Genomic instability is an enabling hallmark of cancer that provides cancer cells a replicative advantage. Accumulation of genomic aberrations can also compromise the genomic integrity and put cells under mitotic stress. Maintaining balance between the instability that gives cancer cells a replicative advantage and the instability that could lead them to mitotic catastrophe is crucial for survival of cancer cells. Mitotic catastrophe can be caused by rapid progression through mitosis before crucial checkpoints are met, resulting in cell death. To overcome this challenge, cancer cells may acquire overexpression of spindle assembly checkpoint (SAC) genes, which can prevent mitotic catastrophe that would occur if they undergo mitosis prematurely. Bub1b is an important part of SAC and inhibits the onset of anaphase until all chromosomes are aligned at the metaphase plate. Our analysis of clinical datasets show Bub1b expression is elevated in breast cancer, compared to normal breast, and is exemplified by a pattern of increasing overexpression in more aggressive variants, such as triple negative breast cancer (TNBC). Bub1b overexpression also correlates with decreased overall survival in patients. Expression analysis of breast cancer cell lines corroborates this clinical data. We hypothesize that the requirement for Bub1b expression indicates a vulnerability of rapidly proliferating breast cancers cells, and the inhibition of Bub1b will result in cell death through mitotic catastrophe.
Using RNA interference with siRNAs we knocked down Bub1b expression in a variety of breast cancer cells. This resulted in significant decrease in cellular viability and clonogenicity in soft agar. Furthermore, analyses of apoptosis using Annexin V and PI costaining showed that Bub1b knockdown results in significantly increased apoptosis and cell death, especially in the TNBC cell line MDA-MB-468. However, the viability and apoptosis levels of the normal breast epithelial cell line, MCF12A, were not affected. Bub1b knockdown resulted in silencing of SAC activity, as measured by Cyclin B levels, and also significantly impaired cell cycle progression in breast cancer cell lines. Additionally, knockdown of Bub1b in breast cancer cell lines demonstrated an additive effect when combined with DNA-damaging agents.
Our data support that Bub1b is a critical player in breast cancer cell survival, by enabling them to overcome mitotic stress. We demonstrate that impairment of Bub1b has detrimental effects on cell cycle and progression of mitosis. Further investigation of the role of Bub1b in promoting successful chromosome alignment and proliferation of breast cancer cells with genomic instability could provide a novel pathway on how cancer overcomes the mitotic stress caused by genomic instability. These studies may also lead to novel therapeutic strategies that combine SAC inhibitors with standard genotoxic treatments.
Citation Format: Dilara Koyuncu, Erik T. Goka, Philip C. Miller, Marc E. Lippman. Knockdown of the spindle assembly checkpoint gene Bub1b results in increased cell death and cell cycle impairment in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 512. doi:10.1158/1538-7445.AM2017-512