An introduction to telomeres and telomerase

It is now more than a dozen years since the enzyme telomerase was discovered, and since that time, key studies have characterized the structural components of the enzyme and the associated telomeric proteins. Since the original discovery of telomerase, a clear association with cancer has been demonstrated. In normal somatic cells the telomeres at the ends of chromosomes shorten with every cell division, whereas in cancer cells telomere length is often maintained by reactivation of the enzyme telomerase. These discoveries have led to the proposal that telomerase expression can be used as a helpful marker for diagnostic and prognostic purposes in humans. Another area of research that has developed as a result of improving knowledge and understanding of the role of telomerase in malignancy is that of cancer therapeutics. This article is an introduction to the field of telomere and telomerase research, with an introduction to recent attempts to develop novel cancer treatments based on telomerase structure and function.     

Regulation of catalytic activity and processivity of human telomerase

The ends of eukaryotic chromosomes are specialized sequences, called telomeres comprising tandem repeats of simple DNA sequences. Those sequences are essential for preventing aberrant recombination and protecting genomic DNA against exonucleolytic DNA degradation. Telomeres are maintained at a stable length by telomerase, an RNA-dependent DNA polymerase. Recently, human telomerase has been recognized as a unique diagnostic marker for human tumors and is potentially a highly selective target for antitumor drugs. In this study, we have examined the major factors affecting the catalytic activity and processivity of human telomerase. Specifically, both the catalytic activity and processivity of human telomerase were modulated by temperature, substrate (dNTP and primer) concentration, and the concentration of K+. The catalytic activity of telomerase increased as temperature (up to 37 degrees C), concentrations of dGTP, primer, and K+ were increased. However, the processivity of human telomerase decreased as temperature, primer concentration, and K+ were increased. Our results support the current model for human telomerase reaction and strengthen the hypothesis that a G-quadruplex structure of telomere DNA plays an important role in the regulation of the telomerase reaction.     

Applications of telomerase research in the fight against cancer

Telomerase, an enzyme that confers immortality upon cells and that is active in the majority of human tumors, has emerged as a powerful new marker and potential prognostic indicator and therapeutic target for cancer. Furthermore, investigations into the biology of telomerase have revealed important clues into the causes of cell death and have made progress toward answering one of the most important questions of cancer research - what gives a tumor cell an advantage over normal cells? In this article, we present the current state of telomerase research and critically assess both its potential and the pitfalls of its application in cancer diagnosis and treatment.     

端粒酶研究的若干进展

端粒酶是一种ＲＮＡ依赖的ＤＮＡ聚合酶。它的生物学功能在于以自身的ＲＮＡ为模板,合成端粒序列,解决了线性染色体的末端复制问题,维持了染色体的稳定性。本文介绍一些新进展：如端粒酶蛋白的ｃＤＮＡ已被克隆;端粒酶活性检测中出现误差的可能原因;端粒酶活性的调控因子ＴＲＦ１及新发现的ＴＲＦ２;端粒酶活性同肿瘤诊断及预后间的关系;端粒酶的激活同肿瘤发生间的关系;反义核酸抑制端粒酶活性的可能性及其可能发生的问题等。     

Telomerase in endocrine and endocrine-dependent tumors

Telomerase, the ribonucleoprotein enzyme that elongates chromosomal ends, or telomeres, is repressed in most normal somatic cells but reactivated in transformed cells to compensate for the progressive erosion of the telomeres during cell divisions. In accordance with this hypothesis, the presence of telomerase activity has been reported in more than 90% of human cancers, whereas most normal tissues or benign tumors contain low or undetectable telomerase activity. Reactivation of telomerase has also been widely reported in endocrine neoplasms and in hormone-related cancers. In the present study, we review the most recent publications on telomerase in these types of tumors. The hormonal regulation of telomerase activity and the possible strategies for cancer therapy based on the inhibition of telomerase has also been discussed.     

有限延伸法检测端粒酶活性

人端粒酶是一种核蛋白体,通过其内含的RNA模板与端粒末端配对把重复端粒片段添加在端粒3＇末端|因此,端粒酶活性与细胞凋亡、衰老、永生化有密切关系,是癌症临床预测诊断的一个生物标签．现有的端粒酶活性检测方法,存在灵敏度低和不易定量等问题．本研究采用错配有限延伸法检测端粒酶活性：在人端粒酶延伸人工合成的游离端粒酶底物时,只加入dATP和dGTP,端粒酶只能把底物延伸4个脱氧核糖核苷酸AGGG．然后加入dNTP,让端粒酶延伸的产物和一条长的引物配对从而延伸出PCR模板|再加入引物进行热启动PCR．PCR后进行非变性PAGE (polyacrylamide gel electrophoresis),得到希望的唯一1条目标带．同时,用不同的端粒酶浓度梯度进行优化,发现有限延伸法检测端粒酶活性的下限达到250个HeLa细胞．     

Telomerase activity in human germline and embryonic tissues and cells

Telomerase is a ribonucleoprotein that synthesizes telomere repeats onto chromosome ends and is involved in maintaining telomere length in germline tissues and in immortal and cancer cells. In the present study, the temporal regulation of expression of telomerase activity was examined in human germline and somatic tissues and cells during development. Telomerase activity was detected in fetal, newborn, and adult testes and ovaries, but not in mature spermatozoa or oocytes. Blastocysts expressed high levels of telomerase activity as did most human somatic tissues at 16–20 weeks of development with the exception of human brain tissue. This activity could no longer be detected in the somatic tissues examined from the neonatal period onward. Neither placenta nor cultured fetal amniocytes contained detectable telomerase activity. Fetal tissues explanted into primary cell culture showed a dramatic decline in telomerase activity which became undetectable after the first passage in vitro. Elucidation of the regulatory pathways involved in the repression of telomerase activity during development may lead to the ability to manipulate telomerase levels and explore the consequences both for cellular aging and for the survival of cancer cells. © 1996 Wiley-Liss, Inc.     

Clonal heterogeneity in telomerase activity and telomere length in tumor-derived cell lines

The ribonucleoprotein, telomerase, is responsible for the maintenance of telomere length in most immortal and cancer cells. Telomerase appears to be a marker of human malignancy with at least 85% of human cancers expressing its activity. In the present study, we examined a series of tumor-derived and in vitro immortalized cell lines for telomerase activity levels, telomere lengths, and expression levels of the RNA and catalytic components of telomerase. We found significant variability in both telomere lengths and telomerase activity in clones from tumor cells. In addition, the levels of telomerase components or telomerase activity were not predictive of telomere length. Data from clonally derived cells suggest that critically shortened telomeres in these tumor-derived cell lines may signal activation of telomerase activity through an increase in the expression of the catalytic subunit of telomerase. Although clones with low telomerase shorten their telomeres over time, their subclones all have high levels of telomerase activity with no telomere shortening. In addition, analysis of early clones for telomerase activity indicates substantial variability, which suggests that activity levels fluctuate in individual cells. Our data imply that cell populations exhibit a cyclic expression of telomerase activity, which may be partially regulated by telomere shortening.     

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…     

Telomerase activity could be used as a marker for neoplastic transformation in gastric adenocarcinoma: but it does not have a prognostic significance

Telomerase activity is responsible for telomere maintenance and is believed to be crucial in most immortal cells and cancer cells; however, its clinicopathological significance in gastric cancer remains to be clarified. The aim of the present study was to assess whether malignant progression of gastric adenocarcinoma correlates with telomerase activity. We also investigated the correlation between telomerase activity and histopathological findings. We examined telomerase activity in tumor specimens and adjacent normal tissues from 43 patients with gastric adenocarcinoma. Telomerase activity was measured quantitatively by the TRAPEZE Gel Based Telomerase Detection Kit. Approximately 98% of the tumor tissues were telomerase positive, but telomerase activity was detected not only in tumor tissues but also in normal gastric mucosa. Although telomerase activity was found to be higher in tumor samples than normal tissue for each subject, we could not find a general cut-off level for telomerase activity in gastric adenocarcinoma. In addition, telomerase activity was not correlated with tumor invasion, lymph node involvement and histological stage. Our results support the idea that telomerase reactivation is a common event in gastric adenocarcinoma and it is not related to histopathological parameters. Since it is difficult to set a cut-off level for this type of cancer, we suggest that the prognostic utility of telomerase assay has not yet reached the clinic in terms of predicting outcome for patients with gastric adenocarcinoma. For the assessment of gastric carcinoma, telomerase activity should be evaluated in both tumor and normal tissues, because normal gastric mucosa samples show appreciable telomerase activity.