The mitotic origin of chromosomal instability

chromosomes, with only 2–6% anaphase CIN-cells displaying lagging chromosomes compared to 12–17% anaphase CIN+ cells (Figure 1B). CIN+ cells also contained multiple lagging chromosomes per anaphase, whereas multiple lagging chromosomes were rarely observed in CIN-cells (not shown). Very few acentric fragments were observed in anaphase for both CIN-and CIN+ cells, and this result was also confirmed by analysis of chromosome spreads from a CIN-(HCT116) and a CIN+ (U251) cell line (Figure 1C). Finally, the rates of anaphase cells containing chromatin bridges, albeit higher than those with acentric fragments and in some cases similar to the rates of lagging chromosomes, were indistinguishable between CIN-and CIN+ cells (Figure 1B). These results demonstrate a paucity of acentric chromosome fragments in CIN+ cells and support the conclusion that lagging chromosomes, rather than chromosome segregation defects arising from chromosome breakage, are the primary phenotypic difference distinguishing CIN-from CIN+ cells. These results are in agreement with previous findings indicating that lagging chromosomes are the most frequent chromosome segregation error in CIN+ cancer cells [9] and represent the first report directly comparing chromosome segregation defects, broken down by type, in CIN-vs. CIN+ cells. It was previously shown that CIN+ cells have increased kinetochore-microtubule attachment stability relative to CIN-cells [4], which leads to persistence of kinetochore-microtubule (kMT) attachment errors and subsequent chromosome mis-segregation [7]. Furthermore, destabilizing kMT attachments by overexpression of microtubule-depolymerizing kinesin-13 proteins reduced the numbers of lagging chromosomes and suppressed CIN in two separate CIN+ cell lines (U2OS and MCF7) [5]. Here, we overexpressed the kinesin-13 protein (GFP–Kif2b) in two additional glioblastoma cell lines that are known to exhibit high rates of chromosome segregation errors. GFP–Kif2b overexpression significantly reduced the frequency of anaphase lagging chromosomes Chromosomal instability (CIN) is a common feature of most human neoplasms and was defined, in a seminal study by Vogelstein and colleagues [1], as persistently elevated rates of whole chromosome mis-segregation. Since then, it was shown that certain errors in mitosis, including defects in the spindle assembly checkpoint [2], sister chromatid cohesion [3], kinetochore-microtubule (kMT) attachments [4,5], and centrosome number [6] can cause chromosome mis-segregation in the form of merotelically attached anaphase lagging chromosomes — chromosomes that lag behind at the spindle equator while all the other chromosomes move toward the spindle poles [7] (Figure 1A). A recent study has suggested that pre-mitotic replication stress generates partially replicated chromosomes during mitosis, and that this results in both numerical and structural chromosome …