Oncogene (2002) 21, 6140 – 6145 ª 2002 Nature Publishing Group All rights reserved 0950 – 9232/02 $25.00 www.nature.com/onc Introduction Kenji Fukasawa*,1 1Department of Cell Biology, University of Cincinnati College of Medicine, PO Box 670521, Cincinnati, Ohio, OH 45267-0521, USA Oncogene (2002) 21, 6140 – 6145. doi:10.1038/sj.onc. Centrosomes have recently attracted considerable 1205771 attention primarily because of their potential impor- tance in carcinogenesis. Chromosome instability is a hallmark of virtually all solid cancers, being a Keywords: centrosome; cancer; chromosome instability formidable force that drives multi-step carcinogenesis: either loss or gain of a single chromosome can simultaneously introduce multiple mutations, which are responsible for acquisition of further malignant The centrosome of animal cells is a small non- phenotypes. The presence of more than two centro- membranous organelle, and is often associated with somes in a cell results in the formation of defective the nuclear membrane. It is composed of a pair of mitotic spindles directed by multiple spindle poles, centrioles and a surrounding electron dense matrix of which in turn increases the chromosome segregation protein aggregates referred to as the pericentriolar errors. This potential role of centrosomes in chromo- material (PCM) (Figure 1, also see Figure 1 in Dutertre some instability, hence in cancer development, is by et al., 2002). Each centriole is cylindrical in shape and no means a new-sprung idea. It was initially built with the nine sets of triplet microtubules. The two proposed by Theodor Boveri (1914). In his book, centrioles are paired in close proximity at one end, and The Origin of Malignant Tumors, he wrote, ‘malig- positioned vertical to each other. Many different nant tumors might be the result of a certain proteins are present in PCM, some of which are abnormal condition of the chromosomes, which permanent constituents of centrosomes, and some are may arise from multipolar mitosis. Abnormal (multi- temporally associated with centrosomes in a cell cycle- polar) mitoses may bring about an immense number dependent manner. of different chromosome combinations, such combi- The centrosome functions as a microtubule organiz- nations as would make a cell into a tumor cell must ing center (MTOC), which establishes polarity and occasionally occur. The more the abnormal (multi- orientation of microtubules during interphase, and polar) divisions that take place in a tissue, the directs assembly of bipolar spindles during mitosis greater of course is the probability that the necessary (Figure 2). Since each daughter cell inherits only one combination (of chromosomes to become malignant centrosome from a mother cell upon cytokinesis, the tumors) shall appear.’ This remarkable proposition cell must duplicate the centrosome prior to the next had long been forgotten until several years ago. In mitosis. The centrosome duplication process in verte- the course of the study to identify the mechanism brate cells consists of several distinct steps: (1) loss of underlying extensive chromosome instability seen in orthogonal configuration and physical separation of mice deficient for p53 tumor suppressor, it was the paired centrioles, (2) synthesis of a daughter found that loss of p53 resulted in abnormal centriole in the vicinity of each preexisting centriole, amplification of centrosomes (Fukasawa et al., and (3) elongation of the daughter centrioles (Figure 1996, 1997). This finding has been further corrobo- 3). Since centrosomes must duplicate only once during rated by the studies using transgenic mice expressing the cell cycle, duplication of centrosomes must proceed the mutant p53 gene (Wang et al., 1998). Since p53 in coordination with other cell cycle events (i.e., DNA is the most frequently mutated gene in human cancer synthesis). Indeed, initiation of centrosome (centriole) (Hollstein et al., 1991; Levine et al., 1991), it is duplication occurs near the G1/S boundary, elongation reasonable to speculate that mutation of p53 at least of daughter centrioles as well as maturation as in part promotes tumor progression via chromosome centrosomes (i.e., recruitment of PCM) continue during instability associated with centrosome amplification. S and G2 phases. Subsequently, it has been found that centrosome abnormality (numerical amplification as well as hypertrophy of centrosomes) is common in human cancer (Pihan et al., 1998; Lingle et al., 1998; Carroll *Correspondence: K Fukasawa, University of Cincinnati College of Medicine, Department of Cell Biology, 3125 Eden Ave. (or PO Box et al., 1999), and these observations have fully 670521), Cincinnati, OH 45267-0521, USA; resurrected Boveri’s century old hypothesis (see E-mail: [email protected] D’Assoro et al., 2002). Introduction K Fukasawa 6141 Figure 1 Simplified diagram of the centrosome. A pair of centrioles is embedded in a cloud of electron dense material known as the pericentriolar material (PCM). Each centriole is a cylindrical structure (*0.2 mm in diameter and 0.2 – 0.5 mm in length) composed of nine identical and equally spaced microtubule arrays, and each array consists of three microtubules. Protein links (shown in red) join adjacent triplets of microtubules. In most animal cells, the PCM acts as the main nucleating site for microtubules (Gould and Borisy, 1977). For more comprehensive information on the ultrastructure of centrosomes, see the review article by Vorobjev and Nadezhdina (1987) Figure 2 The microtubule-nucleating function of centrosomes during the cell cycle. During interphase (left panel), centrosomes (immunostained for g-tubulin; green/yellow dots) organize cytoplasmic microtubule networks (immunostained for a-tubulin; red). Nuclei are shown in blue. During mitosis (center panels), centrosomes become spindle poles (dense green spots in the left row panels, immunostained for a- and b-tubulins), directing the formation of bipolar mitotic spindles. The panels in the center row show condensed chromosomes, and those in the right row show the overlay images. The rightmost panel shows the bipolar metaphase spindles, in which centrosomes are shown in green/yellow, microtubules in red, and DNA in blue Oncogene Introduction K Fukasawa 6142 Mechanisms of chromosome loss and/or gain by cen- arrows). During anaphase, these misaligned centro- trosome hyperamplification somes pull chromosomes out of the bipolar axis. Depending on which daughter cell receive these The important issue to be discussed in the role of isolated chromosomes, aneuploid cells can be generated centrosomes in chromosome instability is the mechan- (Figure 4B, panels e – h, and Figure 4C). istic aspect of how cells either gain or lose Although the exact mechanism of chromosome chromosomes due to the presence of an excess number instability induced by centrosome hyperamplification of centrosomes. Although this is currently not remains to be elucidated, it is likely that both models completely understood and an area of active research, are equally important for generation of aneuploid cells there are data to suggest at least two models: associated with centrosome hyperamplification. (1) Multipolar cell division. For cells to undergo cytokinesis, formation of bipolar mitotic spindles is not required: cells with certain multipolar configurations Mechanisms for cells to possess an excess number of can divide. Especially, division of cells with tripolar centrosomes spindles is frequently observed (Figure 4A). If the daughter cells of the tripolar division inherit a Another important issue to consider when studying chromosome set sufficient to survive in a given centrosomes is to know the origin of excess number of environment, these cells will likely exhibit extensive centrosomes in a given experimental system. There are karyotypic alterations; and at least four possible mechanisms for cells to possess (2) Misalignment of multiple centrosomes on a bipolar more than two centrosomes (or MTOCs) (Figure 5). axis. Many cells with an excess number of centrosomes The first is the deregulation of centrosome can sequester centrosomes to a bipolar axis, which is duplication. There are two major controls imposed likely directed by chromosomes, forming pseudo- on centrosome duplication: one is to ensure the bipolar spindles (Figure 4B, panels a – d). When the proper timing of initiation of duplication, which pseudo-bipolar spindles are formed, cells appear to occurs at late G1 to early S phase of the cell cycle, undergo normal cytokinesis with balanced segregation and the other to suppress re-duplication of centro- of chromosomes. However, a few centrosomes occa- somes once duplicated. If these controls are sionally fail to align on the bipolar axis, yet are abrogated, centrosomes will duplicate multiple times functionally intact, nucleating mitotic spindles that within a single cell cycle, resulting in centrosome capture chromosomes (Figure 4B,C, indicated by hyperamplification. Figure 3 The centrosome (centriole) duplication cycle in animal cells. At the end of mitosis, each daughter cell inherits only one centrosome. In the following cell cycle, the centrosome must duplicate, which is initiated at late G1/early S phase of the cell cycle by the splitting of the centriole pair. This is followed by the formation of daughter centrioles (procentrioles) in the vicinity of each preexisting centriole (shown in red). The procentrioles continue to elongate during S and G2 Oncogene Introduction K Fukasawa 6143 Figure 4 Two models of chromosome