The role of chromatin structure in Alternative Lengthening of Telomeres (ALT): lessons from the Heterochromatin Protein 1 Binding Partner 3 (HP1BP3).

染色质结构在端粒选择性延长 (ALT) 中的作用：异染色质蛋白 1 结合伴侣 3 (HP1BP3) 的经验教训。

• 批准号: MR/X018709/1
• 负责人: Evi Soutoglou
• 金额: $ 83.27万
• 依托单位: University of Sussex
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=MR%2FX018709%2F1
• 关键词: role; chromatin; structure; Alternative; Lengthening

Telomeres are present at both ends of chromosomes and protect the genome form degradation, unnecessary recombination and intrerchromosomal fusion. Telomeres are made up of repetitive sequences. The average length of human telomeres varies from 10 to 15 Kilobase (Kb). Every time the cells go through replication, the chromosome ends are shortened by 25-200 base pairs, a process termed end replication problem. This process acts as a cellular clock and signals end of cell proliferation. Cancer cells overcome this by using two pathways: the telomerase reactivation pathway (TERT) and Alternative Lengthening of Telomeres (ALT). Although many cancers use the TERT pathway, around 4 to 11% of cancers use ALT a pathway that is based on homologous recombination between telomeric repeats and relies on the recruitment of DNA damage response proteins (DDR). These tumours are more aggressive and have poor prognosis. ALT positive cells have a number of characteristics that are not seen in the Telomerase positive cells. They have telomeric DNA that is not attached to the chromosome ends (extra chromosomal DNA) which can be linear or circular (c-circles). They also exert telomeric sister chromatid exchanges and varied telomere length from chromosome to chromosome. Although activation of ALT characterises a substantial fraction of tumours, the lack of complete understanding of the mechanisms of ALT has hindered the development of strategies to selectively kill ALT tumours, despite their aggressive nature and poor prognosis. Telomeres contain highly compacted chromatin called heterochromatin. How chromatin structure contributes to perpetuate the ALT phenotype and whether it can be exploited for treating ALT tumours is unclear. Since mutations in the histone variant H3.3 and its chaperones ATRX and DAXX which are marks of heterochromatin have been observed in ALT tumours, the prevailing view was that ALT is inhibited by chromatin compaction. Recent studies, however, have demonstrated that heterochromatin formation at ALT telomeres contributes to ALT phenotype.In this proposal, we will use the heterochromatin related protein HP1BP3 as a model to study the role of chromatin structure in maintenance of ALT. We will first characterise its role in promoting telomeric stability and recombination and dissect the protein domains that promote each function and how chromatin compaction is involved. Then, we will use unbiased approaches to identify the proteins that their binding is defective upon genetic ablation of HP1BP3. Finally, to broaden the analysis, we will screen for additional epigenetic factors and drugs that selectively kills ALT cancer cells. These approaches will allow us to carefully dissect the epigenetic mechanisms that perpetuate the ALT phenotype and determine how we can engineer a chromatin state or repurpose current epigenetic drugs for selectively treating ALT tumours. Our research will push forward the boundaries of established knowledge about the role of chromatin in genome maintenance and will lay the basis for selective interventions in personalized cancer therapy.

端粒存在于染色体的两端，保护基因组免遭降解、不必要的重组和染色体间融合。端粒由重复序列组成。人类端粒的平均长度为 10 至 15 千碱基 (Kb)。细胞每次复制时，染色体末端都会缩短 25-200 个碱基对，这一过程称为末端复制问题。这个过程充当细胞时钟并发出细胞增殖结束的信号。癌细胞通过两种途径克服这一问题：端粒酶再激活途径（TERT）和端粒替代延长途径（ALT）。尽管许多癌症使用 TERT 途径，但大约 4% 至 11% 的癌症使用 ALT 途径，该途径基于端粒重复之间的同源重组，并依赖于 DNA 损伤反应蛋白 (DDR) 的募集。这些肿瘤更具侵袭性，预后较差。 ALT 阳性细胞具有许多端粒酶阳性细胞所没有的特征。它们具有未附着在染色体末端的端粒 DNA（染色体外 DNA），可以是线性的或环状的（c 环）。它们还进行端粒姐妹染色单体交换，并在染色体之间改变端粒长度。尽管 ALT 激活是大部分肿瘤的特征，但由于缺乏对 ALT 机制的完整了解，阻碍了选择性杀死 ALT 肿瘤的策略的开发，尽管它们具有侵袭性和预后不良。端粒含有高度压缩的染色质，称为异染色质。染色质结构如何有助于维持 ALT 表型以及是否可以利用它来治疗 ALT 肿瘤尚不清楚。由于在 ALT 肿瘤中观察到组蛋白变体 H3.3 及其伴侣 ATRX 和 DAXX（异染色质标记）发生突变，因此普遍认为 ALT 受到染色质压缩的抑制。然而，最近的研究表明，ALT 端粒处的异染色质形成有助于 ALT 表型。在本提案中，我们将使用异染色质相关蛋白 HP1BP3 作为模型来研究染色质结构在 ALT 维持中的作用。我们将首先描述其在促进端粒稳定性和重组中的作用，并剖析促进每种功能的蛋白质结构域以及染色质压缩是如何参与的。然后，我们将使用公正的方法来识别在 HP1BP3 基因消融后其结合有缺陷的蛋白质。最后，为了扩大分析范围，我们将筛选其他表观遗传因素和选择性杀死 ALT 癌细胞的药物。这些方法将使我们能够仔细剖析维持 ALT 表型的表观遗传机制，并确定如何设计染色质状态或重新利用现有的表观遗传药物来选择性治疗 ALT 肿瘤。我们的研究将突破关于染色质在基因组维护中的作用的既定知识的界限，并将为个性化癌症治疗的选择性干预奠定基础。