Involvement of topoisomerase III in telomere-telomere recombination

Telomere maintenance is required for chromosome stability, and telomeres are typically replicated by the action of telomerase. In both mammalian tumor and yeast cells that lack telomerase, telomeres are maintained by an alternative (ALT) recombination mechanism. In yeast, Sgs1p and its associated type IA topoisomerase, Top3p, may work coordinately in removing Holliday junction intermediates from a crossover-producing recombination pathway. Previous studies have also indicated that Sgs1 helicase acts in a telomere recombination pathway. Here we show that topoisomerase III is involved in telomere-telomere recombination. The recovery of telomere recombination-dependent survivors in a telomerase-minus yeast strain was dependent on Top3p catalytic activity. Moreover, the RIF1 and RIF2 genes are required for the establishment of TOP3/SGS1-dependent telomeretelomere recombination. In human Saos-2 ALT cells, human topoisomerase IIIα(hTOP3α) also contributes to telomere recombination. Strikingly, the telomerase activity is clearly enhanced in surviving si-hTOP3α Saos-2 ALT cells. Altogether, the present results suggest a potential role for hTOP3α in dissociating telomeric structures in telomerase-deficient cells, providing therapeutic implications in human tumors.