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The cell cycle regulator Aurora-A kinase presents an attractive target for cancer therapies, though its inhibition is also associated with toxic side effects. To gain a more nuanced understanding of Aurora-A function, we applied shotgun proteomics to identify 407 specific protein partners, including several splicing factors. Supporting a role in alternative splicing, we found that Aurora-A localizes to nuclear speckles, the storehouse of splicing proteins. Aurora-A interacts with and phosphorylates splicing factors both in vitro and in vivo, suggesting that it regulates alternative splicing by modulating the activity of these splicing factors. Consistently, Aurora-A inhibition significantly impacts the alternative splicing of 505 genes, with RNA motif analysis revealing an enrichment for Aurora-A interacting splicing factors. Additionally, we observed a significant positive correlation between the splicing events regulated by Aurora-A and those modulated by its interacting splicing factors. An interesting example is represented by CLK1 exon 4, which appears to be regulated by Aurora-A through SRSF3. Collectively, our findings highlight a broad role of Aurora-A in the regulation of alternative splicing., Competing Interests: CONFLICT OF INTEREST STATEMENT The authors declare no conflict of interest., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Cohesin is key to eukaryotic genome organization and acts throughout the cell cycle in an ATP-dependent manner. The mechanisms underlying cohesin ATPase activity are poorly understood. Here, we characterize distinct steps of the human cohesin ATPase cycle and show that the SMC1A and SMC3 ATPase domains undergo specific but concerted structural rearrangements along this cycle. Specifically, whereas the proximal coiled coil of the SMC1A ATPase domain remains conformationally stable, that of the SMC3 displays an intrinsic flexibility. The ATP-dependent formation of the heterodimeric SMC1A/SMC3 ATPase module (engaged state) favors this flexibility, which is counteracted by NIPBL and DNA binding (clamped state). Opening of the SMC3/RAD21 interface (open-engaged state) stiffens the SMC3 proximal coiled coil, thus constricting together with that of SMC1A the ATPase module DNA-binding chamber. The plasticity of the ATP-dependent interface between the SMC1A and SMC3 ATPase domains enables these structural rearrangements while keeping the ATP gate shut. VIDEO ABSTRACT., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Sister chromatid cohesion is a multi-step process implemented throughout the cell cycle to ensure the correct transmission of chromosomes to daughter cells. Although cohesion establishment and mitotic cohesion dissolution have been extensively explored, the regulation of cohesin loading is still poorly understood. Here, we report that the methyltransferase NSD3 is essential for mitotic sister chromatid cohesion before mitosis entry. NSD3 interacts with the cohesin loader complex kollerin (composed of NIPBL and MAU2) and promotes the chromatin recruitment of MAU2 and cohesin at mitotic exit. We also show that NSD3 associates with chromatin in early anaphase, prior to the recruitment of MAU2 and RAD21, and dissociates from chromatin when prophase begins. Among the two NSD3 isoforms present in somatic cells, the long isoform is responsible for regulating kollerin and cohesin chromatin-loading, and its methyltransferase activity is required for efficient sister chromatid cohesion. Based on these observations, we propose that NSD3-dependent methylation contributes to sister chromatid cohesion by ensuring proper kollerin recruitment and thus cohesin loading., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)

Members of the chromodomain-helicase-DNA binding (CHD) protein family are chromatin remodelers implicated in human pathologies, with CHD6 being one of its least studied members. We discovered a de novo CHD6 missense mutation in a patient clinically presenting the rare Hallermann-Streiff syndrome (HSS). We used genome editing to generate isogenic iPSC lines and model HSS in relevant cell types. By combining genomics with functional in vivo and in vitro assays, we show that CHD6 binds a cohort of autophagy and stress response genes across cell types. The HSS mutation affects CHD6 protein folding and impairs its ability to recruit co-remodelers in response to DNA damage or autophagy stimulation. This leads to accumulation of DNA damage burden and senescence-like phenotypes. We therefore uncovered a molecular mechanism explaining HSS onset via chromatin control of autophagic flux and genotoxic stress surveillance.

LMNA mutations in patients are responsible for a dilated cardiomyopathy. Molecular mechanisms underlying the origin and development of the pathology are unknown. Herein, using mouse pluripotent embryonic stem cells (ESCs) and a mouse model both harboring the p.H222P Lmna mutation, we found early defects in cardiac differentiation of mutated ESCs and dilatation of mutated embryonic hearts at E13.5, pointing to a developmental origin of the disease. Using mouse ESCs, we demonstrated that cardiac differentiation of LmnaH222P/+ was impaired at the mesodermal stage. Expression of Mesp1, a mesodermal cardiogenic gene involved in epithelial-to-mesenchymal transition of epiblast cells, as well as Snai1 and Twist expression, was decreased in LmnaH222P/+ cells compared with WT cells in the course of differentiation. In turn, cardiomyocyte differentiation was impaired. ChIP assay of H3K4me1 in differentiating cells revealed a specific decrease of this histone mark on regulatory regions of Mesp1 and Twist in LmnaH222P/+ cells. Downregulation or inhibition of LSD1 that specifically demethylated H3K4me1 rescued the epigenetic landscape of mesodermal LmnaH222P/+ cells and in turn contraction of cardiomyocytes. Inhibition of LSD1 in pregnant mice or neonatal mice prevented cardiomyopathy in E13.5 LmnaH222P/H222P offspring and adults, respectively. Thus, LSD1 appeared to be a therapeutic target to prevent or cure dilated cardiomyopathy associated with a laminopathy.

Synonymous single nucleotide variants (sSNVs) have been implicated in various genetic disorders through alterations of pre-mRNA splicing, mRNA structure and miRNA regulation. However, their impact on synonymous codon usage and protein translation remains to be elucidated in clinical context. Here, we explore the functional impact of sSNVs in the Sonic Hedgehog (SHH) gene, identified in patients affected by holoprosencephaly, a congenital brain defect resulting from incomplete forebrain cleavage. We identified eight sSNVs in SHH, selectively enriched in holoprosencephaly patients as compared to healthy individuals, and systematically assessed their effect at both transcriptional and translational levels using a series of in silico and in vitro approaches. Although no evidence of impact of these sSNVs on splicing, mRNA structure or miRNA regulation was found, five sSNVs introduced significant changes in codon usage and were predicted to impact protein translation. Cell assays demonstrated that these five sSNVs are associated with a significantly reduced amount of the resulting protein, ranging from 5% to 23%. Inhibition of the proteasome rescued the protein levels for four out of five sSNVs, confirming their impact on protein stability and folding. Remarkably, we found a significant correlation between experimental values of protein reduction and computational measures of codon usage, indicating the relevance of in silico models in predicting the impact of sSNVs on translation. Considering the critical role of SHH in brain development, our findings highlight the clinical relevance of sSNVs in holoprosencephaly and underline the importance of investigating their impact on translation in human pathologies., (© The Author(s) (2020). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

The NIPBL/MAU2 heterodimer loads cohesin onto chromatin. Mutations in NIPBL account for most cases of the rare developmental disorder Cornelia de Lange syndrome (CdLS). Here we report a MAU2 variant causing CdLS, a deletion of seven amino acids that impairs the interaction between MAU2 and the NIPBL N terminus. Investigating this interaction, we discovered that MAU2 and the NIPBL N terminus are largely dispensable for normal cohesin and NIPBL function in cells with a NIPBL early truncating mutation. Despite a predicted fatal outcome of an out-of-frame single nucleotide duplication in NIPBL, engineered in two different cell lines, alternative translation initiation yields a form of NIPBL missing N-terminal residues. This form cannot interact with MAU2, but binds DNA and mediates cohesin loading. Altogether, our work reveals that cohesin loading can occur independently of functional NIPBL/MAU2 complexes and highlights a novel mechanism protective against out-of-frame mutations that is potentially relevant for other genetic conditions., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Transfection is a powerful tool that enables introducing foreign nucleic acids into living cells in order to study the function of a gene product. Ever since the discovery of transfection many side effects or artifacts caused by transfection reagents have been reported. Here, we show that the transfection reagent, JetPRIME alters the localization of the splicing protein SC35 widely used as a nuclear speckle marker. We demonstrate that transfection of plasmids with JetPRIME leads to enlarged SC35 speckles and SC35 cytoplasmic granules. By contrast, transfection of the same plasmid with Lipofectamine 3000 does not have any effect on SC35 localization. The formation of SC35 cytoplasmic granules by JetPRIME-mediated transfection is independent of exogenous expression by plasmid and although similar in morphology they are distinct from P-bodies and stress granules. This method of transfection affected only SC35 and phosphorylated SR proteins but not the nuclear speckles. The JetPRIME-mediated transfection also showed compromised transcription in cells with enlarged SC35 speckles. Our work indicates that the use of JetPRIME alters SC35 localization and can affect gene expression and alternative splicing. Therefore, caution should be exercised when interpreting results after the use of a transient transfection system, particularly when the subject of the study is the function of a protein in the control of gene expression or mRNA splicing., Competing Interests: Declaration of competing interest The authors declare no conflict of interest., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Neural tube defect disorders are developmental diseases that originate from an incomplete closure of the neural tube during embryogenesis. Despite high prevalence-1 out of 3000 live births-their etiology is not yet established and both environmental and genetic factors have been proposed, with a heritability rate of about 60%. Studies in mouse models as well as in human have further suggested a multifactorial pattern of inheritance for neural tube defect disorders. Here, we report results obtained from clinical diagnosis and NGS analysis of a cohort composed of 52 patients. Using a candidate gene panel approach, we identified variants in known genes of planar cell polarity (PCP) pathway, although with higher prevalence than previously reported. Our study also reveals variants in novel genes such as FREM2 and DISP1. Altogether, these results confirm the implication of the PCP genes and involve the FRAS/FREM2 complex and Sonic Hedgehog signaling as novel components in the appearance of NTDs.

Holoprosencephaly is a pathology of forebrain development characterized by high phenotypic heterogeneity. The disease presents with various clinical manifestations at the cerebral or facial levels. Several genes have been implicated in holoprosencephaly but its genetic basis remains unclear: different transmission patterns have been described including autosomal dominant, recessive and digenic inheritance. Conventional molecular testing approaches result in a very low diagnostic yield and most cases remain unsolved. In our study, we address the possibility that genetically unsolved cases of holoprosencephaly present an oligogenic origin and result from combined inherited mutations in several genes. Twenty-six unrelated families, for whom no genetic cause of holoprosencephaly could be identified in clinical settings [whole exome sequencing and comparative genomic hybridization (CGH)-array analyses], were reanalysed under the hypothesis of oligogenic inheritance. Standard variant analysis was improved with a gene prioritization strategy based on clinical ontologies and gene co-expression networks. Clinical phenotyping and exploration of cross-species similarities were further performed on a family-by-family basis. Statistical validation was performed on 248 ancestrally similar control trios provided by the Genome of the Netherlands project and on 574 ancestrally matched controls provided by the French Exome Project. Variants of clinical interest were identified in 180 genes significantly associated with key pathways of forebrain development including sonic hedgehog (SHH) and primary cilia. Oligogenic events were observed in 10 families and involved both known and novel holoprosencephaly genes including recurrently mutated FAT1, NDST1, COL2A1 and SCUBE2. The incidence of oligogenic combinations was significantly higher in holoprosencephaly patients compared to two control populations (P < 10-9). We also show that depending on the affected genes, patients present with particular clinical features. This study reports novel disease genes and supports oligogenicity as clinically relevant model in holoprosencephaly. It also highlights key roles of SHH signalling and primary cilia in forebrain development. We hypothesize that distinction between different clinical manifestations of holoprosencephaly lies in the degree of overall functional impact on SHH signalling. Finally, we underline that integrating clinical phenotyping in genetic studies is a powerful tool to specify the clinical relevance of certain mutations.

Cornelia de Lange syndrome (CdLS) is a dominantly inherited developmental disorder caused by mutations in genes that encode for either structural (SMC1A, SMC3, RAD21) or regulatory (NIPBL, HDAC8) subunits of the cohesin complex. NIPBL represents the major gene of the syndrome and heterozygous mutations can be identified in more than 65% of patients. Interestingly, large portions of these variants were described as somatic mosaicism and often escape standard molecular diagnostics using lymphocyte DNA. Here we discuss the role of somatic mosaicism in CdLS and describe two additional patients with NIPBL mosaicism detected by targeted gene panel or exome sequencing. In order to verify the next generation sequencing data, Sanger sequencing or pyrosequencing on DNA extracted from different tissues were applied. None of the pathogenic variants was originally detected by Sanger sequencing on blood DNA. Patient 1 displays an unusual combination of clinical features: he is cognitively only mildly affected, but shows severe limb reduction defects. Patient 2 presents with a moderate phenotype. Interestingly, Sanger sequencing analysis on fibroblast DNA of this patient did not detect the disease-causing variant previously observed on the same DNA sample by exome sequencing. Subsequent analyses could confirm the variants by Sanger sequencing on buccal mucosa DNA. Notably, this is the first report of a higher mutational load in buccal mucosa than in fibroblast cells of a CdLS patient. Detection of low-level mosaicism is of utmost importance for an accurate molecular diagnosis and a proper genetic counseling of patients with a clinical diagnosis of CdLS. Next-generation sequencing technologies greatly facilitate the detection of low-level mosaicism, which might otherwise remain undetected by conventional sequencing approaches., (Copyright © 2017 Elsevier Masson SAS. All rights reserved.)

Holoprosencephaly (HPE) is a complex genetic disorder of the developing forebrain characterized by high phenotypic and genetic heterogeneity. HPE was initially defined as an autosomal dominant disease, but recent research has shown that its mode of transmission is more complex. The past decade has witnessed rapid development of novel genetic technologies and significant progresses in clinical studies of HPE. In this review, we recapitulate genetic epidemiological studies of the largest European HPE cohort and summarize the novel genetic discoveries of HPE based on recently developed diagnostic methods. Our main purpose is to present different inheritance patterns that exist for HPE with a particular emphasis on oligogenic inheritance and its implications in genetic counseling., (© 2018 Wiley Periodicals, Inc.)

During the prometaphase stage of mitosis, the cell builds a bipolar spindle of microtubules that mechanically segregates sister chromatids between two daughter cells in anaphase. The spindle assembly checkpoint (SAC) is a quality control mechanism that monitors proper attachment of microtubules to chromosome kinetochores during prometaphase. Segregation occurs only when each chromosome is bi-oriented with each kinetochore pair attached to microtubules emanating from opposite spindle poles. Overexpression of the protein kinase Aurora A is a feature of various cancers and is thought to enable tumour cells to bypass the SAC, leading to aneuploidy. Here, we took advantage of a chemical and chemical-genetic approach to specifically inhibit Aurora A kinase activity in late prometaphase. We observed that a loss of Aurora A activity directly affects SAC function, that Aurora A is essential for maintaining the checkpoint protein Mad2 on unattached kinetochores and that inhibition of Aurora A leads to loss of the SAC, even in the presence of nocodazole or Taxol. This is a new finding that should affect the way Aurora A inhibitors are used in cancer treatments.This article has an associated First Person interview with the first authors of the paper., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2018. Published by The Company of Biologists Ltd.)

Cohesin is crucial for genome stability, cell division, transcription and chromatin organization. Its functions critically depend on NIPBL, the cohesin-loader protein that is found to be mutated in >60% of the cases of Cornelia de Lange syndrome (CdLS). Other mutations are described in the cohesin subunits SMC1A, RAD21, SMC3 and the HDAC8 protein. In 25-30% of CdLS cases no mutation in the known CdLS genes is detected. Until now, functional elements in the noncoding genome were not characterized in the molecular etiology of CdLS and therefore are excluded from mutation screening, although the impact of such mutations has now been recognized for a wide range of diseases. We have identified different elements of the noncoding genome involved in regulation of the NIPBL gene. NIPBL-AS1 is a long non-coding RNA transcribed upstream and antisense to NIPBL. By knockdown and transcription blocking experiments, we could show that not the NIPBL-AS1 gene product, but its actual transcription is important to regulate NIPBL expression levels. This reveals a possibility to boost the transcriptional activity of the NIPBL gene by interfering with the NIPBL-AS1 lncRNA. Further, we have identified a novel distal enhancer regulating both NIPBL and NIPBL-AS1. Deletion of the enhancer using CRISPR genome editing in HEK293T cells reduces expression of NIPBL, NIPBL-AS1 as well as genes found to be dysregulated in CdLS.

The coordinated tissue-specific regulation of gene expression is essential for the proper development of all organisms. Mutations in multiple transcriptional regulators cause a group of neurodevelopmental disorders termed "transcriptomopathies" that share core phenotypical features including growth retardation, developmental delay, intellectual disability and facial dysmorphism. Cornelia de Lange syndrome (CdLS) belongs to this class of disorders and is caused by mutations in different subunits or regulators of the cohesin complex. Herein, we report on the clinical and molecular characterization of seven patients with features overlapping with CdLS who were found to carry mutations in chromatin regulators previously associated to other neurodevelopmental disorders that are frequently considered in the differential diagnosis of CdLS. The identified mutations affect the methyltransferase-encoding genes KMT2A and SETD5 and different subunits of the SWI/SNF chromatin-remodeling complex. Complementary to this, a patient with Coffin-Siris syndrome was found to carry a missense substitution in NIPBL. Our findings indicate that mutations in a variety of chromatin-associated factors result in overlapping clinical phenotypes, underscoring the genetic heterogeneity that should be considered when assessing the clinical and molecular diagnosis of neurodevelopmental syndromes. It is clear that emerging molecular mechanisms of chromatin dysregulation are central to understanding the pathogenesis of these clinically overlapping genetic disorders.

Consistent with the diverse roles of the cohesin complex in chromosome biology, mutations in genes encoding cohesin and its regulators are found in different types of cancer and in developmental disorders such as Cornelia de Lange Syndrome. It is so far considered that the defects caused by these mutations result from altered function of cohesin in regulating gene expression during development. Chromatin conformation analyses have established the importance of cohesin for the architecture of developmental gene clusters and in vivo studies in mouse and zebrafish demonstrated how cohesin defects lead to gene misregulation and to malformations similar to the related human syndromes. Here we present our current knowledge on cohesin's involvement in gene expression, highlighting molecular and mechanistic consequences of pathogenic mutations in the Cornelia de Lange syndrome., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Cornelia de Lange syndrome (CdLS) is a well-characterized developmental disorder. The genetic cause of CdLS is a mutation in one of five associated genes (NIPBL, SMC1A, SMC3, RAD21, and HDAC8) accounting for about 70% of cases. To improve our current molecular diagnostic and to analyze some of CdLS candidate genes, we developed and established a gene panel approach. Because recent data indicate a high frequency of mosaic NIPBL mutations that were not detected by conventional sequencing approaches of blood DNA, we started to collect buccal mucosa (BM) samples of our patients that were negative for mutations in the known CdLS genes. Here, we report the identification of three mosaic NIPBL mutations by our high-coverage gene panel sequencing approach that were undetected by classical Sanger sequencing analysis of BM DNA. All mutations were confirmed by the use of highly sensitive SNaPshot fragment analysis using DNA from BM, urine, and fibroblast samples. In blood samples, we could not detect the respective mutation. Finally, in fibroblast samples from all three patients, Sanger sequencing could identify all the mutations. Thus, our study highlights the need for highly sensitive technologies in molecular diagnostic of CdLS to improve genetic diagnosis and counseling of patients and their families., (© 2014 WILEY PERIODICALS, INC.)

Sister chromatid cohesion, which depends on cohesin, is essential for the faithful segregation of replicated chromosomes. Here, we report that splicing complex Prp19 is essential for cohesion in both G2 and mitosis, and consequently for the proper progression of the cell through mitosis. Inactivation of splicing factors SF3a120 and U2AF65 induces similar cohesion defects to Prp19 complex inactivation. Our data indicate that these splicing factors are all required for the accumulation of cohesion factor Sororin, by facilitating the proper splicing of its pre-mRNA. Finally, we show that ectopic expression of Sororin corrects defective cohesion caused by Prp19 complex inactivation. We propose that the Prp19 complex and the splicing machinery contribute to the establishment of cohesion by promoting Sororin accumulation during S phase, and are, therefore, essential to the maintenance of genome stability., (© 2014 The Authors.)

Estradiol signaling is ideally suited for analyzing the molecular and functional linkages between the different layers of information directing transcriptional regulations: the DNA sequence, chromatin modifications, and the spatial organization of the genome. Hence, the estrogen receptor (ER) can bind at a distance from its target genes and engages timely and spatially coordinated processes to regulate their expression. In the context of the coordinated regulation of colinear genes, identifying which ER binding sites (ERBSs) regulate a given gene still remains a challenge. Here, we investigated the coordination of such regulatory events at a 2-Mb genomic locus containing the estrogen-sensitive trefoil factor (TFF) cluster of genes in breast cancer cells. We demonstrate that this locus exhibits a hormone- and cohesin-dependent reduction in the plasticity of its three-dimensional organization that allows multiple ERBSs to be dynamically brought to the vicinity of estrogen-sensitive genes. Additionally, by using triplex-forming oligonucleotides, we could precisely document the functional links between ER engagement at given ERBSs and the regulation of particular genes. Hence, our data provide evidence of a formerly suggested cooperation of enhancers toward gene regulation and also show that redundancy between ERBSs can occur., (Copyright © 2014, American Society for Microbiology. All Rights Reserved.)

Cornelia de Lange syndrome (CdLS) is a dominantly inherited congenital malformation disorder, caused by mutations in the cohesin-loading protein NIPBL for nearly 60% of individuals with classical CdLS, and by mutations in the core cohesin components SMC1A (~5%) and SMC3 (<1%) for a smaller fraction of probands. In humans, the multisubunit complex cohesin is made up of SMC1, SMC3, RAD21 and a STAG protein. These form a ring structure that is proposed to encircle sister chromatids to mediate sister chromatid cohesion and also has key roles in gene regulation. SMC3 is acetylated during S-phase to establish cohesiveness of chromatin-loaded cohesin, and in yeast, the class I histone deacetylase Hos1 deacetylates SMC3 during anaphase. Here we identify HDAC8 as the vertebrate SMC3 deacetylase, as well as loss-of-function HDAC8 mutations in six CdLS probands. Loss of HDAC8 activity results in increased SMC3 acetylation and inefficient dissolution of the ‘used’ cohesin complex released from chromatin in both prophase and anaphase. SMC3 with retained acetylation is loaded onto chromatin, and chromatin immunoprecipitation sequencing analysis demonstrates decreased occupancy of cohesin localization sites that results in a consistent pattern of altered transcription seen in CdLS cell lines with either NIPBL or HDAC8 mutations.

Cohesin complexes mediate sister chromatid cohesion. Cohesin also becomes enriched at DNA double-strand break sites and facilitates recombinational DNA repair. Here, we report that cohesin is essential for the DNA damage-induced G2/M checkpoint. In contrast to cohesin's role in DNA repair, the checkpoint function of cohesin is independent of its ability to mediate cohesion. After RNAi-mediated depletion of cohesin, cells fail to properly activate the checkpoint kinase Chk2 and have defects in recruiting the mediator protein 53BP1 to DNA damage sites. Earlier work has shown that phosphorylation of the cohesin subunits Smc1 and Smc3 is required for the intra-S checkpoint, but Smc1/Smc3 are also subunits of a distinct recombination complex, RC-1. It was, therefore, unknown whether Smc1/Smc3 function in the intra-S checkpoint as part of cohesin. We show that Smc1/Smc3 are phosphorylated as part of cohesin and that cohesin is required for the intra-S checkpoint. We propose that accumulation of cohesin at DNA break sites is not only needed to mediate DNA repair, but also facilitates the recruitment of checkpoint proteins, which activate the intra-S and G2/M checkpoints.

Sister chromatid cohesion depends on cohesin [1-3]. Cohesin associates with chromatin dynamically throughout interphase [4]. During DNA replication, cohesin establishes cohesion [5], and this process coincides with the generation of a cohesin subpopulation that is more stably bound to chromatin [4]. In mitosis, cohesin is removed from chromosomes, enabling sister chromatid separation [6]. How cohesin associates with chromatin and establishes cohesion is poorly understood. By searching for proteins that are associated with chromatin-bound cohesin, we have identified sororin, a protein that was known to be required for cohesion [7]. To obtain further insight into sororin's function, we have addressed when during the cell cycle sororin is required for cohesion. We show that sororin is dispensable for the association of cohesin with chromatin but that sororin is essential for proper cohesion during G2 phase. Like cohesin, sororin is also needed for efficient repair of DNA double-strand breaks in G2. Finally, sororin is required for the presence of normal amounts of the stably chromatin-bound cohesin population in G2. Our data indicate that sororin interacts with chromatin-bound cohesin and functions during the establishment or maintenance of cohesion in S or G2 phase, respectively.

Replicated DNA molecules are physically connected by cohesin complexes from the time of their synthesis in S-phase until they are segregated during anaphase of the subsequent mitosis or meiosis. This sister chromatid cohesion is essential for the biorientation of chromosomes on the mitotic or meiotic spindle. In addition, cohesion is also essential during G2-phase of the cell cycle to allow repair of DNA double-strand breaks by homologous recombination. Although cohesion can normally only be established during S-phase, recent work in yeast has shown that DNA double-strand breaks induce the recruitment of cohesin to the damage site and lead to the de novo formation of cohesion at this site. It is unknown if similar mechanisms operate in higher eukaryotes, but in mammalian cells phosphorylation of the cohesin subunit Smc1 by the protein kinase Atm has been shown to be important for DNA repair. We discuss how cohesin and sister chromatid cohesion might facilitate the repair of damaged DNA.

Background: Sister-chromatid cohesion depends on the cohesin complex whose association with chromatin is mediated by Scc2 and Scc4 in budding yeast. Both cohesin and Scc2 have been conserved from yeast to humans, but no Scc4 orthologs have been identified. Mutation of Scc2 orthologs causes defects in cohesion, transcription, and development, resulting in Cornelia de Lange syndrome in humans., Results: We have identified a family of tetratricopeptide repeat proteins that share weak sequence similarities with yeast Scc4. This family includes MAU-2, which is required for development of the nervous system in Caenorhabditis elegans. We show that the human member of this family is associated with Scc2, is bound to chromatin from telophase until prophase, and is required for association of cohesin with chromatin during interphase. Cells lacking Scc4 lose sister-chromatid cohesion precociously and arrest in prometaphase. Mitotic chromosomes in Scc4-depleted cells lack cohesin, even though the cohesin-protecting proteins Sgo1 and Bub1 are normally enriched at centromeres and separase does not seem to be active., Conclusion: Our data indicate that human Scc4 is required for the association of cohesin with chromatin, which is a prerequisite for the establishment of sister-chromatid cohesion and for chromosome biorientation in mitosis. The proteinaceous machinery that is required for loading of cohesin onto chromatin is therefore conserved from yeast to humans. The finding that Caenorhabditis elegans MAU-2 is an ortholog of Scc4 further supports the notion that the Scc2-Scc4 complex is required for developmental processes in metazoans.

Condensins are heteropentameric complexes that were first identified as structural components of mitotic chromosomes. They are composed of two SMC (structural maintenance of chromosomes) and three non-SMC subunits. Condensins play a role in the resolution and segregation of sister chromatids during mitosis, as well as in some aspects of mitotic chromosome assembly. Two distinct condensin complexes, condensin I and condensin II, which differ only in their non-SMC subunits, exist. Here, we used an RNA interference approach to deplete hCAP-D2, a non-SMC subunit of condensin I, in HeLa cells. We found that the association of hCAP-H, another non-SMC subunit of condensin I, with mitotic chromosomes depends on the presence of hCAP-D2. Moreover, chromatid axes, as defined by topoisomerase II and hCAP-E localization, are disorganized in the absence of hCAP-D2, and the resolution and segregation of sister chromatids are impaired. In addition, hCAP-D2 depletion affects chromosome alignment in metaphase and delays entry into anaphase. This suggests that condensin I is involved in the correct attachment between chromosome kinetochores and microtubules of the mitotic spindle. These results are discussed relative to the effects of depleting both condensin complexes.

Condensins are pentameric complexes that were initially described as being involved in the dynamics of chromosomes during mitosis. It has been recently established that two related complexes (Condensin I and Condensin II) contribute to this process. An increasing sum of studies, using different approaches in various organisms, leads to the paradigm that Condensins are required for the correct segregation of replicated chromosomes by cooperating somehow with Topoisomerase II in sister chromatid resolution. Depending on species and/or experimental studies, these complexes also contribute to some aspects of the assembly and compaction of mitotic chromosomes. Recent studies provided evidences that Condensins and related complexes also function in non-mitotic processes such as replication and transcription. Biochemical studies have highlighted mechanistic aspects of Condensin function and initiated a fine functional dissection of core and regulatory subunits. However, the exact contribution of each subunit remains largely elusive as well as the functional interplay between Condensin I and Condensin II.

To ensure that the genetic information, replicated in the S-phase of the cell cycle, is correctly distributed between daughter cells at mitosis, chromatin duplication and chromosome segregation are highly regulated events. Since the early 1980's, our knowledge of the mechanisms governing these two events has greatly increased due to the use of genetic and biochemical approaches. We present here, first, an overview of the replication process, highlighting molecular aspects involved in coupling replication with chromatin dynamics in mitosis. The second part will present the current understanding of chromosome condensation and segregation during mitosis in higher eukaryotes. Finally, we will underline the links that exist between replication and mitosis.

The 13 S condensin complex plays a crucial role in the condensation and segregation of the two sets of chromosomes during mitosis in vivo as well as in cell-free extracts. This complex, conserved from yeast to human, contains a heterodimer of structural maintenance of chromosome (SMC) family proteins and three additional non-SMC subunits. We have investigated the expression of the non-SMC condensin component XCAP-D2 in Xenopus laevis oocytes. When studied during meiotic maturation, XCAP-D2 starts to accumulate at the time of germinal vesicle breakdown and reaches its maximal amount in metaphase II oocytes. This accumulation is specifically blocked by injection of antisense oligonucleotides. XCAP-D2 antisense-injected oocytes progress normally through meiosis until metaphase II. At this stage, however, chromosomes exhibit architecture defaults, and resolution of sister chromatids is impaired. Surprisingly, in mitotic extracts made from XCAP-D2 knocked-down oocytes, sperm chromatin normally condenses into compacted chromosomes, whereas the amounts of both free and chromosome-bound XCAP-D2 are markedly reduced. This apparent discrepancy is discussed in light of current knowledge on chromosome dynamics.

Cell cycle dynamics and localization of condensins--multiprotein complexes involved in late stages of mitotic chromosome condensation--were studied in Xenopus laevis XL2 cell line. Western blot analysis of synchronized cells showed that the ratio of levels of both pEg7 and XCAP-E to beta-tubulin levels remains almost constant from G1 to M phase. pEg7 and XCAP-E were localized to the mitotic chromosomes and were detected in interphase nuclei. Immunostaining for condensins and nucleolar proteins UBF, fibrillarin and B23 revealed that both XCAP-E and pEg7 are localized in the granular component of the nucleolus. Nucleolar labeling of both proteins is preserved in segregated nucleoli after 6 hours of incubation with actinomycin D (5 mg/ml), but the size of the labeled zone was significantly smaller. The data suggest a novel interphase function of condensin subunits in spatial organization of the nucleolus and/or ribosome biogenesis.

Several antibodies were used to examine the distribution of two condensin members, XCAP-E and XCAP-D2, in the nucleus of Xenopus oocytes. XCAP-D2 was found to be associated with the lampbrush chromosomes. The chromosomal regions containing XCAP-D2 correspond precisely to domains of highly compacted chromatin, suggesting a direct contribution of XCAP-D2 in meiotic chromatin organization. In contrast, XCAP-E was found to be absent from chromosomes but was detected at a high concentration in the granular component of nucleoli. The subnucleolar localization of XCAP-E was further confirmed by double labeling using several nucleolar protein markers. The fate of nucleolar XCAP-E was also followed when changes in the nucleoli morphology were artificially induced. The apparent exclusion of XCAP-E from the ribosomal DNA and its tight association with the granular component in all preparations suggest that it might be sequestrated in nucleoli during early stages of meiosis. Interestingly, both XCAP-D2 and XCAP-E were also detected in Cajal bodies, which are organelles suspected to play a role in the assembly/modification of the RNA transcription and processing machinery. The presence of two condensins in CBs might extend such a role of assembly to chromatin macromolecular components as well.