Suppression of topoisomerase IIalpha expression and function in human cells decreases chromosomal radiosensitivity.

The mechanism behind chromatid break formation is as yet unclear, although it is known that DNA double-strand breaks (DSBs) are the initiating lesions. Chromatid breaks formed in cells in the G2-phase of the cell-cycle disappear ('rejoin') as a function of time between radiation exposure and cell fixation. However, the kinetics of disappearance of chromatid breaks does not correspond to those of DSB rejoining, leading us to seek alternative models. We have proposed that chromatid breaks could be formed indirectly from DSB and that the mechanism involves topoisomerase IIalpha. In support of this hypothesis we have recently shown that frequencies of radiation-induced chromatid breaks are lower in two variant human promyelocytic leukaemic cell lines with reduced topoisomerase IIalpha expression. Here we report that suppression of topoisomerase IIalpha in human hTERT-RPE1 cells, either by its abrogation using specific siRNA or by inhibition of its catalytic activity with the inhibitor ICRF-193, causes a reduction in frequency of chromatid breaks in radiation-exposed cells. The findings support our hypothesis for the involvement of topoisomerase IIalpha in the formation of radiation-induced chromatid breaks, and could help explain inter-individual variation in human chromosomal radiosensitivity; elevation of which has been linked with cancer susceptibility.