Functional characterization of cis-acting elements involved in basal transcription of the human Tbx2 gene: a new insight into the role of Sp1 in transcriptional regulation

Functional interplay of SP family members and nuclear factor Y is essential for transcriptional activation of the human Calreticulin gene

Calreticulin (CALR) is a highly conserved, multifunctional protein involved in a variety of cellular processes including the maintenance of intracellular calcium homeostasis, proper protein folding, differentiation and immunogenic cell death. More recently, a crucial role for CALR in the pathogenesis of certain hematologic malignancies was discovered: in clinical subgroups of acute myeloid leukemia, CALR overexpression mediates a block in differentiation, while somatic mutations have been found in the majority of patients with myeloproliferative neoplasms with nonmutated Janus kinase 2 gene (JAK2) or thrombopoietin receptor gene (MPL). However, the mechanisms underlying CALR promoter activation have insufficiently been investigated so far. By dissecting the core promoter region, we could identify a functional TATA-box relevant for transcriptional activation. In addition, we characterized two evolutionary highly conserved cis-regulatory modules (CRMs) within the proximal promoter each composed of one binding site for the transcription factors SP1 and SP3 as well as for the nuclear transcription factor Y (NFY) and we verified binding of these factors to their cognate sites in vitro and in vivo.

The anti-proliferative function of the TGF-β1 signaling pathway involves the repression of the oncogenic TBX2 by its homologue TBX3

A growing body of work has shown that the highly homologous T-box transcription factors TBX2 and TBX3 play critical but distinct roles in embryonic development and cancer progression. For example, TBX2 and TBX3 are up-regulated in several cancers and recent evidence suggests that whereas TBX2 functions as a pro-proliferative factor, TBX3 inhibits cell proliferation but promotes cancer cell migration and invasion. While the molecular mechanisms regulating these functions of TBX2 and TBX3 are poorly understood we recently reported that the TGF-β1 signaling pathway up-regulates TBX3 expression to mediate, in part, its well described anti-proliferative and pro-migratory roles. The TBX3 targets responsible for these functions were however not identified. Here we reveal for the first time that the TGF-β1 signaling pathway represses TBX2 transcriptionally and we provide a detailed mechanism to show that this is mediated by TBX3. Furthermore, we implicate the down-regulation of TBX2 in the anti-proliferative function of the TGF-β1-TBX3 axis. These findings have important implications for our understanding of the regulation of TBX2 and TBX3 and shed light on the mechanisms involved in the anti-proliferative and pro-migratory roles of TGF-β1.

Genetic analysis of the TBX2 gene promoter in indirect inguinal hernia

PURPOSE

Inguinal hernia is a common disease, majority of which are indirect inguinal hernia (IIH). A positive family history indicates that genetic factors play important roles in the IIH development. To date, genetic causes for IIH remain unknown. T-box transcription factor 2 (TBX2) is a major regulator in the morphogenesis and organogenesis. The human TBX2 gene is widely expressed in fetal and adult tissues, including muscle and connective tissues. Therefore, we speculated that altered TBX2 gene expression may be involved in the IIH formation.

METHODS

IIH patients (n = 129) and ethnic-matched healthy subjects (n = 198) were recruited for this study. The human TBX2 gene promoters were generated with PCR and directly sequenced to identify DNA sequence variants (DSVs). Furthermore, biological functions of the DSVs were examined with reporter gene constructs in cultured cells.

RESULTS

Total six DSVs within the TBX2 gene promoter were identified. A heterozygous DSV (g.59476307G>C) was identified in an IIH patient, but in none of controls, which significantly decreased the TBX2 gene promoter activities. Another heterozygous DSV (g.59476704G>C) was only found in one control, which did not affect TBX2 gene promoter activities. Four DSVs, g.59476316C>A (rs73991913), g.59476415T>C (rs1476781), g.59476510G>C (rs4455026) and g.59476892C>T (rs2286524), all of which were single nucleotide polymorphisms, were found in both IIH patients and controls with similar frequencies.

CONCLUSIONS

Our data suggested that the DSV within the TBX2 gene promoter was implicated in the IIH development as a rare cause.

The NF-Y complex negatively regulates Caenorhabditis elegans tbx-2 expression

T-box genes are frequently expressed in dynamic patterns during animal development, but the mechanisms controlling expression of these genes are not well understood. The Caenorhabditis elegans T-box gene tbx-2 is essential for development of the ABa-derived pharyngeal muscles, specification of neural cell fate in the HSN/PHB lineage, and adaptation in olfactory neurons. The tbx-2 expression pattern is complex, and expression has been described in pharyngeal precursors and body wall muscles during embryogenesis, and amphid sensory neurons and pharyngeal neurons in adults. To examine mechanisms regulating tbx-2 gene expression, we performed an RNAi screen of transcription factor genes in strains containing a Ptbx-2::gfp reporter and identified the Nuclear Factor Y (NF-Y) complex as a negative regulator of tbx-2 expression. NF-Y is a heterotrimeric CCAAT-binding complex consisting of A-C subunits, and reduction of the NF-Y subunits nfya-1, nfyb-1, or nfyc-1 by RNAi or using mutants results in ectopic Ptbx-2::gfp expression in hypodermal seam cells and gut. Mutation of two CCAAT-boxes in the tbx-2 promoter results in a similar pattern of ectopic Ptbx-2::gfp expression, suggesting NF-Y directly represses the tbx-2 promoter. tbx-2 mRNA is moderately increased in nfya-1 null mutants, indicating NF-Y represses expression of endogenous tbx-2. Finally we identify and characterize a second-site mutation that enhances lethality of a temperature sensitive tbx-2 mutant and show that this mutation is a deletion in the nfyb-1 gene. Together, these results identify NF-Y as an important regulator of tbx-2 function in vivo.

Novel and functional sequence variants within the TBX2 gene promoter in ventricular septal defects

Congenital heart disease (CHD) is the most common birth defects in humans. To date, genetic causes for CHD remain largely unknown. T-box transcription factor 2 (TBX2) gene is expressed in the myocardium of atrioventricular canal, outflow tract and inflow tract and plays a critical role in heart chamber formation. Genomic deletion and duplication of TBX2 gene have been associated with cardiac defects. As TBX2 acts in a dose-dependent manner, we hypothesized that DNA sequence variants (DSVs) within TBX2 gene promoter may mediate CHD development by changing TBX2 levels. In this study, TBX2 gene promoter was genetically analyzed in large cohorts of patients with ventricular septal defect (VSD) (n = 324) and ethnic-matched healthy controls (n = 328). Four novel and heterozygous DSVs, g.59477201C > T, g.59477347G > A, g.59477353delG and g.59477371G > A were identified in VSD patients, but in none of controls. Functional analyses revealed that all of the four DSVs significantly decreased transcriptional activities of TBX2 gene promoter. Therefore, our data suggested that the DSVs within TBX2 gene promoter identified in VSD patients may contribute to VSD etiology.

Basal transcription of the human TBX3 gene, a key developmental regulator which is overexpressed in several cancers, requires functional NF-Y and Sp1 sites

TBX3 is a member of the T-box family of genes that encode developmentally important transcription factors. Mutations resulting in decreased levels of functional TBX3 lead to Ulnar-Mammary Syndrome and increased levels of TBX3 have been linked to several cancers. To understand the mechanisms regulating TBX3 expression we have previously cloned the 5'-flanking region of the human TBX3 gene and here we describe cis-elements required for its basal transcription. Using site-directed mutagenesis, luciferase reporter assays and in vitro and in vivo DNA binding experiments we identify a Sp1 element and two CCAAT boxes to be essential for basal TBX3 promoter activity. Our results are consistent with reports that these sites are necessary for efficient basal transcription in genes which lack TATA boxes or an Initiator which we show to be the case for TBX3.

Phosphorylation of histone H3 by protein kinase C signaling plays a critical role in the regulation of the developmentally important TBX2 gene

The mechanism(s) regulating the expression of the TBX2 gene, a key regulator of development, is poorly understood and thus limits an understanding of its function(s). Here we demonstrate that 12-O-tetradecanoylphorbol-13-acetate (TPA) induces TBX2 expression in normal human fibroblasts in a protein kinase C (PKC)-dependent and MAPK-independent manner. Our data further reveal that TPA activates transcription of TBX2 through activating MSK1, which leads to an increase in phosphorylated histone H3 and the recruitment of Sp1 to the TBX2 gene. In addition, TPA was shown to activate MSK1 in a PKC-dependent and MAPK-independent manner. This study is the first to provide evidence that phosphorylation of histone H3 leads to the transcriptional activation of the TBX2 gene and to link MSK1 to PKC.