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Chromosome instability (CIN), a continuous change in the structure or number of chromosomes, is proposed to be a key mechanism driving the genomic changes associated with tumorigenesis. One major cause of CIN in cells is chromosome segregational defects occurring during mitosis. Two such examples are anaphase bridges and multipolar spindles, which are common in most cancer cells and many tumor tissues. Anaphase bridges are chromatin bridges in between separating chromosome masses during anaphase, which may result in gene amplification or loss when breaking. We have found that cigarette smoke condensate (CSC) induced anaphase bridges in cultured primary human cells, which in a short time led to genomic imbalances. The frequency of the induced bridges within the entire population decreased with time, independent of the p53-mediated apoptotic pathway. We also showed that CSC induced DNA double-stranded breaks (DSBs) in cultured cells as well as purified DNA. The reactive oxygen species (ROS) scavenger, 2 deoxyguanosine 5-monophosphate (dGMP) prevented CSC-induced DSBs, anaphase bridge formation and genomic imbalances. Therefore, we propose that CSC induces bridges and genomic imbalances via DNA DSBs. Further analysis in live oral cancer cells shows that cells with anaphase bridges mostly survive and these bridges frequently result in micronuclei formation, indicating that anaphase bridges actively contribute to CIN. Multipolar spindles (MPS) are aberrant mitotic structures when cells divide with greater than two spindle poles, which may result in uneven chromosome segregation. Multipolarity is strongly linked to centrosomal amplification, the mechanism of which remains controversial. We have examined the origin and fate of cells with MPS in real time. In both human embryonic kidney and oral cancer cells, the vast majority of multipolar cells originated from multinucleated cells. The frequency of cytokinesis failure was similar to the frequency of MPS, and each observed bipolar division that ended in a cytokinesis failure led to MPS formation in the subsequent mitosis. While grossly abnormal, these cells are still capable of dividing, often giving rise to a mixed progeny of multinucleated and mononucleated cells. These observations support the model that failure of cytokinesis may be the most common mechanism by which cells form MPS. |
