Characterization of Mitotic Phenotypes Associated with a MYC Synthetic Lethal Compound

Author(s): Qiong Shi, Chenglu Yang, Julia Kalashova, Hongmei Li, Yan Long, Ting Zhang, Xumei Liu, Hong Liu, Thaddeus D. Allen

Therapeutic targeting of MYC directly has proven difficult, prompting indirect strategies using a synthetic lethal drug. Here, we characterize one such drug dimethylfasudil (diMF), a rho-associated protein kinase (ROCK) inhibitor. But this diMF activity is not linked to the synthetic lethal interaction with MYC. Instead, this compound induced multiple cell cycle-related liabilities. These included the early mitotic arrest of cycling cells followed by mitotic catastrophe and the induction of polyploidy in cells that do manage to pass through mitosis. As early as prometaphase, we noted diminished staining for BUB1 kinase, which binds to kinetochores and regulates the mitotic spindle checkpoint and chromosome congression. Kinetochore proteins, such as Centromere Protein F (CENP-F), failed to localize at kinetochores in early mitosis, confirming a deficit in centromere assembly. This, presumably, contributed to the development of anomalies in chromosome segregation in cells treated with diMF. In anaphase cells, the protein regulator of cytokinesis 1 (PRC1), failed to be recruited to the midzone, leading to a cascade of defects that included failed recruitment of the chromosomal passenger protein complex, the central spindlin complex, and polo-like-kinase 1 (PLK1). These observations correlate well with the cell death induced by diMF, which may serve as a prototype MYC synthetic lethal compound to explore synthetic lethal therapy or as a scaffold upon which to build superior compounds. The phenotypes described here serve as examples of MYC synthetic lethal drug effects that can be used to explore and maximize drug discovery programs.