Deciphering the role of heterochromatin in telomere function and maintenance mechanisms

项目概要 端粒是保护线性染色体末端的核蛋白结构，从而 维持基因组稳定性。端粒解决了末端保护和末端复制问题：1) 它们抑制染色体末端的 DNA 损伤，否则类似于断裂的 DNA，2) 因为 染色体末端在复制过程中缩短，端粒充当缓冲序列以防止编码丢失 3）一旦端粒变得太短，它们就不能再抑制DNA损伤，导致 永久性细胞周期停滞（衰老）。这种“有丝分裂时钟”是一个重要的肿瘤抑制屏障， 迫使衰老细胞停止分裂。要癌变，细胞必须获得无限的分裂潜力 通过激活端粒维持机制，或者重新激活端粒酶，该酶 在发育过程中延长端粒，或通过端粒的替代性延长 (ALT) 机制，这是基于重组的。 ALT 癌细胞的特征是端粒聚集成 ALT 相关的 PML 小体 (APB) 作为端粒重组的平台。 ALT 端粒也显示单一染色质， 较低的核小体密度和 ATRX/DAXX 途径的损失，该途径通常将 H3.3 沉积在 端粒。然而，我们发现端粒上的 H3K9 三甲基化促进 APB 的形成，如 以及随后的 ALT 活动。我们在此建议确定哪些染色质调节因子 DNA 损伤和修复基因调节 ALT 细胞中 APB 的形成。

PROJECT SUMMARY Telomeres are nucleoprotein structures that protect the ends of linear chromosomes and thereby maintain genome stability. Telomeres solve both the end-protection and the end-replication problems: 1) They inhibit DNA damage at chromosome ends, which would otherwise resemble broken DNA, 2) Since chromosome ends shorten during replication, telomeres act as buffer sequences to prevent loss of coding regions, 3) Once telomeres become too short, they can no longer inhibit DNA damage, leading to permanent cell cycle arrest (senescence). This “mitotic clock” is a critical tumor-suppressive barrier that forces aging cells to stop dividing. To become cancerous, cells must acquire unlimited division potential by activating a telomere maintenance mechanism, either reactivation of telomerase, the enzyme that elongates telomeres during development, or through the alternative lengthening of telomeres (ALT) mechanism, which is based on recombination. ALT+ cancers cells are characterized by clustering of telomeres into ALT-associated PML bodies (APBs) that serve as platforms for telomere recombination. ALT+ telomeres also display singular chromatin, with a lower nucleosome density and loss of the ATRX/DAXX pathway that normally deposits H3.3 at telomeres. We found, however, that H3K9 trimethylation at telomeres promotes formation of APBs, as well as subsequent ALT activity. We propose here to determine which chromatin regulators as well as DNA damage and repair genes modulate APB formation in ALT+ cells.