Enforced telomere elongation increases the sensitivity of human tumour cells to ionizing radiation

More than 85% of all human cancers possess the ability to maintain chromosome ends, or telomeres, by virtue of telomerase activity. Loss of functional telomeres is incompatible with survival, and telomerase inhibition has been established in several model systems to be a tractable target for cancer therapy. As human tumour cells typically maintain short equilibrium telomere lengths, we wondered if enforced telomere elongation would positively or negatively impact cell survival. We found that telomere elongation beyond a certain length significantly decreased cell clonogenic survival after gamma irradiation. Susceptibility to irradiation was dosage-dependent and increased at telomere lengths exceeding 17 kbp despite the fact that all chromosome ends retained telomeric DNA. These data suggest that an optimal telomere length may promote human cancer cell survival in the presence of genotoxic stress.     

Neural stem cells, tumour stem cells and brain tumours: dangerous relationships?

Neural stem cells (NSC) have been implicated not only in brain development and neurogenesis but also in tumourigenesis. Brain tumour stem cells (BTSC) have been isolated from several paediatric or adult human brain tumours, however their origin is still disputed. This review discusses the normal role of NSC in the adult mammalian brain and their anatomical location. It compares the molecular characteristics and the biological behaviour of NSC/BTSC, and describes the molecular pathways involved in controlling self-renewal and maintenance of adult NSC/BTSC and brain tumour development. It also assesses the current hypotheses about the origin of BTSC and the clinical consequences.     

Tissue context-activated telomerase in human epidermis correlates with little age-dependent telomere loss

Telomerase- and telomere length regulation in normal human tissues is still poorly understood. We show here that telomerase is expressed in the epidermis in situ independent of age but was repressed upon the passaging of keratinocytes in monolayer culture. However, when keratinocytes were grown in organotypic cultures (OTCs), telomerase was re-established, indicating that telomerase activity is not merely proliferation-associated but is regulated in a tissue context-dependent manner in human keratinocytes. While not inducible by growth factors, treatment with the histone deacetylation inhibitor FK228 restored telomerase activity in keratinocytes grown in monolayer cultures. Accordingly, CHIP analyses demonstrated an acetylated, active hTERT promoter in the epidermis in situ and in the epidermis of OTCs but a deacetylated, silenced hTERT promoter with subsequent propagation in monolayer culture suggesting that histone acetylation is part of the regulatory program to guarantee hTERT expression/telomerase activity in the epidermis. In agreement with the loss of telomerase activity, telomeres shortened during continuous propagation in monolayer culture by an average of ～ 70 base pairs (bp) per population doubling (pd). However, telomere erosion varied strongly between different keratinocyte strains and even between individual cells within the same culture, thereby arguing against a defined rate of telomere loss per replication cycle. In the epidermis in situ, as determined from early-passage keratinocytes and tissue sections from different age donors, we calculated a telomere loss of only ～ 25 bp per year. Since we determined the same rate for the non-regenerating melanocytes and dermal fibroblasts, our data suggest that in human epidermis telomerase is a protective mechanism against excessive telomere loss during the life-long regeneration.     

DNA repair factors and telomere-chromosome integrity in mammalian cells

Loss of telomere equilibrium and associated chromosome-genomic instability might effectively promote tumour progression. Telomere function may have contrasting roles: inducing replicative senescence and promoting tumourigenesis and these roles may vary between cell types depending on the expression of the enzyme telomerase, the level of mutations induced, and efficiency/deficiency of related DNA repair pathways. We have identified an alternative telomere maintenance mechanism in mouse embryonic stem cells lacking telomerase RNA unit (mTER) with amplification of non-telomeric sequences adjacent to existing short stretches of telomere repeats. Our quest for identifying telomerase-independent or alternative mechanisms involved in telomere maintenance in mammalian cells has implicated the involvement of potential DNA repair factors in such pathways. We have reported earlier on the telomere equilibrium in scid mouse cells which suggested a potential role of DNA repair proteins in telomere maintenance in mammalian cells. Subsequently, studies by us and others have shown the association between the DNA repair factors and telomere function. Mice deficient in a DNA-break sensing molecule, PARP-1 (poly [ADP]-ribopolymerase), have increased levels of chromosomal instability associated with extensive telomere shortening. Ku80 null cells showed a telomere shortening associated with extensive chromosome end fusions, whereas Ku80+/- cells exhibited an intermediate level of telomere shortening. Inactivation of PARP-1 in p53-/- cells resulted in dysfunctional telomeres and severe chromosome instability leading to advanced onset and increased tumour incidence in mice. Interestingly, haploinsufficiency of PARP-1 in Ku80 null cells causes more severe telomere shortening and chromosome abnormalities compared to either PARP-1 or Ku80 single null cells and Ku80+/-PARP-/- mice develop spontaneous tumours. This overview will focus mainly on the role of DNA repair/recombination and DNA damage signalling molecules such as PARP-1, DNA-PKcs, Ku70/80, XRCC4 and ATM which we have been studying for the last few years. Because the maintenance of telomere function is crucial for genomic stability, our results will provide new insights into the mechanisms of chromosome instability and tumour formation.     

Telomere dysfunction and telomerase activation in cancer--a pathological paradox?

Telomerase is expressed in more than 90% of human cancers. Telomere maintenance by this enzyme is believed to safeguard genomic integrity in neoplastic cells. Nevertheless, many telomerase-expressing tumours exhibit chromosomal instability triggered by short, dysfunctional telomeres, implying that active telomerase is not sufficient for preserving a functional telosomic nucleoprotein complex in cancer cells. We here examine three possible solutions to this ostensible paradox. First, prior to telomerase activation, telomere erosion may have evolved to a level where telomeric repeat sequences are too short to provide a functional substrate for telomerase enzyme activity. Second, mechanisms other than the continuous telomere erosion counteracted by telomerase may contribute to rapid shortening of telomere repeats. Third, dysfunction of telomere-regulating proteins may result in direct telomere uncapping. Moreover, telomerase may contribute to tumour development also through mechanisms unrelated to telomere length maintenance. Taken together, the available data on the role of telomerase in cancer strongly support that inhibition of this enzyme is a feasible strategy for cancer therapy.     

Fanconi DNA repair pathway is required for survival and long-term maintenance of neural progenitors

Although brain development abnormalities and brain cancer predisposition have been reported in some Fanconi patients, the possible role of Fanconi DNA repair pathway during neurogenesis is unclear. We thus addressed the role of fanca and fancg, which are involved in the activation of Fanconi pathway, in neural stem and progenitor cells during brain development and adult neurogenesis. Fanca(-/-) and fancg(-/-) mice presented with microcephalies and a decreased neuronal production in developing cortex and adult brain. Apoptosis of embryonic neural progenitors, but not that of postmitotic neurons, was increased in the neocortex of fanca(-/-) and fancg(-/-) mice and was correlated with chromosomal instability. In adult Fanconi mice, we showed a reduced proliferation of neural progenitor cells related to apoptosis and accentuated neural stem cells exhaustion with ageing. In addition, embryonic and adult Fanconi neural stem cells showed a reduced capacity to self-renew in vitro. Our study demonstrates a critical role for Fanconi pathway in neural stem and progenitor cells during developmental and adult neurogenesis.     

Differentiation rather than aging of muscle stem cells abolishes their telomerase activity

A general feature of stem cells is the ability to routinely proliferate to build, maintain, and repair organ systems. Accordingly, embryonic and germline, as well as some adult stem cells, produce the telomerase enzyme at various levels of expression. Our results show that, while muscle is a largely postmitotic tissue, the muscle stem cells (satellite cells) that maintain this biological system throughout adult life do indeed display robust telomerase activity. Conversely, primary myoblasts (the immediate progeny of satellite cells) quickly and dramatically downregulate telomerase activity. This work thus suggests that satellite cells, and early transient myoblasts, may be more promising therapeutic candidates for regenerative medicine than traditionally utilized myoblast cultures. Muscle atrophy accompanies human aging, and satellite cells endogenous to aged muscle can be triggered to regenerate old tissue by exogenous molecular cues. Therefore, we also examined whether these aged muscle stem cells would produce tissue that is “young” with respect to telomere maintenance. Interestingly, this work shows that the telomerase activity in muscle stem cells is largely retained into old age wintin inbred “long” telomere mice and in wild‐derived short telomere mouse strains, and that age‐specific telomere shortening is undetectable in the old differentiated muscle fibers of either strain. Summarily, this work establishes that young and old muscle stem cells, but not necessarily their progeny, myoblasts, are likely to produce tissue with normal telomere maintenance when used in molecular and regenerative medicine approaches for tissue repair. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

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.     

The relationship between early embryo development and tumourigenesis

With the recent substantial progress in developmental biology and cancer biology, the similarities between early embryo development and tumourigenesis, as well as the important interaction between tumours and embryos become better appreciated. In this paper, we review in detail the embryonic origin of tumour, and the similarities between early embryo development and tumourigenesis with respect to cell invasive behaviours, epigenetic regulation, gene expression, protein profiling and other important biological behaviours. Given an improved understanding of the relationship between early embryo development and tumourigenesis, now we have better and broader resources to attack cancer from the perspective of developmental biology and develop next generation of prognostic and therapeutic approaches for this devastating disease.     

Dietary restriction of amino acids other than methionine prevents oxidative damage during aging: involvement of telomerase activity and telomere length

AimsIt has been suggested that variations in the proportions of some dietary amino acids can slow down aging. In this study, the influence of amino acids other than methionine on aging was investigated.Main methodsRats were fed either with normal (ND) or except methionine, protein restricted diet (PREMD) for 4 months and oxygen radical production, oxidative protein and DNA damage along with telomere length and telomerase activity were evaluated in the liver.Key findingsExcept mitochondrial superoxide production rate, feeding with PREMD significantly decreased the oxygen radical production rate and protein carbonyl levels in the homogenate and mitochondria of 16-month-old rats. Feeding with PREMD prevented 8-OHdG formation in mitochondrial DNA but not in the genomic DNA. Although liver telomerase activities of rats receiving either ND or PREMD seemed to have some variations, these did not reach a statistical significance. Feeding with PREMD conserved the telomere length in the liver. The telomere length of 8- and 16-month-old rats fed PREMD was similar, 16-month-old rats fed ND had telomeres shortened by 36% (p < 0.05).SignificanceLong-term restriction of the amino acids other than methionine may decrease oxygen radical generation and oxidative damage of cellular constituents, and may also prevent telomere shortening in rat liver.     

Immunotherapy of prostate cancer: should we be targeting stem cells and EMT?

Cancer stem cells have been implicated in a number of solid malignancies including prostate cancer. In the case of localised prostate cancer, patients are often treated with surgery (radical prostatectomy) and/or radiotherapy. However, disease recurrence is an issue in about 30% of patients, who will then go on to receive hormone ablation therapy. Hormone ablation therapy is often palliative in a vast proportion of individuals, and for hormone-refractory patients, there are several immunotherapies targeting a number of prostate tumour antigens which are currently in development. However, clinical responses in this setting are inconsistent, and it is believed that the failure to achieve full and permanent tumour eradication is due to a small, resistant population of cells known as ‘cancer stem cells’ (CSCs). The stochastic and clonal evolution models are among several models used to describe cancer development. The general consensus is that cancer may arise in any cell as a result of genetic mutations in oncogenes and tumour suppressor genes, which consequently result in uncontrolled cell growth. The cancer stem cell theory, however, challenges previous opinion and proposes that like normal tissues, tumours are hierarchical and only the rare subpopulation of cells at the top of the hierarchy possess the biological properties required to initiate tumourigenesis. Furthermore, where most cancer models infer that every cell within a tumour is equally malignant, i.e. equally capable of reconstituting new tumours, the cancer stem cell theory suggests that only the rare cancer stem cell component possess tumour-initiating capabilities. Hence, according to this model, cancer stem cells are implicated in both tumour initiation and progression. In recent years, the role of epithelial–mesenchymal transition (EMT) in the advancement of prostate cancer has become apparent. Therefore, CSCs and EMT are both likely to play critical roles in prostate cancer tumourigenesis. This review summarises the current immunotherapeutic strategies targeting prostate tumour antigens taking into account the need to consider treatments that target cancer stem cells and cells involved in epithelial–mesenchymal transition.     

Change of telomerase activity in peripheral blood of patients with head and neck squamous cell carcinoma pre and post curative treatment

BackgroundThere is no clinically applicable tumor marker for head and neck cancers. Telomerase is detected in approximately 90% of all malignant tumors, it may predict poor or favorable outcomes, thus being both a highly attractive biomarker and a target for the development of molecular-based cancer diagnostics, prognostics, and therapeuticsAimPrimary aim was to detect a change of telomerase activity before and after curative treatment.Materials and MethodsPatients with biopsy proven head and neck squamous cell carcinoma, stage I-IVB treated with a curative intent, performance status 0–2 and malignancy at one primary site were included in the study. Telomerase levels were tested in tissue biopsy. Plasma telomerase levels were tested at baseline, 5 days and at 3 months after treatment using ELISA.ResultsRaised plasma telomerase activity was seen in all the patients with cancer at baseline. The mean plasma telomerase level at baseline was 861.4522 ng/ml, at 5 days after completion of curative treatment was 928.92 ng/ml and at 3 months of follow up was 898.87 ng/ml. The mean tissue biopsy telomerase level was 19768.53 ng/mg. There was a significant increase in baseline telomerase levels in cancer patients compared to normals (volunteers) (t = −3.52, p = 0.001).There was a significant increase in plasma levels of telomerase at 3 months compared to baseline values (z = −1.98, p = 0.04). The increase in telomerase level did not correlate with the response of the treatment.ConclusionIn patients with head and neck squamous cell carcinomas treated with a curative intent, the change in levels of telomerase correlates neither with the disease status nor with prognostic factors.     

Telomerase regulation and stem cell behaviour

Telomerase expression is restricted to a few cell types of the adult organism, most notably germ cells and stem/progenitor cells. Telomerase activity in germ cells is sufficient to prevent telomere shortening with age. Stem cells, however, do not have sufficient telomerase to prevent telomere shortening associated with continuous tissue renewal with increasing age. Indeed, telomerase levels in the adult organism are thought to be rate-limiting for longevity. This is supported by rare human syndromes caused by mutations in telomerase components, which are characterized by premature loss of tissue renewal and premature death. More recently, the role of telomerase and telomere length in stem cells is starting to be elucidated.     

Clinical Relevance of Telomere Status and Telomerase Activity in Colorectal Cancer

The role of telomeres and telomerase in colorectal cancer (CRC) is well established as the major driving force in generating chromosomal instability. However, their potential as prognostic markers remains unclear. We investigated the outcome implications of telomeres and telomerase in this tumour type. We considered telomere length (TL), ratio of telomere length in cancer to non-cancer tissue (T/N ratio), telomerase activity and TERT levels; their relation with clinical variables and their role as prognostic markers. We analyzed 132 CRCs and paired non-cancer tissues. Kaplan-Meier curves for disease-free survival were calculated for TL, T/N ratio, telomerase activity and TERT levels. Overall, tumours had shorter telomeres than non-tumour tissues (P < 0.001) and more than 80% of CRCs displayed telomerase activity. Telomere lengths of non-tumour tissues and CRCs were positively correlated (P < 0.001). Considering telomere status and clinical variables, the lowest degree of telomere shortening was shown by tumours located in the rectum (P = 0.021). Regarding prognosis studies, patients with tumours showing a mean TL < 6.35 Kb experienced a significantly better clinical evolution (P < 0.001) and none of them with the highest degree of tumour telomere shortening relapsed during the follow-up period (P = 0.043). The mean TL in CRCs emerged as an independent prognostic factor in the Cox analysis (P = 0.017). Telomerase-positive activity was identified as a marker that confers a trend toward a poor prognosis. In CRC, our results support the use of telomere status as an independent prognostic factor. Telomere status may contribute to explaining the different molecular identities of this tumour type.     

The relationship between ethnicity,social deprivation and late presentation of colorectal cancer

IntroductionTumour staging at time of presentation is an important factor in determining survival in colorectal cancer. The aim of this paper is to investigate the relationship between ethnicity and deprivation in late (Stage IV) presentation of colorectal cancer.MethodsData from the Thames Cancer Registry comprising 77,057 colorectal cancer patients between the years 2000 and 2012 were analysed.ResultsA total of 17,348 patients were identified with complete data, of which 53.9% were male. Patients from a Black Afro/Caribbean background were diagnosed with CRC at a much younger age than the White British group (median age 67 compared with 72, p < 0.001). In multiple regression, ethnicity, deprivation and age were positive predictors of presenting with advanced tumour stage at time of diagnosis. Black patients were more likely to present with Stage IV tumours than white patients (OR 1.37, 95% CI 1.18–1.59, p < 0.001). Social deprivation was also a predictor of Stage IV cancer presentation, with the most deprived group (Quintile 5) 1.26 times more likely to be diagnosed with Stage IV cancer compared with the most affluent group (CI 1.13–1.40, p < 0.001). Sub-group analyses demonstrated that Black & Affluent patients were still at greater risk of Stage IV CRC than their White & Affluent counterparts (OR 1.24, 95% CI 1.11–1.45, p = 0.023). Patients with rectal cancer were less likely to present with Stage IV CRC (OR 0.66, 95% CI 0.61–0.71, p < 0.001).ConclusionRacial and age related disparities exist in tumour presentation in the United Kingdom. Patients from black and socially deprived backgrounds as well as the elderly are more likely to present with advanced tumours at time of diagnosis.     

Telomere length and risk of melanoma,squamous cell carcinoma,and basal cell carcinoma

Background: Telomeres help maintain chromosomal structure and may influence tumorigenesis. We examined the association between telomere length and skin cancer in a clinic-based case-control study of 198 melanoma cases, 136 squamous cell carcinoma (SCC) cases, 185 basal cell carcinoma (BCC) cases, and 372 healthy controls. Methods: Cases were histologically confirmed patients treated at the Moffitt Cancer Center and University of South Florida Dermatology Clinic in Tampa, FL. Controls self-reported no history of cancer and underwent a skin cancer screening exam at study enrollment to rule out the presence of skin cancer. Quantitative real time PCR was used to measure telomere length in peripheral blood samples. Results: Melanoma patients had longer telomeres than controls (odds ratio (OR) = 3.75; 95% confidence interval (CI): 2.02–6.94 for highest versus lowest tertile) (P for trend = <0.0001). In contrast, longer telomere length was significantly inversely associated with SCC (OR = 0.01; 95% CI: 0.00–0.05 for highest versus lowest tertile) (P for trend = <0.0001) and BCC (OR = 0.10; 95% CI: 0.06–0.19 for highest versus lowest tertile) (P for trend = <0.0001). Conclusion: Telomere length may be involved in the development of skin cancer, although the effect on cancer risk differs for melanoma and non-melanoma carcinomas. Our findings suggest that long telomere length is positively associated with melanoma while inversely associated with SCC and BCC.     

Telomeres and telomerase in normal and cancer stem cells

Differences between normal adult tissue stem cells and cancer stem/initiating cells remain poorly defined. For example, it is controversial if cancer stem cells can become fully quiescent, require a stem cell niche, are better at repairing DNA damage than the bulk of the cancer cells, and if and how they regulate symmetric versus asymmetric cell divisions. This minireview will not only provide our personal views to address some of these outstanding questions, but also present evidence that an understanding of telomere dynamics and telomerase activity in normal and cancer stem cells may provide additional insights into how tumors are initiated, and how they should be monitored and treated.