Partial downregulation of MAD1 causes spindle checkpoint inactivation and aneuploidy, but does not confer resistance towards taxol

Maternal genetic polymorphisms in the major mitotic checkpoint genes MAD1L1 and MAD2L1 associated with the risk of survival in abnormal chromosomal fetuses

Background: The genetic etiology of fetal chromosome abnormalities remains unknown, which brings about an enormous burden for patients, families, and society. The spindle assembly checkpoint (SAC) controls the normal procedure of chromosome disjunction and may take part in the process.

Objective: The aim of this study was to explore the association between polymorphisms of MAD1L1 rs1801368 and MAD2L1 rs1283639804, involved in SAC and fetal chromosome abnormalities.

Methods: The case–control study collected 563 cases and 813 health controls to test the genotypes of MAD1L1 rs1801368 and MAD2L1 rs1283639804 polymorphisms by polymerase chain reaction–restrictive fragment length polymorphism methods (PCR-RFLP).

Results:MAD1L1 rs1801368 polymorphism was associated with fetal chromosome abnormalities alone or combined to lower homocysteine (HCY) levels (alone: dominant: OR: 1.75, 95%CI: 1.19–2.57, and p = 0.005; CT vs. CC: OR = 0.73, 95%CI: 0.57–0.94, and p = 0.016; lower HCY: C vs. T: OR = 0.74, 95%CI: 0.57–0.95, and p = 0.02; dominant: OR = 1.75, 95%CI: 0.79–1.92, and p = 0.005). No significant differences were found in other genetic models or subgroups (p > 0.05, respectively). MAD2L1 rs1283639804 polymorphism revealed a sole genotype in the studied population. HCY is significantly associated with fetal chromosome abnormalities in younger groups (OR: 1.78, 95%CI: 1.28–2.47, and p = 0.001).

Conclusion: The results implied that the polymorphism of MAD1L1 rs1801368 may become the susceptibility factor to fetal chromosome abnormalities alone or combined to lower HCY levels but not to MAD2L1 rs1283639804 polymorphism. In addition, HCY significantly affects fetal chromosomal abnormalities in younger women.

Analysis of matched primary and recurrent BRCA1/2 mutation-associated tumors identifies recurrence-specific drivers

Recurrence is a major cause of death among BRCA1/2 mutation carriers with breast (BrCa) and ovarian cancers (OvCa). Herein we perform multi-omic sequencing on 67 paired primary and recurrent BrCa and OvCa from 27 BRCA1/2 mutation carriers to identify potential recurrence-specific drivers. PARP1 amplifications are identified in recurrences (False Discovery Rate q = 0.05), and PARP1 is significantly overexpressed across primary BrCa and recurrent BrCa and OvCa, independent of amplification status. RNA sequencing analysis finds two BRCA2 isoforms, BRCA2-201/Long and BRCA2-001/Short, respectively predicted to be sensitive and insensitive to nonsense-mediated decay. BRCA2-001/Short is expressed more frequently in recurrences and associated with reduced overall survival in breast cancer (87 vs. 121 months; Hazard Ratio = 2.5 [1.18-5.5]). Loss of heterozygosity (LOH) status is discordant in 25% of patient's primary and recurrent tumors, with switching between both LOH and lack of LOH found. Our study reveals multiple potential drivers of recurrent disease in BRCA1/2 mutation-associated cancer, improving our understanding of tumor evolution and suggesting potential biomarkers.

Spindle assembly checkpoint competence in aneuploid canine malignant melanoma cell lines

The spindle assembly checkpoint (SAC) is a surveillance mechanism that prevents unequal segregation of chromosomes during mitosis. Abnormalities in the SAC are associated with chromosome instability and resultant aneuploidy. This study was performed to evaluate the SAC competence in canine malignant melanoma (CMM) using four aneuploid cell lines (CMeC1, CMeC2, KMeC, and LMeC). After treatment with nocodazole, a microtubule disrupting agent, CMeC1, KMeC, and LMeC cells were arrested in M phase, whereas CMeC2 cells were not arrested, and progressed into the next cell cycle phase without cytokinesis. Chromosome spread analysis revealed a significantly increased rate of premature sister chromatid separation in CMeC2 cells. Expression of the phosphorylated form of the SAC regulator, monopolar spindle 1 (Mps1), was lower in CMeC2 cells than in the other CMM cell lines. These results indicate that the SAC is defective in CMeC2 cells, which may partially explain aneuploidy in CMM. Thus, CMeC2 cells may be useful for further studies of the SAC mechanism in CMM and in determining the relationship between SAC incompetence and aneuploidy.

Genetics and Expression Profile of the Tubulin Gene Superfamily in Breast Cancer Subtypes and Its Relation to Taxane Resistance

Taxanes are a class of chemotherapeutic agents that inhibit cell division by disrupting the mitotic spindle through the stabilization of microtubules. Most breast cancer (BC) tumors show resistance against taxanes partially due to alterations in tubulin genes. In this project we investigated tubulin isoforms in BC to explore any correlation between tubulin alterations and taxane resistance. Genetic alteration and expression profiling of 28 tubulin isoforms in 6714 BC tumor samples from 4205 BC cases were analyzed. Protein-protein, drug-protein and alterations neighbor genes in tubulin pathways were examined in the tumor samples. To study correlation between promoter activity and expression of the tubulin isoforms in BC, we analyzed the ChIP-seq enrichment of active promoter histone mark H3K4me3 and mRNA expression profile of MCF-7, ZR-75-30, SKBR-3 and MDA-MB-231 cell lines. Potential correlation between tubulin alterations and taxane resistance, were investigated by studying the expression profile of taxane-sensitive and resistant BC tumors also the MDA-MB-231 cells acquired resistance to paclitaxel. All genomic data were obtained from public databases. Results showed that TUBD1 and TUBB3 were the most frequently amplified and deleted tubulin genes in the BC tumors respectively. The interaction analysis showed physical interactions of α-, β- and γ-tubulin isoforms with each other. The most of FDA-approved tubulin inhibitor drugs including taxanes target only β-tubulins. The analysis also revealed sex tubulin-interacting neighbor proteins including ENCCT3, NEK2, PFDN2, PTP4A3, SDCCAG8 and TBCE which were altered in at least 20% of the tumors. Three of them are tubulin-specific chaperons responsible for tubulin protein folding. Expression of tubulin genes in BC cell lines were correlated with H3K4me3 enrichment on their promoter chromatin. Analyzing expression profile of BC tumors and tumor-adjacent normal breast tissues showed upregulation of TUBA1A, TUBA1C, TUBB and TUBB3 and downregulation of TUBB2A, TUBB2B, TUBB6, TUBB7P pseudogene, and TUBGCP2 in the tumor tissues compared to the normal breast tissues. Analyzing taxane-sensitive versus taxane-resistant tumors revealed that expression of TUBB3 and TUBB6 was significantly downregulated in the taxane-resistant tumors. Our results suggest that downregulation of tumor βIII- and βV-tubulins is correlated with taxane resistance in BC. Based on our results, we conclude that aberrant protein folding of tubulins due to mutation and/or dysfunction of tubulin-specific chaperons may be potential mechanisms of taxane resistance. Thus, we propose studying the molecular pathology of tubulin mutations and folding in BC and their impacts on taxane resistance.

Genetics and Expression Profile of the Tubulin Gene Superfamily in Breast Cancer Subtypes and Its Relation to Taxane Resistance

Taxanes are a class of chemotherapeutic agents that inhibit cell division by disrupting the mitotic spindle through the stabilization of microtubules. Most breast cancer (BC) tumors show resistance against taxanes partially due to alterations in tubulin genes. In this project we investigated tubulin isoforms in BC to explore any correlation between tubulin alterations and taxane resistance. Genetic alteration and expression profiling of 28 tubulin isoforms in 6714 BC tumor samples from 4205 BC cases were analyzed. Protein-protein, drug-protein and alterations neighbor genes in tubulin pathways were examined in the tumor samples. To study correlation between promoter activity and expression of the tubulin isoforms in BC, we analyzed the ChIP-seq enrichment of active promoter histone mark H3K4me3 and mRNA expression profile of MCF-7, ZR-75-30, SKBR-3 and MDA-MB-231 cell lines. Potential correlation between tubulin alterations and taxane resistance, were investigated by studying the expression profile of taxane-sensitive and resistant BC tumors also the MDA-MB-231 cells acquired resistance to paclitaxel. All genomic data were obtained from public databases. Results showed that TUBD1 and TUBB3 were the most frequently amplified and deleted tubulin genes in the BC tumors respectively. The interaction analysis showed physical interactions of α-, β- and γ-tubulin isoforms with each other. The most of FDA-approved tubulin inhibitor drugs including taxanes target only β-tubulins. The analysis also revealed sex tubulin-interacting neighbor proteins including ENCCT3, NEK2, PFDN2, PTP4A3, SDCCAG8 and TBCE which were altered in at least 20% of the tumors. Three of them are tubulin-specific chaperons responsible for tubulin protein folding. Expression of tubulin genes in BC cell lines were correlated with H3K4me3 enrichment on their promoter chromatin. Analyzing expression profile of BC tumors and tumor-adjacent normal breast tissues showed upregulation of TUBA1A, TUBA1C, TUBB and TUBB3 and downregulation of TUBB2A, TUBB2B, TUBB6, TUBB7P pseudogene, and TUBGCP2 in the tumor tissues compared to the normal breast tissues. Analyzing taxane-sensitive versus taxane-resistant tumors revealed that expression of TUBB3 and TUBB6 was significantly downregulated in the taxane-resistant tumors. Our results suggest that downregulation of tumor βIII- and βV-tubulins is correlated with taxane resistance in BC. Based on our results, we conclude that aberrant protein folding of tubulins due to mutation and/or dysfunction of tubulin-specific chaperons may be potential mechanisms of taxane resistance. Thus, we propose studying the molecular pathology of tubulin mutations and folding in BC and their impacts on taxane resistance.

Novel Mad2-targeting miR-493-3p controls mitotic fidelity and cancer cells' sensitivity to paclitaxel

The molecular pathways that contribute to the proliferation and drug response of cancer cells are highly complex and currently insufficiently characterized. We have identified a previously unknown microRNA-based mechanism that provides cancer cells means to stimulate tumorigenesis via increased genomic instability and, at the same time, evade the action of clinically utilized microtubule drugs. We demonstrate miR-493-3p to be a novel negative regulator of mitotic arrest deficient-2 (MAD2), an essential component of the spindle assembly checkpoint that monitors the fidelity of chromosome segregation. The microRNA targets the 3′ UTR of Mad2 mRNA thereby preventing translation of the Mad2 protein. In cancer cells, overexpression of miR-493-3p induced a premature mitotic exit that led to increased frequency of aneuploidy and cellular senescence in the progeny cells. Importantly, excess of the miR-493-3p conferred resistance of cancer cells to microtubule drugs. In human neoplasms, miR-493-3p and Mad2 expression alterations correlated with advanced ovarian cancer forms and high miR-493-3p levels were associated with reduced survival of ovarian and breast cancer patients with aggressive tumors, especially in the paclitaxel therapy arm. Our results suggest that intratumoral profiling of miR-493-3p and Mad2 levels can have diagnostic value in predicting the efficacy of taxane chemotherapy.

Targeting mitotic pathways for endocrine-related cancer therapeutics

A colossal amount of basic research over the past few decades has provided unprecedented insights into the highly complex process of cell division. There is an ever-expanding catalog of proteins that orchestrate, participate and coordinate in the exquisite processes of spindle formation, chromosome dynamics and the formation and regulation of kinetochore microtubule attachments. Use of classical microtubule poisons has still been widely and often successfully used to combat a variety of cancers, but their non-selective interference in other crucial physiologic processes necessitate the identification of novel druggable components specific to the cell cycle/division pathway. Considering cell cycle deregulation, unscheduled proliferation, genomic instability and chromosomal instability as a hallmark of tumor cells, there lies an enormous untapped terrain that needs to be unearthed before a drug can pave its way from bench to bedside. This review attempts to systematically summarize the advances made in this context so far with an emphasis on endocrine-related cancers and the avenues for future progress to target mitotic mechanisms in an effort to combat these dreadful cancers.

An Integrative Drug Repurposing Pipeline: Switching Viral Drugs to Breast Cancer

The growing incidence rate of breast cancer, coupled with cellular chemotherapeutic resistance, has made this disease one of the most prevalent cancers among women worldwide. Despite the recent efforts to understand the underlying cause of the resistance due to mutation, there are no feasible tactics to overcome this bottleneck. This issue could be addressed by the concept of polypharmacology-disguising drugs present in the pharmacopeia for novel purposes (drug repurposing). Of note, we have proposed a multi-modal computational drug-repositioning stratagem to predict drugs possessing anti-proliferative effect. Our results suggest that Ombitasvir, a Hepatitis C NS5B polymerase inhibitor, could be "repurposed" for the control and prevention of beta-tubulin-driven breast cancers. J. Cell. Biochem. 118: 1412-1422, 2017. © 2016 Wiley Periodicals, Inc.

Mitotic arrest deficient-like 1 is correlated with poor prognosis in small-cell lung cancer after surgical resection

Mitotic arrest deficient-like 1 (MAD1L1) whose dysfunction is associated with chromosomal instability plays a pathogenic role in a few human cancers. However, the status of MAD1L1 expression in small-cell lung cancer (SCLC) remains unknown. Immunohistochemistry was used to determine the expression of MAD1L1 protein in 32 lymph node metastasis (LN-M) tissues and 88 primary SCLCs compared with 32 adjacent noncancerous tissues. The associations of MAD1L1 protein expression with the clinicopathologic features and clinical outcomes in patients with SCLC were analyzed. The ratio of MAD1L1 positive expression was higher in primary SCLC tissues (39.8 %) and LN-M tissues (46.9 %) compared with adjacent noncancerous tissues (9.4 %). MAD1L1 positive expression was associated with tumor-node-metastasis (TNM) stage (P = 0.003), International Association for the Study of Lung Cancer (IASLC) stage (P = 0.004), tumor size (P = 0.015), lymph node metastasis (P = 0.014), and recurrence (P < 0.001). Multivariate analysis suggested that MAD1L1 positive expression was an independent factor for overall survival (hazard ratio (HR) 2.002; 95 % confidence interval (CI) 1.065-3.763; P = 0.031) and recurrence-free survival (HR 2.263; 95 % CI 1.197-4.276; P = 0.012). To sum up, MAD1L1 positive expression may be associated with tumour progression and metastasis in SCLCs and may thus serve as a new biomarker for prognosis in these patients.

Cohesion and the aneuploid phenotype in Alzheimer's disease: A tale of genome instability

Neurons are postmitotic cells that are in permanent cell cycle arrest. However, components of the cell cycle machinery that are expressed in Alzheimer's disease (AD) neurons are showing features of a cycling cell and those attributed to a postmitotic cell as well. Furthermore, the unique physiological operations taking place in neurons, ascribed to "core cell cycle regulators" are also key regulators in cell division. Functions of these cell cycle regulators include neuronal migration, axonal elongation, axon pruning, dendrite morphogenesis and synaptic maturation and plasticity. In this review, we focus on cohesion and cohesion related proteins in reference to their neuronal functions and how impaired centromere/cohesion dynamics may connect cell cycle dysfunction to aneuploidy in AD.

The human papillomavirus18 E7 protein inhibits CENP-C binding to α-satellite DNA

Human papillomavirus (HPV) infection leads to aneuploidy, a numerical chromosomal aberration that is caused by dysregulation of chromosomal segregation. We previously found that the E7 proteins of high-risk HPVs, but not of low-risk HPVs, could bind to centromere protein-C (CENP-C). In this study, we first found that CENP-C could bind centromere α-satellite DNAs using ChIP analysis and HA-tagged CENP-C/nuc transfected 293T cells. We then investigated if HA-CENP-C/nuc binding to α-satellite DNAs was affected by the E7 proteins of high- or low-risk HPVs. We found that transfection of the FLAG tagged HPV18 E7 inhibited the binding of HA-CENP-C/nuc to α-satellite DNAs. This finding was confirmed in HeLa S3 cells transfected with siRNA targeted to HPV18 E7 expression. We therefore speculate that altered function of kinetochores as a result of inhibition of CENP-C and α-satellite DNAs binding may be associated with the chromosomal abnormalities observed in HPV18-positive cancers.

The ARF tumor suppressor prevents chromosomal instability and ensures mitotic checkpoint fidelity through regulation of Aurora B

The ARF tumor suppressor is best known for its role in stabilizing p53. This study identifies p53-independent functions of ARF in chromosome segregation and the mitotic checkpoint. Mitotic defects caused by loss of ARF are recapitulated by Aurora B overexpression and rescued by partial depletion of Aurora B.

The ARF tumor suppressor is part of the CDKN2A locus and is mutated or undetectable in numerous cancers. The best-characterized role for ARF is in stabilizing p53 in response to cellular stress. However, ARF has tumor suppressive functions outside this pathway that have not been fully defined. Primary mouse embryonic fibroblasts (MEFs) lacking the ARF tumor suppressor contain abnormal numbers of chromosomes. However, no role for ARF in cell division has previously been proposed. Here we demonstrate a novel, p53-independent role for ARF in the mitotic checkpoint. Consistent with this, loss of ARF results in aneuploidy in vitro and in vivo. ARF^−/− MEFs exhibit mitotic defects including misaligned and lagging chromosomes, multipolar spindles, and increased tetraploidy. ARF^−/− cells exhibit overexpression of Mad2, BubR1, and Aurora B, but only overexpression of Aurora B phenocopies mitotic defects observed in ARF^−/− MEFs. Restoring Aurora B to near-normal levels rescues mitotic phenotypes in cells lacking ARF. Our results define an unexpected role for ARF in chromosome segregation and mitotic checkpoint function. They further establish maintenance of chromosomal stability as one of the additional tumor-suppressive functions of ARF and offer a molecular explanation for the common up-regulation of Aurora B in human cancers.

Epigenetic silencing of DACH1 induces the invasion and metastasis of gastric cancer by activating TGF-β signalling

Gastric cancer (GC) is the fourth most common malignancy in males and the fifth most common malignancy in females worldwide. DACH1 is frequently methylated in hepatic and colorectal cancer. To further understand the regulation and mechanism of DACH1 in GC, eight GC cell lines, eight cases of normal gastric mucosa, 98 cases of primary GC and 50 cases of adjacent non-tumour tissues were examined. Methylation-specific PCR, western blot, transwell assay and xenograft mice were used in this study. Loss of DACH1 expression correlated with promoter region methylation in GC cells, and re-expression was induced by 5-Aza-2′-deoxyazacytidine. DACH1 is methylated in 63.3% (62/98) of primary GC and 38% (19/50) of adjacent non-tumour tissues, while no methylation was found in normal gastric mucosa. Methylation of DACH1 correlated with reduced expression of DACH1 (P < 0.01), late tumour stage (stage III/IV) (P < 0.01) and lymph node metastasis (P < 0.05). DACH1 expression inhibited epithelial–mesenchymal transition and metastasis by inhibiting transforming growth factor (TGF)-β signalling and suppressed GC cell proliferation through inducing G2/M phase arrest. The tumour size is smaller in DACH1-expressed BGC823 cell xenograft mice than in unexpressed group (P < 0.01). Restoration of DACH1 expression also sensitized GC cells to docetaxel. These studies suggest that DACH1 is frequently methylated in human GC and expression of DACH1 was controlled by promoter region methylation. DACH1 suppresses GC proliferation, invasion and metastasis by inhibiting TGF-β signalling pathways both in vitro and in vivo. Epigenetic silencing DACH1 may induce GC cells' resistance to docetaxel.

Proline-rich acidic protein 1 (PRAP1) is a novel interacting partner of MAD1 and has a suppressive role in mitotic checkpoint signalling in hepatocellular carcinoma

Loss of mitotic checkpoint of cells contributes to chromosomal instability and leads to carcinogenesis. Mitotic arrest deficient 1 (MAD1) is a key component in mitotic checkpoint signalling. In this study, we identified a novel MAD1 interacting partner, proline-rich acidic protein 1 (PRAP1), using yeast-two hybrid screening, and investigated its role in mitotic checkpoint signalling in hepatocellular carcinoma (HCC). We demonstrated the physical interaction of PRAP1 with MAD1 and of PRAP1 with MAD1 isoform MAD1β, using a co-immunoprecipitation assay. Moreover, stable expression of PRAP1 in mitotic checkpoint-competent HCC cells, BEL-7402 and SMMC-7721, induced impairment of the mitotic checkpoint (p < 0.01), formation of chromosome bridges (p < 0.01) and aberrant chromosome numbers (p < 0.001). Interestingly, ectopic expression PRAP1 in HCC cells led to significant under-expression of MAD1. In human HCC tumours, 40.4% (23/57) of HCCs showed under-expression of PRAP1 protein as compared with their corresponding non-tumorous livers; up-regulation of MAD1 protein was significantly associated with down-regulation of PRAP1 (p = 0.030). Our data revealed that PRAP1 is a protein interacting partner of MAD1 and that PRAP1 is able to down-regulate MAD1 and suppress mitotic checkpoint signalling in HCC.

TALEN-mediated single-base-pair editing identification of an intergenic mutation upstream of BUB1B as causative of PCS (MVA) syndrome

Cancer-prone syndrome of premature chromatid separation with mosaic variegated aneuploidy [PCS (MVA) syndrome] is a rare autosomal recessive disorder characterized by constitutional aneuploidy and a high risk of childhood cancer. We previously reported monoallelic mutations in the BUB1B gene (encoding BUBR1) in seven Japanese families with the syndrome. No second mutation was found in the opposite allele of any of the families studied, although a conserved BUB1B haplotype and a decreased transcript were identified. To clarify the molecular pathology of the second allele, we extended our mutational search to a candidate region surrounding BUB1B. A unique single nucleotide substitution, G > A at ss802470619, was identified in an intergenic region 44 kb upstream of a BUB1B transcription start site, which cosegregated with the disorder. To examine whether this is the causal mutation, we designed a transcription activator-like effector nuclease-mediated two-step single-base pair editing strategy and biallelically introduced this substitution into cultured human cells. The cell clones showed reduced BUB1B transcripts, increased PCS frequency, and MVA, which are the hallmarks of the syndrome. We also encountered a case of a Japanese infant with PCS (MVA) syndrome carrying a homozygous single nucleotide substitution at ss802470619. These results suggested that the nucleotide substitution identified was the causal mutation of PCS (MVA) syndrome.

BUB1 and BUBR1 inhibition decreases proliferation and colony formation, and enhances radiation sensitivity in pediatric glioblastoma cells

PURPOSE

Glioblastoma (GBM) is a very aggressive and lethal brain tumor with poor prognosis. Despite new treatment strategies, patients' median survival is still lower than 1 year in most cases. The expression of the BUB gene family has demonstrated to be altered in a variety of solid tumors, pointing to a role as putative therapeutic target. The purpose of this study was to determine BUB1, BUB3, and BUBR1 gene expression profiles in glioblastoma and to analyze the effects of BUB1 and BUBR1 inhibition combined or not with Temozolomide and radiation in the pediatric SF188 GBM cell line.

METHODS

For gene expression analysis, 8 cell lines and 18 tumor samples were used. The effect of BUB1 and BUBR1 inhibition was evaluated using siRNA. Apoptosis, cell proliferation, cell cycle kinetics, micronuclei formation, and clonogenic capacity were analyzed after BUB1 and BUBR1 inhibition. Additionally, combinatorial effects of gene inhibition and radiation or Temozolomide (TMZ) treatment were evaluated through proliferation and clonogenic capacity assays.

RESULTS

We report the upregulation of BUB1 and BUBR1 expression and the downregulation of BUB3 in GBM samples and cell lines when compared to white matter samples (p < 0.05). Decreased cell proliferation and colony formation after BUB1 and BUBR1 inhibition were observed, along with increased micronuclei formation. Combinations with TMZ also caused cell cycle arrest and increased apoptosis. Moreover, our results demonstrate that BUB1 and BUBR1 inhibition sensitized SF188 cells to γ-irradiation as shown by decreased growth and abrogation of colony formation capacity.

CONCLUSION

BUB1 and BUBR1 inhibition decreases proliferation and shows radiosensitizing effects on pediatric GBM cells, which could improve treatment strategies for this devastating tumor. Collectively, these findings highlight the potentials of BUB1 and BUBR1 as putative therapeutic targets for glioblastoma treatment.

p31(comet) inactivates the chemically induced Mad2-dependent spindle assembly checkpoint and leads to resistance to anti-mitotic drugs

Mad2 is a key component of the spindle assembly checkpoint (SAC) that delays the onset of anaphase until all kinetochores are attached to the spindle. It binds to Cdc20 and prevents it from promoting destruction of an anaphase inhibitor, Securin. Previously, we showed that a Mad2-binding protein, p31^comet, formed a complex with Mad2 upon the completion of spindle attachment. Here, we showed that the overexpression of p31^comet can abolish the Mad2-dependent SAC that is induced by anti-mitotic drugs, including nocodazole, taxol, and monastrol; these drugs, except monastrol, cause aneuploidy in HeLa cells. In the absence of Eg5, which is a target of monastrol, overexpression of p31^comet caused premature destruction of Securin and premature sister chromatid separation, but it did not cause aneuploidy. These results indicated that Eg5 kinesin function might be required for checkpoint exit and mitotic progression. Moreover, overexpression of p31^comet led to resistance against apoptosis that was induced by nocodazole and taxol in human cells, and taxol resistance was dependent on the p31^comet/Mad2 protein expression level ratio of in cancer cell lines. These results indicated that p31^comet is an indicator of resistance to anti-mitotic drugs in cancer cells.

MicroRNAs: key players of taxane resistance and their therapeutic potential in human cancers

The successful long-term use of taxane for cancer therapy is often prevented by the development of drug resistance in clinic. Thus, exploring the mechanisms involved is a first step towards rational strategies to overcome taxane resistance. Taxane resistance-related microRNA (miRNAs) are under investigation and miRNAs could induce the taxane resistance of tumour cells by regulating cell cycle distribution, survival and/or apoptosis pathways, drug transports, epithelial–mesenchymal transition and cancer stem cell. This article summarizes current research involving miRNAs as regulators of key target genes for tanxanxe chemoresistance and discusses the complex regulatory networks of miRNAs. Also, the authors will envisage future developments towards the potential use of targeting miRNAs as a novel strategy for improving response of tumour patients to taxane. miRNAs play critical roles in taxane chemoresistance and the miRNA-based therapies will be helpful for overcoming drug resistance and developing more effective personalized anti-cancer treatment strategies. Further research studies should be performed to promote therapeutic–clinical use of taxane resistance-related miRNAs in cancer patients, especially in those patients with taxane-resistant cancers.

The MAD1 1673 G → A polymorphism alters the function of the mitotic spindle assembly checkpoint and is associated with a worse response to induction chemotherapy and sensitivity to treatment in patients with advanced epithelial ovarian cancer

BACKGROUND

Mitotic arrest deficient 1 (MAD1), a protein of the mitotic spindle assembly checkpoint (SAC), recognizes MAD2 through two leucine zippers, transporting and activating MAD2, which promotes a metaphase arrest signal. A single nucleotide polymorphism of MAD1 was found to affect the SAC function that could be involved in a poor response to therapeutic agents that alter the dynamics of microtubules.

OBJECTIVE

The aim of this study was to examine the relationship of the polymorphism MAD1 1673 G → A (rs1801368) with the efficiency of the SAC and the generation of aneuploidies and with the therapeutic response of patients with ovarian cancer.

METHODS

The polymorphism was evaluated in 144 healthy individuals and 91 patients. Mitotic arrest and the presence of errors in segregation were analyzed in cultured human lymphocytes treated with nocodazole and paclitaxel. Errors in segregation were also evaluated in 27 biopsies of patients.

RESULTS

Allele frequencies in healthy individuals were G: 50%, A: 50%, whereas in the patients they were G: 38%, A: 62% (P<0.05). The percentage of cells with mitotic arrest was higher among GG cells (P<0.05). The frequency of micronuclei and nondisjunction events increased in AA cells (P<0.05). Tumors from polymorphic patients had a higher percentage of aneuploid cells (P<0.05). The GG patients showed a higher biochemical response, optimal cytoreduction, and sensitivity to the treatment. There were no differences in progression-free or overall survival between both groups.

CONCLUSION

The polymorphism MAD1 1673 G → A affects SAC functionality, increasing the frequency of aneuploid cells. This polymorphism modifies the response to agents that alter the dynamics of microtubules in patients with ovarian cancer.

miR-125b promotes cell death by targeting spindle assembly checkpoint gene MAD1 and modulating mitotic progression

The spindle assembly checkpoint (SAC) is a 'wait-anaphase' mechanism that has evolved in eukaryotic cells in response to the stochastic nature of chromosome-spindle attachments. In the recent past, different aspects of the SAC regulation have been described. However, the role of microRNAs in the SAC is vaguely understood. We report here that Mad1, a core SAC protein, is repressed by human miR-125b. Mad1 serves as an adaptor protein for Mad2 - which functions to inhibit anaphase entry till the chromosomal defects in metaphase are corrected. We show that exogenous expression of miR-125b, through downregulation of Mad1, delays cells at metaphase. As a result of this delay, cells proceed towards apoptotic death, which follows from elevated chromosomal abnormalities upon ectopic expression of miR-125b. Moreover, expressions of Mad1 and miR-125b are inversely correlated in a variety of cancer cell lines, as well as in primary head and neck tumour tissues. We conclude that increased expression of miR-125b inhibits cell proliferation by suppressing Mad1 and activating the SAC transiently. We hypothesize an optimum Mad1 level and thus, a properly scheduled SAC is maintained partly by miR-125b.

Up-regulation of the mitotic checkpoint component Mad1 causes chromosomal instability and resistance to microtubule poisons

The mitotic checkpoint is the major cell cycle checkpoint acting during mitosis to prevent aneuploidy and chromosomal instability, which are hallmarks of tumor cells. Reduced expression of the mitotic checkpoint component Mad1 causes aneuploidy and promotes tumors in mice [Iwanaga Y, et al. (2007) Cancer Res 67:160-166]. However, the prevalence and consequences of Mad1 overexpression are currently unclear. Here we show that Mad1 is frequently overexpressed in human cancers and that Mad1 up-regulation is a marker of poor prognosis. Overexpression of Mad1 causes aneuploidy and chromosomal instability through weakening mitotic checkpoint signaling caused by mislocalization of the Mad1 binding partner Mad2. Cells overexpressing Mad1 are resistant to microtubule poisons, including currently used chemotherapeutic agents. These results suggest that levels of Mad1 must be tightly regulated to prevent aneuploidy and transformation and that Mad1 up-regulation may promote tumors and cause resistance to current therapies.

Involvement of CNOT3 in mitotic progression through inhibition of MAD1 expression

The stability of mRNA influences the dynamics of gene expression. The CCR4-NOT complex, the major deadenylase in mammalian cells, shortens the mRNA poly(A) tail and contributes to the destabilization of mRNAs. The CCR4-NOT complex plays pivotal roles in various physiological functions, including cell proliferation, apoptosis, and metabolism. Here, we show that CNOT3, a subunit of the CCR4-NOT complex, is involved in the regulation of the spindle assembly checkpoint, suggesting that the CCR4-NOT complex also plays a part in the regulation of mitosis. CNOT3 depletion increases the population of mitotic-arrested cells and specifically increases the expression of MAD1 mRNA and its protein product that plays a part in the spindle assembly checkpoint. We showed that CNOT3 depletion stabilizes the MAD1 mRNA, and that MAD1 knockdown attenuates the CNOT3 depletion-induced increase of the mitotic index. Basing on these observations, we propose that CNOT3 is involved in the regulation of the spindle assembly checkpoint through its ability to regulate the stability of MAD1 mRNA.

Phosphorylation of the Ndc80 complex protein, HEC1, by Nek2 kinase modulates chromosome alignment and signaling of the spindle assembly checkpoint

Hec1 contributes to accurate chromosome segregation by mediating spindle assembly checkpoint (SAC) signals and microtubule binding to ensure proper spindle assembly. We found that serine 165 of Hec1 was phosphorylated preferentially at kinetochores of misaligned chromosomes. This phosphorylation is important for SAC maintenance via Mad1/Mad2 localization to kinetochores.

The spindle assemble checkpoint (SAC) is critical for accurate chromosome segregation. Hec1 contributes to chromosome segregation in part by mediating SAC signaling and chromosome alignment. However, the molecular mechanism by which Hec1 modulates checkpoint signaling and alignment remains poorly understood. We found that Hec1 serine 165 (S165) is preferentially phosphorylated at kinetochores. Phosphorylated Hec1 serine 165 (pS165) specifically localized to kinetochores of misaligned chromosomes, showing a spatiotemporal distribution characteristic of SAC molecules. Expressing an RNA interference (RNAi)-resistant S165A mutant in Hec1-depleted cells permitted normal progression to metaphase, but accelerated the metaphase-to-anaphase transition. The S165A cells were defective in Mad1 and Mad2 localization to kinetochores, regardless of attachment status. These cells often entered anaphase with lagging chromosomes and elicited increased segregation errors and cell death. In contrast, expressing S165E mutant in Hec1-depleted cells triggered defective chromosome alignment and severe mitotic arrest associated with increased Mad1/Mad2 signals at prometaphase kinetochores. A small portion of S165E cells eventually bypassed the SAC but showed severe segregation errors. Nek2 is the primary kinase responsible for kinetochore pS165, while PP1 phosphatase may dephosphorylate pS165 during SAC silencing. Taken together, these results suggest that modifications of Hec1 S165 serve as an important mechanism in modulating SAC signaling and chromosome alignment.

Mitosis in vertebrates: the G2/M and M/A transitions and their associated checkpoints

In this review, I stress the importance of direct data and accurate terminology when formulating and communicating conclusions on how the G2/M and metaphase/anaphase transitions are regulated. I argue that entry into mitosis (i.e., the G2/M transition) is guarded by several checkpoint control pathways that lose their ability to delay or stop further cell cycle progression once the cell becomes committed to divide, which in vertebrates occurs in the late stages of chromosome condensation. After this commitment, progress through mitosis is then mediated by a single Mad/Bub-based checkpoint that delays chromatid separation, and exit from mitosis (i.e., completion of the cell cycle) in the presence of unattached kinetochores. When cells cannot satisfy the mitotic checkpoint, e.g., when in concentrations of spindle poisons that prohibit the stable attachment of all kinetochores, they are delayed in mitosis for many hours. In normal cells, the duration of this delay depends on the organism and ranges from ∼4 h in rodents to ∼22 h in humans. Recent live cell studies reveal that under this condition, many cancer cells (including HeLa and U2OS) die in mitosis by apoptosis within ∼24 h, which implies that biochemical studies on cancer cell populations harvested in mitosis after a prolonged mitotic arrest are contaminated with dead or dying cells.

Cell cycle-dependent cytotoxicity and mitotic spindle checkpoint dependency of investigational and approved antimitotic agents

The mitotic spindle checkpoint (SPC) is a highly regulated mechanism in eukaryotic cells that ensures the even distribution of the duplicated genome between daughter cells. Malfunction of the SPC or deregulated expression of SPC regulatory proteins is frequently associated with a poor response to chemotherapeutic agents. We investigated various approved and investigational mitosis-specific agents, including spindle poisons, an Eg5 kinesin inhibitor, inhibitors of polo-like kinase 1 (Plk1) or Aurora-B kinase, a benzamide class HDAC inhibitor and compounds identified in a chemical genetics screen for their cell cycle-dependent cytotoxicities and for their activities toward SPC deficient (HT29, Caco-2, T47D) and SPC proficient human cell lines (A2780, HCT116, SW480). Using the RKOp27 cell system that allows inducible cell cycle arrest by the tunable expression of the cdk inhibitor p27Kip1, we found an exquisite proliferation-dependent cytotoxicity for all compounds except the aurora kinase inhibitor VX-680. Cytotoxicity of the antimitotic compounds was in general higher on SPC proficient than on deficient cells. We found two exceptions, a benzamide HDAC inhibitor which was effective on SPC proficient and deficient cells and an investigational compound, BYK72767, with a yet unknown mode of action. The degree of increased mitotic index was no predictor of cytotoxicity of the compounds nor was the phosphorylation of BubR1. However, SPC deficient cell lines were able to tolerate mitotic arrest for far longer times than SPC proficient cells. We conclude that targeting of SPC deficient cancers with novel antimitotic principles remains a challenge but certain drug classes may be equally efficacious regardless of SPC status.

Mitotic drug targets

Mitosis is the key event of the cell cycle during which the sister chromatids are segregated onto two daughter cells. It is well established that abrogation of the normal mitotic progression is a highly efficient concept for anti-cancer treatment. In fact, various drugs that target microtubules and thus interfere with the function of the mitotic spindle are in clinical use for the treatment of various human malignancies for many years. However, since microtubule inhibitors not only target proliferating cells severe side effects limit their use. Therefore, the identification of novel mitotic drug targets other than microtubules have gained recently much attention. This review will summarize the latest developments on the identification and clinical evaluation of novel mitotic drug targets and will introduce novel concepts for chemotherapy that are based on recent progress in our understanding how mitotic progression is regulated and how anti-mitotic drugs induce tumor cell death.

Analysis of the protein expression changes during taxol-induced apoptosis under translation inhibition conditions

Taxol is currently used in chemotherapeutic treatments of different types of cancers. In this article, we demonstrate that taxol induces apoptosis and translation down-regulation in human embryonic kidney (HEK293T) cells. Antibody arrays are a promising new tool for the analysis of protein levels changes in cells responding to different stimuli. Using this approach, we have identified changes in the expression of 38 proteins (20 down-regulated and 18 up-regulated), implicated in several cellular processes mainly in apoptosis, cell cycle and signal transduction pathways, and also cytoskeleton proteins. Among them, we have confirmed a considerable decrease in the expression of p14(ARF) and a significant increase in the levels of dystrophin and c-Myc. It is known that c-Myc mRNA has an internal ribosome entry segment (IRES) element in its 5'UTR that could regulate its expression under global protein synthesis inhibition conditions. We demonstrate that after taxol treatment, the c-Myc IRES activity is maintained meanwhile cap-dependent activity is inhibited. In addition, an increase in c-Myc mRNA was also observed after taxol treatment. We conclude that taxol-induced c-Myc expression is regulated at both transcriptional and translational levels, the last of them by a mechanism mediated by IRES.

Expression of oncogenic kinase Bcr-Abl impairs mitotic checkpoint and promotes aberrant divisions and resistance to microtubule-targeting agents

Recent findings showed that BRCA1, in addition to its role in DNA damage response, acts as an upstream regulator of genes involved in the mitotic checkpoint regulation, thus protecting against promotion of aberrant divisions and aneuploidy. Moreover, there is also an indication that the BRCA1 protein is downregulated in chronic myeloid leukemia (CML) patients. We have investigated a possible functional relationship between BRCA1 and mitotic checkpoint competence in cells with the same genetic background expressing different levels of Bcr-Abl, an oncogene responsible for CML. Herein, we show that Bcr-Abl strongly downregulates the BRCA1 protein level, which is partially reversed on treatment with imatinib, an inhibitor of Bcr-Abl tyrosine kinase. Bcr-Abl leads to decreased expression of genes involved in the mitotic checkpoint activation--Mad2, Bub1, Bub3, and BubR1, resulting in mitosis perturbances, weakened mitotic checkpoint function, and mitotic slippage after nocodazole treatment. Furthermore, high Bcr-Abl-expressing cells showed also postmitotic checkpoint dysfunctions and inability to effectively arrest in the 4NG1 phase of the cell cycle, which was associated with limited p21 induction. These observations had significant biological consequences, as we found a high level of improper divisions, chromosomal missegregation, and generation of polyploid cells on mitotic checkpoint prolonged activation. Additionally, Bcr-Abl-expressing cells showed resistance to death activated by spindle defects, reversed by imatinib. Our study presents new facts and supports the hypothesis concerning the mutator nature of Bcr-Abl itself. The functional interaction between Bcr-Abl and mitosis dysfunctions, due to compromised mitotic checkpoints, may have important implications for the generation of aneuploidy and CML progression.

The DNA damage response to non-replicating adeno-associated virus: Centriole overduplication and mitotic catastrophe independent of the spindle checkpoint

Adeno-associated virus (AAV) type 2 or UV-inactivated AAV (UV-AAV2) infection provokes a DNA damage response that leads to cell cycle arrest at the G2/M border. p53-deficient cells cannot sustain the G2 arrest, enter prolonged impaired mitosis, and die. Here, we studied how non-replicating AAV2 kills p53-deficient osteosarcoma cells. We found that the virus uncouples centriole duplication from the cell cycle, inducing centrosome overamplification that is dependent on Chk1, ATR and CDK kinases, and on G2 arrest. Interference with spindle checkpoint components Mad2 and BubR1 revealed unexpectedly that mitotic catastrophe occurs independently of spindle checkpoint function. We conclude that, in the p53-deficient cells, UV-AAV2 triggers mitotic catastrophe associated with a dramatic Chk1-dependent overduplication of centrioles and the consequent formation of multiple spindle poles in mitosis. As AAV2 acts through cellular damage response pathways, the results provide information on the role of Chk1 in mitotic catastrophe after DNA damage signaling in general.

p53 suppresses structural chromosome instability after mitotic arrest in human cells

The p53 tumor suppressor inhibits the proliferation of cells which undergo prolonged activation of the mitotic checkpoint. However, the function of this antiproliferative response is not well defined. Here we report that p53 suppresses structural chromosome instability following mitotic arrest in human cells. In both HCT116 colon cancer cells and normal human fibroblasts, DNA breaks occurred during mitotic arrest in a p53-independent manner, but p53 was required to suppress the proliferation and structural chromosome instability of the resulting polyploid cells. In contrast, cells made polyploid without mitotic arrest exhibited neither significant structural chromosome instability nor p53-dependent cell cycle arrest. We also observed that p53 suppressed both the frequency and structural chromosome instability of spontaneous polyploids in HCT116 cells. Furthermore, time-lapse videomicroscopy revealed that polyploidization of p53^−/− HCT116 cells is frequently accompanied by mitotic arrest. These data suggest that a function of the p53-dependent postmitotic response is the prevention of structural chromosome instability following prolonged activation of the mitotic checkpoint. Accordingly, our study suggests a novel mechanism of tumor suppression for p53, as well as a potential role for p53 in the outcome of antimitotic chemotherapy.

Role of prolonged mitotic checkpoint activation in the formation and treatment of cancer

Mitotic abnormalities are a common feature of human cancer cells, and recent studies have provided evidence that such abnormalities may play a causative, rather than merely incidental role, in tumorigenesis. One such abnormality is prolonged activation of the mitotic checkpoint, which can be provoked by a number of the gene changes that drive tumor formation. At the same time, antimitotic chemotherapeutics exert their clinical efficacy through the large-scale induction of prolonged mitotic checkpoint activation, indicating that mitotic arrest is influential in both the formation and treatment of human cancer. However, how this influence occurs is not well understood. In this perspective, we will discuss the current evidence in support of the potential mechanisms by which prolonged activation of the mitotic checkpoint affects both tumorigenesis and antimitotic chemotherapy.

Elevating the frequency of chromosome mis-segregation as a strategy to kill tumor cells

The mitotic checkpoint has evolved to prevent chromosome mis-segregations by delaying mitosis when unattached chromosomes are present. Inducing severe chromosome segregation errors by ablating the mitotic checkpoint causes cell death. Here we have analyzed the consequences of gradual increases in chromosome segregation errors on the viability of tumor cells and normal human fibroblasts. Partial reduction of essential mitotic checkpoint components in four tumor cell lines caused mild chromosome mis-segregations, but no lethality. These cells were, however, remarkably more sensitive to low doses of taxol, which enhanced the amount and severity of chromosome segregation errors. Sensitization to taxol was achieved by reducing levels of Mps1 or BubR1, proteins having dual roles in checkpoint activation and chromosome alignment, but not by reducing Mad2, functioning solely in the mitotic checkpoint. Moreover, we find that untransformed human fibroblasts with reduced Mps1 levels could not be sensitized to sublethal doses of taxol. Thus, targeting the mitotic checkpoint and chromosome alignment simultaneously may selectively kill tumor cells by enhancing chromosome mis-segregations.

The ability to survive mitosis in the presence of microtubule poisons differs significantly between human nontransformed (RPE-1) and cancer (U2OS, HeLa) cells

We used live cell imaging to compare the fate of human nontransformed (RPE-1) and cancer (HeLa, U2OS) cells as they entered mitosis in nocodazole or taxol. In the same field, and in either drug, a cell in all lines could die in mitosis, exit mitosis and die within 10 h, or exit mitosis and survive > or =10 h. Relative to RPE-1 cells, significantly fewer HeLa or U2OS cells survived mitosis or remained viable after mitosis: in nocodazole concentrations that inhibit spindle microtubule assembly, or in 500 nM taxol, 30% and 27% of RPE-1 cells, respectively, died in or within 10 h of exiting mitosis while 90% and 49% of U2OS and 78% and 81% of HeLa died. This was even true for clinically relevant taxol concentrations (5 nM) which killed 93% and 46%, respectively, of HeLa and U2OS cells in mitosis or within 10 h of escaping mitosis, compared to 1% of RPE-1 cells. Together these data imply that studies using HeLa or U2OS cells, harvested after a prolonged block in mitosis with nocodazole or taxol, are significantly contaminated with dead or dying cells. We also found that the relationship between the duration of mitosis and survival is drug and cell type specific and that lethality is related to the cell type and drug used to prevent satisfaction of the kinetochore attachment checkpoint. Finally, work with a pan-caspase inhibitor suggests that the primary apoptotic pathway triggered by nocodazole during mitosis in RPE-1 cells is not active in U2OS cells. Cell Motil. Cytoskeleton 2008. (c) 2008 Wiley-Liss, Inc.

Determinants for the efficiency of anticancer drugs targeting either Aurora-A or Aurora-B kinases in human colon carcinoma cells

The mitotic Aurora kinases, including Aurora-A and Aurora- B, are attractive novel targets for anticancer therapy, and inhibitory drugs have been developed that are currently undergoing clinical trials. However, the molecular mechanisms how these drugs induce tumor cell death are poorly understood. We have addressed this question by comparing the requirements for an efficient induction of apoptosis in response to MLN8054, a selective inhibitor of Aurora-A, and the selective Aurora-B inhibitor ZM447439 in human colon carcinoma cells. By using various isogenic knockout as well as inducible colon carcinoma cell lines, we found that treatment with MLN8054 induces defects in mitotic spindle assembly, which causes a transient spindle checkpoint-dependent mitotic arrest. This cell cycle arrest is not maintained due to the activity of MLN8054 to override the spindle checkpoint. Subsequently, MLN8054-treated cells exit from mitosis and activate a p53-dependent postmitotic G(1) checkpoint, which subsequently induces p21 and Bax, leading to G(1) arrest followed by the induction of apoptosis. In contrast, inhibition of Aurora-B by ZM447439 also interferes with normal chromosome alignment during mitosis and overrides the mitotic spindle checkpoint but allows a subsequent endoreduplication, although ZM447439 potently activates the p53-dependent postmitotic G(1) checkpoint. Moreover, the ZM447439-induced endoreduplication is a prerequisite for the efficiency of the drug. Thus, our results obtained in human colon carcinoma cells indicate that although both Aurora kinase inhibitors are potent inducers of tumor cell death, the pathways leading to the induction of apoptosis in response to these drugs are distinct.

Pharmacologic abrogation of the mitotic spindle checkpoint by an indolocarbazole discovered by cellular screening efficiently kills cancer cells

The mitotic spindle checkpoint represents a signal transduction pathway that prevents the onset of anaphase until all chromosomes are properly aligned on a metaphase plate. Partial inactivation of this checkpoint allows premature separation of sister chromatids and results in aneuploidy, which might contribute to tumorigenesis. Unlike other cell cycle checkpoints, the spindle checkpoint is essential for cell viability, giving rise to the idea that the spindle checkpoint itself might represent a valuable target for anticancer therapy. We used a cell-based screen and identified the indolocarbazole compound Gö6976 as a pharmacologic inhibitor of the spindle checkpoint. Gö6976 potently overrides a spindle checkpoint-mediated mitotic arrest by abrogating the phosphorylation and kinetochore localization of several spindle checkpoint proteins. We identified the Aurora-A and Aurora-B kinases, which have been previously implicated in proper mitotic progression and spindle checkpoint function, as targets for Gö6976. Accordingly, Gö6976 treatment causes severe mitotic abnormalities and chromosome alignment defects, which are not properly detected by the drug-inactivated spindle checkpoint. This results in an aberrant progression of mitosis, leading to apoptosis in various human cancer cell lines, including spindle checkpoint-compromised cancer cells. Thus, our work describes a novel and promising strategy for anticancer treatment that targets the mitotic spindle checkpoint.

Deregulated Aurora-B induced tetraploidy promotes tumorigenesis

High expression of Aurora-B has been observed in various cancers, and inhibition of this kinase has been shown to halt cellular proliferation. However, the mechanism of effect of Aurora-B on cellular transformation has not been fully explored. Here, we show that overexpression of Aurora-B in murine epithelial cells promotes generation of tetraploids. In search of a related mechanism, spectral karyotyping was carried out, showing premature chromatid separation (PCS). Of interest, PCS is a hallmark of Robert's syndrome, which also involves cellular polyploidy and aneuploidy. Sorted tetraploid Aurora-B-overexpressing cells promoted significant mammary epithelial cancers when injected into nude mice, as compared to injection of nonfractionated cells, suggesting that tetraploidy is an important mediator of Aurora-B-induced tumorigenesis. Comparative chromosome hybridization performed on DNA derived from tetraploid cell-induced tumors indicates amplifications and deletions of regions throughout the genome, which include tumor-promoting or tumor-suppressing genes, respectively. Thus, sustained expression of Aurora-B induces tetraploidy, which, in turn, facilitates genomic instability and tumor development in a xenograft model.

Overexpression of UbcH10 alternates the cell cycle profile and accelerate the tumor proliferation in colon cancer

Background

UbcH10 participates in proper metaphase to anaphase transition, and abrogation of UbcH10 results in the premature separation of sister chromatids. To assess the potential role of UbcH10 in colon cancer progression, we analyzed the clinicopathological relevance of UbcH10 in colon cancer.

Methods

We firstly screened the expression profile of UbcH10 in various types of cancer tissues as well as cell lines. Thereafter, using the colon cancer cells line, we manipulated the expression of UbcH10 and evaluated the cell cycle profile and cellular proliferations. Furthermore, the clinicopathological significance of UbcH10 was immunohistologically evaluated in patients with colon cancer. Statistical analysis was performed using the student's t-test and Chi-square test.

Results

Using the colon cancer cells, depletion of UbcH10 resulted in suppression of cellular growth whereas overexpression of UbcH10 promoted the cellular growth and oncogenic cellular growth. Mitotic population was markedly alternated by the manipulation of UbcH10 expression. Immunohistochemical analysis indicated that UbcH10 was significantly higher in colon cancer tissue compared with normal colon epithelia. Furthermore, the clinicopathological evaluation revealed that UbcH10 was associated with high-grade histological tumors.

Conclusion

The results show the clinicopathological significance of UbcH10 in the progression of colon cancer. Thus UbcH10 may act as a novel biomarker in patients with colon cancer.

Clinicopathological relevance of UbcH10 in breast cancer

Abrogated mitotic progression is a common hallmark of cancer. UbcH10, one of the components of the ubiquitin/proteasome pathway, plays a pivotal role in the regulation of mitotic progression. Abnormal UbcH10 activity is reported in certain types of human cancers; its overexpression is occasionally encountered in cancerous tissue compared with adjacent normal tissue. Current studies have suggested the critical role of UbcH10 in the spindle assembly checkpoint and the subsequent accurate separation of sister chromatids, which is orchestrated by a series of molecular interactions governed by the complex and diverse cell cycle machinery. To validate the potential role of UbcH10 in cell proliferation in cancer, we have analyzed the clinicopathological relevance of UbcH10 in progression of breast cancer using a combinatorial approach of human tumor arrays and biochemical analyses. Our results show that the percentage of tested samples which stained positive for UbcH10 in breast cancer tissues is significantly higher compared to the adjacent nonmalignant tissue. Furthermore, results from the clinicopathological analysis have revealed that elevated expression of UbcH10 is associated with higher histological grade tumors. In addition, depletion of UbcH10 by RNA interference in breast cancer cells resulted in decreased cellular proliferation, while overexpression of UbcH10 significantly enhanced cellular growth in breast cancer. Our results suggest a pathological correlation between UbcH10 and cell proliferation in breast cancer. Thus, aberrant UbcH10 activity may induce the dysfunction of proper cell cycle progression and result in the aggressive behavior of tumor cells in patients with breast cancer.

Requirements for protein phosphorylation and the kinase activity of polo-like kinase 1 (Plk1) for the kinetochore function of mitotic arrest deficiency protein 1 (Mad1)

Mitotic arrest deficiency protein 1 (Mad1) is associated with microtubule-unattached kinetochores in mitotic cells and is a component of the spindle assembly checkpoint (SAC). Here, we have studied the phosphorylation of Mad1 and mapped using liquid chromatography-tandem mass spectrometry several phosphorylated amino acids in this protein. One phosphorylated residue, Thr680, was characterized to be important for the kinetochore localization of Mad1 and its SAC function. We also found that in mitotic cells Mad1 co-immunoprecipitated with Plk1. Depletion of cellular Plk1 using small interfering RNAs and inhibition of the kinase activity of Plk1 using a kinase-dead mutant or a small molecule inhibitor attenuated Mad1 phosphorylation and its association with kinetochores. Collectively, these findings indicate mechanistic roles contributed by protein phosphorylation and Plk1 to the SAC activity of Mad1.

Role of a novel splice variant of mitotic arrest deficient 1 (MAD1), MAD1beta, in mitotic checkpoint control in liver cancer

Loss of mitotic checkpoint contributes to chromosomal instability, leading to carcinogenesis. In this study, we identified a novel splicing variant of mitotic arrest deficient 1 (MAD1), designated MAD1beta, and investigated its role in mitotic checkpoint control in hepatocellular carcinoma (HCC). The expression levels of human MAD1beta were examined in hepatoma cell lines and human HCC samples. The functional roles of MAD1beta in relation to the mitotic checkpoint control, chromosomal instability, and binding with MAD2 were assessed in hepatoma cell lines. On sequencing, MAD1beta was found to have deletion of exon 4. It was expressed at both mRNA and protein levels in the nine hepatoma cell lines tested and was overexpressed in 12 of 50 (24%) human HCCs. MAD1beta localized in the cytoplasm, whereas MAD1alpha was found in the nucleus. This cytoplasmic localization of MAD1beta was due to the absence of a nuclear localization signal in MAD1alpha. In addition, MAD1beta was found to physically interact with MAD2 and sequester it in the cytoplasm. Furthermore, expression of MAD1beta induced mitotic checkpoint impairment, chromosome bridge formation, and aberrant chromosome numbers via binding with MAD2. Our data suggest that the novel splicing variant MAD1beta may have functions different from those of MAD1alpha and may play opposing roles to MAD1alpha in mitotic checkpoint control in hepatocarcinogenesis.

Faulty spindle checkpoint and cohesion protein activities predispose oocytes to premature chromosome separation and aneuploidy

Aneuploidy accounts for a major proportion of human reproductive failures, mental and physical anomalies, and neoplasms. To heighten our understanding of normal and abnormal chromosome segregation, additional information is needed about the underlying molecular mechanisms of chromosome segregation. Although many hypotheses have been proposed for the etiology of human aneuploidy, there has not been general acceptance of any specific hypothesis. Moreover, it is important to recognize that many potential mechanisms exist whereby chromosome missegregation may occur. One area for investigating aneuploidy centers on the biochemical changes that take place during oocyte maturation. In this regard, recent results have shown that faulty mRNA of spindle-assembly checkpoint proteins and chromosome cohesion proteins may lead to aneuploidy. Also, postovulatory and in vitro aging of mouse oocytes has been shown to lead to decreased levels of Mad2 transcripts and elevated frequencies of premature centromere separation. The intent of this review is to highlight the major events surrounding chromosome segregation and to present the published results that support the premise that faulty chromosome cohesion proteins and spindle checkpoint proteins compromise accurate chromosome segregation.