Telomere recombination requires the MUS81 endonuclease

Telomerase-negative cancer cells maintain their telomeres via the Alternative Lengthening of Telomeres (ALT) pathway1–3. Although a growing body of evidence demonstrates that the ALT mechanism is a post-replicative telomere recombination process, molecular details of this pathway are largely unknown. Here we demonstrate that MUS81, a DNA structure–specific recombination endonuclease, plays a key role in the maintenance of telomeres in human ALT cells. We find that MUS81 specifically localizes to ALT-associated promyelocytic leukemia nuclear bodies (APBs) and associates with telomeric DNA in ALT cells, which is enriched during G2 phase of the cell cycle. Depletion of MUS81 results in reduction of ALT specific-telomere recombination and leads to proliferation arrest of ALT cells. In addition, the endonuclease activity of MUS81 is required for recombination-based ALT cell survival, and the interaction of MUS81 with TRF2 regulates this enzymatic activity to maintain telomere recombination. Thus, our results suggest that MUS81 is involved in the maintenance of ALT cell survival at least in part by telomere-HR process.