Sister chromatid separation at human telomeric regions

TERRA R-loops connect and protect sister telomeres in mitosis

Resolution of cohesion between sister telomeres in human cells depends on TRF1-mediated recruitment of the polyADP-ribosyltransferase tankyrase to telomeres. In human aged cells, due to insufficient recruitment of TRF1/tankyrase to shortened telomeres, sisters remain cohered in mitosis. This persistent cohesion plays a protective role, but the mechanism by which sisters remain cohered is not well understood. Here we show that telomere repeat-containing RNA (TERRA) holds sister telomeres together through RNA-DNA hybrid (R-loop) structures. We show that a tankyrase-interacting partner, the RNA-binding protein C19orf43, is required for repression of TERRA R-loops. Persistent telomere cohesion in C19orf43-depleted cells is counteracted by RNaseH1, confirming that RNA-DNA hybrids hold sisters together. Consistent with a protective role for persistent telomere cohesion, depletion of C19orf43 in aged cells reduces DNA damage and delays replicative senescence. We propose that the inherent inability of shortened telomeres to recruit R-loop-repressing machinery permits a controlled onset of senescence.

Terpyridine platinum compounds induce telomere dysfunction and chromosome instability in cancer cells

Telomerase/telomere-targeting therapy is a potentially promising approach for cancer treatment because even transient telomere dysfunction can induce chromosomal instability (CIN) and may be a barrier to tumor growth. We recently developed a dual-HAC (Human Artificial Chromosome) assay that enables identification and ranking of compounds that induce CIN as a result of telomere dysfunction. This assay is based on the use of two isogenic HT1080 cell lines, one carrying a linear HAC (containing telomeres) and the other carrying a circular HAC (lacking telomeres). Disruption of telomeres in response to drug treatment results in specific destabilization of the linear HAC. Results: In this study, we used the dual-HAC assay for the analysis of the platinum-derived G4 ligand Pt-tpy and five of its derivatives: Pt-cpym, Pt-vpym, Pt-ttpy, Pt(PA)-tpy, and Pt-BisQ. Our analysis revealed four compounds, Pt-tpy, Pt-ttpy, Pt-vpym and Pt-cpym, that induce a specific loss of a linear but not a circular HAC. Increased CIN after treatment by these compounds correlates with the induction of double-stranded breaks (DSBs) predominantly localized at telomeres and reflecting telomere-associated DNA damage. Analysis of the mitotic phenotypes induced by these drugs revealed an elevated rate of chromatin bridges (CBs) in late mitosis and cytokinesis. These terpyridine platinum-derived G4 ligands are promising compounds for cancer treatment.

RBL2 bi-allelic truncating variants cause severe motor and cognitive impairment without evidence for abnormalities in DNA methylation or telomeric function

RBL2/p130, a member of the retinoblastoma family of proteins, is a key regulator of cell division and propagates irreversible senescence. RBL2/p130 is also involved in neuronal differentiation and survival, and eliminating Rbl2 in certain mouse strains leads to embryonic lethality accompanied by an abnormal central nervous system (CNS) phenotype. Conflicting reports exist regarding a role of RBL2/p130 in transcriptional regulation of DNA methyltransferases (DNMTs), as well as the control of telomere length. Here we describe the phenotype of three patients carrying bi-allelic RBL2-truncating variants. All presented with infantile hypotonia, severe developmental delay and microcephaly. Malignancies were not reported in carriers or patients. Previous studies carried out on mice and human cultured cells, associated RBL2 loss to DNA methylation and telomere length dysregulation. Here, we investigated whether patient cells lacking RBL2 display related abnormalities. The study of primary patient fibroblasts did not detect abnormalities in expression of DNMTs. Furthermore, methylation levels of whole genome DNA, and specifically of pericentromeric repeats and subtelomeric regions, were unperturbed. RBL2-null fibroblasts show no evidence for abnormal elongation by telomeric recombination. Finally, gradual telomere shortening, and normal onset of senescence were observed following continuous culturing of RBL2-mutated fibroblasts. Thus, this study resolves uncertainties regarding a potential non-redundant role for RBL2 in DNA methylation and telomere length regulation, and indicates that loss of function variants in RBL2 cause a severe autosomal recessive neurodevelopmental disorder in humans.

Full length RTEL1 is required for the elongation of the single-stranded telomeric overhang by telomerase

Telomeres cap the ends of eukaryotic chromosomes and distinguish them from broken DNA ends to suppress DNA damage response, cell cycle arrest and genomic instability. Telomeres are elongated by telomerase to compensate for incomplete replication and nuclease degradation and to extend the proliferation potential of germ and stem cells and most cancers. However, telomeres in somatic cells gradually shorten with age, ultimately leading to cellular senescence. Hoyeraal-Hreidarsson syndrome (HHS) is characterized by accelerated telomere shortening and diverse symptoms including bone marrow failure, immunodeficiency, and neurodevelopmental defects. HHS is caused by germline mutations in telomerase subunits, factors essential for its biogenesis and recruitment to telomeres, and in the helicase RTEL1. While diverse phenotypes were associated with RTEL1 deficiency, the telomeric role of RTEL1 affected in HHS is yet unknown. Inducible ectopic expression of wild-type RTEL1 in patient fibroblasts rescued the cells, enabled telomerase-dependent telomere elongation and suppressed the abnormal cellular phenotypes, while silencing its expression resulted in gradual telomere shortening. Our observations reveal an essential role of the RTEL1 C-terminus in facilitating telomerase action at the telomeric 3' overhang. Thus, the common etiology for HHS is the compromised telomerase action, resulting in telomere shortening and reduced lifespan of telomerase positive cells.

Persistent telomere cohesion protects aged cells from premature senescence

Human telomeres are bound by the telomere repeat binding proteins TRF1 and TRF2. Telomere shortening in human cells leads to a DNA damage response that signals replicative senescence. While insufficient loading of TRF2 at shortened telomeres contributes to the DNA damage response in senescence, the contribution of TRF1 to senescence induction has not been determined. Here we show that counter to TRF2 deficiency-mediated induction of DNA damage, TRF1 deficiency serves a protective role to limit induction of DNA damage induced by subtelomere recombination. Shortened telomeres recruit insufficient TRF1 and as a consequence inadequate tankyrase 1 to resolve sister telomere cohesion. Our findings suggest that the persistent cohesion protects short telomeres from inappropriate recombination. Ultimately, in the final division, telomeres are no longer able to maintain cohesion and subtelomere copying ensues. Thus, the gradual loss of TRF1 and concomitant persistent cohesion that occurs with telomere shortening ensures a measured approach to replicative senescence.

Nuclear PARPs and genome integrity

Effective maintenance and stability of our genomes is essential for normal cell division, tissue homeostasis, and cellular and organismal fitness. The processes of chromosome replication and segregation require continual surveillance to insure fidelity. Accurate and efficient repair of DNA damage preserves genome integrity, which if lost can lead to multiple diseases, including cancer. Poly(ADP-ribose) a dynamic and reversible posttranslational modification and the enzymes that catalyze it (PARP1, PARP2, tankyrase 1, and tankyrase 2) function to maintain genome stability through diverse mechanisms. Here we review the role of these enzymes and the modification in genome repair, replication, and resolution in human cells.

Subtelomeric methylation distinguishes between subtypes of Immunodeficiency, Centromeric instability and Facial anomalies syndrome

Human telomeres and adjacent subtelomeres are packaged as heterochromatin. Subtelomeric DNA undergoes methylation during development by DNA methyltransferase 3B (DNMT3B), including the CpG-rich promoters of the long non-coding RNA (TERRA) embedded in these regions. The factors that direct DNMT3B methylation to human subtelomeres and maintain this methylation throughout lifetime are yet unknown. The importance of subtelomeric methylation is manifested through the abnormal telomeric phenotype in Immunodeficiency, Centromeric instability and Facial anomalies (ICF) syndrome type 1 patients carrying mutations in DNMT3B. Patient cells demonstrate subtelomeric hypomethylation, accompanied by elevated TERRA transcription, accelerated telomere shortening and premature senescence of fibroblasts. ICF syndrome can arise due to mutations in at least three additional genes, ZBTB24 (ICF2), CDCA7 (ICF3) and HELLS (ICF4). While pericentromeric repeat hypomethylation is evident in all ICF syndrome subtypes, the status of subtelomeric DNA methylation had not been described for patients of subtypes 2-4. Here we explored the telomeric phenotype in cells derived from ICF2-4 patients with the aim to determine whether ZBTB24, CDCA7 and HELLS also play a role in establishing and/or maintaining human subtelomeric methylation. We found normal subtelomeric methylation in ICF2-4 and accordingly low TERRA levels and unperturbed telomere length. Moreover, depleting the ICF2-4-related proteins in normal fibroblasts did not influence subtelomeric methylation. Thus, these gene products are not involved in establishing or maintaining subtelomeric methylation. Our findings indicate that human subtelomeric heterochromatin has specialized methylation regulation and highlight the telomeric phenotype as a characteristic that distinguishes ICF1 from ICF2-4.

APOL1 risk variants cause podocytes injury through enhancing endoplasmic reticulum stress

Two coding sequence variants (G1 and G2) of Apolipoprotein L1 (APOL1) gene have been implicated as a higher risk factor for chronic kidney diseases (CKD) in African Americans when compared with European Americans. Previous studies have suggested that the APOL1 G1 and G2 variant proteins are more toxic to kidney cells than the wild-type APOL1 G0, but the underlying mechanisms are poorly understood. To determine whether endoplasmic reticulum (ER) stress contributes to podocyte toxicity, we generated human podocytes (HPs) that stably overexpressed APOL1 G0, G1, or G2 (Vec/HPs, G0/HPs, G1/HPs, and G2/HPs). Propidium iodide staining showed that HP overexpressing the APOL1 G1 or G2 variant exhibited a higher rate of necrosis when compared with those overexpressing the wild-type G0 counterpart. Consistently, the expression levels of nephrin and podocin proteins were significantly decreased in the G1- or G2-overexpressing cells despite the maintenance of their mRNA expressions levels. In contrast, the expression of the 78-kDa glucose-regulated protein ((GRP78), also known as the binding Ig protein, BiP) and the phosphorylation of the eukaryotic translation initiation factor 1 (eIF1) were significantly elevated in the G1/HPs and G2/HPs, suggesting a possible occurrence of ER stress in these cells. Furthermore, ER stress inhibitors not only restored nephrin protein expression, but also provided protection against necrosis in G1/HPs and G2/HPs, suggesting that APOL1 risk variants cause podocyte injury partly through enhancing ER stress.

The 4D Nucleome: Genome Compartmentalization in an Evolutionary Context

4D nucleome research aims to understand the impact of nuclear organization in space and time on nuclear functions, such as gene expression patterns, chromatin replication, and the maintenance of genome integrity. In this review we describe evidence that the origin of 4D genome compartmentalization can be traced back to the prokaryotic world. In cell nuclei of animals and plants chromosomes occupy distinct territories, built up from ~1 Mb chromatin domains, which in turn are composed of smaller chromatin subdomains and also form larger chromatin domain clusters. Microscopic evidence for this higher order chromatin landscape was strengthened by chromosome conformation capture studies, in particular Hi-C. This approach demonstrated ~1 Mb sized, topologically associating domains in mammalian cell nuclei separated by boundaries. Mutations, which destroy boundaries, can result in developmental disorders and cancer. Nucleosomes appeared first as tetramers in the Archaea kingdom and later evolved to octamers built up each from two H2A, two H2B, two H3, and two H4 proteins. Notably, nucleosomes were lost during the evolution of the Dinoflagellata phylum. Dinoflagellate chromosomes remain condensed during the entire cell cycle, but their chromosome architecture differs radically from the architecture of other eukaryotes. In summary, the conservation of fundamental features of higher order chromatin arrangements throughout the evolution of metazoan animals suggests the existence of conserved, but still unknown mechanism(s) controlling this architecture. Notwithstanding this conservation, a comparison of metazoans and protists also demonstrates species-specific structural and functional features of nuclear organization.

High Mobility Group A2 protects cancer cells against telomere dysfunction

The non-histone chromatin binding protein High Mobility Group AT-hook protein 2 (HMGA2) plays important roles in the repair and protection of genomic DNA in embryonic stem cells and cancer cells. Here we show that HMGA2 localizes to mammalian telomeres and enhances telomere stability in cancer cells. We present a novel interaction of HMGA2 with the key shelterin protein TRF2. We found that the linker (L1) region of HMGA2 contributes to this interaction but the ATI-L1-ATII molecular region of HMGA2 is required for strong interaction with TRF2. This interaction was independent of HMGA2 DNA-binding and did not require the TRF2 interacting partner RAP1 but involved the homodimerization and hinge regions of TRF2. HMGA2 retained TRF2 at telomeres and reduced telomere-dysfunction despite induced telomere stress. Silencing of HMGA2 resulted in (i) reduced binding of TRF2 to telomere DNA as observed by ChIP, (ii) increased telomere instability and (iii) the formation of telomere dysfunction-induced foci (TIF). This resulted in increased telomere aggregation, anaphase bridges and micronuclei. HMGA2 prevented ATM-dependent pTRF2T188 phosphorylation and attenuated signaling via the telomere specific ATM-CHK2-CDC25C DNA damage signaling axis. In summary, our data demonstrate a unique and novel role of HMGA2 in telomere protection and promoting telomere stability in cancer cells. This identifies HMGA2 as a new therapeutic target for the destabilization of telomeres in HMGA2+ cancer cells.

Mutations in STN1 cause Coats plus syndrome and are associated with genomic and telomere defects

The analysis of individuals with telomere defects may shed light on the delicate interplay of factors controlling genome stability, premature aging, and cancer. We herein describe two Coats plus patients with telomere and genomic defects; both harbor distinct, novel mutations in STN1, a member of the human CTC1-STN1-TEN1 (CST) complex, thus linking this gene for the first time to a human telomeropathy. We characterized the patients' phenotype, recapitulated it in a zebrafish model and rescued cellular and clinical aspects by the ectopic expression of wild-type STN1 or by thalidomide treatment. Interestingly, a significant lengthy control of the gastrointestinal bleeding in one of our patients was achieved by thalidomide treatment, exemplifying a successful bed-to-bench-and-back approach.

Loss of ATRX Suppresses Resolution of Telomere Cohesion to Control Recombination in ALT Cancer Cells

The chromatin-remodeler ATRX is frequently lost in cancer cells that use ALT (alternative lengthening of telomeres) for telomere maintenance, but its function in telomere recombination is unknown. Here we show that loss of ATRX suppresses the timely resolution of sister telomere cohesion that normally occurs prior to mitosis. In the absence of ATRX, the histone variant macroH2A1.1 binds to the poly(ADP-ribose) polymerase tankyrase 1, preventing it from localizing to telomeres and resolving cohesion. The resulting persistent telomere cohesion promotes recombination between sister telomeres, while it suppresses inappropriate recombination between non-sisters. Forced resolution of sister telomere cohesion induces excessive recombination between non-homologs, genomic instability, and impaired cell growth, indicating the ATRX-macroH2A1.1-tankyrase axis as a potential therapeutic target in ALT tumors.

Persistent telomere cohesion triggers a prolonged anaphase

Telomeres use distinct mechanisms (not used by arms or centromeres) to mediate cohesion between sister chromatids. However, the motivation for a specialized mechanism at telomeres is not well understood. Here we show, using fluorescence in situ hybridization and live-cell imaging, that persistent sister chromatid cohesion at telomeres triggers a prolonged anaphase in normal human cells and cancer cells. Excess cohesion at telomeres can be induced by inhibition of tankyrase 1, a poly(ADP-ribose) polymerase that is required for resolution of telomere cohesion, or by overexpression of proteins required to establish telomere cohesion, the shelterin subunit TIN2 and the cohesin subunit SA1. Regardless of the method of induction, excess cohesion at telomeres in mitosis prevents a robust and efficient anaphase. SA1- or TIN2-induced excess cohesion and anaphase delay can be rescued by overexpression of tankyrase 1. Moreover, we show that primary fibroblasts, which accumulate excess telomere cohesion at mitosis naturally during replicative aging, undergo a similar delay in anaphase progression that can also be rescued by overexpression of tankyrase 1. Our study demonstrates that there are opposing forces that regulate telomere cohesion. The observation that cells respond to unresolved telomere cohesion by delaying (but not completely disrupting) anaphase progression suggests a mechanism for tolerating excess cohesion and maintaining telomere integrity. This attempt to deal with telomere damage may be ultimately futile for aging fibroblasts but useful for cancer cells.

SA1 binds directly to DNA through its unique AT-hook to promote sister chromatid cohesion at telomeres

Sister chromatid cohesion relies on cohesin, a complex comprising a tri-partite ring and a peripheral subunit Scc3, which is found as two related isoforms SA1 and SA2 in vertebrates. There is a division of labor between the vertebrate cohesin complexes; SA1-cohesin is required at telomeres and SA2-cohesin at centromeres. Depletion of SA1 has dramatic consequences for telomere function and genome integrity, but the mechanism by which SA1-cohesin mediates cohesion at telomeres is not well understood. Here we dissect the individual contribution of SA1 and the ring subunits to telomere cohesion and show that telomeres rely heavily on SA1 and to a lesser extent on the ring for cohesion. Using chromatin immunoprecipitation we show that SA1 is highly enriched at telomeres, is decreased at mitosis when cohesion is resolved, and is increased when cohesion persists. Overexpression of SA1 alone was sufficient to induce cohesion at telomeres, independent of the cohesin ring and dependent on its unique (not found in SA2) N-terminal domain, which we show binds to telomeric DNA through an AT-hook motif. We suggest that a specialized cohesion mechanism may be required to accommodate the high level of DNA replication-associated repair at telomeres.

Characterization and rescue of telomeric abnormalities in ICF syndrome type I fibroblasts

Mutations in the human DNA methyltransferase 3B (DNMT3B) gene lead to ICF (immunodeficiency, centromeric region instability, and facial anomalies) syndrome type I. We have previously described a telomere-related phenotype in cells from these patients, involving severe hypomethylation of subtelomeric regions, abnormally short telomeres and high levels of telomeric-repeat-containing RNA (TERRA). Here we demonstrate that ICF-patient fibroblasts carry abnormally short telomeres at a low population doubling (PD) and enter senescence prematurely. Accordingly, we attempted to rescue the senescence phenotype by ectopic expression of human telomerase, which led to elongated telomeres with hypomethylated subtelomeres. The senescence phenotype was overcome under these conditions, thus dissociating subtelomeric-DNA hypomethylation per se from the senescence phenotype. In addition, we examined whether the subtelomeric methylation could be restored by expression of a normal copy of full length DNMT3B1 in ICF fibroblasts. Ectopic expression of DNMT3B1 failed to rescue the abnormal hypomethylation at subtelomeres. However, partial rescue of subtelomeric-hypomethylation was achieved by co-expression of DNMT3B1 together with DNA methyltransferase 3-like (DNMT3L), encoding a protein that functions as a stimulator of DNMT3A and DNMT3B. DNMT3B1 and DNMT3L are predominantly expressed during early embryonic development, suggesting that de novo subtelomeric DNA methylation during crucial stages of human embryonic development may be necessary for setting and maintaining normal telomere length.

Cyclin B-dependent kinase 1 regulates human TRF1 to modulate the resolution of sister telomeres

Cyclin B-Cdk1 is a key mediator of mitotic entry; however, little is known about its role in the separation of sister chromatids. Here we report that upon mitotic entry, Cdk1 specifically phosphorylates threonine 371 of TRF1, a telomere binding protein implicated in the regulation of sister telomere cohesion. Such phosphorylation is removed in late mitosis when Cdk1 activity is inhibited, indicative of a tight regulation of T371 phosphorylation. We show that T371 phosphorylation by Cdk1 keeps TRF1 free of chromatin and this phosphorylation is associated with loss of telomere-bound TRF1 and TIN2, and a reduction in telomere heterochromatin. We find that a phosphomimic mutation at T371 of TRF1 induces telomere deprotection, resulting in telomere loss and the formation of telomere fusions, whereas a non-phosphorylatable substitution of T371 blocks sister telomere resolution, promotes micronuclei formation and impairs cell proliferation. Our work suggests that Cdk1 controls TRF1 association with telomeres to facilitate temporal telomere de-protection, which is essential for sister telomere resolution.

Quantitative digital in situ senescence-associated β-galactosidase assay

BACKGROUND

Cellular senescence plays important roles in the aging process of complex organisms, in tumor suppression and in response to stress. Several markers can be used to identify senescent cells, of which the most widely used is the senescence-associated β-galactosidase (SABG) activity. The main advantage of SABG activity over other markers is the simplicity of the detection assay and the capacity to identify in situ a senescent cell in a heterogeneous cell population. Several approaches have been introduced to render the SABG assay quantitative. However none of these approaches to date has proven particularly amenable to quantitative analysis of SABG activity in situ. Furthermore the role of cellular senescence (CS) in vivo remains unclear mainly due to the ambiguity of current cellular markers in identifying CS of individual cells in tissues.

RESULTS

In the current study we applied a digital image analysis technique to the staining generated using the original SABG assay, and demonstrate that this analysis is highly reproducible and sensitive to subtle differences in staining intensities resulting from diverse cellular senescence pathways in culture. We have further validated our method on mouse kidney samples with and without diabetes mellitus, and show that a more accurate quantitative SABG activity with a wider range of values can be achieved at a pH lower than that used in the conventional SABG assay.

CONCLUSIONS

We conclude that quantitative in situ SABG assay, is feasible and reproducible and that the pH at which the reaction is performed should be tailored and chosen, depending on the research question and experimental system of interest.

Reprogramming of telomeric regions during the generation of human induced pluripotent stem cells and subsequent differentiation into fibroblast-like derivatives

Human induced pluripotent stem (hiPS) cells provide therapeutic promises, as well as a potent in vitro model for studying biological processes which take place during human embryonic development and subsequent differentiation in normal and disease states. The epigenetic characteristics of iPS cells are reprogrammed to the embryonic state at which they acquire pluripotency. In addition, telomeres in hiPS cell must elongate sufficiently to provide the necessary replicative potential. Recent studies have demonstrated that the epigenetic characteristics of telomeric and subtelomeric regions are pivotal in regulating telomere length. Here we study telomere length, subtelomeric DNA methylation and telomeric-repeat-containing RNA (TERRA) expression in several hiPS cell clones derived from normal neonatal foreskin fibroblasts. We find that telomeres lengthen significantly in hiPS cells in comparison to the parental fibroblast source, and progressively shorten after differentiation back into fibroblast-like cells, concomitantly with telomerase activation and down-regulation, respectively. Subtelomeres in hiPS cells were found to be generally hypermethylated in comparison to the parental source. However bisulfite analysis revealed that at several subtelomeres examined, methylation levels differed between hiPS clones and that both de novo methylation and demethylation processes occurred during telomere reprogramming. Notably, although subtelomeres were in general very highly methylated, TERRA levels were elevated in hiPS cells, albeit to different degrees in the various clones. TERRA elevation may reflect enhanced stability or impaired degradation in hiPS cells, and/or alternatively, increased transcription from the hypomethylated subtelomeres. We suggest that TERRA may play a role in regulation of appropriate telomere function and length in hiPS cells.

Hypomethylation of subtelomeric regions in ICF syndrome is associated with abnormally short telomeres and enhanced transcription from telomeric regions

Telomeres and adjacent subtelomeric regions are packaged as heterochromatin in many organisms. The heterochromatic features include DNA methylation, histones H3-Lys9 (Lysine 9) and H4-Lys20 (Lysine 20) methylation and heterochromatin protein1 alpha binding. Subtelomeric DNA is hypomethylated in human sperm and ova, and these regions are subjected to de novo methylation during development. In mice this activity is carried out by DNA methyltransferase 3b (Dnmt3b). Mutations in DNMT3B in humans lead to the autosomal-recessive ICF (immunodeficiency, centromeric region instability, facial anomalies) syndrome. Here we show that, in addition to several satellite and non-satellite repeats, the subtelomeric regions in lymphoblastoid and fibroblast cells of ICF patients are also hypomethylated to similar levels as in sperm. Furthermore, the telomeres are abnormally short in both the telomerase-positive and -negative cells, and many chromosome ends lack detectable telomere fluorescence in situ hybridization signals from either one or both sister-chromatids. In contrast to Dnmt3a/b(-/-) mouse embryonic stem cells, increased telomere sister-chromatid exchange was not observed in ICF cells. Hypomethylation of subtelomeric regions was associated in the ICF cells with advanced telomere replication timing and elevated levels of transcripts emanating from telomeric regions, known as TERRA (telomeric-repeat-containing RNA) or TelRNA. The current findings provide a mechanistic explanation for the abnormal telomeric phenotype observed in ICF syndrome and highlights the link between TERRA/TelRNA and structural telomeric integrity.

Chromosome cohesion - rings, knots, orcs and fellowship

Sister-chromatid cohesion is essential for accurate chromosome segregation. A key discovery towards our understanding of sister-chromatid cohesion was made 10 years ago with the identification of cohesins. Since then, cohesins have been shown to be involved in cohesion in numerous organisms, from yeast to mammals. Studies of the composition, regulation and structure of the cohesin complex led to a model in which cohesin loading during S-phase establishes cohesion, and cohesin cleavage at the onset of anaphase allows sister-chromatid separation. However, recent studies have revealed activities that provide cohesion in the absence of cohesin. Here we review these advances and propose an integrative model in which chromatid cohesion is a result of the combined activities of multiple cohesion mechanisms.

Arabidopsis sister chromatids often show complete alignment or separation along a 1.2-Mb euchromatic region but no cohesion "hot spots"

Sister chromatid cohesion is a prerequisite for correct segregation and possibly other functions of replicated chromosomes. Except for yeast, no details are known about arrangement of cohesion sites along interphase chromosomes. Within nuclei of several higher plants, sister chromatids are frequently not aligned at various positions along chromosome arms. Therefore, we tested whether preferential alignment positions ("cohesion hot spots") and constant extension of and distances between aligned sites occur in plants. Along a approximately 1.2-Mb contig from the bottom arm of chromosome 1, the sister chromatid positions of 13 individual BAC inserts were found to be aligned for approximately 67-77% of homologues in 4C Arabidopsis thaliana nuclei. The differences between the 13 BAC positions were not significant at the P < 0.01 level. This suggests variability of alignment positions between cells and indicates the absence of cohesion "hot spots". Similar as for single BACs, FISH with the entire contig indicated complete alignment for approximately 69% and complete separation of sister chromatids for approximately 31% of homologues in 4C nuclei. Partial alignment or separation was barely detectable. When three BAC inserts from a 760-kb region were tested simultaneously, alignment or separation of only the central BAC occurred in 3.3% and 3.5% of replicated chromosomes, respectively. Thus, we assume that sister chromatids can be separated or aligned within a Mb range in differentiated cells. However, the minimum extension of aligned sites or distances between them may (in rare cases) fall below ~500 kb.

Unified mode of centromeric protection by shugoshin in mammalian oocytes and somatic cells

Reductional chromosome segregation in germ cells, where sister chromatids are pulled to the same pole, accompanies the protection of cohesin at centromeres from separase cleavage. Here, we show that mammalian shugoshin Sgo2 is expressed in germ cells and is solely responsible for the centromeric localization of PP2A and the protection of cohesin Rec8 in oocytes, proving conservation of the mechanism from yeast to mammals. However, this role of Sgo2 contrasts with its mitotic role in protecting centromeric cohesin only from prophase dissociation, but never from anaphase cleavage. We demonstrate that, in somatic cells, shugoshin colocalizes with cohesin in prophase or prometaphase, but their localizations become separate when centromeres are pulled oppositely at metaphase. Remarkably, if tension is artificially removed from the centromeres at the metaphase-anaphase transition, cohesin at the centromeres can be protected from separase cleavage even in somatic cells, as in germ cells. These results argue for a unified view of centromeric protection by shugoshin in mitosis and meiosis.

Protein requirements for sister telomere association in human cells

Previous studies in human cells indicate that sister telomeres have distinct requirements for their separation at mitosis. In cells depleted for tankyrase 1, a telomeric poly(ADP-ribose) polymerase, sister chromatid arms and centromeres separate normally, but telomeres remain associated and cells arrest in mitosis. Here, we use biochemical and genetic approaches to identify proteins that might mediate the persistent association at sister telomeres. We use immunoprecipitation analysis to show that the telomeric proteins, TRF1 (an acceptor of PARsylation by tankyrase 1) and TIN2 (a TRF1 binding partner) each bind to the SA1 ortholog of the cohesin Scc3 subunit. Sucrose gradient sedimentation shows that TRF1 cosediments with the SA1-cohesin complex. Depletion of the SA1 cohesin subunit or the telomeric proteins (TRF1 and TIN2) restores the normal resolution of sister telomeres in mitosis in tankyrase 1-depleted cells. Moreover, depletion of TRF1 and TIN2 or SA1 abrogates the requirement for tankyrase 1 in mitotic progression. Our studies indicate that sister telomere association in human cells is mediated by a novel association between a cohesin subunit and components of telomeric chromatin.

Sister chromatid cohesion: the cohesin cleavage model does not ring true

Sister chromatid cohesion is important for high fidelity chromosome segregation during anaphase. Gene products that provide structural components (cohesin complex or cohesin) and regulatory components responsible for cohesion are conserved through eukaryotes. A simple model where cohesion establishment occurs by replication through static cohesin rings and cohesion dissolution occurs by Esp1p/separase mediated cleavage of the cohesin rings (Mcd1p/Rad21p/Scc1p sub-unit cleavage) has become widespread. A growing body of evidence is inconsistent with this ring cleavage model. This review will summarize the evidence showing that cohesin complex is not static but is regulated at multiple cell cycle stages before anaphase in a separase independent manner. Separase is indeed required at anaphase for complete chromosome segregation. However, multiple mechanisms for cohesion dissolution appear to act concurrently during anaphase. Separase is only one such mechanism and its importance varies from organism to organism. The idea that cohesin is a dynamic complex subjected to regulation at various cell cycle stages by multiple mechanisms makes sense in light of the myriad functions in which it has been implicated, such as DNA damage repair, gene silencing and chromosome condensation.

Random homologous pairing and incomplete sister chromatid alignment are common in angiosperm interphase nuclei

The chromosome arrangement in interphase nuclei is of growing interest, e.g., the spatial vicinity of homologous sequences is decisive for efficient repair of DNA damage by homologous recombination, and close alignment of sister chromatids is considered as a prerequisite for their bipolar orientation and subsequent segregation during nuclear division. To study the degree of homologous pairing and of sister chromatid alignment in plants, we applied fluorescent in situ hybridisation with specific bacterial artificial chromosome inserts to interphase nuclei. Previously we found in Arabidopsis thaliana and in A. lyrata positional homologous pairing at random, and, except for centromere regions, sister chromatids were frequently not aligned. To test whether these features are typical for higher plants or depend on genome size, chromosome organisation and/or phylogenetic affiliation, we investigated distinct individual loci in other species. The positional pairing of these loci was mainly random. The highest frequency of sister alignment (in >93% of homologues) was found for centromeres, some rDNA and a few other high copy loci. Apparently, somatic homologous pairing is not a typical feature of angiosperms, and sister chromatid aligment is not obligatory along chromosome arms. Thus, the high frequency of chromatid exchanges at homologous positions after mutagen treatment needs another explanation than regular somatic pairing of homologues (possibly an active search of damaged sites for homology). For sister chromatid exchanges a continuous sister chromatid alignment is not required. For correct segregation, permanent alignment of sister centromeres is sufficient.

Cohesin is dispensable for centromere cohesion in human cells

BACKGROUND

Proper regulation of the cohesion at the centromeres of human chromosomes is essential for accurate genome transmission. Exactly how cohesion is maintained and is then dissolved in anaphase is not understood.

PRINCIPAL FINDINGS

We have investigated the role of the cohesin complex at centromeres in human cells both by depleting cohesin subunits using RNA interference and also by expressing a non-cleavable version of the Rad21 cohesin protein. Rad21 depletion results in aberrant anaphase, during which the sister chromatids separate and segregate in an asynchronous fashion. However, centromere cohesion was maintained before anaphase in Rad21-depleted cells, and the primary constrictions at centromeres were indistinguishable from those in control cells. Expression of non-cleavable Rad21 (NC-Rad21), in which the sites normally cleaved by separase are mutated, resulted in delayed sister chromatid resolution in prophase and prometaphase, and a blockage of chromosome arm separation in anaphase, but did not impede centromere separation.

CONCLUSIONS

These data indicate that cohesin complexes are dispensable for sister cohesion in early mitosis, yet play an important part in the fidelity of sister separation and segregation during anaphase. Cleavage at the separase-sensitive sites of Rad21 is important for arm separation, but not for centromere separation.

Incomplete sister chromatid separation of long chromosome arms

Chromosome segregation ensures the equal partitioning of chromosomes at mitosis. However, long chromosome arms may pose a problem for complete sister chromatid separation. In this paper we report on the analysis of cell division in primary cells from field vole Microtus agrestis, a species with 52 chromosomes including two giant sex chromosomes. Dual chromosome painting with probes specific for the X and the Y chromosomes showed that these long chromosomes are prone to mis-segregate, producing DNA bridges between daughter nuclei and micronuclei. Analysis of mitotic cells with incomplete chromatid separation showed that reassembly of the nuclear membrane, deposition of INner CENtromere Protein (INCENP)/Aurora B to the spindle midzone and furrow formation occur while the two groups of daughter chromosomes are still connected by sex chromosome arms. Late cytokinetic processes are not efficiently inhibited by the incomplete segregation as in a significant number of cell divisions cytoplasmic abscission proceeds while Aurora B is at the midbody. Live-cell imaging during late mitotic stages also revealed abnormal cell division with persistent sister chromatid connections. We conclude that late mitotic regulatory events do not monitor incomplete sister chromatid separation of the large X and Y chromosomes of Microtus agrestis, leading to defective segregation of these chromosomes. These findings suggest a limit in chromosome arm length for efficient chromosome transmission through mitosis.

The influence of a human embryonic stem cell-derived microenvironment on targeting of human solid tumor xenografts

The awareness of the important role that the surrounding tissue microenvironment and stromal response play in the process of tumorigenesis has grown as a result of in vivo models of tumor xenograft growth in immunocompromised mice. In the current study, we used human embryonic stem cells in order to study the interactions of tumor cells with the surrounding microenvironment of differentiated human cell tissues and structures. Several cancer cell types stably expressing an H2A-green fluorescence protein fusion protein, which allowed tracking of tumor cells, were injected into mature teratomas and developed into tumors. The salient findings were: (a) the observation of growth of tumor cells with high proliferative capacity within the differentiated microenvironment of the teratoma, (b) the identification of invasion by tumor cells into surrounding differentiated teratoma structures, and (c) the identification of blood vessels of human teratoma origin, growing adjacent to and within the cancer cell-derived tumor. Mouse embryonic stem cell-derived teratomas also supported cancer cell growth, but provided a less suitable model for human tumorigenesis studies. Anticancer immunotherapy treatment directed against A431 epidermoid carcinoma cell-related epitopes induced the complete regression of A431-derived tumor xenografts following direct i.m. injection in immunocompromised mice, as opposed to corresponding tumors growing within a human embryonic stem cell-derived microenvironment, wherein remnant foci of viable tumor cells were detected and resulted in tumor recurrence. We propose using this novel experimental model as a preclinical platform for investigating and manipulating the stromal response in tumor cell growth as an additional tool in cancer research.

Sister chromatids are often incompletely aligned in meristematic and endopolyploid interphase nuclei of Arabidopsis thaliana

We analyzed whether sister chromatids are continuously aligned in meristematic and endopolyploid Arabidopsis interphase nuclei by studying sister-chromatid alignment at various chromosomal positions. FISH with individual BACs to flow-sorted 4C root and leaf nuclei frequently yielded more than two hybridization signals, indicating incomplete or absent sister-chromatid alignment. Up to 100% of 8C, 16C, and 32C nuclei showed no sister-chromatid alignment at defined positions. Simultaneous FISH with BACs from different chromosomal positions revealed more frequent sister-chromatid alignment in terminal than in midarm positions. Centromeric positions were mainly aligned up to a ploidy level of 16C but became separated or dispersed in 32C nuclei. DNA hypomethylation (of the whole genome) and transcriptional activity (at FWA gene position) did not impair sister-chromatid alignment. Only 6.1% of 4C leaf nuclei showed sister-chromatid separation of the entire chromosome 1 top arm territories. Homozygous transgenic tandem repeat (lac operator) arrays showing somatic homologous pairing more often than average euchromatic loci did not promote an increased frequency of sister-chromatid alignment. The high frequency of separated sister-chromatid arm positions in > or =4C nuclei suggests that sister-chromatid cohesion is variable, dynamic, and not obligatory along the entire chromosome arm in meristematic and differentiated Arabidopsis nuclei.

Loss of hPot1 function leads to telomere instability and a cut-like phenotype

The human telomere binding protein hPot1 binds to the most distal single-stranded extension of telomeric DNA in vitro, and probably in vivo, as well as associating with the double-stranded telomeric DNA binding proteins TRF1 and TRF2 through the bridging proteins PTOP (also known as PIP1 or TINT1) and TIN2. Disrupting either the DNA binding activity of hPot1 or its association with PTOP results in elongated telomeres, suggesting a role for hPot1 in telomere length regulation. However, mutations to POT1 and Cdc13p, the fission and budding yeast genes encoding the structural orthologs of this protein, leads to telomere instability and cell death. Thus, it is possible that the hPot1 protein may also serve to cap and protect telomeres in humans. Indeed, we now find that knocking down the expression of hPot1 in human cells causes apoptosis or senescence, as well as an increase in telomere associations and anaphase bridges, telltale signs of telomere instability. In addition, knockdown cells also displayed chromatin bridges between interphase cells, reminiscent of the cut phenotype that was first described in fission yeast and in which cytokinesis progresses despite a failure of chromatid separation. However, unlike the yeast cut phenotypes, we suggest that the cut-like phenotype observed in hPot1 knockdown cells is a consequence of the fusion of chromosome ends and that this fusion impedes proper chromosomal segregation. We conclude that hPot1 protects chromosome ends from illegitimate recombination, catastrophic chromosome instability, and abnormal chromosome segregation.