Tet1 regulates epigenetic remodeling of the pericentromeric heterochromatin and chromocenter organization in DNA hypomethylated cells

Pericentromeric heterochromatin (PCH), the constitutive heterochromatin of pericentromeric regions, plays crucial roles in various cellular events, such as cell division and DNA replication. PCH forms chromocenters in the interphase nucleus, and chromocenters cluster at the prophase of meiosis. Chromocenter clustering has been reported to be critical for the appropriate progression of meiosis. However, the molecular mechanisms underlying chromocenter clustering remain elusive. In this study, we found that global DNA hypomethylation, 5hmC enrichment in PCH, and chromocenter clustering of Dnmt1-KO ESCs were similar to those of the female meiotic germ cells. Tet1 is essential for the deposition of 5hmC and facultative histone marks of H3K27me3 and H2AK119ub at PCH, as well as chromocenter clustering. RING1B, one of the core components of PRC1, is recruited to PCH by TET1, and PRC1 plays a critical role in chromocenter clustering. In addition, the rearrangement of the chromocenter under DNA hypomethylated condition was mediated by liquid-liquid phase separation. Thus, we demonstrated a novel role of Tet1 in chromocenter rearrangement in DNA hypomethylated cells.


Author summary
The DAPI-dense chromocenter in the nucleus consists of accumulated pericentromeric heterochromatin. Its number, size, shape, and position are quite different among cell types. One of the striking reorganizations of chromocenter is its clustering, which is observed under DNA hypomethylation conditions, such as in female meiotic germ cells. However, how chromocenters gather and accumulate under the specific epigenetic state remained elusive. Here, we tackled this problem using genetically DNA hypomethylated mouse ESCs and epigenome editing tools. We demonstrated that the DNA demethylation-related factor, TET1 plays a critical role in the chromosome clustering by recruiting Polycomb factors. Forced recruitment of TET protein to the pericentromeric region was

Introduction
The chromosomal region around the centromere, which is named pericentromere, is~5% in mice. The pericentromeric region of mouse cells is mainly comprised of a 234 bp long AT-rich repetitive sequence, called major satellite repeats. This region forms constitutive heterochromatin harboring high levels of trimethylation of histone H3 lysine 9 (H3K9me3) and DNA methylation at the C-5 position of cytosine (5mC) [1]. At the interphase of the cell cycle, the pericentromeric heterochromatin (PCH) of chromosomes aggregate and form the chromocenter and are visible as 4 0 6-diamidino-2-phenylindole (DAPI)-dense foci in the nucleus [2]. The organization of chromocenters, including their position, size, and number in the nuclei, are different among individual cell types and are subject to change during their differentiation [3,4]. One of the most striking reorganizations is chromocenter clustering involving the accumulation, and gathering of chromocenters. Chromocenter clustering is observed in germ cells at the meiotic prophase and plays an important role in subsequent meiotic processes, including centromere clustering, homologous chromosome pairing, and synapsis formation [5]. Several factors have been identified to affect chromocenter clustering. For example, MeCP2 and MBD2 localize to the chromocenter and induce clustering during myogenesis, and NANOG is recruited to the chromocenter by SALL1 and controls chromocenter clustering via its transactivation domain in the embryonic stem cells (ESCs) [3,4]. However, previous studies have mainly focused on wild-type ESCs under regular culture conditions and failed to link the clustering mechanism to changes in the epigenetic state.
DNA methylation levels are known to affect the nuclear organization, such as chromocenters [4,6]. Three types of DNA methyltransferases (DNMTs) have been reported to catalyze 5mC in mice: de novo DNA methylase DNMT3A and DNMT3B, and maintenance DNA methylase DNMT1 [7]. Loss of the DNMT family of proteins causes global hypomethylation of the CpG sites. In the wild-type ESCs, 82% of CpG sites were methylated, while it was reduced to 18% and 0.4% in the Dnmt1-KO ESCs and the Dnmt1/3a/3b-TKO (triple knockout of Dnmt1, Dnmt3a, and Dnmt3b) ESCs, respectively [8]. Meanwhile, the TET family comprising TET1, TET2, and TET3 are dioxygenases that convert 5mC to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC) [9]. TET-mediated iterative oxidation is important for the loss of 5mC either through glycosylation by TDG, followed by base excision repair or DNA replication-dependent dilution [9]. In addition, TET proteins have multiple roles, not only in a catalytic activity-dependent but also in an independent manner. For example, TET1 is associated with the SIN3A and NuRD complex as well as pluripotency factors such as NANOG, PRDM14, and LIN28, and regulates the expression of specific genes [10][11][12][13]. Considering that 5hmC enrichment in the PCH of meiotic germ cells became undetectable in Tet1-KO mice, TET1 could access the PCH locus under these circumstances, but its biological function is poorly understood [14]. Meanwhile, the localization of TET1 is known to overlap with polycomb factors [15,16].
Meanwhile, liquid-liquid phase separation (LLPS) is characterized by the spontaneous demixing of a homologous solution into two phases by weak forces, such as hydrophobic and electrostatic (pi/pi, cation/pi, etc.) interactions. LLPS contributes to the formation of membrane-less organelles and functional drops, including stress granules and PcG bodies [25][26][27]. Heterochromatin protein α (HP1α), a major chromocenter protein, has been reported to form LLPS-driven droplets in vitro [28,29]. However, HP1α foci were resistant to the treatment of 1,6-hexanediol, aliphatic alcohol that disrupts the hydrophobic interaction of LLPS [29]. Therefore, established heterochromatin is believed to comprise mostly immobile compartments rather than purely liquid structures.
In this study, we found that ESCs with hypomethylated DNA showed deposition of facultative heterochromatin marks in PCH and chromocenter clustering. Deposition of 5hmC, H2AK119ub, and H3K27me3 in PCH of the DNA hypomethylated ESCs was abrogated by the deletion of Tet1. Moreover, the TET1-PRC1 axis was critical for chromocenter clustering in DNA hypomethylated ESCs. We also demonstrated that DNA hypomethylation-dependent chromocenter clustering is driven by LLPS. Thus, our findings revealed a novel function of Tet1 in the rearrangement of heterochromatin triggered by the loss of DNA methylation.

Dnmt1-KO ESCs
First, we analyzed HP1γ and H3K9me2, which are catalyzed by G9a H3K9 methyltransferase at the meiotic prophase in female germ cells. PCH forms clusters in the nuclei of both male and female germ cells during meiotic prophase. HP1γ and G9a, both of which are highly enriched in the chromocenter in male germ cells, play a critical role in the clustering of PCH and homologous chromosome pairing in male gonocytes [5]. In contrast, although HP1γ showed specific and consistent localization to PCH, H3K9me2 was depleted in the PCH of female germ cells (S1A and S1B Fig). This distinct enrichment pattern of epigenetic marks suggests that the clustering of PCH in male and female germ cells is regulated by different mechanisms.
We then hypothesized that the difference in DNA methylation levels at the meiotic stage would contribute to the sex difference in chromocenter clustering because it affects nuclear architecture [30]. To test this hypothesis, we analyzed the Dnmt1-KO ESCs [8]. PCH of the Dnmt1-KO ESCs was highly clustered to form large chromocenters in the nucleus, and their number was significantly lower than that of the wild-type ESCs (Fig 1A and 1B). This result was consistent with a previous report showing that DNA hypomethylation causes chromocenter clustering [6]. The condensed chromocenter of the Dnmt1-KO ESCs retained a strong enrichment of HP1γ, which is similar to meiotic germ cells (Figs 1A and S1A). In contrast, H3K9me2 was not depleted from the chromocenter of both wild-type and Dnmt1-KO ESCs (S1C and S1D Fig).
Meanwhile, 5hmC was enriched in the PCH of the Dnmt1-KO ESCs, which was also observed in the primordial germ cells (PGCs) during the epigenetic reprogramming phase (S2A Fig) [31]. The relative enrichment score of 5hmC at PCH in the Dnmt1-KO ESCs was about two times higher than that of the wild-type ESCs (1.21 vs. 0.62) (S2B Fig). These results demonstrated that the Dnmt1-KO ESCs exhibited features similar to those of female germ cells, such as global DNA hypomethylation, PCH-5hmC, and clustered chromocenters, to some extent.

The critical function of Tet1 in 5hmC deposition at PCH and chromocenter clustering
Considering that TET1 is critical for the acquisition of PCH-5hmC in PGCs and is highly expressed in ESCs, we focused on the function of TET1 [14]. Immunocytochemical analysis demonstrated that TET1 was slightly but significantly enriched in the chromocenter of the Dnmt1-KO ESCs compared to the wild-type ESCs (S2D and S2E Fig). Taken together, we hypothesized that TET1 was responsible for the gain of 5hmC at PCH and plays a role in chromocenter clustering in DNA hypomethylated cells. To validate this hypothesis, we knocked out Tet1 in the Dnmt1-KO ESCs by CRISPR-mediated mutagenesis, and the depletion of TET1 protein was confirmed by immunocytochemistry and Western blotting analysis (S2D and S2F Fig) [32].
Enrichment of 5hmC in PCH of the Dnmt1/Tet1-DKO ESCs was significantly decreased compared to the Dnmt1-KO ESCs and was comparable to that of the wild-type ESCs (0.54 vs.  that of the wild-type ESCs (Fig 1A and 1B). These results demonstrate that Tet1 is critical for chromocenter clustering in DNA hypomethylated ESCs.

Induction of chromocenter clustering by the forced recruitment of TET1
To address whether TET1 localization to PCH is sufficient to induce chromocenter clustering, we carried out an experiment recruiting TET1 to PCH using the epigenome editing method [34]. This dCas9-SunTag system recruits the TET1 catalytic domain (CD) to single-guide RNA (sgRNA) targeted regions through anti-GCN4 scFv domains by tethering dCas9-fused GCN4 (Fig 2A) [34]. We designed a sgRNA targeting major satellite repeats to recruit TET1-CD to PCH. A whole cassette without sgRNA and scFv-only cassettes were used as negative controls (Fig 2B).
The sgRNA-mediated TET1 recruitment to the PCH was verified by the specific enrichment of GFP (Figs 2C and S3A). As shown in S3B and S3C Fig, 5hmC was strongly enriched in PCH of the TET1-CD recruited ESCs but was absent in PCHs of the negative control and TET1 catalytic dead mutant (CD-Mut)-recruited ESCs. These results indicated that TET1-CD was successfully recruited to PCH in a sgRNA-dependent manner. In the TET1-forced recruited ESCs, chromocenters were clustered, and their numbers were significantly decreased in not only TET1-CD recruited ESCs but also TET1-CD-Mut recruited ESCs (Fig 2C and 2D). These results clearly showed that the recruitment of TET1-CD to PCH was sufficient to induce chromocenter clustering in a catalytic activity-independent manner.

Contribution of PRCs to chromocenter clustering under the DNA hypomethylated state
To address the molecular mechanism of Tet1-mediated chromocenter clustering in the DNA hypomethylated ESCs, first, we analyzed Nanog, which was reported to regulate chromocenter clustering in ESCs [3]. Unfortunately, however obvious changes in neither expression level nor localization of NANOG in the Dnmt1-KO ESCs and the Dnmt1/Tet1-DKO ESCs was not detected (S4A- S4C Fig).
Then, we hypothesized that Polycomb factors would be the core of chromocenter clustering mediated by Tet1 because of their aggregate-forming ability and co-localization with TET1 [15]. TET1 distribution overlaps with PRC2 and H3K27me3 in mouse ESCs [15]. The loss of DNA methylation has been demonstrated to induce PRC2 and H3K27me3 enrichment in PCH [19,35]. Consistently, H3K27me3 at PCH, which was depleted in the wild-type ESCs, was increased in the  Consistently, EZH2 enrichment to PCH was increased in the Dnmt1-KO ESCs and decreased in the Dnmt1/Tet1-DKO ESCs (S5D and S5E Fig). These results suggest that Tet1 plays a role in recruiting PRC2 to PCH under DNA hypomethylated condition.
We then depleted Ezh2, which is the core subunit of PRC2, in the Dnmt1-KO ESCs to test whether PRC2 was critical for chromocenter clustering (S6A Fig H2AK119ub, which was catalyzed by PRC1, was enriched in the PCH of Dnmt1-KO ESCs as previously reported (Fig 3A and 3B) [19]. Both immunostaining and ChIP-qPCR analysis showed that H2AK119ub enrichment in the PCH of Dnmt1-KO ESCs depended on Tet1 ( To test whether PRC1 is involved in chromocenter clustering, we targeted Ring1A and Ring1B, which are core components of PRC1 [36]. It was almost impossible to establish stable Ring1A/1B-DKO ESC lines because of the proliferation defect and differentiated phenotype of Ring1A/1B-depleted ESCs [37]. We then analyzed the Dnmt1/Ring1A/1B-TKO ESCs shortly after the introduction of the CRISPR cassette encoding Cas9 and sgRNAs (S9A Fig).
ESCs with depleted Ring1A/1B should have been detectable as H2AK119ub-negative cells since H2AK119ub synthesis completely depends on RING1A/1B [37]. Although H2AK119ubnegative cells were not detectable in the control wild-type or Dnmt1-KO cells, these cells appeared 4 d after the transfection of the CRISPR cassette ( Fig 3D). These H2AK119ub-negative cells showed a dispersed pattern of chromocenter, and its number was comparable to that of the wild-type ESCs (Fig 3D and 3E). Notably, HP1γ enrichment at PCH did not change in the Dnmt1/Ring1A/1B-TKO ESCs, suggesting that HP1γ was insufficient for chromocenter clustering under DNA hypomethylated condition. Considering that the enrichment of TET1 at PCH was slightly but significantly affected in the Dnmt1/Ring1A/1B-TKO ESCs, PRC1 complex would play some roles to stabilize TET1 at the PCH (S9B and S9C Fig). These results demonstrate that Tet1-dependent localization of PRC1 to PCH is one of the molecular bases of chromocenter clustering in DNA hypomethylated ESCs.

Liquid-liquid Phase separation (LLPS) contributes to chromocenter clustering in the DNA hypomethylated ESCs
the chromocenters. For this purpose, we established ESC lines expressing humanized Kusabira-Orange-1-tagged HP1γ (hKO1-HP1γ), which was specifically localized to PCH [38]. Time-lapse analysis of the Dnmt1-KO ESCs with the hKO1-HP1γ reporter revealed that their chromocenters underwent fusion and round up and produced larger chromocenters (Fig 4A  and S1 Movie). This dynamic change in the nucleus indicated that the chromocenter of DNA hypomethylated cells possessed the property of liquid-like condensates.
LLPS-driven condensation can be disrupted by 1,6-hexanediol (HD) treatment in vivo [27,39]. We treated ESCs with 1,6-HD and monitored the immediate response to chromocenter clustering. The number and shape of the chromocenter of the wild-type ESCs were not affected by 1-min treatment with 1,6-HD, when compared to 2,5-HD, which is an isomer of 1,6-HD and less effective in disrupting LLPS (S10A and S10B Fig). This observation was consistent with a previous study that showed that 1,6-HD had limited effects on the established heterochromatin [29]. In contrast, the number of chromocenters was significantly increased by 1,6-HD in the Dnmt1-KO ESCs (Fig 4B and 4C). Furthermore, the number of chromocenters was reduced to a level comparable to that of the untreated or 2,5-HD-treated Dnmt1-KO ESCs after removal of 1,6-HD (Fig 4B and 4C). Taken together, we conclude that Tet1-dependent chromocenter clustering in DNA hypomethylated ESCs is regulated by LLPS. However, the chromocenter number of the Dnmt1-KO ESCs was still less than that of the wild-type ESCs, even with 1,6-HD treatment (Figs 4C and S10B). It is conceivable that some other physiological interactions, which are resistant to 1,6-HD, play a role in chromocenter clustering in Dnmt1-KO ESCs.
Based on the above results, we propose that Tet1 induces epigenetic reprogramming of PCH under DNA hypomethylated condition, namely, deposition of 5hmC and facultative heterochromatin marks of H2AK119ub and H3K27me3 (Fig 5). Tet1-mediated reprogramming further led to chromocenter clustering through LLPS (Fig 5). This TET1-PRC1 axis is likely the molecular basis of chromocenter reorganization under the DNA hypomethylated condition.

Discussion
DNA in the PCH region is usually highly methylated and is presumably not targeted by TET proteins, as evident through undetectable 5hmC [40]. On the contrary, enrichment of 5hmC in PCH was obvious in the Dnmt1-KO ESCs and wild-type PGCs during epigenetic reprogramming, both of which possessed globally DNA hypomethylated states (S2A Fig) [31]. Although Tet1 and Tet2 are highly expressed in ESCs and PGCs, the depletion of Tet1 resulted in complete abrogation of PCH-5hmC in the Dnmt1-KO ESCs and PGCs (S2A Fig) [14]. The CXXC domain, which can bind to unmethylated DNA, is present in TET1 but not in TET2, and this difference may account for the difference in PCH targeting [41]. On the other hand, chromocenter clustering occurred in the Dnmt1/3a/3b-TKO ESCs and forced recruitment of TET1-CD-Mut to PCH induced its clustering. Therefore, PCH-5hmC was suggested to be a byproduct and not crucial for the chromocenter clustering.
A gain of facultative heterochromatin marks, such as H3K27me3 and H2AK119ub, at PCH, indicates that epigenetic heterochromatin remodeling occurred under the DNA hypomethylated state. We demonstrated that Tet1 is critical for this DNA hypomethylation-linked heterochromatin remodeling. A total of 97.2% of PRC2-binding sites overlapped with TET1 binding sites and 72.2% of PRC2 binding was affected by the depletion of Tet1 [16]. Consistent with these studies, we found that H3K27me3 levels at the PCH were decreased in the Dnmt1/Tet1-DKO ESCs (S5A-S5C Fig).
We also found that PRC1 recruitment to PCH in DNA hypomethylated ESCs depends on Tet1 (Figs 3A, 3B, S7A and S7B). Given that PRC1 binds to H3K27me3 through CBX7 in ESCs, the localization of PRC1 to PCH would be mediated by PRC2 [42]. However, enrichment of H2AK119ub in PCH was more intense than that of H3K27me3, PRC2 depletion showed minor effect on enrichment of H2AK119ub and RING1B, and the phenotype of the chromocenter clustering in the Ring1A/1B-DKO ESCs was more obvious than that of the Ezh2-KO ESCs. Furthermore, forced recruitment of the TET1 catalytic domain, which is associated with PRC1, to PCH induced the chromocenter clustering. Thus, we propose a model in which PRC1 is first recruited to PCH by TET1, followed by the binding of PRC2 to PRC1-mediated H2AK119ub. This model was supported by a study that forced recruitment of MBD-EZH2 to the chromocenter did not lead to H2AK119ub enrichment, but the recruitment of MBD-RING1B induced H3K27me3 enrichment [19]. It has also been reported that 90% of RING1B targets in ESCs correspond to bivalent promoters, which are the main targets of TET1 [9,43].
The PcG body is associated with the enrichment of facultative heterochromatin marks, H2AK119ub and H3K27me3 [22]. The puncta containing RING1B show a liquid-like feature of fusion and fission [21]. CBX2, the canonical PRC1 component, mediates PcG body formation in an LLPS-dependent manner [26,27]. Both clustered chromocenters in the DNA hypomethylated cells and PcG body exhibited PRC1 inclusion and LLPS characteristics (Figs 4B, 4C, S7A and S7B). Therefore, it is reasonable to consider that chromosome clustering in DNA hypomethylated cells relies on the biochemical characteristics of PcG proteins, which have the potential to aggregate into large foci.
PCH is represented by the constitutive heterochromatin marks such as H3K9me3 and HP1. H3K9me3 and HP1 have been known to regulate each other at PCH. For example, Suv39h1/ h2, an enzyme responsible for the H3K9 methylation at the constitutive heterochromatin, plays a pivotal role in HP1 deposition, and HP1 depletion affects the accumulation of H3K9me3 at PCH [35,44]. However, we found only the HP1γ enrichment, but no alteration of H3K9me3 in the PCH of Dnmt1-KO ESC. Thus, the HP1γ enrichment in the PCH of the DNA hypomethylated ESCs was quite likely to be independent of H3K9me3 and was regulated by TET1 (Fig 1A). In contrast, the enrichment of HP1γ in PCH was not affected in the meiotic germ cells of Tet1-KO mice [14]. These results suggest that DNA methylation, Tet1, Suv39h1/ h2, and PRC1/2 are intricately regulated among them and that the epigenetic state of PCH is regulated by distinct mechanisms in different cell types.
Chromocenter clustering is believed to lead to spatial reorganization of the nucleus and affect the microenvironment for gene silencing. Pericentromeric regions are reported to associate with other genomic regions and form a large domain called the pericentromere-associate domain (PAD) [45]. PAD patterns vary in different cell types and dynamically change during differentiation, unlike topologically associating domains (TADs), which are less variable among different cell types [45,46]. Forced recruitment of specific genes to PCH using chromodomain of HP1β resulted in H3K9me3 deposition and transcriptional silencing of that gene. Therefore, chromocenter reorganization, accompanied by chromocenter clustering and its enlargement, may change PAD and provide more space to repress other genomic loci.
Heterochromatin remodeling accompanied by ectopic PRC1 localization to PCH reduces H2AK119ub enrichment in other genomic loci [47]. The high copy number of satellite repeats has been proposed to act as a molecular "sponge", which can harbor epigenetic factors abundantly and affect the enrichment of the proteins in the genome [47]. Considering that ectopic localization of both PRC1 and PRC2 to the chromocenter was observed, re-localization of PRC1 and PRC2 and de-repression in other genomic regions might occur in the DNA hypomethylated ESCs.
Chromocenter clustering occurs at the meiotic prophase and is important for the progression of meiosis in germ cells [5]. Male germ cells enter meiosis under the DNA hypermethylated state, whereas female germ cells initiate meiosis under the DNA hypomethylated state [48]. Since the DNA methylation status of female meiotic germ cells is similar to that of Dnmt1-KO ESCs, Tet1 potentially plays a role in chromocenter clustering during meiosis in female germ cells.
In summary, we identified a novel role of Tet1 in the epigenetic remodeling of PCH and chromocenter clustering through PRC1 recruitment in DNA hypomethylated ESCs. Previous studies on PCH have mainly focused on the factors related to constitutive heterochromatin, such as SUV39H1/2 or HP1 [49,50]. However, our findings demonstrated that TET1 and PRC1, both of which are facultative heterochromatin factors, control the spatial organization of the chromocenter. Global DNA hypomethylation and PRC1 localization to the chromocenter have been reported in pre-implantation embryos and cancer cells [47,51]. TET family proteins would also regulate epigenetic remodeling of the PCH of these cells. Thus, our findings shed light on the field of chromocenter organization.

Generation of knockout cell lines
All knockout ESC lines were established using the CRISPR/Cas9 system. ESCs were transfected with a mixture of px330-puro vectors containing specific sgRNAs targeting each gene (S1 Table). Successful depletion of target genes was verified using genotyping PCR, Western blotting, and immunocytochemical analyses.
As for the depletion of Ring1A/1B, Dnmt1-KO ESCs was transfected with a mixture of px330-Puro vector containing sgRNA targeting Ring1A/1B (S1 Table). For selection, ESCs were cultured with 1 μg/mL puromycin for 2 d. Subsequently, the cells were cultured for recovery without puromycin for 1 d. The cells were then harvested and used for surface spread preparation.

Epigenome editing experiments
The fragment for sgRNA targeting the major satellite repeat (S1 Table) was inserted into the pPlatTET-gRNA2 plasmid (Addgene #82559). The scFv-only vector was generated by removing the TET1-CD domain from this plasmid. The TET1-CD-Mut vector contains nucleotide mutations corresponding to two amino acids critical for Tet1 catalytic activity (H1671Y and D1673A of TET1 protein) [52]. These plasmids were transfected to ESCs using Lipofectamine 2000 (Invitrogen, 11668019). Followed by the culture for 2 d, the cells were fixed for the immunofluorescent analysis, or harvested for surface spread preparation. GFP-positive cells were randomly picked for the following analysis.

Surface spread and immunocytochemistry
ESCs and gonadal cells were dissociated and placed on a glass slide dipped in a fixation solution of 1% paraformaldehyde (PFA) in distilled water (DW) (pH 9.2) containing 0.15% Triton X-100 and 3 mM dithiothreitol. Slides were then incubated overnight at 4˚C for a fixation. The slides were then washed in 0.4% Photoflo (Kodak) in DW and dried at room temperature. The samples were washed with 0.1% Triton X-100/PBS (PBS with Tween 20; PBST) and treated with 0.5% Triton X-100 for 20 min. After washing with PBST, the samples were incubated with 3% BSA and 2% goat serum in PBST (blocking buffer). Samples were then incubated with primary antibodies diluted in blocking buffer, washed, and incubated with appropriate secondary antibodies. Immunofluorescence was visualized using an LSM880 (Carl Zeiss). The enrichment score of each chromosome was calculated by the relative signal intensity of PCH to the entire chromosome using ZEN software (Zeiss). The enrichment score of each cell was determined by an average the enrichment scores of three chromosomes.

Immunocytochemistry using culture slide
For immunostaining against RING1B, TET1, EZH2, and NANOG, ESCs were plated in a culture slide (Corning, #354118) and cultured for 1 d before immunostaining. ESCs were fixed with 4% PFA for 5 min at room temperature, followed by permeabilization with 0.5% Triton X-100/PBS for 20 min. They were then incubated with the primary antibodies diluted in the blocking buffer at 4˚C overnight. The samples were washed with PBST and incubated with the secondary antibodies. Immunofluorescence was visualized using an LSM880 (Carl Zeiss). Relative signal intensity of the chromocenter was determined by dividing the average signal intensity of three to six chromocenters by that of whole nuclear area using ZEN software (Zeiss).

Live-cell imaging
The Dnmt1-KO ESCs stably expressing hKO1-HP1γ was established by the transfection of the linearized CAG-hKO1-HP1γ expressing plasmid. Time-lapse fluorescence microscopy was performed using an upright OLYMPUS confocal microscope (SpinSR10) with an x100 oil immersion lens. Images were taken every 2 min and processed using cellSens (OLYMPUS) and IMARIS (Carl Zeiss) software.

Hexanediol treatment
To disrupt LLPS, 1,6-hexanediol (Nacalai Tesque, 17913-72) or 2,5-hexanediol (TCI, H0100) at a final concentration of 10% was added to the ESC suspension. After 1 min of incubation at room temperature, the cells were placed on a glass slide dipped in the fixative. For the recovery experiment, 1,6-hexanediol treated cells were diluted with a 100-fold volume of ESC culture medium and incubated for 1 h in a humidified incubator at 37˚C, followed by the fixation on a glass slide.