The alternative lengthening of telomeres (ALT) pathway maintains telomere length in a significant fraction of cancers that are associated with poor clinical outcomes. A better understanding of ALT mechanisms is therefore necessary for developing new treatment strategies for ALT cancers. SUMO modification of telomere proteins contributes to the formation of ALT telomere-associated PML bodies (APBs), in which telomeres are clustered and DNA repair proteins are enriched to promote homology-directed telomere DNA synthesis in ALT. However, it is still unknown whether—and if so, how—SUMO supports ALT beyond APB formation. Here, we show that SUMO condensates that contain DNA repair proteins enable telomere maintenance in the absence of APBs. In PML knockout ALT cell lines that lack APBs, we found that SUMOylation is required for manifesting ALT features independent of PML and APBs. Chemically induced telomere targeting of SUMO produces condensate formation and ALT features in PML-null cells. This effect requires both SUMOylation and interactions between SUMO and SUMO interaction motifs (SIMs). Mechanistically, SUMO-induced effects are associated with the accumulation of DNA repair proteins, including Rad52, Rad51AP1, RPA, and BLM, at telomeres. Furthermore, Rad52 can undergo phase separation, enrich SUMO at telomeres, and promote telomere DNA synthesis in collaboration with the BLM helicase in a SUMO-dependent manner. Collectively, our findings suggest that SUMO condensate formation promotes collaboration among DNA repair factors to support ALT telomere maintenance without PML. Given the promising effects of SUMOylation inhibitors in cancer treatment, our findings suggest their potential use in perturbing telomere maintenance in ALT cancer cells.

中文翻译：

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SUMO 促进 DNA 修复蛋白合作，以支持在没有 PML 的情况下替代端粒延长


端粒替代性延长 (ALT) 途径可在相当一部分与不良临床结果相关的癌症中维持端粒长度。因此，更好地了解 ALT 机制对于开发 ALT 癌症的新治疗策略是必要的。端粒蛋白的 SUMO 修饰有助于形成 ALT 端粒相关 PML 体 (APB)，其中端粒聚集，DNA 修复蛋白富集，以促进 ALT 中同源定向的端粒 DNA 合成。然而，目前尚不清楚 SUMO 是否（如果是的话，如何支持 APB 形成之外的 ALT）。在这里，我们证明含有 DNA 修复蛋白的 SUMO 缩合物能够在没有 APB 的情况下维持端粒。在缺乏 APB 的 PML 敲除 ALT 细胞系中，我们发现 SUMO 化对于表现出独立于 PML 和 APB 的 ALT 特征是必需的。 SUMO 的化学诱导端粒靶向在 PML 无效细胞中产生冷凝物形成和 ALT 特征。这种效应需要 SUMO 化以及 SUMO 和 SUMO 相互作用基序 (SIM) 之间的相互作用。从机制上讲，SUMO 诱导的效应与端粒处 DNA 修复蛋白（包括 Rad52、Rad51AP1、RPA 和 BLM）的积累有关。此外，Rad52可以进行相分离，富集端粒上的SUMO，并以SUMO依赖性方式与BLM解旋酶合作促进端粒DNA合成。总的来说，我们的研究结果表明，SUMO 凝聚体的形成促进了 DNA 修复因子之间的协作，以支持 ALT 端粒维持，而无需 PML。鉴于 SUMO 化抑制剂在癌症治疗中的良好作用，我们的研究结果表明它们在扰乱 ALT 癌细胞端粒维持方面具有潜在用途。