EGF repeats of epidermal growth factor­like domain 7 promote endothelial cell activation and tumor escape from the immune system.

The tumor blood vessel endothelium forms a barrier that must be crossed by circulating immune cells in order for them to reach and kill cancer cells. Epidermal growth factor­like domain 7 (Egfl7) represses this immune infiltration by lowering the expression levels of leukocyte adhesion receptors on the surface of endothelial cells. However, the protein domains involved in these properties are not completely understood. Egfl7 is structurally composed of the predicted EMI­, EGF­ and C­terminal domains. The present study aimed to investigate the roles of these different domains in tumor development by designing retroviruses coding for deletion mutants and then infecting 4T1 breast cancer cell populations, which consequently overexpressed the variants. By performing in vitro soft­agar assays, it was found that Egfl7 and its deletion variants did not affect cell proliferation or anchorage­independent growth. When 4T1 cells expressing either the wild­type Egfl7 protein or Egfl7 domain variants were implanted in mice, Egfl7 expression markedly promoted tumor development and deletion of the EGF repeats decreased the tumor growth rate. By contrast, deleting any other domain displayed no significant effect on tumor development. The overexpression of Egfl7 also decreased T cell and natural killer cell infiltration in tumors, as determined by immunofluorescence staining of tumor sections, whereas deletion of the EGF repeats inhibited this effect. Reverse transcription­quantitative PCR analysis of the mechanisms involved revealed that deleting the EGF repeats partially restored the expression levels of vascular cell adhesion molecule 1 and E­selectin, which were suppressed by overexpression of Egfl7 in endothelial cells in vitro. This resulted in a higher number of lymphocytes bound to HUVEC expressing Egfl7­ΔEGF compared with HUVEC expressing wild­type Egfl7, as assessed by fluorescent­THP­1 adhesion assays onto endothelial cells. Overall, the present study demonstrated that the EGF repeats may participate in the protumoral and anti­inflammatory effects of Egfl7.

Endothelial Cell Activation Is Regulated by Epidermal Growth Factor-like Domain 7 (Egfl7) during Inflammation.

Activation of the blood vessel endothelium is a critical step during inflammation. Endothelial cells stimulated by pro-inflammatory cytokines play an essential part in the adhesion and extravasation of circulating leukocytes into inflamed tissues. The endothelial egfl7 gene (VE-statin) represses endothelial cell activation in tumors, and prior observations suggested that it could also participate in the regulation of endothelial cell activation during inflammation. We show here that Egfl7 expression is strongly repressed in mouse lung endothelial cells during LPS- and TNFα-induced inflammation in vivo LPS have a limited effect on Egfl7 expression by endothelial cells in vitro, whereas the pro-inflammatory cytokine TNFα strongly represses Egfl7 expression in endothelial cells. TNFα regulates the egfl7 gene promoter through regions located between -7585 and -5550 bp ahead of the main transcription start site and via an NF-κB-dependent mechanism. Conversely, Egfl7 regulates the response of endothelial cells to TNFα by restraining the induced expression of intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), and E-selectin, resulting in a decreased adhesion of leukocytes onto endothelial cells stimulated by TNFα. Egfl7 regulates the expression of these adhesion molecules through the NF-κB and MEK/Erk pathways, in particular by preventing the proteasome-mediated degradation of IkBα both in non-activated endothelial cells and during activation. Egfl7 is thus an endogenous and constitutive repressor of blood vessel endothelial cell activation in normal and inflammatory conditions and participates in a loop of regulation of activation of these cells by pro-inflammatory cytokines.

Defective DNA single-strand break repair is responsible for senescence and neoplastic escape of epithelial cells.

The main characteristic of senescence is its stability which relies on the persistence of DNA damage. We show that unlike fibroblasts, senescent epithelial cells do not activate an ATM-or ATR-dependent DNA damage response (DDR), but accumulate oxidative-stress-induced DNA single-strand breaks (SSBs). These breaks remain unrepaired because of a decrease in PARP1 expression and activity. This leads to the formation of abnormally large and persistent XRCC1 foci that engage a signalling cascade involving the p38MAPK and leading to p16 upregulation and cell cycle arrest. Importantly, the default in SSB repair also leads to the emergence of post-senescent transformed and mutated precancerous cells. In human-aged skin, XRCC1 foci accumulate in the epidermal cells in correlation with a decline of PARP1, whereas DDR foci accumulate mainly in dermal fibroblasts. These findings point SSBs as a DNA damage encountered by epithelial cells with aging which could fuel the very first steps of carcinogenesis.

The Reelin receptors ApoER2 and VLDLR are direct target genes of HIC1 (Hypermethylated In Cancer 1).

The tumor suppressor gene HIC1 (Hypermethylated In Cancer 1) is located in 17p13.3 a region frequently hypermethylated or deleted in tumors and in a contiguous-gene syndrome, the Miller-Dieker syndrome which includes classical lissencephaly (smooth brain) and severe developmental defects. HIC1 encodes a transcriptional repressor involved in the regulation of growth control, DNA damage response and cell migration properties. We previously demonstrated that the membrane-associated G-protein-coupled receptors CXCR7, ADRB2 and the tyrosine kinase receptor EphA2 are direct target genes of HIC1. Here we show that ectopic expression of HIC1 in U2OS and MDA-MB-231 cell lines decreases expression of the ApoER2 and VLDLR genes, encoding two canonical tyrosine kinase receptors for Reelin. Conversely, knock-down of endogenous HIC1 in BJ-Tert normal human fibroblasts through RNA interference results in the up-regulation of these two Reelin receptors. Finally, through chromatin immunoprecipitation (ChIP) in BJ-Tert fibroblasts, we demonstrate that HIC1 is a direct transcriptional repressor of ApoER2 and VLDLR. These data provide evidence that HIC1 is a new regulator of the Reelin pathway which is essential for the proper migration of neuronal precursors during the normal development of the cerebral cortex, of Purkinje cells in the cerebellum and of mammary epithelial cells. Deregulation of this pathway through HIC1 inactivation or deletion may contribute to its role in tumor promotion. Moreover, HIC1, through the direct transcriptional repression of ATOH1 and the Reelin receptors ApoER2 and VLDLR, could play an essential role in normal cerebellar development.

TRF2 inhibits a cell-extrinsic pathway through which natural killer cells eliminate cancer cells.

Dysfunctional telomeres suppress tumour progression by activating cell-intrinsic programs that lead to growth arrest. Increased levels of TRF2, a key factor in telomere protection, are observed in various human malignancies and contribute to oncogenesis. We demonstrate here that a high level of TRF2 in tumour cells decreased their ability to recruit and activate natural killer (NK) cells. Conversely, a reduced dose of TRF2 enabled tumour cells to be more easily eliminated by NK cells. Consistent with these results, a progressive upregulation of TRF2 correlated with decreased NK cell density during the early development of human colon cancer. By screening for TRF2-bound genes, we found that HS3ST4--a gene encoding for the heparan sulphate (glucosamine) 3-O-sulphotransferase 4--was regulated by TRF2 and inhibited the recruitment of NK cells in an epistatic relationship with TRF2. Overall, these results reveal a TRF2-dependent pathway that is tumour-cell extrinsic and regulates NK cell immunity.

Expression of Egfl7 correlates with low-grade invasive lesions in human breast cancer.

Egfl7 (VE-statin) is specifically expressed by endothelial cells of normal tissues but its expression is deregulated in human cancers. Analysis of expression of Egfl7 protein and transcripts in 211 human breast cancer samples shows that Egfl7 is strongly expressed by breast tumor cells. Egfl7 expression is significantly higher in invasive ductal than in invasive lobular carcinoma. Expression of Egfl7 transcripts is also higher in lower SBR grade lesions and in lesions which are not associated with lymph node invasion. Within the invasive ductal carcinoma sub-population, expression of Egfl7 transcripts is correlated with the SBR score and with the ER+ status. High transcript and Egfl7 protein levels significantly correlate with the absence of axillary lymph node invasion. In lymph nodes, the levels of Egfl7 are correlated with the histological type of the primary lesions; they are higher in ductal than in lobular carcinoma. Egfl7 expression is thus associated with better prognosis factors and with the absence of lymph node invasion in human breast cancer lesions.

Egfl7 promotes tumor escape from immunity.

Egfl7 is an endothelial-specific gene which expression is deregulated in human cancers. We showed that Egfl7 promotes tumor escape from immunity by downregulating the expression of leukocyte adhesion molecules in endothelial cells, thus repressing immune cell extravasation into tumors.

Molecular dissection of the interaction between HIC1 and SIRT1.

HIC1 (Hypermethylated in Cancer 1) is a tumor suppressor gene frequently epigenetically silenced in human cancers. HIC1 encodes a transcriptional repressor involved in the regulation of growth control, cell survival and DNA damage response. The deacetylase SIRT1 regulates the repressive capacity of HIC1 in several fashions. First SIRT1 interacts with the BTB/POZ domain of HIC1 to form a transcriptional repression complex that prevents the transcription of SIRT1 itself. SIRT1 is also responsible of the deacetylation of the lysine 314 of HIC1 that allows its subsequent SUMOylation which in turn favors its interaction with the NuRD complex. To better understand the interplay between HIC1 and SIRT1, we performed co-immunoprecipitation experiments to define the domains essential for the HIC1/SIRT1 interaction. We demonstrated that the isolated four last zinc fingers of HIC1 were capable to interact with SIRT1 and that the amino-acids 610-677 of SIRT1 encompassing the ESA region of the deacetylase were crucial for the HIC1/SIRT1 interaction and HIC1 deacetylation. Finally we demonstrated that this interaction mainly depends on CKII-mediated phosphorylation of SIRT1 serine 659/661 which occurs upon DNA damage. Therefore, our results demonstrate that the activating acetylation to SUMOylation switch of HIC1 is favored by genotoxic stresses to regulate the DNA damage response.

TRF2 controls telomeric nucleosome organization in a cell cycle phase-dependent manner.

Mammalian telomeres stabilize chromosome ends as a result of their assembly into a peculiar form of chromatin comprising a complex of non-histone proteins named shelterin. TRF2, one of the shelterin components, binds to the duplex part of telomeric DNA and is essential to fold the telomeric chromatin into a protective cap. Although most of the human telomeric DNA is organized into tightly spaced nucleosomes, their role in telomere protection and how they interplay with telomere-specific factors in telomere organization is still unclear. In this study we investigated whether TRF2 can regulate nucleosome assembly at telomeres.By means of chromatin immunoprecipitation (ChIP) and Micrococcal Nuclease (MNase) mapping assay, we found that the density of telomeric nucleosomes in human cells was inversely proportional to the dosage of TRF2 at telomeres. This effect was not observed in the G1 phase of the cell cycle but appeared coincident of late or post-replicative events. Moreover, we showed that TRF2 overexpression altered nucleosome spacing at telomeres increasing internucleosomal distance. By means of an in vitro nucleosome assembly system containing purified histones and remodeling factors, we reproduced the short nucleosome spacing found in telomeric chromatin. Importantly, when in vitro assembly was performed in the presence of purified TRF2, nucleosome spacing on a telomeric DNA template increased, in agreement with in vivo MNase mapping.Our results demonstrate that TRF2 negatively regulates the number of nucleosomes at human telomeres by a cell cycle-dependent mechanism that alters internucleosomal distance. These findings raise the intriguing possibility that telomere protection is mediated, at least in part, by the TRF2-dependent regulation of nucleosome organization.

Differential proteomic analysis of human glioblastoma and neural stem cells reveals HDGF as a novel angiogenic secreted factor.

Presence in glioblastomas of cancer cells with normal neural stem cell (NSC) properties, tumor initiating capacity, and resistance to current therapies suggests that glioblastoma stem-like cells (GSCs) play central roles in glioblastoma development. We cultured human GSCs endowed with all features of tumor stem cells, including tumor initiation after xenograft and radio-chemoresistance. We established proteomes from four GSC cultures and their corresponding whole tumor tissues (TTs) and from human NSCs. Two-dimensional difference gel electrophoresis and tandem mass spectrometry revealed a twofold increase of hepatoma-derived growth factor (HDGF) in GSCs as compared to TTs and NSCs. Western blot analysis confirmed HDGF overexpression in GSCs as well as its presence in GSC-conditioned medium, while, in contrast, no HDGF was detected in NSC secretome. At the functional level, GSC-conditioned medium induced migration of human cerebral endothelial cells that can be blocked by anti-HDGF antibodies. In vivo, GSC-conditioned medium induced neoangiogenesis, whereas HDGF-targeting siRNAs abrogated this effect. Altogether, our results identify a novel candidate, by which GSCs can support neoangiogenesis, a high-grade glioma hallmark. Our strategy illustrates the usefulness of comparative proteomic analysis to decipher molecular pathways, which underlie GSC properties.

The receptor tyrosine kinase EphA2 is a direct target gene of hypermethylated in cancer 1 (HIC1).

The tumor suppressor gene hypermethylated in cancer 1 (HIC1), which encodes a transcriptional repressor, is epigenetically silenced in many human tumors. Here, we show that ectopic expression of HIC1 in the highly malignant MDA-MB-231 breast cancer cell line severely impairs cell proliferation, migration, and invasion in vitro. In parallel, infection of breast cancer cell lines with a retrovirus expressing HIC1 also induces decreased mRNA and protein expression of the tyrosine kinase receptor EphA2. Moreover, chromatin immunoprecipitation (ChIP) and sequential ChIP experiments demonstrate that endogenous HIC1 proteins are bound, together with the MTA1 corepressor, to the EphA2 promoter in WI38 cells. Taken together, our results identify EphA2 as a new direct target gene of HIC1. Finally, we observe that inactivation of endogenous HIC1 through RNA interference in normal breast epithelial cells results in the up-regulation of EphA2 and is correlated with increased cellular migration. To conclude, our results involve the tumor suppressor HIC1 in the transcriptional regulation of the tyrosine kinase receptor EphA2, whose ligand ephrin-A1 is also a HIC1 target gene. Thus, loss of the regulation of this Eph pathway through HIC1 epigenetic silencing could be an important mechanism in the pathogenesis of epithelial cancers.

Egfl7 promotes tumor escape from immunity by repressing endothelial cell activation.

Downregulating the leukocyte adhesion molecules expressed by endothelial cells that line tumor blood vessels can limit the entry of immune effector cells into the tumor mass, thereby contributing to tumoral immune escape. Egfl7 (also known as VE-statin) is a secreted protein specifically expressed by endothelial cells in normal tissues and by cancer cells in various human tumors. High levels of Egfl7 correlate with higher tumor grade and poorer prognosis. Here we show that expression of Egfl7 in breast and lung carcinoma cells accelerates tumor growth and metastasis in immunocompetent mice but not in immunodeficient mice. Tumors expressing Egfl7 were infiltrated relatively poorly by immune cells and were characterized by reduced levels of immunostimulatory cytokines [IFN-γ, interleukin-12 (IL-12)] and fewer endothelial adhesion molecules [intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1)]. In vitro studies revealed that Egfl7 inhibited the expression of leukocyte adhesion molecules by endothelial cells, preventing lymphocyte adhesion. In contrast, Egfl7 did not exert any effects on immune cell activation. Human breast cancer lesions expressing high levels of Egfl7 also expressed less ICAM-1 and VCAM-1 in their blood vessels, also indicating an inverse correlation between expression levels of Egfl7 and IFN-γ. Thus, Egfl7 expression in tumors promotes tumor progression by reducing the expression of endothelial molecules that mediate immune cell infiltration. Our findings highlight a novel mechanism through which tumors escape immune control.

Differential regulation of HIC1 target genes by CtBP and NuRD, via an acetylation/SUMOylation switch, in quiescent versus proliferating cells.

The tumor suppressor gene HIC1 encodes a transcriptional repressor involved in regulatory loops modulating P53-dependent and E2F1-dependent cell survival, growth control, and stress responses. Despite its importance, few HIC1 corepressors and target genes have been characterized thus far. Using a yeast two-hybrid approach, we identify MTA1, a subunit of the NuRD complex, as a new HIC1 corepressor. This interaction is regulated by two competitive posttranslational modifications of HIC1 at lysine 314, promotion by SUMOylation, and inhibition by acetylation. Consistent with the role of HIC1 in growth control, we demonstrate that HIC1/MTA1 complexes bind on two new target genes, Cyclin D1 and p57KIP2 in quiescent but not in growing WI38 cells. In addition, HIC1/MTA1 and HIC1/CtBP complexes differentially bind on two mutually exclusive HIC1 binding sites (HiRE) on the SIRT1 promoter. SIRT1 transcriptional activation induced by short-term serum starvation coincides with loss of occupancy of the distal sites by HIC1/MTA1 and HIC1/CtBP. Upon longer starvation, both complexes are found but on a newly identified proximal HiRE that is evolutionarily conserved and specifically enriched with repressive histone marks. Our results decipher a mechanistic link between two competitive posttranslational modifications of HIC1 and corepressor recruitment to specific genes, leading to growth control.

Scavenger chemokine (CXC motif) receptor 7 (CXCR7) is a direct target gene of HIC1 (hypermethylated in cancer 1).

The tumor suppressor gene HIC1 (Hypermethylated in Cancer 1) that is epigenetically silenced in many human tumors and is essential for mammalian development encodes a sequence-specific transcriptional repressor. The few genes that have been reported to be directly regulated by HIC1 include ATOH1, FGFBP1, SIRT1, and E2F1. HIC1 is thus involved in the complex regulatory loops modulating p53-dependent and E2F1-dependent cell survival and stress responses. We performed genome-wide expression profiling analyses to identify new HIC1 target genes, using HIC1-deficient U2OS human osteosarcoma cells infected with adenoviruses expressing either HIC1 or GFP as a negative control. These studies identified several putative direct target genes, including CXCR7, a G-protein-coupled receptor recently identified as a scavenger receptor for the chemokine SDF-1/CXCL12. CXCR7 is highly expressed in human breast, lung, and prostate cancers. Using quantitative reverse transcription-PCR analyses, we demonstrated that CXCR7 was repressed in U2OS cells overexpressing HIC1. Inversely, inactivation of endogenous HIC1 by RNA interference in normal human WI38 fibroblasts results in up-regulation of CXCR7 and SIRT1. In silico analyses followed by deletion studies and luciferase reporter assays identified a functional and phylogenetically conserved HIC1-responsive element in the human CXCR7 promoter. Moreover, chromatin immunoprecipitation (ChIP) and ChIP upon ChIP experiments demonstrated that endogenous HIC1 proteins are bound together with the C-terminal binding protein corepressor to the CXCR7 and SIRT1 promoters in WI38 cells. Taken together, our results implicate the tumor suppressor HIC1 in the transcriptional regulation of the chemokine receptor CXCR7, a key player in the promotion of tumorigenesis in a wide variety of cell types.

An acetylation/deacetylation-SUMOylation switch through a phylogenetically conserved psiKXEP motif in the tumor suppressor HIC1 regulates transcriptional repression activity.

Tumor suppressor HIC1 (hypermethylated in cancer 1) is a gene that is essential for mammalian development, epigenetically silenced in many human tumors, and involved in a complex pathway regulating P53 tumor suppression activity. HIC1 encodes a sequence-specific transcriptional repressor containing five Krüppel-like C(2)H(2) zinc fingers and an N-terminal BTB/POZ repression domain. Here, we show that endogenous HIC1 is SUMOylated in vivo on a phylogenetically conserved lysine, K314, located in the central region which is a second repression domain. K314R mutation does not influence HIC1 subnuclear localization but significantly reduces its transcriptional repression potential, as does the mutation of the other conserved residue in the psiKXE consensus, E316A, or the overexpression of the deSUMOylase SSP3/SENP2. Furthermore, HIC1 is acetylated in vitro by P300/CBP. Strikingly, the K314R mutant is less acetylated than wild-type HIC1, suggesting that this lysine is a target for both SUMOylation and acetylation. We further show that HIC1 transcriptional repression activity is positively controlled by two types of deacetylases, SIRT1 and HDAC4, which increase the deacetylation and SUMOylation, respectively, of K314. Knockdown of endogenous SIRT1 by the transfection of short interfering RNA causes a significant loss of HIC1 SUMOylation. Thus, this dual-deacetylase complex induces either a phosphorylation-dependent acetylation-SUMOylation switch through a psiKXEXXSP motif, as previously shown for MEF2, or a phosphorylation-independent switch through a psiKXEP motif, as shown here for HIC1, since P317A mutation severely impairs HIC1 acetylation. Finally, our results demonstrate that HIC1 is a target of the class III deacetylase SIRT1 and identify a new posttranslational modification step in the P53-HIC1-SIRT1 regulatory loop.

A topological mechanism for TRF2-enhanced strand invasion.

Telomeres can fold into t-loops that may result from the invasion of the 3' overhang into duplex DNA. Their formation is facilitated in vitro by the telomeric protein TRF2, but very little is known regarding the mechanisms involved. Here we reveal that TRF2 generates positive supercoiling and condenses DNA. Using a variety of TRF2 mutants, we demonstrate a strong correlation between this topological activity and the ability to stimulate strand invasion. We also report that these properties require the combination of the TRF-homology (TRFH) domain of TRF2 with either its N- or C-terminal DNA-binding domains. We propose that TRF2 complexes, by constraining DNA around themselves in a right-handed conformation, can induce untwisting of the neighboring DNA, thereby favoring strand invasion. Implications of this topological model in t-loop formation and telomere homeostasis are discussed.

H19 mRNA-like noncoding RNA promotes breast cancer cell proliferation through positive control by E2F1.

The imprinted H19 gene has riboregulatory functions. We show here that H19 transcription is up-regulated during the S-phase of growth-stimulated cells and that the H19 promoter is activated by E2F1 in breast cancer cells. H19 repression by pRb and E2F6 confirms the E2F1-dependent control of the H19 promoter. Consistently, we demonstrate by chromatin immunoprecipitation assays that endogenous E2F1 is recruited to the H19 promoter in vivo. The functionality of E2F promoter sites was further confirmed by gel shift and mutagenesis experiments, revealing that these sites are required for binding and promoter response to E2F1 exogenous expression and serum stimulation. Furthermore, we show that H19 overexpression confers a growth advantage on breast cancer cells released from growth arrest as well as in asynchronously growing cells. The H19 knockdown by small interfering RNA duplexes impedes S-phase entry in both wild-type and stably H19-transfected cells. Based on these findings, we conclude that the H19 RNA is actively linked to E2F1 to promote cell cycle progression of breast cancer cells. This clearly supports the H19 oncogenic function in breast tumor genesis.

The tumor suppressor HIC1 (hypermethylated in cancer 1) is O-GlcNAc glycosylated.

HIC1 (hypermethylated in cancer 1) is a transcriptional repressor containing five Krüppel-like C(2)H(2) zinc fingers and an N-terminal dimerization and autonomous repression domain called BTB/POZ. Here, we demonstrate that full-length HIC1 proteins are modified both in vivo and in vitro with O-linked N-acetylglucosamine (O-GlcNAc). This is a highly dynamic glycosylation found within the cytosolic and the nuclear compartments of eukaryotes. Analysis of [(3)H]Gal-labeled tryptic peptides indicates that HIC1 has three major sites for O-GlcNAc glycosylation. Using C-terminal deletion mutants, we have shown that O-GlcNAc modification of HIC1 proteins occurred preferentially in the DNA-binding domain. Nonglycosylated and glycosylated forms of full-length HIC1 proteins separated by wheat germ agglutinin affinity purification, displayed the same specific DNA-binding activity in electrophoretic mobility shift assays proving that the O-GlcNAc modification is not directly implicated in the specific DNA recognition of HIC1. Intriguingly, N-terminal truncated forms corresponding to BTB-POZ-deleted proteins exhibited a strikingly differential activity, as the glycosylated truncated forms are unable to bind DNA whereas the unglycosylated ones do. Electrophoretic mobility shift assays performed with separated pools of glycosylated and unglycosylated forms of a construct exhibiting only the DNA-binding domain and the C-terminal tail of HIC1 (residues 399-714) and supershift experiments with wheat germ agglutinin or RL-2, an antibody raised against O-GlcNAc residues, fully corroborated these results. Interestingly, these truncated proteins are O-GlcNAc modified in their C-terminal tail (residues 670-711) and not in the DNA-binding domain, as for the full-length proteins. Thus, the O-GlcNAc modification of HIC1 does not affect its specific DNA-binding activity and is highly sensitive to conformational effects, notably its dimerization through the BTB/POZ domain.

The tumor suppressor gene HIC1 (hypermethylated in cancer 1) is a sequence-specific transcriptional repressor: definition of its consensus binding sequence and analysis of its DNA binding and repressive properties.

HIC1 (hypermethylated in cancer 1) is a tumor suppressor gene located at chromosome 17p13.3, a region frequently hypermethylated or deleted in human tumors and in a contiguous-gene syndrome, the Miller-Dieker syndrome. HIC1 is a transcriptional repressor containing five Krüppel-like C(2)H(2) zinc fingers and an N-terminal dimerization and autonomous repression domain called BTB/POZ. Although some of the HIC1 transcriptional repression mechanisms have been recently deciphered, target genes are still to be discovered. In this study, we determined the consensus binding sequence for HIC1 and investigated its DNA binding properties. Using a selection and amplification of binding sites technique, we identified the sequence 5'-(C)/(G)NG(C)/(G)GGGCA(C)/(A) CC-3' as an optimal binding site. In silico and functional analyses fully validated this consensus and highlighted a GGCA core motif bound by zinc fingers 3 and 4. The BTB/POZ domain inhibits the binding of HIC1 to a single site but mediates cooperative binding to a probe containing five concatemerized binding sites, a property shared by other BTB/POZ proteins. Finally, full-length HIC1 proteins transiently expressed in RK13 cells and more importantly, endogenous HIC1 proteins from the DAOY medulloblastoma cell line, repress the transcription of a reporter gene through their direct binding to these sites, as confirmed by chromatin immunoprecipitation experiments. The definition of the HIC1-specific DNA binding sequence as well as the requirement for multiple sites for optimal binding of the full-length protein are mandatory prerequisites for the identification and analyses of bona fide HIC1 target genes.

Identification and developmental expression of the zebrafish orthologue of the tumor suppressor gene HIC1.

Hypermethylated in Cancer 1 (HIC1) is a human tumor suppressor gene located at chromosome 17p13.3 which is frequently hypermethylated and transcriptionally silent in many types of tumors. In addition, its location in the Miller-Dieker syndrome's (MDS) deletion region, its embryonic expression pattern in mice and the phenotype of the HIC1-deficient mice have provided strong evidence for its implication in this contiguous-gene syndrome. HIC1 encodes a five C2H2-type zinc finger transcriptional repressor belonging to the BTB/POZ family. We have isolated the true zebrafish orthologue of human HIC1 since it has a comparable intron-exon structure and since its predicted gene product, ZfHIC1 displays much higher sequence similarities in its overall sequence (737 residues) with human HIC1 (714 residues) than the 454 residues encoded by the only zebrafish HIC1 sequence (AF111712) described so far, which has been renamed ZfHIC1alpha. Notably, the C-terminal end and one zinc finger in the DNA-binding domain are missing in ZfHIC1alpha. As a consequence, ZfHIC1 proteins bind the human HIC1 consensus DNA-binding sequence in vitro, whereas ZfHIC1alpha cannot. Analyses of the expression pattern of ZfHIC1 and of its paralogue ZfHRG22 (HIC1 related gene on chromosome 22) show that they share expression domains with their respective orthologous vertebrate genes. ZfHRG22 is prominently expressed in the brain and in neural tissues. Interestingly, the predominant expression of ZfHIC1 in the mesenchyme of the head, around the nose and the eye and in the branchial arches is possibly consistent with some of the abnormalities seen in the HIC1-deficient mice and provides another clue for the implication of HIC1 in MDS.