Telomeres consist of TTAGGG repeats bound by protein complexes that serve to protect the natural end of linear chromosomes. Most cells maintain telomere repeat lengths by using the enzyme telomerase, although there are some cancer cells that use a telomerase-independent mechanism of telomere extension, termed alternative lengthening of telomeres (ALT). Cells that use ALT are characterized, in part, by the presence of specialized PML nuclear bodies called ALT-associated PML bodies (APBs). APBs localize to and cluster telomeric ends together with telomeric and DNA damage factors, which led to the proposal that these bodies act as a platform on which ALT can occur. However, the necessity of APBs and their function in the ALT pathway has remained unclear. Here, we used CRISPR/Cas9 to delete PML and APB components from ALT-positive cells to cleanly define the function of APBs in ALT. We found that PML is required for the ALT mechanism, and that this necessity stems from APBs' role in localizing the BLM-TOP3A-RMI (BTR) complex to ALT telomere ends. Strikingly, recruitment of the BTR complex to telomeres in a PML-independent manner bypasses the need for PML in the ALT pathway, suggesting that BTR localization to telomeres is sufficient to sustain ALT activity.

中文翻译：

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ALT细胞中端粒长度异质性通过BTR复合物对端粒的PML依赖性定位得以维持。

端粒由与蛋白复合物结合的TTAGGG重复序列组成，这些蛋白复合物可保护线性染色体的天然末端。大多数细胞通过使用端粒酶来维持端粒重复序列的长度，尽管有些癌细胞使用不依赖端粒酶的端粒延伸机制，即端粒的替代性延长（ALT）。使用ALT的细胞部分特征在于存在称为ALT相关PML体（APB）的专门PML核体。APB位于端粒末端并与端粒和DNA损伤因子聚集在一起，这导致提出这样的建议，即这些体充当ALT发生的平台。但是，APB及其在ALT途径中的功能的必要性仍不清楚。这里，我们使用CRISPR / Cas9删除了ALT阳性细胞中的PML和APB成分，从而明确定义了APB在ALT中的功能。我们发现PML是ALT机制所必需的，而这种必要性源于APB在将BLM-TOP3A-RMI（BTR）复合体定位到ALT端粒末端中的作用。引人注目的是，以PML独立的方式将BTR复合物募集至端粒绕过了ALT途径中对PML的需求，这表明BTR定位至端粒足以维持ALT活性。