Mitotic bookmarking by SWI/SNF subunits

For cells to initiate and sustain a differentiated state, it is necessary that a 'memory' of this state is transmitted through mitosis to the daughter cells1-3. Mammalian switch/sucrose non-fermentable (SWI/SNF) complexes (also known as Brg1/Brg-associated factors, or BAF) control cell identity by modulating chromatin architecture to regulate gene expression4-7, but whether they participate in cell fate memory is unclear. Here we provide evidence that subunits of SWI/SNF act as mitotic bookmarks to safeguard cell identity during cell division. The SWI/SNF core subunits SMARCE1 and SMARCB1 are displaced from enhancers but are bound to promoters during mitosis, and we show that this binding is required for appropriate reactivation of bound genes after mitotic exit. Ablation of SMARCE1 during a single mitosis in mouse embryonic stem cells is sufficient to disrupt gene expression, impair the occupancy of several established bookmarks at a subset of their targets and cause aberrant neural differentiation. Thus, SWI/SNF subunit SMARCE1 has a mitotic bookmarking role and is essential for heritable epigenetic fidelity during transcriptional reprogramming.

SWI/SNF-deficient undifferentiated/rhabdoid carcinoma of the gallbladder carrying a POLE mutation in a 30-year-old woman: a case report

BACKGROUND

Undifferentiated carcinoma of the biliary tract are highly aggressive malignancies. In other organs, a subgroup of undifferentiated carcinoma related to SWI/SNF complex-deficiency have been described.

CASE PRESENTATION

A 30-year-old woman presented with rising inflammatory markers (C-reactive protein (CRP)). Ultrasound examination revealed a large tumor of the liver. A computed tomography scan was performed and was primarily interpreted as a tumor-forming liver abscess, possibly caused by gallbladder perforation. Subsequent liver segment resection was performed. Microscopic examination showed an undifferentiated carcinoma with rhabdoid morphology and prominent inflammatory infiltrate in the gallbladder base. With SWI/SNF immunohistochemistry, intact expression of SMARCB1, SMARCA4, ARID1A, but loss of SMARCA2 and PBRM1 was detected. Next-generation-sequencing detected KRAS, PBRM1 and ARID1B mutations, a deleterious splice-site mutation in the POLE-gene and a mutation in the TP53-gene.

CONCLUSIONS

We were able to demonstrate loss of SMARCA2 expression and mutations characteristic of an SWI/SNF-deficient carcinoma in a tumor derived from the gallbladder. This is the first reported case of an undifferentiated carcinoma with rhabdoid features in the gallbladder carrying a POLE mutation and SWI/SNF-deficiency of PBRM1 and SMARCA2.

CENP-C functions in centromere assembly, the maintenance of CENP-A asymmetry and epigenetic age in Drosophila germline stem cells

Germline stem cells divide asymmetrically to produce one new daughter stem cell and one daughter cell that will subsequently undergo meiosis and differentiate to generate the mature gamete. The silent sister hypothesis proposes that in asymmetric divisions, the selective inheritance of sister chromatids carrying specific epigenetic marks between stem and daughter cells impacts cell fate. To facilitate this selective inheritance, the hypothesis specifically proposes that the centromeric region of each sister chromatid is distinct. In Drosophila germ line stem cells (GSCs), it has recently been shown that the centromeric histone CENP-A (called CID in flies)—the epigenetic determinant of centromere identity—is asymmetrically distributed between sister chromatids. In these cells, CID deposition occurs in G₂ phase such that sister chromatids destined to end up in the stem cell harbour more CENP-A, assemble more kinetochore proteins and capture more spindle microtubules. These results suggest a potential mechanism of ‘mitotic drive’ that might bias chromosome segregation. Here we report that the inner kinetochore protein CENP-C, is required for the assembly of CID in G₂ phase in GSCs. Moreover, CENP-C is required to maintain a normal asymmetric distribution of CID between stem and daughter cells. In addition, we find that CID is lost from centromeres in aged GSCs and that a reduction in CENP-C accelerates this loss. Finally, we show that CENP-C depletion in GSCs disrupts the balance of stem and daughter cells in the ovary, shifting GSCs toward a self-renewal tendency. Ultimately, we provide evidence that centromere assembly and maintenance via CENP-C is required to sustain asymmetric divisions in female Drosophila GSCs.

Stem cells can divide in an asymmetric fashion giving rise to two daughter cells with different fates. One daughter remains a stem cell, while the other can differentiate and adopt a new cell fate. Germline stem cells in the testes and ovaries give rise to differentiating daughter cells that eventually form the gametes, eggs and sperm. Here we investigate mechanisms controlling germline stem cell divisions occurring in the ovary of the fruit fly Drosophila melanogaster. Centromeres are epigenetically specified loci on chromosomes that make essential connections to the cell division machinery. Our study is focused on the centromere component CENP-C. We show that CENP-C is critical for the correct assembly of centromeres that occurs prior to cell division in germline stem cells. In addition, we find that CENP-C is asymmetrically distributed between stem and daughter cells, with more CENP-C at stem cell centromeres. Finally, we show that CENP-C depletion in germline stem cells disrupts the balance of stem and daughter cells in the developing ovary, impacting on cell fate. Taken together, we propose that CENP-C level and function at centromeres plays an important role in determining cell fate upon asymmetric division occurring in stem cells.

DOT1L inhibitors block abnormal self-renewal induced by cohesin loss

Acute myeloid leukemia (AML) is a high-risk malignancy characterized by a diverse spectrum of somatic genetic alterations. The mechanisms by which these mutations contribute to leukemia development and how this informs the use of targeted therapies is critical to improving outcomes for patients. Importantly, how to target loss-of-function mutations has been a critical challenge in precision medicine. Heterozygous inactivating mutations in cohesin complex genes contribute to AML in adults by increasing the self-renewal capacity of hematopoietic stem and progenitor cells (HSPCs) by altering PRC2 targeting to induce HOXA9 expression, a key self-renewal transcription factor. Here we sought to delineate the epigenetic mechanism underpinning the enhanced self-renewal conferred by cohesin-haploinsufficiency. First, given the substantial difference in the mutational spectrum between pediatric and adult AML patients, we first sought to identify if HOXA9 was also elevated in children. Next, using primary HSPCs as a model we demonstrate that abnormal self-renewal due to cohesin loss is blocked by DOT1L inhibition. In cohesin-depleted cells, DOT1L inhibition is associated with H3K79me2 depletion and a concomitant increase in H3K27me3. Importantly, we find that there are cohesin-dependent gene expression changes that promote a leukemic profile, including HoxA overexpression, that are preferentially reversed by DOT1L inhibition. Our data further characterize how cohesin mutations contribute to AML development, identifying DOT1L as a potential therapeutic target for adult and pediatric AML patients harboring cohesin mutations.

Smc3 dosage regulates B cell transit through germinal centers and restricts their malignant transformation

During the germinal center (GC) reaction, B cells undergo extensive redistribution of cohesin complex and three-dimensional reorganization of their genomes. Yet, the significance of cohesin and architectural programming in the humoral immune response is unknown. Herein we report that homozygous deletion of Smc3, encoding the cohesin ATPase subunit, abrogated GC formation, while, in marked contrast, Smc3 haploinsufficiency resulted in GC hyperplasia, skewing of GC polarity and impaired plasma cell (PC) differentiation. Genome-wide chromosomal conformation and transcriptional profiling revealed defects in GC B cell terminal differentiation programs controlled by the lymphoma epigenetic tumor suppressors Tet2 and Kmt2d and failure of Smc3-haploinsufficient GC B cells to switch from B cell- to PC-defining transcription factors. Smc3 haploinsufficiency preferentially impaired the connectivity of enhancer elements controlling various lymphoma tumor suppressor genes, and, accordingly, Smc3 haploinsufficiency accelerated lymphomagenesis in mice with constitutive Bcl6 expression. Collectively, our data indicate a dose-dependent function for cohesin in humoral immunity to facilitate the B cell to PC phenotypic switch while restricting malignant transformation.

Combined Cohesin-RUNX1 Deficiency Synergistically Perturbs Chromatin Looping and Causes Myelodysplastic Syndromes

STAG2 encodes a cohesin component and is frequently mutated in myeloid neoplasms, showing highly significant comutation patterns with other drivers, including RUNX1. However, the molecular basis of cohesin-mutated leukemogenesis remains poorly understood. Here we show a critical role of an interplay between STAG2 and RUNX1 in the regulation of enhancer-promoter looping and transcription in hematopoiesis. Combined loss of STAG2 and RUNX1, which colocalize at enhancer-rich, CTCF-deficient sites, synergistically attenuates enhancer-promoter loops, particularly at sites enriched for RNA polymerase II and Mediator, and deregulates gene expression, leading to myeloid-skewed expansion of hematopoietic stem/progenitor cells (HSPC) and myelodysplastic syndromes (MDS) in mice. Attenuated enhancer-promoter loops in STAG2/RUNX1-deficient cells are associated with downregulation of genes with high basal transcriptional pausing, which are important for regulation of HSPCs. Downregulation of high-pausing genes is also confirmed in STAG2-cohesin-mutated primary leukemia samples. Our results highlight a unique STAG2-RUNX1 interplay in gene regulation and provide insights into cohesin-mutated leukemogenesis. SIGNIFICANCE: We demonstrate a critical role of an interplay between STAG2 and a master transcription factor of hematopoiesis, RUNX1, in MDS development, and further reveal their contribution to regulation of high-order chromatin structures, particularly enhancer-promoter looping, and the link between transcriptional pausing and selective gene dysregulation caused by cohesin deficiency.This article is highlighted in the In This Issue feature, p. 747.

BAF60a deficiency uncouples chromatin accessibility and cold sensitivity from white fat browning

Brown and beige fat share a remarkably similar transcriptional program that supports fuel oxidation and thermogenesis. The chromatin-remodeling machinery that governs genome accessibility and renders adipocytes poised for thermogenic activation remains elusive. Here we show that BAF60a, a subunit of the SWI/SNF chromatin-remodeling complexes, serves an indispensable role in cold-induced thermogenesis in brown fat. BAF60a maintains chromatin accessibility at PPARγ and EBF2 binding sites for key thermogenic genes. Surprisingly, fat-specific BAF60a inactivation triggers more pronounced cold-induced browning of inguinal white adipose tissue that is linked to induction of MC2R, a receptor for the pituitary hormone ACTH. Elevated MC2R expression sensitizes adipocytes and BAF60a-deficient adipose tissue to thermogenic activation in response to ACTH stimulation. These observations reveal an unexpected dichotomous role of BAF60a-mediated chromatin remodeling in transcriptional control of brown and beige gene programs and illustrate a pituitary-adipose signaling axis in the control of thermogenesis.

Sirtuin 7 promotes 45S pre-rRNA cleavage at site 2 and determines the processing pathway

In humans, ribosome biogenesis mainly occurs in nucleoli following two alternative pre-rRNA processing pathways differing in the order in which cleavages take place but not by the sites of cleavage. To uncover the role of the nucleolar NAD+-dependent deacetylase sirtuin 7 in the synthesis of ribosomal subunits, pre-rRNA processing was analyzed after sirtinol-mediated inhibition of sirtuin 7 activity or depletion of sirtuin 7 protein. We thus reveal that sirtuin 7 activity is a critical regulator of processing of 45S, 32S and 30S pre-rRNAs. Sirtuin 7 protein is primarily essential to 45S pre-rRNA cleavage at site 2, which is the first step of processing pathway 2. Furthermore, we demonstrate that sirtuin 7 physically interacts with Nop56 and the GAR domain of fibrillarin, and propose that this could interfere with fibrillarin-dependent cleavage. Sirtuin 7 depletion results in the accumulation of 5' extended forms of 32S pre-rRNA, and also influences the localization of fibrillarin. Thus, we establish a close relationship between sirtuin 7 and fibrillarin, which might determine the processing pathway used for ribosome biogenesis.

SWI/SNF Complex-Deficient Soft Tissue Neoplasms: A Pattern-Based Approach to Diagnosis and Differential Diagnosis

Loss of different components of the Switch/sucrose nonfermentable (SWI/SNF) chromatin remodeling complex has been increasingly recognized as a central molecular event driving the initiation and/or dedifferentiation of mostly lethal but histogenetically diverse neoplasms in different body organs. This review summarizes and discusses the morphologic and phenotypic diversity of primary soft tissue neoplasms characterized by SWI/SNF complex deficiency with an emphasis on convergent and divergent cytoarchitectural patterns.

Deletion of the DEF1 gene does not confer UV-immutability but frequently leads to self-diploidization in yeast Saccharomyces cerevisiae

In yeast Saccharomyces cerevisiae, the DEF1 gene is responsible for regulation of many cellular processes including ubiquitin-dependent degradation of DNA metabolism proteins. Recently it has been proposed that Def1 promotes degradation of the catalytic subunit of DNA polymerase δ at sites of DNA damage and regulates a switch to specialized polymerases and, as a consequence, DNA-damage induced mutagenesis. The idea was based substantially on the severe defects in induced mutagenesis observed in the def1 mutants. We describe that UV mutability of def1Δ strains is actually only moderately affected, while the virtual absence of UV mutagenesis in many def1Δ clones is caused by a novel phenotype of the def1 mutants, proneness to self-diploidization. Diploids are extremely frequent (90%) after transformation of wild-type haploids with def1::kanMX disruption cassette and are frequent (2.3%) in vegetative haploid def1 cultures. Such diploids look "UV immutable" when assayed for recessive forward mutations but have normal UV mutability when assayed for dominant reverse mutations. The propensity for frequent self-diploidization in def1Δ mutants should be taken into account in studies of the def1Δ effect on mutagenesis. The true haploids with def1Δ mutation are moderately UV sensitive but retain substantial UV mutagenesis for forward mutations: they are fully proficient at lower doses and only partially defective at higher doses of UV. We conclude that Def1 does not play a critical role in damage-induced mutagenesis.

The Caenorhabditis elegans Ortholog of TDP-43 Regulates the Chromatin Localization of the Heterochromatin Protein 1 Homolog HPL-2

TDP-1 is the Caenorhabditis elegans ortholog of mammalian TDP-43, which is strongly implicated in the etiology of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). We discovered that deletion of the tdp-1 gene results in enhanced nuclear RNA interference (RNAi). As nuclear RNAi in C. elegans involves chromatin changes moderated by HPL-2, a homolog of heterochromatin protein 1 (HP1), we investigated the interaction of TDP-1 and HPL-2. We found that TDP-1 and HPL-2 interact directly and that loss of TDP-1 dramatically alters the chromatin association of HPL-2. We showed previously that deletion of the tdp-1 gene results in transcriptional alterations and the accumulation of double-stranded RNA (dsRNA). These molecular changes are replicated in an hpl-2 deletion strain, consistent with HPL-2 acting in consort with TDP-1 to modulate these aspects of RNA metabolism. Our observations identify novel mechanisms by which HP1 homologs can be recruited to chromatin and by which nuclear depletion of human TDP-43 may lead to changes in RNA metabolism that are relevant to disease.

Cellular senescence regulated by SWI/SNF complex subunits through p53/p21 and p16/pRB pathway

SWI/SNF complex is an evolutionarily well-conserved chromatin-remodeling complex, which is implicated in the nucleosomes removing or sliding, impacting on the DNA repair, replication and genes expression regulation. The SWI/SNF complex consists up to 12 protein subunits. The catalytic subunits are BRG1 or BRM, which are exclusive ATPase subunits. BRG1 has been reported to play an important role in cellular senescence. However, The function of non-catalytic subunits involved in cellular senescence is rarely investigated. Therefore, we focused on the senescence regulation roles of SWI/SNF non-catalytic subunits in cellular senescent model induced by H₂O₂. H₂O₂ treatment was used to induce cellular senescence models in vitro. Screening the candidate subunits involved in this process by comparing the expression levels of SWI/SNF subunits with/without H₂O₂ treatment. Over-expression and knockdown the candidate subunits were utilized to investigate the functions and mechanism of the subunits involved in senescence regulation. The expressions of BAF57, BAF60a and SNF5 were changed significantly after H₂O₂ treatment. Overexpression of the three subunits separately induced cell growth arrest in both HaCaT and GLL19 cells, while knockdown of the subunits separately eased the senescence induced by H₂O₂ treatment. Results further showed that BAF57, BAF60a and SNF5 regulated cellular senescence via both p53/p21 and p16/pRB pathways, and the three subunits all had a directly interaction with p53. These results indicated that BAF57, BAF60a and SNF5 might act as novel pro-senescence factors in both normal and tumor human skin cells. Therefore, inhibiting expression of the three factors might delay the cellular senescence process.

Decreased cohesin in the brain leads to defective synapse development and anxiety-related behavior

Abnormal epigenetic regulation can cause the nervous system to develop abnormally. Here, we sought to understand the mechanism by which this occurs by investigating the protein complex cohesin, which is considered to regulate gene expression and, when defective, is associated with higher-level brain dysfunction and the developmental disorder Cornelia de Lange syndrome (CdLS). We generated conditional Smc3-knockout mice and observed greater dendritic complexity and larger numbers of immature synapses in the cerebral cortex of Smc3+/- mice. Smc3+/- mice also exhibited more anxiety-related behavior, which is a symptom of CdLS. Further, a gene ontology analysis after RNA-sequencing suggested the enrichment of immune processes, particularly the response to interferons, in the Smc3+/- mice. Indeed, fewer synapses formed in their cortical neurons, and this phenotype was rescued by STAT1 knockdown. Thus, low levels of cohesin expression in the developing brain lead to changes in gene expression that in turn lead to a specific and abnormal neuronal and behavioral phenotype.

Cohesin is positioned in mammalian genomes by transcription, CTCF and Wapl

Mammalian genomes are spatially organized by CCCTC-binding factor (CTCF) and cohesin into chromatin loops and topologically associated domains, which have important roles in gene regulation and recombination. By binding to specific sequences, CTCF defines contact points for cohesin-mediated long-range chromosomal cis-interactions. Cohesin is also present at these sites, but has been proposed to be loaded onto DNA elsewhere and to extrude chromatin loops until it encounters CTCF bound to DNA. How cohesin is recruited to CTCF sites, according to this or other models, is unknown. Here we show that the distribution of cohesin in the mouse genome depends on transcription, CTCF and the cohesin release factor Wings apart-like (Wapl). In CTCF-depleted fibroblasts, cohesin cannot be properly recruited to CTCF sites but instead accumulates at transcription start sites of active genes, where the cohesin-loading complex is located. In the absence of both CTCF and Wapl, cohesin accumulates in up to 70 kilobase-long regions at 3'-ends of active genes, in particular if these converge on each other. Changing gene expression modulates the position of these 'cohesin islands'. These findings indicate that transcription can relocate mammalian cohesin over long distances on DNA, as previously reported for yeast cohesin, that this translocation contributes to positioning cohesin at CTCF sites, and that active genes can be freed from cohesin either by transcription-mediated translocation or by Wapl-mediated release.

Zebrafish as a Model to Study Cohesin and Cohesinopathies

The cohesin protein complex regulates multiple cellular events including sister chromatid cohesion and gene expression. Several distinct human diseases called cohesinopathies have been associated with genetic mutations in cohesin subunit genes or genes encoding regulators of cohesin function. Studies in different model systems, from yeast to mouse have provided insights into the molecular mechanisms of action of cohesin/cohesin regulators and their implications in the pathogenesis of cohesinopathies. The zebrafish has unique advantages for embryonic analyses and quantitative gene knockdown with morpholinos during the first few days of development, in contrast to knockouts of cohesin regulators in flies or mammals, which are either lethal as homozygotes or dramatically compensated for in heterozygotes. This has been particularly informative for Rad21, where a role in gene expression was first shown in zebrafish, and Nipbl, where the fish work revealed tissue-specific functions in heart, gut, and limbs, and long-range enhancer-promoter interactions that control Hox gene expression in vivo. Here we discuss the utility of the zebrafish in studying the developmental and pathogenic roles of cohesin.

X-linked TEX11 mutations, meiotic arrest, and azoospermia in infertile men

BACKGROUND

The genetic basis of nonobstructive azoospermia is unknown in the majority of infertile men.

METHODS

We performed array comparative genomic hybridization testing in blood samples obtained from 15 patients with azoospermia, and we performed mutation screening by means of direct Sanger sequencing of the testis-expressed 11 gene (TEX11) open reading frame in blood and semen samples obtained from 289 patients with azoospermia and 384 controls.

RESULTS

We identified a 99-kb hemizygous loss on chromosome Xq13.2 that involved three TEX11 exons. This loss, which was identical in 2 patients with azoospermia, predicts a deletion of 79 amino acids within the meiosis-specific sporulation domain SPO22. Our subsequent mutation screening showed five novel TEX11 mutations: three splicing mutations and two missense mutations. These mutations, which occurred in 7 of 289 men with azoospermia (2.4%), were absent in 384 controls with normal sperm concentrations (P=0.003). Notably, five of those TEX11 mutations were detected in 33 patients (15%) with azoospermia who received a diagnosis of azoospermia with meiotic arrest. Meiotic arrest in these patients resembled the phenotype of Tex11-deficient male mice. Immunohistochemical analysis showed specific cytoplasmic TEX11 expression in late spermatocytes, as well as in round and elongated spermatids, in normal human testes. In contrast, testes of patients who had azoospermia with TEX11 mutations had meiotic arrest and lacked TEX11 expression.

CONCLUSIONS

In our study, hemizygous TEX11 mutations were a common cause of meiotic arrest and azoospermia in infertile men. (Funded by the National Institutes of Health and others.).

The contribution of cohesin-SA1 to gene expression and chromatin architecture in two murine tissues

Cohesin, which in somatic vertebrate cells consists of SMC1, SMC3, RAD21 and either SA1 or SA2, mediates higher-order chromatin organization. To determine how cohesin contributes to the establishment of tissue-specific transcriptional programs, we compared genome-wide cohesin distribution, gene expression and chromatin architecture in cerebral cortex and pancreas from adult mice. More than one third of cohesin binding sites differ between the two tissues and these show reduced overlap with CCCTC-binding factor (CTCF) and are enriched at the regulatory regions of tissue-specific genes. Cohesin/CTCF sites at active enhancers and promoters contain, at least, cohesin-SA1. Analyses of chromatin contacts at the Protocadherin (Pcdh) and Regenerating islet-derived (Reg) gene clusters, mostly expressed in brain and pancreas, respectively, revealed remarkable differences that correlate with the presence of cohesin. We could not detect significant changes in the chromatin contacts at the Pcdh locus when comparing brains from wild-type and SA1 null embryos. In contrast, reduced dosage of SA1 altered the architecture of the Reg locus and decreased the expression of Reg genes in the pancreas of SA1 heterozygous mice. Given the role of Reg proteins in inflammation, such reduction may contribute to the increased incidence of pancreatic cancer observed in these animals.

Meikin is a conserved regulator of meiosis-I-specific kinetochore function

The kinetochore is the crucial apparatus regulating chromosome segregation in mitosis and meiosis. Particularly in meiosis I, unlike in mitosis, sister kinetochores are captured by microtubules emanating from the same spindle pole (mono-orientation) and centromeric cohesion mediated by cohesin is protected in the following anaphase. Although meiotic kinetochore factors have been identified only in budding and fission yeasts, these molecules and their functions are thought to have diverged earlier. Therefore, a conserved mechanism for meiotic kinetochore regulation remains elusive. Here we have identified in mouse a meiosis-specific kinetochore factor that we termed MEIKIN, which functions in meiosis I but not in meiosis II or mitosis. MEIKIN plays a crucial role in both mono-orientation and centromeric cohesion protection, partly by stabilizing the localization of the cohesin protector shugoshin. These functions are mediated mainly by the activity of Polo-like kinase PLK1, which is enriched to kinetochores in a MEIKIN-dependent manner. Our integrative analysis indicates that the long-awaited key regulator of meiotic kinetochore function is Meikin, which is conserved from yeasts to humans.

Epigenetic characterization of the growth hormone gene identifies SmcHD1 as a regulator of autosomal gene clusters

Regulatory elements for the mouse growth hormone (GH) gene are located distally in a putative locus control region (LCR) in addition to key elements in the promoter proximal region. The role of promoter DNA methylation for GH gene regulation is not well understood. Pit-1 is a POU transcription factor required for normal pituitary development and obligatory for GH gene expression. In mammals, Pit-1 mutations eliminate GH production resulting in a dwarf phenotype. In this study, dwarf mice illustrated that Pit-1 function was obligatory for GH promoter hypomethylation. By monitoring promoter methylation levels during developmental GH expression we found that the GH promoter became hypomethylated coincident with gene expression. We identified a promoter differentially methylated region (DMR) that was used to characterize a methylation-dependent DNA binding activity. Upon DNA affinity purification using the DMR and nuclear extracts, we identified structural maintenance of chromosomes hinge domain containing -1 (SmcHD1). To better understand the role of SmcHD1 in genome-wide gene expression, we performed microarray analysis and compared changes in gene expression upon reduced levels of SmcHD1 in human cells. Knock-down of SmcHD1 in human embryonic kidney (HEK293) cells revealed a disproportionate number of up-regulated genes were located on the X-chromosome, but also suggested regulation of genes on non-sex chromosomes. Among those, we identified several genes located in the protocadherin β cluster. In addition, we found that imprinted genes in the H19/Igf2 cluster associated with Beckwith-Wiedemann and Silver-Russell syndromes (BWS & SRS) were dysregulated. For the first time using human cells, we showed that SmcHD1 is an important regulator of imprinted and clustered genes.

Coexistent loss of INI1 and BRG1 expression in a rhabdoid renal cell carcinoma (RCC): implications for a possible role of SWI/SNF complex in the pathogenesis of RCC

In this study, we analyzed the immunohistochemical and molecular profiles of an unusual RCC showed coexistent absence of INI1 and BRG1 expression, rhabdoid morphology, and poor prognosis. Histologically, the tumor had rhabdoid features, which were demonstrated by large round to polygonal cells with eccentric nuclei, prominent nucleoli, and eosinophilic cytoplasm varying from abundant to scanty. Immunohistochemically, the tumor were positive for BRM, PBRM1, ARID1A, CD10, CKpan, Vimentin, carbonic anhydrase IX (CA-IX), and P504S (AMACR) but negative for INI1, BRG1, HMB45, melan A, CK7, CD117, Ksp-cadherin, TFEB, TFE3, and Cathepsin K. We detected all three exons status of the VHL gene of the tumor and observed 1 somatic mutations in 1st exon. Chromosome 3p deletion, coupled with polysomy of chromosome 3 was also found. Based on these findings, it is further indicated that in some cases, rhabdoid RCC may arise from clear cell RCC. SWI/SNF chromatin remodeling complex may be an attractive candidate for being the "second hit" in RCCs and may play an important role during tumor progression. The role of SWI/SNF complex in rhabdoid RCC should be further studied on a larger number of cases.

The sister chromatid cohesion pathway suppresses multiple chromosome gain and chromosome amplification

Gain or loss of chromosomes resulting in aneuploidy can be important factors in cancer and adaptive evolution. Although chromosome gain is a frequent event in eukaryotes, there is limited information on its genetic control. Here we measured the rates of chromosome gain in wild-type yeast and sister chromatid cohesion (SCC) compromised strains. SCC tethers the newly replicated chromatids until anaphase via the cohesin complex. Chromosome gain was measured by selecting and characterizing copper-resistant colonies that emerged due to increased copies of the metallothionein gene CUP1. Although all defective SCC diploid strains exhibited increased rates of chromosome gain, there were 15-fold differences between them. Of all mutants examined, a hypomorphic mutation at the cohesin complex caused the highest rate of chromosome gain while disruption of WPL1, an important regulator of SCC and chromosome condensation, resulted in the smallest increase in chromosome gain. In addition to defects in SCC, yeast cell type contributed significantly to chromosome gain, with the greatest rates observed for homozygous mating-type diploids, followed by heterozygous mating type, and smallest in haploids. In fact, wpl1-deficient haploids did not show any difference in chromosome gain rates compared to wild-type haploids. Genomic analysis of copper-resistant colonies revealed that the "driver" chromosome for which selection was applied could be amplified to over five copies per diploid cell. In addition, an increase in the expected driver chromosome was often accompanied by a gain of a small number of other chromosomes. We suggest that while chromosome gain due to SCC malfunction can have negative effects through gene imbalance, it could also facilitate opportunities for adaptive changes. In multicellular organisms, both factors could lead to somatic diseases including cancer.

Inactivation of the budding yeast cohesin loader Scc2 alters gene expression both globally and in response to a single DNA double strand break

Genome integrity is fundamental for cell survival and cell cycle progression. Important mechanisms for keeping the genome intact are proper sister chromatid segregation, correct gene regulation and efficient repair of damaged DNA. Cohesin and its DNA loader, the Scc2/4 complex have been implicated in all these cellular actions. The gene regulation role has been described in several organisms. In yeast it has been suggested that the proteins in the cohesin network would effect transcription based on its role as insulator. More recently, data are emerging indicating direct roles for gene regulation also in yeast. Here we extend these studies by investigating whether the cohesin loader Scc2 is involved in regulation of gene expression. We performed global gene expression profiling in the absence and presence of DNA damage, in wild type and Scc2 deficient G2/M arrested cells, when it is known that Scc2 is important for DNA double strand break repair and formation of damage induced cohesion. We found that not only the DNA damage specific transcriptional response is distorted after inactivation of Scc2 but also the overall transcription profile. Interestingly, these alterations did not correlate with changes in cohesin binding.

Mechanisms of resistance to therapies targeting BRCA-mutant cancers

Synthetic lethality provides a potential mechanistic framework for the therapeutic targeting of genetic and functional deficiencies in cancers and is now being explored widely. The first clinical exemplification of synthetic lethality in cancer has been the exploitation of inhibitors of poly-(ADP-ribose) polymerase (PARP) for the treatment of cancers with defects in the BRCA1 or BRCA2 tumor suppressor proteins, which are involved in the repair of DNA damage. Although this approach has shown promise, multiple potential resistance mechanisms have been identified. In this Perspective, we discuss these mechanisms and their relevance to the development of selective therapies for BRCA-deficient cancers.

DEK depletion negatively regulates Rho/ROCK/MLC pathway in non-small cell lung cancer

The human DEK proto-oncogene is a nuclear protein with suspected roles in human carcinogenesis. DEK appears to function in several nuclear processes, including transcriptional regulation and modulation of chromatin structure. To investigate the clinicopathological significance of DEK in patients with non-small cell lung cancer (NSCLC), we analyzed DEK immunohistochemistry in 112 NSCLC cases. The results showed that DEK was overexpressed mainly in the nuclear compartment of tumor cells. In squamous cell carcinoma, DEK-positive expression occurred in 47.9% (23/48) of cases, and in lung adenocarcinoma, DEK-positive expression occurred in 67.2% (43/64) of cases and correlated with differentiation, p-TNM stage, and nodal status. Moreover, in lung adenocarcinoma, DEK expression was significantly higher compared with DEK expression in squamous cell carcinoma. Kaplan-Meier analysis showed that patients with low DEK expression had higher overall survival compared with patients with high DEK expression. Depleting DEK expression inhibited cellular proliferation and migration. Furthermore, in DEK-depleted NSCLC cells, we found that RhoA expression was markedly reduced; in conjunction, active RhoA-GTP levels and the downstream effector phosphorylated MLC2 were also reduced. Taken together, DEK depletion inhibited cellular migration in lung cancer cell lines possibly through inactivation of the RhoA/ROCK/MLC signal transduction pathway.

Mcph1-deficient mice reveal a role for MCPH1 in otitis media

Otitis media is a common reason for hearing loss, especially in children. Otitis media is a multifactorial disease and environmental factors, anatomic dysmorphology and genetic predisposition can all contribute to its pathogenesis. However, the reasons for the variable susceptibility to otitis media are elusive. MCPH1 mutations cause primary microcephaly in humans. So far, no hearing impairment has been reported either in the MCPH1 patients or mouse models with Mcph1 deficiency. In this study, Mcph1-deficient (Mcph1^tm1a/tm1a) mice were produced using embryonic stem cells with a targeted mutation by the Sanger Institute's Mouse Genetics Project. Auditory brainstem response measurements revealed that Mcph1^tm1a/tm1a mice had mild to moderate hearing impairment with around 70% penetrance. We found otitis media with effusion in the hearing-impaired Mcph1^tm1a/tm1a mice by anatomic and histological examinations. Expression of Mcph1 in the epithelial cells of middle ear cavities supported its involvement in the development of otitis media. Other defects of Mcph1^tm1a/tm1a mice included small skull sizes, increased micronuclei in red blood cells, increased B cells and ocular abnormalities. These findings not only recapitulated the defects found in other Mcph1-deficient mice or MCPH1 patients, but also revealed an unexpected phenotype, otitis media with hearing impairment, which suggests Mcph1 is a new gene underlying genetic predisposition to otitis media.

The SUMO protease SENP1 is required for cohesion maintenance and mitotic arrest following spindle poison treatment

SUMO conjugation is a reversible posttranslational modification that regulates protein function. SENP1 is one of the six SUMO-specific proteases present in vertebrate cells and its altered expression is observed in several carcinomas. To characterize SENP1 role in genome integrity, we generated Senp1 knockout chicken DT40 cells. SENP1(-/-) cells show normal proliferation, but are sensitive to spindle poisons. This hypersensitivity correlates with increased sister chromatid separation, mitotic slippage, and apoptosis. To test whether the cohesion defect had a causal relationship with the observed mitotic events, we restored the cohesive status of sister chromatids by introducing the TOP2α(+/-) mutation, which leads to increased catenation, or by inhibiting Plk1 and Aurora B kinases that promote cohesin release from chromosomes during prolonged mitotic arrest. Although TOP2α is SUMOylated during mitosis, the TOP2α(+/-) mutation had no obvious effect. By contrast, inhibition of Plk1 or Aurora B rescued the hypersensitivity of SENP1(-/-) cells to colcemid. In conclusion, we identify SENP1 as a novel factor required for mitotic arrest and cohesion maintenance during prolonged mitotic arrest induced by spindle poisons.

Transcriptional repression of Hox genes by C. elegans HP1/HPL and H1/HIS-24

Elucidation of the biological role of linker histone (H1) and heterochromatin protein 1 (HP1) in mammals has been difficult owing to the existence of a least 11 distinct H1 and three HP1 subtypes in mice. Caenorhabditis elegans possesses two HP1 homologues (HPL-1 and HPL-2) and eight H1 variants. Remarkably, one of eight H1 variants, HIS-24, is important for C. elegans development. Therefore we decided to analyse in parallel the transcriptional profiles of HIS-24, HPL-1/-2 deficient animals, and their phenotype, since hpl-1, hpl-2, and his-24 deficient nematodes are viable. Global transcriptional analysis of the double and triple mutants revealed that HPL proteins and HIS-24 play gene-specific roles, rather than a general repressive function. We showed that HIS-24 acts synergistically with HPL to allow normal reproduction, somatic gonad development, and vulval cell fate decision. Furthermore, the hpl-2; his-24 double mutant animals displayed abnormal development of the male tail and ectopic expression of C. elegans HOM-C/Hox genes (egl-5 and mab-5), which are involved in the developmental patterning of male mating structures. We found that HPL-2 and the methylated form of HIS-24 specifically interact with the histone H3 K27 region in the trimethylated state, and HIS-24 associates with the egl-5 and mab-5 genes. Our results establish the interplay between HPL-1/-2 and HIS-24 proteins in the regulation of positional identity in C. elegans males.

Linker histone (H1) and heterochromatin protein 1 (HP1) play central roles in the formation of higher-order chromatin structure and gene expression. Recent studies have shown a physical interaction between H1 and HP1; however, the biological role of histone H1 and HP1 is not well understood. Additionally, the function of HP1 and H1 isoform interactions in any organism has not been addressed, mostly due to the lack of knockout alleles. Here, we investigate the role of HP1 and H1 in development using the nematode C. elegans as a model system. We focus on the underlying molecular mechanisms of gene co-regulation by H1 and HP1. We show that the loss of both HP1 and H1 alters the expression of a small subset of genes. C. elegans HP1 and H1 have an overlapping function in the same or parallel pathways where they regulate a shared target, the Hox genes.

Cohesin-SA1 deficiency drives aneuploidy and tumourigenesis in mice due to impaired replication of telomeres

Cohesin is a protein complex originally identified for its role in sister chromatid cohesion, although increasing evidence portrays it also as a major organizer of interphase chromatin. Vertebrate cohesin consists of Smc1, Smc3, Rad21/Scc1 and either stromal antigen 1 (SA1) or SA2. To explore the functional specificity of these two versions of cohesin and their relevance for embryonic development and cancer, we generated a mouse model deficient for SA1. Complete ablation of SA1 results in embryonic lethality, while heterozygous animals have shorter lifespan and earlier onset of tumourigenesis. SA1-null mouse embryonic fibroblasts show decreased proliferation and increased aneuploidy as a result of chromosome segregation defects. These defects are not caused by impaired centromeric cohesion, which depends on cohesin-SA2. Instead, they arise from defective telomere replication, which requires cohesion mediated specifically by cohesin-SA1. We propose a novel mechanism for aneuploidy generation that involves impaired telomere replication upon loss of cohesin-SA1, with clear implications in tumourigenesis.

The many functions of cohesin--different rings to rule them all?

EMBO J31 9 , 2076 –2089 03 13 2012

EMBO J31 9 , 2090 –2102 03 13 2012

It is well known that somatic and germ cells use different cohesin complexes to mediate sister chromatid cohesion, but why different isoforms of cohesin also co-exist within somatic vertebrate cells has remained a mystery. Two papers in this issue of The EMBO Journal have begun to address this question by analysing mouse cells lacking SA1, an isoform of a specific cohesin subunit.

A unique role of cohesin-SA1 in gene regulation and development

Vertebrates have two cohesin complexes that consist of Smc1, Smc3, Rad21/Scc1 and either SA1 or SA2, but their functional specificity is unclear. Mouse embryos lacking SA1 show developmental delay and die before birth. Comparison of the genome-wide distribution of cohesin in wild-type and SA1-null cells reveals that SA1 is largely responsible for cohesin accumulation at promoters and at sites bound by the insulator protein CTCF. As a consequence, ablation of SA1 alters transcription of genes involved in biological processes related to Cornelia de Lange syndrome (CdLS), a genetic disorder linked to dysfunction of cohesin. We show that the presence of cohesin-SA1 at the promoter of myc and of protocadherin genes positively regulates their expression, a task that cannot be assumed by cohesin-SA2. Lack of SA1 also alters cohesin-binding pattern along some gene clusters and leads to dysregulation of genes within. We hypothesize that impaired cohesin-SA1 function in gene expression underlies the molecular aetiology of CdLS.

A role for cohesin in T-cell-receptor rearrangement and thymocyte differentiation

Cohesin enables post-replicative DNA repair and chromosome segregation by holding sister chromatids together from the time of DNA replication in S phase until mitosis. There is growing evidence that cohesin also forms long-range chromosomal cis-interactions and may regulate gene expression in association with CTCF, mediator or tissue-specific transcription factors. Human cohesinopathies such as Cornelia de Lange syndrome are thought to result from impaired non-canonical cohesin functions, but a clear distinction between the cell-division-related and cell-division-independent functions of cohesion--as exemplified in Drosophila--has not been demonstrated in vertebrate systems. To address this, here we deleted the cohesin locus Rad21 in mouse thymocytes at a time in development when these cells stop cycling and rearrange their T-cell receptor (TCR) α locus (Tcra). Rad21-deficient thymocytes had a normal lifespan and retained the ability to differentiate, albeit with reduced efficiency. Loss of Rad21 led to defective chromatin architecture at the Tcra locus, where cohesion-binding sites flank the TEA promoter and the Eα enhancer, and demarcate Tcra from interspersed Tcrd elements and neighbouring housekeeping genes. Cohesin was required for long-range promoter-enhancer interactions, Tcra transcription, H3K4me3 histone modifications that recruit the recombination machinery and Tcra rearrangement. Provision of pre-rearranged TCR transgenes largely rescued thymocyte differentiation, demonstrating that among thousands of potential target genes across the genome, defective Tcra rearrangement was limiting for the differentiation of cohesin-deficient thymocytes. These findings firmly establish a cell-division-independent role for cohesin in Tcra locus rearrangement and provide a comprehensive account of the mechanisms by which cohesin enables cellular differentiation in a well-characterized mammalian system.

The chromosome passenger complex is required for fidelity of chromosome transmission and cytokinesis in meiosis of mouse oocytes

The existence of two forms of the chromosome passenger complex (CPC) in the mammalian oocyte has meant that its role in female meiosis has remained unclear. Here we use loss- and gain-of function approaches to assess the meiotic functions of one of the shared components of these complexes, INCENP, and of the variable kinase subunits, Aurora B or Aurora C. We show that either the depletion of INCENP or the combined inhibition of Aurora kinases B and C activates the anaphase-promoting complex or cyclosome (APC/C) before chromosomes have properly congressed in meiosis I and also prevents cytokinesis and hence extrusion of the first polar body. Overexpression of Aurora C also advances APC/C activation and results in cytokinesis failure in a high proportion of oocytes, indicative of a dominant effect on CPC function. Together, this points to roles for the meiotic CPC in functions similar to the mitotic roles of the complex: correcting chromosome attachment to microtubules, facilitating the spindle-assembly checkpoint (SAC) function and enabling cytokinesis. Surprisingly, overexpression of Aurora B leads to a failure of APC/C activation, stabilization of securin and consequently a failure of chiasmate chromosomes to resolve - a dominant phenotype that is completely suppressed by depletion of INCENP. Taken together with the differential distribution of Aurora proteins B and C on chiasmate chromosomes, this points to differential functions of the two forms of CPC in regulating the separation of homologous chromosomes in meiosis I.

Epigenetic antagonism between polycomb and SWI/SNF complexes during oncogenic transformation

Epigenetic alterations have been increasingly implicated in oncogenesis. Analysis of Drosophila mutants suggests that Polycomb and SWI/SNF complexes can serve antagonistic developmental roles. However, the relevance of this relationship to human disease is unclear. Here, we have investigated functional relationships between these epigenetic regulators in oncogenic transformation. Mechanistically, we show that loss of the SNF5 tumor suppressor leads to elevated expression of the Polycomb gene EZH2 and that Polycomb targets are broadly H3K27-trimethylated and repressed in SNF5-deficient fibroblasts and cancers. Further, we show antagonism between SNF5 and EZH2 in the regulation of stem cell-associated programs and that Snf5 loss activates those programs. Finally, using conditional mouse models, we show that inactivation of Ezh2 blocks tumor formation driven by Snf5 loss.

Rad21-cohesin haploinsufficiency impedes DNA repair and enhances gastrointestinal radiosensitivity in mice

Approximately half of cancer-affected patients receive radiotherapy (RT). The doses delivered have been determined upon empirical experience based upon average radiation responses. Ideally higher curative radiation doses might be employed in patients with genuinely normal radiation responses and importantly radiation hypersensitive patients would be spared the consequences of excessive tissue damage if they were identified before treatment. Rad21 is an integral subunit of the cohesin complex, which regulates chromosome segregation and DNA damage responses in eukaryotes. We show here, by targeted inactivation of this key cohesin component in mice, that Rad21 is a DNA-damage response gene that markedly affects animal and cell survival. Biallelic deletion of Rad21 results in early embryonic death. Rad21 heterozygous mutant cells are defective in homologous recombination (HR)-mediated gene targeting and sister chromatid exchanges. Rad21+/- animals exhibited sensitivity considerably greater than control littermates when challenged with whole body irradiation (WBI). Importantly, Rad21+/- animals are significantly more sensitive to WBI than Atm heterozygous mutant mice. Since supralethal WBI of mammals most typically leads to death via damage to the gastrointestinal tract (GIT) or the haematopoietic system, we determined the functional status of these organs in the irradiated animals. We found evidence for GIT hypersensitivity of the Rad21 mutants and impaired bone marrow stem cell clonogenic regeneration. These data indicate that Rad21 gene dosage is critical for the ionising radiation (IR) response. Rad21 mutant mice thus represent a new mammalian model for understanding the molecular basis of irradiation effects on normal tissues and have important implications in the understanding of acute radiation toxicity in normal tissues.

Establishment of a mouse model with misregulated chromosome condensation due to defective Mcph1 function

Mutations in the human gene MCPH1 cause primary microcephaly associated with a unique cellular phenotype with premature chromosome condensation (PCC) in early G2 phase and delayed decondensation post-mitosis (PCC syndrome). The gene encodes the BRCT-domain containing protein microcephalin/BRIT1. Apart from its role in the regulation of chromosome condensation, the protein is involved in the cellular response to DNA damage. We report here on the first mouse model of impaired Mcph1-function. The model was established based on an embryonic stem cell line from BayGenomics (RR0608) containing a gene trap in intron 12 of the Mcph1 gene deleting the C-terminal BRCT-domain of the protein. Although residual wild type allele can be detected by quantitative real-time PCR cell cultures generated from mouse tissues bearing the homozygous gene trap mutation display the cellular phenotype of misregulated chromosome condensation that is characteristic for the human disorder, confirming defective Mcph1 function due to the gene trap mutation. While surprisingly the DNA damage response (formation of repair foci, chromosomal breakage, and G2/M checkpoint function after irradiation) appears to be largely normal in cell cultures derived from Mcph1^gt/gt mice, the overall survival rates of the Mcph1^gt/gt animals are significantly reduced compared to wild type and heterozygous mice. However, we could not detect clear signs of premature malignant disease development due to the perturbed Mcph1 function. Moreover, the animals show no obvious physical phenotype and no reduced fertility. Body and brain size are within the range of wild type controls. Gene expression on RNA and protein level did not reveal any specific pattern of differentially regulated genes. To the best of our knowledge this represents the first mammalian transgenic model displaying a defect in mitotic chromosome condensation and is also the first mouse model for impaired Mcph1-function.

Cohesin acetylation speeds the replication fork

Cohesin not only links sister chromatids but also inhibits the transcriptional machinery's interaction with and movement along chromatin. In contrast, replication forks must traverse such cohesin-associated obstructions to duplicate the entire genome in S phase. How this occurs is unknown. Through single-molecule analysis, we demonstrate that the replication factor C (RFC)-CTF18 clamp loader (RFC(CTF18)) controls the velocity, spacing and restart activity of replication forks in human cells and is required for robust acetylation of cohesin's SMC3 subunit and sister chromatid cohesion. Unexpectedly, we discovered that cohesin acetylation itself is a central determinant of fork processivity, as slow-moving replication forks were found in cells lacking the Eco1-related acetyltransferases ESCO1 or ESCO2 (refs 8-10) (including those derived from Roberts' syndrome patients, in whom ESCO2 is biallelically mutated) and in cells expressing a form of SMC3 that cannot be acetylated. This defect was a consequence of cohesin's hyperstable interaction with two regulatory cofactors, WAPL and PDS5A (refs 12, 13); removal of either cofactor allowed forks to progress rapidly without ESCO1, ESCO2, or RFC(CTF18). Our results show a novel mechanism for clamp-loader-dependent fork progression, mediated by the post-translational modification and structural remodelling of the cohesin ring. Loss of this regulatory mechanism leads to the spontaneous accrual of DNA damage and may contribute to the abnormalities of the Roberts' syndrome cohesinopathy.

Cohesin SMC1beta protects telomeres in meiocytes

Meiosis-specific mammalian cohesin SMC1beta is required for complete sister chromatid cohesion and proper axes/loop structure of axial elements (AEs) and synaptonemal complexes (SCs). During prophase I, telomeres attach to the nuclear envelope (NE), but in Smc1beta(-/-) meiocytes, one fifth of their telomeres fail to attach. This study reveals that SMC1beta serves a specific role at telomeres, which is independent of its role in determining AE/SC length and loop extension. SMC1beta is necessary to prevent telomere shortening, and SMC3, present in all known cohesin complexes, properly localizes to telomeres only if SMC1beta is present. Very prominently, telomeres in Smc1beta(-/-) spermatocytes and oocytes loose their structural integrity and suffer a range of abnormalities. These include disconnection from SCs and formation of large telomeric protein-DNA extensions, extended telomere bridges between SCs, ring-like chromosomes, intrachromosomal telomeric repeats, and a reduction of SUN1 foci in the NE. We suggest that a telomere structure protected from DNA rearrangements depends on SMC1beta.

Functional analysis of ScSwi1 and CaSwi1 in invasive and pseudohyphal growth of Saccharomyces cerevisiae

Here we reported that, in Saccharomyces cerevisiae, deleting Swi1 (ScSwi1), a core component in Swi/Snf complex, caused defects of invasive growth, pseudohyphal growth, FLO11 expression, and proper cell separation. Re-introduction of SWI1 into the swi1 mutants could suppress all defects observed. We also showed that overproducing Swi1 could suppress the defect of flo8 cells in pseudohyphal growth in diploids, but not invasive growth in haploids. Overexpression of SWI1 could not bypass the requirement of Ste12 or Tec1 in invasive growth or pseudohyphal growth. We concluded that the Swi/Snf complex was required for FLO11 expression and proper cell separation, and both the FLO8 and STE12 genes should be present for the complex to function for the invasive growth but only the STE12 gene was required for the pseudohyphal growth. Ectopic expression of Candida albicans SWI1 (CaSWI1) could partially complement the defects examined of haploid Scswi1 mutants, but failed to complement the defects examined of diploid Scswi1/ Scswi1 mutants. Overexpressing CaSwi1 mitigated invasive and pseudohyphal growth defects resulting from deletions in the MAP kinase and cAMP pathways. The integrity of S. cerevisiae Swi/Snf complex is required for invasive and filamentous growth promoted by overexpressing CaSwi1.

Heterochromatin and the cohesion of sister chromatids

Heterochromatin, once thought to be the useless junk of chromosomes, is now known to play significant roles in biology. Underlying much of this newfound fame are links between the repressive chromatin structure and cohesin, the protein complex that mediates sister chromatid cohesion. Heterochromatin-mediated recruitment and retention of cohesin to domains flanking centromeres promotes proper attachment of chromosomes to the mitotic and meiotic spindles. Heterochromatin assembled periodically between convergently transcribed genes also recruits cohesin, which promotes a novel form of transcription termination. Heterochromatin-like structures in budding yeast also recruit cohesin. Here the complex appears to regulate transcriptional silencing and recombination between repeated DNA sequences. The link between heterochromatin and cohesin is particularly relevant to human health. In Roberts-SC phocomelia syndrome, heterochromatic cohesion is selectively lost due to mutation of the acetyltransferase responsible for cohesin activation. In this review I discuss recent work that relates to these relationships between heterochromatin and cohesin.

Functional interaction of the retinoblastoma and Ini1/Snf5 tumor suppressors in cell growth and pituitary tumorigenesis

The Ini1 subunit of the SWI/SNF chromatin remodeling complex suppresses formation of malignant rhabdoid tumors in humans and mice. Transduction of Ini1 into Ini1-deficient tumor-derived cell lines has indicated that Ini1 arrests cell growth, controls chromosomal ploidy, and suppresses tumorigenesis by regulating components of the retinoblastoma (Rb) signaling pathway. Furthermore, conditional inactivation of Ini1 in mouse fibroblasts alters the expression of various Rb-E2F-regulated genes, indicating that endogenous Ini1 levels may control Rb signaling in cells. We have reported previously that loss of one allele of Ini1 in mouse fibroblasts results only in a 15% to 20% reduction in total Ini1 mRNA levels due to transcriptional compensation by the remaining Ini1 allele. Here, we examine the effects of Ini1 haploinsufficiency on cell growth and immortalization in mouse embryonic fibroblasts. In addition, we examine pituitary tumorigenesis in Rb-Ini1 compound heterozygous mice. Our results reveal that heterozygosity for Ini1 up-regulates cell growth and immortalization and that exogenous Ini1 down-regulates the growth of primary cells in a Rb-dependent manner. Furthermore, loss of Ini1 is redundant with loss of Rb function in the formation of pituitary tumors in Rb heterozygous mice and leads to the formation of large, atypical Rb(+/-) tumor cells lacking adrenocorticotropic hormone expression. These results confirm in vivo the relationship between Rb and Ini1 in tumor suppression and indicate that Ini1 plays a role in maintaining the morphologic and functional differentiation of corticotrophic cells.

Mrg15 null and heterozygous mouse embryonic fibroblasts exhibit DNA-repair defects post exposure to gamma ionizing radiation

MORF4-related gene on chromosome 15 (MRG15) is a core component of the NuA4/Tip60 histone acetyltransferase complex that modifies chromatin structure. We here demonstrate that Mrg15 null and heterozygous mouse embryonic fibroblasts exhibit an impaired DNA-damage response post gamma irradiation, when compared to wild-type cells. Defects in DNA-repair and cell growth, and delayed recruitment of repair proteins to sites of damage were observed. Formation of phosphorylated H2AX and 53BP1 foci was delayed in Mrg15 mutant versus wild-type cells following irradiation. These data implicate a novel role for MRG15 in DNA-damage repair in mammalian cells.

Rab6 regulates both ZW10/RINT-1 and conserved oligomeric Golgi complex-dependent Golgi trafficking and homeostasis

We used multiple approaches to investigate the role of Rab6 relative to Zeste White 10 (ZW10), a mitotic checkpoint protein implicated in Golgi/endoplasmic reticulum (ER) trafficking/transport, and conserved oligomeric Golgi (COG) complex, a putative tether in retrograde, intra-Golgi trafficking. ZW10 depletion resulted in a central, disconnected cluster of Golgi elements and inhibition of ERGIC53 and Golgi enzyme recycling to ER. Small interfering RNA (siRNA) against RINT-1, a protein linker between ZW10 and the ER soluble N-ethylmaleimide-sensitive factor attachment protein receptor, syntaxin 18, produced similar Golgi disruption. COG3 depletion fragmented the Golgi and produced vesicles; vesicle formation was unaffected by codepletion of ZW10 along with COG, suggesting ZW10 and COG act separately. Rab6 depletion did not significantly affect Golgi ribbon organization. Epistatic depletion of Rab6 inhibited the Golgi-disruptive effects of ZW10/RINT-1 siRNA or COG inactivation by siRNA or antibodies. Dominant-negative expression of guanosine diphosphate-Rab6 suppressed ZW10 knockdown induced-Golgi disruption. No cross-talk was observed between Rab6 and endosomal Rab5, and Rab6 depletion failed to suppress p115 (anterograde tether) knockdown-induced Golgi disruption. Dominant-negative expression of a C-terminal fragment of Bicaudal D, a linker between Rab6 and dynactin/dynein, suppressed ZW10, but not COG, knockdown-induced Golgi disruption. We conclude that Rab6 regulates distinct Golgi trafficking pathways involving two separate protein complexes: ZW10/RINT-1 and COG.

Expansion of chromosome territories with chromatin decompaction in BAF53-depleted interphase cells

Chromosomes are compartmentalized into discrete chromosome territories during interphase in mammalian cells. A chromosome territory is generated by the tendency of chromatin to occupy the smallest shell volume, which is determined by the polymeric properties and interactions of the internal meshwork of the chromatin fiber. Here, we show that BAF53 knockdown by small interfering RNA interference led to the expansion of chromosome territories. This was accompanied by a reduction in chromatin compaction, an increase in the micrococcal nuclease sensitivity of the chromatin, and an alteration in H3-K9 and H3-K79 dimethylation. Interestingly, the BAF53 knockdown cells suffer a cell cycle defect. Despite the significant irregularity and decompaction of the polynucleosomes isolated from the BAF53 knockdown cells, the chromatin loading of H1 and core histones remained unaltered, as did the nucleosome spacing. The histone hyperacetylation and down-regulation of BRG-1, mBrm, and Tip49, the catalytic components of the SWI/SNF complex and the TIP60 complex, respectively, did not expand chromosome territories. These results indicate that BAF53 contributes to the polymeric properties and/or the internal meshwork interactions of the chromatin fiber probably via a novel mechanism.

MPS3 mediates meiotic bouquet formation in Saccharomyces cerevisiae

In meiotic prophase, telomeres associate with the nuclear envelope and accumulate adjacent to the centrosome/spindle pole to form the chromosome bouquet, a well conserved event that in Saccharomyces cerevisiae requires the meiotic telomere protein Ndj1p. Ndj1p interacts with Mps3p, a nuclear envelope SUN domain protein that is required for spindle pole body duplication and for sister chromatid cohesion. Removal of the Ndj1p-interaction domain from MPS3 creates an ndj1 Delta-like separation-of-function allele, and Ndj1p and Mps3p are codependent for stable association with the telomeres. SUN domain proteins are found in the nuclear envelope across phyla and are implicated in mediating interactions between the interior of the nucleus and the cytoskeleton. Our observations indicate a general mechanism for meiotic telomere movements.

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.

Baf60c is a nuclear Notch signaling component required for the establishment of left-right asymmetry

Notch-mediated induction of Nodal at the vertebrate node is a critical step in initiating left-right (LR) asymmetry. In mice and zebrafish we show that Baf60c, a subunit of the Swi/Snf-like BAF chromatin remodeling complex, is essential for establishment of LR asymmetry. Baf60c knockdown mouse embryos fail to activate Nodal at the node and also have abnormal node morphology with mixing of crown and pit cells. In cell culture, Baf60c is required for Notch-dependent transcriptional activation and functions to stabilize interactions between activated Notch and its DNA-binding partner, RBP-J. Brg1 is also required for these processes, suggesting that BAF complexes are key components of nuclear Notch signaling. We propose a critical role for Baf60c in Notch-dependent transcription and LR asymmetry.

The proto-oncogene SET interacts with muscarinic receptors and attenuates receptor signaling

G protein-coupled receptors mediate cell responses to extracellular stimuli and likely function in the context of a larger signal transduction complex. Utilizing the third intracellular loop of a G protein-coupled receptor in glutathione S-transferase pulldown assays from rat brain lysates coupled with high sensitivity detection methods and subsequent functional studies, we report the identification of SET as a regulator of muscarinic receptor signaling. SET is a putative oncogene reported to inhibit protein phosphatase 2A and regulate gene transcription. SET binds the carboxyl region of the M3-muscarinic receptor i3 loop, and endogenous SET co-immunoprecipitates with intact M3 muscarinic receptor expressed in cells. Small interfering RNA knockdown of endogenous SET in Chinese hamster ovary cells stably expressing the M3 muscarinic receptor augmented receptor-mediated mobilization of intracellular calcium by approximately 35% with no change in agonist EC(50), indicating that interaction of SET with the M3 muscarinic receptor reduces its signaling capacity. SET knockdown had no effect on the mobilization of intracellular calcium by the P2-purinergic receptor, ionomycin, or a direct activator of phospholipase C, indicating a specific regulation of M3 muscarinic receptor signaling. These data provide expanded functionality for SET and a previously unrecognized mechanism for regulation of GPCR signaling capacity.

Essential role of chromatin assembly factor-1-mediated rapid nucleosome assembly for DNA replication and cell division in vertebrate cells

Chromatin assembly factor-1 (CAF-1), a complex consisting of p150, p60, and p48 subunits, is highly conserved from yeast to humans and facilitates nucleosome assembly of newly replicated DNA in vitro. To investigate roles of CAF-1 in vertebrates, we generated two conditional DT40 mutants, respectively, devoid of CAF-1p150 and p60. Depletion of each of these CAF-1 subunits led to delayed S-phase progression concomitant with slow DNA synthesis, followed by accumulation in late S/G2 phase and aberrant mitosis associated with extra centrosomes, and then the final consequence was cell death. We demonstrated that CAF-1 is necessary for rapid nucleosome formation during DNA replication in vivo as well as in vitro. Loss of CAF-1 was not associated with the apparent induction of phosphorylations of S-checkpoint kinases Chk1 and Chk2. To elucidate the precise role of domain(s) in CAF-1p150, functional dissection analyses including rescue assays were preformed. Results showed that the binding abilities of CAF-1p150 with CAF-1p60 and DNA polymerase sliding clamp proliferating cell nuclear antigen (PCNA) but not with heterochromatin protein HP1-gamma are required for cell viability. These observations highlighted the essential role of CAF-1-dependent nucleosome assembly in DNA replication and cell proliferation through its interaction with PCNA.

Molecular analysis of mitotic chromosome condensation using a quantitative time-resolved fluorescence microscopy assay

Chromosomes condense during mitotic entry to facilitate their segregation. Condensation is typically assayed in fixed preparations, limiting analysis of contributing factors. Here, we describe a quantitative method to monitor condensation kinetics in living cells expressing GFP fused to a core histone. We demonstrate the utility of this method by using it to analyze the molecular requirements for the condensation of holocentric chromosomes during the first division of the Caenorhabditis elegans embryo. In control embryos, the fluorescence intensity distribution for nuclear GFP:histone changes during two distinct time intervals separated by a plateau phase. During the first interval, primary condensation converts diffuse chromatin into discrete linear chromosomes. After the plateau, secondary condensation compacts the curvilinear chromosomes to form shorter bar-shaped structures. We quantitatively compared the consequences on this characteristic profile of depleting the condensin complex, the mitosis-specific histone H3 kinase Aurora B, the centromeric histone CENP-A, and CENP-C, a conserved protein required for kinetochore assembly. Both condensin and CENP-A play critical but distinct roles in primary condensation. In contrast, depletion of CENP-C slows but does not prevent primary condensation. Finally, Aurora B inhibition has no effect on primary condensation, but slightly delays secondary condensation. These results provide insights into the process of condensation, help resolve apparent contradictions from prior studies, and indicate that CENP-A chromatin has an intrinsic role in the condensation of holocentric chromosomes that is independent of its requirement for kinetochore assembly.