Article
Global Reorganization of the Nuclear Landscape in
Senescent Cells
Graphical Abstract
Authors
Tamir Chandra, Philip Andrew Ewels, ...,
Peter Fraser, Wolf Reik
Correspondence
tamir.chandra@babraham.ac.uk
In Brief
Chandra, Ewels, et al. map changes in
genome organization in cellular
senescence using Hi-C. Contrary to the
believed increase in heterochromatin in
senescence-associated heterochromatic
foci formation, they describe a loss of
local interactions in heterochromatic
regions. This is in agreement with
changes observed in progeria cells.
Highlights
d
SAHF cells show sequence- and LAD-dependent loss of
heterochromatin (HC) structure
d
Senescence HC behavior is mirrored in Hutchinson-Gilford
progeria
d
Senescence-specific spatial clustering of HC leads to a new
model for SAHF formation
d
Comparing ESCs, somatic, and senescent cells links
senescence to differentiation
Chandra et al., 2015, Cell Reports 10, 471–483
February 3, 2015 ª2015 The Authors
http://dx.doi.org/10.1016/j.celrep.2014.12.055
Cell Reports
Article
Global Reorganization of the Nuclear Landscape
in Senescent Cells
Tamir Chandra,1,2,7,* Philip Andrew Ewels,1,7,8 Stefan Schoenfelder,3 Mayra Furlan-Magaril,3 Steven William Wingett,3
Kristina Kirschner,4 Jean-Yves Thuret,5 Simon Andrews,6 Peter Fraser,3 and Wolf Reik1,2
1Epigenetics
Programme, The Babraham Institute, Cambridge CB22 3AT, UK
Wellcome Trust Sanger Institute, Cambridge CB10 1SA, UK
3Nuclear Dynamics Programme, The Babraham Institute, Cambridge CB22 3AT, UK
4Cambridge Institute for Medical Research, University of Cambridge, Cambridge CB2 0XY, UK
5CEA, iBiTec-S, SBIGeM/CNRS FRE3377 I2BC/Université Paris-Sud, Gif-sur-Yvette 91191, France
6Bioinformatics Group, The Babraham Institute, Cambridge CB22 3AT, UK
7Co-first author
8Present address: Department of Biochemistry and Biophysics, Science for Life Laboratory, Stockholm University, Stockholm 106 91,
Sweden
*Correspondence: tamir.chandra@babraham.ac.uk
http://dx.doi.org/10.1016/j.celrep.2014.12.055
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).
2The
SUMMARY
Cellular senescence has been implicated in tumor
suppression, development, and aging and is accompanied by large-scale chromatin rearrangements,
forming senescence-associated heterochromatic
foci (SAHF). However, how the chromatin is reorganized during SAHF formation is poorly understood.
Furthermore, heterochromatin formation in senescence appears to contrast with loss of heterochromatin in Hutchinson-Gilford progeria. We mapped
architectural changes in genome organization in
cellular senescence using Hi-C. Unexpectedly, we
find a dramatic sequence- and lamin-dependent
loss of local interactions in heterochromatin. This
change in local connectivity resolves the paradox
of opposing chromatin changes in senescence and
progeria. In addition, we observe a senescence-specific spatial clustering of heterochromatic regions,
suggesting a unique second step required for SAHF
formation. Comparison of embryonic stem cells
(ESCs), somatic cells, and senescent cells shows a
unidirectional loss in local chromatin connectivity,
suggesting that senescence is an endpoint of
the continuous nuclear remodelling process during
differentiation.
INTRODUCTION
Cellular senescence is an irreversible cell-cycle arrest, originally
described for primary cells after long-term cell culture and attributed to telomere attrition (Hayflick and Moorhead, 1961). More
recently, cellular senescence has been established as a cellular
response to a variety of stresses such as DNA double-strand
breaks or oncogene activation (Di Leonardo et al., 1994; Lin
et al., 1998; Serrano et al., 1997).
Oncogene-induced senescence (OIS) is an intrinsic tumor
suppressor mechanism, involving activation of the key tumor
suppressor pathways p53 and pRB/p16INK4a. Inactivation of
one or both of these pathways is found in the majority of cancers.
Markers of senescence, such as p16 upregulation, are particularly prevalent in benign lesions and are often lost upon malignancy (Braig et al., 2005; Haugstetter et al., 2010; Michaloglou
et al., 2005). Reactivation of p53 in mouse models of liver cancer leads to senescence with subsequent immune clearance of
cancer cells (Xue et al., 2007). A key aspect of the senescence
response implicated in the immune clearance is the senescence-associated secretory phenotype (SASP) (Acosta et al.,
2008; Coppé et al., 2008; Kuilman et al., 2008). SASP is characterized through the secretion of cytokines, which are able
to induce paracrine senescence in neighboring cells (Acosta
et al., 2013). Recent work has implicated cellular senescence
in normal developmental processes (Muñoz-Espı́n et al., 2013;
Storer et al., 2013).
In addition to its role in oncogenesis, a role for senescence in
organismal aging has recently been substantiated; the depletion
of senescent cells has been shown to relieve symptoms in
mouse models of age-related diseases, suggesting that cellular
senescence may be a useful model system for organismal aging
(Baker et al., 2011; López-Otı́n et al., 2013).
Previous work has shown that cellular senescence in human
diploid fibroblasts is accompanied by a large-scale spatial
rearrangement of chromatin, forming nuclear structures known
as senescence-associated heterochromatic foci (SAHF). SAHF
are enriched in constitutive heterochromatic markers, such as
H3K9me3 and HP1 proteins (Narita et al., 2003). However,
SAHF formation does not occur in all senescent cells. The
proportion of cells exhibiting SAHF depends on the method of
senescence induction, ranging from a few percent in replicative
senescence to nearly 90% in c-raf OIS (Jeanblanc et al., 2012). In
contrast, other cellular models of organismal aging such as cells
Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors 471
from Hutchinson-Gilford progeria syndrome (HGPS) patients
show a decrease in heterochromatin and are devoid of SAHF
(Scaffidi and Misteli, 2006; Shumaker et al., 2006). Cellular
models of HGPS and cellular senescence of fibroblasts have
proven to be relevant models for organismal aging. It is therefore
important to understand the seemingly contradictory roles of
heterochromatin in cellular aging and SAHF formation.
We have recently shown that SAHF chromosomes show an
inversion of euchromatin, facultative heterochromatin (fHC), and
constitutive heterochromatin (cHC), with cHC moving to the center of chromosomal territories (see also Figure 4C; Chandra et al.,
2012). This inversion is due to a physical reorientation of the chromatin rather than a redistribution of repressive histone marks,
questioning a causal role for classical heterochromatic marks,
H3K9me3 (cHC) and H3K27me3 (fHC), in the formation of heterochromatin in somatic cells. A key feature of the senescent nucleus
and strong correlate with SAHF formation is the loss of lamin
B1 (Sadaie et al., 2013; Shah et al., 2013). Other factors involved
in SAHF formation, such as the cell-cycle regulator pRb, high
mobility group proteins HMGA1/HMGA2, histone chaperones
HIRA and ASF1a, canonical Wnt signaling, chromatin remodelling
proteins p400 and BRG1, and linker histone H1, have been identified; however, knowledge of how the chromosome structure is
changed is still lacking (Chan et al., 2005; Chicas et al., 2010; Funayama et al., 2006; Narita et al., 2003, 2006; Tu et al., 2013; Ye
et al., 2007a, b; Zhang et al., 2005). More importantly, the function
of SAHF is controversial. Whereas the role of SAHF was initially reported as being tumor and cell-cycle suppressive (Narita et al.,
2003, 2006), recent work has suggested that SAHF may in fact
be proproliferative (Di Micco et al., 2011).
To gain insight into the function of SAHF, we decided to
unravel the physical structure of senescent chromatin in unprecedented detail, combining fluorescence in situ hybridization
(FISH) with Hi-C to map the physical changes that accompany
SAHF formation. We find dramatic changes in both the global
interaction network and local neighborhood of genomic regions.
Surprisingly, we find distinct global changes in the interactions
and compaction of certain classes of lamin-associated domains,
defined by continuous genomic fragments of homogenous guanine-cytosine (GC) content (isochores; Bernardi, 2012). Contrary
to the current view of enhanced heterochromatinization in SAHF
formation, we find a loss of internal structure in constitutive heterochromatic (cHC) regions in cellular senescence. This loss of
internal structure is accompanied by spatial clustering of the
cHC regions. We further show that HGPS cells behave similarly
to senescent cells in their local interaction changes but do not
exhibit the spatial clustering of cHC, suggesting a two-step
mechanism for SAHF formation. Finally, we investigate embryonic stem cells (ESCs) and senescent cells and find a fundamentally opposing local architecture with somatic cells representing
an intermediate state.
RESULTS
Senescence Causes Global Shifts in Local Chromatin
Interactions
To study OIS, we used the WI-38hTERT/GFP-RAF1-ER model
system (Jeanblanc et al., 2012). Senescence is induced by
472 Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors
the activation of the GFP-RAF1-ER kinase by addition of 4-hydroxy-tamoxifen. The nuclear phenotype in these cells is highly
homogenous compared to other OIS systems such as Rasinduced senescence, with a high percentage of SAHF-positive
cells (86%; Figures 1A and S1A). Analyses of bromodeoxyuridine (BrdU) incorporation and abundance of key senescence
markers at 48 hr after induction confirmed an OIS phenotype
(Figures S1A and S1B).
Previous work on the reorganization of chromatin in senescence has revealed a complex change in nuclear architecture; intrachromosomal heterochromatin clusters within SAHF
without redistribution of repressive histone marks along the
linear DNA sequence (Chandra et al., 2012; see also Figure 4C).
These observations suggest a dynamic remodelling of the nucleus driven by spatial changes, as opposed to a change in chromatin identity (Chandra and Narita, 2013). To further understand
these changes in nuclear architecture in growing and senescent
cells, we performed Hi-C, a proximity-based ligation assay that
measures the frequency of close chromatin interactions, as summarized in Figure S1C (Lieberman-Aiden et al., 2009). Two independently grown and fixed batches of cells were sequenced to a
depth of at least 114 million reads. Data were processed using
the HiCUP pipeline (S.W.W. and S.A., unpublished data; Figure S1D). For initial inspection, we plotted Hi-C interaction
heatmaps using HOMER (Figure 1B; Heinz et al., 2010). The
heatmaps show a typical pattern of self-interacting topologically
associated domains (TADs) (Dixon et al., 2012; Nora et al., 2012).
Although the global chromosomal interaction pattern appears
largely unchanged between growing and senescent cells, closer
inspection shows striking differences in the strength of internal
interactions within TADs (Figures 1B and 1C). To further investigate this phenomenon, we calculated the number of interactions
in which both interacting regions are located within a single
TAD versus those spanning the TAD boundaries (Figure 1C).
This revealed a loss of interactions within the TAD and a corresponding gain of cross-boundary interactions in senescent cells
(Figure 1C). To assess the consequences of loss of local contacts for chromatin compaction, we measured the distance
between two FISH probes (separated by 2.2 Mb)—one approximately 100 kb upstream of the TAD and the corresponding
probe inside the TAD (Figures 1D and 1F for probe locations).
We found a substantial shift in the mean distance separating
the probes from 0.57 mm in growing to 0.86 mm in senescent cells
(Wilcoxon-Mann-Whitney test; p = 0.00003). This observation
confirms for this TAD that a loss of local interactions leads to a
change in physical compaction in senescence.
To study this change in local interaction frequency more precisely, we developed a measure to examine the difference in
local versus distal interactions genome wide, which we call
open chromatin index (OCI) (Figure 1E). This analysis counts
the number of interactions with both Hi-C reads within a window
and compares these to a control. Initially, we used interaction
counts across the remainder of the chromosome as a control
but found that results were biased due to the varying lengths
of chromosomes. As an alternative, we used the number of interchromosomal contacts. The distal and interchromosomal interaction counts behaved concordantly (Figure 1C), and the latter
provided a more stable metric. We measured the ratios of these
A
B
WI-38
Growing
WI-38
G TADs
chr18:48Mb-78Mb
5 µm
Growing
Senescence
+ c-raf-ER
hTert
Senescent
D
C
Separation (µm)
Growing
0.5
1.0
Senescence
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Growing
G TADs
G
chr18:58.7Mb-67.7Mb
***
Senescent
S
TAD - internal
Interactions
5 µm
Growing
Senescent
0
TAD - external
Interactions
E
20
40
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80
Normalised Internal Read Count (thousands)
OCI low
Local
cis
trans
Growing
High
OCI high
Distal Low
Local
Low
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Senescent
0
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Normalised External Read Count (thousands)
G
Senescent
G
G
S
20
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15
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5
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9.0
0
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Local
−5
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8.0
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5
Local
Local
Distal
10
15
Distal
Growing
20
48
0
44
−5
40
% GC Content
Distal
G TADs
FISH Probes
log (Growing DNAse Count)
F
−10
36
−10
−5
0
5
Local
10
15
20
Distal
Growing
Figure 1. Senescence Is Accompanied by Local and Global Changes in the Interaction Pattern of the Genome
(A) WI-38 hTERT/GFP-RAF1-ER cells show the characteristic SAHF phenotype upon senescence induction.
(B) Interaction heatmaps for chr18q for growing (top) and senescence (bottom). Strong interactions are shown in red whereas weak interactions are in blue. Green
lines (center) represent topologically associating domains (TADs) determined in growing cells. Highlighted TAD (black rhombus and pink TAD line) suggests a loss
of internal interactions.
(C) Top: genome browser shot of interactions reaching into and from within the depicted TAD. Growing cells show more TAD internal interactions than senescent
cells. Bottom: normalized read counts from highlighted TAD. Internal contacts confirm loss of TAD internal interactions and external interactions increase in
senescence, both in cis (light gray) and in trans (dark gray).
(D) DNA-FISH showing separation between probes located within and adjacent to the highlighted TAD. Separation increases in senescence p = 0.00003 (MannWhitney-Wilcoxon test). Shown to the right are representative confocal microscopy planes of the FISH separation experiment.
(E) Top: schematic of the open chromatin index (OCI), which describes regions changing their ratio between local and distal contacts.
(F) Browser shot of OCI in two biological replicates (G, growing; S, senescence) over highlighted TAD (pink) shows a loss of local interactions.
(G) Scatterplots comparing genome-wide OCI in 200 kb windows between growing and senescence. Points are colored by DNase accessibility as measured in
growing fibroblasts (left) and %GC content (right). A cluster of points can be seen to deviate from the diagonal, which shows a loss of local contacts in
senescence. These regions are the least accessible in the genome and have a low GC content.
Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors 473
interactions in windows across the genome and normalized by
subtracting the median chromosomal value from each window
and then by smoothing values in a rolling 20 Mb window. The
resulting OCI values give insight into the propensity of a region
to form local or distal interactions. Our measurement bears
some resemblance to the previously described interchromosomal contact propensity (ICP) (Kalhor et al., 2012), which itself
has been found to correlate with active marks such as RNA polymerase II occupancy (Kalhor et al., 2012). When visualized over
the TAD shown in Figures 1B and 1C, we find a rise in the OCI in
senescence, suggesting OCI may be a suitable metric to measure a switch from local to distal interactions (Figure 1F). To
explore OCI changes genome wide, we plotted OCI values in
growing and senescence in a scatterplot (Figure 1G). A population of regions appears to follow the behavior of the TAD
described above: an initially low OCI in growing cells with a
rise in senescent cells, indicating a loss of local interactions. It
has been previously suggested that high ICP values could be
affected by proximity to the periphery of chromosomal territories
(Kalhor et al., 2012). To rule out any such effect in our measurements, we calculated the OCI using only cis contacts, counting
any interaction spanning more than 20 Mb as a distal contact
(Figures S1E and S1F). We readily identified the same changing
regions, confirming OCI as a suitable metric to measure changes
in local architecture.
Based on the TAD shown in Figures 1B, 1C, and 1F, a low OCI
in growing cells would suggest a compact structure with strong
local interactions. To test this hypothesis, we correlated OCI with
DNase accessibility in growing cells (Figure 1G). Corroborating
our hypothesis, we found a striking overlap between the leastaccessible regions in growing cells (dark blue) and regions
showing the strongest rise in OCI. Next, we analyzed whether
genomic regions with changing chromosomal interactions
display a characteristic sequence-composition signature. We
highlighted the GC content of each point within the ICP scatterplot and again identified a strong correlation, with those regions
losing internal contacts strongly enriched for low GC content
(Figure 1G).
Using OCI, we have identified regions of chromatin losing internal structure. These regions are the least accessible in the
genome in growing cells and are rich in adenine-thymine (AT)
content.
Sequence Composition Predicts Structural Chromatin
Dynamics in Senescence
The bias in the GC content of regions showing changing OCI
(Figure 1G) led us to investigate the role of sequence composition within senescent nuclear reorganization. We used a recently
published annotation to split the genome into isochores, large
blocks of similar GC content (in GC%: L1 <37; L2 37–41; H1
41–46; H2 46–53; and H3 >53), which show little compositional
heterogeneity (Costantini et al., 2006). Isochores were originally
resolved according to their behavior in density gradient centrifugation, adding a dimension of macromolecular behavior to our
analysis of sequence content (Macaya et al., 1976; Thiery
et al., 1976).
To see how isochores behave in growing and senescent cells,
we plotted the OCI for each isochore (Figures 2A and S2A). We
474 Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors
observe a large difference in the behavior between L-isochores
(L1 and L2) and H-isochores (H1, H2, and H3), with the majority
of regions losing their local interactions within the GC-poor isochores (Figure 2A). Our data extend the role of the sequence
composition in higher-order chromatin dynamics in senescence
to the physical entities of isochores.
TADs are self-interacting domains with boundaries defined by
a change in the directionality bias of interacting fragments. We
calculated the position of TAD boundaries within growing and
senescent cells, as previously described (Dixon et al., 2012).
We found a high 89.3% percent of the domains to be conserved
between growing and senescent cells.
TAD boundary strength and decreased TAD internal contacts
have been shown to be affected by acute loss of a functional
cohesion complex (Sofueva et al., 2013). Interestingly, a recent
report suggests a loss of CTCF at CDKN2A in oncogene-induced
senescence (Hirosue et al., 2012). To test whether TAD boundaries are affected by a change in OCI, we calculated the ratio
of interactions found within TADs versus those spanning a
boundary (Figure 2B). We found a change in insulation strength
that correlates with isochores. L1 TAD boundaries show a striking loss of boundary strength (‘‘opening’’), whereas H2 and H3
TAD boundaries appear to strengthen slightly (‘‘closing’’; Figures 2B and 2C). Thus, whereas the position of TAD boundaries
remains largely unchanged in senescent cells, their quality is
affected by changes in local and distal interactions, with a significant fraction of TADs losing insulation strength and a smaller
fraction gaining insulation strength.
To test the changing TADs for enrichment in genomic features,
we selected regions based on the significance of the difference
in insulation strength (p < 0.05 for opening TADs; p < 0.25 for
closing TADs; Figure 2C). We find opening TADs are enriched
in H3K9me3, late replication timing (RT), and lamina-associated
domains (LADs). TADs with stronger boundary insulation show a
strong underrepresentation in these features and are instead enriched in regions of early replication and H3K36me3.
Sequence Composition and Lamin Association Predict
OCI Increase in Senescence
Recent research has highlighted the importance of LADs and the
loss of LMNB1 for the senescence phenotype (Sadaie et al.,
2013; Shah et al., 2013; Shimi et al., 2011). The level of LMNB1
reduction was shown to predict SAHF-positive cells in a heterogeneous population, and ectopic expression of LMNB1 was able
to reduce the number of SAHF-positive cells (Sadaie et al., 2013).
L1 isochores showed the strongest loss of local interactions (Figure 2A) and TAD isolation strength (Figure 2B) and were strongly
enriched in LADs (Figure 2C). To understand the behavior of
LADs, we plotted the change in average OCI across LADs (Figure S2B). We find a strong correlation between isochore class
and LAD OCI behavior, with L1 LADs losing and H-LADs gaining
local interactions. These observations allow us to predict LAD
behavior by sequence content alone.
To exclude the possibility that this observation is due only to
the enrichment of LADs within AT-rich regions of the genome,
we calculated OCI changes across LAD and inter-LAD regions
of each isochore (Figure 3A). We find that the loss of internal
contacts for L-isochores (a rise in OCI) is LAD dependent, with
A
Distal
10
-10
H1 41-46% GC
L2 37-41% GC
-5
0
5
Senescent
Local
H2 46-53% GC
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Local
Growing
-5
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Internal contacts
= TAD cross-boundary ratio
External contacts
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Op Cl
*
Replication
Timing
Histone
Marks
Op Cl
Op Cl
Early
K4me3
H2
Medium
K36me3
H1
Late
K27me3
H3
0.05
0.10
Growing
Lamin B1
More cross-boundary
interactions in Senescence
Opening
0.05
0.20
0.25
46 - 53
41 - 46
37 - 41
0.05
0.30
%GC
H3
H2
H1
L2
L1
More cross-boundary
interactions in Growing
Senescent
Senescent
TAD cross-boundary ratios
0.20
Closing
TADs
K9me3
L2
L1
0.05
0.10
0.15
0.20
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-8
0
8
log2 Obs / Expected Enrichment
Growing
Figure 2. Sequence Composition Predicts Structural Chromatin Dynamics in Senescence
(A) Scatterplots showing OCI calculated in 200 kb windows for growing and senescence cells, separated by overlap with isochores. The greatest changes can be
seen to occur in the L1 isochores.
(B) Top: schematic of TAD boundary strength calculation. Bottom: scatterplot comparing the cross-boundary ratios over TADs genome wide, colored by isochore. The L-isochores can be seen to gain cross-boundary interactions in senescence.
(C) Top: selection of opening and closing TADs highlighted in scatter plot (a less-stringent cutoff was chosen for closing TADs, in order to reach a comparable
number). Bottom: enrichment for overlap with genomic features (log2 obs/exp; also see Experimental Procedures) for opening (Op) and closing (Cl) TADs.
L-inter LADs showing no change in OCI. This suggests that
the combination of being in an AT-rich L-isochore and a LAD
predicts the dramatic OCI gain in senescence. For H2 and H3
isochores, the OCI change is not dependent on lamin binding
(Figure 3A), suggesting that other features control the behavior
of these LADs.
Senescence is accompanied by a major loss of LMNB1, and
recent chromatin immunoprecipitation sequencing (ChIP-seq)
experiments have identified the majority of the regions losing
LMNB1 occupancy in senescence (Sadaie et al., 2013). Despite
this dramatic loss, a subset of regions not bound by LMNB1 in
growing cells appears to gain binding in senescence (Sadaie
et al., 2013). We calculated the enrichment of these growingand senescent-specific LMNB1 regions across the isochore
LADs and inter-LAD regions (Figure 3B). We observe a strong
enrichment for areas losing LMNB1 in L-isochore laminassociated domains (L-LADs), suggesting the loss of LMNB1
may be involved in the architectural changes we have uncovered. There was no enrichment of LMNB1-gaining regions
in any LAD category, supporting the previous observation
that these regions are unique to senescence. GC-rich interLADs were enriched for senescence-specific LMNB1 binding,
showing the opposite isochore pattern to the loss of lamin. We
tested the overlap between opening and closing TADs and
Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors 475
Change in OCI
A
More local
in Senescence
-10
-5
Inter-LADs
B
More distal
in Senescence
0
5
H3 H2 H1
L2
LADs
L1
H3 H2 H1
L2
L1
10
Early Replication
H3
Medium Replication
> 53% GC
Late Replication
H2
5.3
LMNB1 Grow + Sen
LMMB1 Growing only
H1
41 - 46% GC
LMNB1 Sen only
L2
K9me3
37 - 41% GC
0
K27me3
K36me3
L1
< 37% GC
K4me3
LADs
log2 Obs / Expected Enrichment
46 - 53% GC
-5.3
Inter-LADs
C
D
Distance from Nuclear Periphery
L1-LAD
**
**
5
H2-LAD
FISH Probes
3
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1
0
Senescence
OCI
Local
Growing
OCI
2
Distance (µm)
4
Growing LADs
Chr 18 : 42 - 66 Mb
G
S
H2-LAD
G
S
L1-LAD
E
H2-LAD
Growing
L1-LAD
Senescence
Growing
Focal Plane
Senescence
Distance from
nuclear periphery
5 µm
Figure 3. Combined Sequence Composition and Lamin Association Predict the Strongest OCI-Gaining Regions, which Are Changing Their
Nuclear Positioning
(A) Change in OCI levels in senescence for regions overlapping LADs (purple) and inter-LADs (iLADs, green).
(B) Enrichment of LADs and iLADs for replication timing, LMNB1 regions, and several chromatin marks.
(C) Browser shot indicating genomic location for FISH probes designed against two adjacent H2-LAD and L1-LAD regions (green vertical lines).
(D) Distances of FISH signals to the nuclear periphery.
(E) Left: representative confocal microscopic images of FISH-treated growing and senescent cells. Right: schematic showing the measurements made for the
DNA-FISH from the central focal plane.
476 Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors
growing/senescence-specific LMNB1 regions (Figure S3A). We
find a strong enrichment for the newly forming senescence-specific LADs within TADs exhibiting greater insulation, suggesting
a functional role for the senescence-specific gain of LMNB1.
Our data suggest a role for the chromosomal redistribution of
LMNB1 in the architectural changes we described.
To investigate whether the changes in LAD OCI are accompanied by changes in positioning in the nucleus, we used DNAFISH microscopy to measure the distance of specific LADs to
the nuclear periphery. We designed probes within two adjacent
L1- and H2-LADs (Figure 3C); the L1-LAD showed a significant
increase in the mean distance to the periphery (Figures 3D and
3E; p < 0.006). Interestingly, the H-LAD also showed a significant
change, moving closer to the periphery (Figures 3D and 3E; p <
0.009). We tested another L1-LAD in close proximity to the
CDKN2A locus and observed a similar move away from the
nuclear periphery (Figure S3B; p < 0.002). These observations
suggest that the consequences of chromatin restructuring go
beyond the change in local interactions described by OCI and
could additionally entail a change in nuclear positioning, with
L1 LADs moving away from the periphery and H-LADs moving
closer to it.
Local Changes Are Accompanied by the Formation of
Specific Distal Interactions
Through changes in OCI and TAD insulation strength, we
observe a redistribution of local interactions to more distal
interactions for L-LADs. To test whether these new distal contacts are distributed randomly or form specific interactions, we
mapped the change in normalized interaction count between
different LADs. We calculated all reads linking selected features
while ignoring interactions separated by less than 2 Mb to avoid
any distance effects (Figure 4A). We observe a strong increase in
the number of interactions between L-LADs, with the strongest
gain found between L1-LADs (Figure 4B). In contrast, we do
not observe any major changes in interactions between H-LADs.
Whereas L-LADs lose internal contacts, they increase their interactions between each other across the genome, indicating
that they cluster together in nuclear space. This observation is
reminiscent of the rearrangement of heterochromatin observed
with microscopy during SAHF formation in Ras-induced senescence (Figure 4C; Chandra et al., 2012). We therefore analyzed
whether interactions between domains defined by specific histone marks change in senescence. Consistent with previous microscopy data, we find that H3K9me3 domains come together in
a specific manner during senescence (Figure 4D).
The concerted local loss of internal contacts and global clustering of L-LADs leads us to the model depicted in Figure 4E,
whereby lamin-bound regions detach from the nuclear periphery
and cluster within the nuclear interior. Some GC-rich H2/H3 regions appear to have gained internal structure and relocate to
the periphery, consistent with a scenario of nuclear reshuffling
resulting from LAD relocation.
Analyzing distal contacts of regions dramatically changing
their local structure (L1-LADs and H3K9me3 domains) suggests
a clustering of these regions within the nucleus. Previous work
has speculated that SAHF may represent a special heterochromatic compartment, supported by immunofluorescence studies
using DNA stains such as DAPI which show a very high intensity
signal within the SAHF core. Contrary to this theory, our initial observations using Hi-C data suggest a loss of internal structure
and possible decompaction of DNA enriched in the SAHF core.
The clustering shown here can now explain the perceived expansion and clustering of heterochromatin and provides a new depth
of understanding of the structural organization of senescent
chromatin.
Pre-senescent Replication Timing Predicts Chromatin
Changes
OIS shows an accumulation of cells arrested in S phase. This
has been linked to replication-stress-induced DNA damage
signaling, possibly caused by an initial hyperproliferative burst
following oncogene activation (Bartkova et al., 2006; Di Micco
et al., 2006). Although the relationship between structural chromatin changes and cellular senescence is not well understood,
one key protein required for SAHF formation is pRB, linking
higher-order chromatin changes and replication (Narita et al.,
2003). The interplay between replication timing and SAHF architecture has also been shown by pulse-chase studies, tracking
nucleoside analogue incorporation (Chandra et al., 2012).
Having identified the regions with the most dynamic structural
changes in the Hi-C data, we tested whether these regions show
any change in pre-senescence replication timing (RT). We reanalyzed whole-genome replication timing data from IMR90 ERRas cells, comparing growing cells and presenescent cells
(48 hr after induction of Ras; Chandra et al., 2012). SAHF formation takes longer in ER-Ras cells, and kinetics of senescence
establishment are better described, suggesting that changes
observed at 48 hr would precede SAHF formation (Young
et al., 2009). To compare the two data sets, we measured the
change in normalized replication timing (Figure 5A). We split
the genome into LADs and inter-LADs and then isochores. We
see a shift toward earlier RT for L1-LADs, whereas L1 iLADs
show little change. The scatterplots for the L1 isochore indicates
a split between LADs (green) and iLADs (purple) into late and
early replication timing, respectively (Figure 5B). A subpopulation of L1 LADs appears to be biased toward earlier RT in presenescence. An example of a locus exhibiting this shift can be seen
in Figure 5C, which shows a region surrounding an L1-LAD losing
its internal contact bias and changing in presenescence RT.
We find a correlation between the most significant structural
changes in senescence as revealed by Hi-C and RT changes
in hyperproliferating presenescent cells. This observation may
indicate a role for replication timing in shaping the senescence
chromatin landscape.
Higher-Order Chromatin Dynamics in Senescence
Reflect Changes in Progeria and May Represent the
Endpoint of a Continuous Remodelling Process in
Differentiation
The role of p16-dependent senescence in organismal aging
highlights the importance of cellular senescence as an in vitro
aging model (Baker et al., 2011). Fibroblasts belonging to patients suffering from Hutchinson-Gilford progeria syndrome
(HGPS) undergo premature senescence and also provide an
important model to study aging. However, OIS and HGPS cells
Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors 477
300
250
2 Mb
Interaction Change
200
Features of interest
Interaction Change = (ig × cg) – (is × cs)
H3 LADs
H2 LADs
H1 LADs
L2 LADs
L1 LADs
> 53% GC
46-53% GC
41-46% GC
37-41% GC
< 37% GC
More contacts
in Senescence
B
A
150
100
50
-50
-100
H3 LADs
H2 LADs
H1 LADs
L2 LADs
L1 LADs
D
C
350
Schematic SAHF Structure
300
Less contacts
in Senescence
0
i = Interaction count with features of interest
c = Correction factor based on total Hi-C read count
H3K9me3
H3K27me3
H3K4me3
250
K27me3
K9me3
H3K36me3
200
More contacts
in Senescence
DAPI
Interaction Change
K36me3
150
100
50
-100
H3K4me3
H3K36me3
H3K27me3
H3K9me3
Less contacts
in Senescence
0
-50
E
LADs
Growing
Senescence
Senescence specific LADs
SAHF
Figure 4. Local Changes Are Accompanied by the Formation of Specific Distal Interactions
(A) Schematic showing how the change in interaction strength between features was calculated. g and s supertext denotes growing and senescence. Note the
avoidance of features within 2 Mb.
(B) Change in interaction strength calculated between isochore LADs comparing growing and senescence
(C) Schematic depicting the chromatin organization of histone marks in SAHF as described in Chandra et al. (2012).
(D) Change in interaction strength calculated between regions associated with histone marks.
(E) Model depicting the change of the chromatin architecture. We propose that L-LAD regions with strong local interactions (high OCI) in growing (green) detach
from the nuclear lamina and lose their internal structure (potentially forming the core of the SAHF). Other regions, such as selected H-LADs, move toward the
nuclear periphery.
have distinct chromatin features; for example, SAHF formation is
exclusive to OIS. Whereas OIS is accompanied by a major loss in
LMNB1, premature aging in HPGS is due to the accumulation of
progerin, a mutated version of LMNA/LMNC (Eriksson et al.,
2003). As such, a commonality found within both models is the
destabilization of the nuclear lamina. To investigate the breadth
of our findings regarding local chromatin changes in senescence
and to examine similarities between OIS and HGPS fibroblasts,
we compared our data to recently published Hi-C data sets in
HGPS fibroblasts (McCord et al., 2013).
Unsupervised hierarchical clustering of OCI in growing, senescent, and progeroid fibroblasts split the data into six classes,
478 Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors
which show strong correlation with GC content (Figure 6A). Classes 1 and 5 show similar trends for senescence and progeria and
represent 58% of the genome (Figure S4). Class 1 is GC poor and
highly enriched in L1-LADs (Figure 6B), suggesting a common
loss of internal structure for these regions in both senescence
and progeria. Cluster 5 shows the strongest enrichment of all
clusters for regions gaining LMNB1 in senescence, suggesting
that a common mechanism may lead to the compaction of these
regions.
Whereas the decompaction of heterochromatin for HGPS cells
is consistent with previous observations (Scaffidi and Misteli,
2006; Shumaker et al., 2006), the congruency in changing OCI
Difference in RT : Inter - LADs
0.5
Replication Timing
Growing RT
Pre-Senescence RT
−0.5
H3
H2
H1
L2
B
H3
L1
LADs
H2
H1
L2
L1
Inter LADs
−1.0
−0.5
0.0
Senescence OCI
0.5
1.0
L1
Pre Senesent RT − Growing RT
C
LADs
0.0
0.5
−0.5
Later in
Pre-Senesence
0.0
Earlier in
Pre-Senesence
Difference in RT: LADs
Growing OCI
A
−1.0
−0.5
0.0
0.5
1.0
1.5
80,000,000
Growing RT
85,000,000
90,000,000
Figure 5. Presenescent Replication Timing Predicts Chromatin Changes
(A) Change in replication timing between growing OIS ER-Ras cells and presenescent cells. Regions overlapping LADs are shown in green and iLADs in purple,
divided by isochore.
(B) Scatterplot showing difference between presenescent RT and growing RT versus growing RT for isochore L1; LADs (green) and iLADs (purple) are highlighted.
Points above the horizontal are replicating earlier in presenescent cells.
(C) Browser shot showing a L1-LAD region changing in presenescent RT.
seems to contradict the fact that HGPS cells do not form SAHF,
suggesting that changes in OCI are not solely responsible for the
formation of SAHF. To understand these differences, we mapped the difference in distal interactions between growing cells
and HGPS cells (Figure 6C). Interestingly, we do not observe a
spatial clustering of constitutive heterochromatin in HGPS cells,
suggesting that SAHF formation is a two-step process, with the
initial L1-LAD decompaction shared between cellular senescence and HGPS.
The similar trends in OCI between senescence and progeria
may suggest that the perturbation of the lamina has comparable
consequences, independent of its cause, and that these consequences connect the nuclear changes seen in both cell types.
A role for changes in nuclear lamina interactions and RT dynamics has been described in differentiation (Hiratani et al.,
2004; Peric-Hupkes et al., 2010). Although senescence and differentiation are both being studied thoroughly, there are few
studies focusing on the crosstalk between the two. Senescence
has been identified as a barrier to dedifferentiation with induced
pluripotent stem cell (iPSC) reprogramming (Banito et al., 2009;
Li et al., 2009), and a role for senescence in embryonic development has been highlighted in two recent studies (Muñoz-Espı́n
et al., 2013; Storer et al., 2013). The emerging concept of epigenetic rejuvenation aims to reverse the aging process without dedifferentiating target cells and depends on our ability to discriminate these two processes (Manukyan and Singh, 2012; Rando
and Chang, 2012). To position the higher-order chromatin structure dynamics described above within the wider context of differentiation, we compared our Hi-C data to an ESC Hi-C data set
(Dixon et al., 2012). We plotted the changes in OCI for ESCs,
growing, and senescent cells over LADs, grouped by LAD overlap and isochore (Figure 6D). Remarkably, we find LAD OCI
behavior in senescence to be inversed with respect to the ESC
configuration (Figure 6D). L1-LADs show a similar profile in
both ESCs and growing cells with an inverted profile in senescence, again highlighting the unique behavior of the L1-LADs
for the senescence phenotype. The growing OCI shows an intermediate behavior between ESCs and senescent cells, suggesting a potentially continuous process of higher-order chromatin
structure changes from pluripotency to senescence.
To get a more detailed impression of the dynamics of these
changes, we called OCI domains in ESCs using a hidden Markov
model and tested the conservation of these domains in growing
and senescence cells (Figure 6E). We see a dramatic loss of
the original OCI domain structure in senescence and an intermediate state in growing cells. These data suggest a close relationship between architectural events in differentiation and
senescence.
DISCUSSION
Due to the profound changes seen within DNA-stained SAHFpositive senescent cells, a number of studies have attempted
to capture the key drivers of senescence using epigenomic
and microscopic techniques (Chandra et al., 2012; Cruickshanks
et al., 2013; Sadaie et al., 2013; Shah et al., 2013). Using Hi-C, we
have been able to generate a comprehensive description of
spatial changes within senescent nuclei on a global scale.
Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors 479
g)
er
ia
(a
v
1
G
C
G
C
Cluster
B
2
3
4
5
D
6
%
%
Pr
og
ro
w
in
g
G
A
45%
Early Replication
Medium Replication
LMNB1 - Growing only
41.5%
LMNB1 - Sen only
*
H3 LADs
2
H2 LADs
H1 LADs
3
3
L2 LADs
H3K9me3
2
0
H3K27me3
H3K36me3
1
5
−3
0
Local
-1
6
400
ES Cells
300
200
100
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
150
100
Growing
50
Progeria Histone Interactions
500
0
400
K9me3
K27me3
K36me3
K4me3
300
200
-2
Conservation of ES OCI domains
500
200
H3K4me3
C
E
Frequency
Distal
4
Feature Enrichment
L1 LADs
Frequency
38%
100
0
300
Frequency
1
% GC Content
Late Replication
LMNB1 - Grow & Sen
200
Senescent
100
0
0.0
K4me3
K36me3
K27me3
0.2
0.4
0.6
0.8
1.0
K9me3
Figure 6. Higher-Order Chromatin Dynamics in Senescence Reflect Changes in Progeria and May Represent the Endpoint of a Continuous
Remodelling Process in Differentiation
(A) Hierarchical clustering of OCI values in 1 Mb of genomic windows for growing, senescence, and progeria. Columns to the right show GC content per genomic
window and the average %GC per cluster.
(B) Enrichment of clusters over genomic features. Note that clusters 1 and 5 show similar behavior in growing and senescence.
(C) Change of interactions calculated between regions associated with histone marks. Progeria shows no clustering of H3K9me3 regions or other histone marks
compared to growing cells. Enrichment calculated as shown in Figure 4A.
(D) Average OCI over LADs (top) and iLADs (bottom) split by isochore in ESCs, growing, and senescence.
(E) Conservation of OCI domains called using a hidden Markov model (see Experimental Procedures) in ESCs. The x axis shows percentage of windows within
domains classed as local (0) or distal (1). ESCs show a bimodal distribution as expected. Growing and senescent cells show a decaying conservation of
interaction state within these domains.
SAHF were originally described as a gene-silencing compartment, and cellular senescence has since been associated with
an increase in constitutive heterochromatin (cHC) (Narita et al.,
2003; Reimann et al., 2010). Contrary to these expectations,
we find a dramatic loss of local interactions in the cHC compartment, as described by an increase in OCI. Although this observation challenges the current view of SAHF, it resolves the previously paradoxical relationship between senescence and aging
by correlating both with the loss of heterochromatin observed
in premature and healthy aging (Scaffidi and Misteli, 2006). A
loss of heterochromatin in senescence is also supported by a
recent study showing the activation of satellite repeats early in
cellular senescence, termed senescence-associated distension
of satellites (SADS) (Swanson et al., 2013). If loss of cHC unifies
senescent and aging cells, what is unique to senescence that
enables SAHF formation?
Further to the general loss of heterochromatin, we show
a spatial clustering of decondensing regions in cellular senescence, but not in Hutchinson-Gilford progeria syndrome
(HGPS) cells. These interactions may represent a unique consecutive step required for SAHF formation, leading to a two-step
mechanism where heterochromatin decondensation is followed
by spatial clustering (Figure S5). A two-step mechanism for
480 Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors
SAHF formation has also been proposed in the recent study uncovering SADS, in agreement with our findings (Swanson et al.,
2013).
Whereas we can only speculate about the reason behind the
L-LADs decompaction, one possible mechanism is the recently
described activation of long interspersed nuclear elements
involved in replicative senescence (De Cecco et al., 2013). This
activation and retrotransposition was implicated in reinforcing
the senescence phenotype through DNA damage.
Whereas a higher OCI could suggest an activation of the cHC
compartment within senescence, a deeper investigation into the
relationship with transcriptional changes will be necessary to
better understand SAHF function or to break with the concept
of SAHF as a silencing compartment. For example, it may be
that the retention of repressive chromatin marks such as
H3K9me3 and an increase in HP1 proteins is sufficient for
gene silencing, despite chromatin decondensation.
Our data suggest common chromatin changes between
senescence and progeria; however, we can only speculate
about the upstream mechanisms leading to these similar nuclear
phenotypes. Some possible crosstalks have been described in
the literature, such as the activation of p53 and pRb due to progerin overexpression via a telomeric DNA damage response
(Benson et al., 2010). Likewise, telomere erosion in replicative
senescence was shown to induce progerin expression in normal
fibroblasts (Cao et al., 2011). However, the same study found no
progerin induction in oncogene-induced (telomere-independent)
senescence (Cao et al., 2011). Our study establishes a link between oncogene-induced senescence and HGPS in the loss of
local contacts of GC-poor LADs, suggesting a common destabilization of the nuclear lamina phenocopied by the loss of LMNB1
in senescence and the expression of progerin in HGPS.
In addition to the dramatic changes we see in L-LADs, we
consistently find a GC-rich compartment of the genome gaining
internal contacts in senescence. This compartment is enriched in
H2 and H3 isochores and overlaps those TADs gaining boundary
insulation. When compared to LMNB1 ChIP-seq data, these regions overlap with regions gaining LMNB1 in senescence (Sadaie et al., 2013). Based on our FISH data, they may locate to
the nuclear periphery. It will be interesting to characterize these
senescence-acquired LADs further, especially to see whether
they are a senescence-specific feature or whether they exist in
other cellular states.
The global extent to which regions switch their OCI in senescence is reminiscent of an inversion of the global interaction
pattern. This global response starts to take effect after a few
hours of c-raf induction and affects the whole population of cells
after 48 hr. SAHF formation has been shown to occur downstream of several different cellular stresses. The extent of the interactome change, the response time, and the variety of triggers
leads us to speculate that this nuclear restructuring may be
a fundamental hardwired response of the cell. Our data show
a strong correlation between OCI change and isochores; isochores may have physical properties beyond simple sequence
recognition that allow the genome to rearrange its architecture
during a stress response. A relationship between isochore structure and stress response would have important implications for
evolution and could go some way to explaining the differences
between the integration and fixation rates seen for some repetitive elements (Costantini et al., 2012).
Senescent cells share similarities with terminally differentiated cells, such as those having permanently exited the cell
cycle. Furthermore, effectors of senescence like p53 and p16
have been shown to critically regulate self-renewal in adult
stem cells (Janzen et al., 2006). However, there have been
few studies on the relationship between senescence and
differentiation. The emerging concept of epigenetic rejuvenation aims to reverse the aging process without dedifferentiating
target cells (as opposed to iPSC reprogramming), thereby
avoiding risks associated with pluripotency, such as cancer.
Any realization of epigenetic rejuvenation depends on our ability to discriminate differentiation and senescence (Manukyan
and Singh, 2012). ESCs and senescent cells show globally inverted domains of local compaction whereas somatic cells
show an intermediate state between ESCs and senescence.
Our observations suggest that the remodelling of the higher-order structure we describe for senescence is a continuation
of ESC to somatic differentiation. Based on this preliminary
observation, reversing these architectural changes may reboot
the nuclear architecture in an undifferentiated ESC-like state,
contrary to the concept of rejuvenation. However, a deeper
understanding of distinct architectural features, distinguishing
early differentiation and senescence, could provide a rationale
for rejuvenation approaches.
EXPERIMENTAL PROCEDURES
Hi-C
Hi-C was performed essentially as described in Lieberman-Aiden et al. (2009),
with some modifications described in the Supplemental Experimental
Procedures.
FISH
FISH labeling was performed as described in Bolland et al. (2013). FISH
probes were ordered prelabeled from empire genomics. A list of clones can
be found in the Supplemental Experimental Procedures. FISH data were
analyzed from confocal sections using Volocity software. Top and bottom
focal planes were discarded, and the analysis was restricted to central focal
planes showing a clear FISH signal. Volocity software was used for automated
object detection and distance measurements.
Cell Culture
WI-38hTERT/GFP-RAF1-ER was a generous gift from Carl Mann. Cells were
cultured with 10% fetal bovine serum under 5% O2 and handled as described
in Jeanblanc et al. (2012). Senescent cells were harvested 48 hr after induction
with 4-OH tamoxifen.
Cell Proliferation Assays
BrdU (anti-BrdU; PharMingen) and DAPI staining were carried out as
described in Narita et al. (2003).
Computational Data Analysis
Please refer to the Supplemental Experimental Procedures for details about
the data analysis.
ACCESSION NUMBERS
The ENA accession number for the sequencing data reported in this paper is
PRJEB8073.
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures
and five figures and can be found with this article online at http://dx.doi.org/
10.1016/j.celrep.2014.12.055.
AUTHOR CONTRIBUTIONS
T.C., P.F., and W.R. designed the study. T.C., S.S., M.F.-M., J.-Y.T., and K.K.
performed experiments. T.C., P.A.E., S.W.W., and S.A. analyzed the data.
T.C., P.A.E., and W.R. wrote the manuscript.
ACKNOWLEDGMENTS
This study was funded by the following grants: BBSRC (BB/K010867/1);
BBSRC (BBS/E/B/000C0404); and Wellcome Trust (095645/Z/11/Z). Thanks
to Roland Schwarz for useful discussion and to Jesus Gil, Oliver Bischof, David
Bazett-Jones, David Gilbert, and Ben Pope for critical reading of the manuscript. Figure icons in the graphical abstract were created by OCHA, Freepik
under Creative Commons license 3.0. W.R. is a consultant to Cambridge
Epigenetix Ltd.
Received: July 31, 2014
Revised: November 13, 2014
Accepted: December 22, 2014
Published: January 29, 2015
Cell Reports 10, 471–483, February 3, 2015 ª2015 The Authors 481
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