from impaired survival of CD4+CD8+ thymocytes. Although Cbfb-MYH11 derepressed CD4 expression efficiently in reporter assays, such derepression was less pronounced in vivo. On the other hand, CD4 expression was derepressed and thymocyte development was blocked at DN1 and DN2 stages in E17.5 Cbfb(-/GFP) thymus, with a 20-fold reduction of total thymocyte numbers. Our data suggest that Cbfb-MYH11 suppressed Cbfb in several stages of T-cell development and provide a mechanism for CBFB-MYH11 association with myeloid but not lymphoid leukemia.

Runx3 regulates dendritic epidermal T cell development

The Runx3 transcription factor regulates development of T cells during thymopoiesis and TrkC sensory neurons during dorsal root ganglia neurogenesis. It also mediates transforming growth factor-beta signaling in dendritic cells and is essential for development of skin Langerhans cells. Here, we report that Runx3 is involved in the development of skin dendritic epidermal T cells (DETCs); an important component of tissue immunoregulation. In developing DETCs, Runx3 regulates expression of the alphaEbeta7 integrin CD103, known to affect migration and epithelial retention of DETCs. It also regulates expression of IL-2 receptor beta (IL-2Rbeta) that mediates cell proliferation in response to IL-2 or IL-15. In the absence of Runx3, the reduction in CD103 and IL-2Rbeta expression on Runx3(-/-) DETC precursors resulted in impaired cell proliferation and maturation, leading to complete lack of skin DETCs in Runx3(-/-) mice. The data demonstrate the requirement of Runx3 for DETCs development and underscore the importance of CD103 and IL-2Rbeta in this process. Of note, while Runx3(-/-) mice lack both DETCs and Langerhans cells, the two most important components of skin immune surveillance, the mice did not develop skin lesions under pathogen-free (SPF) conditions.

The genetics of systemic lupus erythematosus: understanding how SNPs confer disease susceptibility

The identification of genes for autoimmune diseases is just the first step towards our understanding of disease pathogenesis. In investigating how mutations, deletions or other types of polymorphic defects occur, it is important to determine the pathways and the mechanisms through which susceptibility leads to disease. In this review I touch on three examples of studies that have attempted to understand the mechanisms of genetic susceptibility in three genes identified recently for systemic lupus erythematosus: PDCD1, PTPN22 and IRF5. We are just beginning to comprehend and much needs to be done.

T-lymphoid, megakaryocyte, and granulocyte development are sensitive to decreases in CBFbeta dosage

The family of core-binding factors includes the DNA-binding subunits Runx1-3 and their common non-DNA-binding partner CBFbeta. We examined the collective role of core-binding factors in hematopoiesis with a hypomorphic Cbfb allelic series. Reducing CBFbeta levels by 3- or 6-fold caused abnormalities in bone development, megakaryocytes, granulocytes, and T cells. T-cell development was very sensitive to an incremental reduction of CBFbeta levels: mature thymocytes were decreased in number upon a 3-fold reduction in CBFbeta levels, and were virtually absent when CBFbeta levels were 6-fold lower. Partially penetrant consecutive differentiation blocks were found among early T-lineage progenitors within the CD4- CD8- double-negative 1 and downstream double-negative 2 thymocyte subsets. Our data define a critical CBFbeta threshold for normal T-cell development, and situate an essential role for core-binding factors during the earliest stages of T-cell development.

Runx-dependent regulation of G-protein gamma3 expression in T-cells

Heterotrimeric G-proteins control diverse biological processes by conveying signals from seven-transmembrane receptors to intracellular effectors. Although their signaling roles were originally ascribed to their GTP-bound alpha-subunits, more recent evidence points to the equally active roles played by their betagamma-dimers. To elucidate the individual contributions of their gamma-subtypes, we used a gene targeting approach to show that mice lacking the gamma3-subtype display a defective T-cell dependent immune response. To identify the cellular basis for this defect, we demonstrated that gamma3-mRNA is strongly induced in activated CD4+ T-cells. To determine the mechanism for this regulated expression, we used several strategies to identify the importance of a Runx consensus sequence element in the first intron of the gamma3 gene and the Runx1 protein. Overall, these data provide the first genetic evidence for the tight regulation and involvement of the G protein gamma3-subtype in mounting an effective immune response in mice.

RUNX3 Is Frequently Inactivated by Dual Mechanisms of Protein Mislocalization and Promoter Hypermethylation in Breast Cancer

A tumor suppressor function has been attributed to RUNX3, a member of the RUNX family of transcription factors. Here, we examined alterations in the expression of three members, RUNX1, RUNX2, and RUNX3, and their interacting partner, CBF-beta, in breast cancer. Among them, RUNX3 was consistently underexpressed in breast cancer cell lines and primary tumors. Fifty percent of the breast cancer cell lines (n = 19) showed hypermethylation at the promoter region and displayed significantly lower levels of RUNX3 mRNA expression (P < 0.0001) and protein (P < 0.001). In primary Singaporean breast cancers, 9 of 44 specimens showed undetectable levels of RUNX3 by immunohistochemistry. In 35 of 44 tumors, however, low levels of RUNX3 protein were present. Remarkably, in each case, protein was mislocalized to the cytoplasm. In primary tumors, hypermethylation of RUNX3 was observed in 23 of 44 cases (52%) and was undetectable in matched adjacent normal breast epithelium. Mislocalization of the protein, with or without methylation, seems to account for RUNX3 inactivation in the vast majority of the tumors. In in vitro and in vivo assays, RUNX3 behaved as a growth suppressor in breast cancer cells. Stable expression of RUNX3 in MDA-MB-231 breast cancer cells led to a more cuboidal phenotype, significantly reduced invasiveness in Matrigel invasion assays, and suppressed tumor formation in immunodeficient mice. This study provides biological and mechanistic insights into RUNX3 as the key member of the family that plays a role in breast cancer. Frequent protein mislocalization and methylation could render RUNX3 a valuable marker for early detection and risk assessment.

Impaired Generation of CD8+ Thymocytes in Ets-1-Deficient Mice

The Ets family of transcription factors function as key regulators of multiple aspects of immune cell development and function. To date, Ets-1 has been implicated in regulating early stages of thymic maturation and lymphocyte function and homeostasis. This report describes a novel role for Ets-1 in supporting later stages of thymic selection, in that positive selection of MHC class I-restricted CD4+CD8+ double-positive thymocytes is markedly inhibited in mice expressing a hypomorphic allele of Ets-1. This effect is thymocyte intrinsic, as Ets-1 mutant thymocytes fail to efficiently generate CD8+ single-positive thymocytes in mixed bone marrow chimeric backgrounds. Although peripheral CD8+ T cells are present in Ets-1 mutant mice, both CD4+ and CD8+ subsets contain an elevated proportion of cells with an effector memory (CD62L-CD44+) phenotype. In addition, while thymic expression of Thy1 is relatively normal, peripheral T cells isolated from Ets-1 mutant mice display a striking loss of Thy1 expression. These data identify Ets-1 as a key transcription factor regulating thymocyte positive selection and lineage commitment of MHC class I-restricted thymocytes.

Groucho/transducin-like Enhancer-of-split (TLE)-dependent and -independent transcriptional regulation by Runx3

Regulation of gene expression by tissue-specific transcription factors involves both turning on and turning off transcription of target genes. Runx3, a runt-domain transcription factor, regulates cell-intrinsic functions by activating and repressing gene expression in sensory neurons, dendritic cells (DC), and T cells. To investigate the mechanism of Runx3-mediated repression in an in vivo context, we generated mice expressing a mutant Runx3 lacking the C-terminal VWRPY, a motif required for Runx3 interaction with the corepressor Groucho/transducin-like Enhancer-of-split (TLE). In contrast with Runx3(-/-) mice, which displayed ataxia due to the death of dorsal root ganglia TrkC neurons, Runx3(VWRPY-/-) mice were not ataxic and had intact dorsal root ganglia neurons, indicating that ability of Runx3 to tether Groucho/TLE is not essential for neurogenesis. In the DC compartment, the mutant protein Runx3(VWRPY-) promoted normally developed skin Langerhans cells but failed to restrain DC spontaneous maturation, indicating that this latter process involves Runx3-mediated repression through recruitment of Groucho/TLE. Moreover, in CD8(+) thymocytes, Runx3(VWRPY-) up-regulated alphaE/CD103-like WT Runx3, whereas unlike wild type, it failed to repress alphaE/CD103 in CD8(+) splenocytes. Thus, in CD8-lineage T cells, Runx3 regulates alphaE/CD103 in opposing regulatory modes and recruits Groucho/TLE to facilitate the transition from activation to repression. Runx3(VWRPY-) also failed to mediate the epigenetic silencing of CD4 gene in CD8(+) T cells, but normally regulated other pan-CD8(+) T cell genes. These data provide evidence for the requirement of Groucho/TLE for Runx3-mediated epigenetic silencing of CD4 and pertain to the mechanism through which other Runx3-regulated genes are epigenetically silenced.

Developmental and molecular characterization of emerging beta- and gammadelta-selected pre-T cells in the adult mouse thymus

The first checkpoint in T cell development, beta selection, has remained incompletely characterized for lack of specific surface markers. We show that CD27 is upregulated in DN3 thymocytes initiating beta selection, concomitant with intracellular TCR-beta expression. Clonal analysis determined that CD27high DN3 cells generate CD4+CD8+ progeny with more than 90% efficiency, faster and more efficiently than the CD27low majority. CD27 upregulation also occurs in gammadelta-selected DN3 thymocytes in TCR-beta-/- mice and in IL2-GFP transgenic reporter mice where GFP marks the earliest emerging TCR-gammadelta cells from DN3 thymocytes. With CD27 to distinguish pre- and postselection DN3 cells, a detailed gene expression analysis defined regulatory changes associated with checkpoint arrest, with beta selection, and with gammadelta selection. gammadelta selection induces higher CD5, Egr, and Runx3 expression as compared to beta selection, but it triggers less proliferation. Our results also reveal differences in Notch/Delta dependence at the earliest stages of divergence between developing alphabeta and gammadelta T-lineage cells.

CD4/CD8 lineage commitment: light at the end of the tunnel?

Two surprisingly clear results have emerged in the past year that suggest that the seemingly intractable problem of CD4/CD8 lineage commitment might eventually be resolved. Manipulating expression of the CD4 and CD8 coreceptors has long been a favorite method to examine the influence of T-cell receptor signalling on lineage commitment. An elegant new twist on this approach now shows that it is all a matter of timing. Thus, termination of CD4 expression after the initiation of positive selection is sufficient to cause complete redirection of class II-restricted thymocytes to the CD8 lineage, which strongly supports quantitative instructive models of lineage commitment. Progress in the field has been significantly hampered by ignorance of the underlying intracellular pathways. Two independent groups, which employed old-fashioned genetics versus new-fangled microarray technology, have now identified the same transcription factor, Th-POK, as a key regulator of alternate lineage commitment. The presence of this factor directs positively selected thymocytes to the CD4 lineage, whereas its absence causes default development to the CD8 lineage.

Commitment issues: linking positive selection signals and lineage diversification in the thymus

The thymus is responsible for the production of CD4+ helper and CD8+ cytotoxic T cells, which constitute the cellular arm of the immune system. These cell types derive from common precursors that interact with thymic stroma in a T-cell receptor (TCR)-specific fashion, generating intracellular signals that are translated into function-specific changes in gene expression. This overall process is termed positive selection, but it encompasses a number of temporally distinct and possibly mechanistically distinct cellular changes, including rescue from apoptosis, initiation of cell differentiation, and commitment to the CD4+ or CD8+ T-cell lineage. One of the puzzling features of positive selection is how specificity of the TCR controls lineage commitment, as both helper and cytolytic T cells utilize the same antigen-receptor components, with the exception of the CD4 or CD8 coreceptors themselves. In this review, we focus on the signals required for positive selection, particularly as they relate to lineage commitment. Identification of genes encoding transcriptional regulators that play a role in T-cell development has led to significant recent advances in the field. We also provide an overview of nuclear factors in this context and, where known, how their regulation is linked to the same TCR signals that have been implicated in initiating and regulating positive selection.

Role of the transcription factor Th-POK in CD4:CD8 lineage commitment

The molecular basis of CD4:CD8 lineage commitment, in particular the mechanism by which the precise correlation between lineage choice and T-cell receptor (TCR) specificity toward class I or II major histocompatibility complex is achieved, remains controversial. Both stochastic/selective and instructive models in various forms have been proposed to explain this correlation. The two main experimental approaches previously employed to elucidate this process have focused on the beginning and end of the process, i.e. the influence of TCR signaling and the alternate transcriptional control of the CD4 and CD8 loci during commitment. The recent finding that the transcription factor Th-POK is necessary and sufficient for CD4 commitment has now provided a direct entry point for studying the intracellular pathways that govern lineage commitment. Here, we review data leading to the identification and characterization of this factor and discuss the implications of these studies in the context of current models of lineage commitment.

Exclusion of RUNX3 as a tumour-suppressor gene in early-onset gastric carcinomas

Recent studies claim a critical role for RUNX3 in gastric epithelial homeostasis. However, conflicting results exist regarding RUNX3 expression in the stomach and its potential role as a tumour-suppressor gene (TSG) in gastric carcinogenesis. Our aim was to evaluate the role of RUNX3 in early-onset gastric carcinomas (EOGCs). We analysed 41 EOGCs for RUNX3 aberrations using loss of heterozygosity (LOH), fluorescence in situ hybridization (FISH) and immunohistochemistry (IHC) analyses. LOH of markers flanking RUNX3 was relatively common, indicating that loss of the gene may play a role in gastric carcinogenesis. However, FISH analysis of selected cases and a panel of 14 gastric carcinoma-derived cell lines showed widespread presence of multiple copies of centromere 1. While RUNX3 copy numbers were generally equal to or fewer than those of centromere 1, at least two copies were present in almost all cells analysed. Accordingly, a subpopulation of tumour cells in 12/37 cases showed RUNX3 protein expression. However, expression was not detected in the adjacent nontumorous mucosa of any case. Together, these observations indicate that chromosome 1 aberrations occur frequently in EOGCs and are reflected in the LOH and IHC patterns found. Our findings refute a role for RUNX3 as a TSG in EOGCs.

Runx3 expression during mouse tongue and palate development

Studies of molecular mechanisms underlying the development of the mammalian oral mucosa have revealed a major involvement of transforming growth factor beta (TGF-beta) and bone morphologic protein (BMP) signaling pathways. Here, we examined the expression of a downstream target of TGF-beta and BMPs, Runx3, in oral mucosa. Runx3 is a runt-related transcription factor that acts as a gastric tumor suppressor and regulator of growth and differentiation in mammalian gastric epithelial cells. Another member of the Runx family in C. elegans, run, is involved in the development of a functional hypodermis and gut. In this report, we examined Runx3 expression using reverse transcription-polymerase chain reaction, immnunohistochemistry, and in situ hybridization and found that Runx3 is expressed in the tongue and palate epithelium of mouse embryos from embryonic day 12.5 to 16.5. The functional relationship between Runx3 and TGF-beta/BMPs signaling in tongue and palate development is discussed.

RUNX3: A new player in myeloid gene expression and immune response

RUNX transcription factors function as scaffolds for interaction with various coregulatory proteins during developmental processes such as hematopoiesis, neurogenesis, and osteogenesis. The current view places RUNX proteins within the TGF-beta signaling pathway, although each one exhibits cell- and tissue-specific functions. In the case of RUNX3, recent data have suggested its function as a tumor suppressor factor and highlighted its involvement in immune cell differentiation and activation. The molecular mechanisms for the pleiotropic effects of Runx3 deficiency are not completely understood. The present article will summarize the known functional activities of RUNX3, emphasizing its role in myeloid cell gene expression and its potential contribution to the migratory and adhesive capabilities of this cell lineage.

The genetics of psoriasis and autoimmunity

Psoriasis is an inflammatory/autoimmune disease and, as with many autoimmune diseases, is associated with alleles from the major histocompatibility complex (MHC). With psoriasis and autoimmune disease, the penetrance of the MHC-associated alleles is never 100%, even for monozygotic twins. This may be because development requires additional environmental and/or genetic modifiers or requires specific T-cell receptor arrangements. Families segregating single or multilocus susceptibility alleles other than the MHC have also been reported. Overlapping genetic locations of loci for different autoimmune diseases have been known for several years and are starting to reveal common genes or genetic variants. These include genes normally involved in preventing spontaneous T-cell activation or proliferation, immune synapse formation, or cytokine production via pathways such as those mediated by NFkappaB and those involved in thymic selection. Autoimmunity may also involve dysregulation of genes or pathways regulated by the RUNX family of transcription factors. RUNX is involved in hematopoietic cell development, development of T cells in the thymus, chromatin remodeling, and gene silencing. Hence, its effect on cells of the immune system may be due to variable changes in gene expression and could account for variable body surface involvement and waxing and waning of disease.

Phosphorylation, acetylation and ubiquitination: the molecular basis of RUNX regulation

The RUNX family members play pivotal roles in normal development and neoplasia. RUNX1 and RUNX2 are essential for hematopoiesis and osteogenesis, respectively, while RUNX3 is involved in neurogenesis, thymopoiesis and functions as a tumor suppressor. Inappropriate levels of RUNX activity are associated with leukemia, autoimmune disease, cleidocranial dysplasia, craniosynostosis and various solid tumors. Therefore, RUNX activity must be tightly regulated to prevent tumorigenesis and maintain normal cell differentiation. Recent work indicates that RUNX activity is controlled by various extracellular signaling pathways, and that phosphorylation, acetylation and ubiquitination are important post-translational modifications of RUNX that affect its stability and activity. Defining the precise roles, these modifications that play in the regulation of RUNX function may reveal not only how the RUNX proteins are regulated but also how they are assembled into other regulatory machineries.

Analyzing expression of perforin, Runx3, and Thpok genes during positive selection reveals activation of CD8-differentiation programs by MHC II-signaled thymocytes

Intrathymic positive selection matches CD4-CD8 lineage differentiation to MHC specificity. However, it is unclear whether MHC signals induce lineage choice or simply select thymocytes of the appropriate lineage. To investigate this issue, we assessed thymocytes undergoing positive selection for expression of the CD8 lineage markers perforin and Runx3. Using both population-based and single-cell RT-PCR analyses, we found large subsets of MHC class II (MHC-II)-signaled thymocytes expressing these genes within the CD4+ 8+ and CD4+ 8(int), but not the CD4+ 8- populations of signaling competent mice. This indicates that MHC-II signals normally fail to impose CD4 differentiation and further implies that the number of mature CD8 single-positive (SP) thymocytes greatly underestimates CD8 lineage choice. We next examined whether MHC-II-restricted CD4+ 8- thymocytes remain competent to initiate CD8 lineage gene expression. In mice in which expression of the tyrosine kinase Zap70 and thereby TCR signaling were impaired selectively in SP thymocytes, MHC-II-signaled CD4+ 8- thymocytes expressed perforin and Runx3 and failed to up-regulate the CD4 marker Thpok. This indicated that impairing TCR signals at the CD4 SP stage switched gene expression patterns from CD4- to CD8-lineage specific. We conclude from these findings that MHC-II-signaled thymocytes remain competent to initiate CD8-specific gene expression even after CD8 down-regulation and that CD4 lineage differentiation is not fixed before the CD4 SP stage.

Runx3 regulates integrin alpha E/CD103 and CD4 expression during development of CD4-/CD8+ T cells

During thymic T cell development, immature CD4+CD8+ double-positive (DP) thymocytes develop either into CD4+CD8- Th cells or CD4-CD8+ CTLs. Differentially expressed primary factors inducing the fate of these cell types are still poorly described. The transcription factor Runx3/AML-2 Runx, runt [corrected] dominant factor; AML, acute myeloid leukemia is expressed specifically during the development of CD8 single-positive (SP) thymocytes, where it silences CD4 expression. Deletion of murine Runx3 results in a reduction of CD8 SP T cells and concomitant accumulation of CD4+CD8+ T cells, which cannot down-regulate CD4 expression in the thymus and periphery. In this study we have investigated the role of Runx3 during thymocyte development and CD4 silencing and have identified integrin alpha(E)/CD103 on CD8 SP T cells as a new potential target gene of Runx3. We demonstrate that Runx3 is necessary not only to repress CD4, but also to induce CD103 expression during development of CD8 SP T cells. In addition, transgenic overexpression of Runx3 reduced CD4 expression during development of DP thymocytes, leading to a reduced number of CD4 SP thymocytes and an increased number of CD8 SP thymocytes. This reversal is not caused by redirection of specific MHC class II-restricted cells to the CD8 lineage. Overexpression of Runx3 also up-regulated CD103 expression on a subpopulation of CD4 SP T cells with characteristics of regulatory T cells. Thus, Runx3 is a main regulator of CD4 silencing and CD103 induction and thus contributes to the phenotype of CD8 SP T cells during thymocyte development.

RUNX3 suppresses gastric epithelial cell growth by inducing p21(WAF1/Cip1) expression in cooperation with transforming growth factor {beta}-activated SMAD

RUNX3 has been suggested to be a tumor suppressor of gastric cancer. The gastric mucosa of the Runx3-null mouse develops hyperplasia due to enhanced proliferation and suppressed apoptosis accompanied by a decreased sensitivity to transforming growth factor beta1 (TGF-beta1). It is known that TGF-beta1 induces cell growth arrest by activating CDKN1A (p21(WAF1)(/Cip1)), which encodes a cyclin-dependent kinase inhibitor, and this signaling cascade is considered to be a tumor suppressor pathway. However, the lineage-specific transcription factor that cooperates with SMADs to induce p21 expression is not known. Here we show that RUNX3 is required for the TGF-beta-dependent induction of p21 expression in stomach epithelial cells. Overexpression of RUNX3 potentiates TGF-beta-dependent endogenous p21 induction. In cooperation with SMADs, RUNX3 synergistically activates the p21 promoter. In contrast, RUNX3-R122C, a mutation identified in a gastric cancer patient, abolished the ability to activate the p21 promoter or cooperate with SMADs. Furthermore, areas in mouse and human gastric epithelium where RUNX3 is expressed coincided with those where p21 is expressed. Our results suggest that at least part of the tumor suppressor activity of RUNX3 is associated with its ability to induce p21 expression.

Reduction of Runx1 Transcription Factor Activity Up-Regulates Fas and Bim Expression and Enhances the Apoptotic Sensitivity of Double Positive Thymocytes

The death or survival of double positive (DP) thymocytes is determined by the strength of their TCR signaling. Of the three Runx family proteins, the DP cells only express the Runx1 transcription factor. We introduced and expressed in murine thymocytes the Runt domain of Runx1, which antagonizes the activity of endogenous Runx1. The Runt transgenic DP thymocytes expressed higher levels of the proapoptotic molecules Fas and Bim compared with the wild-type cells. Furthermore, the Runt transgenic cells were more susceptible to apoptosis induced by the artificial cross-linking of the TCR by the anti-CD3 Ab. This susceptibility was partially abrogated by the lpr/lpr background. In addition, Runx1:HY-TCR-double transgenic DP thymocytes were resistant to the apoptosis induced by the endogenously presented HY Ag. We propose that Runx1 functions to suppress the apoptotic sensitivity of DP thymocytes in the context of TCR signaling.

The mSin3A chromatin-modifying complex is essential for embryogenesis and T-cell development

The corepressor mSin3A is the core component of a chromatin-modifying complex that is recruited by multiple gene-specific transcriptional repressors. In order to understand the role of mSin3A during development, we generated constitutive germ line as well as conditional msin3A deletions. msin3A deletion in the developing mouse embryo results in lethality at the postimplantation stage, demonstrating that it is an essential gene. Blastocysts derived from preimplantation msin3A null embryos and mouse embryo fibroblasts (MEFs) lacking msin3A display a significant reduction in cell division. msin3A null MEFs also show mislocalization of the heterochromatin protein, HP1alpha, without alterations in global histone acetylation. Heterozygous msin3A(+/-) mice with a systemic twofold decrease in mSin3A protein develop splenomegaly as well as kidney disease indicative of a disruption of lymphocyte homeostasis. Conditional deletion of msin3A from developing T cells results in reduced thymic cellularity and a fivefold decrease in the number of cytotoxic (CD8) T cells, while helper (CD4) T cells are unaffected. We show that CD8 development is dependent on mSin3A at a step downstream of T-cell receptor signaling and that loss of mSin3A specifically decreases survival of double-positive and CD8 T cells. Thus, msin3A is a pleiotropic gene which, in addition to its role in cell cycle progression, is required for the development and homeostasis of cells in the lymphoid lineage.

CD4-CD8 lineage commitment: an inside view

The mechanism of CD4-CD8 lineage commitment, which ensures the correlation between T cell receptor specificity and adoption of the T killer or T helper phenotype, has long been the subject of intense debate. Various approaches are slowly elucidating the underlying molecular pathways. Analysis of the function of T cell receptor signaling (the 'top-down' approach) supports the view that differences in signal strength and/or duration 'instruct' alternative commitment. Analysis of the transcriptional regulation of the genes encoding CD4 and CD8 (the 'bottom-up' approach) has identified critical cis-acting elements and their interacting factors. Finally, identification of the transcription factor Th-POK as a central component of the CD4 lineage-determining pathway has provided a new starting point from which to unravel this intriguing process 'from the inside out'.

DNA methylation and the expanding epigenetics of T cell lineage commitment

During their development from progenitors, lymphocytes make a series of cell fate decisions. These decisions reflect and require changes in overall programs of gene expression. To maintain cellular identity, programs of gene expression must be iterated through mitosis in a heritable manner by epigenetic processes, which include DNA methylation, methyl-CpG-binding proteins, histone modifications, transcription factors and higher order chromatin structure. Current evidence is consistent with the notion that DNA methylation acts in concert with other epigenetic processes to limit the probability of aberrant gene expression and to stabilize, rather than to initiate, cell fate decisions. In particular, DNA methylation appears to be a non-redundant repressor of CD8 expression in TCR-gammadelta T cells and Th2 cytokine expression in Th1 and CD8 T cells, and is required to enforce clonally restricted Ly49 and KIR gene expression in NK cells. However, most of our knowledge is derived from in vitro studies, and the importance of DNA methylation in memory cell lineage fidelity in vivo remains to be shown convincingly.

Accelerated chemokine receptor 7-mediated dendritic cell migration in Runx3 knockout mice and the spontaneous development of asthma-like disease

The Runx3 transcription factor is a key regulator of lineage-specific gene expression in several developmental pathways and could also be involved in autoimmunity. We report that, in dendritic cells (DC), Runx3 regulates TGFbeta-mediated transcriptional attenuation of the chemokine receptor CCR7. When Runx3 is lost, i.e., in Runx3 knockout mice, expression of CCR7 is enhanced, resulting in increased migration of alveolar DC to the lung-draining lymph nodes. This increased DC migration and the consequent accumulation of activated DC in draining lymph nodes is associated with the development of asthma-like features, including increased serum IgE, hypersensitivity to inhaled bacterial lipopolysaccharide, and methacholine-induced airway hyperresponsiveness. The enhanced migration of DC in the knockout mice could be blocked in vivo by anti-CCR7 antibodies and by the drug Ciglitazone, known to inhibit CCR7 expression. The data indicate that Runx3 transcriptionally regulates CCR7 and that, when absent, the dysregulated expression of CCR7 in DC plays a role in the etiology of asthmatic conditions that recapitulate clinical symptoms of the human disease. Interestingly, human RUNX3 resides in a region of chromosome 1p36 that contains susceptibility genes for asthma and hypersensitivity against environmental antigens. Thus, mutations in RUNX3 may be associated with increased sensitivity to asthma development.

Receptor signals and nuclear events in CD4 and CD8 T cell lineage commitment

MHC specificity in positive selection is a major determinant in the CD4/CD8 T cell lineage decision. Previous studies support the view that quantitative differences in T cell receptor (TCR) signaling in immature CD4+CD8+ double positive thymocytes leads to an instructive bias in CD4/CD8 T cell lineage commitment that must be re-inforced in subsequent selection steps to ensure that MHC-restricted antigen recognition is linked to appropriate effector functions in mature T cells. Recent work has further defined the TCR signaling pathways involved in this process, but a major effort has been made to identify transcription factors and other regulators of CD4 and CD8 T cell lineage commitment. Methods and screens for detecting changes in gene expression, associated with TCR signaling in positive selection and lineage determination, are starting to provide a better understanding of these complex developmental processes.

Identification and characterisation of two runx2 homologues in zebrafish with different expression patterns

Genome and gene duplications are considered to be the impetus to generate new genes, as the presence of multiple copies of a gene allows for paralogues to adopt novel function. After at least two rounds of genome/gene duplication, the Runt gene family consists of three members in vertebrates, instead of one in invertebrates. One of the family members, Runx2, plays a key role in the development of bone, a tissue that first occurs in vertebrates. The family has thus gained new gene function in the course of evolution. Two Runx2 genes were cloned in the vertebrate model system the zebrafish (Danio rerio). The expression patterns of the two genes differ and their kinetics differ up to four fold. In addition, splice forms exist that are novel when compared with mammals. Together, these findings comprise opportunities for selection and retention of the paralogues towards divergent and possibly new function.

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

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

Transcriptional cross-regulation of RUNX1 by RUNX3 in human B cells

RUNX transcription factors are important in development and in numerous types of human cancer. They act as either transcriptional activators or repressors and can be proto-oncogenes or tumour suppressors. Understanding their regulation and interaction may explain how RUNX factors contribute to such different and often opposing biological processes. We show that RUNX3 regulates RUNX1 expression, contributing to the mutually exclusive expression of RUNX3 and RUNX1 in human B lymphoid cell lines. RUNX3 repressed the RUNX1 P1 promoter by binding specifically to conserved RUNX sites near the transcription start of the promoter. siRNA inhibition of RUNX3 in lymphoblastoid cells resulted in increased RUNX1 expression, indicating that continuous expression of physiological levels of RUNX3 is required to maintain repression. Furthermore, expression of RUNX3 was required for efficient proliferation of B cells immortalized by Epstein-Barr virus. Cross-regulation between different RUNX family members is therefore a means of controlling RUNX protein expression and must now be considered in the interpretation of pathological changes due to loss of RUNX3 tumour suppressor function or following gene duplication or translocation events.

Loss of Runx1 perturbs adult hematopoiesis and is associated with a myeloproliferative phenotype

Homozygous loss of function of Runx1 (Runt-related transcription factor 1 gene) during murine development results in an embryonic lethal phenotype characterized by a complete lack of definitive hematopoiesis. In light of recent reports of disparate requirements for hematopoietic transcription factors during development as opposed to adult hematopoiesis, we used a conditional gene-targeting strategy to effect the loss of Runx1 function in adult mice. In contrast with the critical role of Runx1 during development, Runx1 was not essential for hematopoiesis in the adult hematopoietic compartment, though a number of significant hematopoietic abnormalities were observed. Runx1 excision had lineage-specific effects on B- and T-cell maturation and pronounced inhibition of common lymphocyte progenitor production. Runx1 excision also resulted in inefficient platelet production. Of note, Runx1-deficient mice developed a mild myeloproliferative phenotype characterized by an increase in peripheral blood neutrophils, an increase in myeloid progenitor populations, and extramedullary hematopoiesis composed of maturing myeloid and erythroid elements. These findings indicate that Runx1 deficiency has markedly different consequences during development compared with adult hematopoiesis, and they provide insight into the phenotypic manifestations of Runx1 deficiency in hematopoietic malignancies.

Molecular genetics of T cell development

T cell development is guided by a complex set of transcription factors that act recursively, in different combinations, at each of the developmental choice points from T-lineage specification to peripheral T cell specialization. This review describes the modes of action of the major T-lineage-defining transcription factors and the signal pathways that activate them during intrathymic differentiation from pluripotent precursors. Roles of Notch and its effector RBPSuh (CSL), GATA-3, E2A/HEB and Id proteins, c-Myb, TCF-1, and members of the Runx, Ets, and Ikaros families are critical. Less known transcription factors that are newly recognized as being required for T cell development at particular checkpoints are also described. The transcriptional regulation of T cell development is contrasted with that of B cell development, in terms of their different degrees of overlap with the stem-cell program and the different roles of key transcription factors in gene regulatory networks leading to lineage commitment.

The zinc finger transcription factor Th-POK regulates CD4 versus CD8 T-cell lineage commitment

Development of immature T-cell precursors (thymocytes) to either the CD4 helper or CD8 killer T-cell lineages correlates precisely with their T-cell receptor specificity for major histocompatibility complex class II or class I molecules, respectively, indicating that the process is carefully regulated. Although intensively studied owing to its importance in determining the composition of the mature T-cell compartment and as a general model of binary lineage decisions, the underlying molecular pathways remain obscure. We have previously reported a spontaneous mouse mutant (HD (helper deficient) mice) in which lineage commitment is specifically perturbed without affecting positive selection. Here we show that a point mutation in the zinc finger transcription factor Th-POK (T-helper-inducing POZ/Krüppel-like factor) is responsible for redirection of class-II-restricted thymocytes to the CD8 lineage in HD mice. Furthermore, we demonstrate that constitutive expression of this factor during thymic development leads to redirection of class-I-restricted thymocytes to the CD4 lineage, indicating that Th-POK is a master regulator of lineage commitment.

Dual Functions of Runx Proteins for Reactivating CD8 and Silencing CD4 at the Commitment Process into CD8 Thymocytes

To understand how CD8 expression is regulated during the transition process from CD4+8+ (CD4 and CD8 double positive, DP) to CD4-8+ (CD8 single positive, CD8SP) cells in the thymus, the involvement of Runx proteins in the alteration of chromatin configuration was investigated. Using the chromatin immunoprecipitation assay, we first demonstrated that Runx proteins bind to the stage-specific CD8 enhancer, as well as the CD4 silencer, in CD8SP thymocytes. Among Runx family members, Runx3 expression was initiated in DP thymocytes receiving a positive selection signal and increased in concert with differentiation to the CD8SP stage. Furthermore, reactivation of the CD8 gene, as well as CD4 silencing, was suppressed in positively selected thymocytes of Runx dominant-negative transgenic mice. These results suggest that Runx proteins, especially Runx3, are involved in lineage specification of CD8 T cells and provide important information for understanding the mechanism for the mutually exclusive expression of coreceptors in mature thymocytes.

Overexpression of the Runx3 Transcription Factor Increases the Proportion of Mature Thymocytes of the CD8 Single-Positive Lineage

The Runx family of transcription factors is thought to regulate the differentiation of thymocytes. Runx3 protein is detected mainly in the CD4(-)8(+) subset of T lymphocytes. In the thymus of Runx3-deficient mice, CD4 expression is de-repressed and CD4(-)8(+) thymocytes do not develop. This clearly implicates Runx3 in CD4 silencing, but does not necessarily prove its role in the differentiation of CD4(-)8(+) thymocytes per se. In the present study, we created transgenic mice that overexpress Runx3 and analyzed the development of thymocytes in these animals. In the Runx3-transgenic thymus, the number of CD4(-)8(+) cells was greatly increased, whereas the numbers of CD4(+)8(+) and CD4(+)8(-) cells were reduced. The CD4(-)8(+) transgenic thymocytes contained mature cells with a TCR(high)HSA(low) phenotype. These cells were released from the thymus and contributed to the elevated level of CD4(-)8(+) cells relative to CD4(+)8(-) cells in the spleen. Runx3 overexpression also increased the number of mature CD4(-)8(+) thymocytes in mice with class II-restricted, transgenic TCR and in mice with a class I-deficient background, both of which are favorable for CD4(+)8(-) lineage selection. Thus, Runx3 can drive thymocytes to select the CD4(-)8(+) lineage. This activity is likely to be due to more than a simple silencing of CD4 gene expression.

Shared and distinct roles mediated through C-terminal subdomains of acute myeloid leukemia/Runt-related transcription factor molecules in murine development

AML1/Runx1 is a frequent target of human leukemia-associated gene aberration and encodes a transcription factor with nonredundant biologic functions in initial development of definitive hematopoiesis, T-cell development, and steady-state platelet production. AML1/Runx1 and 2 closely related family genes, AML2/Runx3 and AML3/Runx2/Cbfa1, present in mammals, comprise the Runt-domain transcription factor family. Although they have similar structural and biochemical properties, gene-targeting experiments have identified distinct biologic roles. To directly determine the presence of functional overlap among runt-related transcription factor (Runx) family molecules, we replaced the C-terminal portion of acute myeloid leukemia 1 (AML1) with that derived from its family members, which are variable in contrast to conserved Runt domain, using the gene knock-in method. We found that C-terminal portions of either AML2 or AML3 could functionally replace that of AML1 for myeloid development in culture and within the entire mouse. However, while AML2 substituted for AML1 could effectively rescue lymphoid lineages, AML3 could not, resulting in a smaller thymus and lymphoid deficiency in peripheral blood. Substitution by the C-terminal portion of AML3 also led to high infantile mortality and growth retardation, suggesting that AML1 has as yet unidentified effects on these phenotypes. Thus, the C-terminal portions of Runx family members have both similar and distinct biologic functions.

Dynamic repositioning of CD4 and CD8 genes during T cell development

Although stable repression of CD4 and CD8 genes is a central feature of T cell lineage commitment, we lack detailed information about the timing and mechanism of this repression. Stable gene repression has been linked to the position of genes within the nucleus. Therefore, information about the nuclear position of CD4 and CD8 genes during T cell development could provide insights into both the mechanism of regulation of CD4 and CD8 genes, and the process of lineage commitment. Here, we report that lineage-specific repression of CD4 and CD8 genes is associated with the repositioning of alleles close to heterochromatin. We also provide evidence that the relocalization of CD4 and CD8 genes to heterochromatin can occur as an early response to positive selection signals. We discuss our results in terms of our current knowledge of CD4 and CD8 gene regulation and CD4 versus CD8 lineage commitment.

Control of RUNX2 isoform expression: The role of promoters and enhancers

The three mammalian RUNX genes constitute the family of runt domain transcription factors that are involved in the regulation of a number of developmental processes such as haematopoiesis, osteogenesis and neuronal differentiation. All three genes show a complex temporo-spatial pattern of expression. Since the three proteins are probably mutually interchangeable with regard to function, most of the specificity of each family member seems to be based on a tightly controlled regulation of expression. While RUNX gene expression is driven by two promoters for each gene, the promoter sequence alone does not seem to suffice for a proper expressional control. This review focuses on the available evidence for the existence of such control mechanisms and studies aiming at discovering cis-acting regulatory sequences of the RUNX2 gene.

Runx3 is involved in hair shape determination

Transcriptional regulators of the Runx family play critical roles in normal organ development and, when mutated, lead to genetic diseases and cancer. Runx3 functions during cell lineage decisions in thymopoiesis and neurogenesis and mediates transforming growth factor-beta signaling in dendritic cells. Here, we study the function of Runx3 in the skin and its appendages, primarily the hair follicle, during mouse development. Runx3 is expressed predominantly in the dermal compartment of the hair follicles as they form and during the hair cycle, as well as in the nail and sweat gland skin appendages. Distinct expression is also detected periodically in isolated cells of the epidermis and in melanocytes, populating the hair bulb. Runx3-deficient mice display a perturbation of the normal hair coat, which we show to be due to hair type and hair shape changes. Thus, one of the functions of Runx3 in skin may be to regulate the formation of the epithelial derived structural hair by affecting dermal to epidermal interactions.

Mutation of all Runx (AML1/core) sites in the enhancer of T-lymphomagenic SL3-3 murine leukemia virus unmasks a significant potential for myeloid leukemia induction and favors enhancer evolution toward induction of other disease patterns

SL3-3 murine leukemia virus is a potent inducer of T-lymphomas in mice. Using inbred NMRI mice, it was previously reported that a mutant of SL3-3 with all enhancer Runx (AML1/core) sites disrupted by 3-bp mutations (SL3-3dm) induces predominantly non-T-cell tumors with severely extended latency (S. Ethelberg, J. Lovmand, J. Schmidt, A. Luz, and F. S. Pedersen, J. Virol. 71:7273-7280, 1997). By use of three-color flow cytometry and molecular and histopathological analyses, we have now performed a detailed phenotypic characterization of SL3-3- and SL3-3dm-induced tumors in this mouse strain. All wild-type induced tumors had clonal T-cell receptor beta rearrangements, and the vast majority were CD3(+) CD4(+) CD8(-) T-lymphomas. Such a consistent phenotypic pattern is unusual for murine leukemia virus-induced T-lymphomas. The mutant virus induced malignancies of four distinct hematopoietic lineages: myeloid, T lymphoid, B lymphoid, and erythroid. The most common disease was myeloid leukemia with maturation. Thus, mutation of all Runx motifs in the enhancer of SL3-3 severely impedes viral T-lymphomagenicity and thereby discloses a considerable and formerly unappreciated potential of this virus for myeloid leukemia induction. Proviral enhancers with complex structural alterations (deletions, insertions, and/or duplications) were found in most SL3-3dm-induced T-lymphoid tumors and immature myeloid leukemias but not in any cases of myeloid leukemia with maturation, mature B-lymphoma, or erythroleukemia. Altogether, our results indicate that the SL3-3dm enhancer in itself promotes induction of myeloid leukemia with maturation but that structural changes may arise in vivo and redirect viral disease specificity to induction of T-lymphoid or immature myeloid leukemias, which typically develop with moderately shorter latencies.

Runx1 is expressed in adult mouse hematopoietic stem cells and differentiating myeloid and lymphoid cells, but not in maturing erythroid cells

The transcription factor Runx1 marks all functional hematopoietic stem cells (HSCs) in the embryo and is required for their generation. Mutations in Runx1 are found in approximately 25% of acute leukemias and in familial platelet disorder, suggesting a role for Runx1 in adult hematopoiesis as well. A comprehensive analysis of Runx1 expression in adult hematopoiesis is lacking. Here we show that Runx1 is expressed in functional HSCs in the adult mouse, as well as in cells with spleen colony-forming unit (CFU) and culture CFU capacities. Additionally, we document Runx1 expression in all hematopoietic lineages at the single cell level. Runx1 is expressed in the majority of myeloid cells and in a smaller proportion of lymphoid cells. Runx1 expression substantially decreases during erythroid differentiation. We also document effects of reduced Runx1 levels on adult hematopoiesis.

Loss of Runx3 function in leukocytes is associated with spontaneously developed colitis and gastric mucosal hyperplasia

RUNX transcription factors are key regulators of lineage-specific gene expression and might be involved in autoimmune diseases. Runx3 plays a role during the development of sensory neurons and T cells and regulates transforming growth factor beta (TGF-beta) signaling in dendritic cells. Here, we report that at 4 weeks of age, Runx3 knockout (KO) mice spontaneously develop inflammatory bowel disease (IBD) characterized by leukocyte infiltration, mucosal hyperplasia, formation of lymphoid clusters, and increased production of IgA. Additionally, at a considerably older age (8 months), the KO mice also develop progressive hyperplasia of the gastric mucosa associated with disturbed epithelial differentiation and cellular hyaline degeneration. Analysis of cytokines in the colonic mucosa of Runx3 KO mice revealed a mixed T helper 1/T helper 2 response. By using immunohistochemistry and RNA in situ hybridization, Runx3 expression in the gastrointestinal tract is detected in lymphoid and myeloid populations but not in the epithelium. The data indicate that loss of leukocytic cell-autonomous function of Runx3 results in IBD and gastric lesion in the KO mice. IBD in humans is viewed as a complex genetic disorder. Several susceptibility loci were identified on different human chromosomes including the chromosomal region 1p36 where RUNX3 resides. It is thus tempting to speculate that mutations in RUNX3 may constitute an IBD risk factor in humans.

Stem cell expression of the AML1/ETO fusion protein induces a myeloproliferative disorder in mice

The t(8;21)(q22;q22) translocation, present in 10-15% of acute myeloid leukemia (AML) cases, generates the AML1/ETO fusion protein. To study the role of AML1/ETO in the pathogenesis of AML, we used the Ly6A locus that encodes the well characterized hematopoietic stem cell marker, Sca1, to target expression of AML1/ETO to the hematopoietic stem cell compartment in mice. Whereas germ-line expression of AML1/ETO from the AML1 promoter results in embryonic lethality, heterozygous Sca1(+/AML1-ETO ires EGFP) (abbreviated Sca(+/AE)) mutant mice are born in Mendelian ratios with no apparent abnormalities in growth or fertility. Hematopoietic cells from Sca(+/AE) mice have markedly extended survival in vitro and increasing myeloid clonogenic progenitor output over time. Sca(+/AE) mice develop a spontaneous myeloproliferative disorder with a latency of 6 months and a penetrance of 82% at 14 months. These results reinforce the notion that the phenotype of murine transgenic models of human leukemia is critically dependent on the cellular compartment targeted by the transgene. This model should provide a useful platform to analyze the effect of AML1/ETO on hematopoiesis and its potential cooperation with other mutations in the pathogenesis of leukemia.

RNT-1, the C. elegans homologue of mammalian RUNX transcription factors, regulates body size and male tail development

The rnt-1 gene is the only Caenorhabditis elegans homologue of the mammalian RUNX genes. Several lines of molecular biological evidence have demonstrated that the RUNX proteins interact and cooperate with Smads, which are transforming growth factor-beta (TGF-beta) signal mediators. However, the involvement of RUNX in TGF-beta signaling has not yet been supported by any genetic evidence. The Sma/Mab TGF-beta signaling pathway in C. elegans is known to regulate body length and male tail development. The rnt-1(ok351) mutants show the characteristic phenotypes observed in mutants of the Sma/Mab pathway, namely, they have a small body size and ray defects. Moreover, RNT-1 can physically interact with SMA-4 which is one of the Smads in C. elegans, and double mutant animals containing both the rnt-1(ok351) mutation and a mutation in a known Sma/Mab pathway gene displayed synergism in the aberrant phenotypes. In addition, lon-1(e185) mutants was epistatic to rnt-1(ok351) mutants in terms of long phenotype, suggesting that lon-1 is indeed downstream target of rnt-1. Our data reveal that RNT-1 functionally cooperates with the SMA-4 proteins to regulate body size and male tail development in C. elegans.

TEL/AML1 shows dominant-negative effects over TEL as well as AML1

The TEL/AML1 chimeric gene is generated by the t(12;21) translocation in pre-B cell acute lymphoblastic leukemia. TEL/AML1 consists of the helix-loop-helix (HLH) dimerization domain from TEL and almost the entire of AML1, but loses the ETS DNA-binding domain from TEL. Dominant-negative effects of TEL/AML1 over wild-type-AML1 are believed to trigger the development of this type of leukemia. However, it could also be possible that TEL/AML1 affects wild-type-TEL's molecular and tumor suppressive functions through the HLH domain. To test this possibility, we first confirmed that TEL/AML1 associates with wild-type-TEL. TEL/AML1 neither bound to the ETS-binding consensus site nor repressed transcription through it. Regardless, this prevented wild-type-TEL-induced transcriptional repression. Moreover, TEL/AML1 concomitantly inhibited wild-type-TEL-induced growth suppression and wild-type-AML1-mediated transforming activity in NIH3T3 cells. All these data indicate that TEL/AML1 exerts dominant-interfering effects on both AML1 and TEL, and that expression of TEL/AML1 could result in inactivation of TEL's tumor suppressive functions in t(12;21)-carrying leukemia.

Evolutionary origins of lymphocytes: ensembles of T cell and B cell transcriptional regulators in a cartilaginous fish

The evolutionary origins of lymphocytes can be traced by phylogenetic comparisons of key features. Homologs of rearranging TCR and Ig (B cell receptor) genes are present in jawed vertebrates, but have not been identified in other animal groups. In contrast, most of the transcription factors that are essential for the development of mammalian T and B lymphocytes belong to multigene families that are represented by members in the majority of the metazoans, providing a potential bridge to prevertebrate ancestral roles. This work investigates the structure and regulation of homologs of specific transcription factors known to regulate mammalian T and B cell development in a representative of the earliest diverging jawed vertebrates, the clearnose skate (Raja eglanteria). Skate orthologs of mammalian GATA-3, GATA-1, EBF-1, Pax-5, Pax-6, Runx2, and Runx3 have been characterized. GATA-3, Pax-5, Runx3, EBF-1, Spi-C, and most members of the Ikaros family are shown throughout ontogeny to be 1) coregulated with TCR or Ig expression, and 2) coexpressed with each other in combinations that for the most part correspond to known mouse T and B cell patterns, supporting conservation of function. These results indicate that multiple components of the gene regulatory networks that operate in mammalian T cell and B cell development were present in the common ancestor of the mammals and the cartilaginous fish. However, certain factors relevant to the B lineage differ in their tissue-specific expression patterns from their mouse counterparts, suggesting expanded or divergent B lineage characteristics or tissue specificity in these animals.

Analysis of transcription factor expression during discrete stages of postnatal thymocyte differentiation

Postnatal T lymphocyte differentiation in the thymus is a multistage process involving serial waves of lineage specification, proliferative expansion, and survival/cell death decisions. Although these are believed to originate from signals derived from various thymic stromal cells, the ultimate consequence of these signals is to induce the transcriptional changes that are definitive of each step. To help to characterize this process, high density microarrays were used to analyze transcription factor gene expression in RNA derived from progenitors at each stage of T lymphopoietic differentiation, and the results were validated by a number of appropriate methods. We find a large number of transcription factors to be expressed in developing T lymphocytes, including many with known roles in the control of differentiation, proliferation, or cell survival/death decisions in other cell types. Some of these are expressed throughout the developmental process, whereas others change substantially at specific developmental transitions. The latter are particularly interesting, because stage-specific changes make it increasingly likely that the corresponding transcription factors may be involved in stage-specific processes. Overall, the data presented here represent a large resource for gene discovery and for confirmation of results obtained through other methods.

Origins of lymphocyte developmental programs: transcription factor evidence

This review explores the evolutionary origins of lymphocyte development by focusing on the transcription factors that direct mammalian lymphocyte development today. Gene expression data suggest that the programs to make lymphocytes involve the same transcription factor ensembles in all animals with lymphocytes. Most of these factors, GATA, Runx, PU.1/Spi, EBF/Olf, Ikaros, and Pax-2/5/8 family members, are also encoded in the genomes of animals without lymphocytes. We consider the functions of these factors in animals without lymphocytes in terms of discrete program components, which could have been assembled in a new way to create the lymphocyte developmental program approximately 500 My ago.

Localization of the domains in Runx transcription factors required for the repression of CD4 in thymocytes

The runt family transcription factors Runx1 and Runx3 are expressed in developing murine thymocytes. We show that enforced expression of full-length Runx1 in CD4(-)CD8(-) thymocytes results in a profound suppression of immature CD4/CD8 double-positive thymocytes and mature CD4 single-positive thymocytes compared with controls. This effect arises from Runx1- or Runx3-mediated repression of CD4 expression, and is independent of positively selecting signals. Runx1 is able to repress CD4 in CD4/CD8 double-positive thymocytes, but not in mature splenic T cells. Runx-mediated CD4 repression is independent of association with the corepressors Groucho/TLE or Sin3. Two domains are required for complete Runx-mediated CD4 repression. These are contained within Runx1 aa 212-262 and 263-360. The latter region contains the nuclear matrix targeting sequence, which is highly conserved among runt family transcription factors across species. The presence of the nuclear matrix targeting sequence is required for Runx-mediated CD4 repression, suggesting that Runx transcription factors are stabilized on the CD4 silencer via association with the nuclear matrix.