Pharmacological intervention strategies for affecting telomerase activity: future prospects to treat cancer and degenerative disease

Telomerase enzyme is a ribonucleoprotein maintaining the length of the telomeres by adding G-rich repeats to the end of the eukaryotic chromosomes. Normal human somatic cells, cultured in vitro, have a strictly limited proliferative potential undergoing senescence after about 50-70 population doublings. In contrast, most of the tumor cells have unlimited replicative potential. Although the mechanisms of immortalization are not understood completely at a genetic level, the key role of the telomere/telomerase system in the process is clear. The DNA replication machinery is not able to replicate fully the DNA at the very end of the chromosomes; therefore, about 50-200 nucleotides are lost during each of the replication cycles resulting in a gradual decrease of telomere length. Critically short telomere induces senescence, subsequent crisis and cell death. In tumor cells, however, the telomerase enzyme prevents the formation of critically short telomeres, adding GGTTAG repeats to the 3' end of the chromosomes immortalizing the cells. Immortality is one of the hallmarks of cancer. Besides the catalytic activity dependent telomere maintenance, catalytic activity-independent effects of telomerase may also be involved in the regulation of cell cycle. The telomere/telomerase system offers two possibilities to intervene the proliferative activity of the cell: (1) inhibition the telomere maintenance by inhibiting the telomerase activity; (2) activating the residual telomerase enzyme or inducing telomerase expression. Whilst the former approach could abolish the limitless replicative potential of malignant cells, the activation of telomerase might be utilized for treating degenerative diseases. Here, we review the current status of telomerase therapeutics, summarizing the activities of those pharmacological agents which either inhibit or activate the enzyme. We also discuss the future opportunities and challenges of research on pharmacological intervention of telomerase activity.     

Asparagales Telomerases which Synthesize the Human Type of Telomeres

The order of monocotyledonous plants Asparagales is attractive for studies of telomere evolution as it includes three phylogenetically distinct groups with telomeres composed of TTTAGGG (Arabidopsis-type), TTAGGG (human-type) and unknown alternative sequences, respectively. To analyze the molecular causes of these switches in telomere sequence (synthesis), genes coding for the catalytic telomerase subunit (TERT) of representative species in the first two groups have been cloned. Multiple alignments of the sequences, together with other TERT sequences in databases, suggested candidate amino acid substitutions grouped in the Asparagales TERT synthesizing the human-type repeat that could have contributed to the changed telomere sequence. Among these, mutations in the C motif are of special interest due to its functional importance in TERT. Furthermore, two different modes of initial elongation of the substrate primer were observed in Asparagales telomerases producing human-like repeats, which could be attributed to interactions between the telomerase RNA subunit (TR) and the substrate. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Telomerase with mutated catalytic motifs has dominant negative effects on telomerase activity and inhibits cell growth

Telomerase catalytic subunit (TERT) seems a key factor controlling telomerase activity, telomere length, and cell growth. To further address this issue, we forced expression of a catalytically inactive mutant human TERT (hTERT) in hTERT-immortalised sheep fibroblasts to examine its effects. Expression of mutant hTERT compromised telomerase activity reconstituted by wild-type hTERT in a manner directly attributable to mutant hTERT expression level. High levels of mutant hTERT expression inhibited cell growth with a subset of cells entering replicative senescence. Furthermore, significant telomere attrition was evident in two of three clones with high levels of mutant hTERT expression. Our findings are consistent with the notion that hTERT homodimers are necessarily required to form a functional telomerase complex at the telomere substrate. We also highlight the requirement of a more thorough understanding of telomerase- and telomere-associated factors to understand fully the interplay that governs telomere homeostasis in vitro and in vivo.     

Specific features of telomerase RNA from Hansenula polymorpha

Telomerase, a ribonucleoprotein, is responsible for the maintenance of eukaryotic genome integrity by replicating the ends of chromosomes. The core enzyme comprises the conserved protein TERT and an RNA subunit (TER) that, in contrast, displays large variations in size and structure. Here, we report the identification of the telomerase RNA from thermotolerant yeast Hansenula polymorpha (HpTER) and describe its structural features. We show further that the H. polymorpha telomerase reverse transcribes the template beyond the predicted boundary and adds a nontelomeric dT in vitro. Sequencing of the chromosomal ends revealed that this nucleotide is specifically present as a terminal nucleotide at the 3′ end of telomeres. Mutational analysis of HpTER confirmed that the incorporation of dT functions to limit telomere length in this species.     

Endoplasmic reticulum stress activates telomerase

Telomerase contributes to cell proliferation and survival through both telomere‐dependent and telomere‐independent mechanisms. In this report, we discovered that endoplasmic reticulum (ER) stress transiently activates the catalytic components of telomerase (TERT) expression in human cancer cell lines and murine primary neural cells. Importantly, we show that depletion of hTERT sensitizes cells to undergo apoptosis under ER stress, whereas increased hTERT expression reduces ER stress‐induced cell death independent of catalytically active enzyme or DNA damage signaling. Our findings establish a functional link between ER stress and telomerase, both of which have important implications in the pathologies associated with aging and cancer.     

Dyskeratosis congenita -- a disease of dysfunctional telomere maintenance

Dyskeratosis congenita (DC) is a rare inherited bone marrow failure syndrome associated with abnormalities of the skin, fingernails, and tongue. Other clinical manifestations may include epiphora, lung fibrosis, liver cirrhosis, osteoporosis, and a predisposition to develop a variety of malignancies. The clinical picture often resembles that of a premature aging syndrome and tissues affected are those with a high cell turnover. DC has been linked to mutations in at least four distinct genes, three of which have been identified. The product of these genes, dyskerin, the telomerase RNA (TERC), and the catalytic unit of telomerase (TERT) are part of a ribonucleoprotein complex, the telomerase enzyme, that is essential for the elongation and maintenance of chromosome ends or telomeres. All patients with DC have excessively short telomeres, indicating that the underlying defect in these individuals is an inability to maintain the telomeres. The purpose of the current review is to highlight recent insights into the molecular pathogenesis of DC. We discuss the impact these findings have on our current understanding of telomere function and maintenance, and on the diagnosis, management, and treatment of patients with conditions caused by dysfunctional telomeres.     

端粒酶激活机制研究进展

随着对端粒和端粒酶在衰老和肿瘤中重要性认识的不断深入,端粒酶的激活途径已日益成为这一研究领域的热点.已发现,MYC原癌蛋白(myelocytomatosis virus oncoprotein, MYC)在端粒酶的激活中起关键作用.它可以与TERT基因启动子区域内的E盒结合反式直接激活端粒酶,还可能介导了细胞内其他分子如人乳头瘤病毒E6癌蛋白(human papillomavirus E6 oncoprotein, HPV-E6)和雌激素等对端粒酶的激活.雌激素也可与雌激素受体形成复合物,直接与TERT基因启动子区域内的退化雌激素反应元件结合反式激活端粒酶.此外,腺瘤样结肠息肉癌蛋白(adenomatous polyposis coli protein,APC)、p53及调节蛋白质磷酸化和去磷酸化过程的酶类等亦可能参与了端粒酶活性的激活.     

哺乳动物早期胚胎端粒和端粒酶重编程

端粒位于真核染色体末端,是稳定染色体末端的重要元件。端粒酶(TER)是一种特殊的细胞核糖核蛋白(RNP)反转录酶(RT),其核心酶包括蛋白亚基和RNA元件。在DNA复制过程中的端粒丢失可以被有活性的端粒酶修复回来。哺乳动物端粒酶在发育中受调控,端粒的重编程可能是由于早期胚胎不同时期的端粒酶活性而造成的。因此,研究端粒和端粒酶重编程在早期胚胎发育中是非常重要的。该文综述了端粒和端粒酶的结构和功能,及其与哺乳动物早期胚胎发育的关系,并在此基础上展望了端粒和端粒酶在克隆动物胚胎发育的基础研究。