BORIS, a paralogue of the transcription factor, CTCF, is aberrantly expressed in breast tumours.

BORIS (for brother of the regulator of imprinted sites), a paralogue of the transcription factor, CTCF, is a novel member of the cancer-testis antigen family. The aims of the present study were as follows: (1) to investigate BORIS expression in breast cells and tumours using immunohistochemical staining, western and real-time RT-PCR analyses and (2) assess potential correlation between BORIS levels in tumours with clinical/pathological parameters. BORIS was detected in all 18 inspected breast cell lines, but not in a primary normal breast cell culture. In 70.7% (41 of 58 cases) BORIS was observed in breast tumours. High levels of BORIS correlated with high levels of progesterone receptor (PR) and oestrogen receptor (ER). The link between BORIS and PR/ER was further confirmed by the ability of BORIS to activate the promoters of the PR and ER genes in the reporter assays. Detection of BORIS in a high proportion of breast cancer patients implies potential practical applications of BORIS as a molecular biomarker of breast cancer. This may be important for diagnosis of the condition and for the therapeutic use of BORIS. The ability of BORIS to activate promoters of the RP and ER genes points towards possible involvement of BORIS in the establishment, progression and maintenance of breast tumours.

DNA, but not protein vaccine based on mutated BORIS antigen significantly inhibits tumor growth and prolongs the survival of mice.

The ideal immunological target for cancer vaccine development would meet the criteria of tumor specificity, immunogenicity and vital dependency of the tumor on the functional activities of the antigenic target so as to avoid antigenic loss by mutation. Given that at face value the brother of regulator of imprinted sites (BORIS) transcription factor meets these criteria, we have developed a mutant variant of this molecule (mBORIS) that lacks tumorigenic ability, while retaining immunogenic epitopes that elicits responses against histologically irrelevant tumor cells. Here we compared vaccine strategies employing as an immunogen either mBORIS recombinant protein formulated in a strong Th1-type adjuvant, QuilA or DNA encoding this immunogen along with plasmids expressing interleukin (IL)12/IL18 molecular adjuvants. In both groups of vaccinated mice induction of tumor-specific immunity (antibody response, T-cell proliferation, cytokine production, T-cell cytotoxicity) as well as ability to inhibit growth of the aggressive breast cancer cell line and to prolong survival of vaccinated animals have been tested. We determined that DNA, but not recombinant protein vaccine, induced potent Th1-like T-cell recall responses that significantly inhibited tumor growth and prolongs the survival of vaccinated mice. These studies demonstrate that DNA immunization is superior to recombinant protein strategy and provide a clear guidance for clinical development of a cancer vaccine targeting what appears to be a universal tumor antigen.

Dual role of DNA methylation inside and outside of CTCF-binding regions in the transcriptional regulation of the telomerase hTERT gene.

Expression of hTERT is the major limiting factor for telomerase activity. We previously showed that methylation of the hTERT promoter is necessary for its transcription and that CTCF can repress hTERT transcription by binding to the first exon. In this study, we used electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) to show that CTCF does not bind the methylated first exon of hTERT. Treatment of telomerase-positive cells with 5-azadC led to a strong demethylation of hTERT 5'-regulatory region, reactivation of CTCF binding and downregulation of hTERT. Although complete hTERT promoter methylation was associated with full transcriptional repression, detailed mapping showed that, in telomerase-positive cells, not all the CpG sites were methylated, especially in the promoter region. Using a methylation cassette assay, selective demethylation of 110 bp within the core promoter significantly increased hTERT transcriptional activity. This study underlines the dual role of DNA methylation in hTERT transcriptional regulation. In our model, hTERT methylation prevents binding of the CTCF repressor, but partial hypomethylation of the core promoter is necessary for hTERT expression.

Cell growth inhibition by the multifunctional multivalent zinc-finger factor CTCF.

The 11-zinc finger protein CCTC-binding factor (CTCF) employs different sets of zinc fingers to form distinct complexes with varying CTCF- target sequences (CTSs) that mediate the repression or activation of gene expression and the creation of hormone-responsive gene silencers and of diverse vertebrate enhancer-blocking elements (chromatin insulators). To determine how these varying effects would integrate in vivo, we engineered a variety of expression systems to study effects of CTCF on cell growth. Here we show that ectopic expression of CTCF in many cell types inhibits cell clonogenicity by causing profound growth retardation without apoptosis. In asynchronous cultures, the cell-cycle profile of CTCF-expressing cells remained unaltered, which suggested that progression through the cycle was slowed at multiple points. Although conditionally induced CTCF caused the S-phase block, CTCF can also arrest cell division. Viable CTCF-expressing cells could be maintained without dividing for several days. While MYC is the well-characterized CTCF target, the inhibitory effects of CTCF on cell growth could not be ascribed solely to repression of MYC, suggesting that additional CTS-driven genes involved in growth-regulatory circuits, such as p19ARF, are likely to contribute to CTCF-induced growth arrest. These findings indicate that CTCF may regulate cell-cycle progression at multiple steps within the cycle, and add to the growing evidence for the function of CTCF as a tumor suppressor gene.

CpG methylation regulates the Igf2/H19 insulator.

The differentially methylated 5'-flank of the mouse H19 gene unidirectionally regulates the communication between enhancer elements and gene promoters and presumably represses maternal Igf2 expression in vivo [1-6]. The specific activation of the paternally inherited Igf2 allele has been proposed to involve methylation-mediated inactivation of the H19 insulator function during male germline development [1-4, 6]. Here, we addressed the role of methylation by inserting a methylated fragment of the H19-imprinting control region (ICR) into a nonmethylated episomal H19 minigene construct, followed by the transfection of ligation mixture into Hep3B cells. Individual clones were expanded and analyzed for genotype, methylation status, chromatin conformation, and insulator function. The results show that the methylated status of the H19 ICR could be propagated for several passages without spreading into the episomal vector. Moreover, the nuclease hypersensitive sites, which are typical for the maternally inherited H19 ICR allele [1], were absent on the methylated ICR, underscoring the suggestion that the methylation mark dictates parent of origin-specific chromatin conformations [1] that involve CTCF [2]. Finally, the insulator function was strongly attenuated in stably maintained episomes. Collectively, these results provide the first experimental support that the H19 insulator function is regulated by CpG methylation.

CTCF-binding sites flank CTG/CAG repeats and form a methylation-sensitive insulator at the DM1 locus.

An expansion of a CTG repeat at the DM1 locus causes myotonic dystrophy (DM) by altering the expression of the two adjacent genes, DMPK and SIX5, and through a toxic effect of the repeat-containing RNA. Here we identify two CTCF-binding sites that flank the CTG repeat and form an insulator element between DMPK and SIX5. Methylation of these sites prevents binding of CTCF, indicating that the DM1 locus methylation in congenital DM would disrupt insulator function. Furthermore, CTCF-binding sites are associated with CTG/CAG repeats at several other loci. We suggest a general role for CTG/CAG repeats as components of insulator elements at multiple sites in the human genome.

CTCF is a uniquely versatile transcription regulator linked to epigenetics and disease.

CTCF is an evolutionarily conserved zinc finger (ZF) phosphoprotein that binds through combinatorial use of its 11 ZFs to approximately 50 bp target sites that have remarkable sequence variation. Formation of different CTCF-DNA complexes, some of which are methylation-sensitive, results in distinct functions, including gene activation, repression, silencing and chromatin insulation. Disrupting the spectrum of target specificities by ZF mutations or by abnormal selective methylation of targets is associated with cancer. CTCF emerges, therefore, as a central player in networks linking expression domains with epigenetics and cell growth regulation.

Loss of imprinting of insulin-like growth factor-II in Wilms' tumor commonly involves altered methylation but not mutations of CTCF or its binding site.

Loss of imprinting (LOI) is the most common molecular abnormality in Wilms' tumor (WT), other embryonal cancers, and most other tumor types. LOI in WT involves activation of the normally silent maternal allele of the insulin-like growth factor-II (IGF2) gene, silencing of the normally active maternal allele of the H19 gene, and aberrant methylation of a differentially methylated region (DMR) upstream of the maternal copy of H19. Recently, the transcription factor CTCF, which binds to the H19 DMR, has been implicated in the maintenance of H19 and IGF2 imprinting. Here, we show that mutations in the CTCF gene or in the H19 DMR do not occur at significant frequency in WT, nor is there transcriptional silencing of CTCF. We also confirm that methylation of the H19 DMR in WT with LOI includes the CTCF core consensus site. However, some WTs with normal imprinting of IGF2 also show aberrant methylation of CTCF binding sites, indicating that methylation of these sites is necessary but not sufficient for LOI in WT.

Functional phosphorylation sites in the C-terminal region of the multivalent multifunctional transcriptional factor CTCF.

CTCF is a widely expressed and highly conserved multi-Zn-finger (ZF) nuclear factor. Binding to various CTCF target sites (CTSs) is mediated by combinatorial contributions of different ZFs. Different CTSs mediate distinct CTCF functions in transcriptional regulation, including promoter repression or activation and hormone-responsive gene silencing. In addition, the necessary and sufficient core sequences of diverse enhancer-blocking (insulator) elements, including CpG methylation-sensitive ones, have recently been pinpointed to CTSs. To determine whether a posttranslational modification may modulate CTCF functions, we studied CTCF phosphorylation. We demonstrated that most of the modifications that occur at the carboxy terminus in vivo can be reproduced in vitro with casein kinase II (CKII). Major modification sites map to four serines within the S(604)KKEDS(609)S(610)DS(612)E motif that is highly conserved in vertebrates. Specific mutations of these serines abrogate phosphorylation of CTCF in vivo and CKII-induced phosphorylation in vitro. In addition, we showed that completely preventing phosphorylation by substituting all serines within this site resulted in markedly enhanced repression of the CTS-bearing vertebrate c-myc promoters, but did not alter CTCF nuclear localization or in vitro DNA-binding characteristics assayed with c-myc CTSs. Moreover, these substitutions manifested a profound effect on negative cell growth regulation by wild-type CTCF. CKII may thus be responsible for attenuation of CTCF activity, either acting on its own or by providing the signal for phosphorylation by other kinases and for CTCF-interacting protein partners.

Physical and functional interaction between two pluripotent proteins, the Y-box DNA/RNA-binding factor, YB-1, and the multivalent zinc finger factor, CTCF.

CTCF is a unique, highly conserved, and ubiquitously expressed 11 zinc finger (ZF) transcriptional factor with multiple DNA site specificities. It is able to bind to varying target sequences to perform different regulatory roles, including promoter activation or repression, creating hormone-responsive gene silencing elements, and functional block of enhancer-promoter interactions. Because different sets of ZFs are utilized to recognize different CTCF target DNA sites, each of the diverse DNA.CTCF complexes might engage different essential protein partners to define distinct functional readouts. To identify such proteins, we developed an affinity chromatography method based on matrix-immobilized purified recombinant CTCF. This approach resulted in isolation of several CTCF protein partners. One of these was identified as the multifunctional Y-box DNA/RNA-binding factor, YB-1, known to be involved in transcription, replication, and RNA processing. We examined CTCF/YB-1 interaction by reciprocal immunoprecipitation experiments with anti-CTCF and anti-YB-1 antibodies, and found that CTCF and YB-1 form complexes in vivo. We show that the bacterially expressed ZF domain of CTCF is fully sufficient to retain YB-1 in vitro. To assess possible functional significance of CTCF/YB-1 binding, we employed the very first identified by us, negatively regulated, target for CTCF (c-myc oncogene promoter) as a model in co-transfection assays with both CTCF and YB-1 expression vectors. Although expression of YB-1 alone had no effect, co-expression with CTCF resulted in a marked enhancement of CTCF-driven c-myc transcriptional repression. Thus our findings demonstrate, for the first time, the biological relevance of the CTCF/YB-1 interaction.

Functional association of CTCF with the insulator upstream of the H19 gene is parent of origin-specific and methylation-sensitive.

In mammals, a subset of genes inherit gametic marks that establish parent of origin-dependent expression patterns in the soma ([1] and references therein). The currently most extensively studied examples of this phenomenon, termed genomic imprinting, are the physically linked Igf2 (insulin-like growth factor II) and H19 genes, which are expressed mono-allelically from opposite parental alleles [1] [2]. The repressed status of the maternal Igf2 allele is due to cis elements that prevent the H19 enhancers [3] from accessing the Igf2 promoters on the maternal chromosome [4] [5]. A differentially methylated domain (DMD) in the 5' flank of H19 is maintained paternally methylated and maternally unmethylated [6] [7]. We show here by gel-shift and chromatin immunopurification analyses that binding of the highly conserved multivalent factor CTCF ([8] [9] and references therein) to the H19 DMD is methylation-sensitive and parent of origin-dependent. Selectively mutating CTCF-contacting nucleotides, which were identified by methylation interference within the extended binding sites initially revealed by nuclease footprinting, abrogated the H19 DMD enhancer-blocking property. These observations suggest that molecular mechanisms of genomic imprinting may use an unusual ability of CTCF to interact with a diverse spectrum of variant target sites, some of which include CpGs that are responsible for methylation-sensitive CTCF binding in vitro and in vivo.

Transcriptional repression by the insulator protein CTCF involves histone deacetylases.

The highly conserved zinc-finger protein, CTCF, is a candidate tumor suppressor protein that binds to highly divergent DNA sequences. CTCF has been connected to multiple functions in chromatin organization and gene regulation including chromatin insulator activity and transcriptional enhancement and silencing. Here we show that CTCF harbors several autonomous repression domains. One of these domains, the zinc-finger cluster, silences transcription in all cell types tested and binds directly to the co-repressor SIN3A. Two distinct regions of SIN3A, the PAH3 domain and the extreme C-terminal region, bind independently to this zinc-finger cluster. Analysis of nuclear extract from HeLa cells revealed that CTCF is also capable of retaining functional histone deacetylase activity. Furthermore, the ability of regions of CTCF to retain deacetylase activity correlates with the ability to bind to SIN3A and to repress gene activity. We suggest that CTCF driven repression is mediated in part by the recruitment of histone deacetylase activity by SIN3A.

Negative transcriptional regulation mediated by thyroid hormone response element 144 requires binding of the multivalent factor CTCF to a novel target DNA sequence.

DNA target sites for a "multivalent" 11-zinc-finger CCTC-binding factor (CTCF) are unusually long ( approximately 50 base pairs) and remarkably different. In conjunction with the thyroid receptor (TR), CTCF binding to the lysozyme gene transcriptional silencer mediates the thyroid hormone response element (TRE)-dependent transcriptional repression. We tested whether other TREs, which in addition to the presence of a TR binding site require neighboring sequences for transcriptional function, might also contain a previously unrecognized binding site(s) for CTCF. One such candidate DNA region, previously isolated by Bigler and Eisenman (Bigler, J., and Eisenman, R. N. (1995) EMBO J. 14, 5710-5723), is the TRE-containing genomic element 144. We have identified a new CTCF target sequence that is adjacent to the TR binding site within the 144 fragment. Comparison of CTCF recognition nucleotides in the lysozyme silencer and in the 144 sequences revealed both similarities and differences. Several C-terminal CTCF zinc fingers contribute differently to binding each of these sequences. Mutations that eliminate CTCF binding impair 144-mediated negative transcriptional regulation. Thus, the 144 element provides an additional example of a functionally significant composite "TRE plus CTCF binding site" regulatory element suggesting an important role for CTCF in cooperation with the steroid/thyroid superfamily of nuclear receptors to mediate TRE-dependent transcriptional repression.

Characterization of the chicken CTCF genomic locus, and initial study of the cell cycle-regulated promoter of the gene.

CTCF is a multifunctional transcription factor encoded by a novel candidate tumor suppressor gene (Filippova, G. N., Lindblom, A., Meinke, L. J., Klenova, E. M., Neiman, P. E., Collins, S. J., Doggett, N. D., and Lobanenkov, V. V. (1998) Genes Chromosomes Cancer 22, 26-36). We characterized genomic organization of the chicken CTCF (chCTCF) gene, and studied the chCTCF promoter. Genomic locus of chCTCF contains a GC-rich untranslated exon separated from seven coding exons by a long intron. The 2-kilobase pair region upstream of the major transcription start site contains a CpG island marked by a "Not-knot" that includes sequence motifs characteristic of a TATA-less promoter of housekeeping genes. When fused upstream of a reporter chloramphenicol acetyltransferase gene, it acts as a strong transcriptional promoter in transient transfection experiments. The minimal 180-base pair chCTCF promoter region that is fully sufficient to confer high level transcriptional activity to the reporter contains high affinity binding element for the transcription factor YY1. This element is strictly conserved in chicken, mouse, and human CTCF genes. Mutations in the core nucleotides of the YY1 element reduce transcriptional activity of the minimal chCTCF promoter, indicating that the conserved YY1-binding sequence is critical for transcriptional regulation of vertebrate CTCF genes. We also noted in the chCTCF promoter several elements previously characterized in cell cycle-regulated genes, including the "cell cycle-dependent element" and "cell cycle gene homology region" motifs shown to be important for S/G2-specific up-regulation of cdc25C, cdc2, cyclin A, and Plk (polo-like kinase) gene promoters. Presence of the cell cycle-dependent element/cell cycle gene homology region element suggested that chCTCF expression may be cell cycle-regulated. We show that both levels of the endogenous chCTCF mRNA, and the activity of the stably transfected chCTCF promoter constructs, increase in S/G2 cells.

A widely expressed transcription factor with multiple DNA sequence specificity, CTCF, is localized at chromosome segment 16q22.1 within one of the smallest regions of overlap for common deletions in breast and prostate cancers.

The cellular protooncogene MYC encodes a nuclear transcription factor that is involved in regulating important cellular functions, including cell cycle progression, differentiation, and apoptosis. Dysregulated MYC expression appears critical to the development of various types of malignancies, and thus factors involved in regulating MYC expression may also play a key role in the pathogenesis of certain cancers. We have cloned one such MYC regulatory factor, termed CTCF, which is a highly evolutionarily conserved-11-zinc finger transcriptional factor possessing multiple DNA sequence specificity. CTCF binds to a number of important regulatory regions within the 5' noncoding sequence of the human MYC oncogene, and it can regulate its transcription in several experimental systems. CTCF mRNA is expressed in cells of multiple different lineages. Enforced ectopic expression of CTCF inhibits cell growth in culture. Southern blot analyses and fluorescence in situ hybridization (FISH) with normal human metaphase chromosomes showed that the human CTCF is a single-copy gene situated at chromosome locus 16q22. Cytogenetic studies have pointed out that chromosome abnormalities (deletions) at this locus frequently occur in many different human malignancies, suggesting the presence of one or more tumor suppressor genes in the region. To narrow down their localization, several loss of heterozygosity (LOH) studies of chromosome arm 16q in sporadic breast and prostate cancers have been carried out to define the most recurrent and smallest region(s) of overlap (SRO) for commonly deleted chromosome arm 16q material. For CTCF to be considered as a candidate tumor suppressor gene associated with tumorigenesis, it should localize within one of the SROs at 16q. Fine-mapping of CTCF has enabled us to assign the CTCF gene to about a 2 centiMorgan (cM) interval of 16q22.1 between the somatic cell hybrid breakpoints CY130(D) and CY4, which is between markers D16S186 (16AC16-101) and D16S496 (AFM214zg5). This relatively small region, containing the CTCF gene, overlaps the most frequently observed SROs for common chromosomal deletions found in sporadic breast and prostate tumors. In one of four analyzed paired DNA samples from primary breast cancer patients, we have detected a tumor-specific rearrangement of CTCF exons encoding the 11-zinc-finger domain. Therefore, taken together with other CTCF properties, localization of CTCF to a narrow cancer-associated chromosome region suggests that CTCF is a novel candidate tumor suppressor gene at 16q22.1.

Negative protein 1, which is required for function of the chicken lysozyme gene silencer in conjunction with hormone receptors, is identical to the multivalent zinc finger repressor CTCF.

The transcriptional repressor negative protein 1 (NeP1) binds specifically to the F1 element of the chicken lysozyme gene silencer and mediates synergistic repression by v-ERBA, thyroid hormone receptor, or retinoic acid receptor. Another protein, CCCTC-binding factor (CTCF), specifically binds to 50-bp-long sequences that contain repetitive CCCTC elements in the vicinity of vertebrate c-myc genes. Previously cloned chicken, mouse, and human CTCF cDNAs encode a highly conserved 11-Zn-finger protein. Here, NeP1 was purified and DNA bases critical for NeP1-F1 interaction were determined. NeP1 is found to bind a 50-bp stretch of nucleotides without any obvious sequence similarity to known CTCF binding sequences. Despite this remarkable difference, these two proteins are identical. They have the same molecular weight, and NeP1 contains peptide sequences which are identical to sequences in CTCF. Moreover, NeP1 and CTCF specifically recognize each other's binding DNA sequence and induce identical conformational alterations in the F1 DNA. Therefore, we propose to replace the name NeP1 with CTCF. To analyze the puzzling sequence divergence in CTCF binding sites, we studied the DNA binding of 12 CTCF deletions with serially truncated Zn fingers. While fingers 4 to 11 are indispensable for CTCF binding to the human c-myc P2 promoter site A, a completely different combination of fingers, namely, 1 to 8 or 5 to 11, was sufficient to bind the lysozyme silencer site F1. Thus, CTCF is a true multivalent factor with multiple repressive functions and multiple sequence specificities.

Molecular weight abnormalities of the CTCF transcription factor: CTCF migrates aberrantly in SDS-PAGE and the size of the expressed protein is affected by the UTRs and sequences within the coding region of the CTCF gene.

CTCF belongs to the Zn finger transcription factors family and binds to the promoter region of c-myc. CTCF is highly conserved between species, ubiquitous and localised in nuclei. The endogenous CTCF migrates as a 130 kDa (CTCF-130) protein on SDS-PAGE, however, the open reading frame (ORF) of the CTCF cDNA encodes only a 82 kDa protein (CTCF-82). In the present study we investigate this phenomenon and show with mass-spectra analysis that this occurs due to aberrant mobility of the CTCF protein. Another paradox is that our original cDNA, composed of the ORF and 3'-untranslated region (3'-UTR), produces a protein with the apparent molecular weight of 70 kDa (CTCF-70). This paradox has been found to be an effect of the UTRs and sequences within the coding region of the CTCF gene resulting in C-terminal truncation of CTCF-130. The potential attenuator has been identified and point-mutated. This restored the electrophoretic mobility of the CTCF protein to 130 kDa. CTCF-70, the aberrantly migrating CTCF N-terminus per se, is also detected in some cell types and therefore may have some biological implications. In particular, CTCF-70 interferes with CTCF-130 normal function, enhancing transactivation induced by CTCF-130 in COS6 cells. The mechanism of CTCF-70 action and other possible functions of CTCF-70 are discussed.

An exceptionally conserved transcriptional repressor, CTCF, employs different combinations of zinc fingers to bind diverged promoter sequences of avian and mammalian c-myc oncogenes.

We have isolated and analyzed human CTCF cDNA clones and show here that the ubiquitously expressed 11-zinc-finger factor CTCF is an exceptionally highly conserved protein displaying 93% identity between avian and human amino acid sequences. It binds specifically to regulatory sequences in the promoter-proximal regions of chicken, mouse, and human c-myc oncogenes. CTCF contains two transcription repressor domains transferable to a heterologous DNA binding domain. One CTCF binding site, conserved in mouse and human c-myc genes, is found immediately downstream of the major P2 promoter at a sequence which maps precisely within the region of RNA polymerase II pausing and release. Gel shift assays of nuclear extracts from mouse and human cells show that CTCF is the predominant factor binding to this sequence. Mutational analysis of the P2-proximal CTCF binding site and transient-cotransfection experiments demonstrate that CTCF is a transcriptional repressor of the human c-myc gene. Although there is 100% sequence identity in the DNA binding domains of the avian and human CTCF proteins, the regulatory sequences recognized by CTCF in chicken and human c-myc promoters are clearly diverged. Mutating the contact nucleotides confirms that CTCF binding to the human c-myc P2 promoter requires a number of unique contact DNA bases that are absent in the chicken c-myc CTCF binding site. Moreover, proteolytic-protection assays indicate that several more CTCF Zn fingers are involved in contacting the human CTCF binding site than the chicken site. Gel shift assays utilizing successively deleted Zn finger domains indicate that CTCF Zn fingers 2 to 7 are involved in binding to the chicken c-myc promoter, while fingers 3 to 11 mediate CTCF binding to the human promoter. This flexibility in Zn finger usage reveals CTCF to be a unique "multivalent" transcriptional factor and provides the first feasible explanation of how certain homologous genes (i.e., c-myc) of different vertebrate species are regulated by the same factor and maintain similar expression patterns despite significant promoter sequence divergence.

CTCF, a conserved nuclear factor required for optimal transcriptional activity of the chicken c-myc gene, is an 11-Zn-finger protein differentially expressed in multiple forms.

A novel sequence-specific DNA-binding protein, CTCF, which interacts with the chicken c-myc gene promoter, has been identified and partially characterized (V. V. Lobanenkov, R. H. Nicolas, V. V. Adler, H. Paterson, E. M. Klenova, A. V. Polotskaja, and G. H. Goodwin, Oncogene 5:1743-1753, 1990). In order to test directly whether binding of CTCF to one specific DNA region of the c-myc promoter is important for chicken c-myc transcription, we have determined which nucleotides within this GC-rich region are responsible for recognition of overlapping sites by CTCF and Sp1-like proteins. Using missing-contact analysis of all four nucleotides in both DNA strands and homogeneous CTCF protein purified by sequence-specific chromatography, we have identified three sets of nucleotides which contact either CTCF or two Sp1-like proteins binding within the same DNA region. Specific mutations of 3 of 15 purines required for CTCF binding were designed to eliminate binding of CTCF without altering the binding of other proteins. Electrophoretic mobility shift assay of nuclear extracts showed that the mutant DNA sequence did not bind CTCF but did bind two Sp1-like proteins. When introduced into a 3.3-kbp-long 5'-flanking noncoding c-myc sequence fused to a reporter CAT gene, the same mutation of the CTCF binding site resulted in 10- and 3-fold reductions, respectively, of transcription in two different (erythroid and myeloid) stably transfected chicken cell lines. Isolation and analysis of the CTCF cDNA encoding an 82-kDa form of CTCF protein shows that DNA-binding domain of CTCF is composed of 11 Zn fingers: 10 are of C2H2 class, and 1 is of C2HC class. CTCF was found to be abundant and conserved in cells of vertebrate species. We detected six major nuclear forms of CTCF protein differentially expressed in different chicken cell lines and tissues. We conclude that isoforms of 11-Zn-finger factor CTCF which are present in chicken hematopoietic HD3 and BM2 cells can act as a positive regulator of the chicken c-myc gene transcription. Possible functions of other CTCF forms are discussed.