Mps1 as a link between centrosomes and genomic instability.

Centrosomes are microtubule-organizing centers that must be precisely duplicated before mitosis. Centrosomes regulate mitotic spindle assembly, and the presence of excess centrosomes leads to the production of aberrant mitotic spindles which generate chromosome segregation errors. Many human tumors possess excess centrosomes that lead to the production of abnormal spindles in situ. In some tumors, these extra centrosomes appear before aneuploidy, suggesting that defects in centrosome duplication might promote genomic instability and tumorigenesis. The Mps1 protein kinase is required for centrosome duplication, and preventing the proteasome-dependent degradation of Mps1 at centrosomes increases its local concentration and causes the production of excess centrosomes during a prolonged S-phase. Here, we show that Mps1 degradation is misregulated in two tumor-derived cell lines, and that the failure to appropriately degrade Mps1 correlates with the ability of these cells to produce extra centrosomes during a prolonged S-phase. In the 21NT breast-tumor derived cell line, a mutant Mps1 protein that is normally constitutively degraded can accumulate at centrosomes and perturb centrosome duplication, suggesting that these cells have a defect in the mechanisms that target Mps1 to the proteasome. In contrast, the U2OS osteosarcoma cell line expresses a nondegradable form of Mps1, which we show causes the dose-dependent over duplication of centrioles even at very low levels of expression. Our data demonstrate that defects in Mps1 degradation can occur through multiple mechanisms, and suggest that Mps1 may provide a link between the control of centrosome duplication and genomic instability.