Tumor cell lines with elevated chromosome numbers frequently have correlated elevations of Mps1 expression and these tumors are more dependent on Mps1 activity for their survival than control cell lines. Mps1 is a conserved kinase involved in controlling aspects of chromosome segregation in mitosis and meiosis. The mechanistic explanation for the Mps1-addiction of aneuploid cells is unknown. To address this question, we explored Mps1-dependence in yeast cells with increased sets of chromosomes. These experiments revealed that in yeast, increasing ploidy leads to delays and failures in orienting chromosomes on the mitotic spindle. Yeast cells with elevated numbers of chromosomes proved vulnerable to reductions of Mps1 activity. Cells with reduced Mps1 activity exhibit an extended prometaphase with longer spindles and delays in orienting the chromosomes. One known role of Mps1 is in recruiting Bub1 to the kinetochore in meiosis. We found that the Mps1-addiction of polyploid yeast cells is due in part to its role in Bub1 recruitment. Together, the experiments presented here demonstrate that increased ploidy renders cells more dependent on Mps1 for orienting chromosomes on the spindle. The phenomenon described here may be relevant in understanding why hyper-diploid cancer cells exhibit elevated reliance on Mps1 expression for successful chromosome segregation.

In aneuploid cancer cells, the chromosome segregation apparatus is sensitive to increased chromosome number. The conserved protein kinase, Mps1, is a critical actor of this machinery, orienting the chromosomes properly on the spindle. Abnormally high levels of this kinase have been found in tumors with elevated chromosome number.However, it remains unclear, mechanistically, if and how cells with higher ploidy become dependent upon increased Mps1 levels. To answer these questions, we explored Mps1 dependence in yeast cells with increased sets of chromosomes. We discovered that having more chromosomes affects the ability of cells to orient chromosomes properly.The cells with increased numbers of chromosomes are particularly sensitive to the reduction of Mps1 activity. In mps1 loss of function mutants, cells display an extended prometaphase with a longer spindle and a delay in orienting properly the chromosomes.Altogether, our results suggest that increased numbers of chromosomes render cells more dependent on Mps1 for orienting chromosomes on the spindle. The phenomenon described here may be relevant in understanding why hyperdiploid cancer cells become excessively reliant on high Mps1 expression for successful chromosome segregation. Author summaryMost cells in solid tumors usually carry far more chromosomes than normal cells. Losing or gaining chromosomes during cell division can lead to aneuploidy (an abnormal number of chromosomes), cancer, and other diseases. Mps1 is a master regulator of cell division that is critical to keep the correct number chromosomes in each daughter cell.This master regulator has been shown to target and affect the function of various actors involved in cell division. Abnormally high levels of this master regulator are found in tumors with elevated chromosome numbers. The high levels of this regulator appear to be protecting these tumor cells. To answer if and how cells with higher ploidy become so dependent of Mps1, we generated yeast cells with increased set of chromosomes. Here, we report that cells with elevated chromosome number are particularly sensitive to the reduction of Mps1 level. In cells with higher ploidy and reduced level of Mps1, the progression during cell division is delayed. In the mutant cells, their ability to properly orient and segregate their chromosomes on the spindle is greatly reduced.