Topoisomerase 3α (TOP3A) Dependent Alternative Lengthening of Telomeres (ALT).

Alternative Lengthening of Telomeres (ALT) is a homologous recombination-dependent telomere elongation mechanism utilized by at least 10-15% of all cancers. Here we identified that the DNA topoisomerase, TOP3A is enriched at the telomeres of ALT cells but not at the telomeres of telomerase-positive (Tel) cancer cells. We demonstrate that TOP3A stabilizes the shelterin protein TERF2 in ALT cancer cell lines but not in Tel cells and that long non-coding telomere transcribed RNA (TERRA) enrichment at telomeres depends upon TOP3A. TOP3A also promotes the generation of single-stranded telomeric C-strand (ssTeloC) DNA, which is a recently discovered marker for ALT. Additionally, we found that inducing TOP3A-DNA-protein crosslinks in ALT cells suppresses TERRA enrichment as well as destabilizes TERF2. Taken together these observations uncover the unexplored functions of TOP3A at ALT telomeres and suggest the potential of developing an ALT-specific cancer therapeutic strategy targeting TOP3A.