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

端粒存在于染色体的两端，保护基因组免于降解、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从10到15千碱基（Kb）不等。每次细胞进行复制时，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。这一过程充当细胞时钟，并发出细胞增殖结束的信号。癌细胞通过使用两种途径来克服这一点：端粒酶再活化途径（TERT）和端粒替代延长（ALT）。尽管许多癌症使用TERT途径，但约4%至11%的癌症使用ALT，这是一种基于端粒重复序列之间的同源重组并依赖于DNA损伤反应蛋白（DDR）的募集的途径。这些肿瘤更具侵袭性，预后不良。ALT阳性细胞具有许多在端粒酶阳性细胞中看不到的特征。它们具有不附着于染色体末端的端粒DNA（染色体外DNA），其可以是线性的或环状的（c环）。它们还产生端粒姐妹染色单体交换和染色体间端粒长度的变化。虽然ALT的活化是相当一部分肿瘤的特征，但对ALT机制的缺乏完全理解阻碍了选择性杀死ALT肿瘤的策略的发展，尽管它们具有侵袭性和预后不良。端粒含有高度致密的染色质，称为异染色质。染色质结构如何有助于保持ALT表型，以及它是否可以用于治疗ALT肿瘤尚不清楚。由于在ALT肿瘤中观察到组蛋白变体H3.3及其伴侣蛋白ATRX和DAXX（异染色质的标志物）的突变，因此流行的观点是ALT受到染色质压缩的抑制。然而，最近的研究表明，在ALT端粒异染色质的形成有助于ALT表型，在本建议中，我们将使用异染色质相关蛋白HP 1BP 3作为一个模型，研究染色质结构在ALT维持的作用。我们将首先阐明它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致密化是如何参与的。然后，我们将使用无偏的方法来鉴定在HP 1BP 3基因消融后它们的结合是有缺陷的蛋白质。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使ALT表型永久化的表观遗传机制，并确定我们如何设计染色质状态或重新利用当前的表观遗传药物来选择性治疗ALT肿瘤。我们的研究将推动关于染色质在基因组维护中的作用的现有知识的界限，并将为个性化癌症治疗中的选择性干预奠定基础。

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.