Block in anaphase chromosome separation caused by a telomerase template mutation

RAD51 is required for propagation of the germinal nucleus in Tetrahymena thermophila

RAD51, the eukaryote homolog of the Escherichia coli recA recombinase, participates in homologous recombination during mitosis, meiosis, and in the repair of double-stranded DNA breaks. The Tetrahymena thermophila RAD51 gene was recently cloned, and the in vitro activities and induction of Rad51p following DNA damage were shown to be similar to that of RAD51 from other species. This study describes the pattern of Tetrahymena RAD51 expression during both the cell cycle and conjugation. Tetrahymena RAD51 mRNA abundance is elevated during macronuclear S phase during vegetative cell growth and with both meiotic prophase and new macronuclear development during conjugation. Gene disruption of the macronuclear RAD51 locus leads to severe abnormalities during both vegetative growth and conjugation. rad51 nulls divide slowly and incur rapid deterioration of their micronuclear chromosomes. Conjugation of two rad51 nulls leads to an arrest early during prezygotic development (meiosis I). We discuss the potential usefulness of the ciliates' characteristic nuclear duality for further analyses of the potentially unique roles of Tetrahymena RAD51.

Inhibition of experimental liver cirrhosis in mice by telomerase gene delivery

Accelerated telomere loss has been proposed to be a factor leading to end-stage organ failure in chronic diseases of high cellular turnover such as liver cirrhosis. To test this hypothesis directly, telomerase-deficient mice, null for the essential telomerase RNA (mTR) gene, were subjected to genetic, surgical, and chemical ablation of the liver. Telomere dysfunction was associated with defects in liver regeneration and accelerated the development of liver cirrhosis in response to chronic liver injury. Adenoviral delivery of mTR into the livers of mTR(-/-) mice with short dysfunctional telomeres restored telomerase activity and telomere function, alleviated cirrhotic pathology, and improved liver function. These studies indicate that telomere dysfunction contributes to chronic diseases of continual cellular loss-replacement and encourage the evaluation of "telomerase therapy" for such diseases.

Sequence-specific binding of Taz1p dimers to fission yeast telomeric DNA

The fission yeast (Schizosaccharomyces pombe) taz1 gene encodes a telomere-associated protein. It contains a single copy of a Myb-like motif termed the telobox that is also found in the human telomere binding proteins TRF1 and TRF2, and Tbf1p, a protein that binds to sequences found within the sub-telomeric regions of budding yeast (Saccharomyces cerevisiae) chromosomes. Taz1p was synthesised in vitro and shown to bind to a fission yeast telomeric DNA fragment in a sequence specific manner that required the telobox motif. Like the mammalian TRF proteins, Taz1p bound to DNA as a preformed homodimer. The isolated Myb-like domain was also capable of sequence specific DNA binding, although with less specificity than the full-length dimer. Surprisingly, a protein extract produced from a taz1- fission yeast strain still contained the major telomere binding activity (complex I) we have characterised previously, suggesting that there could be other abundant telomere binding proteins in fission yeast. One candidate, SpX, was also synthesised in vitro, but despite the presence of two telobox domains, no sequence specific binding to telomeric DNA was detected.

Telomeres, telomerase, and myc. An update

Normal human somatic cells have a finite life span in vivo as well as in vitro and retire into senescence after a predictable time. Cellular senescence is triggered by the activation of two interdependent mechanisms. One induces irreversible cell cycle exit involving activation of two tumorsuppressor genes, p53 and pRb, and the proper time point is indicated by a critical shortening of chromosomal ends due to the end-replication problem of DNA synthesis. The development of a malignant cancer cell is only possible when both mechanisms are circumvented. The majority of human cancers and tumor cell lines produce telomerase, a ribonucleoprotein with two components required for core enzyme activity: telomerase RNA (TR) and a telomerase reverse transcriptase protein (TERT). Telomerase adds hexameric DNA repeats (TTAGGG) to telomeric ends and thus compensates the progressive loss of telomeric sequences inherent to DNA replication. While TR of telomerase is present in almost all human cells, human TERT (hTERT) was found rate limiting for telomerase activity. Ectopic expression of hTERT in otherwise mortal human cells induced efficient elongation of telomeres and permanent cell growth. While hTERT-mediated immortalization seems to have no effect on growth potential and cell cycle check points, it bestows an increased susceptibility to experimental transformation. One oncogene that might activate TERT in the natural context is c-myc. Myc genes are frequently deregulated in human tumors and myc overexpression may cause telomerase reactivation and telomere stabilization which, in turn, would allow permanent proliferation. Is this a general strategy of incipient cancer cells to escape senescence? Several recent observations indicate that other scenarios may be conceived as well.

Polyploidy: occurrence in nature, mechanisms, and significance for the megakaryocyte-platelet system

OBJECTIVE

Polyploidy, the state of having greater than the diploid content of DNA, has been recognized in a variety cells. Among these cell types, the megakaryocytes are classified as obligate polyploid cells, developing a polyploid DNA content regularly during the normal life cycle of the organism, while other cells may become polyploid only in response to certain stimuli. The objective of this review is to briefly describe the different cell cycle alterations that may lead to high ploidy, while focusing on the megakaryocyte and the importance of high ploidy to platelet level and function.

MATERIALS AND METHODS

Relevant articles appearing in scientific journals and books published in the United States and in Europe during the years 1910-1999 were used as resources for this review. We selected fundamental studies related to cell cycle regulation as well as studies relevant to the regulation of the endomitotic cell cycle in megakaryocytes. Also surveyed were publications describing the relevance of high ploidy to high platelet count and to platelet reactivity, in normal situations and in a disease state.

RESULTS

Different cells may achieve polyploidy through different alterations in the cell cycle machinery.

CONCLUSIONS

While upregulation of cyclin D3 further augments ploidy in polyploidizing megakaryocytes in vivo, future investigation should aim to explore how normal megakaryocytes may initiate the processes of skipping late anaphase and cytokinesis associated with high ploidy. In humans, under normal conditions, megakaryocyte ploidy correlates with platelet volume, and large platelets are highly reactive. This may not apply, however, to the disease state.

Accelerated telomere shortening in the human inactive X chromosome

Telomeres are nucleoprotein complexes at the end of eukaryotic chromosomes, with important roles in the maintenance of genomic stability and in chromosome segregation. Normal somatic cells lose telomeric repeats with each cell division both in vivo and in vitro. To address a potential role of nuclear architecture and epigenetic factors in telomere-length dynamics, the length of the telomeres of the X chromosomes and the autosomes was measured in metaphases from blood lymphocytes of human females of various ages, by quantitative FISH with a peptide nucleic-acid telomeric probe in combination with an X-chromosome centromere-specific probe. The activation status of the X chromosomes was simultaneously visualized with antibodies against acetylated histone H4. We observed an accelerated shortening of telomeric repeats in the inactive X chromosome, which suggests that epigenetic factors modulate not only the length but also the rate of age-associated telomere shortening in human cells in vivo. This is the first evidence to show a differential rate of telomere shortening between and within homologous chromosomes in any species. Our results are also consistent with a causative role of telomere shortening in the well-documented X-chromosome aneuploidy in aging humans.

Telomerase activity and telomere detection during early bovine development

The ends of mammalian chromosomes are composed of repeated DNA sequences of (TTAGGG)(n) known as telomeres. Telomerase is a ribonucleoprotein that synthesizes telomeric DNA to replenish the 50-200 bp lost during cell replication. Cellular aging and senescence are associated with a lack of telomerase activity and a critical shortening of the telomere. The objectives of this study were to confirm the presence of TTAGGG repeats on the chromosomes of bovine embryos using in situ hybridization and assess the relative amounts of telomerase activity using a telomeric repeat amplification protocol (TRAP) during oocyte maturation and early embryo development. Applying a telomere DNA probe to the chromosomes of blastocysts and adult fibroblasts, telomeres were identified on the terminal ends of the p and q arms of chromosomes in all cells examined. Immature oocytes, matured oocytes, zygotes, 2- to 5-cell embryos, 6- to 8-cell embryos, morulae, and blastocysts were lysed in NP-40 lysis buffer and telomerase activity was assayed using the TRAP assay. Telomerase activity was detected in all developmental stages examined. Relative telomerase activity (based on telomerase internal standards and positive controls) appeared to decrease during oocyte maturation and subsequent development to the 8-cell stage but significantly increased (P < 0.05) by approximately 40-fold at the morula and blastocyst stages. It was concluded that the telomeres of bovine chromosomes contain TTAGGG repeats and that telomerase activity is up-regulated in morulae and blastocysts.

Telomere instability in a human cancer cell line

Telomere maintenance is essential in immortal cancer cells to compensate for DNA lost from the ends of chromosomes, to prevent chromosome fusion, and to facilitate chromosome segregation. However, the high rate of fusion of chromosomes near telomeres, termed telomere association, in many cancer cell lines has led to the proposal that some cancer cells may not efficiently perform telomere maintenance. Deficient telomere maintenance could play an important role in cancer because telomere associations and nondisjunction have been demonstrated to be mechanisms for genomic instability. To investigate this possibility, we have analyzed the telomeres of the human squamous cell carcinoma cell line SQ-9G, which has telomere associations in approximately 75% of the cells in the population. The absence of detectable telomeric repeat sequences at the sites of these telomere associations suggests that they result from telomere loss. The analysis of telomere length by quantitative in situ hybridization demonstrated that, compared to the human squamous cell carcinoma cell line SCC-61 which has few telomere associations, SQ-9G has more extensive heterogeneity in telomere length and more telomeres without detectable telomeric repeat sequences. The dynamics of the changes in telomere length also demonstrated a higher rate of fluctuation in telomere length, both on individual telomeres and coordinately on all telomeres. These results demonstrate that telomere maintenance can play a role in the genomic instability seen in cancer cells.

Probing the structure of multi-stranded guanine-rich DNA complexes by Raman spectroscopy and enzymatic degradation

The multi-stranded DNA complexes formed by the oligonucleotides d(T15G4T2G4), Tel, and d(T15G15), TG, were examined by nuclease digestion and Raman spectroscopy. Both Tel and TG can aggregate to form structures consisting of multiple, parallel-oriented DNA strands with two independent structural domains. Overall, the structures of the TG and Tel aggregates appear similar. According to the Raman data, the majority of bases are in C2'-endo/anti conformation. The interaction of guanines at the 3'-ends in both complexes stabilizes the complexes and protects them from degradation by exonuclease III. The 5'-extensions remain single-stranded and the thymines are accessible to single-strand-specific nuclease digestion. The extent of enzymatic cleavage at the junction at the 5' end of the 15 thymines implies a conformational change between this part of the molecule and the guanine-rich region. The differential enzymatic sensitivity of the complexes suggests there are variations in backbone conformations between TG and Tel aggregates. TG aggregates were more resistant to digestion by DNase I, Mung Bean nuclease, and S1 nuclease than Tel complexes. It is proposed that the lower DNase I sensitivity may be partly due to the more stable backbone exhibited by TG than Tel complexes. Structural uniformity along the guanine core of TG is suggested, as there is no indication of structural discontinuities or protected sites in the guanine-rich regions of TG aggregates. The lower extent of digestion by Mung Bean nuclease at the 3' end implies that these bases are inaccessible to the enzyme. This suggests that there is minimal fraying at the ends, which is consistent with the extreme thermal stability of the TG aggregates.

Uncapping and deregulation of telomeres lead to detrimental cellular consequences in yeast

Telomeres are the protein-nucleic acid structures at the ends of eukaryote chromosomes. Tandem repeats of telomeric DNA are templated by the RNA component (TER1) of the ribonucleoprotein telomerase. These repeats are bound by telomere binding proteins, which are thought to interact with other factors to create a higher-order cap complex that stabilizes the chromosome end. In the budding yeast Kluyveromyces lactis, the incorporation of certain mutant DNA sequences into telomeres leads to uncapping of telomeres, manifested by dramatic telomere elongation and increased length heterogeneity (telomere deregulation). Here we show that telomere deregulation leads to enlarged, misshapen "monster" cells with increased DNA content and apparent defects in cell division. However, such deregulated telomeres became stabilized at their elongated lengths upon addition of only a few functionally wild-type telomeric repeats to their ends, after which the frequency of monster cells decreased to wild-type levels. These results provide evidence for the importance of the most terminal repeats at the telomere in maintaining the cap complex essential for normal telomere function. Analysis of uncapped and capped telomeres also show that it is the deregulation resulting from telomere uncapping, rather than excessive telomere length per se, that is associated with DNA aberrations and morphological defects.

Phosphorylation of histone H3 is required for proper chromosome condensation and segregation

Phosphorylation of histone H3 at serine 10 occurs during mitosis in diverse eukaryotes and correlates closely with mitotic and meiotic chromosome condensation. To better understand the function of H3 phosphorylation in vivo, we created strains of Tetrahymena in which a mutant H3 gene (S10A) was the only gene encoding the major H3 protein. Although both micronuclei and macronuclei contain H3 in typical nucleosomal structures, defects in nuclear divisions were restricted to mitotically dividing micronuclei; macronuclei, which are amitotic, showed no defects. Strains lacking phosphorylated H3 showed abnormal chromosome segregation, resulting in extensive chromosome loss during mitosis. During meiosis, micronuclei underwent abnormal chromosome condensation and failed to faithfully transmit chromosomes. These results demonstrate that H3 serine 10 phosphorylation is causally linked to chromosome condensation and segregation in vivo and is required for proper chromosome dynamics.

Expression of mutated Paramecium telomerase RNAs in vivo leads to templating errors that resemble those made by retroviral reverse transcriptase

Telomeric DNA consists of short, tandemly repeated sequences at the ends of chromosomes. Telomeric DNA in the ciliate Paramecium tetraurelia is synthesized by an error-prone telomerase with an RNA template specific for GGGGTT repeats. We have previously shown that misincorporation of TTP residues at the telomerase RNA templating nucleotide C52 accounts for the 30% GGGTTT repeats randomly distributed in wild-type telomeres. To more completely characterize variable repeat synthesis in P. tetraurelia, telomerase RNA genes mutated at C52 (A, U, and G) were expressed in vivo. De novo telomeric repeats from transformants indicate that the predominant TTP misincorporation error seen in the wild-type telomerase is dependent on the presence of a C residue at template position 52. Paradoxically, the effects of various other telomerase RNA template and alignment region mutations on de novo telomeres include significant changes in fidelity, as well as the synthesis of aberrant, 5-nucleotide telomeric repeats. The occurrence of deletion errors and the altered fidelity of mutated P. tetraurelia telomerase, in conjunction with misincorporation by the wild-type enzyme, suggest that the telomerase RNA template domain may be analogous to homopolymeric mutational hot spots that lead to similar errors by the human immunodeficiency virus proofreading-deficient reverse transcriptase.

Telomere dynamics in a human cancer cell line

Telomere maintenance is thought to be essential for immortalization of human cancer cells to compensate for the loss of DNA from the ends of chromosomes and to prevent chromosome fusion. We have investigated telomere dynamics in the telomerase-positive squamous cell carcinoma cell line SCC-61 by marking the ends of chromosomes with integrated plasmid sequences so that changes in the length of individual telomeres could be monitored. Despite having very short telomeres, SCC-61 has a relatively stable genome and few telomere associations. The marked telomeres in different SCC-61 clones have similar mean lengths which show little change with increasing time in culture. Thus, each marked telomere is maintained at a specific length, which we term the equilibrium mean length (EML). The Gaussian distribution in the length of the marked telomeres demonstrates that telomeres continuously fluctuate in length. Consistent with this observation, the mean lengths of the marked telomere in subclones of these cell lines initially differ, but then gradually return to the EML of the original clone with increasing time in culture. The analysis of a clone with two marked telomeres demonstrated that changes in telomere length can occur on each marked telomere independently or coordinately on both telomeres. These results suggest that the short telomeres in many tumor cell lines do not result from an inability to properly maintain telomeres at a specific length.

Telomere length dynamics and chromosomal instability in cells derived from telomerase null mice

To study the effect of continued telomere shortening on chromosome stability, we have analyzed the telomere length of two individual chromosomes (chromosomes 2 and 11) in fibroblasts derived from wild-type mice and from mice lacking the mouse telomerase RNA (mTER) gene using quantitative fluorescence in situ hybridization. Telomere length at both chromosomes decreased with increasing generations of mTER-/- mice. At the 6th mouse generation, this telomere shortening resulted in significantly shorter chromosome 2 telomeres than the average telomere length of all chromosomes. Interestingly, the most frequent fusions found in mTER-/- cells were homologous fusions involving chromosome 2. Immortal cultures derived from the primary mTER-/- cells showed a dramatic accumulation of fusions and translocations, revealing that continued growth in the absence of telomerase is a potent inducer of chromosomal instability. Chromosomes 2 and 11 were frequently involved in these abnormalities suggesting that, in the absence of telomerase, chromosomal instability is determined in part by chromosome-specific telomere length. At various points during the growth of the immortal mTER-/- cells, telomere length was stabilized in a chromosome-specific man-ner. This telomere-maintenance in the absence of telomerase could provide the basis for the ability of mTER-/- cells to grow indefinitely and form tumors.

DNA molecules can drive the assembly of other DNA molecules into specific four-stranded structures

Single-stranded DNA molecules containing clustered G-repeats can be assembled into various four-stranded structures linked by G-quartets. Here, we report that such molecules can also drive the assembly of other DNA molecules containing G-repeats into specific four-stranded structures. In these assays, the oligonucleotides 5'-CAGGCTGAGCAGGTACGGGGGAGCTGGGGTAGATTGGAATGTAG-3' (oligo D) and 5'-CGGGGGAGCTGGGGT-3' (oligo B), consisting of sequences found in immunoglobulin switch regions, were annealed in a buffer containing K+ and the annealing products were analyzed by polyacrylamide gel electrophoresis. This analysis revealed that whereas annealing of each oligo alone produced four-stranded structures designated D2 and B2, annealing of mixtures containing both oligos produced additional complexes designated D2* and B2*. D2* and B2* were found to contain only D molecules and only B molecules, respectively. The yield of D2* increased and the yield of B2* decreased, as the concentration ratio oligo B/oligo D was increased. These results indicated that B can drive the assembly of D into D2* and D can drive the assembly of B into B2*. Further studies revealed that while the assembly of D2 followed a second order kinetics, the B-driven assembly of D2* followed a first order kinetics. Dimethyl sulfate footprinting indicated that both D2 and D2* are four-stranded structures containing two parallel and two antiparallel chains. In addition, annealing of D mixed with various B mutants showed that only mutants containing two G-clusters can drive the assembly of D2*. Based on these data, we propose that in the process of D2* assembly, a four-stranded intermediate containing B and D is formed and then dissociates into D2* and B in a rate-limiting first order reaction. Driver mechanisms of this type may cause formation of specific four-stranded structures at G-rich chromosomal sites, thereby regulating processes such as recombination and telomere synthesis.

Absence or low number of telomere repeats at junctions of dicentric chromosomes

Human ovarian surface epithelial (HOSE) cells transfected with the E6 and E7 oncogenes of the human papilloma virus (PV) do not express measurable telomerase activity. Relative to untransfected control cells, HOSE-PV cells have an extended in vitro lifespan characterized by a very high frequency of telomeric associations (TAs) of chromosomes. In order to study the role of telomere shortening in the formation of TAs, we studied the telomere length in 120 dicentric chromosomes in HOSE-PV cells by using quantitative fluorescence in situ hybridization. Forty percent of the dicentric chromosomes had no fluorescence signal at the junction site, and in the remainder the fluorescence at the junction was less than at corresponding unjoined ends. These observations support a critical role of telomere shortening in the development of TAs and the subsequent genetic instability observed in a majority of tumor cells.

Telomerase is controlled by protein kinase Calpha in human breast cancer cells

Telomerase, a specialized RNA-directed DNA polymerase that extends telomeres of eukaryotic chromosomes, is repressed in human somatic tissues and becomes activated during tumor progression in most human cancers. To date, little is known about how telomerase is activated and controlled in cancer, although activation is thought to be involved in cancer cell immortalization. Here, we report that human telomerase-associated protein 1 (hTEP1) and the telomerase catalytic subunit (human telomerase reverse transcriptase (hTERT)) are phosphoproteins and that their phosphorylation is a prerequisite for the activation of telomerase in intact human breast cancer cells. Identified by hTEP1 peptide affinity chromatography, protein kinase Calpha mediates the phosphorylation of hTEP1 and hTERT and induces a marked increase in telomerase activity. Thus, phosphorylation of hTEP1 and hTERT by protein kinase Calpha represents an essential step in the generation of a functional telomerase complex in the initiation and maintenance of telomerase activity in human cancer.

Chromosome break-induced DNA replication leads to nonreciprocal translocations and telomere capture

In yeast, broken chromosomes can be repaired by recombination, resulting in nonreciprocal translocations. In haploid cells suffering an HO endonuclease-induced, double-strand break (DSB), nearly 2% of the broken chromosome ends recombined with a sequence near the opposite chromosome end, which shares only 72 bp of homology with the cut sequence. This produced a repaired chromosome with the same 20-kb sequence at each end. Diploid strains were constructed in which the broken chromosome shared homology with the unbroken chromosome only on the centromere-proximal side of the DSB. More than half of these cells repaired the DSB by copying sequences distal to the break from the unbroken template chromosome. All these events were RAD52 dependent. Pedigree analysis established that DSBs occurring in G1 were repaired by a replicative mechanism, producing two identical daughter cells. We discuss the implications of these data in understanding telomerase-independent replication of telomeres, gene amplification, and the evolution of chromosomal ends.

Telomere length regulation--a view from the individual chromosome perspective

Telomeres are specialized structures at chromosome termini implicated in oncogenesis and cellular aging. Since both phenomena are related to variations in telomere length it is of interest to understand mechanisms responsible for telomere length regulation. Recent studies in mammalian cells indicate that specific chromosomes may have specific telomere lengths, suggesting the existence of chromosome-specific factors involved in telomere length regulation. Although these chromosome-specific factors are largely unknown at present, in the mouse evidence suggests a possible role of centromere position in telomere length regulation-telomeres closer to centromeres (i.e., p-arm telomeres) are significantly shorter than their counterparts more distant from centromeres (i.e., q-arm telomeres). The mouse may be a special case because its karyotype consists almost exclusively of acrocentric chromosomes in which p-arm telomeres and centromeres are located immediately adjacent to each other. However, a weak correlation between telomere length and centromere position is observed in the case of nonacrocentric human and Chinese hamster chromosomes, suggesting that the putative centromere position effect might be evolutionarily conserved. Alternatively, telomere length in individual nonacrocentric chromosomes may be affected by the sequence organization of subtelomeric chromosome regions or by some other, currently unknown, factors.

Telomerase in cancer: clinical applications

The biology of telomeres and telomerase has been the subject of intensive investigative effort since it became evident that they play a significant role in two important biological processes, the loss of cellular replicative capacity inherent to organismal ageing and the unrestricted cell proliferation characteristic of carcinogenesis. Telomere shortening in normal cells is a result of DNA replication events, and reduction beyond a critical length is a signal for cellular senescence. One of the cellular mechanisms used to overcome proliferative restriction is the activation of the enzyme telomerase, which replaces the loss of telomeric DNA that occurs at each cell division. Studies have demonstrated that tumours have shorter telomeres than normal tissue and that telomerase is activated in up to 90% of all human cancers while it is present only in a limited range of normal adult tissues. The role of telomerase in the extension of the cellular replicative lifespan has recently been shown by ectopic expression of the enzyme, being consistent with the oncogenesis model whereby the acquisition of an 'immortal' phenotype is a requirement for advanced tumour progression. In this article we review the present knowledge of telomeres and telomerase in cancer and discuss the potential use of this enzyme as a diagnostic and prognostic tumour marker and as a target for cancer therapy.

A hairpin conformation for the 3' overhang of Oxytricha nova telomeric DNA

The solution secondary structure of the Oxytricha nova telomeric 3' overhang, d(T4G4)2, has been investigated by Raman spectroscopy, hydrogen-deuterium exchange kinetics and gel electrophoresis. The electrophoretic mobility of d(T4G4)2 in non-denaturing gels indicates a highly compact conformation, consistent with a hairpin secondary structure. Raman markers show that the d(T4G4)2 hairpin contains equal numbers of C2'-endo/syn and C2'-endo/anti deoxyguanosine conformers, as well as G.G base-pairs of the Hoogsteen type. The hydrogen-deuterium exchange kinetics of d(T4G4)2, monitored by time-resolved Raman spectroscopy, reveal two kinetically distinct classes of guanine imino (N1H) protons. The more slowly exchanging fraction (kN1H(1)=4.6x10(-3) min-1), which represents 50% of N1H groups, is attributed to Hoogsteen-paired residues. The more rapidly exchanging fraction (kN1H(2)>/=0.3 min-1) is attributable to solvent-exposed residues. Raman dynamic probe of the kinetics of guanine C8H-->C8(2)H exchange in d(T4G4)2 reveals modest retardation vis-à-vis dGMP, which rules out quadruplex formation by the telomeric repeat and confirms an ordered secondary structure consistent with a Hoogsteen-paired hairpin. Similar Raman, hydrogen-isotope exchange and electrophoretic mobility experiments on the related telomeric model, dT6(T4G4)2, also reveal a hairpin stabilized by Hoogsteen G.G pairs. Presence of the 5' thymidine tail preceding the Oxytricha telomeric repeat has no apparent effect on the hairpin secondary structure. We propose a molecular model for the hairpin conformation of the Oxytricha nova telomeric repeat and consider its possible roles in mechanisms of telomeric DNA interaction in vitro and telomere function in vivo.

Yeast "operons"

To achieve coordinate gene regulation, yeast (Saccharomyces cerevisiae) appears to have exploited two distinct multifunction "operon" schemas: one, by concatenating originally separate functional domains into single polypeptides, and two, by linking opposite strand genes through common promoter elements. For example, distinct functions found in bacterial operons are often concatenated in yeast. A selective advantage, similar to that for bringing multiple related functions into a single peptide, may also explain the large numbers of yeast opposite-strand, ORF pairs sharing a common regulatory region.

Two types of telomeric chromatin in Tetrahymena thermophila

The telomeric d(GGGGTT).d(AACCCC) repeat tracts (G4T2 repeats) in Tetrahymena thermophila macronuclei were shown previously to be packaged in a non-nucleosomal DNA-protein complex. Here, we demonstrate that these telomeric repeats, together with a short region of the immediately adjacent non-telomeric sequence, exist in two distinct types of chromatin. The non-nucleosomal complex (type I complex) comprises approximately 90 to 97% of telomeric DNA, has no apparent underlying periodic nucleosomal substructure, and includes the whole telomeric tract as well as the immediately adjacent sequence. Type II chromatin, comprising the remaining approximately 3 to 10% of the total telomeric DNA, consists of tightly packed nucleosomes clustered at the inner border of the telomeric tracts, with a periodicity of 154(+/-3) bp. This packing is similar to that of telomeric nucleosomes in vertebrates. However, in contrast to the unstability of vertebrate telomeric mononucleosomes, the T. thermophila mononucleosomes were stable to micrococcal nuclease digestion. During the natural lengthening of the T. thermophila telomeric DNA tracts that occurs in vegetatively dividing cells, the overall ratio of type I and type II chromatin did not change. However, type I complex expanded with the length of the telomeric DNA repeat tract, and the number of telomeric nucleosomes increased from an average of one, up to three to four, per telomeric tract. This finding of telomeric nucleosomes in T. thermophila suggests that the difference between vertebrates and lower eukaryotes in telomeric chromatin structure is quantitative rather than qualitative. We propose that deposition of nucleosomes competes with non-nucleosomal complex formation on telomeric DNA, resulting in a sub-population of chimeric telomeres containing inner nucleosomes abutting a distal, variable length of type I complex.

Characterisation of Leishmania telomeres reveals unusual telomeric repeats and conserved telomere-associated sequence

Characterisation of the telomeres of Leishmania is important for understanding many aspects of the parasitic life of this primitive protozoan and for the completion of the physical map and sequencing of the genome. After sequencing more than 300 telomere-derived clones from Leishmania braziliensis and Leishmania major, a conserved 100 bp sequence was identified immediately adjacent to the telomere at the chromosome end and was named LCTAS (Leishmania conserved telomere-associated sequence). The LCTAS contains two conserved sequence boxes, and is present in all Leishmania species studied. The organisation of the LCTAS in the telomeric region differs between L. braziliensis and L. major: in L. major the LCTASs are tandemly repeated, while in L. braziliensis the LCTAS is present as a single copy per end. Two additional TASs with 1.6 kb and 274 bp repeat structures, which are apparently different to LCTAS, were isolated and mapped onto a L. braziliensis 250 kb multicopy minichromosome and the L. major chromosome 1, respectively. An unusual feature in L. braziliensis is that the telomeric repeats are often comprised of a novel tandem repeat CCCTAACCCGTGGA. A 'slippage' mechanism for LCTAS formation is proposed in this study as an alternative way for the synthesis and maintenance of telomeres and subtelomere regions.

Unusual behavior exhibited by multistranded guanine-rich DNA complexes

The structural properties of oligonucleotides containing two different types of G-rich sequences at the 3'-ends were compared. It is shown that oligonucleotides with uninterrupted runs of guanine residues at the 3'-end, e.g., d(T15G12), form multistranded structures stabilized by guanine-guanine interactions. The chemical and physical properties of these complexes differ from those of the complexes formed by oligonucleotides with telomere-like sequences, e.g., d(T15G4T2G4). In methylation protection and methylation interference experiments, we found all the guanines in complexes formed by d(T15G15) and d(T15G12) to be accessible to methylation. Furthermore, the methylated monomers retain the ability to polymerize. This contrasts with the inaccessibility of the guanines in d(T15G4T2G4) to methylation and the inability of the methylated monomer to form supramolecular structures. The stoichiometry of the complexes arising from the two types of oligonucleotides also differs. The complexes formed by d(T15G15) consist of consecutive integer numbers of DNA strands, whereas complexes formed by telomere-like oligonucleotides contain 1, 2, 4, or multiples of four strands. Magnesium ions favor formation of high molecular weight complexes by d(T15G15) and d(T15G12), but not by d(T15G4T2G4). The d(T15G15) and d(T15G12) complexes have very high thermal stability compared with telomeric complexes. However, at low temperatures, the thymine bases within the telomeric motif, TTGGGGTTGGGG, appear to allow for the formation of stable high-molecular weight species with a longer nonguanine portion.

TRF1 promotes parallel pairing of telomeric tracts in vitro

Human telomeres consist of long arrays of TTAGGG repeats bound to the telomere-specific proteins, TRF1 and TRF2. Here we describe the structure of in vitro complexes formed between telomeric DNA and TRF1 as deduced by electron microscopy. Visualization of TRF1 bound to DNA containing six or 12 tandem TTAGGG repeats revealed a population of DNAs containing a spherical protein complex localized just to the repeats. Mass analysis of the protein complexes suggested binding of TRF1 dimers and tetramers to the TTAGGG repeats. The DNA was not significantly compacted or extended by protein binding. TRF1 formed filamentous structures on longer telomeric repeat arrays (>/=27 repeats) consistent with the presence of an array of bound TRF1 dimers. Unexpectedly, there was a strong propensity for two telomeric tracts to form paired synapses over the TRF1 covered segment. Up to 30% of the TRF1-bound DNAs could be found in a paired configuration with a strong bias for a parallel as contrasted to an antiparallel arrangement. TRF1-induced pairing was confirmed using a ligation assay which detected the formation of DNA multimers dependent on the presence of TRF1 and a 27mer repeat array in the DNA. These findings suggests that this protein may have an architectural role at telomeres. We discuss the possibility that TRF1-dependent changes in the conformation of telomeres are involved in the regulation of telomere length.

The telomere and telomerase: how do they interact?

The tandemly repeated DNA sequence of telomeres is typically specified by the ribonucleoprotein enzyme telomerase. Telomerase copies part of its intrinsic RNA moiety to make one strand of the telomeric repeat DNA. Recent work has led to the concept of a telomere homeostasis system. We have been studying two key physical components of this system: the telomere itself and telomerase. Mutating the template sequence of telomerase RNA caused various phenotypes: (1) mutating specific residues in the ciliate Tetrahymena and two yeasts showed that they are required for critical aspects of telomerase action; (2) certain mutated telomeric sequences caused a previously unreported phenotype, i.e. a strong anaphase block in Tetrahymena micronuclei; and (3) certain template mutations in the telomerase RNA gene of the yeast Kluyveromyces lactis led to unregulated telomere elongation, which in some cases was directly related to loss of binding to K. lactis Rap1p. Using K. lactis carrying alterations in the genes for Rap1p and other silencing components, we proposed a general model for telomere length homeostasis: namely, that the structure and DNA length of the DNA-protein complex that comprises the telomere are key determinants of telomerase access, and hence the frequency of action of telomerase, at the telomere.

Purification, characterization and molecular cloning of TGP1, a novel G-DNA binding protein from Tetrahymena thermophila

G-DNA, a polymorphic family of four-stranded DNA structures, has been proposed to play roles in a variety of biological processes including telomere function, meiotic recombination and gene regulation. Here we report the purification and cloning of TGP1, a G-DNA specific binding protein from Tetrahymena thermophila. TGP1 was purified by three-column chromatographies, including a G-DNA affinity column. Two major proteins (approximately 80 and approximately 40 kDa) were present in the most highly purified column fraction. Renaturation experiments showed that the approximately 80 kDa protein contains TGP1 activity. Biochemical characterization showed that TGP1 is a G-DNA specific binding protein with a preference for parallel G-DNAs. The TGP1/DNA complex has a dissociation constant (Kd) of approximately 2.2 x 10(-8) M and TGP1 can form supershift in gel mobility shift assays. The cDNA coding TGP1 was cloned and sequenced based upon an internal peptide sequence obtained from the approximately 80 kDa protein. Sequence analyses showed that TGP1 is a basic protein with a pI of 10.58, and contains two extensively hydrophilic and basic domains. Homology searches revealed that TGP1 is a novel protein sharing weak similarities with a number of proteins.

Interaction of recombinant Tetrahymena telomerase proteins p80 and p95 with telomerase RNA and telomeric DNA substrates

Telomerase is a specialized reverse transcriptase that catalyzes telomeric repeat addition at the ends of existing telomeres or fragmented chromosomes. Two telomerase proteins from Tetrahymena thermophila, p80 and p95, were identified on the basis of their association with telomerase activity and telomerase RNA. Here we have produced recombinant versions of these proteins to characterize their functions in the ribonucleoprotein. Our findings indicate that the two proteins can form a complex whose association is independent of RNA. Each protein interacts directly with telomerase RNA, but the p80/p95 complex binds RNA with an affinity substantially greater than either protein alone. We have also characterized the DNA binding properties of p95 and show that it interacts with telomeric substrate DNAs with a specificity characteristic of the functionally defined Tetrahymena telomerase substrate anchor site. Together, these findings suggest a model in which protein-nucleic acid interactions separable from the active site contribute to positioning the template and primer, rather than exclusively the direct nucleic acid-active site interaction typical of other polymerases.

Organization of chromosome ends in Ustilago maydis. RecQ-like helicase motifs at telomeric regions

In this study we have established the structure of chromosome ends in the basidiomycete fungus Ustilago maydis. We isolated and characterized several clones containing telomeric regions and found that as in other organisms, they consist of middle repeated DNA sequences. Two principal types of sequence were found: UTASa was highly conserved in nucleotide sequence and located almost exclusively at the chromosome ends, and UTASb was less conserved in nucleotide sequence than UTASa and found not just at the ends but highly interspersed throughout the genome. Sequence analysis revealed that UTASa encodes an open reading frame containing helicase motifs with the strongest homology to RecQ helicases; these are DNA helicases whose function involves the maintenance of genome stability in Saccharomyces cerevisiae and in humans, and the suppression of illegitimate recombination in Escherichia coli. Both UTASa and UTASb contain a common region of about 300 bp located immediately adjacent to the telomere repeats that are also found interspersed in the genome. The analysis of the chromosome ends of U. maydis provides information on the general structure of chromosome ends in eukaryotes, and the putative RecQ helicase at UTASa may reveal a novel mechanism for the maintenance of chromosome stability.

TRF2 protects human telomeres from end-to-end fusions

The mechanism by which telomeres prevent end-to-end fusion has remained elusive. Here, we show that the human telomeric protein TRF2 plays a key role in the protective activity of telomeres. A dominant negative allele of TRF2 induced end-to-end chromosome fusions detectable in metaphase and anaphase cells. Telomeric DNA persisted at the fusions, demonstrating that TTAGGG repeats per se are not sufficient for telomere integrity. Molecular analysis suggested that the fusions represented ligation of telomeres that have lost their single-stranded G-tails. Therefore, TRF2 may protect chromosome ends by maintaining the correct structure at telomere termini. In addition, expression of mutant forms of TRF2 induced a growth arrest with characteristics of senescence. The results raise the possibility that chromosome end fusions and senescence in primary human cells may be caused by loss by TRF2 from shortened telomeres.

Short telomeres on human chromosome 17p

Human chromosomes terminate in a series of T2AG3 repeats, which, together with associated proteins, are essential for chromosome stability. In somatic cells, these sequences are known to be gradually lost through successive cells divisions; however, information about changes on specific chromosomes is not available. Individual telomeres could mediate important biological effects as was shown in yeast, in which loss of a single telomere results in cell-cycle arrest and chromosome loss. We now demonstrate by quantitative fluorescence in situ hybridization (Q-FISH; ref. 7) that the number of T2AG3 repeats on specific chromosome arms is very similar in different tissues from the same donor and varies only to some extent between donors. In all sixteen individuals studied, telomeres on chromosome 17p were shorter than the median telomere length--a finding confirmed by analysis of terminal restriction fragments from sorted chromosomes. These observations provide evidence of chromosome-specific factors regulating the number of T2AG3 repeats in individual telomeres and raise the possibility that the relatively short telomeres on chromosome 17p contribute to the frequent loss of 17p alleles in human cancers.

Telomerase activity: a biomarker of cell proliferation, not malignant transformation

Telomerase activity is readily detected in most cancer biopsies, but not in premalignant lesions or in normal tissue samples with a few exceptions that include germ cells and hemopoietic stem cells. Telomerase activity may, therefore, be a useful biomarker for diagnosis of malignancies and a target for inactivation in chemotherapy or gene therapy. These observations have led to the hypothesis that activation of telomerase may be an important step in tumorigenesis. To test this hypothesis, we studied telomerase activity in isogeneic samples of uncultured and cultured specimens of normal human uroepithelial cells (HUCs) and in uncultured and cultured biopsies of superficial and myoinvasive transitional cell carcinoma (TCC) of the bladder. Our results demonstrated that four of four TCC biopsies, representing both superficial and myoinvasive TCCs, were positive for telomerase activity, but all samples of uncultured HUC were telomerase negative. However, when the same normal HUC samples were established as proliferating cultures in vitro, telomerase activity was readily detected but usually at lower levels than in TCCs. Consistent with the above observation of the telomerase activity in HUCs, telomeres did not shorten during the HUC in vitro lifespan. Demonstration of telomerase in proliferating human epithelial cells in vitro was not restricted to HUCs, because it was also present in prostate and mammary cell cultures. Notably, telomerase activity was relatively low or undetectable in nonproliferating HUC cultures. These data do not support a model in which telomerase is inactive in normal cells and activated during tumorigenic transformation. Rather, these data support a model in which the detection of telomerase in TCC biopsies, but not uncultured HUC samples, reflects differences in proliferation between tumor and normal cells in vivo.

Characterization and cell cycle regulation of the related human telomeric proteins Pin2 and TRF1 suggest a role in mitosis

Telomeres are essential for preserving chromosome integrity during the cell cycle and have been specifically implicated in mitotic progression, but little is known about the signaling molecule(s) involved. The human telomeric repeat binding factor protein (TRF1) is shown to be important in regulating telomere length. However, nothing is known about its function and regulation during the cell cycle. The sequence of PIN2, one of three human genes (PIN1-3) we previously cloned whose products interact with the Aspergillus NIMA cell cycle regulatory protein kinase, reveals that it encodes a protein that is identical in sequence to TRF1 apart from an internal deletion of 20 amino acids; Pin2 and TRF1 may be derived from the same gene, PIN2/TRF1. However, in the cell Pin2 was found to be the major expressed product and to form homo- and heterodimers with TRF1; both dimers were localized at telomeres. Pin2 directly bound the human telomeric repeat DNA in vitro, and was localized to all telomeres uniformly in telomerase-positive cells. In contrast, in several cell lines that contain barely detectable telomerase activity, Pin2 was highly concentrated at only a few telomeres. Interestingly, the protein level of Pin2 was highly regulated during the cell cycle, being strikingly increased in G2+M and decreased in G1 cells. Moreover, overexpression of Pin2 resulted in an accumulation of HeLa cells in G2+M. These results indicate that Pin2 is the major human telomeric protein and is highly regulated during the cell cycle, with a possible role in mitosis. The results also suggest that Pin2/TRF1 may connect mitotic control to the telomere regulatory machinery whose deregulation has been implicated in cancer and aging.

Tales of tails: regulation of telomere length and telomerase activity during lymphocyte development, differentiation, activation, and aging

Telomerase activity and the regulation of telomere length are factors which have been implicated in the control of cellular replication. These variables have been examined during human lymphocyte development, differentiation, activation, and aging. It was found that telomere length of peripheral blood CD4+ T cells decreases with age as well as with differentiation from naive to memory cells in vivo, and decreases with cell division in vitro. These results provide evidence that telomere length correlates with lymphocyte replicative history and residual replicative potential. In contrast, telomere length appears to increase during tonsil B-cell differentiation and germinal center (GC) formation in vivo. It was also found that telomerase activity is highly regulated during T-cell development and B-cell differentiation in vivo, with high levels of telomerase activity expressed in thymocytes and GC B cells, and low levels of telomerase activity in resting mature peripheral blood lymphocytes. Finally, resting lymphocytes retain the ability to upregulate telomerase activity upon activation, and this capacity does not appear to decline with age. Although the precise role of telomerase in lymphocyte function remains to be elucidated, telomerase may contribute to protection from telomere shortening in T and B lymphocytes, and may thus play a critical role in lymphocyte development, differentiation and activation. The future study of telomerase and its regulation of telomere length may enhance our understanding of how the replicative lifespan is regulated in lymphocytes.

Functionally interacting telomerase RNAs in the yeast telomerase complex

The ribonucleoprotein (RNP) enzyme telomerase from Saccharomyces cerevisiae adds telomeric DNA to chromosomal ends in short increments both in vivo and in vitro. Whether or not telomerase functions as a multimer has not been addressed previously. Here we show, first, that following polymerization, the telomerase RNP remains stably bound to its telomeric oligonucleotide reaction product. We then exploit this finding and a previously reported mutant telomerase RNA to demonstrate that, unexpectedly, the S. cerevisiae telomerase complex contains at least two functionally interacting RNA molecules that both act as templates for DNA polymerization. Here, functional telomerase contains at least two active sites.

Reprogramming of telomerase by expression of mutant telomerase RNA template in human cells leads to altered telomeres that correlate with reduced cell viability

Telomerase synthesizes telomeric DNA by copying the template sequence of its own RNA component. In Tetrahymena thermophila and yeast (G. Yu, J. D. Bradley, L. D. Attardi, and E. H. Blackburn, Nature 344:126-131, 1990; M. McEachern and E. H. Blackburn, Nature 376:403-409, 1995), mutations in the template domain of this RNA result in synthesis of mutant telomeres and in impaired cell growth and survival. We have investigated whether mutant telomerase affects the proliferative potential and viability of immortal human cells. Plasmids encoding mutant or wild-type template RNAs (hTRs) of human telomerase and the neomycin resistance gene were transfected into human cells to generate stable transformants. Expression of mutant hTR resulted in the appearance of mutant telomerase activity and in the synthesis of mutant telomeres. Transformed cells were not visibly affected in their growth and viability when grown as mass populations. However, a reduction in plating efficiency and growth rate and an increase in the number of senescent cells were detected in populations with mutant telomeres by colony-forming assays. These results suggest that the presence of mutant telomerase, even if coexpressed with the wild-type enzyme, can be deleterious to cells, likely as a result of the impaired function of hybrid telomeres.

Germ-line effects of a mutator, mu2, in Drosophila melanogaster

A mutator, mu2a, in Drosophila melanogaster potentiates terminal deficiencies. In the female germ line the gamma mutant frequency induced by irradiation of mature oocytes with 5 Gy increases approximately twofold in heterozygotes and 20-fold in homozygotes compared with wild type. The recovery of terminal deficiencies is not limited to breaks close to chromosome ends; high frequencies of deficiencies can be recovered with breakpoints located in centric heterochromatin or near the middle of a chromosome arm. Lesions induced by gamma-rays are repaired slowly in mu2a oocytes, but become "fixed" as terminal deficiencies upon fertilization. A few lesions induced in wild-type females also produce terminal deficiencies. Mutator males do not exhibit an increase in terminal deletions, regardless of the germ cell stage irradiated. In addition, there is no increase in the mutant frequency when mature sperm are irradiated and fertilize eggs produced by mu2a females. The data are consistent with the hypothesis that lesions induced in sperm chromosomes are repaired after fertilization, while lesions induced in oocyte chromosomes are shunted instead to a mechanism that stabilizes broken chromosome ends. We propose that mu2 affects chromosomal structure during oogenesis, thereby modulating DNA repair.

Protein phosphatase 2A inhibits nuclear telomerase activity in human breast cancer cells

Most cancer cells have increased levels of telomerase activity implicated in cell immortalization. Activation of telomerase, a ribonucleoprotein complex, catalyzes the elongation of the ends of mammalian chromosomal DNA (telomeres), the length of which regulates cell proliferation. Currently, how telomerase is regulated in cancer is not yet established. The present study shows that telomerase activity is regulated by protein phosphorylation in human breast cancer cells. Incubation of cell nuclear telomerase extracts with protein phosphatase 2A (PP2A) abolished the telomerase activity; in contrast cytoplasmic telomerase activity was unaffected, and protein phosphatases 1 and 2B were ineffective. Inhibition of telomerase activity by PP2A was both concentration- and time-dependent and was prevented by the protein phosphatase inhibitor okadaic acid. In addition, nuclear telomerase inhibited by PP2A was reactivated by endogenous protein kinase(s) in the presence of ATP, but not in the presence of ATPgammaS. Furthermore, telomerase activity in cultured human breast cancer PMC42 cells was stimulated by okadaic acid, consistent with a role for PP2A in the regulation of telomerase activity in intact cells. These findings suggest that protein phosphorylation reversibly regulates the function of telomerase and that PP2A is a telomerase inhibitory factor in the nucleus of human breast cancer cells.

A telomerase mutant defective in sister chromatid separation at mitosis

The telomere is a functional domain of the chromosome, located at the extreme ends, and is essential for normal chromosome stability. Chromosomes lacking telomeres are inherited improperly, and mutations in the telomeric repeat sequences are thought to lead to senescence and possibly to cancer. The molecular mechanisms maintaining chromosomes by telomeres, however, have been unclear. Results recently reported by Kirk et al, offer an insight into new telomerase function. They have identified a novel telomerase mutation that blocks sister chromatid separation in mitosis.