Suv4-20h2 mediates chromatin compaction and is important for cohesin recruitment to heterochromatin.

Cohesin plays an important role in chromatid cohesion and has additional functions in higher-order chromatin organization and in transcriptional regulation. The binding of cohesin to euchromatic regions is largely mediated by CTCF or the mediator complex. However, it is currently unknown how cohesin is recruited to pericentric heterochromatin in mammalian cells. Here we define the histone methyltransferase Suv4-20h2 as a major structural constituent of heterochromatin that mediates chromatin compaction and cohesin recruitment. Suv4-20h2 stably associates with pericentric heterochromatin through synergistic interactions with multiple heterochromatin protein 1 (HP1) molecules, resulting in compaction of heterochromatic regions. Suv4-20h mutant cells display an overall reduced chromatin compaction and an altered chromocenter organization in interphase referred to as "chromocenter scattering." We found that Suv4-20h-deficient cells display chromosome segregation defects during mitosis that coincide with reduced sister chromatid cohesion. Notably, cohesin subunits interact with Suv4-20h2 both in vitro and in vivo. This interaction is necessary for cohesin binding to heterochromatin, as Suv4-20h mutant cells display substantially reduced cohesin levels at pericentric heterochromatin. This defect is most prominent in G0-phase cells, where cohesin is virtually lost from heterochromatin, suggesting that Suv4-20h2 is involved in the initial loading or maintenance of cohesion subunits. In summary, our data provide the first compelling evidence that Suv4-20h2 plays essential roles in regulating nuclear architecture and ensuring proper chromosome segregation.

Wnt/ß-catenin signalling regulates Sox17 expression and is essential for organizer and endoderm formation in the mouse.

Several signalling cascades are implicated in the formation and patterning of the three principal germ layers, but their precise temporal-spatial mode of action in progenitor populations remains undefined. We have used conditional gene deletion of mouse ß-catenin in Sox17-positive embryonic and extra-embryonic endoderm as well as vascular endothelial progenitors to address the function of canonical Wnt signalling in cell lineage formation and patterning. Conditional mutants fail to form anterior brain structures and exhibit posterior body axis truncations, whereas initial blood vessel formation appears normal. Tetraploid rescue experiments reveal that lack of ß-catenin in the anterior visceral endoderm results in defects in head organizer formation. Sox17 lineage tracing in the definitive endoderm (DE) shows a cell-autonomous requirement for ß-catenin in midgut and hindgut formation. Surprisingly, wild-type posterior visceral endoderm (PVE) in midgut- and hindgut-deficient tetraploid chimera rescues the posterior body axis truncation, indicating that the PVE is important for tail organizer formation. Upon loss of ß-catenin in the visceral endoderm and DE lineages, but not in the vascular endothelial lineage, Sox17 expression is not maintained, suggesting downstream regulation by canonical Wnt signalling. Strikingly, Tcf4/ß-catenin transactivation complexes accumulated on Sox17 cis-regulatory elements specifically upon endoderm induction in an embryonic stem cell differentiation system. Together, these results indicate that the Wnt/ß-catenin signalling pathway regulates Sox17 expression for visceral endoderm pattering and DE formation and provide the first functional evidence that the PVE is necessary for gastrula organizer gene induction and posterior axis development.

SET8 methyltransferase activity during the DNA double-strand break response is required for recruitment of 53BP1.

DNA double-strand breaks (DSBs) activate a signaling pathway known as the DNA damage response (DDR) which via protein-protein interactions and post-translational modifications recruit signaling proteins, such as 53BP1, to chromatin flanking the lesion. Depletion of the SET8 methyltransferase prevents accumulation of 53BP1 at DSBs; however, this phenotype has been attributed to the role of SET8 in generating H4K20 methylation across the genome, which is required for 53BP1 binding to chromatin, prior to DNA damage. Here, we report that SET8 acts directly at DSBs during the DNA damage response (DDR). SET8 accumulates at DSBs and is enzymatically active at DSBs. Depletion of SET8 just prior to the induction of DNA damage abrogates 53BP1's accumulation at DSBs, suggesting that SET8 acts during DDR. SET8's occupancy at DSBs is regulated by histone deacetylases (HDACs). Finally, SET8 is functionally required for efficient repair of DSBs specifically via the non-homologous end-joining pathway (NHEJ). Our findings reveal that SET8's active role during DDR at DSBs is required for 53BP1's accumulation.

Condensin dysfunction in human cells induces nonrandom chromosomal breaks in anaphase, with distinct patterns for both unique and repeated genomic regions.

Condensin complexes are essential for chromosome condensation and segregation in mitosis, while condensin dysfunction, among other pathways leading to chromosomal bridging in mitosis, may play a role in tumor genomic instability, including recently discovered chromotripsis. To characterize potential double-strand breaks specifically occurring in late anaphase, human chromosomes depleted of condensin were analyzed by γ-H2AX ChIP followed by high-throughput sequencing (ChIP-seq). In condensin-depleted cells, the nonrepeated parts of the genome were shown to contain distinct γ-H2AX enrichment zones 75% of which overlapped with known hemizygous deletions in cancers. Furthermore, some tandemly repeated DNA sequences, analyzed separately from the rest of the genome, showed significant γ-H2AX enrichment in condensin-depleted anaphases. The most commonly occurring targets of such enrichment included simple repeats, centromeric satellites, and rDNA. The two latter categories indicate that acrocentric human chromosomes are especially susceptible to breaks upon condensin deficiency. The genomic regions that are specifically destabilized upon condensin dysfunction may constitute a condensin-specific chromosome destabilization pattern.

Essential global role of CDC14 in DNA synthesis revealed by chromosome underreplication unrecognized by checkpoints in cdc14 mutants.

The CDC14 family of multifunctional evolutionarily conserved phosphatases includes major regulators of mitosis in eukaryotes and of DNA damage response in humans. The CDC14 function is also crucial for accurate chromosome segregation, which is exemplified by its absolute requirement in yeast for the anaphase segregation of nucleolar organizers; however the nature of this essential pathway is not understood. Upon investigation of the rDNA nondisjunction phenomenon, it was found that cdc14 mutants fail to complete replication of this locus. Moreover, other late-replicating genomic regions (10% of the genome) are also underreplicated in cdc14 mutants undergoing anaphase. This selective genome-wide replication defect is due to dosage insufficiency of replication factors in the nucleus, which stems from two defects, both contingent on the reduced CDC14 function in the preceding mitosis. First, a constitutive nuclear import defect results in a drastic dosage decrease for those replication proteins that are regulated by nuclear transport. Particularly, essential RPA subunits display both lower mRNA and protein levels, as well as abnormal cytoplasmic localization. Second, the reduced transcription of MBF and SBF-controlled genes in G1 leads to the reduction in protein levels of many proteins involved in DNA replication. The failure to complete replication of late replicons is the primary reason for chromosome nondisjunction upon CDC14 dysfunction. As the genome-wide slow-down of DNA replication does not trigger checkpoints [Lengronne A, Schwob E (2002) Mol Cell 9:1067-1078], CDC14 mutations pose an overwhelming challenge to genome stability, both generating chromosome damage and undermining the checkpoint control mechanisms.

Cooperation of sumoylated chromosomal proteins in rDNA maintenance.

SUMO is a posttranslational modifier that can modulate protein activities, interactions, and localizations. As the GFP-Smt3p fusion protein has a preference for subnucleolar localization, especially when deconjugation is impaired, the nucleolar role of SUMO can be the key to its biological functions. Using conditional triple SUMO E3 mutants, we show that defects in sumoylation impair rDNA maintenance, i.e., the rDNA segregation is defective and the rDNA copy number decreases in these mutants. Upon characterization of sumoylated proteins involved in rDNA maintenance, we established that Top1p and Top2p, which are sumoylated by Siz1p/Siz2p, most likely collaborate with substrates of Mms21p to maintain rDNA integrity. Cohesin and condensin subunits, which both play important roles in rDNA stability and structures, are potential substrates of Mms21, as their sumoylation depends on Mms21p, but not Siz1p and Siz2p. In addition, binding of cohesin and condensin to rDNA is altered in the mms21-CH E3-deficient mutant.

SET8 localization to chromatin flanking DNA damage is dependent on RNF168 ubiquitin ligase.

The DNA damage response (DDR) associated post-translational modifications recruit chromatin remodelers, signaling proteins such as 53BP1 and repair factors to chromatin flanking DNA double strand breaks (DSBs) to promote its repair. Although localization of both RNF168 ubiquitin ligase and SET8 methyltransferase at DSBs is essential for 53BP1's recruitment to DSBs, it is unclear if they do so via the same pathways. Here we report that RNF168 mediates SET8's recruitment to DSBs. Depletion of cellular pool of ubiquitin through proteasome inhibition abolished RNF168 and SET8's localization to DNA damage. Knockdown of RNF8 or RNF168 abolished SET8's recruitment to DNA damage. Moreover, RNF168 and SET8 form stable complexes in vivo. Based on these results we propose a model in which SET8, which despite being a pan-chromatin binding protein, can accumulate several folds at chromatin flanking DSBs through tethering to other proteins that specifically localize to chromatin regions with specific modifications.

Unreplicated DNA in mitosis precludes condensin binding and chromosome condensation in S. cerevisiae.

Condensin is the core activity responsible for chromosome condensation in mitosis. In the yeast S. cerevisiae, condensin binding is enriched at the regions where DNA replication terminates. Therefore, we investigated whether DNA replication completion determines the condensin-binding proficiency of chromatin. In order to fulfill putative mitotic requirements for condensin activity we analyzed chromosome condensation and condensin binding to unreplicated chromosomes in mitosis. For this purpose we used pGAL:CDC6 cdc15-ts cells that are known to enter mitosis without DNA replication if CDC6 transcription is repressed prior to S-phase. Both the condensation of nucleolar chromatin and proper condensin targeting to rDNA sites failed when unreplicated chromosomes were driven in mitosis. We propose that the DNA replication results in structural and/or biochemical changes to replicated chromatin, which are required for two-phase condensin binding and proper chromosome condensation.

The histone chaperone DAXX maintains the structural organization of heterochromatin domains.

BACKGROUND: The death domain-associated protein (DAXX) collaborates with accessory proteins to deposit the histone variant H3.3 into mouse telomeric and pericentromeric repeat DNA. Pericentromeric repeats are the main genetic contributor to spatially discrete, compact, constitutive heterochromatic structures called chromocentres. Chromocentres are enriched in the H3K9me3 histone modification and serve as integral, functionally important components of nuclear organization. To date, the role of DAXX as an H3.3-specific histone chaperone has been investigated primarily using biochemical approaches which provide genome-wide views on cell populations and information on changes in local chromatin structures. However, the global chromatin and subnuclear reorganization events that coincide with these changes remain to be investigated. RESULTS: Using electron spectroscopic imagine (ESI), a specialized form of energy-filtered transmission electron microscopy that allows us to visualize chromatin domains in situ with high contrast and spatial resolution, we show that in the absence of DAXX, H3K9me3-enriched domains are structurally altered and become uncoupled from major satellite DNA. In addition, the structural integrity of nucleoli and the organization of ribosomal DNA (rDNA) are disrupted. Moreover, the absence of DAXX leads to chromatin that is more sensitive, on a global level, to micrococcal nuclease digestion. CONCLUSIONS: We identify a novel role of DAXX as a major regulator of subnuclear organization through the maintenance of the global heterochromatin structural landscape. As well, we show, for the first time, that the loss of a histone chaperone can have severe consequences for global nuclear organization.