RESEARCH ARTICLEOpen Access
Characterization of constitutive CTCF/cohesin loci:
a possible role in establishing topological
domains in mammalian genomes
Yuanyuan Li1, Weichun Huang1, Liang Niu1, David M Umbach1, Shay Covo2and Leping Li1*
Background: Recent studies suggested that human/mammalian genomes are divided into large, discrete domains
that are units of chromosome organization. CTCF, a CCCTC binding factor, has a diverse role in genome regulation
including transcriptional regulation, chromosome-boundary insulation, DNA replication, and chromatin packaging. It
remains unclear whether a subset of CTCF binding sites plays a functional role in establishing/maintaining
chromatin topological domains.
Results: We systematically analysed the genomic, transcriptomic and epigenetic profiles of the CTCF binding sites
in 56 human cell lines from ENCODE. We identified ~24,000 CTCF sites (referred to as constitutive sites) that were
bound in more than 90% of the cell lines. Our analysis revealed: 1) constitutive CTCF loci were located in
constitutive open chromatin and often co-localized with constitutive cohesin loci; 2) most constitutive CTCF loci
were distant from transcription start sites and lacked CpG islands but were enriched with the full-spectrum CTCF
motifs: a recently reported 33/34-mer and two other potentially novel (22/26-mer); 3) more importantly, most
constitutive CTCF loci were present in CTCF-mediated chromatin interactions detected by ChIA-PET and these
pair-wise interactions occurred predominantly within, but not between, topological domains identified by Hi-C.
Conclusions: Our results suggest that the constitutive CTCF sites may play a role in organizing/maintaining the
recently identified topological domains that are common across most human cells.
Keywords: CTCF, Cohesin, Constitutive binding site, Chromatin interaction, Topological domain
The CCCTC-binding factor (CTCF) is a C2H2-zinc finger
protein with eleven zinc fingers that display close to 100%
similarity between mouse, chicken, and human . CTCF
has a versatile role in genome regulation including tran-
scriptional regulation, e.g., c-Myc [2,3], X chromosome in-
activation , allele-specific silencing at imprinted loci
such as Igf2/H19 [5-8], and regulation of expression of
lineage-specific gene clusters such as the β-globin locus
 and the MHC class II locus . Recently, CTCF has
been implicated in splicing through its action on local
RNA polymerase II pausing , trinucleotide repeat in-
stability [12,13], DNA replication [14,15], and nucleosome
positioning [16,17]. Because of these diverse functional
roles in genome regulation, CTCF has been dubbed the
“Master Weaver” of the genome .
CTCF sometimes co-localizes with cohesin [19,20].
Cohesin, a multi-subunit complex, consists of a hete-
rodimer of SMC (structure maintenance of chromosomes)
proteins, SMC1 (structural maintenance of chromosomes
1) and SMC3 (structural maintenance of chromosomes 3),
with Rad21 [RAD21 homolog (S. pombe) also known as
Scc1] and STAG (also known as Scc3). Cohesin was ini-
tially identified for its role in sister chromatid cohesion
[14,21,22] but has been implicated in regulation of gene
expression [19,20,23-27] and DNA replication [28,29].
Schmidt et al. have shown that cohesin can also bind to
thousands of sites independent of CTCF .
The first genome-wide study of the CTCF binding in hu-
man cell lines identified ~14,000 CTCF binding sites .
Most of these sites were located far from the annotated
transcriptional start sites (TSS). A subsequent analysis of
* Correspondence: firstname.lastname@example.org
1Biostatistics Branch, National Institute of Environmental Health Sciences,
Research Triangle Park, Durham, NC 27709, USA
Full list of author information is available at the end of the article
© 2013 Li et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.
Li et al. BMC Genomics 2013, 14:553
CTCF binding in three human cell lines showed that they
had around 40-60% of the CTCF binding sites in common
. Recently, Chen et al.  identified ~28,000 constitu-
tive CTCF sites in 19 human cell types and showed that a
large proportion of the variable CTCF binding between dif-
ferent cell types is linked to differential DNA methylation.
A study of the evolution of CTCF binding in six repre-
sentative mammals identified thousands of highly con-
served, robust and tissue-independent CTCF binding
sites . Those studies suggest that, unlike many other
transcription factors/proteins, a substantial portion of
CTCF sites may be bound across multiple cell lines, i.e.,
The canonical CTCF binding motif is 16 to 20 base pairs
(bp) long . Earlier studies suggested CTCF may be cap-
able of binding to sequences of as long as 40–60 bp [2,3].
Footprinting of CTCF binding to the amyloid precursor
protein (APP) promoter confirmed that CTCF can bind to
a 40-bp fragment . Recently, Schmidt et al.  identi-
fied a 33/34-mer full-spectrum CTCF motif in a subset of
evolutionarily conserved CTCF binding loci.
Given its diverse functional roles in genome regulation,
different “classes” of CTCF binding sites might exist where
each class has a unique co-factor (or a combination of co-
factors) and/or has different binding specificity (e.g., ca-
nonical vs. full-spectrum). In this computational study, we
examine the functional relevance of a unique “class” of
CTCF binding sites – those that were constitutively bound
across multiple human cell lines and co-localized with
constitutively bound cohesin loci. We operationally refer
to a binding site that was bound by a protein in 90% or
more of available cell lines as a “constitutive” site.
Genome-wide CTCF-mediated chromatin interactions
have been mapped using Chromatin Interaction Analysis
Paired-End Tags (ChIA-PET) in mouse ES cells  and in
K562 and MCF7 cell lines (data available at the ENCODE
portal on UCSC genome browser). ChIA-PET identifies
specific long-range chromatin interactions where widely
separated genomic regions are brought to spatial proximity
mediated by a protein detected by ChIP .
Recently, Guelen et al.  identified ~1300 sharply
defined large domains defined by interactions with nu-
clear lamina components. Similarly, Dixon et al. 
identified two to three thousand topological domains
in multiple cell and tissue types by Hi-C . Both
studies suggested that human/mammalian genomes are
partitioned into large, discrete domains that are units
of chromosome organization. Dixon et al.  and
Meuleman et al.  further proposed that these topo-
logical domains are common across different cell types
and highly conserved across species. Both studies found
that boundaries are enriched with CTCF sites. Those
studies reinforce the notion that CTCF plays an import-
ant role in genome organization.
Using ChIP-seq (chromatin immunoprecipitation with
sequencing) datasets from ENCODE  (http://genome.
ucsc.edu/ENCODE/), we identified ~24,000 constitutive
CTCF binding sites, ~12,000 of which further co-localized
with constitutive cohesin loci. Our computational analysis
further revealed that the CTCF-mediated chromatin inter-
action regions detected by ChIA-PET in both K562 and
MCF7 cell lines were enriched with sites where constitu-
tive CTCF and constitutive cohesin co-localized . Fur-
thermore, we found that these CTCF-mediated chromatin
interactions were predominantly within topological do-
mains rather than between them. Those results suggest
that sites with constitutive CTCF plus constitutive cohesin
may be involved in establishing/maintaining global chro-
matin structure that is common across cell lines [23,24].
ENCODE  makes available ChIP-seq data for CTCF as
well as other proteins, with multiple cell lines available for
some proteins. These data allowed us to identify CTCF
binding sites within CTCF peaks and within ChIP-seq
peaks for other proteins. We refer to a genomic site that is
bound by a protein in more than 90% of cell lines as a “con-
stitutive” site (Figure 1). (Our choice of 90% was arbitrary
but designed to balance stringency with possible false nega-
tives). We used those CTCF binding sites as surrogates
both to identify constitutive CTCF loci and to identify, for
other proteins, their constitutive loci that overlapped with
constitutive CTCF loci (Figure 1).
Because all analyses involved CTCF binding sites in
terms of constitutive sites, we simply refer to constitutive
CTCF sites in CTCF peaks as constitutive CTCF sites,
with CTCF here being the protein, not the motif.
Throughout the manuscript, we used ‘c’ in front a protein
to denote constitutive and ‘/’ between proteins to denote
co-localization or overlap. For instance, a cCTCF/cRad21
site is a constitutive CTCF site within a CTCF peak that
also lies within a constitutive Rad21 peak. We also use
‘site’ to indicate a motif site, as distinct from a ‘peak’ from
ChIP-seq. We also refer to a broader region near a binding
site as a locus.
Constitutive CTCF sites
We identified 458,251 unique CTCF binding sites in the
112 human CTCF ChIP-seq datasets representing 56 cell
lines (Figure 2). Over half of these sites (241,300 or 53%)
were identified only in a single cell line, suggesting that
those CTCF sites may be cell-line specific. Recently, Wang
et al.  found that differential DNA methylation of the
CTCF binding sites may count for a considerable portion
of cell-selective regulation of CTCF binding. A substantial
number (23,709) of CTCF sites were identified in more
than 90% (≥51 of 56) of the cell lines, suggesting that
Li et al. BMC Genomics 2013, 14:553
Page 2 of 12
those CTCF sites may be involved in some fundamental
biological process common to most or all cell lines. One
might consider the CTCF sites bound in all cell lines as
the constitutive sites (5,611 in our data). ChIP-seq data is
imperfect, however, so we regarded that criterion as too
stringent – especially for questions of co-localization. Our
goal was to probe the functional relevance of certain clas-
ses of CTCF sites, not to identify individual sites.
Constitutive CTCF/cohesin sites
To characterize co-localization with cohesin, we pre-
dicted the CTCF binding sites in Rad21 peaks from 15
ChIP-seq datasets in six cell lines and in Smc3 peaks
from four ENCODE ChIP-seq datasets in four cell lines
(Gm12878, Helas3, Hepg2, and K562 cell lines). We
identified 22,055 constitutive CTCF sites in Rad21 ChIP-
seq peaks and 16,704 constitutive CTCF sites in Smc3
Figure 1 A cartoon illustrating the various constitutive binding sites. The blue, orange, and green horizontal bars represent the 200 bp
ChIP-seq peaks for CTCF, Rad21, and Smc3 proteins, respectively. There were 56, 6, and 4 human cell lines for which CTCF, Rad21, and Smc3
ChIP-seq data were available. We first predicted the CTCF binding site in the peaks, represented by the black rectangles (see Method). We define
as a “constitutive” CTCF site any CTCF binding site found in more than 90% of the cell lines. We use ‘c’ in front of a protein to indicate
constitutive, ‘/’ between proteins to indicate co-localization/overlap. A cRad21 site that overlaps with a cSmc3 site is referred to as a cRad21/
cSmc3 or cCohesin site, similarly for cCTCF/cCohesin sites.
Figure 2 Distribution CTCF binding sites according to the number of cell lines in which binding at the site was present. Those at the
right side of the plot are considered constitutive CTCF sites.
Li et al. BMC Genomics 2013, 14:553
Page 3 of 12
peaks (Table 1). Ninety per cent of the cSmc3 sites over-
lapped with the cRad21 sites. The majority of cRad21 sites
(76%) and of cSmc3 sites (79%) overlapped with cCTCF
sites (Table 1). We refer to the constitutive CTCF sites in
CTCF peaks that overlap with the constitutive CTCF sites
in both Rad21 and Smc3 peaks as the cCTCF/cCohesin
sites (intersection of the three) (Table 1). The association
of CTCF with cohesin is known [19,20], however, we re-
fined that result by showing that this association was
strongest when both are constitutive (Additional file 1,
Additional file 2: Table S1-S3 and Additional file 3: Figures
S1 and S2).
Among the 23,709 cCTCF sites, 12,014 overlapped
with cCohesin (cRad21/cSmc3) sites. Only 925 of the
remaining 11,065 cCTCF sites did not overlap with any
Rad21/Smc3 (cohesin) sites in the four cell lines with
Smc3 ChIP-seq data available. We refer to these 925
sites as the cCTCF without any cohesin sites or cCTCF-
non-cohesin sites and contrast their properties with
those of cCTCF/cCohesin sites. Those cCTCF sites that
overlap with cohesin loci in one, two or three of the four
cell lines are not classified as either cCTCF/cCohesin or
cCTCF-non-cohesin. (Additional file 4 list all sites that
were constitutive by our criterion).
cCTCF sites are more conserved than non-constitutive
To see if cCTCF sites are more likely to be conserved than
non-constitutive CTCF sites, we extracted the multiZ
alignments of both the 23,709 human cCTCF and an
equal number of randomly selected non-constitutive hu-
man CTCF sites and scanned for CTCF binding sites (see
Methods). As expected, the cCTCF sites are twice more
likely to be conserved among species than the con-
constitutive CTCF sites (Figure 3). On average, cCTCF
sites could be found in 20 species compared to 10 for
non-constitutive CTCF sites.
Most cCTCF/cCohesin sites are distant from promoters
and lack CpG islands
Most cCTCF/cCohesin sites were located in intergenic
or intronic regions, away from transcription start sites
(TSS) and without CpG islands (Figure 4). The lack of
CpG islands near the cCTCF/cCohesin sites is consistent
with the finding that a considerable portion of the cell-
type variable CTCF binding sites are subject to differen-
tial DNA methylation . DNA methylation in CpG
islands near CTCF binding sites affects CTCF binding
and, subsequently, transcription regulation of the nearby
enhancer elements [5,41]. Together, those results suggest
that the role of the cCTCF/cCohesin sites may be struc-
tural rather than transcriptional. In contrast, the cCTCF-
non-cohesin sites tend to be located near TSS and in CpG
islands (Figure 4).
cCTCF/cCohesin loci are associated with constitutive
We examined other proteins for possible association with
cCTCF loci. In each cell line separately, we counted how
often a protein had the center of its ChIP-seq peak
within ± 200 bp from CTCF sites in three classes:
the 12,014 cCTCF/cCohesin sites, the 925 cCTCF-non-
cohesin sites, and all CTCF sites bound in the given
cell line but excluding the cCTCF sites. We used all
ENCODE TFBS datasets (encodeHaib, encodeSydh,
encodeUTA, and encodeUW), histone modification
datasets (encodeHistoneBroad, encodeHistoneUW, en-
codeHistoneUTA), and open chromatin datasets (enco-
deDukeDNase, encodeUWDNase, and encodeUNCFaire).
The total number of histone marks analysed was 11
Table 1 Total CTCF binding sites in various classes of
Number of cell lines
Any CTCFAny56 458,251
CTCF in one cell line
1 56 241,300
≥51/6 56/6 16,689
1To be declared constitutive, a site had to be identified in ≥90% of available
Figure 3 Boxplots of the number of species in which CTCF
binding sites are found in the 46-way multiZ alignments for
23,709 cCTCF (green) and 23,709 randomly selected
non-constitutive CTCF (blue) sites.
Li et al. BMC Genomics 2013, 14:553
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(Additional file 1). CTCF, Rad21, and Smc3were not in-
cluded in the analysis. In total, we included 1011 ChIP-seq
datasets representing 68 factors in this analysis and com-
bined results from all cell lines for the same factor/feature.
On average, 74% of the cCTCF/cCohesin sites over-
lapped with DNase I [42,43] peaks compared to only
44% of the cCTCF-non-cohesin sites or 15% of all
CTCF sites bound in a given cell line excluding cCTCF
sites (Figure 5A); the same pattern held with FAIRE
[44,45] peaks (Figure 5B). These results together indi-
cate that cCTCF/cCohesin sites are highly associated
with open chromatin. H2A.z was also more likely to be
associated with the cCTCF compared to all CTCF sites
bound in a given cell line excluding cCTCF sites
(Figure 5C). Several factors, especially those that tend
to be associated with actively transcribed genes or lo-
cated at the proxy promoter such as H3k4me3 were
more likely to be associated with the cCTCF-non-
cohesin sites (Additional file 3: Figure S3).
cCTCF/cCohesin loci are enriched with the 33/34-mer and
The motif logo of the extended cCTCF/cCohesin sites
(30bp at each side) showed information flanking the 16-
bp core CTCF motif (data not shown), indicative of the
existence of additional motifs. To discover those motifs,
we used the 12,014 cCTCF/cCohesin sequences. At each
of the 30+30+16-5+1=72 positions, we counted the
number of sequences in which each of the 1,024 possible
k-mers (k=5) occurred as in . The counts were then
ranked and the top 50 k-mers at each side of the core
CTCF motif were combined separately to create the
composite motif for the side using the position-specific
k-mer frequency as the weight.
This process identified a 33/34-mer full-spectrum
CTCF motif (Figure 6) that was reported previously .
In addition, we identified two potentially novel motifs at
the other side of the core CTCF motif (Figure 6). It is
unclear if the pattern represents two separate motifs
Figure 4 Genomic distributions of the 12,014 cCTCF/cCohesin and the 925 cCTCF-cCohesin sites. (A) Pie charts. If a binding site is
within ±5 kb from the transcription start site (TSS) of a reference gene (GRCh37), it was annotated as ‘TSS’. Otherwise, if it is located between the
TSS and the transcription end site of a gene, it was annotated as ‘in gene’. All others were annotated as “others”. (B) The density of CpG islands.
For each CTCF binding site, we computed the cumulative density of the CpG islands within ±2.5 kb centered at the site.
Figure 5 Box plots of the fraction of overlap within each cell line between classes of CTCF sites and open chromatin (as assessed by
either DNase I or by FAIRE) or H2A.z. The classes of CTCF sites are: cCTCF/cCohesin sites (black), cCTCF-non-cohesin sites (red), or all CTCF sites
bound in each cell line excluding cCTCF sites (blue). Each point in a box plot represents a cell line.
Li et al. BMC Genomics 2013, 14:553
Page 5 of 12
(denoted the 20-mer and the 26-mer) or one single
motif (the 20+26-mer). The extra motif in the 20-mer
resembled the GAGA-binding motif (GAGA) whereas
the extra motif in the 26-mer resembled that for CAF1
(chromatin assembly factor) according to STAMP
(http://www.benoslab.pitt.edu/stamp/). All of these full-
spectrum motifs showed higher enrichment among
cCTCF/cCohesin loci compared to either cCTCF-non-
cohesin or all CTCF loci (Table 2). It is not clear if the
additional motifs (20-/26-mer) at the right side of the
canonical motif are bound by CTCF or by a co-factor.
This question needs to be determined experimentally.
Recently, additional CTCF motifs such as (C6D, C7D,
C8D, and U5C7D) have been reported . However, the
proportions of those motifs in the context of the core
motif are low ranging from 0.3% to 4% in CD43 cells. We
estimated that the proportions of those motifs in both
cCTCF and non-constitutive CTCF loci were ~0.5%.
cCTCF is enriched in CTCF-mediated chromatin interactions
Several studies have implicated CTCF in mediating long-
range chromatin interactions [18-20,23-26,48,49]. The
genome-wide CTCF-mediated chromatin interactions in
cell lines K562 and MCF7 have been mapped using Chro-
matin Interaction Analysis Paired-End Tags (ChIA-PET)
from ENCODE/GIS-Ruan. In cell line K562, we identified
105,041 unique CTCF binding sites (combined from repli-
cates) in CTCF ChIP-seq peaks, among which 23,577 were
cCTCF sites. The proportion of ChIA-PET interaction
sites among these cCTCF sites was significantly higher
than the corresponding proportion among the non-
constitutive CTCF binding sites (odds ratio=7.7, p-value≈0,
one-sided Fisher exact test) (Table 3a). This enrichment
was also seen for cell line MCF7 (odds ratio=8.5, p-
value≈0, one-sided Fisher exact test) (Table 3b). The major-
ity of the cCTCF sites (60% and 82%, in K562 and MCF7,
respectively) overlapped with the chromatin interaction re-
gions compared to only 16% and 35% of the non-
constitutive CTCF sites. Similarly, we found that among all
CTCF-mediated interactions, more interactions involved
cCTCF than the non-constitutive CTCF in both K562 and
MCF7 cell lines (Table 4), respectively, despite there being
2.5 and 1.9 times more non-constitutive CTCF sites than
cCTCF sites in those cell lines. Together those results sug-
gest that chromatin interactions may be mediated largely
by cCTCF sites.
Because of the strong association between cCTCF and
cCohesin in K562 and MCF7 cell lines, we found that the
odds of ChIA-PET detected interactions were approxi-
mately 3 times greater among the cCTCF/cCohesin sites
than among the cCTCF sites without cCohesin (p-value≈0)
Interplay between topological domains and cCTCF sites
Two recent studies [23,36] suggested that human/mam-
malian genomes are divided into large, discrete domains
that are units of chromosome organization. Dixon et al.
 further proposed that the topological domains are
common across different cell types and highly conserved
across species. Those results, together with our ChIA-
PET results, suggest that chromatin topological domains
and CTCF-mediated chromatin interactions may be in-
Indeed, we found that 87% of the CTCF-mediated
interactions from ChIA-PET in either K562 or MCF7
cell line were within the topological domains of H1hesc
cell line (mean and median domain lengths 852 kb
and 680 kb) . Importantly, among those CTCF-
mediated intrachromosomal interactions that were in
og o l f i t oMf i t oM
Figure 6 Motif logos of the 33/34-mer and 20/26-mer motifs.
Table 2 Proportions of the 33/34-mer and the 20/26-mer
motifs in various classes of CTCF loci
MotifcCTCF/cCohesin cCTCF-non-cohesinAll CTCF sites
20-mer 8.2%3.7% 1.2%
Li et al. BMC Genomics 2013, 14:553
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the topological domains, 77% to 81% had both regions
of the pair in the same domain, suggesting that the
CTCF-mediated chromatin interactions are predomin-
antly within a domain. More than 90% of both cCTCF
and non-constitutive CTCF sites were located inside the
topological domains. However, the non-constitutive
CTCF sites tended to be uniformly distributed within
the domain whereas the cCTCF sites appear to be
enriched near the boundaries of the domains (Figure 7).
As a control, we selected five other ENCODE TFBS
ChIP-seq datasets with the highest number of peaks
(Bach1, Cebpb, cJund, Max, and Sin3a for K562 and
Cebpb, Corest, Mafk, Maz and Mxil1 for Imr90) and
computed the distances between the ChIP-seq peaks
and the nearest topological domains. Similar to the non-
constitutive CTCF sites, the distribution of the distances
for the five proteins in each cell is more uniform than the
corresponding distribution for cCTCF (Figure 7).
CTCF is a multi-functional protein that has been impli-
cated in transcriptional regulation, insulation, DNA repli-
cation, X-chromosome inactivation, splicing chromatin
packaging and many others . CTCF binding sites are
widespread in genomes from fly to humans [1,18]. Earlier,
several genome-wide studies identified ~14,000 to ~27,000
CTCF binding sites in several human cell lines. Those
studies also showed that 40-60% of the CTCF sites in the
cell lines studied were invariant to cell types [17,31]. Many
CTCF binding sites were also computationally identified
 and found to be conserved [17,31,33,50,51]. However,
it remained unclear how many CTCF binding sites are
present in the human genome and what proportion of
them is constitutively bound across most cell lines/tissues.
A comprehensive CTCF binding site database containing
more than 15 million sequences in 10 species has been re-
cently updated to include long-range chromatin interaction
data mediated by CTCF , thereby facilitating analyses
like ours in non-human species.
Our analysis of 112 ENCODE CTCF ChIP-seq datasets
representing 56 human cell lines suggests that there might
be as many as 450,000 CTCF binding sites in the human
genome. Nearly half were found in CTCF peaks in only
one of the 56 cell lines. About a quarter million of the
Table 3 Proportions of various CTCF binding sites that contained within the CTCF-mediated interaction regions from
Sites in interaction
Comparison Total sitesOdds ratio
cCTCF 23,577 14,1780.60
All CTCF excluding cCTCF81,46413,3160.16
cCTCF excluding cCohesin11,5635,4640.47
All CTCF excluding cCTCF 67,75223,7010.35
cCTCF/cCohesin 12,00110,796 0.90
cCTCF excluding cCohesin 11,6408,6020.74
Table 4 Proportions of CTCF-mediated interactions
involving cCTCF and non-constitutive CTCF sites
Category of interaction
CTCF type in region 1/2CTCF type in region 2/1
cCTCFAny CTCF excluding cCTCF8,991 (35.5%)
Any CTCF excluding cCTCFAny CTCF excluding cCTCF 2,567 (10.1%)
Any CTCF excluding cCTCFNeither1,627 (6.4%)
(B) MCF7 replicate 1
cCTCFNeither 6,590 (13.1%)
cCTCFAny CTCF excluding cCTCF17,601 (34.9%)
Any CTCF excluding cCTCFAny CTCF excluding cCTCF6,738 (13.3%)
Any CTCF excluding cCTCFNeither4,265 (8.5%)
(C) MCF7 replicate 2
cCTCF Neither2,332 (11.6%)
cCTCFAny CTCF excluding cCTCF6,693 (33.2%)
Any CTCF excluding cCTCFAny CTCF excluding cCTCF2,219 (11.0%)
Any CTCF excluding cCTCFNeither 1,469 (7.3%)
Li et al. BMC Genomics 2013, 14:553
Page 7 of 12
CTCF sites were found in CTCF peaks in more than one
of the 56 cell lines. Moreover, ~24,000 CTCF binding sites
were found in CTCF peaks in more than 90% (at least 51
of 56) cell lines, suggesting that those constitutive CTCF
sites may be implicated in some fundamental biological
process/function for most or all cell lines.
Of course, the exact numbers of cCTCF sites identified
by our methods depend on thresholds used for making de-
cisions. In our analysis, we trimmed/extended all peaks to
200bp in length from the center. Using 300bp instead in-
creased by 1,640 the number of CTCF sites declared con-
stitutive. Including these additional sites in our analysis of
ChIA-PET interactions yielded results substantially the
same as those in Table 4. In our analysis, a p-value cut-off
of 0.0005 on the PWM score identified a CTCF binding
site in 80-95% of the peaks. Adjusting the cut-off would
certainly affect the number of CTCF sites identified and
declared constitutive; but, like changing the peak length,
changing this cut-off seems unlikely to influence our re-
sults about enrichment and our overall conclusions about
the role of cCTCF sites.
Because many datasets used in our analysis were from
cancer cell lines which often carry genetic and chromatin
aberrations, we looked for evidence that cCTCF sites
might diverge between cancer and normal cell lines. We
identified 27,735, 28,662, and 27,774 cCTCF sites in re-
cently deposited CTCF ChIP-seq from 23 cancer cell lines,
20 normal cell lines, and 19 cell lines with unknown kar-
yotypes, respectively . Not only did these three groups
have similar numbers of cCTCF sites, they had 19,279
(80.5% to 83.2%) cCTCF sites in common, indicating that
cell origins have little effect on the number or locations of
The nature of ChIP-seq experiments is to capture a
snapshot of protein binding in time. Thus, the sites that
we define as constitutive because they are bound in over
90% of cell lines are likely sites where a protein spends
most time in the bound state -- perhaps an individual
binding event of long duration or perhaps frequent
bouts of binding/unbinding with the bound state pre-
dominating. Long-duration binding might be attributed
to strong binding whereas frequent binding/unbinding
would not be. Thus, the constitutive sites that we detect
should not correspond exactly to sites with strong bind-
ing, though different binding motifs (canonical vs. full-
spectrum) might be correlated with binding strength.
On the other hand, one can imagine sites in the genome
where a protein bound relatively briefly but the site is
bound at some time in every tissue or cell line. Such a
site would theoretically meet our definition of ‘constitu-
tive’ but would go undetected by our analysis as ChIP-
seq snapshots would be virtually impossible to capture
short-term binding at the same site in multiple cell lines.
Strong binding may occur at constitutive sites, but it
may not be the only explanation for their existence. We
recently developed an alternative method for identifying
constitutive sites using peak data only (without motif
search) (manuscript in preparation). We identified consti-
tutive sites for 22 factors with ChIP-seq data in more than
six cell lines. We found that the proportions of constitu-
tive sites vary between different factors from a few to
many thousands. It is unlikely that factors that bind to
the highest number of constitutive sites (e.g., CTCF and
Rad21) are strong binders whereas those that bind to the
fewest constitutive sites (e.g., JunD) are weaker binders.
We also found that gene ontology analysis of the target
genes of the constitutive Pol II sites are highly enriched
with biological processes such as metabolism and cell cycle
(data not shown). Together, those results strongly suggest
that the constitutive sites are biologically meaningful.
Because of CTCF’s diverse roles in genome regulation,
different “classes” of CTCF binding sites might exist to
Figure 7 Kernel density estimate of the distances (in bp) between CTCF sites (cCTCF in green and all CTCF sites excluding the cCTCF in
red) and their nearest topological domain boundaries. Since more than 90% of the CTCF sites are located inside topological domains, only
CTCF sites located inside topological domains were included. We standardized all distances to the median length of all domains in each cell line
(680 kb in H1hesc and 840 kb in IMR90). The distances from the nearest boundary at maximum density (vertical lines) are ~19 kb and 25 kb for
H1hesc and IMR90 cell lines, respectively.
Li et al. BMC Genomics 2013, 14:553
Page 8 of 12
carry out different functional roles. Such classes might
differ in their co-factors and/or binding strength and speci-
ficity (e.g., canonical vs. full-spectrum motifs). In this study,
we focused on the class of CTCF binding sites that are
constitutively bound and co-localized with the constitutive
cohesin loci and compared it to a class of constitutive
CTCF binding sites without cohesin. We examined the
genomic features, transcriptional landscape and epigenetic
environments of those sites to gain insights into their func-
tional relevance. Our analysis not only included many
more datasets but also was more comprehensive than the
earlier analyses of CTCF binding sites [16,31,32,51,53].
We identified ~12,000 constitutive CTCF binding sites
co-localized with constitutive cohesin loci. The majority of
these cCTCF/cCohesin sites were located ≥ 5 kb from the
TSS in introns or in intergenic regions that lacked CpG
islands. Furthermore, the cCTCF/cCohesin loci were
enriched in H3k4me1 mark with well-positioned nucleo-
somes (Additional file 1). A substantial number of the
cCTCF sites overlapped with cohesin in one or more cell
lines without meeting the criterion that the corresponding
Rad21 and Smc3 peaks were in ≥ 90% of available cell
lines. In contrast, few cCTCF sites did not co-localize with
cohesin loci in any cell line.
Our analysis of the constitutive sites is limited by the
number of cell lines studied; some factors have data from
only a limited number of cell lines. As data from add-
itional cell lines become available, some of the cCTCF/
cCohesin sites will no longer be designated as constitutive.
Although the cCTCF sites were found in at least 51 of the
56 cell lines, constitutive cohesin was defined via Rad21
and Smc3 peaks, which were identified in only 6 and 4 cell
Numerous studies have shown that CTCF cooperates
with cohesin to contribute to DNA loop formation to
thereby regulate gene expression and chromatin interac-
tions [18-20,23-26,48,49,54], DNA replication , RNA
pol II pausing . Our computational analysis revealed
that the strength of association between CTCF and co-
hesin increases when both sites/loci were constitutive,
similarly for CTCF and Znf143 (Additional file 1 and
Additional file 2: Table S2), and for CTCF, cohesin, and
Znf143 (Additional file 1 and Additional file 2: Table S3).
A footprinting study of CTCF binding to the pro-
moter of the APP gene showed that the binding of
the full-length CTCF protein generated a DNase I pro-
tected region covering 40 bp . Subsequent motif
analysis  in a set of evolutionarily conserved CTCF
sites identified ~5,000 33/34-mer full-spectrum CTCF
binding sites. We independently identified the same
33/34-mer motifs in the set of cCTCF/cCohesin loci.
Furthermore, we also identified two potentially novel
20/26-mer CTCF motifs (Figure 6). Whether those full-
spectrum motifs function in transcriptional regulation
or in mediating chromatin-chromatin interactions, or
both, remains unclear.
Our analysis in cancer cell lines K562 and MCF7 further
revealed that the majority of the cCTCF sites were located
in the CTCF-mediated chromatin interactions from
ChIA-PET . The proportion of the cCTCF sites in the
chromatin interactions was higher for those cCTCF sites
that overlapped with cCohesin loci than for those that did
not. These results suggest that the genomic loci that are
constitutively co-bound by both CTCF and cohesin may
be involved in establishing or maintaining the “common”
or “ground state” chromatin architecture in most human
cell lines (Figure 8). This idea is consistent with the finding
that the overall topological domain structure between cell/
tissue types or across species is largely unchanged .
Hu et al. further suggested that the geometric shapes of
the topological domains are strongly correlated with sev-
eral genomic and epigenetic features . We found that
most CTCF-mediated interactions from ChIA-PET 
involved cCTCF and were within a domain. It is conceiv-
able that the cCTCF/cCohesin sites are an integral part of
the large, discrete domains [23,24], possibly mediating/
maintaining the sub-domain structures within a domain.
Using ENCODE ChIP-seq data we identified ~450,000
CTCF binding sites in CTCF peaks from 56 cell lines. We
also identified ~24,000 cCTCF and ~12,000 cCTCF/
cCohesin binding sites. The cCTCF sites were located in
unique genomic environments and were over-represented
in CTCF-mediated global chromatin interactions that are
predominantly within, but not between, the proposed to-
pological domains. We suggest that CTCF and cohesin co-
operate in those loci to establish/maintain the “common”
chromatin structure in most human cell lines.
We downloaded all ChIP-seq data defined as either
narrow or broad peaks from the ENCODE portal at
the UCSC genome browser (http://genome.ucsc.edu/EN-
CODE/downloads.html). We extended/trimmed all TFBS
ChIP-seq peaks to 200bp in length from the center of the
peak. All genomic sequence data, such as CpG islands,
were downloaded from the UCSC genome browser. All
data were in GRCh37 (hg19) assembly.
Predicting CTCF binding sites in ChIP-seq data
For each ChIP-seq dataset, we predicted the location of a
CTCF binding site in each peak using the GADEM soft-
ware . The position weight matrix (PWM) model for
CTCF was obtained from earlier de novo analysis  of
the CTCF ChIP-seq datasets . The p-value for the
PWM score cutoff was set to 0.0005 to identify well-
Li et al. BMC Genomics 2013, 14:553
Page 9 of 12
defined CTCF sites. For each ChIP-seq peak, we selected
the single highest scoring site that passed the p-value cut-
off for the PWM score as the binding site. We found a
CTCF binding site in 80-95% of the CTCF ChIP-seq peaks
in each dataset. We combined binding sites from replicate
experiments for any cell line and retained only the unique
ones. Similarly, we predicted the CTCF binding sites in all
other ChIP-seq datasets from ENCODE. The coordinates
of all unique CTCF binding sites identified in the 112
CTCF ChIP-seq datasets in 56 cell lines are provided in
Additional file 5.
Motif conservation analysis
We extracted the 46-way multiZ alignments (hg19) for the
23,709 cCTCF binding sites (16 bp) plus the 10bp flanking
regions using the Galaxy “Extract MAF blocks given a set
of genomic intervals” tool (http://main.g2.bx.psu.edu/).
Multiple blocks in the Galaxy output were merged using
a custom python code. For each multiZ alignment, we
scanned each sequence in the alignment for a CTCF bind-
ing site using the same PWM and p-value (0.0005) cutoff
as before. We then counted the number of sequences
(equivalently, species) in each alignment containing a
CTCF binding site and used the number as a surrogate for
conservation. Similarly, we randomly selected 23,709 non-
constitutive CTCF binding sites that were identified in 2–
10 cell lines and repeated the above analysis.
Overlap with ChIP-PET interaction data
We downloaded genome-wide CTCF-mediated chromatin
interactions identified by ChIA-PET in K562 and MCF7
cell lines from ENCODE/GIS-Ruan (http://hgdownload.
isChiaPet/). The CTCF binding sites used were those
we predicted from ENCODE CTCF ChIP-seq peaks in
K562 and MCF7 as described above. There were 25,304
unique CTCF-mediated interactions from cell line
K562. For cell line MCF7, two replicate experiments
yielded 50,498 and 20,140 unique CTCF-mediated in-
teractions, respectively. Each interaction is defined by a
pair of genomic coordinates, referred to herein as re-
gion1 and region2, respectively. Since is an interaction
has no directionality, the order of the two regions is ir-
relevant. It is worth pointing out that a region in one
interaction pair may overlap with region(s) in another.
Proportions of CTCF sites in ChIA-PET detected interactions
We found 23,577 cCTCF and 81,464 CTCF excluding
cCTCF (non-constitutive CTCF) binding sites in cell
line K562 based on the ChIP-seq data. We then counted
the number of cCTCF and non-constitutive CTCF sites
contained within any regions in the ChIA-PET inter-
Proportion of ChIA-PET detected interactions involving
A ChIA-PET interaction region may contain a cCTCF
site, a non-constitutive CTCF site, or neither. Thus, a
ChIA-PET interaction pair may be one of six possible
types: cCTCF and cCTCF, cCTCF and non-constitutive
CTCF, cCTCF and neither, non-constitutive CTCF and
non-constitutive CTCF, non-constitutive CTCF and nei-
ther, neither and neither (Table 4). Since a region in the
interaction pair may contain multiple cCTCF and/or
Figure 8 A proposed model of role of cCTCF loci in chromatin structure. CTCF, cohesin (not shown) and possibly other factors such as
Znf143 and mediator  (not shown) mediate long-range chromatin interactions through the constitutive CTCF sites. The cCTCF-mediated
interactions organize/maintain the topological domains  that are units of chromosome organization. In this conceptual diagram, only
constitutive CTCF sites are shown (purple ovals) and the topological domains are coloured individually.
Li et al. BMC Genomics 2013, 14:553
Page 10 of 12
non-constitutive CTCF sites, we assigned all possible
types of interactions possible for the pair of regions and
counted them proportionally. For example, if region 1
contained one cCTCF and one non-constitutive CTCF
site and region 2 contained one non-constitutive CTCF
site, we assigned one-half count for cCTCF and non-
constitutive CTCF interaction and one-half count for
non-constitutive CTCF and non-constitutive cCTCF
interaction. This way, each interaction pair in the ori-
ginal ChIA-PET data contributes equally and the sum
of the counts equals to the total number of interaction
pairs in the original ChIA-PET data.
Additional file 1: Supplementary Text.
Additional file 2: Table S1. Summary results for CTCF, Rad21, Smc3 and
Znf143. Table S2. Comparison of the proportion of overlap (fraction of bins
containing peaks for both proteins among bins containing peaks for at least
one of the proteins) among constitutive sites versus non-constitutive sites.
Table S3. Association between CTCF and Rad21/Smc3 and Rad21/Znf143
Additional file 3: Figure S1. The Venn diagrams showing the pair-wise
overlap between CTCF and Rad21, Smc3, and Znf143 when both are
constitutive or non-constitutive. Counts provide for each region in the
Venn diagrams. Figure S2. The Venn diagrams showing the trio-wise
overlap between CTCF and Rad21 and Smc3, Rad21 and Znf143, and Znf143
and Smc3 when both are constitutive or non-constitutive. Figure S3. Box
plots of the fraction of overlap within each cell line between classes of CTCF
sites and various factors/features. Figure S4. Density of epigenetic marks
cCTCF/cCohesin loci (top panels, black) and the cCTCF-Cohesin loci (bottom
panels, red) in Gm12878, Helas3, Hepg2, and K562 cell lines.
Additional file 4: Coordinates of constitutive CTCF binding sites.
Additional file 5: Coordinates of all unique CTCF binding sites
identified in the 112 CTCF ChIP-seq datasets in 56 cell lines. The first
three columns list the coordinates whereas the third column lists the strand
in which the site was found. The last column lists the number of cell lines in
which the site was identified. All coordinates are in GRCh37 build.
CTCF: CCCTC binding factor; Rad21: RAD21 homolog (S. pombe);
Smc1: Structure maintenance of chromosomes 1; Smc3: Structure
maintenance of chromosomes 3; cCTCF: Constitutive CTCF sites;
cRad21: Constitutive Rad21 site; cSmc3: Constitutive Smc3 site; cCTCF/
cCohesin: Constitutive CTCF sites that overlap with constitutive cohesin loci;
cCTCF-non-cohesin: Constitutive CTCF sites that did not overlap with either
Rad21 or Smc3 loci.
The authors declare that they have no competing interests.
LL conceived the study, YL, WH, LN and LL performed the analyses, and
DMU and SC were involved in design, analysis and interpretation of data. All
authors contributed and approved the final manuscript for publication.
We thank Karen Adelman, Kasia Bebenek, Tom Kunkel, Daniel Menendez,
Mike Resnick, and Paul Wade for critical comments and suggestions in the
earlier version of the manuscript. We thank the Computational Biology
Facility at NIEHS for computing time and support.
This research was supported by Intramural Research Program of the NIH,
National Institute of Environmental Health Sciences (ES101765).
1Biostatistics Branch, National Institute of Environmental Health Sciences,
Research Triangle Park, Durham, NC 27709, USA.2Department of Plant
Pathology and Microbiology, Robert H. Smith Faculty of Agriculture, Food
and Environment, Hebrew University, Rehovot, Israel.
Received: 14 March 2013 Accepted: 26 July 2013
Published: 14 August 2013
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Cite this article as: Li et al.: Characterization of constitutive CTCF/
cohesin loci: a possible role in establishing topological domains in
mammalian genomes. BMC Genomics 2013 14:553.
Li et al. BMC Genomics 2013, 14:553
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