Telomere dysfunction increases mutation rate and genomic instability

Dual roles of telomere dysfunction in initiation and suppression of tumorigenesis

Human carcinomas arise through the acquisition of genetic changes that endow precursor cancer cells with a critical threshold of cancer-relevant genetic lesions. This complex genomic alterations confer upon precursor cancer cells the ability to grow indefinitely and to metastasize to distant sites. One important mechanism underlying a cell's tumorigenic potential is the status of its telomere. Telomeres are G-rich simple repeat sequences that serve to prevent chromosomal ends from being recognized as DNA double-strand breaks (DSBs). Dysfunctional telomeres resemble DSBs, leading to the formation of dicentric chromosomes that fuel high degrees of genomic instability. In the setting of an intact p53 pathway, this instability promotes cellular senescence, a potent tumor suppressor mechanism. However, rare cells that stochastically lose p53 function emerge from this sea of genomic instability and progress towards cancer. In this review, we describe the use of mouse models to probe the impact of dysfunctional telomeres on tumor initiation and suppression.

Retrosequence formation restructures the yeast genome

Retrosequences generated by reverse transcription of mRNA transcripts have a substantial influence on gene expression patterns, generation of novel gene functions, and genome organization. The Ty1 retrotransposon is a major source of RT activity in the yeast, Saccharomyces cerevisiae, and Ty1 retromobility is greatly elevated in strains lacking telomerase. We report that Ty1-dependent formation of retrosequences derived from single-copy gene transcripts is progressively elevated as yeast cells senesce in the absence of telomerase. Retrosequences are frequently fused to Ty1 sequences, and occasionally to sequences from other mRNA transcripts, forming chimeric pseudogenes. Efficient retrosequence formation requires the homologous recombination gene RAD52. Selection for retrosequence formation is correlated with a high frequency of chromosome rearrangements in telomerase-negative yeast. Ty1-associated retrosequences were present at the breakpoint junctions of four chromosomes analyzed in detail. Our results support a role for reverse transcripts in promoting chromosome rearrangements.

The Paf1 complex promotes displacement of histones upon rapid induction of transcription by RNA polymerase II

BACKGROUND

The yeast Paf1 protein complex is required for efficient transcription elongation by RNA polymerase II (RNA pol II), but the precise role of the complex has been unclear.

RESULTS

Here we show that depletion of the Ctr9 or Paf1 component of the Paf1 complex delays the loss of histones from the GAL1 gene upon induction. This delay in histone removal is accompanied by a decrease in association of RNA pol II with GAL1 and altered distribution of the polymerase along the locus.

CONCLUSION

These observations may explain why initial induction of GAL transcripts is reduced in Ctr9- or Paf1-deficient cells, and is consistent with a model suggesting that the Paf1 complex and the histone modifications that it mediates increase efficiency of transcriptional elongation by promoting nucleosomal destabilization and histone removal.

Protection against chromosome degradation at the telomeres

Telomeres, the ends of linear chromosomes, contain repeated TG-rich sequences which, in dividing cells, must be constantly replenished in order to avoid chromosome erosion and, hence, genomic instability. Moreover, unprotected telomeres are prone to end-to-end fusions. Telomerase, a specialized reverse transcriptase with a built-in RNA template, or, in the absence of telomerase, alternative pathways of telomere maintenance are required for continuous cell proliferation in actively dividing cells as well as in cancerous cells emerging in deregulated somatic tissues. The challenge is to keep these free DNA ends masked from the nucleolytic attacks that will readily operate on any DNA double-strand break in the cell, while also allowing the recruitment of telomerase at intervals. Specialized telomeric proteins, as well as DNA repair and checkpoint proteins with a dual role in telomere maintenance and DNA damage signaling/repair, protect the telomere ends from degradation and some of them also function in telomerase recruitment or other aspects of telomere length homeostasis. Phosphorylation of some telomeric proteins by checkpoint protein kinases appears to represent a mode of regulation of telomeric mechanisms. Finally, recent studies have allowed starting to understand the coupling between progression of the replication forks through telomeric regions and the subsequent telomere replication by telomerase, as well as retroaction of telomerase in cis on the firing of nearby replication origins.

Localization of telomeres and telomere-associated proteins in telomerase-negative Saccharomyces cerevisiae

Cells lacking telomerase cannot maintain their telomeres and undergo a telomere erosion phase leading to senescence and crisis in which most cells become nonviable. On rare occasions survivors emerge from these cultures that maintain their telomeres in alternative ways. The movement of five marked telomeres in Saccharomyces cerevisiae was followed in wild-type cells and through erosion, senescence/crisis and eventual survival in telomerase-negative (est2::HYG) yeast cells. It was found that during erosion, movements of telomeres in est2::HYG cells were indistinguishable from wild-type telomere movements. At senescence/crisis, however, most cells were in G(2) arrest and the nucleus and telomeres traversed back and forth across the bud neck, presumably until cell death. Type I survivors, using subtelomeric Y' amplification for telomere maintenance, continued to show this aberrant telomere movement. However, Type II survivors, maintaining telomeres by a sudden elongation of the telomere repeats, became indistinguishable from wild-type cells, consistent with growth properties of the two types of survivors. When telomere-associated proteins Sir2p, Sir3p and Rap1p were tagged, the same general trend was seen-Type I survivors retained the senescence/crisis state of protein localization, while Type II survivors were restored to wild type.

A screen for suppressors of gross chromosomal rearrangements identifies a conserved role for PLP in preventing DNA lesions

Genome instability is a hallmark of cancer cells. One class of genome aberrations prevalent in tumor cells is termed gross chromosomal rearrangements (GCRs). GCRs comprise chromosome translocations, amplifications, inversions, deletion of whole chromosome arms, and interstitial deletions. Here, we report the results of a genome-wide screen in Saccharomyces cerevisiae aimed at identifying novel suppressors of GCR formation. The most potent novel GCR suppressor identified is BUD16, the gene coding for yeast pyridoxal kinase (Pdxk), a key enzyme in the metabolism of pyridoxal 5′ phosphate (PLP), the biologically active form of vitamin B6. We show that Pdxk potently suppresses GCR events by curtailing the appearance of DNA lesions during the cell cycle. We also show that pharmacological inhibition of Pdxk in human cells leads to the production of DSBs and activation of the DNA damage checkpoint. Finally, our evidence suggests that PLP deficiency threatens genome integrity, most likely via its role in dTMP biosynthesis, as Pdxk-deficient cells accumulate uracil in their nuclear DNA and are sensitive to inhibition of ribonucleotide reductase. Since Pdxk links diet to genome stability, our work supports the hypothesis that dietary micronutrients reduce cancer risk by curtailing the accumulation of DNA damage and suggests that micronutrient depletion could be part of a defense mechanism against hyperproliferation.

Cells must ensure the integrity of genetic information before cellular division. Loss of genome integrity is particularly germane to tumorigenesis, where it is thought to contribute to the rapid evolution of the malignant cell towards the fully cancerous phenotype. It is therefore imperative that we understand fully how cells maintain the integrity of the genome and how it is lost during tumorigenesis. In this study, we developed an assay that allowed us to systematically interrogate each gene of the budding yeast S. cerevisiae for its respective contribution to genome integrity. We report the identification of nine novel genes that increase the rate of genome instability in yeast when deleted. To our surprise, one of the genes we identified encodes the enzyme pyridoxal kinase, which acts in the metabolism of vitamin B6. We show that pyridoxal kinase influences genome stability by promoting the conversion of dietary vitamin B6 into its biologically active form, pyridoxal 5′ phosphate. Our work indicates that vitamin B6 metabolites are critical to maintain genome stability and supports a long-standing model, which hypothesizes that vitamin B6 reduces cancer risk by curtailing genome rearrangements.

Genetic Mechanisms and Aberrant Gene Expression during the Development of Gastric Intestinal Metaplasia and Adenocarcinoma

Gastric adenocarcinoma occurs via a sequence of molecular events known as the Correa's Cascade which often progresses over many years. Gastritis, typically caused by infection with the bacterium H. pylori, is the first step of the cascade that results in gastric cancer; however, not all cases of gastritis progress along this carcinogenic route. Despite recent antibiotic intervention of H. pylori infections, gastric adenocarcinoma remains the second most common cause of cancer deaths worldwide. Intestinal metaplasia is the next step along the carcinogenic sequence after gastritis and is considered to be a precursor lesion for gastric cancer; however, not all patients with intestinal metaplasia develop adenocarcinoma and little is known about the molecular and genetic events that trigger the progression of intestinal metaplasia into adenocarcinoma. This review aims to highlight the progress to date in the genetic events involved in intestinal-type gastric adenocarcinoma and its precursor lesion, intestinal metaplasia. The use of technologies such as whole genome microarray analysis, immunohistochemical analysis and DNA methylation analysis has allowed an insight into some of the events which occur in intestinal metaplasia and may be involved in carcinogenesis. There is still much that is yet to be discovered surrounding the development of this lesion and how, in many cases, it develops into a state of malignancy.

Telomere instability and cancer

Telomeres are required to preserve genome integrity, chromosome stability, nuclear architecture and chromosome pairing during meiosis. Given that telomerase activity is limiting or absent in most somatic tissues, shortening of telomeres during development and aging is the rule. In vitro, telomere length operates as a mechanism to prevent uncontrolled cell growth and therefore defines the proliferation potential of a cell. In vitro, in somatic cells that have lost proliferation control, shortening of telomeres becomes the main source of genome instability leading to genetic or epigenetic changes that may allow cells to become immortal and to acquire tumor phenotypes. In vivo, mice models have indisputably shown both the protective and the promoting role of very short telomeres in cancer development. In humans, although telomere shortening and other types of telomere dysfunction probably contribute to the genome instability often detected in tumors, the specific contributions of such instability to the development of cancer remain largely undetermined.

Structure of the Hybrid-2 type intramolecular human telomeric G-quadruplex in K+ solution: insights into structure polymorphism of the human telomeric sequence

Formation of the G-quadruplex in the human telomeric sequence can inhibit the activity of telomerase, thus the intramolecular telomeric G-quadruplexes have been considered as an attractive anticancer target. Information of intramolecular telomeric G-quadruplex structures formed under physiological conditions is important for structure-based drug design. Here, we report the first structure of the major intramolecular G-quadruplex formed in a native, non-modified human telomeric sequence in K⁺ solution. This is a hybrid-type mixed parallel/antiparallel-G-stranded G-quadruplex, one end of which is covered by a novel T:A:T triple capping structure. This structure (Hybrid-2) and the previously reported Hybrid-1 structure differ in their loop arrangements, strand orientations and capping structures. The distinct capping structures appear to be crucial for the favored formation of the specific hybrid-type intramolecular telomeric G-quadruplexes, and may provide specific binding sites for drug targeting. Our study also shows that while the hybrid-type G-quadruplexes appear to be the major conformations in K⁺ solution, human telomeric sequences are always in equilibrium between Hybrid-1 and Hybrid-2 structures, which is largely determined by the 3′-flanking sequence. Furthermore, both hybrid-type G-quadruplexes suggest a straightforward means for multimer formation with effective packing in the human telomeric sequence and provide important implications for drug targeting of G-quadruplexes in human telomeres.

Structure of a human telomeric DNA sequence stabilized by 8-bromoguanosine substitutions, as determined by NMR in a K+ solution

The structure of human telomeric DNA is controversial; it depends upon the sequence contexts and the methodologies used to determine it. The solution structure in the presence of K(+) is particularly interesting, but the structure is yet to be elucidated, due to possible conformational heterogeneity. Here, a unique strategy is applied to stabilize one such structure in a K(+) solution by substituting guanosines with 8-bromoguanosines at proper positions. The resulting spectra are cleaner and led to determination of the structure at a high atomic resolution. This demonstrates that the application of 8-bromoguanosine is a powerful tool to overcome the difficulty of nucleic acid structure determination arising from conformational heterogeneity. The obtained structure is a mixed-parallel/antiparallel quadruplex. The structure of telomeric DNA was recently reported in another study, in which stabilization was brought about by mutation and resultant additional interactions [Luu KN, Phan AT, Kuryavyi V, Lacroix L & Patel DJ (2006) Structure of the human telomere in K(+) solution: an intramolecular (3+1) G-quadruplex scaffold. J Am Chem Soc 128, 9963-9970]. The structure of the guanine tracts was similar between the two. However, a difference was seen for loops connecting guanine tracts, which may play a role in the higher order arrangement of telomeres. Our structure can be utilized to design a small molecule which stabilizes the quadruplex. This type of molecule is supposed to inhibit a telomerase and thus is expected to be a candidate anticancer drug.

Murine but not human mesenchymal stem cells generate osteosarcoma-like lesions in the lung

Murine mesenchymal stem cells are capable of differentiation into multiple cell types both in vitro and in vivo and may be good candidates to use as cell therapy for diseased or damaged organs. We have previously reported a method of enriching a population of murine MSCs that demonstrated a diverse differentiation potential both in vitro and in vivo. In this study, we show that this enriched population of murine mesenchymal stem cells embolize within lung capillaries following systemic injection and then rapidly expand within, and invade into, the lung parenchyma, forming tumor nodules. These lesions rarely contain cells bearing the immunohistochemical characteristics of lung epithelium, but they do show the characteristics of immature bone and cartilage that resembles exuberant fracture callus or well-differentiated osteosarcoma. Our findings indicate that murine mesenchymal stem cells can behave in a manner similar to tumor cells, with dysregulated growth and aberrant differentiation within the alveolar microenvironment after four passages. We demonstrate that unlike human MSCs, MSCs from different mouse strains can acquire chromosomal abnormalities after only a few in vitro passages. Moreover, other parameters, such as mouse strain used, might also play a role in the induction of these tumors. These findings might be clinically relevant for future stem cell therapy studies. Disclosure of potential conflicts of interest is found at the end of this article.

Quantitative proteomic analysis of human breast epithelial cells with differential telomere length

Telomeres play important functional roles in cell proliferation, cell cycle regulation, and genetic stability, in which telomere length is critical. In this study, quantitative proteome comparisons for the human breast epithelial cells with short and long telomeres (184-hTERTL vs. 184-hTERTS and 90P-hTERTL vs. 90P-hTERTS), resulting from transfection of the human telomerase reverse transcriptase (hTERT) gene, were performed using cleavable isotope-coded affinity tags. More than 2000 proteins were quantified in each comparative experiment, with approximately 77% of the proteins identified in both analyses. In the cells with long telomeres, significant and consistent alterations were observed in metabolism (amino acid, nucleotide, and lipid metabolism), genetic information transmission (transcription and translation regulation, spliceosome and ribosome complexes), and cell signaling. Interestingly, the DNA excision repair pathway is enhanced, while integrin and its ligands are downregulated in the cells with long telomeres. These results may provide valuable information related to telomere functions.

Photochemical approach to probing different DNA structures

The various conformations of DNA--the A, B, and Z forms, the protein-induced DNA kink, and the G-quartet form--are thought to play important biological roles in processes such as DNA replication, gene expression and regulation, and the repair of DNA damage. The investigation of local DNA conformational changes associated with biological events is therefore essential for understanding the function of DNA. In this Minireview, we discuss the use of photochemical dehalogenation of 5-halouracil-containing DNA to probe the structure of DNA. Hydrogen abstraction by the resultant uracil-5-yl radicals is atom-specific and highly dependent on the structure of the DNA, suggesting that this photochemical approach could be applied as a probe of DNA conformations in living cells.

Telomerase expression in lung preneoplasia and neoplasia

Telomeres are specialized structures at eukaryotic chromosomes ends, which role is to prevent them from degradation, end to-end fusion and rearrangement. However, they shorten after each cellular division because of an incomplete DNA replication, acting in normal somatic cells as a mitotic clock for permanent proliferation arrest or senescence entry. Short telomeres are perceived as damaged DNA leading to p53/ATM pathway activation. In tumoral cells, a ribonucleoprotein complex termed telomerase enables telomere elongation. This complex, composed of 2 main components, the telomerase RNA component or hTR, the RNA template for telomere synthesis, and telomerase reverse transcriptase, the catalytic subunit, is reactivated in the majority of cancers, including those of the lung. In this review, we briefly present the main results on telomerase expression in various histological types of lung carcinoma and in bronchial carcinogenesis along with telomere attrition. Inhibition of one of the main components of the enzyme or limitation of telomere access by telomerase represent novel targets for cancer therapies and chemoprevention in high risk patients of lung cancer.

Inverted duplication pattern in anaphase bridges confirms the breakage-fusion-bridge (BFB) cycle model for 11q13 amplification

The homogeneously staining region (hsr) involving chromosome band 11q13 includes amplified genes from this chromosome segment and carries a relatively poor prognosis in oral squamous cell carcinomas (OSCC), with shorter time to recurrence and reduced overall survival. We previously identified an inverted duplication pattern of genes within the 11q13 hsr in OSCC cells, supporting a breakage-fusion-bridge (BFB) cycle model for gene amplification. To validate our hypothesis that 11q13 gene amplification in OSCC occurs via BFB cycles, we carried out fluorescence in situ hybridization (FISH) using probes for band 11q13 on 29 OSCC cell lines. We demonstrate that all OSCC cell lines with 11q13 amplification express a significantly higher frequency of anaphase bridges containing 11q13 sequences compared to cell lines without amplification, providing further experimental evidence that 11q13 gene amplification in OSCC cells occurs via BFB cycles. Elucidation of mechanisms responsible for initiating and promoting gene amplification provides opportunities to identify new biomarkers to aid in the diagnosis and prognosis of oral cancer, and may be useful for developing novel therapeutic strategies for patients with OSCC.

Structure of the intramolecular human telomeric G-quadruplex in potassium solution: a novel adenine triple formation

We report the NMR solution structure of the intramolecular G-quadruplex formed in human telomeric DNA in K⁺. The hybrid-type telomeric G-quadruplex consists of three G-tetrads linked with mixed parallel–antiparallel G-strands, with the bottom two G-tetrads having the same G-arrangement (anti:anti:syn:anti) and the top G-tetrad having the reversed G-arrangement (syn:syn:anti:syn). The three TTA loop segments adopt different conformations, with the first TTA assuming a double-chain-reversal loop conformation, and the second and third TTA assuming lateral loop conformations. The NMR structure is very well defined, including the three TTA loops and the two flanking sequences at 5′- and 3′-ends. Our study indicates that the three loop regions interact with the core G-tetrads in a specific way that defines and stabilizes the unique human telomeric G-quadruplex structure in K⁺. Significantly, a novel adenine triple platform is formed with three naturally occurring adenine residues, A21, A3 and A9, capping the top tetrad of the hybrid-type telomeric G-quadruplex. This adenine triple is likely to play an important role in the formation of a stable human telomeric G-quadruplex structure in K⁺. The unique human telomeric G-quadruplex structure formed in K⁺ suggests that it can be specifically targeted for anticancer drug design.

DNA breaks are masked by multiple Rap1 binding in yeast: implications for telomere capping and telomerase regulation

Eukaryotic cells distinguish their chromosome ends from accidental DNA double-strand breaks by packaging them in a protective structure referred to as the telomere "cap." Here we investigate the nature of the telomere cap by examining events at DNA breaks generated adjacent to either natural telomeric sequences (TG repeats) or arrays of Rap1-binding sites that vary in length. Although DNA breaks adjacent to either short or long telomeric sequences are efficiently converted into stable telomeres, they elicit very different initial responses. Short telomeric sequences (80 base pair [bp]) are avidly bound by Mre11, as well as the telomere capping protein Cdc13 and telomerase enzyme, consistent with their rapid telomerase-dependent elongation. Surprisingly, little or no Mre11 binding is detected at long telomere tracts (250 bp), and this is correlated with reduced Cdc13 and telomerase binding. Consistent with these observations, ends with long telomere tracts undergo strongly reduced exonucleolytic resection and display limited binding by both Rpa1 and Mec1, suggesting that they fail to elicit a checkpoint response. Rap1 binding is required for end concealment at long tracts, but Rif proteins, yKu, and Cdc13 are not. These results shed light on the nature of the telomere cap and mechanisms that regulate telomerase access at chromosome ends.

The role of the nonhomologous end-joining DNA double-strand break repair pathway in telomere biology

Double-strand breaks are a cataclysmic threat to genome integrity. In higher eukaryotes the predominant recourse is the nonhomologous end-joining (NHEJ) double-strand break repair pathway. NHEJ is a versatile mechanism employing the Ku heterodimer, ligase IV/XRCC4 and a host of other proteins that juxtapose two free DNA ends for ligation. A critical function of telomeres is their ability to distinguish the ends of linear chromosomes from double-strand breaks, and avoid NHEJ. Telomeres accomplish this feat by forming a unique higher order nucleoprotein structure. Paradoxically, key components of NHEJ associate with normal telomeres and are required for proper length regulation and end protection. Here we review the biochemical mechanism of NHEJ in double-strand break repair, and in the response to dysfunctional telomeres. We discuss the ways in which NHEJ proteins contribute to telomere biology, and highlight how the NHEJ machinery and the telomere complex are evolving to maintain genome stability.

Mrc1, a non-essential DNA replication protein, is required for telomere end protection following loss of capping by Cdc13, Yku or telomerase

Proteins involved in telomere end protection have previously been identified. In Saccharomyces cerevisiae, Cdc13, Yku and telomerase, mainly, prevent telomere uncapping, thus providing telomere stability and avoiding degradation and death by senescence. Here, we report that in the absence of Mrc1, a component of the replication forks, telomeres of cdc13 or yku70 mutants exhibited increased degradation, while telomerase-negative cells displayed accelerated senescence. Moreover, deletion of MRC1 increased the single-strandedness of the telomeres in cdc13-1 and yku70Delta mutant strains. An mrc1 deletion strain also exhibited slight but stable telomere shortening compared to a wild-type strain. Loss of Mrc1's checkpoint function alone did not provoke synthetic growth defects in combination with the cdc13-1 mutation. Combinations between the cdc13-1 mutation and deletion of either TOF1 or PSY2, coding for proteins physically interacting with Mrc1, also resulted in a synthetic growth defect. Thus, the present data suggest that non-essential components of the DNA replication machinery, such as Mrc1 and Tof1, may have a role in telomere stability in addition to their role in fork progression.

Photochemical determination of different DNA structures

The various conformations of DNA are thought to have important biological roles. Investigation of the local DNA conformational changes associated with biological events is therefore essential to an understanding of the functions of DNA. We have reported the photoreactivities of 5-halouracil in the five characteristic local DNA structures: the A, B and Z forms, protein-induced DNA kinks and the G-quadruplex form. These studies demonstrate the detailed relationships between the local DNA structures and the photochemical products of photoinduced hydrogen abstraction by the resulting uracil-5-yl radicals, and show that this photochemical method can be used to detect DNA structures. Here, we describe in detail procedures that have been developed in our laboratory for probing DNA conformations by product analysis of photoirradiated 5-halouracil-containing DNA. The protocol includes the preparation of 5-halouracil-containing DNA and the characterization of the photoproducts, and it can be completed in 2 weeks.

Break-induced replication and recombinational telomere elongation in yeast

When a telomere becomes unprotected or if only one end of a chromosomal double-strand break succeeds in recombining with a template sequence, DNA can be repaired by a recombination-dependent DNA replication process termed break-induced replication (BIR). In budding yeasts, there are two BIR pathways, one dependent on the Rad51 recombinase protein and one Rad51 independent; these two repair processes lead to different types of survivors in cells lacking the telomerase enzyme that is required for normal telomere maintenance. Recombination at telomeres is triggered by either excessive telomere shortening or disruptions in the function of telomere-binding proteins. Telomere elongation by BIR appears to often occur through a "roll and spread" mechanism. In this process, a telomeric circle produced by recombination at a dysfunctional telomere acts as a template for a rolling circle BIR event to form an elongated telomere. Additional BIR events can then copy the elongated sequence to all other telomeres.

Telomere dysfunction drives increased mutation by error-prone polymerases Rev1 and zeta in Saccharomyces cerevisiae

Using a model system, we have shown that replicative senescence is accompanied by a 16-fold increase in base substitution and frameshift mutations near a chromosome end. The increase was dependent on error-prone polymerases required for the mutagenic response to DNA lesions that block the replication fork.

Regulation of telomere elongation by the cyclin-dependent kinase CDK1

Telomere elongation is cell-cycle regulated and requires the coordinated activity of proteins involved in the DNA damage response. We used an assay that detects de novo telomere addition to examine the role of the cyclin-dependent kinase Cdk1 (Cdc28) in cell-cycle-specific telomere elongation. Inhibition of an ATP analog-sensitive allele of Cdk1 completely blocked the addition of telomere repeats. Mutations in Rif2 and DNA polymerase alpha that cause increased telomere elongation were unable to compensate for the loss of Cdk1 activity, suggesting Cdk1 activity is required for an early step in telomere addition. Mutations in DNA repair proteins that act with Cdk1 at double-strand breaks also prevented telomere elongation. Cdk1 activity was required for the generation of 3' single-strand overhangs at both native and de novo telomeres. We propose Cdk1 activity controls the timing of telomere elongation by regulating the single-strand overhang at chromosome ends.

Telomerase with mutated catalytic motifs has dominant negative effects on telomerase activity and inhibits cell growth

Telomerase catalytic subunit (TERT) seems a key factor controlling telomerase activity, telomere length, and cell growth. To further address this issue, we forced expression of a catalytically inactive mutant human TERT (hTERT) in hTERT-immortalised sheep fibroblasts to examine its effects. Expression of mutant hTERT compromised telomerase activity reconstituted by wild-type hTERT in a manner directly attributable to mutant hTERT expression level. High levels of mutant hTERT expression inhibited cell growth with a subset of cells entering replicative senescence. Furthermore, significant telomere attrition was evident in two of three clones with high levels of mutant hTERT expression. Our findings are consistent with the notion that hTERT homodimers are necessarily required to form a functional telomerase complex at the telomere substrate. We also highlight the requirement of a more thorough understanding of telomerase- and telomere-associated factors to understand fully the interplay that governs telomere homeostasis in vitro and in vivo.

Telomeres and chromosome instability

Genomic instability has been proposed to play an important role in cancer by accelerating the accumulation of genetic changes responsible for cancer cell evolution. One mechanism for chromosome instability is through the loss of telomeres, which are DNA-protein complexes that protect the ends of chromosomes and prevent chromosome fusion. Telomere loss can occur as a result of exogenous DNA damage, or spontaneously in cancer cells that commonly have a high rate of telomere loss. Mouse embryonic stem cells and human tumor cell lines that contain a selectable marker gene located immediately adjacent to a telomere have been used to investigate the consequences of telomere loss. In both cell types, telomere loss is followed by either the addition of a new telomere on to the end of the broken chromosome, or sister chromatid fusion and prolonged breakage/fusion/bridge (B/F/B) cycles that result in DNA amplification and large terminal deletions. The regions amplified by B/F/B cycles can then be transferred to other chromosomes, either through the formation of double-minute chromosomes that reintegrate at other sites, or through end-to-end fusions between chromosomes. B/F/B cycles eventually end when a chromosome acquires a new telomere by one of several mechanisms, the most common of which is translocation, which can involve either nonreciprocal transfer or duplication of all or part of an arm of another chromosome. Telomere acquisition involving nonreciprocal translocations results in the loss of a telomere on the donor chromosome, which subsequently becomes unstable. In contrast, translocations involving duplications do not destabilize the donor chromosome, although they result in allelic imbalances. Thus, the loss of a single telomere can generate a wide variety of chromosome alterations commonly associated with human cancer, not only on the chromosome that originally lost its telomere, but other chromosomes as well. Factors promoting spontaneous telomere loss and the resulting B/F/B cycles are therefore likely to be important in generating the karyotypic changes associated with human cancer.

Genetic variation, nucleotide diversity, and linkage disequilibrium in seven telomere stability genes suggest that these genes may be under constraint

To maintain chromosomal integrity and to protect the ends of chromosomes against recognition as damaged DNA, end-to-end fusion, or recombination, a coordinated set of genes is required to stabilize the telomere. We surveyed common genetic variation in seven genes that are vital to telomere stability (TERT, POT1, TNKS, TERF1, TINF2, TERF2, and TERF2IP) and validated single nucleotide polymorphisms (SNPs) in four different ethnic groups (n=118 total). Overall, our data show limited degrees of nucleotide diversity in comparison with data from other gene families. We observed that these genes are highly conserved in sequence between species, and that for nearly all of the coding SNPs the most common allele is ancestral (i.e., it is observed in primate sequences). Our findings support the hypothesis that genetic variation in a pathway that is critical for telomere stability may be under constraint. These data establish a foundation for further investigation of these genes in population-genetics, evolution, and disease-association studies.

Telomere length variation in biliary tract metaplasia, dysplasia, and carcinoma

Biliary tract carcinoma, including carcinoma of the gallbladder, intrahepatic bile ducts (cholangiocarcinoma), and extrahepatic bile ducts, affects 7500 people in the United States annually, and has an overall 32% 5-year survival rate for disease limited to the mucosa, and a dismal 10% 5-year survival for more advanced disease. The identification of factors involved in the pathogenesis and progression of biliary tract carcinoma is critical for devising effective methods of screening and treatment. Recent evidence suggests that reduction of the length of telomeres, which normally help maintain chromosomal stability, may promote the development and progression of a variety of carcinomas. Using a novel, recently validated telomere fluorescence in situ hybridization method, we examined telomere length in normal and inflamed gallbladder epithelium, metaplasia and dysplasia of the gallbladder, and biliary tract carcinoma to determine whether telomere shortening is associated with neoplastic progression in the biliary tract. Although normal and inflamed gallbladder epithelium demonstrated uniform normal telomere lengths, over half of all metaplastic lesions demonstrated shortened telomeres, supporting prior evidence that metaplastic lesions of the gallbladder are pre-neoplastic. Dysplastic epithelium and invasive carcinomas demonstrated almost universally abnormally short telomeres, indicating that telomere shortening occurs at an early, preinvasive stage of cancer development. In addition, invasive adenocarcinoma of the biliary tract frequently demonstrated intratumoral heterogeneity of telomere lengths. We conclude that telomere shortening is a consistent and early finding in the development of biliary tract carcinoma.

Telomeres and telomerase in cancer stem cells

Alterations in telomere dynamics both suppress and facilitate malignant transformation by regulating genomic stability and cell lifespan. Checkpoints induced by telomere dysfunction play a major role in tumour suppression, whereas telomere shortening contributes to the initiation of cancer by inducing chromosomal instability. Since stem cells are exposed to various tumourigenic agents and stresses throughout their lifetime, the ageing stem cell is a major target of malignant transformation. This review summarises our knowledge of telomere length and telomerase activity in stem cells during ageing and carcinogenesis.

Arsenic in the aetiology of cancer

Arsenic, one of the most significant hazards in the environment affecting millions of people around the world, is associated with several diseases including cancers of skin, lung, urinary bladder, kidney and liver. Groundwater contamination by arsenic is the main route of exposure. Inhalation of airborne arsenic or arsenic-contaminated dust is a common health problem in many ore mines. This review deals with the questions raised in the epidemiological studies such as the dose-response relationship, putative confounders and synergistic effects, and methods evaluating arsenic exposure. Furthermore, it describes the metabolic pathways of arsenic, and its biological modes of action. The role of arsenic in the development of cancer is elucidated in the context of combined epidemiological and biological studies. However, further analyses by means of molecular epidemiology are needed to improve the understanding of cancer aetiology induced by arsenic.

Human telomeric sequence forms a hybrid-type intramolecular G-quadruplex structure with mixed parallel/antiparallel strands in potassium solution

Human telomeric DNA consists of tandem repeats of the sequence d(TTAGGG). The formation and stabilization of DNA G-quadruplexes in the human telomeric sequence have been shown to inhibit the activity of telomerase, thus the telomeric DNA G-quadruplex has been considered as an attractive target for cancer therapeutic intervention. However, knowledge of the intact human telomeric G-quadruplex structure(s) formed under physiological conditions is a prerequisite for structure-based rational drug design. Here we report the folding structure of the human telomeric sequence in K+ solution determined by NMR. Our results demonstrate a novel, unprecedented intramolecular G-quadruplex folding topology with hybrid-type mixed parallel/antiparallel G-strands. This telomeric G-quadruplex structure contains three G-tetrads with mixed G-arrangements, which are connected consecutively with a double-chain-reversal side loop and two lateral loops, each consisting of three nucleotides TTA. This intramolecular hybrid-type telomeric G-quadruplex structure formed in K+ solution is distinct from those reported on the 22 nt Tel22 in Na+ solution and in crystalline state in the presence of K+, and appears to be the predominant conformation for the extended 26 nt telomeric sequence Tel26 in the presence of K+, regardless of the presence or absence of Na+. Furthermore, the addition of K+ readily converts the Na+-form conformation to the K+-form hybrid-type G-quadruplex. Our results explain all the reported experimental data on the human telomeric G-quadruplexes formed in the presence of K+, and provide important insights for understanding the polymorphism and interconversion of various G-quadruplex structures formed within the human telomeric sequence, as well as the effects of sequence and cations. This hybrid-type G-quadruplex topology suggests a straightforward pathway for the secondary structure formation with effective packing within the extended human telomeric DNA. The hybrid-type telomeric G-quadruplex is most likely to be of pharmacological relevance, and the distinct folding topology of this G-quadruplex suggests that it can be specifically targeted by G-quadruplex interactive small molecule drugs.

A genome-wide screen identifies the evolutionarily conserved KEOPS complex as a telomere regulator

Telomere capping is the essential function of telomeres. To identify new genes involved in telomere capping, we carried out a genome-wide screen in Saccharomyces cerevisiae for suppressors of cdc13-1, an allele of the telomere-capping protein Cdc13. We report the identification of five novel suppressors, including the previously uncharacterized gene YML036W, which we name CGI121. Cgi121 is part of a conserved protein complex -- the KEOPS complex -- containing the protein kinase Bud32, the putative peptidase Kae1, and the uncharacterized protein Gon7. Deletion of CGI121 suppresses cdc13-1 via the dramatic reduction in ssDNA levels that accumulate in cdc13-1 cgi121 mutants. Deletion of BUD32 or other KEOPS components leads to short telomeres and a failure to add telomeres de novo to DNA double-strand breaks. Our results therefore indicate that the KEOPS complex promotes both telomere uncapping and telomere elongation.

The new models of the human telomere d[AGGG(TTAGGG)3] in K+ solution

The human telomeric sequence d[AGGG(TTAGGG)(3)] has been found to form different types of G-quadruplex structures. NMR revealed that in Na(+) solution this 22 nucleotide (nt) sequence exhibits an antiparallel structure, whereas crystallographic studies in the presence of K(+) showed a dramatically different parallel structure. The structure of this 22 nt sequence in the presence of K(+) has drawn intense interest as the intracellular K(+) concentration is greater than that of Na(+). However, the question of the type of structure for the 22 nt telomeric sequence in K(+) solution remains open. In this study, we substituted the Gs in the sequence with 8-bromoguanine and examined the resultant structures and thermal stabilities by circular dichroism (CD) spectroscopy. The results suggest that the 22 nt in K(+) solution exists as a mixture of mixed-parallel/antiparallel and chair-type G-quadruplex. To date, the exact structure of human telomeric G-quadruplex in K(+) solution is extremely controversial. The present study provides valuable information for understanding the discrepancies between the crystal and solution studies. We discuss the possible implications of the structure in understanding higher-order telomeric DNA structure and T-loop formation.

Critically short telomeres in acute myeloid leukemia with loss or gain of parts of chromosomes

Telomeres, nucleoprotein complexes at chromosome ends, protect chromosomes against end-to-end fusion. Previous in vitro studies in human fibroblast models indicated that telomere dysfunction results in chromosome instability. Loss of telomere function can result either from critical shortening of telomeric DNA or from loss of distinct telomere-capping proteins. It is less clear whether telomere dysfunction has an important role in human cancer development in vivo. Acute myeloid leukemia (AML) is a good model to study mechanisms that generate chromosome instability in human cancer development because distinct groups of AML are characterized either by aberrations that theoretically could result from telomere dysfunction (terminal deletions, gains/losses of chromosome parts, nonreciprocal translocations), or aberrations that are unlikely to result from telomere dysfunction (e.g., reciprocal translocations or inversions). Here we demonstrate that AML with multiple chromosome aberrations that theoretically could result from telomere dysfunction is invariably characterized by critically short telomeres. Short telomeres in this group are not associated with low telomerase activity or decreased expression of essential telomeric capping proteins TRF2 and POT1. In contrast, telomerase activity levels are significantly higher in AML with short telomeres. Notably, short telomeres in the presence of high telomerase may relate to significantly higher expression of TRF1, a negative regulator of telomere length. Our observations suggest that, consistent with previous in vitro fibroblast models, age-related critical telomere shortening may have a role in generating chromosome instability in human AML development.

Telomeres, interstitial telomeric repeat sequences, and chromosomal aberrations

Telomeres are specialized nucleoproteic complexes localized at the physical ends of linear eukaryotic chromosomes that maintain their stability and integrity. The DNA component of telomeres is characterized by being a G-rich double stranded DNA composed by short fragments tandemly repeated with different sequences depending on the species considered. At the chromosome level, telomeres or, more properly, telomeric repeats--the DNA component of telomeres--can be detected either by using the fluorescence in situ hybridization (FISH) technique with a DNA or a peptide nucleic acid (PNA) (pan)telomeric probe, i.e., which identifies simultaneously all of the telomeres in a metaphase cell, or by the primed in situ labeling (PRINS) reaction using an oligonucleotide primer complementary to the telomeric DNA repeated sequence. Using these techniques, incomplete chromosome elements, acentric fragments, amplification and translocation of telomeric repeat sequences, telomeric associations and telomeric fusions can be identified. In addition, chromosome orientation (CO)-FISH allows to discriminate between the different types of telomeric fusions, namely telomere-telomere and telomere-DNA double strand break fusions and to detect recombination events at the telomere, i.e., telomeric sister-chromatid exchanges (T-SCE). In this review, we summarize our current knowledge of chromosomal aberrations involving telomeres and interstitial telomeric repeat sequences and their induction by physical and chemical mutagens. Since all of the studies on the induction of these types of aberrations were conducted in mammalian cells, the review will be focused on the chromosomal aberrations involving the TTAGGG sequence, i.e., the telomeric repeat sequence that "caps" the chromosomes of all vertebrate species.

Formation of the G-quadruplex and i-motif structures in retinoblastoma susceptibility genes (Rb)

The formation of G-quadruplex and i-motif structures in the 5′ end of the retinoblastoma (Rb) gene was examined using chemical modifications, circular dichroism (CD) and fluorescence spectroscopy. It was found that substitutions of 8-methylguanine at positions that show syn conformations in antiparallel G-quadruplexes stabilize the structure in the G-rich strand. The complementary C-rich 18mer forms an i-motif structure, as suggested by CD spectroscopy. Based on the C to T mutation experiments, C bases participated in the C–C⁺ base pair of the i-motif structure were determined. Experiments of 2-aminopurine (2-AP) substitution reveal that an increase of fluorescence in the G-quadruplex relative to duplex is attributed to unstacked 2-AP within the loop of G-quadruplex. The fluorescence experiments suggest that formation of the G-quadruplex and i-motif can compete with duplex formation. Furthermore, a polymerase arrest assay indicated that formation the G-quadruplex structure in the Rb gene acts as a barrier in DNA synthesis.

A role for Brca1 in chromosome end maintenance

The role of BRCA1 in breast and ovarian tumor suppression has been primarily ascribed to the maintenance of genome integrity. BRCA1 interacts with components of the non-homologous end-joining pathway previously shown to play a role in telomere maintenance in yeast. Here, we provide evidence that links Brca1 with telomere integrity. Brca1(-/-) T-cells display telomere dysfunction in both loss of telomere repeats as well as defective telomere capping. Loss of Brca1 synergizes with p53 deficiency in the onset and frequency of tumorigenesis. Karyotyping of tBrca1(-/-)p53(-/-) thymic lymphomas revealed the presence of telomere dysfunction accompanied by clonal chromosomal translocations. The telomere dysfunction phenotype in Brca1-deficient cells suggests that loss of telomere integrity might contribute to chromosome end dysfunction and permit the formation of potentially oncogenic translocations.

Effects of hTERT on metal ion-induced genomic instability

There is currently a great interest in delayed chromosomal and other damaging effects of low-dose exposure to a variety of pollutants which appear collectively to act through induction of stress-response pathways related to oxidative stress and ageing. These have been studied mostly in the radiation field but evidence is accumulating that the mechanisms can also be triggered by chemicals, especially heavy metals. Humans are exposed to metals, including chromium (Cr) (VI) and vanadium (V) (V), from the environment, industry and surgical implants. Thus, the impact of low-dose stress responses may be larger than expected from individual toxicity projections. In this study, a short (24 h) exposure of human fibroblasts to low doses of Cr (VI) and V (V) caused both acute chromosome damage and genomic instability in the progeny of exposed cells for at least 30 days after exposure. Acutely, Cr (VI) caused chromatid breaks without aneuploidy while V (V) caused aneuploidy without chromatid breaks. The longer-term genomic instability was similar but depended on hTERT positivity. In telomerase-negative hTERT- cells, Cr (VI) and V (V) caused a long lasting and transmissible induction of dicentric chromosomes, nucleoplasmic bridges, micronuclei and aneuploidy. There was also a long term and transmissible reduction of clonogenic survival, with an increased beta-galactosidase staining and apoptosis. This instability was not present in telomerase-positive hTERT+ cells. In contrast, in hTERT+ cells the metals caused a persistent induction of tetraploidy, which was not noted in hTERT- cells. The growth and survival of both metal-exposed hTERT+ and hTERT- cells differed if they were cultured at subconfluent levels or plated out as colonies. Genomic instability is considered to be a driving force towards cancer. This study suggests that the type of genomic instability in human cells may depend critically on whether they are telomerase-positive or -negative and that their sensitivities to metals could depend on whether they are clustered or diffuse.

Viral carcinogenesis and genomic instability

Oncogenes encoded by human tumor viruses play integral roles in the viral conquest of the host cell by subverting crucial and relatively non-redundant regulatory circuits that regulate cellular proliferation, differentiation, apoptosis and life span. Human tumor virus oncoproteins can also disrupt pathways that are necessary for the maintenance of the integrity of host cellular genome. Some viral oncoproteins act as powerful mutator genes and their expression dramatically increases the incidence of host cell mutations with every round of cell division. Others subvert cellular safeguard mechanisms intended to eliminate cells that have acquired abnormalities that interfere with normal cell division. Viruses that encode such activities can contribute to initiation as well as progression of human cancers.

Chromosome healing byde novotelomere addition inSaccharomyces cerevisiae

The repair of spontaneous or induced DNA damage by homologous recombination (HR) in Saccharomyces cerevisiae will suppress chromosome rearrangements. Alternative chromosome healing pathways can result in chromosomal instability. One of these pathways is de novo telomere addition where the end of a broken chromosome is stabilized by telomerase-dependent addition of telomeres at non-telomeric sites. De novo telomere addition requires the recruitment of telomerase to chromosomal targets. Subsequently, annealing of the telomerase reverse transcriptase RNA-template (guide RNA) at short regions of homology is followed by extension of the nascent 3'-end of the broken chromosome to copy a short region of the telomerase guide RNA; multiple cycles of this process yield the new telomere. Proteins including Pif1 helicase, the single-stranded DNA-binding protein Cdc13 and the Ku heterocomplex are known to participate in native telomere functions and also regulate the de novo telomere addition reaction. Studies of the sequences added at de novo telomeres have lead to a detailed description of the annealing-extension-dissociation cycles that copy the telomerase guide RNA, which can explain the heterogeneity of telomeric repeats at de novo and native telomeres in S. cerevisiae.

High incidence of rapid telomere loss in telomerase-deficient Caenorhabditis elegans

Telomerase is essential to maintain telomere length in most eukaryotes. Other functions for telomerase have been proposed but molecular mechanisms remain unclear. We studied Caenorhabditis elegans with a mutation in the trt-1 telomerase reverse transcriptase gene. Mutant animals showed a progressive decrease in brood size and typically failed to reproduce after five generations. Using PCR analysis to measure the length of individual telomere repeat tracks on the left arm of chromosome V we observed that trt-1 mutants lost ∼125bp of telomeric DNA per generation. Chromosome fusions involving complex recombination reactions were observed in late generations. Strikingly, trt-1 mutant animals displayed a high frequency of telomeres with many fewer repeats than average. Such outlying short telomeres were not observed in mrt-2 mutants displaying progressive telomere loss very similar to trt-1 mutants. We speculate that, apart from maintaining the average telomere length, telomerase is required to prevent or repair sporadic telomere truncations that are unrelated to the typical ‘end-replication’ problems.

Telomere DNA content and allelic imbalance demonstrate field cancerization in histologically normal tissue adjacent to breast tumors

Cancer arises from an accumulation of mutations that promote the selection of cells with progressively malignant phenotypes. Previous studies have shown that genomic instability, a hallmark of cancer cells, is a driving force in this process. In the present study, two markers of genomic instability, telomere DNA content and allelic imbalance, were examined in two independent cohorts of mammary carcinomas. Altered telomeres and unbalanced allelic loci were present in both tumors and surrounding histologically normal tissues at distances at least 1 cm from the visible tumor margins. Although the extent of these genetic changes decreases as a function of the distance from the visible tumor margin, unbalanced loci are conserved between the surrounding tissues and the tumors, implying cellular clonal evolution. Our results are in agreement with the concepts of "field cancerization" and "cancer field effect," concepts that were previously introduced to describe areas within tissues consisting of histologically normal, yet genetically aberrant, cells that represent fertile grounds for tumorigenesis. The finding that genomic instability occurs in fields of histologically normal tissues surrounding the tumor is of clinical importance, as it has implications for the definition of appropriate tumor margins and the assessment of recurrence risk factors in the context of breast-sparing surgery.

Telomere dysfunction drives chromosomal instability in human mammary epithelial cells

The development of genomic instability is an important step toward generating the multiple genetic changes required for cancer. Telomere dysfunction is one of the factors that contribute to tumorigenesis. Telomeres shorten with each cell division in the absence of telomerase. Human mammary epithelial cells (HMECs) obtained from normal human tissue demonstrate two growth phases. After an initial phase of active growth, HMECs exhibit a growth plateau termed selection. However, some cells can emerge from this growth plateau by spontaneously losing expression of the p16(INK4a) protein. These post-selection HMECs are capable of undergoing an additional 20-50 population doublings in culture. Continued proliferation of these post-selection HMECs leads to further telomere erosion, loss of the capping function, and the appearance of end-to-end chromosome fusions that can enter bridge-fusion-breakage (BFB) cycles, generating massive chromosomal instability before terminating in a population growth plateau termed agonescence. We have found that the chromosome arms carrying the shortest telomeres are those involved in telomere-telomere type rearrangements. In addition, we found that the risk of a particular chromosome being unstable differs between individuals. Most importantly, we identified sister chromatid fusion as a first event in generating genomic instability in HMECs. During post-selection HMEC growth, double strand breaks are generated by both fused chromosomes as well as individual chromosomes with fused chromatids entering BFB cycles. These broken chromosome extremities are able to join other broken ends or eroded telomeres, producing massive chromosomal instability at the later passages of the cell culture. This article contains Supplementary Material available at http://www.interscience.wiley.com/jpages/1045-2257/suppmat.

Cloning and analysis of the AWA1 gene of a nonfoaming mutant of a sake yeast

Almost all sake yeasts form a thick foam layer on sake mash during fermentation. To reduce the amount of foam, nonfoaming mutants were bred from foam-forming sake yeasts. To elucidate the mechanism of this foam formation, we have cloned a gene from a foam-forming sake yeast that confers foam-forming ability to a nonfoaming mutant. This gene, named AWA1, encodes a glycosylphosphatidylinositol (GPI) anchor protein that is localized to the cell wall and is required for cell surface hydrophobicity. In this paper, we describe the genomic analysis of the AWA1 gene in a nonfoaming mutant strain K701 derived from a foam-forming sake yeast strain K7. K701-AWA1 was cloned in a cosmid and its sequence was compared with that of K7-AWA1. Although the 5' half of K701-AWA1 was identical to that of K7-AWA1, the 3' half of K701-AWA1 was different from that of K7-AWA1, resulting in a loss of the C-terminal hydrophobic sequence of Awa1p. Since this sequence is considered to be required for the anchoring of Awa1p to the cell wall, K7-Awa1p could not confer both cell surface hydrophobicity and foam-forming ability to strain K701 cells. Since the change found in K701-AWA1 was not a point mutation but a larger scale event, we analyzed chromosome rearrangement by pulsed-field gel electrophoresis Southern blot analyses. The results suggest that the left subtelomeric region of chromosome IX in strain K7 was translocated to the AWA1 gene in chromosome XV by a nonreciprocal recombination.

Modeling aging and cancer in the telomerase knockout mouse

The telomerase deficient mouse has been invaluable in providing insights into basic questions pertaining to consequences of telomere dysfunction during aging and cancer in the context of the mammalian organism. Studies using this mouse model have demonstrated that cellular responses to telomere dysfunction are fundamentally conserved in both humans and mice, and that the tight regulation of telomere length and telomerase activity in somatic cells may be important in mediating the balance between aging and cancer. Here, I discuss the use of the telomerase null mouse for understanding the contrasting roles of telomeres and telomerase in organismal aging and cancer.

Saccharomyces cerevisiae Mre11 is a high-affinity G4 DNA-binding protein and a G-rich DNA-specific endonuclease: implications for replication of telomeric DNA

In Saccharomyces cerevisiae, Mre11p/Rad50p/Xrs2p (MRX) complex plays a vital role in several nuclear processes including cellular response to DNA damage, telomere length maintenance, cell cycle checkpoint control and meiotic recombination. Telomeres are comprised of tandem repeats of G-rich DNA and are incorporated into non-nucleosomal chromatin. Although the structure of the yeast telomeric DNA is poorly understood, it has been suggested that the G-rich sequences can fold into G4 DNA, which has been shown to inhibit DNA synthesis by telomerase. However, little is known about the factors and mechanistic aspects of the generation of appropriate termini for DNA synthesis by telomerase. Here, we show that S.cerevisiae Mre11 protein (ScMre11p) possesses substantially higher binding affinity for G4 DNA, over single- or double-stranded DNA, and binding was inhibited by poly(dG) or porphyrin. Binding of ScMre11p to G4 DNA was most robust, compared with G2' DNA and the resulting protein-DNA complexes were strikingly very resistant to dissociation by NaCl. Remarkably, binding of ScMre11p to G4 DNA and G-rich single-stranded DNA was accompanied by the endonucleolytic cleavage at sites flanking the array of G residues and G-quartets in Mn2+-dependent manner. Collectively, these results suggest that ScMre11p is likely to play a major role in generating appropriate substrates for DNA synthesis by telomerase and telomere-binding proteins. We discuss the implications of these findings with regard to telomere length maintenance by telomerase-dependent and independent mechanisms.

The loss of a single telomere can result in instability of multiple chromosomes in a human tumor cell line

Spontaneous telomere loss has been proposed as an important mechanism for initiating the chromosome instability commonly found in cancer cells. We have previously shown that spontaneous telomere loss in a human cancer cell line initiates breakage/fusion/bridge (B/F/B) cycles that continue for many cell generations, resulting in DNA amplification and translocations on the chromosome that lost its telomere. We have now extended these studies to determine the effect of the loss of a single telomere on the stability of other chromosomes. Our study showed that telomere acquisition during B/F/B cycles occurred mainly through translocations involving either the nonreciprocal transfer or duplication of the arms of other chromosomes. Telomere acquisition also occurred through small duplications involving the subtelomeric region of the other end of the same chromosome. Although all of these mechanisms stabilized the chromosome that lost its telomere, they differed in their consequences for the stability of the genome as a whole. Telomere acquisition involving nonreciprocal translocations resulted in the loss of a telomere on the donor chromosome, which consequently underwent additional translocations, isochromosome formation, or complete loss. In contrast, telomere acquisition involving duplications stabilized the genome, although the large duplications created substantial allelic imbalances. Thus, the loss of a single telomere can generate a variety of chromosome alterations commonly associated with human cancer, not only on a chromosome that loses its telomere but also on other chromosomes. Factors promoting telomere loss are therefore likely to have an important role in generating the karyotype evolution associated with human cancer.

An increase in telomere sister chromatid exchange in murine embryonic stem cells possessing critically shortened telomeres

Telomerase deficiency leads to a progressive loss of telomeric DNA that eventually triggers cell apoptosis in human primary cells during prolonged growth in culture. Rare survivors can maintain telomere length through either activation of telomerase or recombination-based telomere lengthening, and thus proliferate indefinitely. We have explored the possibility that telomeres may be maintained through telomere sister chromatid exchange (T-SCE) in murine telomere reverse transcriptase-deficient (mTert-/-) splenocytes and ES cells. Because telomerase deficiency leads to gradual loss of telomeric DNA in mTert-/- splenocytes and ES cells and eventually to chromosomes with telomere signal-free ends (SFEs), we examined these cell types for evidence of sister chromatid exchange at telomeres, and observed an increase in T-SCEs only in a subset of mTert-/- splenocytes or ES cells that possessed multiple SFEs. Furthermore, T-SCEs were more often detected in ES cells than in splenocytes that harbored a similar frequency of SFEs. In mTert heterozygous (mTert+/-) ES cells or splenocytes, which are known to exhibit a decrease in average telomere length but no SFEs, no increase in T-SCE was observed. In addition to T-SCE, other genomic rearrangements (i.e., SCE) were also significantly increased in mTert-/- ES cells possessing critically short telomeres, but not in splenocytes. Our results suggest that animals and cell culture differ in their ability to carry out genomic rearrangements as a means of maintaining telomere integrity when telomeres become critically shortened.