The spindle checkpoint protects cells from aneuploidy by monitoring the status of the kinetochore-microtubule attachment. When cells have defects in this mitotic checkpoint pathway and in kinetochore-microtubule attachment, substantial aneuploidy is thought to result. We report here a novel type of caspase-independent apoptosis that occurs during mitosis_CIMA (caspase-independent mitotic apoptosis). Simultaneous depletion of BUB1, but not MAD2, and treatment with nocodazole (a microtubule-depolymerizing drug), paclitaxel (Taxol, a microtubule-stabilizing drug), or 17-AAG (17-allylaminogeldanamycin, an HSP90 inhibitor) induced DNA fragmentation in cells during early mitosis. Because the common defects caused by the microtubule inhibitors and 17-AAG are those in kinetochore-microtubule attachment, we conclude that CIMA occurs when the kinetochore-microtubule attachment is altered and BUB1 function is altered. This novel mitotic cell death appeared to be independent of caspase activation. We found that AIF (apoptosis-inducing factor) and EndoG (endonuclease G), which are effectors of caspase-independent apoptosis, are released from mitochondria during the activation of CIMA, and that DNA fragmentation is dependent on EndoG. Our results suggest that BUB1 is a critical factor in regulating CIMA. We propose that CIMA protects cells from aneuploidy by inducing the death of cells prone to substantial chromosome missegregation.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]