端粒结合蛋白与端粒长度调节

端粒结合蛋白与端粒长度调节郑晓飞王升启孙志贤（军事医学科学院放射医学研究所,北京１００８５０）关键词端粒端粒结合蛋白端粒是真核细胞染色体的末端序列,其功能是保持染色体的稳定性。端粒ＤＮＡ的长短和稳定性决定了细胞的寿命,并与细胞的癌变和衰老有关。端粒Ｄ...     

端粒结合蛋白与端粒长度调控

真核细胞端粒DNA序列的丢失与细胞的衰老及凋亡有关.端粒酶的激活可维持端粒长度并使细胞获得无限增殖的能力.端粒结合蛋白则可能通过调节端粒酶或其他相关因子的行为参与对端粒长度的调控.近年有关端粒结合蛋白的研究取得了突破性进展并在此基础上建立了端粒长度调控模型.     

端粒及端粒酶的研究进展

端粒是真核细胞染色体末端的特有结构,是由端粒结合蛋白和一段重复序列的端粒DNA组成的一个高度精密的复合体,在维持染色体末端稳定性,避免染色体被核酸酶降解等方面起着重要的作用。端粒的长度、结构及组织形式受多种端粒结合因子的调控。由于端粒的重要性,在哺乳动物细胞里,端粒的长度或端粒结构变化与癌症发生及细胞衰老有密切的关系。由于末端复制问题的存在,随着细胞分裂次数的增加,端粒不断缩短,细胞不可避免的走向衰老或凋亡。由于在细胞分裂过程中端粒长度的不断缩短与细胞分裂代数增加具有相关性,即端粒长度反应了细胞的分裂次数,因此有人将端粒形象的比喻为生物时钟。在90%的癌细胞中,端粒酶被重新激活,以此来维持端粒的长度,使细胞走向永生化。简要综述了端粒、端粒酶及端粒酶结合蛋白的最新研究进展。     

端粒DNA与端粒酶

端粒ＤＮＡ与端粒酶王勖焜,黄熙泰（南开大学生物化学与分子生物学系,天津３０００７１）关键词端粒ＤＮＡ,端粒酶,端粒结合蛋白端粒（Ｔｅｌｏｍｅｒｅ）是指真核细胞线性染色体末端的ＤＮＡ顺序。细胞正常的ＤＮＡ复制均需引物,这样真核生物线性染色体ＤＮＡ形式就...     

端粒保护蛋白TRF2的研究进展

端粒是染色体末端的核蛋白结构。染色体末端重复的端粒DNA可以规避不适当的DNA损伤反应(DNA damage response, DDR)的激活,维持染色体的稳定性,端粒的缺失会引起染色体融合并导致细胞的衰老及死亡。端粒特异性蛋白复合物Shelterin在保护端粒完整性方面具有重要作用。在这个复合体中,端粒结合因子2 (telomeric-repeat binding factor 2, TRF2)在维持端粒稳定、防止端粒染色体末端融合以及端粒染色体复制过程中发挥关键作用。该文综述了TRF2介导的保护染色体末端的多方面的机制。     

端粒长度及变化规律

端粒是位于染色体末段的一种特殊结构,具有维持染色体稳定的功能。不同的细胞,同一细胞的不同染色体具有不同的端粒长度;端粒长度是由基因控制的,并受到各种外界因素的影响;端粒长度随细胞的增殖与分化而缩短或者延长,细胞内在的端粒长度调控机制控制着端粒长度在一定的范围内变化。本文还分析了在端粒长度研究方面存在的问题。     

端粒保护蛋白

端粒保护蛋白（pmtection of telomere 1,PoT1）是存在于人和裂殖酵母的端粒相关蛋白,特异性地与端粒单链DNA相结合。人POT1基因位于7号染色体上,由22个外显子组成,其中4个外显子属于跳跃外显子,可形成5个剪接变异体。POT1的功能在于维持端粒的稳定,通过TRF1．TIN2．PIP1-POT通路调节端粒长度。     

ALT- 端粒延长替代机制

端粒长度和结构的稳定与肿瘤及衰老的发生密切相关, 端粒维持机制是细胞增殖的必要条件, 端粒维持机制的激活是肿瘤细胞演化过程中的一个重要环节。这种端粒维持机制可能是通过重新激活端粒酶, 使细胞快速增殖。在端粒酶失活或不足的情况下, 也存在着一种或多种维持和增加端粒长度的机制, 统称为端粒延长替代机制(Alterative lengthening of telomere, ALT)。其特点包括: 具有不均一的端粒长度, 存在与ALT相关的PML小体(APBs)以及同源重组增加。ALT细胞内存在的ALT相关蛋白及异常活跃的同源重组为ALT机制的激活和维持提供了可能。文章综述了ALT的特征性表型、与端粒酶的相关性及其可能的发生机制。对ALT机制的深入研究将有利于阐明衰老与肿瘤之间的辩证关系。     

端粒位置效应与人类衰老

端粒随细胞分裂进行性缩短不但防止了人类肿瘤的发展,而且与人类的衰老密切相关。另外,端粒中存在一种特殊的现象：端粒位置效应,它首先在酵母中发现,表现为靠近端粒序列附近的基因表达因端粒的位置效应而沉默。在人类细胞中也存在端粒位置效应,并且有多种因子参与此效应,它可能对细胞生长停止、肿瘤以及衰老发生时等许多随端粒缩短密切相关基因的程序性表达产生重要作用。     

定量荧光原位杂交测定端粒长度

目的:应用定量荧光原位杂交(Q-FISH)方法测定端粒长度。方法:选取4种端粒长度均一的标准细胞株采用Q-FISH的方法做出荧光亮度与端粒长度的标准曲线,从而得出实验细胞株的端粒长度,与DNA印迹法测定末端限制性片段(TRF)长度进行二者之间的相关性分析。结果:检测荧光强度的最佳线性曝光时间为400ms,相对于DNA印迹法,定量荧光原位杂交(Q-FISH)法所需标本量少,实验周期短,端粒长度结果与Southern杂交法具有很好的相关性。结论:采用定量荧光原位杂交方法测端粒长度具有重复性好、精确可靠的特点,适用于对珍贵标本的端粒改变进行分析。     

Telomere dysfunction and stem cell ageing

Ageing is characterized by a decline in organ maintenance and repair. Adult stem cells contribute to tissue repair and organ maintenance. Thus it is conceivable that ageing is partly due to a decline of stem cell function. At molecular level, ageing is associated with an accumulation of damage affecting DNA, proteins, membranes, and organelles, as well as the formation of insoluble protein aggregates. Telomere shortening represents a cell intrinsic mechanism, which contributes to the accumulation of DNA damage during cellular ageing. Telomere dysfunction in response to critical telomere shortening induces DNA damage checkpoints that lead to cell cycle arrest and/or cell death. Checkpoint responses induced by telomere dysfunction have mostly been studied in somatic cells but there are emerging data on cell intrinsic checkpoints that impair the maintenance and function of adult stem cell in response to telomere dysfunction. Moreover, telomere dysfunction induces alterations in the stem cell environment that limit the function of adult stem cells. In this review we summarize our current knowledge on the role of telomere dysfunction in adult stem cell ageing.     

Telomere dynamics in human cells

Human telomeres are intrinsically dynamic structures, with multiple biological processes operating to generate substantial length heterogeneity. Processes that operate specifically at the terminus, that include the end-replication problem coupled with C-strand resection, result in gradual telomere erosion with ongoing cell division. Rates of telomere erosion can be modulated by cell culture conditions and pleiotropic effects. Other processes, that are not consistent with the end replication problem, generate sporadic large-scale changes in telomere length. These events are detected in normal human cells and tissues; the severely truncated telomeres that result are potentially fusogenic and may lead to the types of genetic rearrangements that typify early-stage neoplasia. The processes that underlie sporadic telomeric deletion are unclear, but may include intra-allelic recombination within the T-loop structure, unequal sister chromatid exchange and replication fork stalling. The relative contributions of these processes in the generation of the heterogeneous telomere length profiles observed in human cells are discussed.     

Telomere length regulation in budding yeasts

Telomeres are the nucleoprotein caps of chromosomes. Their length must be tightly regulated in order to maintain the stability of the genome. This is achieved by the intricate network of interactions between different proteins and protein–RNA complexes. Different organisms use various mechanisms for telomere length homeostasis. However, details of these mechanisms are not yet completely understood. In this review we have summarized our latest achievements in the understanding of telomere length regulation in budding yeasts.     

Telomere stability and telomerase in mesenchymal stem cells

Telomeres are repetitive genetic material that cap and thereby protect the ends of chromosomes. Each time a cell divides, telomeres get shorter. Telomere length is mainly maintained by telomerase. This enzyme is present in high concentrations in the embryonic stem cells and in fast growing embryonic cells, and declines with age. It is still unclear to what extent there is telomerase in adult stem cells, but since these are the founder cells of cells of all the tissues in the body, understanding the telomere dynamics and expression of telomerase in adult stem cells is very important. In the present communication we focus on telomere expression and telomere length in stem cells, with a special focus on mesenchymal stem cells. We consider different mechanisms by which stem cells can maintain telomeres and also focus on the dynamics of telomere length in mesenchymal stem cells, both the overall telomere length and the telomere length of individual chromosomes.     

FEN1 Ensures Telomere Stability by Facilitating Replication Fork Re-initiation

Telomeres are terminal repetitive DNA sequences whose stability requires the coordinated actions of telomere-binding proteins and the DNA replication and repair machinery. Recently, we demonstrated that the DNA replication and repair protein Flap endonuclease 1 (FEN1) is required for replication of lagging strand telomeres. Here, we demonstrate for the first time that FEN1 is required for efficient re-initiation of stalled replication forks. At the telomere, we find that FEN1 depletion results in replicative stress as evidenced by fragile telomere expression and sister telomere loss. We show that FEN1 participation in Okazaki fragment processing is not required for efficient telomere replication. Instead we find that FEN1 gap endonuclease activity, which processes DNA structures resembling stalled replication forks, and the FEN1 interaction with the RecQ helicases are vital for telomere stability. Finally, we find that FEN1 depletion neither impacts cell cycle progression nor in vitro DNA replication through non-telomeric sequences. Our finding that FEN1 is required for efficient replication fork re-initiation strongly suggests that the fragile telomere expression and sister telomere losses observed upon FEN1 depletion are the direct result of replication fork collapse. Together, these findings suggest that other nucleases compensate for FEN1 loss throughout the genome during DNA replication but fail to do so at the telomere. We propose that FEN1 maintains stable telomeres by facilitating replication through the G-rich lagging strand telomere, thereby ensuring high fidelity telomere replication.     

Synthetic Nucleosides and Nucleotides. 43. Inhibition of Vertebrate Telomerases by Carbocyclic Oxetanocin G (C.OXT-G) Triphosphate Analogues and Influence of C.OXT-G Treatment on Telomere Length in Human HL60 Cells

Telomerase, responsible for telomere synthesis, is expressed in ～ 90% of human tumor cells but seldom in normal somatic cells. In this study, inhibition by carbocyclic oxetanocin G triphosphate (C.OXT-GTP) and its analogues was investigated in order to clarify the susceptibility of telomerase to various nucleotide analogues. C.OXT-GTP competitively inhibited telomerase activity with respect to dGTP. However, C.OXT-GTP had a potent inhibitory effect on DNA polymerase α. It was examined whether the nucleoside (C.OXT-G) was able to alter telomere length in cultured human HL60 cells. Contrary to expectation, long-term treatment with 10 μM C.OXT-G was found to cause telomere lengthening.     

Telomere and telomerase in oncology

Telomere and cell replicative senescenceTelomeres, which are located at the end of chro-mosome, are crucial to protect chromosome againstdegeneration, rearrangment and end to end fusion[1].Human telomeres are tandemly repeated units of thehexanucleotide TTAGGG. The estimated length oftelomeric DNA varies from 2 to 20 kilo base pairs,depending on factors such as tissue type and hu-man age. The buck of telomeric DNA is double-stranded, but the end of telomeric DNA consists of3' overhang of…     

端粒与衰老及心脑血管疾病的研究进展

端粒是真核细胞染色体末端的一种保护性结构,在维持染色体末端稳定性等方面起重要作用。端粒被认为是细胞衰老的生物钟。研究证明端粒的长度随着人体的衰老呈进行性缩短。近年来,分子流行病学研究表明端粒的长度与人的寿命呈负相关。越来越多的相关研究发现,端粒的长度与许多衰老相关的疾病密切相关。原发性高血压（esential hypertension,EH）、冠状动脉粥样硬化（coronary atherosclerosis,CA）、心力衰竭（heart failure,HF）和脑卒中（stroke）等心脑血管疾病的发生发展过程中都伴有端粒长度的改变。影响端粒长度的因素有很多,包括遗传因素和非遗传因素,其中有关端粒长度和非遗传因素的关系还不确定。另外,端粒是否可以作为衰老及其相关疾病的预测因子还没有定论。本文现就端粒在人类疾病中的相关研究进展作一简要综述。     

TERRA介导的端粒维持

真核细胞线状染色体末端特殊结构被称为端粒,而端粒维持对于生命体来说具有十分重要的意义,其维持机制也十分复杂.端粒酶可以通过其具有的特殊逆转录酶特性,利用自身的RNA模板(TERC)以及具有催化功能的蛋白质亚基(TERT)延长端粒,维持其长度.本文着重综述端粒TERRA (telomeric repeat-containing RNA)对端粒维持的影响及其作用机制.首先介绍端粒维持与细胞存活老化之间的关系;其次,阐述TERRA的结构及其转录特性,TERRA依赖的DNA∶RNA杂合体和R-loop形成和结构特点,TERRA结合蛋白及其作用;进而讨论依赖于TERRA的端粒维护分子机制以及在生命过程中的意义.     

Telomere configuration influences the choice of telomere maintenance pathways

Telomere maintenance is required for chromosome stability, and telomeres are typically replicated by the action of telomerase. In yeast cells that lack telomerase, telomeres are maintained by alternative type I and type II recombination mechanisms. Previous studies identified several proteins to control the choice between two types of recombinations. Here, we demonstrate that configuration of telomeres also plays a role to determine the fate of telomere replication in progeny. When diploid yeasts from mating equip with a specific type of telomeric structure in their genomes, they prefer to maintain this type of telomere replication in their descendants. While inherited telomere structure is easier to be utilized in progeny at the beginning stage, the telomeres in type I diploids can gradually switch to the type II cells in liquid culture. Importantly, the TLC1/tlc1 yeast cells develop type II survivors suggesting that haploid insufficiency of telomerase RNA component, which is similar to a type of dyskeratosis congenital in human. Altogether, our results suggest that both protein factors and substrate availability contribute to the choice among telomere replication pathways in yeast.