Precocious activation of APC/C-Cdh1 at pre-anaphase causes genome instability

Faithful chromosome segregation and thereby accurate gene transmission are crucial for all organisms. Until proper attachment of the mitotic spindle to the kinetochore is established, the ubiquitin ligase (E3) Cdc20-activated APC/C (anaphase promoting complex/cyclosome) is repressed by the spindle assembly checkpoint (SAC) and sister chromatin cohesion is protected. Mutants defective in SAC fail to arrest at metaphase even in the presence of damaged microtubules. Interestingly, a similar phenomenon occurs in yeast cells defective in Bub2, a negative factor of the mitotic exit network (MEN), which is required for telophase onset, although its precise molecular mechanism is unknown. Here, we show that chromosome missegregation occurs frequently in bub2∆ cells in the presence of damaged microtubules. The loss of Bub2 caused precocious activation of APC/C-Cdh1/Hct1 at pre-anaphase, leading to securin degradation and then separase-mediated cohesin cleavage. Overexpression of CDH1 and CDC14, encoding Cdc14 phosphatase, at pre-anaphase similarly caused chromosome missegregation. Thus, sequential activation of APC/C-Cdc20 and then APC/C-Cdh1 is critical for precise chromosome segregation and precocious activation of APC/C-Cdh1 at pre-anaphase causes genomic instability. Since degradation of human securin is also mediated by APC/C-Cdc20 and APC/C-Cdh1, this study predicts that precocious activation APC/C-Cdh1 in human cells similarly causes genomic instability, and thereby cell death or tumorigenesis

APC/C-Cdh1-dependent anaphase and telophase progression during mitotic slippage

<p>Abstract</p> <p>Background</p> <p>The spindle assembly checkpoint (SAC) inhibits anaphase progression in the presence of insufficient kinetochore-microtubule attachments, but cells can eventually override mitotic arrest by a process known as mitotic slippage or adaptation. This is a problem for cancer chemotherapy using microtubule poisons.</p> <p>Results</p> <p>Here we describe mitotic slippage in yeast <it>bub2Δ </it>mutant cells that are defective in the repression of precocious telophase onset (mitotic exit). Precocious activation of anaphase promoting complex/cyclosome (APC/C)-Cdh1 caused mitotic slippage in the presence of nocodazole, while the SAC was still active. APC/C-Cdh1, but not APC/C-Cdc20, triggered anaphase progression (securin degradation, separase-mediated cohesin cleavage, sister-chromatid separation and chromosome missegregation), in addition to telophase onset (mitotic exit), during mitotic slippage. This demonstrates that an inhibitory system not only of APC/C-Cdc20 but also of APC/C-Cdh1 is critical for accurate chromosome segregation in the presence of insufficient kinetochore-microtubule attachments.</p> <p>Conclusions</p> <p>The sequential activation of APC/C-Cdc20 to APC/C-Cdh1 during mitosis is central to accurate mitosis. Precocious activation of APC/C-Cdh1 in metaphase (pre-anaphase) causes mitotic slippage in SAC-activated cells. For the prevention of mitotic slippage, concomitant inhibition of APC/C-Cdh1 may be effective for tumor therapy with mitotic spindle poisons in humans.</p