Biomedical subjects

William S Saunders

Publications and source records attributed to William S Saunders.

11 recordsLinked to original sources

PubMed2007

Relationship between FRA11F and 11q13 gene amplification in oral cancer.

Common fragile sites (CFS) are nonstaining gaps or breaks in chromosomes that are expressed under conditions inducing replicative stress. CFS have been suggested to play a role in epithelial cancers by their association with loss of heterozygosity, loss of gene expression, and/or gene amplification in the form of homogeneously staining regions (hsrs). In oral squamous-cell carcinomas (OSCC), amplification of chromosomal band 11q13 occurs in the form of an hsr. We suggested previously that CFS flanking 11q13 may be susceptible to breakage induced by tobacco or other carcinogens and/or human papillomavirus, promoting formation of the 11q13 amplicon. Examination of OSCC cell lines with 11q13 amplification using fluorescence in situ hybridization showed loss of FRA11F sequences, whereas cell lines without 11q13 amplification displayed an intact FRA11F site. Cell lines with more complex 11q rearrangements expressed FRA11F in the form of an inverted duplication, characteristic of breakage-fusion-bridge cycles. Our findings suggest that gene amplification involving chromosomal band 11q13 in OSCC may be initiated by breakage at FRA11F.

Carcinoma, Squamous Cell↗

PubMed2006

Overexpression of Cdc20 leads to impairment of the spindle assembly checkpoint and aneuploidization in oral cancer.

Defects in the spindle assembly checkpoint are thought to be responsible for an increased rate of aneuploidization during tumorigenesis. Despite a plethora of information on the correlation between BUB-MAD gene expression levels and defects in the spindle checkpoint, very little is known about alteration of another important spindle checkpoint protein, Cdc20, in human cancer and its role in tumor aneuploidy. We observed overexpression of CDC20 in several oral squamous cell carcinoma (OSCC) cell lines and primary head and neck tumors and provide evidence that such overexpression of CDC20 is associated with premature anaphase promotion, resulting in mitotic abnormalities in OSCC cell lines. We also reconstituted the chromosomal instability phenotype in a chromosomally stable OSCC cell line by overexpressing CDC20. Thus, abnormalities in the cellular level of Cdc20 may deregulate the timing of anaphase promoting complex (APC/C) in promoting premature anaphase, which often results in aneuploidy in the tumor cells.

Anaphase-Promoting Complex-Cyclosome↗

PubMed2005

Cik1 targets the minus-end kinesin depolymerase kar3 to microtubule plus ends.

Kar3, a Saccharomyces cerevisiae Kinesin-14, is essential for karyogamy and meiosis I but also has specific functions during vegetative growth. For its various roles, Kar3 forms a heterodimer with either Cik1 or Vik1, both of which are noncatalytic polypeptides. Here, we present the first biochemical characterization of Kar3Cik1, the kinesin motor that is essential for karyogamy. Kar3Cik1 depolymerizes microtubules from the plus end and promotes robust minus-end-directed microtubule gliding. Immunolocalization studies show that Kar3Cik1 binds preferentially to one end of the microtubule, whereas the Kar3 motor domain, in the absence of Cik1, exhibits significantly higher microtubule lattice binding. Kar3Cik1-promoted microtubule depolymerization requires ATP turnover, and the kinetics fit a single exponential function. The disassembly mechanism is not microtubule catastrophe like that induced by the MCAK Kinesin-13s. Soluble tubulin does not activate the ATPase activity of Kar3Cik1, and there is no evidence of Kar3Cik1(.)tubulin complex formation as observed for MCAK. These results reveal a novel mechanism to regulate microtubule depolymerization. We propose that Cik1 targets Kar3 to the microtubule plus end. Kar3Cik1 then uses its minus-end-directed force to depolymerize microtubules from the plus end, with each tubulin-subunit release event tightly coupled to one ATP turnover.

Adenosine Triphosphate↗

PubMed2005

Spindle multipolarity is prevented by centrosomal clustering.

Most tumor cells are characterized by increased genomic instability and chromosome segregational defects, often associated with hyperamplification of the centrosome and the formation of multipolar spindles. However, extra centrosomes do not always lead to multipolarity. Here, we describe a process of centrosomal clustering that prevented the formation of multipolar spindles in noncancer cells. Noncancer cells needed to overcome this clustering mechanism to allow multipolar spindles to form at a high frequency. The microtubule motor cytoplasmic dynein was a critical part of this coalescing machinery, and in some tumor cells overexpression of the spindle protein NuMA interfered with dynein localization, promoting multipolarity.

Antigens, Nuclear↗

PubMed2004

Cigarette smoke induces anaphase bridges and genomic imbalances in normal cells.

Exposure to cigarette smoke has long been linked to carcinogenesis, but the emphasis has been placed on mutational changes in the DNA sequence caused by the carcinogens in smoke. Here, we report an additional role for cigarette smoke exposure in contributing to chromosomal aberrations in cells. We have found that cigarette smoke condensate (CSC) induces anaphase bridges in cultured human cells, which in a short time lead to genomic imbalances. The frequency of the induced bridges within the entire population decreases with time, and this decrease is not dependent upon the p53-mediated apoptotic pathway. Additionally, we show that CSC induces DNA double stranded breaks (DSBs) in cultured cells and purified DNA. The reactive oxygen species (ROS) scavenger, 2' deoxyguanosine 5'-monophosphate (dGMP) prevents CSC-induced DSBs, anaphase bridge formation and genomic imbalances. Therefore, we propose that CSC induces bridges and genomic imbalances via DNA DSBs. Furthermore, since the amount of CSC added to the cultures was substantially less than that extracted from a single cigarette, our results show that even low levels of cigarette smoke can cause irreversible changes in the chromosomal constitution of cultured cells.

Antioxidants↗

PubMed2004

Resolution of anaphase bridges in cancer cells.

Chromosomal instability is a key step in the generation of the cancer cell karyotype. An indicator of unstable chromosomes is the presence of chromatin bridges during anaphase. We examined in detail the fate of anaphase bridges in cultured oral squamous cell carcinoma cells in real-time. Surprisingly, chromosomes in bridges typically resolve by breaking into multiple fragments. Often these fragments give rise to micronuclei (MN) at the end of mitosis. The formation of MN is shown to have important consequences for the cell. We found that MN have incomplete nuclear pore complex (NPC) formation and nuclear import defects and the chromatin within has greatly reduced transcriptional activity. Thus, a major consequence of the presence of anaphase bridges is the regular sequestration of chromatin into genetically inert MN. This represents another source of ongoing genetic instability in cancer cells.

Anaphase↗

PubMed2004

The occurrence of chromosome segregational defects is an intrinsic and heritable property of oral squamous cell carcinoma cell lines.

Chromosomal segregational defects are commonly observed in cancer cells and are an important source of genetic instability. It is currently unknown whether these mitotic defects are the result of a subpopulation of defective cells or reflect characteristics of the population of cells as a whole. In this study, we compared chromosomal segregational defects in two oral squamous cell carcinoma cell lines and five single-cell clones from each of those cell lines. We used immunofluorescence microscopy to quantitate the occurrence of multipolar metaphase spindles, lagging chromosomes at metaphase and anaphase, and anaphase bridges. We conclude that chromosome segregational defects in these cancer cell lines represent an intrinsic and inherited tendency toward segregational defects in the general cell population, rather than the existence of a subpopulation of cells with segregational defects.

Carcinoma, Squamous Cell↗

PubMed2004

Chromosomal instability and marker chromosome evolution in oral squamous cell carcinoma.

Squamous cell carcinoma of the head and neck and its subset, oral squamous cell carcinoma (OSCC), arise through a multistep process of genetic alterations as a result of exposure to environmental agents, such as tobacco smoke, alcoholic beverages, and viruses, including human papillomavirus. We and others have shown that the karyotypes of OSCC are near-triploid and contain multiple structural and numerical abnormalities. However, despite a background of clonal chromosomal aberrations, individual cells within a culture express many nonclonal numerical and structural abnormalities, termed chromosomal instability (CIN). To evaluate CIN in oral cancer cells, we isolated clones from two OSCC cell lines and carried out classical cytogenetic analysis, fluorescence in situ hybridization using centromere-specific probes, and spectral karyotyping. We observed variation in chromosome number within clones and between clones of the same cell line. Although similar numbers of centromeric signals for a particular chromosome were present, "homologs" of a chromosome varied structurally from cell to cell (marker chromosome evolution) as documented by classical and spectral karyotyping. In addition to the numerical chromosome variations within a clone, we observed marker chromosome evolution by structural chromosome alterations. It appears that both intrinsic structural alterations and extrinsic cytoskeletal factors influence chromosome segregation, resulting in individual tumor cells that express unique karyotypes. We show that CIN and marker chromosome evolution are essential acquired features of neoplastic cells. Proliferation of this heterogeneous cell population may provide some cells with the ability to evade standard therapies.

Biomarkers↗

PubMed2003

Large-scale functional genomic analysis of sporulation and meiosis in Saccharomyces cerevisiae.

We have used a single-gene deletion mutant bank to identify the genes required for meiosis and sporulation among 4323 nonessential Saccharomyces cerevisiae annotated open reading frames (ORFs). Three hundred thirty-four sporulation-essential genes were identified, including 78 novel ORFs and 115 known genes without previously described sporulation defects in the comprehensive Saccharomyces Genome (SGD) or Yeast Proteome (YPD) phenotype databases. We have further divided the uncharacterized sporulation-essential genes into early, middle, and late stages of meiosis according to their requirement for IME1 induction and nuclear division. We believe this represents a nearly complete identification of the genes uniquely required for this complex cellular pathway. The set of genes identified in this phenotypic screen shows only limited overlap with those identified by expression-based studies.

Genes, Fungal↗

PubMed2002

The FEAR factor.

The decision to exit from mitosis is a fateful one, and Amon and colleagues have shown that it involves both an early and a late component. The FEAR network initiates the exit machinery, while the previously described MEN pathway maintains it.

Anaphase↗

PubMedDate not supplied

The FEAR Before MEN: networks of mitotic exit.

Variations in the normal regulation of the mitotic cell cycle can lead to such global cellular changes as differential development or malignant transformation. Studies on the control of mitosis are particularly important to discover the details of the basic mechanisms responsible for normal cell division, as well as to learn about strategies employed by cancerous cells to indefinitely proliferate. The past years have brought noteworthy progress in elucidating the molecular pathways that regulate crucial events during mitosis such as: chromosome condensation, formation of the mitotic spindle, chromosome segregation, cytokinesis, and disassembly of the mitotic spindle.

Cell Cycle Proteins↗