Spindle Assembly Checkpoint Genes Research Articles

Research progress on spindle assembly checkpoint gene BubR1

BubR1 gene is a homologue of the mitotic checkpoint gene Mad3 in budding yeast which is highly conserved in mammalian. BubR1 protein is a key component mediating spindle assembly checkpoint activation. BubR1 safeguards accurate chromosome segregation during cell division by monitoring kinetochore-microtubule attachments and kinetochore tension. There is a dose-dependent effect between the level of BubR1 expression and the function of spindle assembly checkpoint. BubR1-deficient would lead to mitotic progression with compromised spindle assembly checkpoint because cells become progressively aneuploid. Recently, it has been reported that BubR1 also plays important roles in meiotic, DNA damage response, cancer, infertility, and early aging. This review briefly summarizes the current progresses in studies of BubR1 function.

Abstract 4186: Survival time for patients with glioblastomas is highly correlated with increased expression of major mitotic checkpoint genes

Abstract Glioblastomas are the most common and deadly CNS neoplasms. Survival time correlates with pathological grade; most World Health Organization grade III and grade IV cases die within one year of diagnosis, whereas lower grades sometimes live for years. With the goal of optimizing objective prognostic molecular staging methods for gliomas, we examined three independent microarray datasets with a total of 6 normal, 14 grade I- II, 13 grade III and 13 grade IV gliomas, using a cross-platform analysis suite (http://www.webarraydb.org). We noticed that a number of mitotic spindle assembly checkpoint (SAC) genes had transcript levels significantly altered across different pathological grades. SAC genes ensure that accurate chromosome segregation is maintained during mitosis. Previous studies have shown failure of the mitotic spindle assembly checkpoint (SAC) leads to genomic instability and neoplastic transformation. Several studies of malignant tumor suggested that increased mRNA levels of the mitotic checkpoint correlates with tumor progression, including in breast cancer and bladder cancer. Eight major mitotic spindle checkpoint genes that had significant increases in gene expression levels with increasing glioma grade in the microarray data (p < 0.001, ANOVA) were selected for quantitative reverse transcription PCR (qRT-PCR) analysis in 40 additional glioma tumors and 6 normal brain tissues; BUB1, BUB1B, BUB3, CENPE, MAD1L1, MAD2L1, CDC20, and TTK. One of these genes, CDC20, is already known to be differentially expressed across grades and inclusion allowed the relative performance of other genes to be assessed. In our study, the average mRNA transcript levels for all of these checkpoint genes increased with increasing grade. The correlations with grade had a significance level of p<0.05 (ANOVA) for BUB1, BUB1B, CENPE, MAD1L1 and CDC20. Mutations or epigenetic changes in these five genes have not yet been noted to play a significant role in the causation of chromosomal instability in glioblastomas, nor in most other cancers, an exception being BUB1mutations in some cases of colorectal cancer. The consistently high level of these particular transcripts in aggressive gliomas is perhaps due to compensation for other defects in the cell machinery that would otherwise be catastrophic. Kaplan-Meier analysis showed that BUB1B and CDC20 were independent prognostic indicators for overall survival (p<0.05). Further regression analysis indicated that changes in expression of BUB1B and CDC20, and perhaps CENPE, rival glioma grade as excellent prognosticators of survival. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4186. doi:10.1158/1538-7445.AM2011-4186

Spindle assembly checkpoint genes reveal distinct as well as overlapping expression that implicates MDF-2/Mad2 in postembryonic seam cell proliferation in Caenorhabditis elegans

BackgroundThe spindle assembly checkpoint (SAC) delays anaphase onset by inhibiting the activity of the anaphase promoting complex/cyclosome (APC/C) until all of the kinetochores have properly attached to the spindle. The importance of SAC genes for genome stability is well established; however, the roles these genes play, during postembryonic development of a multicellular organism, remain largely unexplored.ResultsWe have used GFP fusions of 5' upstream intergenic regulatory sequences to assay spatiotemporal expression patterns of eight conserved genes implicated in the spindle assembly checkpoint function in Caenorhabditis elegans. We have shown that regulatory sequences for all of the SAC genes drive ubiquitous GFP expression during early embryonic development. However, postembryonic spatial analysis revealed distinct, tissue-specific expression of SAC genes with striking co-expression in seam cells, as well as in the gut. Additionally, we show that the absence of MDF-2/Mad2 (one of the checkpoint genes) leads to aberrant number and alignment of seam cell nuclei, defects mainly attributed to abnormal postembryonic cell proliferation. Furthermore, we show that these defects are completely rescued by fzy-1(h1983)/CDC20, suggesting that regulation of the APC/CCDC20 by the SAC component MDF-2 is important for proper postembryonic cell proliferation.ConclusionOur results indicate that SAC genes display different tissue-specific expression patterns during postembryonic development in C. elegans with significant co-expression in hypodermal seam cells and gut cells, suggesting that these genes have distinct as well as overlapping roles in postembryonic development that may or may not be related to their established roles in mitosis. Furthermore, we provide evidence, by monitoring seam cell lineage, that one of the checkpoint genes is required for proper postembryonic cell proliferation. Importantly, our research provides the first evidence that postembryonic cell division is more sensitive to SAC loss, in particular MDF-2 loss, than embryonic cell division.

Arecoline arrests cells at prometaphase by deregulating mitotic spindle assembly and spindle assembly checkpoint: Implication for carcinogenesis

One apparent feature of cancerous cells is genomic instability, which may include various types of chromosomal aberrations, such as translocation, aneuploidy, and the presence of micronuclei inside the cells. Mutagenic factors that promote the emergence of genomic instability are recognized as risk factors for the development of human malignancies. In Asia, betel quid (BQ) chewing is one of such risk factors for oral cancer. Areca nut is an essential constitute of BQ and is declared as a group I carcinogen by the International Agency for Research on Cancer. However, the molecular and cellular mechanisms regarding the carcinogenicity of areca nut are not fully explored. Here we reported that arecoline, a major alkaloid of areca nut, could arrest cells at prometaphase with large amounts of misaligned chromosomes. This prometaphase arrest was evidenced by condensed chromosome pattern, increased histone H3 phosphorylation, and accumulation of mitotic proteins, including aurora A and cyclin B(1). To investigate the molecular mechanisms accounting for arecoline-induced prometaphase arrest, we found that arecoline could stabilize mitotic spindle assembly, which led to distorted organization of mitotic spindles, misalignment of chromosomes, and up-regulation of spindle assembly checkpoint (SAC) genes. The SAC proteins BubR1 and Mps1 were differentially modified between the cells treated with arecoline and nocodazole. This together with aurora A overexpression suggested that SAC might be partly suppressed by arecoline. As a result, the arecoline-exposed cells might produce progeny that contained various chromosomal aberrations and exhibited genomic instability.

Irc15 Is a Microtubule-Associated Protein that Regulates Microtubule Dynamics in Saccharomyces cerevisiae

Microtubules are polymers composed of alpha-beta tubulin heterodimers that assemble into microtubules. Microtubules are dynamic structures that have periods of both growth and shrinkage by addition and removal of subunits from the polymer. Microtubules stochastically switch between periods of growth and shrinkage, termed dynamic instability. Dynamic instability is coupled to the GTPase activity of the beta-tubulin subunit of the tubulin heterodimer. Microtubule dynamics are regulated by microtubule-associated proteins (MAPs) that interact with microtubules to regulate dynamic instability. MAPs in budding yeast have been identified that bind microtubule ends (Bim1), that stabilize microtubule structures (Stu2), that bundle microtubules by forming cross-bridges (Ase1), and that interact with microtubules at the kinetochore (Cin8, Kar3, Kip3). IRC15 was previously identified in four different genetic screens for mutants affecting chromosome transmission or repair [11-14]. Here we present evidence that Irc15 is a microtubule-associated protein, localizing to microtubules in vivo and binding to purified microtubules in vitro. Irc15 regulates microtubule dynamics in vivo and loss of IRC15 function leads to delayed mitotic progression, resulting from failure to establish tension between sister kinetochores.

SPDL-1 functions as a kinetochore receptor for MDF-1 in Caenorhabditis elegans

The spindle assembly checkpoint (SAC) ensures faithful chromosome segregation by delaying anaphase onset until all sister kinetochores are attached to bipolar spindles. An RNA interference screen for synthetic genetic interactors with a conserved SAC gene, san-1/MAD3, identified spdl-1, a Caenorhabditis elegans homologue of Spindly. SPDL-1 protein localizes to the kinetochore from prometaphase to metaphase, and this depends on KNL-1, a highly conserved kinetochore protein, and CZW-1/ZW10, a component of the ROD–ZW10–ZWILCH complex. In two-cell–stage embryos harboring abnormal monopolar spindles, SPDL-1 is required to induce the SAC-dependent mitotic delay and localizes the SAC protein MDF-1/MAD1 to the kinetochore facing away from the spindle pole. In addition, SPDL-1 coimmunoprecipitates with MDF-1/MAD1 in vivo. These results suggest that SPDL-1 functions in a kinetochore receptor of MDF-1/MAD1 to induce SAC function.

Epigenetic regulation of gene expression by Drosophila Myb and E2F2–RBF via the Myb–MuvB/dREAM complex

The Drosophila Myb oncoprotein, the E2F2 transcriptional repressor, and the RBF and Mip130/LIN-9 tumor suppressor proteins reside in a conserved Myb-MuvB (MMB)/dREAM complex. We now show that Myb is required in vivo for the expression of Polo kinase and components of the spindle assembly checkpoint (SAC). Surprisingly, the highly conserved DNA-binding domain was not essential for assembly of Myb into MMB/dREAM, for transcriptional regulation in vivo, or for rescue of Myb-null mutants to adult viability. E2F2, RBF, and Mip130/LIN-9 acted in opposition to Myb by repressing the expression of Polo and SAC genes in vivo. Remarkably, the absence of both Myb and Mip130, or of both Myb and E2F2, caused variegated expression in which high or low levels of Polo were stably inherited through successive cell divisions in imaginal wing discs. Restoration of Myb resulted in a uniformly high level of Polo expression similar to that seen in wild-type tissue, whereas restoration of Mip130 or E2F2 extinguished Polo expression. Our results demonstrate epigenetic regulation of gene expression by Myb, Mip130/LIN-9, and E2F2-RBF in vivo, and also provide an explanation for the ability of Mip130-null mutants to rescue the lethality of Myb-null mutants in vivo.

Spindle assembly checkpoint gene mdf-1 regulates germ cell proliferation in response to nutrition signals in C. elegans

When newly hatched Caenorhabditis elegans larvae are starved, their primordial germ cells (PGCs) arrest in the post-S phase. This starvation-induced PGC arrest is mediated by the DAF-18/PTEN-AKT-1/PKB nutrient-sensing pathway. Here, we report that the conserved spindle assembly checkpoint (SAC) component MDF-1/MAD1 is required for the PGC arrest. We identified 2 Akt kinase phosphorylation sites on MDF-1. Expression of a non-phosphorylatable mutant MDF-1 partially suppressed the defect in the starvation-induced PGC arrest in L1 larvae lacking DAF-18, suggesting that MDF-1 regulates germ cell proliferation as a downstream target of AKT-1, thereby demonstrating a functional link between cell-cycle regulation by the SAC components and nutrient sensing by DAF-18-AKT-1 during post-embryonic development. The phosphorylation status of MDF-1 affects its binding to another SAC component, MDF-2/MAD2. The loss of MDF-2 or another SAC component also caused inappropriate germ cell proliferation, but the defect was less severe than that caused by mdf-1 hemizygosity, suggesting that MDF-1 causes the PGC arrest by two mechanisms, one involving MDF-2 and another that is independent of other SAC components.

Kinetochore structure and spindle assembly checkpoint signaling in the budding yeast, Saccharomyces Cerevisiae

The Spindle Assembly Checkpoint (SAC) delays the onset of anaphase until every chromosome is properly bioriented at the spindle equator. Mutations in SAC genes have been found in tumors and compromised SAC function can increase the incidence of some carcinomas in mice, providing further links between cancer etiology, chromosome segregation defects and aneuploidy. Here we review recent developments in our understanding of SAC control with particular emphasis on the role of the kinetochore, the nature of the tension sensing mechanism and the possibility that the SAC encompasses more than just stabilization of securin and/or cyclin-B via inhibition of the APC/C to delay anaphase initiation. Our primary emphasis is on the SAC in the budding yeast Saccharomyces cerevisiae. However, relevant findings in other cells are also discussed to highlight the generally conserved nature of SAC signaling mechanisms.

Aneuploidy and cancer

The cell's euploid status is influenced by, amongst other mechanisms, an intact spindle assembly checkpoint (SAC), an accurate centrosome cycle, and proper cytokinesis. Studies in mammalian cells suggest that dysregulated SAC function, centrosome cycle, and cytokinesis can all contribute significantly to aneuploidy. Of interest, human cancers are frequently aneuploid and show altered expression in SAC genes. The SAC is a multi-protein complex that monitors against mis-segregation of sister chromatids. Several recent experimental mouse models have suggested a link between weakened SAC and in vivo tumorigenesis. Here, we review in brief some mechanisms which contribute to cellular aneuploidy and offer a perspective on the relationship between aneuploidy and human cancers.

A CERTain Role for Ceramide in Taxane-Induced Cell Death

An unexpected benefit of functional genomic screens is that at times they answer questions that they were not designed to ask. A siRNA screen reported by Swanton et al. in this issue of Cancer Cell reveals that silencing of spindle assembly checkpoint genes facilitates mitotic slippage, resulting in escape from taxane-induced cell death, aneuploidy, and chromosomal instability, hallmarks of taxane resistance. Unexpectedly, the screen disclosed that the sphingolipid ceramide is a key regulator of the taxane-mediated spindle assembly checkpoint and taxane-induced cell death. Ceramide metabolism thus serves as a legitimate target for modulation of taxane effect on tumors.

Novelmad2Alleles Isolated in aSchizosaccharomyces pombeγ-Tubulin Mutant Are Defective in Metaphase Arrest Activity, but Remain Functional for Chromosome Stability in Unperturbed Mitosis

A previously isolated fission yeast gamma-tubulin mutant containing apparently stabilized microtubules proliferated at an approximately identical rate as wild type, yet the mutant mitosis spindle dynamics were aberrant, particularly the kinetochore microtubule dynamics. Progression through mitosis in the mutant, however, resulted in mostly accurate chromosome segregation. In the absence of the spindle assembly checkpoint gene, mad2+, the spindle dynamics in the gamma-tubulin mutant were greatly compromised, leading to a high incidence of chromosome missegregation. Unlike in wild-type cells, green fluorescent protein (GFP)-tagged Mad2 protein often accumulated near one of the poles of an elongating spindle in the gamma-tubulin mutant. We isolated novel mad2 mutants that were defective in arresting mitotic progression upon gross perturbation of the spindle formation but remained functional for the viability of the gamma-tubulin mutant. Further, the mad2 mutations did not appreciably destabilize minichromosomes in unperturbed mitoses. When overexpressed ectopically, these mutant Mad2 proteins sequestered wild-type Mad2, preventing its function in mitotic checkpoint arrest, but not in minichromosome stability. These results indicated that the Mad2 functions required for checkpoint arrest and chromosome stability in unperturbed mitosis are genetically discernible. Immunoprecipitation studies demonstrated that GFP-fused mutant Mad2 proteins formed a Mad1-containing complex with altered stability compared to that formed with wild-type Mad2, providing clues to the novel mad2 mutant phenotype.

Components of the Spindle Assembly Checkpoint Regulate the Anaphase-Promoting Complex During Meiosis inCaenorhabditis elegans

Temperature-sensitive mutations in subunits of the Caenorhabditis elegans anaphase-promoting complex (APC) arrest at metaphase of meiosis I at the restrictive temperature. Embryos depleted of the APC co-activator FZY-1 by RNAi also arrest at this stage. To identify regulators and potential substrates of the APC, we performed a genetic suppressor screen with a weak allele of the APC subunit MAT-3/CDC23/APC8, whose defects are specific to meiosis. Twenty-seven suppressors that resulted in embryonic viability and larval development at the restrictive temperature were isolated. We have identified the molecular lesions in 18 of these suppressors, which correspond to five genes. In addition to a single intragenic suppressor, we found mutations in the APC co-activator fzy-1 and in three spindle assembly checkpoint genes, mdf-1, mdf-2, and mdf-3/san-1, orthologs of Mad1, Mad2, and Mad3, respectively. Reduction-of-function alleles of mdf-2 and mdf-3 suppress APC mutants and exhibit pleiotropic phenotypes in an otherwise wild-type background. Analysis of a single separation-of-function allele of mdf-1 suggests that MDF-1 has a dual role during development. These studies provide evidence that components of the spindle assembly checkpoint may regulate the metaphase-to-anaphase transition in the absence of spindle damage during C. elegans meiosis.

Mad2 and spindle assembly checkpoint function during meiosis I in mammalian oocytes.

During mammalian mitosis, a proofreading network called the spindle assembly checkpoint (SAC) is indispensable for ensuring the fidelity of chromosome segregation. An inhibitory SAC signal is deputed to inhibits mitotic cell-cycle progression in response to misaligned chromosomes until such imperfections are rectified thereby ensuring equitable chromosome partitioning to daughter cells. Amongst the cast of SAC proteins, mitotic arrest deficient 2 (Mad2) plays a leading role in transducing the SAC signal. The aneuploidy and cancer predispositions of individuals who harbour genetic mutations in SAC genes emphasise the in vivo significance of this surveillance mechanism. In humans, congenital aneuploidies such as Down's syndrome demonstrate an exponential increase with advancing female age. Although largely the result of female meiosis I errors, the molecular entities that succumb with age in oocytes remain elusive. Declining oocyte SAC function could plausibly contribute to such errors. Until recently however, convincing evidence for a functional SAC in mammalian oocytes during meiosis I was unforthcoming. Here I review the evidence regarding the SAC in female mammalian meiosis I and how our understanding of this system has evolved in recent years. This review will focus on Mad2 as this is the SAC protein that has been most comprehensively investigated.

Diverse Functions of Spindle Assembly Checkpoint Genes in Saccharomyces cerevisiae

The spindle assembly checkpoint regulates the metaphase-to-anaphase transition from yeast to humans. We examined the genetic interactions with four spindle assembly checkpoint genes to identify nonessential genes involved in chromosome segregation, to identify the individual roles of the spindle assembly checkpoint genes within the checkpoint, and to reveal potential complexity that may exist. We used synthetic genetic array (SGA) analysis using spindle assembly checkpoint mutants mad1, mad2, mad3, and bub3. We found 228 synthetic interactions with the four spindle assembly checkpoint mutants with substantial overlap in the spectrum of interactions between mad1, mad2, and bub3. In contrast, there were many synthetic interactions that were common to mad1, mad2, and bub3 that were not shared by mad3. We found shared interactions between pairs of spindle assembly checkpoint mutants, suggesting additional complexity within the checkpoint and unique interactions for all of the spindle assembly checkpoint genes. We show that most genes in the interaction network, including ones with unique interactions, affect chromosome transmission or microtubule function, suggesting that the complexity of interactions reflects diverse roles for the checkpoint genes within the checkpoint. Our analysis expands our understanding of the spindle assembly checkpoint and identifies new candidate genes with possible roles in chromosome transmission and mitotic spindle function.

The A78V Mutation in the Mad3-like Domain ofSchizosaccharomyces pombeBub1p Perturbs Nuclear Accumulation and Kinetochore Targeting of Bub1p, Bub3p, and Mad3p and Spindle Assembly Checkpoint Function

During mitosis, the spindle assembly checkpoint (SAC) responds to faulty attachments between kinetochores and the mitotic spindle by imposing a metaphase arrest until the defect is corrected, thereby preventing chromosome missegregation. A genetic screen to isolate SAC mutants in fission yeast yielded point mutations in three fission yeast SAC genes: mad1, bub3, and bub1. The bub1-A78V mutant is of particular interest because it produces a wild-type amount of protein that is mutated in the conserved but uncharacterized Mad3-like region of Bub1p. Characterization of mutant cells demonstrates that the alanine at position 78 in the Mad3-like domain of Bub1p is required for: 1) cell cycle arrest induced by SAC activation; 2) kinetochore accumulation of Bub1p in checkpoint-activated cells; 3) recruitment of Bub3p and Mad3p, but not Mad1p, to kinetochores in checkpoint-activated cells; and 4) nuclear accumulation of Bub1p, Bub3p, and Mad3p, but not Mad1p, in cycling cells. Increased targeting of Bub1p-A78V to the nucleus by an exogenous nuclear localization signal does not significantly increase kinetochore localization or SAC function, but GFP fused to the isolated Bub1p Mad 3-like accumulates in the nucleus. These data indicate that Bub1p-A78V is defective in both nuclear accumulation and kinetochore targeting and that a threshold level of nuclear Bub1p is necessary for the nuclear accumulation of Bub3p and Mad3p.

HBUB1 defects in leukemia and lymphoma cells.

Tumorigenesis is a multi-step process involving a series of changes of cellular genes. Most solid tumors and hematopoietic malignancies often show abnormal chromosome numbers, the aneuploidy. The chromosomal aneuploidy keeps cells in the state of chromosomal instability (CIN) that will increase the mutation rate of cells affected and thus push them deeper into the process of tumorigenesis. The yeast genetic studies showed that normal distribution of chromosome during mitosis is under the surveillance of a set of genes, the spindle assembly checkpoint genes, that include the BUB and MAD gene groups and MPS. In some colorectal cancers with CIN it was found to have hBUB1 gene mutated and the mutated gene functions dominantly. We have examined a series of breast cancer cell lines with or without CIN for the hBUB1 gene mutation and found none. However, we detected various degrees of deletion in the coding sequences of the hBUB1 gene in cells from T lymphoblastic leukemia cell lines, Molt3 and Molt4, and cells from some acute lymphoblastic leukemia and Hodgkin's lymphoma patients. So far the lesions of deletion are in the kinetochore localization domain of the hBUB1 gene that may explain why the deletion lesions in the BUB1 gene cause aneuploidy in lymphoma and lymphoma cells. The deletions are heterozygous in nature. Like the mutated hBUB1 gene in colorectal cancer, the mutant hBUB1 cDNA from lymphoblastic leukemia cells behaves dominantly.

Kinesins klp5(+) and klp6(+) are required for normal chromosome movement in mitosis.

Proper mitotic chromosome segregation requires dynamic interactions between spindle microtubules and kinetochores. Here we demonstrate that two related fission yeast kinesins, klp5(+) and klp6(+), are required for normal chromosome segregation in mitosis. Null mutants frequently lack a normal metaphase chromosome alignment. Chromosome pairs move back and forth along the spindle for an extended period prior to sister chromatid separation, a phenotype reminiscent of the loss of CENP-E in metazoans. Ultimately, sister chromatids segregate, regardless of chromosome position along the spindle, and viable daughter cells are usually produced. The initiation of anaphase B is sometimes delayed, but the rate of spindle elongation is similar to wildtype. Despite a delay, anaphase B often begins before anaphase A is completed. The klp5Delta and klp6Delta null mutants are synthetically lethal with a deletion of the spindle assembly checkpoint gene, bub1(+), several mutants in components of the anaphase promoting complex, and a cold sensitive allele of the kinetochore and microtubule-binding protein, Dis1p. Klp5p-GFP and Klp6p-GFP localize to kinetochores from prophase to the onset of anaphase A, but relocalize to the spindle midzone during anaphase B. These data indicate that Klp5p and Klp6p are kinetochore kinesins required for normal chromosome movement in prometaphase.

Association Between Spindle Assembly Checkpoint Gene Expression and Maternal Age in Human Oocytes

Objective: Maternal aging remains the overwhelming factor in the aetiology of human aneuploidy in assisted reproduction. Defects in cell cycle checkpoint genes may play a role in its development. The spindle assembly checkpoint modulates the timing of anaphase initiation in response to the improper alignment of chromosomes at metaphase. MAD2 and BUB1 genes encode conserved kinetochore-associated proteins believed to be major components of this regulatory pathway. A failure in this surveillance system could lead to genomic instability, which may underlie the increased incidence of aneuploidy in oocytes of older women.Design: The concentrations of MAD2 and BUB1 transcripts in human oocytes at GV, MI and MII stages of maturation were determined by real-time rapid cycle fluorescent RT-PCR. The incidence of aneuploidy was established by multi-probe FISH analysis.Materials and Methods: Nonviable human mature and immature oocytes (n=68) were obtained from patients undergoing IVF. Rapid fluorescence monitored cycling was used to examine expression levels of the checkpoint and housekeeping genes in individual human oocytes. Unknown concentrations were extrapolated from standards co-amplified producing a standard curve. Polar body biopsy, fixation and multi-probe FISH analysis was performed to determine chromosomal status of MII oocytes.Results: Regression analysis confirmed that a significant linear correlation exists between maternal age and MAD2 transcript concentration for GV, MI and MII stages of maturation. Likewise, a similar trend emerged when the BUB1 transcript copy numbers were plotted versus maternal age but only for GV and MI stages. No association was apparent between maternal age and the concentration of β-actin mRNA. Experiments are in progress comparing the incidence of aneuploidy and checkpoint transcript concentrations. However, preliminary results suggest that an association may exist between aneuploidy status and MAD2 transcript concentrations.Conclusions: The observations reported here suggest that the MAD2 and BUB1 transcripts may degrade as the oocyte ages. Since the concentration of housekeeping transcripts remained constant throughout each stage of maturation examined, it appears that this phenomenon is not universal of all transcripts in oocytes from older women. Potentially, the degradation of these messages may impair spindle checkpoint function in older oocytes and be a contributing factor in age-related aneuploidy. Understanding the checkpoint defects associated with chromosomal abnormalities may have profound therapeutic consequences for older women undergoing assisted reproduction. Objective: Maternal aging remains the overwhelming factor in the aetiology of human aneuploidy in assisted reproduction. Defects in cell cycle checkpoint genes may play a role in its development. The spindle assembly checkpoint modulates the timing of anaphase initiation in response to the improper alignment of chromosomes at metaphase. MAD2 and BUB1 genes encode conserved kinetochore-associated proteins believed to be major components of this regulatory pathway. A failure in this surveillance system could lead to genomic instability, which may underlie the increased incidence of aneuploidy in oocytes of older women. Design: The concentrations of MAD2 and BUB1 transcripts in human oocytes at GV, MI and MII stages of maturation were determined by real-time rapid cycle fluorescent RT-PCR. The incidence of aneuploidy was established by multi-probe FISH analysis. Materials and Methods: Nonviable human mature and immature oocytes (n=68) were obtained from patients undergoing IVF. Rapid fluorescence monitored cycling was used to examine expression levels of the checkpoint and housekeeping genes in individual human oocytes. Unknown concentrations were extrapolated from standards co-amplified producing a standard curve. Polar body biopsy, fixation and multi-probe FISH analysis was performed to determine chromosomal status of MII oocytes. Results: Regression analysis confirmed that a significant linear correlation exists between maternal age and MAD2 transcript concentration for GV, MI and MII stages of maturation. Likewise, a similar trend emerged when the BUB1 transcript copy numbers were plotted versus maternal age but only for GV and MI stages. No association was apparent between maternal age and the concentration of β-actin mRNA. Experiments are in progress comparing the incidence of aneuploidy and checkpoint transcript concentrations. However, preliminary results suggest that an association may exist between aneuploidy status and MAD2 transcript concentrations. Conclusions: The observations reported here suggest that the MAD2 and BUB1 transcripts may degrade as the oocyte ages. Since the concentration of housekeeping transcripts remained constant throughout each stage of maturation examined, it appears that this phenomenon is not universal of all transcripts in oocytes from older women. Potentially, the degradation of these messages may impair spindle checkpoint function in older oocytes and be a contributing factor in age-related aneuploidy. Understanding the checkpoint defects associated with chromosomal abnormalities may have profound therapeutic consequences for older women undergoing assisted reproduction.

The S. pombe zfs1 gene is required to prevent septation if mitotic progression is inhibited.

Schizosaccharomyces pombe cdc16p is required to limit the cell to forming a single division septum per cell cycle; the heat-sensitive loss-of-function mutant cdc16-116 completes mitosis, and then undergoes multiple rounds of septum formation without cell cleavage. cdc16p is a homologue of Saccharomyces cerevisiae BUB2p, and has also been implicated in the spindle assembly checkpoint function in S. pombe. To identify other proteins involved in regulating septum formation, we have screened for multicopy suppressors of the cdc16-116 mutation. In this paper, we describe one of these suppressors, zfs1. The null allele (zfs1-D1) is viable. However, at low temperatures it divides at a reduced size, while at higher temperatures, it partially suppresses heat sensitive mutants in genes signalling the onset of septum formation. Zfs1-D1 cells show an increased rate of chromosome loss during exponential growth. Moreover, if assembly of the spindle is prevented, zfs1-D1 cells do not arrest normally, but the activity of cdc2p kinase decays, and cells form a division septum without completing a normal mitosis. We conclude that zfs1 function is required to prevent septum formation and exit from mitosis if the mitotic spindle is not assembled. The suppression of cdc16-116 by zfs1 is independent of dma1 function and the spindle assembly checkpoint genes mad2 and mph1. The genetic interactions of zfs1 with genes regulating septum formation suggest that it may be a modulator of the signal transduction network controlling the onset of septum formation and exit from mitosis.