Key Enzyme TOP3A Found to Control Cancer's Telomere Survival Trick

**Why this matters:** At least 10–15% of all cancers, prominently osteosarcoma (~45%) and glioblastoma (~33%), survive without telomerase by using ALT — a homologous-recombination pathway that keeps telomeres long enough for unlimited proliferation. Unlike telomerase inhibitors, there are currently no approved drugs targeting ALT, making mechanistic discoveries in this area directly relevant to developing new cancer therapies.

**What was studied:** This NCI study investigated the role of topoisomerase IIIα (TOP3A), a type IA enzyme normally involved in untangling DNA during replication and resolving Holliday junctions, at ALT telomeres. The researchers used immunofluorescence microscopy, proximity ligation assays, RNA FISH, native telomere FISH, western blotting, cycloheximide chase experiments, cell-cycle analysis, and metaphase chromosome spreads in ALT cell lines (U2OS, SAOS2, HU09) compared to telomerase-positive controls (SJSA-1, HT1080). They also employed siRNA-mediated TOP3A knockdown and overexpression of a self-poisoning TOP3A mutant (R364W) that generates irreversible DNA-protein crosslinks.

**Key results:** TOP3A colocalized with the shelterin protein TERF2 in 93% of ALT U2OS cells but was absent from telomeres in telomerase-positive cells. TOP3A knockdown dismantled ALT-associated PML nuclear bodies (APBs) and reduced BLM helicase recruitment to telomeres. It also dramatically decreased single-stranded C-strand telomeric DNA (ssTeloC) signals — a hallmark of ALT — and TERRA RNA foci at telomeres without reducing TERRA transcription, indicating a recruitment rather than expression defect. Western blotting and cycloheximide chase experiments showed that TOP3A loss selectively destabilized shelterin components (TERF2, TERF1, POT1) and BTRR complex members (BLM, RMI1) specifically in ALT cells. Chromosome spreads revealed fragile, smeared, and ultrabright telomeres after TOP3A depletion. Critically, introducing the toxic R364W TOP3A mutant, which forms DNA-protein crosslinks, similarly disrupted TERRA foci and reduced TERF2 levels, suggesting that trapping TOP3A on DNA is as damaging to ALT telomeres as removing it entirely.

**Implications:** TOP3A appears to resolve complex DNA intermediates — Holliday junctions, D-loop extensions, hypernegative supercoils — generated during ALT-associated break-induced replication. Because its telomeric function is ALT-specific and dispensable in telomerase-positive cells, it represents an attractive drug target with a potentially favorable therapeutic window. The self-poisoning R364W mutant concept further suggests that small molecules that trap TOP3A as a cleavage complex could selectively kill ALT cancers.

**Caveats:** All experiments were conducted in cell lines; in vivo validation in animal models is needed. The molecular mechanism by which TOP3A stabilizes shelterin protein levels (whether via direct interaction, protection from proteasomal degradation, or another route) remains to be fully elucidated. It is also unclear whether TOP3A's role in ALT extends to all ALT cancer subtypes beyond osteosarcoma-derived lines.