The role of Wee1 and Myt1 in breast cancer

  • Author / Creator
    Lewis, Cody Wayne
  • To ensure faithful cell division, entry into mitosis must be inhibited in cells with damage or under-replicated DNA. Wee1 and Myt1 are two partially redundant kinases that inhibit mitotic entry through the phosphorylation of the Cdk1/cyclin B complex. Cdk1 is an essential mitotic kinase that regulates essentially all mitotic processes including chromosome condensation and nuclear envelop breakdown. Ectopic Cdk1 activation causes cells to prematurely enter mitosis with under-replicated DNA leading to chromosome fragmentation. Moreover, failure to inhibit Cdk1 during the metaphase to anaphase transition can induce a mitotic arrest. Centromere fragmentation and mitotic arrest induce cell death by mitotic catastrophe.
    Mitotic catastrophe is a common mode of cell death that occurs in tumour cells in response to various genotoxic therapies including irradiation. However, many cancer cells upregulate Wee1 expression, which promotes cancer cell survival through Cdk1 inhibition. To enhance the efficacy of genotoxic anticancer therapies, the Wee1 inhibitor Adavosertib was developed. Adavosertib is currently being tested in the clinic against various cancer types alone and in combination with different genotoxic agents.
    Our lab has found that monotreatment with Adavosertib is enough to induce cell death in a subset of cancer cells. In these cells, Adavosertib has two major effects on the cell cycle: premature mitotic entry leading to chromosome fragmentation and prolonged mitotic arrest, which is not dependent on chromosome fragmentation. Cell sensitivity to Adavosertib is directly correlated with Myt1 protein expression; cells with high Myt1 protein levels are resistant to Adavosertib (and vice versa). Likewise, Myt1-induced overexpression reduces cell sensitivity to Adavosertib. Furthermore, cells selective for Adavosertib resistance have upregulated Myt1 protein levels. Adavosertib resistant cells have less in vitro Cdk1 activity, do not undergo premature mitosis or centromere fragmentation, and do not arrest in mitosis following Wee1 inhibition, suggesting that Adavosertib resistance is mediated through reduced Cdk1 activity.
    Cdk1 phosphorylation on Y15 is catalyzed by Wee1 and to a lesser extent Myt1 whereas Cdk1 phosphorylation on T14 is strictly regulated by Myt1. We found that Adavosertib treatment reduced pY15-Cdk1 levels but not pT14-Cdk1 levels, which suggests that Adavosertib inhibits Wee1 but not Myt1 activity. siRNA knockdown of Myt1 sensitizes Adavosertib resistant cells to Wee1 inhibition; these cells have increased in vitro Cdk1 activity, are prone to premature mitosis, undergo centromere fragmentation, and arrest in mitosis. This data confirm that Myt1 is an important driver of Adavosertib resistance.
    Our data shows that Myt1 is overexpressed in breast cancer tissue and that high Myt1 levels are associated with a worse clinical outcome. Myt1 is also reported to be overexpressed in other cancer types including colorectal, lung, and head and neck cancers. Currently, there are no selective Myt1 small molecule inhibitors available for preclinical or clinical use, but our data provides a rationale for the development of such inhibitors.
    Given the lack of selective small molecule Myt1 inhibitors, we investigated alternative treatment strategies for enhancing Adavosertib sensitivity in cancer cells. We focused our study on small molecule inhibitors that were either approved for clinical used or had the potential for clinical use. Checkpoint kinases such as ATR and Chk1 function in a parallel pathway to Wee1 and Myt1. Several ATR and Chk1 inhibitors are being tested in the clinic. Both ATR and Chk1 inhibitors induce premature mitosis and synergistic cancer cell killing when used in combination with Adavosertib. Additionally, antimitotic agents that delay mitotic exit (e.g. paclitaxel and FTI inhibitor L-744-832) were also found to sensitize cancer cells to Adavosertib by inhibiting mitotic exit. Importantly, combination treatments with Adavosertib were found to be effective in cells that overexpressed Myt1. Our data highlight potential avenues for overcoming Myt1 induced Adavosertib resistance.

  • Subjects / Keywords
  • Graduation date
    Spring 2020
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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