Telomere length control by shelterin

描述（由申请人提供）：人类癌变的关键步骤之一是激活端粒维持系统，使转化细胞持续增殖。人类原代细胞的端粒随着每次细胞分裂而缩短，这种损耗最终导致细胞周期阻滞，从而限制了新出现的致瘤性克隆的增殖能力。众所周知，恶性细胞通过激活端粒酶来克服这一障碍，端粒酶是一种RNP逆转录酶，它在染色体末端合成端粒DNA。在之前的资助期内，我们已经描述了癌细胞中控制染色体末端端粒酶作用的调控途径。这种端粒长度的调节依赖于端粒特异性蛋白复合物，庇护蛋白，它以顺式方式抑制端粒酶。庇护蛋白在端粒上的积累与端粒重复序列的长度成正比，导致端粒越长，端粒酶的抑制作用越大。在下一个资助期，我们将进一步明确庇护蛋白（AIM 1）抑制端粒酶的机制。AIM 2的工作将集中在端粒酶介导的人肿瘤细胞系端粒延伸的正调控上。虽然端粒酶组分的表达和酶的生物发生已被详细分析，但对于端粒酶如何被募集到染色体末端，这种募集是如何被调节的，以及端粒酶是否需要额外的激活步骤，我们知之甚少。我们的初步数据揭示了端粒酶介导的端粒延伸的推定阳性调节因子，我们建议进一步定义这些途径和其他类似的途径。在第二个新方向，我们将关注端粒缩短（AIM 3）的机制和调控，尽管它作为一种推定的肿瘤抑制途径很重要，但人们对其知之甚少。人类端粒的损耗比预期的“末端复制问题”要快得多，但这种加速末端序列丢失的过程尚不清楚。我们的初步数据表明，端粒磨损涉及庇护蛋白调节的富含c的端粒DNA链的降解。我们建议在（前）恶性细胞中鉴定负责端粒缩短的基因，并研究这一加工步骤的调控。预计对端粒长度控制的分子基础的深入研究将为各种人类癌症的治疗干预提供机会，包括白血病、淋巴瘤和大多数癌症。

DESCRIPTION (provided by applicant): One of the key steps in human carcinogenesis is the activation of a telomere maintenance system that allows the continued proliferation of transformed cells. The telomeres of primary human cells shorten with each cell division and this attrition eventually induces cell cycle arrest, thus limiting the proliferative capacity of emerging tumorigenic clones. Malignant cells are known to overcome this barrier through the activation of telomerase, the RNP reverse transcriptase that synthesizes telomeric DNA onto chromosome ends. In the preceding funding period, we have described a regulatory pathway in cancer cells that controls the action of telomerase at individual chromosome ends. This telomere length regulation depends on the telomere specific protein complex, shelterin, which acts in cis to inhibit telomerase. Shelterin's accumulation at telomeres is proportional to the length of the telomeric repeat array, resulting in a greater inhibition of telomerase at longer telomeres. In the next funding period, we will further define the mechanism of telomerase inhibition by shelterin (AIM 1). The work in AIM 2 will focus on the positive regulation of telomerase-mediated telomere extension in human tumor cell lines. Whereas the expression of telomerase components and the biogenesis of the enzyme has been analyzed in detail, little is known about how telomerase is recruited to chromosome ends, how this recruitment is regulated, and whether telomerase requires additional activation steps. Our preliminary data has revealed putative positive regulators of telomerase-mediated telomere elongation and we propose to define these pathways and others like it further. In a second new direction, we will focus on the mechanism and regulation of telomere shortening (AIM 3), which is poorly understood, despite its importance as a presumed tumor suppressor pathway. The attrition of human telomeres is much faster than expected from the `end-replication problem' but the process underlying this accelerated terminal sequence loss is not known. Our preliminary data suggests that telomere attrition involves shelterin regulated degradation of the C-rich telomeric DNA strand. We propose to identify the genes responsible for telomere shortening in (pre-) malignant cells and study the regulation of this processing step. It is anticipated that insights into the molecular basis of telomere length control will provide opportunities for therapeutic intervention in a wide variety of human cancers, including leukemias, lymphomas, and most carcinomas.