Mitotic entry upon Topo II catalytic inhibition is controlled by Chk1 and Plk1

Cell cycle responses to Topoisomerase II inhibition: Molecular mechanisms and clinical implications

DNA Topoisomerase IIA (Topo IIA) is an enzyme that alters the topological state of DNA and is essential for the separation of replicated sister chromatids and the integrity of cell division. Topo IIA dysfunction activates cell cycle checkpoints, resulting in arrest in either the G2-phase or metaphase of mitosis, ultimately triggering the abscission checkpoint if non-disjunction persists. These events, which directly or indirectly monitor the activity of Topo IIA, have become of major interest as many cancers have deficiencies in Topoisomerase checkpoints, leading to genome instability. Recent studies into how cells sense Topo IIA dysfunction and respond by regulating cell cycle progression demonstrate that the Topo IIA G2 checkpoint is distinct from the G2-DNA damage checkpoint. Likewise, in mitosis, the metaphase Topo IIA checkpoint is separate from the spindle assembly checkpoint. Here, we integrate mechanistic knowledge of Topo IIA checkpoints with the current understanding of how cells regulate progression through the cell cycle to accomplish faithful genome transmission and discuss the opportunities this offers for therapy.

The Implication of Topoisomerase II Inhibitors in Synthetic Lethality for Cancer Therapy

DNA topoisomerase II (Top2) is essential for all eukaryotic cells in the regulation of DNA topology through the generation of temporary double-strand breaks. Cancer cells acquire enhanced Top2 functions to cope with the stress generated by transcription and DNA replication during rapid cell division since cancer driver genes such as Myc and EZH2 hijack Top2 in order to realize their oncogenic transcriptomes for cell growth and tumor progression. Inhibitors of Top2 are therefore designed to target Top2 to trap it on DNA, subsequently causing protein-linked DNA breaks, a halt to the cell cycle, and ultimately cell death. Despite the effectiveness of these inhibitors, cancer cells can develop resistance to them, thereby limiting their therapeutic utility. To maximize the therapeutic potential of Top2 inhibitors, combination therapies to co-target Top2 with DNA damage repair (DDR) machinery and oncogenic pathways have been proposed to induce synthetic lethality for more thorough tumor suppression. In this review, we will discuss the mode of action of Top2 inhibitors and their potential applications in cancer treatments.

MCPH1 Lack of Function Enhances Mitotic Cell Sensitivity Caused by Catalytic Inhibitors of Topo II

The capacity of Topoisomerase II (Topo II) to remove DNA catenations that arise after replication is essential to ensure faithful chromosome segregation. Topo II activity is monitored during G2 by a specific checkpoint pathway that delays entry into mitosis until the chromosomes are properly decatenated. Recently, we demonstrated that the mitotic defects that are characteristic of cells depleted of MCPH1 function, a protein mutated in primary microcephaly, are not a consequence of a weakened G2 decatenation checkpoint response. However, the mitotic defects could be accounted for by a minor defect in the activity of Topo II during G2/M. To test this hypothesis, we have tracked at live single cell resolution the dynamics of mitosis in MCPH1 depleted HeLa cells upon catalytic inhibition of Topo II. Our analyses demonstrate that neither chromosome alignment nor segregation are more susceptible to minor perturbation in decatenation in MCPH1 deficient cells, as compared with control cells. Interestingly, MCPH1 depleted cells were more prone to mitotic cell death when decatenation was perturbed. Furthermore, when the G2 arrest that was induced by catalytic inhibition of Topo II was abrogated by Chk1 inhibition, the incidence of mitotic cell death was also increased. Taken together, our data suggest that the MCPH1 lack of function increases mitotic cell hypersensitivity to the catalytic inhibition of Topo II.