Search for in vivo somatic mutations in the mitotic checkpoint gene, hMAD1, in human lung cancers

CENPE Inhibition Leads to Mitotic Catastrophe and DNA Damage in Medulloblastoma Cells

Simple Summary

Medulloblastoma (MB) is the most frequent brain tumor in children. The standard treatment consists in surgery, followed by radiotherapy and chemotherapy. These therapies are only partially effective, since many patients still die and those who survive suffer from neurological and endocrine disorders. Therefore, more effective therapies are needed. CENPE is a gene critical for normal proliferation and survival of neural progenitors. Since there is evidence that MB cells are very similar to neural progenitors, we hypothesized that CENPE could be an effective target for MB treatment. In MB cell lines, CENPE depletion induced defects in division and resulted in cell death. To consolidate CENPE as a target for MB treatment, we tested GSK923295, a specific inhibitor already in clinical trials for other cancer types. GSK923295 induced effects similar to CENPE depletion at low nM levels, supporting the idea that CENPE’s inhibition could be a viable strategy for MB treatment.

Abstract

Medulloblastoma (MB) is the most frequent brain tumor in children. The standard treatment consists in surgery, followed by radiotherapy and chemotherapy. These therapies are only partially effective since many patients still die and those who survive suffer from neurological and endocrine disorders. Therefore, more effective therapies are needed. Primary microcephaly (MCPH) is a rare disorder caused by mutations in 25 different genes. Centromere-associated protein E (CENPE) heterozygous mutations cause the MCPH13 syndrome. As for other MCPH genes, CENPE is required for normal proliferation and survival of neural progenitors. Since there is evidence that MB shares many molecular features with neural progenitors, we hypothesized that CENPE could be an effective target for MB treatment. In ONS-76 and DAOY cells, CENPE knockdown induced mitotic defects and apoptosis. Moreover, CENPE depletion induced endogenous DNA damage accumulation, activating TP53 or TP73 as well as cell death signaling pathways. To consolidate CENPE as a target for MB treatment, we tested GSK923295, an allosteric inhibitor already in clinical trial for other cancer types. GSK923295, induced effects similar to CENPE depletion with higher penetrance, at low nM levels, suggesting that CENPE’s inhibition could be a therapeutic strategy for MB treatment.

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.

Cell cycle inhibitors for the treatment of NSCLC

INTRODUCTION

Lung cancer remains to be the leading cause of cancer-related death worldwide. Treatment of lung cancer still poses a significant challenge. Cell cycle is a tightly integrated process and is frequently aberrant in lung cancer. Cell cycle inhibitors have emerged as novel therapeutics, in anticipation of overcoming the unrestricted cell division and growth in lung cancer.

AREAS COVERED

In this article, we first address the potential roles of cell cycle proteins and cell cycle deregulation in the development of lung cancer. The review then provides an overview for several major categories of cell cycle inhibitors with particular attention to their tolerability and disease control in early phases of lung cancer trials.

EXPERT OPINION

Targeted agents against different components of cell cycle regulation, such as cyclin-dependent kinase, polo-like kinase, checkpoint kinase and aurora kinase, are currently in clinical development for lung cancer management. Their clinical benefits remain to be defined. When evaluated as single agents in lung cancer, cell cycle inhibitors are often associated with limited clinical activity and tolerable toxicities. The key challenges in the drug development are to understand resistance mechanisms and to identify predictive biomarkers that can potentially guide patient selection and optimize the utility of these targeted inhibitors.

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.

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.

Tumor suppressors and cell-cycle proteins in lung cancer

The cell cycle is the cascade of events that allows a growing cell to duplicate all its components and split into two daughter cells. Cell cycle progression is mediated by the activation of a highly conserved family of protein kinases, the cyclin-dependent kinases (CDKs). CDKs are also regulated by related proteins called cdk inhibitors grouped into two families: the INK4 inhibitors (p16, p15, p19, and p18) and the Cip/Kip inhibitors (p21, p27, and p53). Several studies report the importance of cell-cycle proteins in the pathogenesis and the prognosis of lung cancer. This paper will review the most recent data from the literature about the regulation of cell cycle. Finally, based essentially on the data generated in our laboratory, the expression, the diagnostic, and prognostic significance of cell-cycle molecules in lung cancer will be examined.

Mitotic arrest defective protein 2 expression abnormality and its clinicopathologic significance in human osteosarcoma

Osteosarcoma is the most common primary malignancy of bone. Overexpression of mitotic arrest defective protein 2 (MAD2) is found in many human neoplasms, but its role in the oncogenesis of osteosarcoma is an untouched topic. The objective of this research was to observe the expression of MAD2 in human osteosarcoma and explore its clinicopathologic significance. MAD2 expression was analyzed in 48 primary osteosarcoma cases (19 osteoblastic osteosarcomas, 17 chondroblastic osteosarcomas and 12 fibroblastic osteosarcomas) using immunohistochemistry. A total of 20 normal bone specimens formed a control group. MAD2 was commonly overexpressed in human osteosarcoma. Immunopositivity was higher in tumors with lower differentiation and higher clinical stage. Increased expression of MAD2 was associated with earlier metastasis and poorer survival. Our findings provide evidence that MAD2 contributes to the pathogenesis and development of human osteosarcoma, Testing may have a clinical role in predicting prognosis, selecting appropriate chemotherapeutic strategies and providing novel strategies for osteosarcoma therapy.

Mitotic chromosomal instability and cancer: mouse modelling of the human disease

The stepwise progression from an early dysplastic lesion to full-blown metastatic malignancy is associated with increases in genomic instability. Mitotic chromosomal instability - the inability to faithfully segregate equal chromosome complements to two daughter cells during mitosis - is a widespread phenomenon in solid tumours that is thought to serve as the fuel for tumorigenic progression. How chromosome instability (CIN) arises in tumours and what consequences it has are still, however, hotly debated issues. Here we review the recent literature with an emphasis on models that recapitulate observations from human disease.

Role of cell cycle regulators in lung carcinogenesis

Cellular division is an ordered, tightly regulated process involving multiple checkpoints that assess extracellular growth signals, cell size and DNA integrity. Progression throughout the cell cycle is based on the activation of different CDK-cyclin complexes that prevent cells from entering into a new phase until thay have successfully complete the previous one. In addition, a series of cell cycle checkpoints are designed to preserve genome integrity and chromosomal stability. Neoplastic lung cells develop the ability to bypass several of these checkpoints, and tumor cell proliferation is frequently associated with genetic or epigenetic alterations in key regulators of the cell cycle. The goal of this review is to summarize the knowledge about the dysregulation of major cell cycle regulators in lung cancer pathogenesis and to discuss the use of these proteins as targets for therapeutic intervention.

Expression of mitotic checkpoint proteins BUB1B and MAD2L1 in salivary duct carcinomas

OBJECTIVE

Defects in the mitotic checkpoint lead to aneuploidy and might facilitate tumorigenesis. However, the ploidy status in salivary duct carcinoma (SDC) has been reported to play limited role in prediction of prognosis. Thus, we need more reliable markers to reflect the rapid tumor progression in SDCs. We aimed here to investigate the expression of mitotic checkpoint proteins benzimidazole 1 homolog beta (BUB1B) and mitosis arrest-deficient 2 like 1 (MAD2L1) in SDCs and to determine their possible role as surrogate prognostic markers.

METHODS

We analyzed the clinical courses, pathologic findings and immunohistochemical profiles of mitotic checkpoint proteins (BUB1B and MAD2L1) in 27 pathologically confirmed SDCs. The expression status of BUB1B and MAD2L1 was compared with clinicopathologic factors and other molecular markers, such as TGF-beta, c-erb-B2, androgen receptor, vascular endothelial growth factor, and epidermal growth factor receptor, for prognostic significance.

RESULTS

High BUB1B expression was detected in 25.9% of subjects, and high MAD2L1 expression was in 55.6% of subjects. However, survival analysis revealed that mitotic checkpoint expression did not have prognostic significance in SDCs, nor did the other studied markers. Rather, the clinical variable of N classification at diagnosis (in N+ status, hazard ratio 5.19, 95% CI 1.26-21.32 for disease-free survival and hazard ratio 7.18, 95% CI 1.09-46.99 for overall survival) was strongly associated with survival and prognosis based on the Cox proportional hazard model.

CONCLUSIONS

Mitotic checkpoint proteins appeared to play a limited role in predicting prognosis in SDCs. Further study is required to elucidate the exact role of mitotic checkpoint proteins in SDCs.

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.

Clinicopathologic significance of mitotic arrest defective protein 2 overexpression in hepatocellular carcinoma

Mitotic arrest defective protein 2 (MAD2) gene plays a central role in the mitotic checkpoint. Elevated MAD2 expression was observed in a number of human malignancies; its role in the development of hepatocellular carcinoma is still not understood and is controversial. The purpose of this study was to investigate the clinicopathologic significance of MAD2 expression in hepatocellular carcinoma. The MAD2 protein and its messenger RNA levels were measured in hepatocellular carcinomas, high-grade dysplastic nodules, and their paired nontumorous liver tissues by quantitative real-time polymerase chain reaction, Western blot, and immunohistochemistry. The results showed that MAD2 at both messenger RNA and protein levels was overexpressed in 8 of 9 high-grade dysplastic nodules and in 51 of 58 hepatocellular carcinomas, including 12 of 14 unifocal small hepatocellular carcinomas. There was a tendency for MAD2 expression to increase in the process of this multistep carcinogenesis. A significantly high tumor MAD2 immunostaining was associated with the progression of histologic grade and the overall low survival. In conclusion, MAD2 is overexpressed frequently in hepatocellular carcinoma, including high-grade dysplastic nodules and early-stage small hepatocellular carcinoma, indicating that overexpression of MAD2 plays a role in the development and progression of hepatocellular carcinoma. It may be an early event in hepatocarcinogenesis and could be used as a potential prognostic indicator.

Mitotic checkpoint gene MAD1 in hepatocellular carcinoma is associated with tumor recurrence after surgical resection

OBJECTIVE

Underlying mechanism of mitotic checkpoint gene mitosis arrest deficiency 1 (MAD1) in human hepatocellular carcinoma (HCC) is rarely known.

MATERIALS AND METHODS

We studied genetic change of the MAD1 gene as well as protein expression in 44 HCC and their associated non-cancerous surrounding liver tissues.

RESULTS

Genotype AG of MAD1 G-1849 A promoter was highly significant in microscopic vascular invasion than other genotypes (P = 0.006). Moreover, the mean tumor size of HCC with genotype AG (7.71 cm) was significantly larger than those of other genotypes (AA, 4.41 cm; GG, 4.59 cm; P = 0.033). After a median follow-up of 22 months, 18 (41%) of the 44 patients relapsed. Eleven (32.4%) of 34 with MAD1 protein expression and 7 (70%) of 10 with no expression of MAD1 protein showed tumor recurrence. The incidence of tumor recurrence in patients with the lost MAD1 expression was significantly higher than in those with the expressed MAD1 protein (P = 0.011).

CONCLUSION

These results suggest that MAD1 promoter genotype may be involved in tumor progression. Moreover, the loss of MAD1 protein expression may be related to the tumor recurrence after surgical resection of HCC.

The spindle assembly checkpoint: Preventing chromosome mis-segregation during mitosis and meiosis

Aneuploidy is a common feature of many cancers, suggesting that genomic stability is essential to prevent tumorigenesis. Also, during meiosis, chromosome non-disjunction produces gamete imbalance and when fertilized result in developmental arrest or severe birth defects. The spindle assembly checkpoint prevents chromosome mis-segregation during both mitosis and meiosis. In mitosis, this control system monitors kinetochore-microtubule attachment while in meiosis its role is still unclear. Interestingly, recent data suggest that defects in the spindle assembly checkpoint are unlikely to cause cancer development but might facilitate tumour progression. However, in meiosis a weakened checkpoint could contribute to age-related aneuploidy found in humans.

In silico whole-genome scanning of cancer-associated nonsynonymous SNPs and molecular characterization of a dynein light chain tumour variant

Last decade has led to the accumulation of large amounts of data on cancer genetics, opening an unprecedented access to the mapping of cancer genes in the human genome. Single-nucleotide polymorphisms (SNPs), the most common form of DNA variation in humans, emerge as an invaluable tool for cancer association studies. These genotypic markers can be used to assay how alleles of candidate genes correlate with the malignant phenotype, and may provide new clues into the genetic modifications that characterize cancer onset. In this cancer-oriented study, we detail an SNP mining strategy based on the analysis of expressed sequence tags among publicly available databases. Our whole-genome approach provides a comprehensive and unbiased description of nonsynonymous SNPs (nsSNPs) in tumoral versus normal tissues. To gain further insights into the possible relationships between genetic variation and altered phenotype, locations of a subset of nsSNPs were mapped onto protein domains known to be critical for protein function. Computational methods were also used to predict the potential impact of these cancer-associated nsSNPs on protein structure and function. We illustrate our approach through the detailed biochemical and structural characterization of a previously unknown cancer-associated mutation (G79C) affecting the 8 kDa dynein light chain (DNCL1).

On the road to cancer: aneuploidy and the mitotic checkpoint

Abnormal chromosome content - also known as aneuploidy - is the most common characteristic of human solid tumours. It has therefore been proposed that aneuploidy contributes to, or even drives, tumour development. The mitotic checkpoint guards against chromosome mis-segregation by delaying cell-cycle progression through mitosis until all chromosomes have successfully made spindle-microtubule attachments. Defects in the mitotic checkpoint generate aneuploidy and might facilitate tumorigenesis, but more severe disabling of checkpoint signalling is a possible anticancer strategy.

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

The mitotic spindle assembly checkpoint ensures proper chromosome segregation during mitosis by inhibiting the onset of anaphase until all kinetochores are attached to the mitotic spindle and tension across the kinetochores is generated. Here, we report that the stable partial downregulation of the spindle checkpoint gene MAD1, which is observed in human cancer, leads to a functional inactivation of the spindle checkpoint resulting in gross aneuploidy. Interestingly, although Mad1 is thought to act as a kinetochore based activator of Mad2 during checkpoint activation, we show that normal levels of Mad2, but not of Mad1, are required for preventing premature sister chromatid separation and for maintaining the timing of an undisturbed mitosis, suggesting a Mad1 independent function of Mad2 that operates independent of its checkpoint function. Most significantly, a partial repression of either MAD1 or MAD2 confers resistance to nocodazole, a drug that inhibits microtubule attachment. In contrast, sensitivity to clinically relevant drugs like taxol or monastrol that inhibit the generation of tension across kinetochores is not modulated by partial downregulation of MAD1, suggesting a functional bifurcation of spindle checkpoint dependent apoptotic pathways.

Throwing new light on lung cancer pathogenesis: updates on three recent topics

Lung cancers have become the leading cause of cancer deaths in Japan, claiming more than 55 000 lives annually. Unfortunately, substantial improvement in terms of cure rates has not been achieved over the last two decades, although during the same period of time in-depth basic knowledge of the molecular mechanisms, which underlies carcinogenesis and progression of this deadly group of neoplasms, has accumulated at an amazing pace. It has consequently become evident that they have many shared but also distinct features, when comparisons are made not only with other common epithelial cancers of adults, such as colon cancer, but also within the various histologic types of lung cancers themselves. This review article provides an up-date on cutting-edge research into the following three different topics, from which important new insights have been obtained. The first concerns genetic instability, especially chromosome instability, and checkpoint failure in lung cancers. Second, we deal with EGFR mutations, which shows revealing specificities in various aspects. Finally, advances in the expression profiling analysis of both transcriptomes and proteomes of lung cancers are summarized.

Localization of Mitotic Arrest Deficient 1 (MAD1) in Mouse Oocytes During the First Meiosis and Its Functions as a Spindle Checkpoint Protein1

The present study was designed to investigate the localization of mitotic arrest deficient 1 (MAD1) in mouse oocytes during meiotic maturation and its relationship with kinetochores, chromosomes, and microtubules. Oocytes at various stages during the first meiosis were fixed and immunostained for MAD1, kinetochores, microtubules, and chromosomes. The stained oocytes were examined by confocal microscopy. Some oocytes were treated with nocodazole or Taxol before examination. The anti-MAD1 antibody was injected into the oocytes at the germinal vesicle (GV) stage for examination of chromosome alignment and spindle formation. It was found that MAD1 was present in the oocytes from the GV to prometaphase I stages around the nuclei. When the oocytes reached the metaphase I (M-I) to metaphase II (M-II) stages, MAD1 was mainly localized at the spindle poles. However, MAD1 relocated to the vicinity of the chromosomes when spindles were disassembled by nocodazole or cooling, and the relocated MAD1 moved back to the spindle poles during spindle recovery. Taxol treatment did not affect the MAD1 localization. Although anti-MAD1 antibody injection did not affect nuclear maturation, significantly higher proportions of injected oocytes had misaligned chromosomes when the oocytes reached the M-I to M-II stages. The results of the present study indicate that MAD1 is present in mouse oocytes at all stages during the first meiosis and that it participates in spindle checkpoint during meiosis. However, MAD1 could not check misaligned chromosomes during spindle recovery after the spindles were destroyed by drug or cooling, which caused some chromosomes to scatter in the oocytes.

Inactivation of the RRB1-Pescadillo pathway involved in ribosome biogenesis induces chromosomal instability

Since chromosomal instability (CIN) is a hallmark of most cancer cells, it is essential to identify genes whose alteration results into this genetic instability. Using a yeast CIN indicator strain, we show that inactivation of the YMR131c/RRB1 gene, which is involved in early ribosome assembly and whose expression is induced when the spindle checkpoint is activated, alters chromosome segregation and blocks mitosis at the metaphase/anaphase transition. We demonstrate that RRB1 interacts with YPH1 (yeast pescadillo homologue 1) and other members of the Yph1 complex, RPL3, ERB1 and ORC6, involved in ribosome biogenesis and DNA replication. Transient depletion of the human homologues GRWD, Pescadillo, Rpl3, Bop1 and Orc6L resulted in an increase of abnormal mitoses with appearance of binucleate or hyperploid cells, of cells with multipolar spindles and of aberrant metaphase plates. If deregulation of proteins involved in ribosome biogenesis, commonly observed in malignant tumors, could contribute to cancer through an aberrant protein synthesis, our study demonstrates that alteration of proteins linking ribosome biogenesis and DNA replication may directly cause CIN.

The spindle checkpoint: from normal cell division to tumorigenesis

Faithful chromosome segregation during each cell division is regulated by the spindle checkpoint. This surveillance mechanism monitors kinetochore-microtubule attachment and the integrity of the mitotic apparatus, delaying mitotic exit until all chromosomes are properly aligned at the metaphase plate. Failure of this mechanism can generate gross aneuploidy. Since its discovery, mutations in genes involved in the spindle checkpoint response were predicted to be serious candidates for the chromosomal instability phenotype observed in many tumors. During the last few years, significant advances have been made in understanding the molecular basis of the spindle checkpoint. However, many studies of tumor cell lines and primary cancer isolates have failed to show a direct correlation with mutations in spindle checkpoint components. Nevertheless, it was shown that many tumor cells have an abnormal spindle checkpoint. Therefore, better understanding of the molecular mechanisms involved in regulation of spindle checkpoint response are expected to provide important clues regarding the mechanisms underlying the emergence of neoplasia.

Detailed characterization of a homozygously deleted region corresponding to a candidate tumor suppressor locus at distal 17p13.3 in human lung cancer

17p13.3 is one of the chromosomal regions most frequently affected by allelic loss in a variety of human neoplasms including lung cancer. A number of loss of heterozygosity (LOH) analyses have suggested the existence of a tumor suppressor gene at 17p13.3, distal to the p53 locus at 17p13.1. In the present study, we characterized a homozygous deletion at 17p13.3 in a small cell lung cancer cell line by constructing a bacterial artificial chromosome (BAC) contig and a restriction map surrounding the region, as well as by utilizing publicly available draft sequences. We defined the breakpoint, assigned and analysed two known genes, 14-3-3 epsilon and CRK, and a novel gene LOST1 within or at the end of the homozygous deletion of about 170 kb in size. Marked reduction of LOST1 expression was detected in 69% (11/16) of lung cancer specimens by quantitative real-time RT-PCR, while significant DNA hypermethylation was observed at the 5' end of the LOST1 gene in four of six lung cancer cell lines with negligible LOST1 expression. We also show here that a polymorphic marker D17S1174, which resides within the homozygous deletion, was apparently located in the middle of the minimum LOH region, providing further supportive evidence for the presence of a tumor suppressor gene(s) in this region.

Polo-like kinase 1 (PLK1) is overexpressed in primary colorectal cancers

PLK (polo-like kinase), the human counterpart of polo in Drosophila melanogaster and of CDC5 in Saccharomyces cerevisiae, belongs to a family of serine/threonine kinases. It is intimately involved in spindle formation and chromosome segregation during mitosis. The purpose of this study was to determine whether PLK1 is overexpressed in primary colorectal cancer specimens as compared with normal colon mucosa and to assess its relation to other kinases as a potential new tumor marker. In the present study, immunohistochemical analyses were performed of PLK1 expression in 78 primary colorectal cancers as well as 15 normal colorectal specimens. Furthermore, we examined the relationship between other kinases, Aurora-A and Aurora-C, and PLK1 expression. In normal colon mucosa, some crypt cells showed weakly positive staining for PLK1 in 13 out of 15 cases, the remaining cases being negative. Elevated expression of PLK1 was observed in 57 (73.1%) of the colorectal cancers, statistically significant associations being evident with pT (primary tumor invasion) (P=0.0006, Mann-Whitney U test), pN (regional lymph nodes) (P=0.008, chi2 test) and the Dukes' classification (P=0.0005, Mann-Whitney U test). Mean proliferating cell nuclear antigen-labeling index was 52.3%, with a range of 24.1% to 77.3%. Values for lesions with high and low PLK1 expression were 54.7+/-10.3% (mean+/-SD) and 45.9+/-11.9% (P=0.002, Student's t test). PLK1 was significantly associated with Aurora-A, but PLK1 staining was more diffuse and extensive than for Aurora-A or Aurora-C. Interestingly, PLK1 overexpression was significantly associated with p53 accumulation in colorectal cancers. Our results suggest overexpression of PLK1 might be of pathogenic, prognostic and proliferative importance, so that this kinase might have potential as a new tumor marker for colorectal cancers.

The promoter region of the human BUBR1 gene and its expression analysis in lung cancer

Mitotic checkpoint impairment is present in human lung cancers with chromosomal instability (CIN). Spindle-checkpoint genes have been reported to be mutated in several human cancers, but these mutations are infrequent. Recent reports suggest that the hBUBR1 gene may play an important role in mitotic checkpoint control and in mitotic checkpoint impairment in human cancers. We analyzed the expression of hBUBR1 in lung cancer cell lines using real time quantitative RT-PCR. The expression of BUBR1 was found to be up-regulated in all of these cell lines. In addition, we cloned and characterized the promotor region of hBUBR1 and determined its genomic structure, which includes 23 exons. The open reading frame (ORF) of the hBUBR1 gene comprises exons 1 through 23. There are GC-rich regions located at the flanking region and about 150 bp upstream from exon 1. The promoter region (424 bp upstream from exon 1) showed promoter activity and includes multiple transcription factor consensus binding motifs, including those for Sp1, Nkx-2, CdxA, SRY, MyoD, Ik-2, HNF-3b, Staf, Oct-1, Nkx-2, v-Myb, and AML 1a. Multiple pathways leading to activation of those binding factors may contribute to hBUBR1 gene transcription. Knowledge of the genomic structure and the promoter region of the hBUBR1 gene will facilitate investigation of its role in mitotic checkpoint control and tumor progression in human cancers.

Genetic alterations of multiple tumor suppressors and oncogenes in the carcinogenesis and progression of lung cancer

Lung cancer has become the leading cause of cancer death in many economically well-developed countries. Recent molecular biological studies have revealed that overt lung cancers frequently develop through sequential morphological steps, with the accumulation of multiple genetic and epigenetic alterations affecting both tumor suppressor genes and dominant oncogenes. Cell cycle progression needs to be properly regulated, while cells have built-in complex and minute mechanisms such as cell cycle checkpoints to maintain genomic integrity. Genes in the p16INK4A-RB and p14ARF-p53 pathways appear to be a major target for genetic alterations involved in the pathogenesis of lung cancer. Several oncogenes are also known to be altered in lung cancer, leading to the stimulation of autocrine/paracrine loops and activation of multiple signaling pathways. It is widely acknowledged that carcinogens in cigarette smoke are deeply involved in these multiple genetic alterations, mainly through the formation of DNA adducts. A current understanding of the molecular mechanisms of lung cancer pathogenesis and progression is presented in relation to cigarette smoking, an absolute major risk factor for lung cancer development, by reviewing genetic alterations of various tumor suppressor genes and oncogenes thus far identified in lung cancer, with brief summaries of their functions and regulation.

Chromosome instability in human lung cancers: possible underlying mechanisms and potential consequences in the pathogenesis

Chromosomal abnormality is one of the hallmarks of neoplastic cells, and the persistent presence of chromosome instability (CIN) has been demonstrated in human cancers, including lung cancer. Recent progress in molecular and cellular biology as well as cytogenetics has shed light on the underlying mechanisms and the biological and clinical significance of chromosome abnormalities and the CIN phenotype. Chromosome abnormalities can be classified broadly into numerical (i.e., aneuploidy) and structural alterations (e.g., deletion, translocation, homogenously staining region (HSR), double minutes (DMs)). However, both alterations usually occur in the same cells, suggesting some overlap in their underlying mechanisms. Missegregation of chromosomes may result from various causes, including defects of mitotic spindle checkpoint, abnormal centrosome formation and failure of cytokinesis, while structural alterations of chromosomes may be caused especially by failure in the repair of DNA double-strand breaks (DSBs) due to the impairment of DNA damage checkpoints and/or DSB repair systems. Recent studies also suggest that telomere erosion may be involved. The consequential acquisition of the CIN phenotype would give lung cancer cells an excellent opportunity to efficiently alter their characteristics so as to be more malignant and suitable to their microenvironment, thereby gaining a selective growth advantage.

Aberrant expression of cell-cycle regulator cyclin D1 in breast cancer is related to chromosomal genomic instability

To account for the accumulation of genomic alterations required for tumor progression, it has been suggested that the genomes of cancer cells are unstable and that this instability results from defective mutators (the "mutator phenotype" theory). To examine the hypothesis that abnormal cell-cycle regulators act as the mutators contributing to genomic instability, the present study, based on primary tumor tissues from 71 patients with breast cancer, was performed to determine whether there was an association between aberrant expression of cell-cycle regulators (cyclin A, cyclin D1, cyclin E, RB1, p21, and p27) and chromosomal instability. Comparative genomic hybridization was used to measure chromosomal changes, reflecting genomic instability in individual tumors, whereas immunohistochemistry was used to detect aberrant expression of cell-cycle regulators. Overexpression of cyclin D1 was found to be significantly correlated with increased chromosomal instability (defined as harboring more than 7 chromosomal changes), with 63% of tumors overexpressing and 27% of tumors not overexpressing, with cyclin D1 showing chromosomal instability (P < 0.05). Interestingly, this relationship was independent of cell outgrowth (as detected by the proliferation marker Ki-67) and was particularly significant in tumors not expressing p27 or in tumors with detectable RB1. These results suggest that cyclin D1 plays an alternative role in the regulation of genomic stability.

Aberrant hypermethylation of the CHFR prophase checkpoint gene in human lung cancers

The CHFR gene, which was recently cloned by Scolnick and Halazonetis in search for a novel mitotic checkpoint gene with fork-head association motifs, has been suggested to play a key role in the mitotic prophase checkpoint. In this study, we demonstrated tumor-specific aberrant hypermethylation of the promoter region of the CHFR gene in a significant fraction of lung cancers in association with loss of detectable levels of CHFR transcripts. Aberrant hypermethylation was observed in seven of 37 primary lung cancer cases. Treatment with the demethylating agent 5-aza-2'-deoxycytidine restored expression of the CHFR gene in lung cancer cell lines exhibiting aberrant hypermethylation and loss of its expression. In contrast, genetic alterations were found to be infrequent in lung cancers. This is the first description of aberrant hypermethylation of the CHFR gene in any type of human cancer, and provides further evidence of the involvement of multiple checkpoint alterations in lung cancer.

Frequent impairment of the spindle assembly checkpoint in hepatocellular carcinoma

BACKGROUND

Chromosomal instability (CI) leading to aneuploidy is one form of genetic instability, a characteristic feature of various types of cancers. Recent work has suggested that CI can be induced by a spindle assembly checkpoint defect. The aim of the current study was to determine the frequency of a defect of the checkpoint in hepatocellular carcinoma (HCC) and to establish whether alterations of genes encoding the checkpoint were associated with CI in HCC.

METHODS

Aneuploidy and the function of the spindle assembly checkpoint were examined using DNA flow cytometry and morphologic analysis with microtubule disrupting drugs. To explore the molecular basis, the authors examined the expression and alterations of the mitotic checkpoint gene, BUB1, using Northern hybridization and direct sequencing in 8 HCC cell lines and 50 HCC specimens. Furthermore, the authors examined the alterations of other mitotic checkpoint genes, BUBR1, BUB3, MAD2B, and CDC20, using direct sequencing in HCC cell lines with aneuploidy.

RESULTS

An impaired spindle assembly checkpoint was found in five (62.5%) of the eight aneuploid cell lines. Transcriptional expressions of the BUB1 gene appeared in all cell lines. While some polymorphic base changes were noted in BUB1, BUBR1, and CDC20, no mutations responsible for impairment of the mitotic checkpoint were found in either the HCC cell lines or HCC specimens, which suggests that these genes did not seem to be involved in tumor development in HCC.

CONCLUSIONS

The loss of spindle assembly checkpoint occurred with a high frequency in HCC with CI. However, other mechanisms might also contribute to CI in HCC.

Persistent increase in chromosome instability in lung cancer: possible indirect involvement of p53 inactivation

Karyotype and fluorescence in situ hybridization analyses have demonstrated the frequent presence of an altered static state of the number of chromosomes (ie, aneuploidy) in lung cancer, but it has not been directly established whether aneuploidy is in fact associated with a persistent increase in the rate of chromosomal losses and gains (ie, chromosome instability, or CIN). The study presented here used a panel of 10 lung cancer cell lines to provide for the first time direct evidence that CIN is a common feature in lung cancer cell lines in association with the presence of significant aneuploidy. In addition, we found that the CIN phenotype correlates well with the presence of p53 mutations. However, human papilloma virus 16-E6-directed inactivation of p53 in a representative non-CIN lung cancer cell line did not result in the induction of CIN, at least up to the 25th generation, suggesting that inactivation of p53 itself is unlikely to directly induce CIN in lung cancer cells. Interestingly, however, significant CIN could be induced in conjunction with the generation of aneuploid populations when the mitotic spindle formation was transiently abrogated in p53-inactivated cells. These results suggest that inactivation of p53 may allow lung cancer cells to go through an inappropriate second division cycle under certain forms of mitotic stresses, which would result in the induction of the CIN phenotype in conjunction with the generation of aneuploidy.

Mutations in the mitotic check point gene, MAD1L1, in human cancers

Aneuploidy is a characteristic of the majority of human cancers, and recent studies suggest that defects of mitotic checkpoints play a role in carcinogenesis. MAD1L1 is a checkpoint gene, and its dysfunction is associated with chromosomal instability. Rare mutations of this gene have been reported in colon and lung cancers. We examined a total of 44 cell lines (hematopoietic, prostate, osteosarcoma, breast, glioblastoma and lung) and 133 fresh cancer cells (hematopoietic, prostate, breast and glioblastoma) for alterations of MAD1L1 by RT-PCR-SSCP and nucleotide sequencing. Eight mutations consisting of missense, nonsense and frameshift mutations were found, together with a number of nucleotide polymorphisms. All the alterations in cell lines were heterozygous. Frequency of mutations was relatively high in prostate cancer (2/7 cell lines and 2/33 tumor specimens). We placed a mutant truncated MAD1L1, found in a lymphoma sample, into HOS, Ht161 and SJSA cell lines and found that it was less inhibitory than wild type MAD1L1 at decreasing cell proliferation. Co-expression experiments showed that the mutant form had a dominant-negative effect. Furthermore, this mutant impaired the mitotic checkpoint as shown by decreased mitotic indices in HOS cells expressing mutant MAD1L1 after culture with the microtubule-disrupting agent, nocodazole. Our results suggest a pathogenic role of MAD1L1 mutations in various types of human cancer.

Molecular analysis of the mitotic checkpoint genes BUB1, BUBR1 and BUB3 in human lung cancers

Our previous studies showed that mitotic checkpoint impairment is present in about 40% of human lung cancer cell lines but that mutations in the MAD mitotic checkpoint genes are infrequent. In the present study, we examined 44 lung cancer cases for the potential involvement of the other gene family involved in the mitotic checkpoint, i.e. BUB. We found that the BUB gene family members including BUB1, BUBR1 and BUB3 are not frequent targets for mitotic checkpoint defects in lung cancers, if present at all. Further studies are thus warranted to elucidate the molecular basis for the acquisition of mitotic checkpoint defects in order to better understand the molecular pathogenesis of lung cancers.

Pleiotropic effects of HTLV type 1 Tax protein on cellular metabolism: mitotic checkpoint abrogation and NF-kappaB activation

Tax protein expressed by human T cell leukemia virus type 1 (HTLV-1) is a strong trans-activator of its own LTR promoter; it also affects the function of multiple cellular genes involved in cell cycle control and transcription. One way in which Tax exerts its pleiotropic effects is through protein-protein interaction with cellular cofactors. By using yeast two-hybrid technology, we have isolated several cellular proteins that bind to Tax. Two of these are MAD1, a mitotic checkpoint control protein, and TXBP151, a suppressor of tumor necrosis factor alpha-induced apoptosis. Here we discuss findings describing the role of MAD1 in exit of cells from mitosis and TXBP151 in NF-kappaB activation.

Chromosome missegregation and apoptosis in mice lacking the mitotic checkpoint protein Mad2

The initiation of chromosome segregation at anaphase is linked by the spindle assembly checkpoint to the completion of chromosome-microtubule attachment during metaphase. To determine the function of the mitotic checkpoint protein Mad2 during normal cell division and when mitosis goes awry, we have knocked out Mad2 in mice. We find that E5.5 embryonic cells lacking Mad2, like mad2 yeast, grow normally but are unable to arrest in response to spindle disruption. At E6.5, the cells of the epiblast begin rapid cell division and the absence of a checkpoint results in widespread chromosome missegregation and apoptosis. In contrast, the postmitotic trophoblast giant cells survive without Mad2. Thus, the spindle assembly checkpoint is required for accurate chromosome segregation in mitotic mouse cells, and for embryonic viability, even in the absence of spindle damage.