Beth A. Weaver

On the road to cancer: Aneuploidy and the mitotic checkpoint

Abnormal chromosome content — also known as aneuploidy — is the most common characteristic of human solid tumours. It has therefore been proposed that aneuploidy contributes to, or even drives, tumour development. The mitotic checkpoint guards against chromosome mis-segregation by delaying cell-cycle progression through mitosis until all chromosomes have successfully made spindle-microtubule attachments. Defects in the mitotic checkpoint generate aneuploidy and might facilitate tumorigenesis, but more severe disabling of checkpoint signalling is a possible anticancer strategy.

Centromere-associated protein-E is essential for the mammalian mitotic checkpoint to prevent aneuploidy due to single chromosome loss

Centromere-associated protein-E (CENP-E) is an essential mitotic kinesin that is required for efficient, stable microtubule capture at kinetochores. It also directly binds to BubR1, a kinetochore-associated kinase implicated in the mitotic checkpoint, the major cell cycle control pathway in which unattached kinetochores prevent anaphase onset. Here, we show that single unattached kinetochores depleted of CENP-E cannot block entry into anaphase, resulting in aneuploidy in 25% of divisions in primary mouse fibroblasts in vitro and in 95% of regenerating hepatocytes in vivo. Without CENP-E, diminished levels of BubR1 are recruited to kinetochores and BubR1 kinase activity remains at basal levels. CENP-E binds to and directly stimulates the kinase activity of purified BubR1 in vitro. Thus, CENP-E is required for enhancing recruitment of its binding partner BubR1 to each unattached kinetochore and for stimulating BubR1 kinase activity, implicating it as an essential amplifier of a basal mitotic checkpoint signal.

ZW10 links mitotic checkpoint signaling to the structural kinetochore

The mitotic checkpoint ensures that chromosomes are divided equally between daughter cells and is a primary mechanism preventing the chromosome instability often seen in aneuploid human tumors. ZW10 and Rod play an essential role in this checkpoint. We show that in mitotic human cells ZW10 resides in a complex with Rod and Zwilch, whereas another ZW10 partner, Zwint-1, is part of a separate complex of structural kinetochore components including Mis12 and Ndc80–Hec1. Zwint-1 is critical for recruiting ZW10 to unattached kinetochores. Depletion from human cells or Xenopus egg extracts is used to demonstrate that the ZW10 complex is essential for stable binding of a Mad1–Mad2 complex to unattached kinetochores. Thus, ZW10 functions as a linker between the core structural elements of the outer kinetochore and components that catalyze generation of the mitotic checkpoint-derived “stop anaphase” inhibitor.

Aneuploidy: Instigator and Inhibitor of Tumorigenesis

Aneuploidy, an aberrant chromosome number, has been recognized as a common characteristic of cancer cells for more than 100 years and has been suggested as a cause of tumorigenesis for nearly as long. However, this proposal had remained untested due to the difficulty of selectively generating aneuploidy without causing other damage. Using Cenp-E heterozygous animals, which develop whole chromosome aneuploidy in the absence of other defects, we have found that aneuploidy promotes tumorigenesis in some contexts and inhibits it in others. These findings confirm that aneuploidy can act oncogenically and reveal a previously unsuspected role for aneuploidy as a tumor suppressor.

How Taxol/paclitaxel kills cancer cells

Taxol (generic name paclitaxel) is a microtubule-stabilizing drug that is approved by the Food and Drug Administration for the treatment of ovarian, breast, and lung cancer, as well as Kaposis sarcoma. It is used off-label to treat gastroesophageal, endometrial, cervical, prostate, and head and neck cancers, in addition to sarcoma, lymphoma, and leukemia. Paclitaxel has long been recognized to induce mitotic arrest, which leads to cell death in a subset of the arrested population. However, recent evidence demonstrates that intratumoral concentrations of paclitaxel are too low to cause mitotic arrest and result in multipolar divisions instead. It is hoped that this insight can now be used to develop a biomarker to identify the ∼50% of patients that will benefit from paclitaxel therapy. Here I discuss the history of paclitaxel and our recently evolved understanding of its mechanism of action.

Cytotoxicity of Paclitaxel in Breast Cancer Is due to Chromosome Missegregation on Multipolar Spindles

The chemotherapy drug paclitaxel causes tumor regression and cell death by inducing high rates of chromosome missegregation on multipolar spindles. The Secret Life of Paclitaxel The classic chemotherapy drug paclitaxel is a standard part of treatment for breast cancer and other malignancies. Although it is commonly understood to act as a microtubule poison and lead to mitotic arrest, this knowledge is largely based on studies of cells in culture, with drug concentrations that may not be realistic. Now, Zasadil and coauthors measured the concentration of paclitaxel in real patients undergoing treatment with the drug, and then investigated the response of cancer cells to paclitaxel at these lower and more realistic concentrations. Unexpectedly, the cells treated under these conditions did not undergo mitotic arrest, but instead proceeded through mitosis with abnormal spindles, resulting in chromosome missegregation, which leads to tumor cell death. This intriguing discovery demonstrates that we may not know as much as we thought about the effects of one of our most common chemotherapy drugs. In addition, the findings from this study may lead to clinical applications both in optimizing the selection of chemotherapy drug combinations and in determining which patients are likely to respond to paclitaxel treatment. The blockbuster chemotherapy drug paclitaxel is widely presumed to cause cell death in tumors as a consequence of mitotic arrest, as it does at concentrations routinely used in cell culture. However, we determine here that paclitaxel levels in primary breast tumors are well below those required to elicit sustained mitotic arrest. Instead, cells in these lower concentrations of drug proceed through mitosis without substantial delay and divide their chromosomes on multipolar spindles, resulting in chromosome missegregation and cell death. Consistent with these cell culture data, most mitotic cells in primary human breast cancers contain multipolar spindles after paclitaxel treatment. Contrary to the previous hypothesis, we find that mitotic arrest is dispensable for tumor regression in patients. These results demonstrate that mitotic arrest is not responsible for the efficacy of paclitaxel, which occurs because of chromosome missegregation on highly abnormal, multipolar spindles. This mechanistic insight may be used to improve selection of future antimitotic drugs and to identify a biomarker with which to select patients likely to benefit from paclitaxel.

Chromosome missegregation rate predicts whether aneuploidy will promote or suppress tumors

Significance Aneuploidy, an abnormal chromosome content that commonly occurs because of errors in chromosome segregation, can promote or suppress tumor formation. What determines how aneuploidy influences tumorigenesis has remained unclear. Here we show that the rate of chromosome missegregation, rather than the level of accumulated aneuploidy, determines the effect on tumors. Increasing the rate of chromosome missegregation beyond a certain threshold suppresses tumors by causing cell death. Increasing errors of chromosome segregation did not affect tumor formation caused by genetic mutations that do not themselves alter chromosome inheritance. These results suggest that accelerating chromosome missegregation in chromosomally unstable tumors may be a useful strategy therapeutically. Aneuploidy, a chromosome content other than a multiple of the haploid number, is a common feature of cancer cells. Whole chromosomal aneuploidy accompanying ongoing chromosomal instability in mice resulting from reduced levels of the centromere-linked motor protein CENP-E has been reported to increase the incidence of spleen and lung tumors, but to suppress tumors in three other contexts. Exacerbating chromosome missegregation in CENP-E+/− mice by reducing levels of another mitotic checkpoint component, Mad2, is now shown to result in elevated cell death and decreased tumor formation compared with reduction of either protein alone. Furthermore, we determine that the additional contexts in which increased whole-chromosome missegregation resulting from reduced CENP-E suppresses tumor formation have a preexisting, elevated basal level of chromosome missegregation that is exacerbated by reduction of CENP-E. Tumors arising from primary causes that do not generate chromosomal instability, including loss of the INK4a tumor suppressor and microsatellite instability from reduction of the DNA mismatch repair protein MLH1, are unaffected by CENP-E–dependent chromosome missegregation. These findings support a model in which low rates of chromosome missegregation can promote tumorigenesis, whereas missegregation of high numbers of chromosomes leads to cell death and tumor suppression.

The Aneuploidy Paradox in Cell Growth and Tumorigenesis

Aneuploidy, an abnormal chromosome number, is a frequent characteristic of malignant cells, leading to the suggestion that aneuploidy drives tumorigenesis. In a recent issue of Science, Williams et al. identified a paradoxical relationship between aneuploidy and its linkage to tumorigenesis: chromosome gains in nontransformed cells are antiproliferative, despite frequently occurring in human tumors.

The role of aneuploidy in promoting and suppressing tumors

Impaired mitotic checkpoint signaling can both promote and suppress tumors. The mitotic checkpoint targets Cdc20, the specificity factor of the ubiquitin ligase that promotes anaphase by targeting cyclin B and securin for destruction. In this issue, Li et al. (2009. J. Cell Biol. doi:10.1083/jcb.200904020) use gene replacement to produce mice expressing a Cdc20 mutant that cannot be inhibited by the mitotic checkpoint. In addition to the expected aneuploidy, these animals have a high tumor incidence that is likely caused by persistent aneuploidy coupled with nonmitotic functions of mutant Cdc20.

Up-regulation of the mitotic checkpoint component Mad1 causes chromosomal instability and resistance to microtubule poisons

The mitotic checkpoint is the major cell cycle checkpoint acting during mitosis to prevent aneuploidy and chromosomal instability, which are hallmarks of tumor cells. Reduced expression of the mitotic checkpoint component Mad1 causes aneuploidy and promotes tumors in mice [Iwanaga Y, et al. (2007) Cancer Res 67:160–166]. However, the prevalence and consequences of Mad1 overexpression are currently unclear. Here we show that Mad1 is frequently overexpressed in human cancers and that Mad1 up-regulation is a marker of poor prognosis. Overexpression of Mad1 causes aneuploidy and chromosomal instability through weakening mitotic checkpoint signaling caused by mislocalization of the Mad1 binding partner Mad2. Cells overexpressing Mad1 are resistant to microtubule poisons, including currently used chemotherapeutic agents. These results suggest that levels of Mad1 must be tightly regulated to prevent aneuploidy and transformation and that Mad1 up-regulation may promote tumors and cause resistance to current therapies.