The α isoform of topoisomerase II is required for hypercompaction of mitotic chromosomes in human cells

As proliferating cells transit from interphase into M-phase, chromatin undergoes extensive reorganization, and topoisomerase (topo) II alpha, the major isoform of this enzyme present in cycling vertebrate cells, plays a key role in this process. In this study, a human cell line conditional null mutant for topo II alpha and a derivative expressing an auxin-inducible degron (AID)-tagged version of the protein have been used to distinguish real mitotic chromosome functions of topo II alpha from its more general role in DNA metabolism and to investigate whether topo II beta makes any contribution to mitotic chromosome formation. We show that topo II beta does contribute, with endogenous levels being sufficient for the initial stages of axial shortening. However, a significant effect of topo II alpha depletion, seen with or without the co-depletion of topo II beta, is the failure of chromosomes to hypercompact when delayed in M-phase. This requires much higher levels of topo II protein and is impaired by drugs or mutations that affect enzyme activity. A prolonged delay at the G2/M border results in hyperefficient axial shortening, a process that is topo II alpha-dependent. Rapid depletion of topo II alpha has allowed us to show that its function during late G2 and M-phase is truly required for shaping mitotic chromosomes.