Mutation and expression analysis of human BUB1 and BUB1B in aneuploid breast cancer cell lines

Genetic and transcriptional analysis of spindle checkpoint genes in bone marrow failure patients

The evolution of bone marrow failure syndromes such as aplastic anemia (AA) to clonal hematologic diseases such as myelodysplastic syndrome is well recognized. Cytogenetic abnormalities are commonly seen late events, particularly aneuploidy of chromosomes 7 and 8. A proportion of bone marrow failure patients may also develop aneuploidy that is detectable by fluorescence in situ hybridization but not by standard cytogenetic analysis. The molecular basis for aneuploidy in this setting is currently unknown but may include abnormalities in the mitotic spindle checkpoint. For this reason, we searched for mutations in the mitotic spindle checkpoint genes hBUB1 and hMAD2, and also examined the expression of hBUB1 in cells of bone marrow failure patients. No pathogenic mutations were found in 59 patients. Of 170 bone marrow failure patients, less than one-third expressed hBUB1 transcript. Gene expression profiling confirmed a significant down-regulation of hBUB1 message in patients. We conclude that mutations in mitotic spindle checkpoint genes do not account for aneuploidy in marrow failure states. However, we cannot exclude epigenetic inactivation of hBUB1 as a potential mechanism in some patients.

AURORA-A amplification overrides the mitotic spindle assembly checkpoint, inducing resistance to Taxol

The serine-threonine kinase gene AURORA-A is commonly amplified in epithelial malignancies. Here we show that elevated Aurora-A expression at levels that reflect cancer-associated gene amplification overrides the checkpoint mechanism that monitors mitotic spindle assembly, inducing resistance to the chemotherapeutic agent paclitaxel (Taxol). Cells overexpressing Aurora-A inappropriately enter anaphase despite defective spindle formation, and the persistence of Mad2 at the kinetochores, marking continued activation of the spindle assembly checkpoint. Mitosis is subsequently arrested by failure to complete cytokinesis, resulting in multinucleation. This abnormality is relieved by an inhibitory mutant of BUB1, linking the mitotic abnormalities provoked by Aurora-A overexpression to spindle checkpoint activity. Consistent with this conclusion, elevated Aurora-A expression causes resistance to apoptosis induced by Taxol in a human cancer cell line.

Characterization of regions in hsMAD1 needed for binding hsMAD2. A polymorphic change in an hsMAD1 leucine zipper affects MAD1-MAD2 interaction and spindle checkpoint function

In eukaryotes, the mitotic spindle assembly checkpoint provides a monitor for the fidelity of chromosomal segregation. In this context, the mitotic arrest deficiency protein 2 (MAD2) censors chromosomal mis-segregation by monitoring microtubule attachment/tension, a role that requires its attachment to kinetochores. Studies in yeast have shown that binding of MAD1 to MAD2 is important for the checkpoint function of the latter. The interactions between human MAD1 (hsMAD1) and human MAD2 (hsMAD2) have, however, remained poorly characterized. Here we report that two leucine zipper domains (amino acids 501-522 and 557-571) in hsMAD1 are required for its contact with hsMAD2. Interestingly, in several cancer cell lines, we noted the frequent presence of a coding single nucleotide Arg to His polymorphism at codon 558 located within the second leucine zipper of hsMAD1. We found that hsMAD1H558 is less proficient than hsMAD1R558 in binding hsMAD2 and in enforcing mitotic arrest. We also document a first example of loss-of-heterozygosity for a spindle checkpoint gene (at the hsMAD1 558 locus) in a human breast cancer. Based on our findings, it is possible that hsMAD1H558 could be an at-risk polymorphism that contributes to attenuated spindle checkpoint function in human cells.

G(1) and G(2) cell-cycle arrest following microtubule depolymerization in human breast cancer cells

Microtubule-depolymerizing agents are widely used to synchronize cells, screen for mitotic checkpoint defects, and treat cancer. The present study evaluated the effects of these agents on normal and malignant human breast cell lines. After treatment with 1 microM nocodazole, seven of ten breast cancer lines (type A cells) arrested in mitosis, whereas the other three (type B cells) did not. Similar effects were observed with 100 nM vincristine or colchicine. Among five normal mammary epithelial isolates, four exhibited type A behavior and one exhibited type B behavior. Further experiments revealed that the type B cells exhibited a biphasic dose-response curve, with mitotic arrest at low drug concentrations (100 nM nocodazole or 6 nM vincristine) that failed to depolymerize microtubules and a p53-independent p21(waf1/cip1)-associated G(1) and G(2) arrest at higher concentrations (1 microM nocodazole or 100 nM vincristine) that depolymerized microtubules. Collectively, these observations provide evidence for coupling of premitotic cell-cycle progression to microtubule integrity in some breast cancer cell lines (representing a possible "microtubule integrity checkpoint") and suggest a potential explanation for the recently reported failure of some cancer cell lines to undergo nocodazole-induced mitotic arrest despite intact mitotic checkpoint proteins.

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.

Prevalent loss of mitotic spindle checkpoint in adult T-cell leukemia confers resistance to microtubule inhibitors

Human T-cell leukemia virus type I (HTLV-I) is the causative agent for adult T-cell leukemia (ATL). Molecularly, ATL cells have extensive aneugenic abnormalities that occur, at least in part, from cell cycle dysregulation by the HTLV-I-encoded Tax oncoprotein. Here, we compared six HTLV-I-transformed cells to Jurkat and primary peripheral blood mononuclear cells (PBMC) in their responses to treatment with microtubule inhibitors. We found that both Jurkat and PBMCs arrested efficiently in mitosis when treated with nocodazole. By contrast, all six HTLV-I cells failed to arrest comparably in mitosis, suggesting that ATL cells have a defect in the mitotic spindle assembly checkpoint. Mechanistically, we observed that in HTLV-I Tax-expressing cells human spindle assembly checkpoint factors hsMAD1 and hsMAD2 were mislocated from the nucleus to the cytoplasm. This altered localization of hsMAD1 and hsMAD2 correlated with loss of mitotic checkpoint function and chemoresistance to microtubule inhibitors.

Expression of hBUB1 in acute myeloid leukemia

hBUB1 gene is a component of the mitotic checkpoint that monitors proper assembly of the mitotic spindles and the alteration of the hBUB1 gene has been found to be associated with chromosomal instability in some tumor cell lines. We analyzed the coding region of the hBUB1 gene for mutations and its expression in 92 acute myeloid leukemia (AML) specimens and five hematopoietic cell lines. We also used Southern hybridization to analyze the genomic DNA of those cases, which had aberrant transcription to confirm the lesion. A thymine/cytosine polymorphism at 8 bp upstream of the 5' splice acceptor site of exon 10 was observed in Raji cell line and two AML specimens without a resultant change in the expression of hBUB1. Reduced expression and aberrant transcription of the hBUB1 gene, which may affect the control of mitotic checkpoint, were detected in AML specimens by reverse transcription-polymerase chain reaction (RT-PCR) analysis. Our study suggests that the mutation of the hBUB1 gene is a rare event in AML, and further studies are necessary to clarify its role in leukemia.

Molecular analyses of the mitotic checkpoint components hsMAD2, hBUB1 and hBUB3 in human cancer

During the metaphase-anaphase transition, the spindle checkpoint prevents segregation of chromosomes if the spindle assembly is perturbed. Critical components of this checkpoint are the MAD and BUB families of proteins, which prevent the proteolysis of Pds1 and B cyclins, producing mitotic arrest. In the present study, we first intended to resolve the role of the hsMAD2 gene in human cancer by determining the potential presence of hsMAD2 mutations in 44 primary bladder tumors, 42 soft-tissue sarcomas and 10 hepatocellular carcinomas. The entire coding region of the hsMAD2 gene was analyzed using PCR-SSCP and sequencing. One of the bladder tumor samples showed a point mutation consisting of a transition, ATC-->GTC (Ile-->Val) in codon 190 of hsMAD2. However, no differences were found in the mitotic arrest between cells transfected with mutant and wild-type MAD2 cDNA. We also identified mobility shifts in hsMAD2 in both normal and tumor DNA in 3 bladder tumors, 3 soft-tissue sarcomas and 1 hepatocellular carcinoma, consistent with a polymorphism at codon 143, CCA-->CCG (Pro-->Pro). Another polymorphism was identified in a hepatocellular carcinoma case at codon 22, GAG-->GAA (Glu-->Glu). In addition, a subgroup of 67 primary tumors was analyzed by Southern blot hybridization. No deletion or visible re-arrangements were detected by comparing tumor and normal DNA band signals. Two other important components of the spindle mitotic checkpoint, hBUB1 and hBUB3, were also screened for mutations: hBUB1 in 43 bladder tumors and 9 bladder cell lines and hBUB3 only in the cell lines. Two polymorphisms were found in hBUB1 at positions 144, CAG-->CAA (Gln-->Gln) in 1 primary tumor and 1 bladder cell line, and 913 (ATC-->ATT, Ile-->Ile) in 1 primary tumor. We did not find sequence alterations in hBUB3. These results suggest that mutations of the hsMAD2, hBUB1 and hBUB3 genes are very rare in bladder tumors and that hsMAD2 alterations are also infrequent in soft-tissue sarcomas and hepatocellular carcinomas.

Aneuploid colon cancer cells have a robust spindle checkpoint

Colon cancer cells frequently display minisatellite instability (MIN) or chromosome instability (CIN). While MIN is caused by mismatch repair defects, the lesions responsible for CIN are unknown. The observation that CIN cells fail to undergo mitotic arrest following spindle damage suggested that mutations in spindle checkpoint genes may account for CIN. However, here we show that CIN cells do undergo mitotic arrest in response to spindle damage. Although the maximum mitotic index achieved by CIN lines is diminished relative to MIN lines, CIN cells clearly have a robust spindle checkpoint. Consistently, mutations in spindle checkpoint genes are rare in human tumours. In contrast, the adenomatous polyposis coli (APC) gene is frequently mutated in CIN cells. Significantly, we show here that expression of an APC mutant in MIN cells reduces the mitotic index following spindle damage to a level observed in CIN cells, suggesting that APC dysfunction may contribute to CIN.