Short dysfunctional telomeres impair tumorigenesis in the INK4a(delta2/3) cancer-prone mouse

Recent advances in cancer research: mouse models of tumorigenesis

Over the past 20 years, cancer research has gained major insights into the complexity of tumor development, in particular into the molecular mechanisms that underlie the progressive transformation of normal cells into highly malignant derivatives. It is estimated that the transformation of a normal cell to a malignant tumor cell is dependent upon a small number of genetic alterations, estimated to be within the range of four to seven rate-limiting events. Critical events in the evolution of neoplastic disease include the loss of proliferative control, the failure to undergo programmed cell death (apoptosis), the onset of neoangiogenesis, tissue remodeling, invasion of tumor cells into surrounding tissue and, finally, metastatic dissemination of tumor cells to distant organs. In patients, the molecular analysis of these multiple steps is hampered by the unavailability of tumor biopsies from all tumor stages. In contrast, mouse models of tumorigenesis allow the reproducible isolation of all tumor stages, including normal tissue, which are then amenable to pathological, genetic and biochemical analyses and, hence, have been instrumental in investigating cancer-related genes and their role in carcinogenesis. In this review, we discuss mouse tumor models that have contributed substantially to the identification and characterization of novel tumor pathways. In particular, we focus on transgenic and knockout mouse models that closely mimic human cancer and thus can be used as model systems for cancer research.

Constitutive telomerase expression promotes mammary carcinomas in aging mice

Telomerase is up-regulated in the vast majority of human cancers and serves to halt the progressive telomere shortening that ultimately blocks would-be cancer cells from achieving a full malignant phenotype. In contrast to humans, the laboratory mouse possesses long telomeres and, even in early generation telomerase-deficient mice, the level of telomere reserve is sufficient to avert telomere-based checkpoint responses and to permit full malignant progression. These features in the mouse provide an opportunity to determine whether enforced high-level telomerase activity can serve functions that extend beyond its ability to sustain telomere length and function. Here, we report the generation and characterization of transgenic mice that express the catalytic subunit of telomerase (mTERT) at high levels in a broad variety of tissues. Expression of mTERT conferred increased telomerase enzymatic activity in several tissues, including mammary gland, splenocytes, and cultured mouse embryonic fibroblasts. In mouse embryonic fibroblasts, mTERT overexpression extended telomere lengths but did not prevent culture-induced replicative arrest, thus reinforcing the view that this phenomenon is not related to occult telomere shortening. Robust telomerase activity, however, was associated with the spontaneous development of mammary intraepithelial neoplasia and invasive mammary carcinomas in a significant proportion of aged females. These data indicate that enforced mTERT expression can promote the development of spontaneous cancers even in the setting of ample telomere reserve.

Identification of telomere-dependent "senescence-like" arrest in mouse embryonic fibroblasts

In contrast to human primary cells, mouse embryonic fibroblasts (MEF) do not show telomere shortening-mediated replicative senescence due to the fact that they have telomerase activity and show sufficiently long telomeres. Instead, it is now generally accepted that the "senescence-like" arrest that occurs in MEF after 5-10 divisions in culture is mediated by telomere-length-independent mechanisms generally referred to as stress. Using telomerase-deficient MEF Terc(-/-), we show here that telomere shortening to a critical length leads to a premature senescence-like arrest in MEF, as well as has a negative effect on spontaneous immortalization. Similarly, elimination of the telomere end-capping protein Ku86 also leads to a premature senescence-like arrest and has a negative effect on spontaneous immortalization. Both Terc(-/-) MEF with short telomeres and Ku86(-/-) MEF show dysfunctional telomeres, as indicated by similarly increased frequencies of end-to-end fusions. These results suggest that loss of telomere function is a general mechanism leading to cell arrest. These observations also indicate that telomere dysfunction is interfering with successful cell division and thus interferes with tumor formation. In summary, we have identified here two different ways to induce a telomere-dependent senescence-like arrest in MEF.

Telomerase recognizes its template by using an adjacent RNA motif

Telomerase adds telomeric repeats to chromosome 3' ends, forestalling the cellular senescence, apoptosis, and genomic instability that result from telomere loss caused by incomplete DNA replication. The telomerase ribonucleoprotein is dedicated to synthesis of tandem, simple-sequence repeats by virtue of its specialization for copying only a specific template region within the integral RNA. Here, using circularly permuted variants of Tetrahymena thermophila telomerase RNA, we identify the features that allow recognition of the template region within the RNA. We engineered a template-less telomerase ribonucleoprotein that can position and reverse transcribe an exchangeable RNA oligonucleotide template accurately. Only a short "template-recognition" element sequence tag is required to direct efficient use of adjacent 5' residues as a template for telomeric repeat synthesis. Our findings reveal molecular requirements for template selection by telomerase and physically resolve templating from other RNA functions in catalysis.

Reconstitution of hTERT restores tumorigenicity in melanoma-derived c-Myc low-expressing clones

c-Myc is involved in the control of telomerase activity through its ability to induce the expression of the catalytic subunit of the enzyme, the human telomerase reverse transcriptase (hTERT). Our aim was to study whether telomerase plays a critical role in c-Myc-dependent tumorigenicity of melanoma cells. By using M14-derived clones, expressing low levels of c-Myc, we demonstrated that the down-regulation of c-Myc reduced cell proliferation rate, cloning efficiency and tumorigenicity and increased the apoptotic rate. Decreased tumorigenic potential correlated with reduced hTERT gene expression, telomerase activity and telomere shortening. Introduction of wild-type hTERT into these cells increased their proliferation rate and partially re-established their tumorigenic potential, at early passages, even though the apoptotic rate of the population remained unaltered. After several in vitro passages, hTERT-mediated cell proliferation made the tumorigenic potential of the c-Myc low-expressing clones comparable to that of the M14 parental line. Over-expression of the mutant biologically inactive hTERT did not drive cells to proliferate. In conclusion, our results demonstrate that the reconstitution of high levels of telomerase activity reverses the low tumorigenicity due to low c-Myc expression.

Mammalian Ku86 mediates chromosomal fusions and apoptosis caused by critically short telomeres

Here we analyze the functional interaction between Ku86 and telomerase at the mammalian telomere by studying mice deficient for both proteins. We show that absence of Ku86 prevents the end-to-end chromosomal fusions that result from critical telomere shortening in telomerase-deficient mice. In addition, Ku86 deficiency rescues the male early germ cell apoptosis triggered by short telomeres in these mice. Together, these findings define a role for Ku86 in mediating chromosomal instability and apoptosis triggered by short telomeres. In addition, we show here that Ku86 deficiency results in telomerase-dependent telomere elongation and in the fusion of random pairs of chromosomes in telomerase-proficient cells, suggesting a model in which Ku86 keeps normal-length telomeres less accessible to telomerase-mediated telomere lengthening and to DNA repair activities.

Telomerase inhibition and the future management of head-and-neck cancer

Telomeres are tandem repeats of DNA associated with specific proteins. These structures cap eukaryotic chromosomes and maintain the integrity of the chromosome ends. In the germline, telomeres are maintained by the enzyme telomerase, but in normal somatic cells the enzyme's activity is low or undetectable. Human tumours, including squamous-cell carcinoma of the head and neck (SCCHN), need telomerase to maintain telomere function; inhibition of the enzyme can lead to apoptosis. Furthermore, because most tumour cells have very short telomeres, they are more likely to succumb to telomerase inhibition than normal cells. Telomerase is therefore a potential selective anticancer target. The telomere is also involved in the repair of DNA double strand breaks, and telomere dysfunction provokes radiosensitivity. In this review we consider whether manipulation of telomere function may selectively sensitise SCCHN to radiotherapy and discuss the possible pitfalls. We also assess how some conventional treatments may affect the subsequent use of telomerase inhibitors.

Natural and pharmacological regulation of telomerase

The extremities of eukaryotic chromosomes are called telomeres. They have a structure unlike the bulk of the chromosome, which allows the cell DNA repair machinery to distinguish them from 'broken' DNA ends. But these specialised structures present a problem when it comes to replicating the DNA. Indeed, telomeric DNA progressively erodes with each round of cell division in cells that do not express telomerase, a specialised reverse transcriptase necessary to fully duplicate the telomeric DNA. Telomerase is expressed in tumour cells but not in most somatic cells and thus telomeres and telomerase may be proposed as attractive targets for the discovery of new anticancer agents.

Many ways to telomere dysfunction: in vivo studies using mouse models

The existence of a capping structure at the extremities of chromosomes was first deduced in the 1930s by Herman Müller (Müller, 1938), who showed that X-irradiation of Drosophila rarely resulted in terminal deletions or inversions of chromosomes, suggesting that chromosome ends have protective structures that distinguish them from broken chromosomes, which he named telomeres. In this review, we will focus on mammalian telomeres and, in particular, on the analysis of different mouse models for proteins that are important for telomere function, such as telomerase and various telomere-binding proteins. These murine models are helping us to understand the consequences of telomere dysfunction for cancer, aging and DNA repair, as well as, the molecular mechanisms by which telomeres exert their protective function.

Telomeres, aging and cancer: in search of a happy ending

Telomeres are distinctive structures, composed of a repetitive DNA sequence and associated proteins, that cap the ends of linear chromosomes. Telomeres are essential for maintaining the integrity and stability of eukaryotic genomes. In addition, under some circumstances, telomeres can influence cellular gene expression. In mammals, the length, structure, and function of telomeres have been proposed to contribute to cellular and organismal phenotypes associated with cancer and aging. Here, we discuss what is known about the basis for the links between telomeres, aging and cancer, and some of the known and proposed consequences of telomere dysfunction and maintenance for mammalian cells and organisms.

Balancing instability: dual roles for telomerase and telomere dysfunction in tumorigenesis

Telomere shortening and telomerase activation both occur in human tumors. Telomere shortening has been proposed to have two conflicting roles in tumorigenesis: tumor suppression and initiation of chromosomal instability. Similarly, while telomerase activation is suggested to be necessary for tumor growth, telomerase may help to stabilize genomic instability. Here we review what is known about these conflicting roles and propose a framework to understand the role of telomerase in cancer progression.

Telomere shortening and cell fates in mouse models of neoplasia

Cell division in the absence of telomerase leads to telomere shortening that can activate checkpoint responses and impair chromosomal stability. The absence of telomerase in primary human cells and its near universal reactivation in human cancers has highlighted the importance of telomere shortening and telomerase reactivation during tumor development. Data from telomerase-deficient mouse models of cancer have indicated that telomere shortening can exert profoundly different influences on cell fates in developing cancers, limiting tumorigenesis by enhancing cell death or facilitating carcinogenesis by compromising chromosomal stability. These alternate fates depend on the integrity of the p53 pathway and on cell type.

A highly selective telomerase inhibitor limiting human cancer cell proliferation

Telomerase, the ribonucleoprotein enzyme maintaining the telomeres of eukaryotic chromosomes, is active in most human cancers and in germline cells but, with few exceptions, not in normal human somatic tissues. Telomere maintenance is essential to the replicative potential of malignant cells and the inhibition of telomerase can lead to telomere shortening and cessation of unrestrained proliferation. We describe novel chemical compounds which selectively inhibit telomerase in vitro and in vivo. Treatment of cancer cells with these inhibitors leads to progressive telomere shortening, with no acute cytotoxicity, but a proliferation arrest after a characteristic lag period with hallmarks of senescence, including morphological, mitotic and chromosomal aberrations and altered patterns of gene expression. Telomerase inhibition and telomere shortening also result in a marked reduction of the tumorigenic potential of drug-treated tumour cells in a mouse xenograft model. This model was also used to demonstrate in vivo efficacy with no adverse side effects and uncomplicated oral administration of the inhibitor. These findings indicate that potent and selective, non-nucleosidic telomerase inhibitors can be designed as novel cancer treatment modalities.

Cell senescence and cancer

Historically, the senescent state has been associated with, and was named after, the cell-cycle arrest that occurs after cells have undergone an intrinsically defined number of divisions in vitro. More recently, however, it has been shown that extrinsic factors, including those encountered in normal tissue-culture environments, can prematurely induce an indistinguishable senescent phenotype. In this review, we discuss the pathways of cell senescence, the mechanisms involved and the role that these pathways have in regulating the initiation and progression of cancer.

Impaired nonhomologous end-joining provokes soft tissue sarcomas harboring chromosomal translocations, amplifications, and deletions

Although nonhomologous end-joining (NHEJ) deficiency has been shown to accelerate lymphoma formation in mice, its role in suppressing tumors in cells that do not undergo V(D)J recombination is unclear. Utilizing a tumor-prone mouse strain (ink4a/arf(-/-)), we examined the impact of haploinsufficiency of a NHEJ component, DNA ligase IV (Lig4), on murine tumorigenesis. We demonstrate that lig4 heterozygosity promotes the development of soft-tissue sarcomas that possess clonal amplifications, deletions, and translocations. That these genomic alterations are relevant in tumorigenesis is supported by the finding of frequent mdm2 amplification, a known oncogene in human sarcoma. Together, these findings support the view that loss of a single lig4 allele results in NHEJ activity being sufficiently reduced to engender chromosomal aberrations that drive non-lymphoid tumorigenesis.

Arsenic inhibition of telomerase transcription leads to genetic instability

Arsenic is effective in the treatment of acute promyelocytic leukemia. Paradoxically, it is also carcinogenic. In the process of elucidating a mechanism of arsenic resistance in a leukemia cell line, NB4, we discovered that arsenic exposure causes chromosomal abnormalities, with a preponderance of end-to-end fusions. These chromosomal end fusions suggested that telomerase activity may be inhibited by arsenic. We found that arsenic inhibits transcription of the hTERT gene, which encodes the reverse transcriptase subunit of human telomerase. This effect may in part be explained by decreased c-Myc and Sp1 transcription factor activities. Decreased telomerase activity leads to chromosomal end lesions, which promote either genomic instability and carcinogenesis or cancer cell death. These phenomena may explain the seemingly paradoxical carcinogenic and antitumor effects of arsenic.

Telomerase and cancer: time to move from a promising target to a clinical reality

The past 25 years have seen unparalleled advances in our understanding of the molecular basis of cancer. As a result, novel molecular targets have been identified that provide great potential for the development of new cancer diagnostics and therapies. Four key features of cancer cells distinguish them from their normal counterparts: loss of cell-cycle regulation, loss of control over invasion and metastasis, failure of apoptotic mechanisms, and bypass of senescence. This review examines our understanding of the bypass of senescence and the process of immortalization during carcinogenesis. In addition, the realistic opportunities for telomerase in cancer diagnostics and the challenges faced in clinical trial design for telomerase therapeutics are discussed.

Cellular senescence, cancer and aging: the telomere connection

Telomeres are the repetitive DNA sequences and specialized proteins that form the distinctive structure that caps the ends of linear chromosomes. Telomeres allow cells to distinguish the chromosome ends from double strand DNA breaks. The telomeric structure prevents the degradation or fusion of chromosome ends, and thus is essential for maintaining the integrity and stability of eukaryotic genomes. In addition, and perhaps less widely appreciated, telomeres may also indirectly influence gene expression. The length, structure and organization of telomeres are regulated by a host of telomere-associated proteins, and can be influenced by basic cellular processes such as cell proliferation, differentiation, and DNA damage. In mammalian cells, telomere length and/or telomere structure have been linked to both cancer and aging. Here, we briefly review what is known about mammalian telomeres and the proteins that associate with them, and discuss the cellular and organismal consequences of telomere dysfunction and the evidence that cells with dysfunctional telomeres can contribute to cancer and aging phenotypes.

Telomerase and cancer

The past few years have brought a flood of new information to the telomerase field. The identification of multiple components of both the telomere and telomerase, the understanding of the importance of telomere maintenance to the long term viability of cells, and the demonstration of the utility of telomerase inhibition in limiting tumor cell growth all convene to provide great enthusiasm for the prospects of targeting the telomerase enzyme in cancer. However, there is clearly much to be learned. Because tumor cells evolve under powerful selection, the emergence of non-telomerase based mechanisms for telomere maintenance should be examined closely. Additionally, the nature of telomerase regulation is currently only poorly understood. More work on the tumor specific regulation of telomerase activity might provide either more opportunities for telomerase inhibition, or more skepticism, as a tumor cell might possess mechanisms for upregulating telomerase activity in the presence of inhibitors. The potential for such regulation has already been observed in certain cell types (46). Currently, the field is intensively investigating the biology and applications of telomere and telomerase biology. In it are great hopes that these fundamental cellular processes might be manipulated to success in the treatment of cancer.

Restoration of telomerase activity rescues chromosomal instability and premature aging in Terc-/- mice with short telomeres

Reconstitution of telomerase activity is proposed as a potential gene therapy to prevent, or rescue, age-related diseases produced by critical telomere shortening. However, it is not known whether or not short telomeres are irreversibly damaged. We addressed this by re-introducing telomerase in late generation telomerase-deficient mice, Terc-/-, which have short telomeres and show severe proliferative defects. For this, we have crossed these mice with Terc+/- mice and analyzed telomere length, chromosomal instability and premature aging of the progeny. The Terc-/- progeny had one set of chromosomes with normal telomeres, whereas the other set remained with critically short telomeres; these mice presented chromosomal instability and premature aging. In contrast, Terc+/- progeny showed all chromosomes with detectable telomeres, and did not show chromosomal instability or premature aging. These results prove that critically short telomeres can be rescued by telomerase, and become fully functional, thus rescuing premature aging. This has important implications for the future design of telomerase-based gene therapy of age-related diseases.

Mutant and genetically modified mice as models for studying the relationship between aging and carcinogenesis

Increased interest is emerging in using mouse models to assess the genetics of aging and age-related diseases, including cancer. However, only limited information is available regarding the relationship between aging and spontaneous tumor development in genetically modified mice. Analysis of various transgenic and knockout rodent models with either a shortened or an extended life span, provides a unique opportunity to evaluate interactions of genes involved in the aging process and carcinogenesis. There are only a few models which show life span extension. Ames dwarf mutant mice, p66(-/-) knockout mice, alpha MUPA and MGMT transgenic mice live longer than wild-type strains. The incidence of spontaneous tumors in these mutant mice was usually similar to those in controls, whereas the latent period of tumor development was increased. Practically all models of accelerated aging showed increased incidence and shorter latency of tumors. This phenomenon has been observed in animals which display a phenotype that more closely resembles natural aging, and in animals which manifest only some features of the normal aging process. These observations are in agreement with an earlier established positive correlation between tumor incidence and the rate of tumor incidence increase associated with aging and the aging rate in a population. Thus, genetically modified animals are a valuable tool in unravelling mechanisms underlying aging and cancer. Systemic evaluation of newly generated models should include onco-gerontological studies.

Role of the intermediate filament protein vimentin in delaying senescence and in the spontaneous immortalization of mouse embryo fibroblasts

Because knockout of the vimentin gene in mice did not produce an immediately obvious, overt, or lethal specific phenotype, the conjecture was made that the mutation affects some subtle cellular functions whose loss manifests itself only when the mutant animals are exposed to stress. In order to substantiate this idea in a tractable in vitro system, primary embryo fibroblasts from wildtype (V(+/+)) and vimentin-knockout (V(-/-)) mice were compared with regard to their growth behavior under the pseudophysiologic conditions of conventional cell culture. Whereas in the course of serial transfer, the V(+/+) fibroblasts progressively reduced their growth potential, passed through a growth minimum around passage 12 (crisis), and, as immortalized cells, resumed faster growth, the V(-/-) fibroblasts also cut down their growth rate but much earlier, and they either did not immortalize or did so at an almost undetectable rate. Cells withdrawing from the cell cycle showed increased concentrations of reactive oxygen species and signs of oxidative damage: enlarged and flattened morphology, large nuclear volume, reinforced stress fiber system as a result of increased contents of actin and associated proteins, prominent extracellular matrix, and perinuclear masses of pathological forms of mitochondria with low membrane potential. The differences in the cell cycle behavior of the V(+/+) and V(-/-) cells in conjunction with the morphologic changes observed in mitotically arrested cells suggests a protective function of vimentin against oxidative cell damage. Because vimentin exhibits affinity for and forms crosslinkage products with recombinogenic nuclear as well as mitochondrial DNA in intact cells, it is credible to postulate that vimentin plays a role in the recombinogenic repair of oxidative damage inflicted on the nuclear and mitochondrial genome throughout the cells' replicative lifespan. Recombinational events mediated by vimentin also appear to take place when the cells pass through the genetically unstable state of crisis to attain immortality. The residual immortalization potential of V(-/-) fibroblasts might be attributable to their capacity to synthesize, in place of vimentin, the tetrameric form of a lacZ fusion protein carrying, in addition to a nuclear localization signal, the N-terminal 59 amino acids of vimentin and thus its DNA-binding site. On the basis of these results and considerations, a major biologic role of vimentin may be to protect animals during development and postnatal life against genetic damage and, because of its contribution to the plasticity of the genome, to allow them to respond to environmental challenges.

Mechanisms of telomerase induction during vascular smooth muscle cell proliferation

Telomeres are primarily controlled by a highly specialized DNA polymerase termed telomerase. Recent studies have demonstrated that introduction of the telomerase catalytic component (TERT) into telomerase-negative cells activates telomerase and extends cell life span, whereas mice lacking telomerase activity revealed impaired cell proliferation in some organs as well as reduced tumorigenesis. These reports suggest that telomerase plays an important role in long-term cell viability and cell proliferation. However, the mechanism or mechanisms by which telomerase is induced or regulated remains to be elucidated. We report here that primary vascular smooth muscle cells (VSMCs) express telomerase and that increased telomerase activity correlates with cell proliferation. Inhibition of telomerase diminished growth of VSMCs, which suggests a crucial role for telomerase activation in the regulation of VSMC proliferation. We propose a novel model whereby telomerase is first activated in the cytoplasm before cell proliferation, followed by accumulation of activity in the nucleus during the logarithmic phase of cell growth. Activation of telomerase in VSMCs was linked to phosphorylation of TERT. The protein kinase inhibitor H7 suppressed the activation of telomerase in the cytoplasm and also inhibited the accumulation of TERT as well as telomerase activity in the nucleus. These data suggest that posttranslational modification of TERT by phosphorylation is important for activation and accumulation of telomerase into the nucleus in the process of VSMC proliferation.

Overall survival in aggressive B-cell lymphomas is dependent on the accumulation of alterations in p53, p16, and p27

Different studies have already shown that the isolated inactivation of p21, p16, or p27 cyclin-dependent kinase inhibitors (CKIs) is associated with increased growth fraction, tumor progression, or decreased overall survival in cases of non-Hodgkin's lymphoma. In this study we linked molecular study of the p53 and p16 genes with immunohistochemical analysis of p27 expression in a group of aggressive B-cell lymphomas [large B-cell lymphomas (LBCLs) and Burkitt's lymphomas]. This was done to analyze the relationship between p53 and p16 silencing, p27 anomalous overexpression, and clinical follow-up, testing the hypothesis that the accumulation of CKI alterations could confer to the tumors a higher aggressivity. In a group of 62 patients, p53 inactivation as a result of mutation was observed in 11 cases (18%) and p16 silencing was seen in 27 cases (43.5%) as a result of methylation (20 of 62), 9p21 deletion (7 of 44), or p16 mutation (2 of 62). The simultaneous inactivation of p53 and p16 was detected exclusively in five LBCL cases. Anomalous expression of p27, which has been proven to be associated with the absence of p27/CDK2 complexes and the formation of p27/cyclin D3 complexes where p27 is inactivated, was detected in 19 of 61 cases (31%). Cases characterized by p27 anomalous expression display concurrent inactivation of p21 (provided by p53 mutations) and/or p16 CKIs in 11 of 14 LBCL cases (P = 0.040). When the relationship between the association of inactivated CKIs and overall survival was considered, a significant relationship was found between a lower overall survival probability and an increased number of inactivated CKIs in LBCL cases, with the worst prognosis for the cases displaying concurrent p53, p16, and p27 alterations. This proves that simultaneous inactivation of different tumor suppressor pathways does indeed take place, and that tumor aggressiveness takes advantage of this CKI-concerted silencing. In this same series of data, Burkitt's lymphoma patients seem to behave in a different way than LBCLs, with p53 and p16 alteration being mutually exclusive and the association with p27 anomalous expression not being clinically significant. These facts seem to support that the additive effect of the inactivation of different CKIs could be dependent of the histological type.

Telomere dysfunction triggers developmentally regulated germ cell apoptosis

Telomere dysfunction results in fertility defects in a number of organisms. Although data from fission yeast and Caenorhabditis elegans suggests that telomere dysfunction manifests itself primarily as defects in proper meiotic chromosome segregation, it is unclear how mammalian telomere dysfunction results in germ cell death. To investigate the specific effects of telomere dysfunction on mammalian germ cell development, we examined the meiotic progression and germ cell apoptosis in late generation telomerase null mice. Our results indicate that chromosome asynapsis and missegregation are not the cause of infertility in mice with shortened telomeres. Rather, telomere dysfunction is recognized at the onset of meiosis, and cells with telomeric defects are removed from the germ cell precursor pool. This germ cell telomere surveillance may be an important mechanism to protect against the transmission of dysfunctional telomeres and chromosomal abnormalities.

Modeling chromosomal instability and epithelial carcinogenesis in the telomerase-deficient mouse

Human carcinomas are intimately linked to advancing age. These cancers have complex cytogenetic profiles, including aneuploidy and chromosomal structural aberrations. While aged humans sustain a high rate of carcinomas, mice bearing common tumor suppressor gene mutations typically develop soft tissue sarcomas and lymphomas. One marked species distinction between human and mouse that bears on the predisposition to carcinogenesis lies in the radical differences in length and regulation of the telomere, nucleoprotein complexes that cap the ends of eukaryotic chromosomes. Recent cancer modeling studies in the telomerase knockout p53 mutant mice revealed that telomere dynamics might be relevant to carcinogenesis. In these mice, there is a shift in the tumor spectrum towards epithelial carcinomas, and these cancers emerge with complex cytogenetic profiles classical for human carcinomas. In this review, we suggest that the mechanism of fusion-bridge-breakage-translocation, triggered by critically short telomeres, may be one of the generators of genomic instability commonly seen in human carcinomas.

Telomere dysfunction and evolution of intestinal carcinoma in mice and humans

Telomerase activation is a common feature of advanced human cancers and facilitates the malignant transformation of cultured human cells and in mice. These experimental observations are in accord with the presence of robust telomerase activity in more advanced stages of human colorectal carcinogenesis. However, the occurrence of colon carcinomas in telomerase RNA (Terc)-null, p53-mutant mice has revealed complex interactions between telomere dynamics, checkpoint responses and carcinogenesis. We therefore sought to determine whether telomere dysfunction exerts differential effects on cancer initiation versus progression of mouse and human intestinal neoplasia. In successive generations of ApcMin Terc-/- mice, progressive telomere dysfunction led to an increase in initiated lesions (microscopic adenomas), yet a significant decline in the multiplicity and size of macroscopic adenomas. That telomere dysfunction also contributes to human colorectal carcinogenesis is supported by the appearance of anaphase bridges (a correlate of telomere dysfunction) at the adenoma-early carcinoma transition, a transition recognized for marked chromosomal instability. Together, these data are consistent with a model in which telomere dysfunction promotes the chromosomal instability that drives early carcinogenesis, while telomerase activation restores genomic stability to a level permissive for tumor progression. We propose that early and transient telomere dysfunction is a major mechanism underlying chromosomal instability of human cancer.

How long should telomeres be?

What began as a study of the "end-replication problem" took on a new dimension as it became clear that telomeres are a "molecular clock" of replication in human somatic cells. Here we review the biology of telomeres in vitro and in vivo, in mice and humans. We suggest that, in humans, telomeres are involved in the biology of aging and pathobiology of disorders of aging, including cancer and cardiovascular disease. We also propose that the underlying dynamics of telomere biology is in line with broad principles of evolutionary theories.

Deficiency of colonic telomerase in ulcerative colitis

OBJECTIVE

GI epithelial cells express telomerase, a ribonucleoprotein that prevents telomeric shortening in proliferating cells. Telomerase levels are high in cancer, but little is known about telomerase expression in other diseases. We, therefore, designed experiments to determine telomerase expression in different colonic segments and to compare this with corresponding segments in patients with ulcerative colitis. Colorectal cancers and adenomatous polyps were included as disease controls.

METHODS

In total, telomerase expression was determined in colonic tissues obtained from 62 patients. Twenty-five patients had ulcerative colitis, 21 had normal colons, 11 had colorectal cancer, and nine had adenomatous polyps. Endoscopic biopsies were collected prospectively at colonoscopy, processed for telomerase assays (Telomeric Repeat Amplification Protocol), hematoxylin and eosin staining, and scored for inflammation.

RESULTS

Telomerase activity is expressed in arbitrary units (median 95% confidence interval). In the normal colon, telomerase activity in the cecum, transverse, sigmoid, and rectum was 255 (171-449), 707 (374-895), 561 (468-1426), and 563 (402-846), respectively. Telomerase was higher in the distal three segments when compared with the cecum (p = 0.005). In ulcerative colitis, there was a marked decrease in telomerase activity in the cecum 152 (59-272), p = 0.04, transverse 180 (129-365), p < 0.001, sigmoid 352 (114-464), p = 0.005, and rectum 180 (70-337), p = 0.001 when compared with normals. Telomerase activity correlated negatively with inflammation (r = -0.32, p = 0.001) and was also decreased in microscopically normal areas. Cancers expressed high levels of telomerase.

CONCLUSIONS

Colonic mucosal expression of telomerase is reduced in ulcerative colitis. Levels are low even in microscopically normal mucosa, suggesting that telomerase deficiency may contribute to the pathogenesis of the disease.

Telomere dysfunction alters the chemotherapeutic profile of transformed cells

Telomerase inhibition has been touted as a novel cancer-selective therapeutic goal based on the observation of high telomerase levels in most cancers and the importance of telomere maintenance in long-term cellular growth and survival. Here, the impact of telomere dysfunction on chemotherapeutic responses was assessed in normal and neoplastic cells derived from telomerase RNA null (mTERC(-/-)) mice. Telomere dysfunction, rather than telomerase per se, was found to be the principal determinant governing chemosensitivity specifically to agents that induced double-stranded DNA breaks (DSB). Enhanced chemosensitivity in telomere dysfunctional cells was linked to therapy-induced fragmentation and multichromosomal fusions, whereas telomerase reconstitution restored genomic integrity and chemoresistance. Loss of p53 function muted the cytotoxic effects of DSB-inducing agents in cells with telomere dysfunction. Together, these results point to the combined use of DSB-inducing agents and telomere maintenance inhibition as an effective anticancer therapeutic approach particularly in cells with intact p53-dependent checkpoint responses.

The Myc/Max/Mad network and the transcriptional control of cell behavior

The Myc/Max/Mad network comprises a group of transcription factors whose distinct interactions result in gene-specific transcriptional activation or repression. A great deal of research indicates that the functions of the network play roles in cell proliferation, differentiation, and death. In this review we focus on the Myc and Mad protein families and attempt to relate their biological functions to their transcriptional activities and gene targets. Both Myc and Mad, as well as the more recently described Mnt and Mga proteins, form heterodimers with Max, permitting binding to specific DNA sequences. These DNA-bound heterodimers recruit coactivator or corepressor complexes that generate alterations in chromatin structure, which in turn modulate transcription. Initial identification of target genes suggests that the network regulates genes involved in the cell cycle, growth, life span, and morphology. Because Myc and Mad proteins are expressed in response to diverse signaling pathways, the network can be viewed as a functional module which acts to convert environmental signals into specific gene-regulatory programs.

Disruption of the Cockayne syndrome B gene impairs spontaneous tumorigenesis in cancer-predisposed Ink4a/ARF knockout mice

Cells isolated from individuals with Cockayne syndrome (CS) have a defect in transcription-coupled DNA repair, which rapidly corrects certain DNA lesions located on the transcribed strand of active genes. Despite this DNA repair defect, individuals with CS group A (CSA) or group B (CSB) do not exhibit an increased spontaneous or UV-induced cancer rate. In order to investigate the effect of CSB deficiency on spontaneous carcinogenesis, we crossed CSB(-/-) mice with cancer-prone mice lacking the p16(Ink4a)/p19(ARF) tumor suppressor locus. CSB(-/-) mice are sensitive to UV-induced skin cancer but show no increased rate of spontaneous cancer. CSB(-/-) Ink4a/ARF(-/-) mice developed 60% fewer tumors than Ink4a/ARF(-/-) animals and demonstrated a longer tumor-free latency time (260 versus 150 days). Moreover, CSB(-/-) Ink4a/ARF(-/-) mouse embryo fibroblasts (MEFs) exhibited a lower colony formation rate after low-density seeding, a lower rate of H-Ras-induced transformation, slower proliferation, and a lower mRNA synthesis rate than Ink4a/ARF(-/-) MEFs. CSB(-/-) Ink4a/ARF(-/-) MEFs were also more sensitive to UV-induced p53 induction and UV-induced apoptosis than were Ink4a/ARF(-/-) MEFs. In order to investigate whether the apparent antineoplastic effect of CSB gene disruption was caused by sensitization to genotoxin-induced (p53-mediated) apoptosis or by p53-independent sequelae, we also generated p53(-/-) and CSB(-/-) p53(-/-) MEFs. The CSB(-/-) p53(-/-) MEFs demonstrated lower colony formation efficiency, a lower proliferation rate, a lower mRNA synthesis rate, and a higher rate of UV-induced cell death than p53(-/-) MEFs. Collectively, these results indicate that the antineoplastic effect of CSB gene disruption is at least partially p53 independent; it may result from impaired transcription or from apoptosis secondary to environmental or endogenous DNA damage.

Telomerase: biology and phase I trials

In normal somatic cells, the ends of chromosomes (the telomeres) shorten with each cell division. By contrast, in tumour cells, telomere length is maintained, generally through the reactivation of the reverse transcriptase enzyme, telomerase. At least three applications relating to telomeres and telomerase have been proposed: in cancer diagnosis and prognosis (especially through measurements of the catalytic component of telomerase, hTERT) and as a means of monitoring tumour response to therapy; as an aid to tissue engineering; and inhibition as a cancer therapeutic strategy. Mouse knockout, hTERT dominant negative, and antisense experiments suggest that telomerase inhibitors will confer anticancer activity, especially in tumours with short telomeres. Inhibitory strategies have focused on antisense molecules, inhibitors of reverse transcriptases, and small molecules able to interact with and stabilise four-stranded (G-quadruplex) structures formed by telomeres. Clinical trials involving telomerase inhibitors require careful consideration compared to those looking at conventional anticancer cytotoxic drugs.

Telomerase activation cooperates with inactivation of p16 in early head and neck tumorigenesis

Alteration of the p16/pRb pathway may cooperate with telomerase activation during cellular immortalization and tumour progression. We studied p16 expression status by immunohistochemistry and telomerase activity using the TRAP assay in 21 premalignant lesions of the head and neck epithelium as well as 27 squamous-cell carcinomas. We also examined expression of other components of the pathway (cyclin D1 and pRb) as well as presence of human papillomavirus genomes which can target these molecules. 4 of 9 mild dysplastic lesions (44%), 8 of 12 moderate/severe dysplastic lesions (67%), and 25 of 27 squamous-cell carcinomas (92%) demonstrated high telomerase activity (P = 0.009). There was a parallel increase with severity of lesions for the trend in proportions of cases demonstrating p16 inactivation or cyclin D1 overexpression (P = 0.02 and P = 0.01, respectively). For Ki67, a marker of cell proliferation, this trend was not significant (P = 0.08). Human papillomavirus infection was only found in 4 cases among the 48 samples tested (8.3%). In conclusion, progression of disease is accompanied by a parallel and continuous increase in telomerase activity and alterations in cell cycle regulators (p16, cyclin D1), as proposed by in vitro models.

Translating basic aging research into geriatric health care

Aging processes are amenable to molecular genetic analyses. Two aspects of such research have been selected for discussion in this paper because of current great interest and their relevance to human aging. Studies on telomeres have revealed new insights on the control of cellular replicative senescence and provided a means to extend the cell's life span during in vitro cultivation. Emerging studies on genetic biomarkers have identified genes that appear to be associated with longevity or with risk factors for aging-related diseases, and raised considerations of ways to reduce disease expression. An interchange between basic scientists and clinicians would encourage new thoughts on the feasibility of translating these fundamental studies into interventions that promote healthier longevity.

Tumour immunotherapy: new tools, new treatment modalities and new T-cell antigens

Tumour immunology has seen many exciting developments in the last few years. In addition to tumour antigens that are defined by antitumour T- and B-cell responses in patients, the human telomerase reverse transcriptase has been identified by 'reverse immunology' as the first truly universal tumour antigen. Molecular remission has been associated with a cancer vaccine that targets the clonal idiotype of B-cell malignancies, and sophisticated cellular vaccines (including fusions of tumour cells and antigen-presenting cells) have demonstrated promising results. Moreover, our capabilities of measuring immunity have been significantly enhanced by novel technology, such as major histocompatibility complex (MHC)-peptide tetramers and ELISPOT analysis. We are now capable of tracking antigen-specific T cells at a single cell level. This review will analyse recent developments and highlight some important issues that need to be addressed in the future.

Telomere maintenance mechanisms as a target for drug development

The shortening of the telomeric DNA sequences at the ends of chromosomes is thought to play a critical role in regulating the lifespan of human cells. Since all dividing cells are subject to the loss of telomeric sequences, cells with long proliferative lifespans need mechanisms to maintain telomere integrity. It appears that the activation of the enzyme telomerase is the major mechanism by which these cells maintain their telomeres. The proposal that a critical step in the process of the malignant transformation of cells is the upregulation of expression of telomerase has made this enzyme a potentially useful prognostic and diagnostic marker for cancer, as well as a new target for therapeutic intervention for the treatment of patients with cancer. It is now clear that simply inhibiting telomerase may not result in the anticancer effects that were originally hypothesized. While telomerase may not be the universal target for cancer therapy, we certainly believe that targeting the telomere maintenance mechanisms will be important in future research aimed toward a successful strategy for curing cancer.

Recognition of 2'-deoxy-l-ribonucleoside 5'-triphosphates by human telomerase

Telomerase is classified as one of the reverse transcriptases (RTs). To clarify whether l-enantiomers of natural 2'-deoxyribonucleoside 5'-triphosphates (dNTPs) are recognized by human telomerase, a quantitative telomerase assay based on the "stretch PCR" method was developed and used for kinetic analysis of the inhibitory effects of these compounds on the enzyme. Among the four l-enantiomers of dNTPs, l-dTTP and l-dGTP inhibited telomerase activity and the others showed slight or no inhibitory effect. Lineweaver-Burk plot analysis showed that the inhibition modes of l-dTTP and l-dGTP were partially competitive (mixed type) and competitive with the corresponding substrate dNTP, respectively. However, the K(i) values of l-dTTP and l-dGTP (21 and 15 microM) were several times larger than the K(m) values (3-6 microM). These results suggest that the active site of telomerase is not able to discriminate strictly the chirality of dNTPs, although it is more discriminatory than HIV-1 RT.

Telomerase and human tumorigenesis

Human cancer cells, unlike their normal counterparts, have shed the molecular restraints to limited cell growth and are immortal. Exactly how cancer cells manage this at the molecular level is beginning to be understood. Human cells must overcome two barriers to cellular proliferation. The first barrier, referred to as senescence, minimally involves the p53 and Rb tumor-suppressor pathways. Inactivation of these pathways results in some extension of lifespan. However, inactivation of these pathways is insufficient for immortalization. As normal cells undergo repeated rounds of DNA replication, their telomeres shorten due to the inability of traditional DNA polymerases to completely replicate the end of the chromosomal DNA. This shortening continues until the cells reach a second proliferative block referred to as crisis, which is characterized by chromosomal instability, end-to-end fusions, and cell death. Stabilization of the telomeric DNA through either telomerase activation or the activation of the alternative mechanism of telomere maintenance (ALT) is essential if the cells are to survive and proliferate indefinitely. Conversely, loss of telomere stabilization by an already-immortalized cell results in loss of immortality and cell death. Together this indicates that telomere maintenance is a critical component of immortality. In this review we attempt to describe our current understanding of the role of telomere maintenance in senescence, crisis, and tumorigenesis.

The age of cancer

A striking link exists between advanced age and increased incidence of cancer. Here I review how several of the age-related molecular and physiological changes might act in concert to promote cancer, and in particular epithelial carcinogenesis. Experimental data indicate that the aged, cancer-prone phenotype might represent the combined pathogenetic effects of mutation load, epigenetic regulation, telomere dysfunction and altered stromal milieu. Further verification of the role of these effects should in turn lead to the design of effective therapeutics for the treatment and prevention of cancer in the aged.

Telomerase-deficient mice with short telomeres are resistant to skin tumorigenesis

Inhibition of telomerase is proposed to limit the growth of cancer cells by triggering telomere shortening and cell death. Telomere maintenance by telomerase is sufficient, in some cell types, to allow immortal growth. Telomerase has been shown to cooperate with oncogenes in transforming cultured primary human cells into neoplastic cells, suggesting that telomerase activation contributes to malignant transformation. Moreover, telomerase inhibition in human tumour cell lines using dominant-negative versions of TERT leads to telomere shortening and cell death. These findings have led to the proposition that telomerase inhibition may result in cessation of tumour growth. The absence of telomerase from most normal cells supports the potential efficacy of anti-telomerase drugs for tumour therapy, as its inhibition is unlikely to have toxic effects. Mice deficient for Terc RNA (encoding telomerase) lack telomerase activity, and constitute a model for evaluating the role of telomerase and telomeres in tumourigenesis. Late-generation Terc-/- mice show defects in proliferative tissues and a moderate increase in the incidence of spontaneous tumours in highly proliferative cell types (lymphomas, teratocarcinomas). The appearance of these tumours is thought to be a consequence of chromosomal instability in these mice. These observations have challenged the expected effectiveness of anti-telomerase-based cancer therapies. Different cell types may nonetheless vary in their sensitivity to the chromosomal instability produced by telomere loss or to the activation of telomere-rescue mechanisms. Here we show that late-generation Terc-/- mice, which have short telomeres and are telomerase-deficient, are resistant to tumour development in multi-stage skin carcinogenesis. Our results predict that an anti-telomerase-based tumour therapy may be effective in epithelial tumours.

Mice without telomerase: what can they teach us about human cancer?

Unicellular organisms, human cells and mice have provided insights into the processes of senescence, crisis, genomic instability and cancer in humans. Here, Artandi and DePinho discuss how studies in mice have uncovered a complex interplay between the ARF-p53 pathway, genomic instability due to telomere dysfunction, and the suppression or promotion of cancer.

Telomere dynamics in cancer progression and prevention: fundamental differences in human and mouse telomere biology

Cells from the telomerase knockout mouse immortalize with an approximately ten million-fold greater frequency than human cells. In this commentary, Wright and Shay discuss the implications of this difference between mice and men and its relationship to cancer.

The role of telomerase expression and telomere length maintenance in human and mouse

The molecular regulation of telomere length has been well elucidated by a series of elegant studies over the past decade. More recently, experimental evidence has accrued that addresses the challenging question of if and how telomere length regulation may contribute to normal human aging or to human disease. Recent studies in mice have provided a mammalian precedent indicating that telomerase deficiency can lead to in vivo dysfunction, most probably as a consequence of progressive telomere shortening. In humans, the evidence that telomere shortening might lead to in vivo dysfunction is far less direct, although the recent description of telomerase deficiency and telomere shortening associated with the DKC syndrome is suggestive of such a link. Methodologies exist and continue to be developed that are increasingly capable of manipulating telomerase activity and telomere length in human cells. It remains to be determined whether scientifically rigorous and (equally important) medically ethical approaches will emerge to directly assess the ability of telomere length modulation to correct functional disorders of human cellular function ex vivo or more challenging still, in vivo.

Role of telomerase in normal and cancer cells

Shortening of the telomeric DNA at chromosome ends is postulated to limit the lifespan of human cells. In contrast, activation of telomerase, the enzyme that synthesizes telomeric DNA, is proposed to be an essential step in cancer cell immortalization and cancer progression. This review discusses the structure and function of telomeres and telomerase, the role of telomerase in cell immortalization, and the effects of telomerase inactivation on normal and cancer cells. Moreover, data on the experimental use of telomerase assays for cancer detection and diagnosis are reviewed. Finally, the review considers the evidence regarding whether telomerase inhibitors could be used to treat human cancers.

Mammalian telomeres and telomerase

New features of mammalian telomeres and telomerase have been identified. Telomeres form t-loops, which engage the 3' single-stranded DNA overhang in an interaction with double-stranded telomeric repeats. Mammalian telomerases contain an RNA H/ACA motif and associated protein(s) shared with H/ACA family of small nucleolar ribonucleoproteins. Essential roles for telomerase in the sustained viability of cultured tumor cells and in the normal proliferative capacity of human somatic cells have been demonstrated.