Chromosome instability (CIN) is a hallmark of many tumors and correlates with the presence of extra centrosomes1-4. However, a direct mechanistic link between extra centrosomes and CIN has not been established. It has been proposed that extra centrosomes generate CIN by promoting multipolar anaphase, a highly abnormal division that produces 3 or more aneuploid daughter cells. Here, we use long-term live-cell imaging to demonstrate that cells with multiple centrosomes rarely undergo multipolar cell divisions, and the progeny of these divisions are typically inviable. Thus, multipolar divisions cannot explain observed rates of CIN. By contrast, we observe that CIN cells with extra centrosomes routinely undergo bipolar cell divisions, but display a significantly elevated frequency of lagging chromosomes during anaphase. To define the mechanism underlying this mitotic defect, we generated cells that differ only in their centrosome number. We demonstrate that extra centrosomes alone are sufficient to promote chromosome missegregation during bipolar cell division. These segregation errors are a consequence of cells passing through a transient ‘multipolar spindle intermediate’ in which merotelic kinetochore-microtubule attachment errors accumulate prior to centrosome clustering and anaphase. These findings provide a direct mechanistic link between extra centrosomes and CIN, two common characteristics of solid tumors. We propose that this mechanism may be a common underlying cause of CIN in human cancer.

Multiple centrosomes in tumor cells create the potential for multipolar divisions that can lead to aneuploidy and cell death. Nevertheless, many cancer cells successfully divide because of mechanisms that suppress multipolar mitoses. A genome-wide RNAi screen in Drosophila S2 cells and a secondary analysis in cancer cells defined mechanisms that suppress multipolar mitoses. In addition to proteins that organize microtubules at the spindle poles, we identified novel roles for the spindle assembly checkpoint, cortical actin cytoskeleton, and cell adhesion. Using live cell imaging and fibronectin micropatterns, we found that interphase cell shape and adhesion pattern can determine the success of the subsequent mitosis in cells with extra centrosomes. These findings may identify cancer-selective therapeutic targets: HSET, a normally nonessential kinesin motor, was essential for the viability of certain extra centrosome-containing cancer cells. Thus, morphological features of cancer cells can be linked to unique genetic requirements for survival.[Keywords: Centrosomes; mitosis; actin; adhesion; cancer; cell cycle] Supplemental material is available at http://www.genesdev.org.

Centrosome amplification has long been recognized as a feature of human tumors, however its role in tumorigenesis remains unclear 1. Centrosome amplification is poorly tolerated by non-transformed cells, and, in the absence of selection, extra centrosomes are spontaneously lost 2. Thus, the high frequency of centrosome amplification, particularly in more aggressive tumors 3, raises the possibility that extra centrosomes could, in some contexts, confer advantageous characteristics that promote tumor progression. Using a three-dimensional model system and other approaches to culture human mammary epithelial cells, we find that centrosome amplification triggers cell invasion. This invasive behavior is similar to that induced by overexpression of the breast cancer oncogene ErbB2 4 and indeed enhances invasiveness triggered by ErbB2. We show that, through increased centrosomal microtubule nucleation, centrosome amplification increases Rac1 activity, which disrupts normal cell-cell adhesion and promotes invasion. These findings demonstrate that centrosome amplification, a structural alteration of the cytoskeleton, can promote features of malignant transformation.

Germinal centers (GC) are sites of intense B cell proliferation, central for T cell dependent antibody responses. However, the role of MYC, a key cell cycle regulator, in this process has been questioned. Here, we identified MYC positive B cell subpopulations in immature and mature GCs, and show through genetic ablation of Myc that they play indispensable roles in GC formation and maintenance. The identification of these functionally critical cellular subsets has important implications for human B cell lymphomagenesis, which mostly originates from GC B cells and frequently involves MYC chromosomal translocations. As these translocations are generally dependent on transcription of the recombining partner loci, the MYC positive GC subpopulations may be at a particularly high risk for malignant transformation.

Centrosome amplification is a hallmark of cancer. However, despite significant progress in recent years, we are still far from understanding how centrosome amplification affects tumorigenesis. Boveri's hypothesis formulated more than 100 years ago was that aneuploidy induced by centrosome amplification promoted tumorigenesis. Although the hypothesis remains appealing 100 years later, it is also clear that the role of centrosome amplification in cancer is more complex than initially thought. Here, we review how centrosome abnormalities are generated in cancer and the mechanisms cells employ to adapt to centrosome amplification, in particular centrosome clustering. We discuss the different mechanisms by which centrosome amplification could contribute to tumour progression and the new advances in the development of therapies that target cells with extra centrosomes.

Centrioles are microtubule-derived structures that are essential to form centrosomes, cilia and flagella. The centrosome is the major microtubule organiser in animal cells, participating in a variety of processes from cell polarization to cell division, while cilia and flagella contribute to several mechanisms in eukaryotic cells from motility to sensing. Although it was suggested more than a century ago that these microtubule-derived structures are involved in human disease, the molecular bases of this association have only recently been discovered. Surprisingly, there is very little overlap between the genes affected in the different diseases, suggesting there are tissue-specific requirements for these microtubule-derived structures. Knowledge of these requirements and disease mechanisms has opened new avenues for therapeutical strategies. Here, we give an overview of recent developments in this field focusing on cancer, diseases of brain development and ciliopathies.

Precise control of centrosome number is crucial for bipolar spindle assembly and accurate transmission of genetic material to daughter cells. Failure to properly control centrosome number results in supernumerary centrosomes, which are frequently found in cancer cells. This presents a paradox: during mitosis, cells with more than two centrosomes are prone to multipolar mitoses and cell death, however, cancer cells possessing extra centrosomes usually divide successfully. One mechanism frequently utilized by cancer cells to escape death caused by multipolar mitoses is the clustering of supernumerary centrosomes into bipolar arrays. An understanding of the molecular mechanisms by which cancer cells can suppress multipolar mitoses is beginning to emerge. Here, we review what's currently known about centrosome clustering mechanisms and discuss potential strategies to target these mechanisms for the selective killing of cancer cells.

Tandemly repeated ribosomal DNA (rDNA) arrays are among the most evolutionary dynamic loci of eukaryotic genomes. The loci code for essential cellular components, yet exhibit extensive copy number (CN) variation within and between species. CN might be partly determined by the requirement of dosage balance between the 5S and 45S rDNA arrays. The arrays are nonhomologous, physically unlinked in mammals, and encode functionally interdependent RNA components of the ribosome. Here we show that the 5S and 45S rDNA arrays exhibit concerted CN variation (cCNV). Despite 5S and 45S rDNA elements residing on different chromosomes and lacking sequence similarity, cCNV between these loci is strong, evolutionarily conserved in humans and mice, and manifested across individual genotypes in natural populations and pedigrees. Finally, we observe that bisphenol A induces rapid and parallel modulation of 5S and 45S rDNA CN. Our observations reveal a novel mode of genome variation, indicate that natural selection contributed to the evolution and conservation of cCNV, and support the hypothesis that 5S CN is partly determined by the requirement of dosage balance with the 45S rDNA array. We suggest that human disease variation might be traced to disrupted rDNA dosage balance in the genome.nucleolus | ribosome | gene dosage balance | concerted evolution | bisphenol A R epeated gene arrays have provided unique challenges to genetic and genome analyses, and have remained among the most elusive components of eukaryotic genomes (1, 2). Tandemly repeated loci of high copy number (CN) are labile, evolutionary dynamic, often subjected to concerted evolution of DNA sequences, and display abundant CN variation that emerges from high rates of repeat expansion and contraction (1). Moreover, natural selection contributes to the determination of gene CN, shaping rapid gene amplification in cancer, balanced gene loss after whole genome duplication, and optimal gene CN in locally adapted populations (3-5). For example, higher CN of the amylase gene is present in populations with starch-rich diets across organisms as diverse as humans, dogs, and fungi (3, 6, 7). Remarkably, gene CN may also be developmentally amplified in specific tissues to ensure rates of transcription in genes with high transcriptional demands (8,9). This is the case, for instance, of the chorion genes in Drosophila, which are amplified up to 80 fold in ovarian cells (10).The ribosomal DNA (rDNA) arrays display substantial CN variation within and between species (2, 11-16). The variation is functionally relevant with rDNA CN polymorphism modifying chromatin states and gene expression across the genome in humans and flies (17)(18)(19). In mammals, the rDNA arrays are dispersed across several chromosomes, and encode the four rRNAs that account for more than 60% of all transcription in the cell (20,21). Transcription of rDNA loci varies with cell and tissue type and is epigenetically regulated with allelic specificity (22, 23). The four rRNAs are indispensable structural and catalyti...