Unique functions of mammalian DNA-topoisomerases II␣ and -␤ 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 II␣ mostly outside the nucleus. In these cells topoisomerase II␤ showed a normal nuclear localization. However, at mitosis it diffused away from the chromatin despite the nuclear lack of the ␣-isoform. 80% of these cells performed chromosome condensation and disjunction with the aid of cytosolic topoisomerase II␣, 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 II␣ 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 II␣ is essential for chromosome condensation/disjunction and (ii) that topoisomerase II␤ does not adopt these functions.

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