Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129, USA.
Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129, USA; Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA.
Mol Cell. 2021 Mar 4;81(5):1027-1042.e4. doi: 10.1016/j.molcel.2020.12.030. Epub 2021 Jan 15.
Alternative lengthening of telomeres (ALT) is mediated by break-induced replication (BIR), but how BIR is regulated at telomeres is poorly understood. Here, we show that telomeric BIR is a self-perpetuating process. By tethering PML-IV to telomeres, we induced telomere clustering in ALT-associated PML bodies (APBs) and a POLD3-dependent ATR response at telomeres, showing that BIR generates replication stress. Ablation of BLM helicase activity in APBs abolishes telomere synthesis but causes multiple chromosome bridges between telomeres, revealing a function of BLM in processing inter-telomere BIR intermediates. Interestingly, the accumulation of BLM in APBs requires its own helicase activity and POLD3, suggesting that BIR triggers a feedforward loop to further recruit BLM. Enhancing BIR induces PIAS4-mediated TRF2 SUMOylation, and PIAS4 loss deprives APBs of repair proteins and compromises ALT telomere synthesis. Thus, a BLM-driven and PIAS4-mediated feedforward loop operates in APBs to perpetuate BIR, providing a critical mechanism to extend ALT telomeres.
端粒的替代性延长（ALT）是由断裂诱导复制（BIR）介导的，但 BIR 如何在端粒处被调控还知之甚少。在这里，我们表明端粒 BIR 是一个自我延续的过程。通过将 PML-IV 锚定在端粒上，我们在 ALT 相关的 PML 体（APB）中诱导了端粒聚集，并在端粒处引发了依赖于 POLD3 的 ATR 反应，表明 BIR 会产生复制应激。在 APB 中消融 BLM 解旋酶活性会破坏端粒合成，但会导致端粒之间出现多个染色体桥，揭示了 BLM 在处理端粒间 BIR 中间体方面的功能。有趣的是，BLM 在 APB 中的积累需要其自身的解旋酶活性和 POLD3，表明 BIR 触发了正反馈回路，以进一步募集 BLM。增强 BIR 会诱导 PIAS4 介导的 TRF2 SUMO 化，而 PIAS4 的缺失会剥夺 APB 中的修复蛋白，并损害 ALT 端粒合成。因此，BLM 驱动和 PIAS4 介导的正反馈回路在 APB 中运作，以维持 BIR，为扩展 ALT 端粒提供了一个关键机制。
