DNA topoisomerase II cleavage of telomeres in vitro and in vivo

The radiomimetic compound streptonigrin induces persistent telomere dysfunction in mammalian cells

We analyzed the chromosomal aberrations involving telomeres in the progeny of mammalian cells exposed to the radiomimetic compound streptonigrin (SN) in order to determine if this antineoplastic drug induces long-term telomere instability. To this end, rat cells (ADIPO-P2 cell line, derived from adipose cells from Sprague-Dawley rat) were treated with a single concentration of SN (100ng/ml), and chromosomal aberrations were analyzed 18h and 10 and 15 days after treatment by using PNA-FISH with a pan-telomeric probe [Cy3-(CCCTAA)3] to detect (TTAGGG)n repeats. Cytogenetic analysis revealed a higher frequency of telomere dysfunction-related aberrations (additional telomeric FISH signals, extra-chromosomal telomeric FISH signals, and telomere FISH signal loss and duplications) in SN-exposed cultures vs. untreated cultures at every time points analyzed. The yield of SN-induced aberrations remained very similar at 18h, 10 days as well as 15 days after treatment. Thus, our data demonstrate that SN induces persistent telomere dysfunction in mammalian cells. Moreover, we found that the level of telomerase activity in SN-treated cells was significantly lower (up to 77%) than that of untreated control cells at each time points analyzed. This fact suggests that telomerase could be involved in SN-induced telomere dysfunction.

Nucleostemin prevents telomere damage by promoting PML-IV recruitment to SUMOylated TRF1

A novel telomere-protection mechanism relies upon nucleostemin-mediated recruitment of PML-IV to telomeres and subsequent recruitment of RAD51 in both ALT and telomerase-active cells.

Continuously dividing cells must be protected from telomeric and nontelomeric DNA damage in order to maintain their proliferative potential. Here, we report a novel telomere-protecting mechanism regulated by nucleostemin (NS). NS depletion increased the number of telomere damage foci in both telomerase-active (TA⁺) and alternative lengthening of telomere (ALT) cells and decreased the percentage of damaged telomeres associated with ALT-associated PML bodies (APB) and the number of APB in ALT cells. Mechanistically, NS could promote the recruitment of PML-IV to SUMOylated TRF1 in TA⁺ and ALT cells. This event was stimulated by DNA damage. Supporting the importance of NS and PML-IV in telomere protection, we demonstrate that loss of NS or PML-IV increased the frequency of telomere damage and aberration, reduced telomeric length, and perturbed the TRF2^ΔBΔM-induced telomeric recruitment of RAD51. Conversely, overexpression of either NS or PML-IV protected ALT and TA⁺ cells from telomere damage. This work reveals a novel mechanism in telomere protection.

Telomere dysfunction induced by chemotherapeutic agents and radiation in normal human cells

The number of long-term survivors of patients with various malignancies (>5 years) is increasing mainly owing to advances in cancer therapeutics, but long-term side effects of the cancer treatment in this population have emerged as an important health and socio-economical issue. Telomeres and telomerase are known to be essential for regulation of cellular life-span and maintenance of genomic stability, and earlier studies have demonstrated that cancer patients who receive chemotherapy have shorter telomeres in their blood cells, indicating accelerated telomere erosion and a potential contribution of telomere loss to late side-effects. Little is currently known about the effect of chemotherapeutic agents and radiation on telomere dynamics including potential effects on telomere length, structure, function, telomerase activity, and telomere shelterin proteins in normal human cells. In the present study, we had addressed this issue experimentally. The treatment of normal human T lymphocytes and fibroblasts with chemotherapeutic agents doxorubicin (DOX) or etoposide (VP16) led to significant shortening of telomeres, down-regulation of telomerase activity, and diminished expression of telomerase reverse transcriptase (hTERT) and the telomere binding proteins TPP1 and POT1. More importantly, telomere dysfunction was observed in cells treated with DOX or VP16. Furthermore, all the above alterations were similarly found in the cells receiving γ-irradiation. Taken together, both chemotherapy and radiotherapy significantly impair telomere maintenance and function in normal human cells. Conceivably telomere dysfunction causes shortened life-span and genomic instability of normal human cells, and thereby contributes to tissue/organ damage and secondary malignancies in long-term survivors of cancer.

Telomerase-associated protein 1, HSP90, and topoisomerase IIalpha associate directly with the BLM helicase in immortalized cells using ALT and modulate its helicase activity using telomeric DNA substrates

The BLM helicase associates with the telomere structural proteins TRF1 and TRF2 in immortalized cells using the alternative lengthening of telomere (ALT) pathways. This work focuses on identifying protein partners of BLM in cells using ALT. Mass spectrometry and immunoprecipitation techniques have identified three proteins that bind directly to BLM and TRF2 in ALT cells: telomerase-associated protein 1 (TEP1), heat shock protein 90 (HSP90), and topoisomerase IIalpha (TOPOIIalpha). BLM predominantly co-localizes with these proteins in foci actively synthesizing DNA during late S and G(2)/M phases of the cell cycle when ALT is thought to occur. Immunoprecipitation studies also indicate that only HSP90 and TOPOIIalpha are components of a specific complex containing BLM, TRF1, and TRF2 but that this complex does not include TEP1. TEP1, TOPOIIalpha, and HSP90 interact directly with BLM in vitro and modulate its helicase activity on telomere-like DNA substrates but not on non-telomeric substrates. Initial studies suggest that knockdown of BLM in ALT cells reduces average telomere length but does not do so in cells using telomerase.

Accelerated senescence: an emerging role in tumor cell response to chemotherapy and radiation

Treatment of malignancies with chemotherapeutic drugs and/or radiotherapy is designed to eliminate the disease by depriving the tumor cell of its reproductive potential. Frequently, the desired effect of cell killing is achieved through the promotion of apoptosis; however, accumulating evidence suggests that apoptosis may not be the exclusive or even primary mechanism whereby tumor cells lose their self-renewal capacity after radiation or drug treatment, particularly in the case of solid tumors. While failure to undergo apoptosis in response to chemotherapeutic drugs or radiation may represent a mechanism of drug and radiation resistance, particularly in the case of leukemias and lymphomas, it is gradually being recognized that in the case of solid tumors, loss of reproductive capacity can occur through alternative pathways including reproductive cell death or mitotic catastrophe, through autophagic cell death, and as described below, through a terminally arrested state similar to replicative senescence. Studies building upon the phenomenon of replicative senescence in normal cells approaching the limit of their reproductive potential have identified a comparable senescence-like arrest as a component of the tumor cell response to chemotherapeutic drugs and radiation. This response, which has been termed "premature senescence", "senescence-like growth arrest", "stress-induced premature senescence", and "accelerated senescence", can also result from supraphysiological mitogenic signaling, sub-optimal culture conditions, and ectopic expression of oncogenes. Here, we will use the term "accelerated senescence" in our consideration of the morphological, biochemical, and molecular aspects of treatment-induced senescence, its relationship to classical replicative senescence, its prevalence in clinical specimens and the implications of accelerated senescence for the outcome of cancer therapy.

Tumour-cell apoptosis after cisplatin treatment is not telomere dependent

Cisplatin is a major chemotherapeutic agent, especially for the treatment of neuroblastoma. Telomeres with their sequence (TTAGGG)n are probable targets for cisplatin intrastrand cross-linking, but the role of telomeres in mediating cisplatin cytotoxicity is not clear. After exposure to cisplatin as single dose or continuous treatment, we found no loss of telomeres in either SHSY5Y neuroblastoma cells (telomere length, approximately 4 kbp), HeLa 229 cells (telomere length, 20 kbp) or in the acute lymphoblastic T cell line 1301 (telomere length, approximately 80 kbp). There was no induction of telomeric single strand breaks, telomeric overhangs were not degraded and telomerase activity was down-regulated only after massive onset of apoptosis. In contrast, cisplatin induced a delayed formation of DNA strand breaks and induced DNA damage foci containing gamma-H2A.X at nontelomeric sites. Interstitial DNA damage appears to be more important than telomere loss or telomeric damage as inducer of the signal pathway towards apoptosis and/or growth arrest in cisplatin-treated tumour cells.

Telomeres and telomerase as targets for anticancer drug development

In most human cancers, the telomere erosion problem has been bypassed through the activation of a telomere maintenance system (usually activation of telomerase). Therefore, telomere and telomerase are attractive targets for anti-cancer therapeutic interventions. Here, we review a large panel of strategies that have been explored to date, from small inhibitors of the catalytic sub-unit of telomerase to anti-telomerase immunotherapy and gene therapy. The many positive results that are reported from anti-telomere/telomerase assays suggest a prudent optimism for a possible clinical application in a close future. However, we discuss some of the main limits for these approaches of antitumour drug development and why significant work remains before a clinically useful drug can be proposed to patients.

Induction of unique structural changes in guanine-rich DNA regions by the triazoloacridone C-1305, a topoisomerase II inhibitor with antitumor activities

We recently reported that the antitumor triazoloacridone, compound C-1305, is a topoisomerase II poison with unusual properties. In this study we characterize the DNA interactions of C-1305 in vitro, in comparison with other topoisomerase II inhibitors. Our results show that C-1305 binds to DNA by intercalation and possesses higher affinity for GC- than AT-DNA as revealed by surface plasmon resonance studies. Chemical probing with DEPC indicated that C-1305 induces structural perturbations in DNA regions with three adjacent guanine residues. Importantly, this effect was highly specific for C-1305 since none of the other 22 DNA interacting drugs tested was able to induce similar structural changes in DNA. Compound C-1305 induced stronger structural changes in guanine triplets at higher pH which suggested that protonation/deprotonation of the drug is important for this drug-specific effect. Molecular modeling analysis predicts that the zwitterionic form of C-1305 intercalates within the guanine triplet, resulting in widening of both DNA grooves and aligning of the triazole ring with the N7 atoms of guanines. Our results show that C-1305 binds to DNA and induces very specific and unusual structural changes in guanine triplets which likely plays an important role in the cytotoxic and antitumor activity of this unique compound.

Telomeric DNA Damage by Topoisomerase I

Topoisomerase I adjusts torsional stress in the genome by breaking and resealing one strand of the helix through a transient covalent coupling between enzyme and DNA. Camptothecin, a specific topoisomerase I poison, traps this covalent intermediate, thereby damaging the genome. Here we examined the activity of topoisomerase I at telomeric repeats to determine whether telomere structures are targets for DNA damage. We show that topoisomerase I is catalytically active in cleaving the G-rich telomeric strand in vitro in the presence of camptothecin but not in cleaving the C-rich strand. The topoisomerase I cleavage site is 5'-TT (downward arrow) AGGG-3' (cleavage site marked by the downward arrow). We also show that endogenous topoisomerase I can access telomeric DNA in vivo and form camptothecin-dependent covalent complexes. Therefore, each telomeric repeat represents a potential topoisomerase I cleavage site in vivo. Because telomere structures are comprised of a large number of repeats, telomeres in fact represent a high concentration of nested topoisomerase I sites. Therefore, more telomeric DNA damage by camptothecin could occur in cells with longer telomeres when cells possess equivalent levels of topoisomerase I. The evidence presented here suggests that DNA damage at telomeric repeats by topoisomerase I is a prominent feature of cell killing by camptothecin and triggers camptothecin-induced apoptosis.

DNA damage transiently increases TRF2 mRNA expression and telomerase activity

Telomerase activity transiently increases when HL60 cells are treated with the topoisomerase II inhibitor etoposide. A quantitative assessment revealed that telomerase is activated by etoposide treatment in a number of cell lines and that the increase is reversible after withdrawal of etoposide from the cell culture. Telomerase activation correlated with the occurrence of DNA damage but not with cell cycle arrest. We did not detect any transcriptional upregulation of hTERT mRNA, suggesting a post-transcriptional mechanism of telomerase activation. Furthermore, the mRNA expression of the telomere binding protein TRF2 was upregulated early and reversibly after etoposide treatment. TRF1 mRNA expression levels were unchanged after DNA damage, but increased when the cells accumulated in the G2/M phase. The data show that the telosome reacts after DNA damage by upregulating telomerase activity and TRF2 expression in malignant cells. It has previously been shown that overexpression of TRF2 can repress senescence signals arising from critically shortened telomeres. We show here that TRF2 is upregulated by undirected DNA damage that also affects the telomeric DNA. These data suggest that upregulation of telomerase activity and TRF2 expression might act as antiapoptotic mechanisms in the DNA-damage response of malignant cells.

Telomerase inhibition as cancer therapy

A number of different approaches have been developed to inhibit telomerase activity in human cancer cells. Different components and types of inhibitors targeting various regulatory levels have been regarded as useful for telomerase inhibition. Most methods, however, rely on successive telomere shortening. This process is very slow and causes a long time lag between the onset of inhibition and the occurrence of senescence or apoptosis as a reversal of the immortal phenotype. Many telomerase inhibitors seem to be most efficient when combined with conventional chemotherapeutics. There are some promising approaches that seem to circumvent the slow way of telomere shortening and induce fast apoptosis in treated tumor cells. It has been demonstrated that telomerase may be involved in triggering apoptosis, but the underlying molecular mechanism remains unclear.

Extracellular superoxide dismutase is a major antioxidant in human fibroblasts and slows telomere shortening

There is good evidence that telomere shortening acts as a biological clock in human fibroblasts, limiting the number of population doublings a culture can achieve. Oxidative stress also limits the growth potential of human cells, and recent data show that the effect of mild oxidative stress is mediated by a stress-related increased rate of telomere shortening. Thus, fibroblast strains have donor-specific antioxidant defense, telomere shortening rate, and growth potential. We used low-density gene expression array analysis of fibroblast strains with different antioxidant potentials and telomere shortening rates to identify gene products responsible for these differences. Extracellular superoxide dismutase was identified as the strongest candidate, a correlation that was confirmed by Northern blotting. Over-expression of this gene in human fibroblasts with low antioxidant capacity increased total cellular superoxide dismutase activity, decreased the intracellular peroxide content, slowed the telomere shortening rate, and elongated the life span of these cells under normoxia and hyperoxia. These results identify extracellular superoxide dismutase as an important antioxidant gene product in human fibroblasts, confirm the causal role of oxidative stress for telomere shortening, and strongly suggest that the senescence-like arrest under mild oxidative stress is telomere-driven.

The telomere protein Taz1 is required to prevent and repair genomic DNA breaks

One fundamental function of telomeres is to prevent the ends of chromosomes from being sensed and treated as DNA damage. Here we present evidence for additional roles of telomeres in promoting proper chromosome segregation and DNA repair. We find that the fission yeast telomere protein Taz1p is required for cell cycle progression at 20 degrees C, a temperature at which taz1Delta cells exhibit a G(2)/M DNA damage checkpoint delay, chromosome missegregation, and DNA double-strand breaks (DSBs). Spindle assembly checkpoint components and a checkpoint-independent function of Rad3p are required for taz1Delta cells to survive at 20 degrees C. Disruption of topoisomerase II activity suppresses the cold sensitivity of taz1Delta cells, suggesting a scenario in which telomeric entanglement is the primary defect. Furthermore, hypersensitivity to treatments that induce DSBs suggests that Taz1p is involved in DSB repair. Our observations imply roles for Taz1p-containing telomeres in preventing and repairing DNA breaks throughout the genome.

Interaction in vitro of type III intermediate filament proteins with supercoiled plasmid DNA and modulation of eukaryotic DNA topoisomerase I and II activities

To further characterize the interaction of cytoplasmic intermediate filament (cIF) proteins with supercoiled (sc)DNA, and to support their potential function as complementary nuclear matrix proteins, the type III IF proteins vimentin, glial fibrillary acidic protein, and desmin were analyzed for their capacities to interact with supercoiled plasmids containing a bent mouse gamma-satellite insert or inserts capable of non-B-DNA transitions into triplex, Z, and cruciform DNA, that is, DNA conformations typically bound by nuclear matrices. While agarose gel electrophoresis revealed a rough correlation between the superhelical density of the plasmids and their affinity for cIF proteins as well as cIF protein-mediated protection of the plasmid inserts from S1 nucleolytic cleavage, electron microscopy disclosed binding of the cIF proteins to DNA strand crossovers in the plasmids, in accordance with their potential to interact with both negatively and positively supercoiled DNA. In addition, the three cIF proteins were analyzed for their effects on eukaryotic DNA topoisomerases I and II. Possibly because cIF proteins interact with the same plectonemic and paranemic scDNA conformations also recognized by topoisomerases, but select the major groove of DNA for binding in contrast to topoisomerases that insert into the minor groove, the cIF proteins were able to stimulate the enzymes in their supercoil-relaxing activity on both negatively and positively supercoiled plasmids. The stimulatory effect was considerably stronger on topoisomerase I than on topoisomerase II. Moreover, cIF proteins assisted topoisomerases I and II in overwinding plasmid DNA with the formation of positive supercoils. Results obtained with the N-terminal head domain of vimentin harboring the DNA binding region and terminally truncated vimentin proteins indicated the involvement of both protein-DNA and protein-protein interactions in these activities. Based on these observations, it seems conceivable that cIF proteins participate in the control of the steady-state level of DNA superhelicity in the interphase nucleus in conjunction with such topoisomerase-controlled processes as DNA replication, transcription, recombination, maintenance of genome stability, and chromosome condensation and segregation.

Rapid upregulation of telomerase activity in human leukemia HL-60 cells treated with clinical doses of the DNA-damaging drug etoposide

The enzyme telomerase is implicated in cellular resistance to apoptosis, but the mechanism for this resistance remains to be elucidated. The ability of telomerase to synthesize new DNA at telomeres suggests that this enzyme might function in the repair of double-stranded DNA breaks. To distinguish the effects of double-stranded DNA break damage and apoptosis on human telomerase activity, we treated the HL-60 human hematopoietic cancer cell line with clinical doses of the chemotherapeutic drug etoposide (0.5 to 5 microM), which allowed us to distinguish between events associated with DNA damage-induced cell cycle arrest, and events associated with apoptosis. Large (three- to seven-fold) upregulation of telomerase activity occurred soon after etoposide treatment (3 h) in S/G2/M-arresting populations; this upregulation was abolished at onset of apoptotic cell death. No upregulation of telomerase activity was observed in cells treated with a larger dose of etoposide (5 microM) that caused cells to undergo rapid apoptosis without intervening cell cycle arrests. These observations are consistent with a possible role for telomerase upregulation during the DNA damage response.

A major breakpoint cluster domain in murine radiation-induced acute myeloid leukemia

Cytogenetic and molecular studies have provided evidence of the clustering of chromosome 2 deletion breakpoints in radiation-induced murine acute myeloid leukemia (AML). Moreover, clustering occurs in at least two fragile domains rich in telomere-like arrays. Here we describe a physical map of the distal breakpoint cluster and confirm the presence of inverted head-to-head telomeric sequence arrays. These potentially recombinogenic sequences were not, however, the direct focus for post-irradiation chromosome breakage in AML. Instead, the two arrays bordered a 2.5-kb sequence with properties expected of a nuclear matrix attachment region (MAR). The putative MAR co-localized in the fragile domain with genes important to the hemopoietic system (leukocyte tyrosine kinase, zinc finger protein 106, erythrocyte protein band 4.2, and beta(2)-microglobulin (beta2m)); the beta2m subdomain was a particular focus of breakage. On the basis of these and other data, we suggest that AML-associated chromosome 2 fragility in the mouse is a consequence of domain-specific fragility in genomic domains containing numerous genes critical to the hemopoietic system. Recorded with the permission of the controller of Her Majesty's Stationery Office. Published by Wiley-Liss, Inc.

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.

Biological basis for chemo-radiotherapy interactions

For over 10 years, chemo-radiotherapeutic combinations have been used to treat locally advanced epithelial tumours. The rationale for these combinations relies on spatial cooperation or interaction between modalities. Interactions may take place (i) at the molecular level, with altered DNA repair or modification of the lesions induced by drugs or radiation, (ii) at the cellular level, notably through cytokinetic cooperation arising from differential sensitivity of the various compartments of the cell cycle to the drug or radiation, and (iii) at the tissue level, including reoxygenation, increased drug uptake or inhibition of repopulation or angiogenesis. Some mechanisms underlying interaction of radiation with cis-diammino-platinum (II) (cis-Pt), 5-fluoro-2'-deoxyuridine (5-FU), taxanes and gemcitabine are described. It is shown how various mechanisms including cell synchronisation and reoxygenation concur to paclitaxel-induced radiosensitisation. In the future, specific targeting of tumours, for example, with the epidermal growth factor receptor (EGFR) or angiogenesis inhibitors, should be achieved in order to increase the therapeutic index.

Telomeres and replicative senescence: Is it only length that counts?

Telomeres are well established as a major 'replicometer', counting the population doublings in primary human cell cultures and ultimately triggering replicative senescence. However, neither is the pace of this biological clock inert, nor is there a fixed threshold telomere length acting as the universal trigger of replicative senescence. The available data suggest that opening of the telomeric loop and unscheduled exposure of the single-stranded G-rich telomeric overhang might act like a semaphore to signal senescent cell cycle arrest. Short telomere length, telomeric single-strand breaks, low levels of loop-stabilizing proteins, or other factors may trigger this opening of the loop. Thus, both telomere shortening and the ultimate signalling into senescence are able to integrate different environmental and genetic factors, especially oxidative stress-mediated damage, which might otherwise become a thread to genomic stability.

A novel topoisomerase II poison GL331 preferentially induces DNA cleavage at (C/G)T sites and can cause telomere DNA damage

PURPOSE

Topoisomerase II (Topo II) preferentially cuts DNA at alternating purine-pyrimidine repeats. Different Topo II poisons may affect Topo II to produce distinct drug-specific DNA cleavage patterns. GL331 is a new podophyllotoxin derivative exhibiting potent Topo II-poisoning activity. Therefore, the sequence selectivity of GL331-induced DNA cleavage was determined.

METHODS

Human gastric adenocarcinoma SC-M1 cells were treated with GL331, and the resultant DNA fragments were isolated by SDS-K+ precipitation. These DNA fragments were further cloned and sequenced to exhibit GL331-induced DNA cleavage sites. In addition, the telomere damage was detected by Southern blot analyses using a (TTAGGG)4 probe. GL331's effect on telomerase was examined using the TRAP assay.

RESULTS

The selective sequences of GL331-induced DNA cleavage were analyzed. The first nucleotide 3'-terminal to the cleavage sites was preferentially C or G and followed by the second nucleotide T. More than 50% of GL331-induced DNA cleavage fragments exhibited AT-rich sequences in the first 20 nucleotides. In addition, the telomeric damage was observed both from GL331-treated SC-M1 cells and in vitro incubation of genomic DNA with GL331 and purified human Topo II. Although GL331 treatment reduced cellular telomerase activity, in vitro reaction data suggested that GL331 was not a telomerase inhibitor.

CONCLUSION

GL331 preferentially induced Topo II-mediated DNA cleavage at (C/G)T sites. Because the telomeric repeat sequence contains GL331's GT preference site, the telomere was identified as one of the targets of GL331-induced DNA damage.

Sequence-specific recognition and cleavage of telomeric repeat (TTAGG)(n) by endonuclease of non-long terminal repeat retrotransposon TRAS1

The telomere of the silkworm Bombyx mori consists of (TTAGG/CCTAA)(n) repeats and harbors a large number of telomeric repeat-specific non-long terminal repeat retrotransposons, such as TRAS1 and SART1. To understand how these retrotransposons recognize and integrate into the telomeric repeat in a sequence-specific manner, we expressed the apurinic-apryrimidinic endonuclease-like endonuclease domain of TRAS1 (TRAS1 EN), which is supposed to digest the target DNA, and characterized its enzymatic properties. Purified TRAS1 EN could generate specific nicks on both strands of the telomeric repeat sequence between T and A of the (TTAGG)(n) strand (bottom strand) and between C and T of the (CCTAA)(n) strand (top strand). These sites are consistent with insertion sites expected from the genomic structure of boundary regions of TRAS1. Time course studies of nicking activities on both strands revealed that the cleavages on the bottom strand preceded those on the top strand, supporting the target-primed reverse transcription model. TRAS1 EN could cleave the telomeric repeats specifically even if it was flanked by longer tracts of nontelomeric sequence, indicating that the target site specificity of the TRAS1 element was mainly determined by its EN domain. Based on mutation analyses, TRAS1 EN recognizes less than 10 bp around the initial cleavage site (upstream 7 bp and downstream 3 bp), and the GTTAG sequence especially is essential for the cleavage reaction on the bottom strand (5'. TTAGGTT downward arrow AGG. 3'). TRAS1 EN, the first identified endonuclease digesting telomeric repeats, may be used as a genetic tool to shorten the telomere in insects and some other organisms.

Interaction of human DNA topoisomerase I with G-quartet structures

Because of their role in the control of the topological state of DNA, topoisomerases are ubiquitous and vital enzymes, which participate in nearly all events related to DNA metabolism including replication and transcription. We show here that human topoisomerase I (Topo I) plays an unexpected role of 'molecular matchmaker' for G-quartet formation. G-quadruplexes are multi-stranded structures held together by square planes of four guanines ('G-quartets') interacting by forming Hoogsteen hydrogen bonds. Topo I is able to promote the formation of four-stranded intermolecular DNA structures when added to single-stranded DNA containing a stretch of at least five guanines. We provide evidence that these complexes are parallel G-quartet structures, mediated by tetrads of hydrogen-bonded guanine. In addition, Topo I binds specifically to pre-formed parallel and anti-parallel G4-DNA.

Role of oxidative stress in telomere length regulation and replicative senescence

Replicative senescence is tied into organismal aging processes in more than one respect, and telomeres appear to be the major trigger of replicative senescence under many conditions in vitro and in vivo. However, the structure-function relationships in telomeres, the mechanisms of telomere shortening with advancing replicative age, and the regulation of senescence by telomeres are far from understood. Combining recent data on telomere structure, function of telomere-binding proteins, and sensitivity of telomeres to oxidative damage, an integrative model of telomere shortening and signaling is developed. The model suggests that t-loop formation hinders access of repair proteins to telomeres, leading to accumulation of a basic sites and single-strand breaks. These might contribute to accelerated telomere shortening by transient stalling of replication as well as, if present in high concentrations, to a relief of torsional tension which might destabilize the telomeric loop structure. As a result, the single-stranded G-rich overhang, which is present at the very ends of telomeres but is normally protected at the base of the telomeric loop, will be exposed to the nucleoplasm. Free G-rich telomeric single strands are a strong inductor of the p53 pathway, and exposure of the overhangs seems to be the first step in the signal transduction cascade to replicative senescence.

Telomerase: a therapeutic target for the third millennium?

Telomerase offers the potential opportunity to control cell proliferation by interfering with a totally new and unique biological process which is cell senescence. The aim of this review is to impartially present the state of the art in telomerase with the pros and the cons of the current scientific situation of this fast-growing and fascinating topic for answering the key question asked by experimental and medical oncologists: Will telomerase be a therapeutic target for the third millenium? The most convincing argument (which is a scientifically documented one) for going ahead with this target is obviously the strong correlation existing between the level and frequency of telomerase expression and the malignant properties of tumors. This has been now largely documented in established tumor cell lines and fresh tumor samples obtained from patients. Noteworthy is the very important difference of telomerase expression between malignant and normal tissues. This difference is much higher than those observed for classical enzymatic targets of chemotherapy such as thymidylate synthetase, dihydrofolate reductase and topoisomerases. If this translates to the clinical situation, telomerase inhibitors might display a good selectivity for tumor cells with a minimal toxicity for normal tissues. The most appealing criticism (which is still purely speculative) is obviously the clinical relevance of inhibiting telomerase in cancer patients. According to the paradigm currently proposed for telomeres and telomerases, it can be predicted that telomerase inhibition will not affect a tumor until its telomeres reach the critical size for entering senescence. This means that during anti-telomerase therapy, the tumor cells will continue grow undergoing 20-30 divisions until the telomeres reach a critical size leading to tumor senescence. Does this make sense, especially in patients with advanced tumors at the beginning of the therapy? Ultimately, the definitive answer to the question will not come from intellectual speculation but from the properties of telomerase inhibitors, first in tumor bearing animals, then finally in cancer patients! Several institutions are very active in the development of telomerase inhibitors. Different stategies are used: direct inhibition of telomerase, interference with telomeres (G quartets), interaction with other proteins involved in the regulation of telomerase and telomeres.