Cloning and characterization of rad21 an essential gene of Schizosaccharomyces pombe involved in DNA double-strand-break repair

The rice OsRad21-4, an orthologue of yeast Rec8 protein, is required for efficient meiosis

In yeast, Rad21/Scc1 and its meiotic variant Rec8 are key players in the establishment and subsequent dissolution of sister chromatid cohesion for mitosis and meiosis, respectively, which are essential for chromosome segregation. Unlike yeast, our identification revealed that the rice genome has 4 RAD21-like genes that share lower than 21% identity at polypeptide levels, and each is present as a single copy in this genome. Here we describe our analysis of the function of OsRAD21-4 by RNAi. Western blot analyses indicated that the protein was most abundant in young flowers and less in leaves and buds but absent in roots. In flowers, the expression was further defined to premeiotic pollen mother cells (PMCs) and meiotic PMCs of anthers. Meiotic chromosome behaviors were monitored from male meiocytes of OsRAD21-4-deficient lines mediated by RNAi. The male meiocytes showed multiple aberrant events at meiotic prophase I, including over-condensation of chromosomes, precocious segregation of homologues and chromosome fragmentation. Fluorescence in situ hybridization experiments revealed that the deficient lines were defective in homologous pairing and cohesion at sister chromatid arms. These defects resulted in unequal chromosome segregation and aberrant spore generation. These observations suggest that OsRad21-4 is essential for efficient meiosis.

DNA damage responses and their many interactions with the replication fork

The cellular response to DNA damage is composed of cell cycle checkpoint and DNA repair mechanisms that serve to ensure proper replication of the genome prior to cell division. The function of the DNA damage response during DNA replication in S-phase is critical to this process. Recent evidence has suggested a number of interrelationships of DNA replication and cellular DNA damage responses. These include S-phase checkpoints which suppress replication initiation or elongation in response to DNA damage. Also, many components of the DNA damage response are required either for the stabilization of, or for restarting, stalled replication forks. Further, translesion synthesis permits DNA replication to proceed in the presence of DNA damage and can be coordinated with subsequent repair by homologous recombination (HR). Finally, cohesion of sister chromatids is established coincident with DNA replication and is required for subsequent DNA repair by homologous recombination. Here we review these processes, all of which occur at, or are related to, the advancing replication fork. We speculate that these multiple interdependencies of DNA replication and DNA damage responses integrate the many steps necessary to ensure accurate duplication of the genome.

Correlation of invasion and metastasis of cancer cells, and expression of the RAD21 gene in oral squamous cell carcinoma

Although RAD21 is involved in the repair of double-strand breaks in DNA and is essential for mitotic growth, its role in cancer has been unclear. In this study, the relevance of RAD21 gene expression to the invasion and metastasis of oral squamous cell carcinoma was clarified using laser microdissection and real-time polymerase chain reaction (PCR). Using two different metastatic potential oral squamous cells [high-metastatic-potential squamous cell carcinoma cells (SAS-Ly) and low-metastatic-potential squamous cell carcinoma cells (SAS)], the relation of RAD21 gene expression to apoptosis, invasion, and metastasis was examined. The results showed that RAD21 gene expression was significantly decreased in oral squamous cell carcinoma when it expressed the INFbeta and INFgamma invasion patterns in comparison with the INFalpha invasion pattern (p<0.01). In addition, in comparison with SAS cells, SAS-Ly cells indicated tolerance to cell death induced by an apoptosis induction reagent, while the expression level of the RAD21 gene in SAS cells was increased by the apoptosis induction reagent. However, in SAS-Ly cells, the reagent induced no significant difference. Our findings indicate that the RAD21 gene was closely related to the invasion and metastasis of cancer cells.

Characterization of the three Arabidopsis thaliana RAD21 cohesins reveals differential responses to ionizing radiation

The RAD21/REC8 gene family has been implicated in sister chromatid cohesion and DNA repair in several organisms. Unlike most eukaryotes, Arabidopsis thaliana has three RAD21 gene homologues, and their cloning and characterization are reported here. All three genes, AtRAD21.1, AtRAD21.2, and AtRAD21.3, are expressed in tissues rich in cells undergoing cell division, and AtRAD21.3 shows the highest relative level of expression. An increase in steady-state levels of AtRAD21.1 transcript was also observed, specifically after the induction of DNA damage. Phenotypic analysis of the atrad21.1 and atrad21.3 mutants revealed that neither of the single mutants was lethal, probably due to the redundancy in function of the AtRAD21 genes. However, AtRAD21.1 plays a critical role in recovery from DNA damage during seed imbibition, prior to germination, as atrad21.1 mutant seeds are hypersensitive to radiation damage.

Schizosaccharomyces pombe mst2+ encodes a MYST family histone acetyltransferase that negatively regulates telomere silencing

Histone acetylation and deacetylation are associated with transcriptional activity and the formation of constitutively silent heterochromatin. Increasingly, histone acetylation is also implicated in other chromosome transactions, including replication and segregation. We have cloned the only Schizosaccharomyces pombe MYST family histone acetyltransferase genes, mst1(+) and mst2(+). Mst1p, but not Mst2p, is essential for viability. Both proteins are localized to the nucleus and bound to chromatin throughout the cell cycle. Deltamst2 genetically interacts with mutants that affect heterochromatin, cohesion, and telomere structure. Mst2p is a negative regulator of silencing at the telomere but does not affect silencing in the centromere or mating type region. We generated a census of proteins and histone modifications at wild-type telomeres. A histone acetylation gradient at the telomeres is lost in Deltamst2 cells without affecting the distribution of Taz1p, Swi6p, Rad21p, or Sir2p. We propose that the increased telomeric silencing is caused by histone hypoacetylation and/or an increase in the ratio of methylated to acetylated histones. Although telomere length is normal, meiosis is aberrant in Deltamst2 diploid homozygote mutants, suggesting that telomeric histone acetylation contributes to normal meiotic progression.

Recombination protein Tid1p controls resolution of cohesin-dependent linkages in meiosis in Saccharomyces cerevisiae

Sister chromatid cohesion and interhomologue recombination are coordinated to promote the segregation of homologous chromosomes instead of sister chromatids at the first meiotic division. During meiotic prophase in Saccharomyces cerevisiae, the meiosis-specific cohesin Rec8p localizes along chromosome axes and mediates most of the cohesion. The mitotic cohesin Mcd1p/Scc1p localizes to discrete spots along chromosome arms, and its function is not clear. In cells lacking Tid1p, which is a member of the SWI2/SNF2 family of helicase-like proteins that are involved in chromatin remodeling, Mcd1p and Rec8p persist abnormally through both meiotic divisions, and chromosome segregation fails in the majority of cells. Genetic results indicate that the primary defect in these cells is a failure to resolve Mcd1p-mediated connections. Tid1p interacts with recombination enzymes Dmc1p and Rad51p and has an established role in recombination repair. We propose that Tid1p remodels Mcd1p-mediated cohesion early in meiotic prophase to facilitate interhomologue recombination and the subsequent segregation of homologous chromosomes.

SMC complexes in bacterial chromosome condensation and segregation

Bacterial chromosomes segregate via a partition apparatus that employs a score of specialized proteins. The SMC complexes play a crucial role in the chromosome partitioning process by organizing bacterial chromosomes through their ATP-dependent chromatin-compacting activity. Recent progress in the composition of these complexes and elucidation of their structural and enzymatic properties has advanced our comprehension of chromosome condensation and segregation mechanics in bacteria.

Changes in gene expression profiles induced by parvovirus H-1 in human gastric cancer cells

OBJECTIVE

The autonomous parvovirus H-1 exhibits preferential toxicity for transformed or tumor cells. The precise molecular mechanism of H-1 virus-associated cytotoxicity is not fully understood. The present study aimed at gaining more information about parvovirus-induced cellular disturbances.

METHODS

The H-1 virus-sensitive human gastric cancer cell line HGC27 was analyzed in the present study. cDNA microarrays were used to determine the global cellular gene expression changes which occur during the process of H-1 virus-induced death of HGC27 cells. A subset of differential expressed genes was further tested by RT-PCR and Northern blot analyzes.

RESULTS

A total of 920 genes belonging to various functional groups were found to be differentially expressed in H-1 virus- versus mock-infected cells in cDNA microarrays. Among them, 363 genes were upregulated, whilst 557 genes were downregulated. The differential expressions of some of these genes were further confirmed by RT-PCR and Northern blot analysis.

CONCLUSION

Some of genes known to be involved in cell signal transduction, apoptosis, DNA replication, DNA repair, DNA binding and transcription were differentially expressed after parvovirus H-1 infection, they might play a role in H-1 virus-induced gastric cancer cell death. These genes represent interesting candidates to be tested at the functional level for their contribution to the disturbances triggered by H-1 virus in tumor cells.

Dynamic molecular linkers of the genome: the first decade of SMC proteins

Structural maintenance of chromosomes (SMC) proteins are chromosomal ATPases, highly conserved from bacteria to humans, that play fundamental roles in many aspects of higher-order chromosome organization and dynamics. In eukaryotes, SMC1 and SMC3 act as the core of the cohesin complexes that mediate sister chromatid cohesion, whereas SMC2 and SMC4 function as the core of the condensin complexes that are essential for chromosome assembly and segregation. Another complex containing SMC5 and SMC6 is implicated in DNA repair and checkpoint responses. The SMC complexes form unique ring- or V-shaped structures with long coiled-coil arms, and function as ATP-modulated, dynamic molecular linkers of the genome. Recent studies shed new light on the mechanistic action of these SMC machines and also expanded the repertoire of their diverse cellular functions. Dissecting this class of chromosomal ATPases is likely to be central to our understanding of the structural basis of genome organization, stability, and evolution.

Psc3 cohesin of Schizosaccharomyces pombe: cell cycle analysis and identification of three distinct isoforms

Cohesins are a group of proteins that function to mediate correct chromosome segregation, DNA repair and meiotic recombination. This report presents the amino acid sequence for the Schizosaccharomyces pombe cohesin Psc3 based on the translation of the cDNA sequence, showing that the protein is smaller than previously predicted. Interestingly, comparison of the amino acid and DNA coding sequences of Psc3 with fission yeast Rec11 meiotic region-specific recombination activator shows that both intron positioning within the genes and the amino-terminal half of the two proteins are highly conserved. We demonstrate that although the intergenic region upstream of the psc3+ start codon contains a consensus sequence for the cell-cycle regulatory MluI cell-cycle box, psc3+ transcription is not differentially regulated during the mitotic cell cycle. Finally, we demonstrate that an epitope-tagged version of Psc3 undergoes no major changes during the mitotic cell cycle. However, instead we identify at least three distinct isoforms of Psc3, suggesting that post-translational modification of Psc3 contributes to the regulation of cohesion function.

Suppression of RAD21 gene expression decreases cell growth and enhances cytotoxicity of etoposide and bleomycin in human breast cancer cells

A genome-wide case-control association study done in our laboratory has identified a single nucleotide polymorphism located in RAD21 as being significantly associated with breast cancer susceptibility. RAD21 is believed to function in sister chromatid alignment as part of the cohesin complex and also in double-strand break (DSB) repair. Following our initial finding, expression studies revealed a 1.25- to 2.5-fold increased expression of this gene in several human breast cancer cell lines as compared with normal breast tissue. To determine whether suppression of RAD21 expression influences cellular proliferation, RNA interference technology was used in breast cancer cell lines MCF-7 and T-47D. Proliferation of cells treated with RAD21-specific small inhibitory RNA (siRNA) was significantly reduced as compared with mock-transfected cells and cells transfected with a control siRNA (Lamin A/C). This inhibition of proliferation correlated with a significant reduction in the expression of RAD21 mRNA and with an increased level of apoptosis. Moreover, MCF-7 cell sensitivity to two DNA-damaging chemotherapeutic agents, etoposide and bleomycin, was increased after inhibition of RAD21 expression with a dose reduction factor 50 (DRF50) of 1.42 and 3.71, respectively. At the highest concentrations of etoposide and bleomycin administered, cells transfected with a single siRNA duplex targeted against RAD21 showed 57% and 60% survival as compared with control cells, respectively. Based on these findings, we conclude that RAD21 is a novel target for developing cancer therapeutics that can potentially enhance the antitumor activity of chemotherapeutic agents acting via induction of DNA damage.

Sororin, a substrate of the anaphase-promoting complex, is required for sister chromatid cohesion in vertebrates

We have identified a regulator of sister chromatid cohesion in a screen for cell cycle-controlled proteins. This 35 kDa protein is degraded through anaphase-promoting complex (APC)-dependent ubiquitination in G1. The protein is nuclear in interphase cells, dispersed from the chromatin in mitosis, and interacts with the cohesin complex. In Xenopus embryos, overexpression of the protein causes failure to resolve and segregate sister chromatids in mitosis and an increase in the level of cohesin associated with metaphase chromosomes. In cultured cells, depletion of the protein causes mitotic arrest and complete failure of sister chromatid cohesion. This protein is thus an essential, cell cycle-dependent mediator of sister chromatid cohesion. Based on sequence analysis, this protein has no apparent orthologs outside of the vertebrates. We speculate that the protein, which we have named sororin, regulates the ability of the cohesin complex to mediate sister chromatid cohesion, perhaps by altering the nature of the interaction of cohesin with the chromosomes.

The role of SMC proteins in the responses to DNA damage

The SMC proteins form the cores of three protein complexes in eukaryotes, cohesin, condensin and the Smc5-6 complex. Cohesin holds sister chromatids together after DNA replication and is involved in both the repair of double-strand breaks by homologous recombination and the intra-S-phase checkpoint. Condensin assists in the condensation of chromosomes at mitosis and also has a role in checkpoint control pathways. The Smc5-6 complex is involved in a variety of DNA repair and damage response pathways by as yet unknown mechanisms, but is also associated with repair by homologous recombination.

The structure and function of SMC and kleisin complexes

Protein complexes consisting of structural maintenance of chromosomes (SMC) and kleisin subunits are crucial for the faithful segregation of chromosomes during cell proliferation in prokaryotes and eukaryotes. Two of the best-studied SMC complexes are cohesin and condensin. Cohesin is required to hold sister chromatids together, which allows their bio-orientation on the mitotic spindle. Cleavage of cohesin's kleisin subunit by the separase protease then triggers the movement of sister chromatids into opposite halves of the cell during anaphase. Condensin is required to organize mitotic chromosomes into coherent structures that prevent them from getting tangled up during segregation. Here we describe the discovery of SMC complexes and discuss recent advances in determining how members of this ancient protein family may function at a mechanistic level.

Mcl1p is a polymerase alpha replication accessory factor important for S-phase DNA damage survival

Mcl1p is an essential fission yeast chromatin-binding protein that belongs to a family of highly conserved eukaryotic proteins important for sister chromatid cohesion. The essential function is believed to result from its role as a Pol1p (polymerase alpha) accessory protein, a conclusion based primarily on analogy to Ctf4p's interaction with Pol1p. In this study, we show that Mcl1p also binds to Pol1p with high affinity for the N terminus of Pol1p during S phase and DNA damage. Characterization of an inducible allele of mcl1+, (nmt41)mcl1-MH, shows that altered expression levels of Mcl1p lead to sensitivity to DNA-damaging agents and synthetic lethality with the replication checkpoint mutations rad3Delta, rqh1Delta, and hsk1-1312. Further, we find that the overexpression of the S-phase checkpoint kinase, Cds1, or the loss of Hsk1 kinase activity can disrupt Mcl1p's interaction with chromatin and Pol1p during replication arrest with hydroxyurea. We take these data to mean that Mcl1p is a dynamic component of the polymerase alpha complex during replication and is important for the replication stress response in fission yeast.

DNA damage response pathway uses histone modification to assemble a double-strand break-specific cohesin domain

The postreplicative repair of double-strand breaks (DSBs) is thought to require sister chromatid cohesion, provided by the cohesin complex along the chromosome arms. A further specialized role for cohesin in DSB repair is suggested by its de novo recruitment to regions of DNA damage in mammals. Here, we show in budding yeast that a single DSB induces the formation of a approximately 100 kb cohesin domain around the lesion. Our analyses suggest that the primary DNA damage checkpoint kinases Mec1p and Tel1p phosphorylate histone H2AX to generate a large domain, which is permissive for cohesin binding. Cohesin binding to the phospho-H2AX domain is enabled by Mre11p, a component of a critical repair complex, and Scc2p, a component of the cohesin loading machinery that is necessary for sister chromatid cohesion. We also provide evidence that the DSB-induced cohesin domain functions in postreplicative repair.

Postreplicative recruitment of cohesin to double-strand breaks is required for DNA repair

Chromosome stability depends on accurate chromosome segregation and efficient DNA double-strand break (DSB) repair. Sister chromatid cohesion, established during S phase by the protein complex cohesin, is central to both processes. In the absence of cohesion, chromosomes missegregate and G2-phase DSB repair fails. Here, we demonstrate that G2-phase repair also requires the presence of cohesin at the damage site. Cohesin components are shown to be recruited to extended chromosome regions surrounding DNA breaks induced during G2. We find that in the absence of functional cohesin-loading proteins (Scc2/Scc4), the accumulation of cohesin at DSBs is abolished and repair is defective, even though sister chromatids are connected by S phase generated cohesion. Evidence is also provided that DSB induction elicits establishment of sister chromatid cohesion in G2, implicating that damage-recruited cohesin facilitates DNA repair by tethering chromatids.

Rad62 protein functionally and physically associates with the smc5/smc6 protein complex and is required for chromosome integrity and recombination repair in fission yeast

Smc5 and Smc6 proteins form a heterodimeric SMC (structural maintenance of chromosome) protein complex like SMC1-SMC3 cohesin and SMC2-SMC4 condensin, and they associate with non-SMC proteins Nse1 and Nse2 stably and Rad60 transiently. This multiprotein complex plays an essential role in maintaining chromosome integrity and repairing DNA double strand breaks (DSBs). This study characterizes a Schizosaccharomyces pombe mutant rad62-1, which is hypersensitive to methyl methanesulfonate (MMS) and synthetically lethal with rad2 (a feature of recombination mutants). rad62-1 is hypersensitive to UV and gamma rays, epistatic with rhp51, and defective in repair of DSBs. rad62 is essential for viability and genetically interacts with rad60, smc6, and brc1. Rad62 protein physically associates with the Smc5-6 complex. rad62-1 is synthetically lethal with mutations in the genes promoting recovery from stalled replication, such as rqh1, srs2, and mus81, and those involved in nucleotide excision repair like rad13 and rad16. These results suggest that Rad62, like Rad60, in conjunction with the Smc5-6 complex, plays an essential role in maintaining chromosome integrity and recovery from stalled replication by recombination.

Nse1, Nse2, and a novel subunit of the Smc5-Smc6 complex, Nse3, play a crucial role in meiosis

The structural maintenance of chromosomes (SMC) family of proteins play key roles in the organization, packaging, and repair of chromosomes. Cohesin (Smc1+3) holds replicated sister chromatids together until mitosis, condensin (Smc2+4) acts in chromosome condensation, and Smc5+6 performs currently enigmatic roles in DNA repair and chromatin structure. The SMC heterodimers must associate with non-SMC subunits to perform their functions. Using both biochemical and genetic methods, we have isolated a novel subunit of the Smc5+6 complex, Nse3. Nse3 is an essential nuclear protein that is required for normal mitotic chromosome segregation and cellular resistance to a number of genotoxic agents. Epistasis with Rhp51 (Rad51) suggests that like Smc5+6, Nse3 functions in the homologous recombination based repair of DNA damage. We previously identified two non-SMC subunits of Smc5+6 called Nse1 and Nse2. Analysis of nse1-1, nse2-1, and nse3-1 mutants demonstrates that they are crucial for meiosis. The Nse1 mutant displays meiotic DNA segregation and homologous recombination defects. Spore viability is reduced by nse2-1 and nse3-1, without affecting interhomolog recombination. Finally, genetic interactions shared by the nse mutants suggest that the Smc5+6 complex is important for replication fork stability.

Separase-mediated cleavage of cohesin at interphase is required for DNA repair

Sister chromatids are held together by cohesins. At anaphase, separase is activated by degradation of its inhibitory partner, securin. Separase then cleaves cohesins, thus allowing sister chromatid separation. Fission yeast securin (Cut2) has destruction boxes and a separase (Cut1) interaction site in the amino and carboxyl terminus, respectively. Here we show that securin is essential for separase stability and also for proper repair of DNA damaged by ultraviolet, X-ray and gamma-ray irradiation. The cut2(EA2) mutant is defective in the repair of ultraviolet damage lesions, although the DNA damage checkpoint is activated normally. In double mutant analysis of ultraviolet sensitivity, checkpoint kinase chk1 (ref. 9) and excision repair rad13 (ref. 10) mutants were additive with cut2(EA2), whereas recombination repair rhp51 (ref. 11) and cohesin subunit rad21 (ref. 12) mutants were not. Cohesin was hyper-modified on ultraviolet irradiation in a Rad3 kinase-dependent way. Experiments using either mutant cohesin that cannot be cleaved by separase or a protease-dead separase provide evidence that this DNA repair function of securin-separase acts through the cleavage of cohesin. We propose that the securin-separase complex might aid DNA repair by removing local cohesin in interphase cells.

Nse1, Nse2, and a novel subunit of the Smc5-Smc6 complex, Nse3, play a crucial role in meiosis

The structural maintenance of chromosomes (SMC) family of proteins play key roles in the organization, packaging, and repair of chromosomes. Cohesin (Smc1+3) holds replicated sister chromatids together until mitosis, condensin (Smc2+4) acts in chromosome condensation, and Smc5+6 performs currently enigmatic roles in DNA repair and chromatin structure. The SMC heterodimers must associate with non-SMC subunits to perform their functions. Using both biochemical and genetic methods, we have isolated a novel subunit of the Smc5+6 complex, Nse3. Nse3 is an essential nuclear protein that is required for normal mitotic chromosome segregation and cellular resistance to a number of genotoxic agents. Epistasis with Rhp51 (Rad51) suggests that like Smc5+6, Nse3 functions in the homologous recombination based repair of DNA damage. We previously identified two non-SMC subunits of Smc5+6 called Nse1 and Nse2. Analysis of nse1-1, nse2-1, and nse3-1 mutants demonstrates that they are crucial for meiosis. The Nse1 mutant displays meiotic DNA segregation and homologous recombination defects. Spore viability is reduced by nse2-1 and nse3-1, without affecting interhomolog recombination. Finally, genetic interactions shared by the nse mutants suggest that the Smc5+6 complex is important for replication fork stability.

Genetic biomarkers of therapeutic radiation sensitivity

The occurrence of acute or late normal tissue reactions after therapeutic radiotherapy and cellular responses in in vitro radiosensitivity assays do not correlate well suggesting that to date no one test system is suitable for predicting the risk or severity of such reactions. New insights into the underlying molecular mechanisms of this sensitivity are coming from studies that assess associations between common polymorphisms in DNA damage detection and repair genes and the development of adverse reactions to radiotherapy. The presence of such variants may alter protein function and an individual's capacity to repair damaged DNA modifying the response of the normal tissue. Polymorphisms in the XRCC1, ATM, hHR21 and TGFbeta1 genes have been shown to be associated with an increased risk of developing an adverse normal tissue reaction to radiotherapy, whilst one variant in the ATM gene has been reported to be radioprotective. Functional studies, taking into account either the haplotypes or the combined genotypes when multiple polymorphisms in a gene are present, will be necessary to establish the mechanistic basis of these associations. In the future association studies can only benefit from the analysis of multiple genes in large, well-characterized cohorts in particular to identify genetic factors that might specifically influence the temporal occurrence of these adverse reactions.

Fission yeast Sap1 protein is essential for chromosome stability

Sap1 is a dimeric sequence-specific DNA binding-protein, initially identified for its role in mating-type switching of the fission yeast Schizosaccharomyces pombe. The protein is relatively abundant, around 10,000 dimers/cell, and is localized in the nucleus. sap1+ is essential for viability, and transient overexpression is accompanied by rapid cell death, without an apparent checkpoint response and independently of mating-type switching. Time lapse video microscopy of living cells revealed that the loss of viability is accompanied by abnormal mitosis and chromosome fragmentation. Overexpression of the C terminus of Sap1 induces minichromosome loss associated with the "cut" phenotype (uncoupling mitosis and cytokinesis). These phenotypes are favored when the C terminus of Sap1 is overexpressed during DNA replication. Fluorescence in situ hybridization experiments demonstrated that the cut phenotype is related to precocious centromere separation, a typical marker for loss of cohesion. We propose that Sap1 is an architectural chromatin-associated protein, required for chromosome organization.

Sister chromatid separation at human telomeric regions

Telomeres are nucleoprotein complexes located at chromosome ends, vital for preserving chromosomal integrity. Telomeric DNA shortens with progressive rounds of cell division, culminating in replicative senescence. Previously we have reported, on the basis of fluorescent in situ hybridization, that several human telomeric regions display solitary signals (singlets) in metaphase cells of presenescent fibroblasts, in comparison to other genomic regions that hybridize as twin signals (doublets). In the current study, we show that an additional 12 out of 12 telomeric regions examined also display metaphase singlet signals in pre-senescent cells, and that excess telomere-metaphase singlets also occur in earlier passage cells harvested from elderly individuals. In cancer cell lines expressing telomerase and in pre-senescent fibroblasts ectopically expressing hTERT, this phenomenon is abrogated. Confocal microscope image analysis showed that the telomere metaphase singlets represent regions that have replicated but not separated; this is presumably because of persistent cohesion. The introduction of mutations that interfere with the normal dissolution of cohesion at the metaphase to anaphase transition induced the cut (chromosomes untimely torn) phenotype in early passage fibroblasts, with predominantly telomeric rather than centromeric DNA, present on the chromatin bridges between the daughter nuclei. These results suggest that telomeric regions in animal cells may potentially be sites of persistent cohesion, and that this cohesion may be the basis for an observed excess of fluorescent in situ hybridization metaphase singlets at telomeres. Persistent cohesion at telomeres may be associated with attempted DNA repair or chromosomal abnormalities, which have been described in pre-senescent cells.

Coordination of DNA damage responses via the Smc5/Smc6 complex

The detection of DNA damage activates DNA repair pathways and checkpoints to allow time for repair. Ultimately, these responses must be coordinated to ensure that cell cycle progression is halted until repair is completed. Several multiprotein complexes containing members of the structural maintenance of chromosomes family of proteins have been described, including the condensin and cohesin complexes, that are critical for chromosomal organization. Here we show that the Smc5/Smc6 (Smc5/6) complex is required for a coordinated response to DNA damage and normal chromosome integrity. Fission yeast cells lacking functional Smc6 initiate a normal checkpoint response to DNA damage, culminating in the phosphorylation and activation of the Chk1 protein kinase. Despite this, cells enter a lethal mitosis, presumably without completion of DNA repair. Another subunit of the complex, Nse1, is a conserved member of this complex and is also required for this response. We propose that the failure to maintain a checkpoint response stems from the lack of ongoing DNA repair or from defective chromosomal organization, which is the signal to maintain a checkpoint arrest. The Smc5/6 complex is fundamental to genome integrity and may function with the condensin and cohesin complexes in a coordinated manner.

Condensin and cohesin: more than chromosome compactor and glue

Two related protein complexes, cohesin and condensin, are essential for separating identical copies of the genome into daughter cells during cell division. Cohesin glues replicated sister chromatids together until they split at anaphase, whereas condensin reorganizes chromosomes into their highly compact mitotic structure. Unexpectedly, mutations in the subunits of these complexes have been uncovered in genetic screens that target completely different processes. Exciting new evidence is emerging that cohesin and condensin influence crucial processes during interphase, and unforeseen aspects of mitosis. Each complex can perform several roles, and individual subunits can associate with different sets of proteins to achieve diverse functions, including the regulation of gene expression, DNA repair, cell-cycle checkpoints and centromere organization.

Depletion of Drad21/Scc1 in Drosophila cells leads to instability of the cohesin complex and disruption of mitotic progression

BACKGROUND

The coordination of cell cycle events is necessary to ensure the proper duplication and dissemination of the genome. In this study, we examine the consequences of depleting Drad21 and SA, two non-SMC subunits of the cohesin complex, by dsRNA-mediated interference in Drosophila cultured cells.

RESULTS

We have shown that a bona fide cohesin complex exists in Drosophila embryos. Strikingly, the Drad21/Scc1 and SA/Scc3 non-SMC subunits associate more intimately with one another than they do with the SMCs. We have observed defects in mitotic progression in cells from which Drad21 has been depleted: cells delay in prometaphase with normally condensed, but prematurely separated, sister chromatids and with abnormal spindle morphology. Much milder defects are observed when SA is depleted from cells. The dynamics of the chromosome passenger protein, INCENP, are affected after Drad21 depletion. We have also made the surprising observation that SA is unstable in the absence of Drad21; however, we have shown that the converse is not true. Interference with Drad21 in living Drosophila embryos also has deleterious effects on mitotic progression.

CONCLUSIONS

We conclude that Drad21, as a member of a cohesin complex, is required in Drosophila cultured cells and embryos for proper mitotic progression. The protein is required in cultured cells for chromosome cohesion, spindle morphology, dynamics of a chromosome passenger protein, and stability of the cohesin complex, but apparently not for normal chromosome condensation. The observation of SA instability in the absence of Drad21 implies that the expression of cohesin subunits and assembly of the cohesin complex will be tightly regulated.

SMC protein complexes and the maintenance of chromosome integrity

Structural maintenance of chromosomes (SMC) family proteins have attracted much attention for their unique protein structure and critical roles in mitotic chromosome organization. Elegant genetic and biochemical studies in yeast and Xenopus identified two different SMC heterodimers in two conserved multiprotein complexes termed 'condensin' and 'cohesin'. These complexes are required for mitotic chromosome condensation and sister chromatid cohesion, respectively, both of which are prerequisite to accurate segregation of chromosomes. Although structurally similar, the SMC proteins in condensin and cohesin appear to have distinct functions, whose specificity and cell cycle regulation are critically determined by their interactions with unique sets of associated proteins. Recent studies of subcellular localization of SMC proteins and SMC-containing complexes, identification of their interactions with other cellular factors, and discovery of new SMC family members have uncovered unexpected roles for SMC proteins and SMC-containing complexes in different aspects of genome functions and chromosome organization beyond mitosis, all of which are critical for the maintenance of chromosome integrity.

DNA structure dependent checkpoints as regulators of DNA repair

Checkpoint proteins were initially identified because their loss of function resulted in defects in cell cycle arrest in response to genotoxic treatments. Initially, the analysis of checkpoint pathways concentrated on their function as signal transducers and how the checkpoint signals were communicated to the core cell cycle machinery and transcriptional apparatus. Although some of the early genetic analysis indicated a complex relationship between DNA replication, DNA repair and the checkpoint pathways, it is only now becoming apparent that checkpoint proteins regulate multiple DNA repair and replication functions. Furthermore, recent data suggest that some checkpoint proteins may participate directly in DNA repair events. In this review I summarise the current models for DNA structure-dependent checkpoint activation and review the evidence linking checkpoint proteins both directly and indirectly to DNA repair.

Linking sister chromatid cohesion and apoptosis: role of Rad21

Rad21 is one of the major cohesin subunits that holds sister chromatids together until anaphase, when proteolytic cleavage by separase, a caspase-like enzyme, allows chromosomal separation. We show that cleavage of human Rad21 (hRad21) also occurs during apoptosis induced by diverse stimuli. Induction of apoptosis in multiple human cell lines results in the early (4 h after insult) generation of 64- and 60-kDa carboxy-terminal hRad21 cleavage products. We biochemically mapped an apoptotic cleavage site at residue Asp-279 (D(279)) of hRad21. This apoptotic cleavage site is distinct from previously described mitotic cleavage sites. hRad21 is a nuclear protein; however, the cleaved 64-kDa carboxy-terminal product is translocated to the cytoplasm early in apoptosis before chromatin condensation and nuclear fragmentation. Overexpression of the 64-kDa cleavage product results in apoptosis in Molt4, MCF-7, and 293T cells, as determined by TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) and Annexin V staining, assaying of caspase-3 activity, and examination of nuclear morphology. Given the role of hRad21 in chromosome cohesion, the cleaved C-terminal product and its translocation to the cytoplasm may act as a nuclear signal for apoptosis. In summary, we show that cleavage of a cohesion protein and translocation of the C-terminal cleavage product to the cytoplasm are early events in the apoptotic pathway and cause amplification of the cell death signal in a positive-feedback manner.

Specific recruitment of human cohesin to laser-induced DNA damage

Cohesin is a conserved multiprotein complex that plays an essential role in sister chromatid cohesion. During interphase, cohesin is required for the establishment of cohesion following DNA replication. Because cohesin mutants resulted in increased sensitivity to DNA damage, a role for cohesin in DNA repair was also suggested. However, it was unclear whether this was due to general perturbation of cohesion or whether cohesin has a specialized role at the damage site. We therefore used a laser microbeam to create DNA damage at discrete sites in the cell nucleus and observed specific in vivo assembly of proteins at these sites by immunofluorescent detection. We observed that human cohesin is recruited to the damage site immediately after damage induction. Analysis of mutant cells revealed that cohesin recruitment to the damage site is dependent on the DNA double-strand break repair factor Mre11/Rad50 but not ATM or Nbs1. Consistently, Mre11/Rad50 and cohesin interact with each other in an interphase-specific manner. This interaction peaks in S/G(2) phase, during which cohesin is recruited to the DNA damage. Our results demonstrate the S/G(2)-specific and Mre11/Rad50-dependent recruitment of human cohesin to DNA damage, suggesting a specialized subfunction for cohesin in cell cycle-specific DNA double strand break repair.

The Coprinus cinereus adherin Rad9 functions in Mre11-dependent DNA repair, meiotic sister-chromatid cohesion, and meiotic homolog pairing

Mitotic sister-chromatid cohesion (SCC) is known to depend in part on conserved proteins called adherins, which although necessary for SCC are not themselves localized between sister chromatids. We have examined mitotic DNA-repair and meiotic chromosome behavior in the Coprinus cinereus adherin mutant rad9-1. Genetic pathway analysis established that Rad9 functions in an Mre11-dependent pathway of DNA repair. Using fluorescence in situ hybridization, we found that the rad9-1 mutant is defective in the establishment of meiotic homolog pairing at both interstitial and subtelomeric sites but in the maintenance of pairing at only interstitial loci. To determine the role of Rad9 in meiotic SCC, we hybridized nuclear spreads simultaneously with a homolog-specific probe and a probe that recognizes both members of a homologous pair. We found that Rad9 is required for wild-type levels of meiotic SCC, and that nuclei showing loss of cohesion were twice as likely also to fail at homolog pairing. To ask whether the contribution of Rad9 to homolog pairing is solely in the establishment of SCC, we examined a rad9-1;msh5-22 double mutant, in which premeiotic DNA replication is inhibited. The msh5-22 mutation partially suppressed the deleterious effects of the rad9-1 mutation on homolog pairing; however, pairing in the double mutant still was significantly lower than in the msh5-22 single mutant control. Because the role of Rad9 in homolog pairing is not obviated by the absence of a sister chromatid, we conclude that adherins have one or more early meiotic functions distinct from the establishment of cohesion.

mcl1+, the Schizosaccharomyces pombe homologue of CTF4, is important for chromosome replication, cohesion, and segregation

The fission yeast minichromosome loss mutant mcl1-1 was identified in a screen for mutants defective in chromosome segregation. Missegregation of the chromosomes in mcl1-1 mutant cells results from decreased centromeric cohesion between sister chromatids. mcl1+ encodes a beta-transducin-like protein with similarity to a family of eukaryotic proteins that includes Ctf4p from Saccharomyces cerevisiae, sepB from Aspergillus nidulans, and AND-1 from humans. The previously identified fungal members of this protein family also have chromosome segregation defects, but they primarily affect DNA metabolism. Chromosomes from mcl1-1 cells were heterogeneous in size or structure on pulsed-field electrophoresis gels and had elongated heterogeneous telomeres. mcl1-1 was lethal in combination with the DNA checkpoint mutations rad3delta and rad26delta, demonstrating that loss of Mcl1p function leads to DNA damage. mcl1-1 showed an acute sensitivity to DNA damage that affects S-phase progression. It interacts genetically with replication components and causes an S-phase delay when overexpressed. We propose that Mcl1p, like Ctf4p, has a role in regulating DNA replication complexes.

The many functions of SMC proteins in chromosome dynamics

Members of the structural maintenance of chromosomes (SMC) family share a characteristic design and configuration of protein domains that provides the molecular basis for the various functions of this family in chromosome dynamics. SMC proteins have a role in chromosome condensation, sister-chromatid cohesion, DNA repair and recombination, and gene dosage compensation, and they function in somatic and meiotic cells. As more is learned about how their unique design affects their function, a picture of a dynamic and varied protein family is emerging.

A chromatin remodelling complex that loads cohesin onto human chromosomes

Nucleosomal DNA is arranged in a higher-order structure that presents a barrier to most cellular processes involving protein DNA interactions. The cellular machinery involved in sister chromatid cohesion, the cohesin complex, also requires access to the nucleosomal DNA to perform its function in chromosome segregation. The machineries that provide this accessibility are termed chromatin remodelling factors. Here, we report the isolation of a human ISWI (SNF2h)-containing chromatin remodelling complex that encompasses components of the cohesin and NuRD complexes. We show that the hRAD21 subunit of the cohesin complex directly interacts with the ATPase subunit SNF2h. Mapping of hRAD21, SNF2h and Mi2 binding sites by chromatin immunoprecipitation experiments reveals the specific association of these three proteins with human DNA elements containing Alu sequences. We find a correlation between modification of histone tails and association of the SNF2h/cohesin complex with chromatin. Moreover, we show that the association of the cohesin complex with chromatin can be regulated by the state of DNA methylation. Finally, we present evidence pointing to a role for the ATPase activity of SNF2h in the loading of hRAD21 on chromatin.

Caspase proteolysis of the cohesin component RAD21 promotes apoptosis

Caspases are a conserved family of proteases that play a critical role in the execution of apoptosis by cleaving key cellular proteins at Asp residues and modifying their function. Using an expression cloning strategy we recently developed, we isolated human RAD21/SCC1/MCD1 as a novel caspase substrate. RAD21 is a component of the cohesin complex that holds sister chromatids together during mitosis and repairs double-strand DNA breaks. Interestingly, RAD21 is cleaved by a caspase-like Esp1/separase at the onset of anaphase to trigger sister chromatid separation. Here, we demonstrate that human RAD21 is preferentially cleaved at Asp(279) by caspases-3 and -7 in vitro to generate two major proteolytic products of approximately 65 and 48 kDa. Moreover, we show that RAD21 is specifically proteolyzed by caspases into a similarly sized 65-kDa carboxyl-terminal product in cells undergoing apoptosis in response to diverse stimuli. We also demonstrate that caspase proteolysis of RAD21 precedes apoptotic chromatin condensation and has important functional consequences, viz. the partial removal of RAD21 from chromatin and the production of a proapoptotic carboxyl-terminal cleavage product that amplifies the cell death signal. Taken together, these findings point to an entirely novel function of RAD21 in the execution of apoptosis.

The Schizosaccharomyces pombe rad60 gene is essential for repairing double-strand DNA breaks spontaneously occurring during replication and induced by DNA-damaging agents

To identify novel genes involved in DNA double-strand break (DSB) repair, we previously isolated Schizosaccharomyces pombe mutants which are hypersensitive to methyl methanesulfonate (MMS) and synthetic lethals with rad2. This study characterizes one of these mutants, rad60-1. The gene that complements the MMS sensitivity of this mutant was cloned and designated rad60. rad60 encodes a protein with 406 amino acids which has the conserved ubiquitin-2 motif found in ubiquitin family proteins. rad60-1 is hypersensitive to UV and gamma rays, epistatic to rhp51, and defective in the repair of DSBs caused by gamma-irradiation. The rad60-1 mutant is also temperature sensitive for growth. At the restrictive temperature (37 degrees C), rad60-1 cells grow for several divisions and then arrest with 2C DNA content; the arrested cells accumulate DSBs and have a diffuse and often aberrantly shaped nuclear chromosomal domain. The rad60-1 mutant is a synthetic lethal with rad18-X, and expression of wild-type rad60 from a multicopy plasmid partially suppresses the MMS sensitivity of rad18-X cells. rad18 encodes a conserved protein of the structural maintenance of chromosomes (SMC) family (A. R. Lehmann, M. Walicka, D. J. Griffiths, J. M. Murray, F. Z. Watts, S. McCready, and A. M. Carr, Mol. Cell. Biol. 15:7067-7080, 1995). These results suggest that S. pombe Rad60 is required to repair DSBs, which accumulate during replication, by recombination between sister chromatids. Rad60 may perform this function in concert with the SMC protein Rad18.

The fission yeast NIMA kinase Fin1p is required for spindle function and nuclear envelope integrity

NIMA kinases appear to be the least functionally conserved mitotic regulators, being implicated in chromosome condensation in fungi and in spindle function in metazoans. We demonstrate here that the fission yeast NIMA homologue, Fin1p, can induce profound chromosome condensation in the absence of the condensin and topoisomerase II, indicating that Fin1p-induced condensation differs from mitotic condensation. Fin1p expression is transcriptionally and post-translationally cell cycle-regulated, with Fin1p kinase activity maximal from the metaphase-anaphase transition to G(1). Fin1p is localized to the spindle pole body and fin1Delta cells are hypersensitive to anti-microtubule drugs, synthetically lethal with a number of spindle mutants and require the spindle checkpoint for viability. Moreover, fin1Delta cells show unusual and extensive elaborations of the nuclear envelope. These data support a role for Fin1p in spindle function and nuclear envelope transactions at or after the metaphase-anaphase transition that may be generally applicable to other NIMA-family members.

SMC1 is a downstream effector in the ATM/NBS1 branch of the human S-phase checkpoint

Structural maintenance of chromosomes (SMC) proteins (SMC1, SMC3) are evolutionarily conserved chromosomal proteins that are components of the cohesin complex, necessary for sister chromatid cohesion. These proteins may also function in DNA repair. Here we report that SMC1 is a component of the DNA damage response network that functions as an effector in the ATM/NBS1-dependent S-phase checkpoint pathway. SMC1 associates with BRCA1 and is phosphorylated in response to IR in an ATM- and NBS1-dependent manner. Using mass spectrometry, we established that ATM phosphorylates S957 and S966 of SMC1 in vivo. Phosphorylation of S957 and/or S966 of SMC1 is required for activation of the S-phase checkpoint in response to IR. We also discovered that the phosphorylation of NBS1 by ATM is required for the phosphorylation of SMC1, establishing the role of NBS1 as an adaptor in the ATM/NBS1/SMC1 pathway. The ATM/CHK2/CDC25A pathway is also involved in the S-phase checkpoint activation, but this pathway is intact in NBS cells. Our results indicate that the ATM/NBS1/SMC1 pathway is a separate branch of the S-phase checkpoint pathway, distinct from the ATM/CHK2/CDC25A branch. Therefore, this work establishes the ATM/NBS1/SMC1 branch, and provides a molecular basis for the S-phase checkpoint defect in NBS cells.

Condensin and cohesin display different arm conformations with characteristic hinge angles

Structural maintenance of chromosomes (SMC) proteins play central roles in higher-order chromosome dynamics from bacteria to humans. In eukaryotes, two different SMC protein complexes, condensin and cohesin, regulate chromosome condensation and sister chromatid cohesion, respectively. Each of the complexes consists of a heterodimeric pair of SMC subunits and two or three non-SMC subunits. Previous studies have shown that a bacterial SMC homodimer has a symmetrical structure in which two long coiled-coil arms are connected by a flexible hinge. A catalytic domain with DNA- and ATP-binding activities is located at the distal end of each arm. We report here the visualization of vertebrate condensin and cohesin by electron microscopy. Both complexes display the two-armed structure characteristic of SMC proteins, but their conformations are remarkably different. The hinge of condensin is closed and the coiled-coil arms are placed close together. In contrast, the hinge of cohesin is wide open and the coiled-coils are spread apart from each other. The non-SMC subunits of both condensin and cohesin form a globular complex bound to the catalytic domains of the SMC heterodimers. We propose that the "closed" conformation of condensin and the "open" conformation of cohesin are important structural properties that contribute to their specialized biochemical and physiological functions.

Fission yeast Pds5 is required for accurate chromosome segregation and for survival after DNA damage or metaphase arrest

Sister chromatid cohesion, which is established during the S phase of the eukaryotic cell cycle and persists until the onset of anaphase, is essential for the maintenance of genomic integrity. Cohesion requires the multi-protein complex cohesin, as well as a number of accessory proteins including Pds5/BIMD/Spo76. In the budding yeast Saccharomyces cerevisiae Pds5 is an essential protein that localises to chromosomes in a cohesin-dependent manner. Here we describe the characterisation in the fission yeast Schizosaccharomyces pombe of pds5(+), a novel, non-essential orthologue of S. cerevisiae PDS5. The S. pombe Pds5 protein was localised to punctate nuclear foci in a manner that was dependent on the Rad21 cohesin component. This, together with additional genetic evidence, points towards an involvement of S. pombe Pds5 in sister chromatid cohesion. S. pombe pds5 mutants were hypersensitive to DNA damage and to mitotic metaphase delay, but this sensitivity was apparently not due to precocious loss of sister chromatid cohesion. These cells also suffered increased spontaneous chromosome loss and meiotic defects and their viability was dependent on the spindle checkpoint protein Bub1. Thus, while S. pombe Pds5 has an important cohesin-related role, this differs significantly from that of the equivalent budding yeast protein.

Disseminating the genome: joining, resolving, and separating sister chromatids during mitosis and meiosis

The separation of sister chromatids at the metaphase to anaphase transition is one of the most dramatic of all cellular events and is a crucial aspect of all sexual and asexual reproduction. The molecular basis for this process has until recently remained obscure. New research has identified proteins that hold sisters together while they are aligned on the metaphase plate. It has also shed insight into the mechanisms that dissolve sister chromatid cohesion during both mitosis and meiosis. These findings promise to provide insights into defects in chromosome segregation that occur in cancer cells and into the pathological pathways by which aneuploidy arises during meiosis.

The molecular basis of sister-chromatid cohesion

The replicated copies of each chromosome, the sister chromatids, are attached prior to their segregation in mitosis and meiosis. This association or cohesion is critical for each sister chromatid to bind to microtubules from opposite spindle poles and thus segregate away from each other at anaphase of mitosis or meiosis II. The cohesin protein complex is essential for cohesion in both mitosis and meiosis, and cleavage of one of the subunits is sufficient for loss of cohesion at anaphase. The localization of the cohesin complex and other cohesion proteins permits evaluation of the positions of sister-chromatid associations within the chromosome structure, as well as the relationship between cohesion and condensation. Recently, two key riddles in the mechanism of meiotic chromosome segregation have yielded to molecular answers. First, analysis of the cohesin complex in meiosis provides molecular support for the long-standing hypothesis that sister-chromatid cohesion links homologs in meiosis I by stabilizing chiasmata. Second, the isolation of the monopolin protein that controls kinetochore behavior in meiosis I defines a functional basis by which sister kinetochores are directed toward the same pole in meiosis I.

A potential role for human cohesin in mitotic spindle aster assembly

The cohesin multiprotein complex containing SMC1, SMC3, Scc3 (SA), and Scc1 (Rad21) is required for sister chromatid cohesion in eukaryotes. Although metazoan cohesin associates with chromosomes and was shown to function in the establishment of sister chromatid cohesion during interphase, the majority of cohesin was found to be off chromosomes and reside in the cytoplasm in metaphase. Despite its dissociation from chromosomes, however, microinjection of an antibody against human SMC1 led to disorganization of the metaphase plate and cell cycle arrest, indicating that human cohesin still plays an important role in metaphase. To address the mitotic function of human cohesin, the subcellular localization of cohesin components was reexamined in human cells. Interestingly, we found that cohesin localizes to the spindle poles during mitosis and interacts with NuMA, a spindle pole-associated factor required for mitotic spindle organization. The interaction with NuMA persists during interphase. Similar to NuMA, a significant amount of cohesin was found to associate with the nuclear matrix. Furthermore, in the absence of cohesin, mitotic spindle asters failed to form in vitro. Our results raise the intriguing possibility that in addition to its well demonstrated function in sister chromatid cohesion, cohesin may be involved in spindle assembly during mitosis.

Fission yeast Rad50 stimulates sister chromatid recombination and links cohesion with repair

To study the role of Rad50 in the DNA damage response, we cloned and deleted the Schizosaccharomyces pombe RAD50 homologue. The deletion is sensitive to a range of DNA-damaging agents and shows dynamic epistatic interactions with other recombination-repair genes. We show that Rad50 is necessary for recombinational repair of the DNA lesion at the mating-type locus and that rad50Delta shows slow DNA replication. We also find that Rad50 is not required for slowing down S phase in response to hydroxy urea or methyl methanesulfonate (MMS) treatment. Interestingly, in rad50Delta cells, the recombination frequency between two homologous chromosomes is increased at the expense of sister chromatid recombination. We propose that Rad50, an SMC-like protein, promotes the use of the sister chromatid as the template for homologous recombinational repair. In support of this, we found that Rad50 functions in the same pathway for the repair of MMS-induced damage as Rad21, the homologue of the Saccharomyces cerevisiae Scc1 cohesin protein. We speculate that Rad50 interacts with the cohesin complex during S phase to assist repair and possibly re-initiation of replication after replication fork collapse.

Scc1/Rad21/Mcd1 is required for sister chromatid cohesion and kinetochore function in vertebrate cells

Proteolytic cleavage of the cohesin subunit Scc1 is a consistent feature of anaphase onset, although temporal differences exist between eukaryotes in cohesin loss from chromosome arms, as distinct from centromeres. We describe the effects of genetic deletion of Scc1 in chicken DT40 cells. Scc1 loss caused premature sister chromatid separation but did not disrupt chromosome condensation. Scc1 mutants showed defective repair of spontaneous and induced DNA damage. Scc1-deficient cells frequently failed to complete metaphase chromosome alignment and showed chromosome segregation defects, suggesting aberrant kinetochore function. Notably, the chromosome passenger INCENP did not localize normally to centromeres, while the constitutive kinetochore proteins CENP-C and CENP-H behaved normally. These results suggest a role for Scc1 in mitotic regulation, along with cohesion.

Novel meiosis-specific isoform of mammalian SMC1

Structural maintenance of chromosomes (SMC) proteins fulfill pivotal roles in chromosome dynamics. In yeast, the SMC1-SMC3 heterodimer is required for meiotic sister chromatid cohesion and DNA recombination. Little is known, however, about mammalian SMC proteins in meiotic cells. We have identified a novel SMC protein (SMC1beta), which-except for a unique, basic, DNA binding C-terminal motif-is highly homologous to SMC1 (which may now be called SMC1alpha) and is not present in the yeast genome. SMC1beta is specifically expressed in testes and coimmunoprecipitates with SMC3 from testis nuclear extracts, but not from a variety of somatic cells. This establishes for mammalian cells the concept of cell-type- and tissue-specific SMC protein isoforms. Analysis of testis sections and chromosome spreads of various stages of meiosis revealed localization of SMC1beta along the axial elements of synaptonemal complexes in prophase I. Most SMC1beta dissociates from the chromosome arms in late-pachytene-diplotene cells. However, SMC1beta, but not SMC1alpha, remains chromatin associated at the centromeres up to metaphase II. Thus, SMC1beta and not SMC1alpha is likely involved in maintaining cohesion between sister centromeres until anaphase II.

Novel DNA sequence variants in the hHR21 DNA repair gene in radiosensitive cancer patients

PURPOSE

Radiation therapy is an important treatment modality for oncology patients. DNA sequence variants have so far been identified in only a few genes in radiosensitive cancer patients. Patients known to be clinically radiosensitive were tested for mutation of a gene involved in DNA double-strand break repair and sister chromatid cohesion--hHR21.

METHODS AND MATERIALS

Clinically radiation-sensitive patients were accrued to the study after giving informed consent. Blood samples were obtained and lymphoblastoid cell lines established. Reverse transcriptase-polymerase chain reaction (RT-PCR) was performed to amplify the hHR21 gene, and the DNA product was sequenced to identify any genetic abnormalities. Northern blot analysis, cell survival, and growth assays were performed on control cells and cells with hHR21 variants, and a restriction digest assay was developed to screen for carriers of a detected gene variant.

RESULTS

The DNA sequence of the hHR21 gene was determined in 19 radiation-sensitive cancer patients. In 6 of the 19 patients, a thymidine (T) to cytosine (C) transition was detected at position 1440 of the hHR21 open reading frame (T1440C). This variant did not alter the amino acid sequence and was likely to be a polymorphism. One patient with a particularly severe radiation reaction had a second sequence variant immediately adjacent to the first. This was a guanine (G) to adenine (A) transition (G1441A), resulting in a change of the amino acid sequence (glycine --> arginine) in a portion of the protein conserved in evolution. This suggests that this DNA alteration may be biologically significant. Restriction digest with the HpaII enzyme confirmed the presence of both sequence variants on the same allele.

CONCLUSIONS

We describe the first two DNA sequence variants ever found in the hHR21 gene, in patients with clinical radiation hypersensitivity. Although no direct evidence for the involvement of hHR21 alterations in the radiosensitivity of the cancer patients examined has been demonstrated, the possibility exists that homozygous mutations or other mutations of this gene could contribute to radiosensitivity. A simple test is described that could be applied to screening for these variants in relevant populations.