Telomeres and their possible role in chromosome stabilization

Effects chronic administration of corticosterone and estrogen on HPA axis activity and telomere length in brain areas of female rats

Chronic stress is related to the acceleration of telomere shortening. Recent work showed a correlation between chronic psychosocial stress and reduced telomere length in certain cells. The exposure of T lymphocytes to cortisol promoted a significant reduction in telomerase activity. Although stress can promote changes in telomere length, whether increased glucocorticoid concentrations alter telomere length in brain tissue cells is unclear. In addition to modulating the activity of the stress system, estrogen also influences telomere length. The objective of this study was to verify whether chronic exposure to glucocorticoids promotes changes in the telomere length of encephalic areas involved in the control of HPA axis activity and whether estrogen affects these changes. Wistar rats were ovariectomized and treated with estradiol cypionate [(50 or 100 μg/kg, subcutaneously)] or oil and 20 mg/kg corticosterone or vehicle (isotonic saline with 2% Tween 80, subcutaneously) for 28 days. On the day after the end of the hormonal treatment, the animals were euthanized for collection of blood, brain and pituitary gland samples. Estrogen modulated the activity of the HPA axis. CRH, AVP and POMC mRNA levels were reduced by estrogen. At least in doses and treatment time used, there was no correlation between effects of exposure to glucocorticoids and estrogen on telomere length in the brain areas of female rats. However, estrogen treatment reduced the telomere length in the central amygdala and dorsal hippocampus, but not in the PVN, indicating a variation of reaction of telomeres for estrogen in different brain areas.

Identifying the biomarker potential of telomerase activity and shelterin complex molecule, telomeric repeat binding factor 2 (TERF2), in multiple myeloma

Telomere length (TL) is maintained by telomere capping protein complex called shelterin complex. We studied the possible involvement and biomarker potential of shelterin complex molecules in naive multiple myeloma (MM) patients and controls. TL, relative telomerase activity (RTA), real-time PCR and Western blotting were performed in bonemarrow sample of 70 study subjects (patients = 50; controls = 20). Significantly lowered mean TL, increased RTA and higher mRNA expression of shelterin molecules were observed in patients, while PIN2/TERF1 interacting telomerase inhibitor 1 (PINX1) showed lower mRNA expression. Significantly increased protein expression of telomeric repeat binding factor 2 (TERF2), protection of telomeres 1, adrenocortical dysplasia homolog, Tankyrase 1 and telomere reverse transcriptase were observed in MM patients. Significant correlation was observed among genes and of genes with clinical parameters. In conclusion, our findings showed alteration of these molecules at mRNA and protein levels suggested their involvement in disease progression. Optimal sensitivity and specificity of TERF2 and RTA on receiver operating characteristics curve analysis and univariate analysis demonstrated their biomarkers potential in better prediction of disease course.

Holokinetic centromeres and efficient telomere healing enable rapid karyotype evolution

Species with holocentric chromosomes are often characterized by a rapid karyotype evolution. In contrast to species with monocentric chromosomes where acentric fragments are lost during cell division, breakage of holocentric chromosomes creates fragments with normal centromere activity. To decipher the mechanism that allows holocentric species an accelerated karyotype evolution via chromosome breakage, we analyzed the chromosome complements of irradiated Luzula elegans plants. The resulting chromosomal fragments and rearranged chromosomes revealed holocentromere-typical CENH3 and histone H2AThr120ph signals as well as the same mitotic mobility like unfragmented chromosomes. Newly synthesized telomeres at break points become detectable 3 weeks after irradiation. The presence of active telomerase suggests a telomerase-based mechanism of chromosome healing. A successful transmission of holocentric chromosome fragments across different generations was found for most offspring of irradiated plants. Hence, a combination of holokinetic centromere activity and the fast formation of new telomeres at break points enables holocentric species a rapid karyotype evolution involving chromosome fissions and rearrangements.

Biology of telomeres: importance in etiology of esophageal cancer and as therapeutic target

The purpose of this review is to highlight the importance of telomeres, the mechanisms implicated in their maintenance, and their role in the etiology as well as the treatment of human esophageal cancer. We will also discuss the role of telomeres in the maintenance and preservation of genomic integrity, the consequences of telomere dysfunction, and the various factors that may affect telomere health in esophageal tissue predisposing it to oncogenesis. There has been growing evidence that telomeres, which can be affected by various intrinsic and extrinsic factors, contribute to genomic instability, oncogenesis, as well as proliferation of cancer cells. Telomeres are the protective DNA-protein complexes at chromosome ends. Telomeric DNA undergoes progressive shortening with age leading to cellular senescence and/or apoptosis. If senescence/apoptosis is prevented as a consequence of specific genomic changes, continued proliferation leads to very short (ie, dysfunctional) telomeres that can potentially cause genomic instability, thus, increasing the risk for activation of telomere maintenance mechanisms and oncogenesis. Like many other cancers, esophageal cancer cells have short telomeres and elevated telomerase, the enzyme that maintains telomeres in most cancer cells. Homologous recombination, which is implicated in the alternate pathway of telomere elongation, is also elevated in Barrett's-associated esophageal adenocarcinoma. Evidence from our laboratory indicates that both telomerase and homologous recombination contribute to telomere maintenance, DNA repair, and the ongoing survival of esophageal cancer cells. This indicates that telomere maintenance mechanisms may potentially be targeted to make esophageal cancer cells static. The rate at which telomeres in healthy cells shorten is determined by a number of intrinsic and extrinsic factors, including those associated with lifestyle. Avoidance of factors that may directly or indirectly injure esophageal tissue including its telomeric and other genomic DNA can not only reduce the risk of development of esophageal cancer but may also have positive impact on overall health and lifespan.

Targeting homologous recombination and telomerase in Barrett's adenocarcinoma: impact on telomere maintenance, genomic instability and tumor growth

Homologous recombination (HR), a mechanism to accurately repair DNA in normal cells, is deregulated in cancer. Elevated/deregulated HR is implicated in genomic instability and telomere maintenance, which are critical lifelines of cancer cells. We have previously shown that HR activity is elevated and significantly contributes to genomic instability in BAC. The purpose of this study was to evaluate therapeutic potential of HR inhibition, alone and in combination with telomerase inhibition, in BAC. We demonstrate that telomerase inhibition in BAC cells increases HR activity, RAD51 expression, and association of RAD51 to telomeres. Suppression of HR leads to shorter telomeres as well as markedly reduced genomic instability in BAC cells over time. Combination of HR suppression (whether transgenic or chemical) with telomerase inhibition, causes a significant increase in telomere attrition and apoptotic death in all BAC cell lines tested, relative to either treatment alone. A subset of treated cells also stain positive for β-galactosidase, indicating senescence. The combined treatment is also associated with decline in S-phase and a strong G2/M arrest, indicating massive telomere attrition. In a subcutaneous tumor model, the combined treatment resulted in the smallest tumors, which were even smaller (P=0.001) than those resulted from either treatment alone. Even the tumors removed from these mice had significantly reduced telomeres and evidence of apoptosis. We therefore conclude that although telomeres are elongated by telomerase, elevated RAD51/HR assist in their maintenance/stabilization in BAC cells. Telomerase inhibitor prevents telomere elongation but induces RAD51/HR, which contribute to telomere maintenance/stabilization and prevention of apoptosis, reducing the efficacy of treatment. Combining HR inhibition with telomerase, makes telomeres more vulnerable to degradation and significantly increases/expedites their attrition, leading to apoptosis. We therefore demonstrate that a therapy, targeting HR and telomerase, has potential to prevent both the tumor growth and genomic evolution in BAC.

Cancer models, genomic instability and somatic cellular Darwinian evolution

The biology of cancer is critically reviewed and evidence adduced that its development can be modelled as a somatic cellular Darwinian evolutionary process. The evidence for involvement of genomic instability (GI) is also reviewed. A variety of quasi-mechanistic models of carcinogenesis are reviewed, all based on this somatic Darwinian evolutionary hypothesis; in particular, the multi-stage model of Armitage and Doll (Br. J. Cancer 1954:8;1-12), the two-mutation model of Moolgavkar, Venzon, and Knudson (MVK) (Math. Biosci. 1979:47;55-77), the generalized MVK model of Little (Biometrics 1995:51;1278-1291) and various generalizations of these incorporating effects of GI (Little and Wright Math. Biosci. 2003:183;111-134; Little et al. J. Theoret. Biol. 2008:254;229-238).

Telomere maintenance in laser capture microdissection-purified Barrett's adenocarcinoma cells and effect of telomerase inhibition in vivo

PURPOSE

The aims of this study were to investigate telomere function in normal and Barrett's esophageal adenocarcinoma (BEAC) cells purified by laser capture microdissection and to evaluate the effect of telomerase inhibition in cancer cells in vitro and in vivo.

EXPERIMENTAL DESIGN

Epithelial cells were purified from surgically resected esophagi. Telomerase activity was measured by modified telomeric repeat amplification protocol and telomere length was determined by real-time PCR assay. To evaluate the effect of telomerase inhibition, adenocarcinoma cell lines were continuously treated with a specific telomerase inhibitor (GRN163L) and live cell number was determined weekly. Apoptosis was evaluated by Annexin labeling and senescence by beta-galactosidase staining. For in vivo studies, severe combined immunodeficient mice were s.c. inoculated with adenocarcinoma cells and following appearance of palpable tumors, injected i.p. with saline or GRN163L.

RESULTS

Telomerase activity was significantly elevated whereas telomeres were shorter in BEAC cells relative to normal esophageal epithelial cells. The treatment of adenocarcinoma cells with telomerase inhibitor, GRN163L, led to loss of telomerase activity, reduction in telomere length, and growth arrest through induction of both the senescence and apoptosis. GRN163L-induced cell death could also be expedited by addition of the chemotherapeutic agents doxorubicin and ritonavir. Finally, the treatment with GRN163L led to a significant reduction in tumor volume in a subcutaneous tumor model.

CONCLUSIONS

We show that telomerase activity is significantly elevated whereas telomeres are shorter in BEAC and suppression of telomerase inhibits proliferation of adenocarcinoma cells both in vitro and in vivo.

Telomerase inhibitor GRN163L inhibits myeloma cell growth in vitro and in vivo

Human telomerase, the reverse transcriptase which extends the life span of a cell by adding telomeric repeats to chromosome ends, is expressed in most cancer cells but not in the majority of normal somatic cells. Inhibition of telomerase therefore holds great promise as anticancer therapy. We have synthesized a novel telomerase inhibitor GRN163L, a lipid-attached phosphoramidate oligonucleotide complementary to template region of the RNA subunit of telomerase. Here, we report that GRN163L is efficiently taken up by human myeloma cells without any need of transfection and is resistant to nucleolytic degradation. The exposure of myeloma cells to GRN163L led to an effective inhibition of telomerase activity, reduction of telomere length and apoptotic cell death after a lag period of 2-3 weeks. Mismatch control oligonucleotides had no effect on growth of myeloma cells. The in vivo efficacy of GRN163L was confirmed in two murine models of human multiple myeloma. In three independent experiments, significant reduction in tumor cell growth and better survival than control mice was observed. Furthermore, GRN163L-induced myeloma cell death could be significantly enhanced by Hsp90 inhibitor 17AAG. These data provide the preclinical rationale for clinical evaluation of GRN163L in myeloma and in combination with 17AAG.

Genetic and epigenetic features in radiation sensitivity

Recent progress especially in the field of gene identification and expression has attracted greater attention to genetic and epigenetic susceptibility to cancer, possibly enhanced by ionising radiation. It has been proposed that the occurrence and severity of the adverse reactions to radiation therapy are also influenced by such genetic susceptibility. This issue is especially important for radiation therapists since hypersensitive patients may suffer from adverse effects in normal tissues following standard radiation therapy, while normally sensitive patients could receive higher doses of radiation offering a better likelihood of cure for malignant tumours. This paper, the first of two parts, reviews the main mechanisms involved in cell response to ionising radiation. DNA repair machinery and cell signalling pathways are considered and their role in radiosensitivity is analysed. The implication of non-targeted and delayed effects in radiosensitivity is also discussed.

Bystander effects in radiation-induced genomic instability

Exposure of GM10115 hamster-human hybrid cells to X-rays can result in the induction of chromosomal instability in the progeny of surviving cells. This instability manifests as the dynamic production of novel sub-populations of cells with unique cytogenetic rearrangements involving the "marker" human chromosome. We have used the comet assay to investigate whether there was an elevated level of endogenous DNA breaks in chromosomally unstable clones that could provide a source for the chromosomal rearrangements and thus account for the persistent instability observed. Our results indicate no significant difference in comet tail measurement between non-irradiated and radiation-induced chromosomally unstable clones. Using two-color fluorescence in situ hybridization we also investigated whether recombinational events involving the interstitial telomere repeat-like sequences in GM10115 cells were involved at frequencies higher than random processes would otherwise predict. Nine of 11 clones demonstrated a significantly higher than expected involvement of these interstitial telomere repeat-like sequences at the recombination junction between the human and hamster chromosomes. Since elevated levels of endogenous breaks were not detected in unstable clones we propose that epigenetic or bystander effects (BSEs) lead to the activation of recombinational pathways that perpetuate the unstable phenotype. Specifically, we expand upon the hypothesis that radiation induces conditions and/or factors that stimulate the production of reactive oxygen species (ROS). These reactive intermediates then contribute to a chronic pro-oxidant environment that cycles over multiple generations, promoting chromosomal recombination and other phenotypes associated with genomic instability.

Chromosomal aberrations: formation, identification and distribution

Chromosomal aberrations (CA) are the microscopically visible part of a wide spectrum of DNA changes generated by different repair mechanisms of DNA double strand breaks (DSB). The method of fluorescence in situ hybridisation (FISH) has uncovered unexpected complexities of CA and this will lead to changes in our thinking about the origin of CA. The inter- and intrachromosomal distribution of breakpoints is generally not random. CA breakpoints occur preferentially in active chromatin. Deviations from expected interchromosomal distributions of breakpoints may result from the arrangement of chromosomes in the interphase nucleus and/or from different sensitivities of chromosomes with respect to the formation of CA. Telomeres and interstitial telomere repeat like sequences play an important role in the formation of CA. Subtelomeric regions are hot spots for the formation of symmetrical exchanges between homologous chromatids and cryptic aberrations in these regions are associated with human congenital abnormalities.

Wiedemann-Rautenstrauch (neonatal progeroid) syndrome: new case with normal telomere length in skin fibroblasts

Wiedemann-Rautenstrauch (neonatal progeroid) syndrome is an autosomal recessive condition with characteristic appearance of premature aging present at birth (aged face, natal teeth, and wrinkled skin). Other features of the syndrome are generalized lipoatrophy with specific fat accumulation in the lateral suprabuttock region, hypotrichosis, macrocephaly (pseudohydrocephalus), and mental retardation. We report on a new case that demonstrates all typical features of the syndrome. The girl is now 16 years and 10 months old and has had follow-up from birth. We measured terminal restriction fragment (TRF) length to evaluate whether the patient's premature aging process is accompanied by shortening of telomere length in her cultured fibroblasts. Mean TRF of 13.5 kb found in our patient's fibroblasts is not shortened as compared to that of normal fibroblasts. Our results differ from those observed in Hutchinson-Gilford progeria.

Telomerase activity as a prognostic factor in neuroblastomas

Neuroblastoma is the most common extracranial solid tumor of early childhood. This tumor demonstrates significant heterogeneity with respect to pathologic, genetic, and clinical features. The outcome varies from spontaneous regression or maturation to rapid progression, despite aggressive therapy. Prognostic factors have been found that identify those tumors which have a high probability of aggressive behavior; these factors include unfavorable histology, MYCN copy number, deletions of the short arm of chromosome 1, DNA content, and TRK-A (high-affinity receptor protein for nerve growth factor) expression. Recent studies have suggested that high levels of telomerase activity also correlate with poor clinical outcome. We investigated this relationship in 40 patients with untreated neuroblastoma, using a PCR-ELISA assay for telomerase activity. In these patients, 23 tumors had no or minimal telomerase activity whereas 15 had high levels of activity. In two tumors, telomerase activity was not assessable. There was significant correlation between the telomerase activity and MYCN copy number, 1p deletions, and TRK-A expression, as well as patient age, clinical stage, and outcome. The histological classification of the tumors was not significantly different between the two groups, being predominantly unfavorable by the Shimada classification. In addition, for 17 patients tumor tissue was assessed for telomerase activity post-chemotherapy. In those cases where the tumor was negative for telomerase activity before and after chemotherapy, the patients uniformly did well. In cases where the tumor was positive before and negative or weakly positive after treatment, two of the seven patients did well clinically. However, in cases that were positive after chemotherapy, all had recurrence or died. In conclusion, telomerase activity appears to be a prognostic factor for neuroblastomas. In addition, assessment of tumors post-chemotherapy may be a further indicator of clinical outcome.

Enzymatic activity of endogenous telomerase associated with intact nuclei from human leukemia CEM cells

Telomerase, a telomere-specific DNA polymerase and novel target for chemotherapeutic intervention, is found in many types of cancers. Telomerase activity is typically assayed using an exogenous primer and cellular extracts as the source of enzyme. Since the nuclear organization might affect telomerase function, we developed a system in which telomerase in intact nuclei catalyzes primer extension. Telomerase activity in isotonically isolated nuclei from human CEM cells shows low processivity (addition of up to four TTAGGG repeats). In contrast, telomerase activity which leaks into a 500 g postnuclear supernatant and the activity in a CHAPS extract are highly processive. The nucleotide inhibitor, 7-deaza-dGTP, seems to be more inhibitory against the nuclei-associated enzyme compared to telomerase from cytoplasmic extracts. However, 7-deaza-dATP and ddGTP are less inhibitory against nuclei-associated telomerase. The results suggest that the association of telomerase with the nuclear chromatin affects telomerase activity. Examination of telomerase activity in a more natural nuclear environment may shed new light on the telomerase function and provide a useful system for the evaluation of new telomerase inhibitors.

Recombination and its roles in DNA repair, cellular immortalization and cancer

Genetic recombination is the creation of new gene combinations in a cell or gamete, which differ from those of progenitor cells or parental gametes. In eukaryotes, recombination may occur at mitosis or meiosis. Mitotic recombination plays an indispensable role in DNA repair, which presumably directed its early evolution; the multiplicity of recombination genes and pathways may be best understood in this context, although they have acquired important additional functions in generating diversity, both somatically (increasing the immune repertoire) and in germ line (facilitating evolution). Chromosomal homologous recombination and HsRad51 recombinase expression are increased in both immortal and preimmortal transformed cells, and may favor the occurrence of multiple oncogenic mutations. Tumorigenesis in vivo is frequently associated with karyotypic instability, locus-specific gene rearrangements, and loss of heterozygosity at tumor suppressor loci - all of which can be recombinationally mediated. Genetic defects which increase the rate of somatic mutation (several of which feature elevated recombination) are associated with early incidence and high risk for a variety of cancers. Moreover, carcinogenic agents appear to quite consistently stimulate homologous recombination. If cells with high recombination arise, either spontaneously or in response to "recombinogens," and predispose to the development of cancer, what selective advantage could favor these cells prior to the occurrence of growth-promoting mutations? We propose that the augmentation of telomere-telomere recombination may provide just such an advantage, to hyper-recombinant cells within a population of telomerase-negative cells nearing their replicative (Hayflick) limit, by extending telomeres in some progeny cells and thus allowing their continued proliferation.

PCR and FISH analysis of a ring Y chromosome

A newborn male infant presented with midshaft hypospadias, chordee, and undescended left testis. Both gonads lacked the tunica albuginea and appeared to be adjacent to structures resembling fallopian tubes. On biopsy, there was marked dysgenesis of both gonads, with a paucity of testicular tubules and foci of ovarian-like stroma. Peripheral blood karyotype was 46,X,mar(Y) [39]/45,X [5]. Right gonadal biopsy material showed the same mosaicism but with a higher proportion of 45,X cells (46%). PCR and FISH analyses with primers/probes from different Yp, Yq, and Ycen loci defined the structure of the marker Y as a probable complex ring with breakpoints in Yq11.21 (very close to the centromere) and in Yp11.32 (the pseudoautosomal region). Based on the phenotype and the laboratory findings, the prognosis given to the patient was for short stature and azoospermia without an increased risk for gonadoblastomas.

Paracentric inversion involving the long arm of chromosome 9 resulting in deletion of abl gene

We report on a new chromosomal finding in a newborn male with hypertelorism, apparently low-set malformed ears with patent canal, micrognathia with narrow high-arched palate, bilateral webbing of neck with low posterior hairline, widely spaced nipples, and complex heart anomalies. Initially, what appeared to be a simple paracentric inversion of the long arm of chromosome 9, that is, 46,XY, inv(9)(q31q34) by routine GTG-banding technique was later determined to be a paracentric inversion with deletion of the band 9q34.1 by FISH technique using an abl unique sequence DNA probe. Thus the cytogenetic diagnosis was modified to 46,XY,der(9) inv(9)(q31q34.1)del(q34.1). Nevertheless, the presence of telomeric repeat sequences in the inverted chromosome 9 suggests that either healing has occurred by adding [TTAGGG]n sequences to the non-telomeric end (q31) by the enzyme telomerase or telomeric sequences were not affected during this inversion process. This abnormality is a rare occurrence and has never been reported before either because of a high rate of lethality or it has been undetected by routine cytogenetic techniques. The other abnormal cases with apparent paracentric inversions could also have a complex nature with congenital anomalies associated with loss of "few" DNA sequences as exemplified here.

Reduced telomere length in ataxia-telangiectasia fibroblasts

Chromosomal instability with a high frequency of telomere fusion is characteristic of ataxia-telangiectasia cells both in vivo and in vitro. We have measured telomere length and found it to be consistently reduced in both diploid and SV40-transformed cells A-T fibroblasts, relative to control cells. We examined a few possible mechanisms which might account for telomeric length reduction, including telomerase activity in transformed cells and endogenous nuclease activities, but found no differences between A-T and control cells in these parameters.

A de novo interstitial deletion of chromosome 15 band q25 as revealed by FISH-technique

We report a new chromosomal finding in a 20 month-old girl. The minor clinical features included: moderate mental retardation, microcephaly, mild hypotonia and hypertelorism. Initially, what appeared to be a terminal deletion of the long arm of one chromosome 15 [15q26-->qter] was determined to be an interstitial deletion involving band 15q25 as revealed by FISH-technique, showing the presence of intact telomeric hybridization signals. The cytogenetic diagnosis was thus modified to 46,XX, del (15) (pter-->q24::q26--> qter). Nevertheless, the function of the enzyme telomerase should not be ignored, as healing could occur following such terminal deletions. Consequently, it will remain a difficult task to distinguish terminal deletions from those that are interstitial.

Characterization of 7q--by FISH technique of a case with acute myelogenous leukemia evolving from agnogenic myeloid metaplasia

We present a new case of acute myelogenous leukemia (AML) which evolved from agnogenic myeloid metaplasia (AMM). Routine cytogenetic techniques revealed a terminal deletion of one chromosome 7 (del (7) (q21)). When metaphases were hybridized with the 7q specific telomeric probe, a signal was detected at the distal q arm of the deleted chromosome 7 suggesting an interstitial deletion and the cytogenetic diagnosis was changed to 46,XY, del(7) (q21.1q36.2). Apparently, precise characterization of such deletions may have gone undetected, owing to the lack of technology, and deletions that have been previously regarded as terminal may very well be interstitial. Undoubtedly, precise identification of genetic abnormalities may lead to a better classification which may help to assign patients into sub-subgroups within the specific chromosomal abnormalities to initiate better therapeutic modalities and decisions.

A role for genomic instability in cellular radioresistance?

Inherent cellular radioresistance plays a critical role in the failure of radiotherapy. Although the consequences of radioresistance are well known, the molecular, biological, and cellular bases of radioresistance remain a mystery. We propose that genomic instability, the increased rate of acquisition of alterations in the mammalian genome, can directly modulate cells' sensitivity to radiation. In particular, destabilization of chromosomes occurring as a consequence of genomic instability may result in enhanced 'plasticity of the genome'. This increased plasticity of the genome allows cells to better adapt to changes in local environment(s) during tumor progression, or improve cell survival following exposure to DNA damage encountered during radiotherapy protocols, thereby contributing to radioresistant cell populations found in tumors both before and after radiotherapy.