Essential mitotic functions of DNA topoisomerase II alpha are not adopted by topoisomerase II beta in human h69 cells

Unique functions of mammalian DNA-topoisomerases II alpha and -beta are sug gested by their distinct cellular distribution and chromatin binding at mit osis, Here, we studied H69-VP cells that, due to a homozygous mutation, exp ress topoisomerase II alpha mostly outside the nucleus. In these cells topo isomerase II beta showed a normal nuclear localization, However, at mitosis it diffused away from the chromatin despite the nuclear lack of the alpha- isoform. 80% of these cells performed chromosome condensation and disjuncti on with the aid of cytosolic topoisomerase II alpha, which bound to the mit otic 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 underg o disjunction and cytokinesis, Parental H69 cells expressing topoisomerase II alpha inside the nucleus exhibited high affinity binding of the enzyme t o the mitotic chromatin, Their genotype was mostly diploid and stable. We c onclude (i) that high affinity chromatin binding of topoisomerase II alpha is essential for chromosome condensation/disjunction and (ii) that topoisom erase II beta does not adopt these functions.