Mitotic slippage underlies the relationship between p53 dysfunction and the induction of large micronuclei by colcemid

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Micronuclei induced by aneugens are larger than those induced by clastogens in both in vitro and in vivo micronucleus (MN) assays. p53 dysfunction increases the formation of large micronuclei following treatment with aneugens; this study sought to identify the mechanisms responsible for this. Treatment with colcemid, both a mitotic inhibitor and an aneugen, induced MN containing two or more chromosomes more frequently in NH32 cells, in which p53 function is compromised, than in TK6 cells, in which p53 is functional. This indicates that p53 dysfunction enhances aneugen-induced chromosome loss or perturbs apoptosis, resulting in the formation of large MN. To examine the former hypothesis, the incidence of chromosome malsegregation in colcemid-treated TK6 and NH32 cells was compared using the cytokinesis-block MN assay. The incidence of chromosome non-disjunction was higher in NH32 cells than in TK6 cells, whereas the incidence of MN containing two or more chromosomes was similar between the two cell lines. To address the involvement of apoptosis in cell cycle progression, examination of chromosome 8 distribution revealed that more mononuclear NH32 than TK6 cells were tetraploid after prolonged mitotic inhibition, which indicated that the more number of NH32 cells may have bypassed the spindle assembly checkpoint via mitotic slippage and progression into the next interphase. Cells that underwent mitotic slippage were likely to contain lagging chromosomes formed via chromosome malsegregation, resulting in MN separated from the main nucleus. The number of TK6 cells containing large MN following colcemid treatment was increased by treatment with a caspase inhibitor in a dose-dependent manner, indicating that TK6 cells with MN normally undergo apoptosis. In conclusion, these findings indicate that mitotic slippage and perturbed apoptosis contribute to the induction of large MN in p53-compromised cells following treatment with colcemid.

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