Essential mitotic functions of DNA topoisomerase IIα are not adopted by topoisomerase IIβ in human H69 cells

Abstract

Unique functions of mammalian DNA-topoisomerases IIα and -β are suggested by their distinct cellular distribution and chromatin binding at mitosis. Here, we studied H69-VP cells that, due to a homozygous mutation, express topoisomerase IIα mostly outside the nucleus. In these cells topoisomerase IIβ showed a normal nuclear localization. However, at mitosis it diffused away from the chromatin despite the nuclear lack of the α- isoform. 80% of these cells performed chromosome condensation and disjunction with the aid of cytosolic topoisomerase IIα, which bound to the mitotic chromatin with low affinity. However, the genotype of these cells was highly polyploid indicating an increased rate of non-disjunction. In 20% of the mutant cells neither topoisomerase II isoform was bound to the mitotic chromatin, which appeared as an unstructured DNA spheroid unable to undergo disjunction and cytokinesis. Parental H69 cells expressing topoisomerase IIα inside the nucleus exhibited high affinity binding of the enzyme to the mitotic chromatin. Their genotype was mostly diploid and stable. We conclude (i) that high affinity chromatin binding of topoisomerase IIα is essential for chromosome condensation/disjunction and (ii) that topoisomerase IIβ does not adopt these functions.