Downregulation of Bub1 and a Defective Spindle Checkpoint in Acute Myeloid Leukemia.

Chromosomal aberrations are common in leukemia. The spindle assembly checkpoint (SAC) is a surveillance mechanism responsible to ensure proper chromosome segregation. Unattached chromosomes to the mitotic spindle or lack of spindle tension are sensed by the SAC leading to metaphase arrest. The activated SAC inhibits the ordered proteolysis of regulatory proteins and thereby prevents mitotic progression providing time for the repair of spindle defects. Ubiquitin-dependent proteolysis of cell cycle regulators is a central mechanism of cell cycle control. The anaphase-promoting complex (APC) is an essential ubiquitin-ligase responsible for mitotic progression. The activated SAC inhibits the APC preventing degradation of cyclin B1 and the anaphase inhibitor securin and thereby delaying chromosome separation. We found this mitotic delay mechanism to be defective in acute myeloid leukemia (AML). Leukemia cells that were exposed to the microtubule disrupting agent nocodazole were not capable of preventing anaphase onset. This is indicated by a low mitotic index and degradation of the APC targets securin and cyclin B1 in nocodazole treated Kasumi-1 cells compared to the SAC competent burkitt lymphoma cell line DG-75. Thus, unattached chromosomes were unable to induce prolonged metaphase arrest in these cells. They undergo apoptosis instead due to the complete absence of the mitotic spindle. Similar results with defective mitotic arrest and enhanced apoptosis were obtained for several other leukemia cell lines. In vivo SAC malfunction in AML can favor the generation of aneuploidy, consistent with the numerical aberrations observed in these cells. In order to identify the underlying defect we screened for mutations or altered expression of checkpoint proteins and identified the SAC component Bub1 to be posttranscriptionally downregulated in all tested AML cell lines. Our studies suggest that impaired SAC function is an important mechanism to create genetic instability and thus may contribute to rise and progression of AML.