Supernumerary centrosomes and cancer: Boveri's hypothesis resurrected

Integrating the DNA damage and protein stress responses during cancer development and treatment

During evolution, cells have developed a wide spectrum of stress response modules to ensure homeostasis. The genome and proteome damage response pathways constitute the pillars of this interwoven 'defensive' network. Consequently, the deregulation of these pathways correlates with ageing and various pathophysiological states, including cancer. In the present review, we highlight: (1) the structure of the genome and proteome damage response pathways; (2) their functional crosstalk; and (3) the conditions under which they predispose to cancer. Within this context, we emphasize the role of oncogene-induced DNA damage as a driving force that shapes the cellular landscape for the emergence of the various hallmarks of cancer. We also discuss potential means to exploit key cancer-related alterations of the genome and proteome damage response pathways in order to develop novel efficient therapeutic modalities. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Functions and dysfunctions of the mammalian centrosome in health, disorders, disease, and aging

Since its discovery well over 100 years ago (Flemming, in Sitzungsber Akad Wissensch Wien 71:81-147, 1875; Van Beneden, in Bull Acad R Belg 42:35-97, 1876) the centrosome is increasingly being recognized as a most impactful organelle for its role not only as primary microtubule organizing center (MTOC) but also as a major communication center for signal transduction pathways and as a center for proteolytic activities. Its significance for cell cycle regulation has been well studied and we now also know that centrosome dysfunctions are implicated in numerous diseases and disorders including cancer, Alstrom syndrome, Bardet-Biedl syndrome, Huntington's disease, reproductive disorders, and several other diseases and disorders. The present review is meant to build on information presented in the previous review (Schatten, in Histochem Cell Biol 129:667-686, 2008) and to highlight functions of the mammalian centrosome in health, and dysfunctions in disorders, disease, and aging with six sections focused on (1) centrosome structure and functions, and new insights into the role of centrosomes in cell cycle progression; (2) the role of centrosomes in tumor initiation and progression; (3) primary cilia, centrosome-primary cilia interactions, and consequences for cell cycle functions in health and disease; (4) transitions from centrosome to non-centrosome functions during cellular polarization; (5) other centrosome dysfunctions associated with the pathogenesis of human disease; and (6) centrosome functions in oocyte germ cells and dysfunctions in reproductive disorders and reproductive aging.

Defect driven shapes in nematic droplets: analogies with cell division

Building on the striking similarity between the structure of the spindle during mitosis in living cells and nematic textures in confined liquid crystals, we use a continuum model of two-dimensional nematic liquid crystal droplets to examine the physical aspects of cell division. The model investigates the interplay between bulk elasticity of the microtubule assembly, described as a nematic liquid crystal, and surface elasticity of the cell cortex, modeled as a bounding flexible membrane, in controlling cell shape and division. The centrosomes at the spindle poles correspond to the cores of the topological defects required to accommodate nematic order in a closed geometry. We map out the progression of both healthy bipolar and faulty multi-polar division as a function of an effective parameter that incorporates active processes and controls centrosome separation. A robust prediction, independent of energetic considerations, is that the transition from a single cell to daughters cells occurs at critical value of this parameter. Our model additionally suggests that microtubule anchoring at the cell cortex may play an important role for successful bipolar division. This can be tested experimentally by regulating microtubule anchoring.

Maintaining Genome Stability in Defiance of Mitotic DNA Damage

The implementation of decisions affecting cell viability and proliferation is based on prompt detection of the issue to be addressed, formulation and transmission of a correct set of instructions and fidelity in the execution of orders. While the first and the last are purely mechanical processes relying on the faithful functioning of single proteins or macromolecular complexes (sensors and effectors), information is the real cue, with signal amplitude, duration, and frequency ultimately determining the type of response. The cellular response to DNA damage is no exception to the rule. In this review article we focus on DNA damage responses in G2 and Mitosis. First, we set the stage describing mitosis and the machineries in charge of assembling the apparatus responsible for chromosome alignment and segregation as well as the inputs that control its function (checkpoints). Next, we examine the type of issues that a cell approaching mitosis might face, presenting the impact of post-translational modifications (PTMs) on the correct and timely functioning of pathways correcting errors or damage before chromosome segregation. We conclude this essay with a perspective on the current status of mitotic signaling pathway inhibitors and their potential use in cancer therapy.

Single-cell analyses of transcriptional heterogeneity during drug tolerance transition in cancer cells by RNA sequencing

The acute cellular response to stress generates a subpopulation of reversibly stress-tolerant cells under conditions that are lethal to the majority of the population. Stress tolerance is attributed to heterogeneity of gene expression within the population to ensure survival of a minority. We performed whole transcriptome sequencing analyses of metastatic human breast cancer cells subjected to the chemotherapeutic agent paclitaxel at the single-cell and population levels. Here we show that specific transcriptional programs are enacted within untreated, stressed, and drug-tolerant cell groups while generating high heterogeneity between single cells within and between groups. We further demonstrate that drug-tolerant cells contain specific RNA variants residing in genes involved in microtubule organization and stabilization, as well as cell adhesion and cell surface signaling. In addition, the gene expression profile of drug-tolerant cells is similar to that of untreated cells within a few doublings. Thus, single-cell analyses reveal the dynamics of the stress response in terms of cell-specific RNA variants driving heterogeneity, the survival of a minority population through generation of specific RNA variants, and the efficient reconversion of stress-tolerant cells back to normalcy.

Comparison of the aneugenic properties of nocodazole, paclitaxel and griseofulvin in vitro. Centrosome defects and alterations in protein expression profiles

We have comparatively investigated the aneugenic activity of two anticancer drugs, nocodazole (NOC) and paclitaxel (PTX), and the antifungal griseofulvin with promising role in cancer treatment (GF), which affect microtubule dynamics in different ways. The comparison was achieved in HFFF2 human fibroblasts, MCF-7 human breast cancer cells and C2C12 mouse myoblasts, and focused on three issues: (i) induction of chromosome delay by estimation of MN frequency using CREST analysis; (ii) disturbance of spindle organization with Aurora-A/β-tubulin immunofluorescence; and (iii) alterations in the expression of Aurora-A, β- and γ-tubulin by western blotting. They induced chromosome delay, provoked metaphase arrest and promoted microtubule disorganization, reflecting their common characteristic of generating aneuploidy. In particular, NOC induced mainly monopolar metaphases, although PTX induced only multipolar metaphases. GF generated different types of abnormal metaphases, exhibiting cell specificity. Additionally, NOC decreased the expression of Aurora-A and β-tubulin, while the opposite held true for PTX and GF. γ-Tubulin expression was not modulated owing to NOC treatment, whereas PTX and GF increased γ-tubulin expression. Our findings throw a light on the manifestation of the aneugenicity of the studied compounds through centrosome proliferation/separation and protein expression, reflecting their different effects on microtubule dynamics.

Effect of chromosome instability on the maintenance and differentiation of human embryonic stem cells in vitro and in vivo

The therapeutic potential of human embryonic stem cells (hESCs) has long been appreciated, and the recent FDA approval of hESC derivatives for cell-based therapy encourages the clinical application of hESCs. Here, using CHA3-hESCs with normal and abnormal karyotypes, we report the importance of maintaining normal chromosomes during in vitro culture and the differentiation of hESCs for minimization of posttransplantation complications. We found that undifferentiated CHA3-hESCs with trisomy chromosome 12 undergo abnormal cell division with multiple spindles in comparison to the bipolar cell division of the karyotypically normal CHA3-hESCs. Transplanted karyotypically abnormal CHA3-hESC derivatives formed a tumor-like tissue 6weeks after transplantation in two out of seven mice tested. Our results demonstrate that the preservation of normal chromosomes is indispensable for maintaining the true properties of hESCs in vitro and abolishing adverse effects posttransplantation. Thus, the development of optimized techniques for stabilizing the chromosome state during in vitro hESC culture is a prerequisite for the therapeutic application of hESCs.

Centrioles: active players or passengers during mitosis?

Centrioles are cylinders made of nine microtubule (MT) triplets present in many eukaryotes. Early studies, where centrosomes were seen at the poles of the mitotic spindle led to their coining as "the organ for cell division". However, a variety of subsequent observational and functional studies showed that centrosomes might not always be essential for mitosis. Here we review the arguments in this debate. We describe the centriole structure and its distribution in the eukaryotic tree of life and clarify its role in the organization of the centrosome and cilia, with an historical perspective. An important aspect of the debate addressed in this review is how centrioles are inherited and the role of the spindle in this process. In particular, germline inheritance of centrosomes, such as their de novo formation in parthenogenetic species, poses many interesting questions. We finish by discussing the most likely functions of centrioles and laying out new research avenues.

Role of APC and its binding partners in regulating microtubules in mitosis

Adenomatous polyposis coli (APC) is a multifunctional protein commonly mutated in colon cancer. APC contains binding sites for multiple proteins with diverse roles in signaling and the structural and functional organization of cells. Recent evidence suggests roles for APC and some of its binding partners in regulating microtubules in mitosis. APC localizes to three key locations in mitosis: kinetochores, the cortex and centrosomes. Here, we discuss possible mechanisms for APC function at these sites and suggest new pathways by which APC mutations promote tumorigenesis.

Nucleation capacity and presence of centrioles define a distinct category of centrosome abnormalities that induces multipolar mitoses in cancer cells

Analysis of centrosome number and structure has become one means of assessing the potential for aberrant chromosome segregation and aneuploidy in tumor cells. Centrosome amplification directly causes multipolar catastrophic mitoses in mouse embryonic fibroblasts (MEFs) deficient for the tumor suppressor genes Brca1 or Trp53. We observed supernumerary centrosomes in cell lines established from aneuploid, but not from diploid, colorectal carcinomas; however, multipolar mitoses were never observed. This discrepancy prompted us to thoroughly characterize the centrosome abnormalities in these and other cancer cell lines with respect to both structure and function. The most striking result was that supernumerary centrosomes in aneuploid colorectal cancer cell lines were unable to nucleate microtubules, despite the presence of gamma-tubulin, pericentrin, PLK1, and AURKA. Analysis by scanning electron microscopy revealed that these supernumerary structures are devoid of centrioles, a result significantly different from observations in aneuploid pancreatic cancer cell lines and in Trp53 or Brca1 deficient MEFs. Thus, multipolar mitoses are dependent upon the ability of extra gamma-tubulin containing structures to nucleate microtubules, and this correlated with the presence of centrioles. The assessment of centrosome function with respect to chromosome segregation must therefore take into consideration the presence of centrioles and the capacity to nucleate microtubules. The patterns and mechanisms of chromosomal aberrations in hematologic malignancies and solid tumors are fundamentally different. The former is characterized by specific chromosome translocations, whose consequence is the activation of oncogenes. Most carcinomas, however, reveal variations in the nuclear DNA content. The observed genomic imbalances and gross variations in chromosome number can result from unequal chromosome segregation during mitotic cell division. It is therefore fundamental to elucidate mechanisms involved in distribution of the genome to daughter cells. Prior to cell division, the centrosome organizes microtubules and the mitotic spindle. Deciphering the consequences of alterations in centrosome number, structure, and function is an important step towards understanding how a diploid genome is maintained. Although extra centrosomes have now been observed in carcinomas and were correlated with aneuploidy, a careful functional investigation of these structures and their role in generating chromosome imbalances may lead to the identification of distinct mechanistic pathways of genomic instability. Understanding these pathways will also be important in determining whether they are potential molecular targets of therapeutic intervention.

Gamma-tubulin-containing abnormal centrioles are induced by insufficient Plk4 in human HCT116 colorectal cancer cells

Cancer cells frequently induce aberrant centrosomes, which have been implicated in cancer initiation and progression. Human colorectal cancer cells, HCT116, contain aberrant centrioles composed of disorganized cylindrical microtubules and displaced appendages. These cells also express unique centrosome-related structures associated with a subset of centrosomal components, including gamma-tubulin, centrin and PCM1. During hydroxyurea treatment, these abnormal structures become more abundant and undergo a change in shape from small dots to elongated fibers. Although gamma-tubulin seems to exist as a ring complex, the abnormal structures do not support microtubule nucleation. Several lines of evidence suggest that the fibers correspond to a disorganized form of centriolar microtubules. Plk4, a mammalian homolog of ZYG-1 essential for initiation of centriole biogenesis, is not associated with the gamma-tubulin-specific abnormal centrosomes. The amount of Plk4 at each centrosome was less in cells with abnormal centrosomes than cells without gamma-tubulin-specific abnormal centrosomes. In addition, the formation of abnormal structures was abolished by expression of exogenous Plk4, but not SAS6 and Cep135/Bld10p, which are downstream regulators required for the organization of nine-triplet microtubules. These results suggest that HCT116 cells fail to organize the ninefold symmetry of centrioles due to insufficient Plk4.

The small organic compound HMN-176 delays satisfaction of the spindle assembly checkpoint by inhibiting centrosome-dependent microtubule nucleation

HMN-176 is a potential new cancer therapeutic known to retard the proliferation of tumor cell lines. Here, we show that this compound inhibits meiotic spindle assembly in surf clam oocytes and delays satisfaction of the spindle assembly checkpoint in human somatic cells by inducing the formation of short and/or multipolar spindles. HMN-176 does not affect centrosome assembly, nuclear envelope breakdown, or other aspects of meiotic or mitotic progression, nor does it affect the kinetics of Spisula or mammalian microtubule (MT) assembly in vitro. Notably, HMN-176 inhibits the formation of centrosome-nucleated MTs (i.e., asters) in Spisula oocytes and oocyte extracts, as well as from isolated Spisula or mammalian centrosomes in vitro. Together, these results reveal that HMN-176 is a first-in-class anticentrosome drug that inhibits proliferation, at least in part, by disrupting centrosome-mediated MT assembly during mitosis.

The consequences of tetraploidy and aneuploidy

Polyploidy, an increased number of chromosome sets, is a surprisingly common phenomenon in nature, particularly in plants and fungi. In humans, polyploidy often occurs in specific tissues as part of terminal differentiation. Changes in ploidy can also result from pathophysiological events that are caused by viral-induced cell fusion or erroneous cell division. Tetraploidization can initiate chromosomal instability (CIN), probably owing to supernumerary centrosomes and the doubled chromosome mass. CIN, in turn, might persist or soon give way to a stably propagating but aneuploid karyotype. Both CIN and stable aneuploidy are commonly observed in cancers. Recently, it has been proposed that an increased number of chromosome sets can promote cell transformation and give rise to an aneuploid tumor. Here, we review how tetraploidy can occur and describe the cellular responses to increased ploidy. Furthermore, we discuss how the specific physiological changes that are triggered by polyploidization might be used as novel targets for cancer therapy.

Delocalization of the microtubule motor Dynein from mitotic spindles by the human papillomavirus E7 oncoprotein is not sufficient for induction of multipolar mitoses

Dynein is a minus end-directed microtubule motor that transports numerous cargoes throughout the cell. During mitosis, dynein motor activity is necessary for the positioning of spindle microtubules and has also been implicated in inactivating the spindle assembly checkpoint. Mutations in dynein motor and/or accessory proteins are associated with human disease, including cancer, and the delocalization of dynein from mitotic spindles has been correlated with an increased incidence of multipolar spindle formation in some cancer cells that contain supernumerary centrosomes. The high-risk human papillomavirus type 16 (HPV16) E7 oncoprotein induces centrosome overduplication and has been shown to cause multipolar mitotic spindle formation, a diagnostic hallmark of HPV-associated neoplasias. Here, we show that HPV16 E7 expression leads to an increased population of mitotic cells with dynein delocalized from the mitotic spindle. This function maps to sequences of HPV16 E7 that are distinct from the region necessary for centrosome overduplication. However, contrary to previous reports, we provide evidence that dynein delocalization by HPV16 E7 is neither necessary nor sufficient to cause the formation of multipolar mitoses.

Association of loss of BRCA1 expression with centrosome aberration in human breast cancer

PURPOSE

Centrosome aberration in number and/or size is reportedly often observed in human breast cancer. The aim of this study was to investigate the relationship between centrosome aberration and chromosomal instability as well as the expression of centrosome regulators such as BRCA1, Aurora-A, and p53.

METHODS

Centrosome aberration in number and size was determined immunohistochemically using the anti-gamma-tubulin antibody, and chromosomal instability was evaluated by fluorescence in situ hybridization analysis of chromosomes 1, 11, and 17 in paraffin sections from 50 human breast cancers. Immunohistochemical examination of BRCA1, Aurora-A, and p53 was also performed to examine the relationship of their expression with centrosome aberration.

RESULTS

Percentage of tumor cells with centrosome aberration in size varied from 0.9 to 30.4% (median 9.5%) and in number it varied from 0.5 to 86.5% (median 34.5%) in each tumor. No significant association in number or size, however, was observed between chromosomal instability and centrosome aberration. Numerical centrosome aberration was significantly associated with negative BRCA1 expression (P = 0.001). Breast tumors (n = 3) from patients with a proven BRCA1 germline mutation also showed a significant relationship with numerical centrosome aberration (P = 0.011). On the other hand, expression of Aurora-A or p53 was not significantly associated with centrosome aberration in either number or size.

CONCLUSIONS

Centrosome aberration is not associated with chromosomal instability, indicating the importance of other mechanisms in the induction of chromosomal instability in human breast cancer. BRCA1, but not Aurora-A and p53, is significantly involved in the pathogenesis of centrosome aberration.

Centrosome amplification can initiate tumorigenesis in flies

Centrosome amplification is a common feature of many cancer cells, and it has been previously proposed that centrosome amplification can drive genetic instability and so tumorigenesis. To test this hypothesis, we generated Drosophila lines that have extra centrosomes in approximately 60% of their somatic cells. Many cells with extra centrosomes initially form multipolar spindles, but these spindles ultimately become bipolar. This requires a delay in mitosis that is mediated by the spindle assembly checkpoint (SAC). As a result of this delay, there is no dramatic increase in genetic instability in flies with extra centrosomes, and these flies maintain a stable diploid genome over many generations. The asymmetric division of the larval neural stem cells, however, is compromised in the presence of extra centrosomes, and larval brain cells with extra centrosomes can generate metastatic tumors when transplanted into the abdomens of wild-type hosts. Thus, centrosome amplification can initiate tumorigenesis in flies.

The mammalian centrosome and its functional significance

Primarily known for its role as major microtubule organizing center, the centrosome is increasingly being recognized for its functional significance in key cell cycle regulating events. We are now at the beginning of understanding the centrosome’s functional complexities and its major impact on directing complex interactions and signal transduction cascades important for cell cycle regulation. The centrosome orchestrates entry into mitosis, anaphase onset, cytokinesis, G1/S transition, and monitors DNA damage. Recently, the centrosome has also been recognized as major docking station where regulatory complexes accumulate including kinases and phosphatases as well as numerous other cell cycle regulators that utilize the centrosome as platform to coordinate multiple cell cycle-specific functions. Vesicles that are translocated along microtubules to and away from centrosomes may also carry enzymes or substrates that use centrosomes as main docking station. The centrosome’s role in various diseases has been recognized and a wealth of data has been accumulated linking dysfunctional centrosomes to cancer, Alstrom syndrome, various neurological disorders, and others. Centrosome abnormalities and dysfunctions have been associated with several types of infertility. The present review highlights the centrosome’s significant roles in cell cycle events in somatic and reproductive cells and discusses centrosome abnormalities and implications in disease.

Impaired p53 function leads to centrosome amplification, acquired ERalpha phenotypic heterogeneity and distant metastases in breast cancer MCF-7 xenografts

In this study, we establish an MCF-7 xenograft model that mimics the progression of human breast carcinomas typified by loss of p53 integrity, development of centrosome amplification, acquired estrogen receptor (ERalpha) heterogeneity, overexpression of Mdm2 and metastatic spread from the primary tumor to distant organs. MCF-7 cells with abrogated p53 function (vMCF-7(Dnp53)) maintained nuclear ERalpha expression and normal centrosome characteristics in vitro. However, following mitogen stimulation, they developed centrosome amplification and a higher frequency of aberrant mitotic spindles. Centrosome amplification was dependent on cdk2/cyclin activity since treatment with the small molecule inhibitor SU9516 suppressed centriole reduplication. In contrast to the parental MCF-7 cells, when introduced into nude mice as xenografts, tumors derived from the vMCF-7(DNp53) cell line developed a strikingly altered phenotype characterized by increased tumor growth, higher tumor histopathology grade, centrosome amplification, loss of nuclear ERalpha expression, increased expression of Mdm-2 oncoprotein and resistance to the antiestrogen tamoxifen. Importantly, while MCF-7 xenografts did not develop distant metastases, primary tumors derived from vMCF-7(DNp53) cells gave rise to lung metastases. Taken together, these observations indicate that abrogation of p53 function and consequent deregulation of the G1/S cell cycle transition leads to centrosome amplification responsible for breast cancer progression.

Human papillomavirus type 16 E7 oncoprotein associates with the centrosomal component gamma-tubulin

Expression of a high-risk human papillomavirus (HPV) E7 oncoprotein is sufficient to induce aberrant centrosome duplication in primary human cells. The resulting centrosome-associated mitotic abnormalities have been linked to the development of aneuploidy. HPV type 16 (HPV16) E7 induces supernumerary centrosomes through a mechanism that is at least in part independent of the inactivation of the retinoblastoma tumor suppressor pRb and is dependent on cyclin-dependent kinase 2 activity. Here, we show that HPV16 E7 can concentrate around mitotic spindle poles and that a small pool of HPV16 E7 is associated with centrosome fractions isolated by sucrose density gradient centrifugation. The targeting of HPV16 E7 to the centrosome, however, was not sufficient for centrosome overduplication. Nonetheless, we found that HPV16 E7 can associate with the centrosomal regulator gamma-tubulin and that the recruitment of gamma-tubulin to the centrosome is altered in HPV16 E7-expressing cells. Since the association of HPV16 E7 with gamma-tubulin is independent of pRb, p107, and p130, our results suggest that the association with gamma-tubulin contributes to the pRb/p107/p130-independent ability of HPV16 E7 to subvert centrosome homeostasis.

Haploinsufficiency of poly(ADP-ribose) polymerase-1-mediated poly(ADP-ribosyl)ation for centrosome duplication

The centrosome plays a vital role in maintaining chromosomal stability. Known as the microtubule organizing center, the centrosome is involved in the formation of spindle poles during mitosis, which ensures the distribution of the correct number of chromosomes to daughter cells. Aberrant centrosome duplication could cause centrosome amplification and chromosomal instability. We have previously shown that poly(ADP-ribose) polymerase-1 (PARP-1) is important for centrosome function and chromosomal stability. In this study, we used PARP-1(+/+), PARP-1(+/-) and PARP-1(-/-) primary mouse embryonic fibroblasts and found that the level of PARP-1 gene dosage correlates with PARP activity and the in vivo level of poly(ADP-ribosyl)ation, which could explain the mechanism by which PARP-1 haploinsufficiency affects centrosome duplication and chromosomal stability. Our results emphasize that correct regulation of poly(ADP-ribosyl)ation levels in vivo is important for maintenance of proper centrosome duplication and chromosomal stability.

What is the function of centrioles?

The function of centrioles has been controversial and remains incompletely resolved. This is because centrioles, in and of themselves, do not directly perform any physiological activity. Instead, their role is only to act as a jig or breadboard onto which other functional structures can be built. Centrioles are primarily involved in forming two structures-centrosomes and cilia. Centrioles bias the position of spindle pole formation, but because spindle poles can self-organize, the function of the centriole in mitosis is not obligatory. Consequently, lack of centrioles does not generally prevent mitosis, although recent experiments suggest acentriolar spindles have reduced fidelity of chromosome segregation. In contrast, centrioles are absolutely required for the assembly of cilia, including primary cilia that act as cellular antennae. Consistent with this requirement, it is now becoming clear that many ciliary diseases, including nephronophthisis, Bardet-Biedl syndrome, Meckel Syndrome, and Oral-Facial-Digital syndrome, are caused by defects in centriole-associated proteins.

Aurora A and B Overexpression and Centrosome Amplification in Early Estrogen-Induced Tumor Foci in the Syrian Hamster Kidney: Implications for Chromosomal Instability, Aneuploidy, and Neoplasia

Estrogen-induced Syrian hamster tumors in the kidney represent a useful model to gain insight into the role of estrogens in oncogenic processes. We provided evidence that early tumor foci in the kidney arise from interstitial ectopic uterine-like germinal stem cells, and that early tumor foci and well-established tumors are highly aneuploid (92-94%). The molecular mechanisms whereby estrogens mediate this process are unclear. Here, we report that estrogen treatment induced significant increases in Aurora A protein expression (8.7-fold), activity (2.6-fold), mRNA (6.0-fold), and Aurora B protein expression (4.6-fold) in tumors, compared with age-matched cholesterol-treated kidneys. Immunohistochemistry revealed that this increase in Aurora A and B protein expression was essentially confined to cells within early and large tumor foci at 3.5 and 6 months of estrogen treatment, respectively. Upon estrogen withdrawal or coadministration of tamoxifen for 10 days, a 78% to 79% and 81% to 64% reduction in Aurora A and B expression, respectively, were observed in primary tumors compared with tumors continuously exposed to estrogens. These data indicate that overexpressed Aurora A and B in these tumors are under estrogen control via estrogen receptor alpha. Aurora A coenriched with the centrosome fraction isolated from tumors in the kidney. Centrosome amplification (number and area/cell) was detected in early tumor foci and large tumors but not in adjacent uninvolved or age-matched control kidneys. Taken together, these data indicate that persistent overexpression of Aurora A and B is under estrogen control, and is coincident with centrosome amplification, chromosomal instability, and aneuploidy, and represent an important mechanism driving tumorigenesis.

Aneugenic potential of the nitrogen mustard analogues melphalan, chlorambucil and p-N,N-bis(2-chloroethyl)aminophenylacetic acid in cell cultures in vitro

Melphalan (MEL), chlorambucil (CAB) and p-N,N-bis(2-chloroethyl)aminophenylacetic acid (PHE) are nitrogen mustard analogues, which are clinically used as chemotherapeutic agents. They also exert carcinogenic activity. The aim of this study was to investigate the aneugenic potential of the above drugs and the possible mechanism responsible for this activity. The Cytokinesis Block Micronucleus (CBMN) assay in combination with fluorescence in situ hybridization (FISH) was used in human lymphocyte cultures to evaluate micronucleus (MN) frequency. Pancentromeric probe (alpha-satellite) was applied to identify chromosomes in micronuclei and an X-chromosome specific centromeric probe was used to asses micronucleation and non-disjunction of this chromosome in binucleated cells. The effect of the above compounds on the organization of mitotic apparatus, as a possible target of chemicals with aneugenic potential, was investigated in C(2)C(12) mouse cell line by double immunofluorescence of alpha- and gamma-tubulin. We found that the studied drugs increased MN frequency in a linear dose-dependent manner primarily by chromosome breakage and in a lesser extent by an aneugenic mechanism. Non-disjunction and micronucleation of X-chromosome were also induced. Abnormal metaphase cells were linearly increased with concentration and characterized by abnormal centrosome number. Interphase cells with micronuclei and abnormal centrosome number were also observed. Since nitrogen mustards are highly reactive agents, with low selectivity and form covalent bonds with different nucleophilic sites in proteins and nucleic acids, it is reasonable to consider that one possible pathway for nitrogen mustard analogues to exert their aneugenic activity is through reaction with nucleophilic moieties of proteins or genes that are involved in the duplication and/or separation of centrosomes, resulting in abnormal centrosome number. Based on our results the carcinogenicity of nitrogen mustard analogues studied may be attributed not only to their activity to trigger gene mutation and chromosome breakage, but also to their aneugenic potential. Further studies are warranted to clarify the above two hypotheses.

A mechanistic view on the evolutionary origin for centrin-based control of centriole duplication

Mounting evidence implicates the protein centrin as a key regulator of centriole duplication, yet it remains to be determined just how centrin functions in this process. Recent studies suggest that centrin exerts both spatial and temporal control over centriole duplication through its role as a component of centriole precursor structures and through periodic cell-cycle specific changes in its abundance. Here, an overview of centrin and its role in centrosome dynamics is presented. Finally, a speculative model for just how centrin may operate to control centriole duplication is proposed with the intention to stimulate future advances in this area. This model provides an evolutionary basis for the preservation of essential features of the yeast spindle pole body (SPB) with the origin of the complex structure of the mammalian centriole.

Altered cellular distribution and subcellular sorting of gamma-tubulin in diffuse astrocytic gliomas and human glioblastoma cell lines

Centrosome amplification is a pivotal mechanism underlying tumorigenesis but its role in gliomas is underinvestigated. The present study specifically examines the expression and distribution of the centrosome-associated cytoskeletal protein gamma-tubulin in 56 primary diffuse astrocytic gliomas (grades II-IV) and in 4 human glioblastoma cell lines (U87MG, U118MG, U138MG, and T98G). Monoclonal anti-peptide antibodies recognizing epitopes in C-terminal or N-terminal domains of the gamma-tubulin molecule were used in immunohistochemical, immunofluorescence, and immunoblotting studies. In tumors in adults (n = 46), varying degrees of localization were detected in all tumor grades, but immunoreactivity was significantly increased in high-grade anaplastic astrocytomas and glioblastomas multiforme as compared to low-grade diffuse astrocytomas (p = 0.0001). A similar trend was noted in diffuse gliomas in children but the sample of cases was too small as to be statistically meaningful. Two overlapping patterns of ectopic cellular localization were identified in both primary tumors and glioblastoma cell lines: A punctate pattern, in which gamma-tubulin was partially co-distributed with pericentrin in the pericentriolar region, and a diffuse pattern, independent of pericentrin staining, denoting a soluble pool of gamma-tubulin. Cellular gamma-tubulin was detected in both soluble and insoluble (nocodazole-resistant) fractions of glioblastoma cells. Divergent localizations of gamma-tubulin and pericentrin suggest a differential distribution of these 2 centrosome-associated proteins in glioblastoma cell lines. Our results indicate that overexpression and ectopic cellular distribution of gamma-tubulin in astrocytic gliomas may be significant in the context of centrosome protein amplification and may be linked to tumor progression and anaplastic potential.

The influence of different chromosomal aberrations on molecular cytogenetic parameters in chronic lymphocytic leukemia

B-cell chronic lymphocytic leukemia (B-CLL) is the most common leukemia of adults in Western countries. The most frequent recurring chromosomal aberrations identified in B-CLL patients are trisomy 12 and deletions of 13q, 17p, and 11q. Cases with deletions of 11q and 17p have a poor prognosis, whereas cases with deletions in 13q have a favorable prognosis. It was previously shown that CLL patients with trisomy 12 and del(13)(q14) have a higher rate of asynchronous replication of normal structural genes when compared to those with normal karyotypes. We studied the replication pattern of the structural locus 21q22 and the imprinted gene SNRPN and its telomere (15qter) and the random aneuploidy of chromosomes 9 and 18 in CLL patients with trisomy 12 and deletions of 11q and 17p, and compared the results to those of CLL patients without these aberrations and to healthy controls. Random aneuploidy rate was higher in the group of patients with trisomy 12 as compared to all other groups. The replication pattern with higher asynchronous pattern was found in both aberration groups compared to the CLL patients without the aberrations and to the control group with involvement of 21q22 and 15qter, whereas the highest synchronous group was found in the 2 aberrations CLL patient groups compared to the other groups with the imprinted locus SNRPN. The existence and significance of chromosomal aberrations in CLL have a deleterious effect on the processes of cell cycle and gene replication and may have biological and prognostic implications.

Induction of centrosome amplification in p53 siRNA-treated human fibroblast cells by radiation exposure

Centrosome amplification can be detected in the tissues of p53(-/-) mice. In contrast, loss of p53 does not induce centrosome amplification in cultured human cells. However, examination of human cancer tissues and cultured cells has revealed a significant correlation between loss or mutational inactivation of p53 and occurrence of centrosome amplification, supporting the notion that p53 mutation alone is insufficient to induce centrosome amplification in human cells, and that additional regulatory mechanisms are involved. It has recently been shown that gamma irradiation of tumor cells induces centrosome amplification. However, the precise mechanism of radiation-induced centrosome amplification is not fully understood. In the present study, CCD32SK diploid normal human fibroblasts were transfected transiently with short interfering RNA (siRNA) specific for human p53 (CCD/p53i). There was a small increase in the frequency of centrosome amplification in CCD/p53i cells (4.0%) without irradiation. In contrast, CCD/p53i cells after 5-Gy irradiation showed a marked increase in abnormal nuclear shapes and pronounced amplification of centrosomes (46.0%). At 12 h after irradiation, irradiated CCD/p53i cells were arrested in G(2) phase. By laser scanning cytometry, abnormal mitosis with amplified centrosomes was observed frequently in the accumulating G(2)/M population at 48 h after irradiation. In the present study, we found that siRNA-mediated silencing of p53 in normal human fibroblasts, together with DNA damage by irradiation, efficiently induced centrosome amplification and nuclear fragmentation, but these phenomena were not observed with either siRNA-mediated silencing of p53 or irradiation alone.

Aneuploidy, stem cells and cancer

Telomeres which protect the individual chromosomes from disintegration, end-to-end fusion and maintain the genomic integrity during the somatic cell divisions play an important role in cellular aging. Aging and cancer development are linked with each other because cancer is considered a group of complex genetic diseases that develop in old cells and, in both, telomere attrition is involved. Numeric chromosome imbalance also known as aneuploidy is the hallmark of most solid tumors, whether spontaneous or induced by carcinogens. We provide evidence in support of the hypothesis that telomere attrition is the earliest genetic alteration responsible for the induction of aneuploidy. Dysfunctional telomeres are highly recombinogenic leading to the formation of dicentric chromosomes. During cell divisions, such complex chromosome alterations undergo breakage fusion bridge cycles and may lead to loss of heterozygosity (LOH) and gene amplification. Furthermore, we have provided evidence in support of the hypothesis that all types of cancer originate in the organ- or tissue-specific stem cells present in a particular organ. Cancer cells and stem cells share many characteristics, such as, self-renewal, migration, and differentiation. Metaphases with abnormal genetic constitution present in the lymphocytes of cancer patients and in some of their asymptomatic family members may have been derived from the organ-specific stem cells. In addition, evidence and discussion has been presented for the existence of cancer-specific stem cells. Successful treatment of cancer, therefore, should be directed towards these cancer stem cells.

p16(INK4a) prevents centrosome dysfunction and genomic instability in primary cells

Aneuploidy, frequently observed in premalignant lesions, disrupts gene dosage and contributes to neoplastic progression. Theodor Boveri hypothesized nearly 100 years ago that aneuploidy was due to an increase in centrosome number (multipolar mitoses) and the resultant abnormal segregation of chromosomes. We performed immunocytochemistry, quantitative immunofluorescence, karyotypic analysis, and time-lapse microscopy on primary human diploid epithelial cells and fibroblasts to better understand the mechanism involved in the production of supernumerary centrosomes (more than two microtubule nucleating bodies) to directly demonstrate that the presence of supernumerary centrosomes in genomically intact cells generates aneuploid daughter cells. We show that loss of p16^INK4a generates supernumerary centrosomes through centriole pair splitting. Generation of supernumerary centrosomes in human diploid epithelial cells was shown to nucleate multipolar spindles and directly drive production of aneuploid daughter cells as a result of unequal segregation of the genomic material during mitosis. Finally, we demonstrate that p16^INK4a cooperates with p21 through regulation of cyclin-dependent kinase activity to prevent centriole pair splitting. Cells with loss of p16^INK4a activity have been found in vivo in histologically normal mammary tissue from a substantial fraction of healthy, disease-free women. Demonstration of centrosome dysfunction in cells due to loss of p16^INK4a suggests that, under the appropriate conditions, these cells can become aneuploid. Gain or loss of genomic material (aneuploidy) may provide the necessary proproliferation and antiapoptotic mechanisms needed for the earliest stages of tumorigenesis.

Here the authors show that aneuploidy (where a cell has an abnormal number of chromosomes) can be caused by cells having an abnormal number of centrosomes, which leads to asymmetric cell division.

Rootletin interacts with C-Nap1 and may function as a physical linker between the pair of centrioles/basal bodies in cells

Rootletin, a major structural component of the ciliary rootlet, is located at the basal bodies and centrosomes in ciliated and nonciliated cells, respectively. Here we investigated its potential role in the linkage of basal bodies/centrioles and the mechanism involved in such linkages. We show that rootletin interacts with C-Nap1, a protein restricted at the ends of centrioles and functioning in centrosome cohesion in interphase cells. Their interaction in vivo is supported by their colocalization at the basal bodies/centrioles and coordinated association with the centrioles during the cell cycle. Ultrastructural examinations demonstrate that rootletin fibers connect the basal bodies in ciliated cells and are present both at the ends of and in between the pair of centrioles in nonciliated cells. The latter finding stands in contrast with C-Nap1, which is present only at the ends of the centrioles. Transient expression of C-Nap1 fragments dissociated rootletin fibers from the centrioles, resulting in centrosome separation in interphase. Overexpression of rootletin in cells caused multinucleation, micronucleation, and irregularity of nuclear shape and size, indicative of defects in chromosome separation. These data suggest that rootletin may function as a physical linker between the pair of basal bodies/centrioles by binding to C-Nap1.

Random aneuploidy in neoplastic and pre-neoplastic diseases, multiple myeloma, and monoclonal gammopathy

In this study we evaluated the aneuploidy rate of cells from patients considered to have a premalignant condition (monoclonal gammopathy or MGUS) and patients with multiple myeloma, as well as healthy controls. By applying a fluorescence situ hybridization technique, we estimated the random aneuploidy rate of alpha-satellite (centromeres) probes from chromosomes 9 and 18. The monosomy and total aneuploidy rates were higher in the two study groups compared to the control group. The monosomy rate was significantly higher in the MGUS group compared to the group with chromosome 18 alpha-satellite probes, a finding that was reported before in preneoplastic conditions. Our results support the cancer aneuploidy theory that carcinogenesis is initiated by a random aneuploidy, which is induced either spontaneously or by a carcinogen. The resulting karyotype instability sets a chain reaction of aneuploidization, which generates even more abnormal and eventually cancer-specific combinations and rearrangements of chromosomes.

Altered centrosomes in ataxia-telangiectasia cells and rapamycin-treated Chinese hamster cells

Rapamycin induces chromosome malsegregation in mammalian cell lines and yeast. Previous studies indicate that the function impaired in ataxia-telangiectasia (A-T) patients is necessary for both the growth inhibition and the chromosome malsegregation induced by rapamycin, and that treating the non-tumorigenic Chinese hamster cell line CHEF/18 with rapamycin results in supernumerary centrosomes and multipolar spindles. In this paper we report that lymphoblastoid cell lines established from A-T patients as well as hamster A-T-like cells are more resistant to rapamycin than the respective normal cell lines. Two cell lines derived from Nijmegen Breakage Syndrome (NBS) patients, who have clinical symptoms similar to those of A-T but a different molecular defect, were not resistant to rapamycin. Both A-T lymphoblastoid cells and A-T-like fibroblasts had giant centrosomes formed by more than two areas of gamma-tubulin-reacting material. Such giant centrosomes were also observed in CHEF/18 cells after prolonged treatment with rapamycin. Formation of giant centrosomes, possibly due to the coalescence of supernumerary centrosomes, was associated with increased aneuploidy in treated cells. Expression analysis of cell-cycle regulatory genes in rapamycin-treated human lymphoblastoid cells indicated that rapamycin decreased the expression of the tumor suppressor gene GADD45. The levels of RB, p21 and p53 mRNA were also decreased, although to a lesser extent. As rapamycin is often used as an immunosuppressant in pediatric transplant patients, these data indicate that caution should be taken, especially when the drug is given for prolonged periods of time.

Transcriptional control of BubR1 by p53 and suppression of centrosome amplification by BubR1

Elimination of the regulatory mechanism underlying numeral homeostasis of centrosomes, as seen in cells lacking p53, results in abnormal amplification of centrosomes, which increases the frequency of chromosome segregation errors, and thus contributes to the chromosome instability frequently observed in cancer cells. We have previously reported that p53(-/-) mouse cells in prolonged culture undergo genomic convergence similar to that observed during tumor progression; early-passage p53(-/-) cells are karyotypically heterogeneous due to extensive chromosome instability associated with centrosome amplification, while late-passage p53(-/-) cells are aneuploid yet karyotypically homogeneous and chromosomally stable. Moreover, they contain numerically normal centrosomes. Through the microarray analysis of early- and late-passage p53(-/-) cells, we identified the BubR1 spindle checkpoint protein, which plays a critical role in suppression of centrosome amplification and stabilization of chromosomes in late-passage p53(-/-) cells. Up-regulation of BubR1 augments the checkpoint function, which effectively senses the spindle/chromosome aberrations associated with centrosome amplification. We further found that BubR1 transcription is largely controlled by p53. In early-passage p53(-/-) cells, BubR1 expression is low and the checkpoint function in response to microtubule toxin is considerably compromised. In late-passage cells, however, regaining of BubR1 expression restores the checkpoint function to mitotic aberrations caused by microtubule toxin. Our studies demonstrate the molecular aspect of genomic convergence in cultured cells, providing critical information for understanding the stepwise progression of tumors.

A mutation in the centriole-associated protein centrin causes genomic instability via increased chromosome loss in Chlamydomonas reinhardtii

BACKGROUND

The role of centrioles in mitotic spindle function remains unclear. One approach to investigate mitotic centriole function is to ask whether mutation of centriole-associated proteins can cause genomic instability.

RESULTS

We addressed the role of the centriole-associated EF-hand protein centrin in genomic stability using a Chlamydomonas reinhardtii centrin mutant that forms acentriolar bipolar spindles and lacks the centrin-based rhizoplast structures that join centrioles to the nucleus. Using a genetic assay for loss of heterozygosity, we found that this centrin mutant showed increased genomic instability compared to wild-type cells, and we determined that the increase in genomic instability was due to a 100-fold increase in chromosome loss rates compared to wild type. Live cell imaging reveals an increased rate in cell death during G1 in haploid cells that is consistent with an elevated rate of chromosome loss, and analysis of cell death versus centriole copy number argues against a role for multipolar spindles in this process.

CONCLUSION

The increased chromosome loss rates observed in a centrin mutant that forms acentriolar spindles suggests a role for centrin protein, and possibly centrioles, in mitotic fidelity.

The de novo centriole assembly pathway in HeLa cells: cell cycle progression and centriole assembly/maturation

It has been reported that nontransformed mammalian cells become arrested during G1 in the absence of centrioles (Hinchcliffe, E., F. Miller, M. Cham, A. Khodjakov, and G. Sluder. 2001. Science. 291:1547-1550). Here, we show that removal of resident centrioles (by laser ablation or needle microsurgery) does not impede cell cycle progression in HeLa cells. HeLa cells born without centrosomes, later, assemble a variable number of centrioles de novo. Centriole assembly begins with the formation of small centrin aggregates that appear during the S phase. These, initially amorphous "precentrioles" become morphologically recognizable centrioles before mitosis. De novo-assembled centrioles mature (i.e., gain abilities to organize microtubules and replicate) in the next cell cycle. This maturation is not simply a time-dependent phenomenon, because de novo-formed centrioles do not mature if they are assembled in S phase-arrested cells. By selectively ablating only one centriole at a time, we find that the presence of a single centriole inhibits the assembly of additional centrioles, indicating that centrioles have an activity that suppresses the de novo pathway.

Characterisation of breast fine-needle aspiration biopsies by centrosome aberrations and genomic instability

Recent studies have suggested that aneuploidy in malignant tumours could be a consequence of centrosome aberrations. Using immunofluorescence analysis with an antibody against γ-tubulin and DNA image cytometry, we measured centrosome aberrations and DNA ploidy patterns in fine-needle aspiration biopsies (FNABs) of 58 breast lesions. Benign lesions did not show any centrosome aberrations. DNA diploid carcinomas showed a mean percentage of cells with centrosomal defects of 2.1%. The aneuploid invasive carcinomas could be divided into two subgroups by their significantly (P=0.0003) different percentage of cells with centrosome aberrations (2.0 and 10.3%, respectively) and their significantly (P=0.0003) different percentage of cells with nonmodal DNA content values determined by the Stemline Scatter Index (SSI), a measure of genomic instability. The percentage of cells with centrosome aberrations demonstrated a positive, linear correlation with the corresponding SSI (r=0.82, P<0.0001) and loss of tissue differentiation (r=0.78, P<0.0001). Our results indicate the percentage of cells with centrosome aberrations as being sufficient to divide the investigated tumours into three significantly different groups: benign lesions with no centrosomal aberrations, and two malignant tumour types with mean values of 2.1 and 9.6% of centrosomal defects, respectively. Together, these results demonstrate that centrosome aberrations correlate with genomic instability and loss of tissue differentiation. Furthermore, this study shows the feasibility of centrosomal analysis in FNAB of the breast and suggests centrosomal aberrations as possessing diagnostic and prognostic value.

The centrosome in human genetic disease

The centrosome is an indispensable component of the cell-cycle machinery of eukaryotic cells, and the perturbation of core centrosomal or centrosome-associated proteins is linked to cell-cycle misregulation and cancer. Recent work has expanded our understanding of the functional complexity and importance of this organelle. The centrosomal localization of proteins that are involved in human genetic disease, and the identification of novel centrosome-associated proteins, has shown that numerous, seemingly unrelated, cellular processes can be perturbed by centrosomal dysfunction. Here, we review the mechanistic relationship between human disease phenotypes and the function of the centrosome, and describe some of the newly-appreciated functions of this organelle in animal cells.

Centrosomal amplification and spindle multipolarity in cancer cells

Recent developments have highlighted the important role centrosomal defects play in the cellular changes associated with tumorigenesis. This article reviews recent developments addressing the impact of numerical centrosomal amplification on chromosomal segregational defects in the cancer cell. Probably, the most significant is the change to the structure of the spindle that leads to increased numbers of spindle poles and abnormal partitioning of the chromosomes in mitosis. I address how centrosomal changes are initiated and how they may lead to spindle multipolarity.

Estrogen mediates Aurora-A overexpression, centrosome amplification, chromosomal instability, and breast cancer in female ACI rats

Estrogens play a crucial role in the causation and development of sporadic human breast cancer (BC). Chromosomal instability (CIN) is a defining trait of early human ductal carcinoma in situ (DCIS) and is believed to precipitate breast oncogenesis. We reported earlier that 100% of female ACI (August/Copenhagen/Irish) rats treated with essentially physiological serum levels of 17beta-estradiol lead to mammary gland tumors with histopathologic, cellular, molecular, and ploidy changes remarkably similar to those seen in human DCIS and invasive sporadic ductal BC. Aurora-A (Aur-A), a centrosome kinase, and centrosome amplification have been implicated in the origin of aneuploidy via CIN. After 4 mo of estradiol treatment, levels of Aur-A and centrosomal proteins, gamma-tubulin and centrin, rose significantly in female ACI rat mammary glands and remained elevated in mammary tumors at 5-6 mo of estrogen treatment. Centrosome amplification was initially detected at 3 mo of treatment in focal dysplasias, before DCIS. At 5-6 mo, 90% of the mammary tumor centrosomes were amplified. Comparative genomic hybridization revealed nonrandom amplified chromosome regions in seven chromosomes with a frequency of 55-82% in 11 primary tumors each from individual rats. Thus, we report that estrogen is causally linked via estrogen receptor alpha to Aur-A overexpression, centrosome amplification, CIN, and aneuploidy leading to BC in susceptible mammary gland cells.

Identification and Characterization of a Novel Cytoplasm Protein ICF45 That Is Involved in Cell Cycle Regulation

A novel cytoplasm protein, interphase cytoplasm foci protein 45 kDa (ICF45), was identified by screening the cDNA expression library for HeLa cells with serum from an autoimmune patient. The complete cDNA sequence of ICF45 was determined to be 1.32 kb and to encode 298 amino acids with an apparent molecular mass of 45 kDa. The ICF45 transcripts were detected in different tissues and were relatively rich in human liver and lung tissues but scarce in brain tissue. Immunofluorescence with anti-ICF45-specific antibodies demonstrated that ICF45 is strongly expressed in interphase and cannot be seen in mitosis. The subcellular localization of ICF45 and fusion proteins GFP-ICF45, ICF45-GFP, and HA-ICF45 showed ICF45 centralized into 1-2 dots in the cytoplasm and always near the nuclear membrane. The staining foci of ICF45 appeared to be slightly larger than centrosomes and in some cases were found to colocalize with centrosomes. After effectively silencing the ICF45 by RNAi, the growth and proliferation of the cells were significantly inhibited, and p53 was detected to be up-regulated. The silencing of ICF45 also resulted in an appearance of polycentrosome and multinuclear cells, which finally went to apoptosis. Our results suggest that ICF45 is a highly conserved novel protein, which is expressed in a cell cycle-dependent manner and seemed to be involved in cell cycle progression and cell proliferation.

Induction of centrosome amplification and chromosome instability in p53-null cells by transient exposure to subtoxic levels of S-phase-targeting anticancer drugs

Chromosome instability (CIN) is one of the most important phenotypes in tumor progression, introducing multiple mutations required for acquisition of further malignant characteristics. Abnormal amplification of centrosomes, which is frequently observed in human cancer, has been shown to contribute to CIN by increasing the frequency of mitotic defects. Here, we show that transient exposure to subtoxic concentrations of commonly used anticancer drugs that target DNA synthesis induces centrosome amplification in cells lacking p53 tumor suppressor protein, by allowing continuous centrosome duplication in the absence of DNA synthesis. When these cells are released from cell cycle arrest by removal of drugs, cells suffer extensive destabilization of chromosomes. Considering that p53 is the most frequently mutated gene in human cancer and that CIN is known to be associated with acquisition of malignant phenotypes, our observations may explain why recurrent tumors, after chemotherapy, often exhibit more malignant characteristics than the original tumors. The tumor cells that are exposed to subtoxic levels of DNA synthesis-targeting drugs will be arrested and undergo centrosome amplification. Upon cessation of chemotherapy, these cells will re-enter cell cycling, and experience extensive CIN due to the presence of amplified centrosomes. This in turn promotes generation of tumor cells equipped with further malignant characteristics.