Adaptation of the spindle-assembly checkpoint

Abstract

Antimicrotubule agents are commonly used chemotherapeutic agents that kill cancer cells by activating the spindle assembly checkpoint (SAC). However, some cancer cells can adapt to the checkpoint and exit mitosis without chromosome segregation. Checkpoint adaptation not only limits the effectiveness of antimicrotubule drugs but also increases chromosomal instability. Understanding the mechanism of the SAC adaptation is important for improving treatment effectiveness. It is generally believed that the adaptation is caused by “leaky” activity of APC/C^CDC20, targeting cyclin B1 for slow degradation. However, the exact underlying mechanism remains mysterious. In the first study, I showed that progressive weakening of the SAC is responsible for the activation of APC/C^CDC20 following prolonged mitotic block. Stabilization of mitotic checkpoint complex level (MCC) by depletion of p31^comet could prevent from checkpoint adaptation and shift cell fates to apoptosis. By re-introducing different p31^comet mutants into p31^comet-deficient cells, I found that regulation of MCC production but not MCC disassembly was the rate-limiting step in regulating MCC abundance. During prolonged mitotic block, the signal intensities of MAD1 and MAD2 at kinetochore decreased, indicating that MCC production may gradually diminished with time, resulting in SAC adaptation. In the second study, I showed that chloroquine could promote adaptation of the SAC in different cell lines. Although chloroquine is a well-known autophagy inhibitor, it induced SAC adaption through an autophagy-independent pathway. When MCC was stabilized by depletion of p31^comet, chloroquine could not trigger the checkpoint adaptation. These data suggested that chloroquine probably induced SAC adaptation by disturbing MCC dynamics. Although the exact mechanism of chloroquine-induced checkpoint adaptation remained to be deciphered in the future, I found that chromosome condensation is important in regulating adaption of the SAC. Targeting chromosome condensation may be able to enhance treatment efficacy in the future.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology