Topoisomerase IIIα is required for normal proliferation and telomere stability in alternative lengthening of telomeres

Topoisomerase (Topo) III alpha associates with BLM helicase, which is proposed to be important in the alternative lengthening of telomeres (ALT) pathway that allows telomere recombination in the absence of telomerase. Here, we show that human Topo III alpha colocalizes with telomeric proteins at ALT-associated promyelocytic bodies from ALT cells. In these cells, Topo III alpha immunoprecipitated with telomere binding protein (TRF) 2 and BLM and was shown to be associated with telomeric DNA by chromatin immunoprecipitation, suggesting that these proteins form a complex at telomere sequences. Topo III alpha depletion by small interfering RNA reduced ALT cell survival, but did not affect telomerase-positive cell lines. Moreover, repression of Topo III alpha expression in ALT cells reduced the levels of TRF2 and BLM proteins, provoked a strong increase in the formation of anaphase bridges, induced the degradation of the G-overhang signal, and resulted in the appearance of DNA damage at telomeres. In contrast, telomere maintenance and TRF2 levels were unaffected in telomerase-positive cells. We conclude that Topo III alpha is an important telomere-associated factor, essential for telomere maintenance and chromosome stability in ALT cells, and speculate on its potential mechanistic function.