Skip Navigation


Carcinogenesis Advance Access originally published online on January 27, 2007
Carcinogenesis 2007 28(5):899-912; doi:10.1093/carcin/bgm019
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
28/5/899    most recent
bgm019v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrow Search for citing articles in:
ISI Web of Science (2)
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Pérez de Castro, I.
Right arrow Articles by Malumbres, M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Pérez de Castro, I.
Right arrow Articles by Malumbres, M.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

A census of mitotic cancer genes: new insights into tumor cell biology and cancer therapy

Ignacio Pérez de Castro, Guillermo de Cárcer and Marcos Malumbres*

Cell Division and Cancer Group, Centro Nacional de Investigaciones Oncológicas, Melchor Fernández Almagro 3, E-28029 Madrid, Spain

* To whom correspondence should be addressed. Tel: +34 91 2246900; Fax: +34 91 7328033; Email: malumbres{at}cnio.es


   Abstract
Top
 Abstract
Oncogenes and tumor suppressors...
 Molecular regulation of mitosis
 Alteration of mitotic regulators...
 Experimental evidence on mitotic...
 CIN and cancer: conceptual...
 References
 
Tumor cell proliferation is frequently associated with genetic or epigenetic alterations in key regulators of the cell cycle. Most known oncogenes and tumor suppressors target entry into the cell cycle and control the G1/S transition. However, tumor-associated alterations in spindle formation or chromosome segregation are also frequent and may result in chromosomal instability. In fact, a few centrosomal or mitotic proteins such as aurora A, polo-like kinase 1 and PTTG1 (securin) have been reported to act as oncogenes. Some spindle checkpoint regulators such as the BUB kinases or MAD2 protect cells from aberrant chromosome segregation and may therefore function as suppressors of malignant transformation. However, few cancer-associated mutations in these or other mitotic regulators have been described thus far and many of these molecules do not fit into the classical definition of ‘oncogenes’ or ‘tumor suppressor genes’. In some cases, both over-expression and decreased expression of these genes result in mitotic arrest. Moreover, some mitotic regulators such as MAD2 are either up- or down-regulated depending on the tumor types and, in both cases, these alterations result in chromosomal imbalances and tumor development. Minor changes in protein levels that do not compromise cell viability might therefore be sufficient to dysregulate the mitotic cycle and induce genomic instability. Despite the limited knowledge on the molecular basis of these processes, the clinical success of mitotic poisons such as taxanes reinforces the interest in these molecules, their involvement in human cancer and the therapeutic opportunities to modulate their function in cancer treatment.

Abbreviations: APC/C, anaphase-promoting complex/cyclosome; CDK, cyclin-dependent kinase; FZR1, fizzy-related 1; PLK1, polo-like kinase 1; SAC, spindle assembly checkpoint


   Oncogenes and tumor suppressors in the cell cycle
Top
 Abstract
 Oncogenes and tumor suppressors...
Molecular regulation of mitosis
 Alteration of mitotic regulators...
 Experimental evidence on mitotic...
 CIN and cancer: conceptual...
 References
 
More than 100 different diseases are included under the term ‘cancer’. All these diseases share a reduced number of unique and specific properties (1). Some of these properties, such as limitless replicative proliferation, self-sufficiency in growth signals and insensitivity to growth inhibitory signals, may be a direct consequence of deregulated cell cycles (2–4). In fact, cell cycle alterations resulting in unscheduled proliferation are frequently associated with cancer. Most of these alterations target key regulators of G1 progression and the G1/S transition such as the components of the so-called p16INK4A-CDK4-pRB pathway (2,4). These alterations include over-expression of cyclins (mainly D- and E-type cyclins) and cyclin-dependent kinases (CDKs) such as CDK4 and CDK6. CDK inhibitors (mainly p16INK4A, p15INK4B and p27KIP1) or CDK substrates such as the retinoblastoma protein (pRB) are also often inactivated. In most cases, dysregulation of these genes is a consequence of chromosome alterations [amplification of cyclin D1 (CCND1) or CDK4, translocation of CDK6 and deletion of the genes encoding p16INK4A and pRB], promoter hypermethylation (pRB, p16INK4A and p15INK4B) or specific point mutations as described in the CDK4, CDK6 and p16INK4A genes. In other cases, inactivation of tumor suppressors such as p27KIP1 is a consequence of genetic alterations in their proteolytic pathway. The high frequency of these alterations in human tumors suggests that dysregulation of the pathways controlling entry into the cell cycle and commitment to DNA replication are essential to allow unscheduled proliferation of cancer cells (4).

Little is known about the involvement of other cell cycle regulators in tumorigenesis. During the last few years, various mutations have been identified that do not provoke a direct increase in cell proliferation, but rather target specific cell cycle regulators involved in progression through mitosis. Although these alterations do not directly promote unscheduled proliferation, they probably induce chromosome aberrations that may contribute to a transformed phenotype. Molecular biology and genetic studies suggest that subtle alterations on the protein levels of these mitotic regulators, generally not detected by routine molecular pathology screenings, might provoke mitotic aberrations with significant consequences in malignant transformation. We will review these alterations in mitotic regulators here and will discuss how they may participate in the malignant phenotype.


   Molecular regulation of mitosis
Top
 Abstract
 Oncogenes and tumor suppressors...
 Molecular regulation of mitosis
Alteration of mitotic regulators...
 Experimental evidence on mitotic...
 CIN and cancer: conceptual...
 References
 
During mitosis, duplicated genetic material and centrosomes are equally distributed between the two daughter cells. The morphological changes required for this process have traditionally been used to define the different stages throughout mitosis (Figure 1). At prophase, chromatin condenses into chromosomes and the nuclear envelope breaks down. During prometaphase, a massive reorganization of the cytoskeleton results in the generation of the bipolar spindle where chromosomes are attached. In order to organize this bipolar spindle, centrosomes have been duplicated previously in a ‘centrosome cycle’, which parallels the ‘chromatin’ cell cycle and includes duplication (during S phase), segregation and maturation (at the G2/M transition) of the centrosomes (5). These centrosomes function as a pair of microtubule-organizing centers that migrate to opposite poles of the cells and are essential for proper spindle formation. At metaphase, chromosomes are bound to the plus ends of the microtubules through their kinetochores and are aligned at the ‘metaphase plate’ in the center of the mitotic spindle. Segregation of the two sets of chromosomes occurs during anaphase after loss of the sister chromatid cohesion. Finally, in telophase, chromosomes decondense and the two new nuclei are formed after reconstruction of the nuclear envelope. Once the two new nuclei are separated, the cell undergoes cytokinesis to divide the cytoplasm and separate the two daughter cells (Figure 1).


Figure 1
View larger version (37K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 1. A cellular and molecular view of mitosis. Major changes in chromosome and spindle structure as well as the mitotic checkpoints that regulate transition through the different stages of mitosis are indicated. Some of the representative regulators altered in human cancer are indicated. The involvement of APC/C ubiquitin ligases during different mitotic stages is also indicated with thick arrows. Pictures represent NIH 3T3 cells at different stages during mitosis. DNA is stained with 4',6-diamino-2-phenylindole (blue); microtubules (red) and PLK1 (green) are detected using anti-{alpha}-tubulin or anti-PLK1-specific antibodies.

 
Protein phosphorylation and degradation
From a molecular point of view, the majority, if not all, of mitotic regulators are present at the end of G2 and are ready to act throughout mitosis. Their activity is primarily regulated by phosphorylation and proteolysis, although other lesser known post-translational regulatory mechanisms such as sumoylation and acetylation (6,7) are also involved. Several kinases and phosphatases have been identified that regulate the centrosomal and mitotic cycles (8,9). The CDK1 (also known as CDC2) is one of the master regulators of mitosis as it is involved in the centrosome cycle and early mitotic events. CDK1 requires binding to A- or B-type cyclins and is further regulated by phosphorylation and dephosphorylation events (8,10). CDK1 is inactivated by the inhibitory kinases WEE1 and MYT1 by phosphorylation of specific residues at its N-terminus. Activation of CDK1 requires phosphorylation at the T loop by the CDK-activating kinase, as well as elimination of the N-terminal inhibitory phosphates by CDC25 phosphatases. Active CDK1–cyclin complexes phosphorylate >70 substrates during G2 and early mitosis triggering centrosome separation, Golgi dynamics, nuclear envelope breakdown and chromosome condensation among other processes (10). Additional mitotic kinases of the aurora, polo and nek protein families participate in the centrosome cycle by phosphorylating specific substrates such as Abnormal Spindle Microcephaly Associated Homolog (ASPM) and Centrosomal Nek2-associated protein 1 (CNAP1) (11–13). Similarly, proline-directed kinases such as CDK1 cooperate with aurora and polo kinases to phosphorylate various histones involved in chromosome condensation including histones H1 and H3, as well as other proteins involved in chromosome condensation such as topoisomerase II (TOP2A) and the complex known as condensin (10–12,14). The effect of phosphorylation by these kinases is counteracted by several phosphatases such as the general phosphatases PP1 and PP2 or the proline-directed phosphatases CDC14A and CDC14B (15).

The activity of cell cycle kinases and other mitotic regulators is tightly controlled by ubiquitin-mediated proteolysis (16). Progression through mitosis is intimately associated with the activity of the anaphase-promoting complex/cyclosome (APC/C), the major mitotic ubiquitin ligase that controls the timely degradation of several mitotic regulators such as mitotic cyclins or aurora and polo kinases (17,18). Substrate specificity is provided by two regulatory cofactors of the APC/C: CDC20 (also known as fizzy in Drosophila) and FZR1 (fizzy-related 1 or Cdh1). Whereas APC/C–CDC20 activity is controlled by the spindle assembly checkpoint (SAC) during early mitosis (see below), APC/C–FZR1 complexes are activated in late mitosis and remain active through G1 phase (19). Once all chromosomes contact the bipolar spindle and move to the metaphase plate, APC/C–CDC20 degrades B-type cyclins and PTTG1 triggering the cleavage of cohesins by separase (ESPL1) and the separation of sister chromatids. FZR1, on the other hand, targets for degradation of additional APC/C substrates such as polo-like kinase 1 (PLK1), aurora A, survivin (also known as BIRC5), NEK2, CDC20 and SKP2 later from anaphase to the following G1 phase. FZR1 levels are relatively constant during the cell cycle and its activity is mainly regulated by cell cycle-dependent phosphorylation. Thus, phosphorylation of FZR1 by CDKs during S, G2 and early M phases inhibits its binding to APC/C (20,21), whereas its dephosphorylation by CDC14 in late M and G1 phases allows binding to APC/C and activation of the complex (21,22). Other ubiquitin ligases have been shown to function in mitosis regulation. For example, CHFR seems to be critical for the early phases of mitosis since it plays an important role in the antephase checkpoint (Figure 1), which arrests the cell cycle in the presence of certain stresses before the cell commits to mitosis (23). Although it was originally proposed that CHFR establishes this checkpoint by targeting specific mitotic proteins for degradation (24,25), recent evidences show that the CHFR-dependent checkpoint requires ubiquitination but not proteasome activity (26,27).

In addition to kinases, phosphatases and proteolytic molecules, mitotic progression requires a variety of other biochemical functions including ATPase-driven motors such as dyneins, kinesins and related proteins (28). These and other microtubule-associated and kinetochore-associated proteins are involved in spindle dynamics and chromosome movements that facilitate the proper separation of DNA material into two daughter cells (29).

Mitotic checkpoints and chromosome segregation
A number of different mitotic checkpoints that arrest mitotic progression in response to cell cycle dysfunction have been described based on the existence of signaling pathways (30). A previous G2 checkpoint does not allow mitosis entry if DNA is damaged (31,32). This cell cycle checkpoint involves sensor proteins, such as Ataxia Telangiectasia Mutated (ATM) and Ataxia Telangiectasia and RAD3 Related (ATR), which detect DNA damage and trigger a cascade of signals through the CHK1 and CHK2 kinases and the p53 pathway among others. These DNA damage signaling pathways control both G1/S and G2/M transitions and their involvement in human cancer is well established (31). Therefore, these checkpoints will not be further discussed here.

The major checkpoint that controls mitotic progression is known as the SAC or mitotic checkpoint (33,34). This signaling pathway ensures the proper alignment of the chromosomes at the metaphase plate prior to chromosome segregation. The SAC is activated in every cell cycle immediately upon entry into mitosis and functions to delay anaphase until all chromosomes are properly attached at the metaphase plate (Figure 2). The inhibitory signal comes from the unattached kinetochores, and induces the recruitment of checkpoint proteins such as MAD2, BUBR1 (also known as BUB1B), BUB3 and TTK (Mps1). Although the origin of this signal is not properly understood, additional protein complexes such as the one formed by aurora B (AURKB), survivin and INCENP are known to participate by sensing tension between sister centromeres (35,36). Upon binding to the kinetochore protein CENPE, the kinase BUBR1 is activated and recruits the MAD1–MAD2 heterodimer in collaboration with HEC1 and the ZW10–ZWINT–ZWILCH complex (37,38). Activated BUB1B and/or MAD2 tightly sequester CDC20, preventing the activation of the APC/C. Upon proper chromosome alignment, MAD2 is released from the complex, resulting in the activation of APC/C–CDC20, which targets PTTG1 and cyclin B1 for degradation. The elimination of PTTG1 activates separase, which in turn cleaves the cohesin complexes that keep together sister chromatids (Figure 2). On the other hand, degradation of cyclin B1 results in CDK1 inactivation, which is required for mitotic exit (33).


Figure 2
View larger version (29K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 2. Molecular players at the SAC. Sister chromatid cohesion is maintained until metaphase by cohesin complexes, whose stability is ensured by a signaling pathway that sensors unattached kinetochores. Lack of attachment is sensored by kinases such as BUB1, BUBR1 or TTK and microtubule motors such as CENPE. These proteins recruit MAD1/MAD2 heterodimers. MAD2 cycles rapidly to sequester CDC20 and inhibits APC/C activity. After microtubule attachment, MAD2 is released from APC/C–CDC20 complexes resulting in APC/C activation, ubiquitination (Ub) and destruction of PTTG1 and the subsequent activation of the protease separase that destroys cohesin complexes. Tension is then generated between the bi-oriented kinetochores resulting in sister chromatid separation during anaphase. Pictures represent NIH 3T3 cells at metaphase or anaphase. DNA (blue) and microtubules (red) are detected as described in Figure 1.

 
In addition to the SAC, additional checkpoints that ensure proper progression throughout mitosis have been proposed (Figure 1). The antephase checkpoint, defined by CHFR, delays entry into metaphase when centrosome separation is inhibited by cellular stress (23). Two cytokinesis checkpoints that prevent cell division in response to misaligned chromosomes have also been proposed in yeast. One of them is modulated by the function of EB1, a microtubule-binding protein (39). The second cytokinesis checkpoint, known as the NoCut pathway, depends on aurora B and the anillin-related proteins Boi1 and Boi2 and delays the completion of cytokinesis in cells with spindle mid-zone defects (40). The signaling pathways that control these three checkpoints are not well understood and require further study.

Exit from mitosis and cytokinesis
The proper exit from mitosis requires the spatial and temporal coordination of several processes including inactivation of CDKs, the onset of anaphase, the disassembly of the spindle and, finally, cytokinesis. Some of the molecular mechanisms involved in these processes are included in three different pathways described in yeast: the CDC14 early anaphase release (FEAR), the mitotic exit networks and the NoCut pathway (40–43). The CDC14 phosphatase is the main player in the FEAR and mitotic exit network pathways and has a critical role in many late mitotic events in yeast (44). Although the ability of CDC14 homologs to antagonize mitotic CDK activity is probably conserved in all eukaryotes, other CDC14 functions described in yeast appear to differ significantly between species (42,44). It has been speculated that, in mammalian cells, chromosome passenger proteins such as INCENP and aurora B could function similarly to the FEAR network in coordinating accurate chromosome segregation with later mitotic events (42). Interestingly, it has been recently reported that aurora B is required for the NoCut pathway, a checkpoint-like network that prevents chromosome breakage by linking completion of cytokinesis to spindle mid-zone function (40).

The central spindle assembly, another process relevant to cytokinesis, requires the action of the microtubule-associated protein PRC1 (protein regulator of cytokinesis 1) and the centralspindlin complex (45). This complex consists of a Rho family GAP, RACGAP1 (also called MgcRacGAP) and the kinesin-like protein KIF23 (also called MKLP1). Both PRC1 and the centralspindlin complex are regulated by phosphorylation. Thus, inactivation of CDK1 at the end of mitosis activates PRC1. Aurora B and PLK1 kinases, on the other hand, associate with another kinesin-like protein, KIF20A (also called MKLP2), and seem to be required for centralspindlin function at the central spindle (46,47). During furrow positioning and initiation, which is facilitated by the central spindle, the main players seem to be aurora B and the RhoA pathway, which includes this small GTPase as well as its exchange factor, ECT2, and the downstream effector Rho-associated, coiled- coil-containing protein kinase 1 (ROCK1) (45). The contractile ring, composed of actin, myosin II, formin and septins among many other structural and regulatory proteins, is assembled at the future cleavage site (48). Finally, contractile force is generated upon the phosphorylation of non-muscle myosin II, the principal motor responsible for cytokinesis, by ROCK1 and citron kinase (49) resulting in complete cell division into two daughter cells.


   Alteration of mitotic regulators in human cancer
Top
 Abstract
 Oncogenes and tumor suppressors...
 Molecular regulation of mitosis
 Alteration of mitotic regulators...
Experimental evidence on mitotic...
 CIN and cancer: conceptual...
 References
 
One century ago, Theodore Boveri predicted that chromosome alterations may be associated with cancer development and progression (50). In the last few years, a significant number of genetic alterations in mitotic regulators have been reported (Table I). As predicted, molecular studies show that these mutations induce genetic instability and, in fact, many of these alterations are associated with human tumors with a chromosome instability (CIN) phenotype (51). So far, >20 mitotic regulators have been found to be mutated in human cancer by genetic or epigenetic means (Table I). Genetic alterations include DNA amplification (such as AURKA and its regulators TPX2, FOXM1 and CENPF) and chromosomal translocations affecting the expression of particular genes (NUMA1, CEP110, Ninein, NUP98, PCM1 and the nucleophosmin gene NPM1). Deletions in the NPM1 and LATS1 genes also occur in specific tumor types. In addition, tumor-associated point mutations have been reported in NPM1 and PLK1, as well as in several SAC regulators such as the BUB1 and BUBR1 kinases and the kinetochore proteins KNTC1 (also known as ROD), ZW10, ZWILCH, MAD1 and MAD2 (also known as MAD1L1 and MAD2L1, respectively). In some particular cases, normal expression of mitotic proteins is altered by epigenetic means as described for LATS1, LATS2, CHFR and RASSF1.


View this table:
[in this window]
[in a new window]

  		Table I. Alteration of mitotic regulators in human cancer

 
In addition to these genetic and epigenetic alterations, many more mitotic genes display cancer-associated altered expression (Table I). These molecules include proteins regulating pre-mitotic events (such as B-type cyclins, CHFR, CDK1 and FOXM1, a transcription factor that modulates the expression of many other mitotic genes), centrosome kinases (aurora A, NEK2 and PLK1), proteolysis regulatory proteins (including CDC20 and RASSF1), structural proteins (such as H2AFX, CENPF and PCM1), SAC components (BUB kinases, MAD1, MAD2 and TTK, among others) and other proteins involved in the exit from mitosis (ECT2 and PRC1). Interestingly, the SAC is the major target of mitotic alterations (Figure 3), suggesting the importance of this mitotic checkpoint in protecting cells from malignant transformation.


Figure 3
View larger version (19K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Fig. 3. Summary of mitotic alterations in cancer categorized by molecular or cellular function. Numbers reflect the percentage of molecules listed in Table I associated to specific cellular or molecular functions. Mitotic checkpoints, and specifically the SAC, are the major targets for tumor-associated alterations. Protein kinases (mostly centrosomal and checkpoint kinases) are significantly represented, suggesting diverse therapeutic uses in cancer.

 
The signature of CIN
As mentioned above, CIN is a hallmark of many tumor types and alteration of mitotic regulators seems to be more frequent among CIN+ cancers. In fact, CIN has been proposed as a driving force in tumor initiation since it can be detected in the early stages of tumorigenesis (52). The molecular mechanisms underlying CIN were poorly understood until very recently when a CIN signature associated with cancer has been described (53). Out of the 70 genes included in that signature, 29 of them can be considered mitotic regulators according to their function. Some of these molecules participate in the dynamic structural changes required for mitosis (CKAP5, NACPH, NCAPD2 and H2AFX), although most of them regulate progression through the different mitotic stages (e.g. CDK1, FOXM1, AURKA, CDC20, PTTG1, MAD2L1, ZWINT and PRC1). Interestingly, many of these genes are involved in the regulation of the centrosome cycle (CDK1 and AURKA) or in SAC (AURKB, MAD2L1, PTTG1, ZWINT and CDC20). In fact, these genes account for more than half of the genes represented in Table I, indicating that the altered expression of mitotic genes, in general, is associated with CIN rather than cell proliferation. It is important to note that altered expression of mitotic genes does not necessarily correlate with cell proliferation indicators such as the ‘mitotic index’ used to quantify dividing cells in histology sections. In contrast, expression of G1/S regulators or DNA replication molecules such as cyclin E, p27KIP1 or MCM proteins is usually a better predictor of cell proliferation (4).

Not all human tumor types display a CIN phenotype. CIN is observed in colon, breast, lung and prostate cancer, whereas it does not appear in hematopoietic tumors such as chronic myelogenic leukemia or acute lymphoblastic leukemia. On the other hand, hematopoietic malignancies are rarely associated with alterations in mitotic regulators. Importantly, in those tumor types where CIN is present, there is a significant correlation between the CIN phenotype and poor prognosis, suggesting that chromosome imbalances might specifically contribute to aggressive or metastatic cancer (53).

Interestingly, there are some cases where both the up- and the down-regulation of specific mitotic regulators result in CIN. An example is MAD2, which is either up- or down-regulated in some specific tumor cells, provoking defective SAC and chromosomal imbalances (54). These data suggest that subtle changes in the level of expression of specific mitotic regulators might have important consequences in genomic stability. Even more importantly, either up-regulation or down-regulation of the same molecule may result in similar genomic aberrations. As these changes in expression level are not routinely detected in clinical settings, defining these proteins as having either oncogenic or tumor suppressor properties is difficult and a challenge in this area of cancer research.


   Experimental evidence on mitotic deregulation and cancer
Top
 Abstract
 Oncogenes and tumor suppressors...
 Molecular regulation of mitosis
 Alteration of mitotic regulators...
 Experimental evidence on mitotic...
CIN and cancer: conceptual...
 References
 
In agreement with the observed effect of mitotic dysregulation in human cancer, over-expression or down-regulation of specific mitotic regulators promotes genetic instability in cultured cells. For example, up-regulation or under-expression of PIN1 (55), NEK2 (56), auroras A and B (57–60), TPX2 (57) or FOXM1 (61), among others, leads to mitotic abnormalities including G2/M arrest, centrosome maturation and centrosome numeric defects, lagging chromosomes, improper spindle orientation, lack of spindle checkpoint and cytokinesis failure. In some cases, over-expression of these genes induces cellular transformation in mouse fibroblasts. PLK1 over-expression results in foci formation and the resulting transformed clones grow in soft agar and, more importantly, form tumors in nude mice (62). Similar results have been obtained with aurora kinases A and B, PTTG1, TPX2, PIN1, UBC2E, BUB1 and ECT2 (63–72). On the other hand, re-expression of mitotic regulators inactivated in human cancer (RASSF1, CHFR, LATS1 and PMS2) reverts some oncogenic properties in tumor cells (9).

Mouse models: heterozygous mice make the difference
Genetically engineered mice have recently provided solid evidence of the critical role of mitotic regulators in tumorigenesis (73). Over the last few years, a number of mouse models have been generated for specific mitotic genes. Interestingly, many of them develop a cancer-associated phenotype (Table II). Over-expression of aurora A or Pin1 in the mammary gland results in breast hyperplasia or tumors accompanied with genetic instability or centrosome amplification (65,74). The tumor-associated phenotype of over-expressing aurora A is worsened in a p53+/– background (74,75), suggesting that the p53 pathway protects cells from malignant transformation by inducing apoptosis upon CIN.


View this table:
[in this window]
[in a new window]

  		Table II. Tumor mouse models of mitotic regulators

 
In addition to these transgenic models, the function of the endogenous molecules has been investigated using gene targeting in the mouse. Ablation of some SAC proteins, such as Bub1, Bub1b, Bub3 and Mad2, results in embryonic lethality linked to massive chromosome mis-segregation, lagging chromosomes and/or apoptosis (76–80). Interestingly, partial deficiency in some of these molecules (heterozygous models) provokes a cancer-prone phenotype by themselves or in collaboration with other genetic abnormalities or tumor induction treatments. For example, Mad2 heterozygous mice display an increased incidence of papillary lung adenocarcinomas when compared with control animals (81) and Bub3 heterozygous mice are more susceptible to 7,12-dimethylbenzanthracene (DMBA)-induced lung adenocarcinomas than their wild-type littermates (82). However, despite accumulating aneuploid cells, these mice are not prone to spontaneous tumors even when they are crossed with p53 or pRb heterozygous knockout mice (83). Similar results are observed in Bub1b hypomorphic mice (~10% of normal BubR1 protein levels) as only a small percentage of mice develop spontaneous tumors (80). However, these mice display a significantly higher susceptibility to carcinogen-induced tumors (80,84).

In contrast to SAC proteins, both CHFR and RASSF1 are not required for cell survival or proliferation (85,86). Both genes are inactivated in different human tumor types as a consequence of the hypermethylation of their promoter (Table I). In agreement with these data, mice deficient for any of these genes are predisposed to the development of spontaneous and carcinogen-induced tumors (85,86). Chfr-null cells display higher amounts of aurora A and Plk1, which might be involved in their CIN phenotype (85). Depletion of Rassf1a, on the other hand, provokes premature APC/C activation that results in accelerated degradation of mitotic cyclins, and causes a cell division defect characterized by centrosome abnormalities and multipolar spindles (87).

Ablation of some additional mitotic regulators also results in tumor susceptibility in the mouse. Mice lacking H2afx, a critical mitotic molecule involved in DNA repair, also show genetic instability (88). Furthermore, H2AFX heterozygosity enhances, as in the case of SAC genes, susceptibility to cancer (89). A similar situation has been reported for Plk4 and Npm1, two proteins involved in centrosome function (90,91). Plk4 and Npm1 are haplo-insufficient for tumor suppression since heterozygous mice display a significant increase in tumor susceptibility without losing the wild-type allele (91,92). Mice deficient in Lats1, a serine/threonine kinase involved in mitotic exit, develop soft tissue sarcomas and ovarian stromal cell tumors and are highly sensitive to carcinogenic treatments (93). Interestingly, these models parallel the reduced expression of this gene due to allelic loss and promoter hypermethylation in human soft tissue sarcomas and breast cancers (94,95).

Finally, genetic elimination of the transcription factor FoxM1 suggests putative therapeutic value for the inactivation of some mitotic proteins. Specific inhibition of FoxM1 in the mouse lung prior to the induction of tumors with urethane significantly diminished the size and number of lung adenomas (96). Since FOXM1 is involved in the transcriptional regulation of some mitotic proteins (61,97), these results suggest that inhibition of the proper targets might provide therapeutic advantages to arrest tumor cells.


   CIN and cancer: conceptual and therapeutic implications
Top
 Abstract
 Oncogenes and tumor suppressors...
 Molecular regulation of mitosis
 Alteration of mitotic regulators...
 Experimental evidence on mitotic...
 CIN and cancer: conceptual...
References
 
Cancer epidemiology studies show that abnormal expression of mitotic genes is quite frequent in different tumor types and correlates with CIN and poor prognosis (53). These data are in agreement with the phenotype of specific mouse models. Thus, complete inactivation of the spindle checkpoint regulator Mad2 induces mitotic defects incompatible with cell survival and proliferation, whereas the deletion of one allele results in cancer development without the loss of the second allele (81). Molecular studies suggest that both up- and down-regulation of some mitotic regulators induce similar aberrant mitotic cycles and lead to genomic instability. Similarly, not only MAD2 deficiency but also MAD2 over-expression is linked to tumor development in both humans and mouse models (54,98). These data suggest that subtle variations in mitotic protein levels may have oncogenic effects, whereas complete mutation or elimination of these proteins may not be compatible with cell survival. In other cases, mitotic regulators may be relevant in tumor development as ‘modifier’ genes, which serve to enhance or suppress oncogenic phenotypes induced by other mutations. At least three mitotic regulators, aurora A, NUMA1 and MAD1, have allelic variants that confer increased tumor susceptibility to their carriers (99–104).

The crucial role of several mitotic kinases in cell cycle progression has lead to an increased interest in the development of small-molecule inhibitors for aurora or polo family members in cancer treatment (9,105,106). In addition, many anti-tumor drugs currently used in the clinic, such as the taxanes and the vinca alkaloids, inhibit the cell cycle by altering the mitotic spindle (107). It is believed that these drugs, by reducing microtubule dynamics, keep the SAC in an active state, and that sustained SAC activation is often followed by cell death (108–110). Complete inhibition of the mitotic checkpoint is lethal to individual cells as it has been demonstrated by significantly reducing MAD2 or BUB1B levels in tumor cell lines (81,111). These results have led to the proposal of using SAC inhibitors for cancer therapy. In fact, abrogation of the DNA damage checkpoint (112) is being considered as a general strategy in cancer therapy and similar reasoning applies for the abrogation of the mitotic checkpoint. This hypothesis has been validated since some new-class TTK inhibitors have been identified that specifically override the mitotic arrest induced by spindle poisons (113,114). Mitotic regulators therefore offer a wide range of opportunities for drug design and the induction of tumor cell-specific death (9,110). Most current therapeutic studies have focused on cell cycle kinases since this biochemical activity has been traditionally taken as one of the best options in rational anti-tumoral drug design. Although alteration of kinases accounts for one-fourth of the mitotic proteins associated with cancer (Figure 3), further studies will undoubtedly extend current therapeutic strategies to other biochemical activities. As a relevant example, inhibition of the mitotic kinesin KSP (also known as Eg5) by small molecules has been reported to provide unique therapeutic advantages (115,116).

In summary, although unscheduled cell proliferation is usually promoted by alteration of G1/S regulators, further dysregulation of mitotic proteins provides additional advantages to tumor cells. An integrated examination of biochemical studies, animal models and a multivariant analysis of molecular alterations in human cancer will significantly improve our knowledge on chromosome segregation and genomic stability. Understanding the molecular basis of these pathways will undoubtedly help in the development of new therapies against cancer progression and metastasis.

Note added in Proof: A recent screen for mutations in more than 200 human cancers (Greenman et al., 2007 Nature 2007; 446, 153-158) has identified additional mutations in mitotic kinases including Aurora and Polo kinases, several NEK proteins, LATS1-2, TLK1-2, and TTK, among others.


   Acknowledgments
 
G. de C. and I.P. de C. are supported by Ramón y Cajal contracts from the Spanish Ministry of Education and Science. This work was funded by grants from the Fondo Investigaciones Sanitarias (PI051186) (G. de C.), the Fundación Ramón Areces, The human factor, mobility and Marie Curie activities of the European Commission (017448-IRG7) and Ministry of Education and Science (SAF2004-07459) (I. P. de C.) and from the Fundación Científica de la Asociación Española contra el Cáncer, Fundación Médica Mutua Madrileña and the Ministry of Science (SAF2006-05186) (M.M.). The Cell Division and Cancer Group is partially supported by the OncoCycle programme from the Comunidad de Madrid (S-BIO-0283-2006).

Conflict of Interest Statement: None declared.


   References
Top
 Abstract
 Oncogenes and tumor suppressors...
 Molecular regulation of mitosis
 Alteration of mitotic regulators...
 Experimental evidence on mitotic...
 CIN and cancer: conceptual...
 References
 

  1. Hanahan D, et al. (2000) The hallmarks of cancer. Cell 100:57–70.[CrossRef][ISI][Medline]
  2. Massague J. (2004) G1 cell-cycle control and cancer. Nature 432:298–306.[CrossRef][Medline]
  3. Sherr CJ. (2000) The Pezcoller lecture: cancer cell cycles revisited. Cancer Res. 60:3689–3695.[Abstract/Free Full Text]
  4. Malumbres M, et al. (2001) To cycle or not to cycle: a critical decision in cancer. Nat. Rev. Cancer 1:222–231.[CrossRef][Medline]
  5. Doxsey S, et al. (2005) Centrosome control of the cell cycle. Trends Cell Biol. 15:303–311.[CrossRef][ISI][Medline]
  6. Herceg Z, et al. (2005) Rendez-vous at mitosis: TRRAPed in the chromatin. Cell Cycle 4:383–387.[ISI][Medline]
  7. Gutierrez GJ, et al. (2006) Ubiquitin and SUMO systems in the regulation of mitotic checkpoints. Trends Biochem. Sci. 31:324–332.[CrossRef][ISI][Medline]
  8. Nigg EA. (2001) Mitotic kinases as regulators of cell division and its checkpoints. Nat. Rev. Mol. Cell Biol. 2:21–32.[CrossRef][ISI][Medline]
  9. Malumbres M. (2006) Therapeutic opportunities to control tumor cell cycles. Clin. Transl. Oncol. 8:399–408.[Medline]
  10. Malumbres M, et al. (2005) Mammalian cyclin-dependent kinases. Trends Biochem. Sci. 30:630–641.[CrossRef][ISI][Medline]
  11. Barr FA, et al. (2004) Polo-like kinases and the orchestration of cell division. Nat. Rev. Mol. Cell Biol. 5:429–440.[CrossRef][ISI][Medline]
  12. Meraldi P, et al. (2004) Aurora kinases link chromosome segregation and cell division to cancer susceptibility. Curr. Opin. Genet. Dev. 14:29–36.[CrossRef][ISI][Medline]
  13. Hayward DG, et al. (2006) Nek2 kinase in chromosome instability and cancer. Cancer Lett. 237:155–166.[CrossRef][ISI][Medline]
  14. Hirano T. (2006) At the heart of the chromosome: SMC proteins in action. Nat. Rev. Mol. Cell Biol. 7:311–322.[CrossRef][ISI][Medline]
  15. Trinkle-Mulcahy L, et al. (2006) Mitotic phosphatases: no longer silent partners. Curr. Opin. Cell Biol. 18:623–631.[CrossRef][ISI][Medline]
  16. Pines J. (2006) Mitosis: a matter of getting rid of the right protein at the right time. Trends Cell Biol. 16:55–63.[CrossRef][ISI][Medline]
  17. Tyers M, et al. (2000) Proteolysis and the cell cycle: with this RING I do thee destroy. Curr. Opin. Genet. Dev. 10:54–64.[CrossRef][ISI][Medline]
  18. Peters JM. (2006) The anaphase promoting complex/cyclosome: a machine designed to destroy. Nat. Rev. Mol. Cell Biol. 7:644–656.[CrossRef][ISI][Medline]
  19. Nakayama KI, et al. (2006) Ubiquitin ligases: cell-cycle control and cancer. Nat. Rev. Cancer 6:369–381.[CrossRef][ISI][Medline]
  20. Zachariae W, et al. (1998) Control of cyclin ubiquitination by CDK-regulated binding of Hct1 to the anaphase promoting complex. Science 282:1721–1724.[Abstract/Free Full Text]
  21. Jaspersen SL, et al. (1999) Inhibitory phosphorylation of the APC regulator Hct1 is controlled by the kinase Cdc28 and the phosphatase Cdc14. Curr. Biol. 9:227–236.[CrossRef][ISI][Medline]
  22. Visintin R, et al. (1998) The phosphatase Cdc14 triggers mitotic exit by reversal of Cdk-dependent phosphorylation. Mol. Cell 2:709–718.[CrossRef][ISI][Medline]
  23. Scolnick DM, et al. (2000) Chfr defines a mitotic stress checkpoint that delays entry into metaphase. Nature 406:430–435.[CrossRef][Medline]
  24. Kang D, et al. (2002) The checkpoint protein Chfr is a ligase that ubiquitinates Plk1 and inhibits Cdc2 at the G2 to M transition. J. Cell Biol. 156:249–259.[Abstract/Free Full Text]
  25. Chaturvedi P, et al. (2002) Chfr regulates a mitotic stress pathway through its RING-finger domain with ubiquitin ligase activity. Cancer Res. 62:1797–1801.[Abstract/Free Full Text]
  26. Bothos J, et al. (2003) The Chfr mitotic checkpoint protein functions with Ubc13-Mms2 to form Lys63-linked polyubiquitin chains. Oncogene 22:7101–7107.[CrossRef][ISI][Medline]
  27. Matsusaka T, et al. (2004) Chfr acts with the p38 stress kinases to block entry to mitosis in mammalian cells. J. Cell Biol. 166:507–516.[Abstract/Free Full Text]
  28. Sharp DJ, et al. (2000) Roles of motor proteins in building microtubule-based structures: a basic principle of cellular design. Biochim. Biophys. Acta 1496:128–141.[Medline]
  29. McIntosh JR, et al. (2002) Chromosome-microtubule interactions during mitosis. Annu. Rev. Cell Dev. Biol. 18:193–219.[CrossRef][ISI][Medline]
  30. Cortez D, et al. (2000) Conducting the mitotic symphony. Nature 406:354–356.[Medline]
  31. Bartek J, et al. (2004) Checking on DNA damage in S phase. Nat. Rev. Mol. Cell Biol. 5:792–804.[CrossRef][ISI][Medline]
  32. Harrison JC, et al. (2006) Surviving the breakup: the DNA damage checkpoint. Annu. Rev. Genet. 40:209–235.[CrossRef][ISI][Medline]
  33. Lew DJ, et al. (2003) The spindle assembly and spindle position checkpoints. Annu. Rev. Genet. 37:251–282.[CrossRef][ISI][Medline]
  34. Rudner AD, et al. (1996) The spindle assembly checkpoint. Curr. Opin. Cell Biol. 8:773–780.[CrossRef][ISI][Medline]
  35. Ditchfield C, et al. (2003) Aurora B couples chromosome alignment with anaphase by targeting BubR1, Mad2, and Cenp-E to kinetochores. J. Cell Biol. 161:267–280.[Abstract/Free Full Text]
  36. Pinsky BA, et al. (2005) The spindle checkpoint: tension versus attachment. Trends Cell Biol. 15:486–493.[CrossRef][ISI][Medline]
  37. Karess R. (2005) Rod-Zw10-Zwilch: a key player in the spindle checkpoint. Trends Cell Biol. 15:386–392.[CrossRef][ISI][Medline]
  38. Lin YT, et al. (2006) Hec1 sequentially recruits Zwint-1 and ZW10 to kinetochores for faithful chromosome segregation and spindle checkpoint control. Oncogene 25:6901–6914.[CrossRef][ISI][Medline]
  39. Muhua L, et al. (1998) A cytokinesis checkpoint requiring the yeast homologue of an APC-binding protein. Nature 393:487–491.[CrossRef][Medline]
  40. Norden C, et al. (2006) The NoCut pathway links completion of cytokinesis to spindle midzone function to prevent chromosome breakage. Cell 125:85–98.[CrossRef][ISI][Medline]
  41. Bosl WJ, et al. (2005) Mitotic-exit control as an evolved complex system. Cell 121:325–333.[CrossRef][ISI][Medline]
  42. Stegmeier F, et al. (2004) Closing mitosis: the functions of the Cdc14 phosphatase and its regulation. Annu. Rev. Genet. 38:203–232.[CrossRef][ISI][Medline]
  43. Bardin AJ, et al. (2001) Men and sin: what's the difference? Nat. Rev. Mol. Cell Biol. 2:815–826.[CrossRef][ISI][Medline]
  44. Seshan A, et al. (2004) Linked for life: temporal and spatial coordination of late mitotic events. Curr. Opin. Cell Biol. 16:41–48.[CrossRef][ISI][Medline]
  45. Glotzer M. (2005) The molecular requirements for cytokinesis. Science 307:1735–1739.[Abstract/Free Full Text]
  46. Gruneberg U, et al. (2004) Relocation of aurora B from centromeres to the central spindle at the metaphase to anaphase transition requires MKlp2. J. Cell Biol. 166:167–172.[Abstract/Free Full Text]
  47. Neef R, et al. (2003) Phosphorylation of mitotic kinesin-like protein 2 by polo-like kinase 1 is required for cytokinesis. J. Cell Biol. 162:863–875.[Abstract/Free Full Text]
  48. Glotzer M. (2004) Cleavage furrow positioning. J. Cell Biol. 164:347–351.[Abstract/Free Full Text]
  49. Matsumura F. (2005) Regulation of myosin II during cytokinesis in higher eukaryotes. Trends Cell Biol. 15:371–377.[CrossRef][ISI][Medline]
  50. Boveri T. (1914) Zur Frage Der Entstehung Maligner Tumoren(Gustav Fischer Verlag, Jena, Germany).
  51. Kops GJ, et al. (2005) On the road to cancer: aneuploidy and the mitotic checkpoint. Nat. Rev. Cancer 5:773–785.[CrossRef][ISI][Medline]
  52. Shih IM, et al. (2001) Evidence that genetic instability occurs at an early stage of colorectal tumorigenesis. Cancer Res. 61:818–822.[Abstract/Free Full Text]
  53. Carter SL, et al. (2006) A signature of chromosomal instability inferred from gene expression profiles predicts clinical outcome in multiple human cancers. Nat. Genet. 38:1043–1048.[CrossRef][ISI][Medline]
  54. Hernando E, et al. (2004) Rb inactivation promotes genomic instability by uncoupling cell cycle progression from mitotic control. Nature 430:797–802.[CrossRef][Medline]
  55. Yeh ES, et al. (2006) The loss of PIN1 deregulates cyclin E and sensitizes mouse embryo fibroblasts to genomic instability. J. Biol. Chem. 281:241–251.[Abstract/Free Full Text]
  56. Prigent C, et al. (2005) Drosophila nek2 protein kinase knockdown leads to centrosome maturation defects while overexpression causes centrosome fragmentation and cytokinesis failure. Exp. Cell Res. 303:1–13.[ISI][Medline]
  57. De Luca M, et al. (2006) A functional interplay between aurora-A, Plk1 and TPX2 at spindle poles: Plk1 controls centrosomal localization of aurora-A and TPX2 spindle association. Cell Cycle 5:296–303.[ISI][Medline]
  58. Yang H, et al. (2005) Mitotic requirement for aurora A kinase is bypassed in the absence of aurora B kinase. FEBS Lett. 579:3385–3391.[CrossRef][ISI][Medline]
  59. Dutertre S, et al. (2003) Aurora-A overexpression leads to override of the microtubule-kinetochore attachment checkpoint. Mol. Interv. 3:127–130.[Abstract/Free Full Text]
  60. Anand S, et al. (2003) Aurora-A amplification overrides the mitotic spindle assembly checkpoint, inducing resistance to Taxol. Cancer Cell 3:51–62.[CrossRef][ISI][Medline]
  61. Laoukili J, et al. (2005) FoxM1 is required for execution of the mitotic programme and chromosome stability. Nat. Cell Biol. 7:126–136.[CrossRef][ISI][Medline]
  62. Smith MR, et al. (1997) Malignant transformation of mammalian cells initiated by constitutive expression of the polo-like kinase. Biochem. Biophys. Res. Commun. 234:397–405.[CrossRef][ISI][Medline]
  63. Musio A, et al. (2003) Inhibition of BUB1 results in genomic instability and anchorage-independent growth of normal human fibroblasts. Cancer Res. 63:2855–2863.[Abstract/Free Full Text]
  64. Pallante P, et al. (2005) UbcH10 overexpression may represent a marker of anaplastic thyroid carcinomas. Br. J. Cancer 93:464–471.[CrossRef][ISI][Medline]
  65. Suizu F, et al. (2006) Pin1 regulates centrosome duplication, and its overexpression induces centrosome amplification, chromosome instability, and oncogenesis. Mol. Cell Biol. 26:1463–1479.[Abstract/Free Full Text]
  66. Ma Y, et al. (2006) Expression of targeting protein for xklp2 associated with both malignant transformation of respiratory epithelium and progression of squamous cell lung cancer. Clin. Cancer Res. 12:1121–1127.[Abstract/Free Full Text]
  67. Hamid T, et al. (2005) Ectopic expression of PTTG1/securin promotes tumorigenesis in human embryonic kidney cells. Mol. Cancer 4:3.[CrossRef][Medline]
  68. Gonzalez S, et al. (2006) Oncogenic activity of Cdc6 through repression of the INK4/ARF locus. Nature 440:702–706.[CrossRef][Medline]
  69. Bischoff JR, et al. (1998) A homologue of Drosophila aurora kinase is oncogenic and amplified in human colorectal cancers. EMBO J. 17:3052–3065.[CrossRef][ISI][Medline]
  70. Tatsuka M, et al. (2005) Overexpression of aurora-A potentiates HRAS-mediated oncogenic transformation and is implicated in oral carcinogenesis. Oncogene 24:1122–1127.[CrossRef][ISI][Medline]
  71. Kanda A, et al. (2005) Aurora-B/AIM-1 kinase activity is involved in Ras-mediated cell transformation. Oncogene 24:7266–7272.[CrossRef][ISI][Medline]
  72. Miki T, et al. (1993) Oncogene ect2 is related to regulators of small GTP-binding proteins. Nature 362:462–465.[CrossRef][Medline]
  73. Baker DJ, et al. (2005) The mitotic checkpoint in cancer and aging: what have mice taught us? Curr. Opin. Cell Biol. 17:583–589.[CrossRef][ISI][Medline]
  74. Zhang D, et al. (2004) Cre-loxP-controlled periodic aurora-A overexpression induces mitotic abnormalities and hyperplasia in mammary glands of mouse models. Oncogene 23:8720–8730.[CrossRef][ISI][Medline]
  75. Wang X, et al. (2006) Overexpression of aurora kinase A in mouse mammary epithelium induces genetic instability preceding mammary tumor formation. Oncogene 25:7148–7158.[CrossRef][ISI][Medline]
  76. Dobles M, et al. (2000) Chromosome missegregation and apoptosis in mice lacking the mitotic checkpoint protein Mad2. Cell 101:635–645.[CrossRef][ISI][Medline]
  77. Burds AA, et al. (2005) Generating chromosome instability through the simultaneous deletion of Mad2 and p53. Proc. Natl Acad. Sci. USA 102:11296–11301.[Abstract/Free Full Text]
  78. Kalitsis P, et al. (2000) Bub3 gene disruption in mice reveals essential mitotic spindle checkpoint function during early embryogenesis. Genes Dev. 14:2277–2282.[Abstract/Free Full Text]
  79. Wang Q, et al. (2004) BUBR1 deficiency results in abnormal megakaryopoiesis. Blood 103:1278–1285.[Abstract/Free Full Text]
  80. Baker DJ. (2004) BubR1 insufficiency causes early onset of aging-associated phenotypes and infertility in mice. Nat. Genet. 36:744–749.[CrossRef][ISI][Medline]
  81. Michel LS, et al. (2001) MAD2 haplo-insufficiency causes premature anaphase and chromosome instability in mammalian cells. Nature 409:355–359.[CrossRef][Medline]
  82. Babu JR. (2003) Rae1 is an essential mitotic checkpoint regulator that cooperates with Bub3 to prevent chromosome missegregation. J. Cell Biol. 160:341–353.[Abstract/Free Full Text]
  83. Kalitsis P, et al. (2005) Increased chromosome instability but not cancer predisposition in haploinsufficient Bub3 mice. Genes Chromosomes Cancer 44:29–36.[CrossRef][ISI][Medline]
  84. Dai W, et al. (2004) Slippage of mitotic arrest and enhanced tumor development in mice with BubR1 haploinsufficiency. Cancer Res. 64:440–445.[Abstract/Free Full Text]
  85. Yu X, et al. (2005) Chfr is required for tumor suppression and aurora A regulation. Nat. Genet. 37:401–406.[CrossRef][ISI][Medline]
  86. Tommasi S, et al. (2005) Tumor susceptibility of Rassf1a knockout mice. Cancer Res. 65:92–98.[Abstract/Free Full Text]
  87. Song MS, et al. (2004) The tumour suppressor RASSF1A regulates mitosis by inhibiting the APC-Cdc20 complex. Nat. Cell Biol. 6:129–137.[CrossRef][ISI][Medline]
  88. Celeste A, et al. (2002) Genomic instability in mice lacking histone H2AX. Science 296:922–927.[Abstract/Free Full Text]
  89. Celeste A, et al. (2003) H2AX haploinsufficiency modifies genomic stability and tumor susceptibility. Cell 114:371–383.[CrossRef][ISI][Medline]
  90. Hudson JW, et al. (2001) Late mitotic failure in mice lacking Sak, a polo-like kinase. Curr. Biol. 11:441–446.[CrossRef][ISI][Medline]
  91. Grisendi S, et al. (2005) Role of nucleophosmin in embryonic development and tumorigenesis. Nature 437:147–153.[CrossRef][Medline]
  92. Ko MA, et al. (2005) Plk4 haploinsufficiency causes mitotic infidelity and carcinogenesis. Nat. Genet. 37:883–888.[CrossRef][ISI][Medline]
  93. St John MA, et al. (1999) Mice deficient of Lats1 develop soft-tissue sarcomas, ovarian tumours and pituitary dysfunction. Nat. Genet. 21:182–186.[CrossRef][ISI][Medline]
  94. Takahashi Y, et al. (2005) Down-regulation of LATS1 and LATS2 mRNA expression by promoter hypermethylation and its association with biologically aggressive phenotype in human breast cancers. Clin. Cancer Res. 11:1380–1385.[Abstract/Free Full Text]
  95. Hisaoka M, et al. (2002) Molecular alterations of h-warts/LATS1 tumor suppressor in human soft tissue sarcoma. Lab. Invest. 82:1427–1435.[ISI][Medline]
  96. Kim IM, et al. (2006) The Forkhead Box m1 transcription factor stimulates the proliferation of tumor cells during development of lung cancer. Cancer Res. 66:2153–2161.[Abstract/Free Full Text]
  97. Wang IC, et al. (2005) Forkhead box M1 regulates the transcriptional network of genes essential for mitotic progression and genes encoding the SCF (Skp2-Cks1) ubiquitin ligase. Mol. Cell Biol. 25:10875–10894.[Abstract/Free Full Text]
  98. Sotillo R, et al. (2007) Mad2 overexpression promotes aneuploidy and tumorigenesis in mice. Cancer Cell 11:9–23.[CrossRef][ISI][Medline]
  99. Ewart-Toland A, et al. (2003) Identification of Stk6/STK15 as a candidate low-penetrance tumor-susceptibility gene in mouse and human. Nat. Genet. 34:403–412.[CrossRef][ISI][Medline]
  100. Kimura MT, et al. (2005) Two functional coding single nucleotide polymorphisms in STK15 (aurora-A) coordinately increase esophageal cancer risk. Cancer Res. 65:3548–3554.[Abstract/Free Full Text]
  101. Ewart-Toland A, et al. (2005) Aurora-A/STK15 T+91A is a general low penetrance cancer susceptibility gene: a meta-analysis of multiple cancer types. Carcinogenesis 26:1368–1373.[Abstract/Free Full Text]
  102. Sun T, et al. (2004) Functional Phe31Ile polymorphism in aurora A and risk of breast carcinoma. Carcinogenesis 25:2225–2230.[Abstract/Free Full Text]
  103. Iwanaga Y, et al. (2002) Characterization of regions in hsMAD1 needed for binding hsMAD2. A polymorphic change in an hsMAD1 leucine zipper affects MAD1-MAD2 interaction and spindle checkpoint function. J. Biol. Chem. 277:31005–31013.[Abstract/Free Full Text]<