Chromosome segregation errors: a double-edged sword - TI Pharma

Introduction
Equal segregation of the genetic material to both daughter cells upon cell division depends
on the proper functioning of the mitotic checkpoint 441 . The mitotic checkpoint delays mitosis
until all chromosomes are attached to the mitotic spindle. Defects in this checkpoint signaling
result in chromosome segregation errors, which can cause aneuploidy and DNA damage
389,390,532 , two hallmarks of cancer 237 . Mps1, a key player in the mitotic checkpoint, is a kinase
originally found to be important in budding yeast spindle pole body duplication 137 , spindle
formation 571 and the mitotic or ‘spindle assembly’ checkpoint 140 . In contrast to Mps1 function in
centrosome duplication and subsequently bipolar spindle formation 143,146-148 , its role in mitotic
checkpoint signaling has been confirmed in higher eukaryotes already for over a decade 124,142,143 .
Introduction of various mutations in the Mps1 kinase domain of mammalian cells has revealed
the essential function of the kinase activity of Mps1 in proper mitotic checkpoint functioning 116-119 .
Moreover, in human cells, Mps1 kinase activity is required for proper alignment of the chromosomes
at the spindle equator by promoting correction of erroneous kinetochore-microtubule
attachments through regulation of Aurora B activity 116,118,123 , another important mitotic kinase 572 .
Because of its essential roles in the mitotic checkpoint and chromosome alignment, complete
absence of Mps1 activity results in cell death within several rounds of cell division 116,118,123,299,573 .
Moreover, partial shRNA-based depletion of Mps1 protein levels results in enhanced sensitivity to
low doses of the microtubule targeting chemotherapeutic paclitaxel (taxol) in human tumor cells 299 ,
whereas immortalized human fibroblasts display less sensitivity to this combination treatment.
Interestingly, Mps1 levels have been found to be strongly elevated in breast cancer cells that
harbor an abnormal chromosome count 322 . Reducing Mps1 levels by RNAi specifically killed these
cancer cells, but did not affect cell viability of isogenic untransformed breast epithelial cells 324 .
These observations have led to a search for more specific Mps1 kinase inhibitors that could be useful
in the treatment of cancer. Various small molecule inhibitors have been developed that target Mps1
kinase activity (reviewed in 167,574 ). However, some of these inhibitors, such as SP600125, Reversine,
staurosporine, Mps1 IN-2 and compound B13, have been shown to have additional targets besides
Mps1, which make them less suitable in target validation studies on the role of Mps1 in cancer
123,550,563,575-577 . Other inhibitors, such as Mps1 IN-1, potently inhibit Mps1, but only do so at relatively
high doses 123 . Two Mps1 inhibitors, NMS-P715 and MPI-0479605, have been shown to exhibit
anti-tumor activity using xenograft studies in mice, but only limited inhibition of tumor growth
has been observed 176,578 . These limitations demonstrate the need for more potent and specific
Mps1 small molecule inhibitors to assess the utility of Mps1 as a target in anti-cancer treatment.
Here, we characterize a previously unrecognized specific small molecule inhibitor of Mps1,
compound-5 579 , which potently binds to the ATP binding pocket of Mps1 with a long residence time
and is active in mice. Using this inhibitor, we can inhibit Mps1 in cells at extremely low doses and
recapitulate the synergistic effects of partial Mps1 depletion and taxol on tumor cell viability 299 .
We find that Mps1 inhibition has a more prominent effect on cell viability of transformed cells,
compared to healthy, untransformed cells, which makes Mps1 inhibition an interesting candidate for
anti-cancer treatment.
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Mps1 inhibition kills aneuploid cells
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