Solid tumors can be highly aneuploid and many display high rates of chromosome missegregation in a phenomenon called chromosomal instability (CIN). In principle, aneuploidy is the consequence of CIN, but the relationship between CIN and aneuploidy has not been clearly defined. In this study, we use live cell imaging and clonal cell analyses to evaluate the fidelity of chromosome segregation in chromosomally stable and unstable human cells. We show that improper microtubule–chromosome attachment (merotely) is a cause of chromosome missegregation in unstable cells and that increasing chromosome missegregation rates by elevating merotely during consecutive mitoses generates CIN in otherwise stable, near-diploid cells. However, chromosome missegregation compromises the proliferation of diploid cells, indicating that phenotypic changes that permit the propagation of nondiploid cells must combine with elevated chromosome missegregation rates to generate aneuploid cells with CIN.

SummaryMost solid tumors are aneuploid and many frequently mis-segregate chromosomes. This chromosomal instability is commonly caused by persistent maloriented attachment of chromosomes to spindle microtubules. Chromosome segregation requires stable microtubule attachment at kinetochores, yet those attachments must be sufficiently dynamic to permit correction of malorientations. How this balance is achieved is unknown, and the permissible boundaries of attachment stability versus dynamics essential for genome stability remain poorly understood. Here we show that two microtubule-depolymerizing kinesins, Kif2b and MCAK, stimulate kinetochore-microtubule dynamics during distinct phases of mitosis to correct malorientations. Few-fold reductions in kinetochore-microtubule turnover, particularly in early mitosis, induce severe chromosome segregation defects. In addition, we show that stimulation of microtubule dynamics at kinetochores restores chromosome stability to chromosomally unstable tumor cell lines, establishing a causal relationship between deregulation of kinetochore-microtubule dynamics and chromosomal instability. Thus, temporal control of microtubule attachment to chromosomes during mitosis is central to genome stability in human cells.

Most solid tumors are aneuploid, having a chromosome number that is not a multiple of the haploid number, and many frequently mis-segregate whole chromosomes in a phenomenon called chromosomal instability (CIN). CIN positively correlates with poor patient prognosis, indicating that reduced mitotic fidelity contributes to cancer progression by increasing genetic diversity among tumor cells. Here, we review the mechanisms underlying CIN, which include defects in chromosome cohesion, mitotic checkpoint function, centrosome copy number, kinetochore–microtubule attachment dynamics, and cell-cycle regulation. Understanding these mechanisms provides insight into the cellular consequences of CIN and reveals the possibility of exploiting CIN in cancer therapy.

After chromosome missegregation, the growth of nondiploid cells is inhibited thanks to a p53-dependent mechanism.

Most solid tumors are aneuploid, and many missegregate chromosomes at high rates in a phenomenon called chromosomal instability (CIN). CIN reflects the erosion of mitotic fidelity, and it correlates with poor patient prognosis and drug resistance. The most common mechanism causing CIN is the persistence of improper kinetochore-microtubule attachments called merotely. Chromosomes with merotelic kinetochores often manifest as lagging chromosomes in anaphase, suggesting that lagging chromosomes fail to segregate properly. However, it remains unknown whether the lagging chromosomes observed in anaphase segregate to the correct or incorrect daughter cell. To address this question, we tracked the segregation of a single human chromosome during cell division by using LacI-GFP to target an integrated LacO array. By scoring the distribution of each sister chromatid during mitosis, we show that a majority of lagging chromosomes in anaphase segregate to the correct daughter cell. Instead, sister chromatids that segregate erroneously frequently do so without obvious evidence of lagging during anaphase. This outcome is expected if sister kinetochores on a chromosome bind microtubules oriented toward the same spindle pole, and we find evidence for syntelic kinetochore attachments in cells after treatments that increase missegregation rates. Thus, lagging chromosomes in anaphase are symptomatic of defects in kinetochore-microtubule attachment dynamics that cause chromosome missegregation associated with CIN, but the laggards rarely missegregate.aneuploidy | syntely | MCAK | micronuclei | genome instability S olid tumors are frequently aneuploid and many missegregate chromosomes at high rates in a phenomenon called chromosomal instability (CIN; refs. 1 and 2). CIN is associated with poor patient prognosis, and various studies have shown that it correlates with advanced tumor stage including acquisition of metastatic potential and drug resistance (3-5). It has been proposed that by frequently changing the karyotype of tumor cells, that CIN provides an agent of change that drives the evolution of tumor cell phenotypes (3-8). The treatment difficulties encountered in advanced stage tumors underscores the importance of determining the mechanisms of CIN and how they contribute to tumor growth.Various mechanisms have been proposed to cause CIN including dysfunction of the spindle assembly checkpoint, defects in sister chromatid cohesion, and defects in the attachment of chromosomes to spindle microtubules (2). Recently, live cell imaging demonstrated that the most common cause of CIN is the persistence of errors in the attachment of spindle microtubules to chromosomes (9, 10). Microtubules bind to chromosomes at specialized structures called kinetochores. Each chromosome has a pair of kinetochores, and faithful chromosome segregation arises when single kinetochores bind microtubules oriented toward only one spindle pole resulting in the biorientation of chromosomes on the spindle. However, errors in the orientation of kinetochore-mic...