Most tumor cells are characterized by increased genomic instability and chromosome segregational defects, often associated with hyperamplification of the centrosome and the formation of multipolar spindles. However, extra centrosomes do not always lead to multipolarity. Here, we describe a process of centrosomal clustering that prevented the formation of multipolar spindles in noncancer cells. Noncancer cells needed to overcome this clustering mechanism to allow multipolar spindles to form at a high frequency. The microtubule motor cytoplasmic dynein was a critical part of this coalescing machinery, and in some tumor cells overexpression of the spindle protein NuMA interfered with dynein localization, promoting multipolarity.

Chromosomal instability is a key step in the generation of the cancer cell karyotype. An indicator of unstable chromosomes is the presence of chromatin bridges during anaphase. We examined in detail the fate of anaphase bridges in cultured oral squamous cell carcinoma cells in real-time. Surprisingly, chromosomes in bridges typically resolve by breaking into multiple fragments. Often these fragments give rise to micronuclei (MN) at the end of mitosis. The formation of MN is shown to have important consequences for the cell. We found that MN have incomplete nuclear pore complex (NPC) formation and nuclear import defects and the chromatin within has greatly reduced transcriptional activity. Thus, a major consequence of the presence of anaphase bridges is the regular sequestration of chromatin into genetically inert MN. This represents another source of ongoing genetic instability in cancer cells.