Telomerase Biology Research Articles

Meeting Report: The Role of Telomeres and Telomerase in Cancer

The role of telomeres and telomerase in cancer is an area of much recent interest. This conference sponsored by the American Association of Cancer Research provided a timely opportunity to bring together basic and clinical scientists interested in the field of telomeres and telomerase cancer biology. The meeting included over 250 attendees with 150 oral and poster presentations focused on understanding telomere and telomerase biology for the development of cancer therapeutics. The meeting chairpersons were Dr. Jerry W. Shay, University of Texas Southwestern Medical Center, Dallas, TX; Dr. Elizabeth H. Blackburn, University of California San Francisco, CA; and Dr. Maria A. Blasco, Spanish National Cancer Center, Madrid, Spain. The meeting provided an update on the field, pointing to areas in which our knowledge is deficient, and explored how the most promising areas may be advanced into translational research. This conference brought together cell and molecular biologists with clinicians interested in fundamental cancer mechanisms as they relate to telomeres and telomerase. The symposium consisted of formal presentations by prominent scientists working in these areas and by participants selected from submitted abstracts. In addition, there were two poster sessions. Whereas there were many basic research advances presented, the focus of this overview will be the areas of clinical advances.

HTERT, the Catalytic Component of Telomerase, Is Down-Regulated in the Hematopoietic Stem Cells of Patients with Chronic Myeloid Leukemia.

Telomere shortening and increased telomerase activity is associated with disease progression in chronic myeloid leukemia (CML). In order to gain insight into the biology of telomerase and its regulation in CML we have evaluated the expression levels of the components of telomerase, its regulators, and several telomeric associated proteins in the hematopoietic stem cells (HSC)/leukemic blast cells of 21 patients with this disorder, as compared to the HSC population of a group of healthy individuals, using quantitative real time PCR. In this study we have made the surprising observation that hTERT, the catalytic component of telomerase, is down-regulated in the CD34+/blast cells of the majority of CML patients in chronic phase (CP) (7 out of 9, p=0.0339) relative to CD34+ cells from normal controls. Furthermore hTERT was found to be significantly down-regulated in 2 out of 3 patients in accelerated phase (AP) and in 9 out of 9 patients in blast crisis (BC) relative to normal controls (p=0.0003). The expression levels of hTR, the telomerase RNA template, and the telomeric- associated proteins TEP1, TRF1, TRF2 and tankyrase were all up-regulated in the CD34+/blast cells of the majority of the CML patients in CP and AP, relative to normal control CD34+ cells. With the exception of TEP1, the expression levels of all these factors were highest in CP CML and, in the majority of patients, decreased during disease progression. We speculate that the up-regulation of tankyrase (positive regulator of telomere length) and TRF2 (protects critically shortened telomeres) may contribute to telomere maintenance in CML. The expression levels of hTERT and c-myc, a positive regulator of hTERT expression, correlate in all stages of CML (R2=0.623). The expression levels of c-myc decreased with disease progression, falling below the levels observed in normal CD34+ cells in the majority of patients in BC. The expression levels of hTERT were increased in the CD34+ cells of three additional CP CML patients in remission following successful treatment with imatinib, relative to both untreated CP CML patients (p=0.0126) and to normal individuals, reflecting a restoration of haematopoiesis towards normal. In CML the turnover of all types of Ph+ stem and progenitor cells is known to be increased. We suggest that the reduced (and presumably inadequate) expression levels of hTERT in CML CD34+/blast cells, and the consequent telomere shortening with each division, is the direct cause of the shortened telomeres frequently observed in CML patients. These data challenge the widely accepted view that hTERT expression is always increased in tumorigenesis. It is possible that the Bcr-Abl fusion product may play a role in the down-regulation of hTERT observed in the HSC of CML patients in this study.

Replicative senescence in sheep fibroblasts is a p53 dependent process

Studies on telomere and telomerase biology are fundamental to the understanding of human ageing, and age-related diseases such as cancer. However, human studies are hampered by the lack of fully reflective animal model systems. Here we describe basic studies of telomere length and telomerase activity in sheep tissues and cells. Terminal restriction fragment lengths from sheep tissues ranged from 9 to 23kb, with telomerase activity present in testis but suppressed in somatic tissues. Sheep fibroblasts had a finite lifespan in culture, after which the cells entered senescence. During in vitro growth the mean terminal restriction fragment lengths decreased in size at a rate of 210 and 350bp per population doubling (PD). Senescent skin fibroblasts had increased levels of p53 and p21WAF1 compared to young cells. Incubation of senescent cells with siRNA duplexes specific for p53 suppressed p53 expression and allowed the cells to re-enter the cell cycle. Five PDs beyond senescence the siRNA-treated cells reached a second proliferative barrier. This study shows that telomere biology in sheep is similar to that in humans, with senescence in sheep GM03550 fibroblasts being a telomere-driven, p53-(p21WAF1)-dependent process. Therefore sheep may represent an alternative model system for studying telomere biology, replicative senescence, and by implication human ageing.

Mouse models for telomere and telomerase biology.

Telomeres serve a critical role in maintenance of genomic stability in all eukaryotes, from yeast to human. The maintenance of telomeres is achieved by the telomerase complex, which is largely composed of telomerase reverse transcriptase (TERT) and telomerase RNA component (TERC). A variety of mouse models have provided valuable insights into the relationship between the telomerase complex and telomere dysfunction at the organismal level and helped understand their biological significance in human. Recently, in addition to its role in maintenance of the telomeres, novel functions of the telomerase complex have been emerging. In this review, studies of all gene-targeted or transgenic mouse models so far generated for telomerase and telomere biology are comprehensively described, and potential novel functions of telomerase are briefly discussed.

Telomerase and tumorigenesis

The unique biology of telomeres and telomerase plays important roles in many aspects of mammalian cell physiology. Over the past decade, several lines of evidence have confirmed that the maintenance of telomeres and telomerase participate actively in the pathogenesis of human cancer. Specifically, activation of telomerase is strongly associated with cancer, and recent observations confirm that telomeres and telomerase perform important roles in both suppressing and facilitating malignant transformation by regulating genomic stability and cell lifespan. In addition, recent evidence suggests that telomerase activation contributes to tumorigenesis independently of its role in maintaining telomere length. Here we review recent developments in our understanding of the relationships among telomeres, telomerase, and cancer.

Telomerase: a potential diagnostic and therapeutic tool in canine oncology.

In recent years there has been considerable interest in telomerase as a target for therapeutic intervention in oncology. This largely stems from the vast number of studies that have demonstrated expression and activity of the enzyme telomerase in the majority of human cancer tissues with little or no activity detectable in normal somatic tissues. These studies have led to an interest in the role of telomerase in cancers associated with domesticated species, in particular tumors that affect dogs. This article reviews the biology of telomerase and the biological significance of telomerase activity in canine tumors and discusses the clinical implications of telomerase expression in canine cancers with regard to therapeutics and diagnostics.

Maria Blasco

Maria Blasco (Fig. 1) received her PhD in 1993 from the Centre of Molecular Biology at the University of Madrid (Autonoma), Spain, where she worked with Margarita Salas on the end-replication problem of a bacteriophage – how terminal proteins help to prevent loss of genetic information arising through incomplete DNA replication. The same year, she moved to Cold Spring Harbor Laboratory (CSHL), New York, to join Carol Greider, who was also working on the end-replication problem, but in eukaryotes. There, she spent three years as a postdoc studying the regulation of mouse telomerase – the enzyme thought to counteract the whittling down of the ends of chromosomes by adding repeat DNA sequences to the telomere. For the past five years, she has been back in Madrid, running her own group and exploring the biology of telomerase in mice at the Dept of Immunology and Oncology at the National Centre of Biotechnology, part of the Spanish Council for Scientific Research.

Telomerase inhibitors as anticancer therapy.

Telomerase inhibitors have been touted as a novel cancer specific therapy, as most tumor cells have high expression of telomerase, whereas most normal somatic cells express low or undetectable levels of telomerase. Continued proliferation of tumor cells requires activation of telomerase to maintain chromosomal stability and extend life span, because telomerase elongates telomere length and rewinds the cellular mitotic clock. Conversely, shortening of telomeres by inhibition of telomerase activity induces growth arrest (senescence) and apoptosis in tumor cells. Moreover, it has been reported that inhibition of telomerase increases the susceptibility of tumor cells to apoptosis induced by anticancer agents. Thus, telomerase inhibitors could be used as an adjuvant with conventional therapy. However, there are also several potential limitations of telomerase inhibition as a therapeutic strategy. For example, there is a lag phase between telomerase inhibition and telomere shortening, with growth arrest and cell death. In this review, we will discuss the basic biology of telomeres and telomerase as a platform for the development of treatments based upon inhibition of telomerase activity.

Prospects for anti-neoplastic therapies based on telomere biology.

The maintenance of specialized nucleoprotein structures at the ends of human chromosomes called telomeres is essential for chromosome stability, and plays a fundamental role in the regulation of cellular lifespan. Without new synthesis of telomeres, chromosome ends shorten with progressive cell division, eventually triggering either replicative senescence or apoptosis when telomere length becomes critically short. The regulation of telomerase activity in human cells plays a significant role in the development of cancer. Telomerase is tightly repressed in the vast majority of normal human somatic cells but becomes activated during cell immortalization and in cancers. Recent work has demonstrated that inhibiting or targeting telomerase shows promise as a novel anti-neoplastic strategy; however, the biology of telomeres and telomerase predict that such approaches will differ in important ways from traditional cytotoxic drug therapies. Understanding telomerase biology may eventually lead to several types of clinically effective, telomerase-based therapies for neoplastic disease.

Telomere Lengths and Telomerase Activity in Dog Tissues: A Potential Model System to Study Human Telomere and Telomerase Biology

Studies on telomere and telomerase biology are fundamental to the understanding of aging and age-related diseases such as cancer. However, human studies have been hindered by differences in telomere biology between humans and the classical murine animal model system. In this paper, we describe basic studies of telomere length and telomerase activity in canine normal and neoplastic tissues and propose the dog as an alternative model system. Briefly, telomere lengths were measured in normal canine peripheral blood mononuclear cells (PBMCs), a range of normal canine tissues, and in a panel of naturally occurring soft tissue tumours by terminal restriction fragment (TRF) analysis. Further, telomerase activity was measured in canine cell lines and multiple canine tissues using a combined polymerase chain reaction/enzyme-linked immunosorbent assay method. TRF analysis in canine PBMCs and tissues demonstrated mean TRF lengths to range between 12 and 23 kbp with heterogeneity in telomere lengths being observed in a range of normal somatic tissues. In soft tissue sarcomas, two subgroups were identified with mean TRFs of 22.2 and 18.2 kbp. Telomerase activity in canine tissue was present in tumour tissue and testis with little or no activity in normal somatic tissues. These results suggest that the dog telomere biology is similar to that in humans and may represent an alternative model system for studying telomere biology and telomerase-targeted anticancer therapies.

Telomerase activation, cellular immortalization and cancer

The maintenance of specialized nucleoprotein structures termed telomeres is essential for chromosome stability. Without new synthesis of telomeres at chromosome ends the chromosomes shorten with progressive cell division, eventually triggering either replicative senescence or apoptosis when telomere length becomes critically short. The regulation of telomerase activity in human cells plays a significant role in the development of cancer. Telomerase is tightly repressed in the vast majority of normal human somatic cells but becomes activated during cellular immortalization and in cancers. While the mechanisms for telomerase activation in cancers have not been fully defined, they include telomerase catalytic subunit gene (hTERT) amplification and trans-activation of the hTERT promoter by the myc oncogene product. Ectopic expression of hTERT is sufficient to restore telomerase activity in cells that lack the enzyme and can immortalize many cell types. Understanding telomerase biology will eventually lead to several clinically relevant telomerase-based therapies. These applications include inhibiting or targeting telomerase as a novel antineoplastic strategy and using cells immortalized by telomerase for therapeutic applications.

Telomerase in Cancer Diagnosis and Therapy

Curing cancers is one of the most challenging tasks of modern medicine. The major problem is the heterogeneity of human tumours and thus finding a 'universal' target for cancer treatment. The discovery that the expression of the enzyme telomerase is a hallmark of immortality and cancer, and that it is found in the majority (>85%) of human tumours but is repressed in most normal cells, has therefore caused considerable excitement. These observations led to the design of potential telomerase inhibitors and ideas about targeting telomerase in the clinic. To date, several classes of telomerase inhibitory agents have been identified and are in preclinical development. However, the approach has not yet been tested clinically. Because of the proposed function of telomerase, and the understanding that replicative cell senescence or cell death result from progressive telomere shortening during successive cell divisions, even complete enzyme inhibition will not produce immediate cell death. Designing clinical trials for promising telomerase inhibitors requires consideration of the novel mechanism of action of these drugs. A lag period between initiation of treatment and occurrence of effects is likely, and thus anti-telomerase therapy might best be given in adjuvant treatment protocols after initial tumour debulking therapy and in combination with other cytostatic agents. The available knowledge of telomerase biology and its association with human tumours suggests that telomerase inhibition might prove a valuable addition to current cancer treatment regimens.