CDK5RAP2 Is Required For Spindle Assembly Checkpoint Function

Cell cycle checkpoints are responsible for monitoring and responding to DNA damage,replication stress, and proper chromosome segregation. Checkpoint signaling pathways are oftencomposed of tumor predisposition inherited genes and/or tumor suppressor genes. Checkpointdefects may lead to genome instability, proneness to mutations, and eventually tumorpredisposition. The spindle assembly checkpoint (SAC) ensures proper segregation of sisterchromatids, preventing aneuploidy. Proteins essential for SAC control include MAD1, MAD2,BUB1, BUBR1, and MPS1. Both BUBR1 and MAD2 block APC (anaphase promoting complex)activation through sequestering the APC activator CDC20, thus compromising APC-mediateddegradation of cyclin B and securin, and halting separation of sister chromatids.Primary microcephaly (MCPH) is an autosomal recessive disease with neurodevelopmental.CDK5RAP2 (Cyclin-dependent kinase 5 regulatory associated protein 2) is one of the fivecausative genes identified by far. My dissertation research attempted to dissect its function in theSAC control. I have found that inhibition of CDK5RAP2 expression causes chromosome missegregation,fails to maintain the spindle checkpoint, and is associated with reduced expression ofthe spindle checkpoint proteins BUBR1 and MAD2 and an increase in chromatin-associatedCDC20. CDK5RAP2 resides on the BUBR1 and MAD2 promoters and positively regulates theirtranscription. Furthermore, CDK5RAP2-knockdown cells have increased resistance to paclitaxeland doxorubicin, and this resistance is partially rescued upon restoration of CDK5RAP2expression. Cancer cells cultured in the presence of paclitaxel or doxorubicin exhibitdramatically decreased CDK5RAP2 levels. These results suggest that CDK5RAP2 is required forspindle checkpoint function and is a common target in paclitaxel and doxorubicin resistance.Manipulation of CDK5RAP2 expression may impact on the therapeutic efficacy of paclitaxel anddoxorubicin, and CDK5RAP2 mutation-mediated spindle checkpoint defect may lead to MCPH.