Chromosome instability (CIN) is a hallmark of many tumors and correlates with the presence of extra centrosomes1-4. However, a direct mechanistic link between extra centrosomes and CIN has not been established. It has been proposed that extra centrosomes generate CIN by promoting multipolar anaphase, a highly abnormal division that produces 3 or more aneuploid daughter cells. Here, we use long-term live-cell imaging to demonstrate that cells with multiple centrosomes rarely undergo multipolar cell divisions, and the progeny of these divisions are typically inviable. Thus, multipolar divisions cannot explain observed rates of CIN. By contrast, we observe that CIN cells with extra centrosomes routinely undergo bipolar cell divisions, but display a significantly elevated frequency of lagging chromosomes during anaphase. To define the mechanism underlying this mitotic defect, we generated cells that differ only in their centrosome number. We demonstrate that extra centrosomes alone are sufficient to promote chromosome missegregation during bipolar cell division. These segregation errors are a consequence of cells passing through a transient ‘multipolar spindle intermediate’ in which merotelic kinetochore-microtubule attachment errors accumulate prior to centrosome clustering and anaphase. These findings provide a direct mechanistic link between extra centrosomes and CIN, two common characteristics of solid tumors. We propose that this mechanism may be a common underlying cause of CIN in human cancer.

Whether whole-chromosome aneuploidy promotes tumorigenesis has been controversial, in large part because of the paucity of insight into underlying mechanisms. Here we identify a mechanism by which mitotic chromosome segregation errors generate DNA breaks via the formation of structures called micronuclei. Whole chromosome-containing micronuclei form when mitotic errors produce lagging chromosomes. We tracked the fate of newly generated micronuclei and found that they undergo defective and asynchronous DNA replication, resulting in DNA damage and frequently pulverization of the chromosome in the micronucleus. Micronuclei can persist in cells over several generations but the chromosome in the micronucleus can also be distributed to daughter nuclei. Thus, chromosome segregation errors potentially lead to mutations and chromosome rearrangements that can integrate into the genome. Pulverization of chromosomes in micronuclei may also be one explanation for “chromothripsis” in cancer and developmental disorders, where isolated chromosomes or chromosome arms undergo massive local DNA breakage and rearrangement.