Targeting telomeres and telomerase

Isolation of G-quadruplex DNA using NMM-sepharose affinity chromatography

DNA can adopt a variety of non-standard conformations, including structures known as G-quadruplexes (G4-DNA), which consist of stacked tetrads of guanines. There are growing indications that G4-DNA is of biological importance, including evidence that it plays roles in telomere function, DNA recombination and the regulation of transcription and translation. However, it has been difficult to obtain direct, physical evidence for the presence of G-quadruplex DNA in vivo due, in part, to a lack of tools for G4-DNA identification. Here, we describe a method for coupling the G4-DNA binding ligand N-methyl mesoporphyrin IX (NMM) to a Sepharose resin, and demonstrate the ability of the resin to bind tightly and selectively to DNA oligonucleotides with the capacity to form G4-DNA. This technique might also be extended to examine genomic distributions of G4-DNA isolated from in vivo sources.

G4-FID: a fluorescent DNA probe displacement assay for rapid evaluation of quadruplex ligands

Currently, small molecules able to interact specifically with G-quadruplex DNA (G-quadruplex ligands) are intensively studied since they appear to inhibit the growth of cancer cells via an unprecedented mode of action based on structural perturbation of telomeres. It is thus of crucial interest to develop methods that enable easy, rapid and reliable detection of valuable candidates. Herein, we would like to report on the G4-FID assay, a simple fluorescence screening method based on the displacement of a DNA light-up probe (thiazole orange) from both quadruplex and duplex DNA, therefore allowing a concomitant evaluation of the quadruplex-affinity and quadruplex- over duplex-selectivity of the tested candidates.

Quantitative analysis of small molecule-nucleic acid interactions with a biosensor surface and surface plasmon resonance detection

Surface plasmon resonance (SPR) technology with biosensor surfaces has become a widely-used tool for the study of nucleic acid interactions without any labeling requirements. The method provides simultaneous kinetic and equilibrium characterization of the interactions of biomolecules as well as small molecule-biopolymer binding. SPR monitors molecular interactions in real time and provides significant advantages over optical or calorimetic methods for systems with strong binding coupled to small spectroscopic signals and/or reaction heats. A detailed and practical guide for nucleic acid interaction analysis using SPR-biosensor methods is presented. Details of the SPR technology and basic fundamentals are described with recommendations on the preparation of the SPR instrument, sensor chips, and samples, as well as extensive information on experimental design, quantitative and qualitative data analysis and presentation. A specific example of the interaction of a minor-groove-binding agent with DNA is evaluated by both kinetic and steady-state SPR methods to illustrate the technique. Since the molecules that bind cooperatively to specific DNA sequences are attractive for many applications, a cooperative small molecule-DNA interaction is also presented.

G-quadruplexes: from guanine gels to chemotherapeutics

G-quartets are square planar arrangements of four guanine bases, which can form extraordinarily stable stacks when present in nucleic acid sequences. Such G-quadruplex structures were long regarded as an in vitro phenomenon, but the widespread presence of suitable sequences in genomes and the identification of proteins that stabilize, modify, or resolve these nucleic acid structures have provided circumstantial evidence for their physiological relevance. The therapeutic potential of small molecules that can stabilize or disrupt G-quadruplex structures has invigorated the field in recent years. Here we review some of the key observations that support biological functions for G-quadruplex DNA as well as the techniques and tools that have enabled researchers to probe these structures and their interactions with proteins and small molecules.

Human telomeric G-quadruplex: structures of DNA and RNA sequences

Telomeres play an important role in cellular aging and cancer. Human telomeric DNA and RNA G-rich sequences are capable of forming a four-stranded structure, known as the G-quadruplex. Such a structure might be important for telomere biology and a good target for drug design. This minireview describes the structural diversity or conservation of DNA and RNA human telomeric G-quadruplexes, discusses structural views on targeting these G-quadruplexes and presents some future challenges for structural studies.

Selectivity in small molecule binding to human telomeric RNA and DNA quadruplexes

Quadruplex RNAs are less well understood than their DNA counterparts, yet of potentially high biological relevance. The interactions of several quadruplex-binding ligands with telomeric RNA quadruplexes are reported and compared with their binding to the analogous DNA quadruplexes.

Synthesis and evaluation of fused bispyrimidinoacridines as novel pentacyclic analogues of quadruplex-binder BRACO-19

The present article reports on the design and the synthesis of a series of mono- and bis-pyrimidinoacridines and their evaluation as a novel family of quadruplex-binders. It is shown that bispyrimidinoacridines represent an interesting compromise between easy synthetic access and efficiency in terms of quadruplex interaction (both affinic and selective), as judged by G4-FID assay and molecular modelling. The present study also highlights that control of the pi-stacking interactions taking place between the ligand and the accessible G-tetrad of a quadruplex-DNA is indeed essential for good recognition but not exclusively (key role of direct and water-mediated H-bonds). The introduction of additional amino side chains, valuable in the acridine series, results here in steric perturbations of the ligand/quadruplex recognition and lowers the quadruplex/duplex selectivity.

Molecular models for intrastrand DNA G-quadruplexes

BACKGROUND

Independent surveys of human gene promoter regions have demonstrated an overrepresentation of G(3)X(n1)G3X(n2)G(3)X(n3)G(3) motifs which are known to be capable of forming intrastrand quadruple helix structures. In spite of the widely recognized importance of G-quadruplex structures in gene regulation and growing interest around this unusual DNA structure, there are at present only few such structures available in the Nucleic Acid Database. In the present work we generate by molecular modeling feasible G-quadruplex structures which may be useful for interpretation of experimental data.

RESULTS

We have used all quadruplex DNA structures deposited in the Nucleic Acid Database in order to select a list of fragments entailing a strand of three adjacent G's paired with another strand of three adjacent G's separated by a loop of one to four residues. These fragments were further clustered and representative fragments were finally selected. Further fragments were generated by assemblying the two strands of each fragment with loops from different fragments whenever the anchor G's were superimposable. The fragments were used to assemble G quadruplex based on a superimposability criterion.

CONCLUSION

Molecular models have been generated for a large number of G(3)X(n1)G(3)X(n2)G3X(n3)G(3) sequences. For a given sequence not all topologies are possible with the available repertoire of fragments due to steric hindrance and low superimposability. Since all molecular models are generated by fragments coming from observed quadruplex structures, molecular models are in principle reliable and may be used for interpretation of experimental data. Some examples of applications are given.

Anticancer activity of CX-3543: a direct inhibitor of rRNA biogenesis

Hallmark deregulated signaling in cancer cells drives excessive ribosome biogenesis within the nucleolus, which elicits unbridled cell growth and proliferation. The rate-limiting step of ribosome biogenesis is synthesis of rRNA (building blocks of ribosomes) by RNA Polymerase I (Pol I). Numerous kinase pathways and products of proto-oncogenes can up-regulate Pol I, whereas tumor suppressor proteins can inhibit rRNA synthesis. In tumorigenesis, activating mutations in certain cancer-associated kinases and loss-of-function mutations in tumor suppressors lead to deregulated signaling that stimulates Pol I transcription with resultant increases in ribosome biogenesis, protein synthesis, cell growth, and proliferation. Certain anticancer therapeutics, such as cisplatin and 5-fluorouracil, reportedly exert, at least partially, their activity through disruption of ribosome biogenesis, yet many prime targets for anticancer drugs within the ribosome synthetic machinery of the nucleolus remain largely unexploited. Herein, we describe CX-3543, a small molecule nucleolus-targeting agent that selectively disrupts nucleolin/rDNA G-quadruplex complexes in the nucleolus, thereby inhibiting Pol I transcription and inducing apoptosis in cancer cells. CX-3543 is the first G-quadruplex interactive agent to enter human clinical trials, and it is currently under evaluation against carcinoid/neuroendocrine tumors in a phase II clinical trial.

A molecular model for drug binding to tandem repeats of telomeric G-quadruplexes

The extreme 3'-ends of human telomeres consist of 150-250 nucleotides of single-stranded DNA sequence together with associated proteins. Small-molecule ligands can compete with these proteins and induce a conformational change in the DNA to a four-stranded quadruplex arrangement, which is also no longer a substrate for the telomerase enzyme. The modified telomere ends provide signals to the DNA-damage-response system and trigger senescence and apoptosis. Experimental structural data are available on such quadruplex complexes comprising up to four telomeric DNA repeats, but not on longer systems that are more directly relevant to the single-stranded overhang in human cells. The present paper reports on a molecular modelling study that uses Molecular Dynamics simulation methods to build dimer and tetramer quadruplex repeats. These incorporate ligand-binding sites and are models for overhang-ligand complexes.

The G-quadruplex ligand telomestatin impairs binding of topoisomerase IIIalpha to G-quadruplex-forming oligonucleotides and uncaps telomeres in ALT cells

In Alternative Lengthening of Telomeres (ALT) cell lines, specific nuclear bodies called APBs (ALT-associated PML bodies) concentrate telomeric DNA, shelterin components and recombination factors associated with telomere recombination. Topoisomerase IIIalpha (Topo III) is an essential telomeric-associated factor in ALT cells. We show here that the binding of Topo III to telomeric G-overhang is modulated by G-quadruplex formation. Topo III binding to G-quadruplex-forming oligonucleotides was strongly inhibited by telomestatin, a potent and specific G-quadruplex ligand. In ALT cells, telomestatin treatment resulted in the depletion of the Topo III/BLM/TRF2 complex and the disruption of APBs and led to the segregation of PML, shelterin components and Topo III. Interestingly, a DNA damage response was observed at telomeres in telomestatin-treated cells. These data indicate the importance of G-quadruplex stabilization during telomere maintenance in ALT cells. The function of TRF2/Topo III/BLM in the resolution of replication intermediates at telomeres is discussed.

Arrangements of human telomere DNA quadruplex in physiologically relevant K+ solutions

The arrangement of the human telomeric quadruplex in physiologically relevant conditions has not yet been unambiguously determined. Our spectroscopic results suggest that the core quadruplex sequence G₃(TTAG₃)₃ forms an antiparallel quadruplex of the same basket type in solution containing either K⁺ or Na⁺ ions. Analogous sequences extended by flanking nucleotides form a mixture of the antiparallel and hybrid (3 + 1) quadruplexes in K⁺-containing solutions. We, however, show that long telomeric DNA behaves in the same way as the basic G₃(TTAG₃)₃ motif. Both G₃(TTAG₃)₃ and long telomeric DNA are also able to adopt the (3 + 1) quadruplex structure: Molecular crowding conditions, simulated here by ethanol, induced a slow transition of the K⁺-stabilized quadruplex into the hybrid quadruplex structure and then into a parallel quadruplex arrangement at increased temperatures. Most importantly, we demonstrate that the same transitions can be induced even in aqueous, K⁺-containing solution by increasing the DNA concentration. This is why distinct quadruplex structures were detected for AG₃(TTAG₃)₃ by X-ray, nuclear magnetic resonance and circular dichrosim spectroscopy: Depending on DNA concentration, the human telomeric DNA can adopt the antiparallel quadruplex, the (3 + 1) structure, or the parallel quadruplex in physiologically relevant concentrations of K⁺ ions.

Synthetic strategies towards ruthenium-porphyrin conjugates for anticancer activity

The conjugation of porphyrins to metal fragments is a strategy for making new compounds that are expected to combine the phototoxicity and the tumour-localization properties of the porphyrin chromophore with the cytotoxicity of the metal fragment for additive antitumour effect. We report here the preparation of new classes of porphyrin-ruthenium conjugates with potential bio-medical applications. Ruthenium was chosen because several Ru compounds have shown promising anticancer activity. The conjugation with the porphyrin moiety was accomplished either through peripheral pyridyl rings (e.g.meso-4'-tetrapyridylporphyrin, 4'TPyP) or through bpy units (e.g.meso-(p-bpy-phenyl)porphyrins, bpy(n)-PPs, n = 1-4). The number of Ru fragments attached to the porphyrins ranges from 1 to 4 and the total charge of the conjugates from -4 to +8. Different types of peripheral fragments, both Ru(III) and Ru(II), have been used: in some cases they are structurally similar to established anticancer compounds. Examples are [Na](4)[4'TPyP{trans-RuCl(4)(dmso-S)}(4)] (2), that bears four NAMI-type Ru(III) fragments, or [4'TPyP{Ru([9]aneS3)(en)}(4)][CF(3)SO(3)](8) (3) and [bpy(4)-PP{Ru([9]aneS3)(dmso-S)}(4)][CF(3)SO(3)](8) (9) (en = ethane-1,2-diamine, [9]aneS3 = 1,4,7-trithiacyclononane) that have four half-sandwich Ru(II) compounds. The Ru fragments may either contain one or more labile ligands, such as in 2 or in 9, or be coordinatively saturated and substitutionally inert, such as in 3 or in [bpy(4)-PP{Ru([12]aneS4)}(4)][CF(3)SO(3)](8) (11) ([12]aneS4 = 1,4,7,10-tetrathiacyclododecane). Most of the ruthenium-porphyrin conjugates described in this work are soluble--at least moderately--in aqueous solution and are thus suitable for biological investigations, in particular for cytotoxicity and photo-cytotoxicity tests.

Sequence variant (CTAGGG)n in the human telomere favors a G-quadruplex structure containing a G.C.G.C tetrad

Short contiguous arrays of variant CTAGGG repeats in the human telomere are unstable in the male germline and somatic cells, suggesting formation of unusual structures by this repeat type. Here, we report on the structure of an intramolecular G-quadruplex formed by DNA sequences containing four human telomeric variant CTAGGG repeats in potassium solution. Our results reveal a new robust antiparallel G-quadruplex fold involving two G-tetrads sandwiched between a G.C base pair and a G.C.G.C tetrad, which could represent a new platform for drug design targeted to human telomeric DNA.

Selectivity of small molecule ligands for parallel and anti-parallel DNA G-quadruplex structures

We report CD, ESI-MS and molecular modelling studies of ligand binding interactions with DNA quadruplex structures derived from the human telomeric repeat sequence (h-Tel) and the proto-oncogenic c-kit promoter sequence. These sequences form anti-parallel (both 2 + 2 and 3 + 1) and parallel conformations, respectively, and demonstrate distinctively different degrees of structural plasticity in binding ligands. With h-Tel, we show that an extended heteroaromatic 1,4-triazole (TRZ), designed to exploit pi-stacking interactions and groove-specific contacts, shows some selectivity for parallel folds, however, the polycyclic fluorinated acridinium cation (RHPS4), which is a similarly potent telomerase inhibitor, shows selectivity for anti-parallel conformations implicating favourable interactions with lateral and diagonal loops. In contrast, the unique c-kit parallel-stranded quadruplex shows none of the structural plasticity of h-Tel with either ligand. We show by quantitative ESI-MS analysis that both sequences are able to bind a ligand on either end of the quadruplex. In the case of h-Tel the two sites have similar affinities, however, in the case of the c-kit quadruplex the affinities of the two sites are different and ligand-dependent. We demonstrate that two different small molecule architectures result in significant differences in selectivity for parallel and anti-parallel quadruplex structures that may guide quadruplex targeted drug-design.

Interactions of ruthenium coordination cubes with DNA

The interactions of four octacationic ruthenium coordination cubes with duplex and quadruplex (telomeric and c-myc) DNA have been studied by FID and SPR, showing these cubes to bind strongly to tetrastranded DNA structures.

Structures of telomerase subunits provide functional insights

BACKGROUND

Telomerase continues to generate substantial attention both because of its pivotal roles in cellular proliferation and aging and because of its unusual structure and mechanism. By replenishing telomeric DNA lost during the cell cycle, telomerase overcomes one of the many hurdles facing cellular immortalization. Functionally, telomerase is a reverse transcriptase, and it shares structural and mechanistic features with this class of nucleotide polymerases. Telomerase is a very unusual reverse transcriptase because it remains stably associated with its template and because it reverse transcribes multiple copies of its template onto a single primer in one reaction cycle.

SCOPE OF REVIEW

Here, we review recent findings that illuminate our understanding of telomerase. Even though the specific emphasis is on structure and mechanism, we also highlight new insights into the roles of telomerase in human biology.

GENERAL SIGNIFICANCE

Recent advances in the structural biology of telomerase, including high resolution structures of the catalytic subunit of a beetle telomerase and two domains of a ciliate telomerase catalytic subunit, provide new perspectives into telomerase biochemistry and reveal new puzzles.

Bis-anthracenyl isoxazolyl amides have enhanced anticancer activity

Dimeric analogs of Anthracenyl Isoxazole Amides (AIMs) (the designation AIM is in honor of the memory of Professor Albert I. Meyers) were prepared and dimer 6 exhibited the highest efficacy to date for this class of anti-tumor compounds against the human glioma Central Nervous System cell line SNB-19.

A microfluidic platform to synthesize a G-quadruplex binding ligand

An aromatic triarylpyridine chromophore promotes pi-stacking interactions with the terminal G-tetrad in quadruplex DNA, stabilizing the structure and presenting a pathway towards cancer treatment by inhibition of telomerase. An interesting parent compound in this class is the dimethylamino functionalised 4'-aryl-2,6-bis(4-aminophenyl)pyridine. However, access to this compound using traditional batch synthetic methodology is limited, due to thermodynamic and kinetic constraints. A novel approach to the synthesis of this compound has been developed, involving dynamic thin films, overcoming a series of competing reactions, effectively controlling chemical reactivity and selectivity.

G-quadruplex nucleic acids as therapeutic targets

Nucleic acid sequences containing several short runs of guanine nucleotides can form complex higher order structures, termed quadruplexes. Their occurrence has been most extensively characterised at the telomeric ends of eukaryotic chromosomes, whose DNA comprises such sequences, and where the extreme 3' ends are single-stranded. This enables relatively facile formation of quadruplex arrangements under the influence of a quadruplex-selective small molecule to compete effectively with telomeric protein-DNA interactions. Occurrences of quadruplexes within the human and other genomes have been mapped by bioinformatics surveys, which have revealed over-representations in promoter regions, especially of genes involved in replication, such as oncogenes, as well as in 5'UTR regions. The highly distinctive nature of quadruplex topologies suggests that they can act as novel therapeutic targets, for example in the selective inhibition of transcription of a given oncogene, using designed small molecules to stabilise a particular quadruplex. This offers the prospect of an alternative to, for example, direct kinase targeting with small molecules, without the attendant issues of active-site resistance. We survey here the basis of these approaches, together with current progress, and discuss the mechanistic issues posed by quadruplex targeting.

Biaryl polyamides as a new class of DNA quadruplex-binding ligands

We report a novel class of biaryl polyamides highly selective for G-quadruplex DNA, and with significant cytotoxicity in several cancer cell lines; they form planar U-shaped structures that match the surface area dimensions of a terminal G-quartet in quadruplex structures rather than the grooves of duplex DNA.

The structures of quadruplex nucleic acids and their drug complexes

Quadruplex nucleic acids are four-stranded structures formed from short tracts of G-rich sequence associating together. Their formation from eukaryotic telomeric DNA sequences is well established, at least in vitro, and has more recently been the focus of attention as novel anticancer targets since their formation inhibits the telomerase complex from maintaining telomere length in cancer cells. Structural studies have revealed a diversity of topologies for telomeric quadruplexes, which are sensitive to the nature of the cations present, to the flanking sequences, and probably also to concentration. The small number of structures determined for ligand-quadruplex complexes have to date shown a marked preference for the quadruplex component to have all strands in a parallel folding arrangement, which maximises the planar surface available for ligand binding. Informatics has revealed that there are a large number of quadruplex sequences present in human and other genomes, both eukaryotic and prokaryotic, with over-representation in upstream promoter regions. Structures for a few of these are now available, which emphasise the role played by loop sequences in determining fold.

2,6-Diphenylthiazolo[3,2-b][1,2,4]triazoles as telomeric G-quadruplex stabilizers

The design and synthesis of 2,6-diphenylthiazolo[3,2-b][1,2,4]triazoles characterized by a large aromatic building block bearing cationic side chains are reported. These molecules are evaluated as telomeric G-quadruplex stabilizers and for their selectivity towards duplex DNA by competition experiments. Two compounds (14a, 19) were found active with high selectivity for telomeric G-quadruplex over duplex DNA.

Dramatic effect of single-base mutation on the conformational dynamics of human telomeric G-quadruplex

Guanine-rich DNA sequences can form G-quadruplexes. These four-stranded structures are known to form in several genomic regions and to influence certain biological activities. Sometimes, the instability of G-quadruplexes causes the abnormal biological processes. Mutation is a culprit for the destabilization of G-quadruplexes, but the details of mutated G-quadruplexes are poorly understood. In this article, we investigated the conformational dynamics of single-base mutated human telomeric G-quadruplexes in the presence of K(+) with single-molecule FRET spectroscopy. We observed that the replacement of single guanine by thymine in a G-track induces various folded structures, i.e. structural polymorphism. Moreover, direct observation of their dynamics revealed that a single-base mutation causes fast unfolding of folded states under physiological conditions. Furthermore, we found that the degree of destabilization varies according to mutation positions. When the central guanine of a G-track is replaced, the G-quadruplexes unfold quickly at any K(+) concentrations and temperature. Meanwhile, outer-quartet mutated G-quadruplexes have heterogeneous dynamics at intermediate K(+) concentrations and longstanding folded states at high K(+) concentrations. Several factors such as base-stacking interaction and K(+) coordination are responsible for the different dynamics according to the mutation position.

A novel small molecule that alters shelterin integrity and triggers a DNA-damage response at telomeres

We describe a novel synthetic small molecule which shows an unprecedented stabilization of the human telomeric G-quadruplex with high selectivity relative to double-stranded DNA. We report that this compound can be used in vitro to inhibit telomerase activity and to uncap human POT1 (protection of telomeres 1) from the telomeric G-overhang. We also show that the small molecule G-quadruplex binder induces a partial alteration of shelterin through POT1 uncapping from telomeres in human HT1080 cancer cells and the presence of gammaH2AX foci colocalized at telomeres.

Structure of the human telomere in K+ solution: a stable basket-type G-quadruplex with only two G-tetrad layers

Previously, it has been reported that human telomeric DNA sequences could adopt in different experimental conditions four different intramolecular G-quadruplexes each involving three G-tetrad layers, namely, Na(+) solution antiparallel-stranded basket form, K(+) crystal parallel-stranded propeller form, K(+) solution (3 + 1) Form 1, and K(+) solution (3 + 1) Form 2. Here we present a new intramolecular G-quadruplex adopted by a four-repeat human telomeric sequence in K(+) solution (Form 3). This structure is a basket-type G-quadruplex with only two G-tetrad layers: loops are successively edgewise, diagonal, and edgewise; glycosidic conformations of guanines are syn x syn x anti x anti around each tetrad. Each strand of the core has both a parallel and an antiparallel adjacent strands; there are one narrow, one wide, and two medium grooves. Despite the presence of only two G-tetrads in the core, this structure is more stable than the three-G-tetrad intramolecular G-quadruplexes previously observed for human telomeric sequences in K(+) solution. Detailed structural elucidation of Form 3 revealed extensive base pairing and stacking in the loops capping both ends of the G-tetrad core, which might explain the high stability of the structure. This novel structure highlights the conformational heterogeneity of human telomeric DNA. It establishes a new folding principle for G-quadruplexes and suggests new loop sequences and structures for targeting in human telomeric DNA.

Stabilizing parallel G-quadruplex DNA by a new class of ligands: two non-planar alkaloids through interaction in lateral grooves

Human DNA sequences consisting of tandem guanine (G) nucleotides can fold into a four-stranded structure named G-quadruplex via Hoogsteen hydrogen bonding. As the sequences forming G-quadruplex exist in essential regions of eukaryotic chromosomes and are involved in many important biological processes, the study of their biological functions has currently become a hotspot. Compounds selectively binding and stabilizing G-quadruplex structures have the potential to inhibit telomerase activity or alter oncogene expression levels and thus may act as antitumor agents. Most of reported G-quadruplex ligands generally have planar structures which stabilize G-quadruplex by pi-pi stacking. However, based on a pharmacophore-based virtual screening two non-planar G-quadruplex ligands were found. These two ligands exhibit good capability for G-quadruplex stabilization and prefer binding to paralleled G-quadruplex rather than to duplex DNA. The binding of these ligands to G-quadruplex may result from groove binding at a 2:1 stoichiometry. These results have shown that planar structures are not essential for G-quadruplex stabilizers, which may represent a new class of G-quadruplex-targeted agents as potential antitumor drugs.

Cell proliferation in the presence of telomerase

Background

Telomerase, which is active early in development and later in stem and germline cells, is also active in the majority of human cancers. One of the known functions of telomerase is to extend the ends of linear chromosomes, countering their gradual shortening at each cell division due to the end replication problem and postreplication processing. Telomerase concentration levels vary between different cell types as well as between different tumors. In addition variable telomerase concentrations will exist in different cells in the same tumor when telomerase inhibitors are used, because of limitations of drug delivery in tissue. Telomerase extends short telomeres more frequently than long telomeres and the relation between the extension frequency and the telomere length is nonlinear.

Methodolgy/Principal Findings

Here, the biological data of the nonlinear telomerase-telomere dynamics is incorporated in a mathematical theory to relate the proliferative potential of a cell to the telomerase concentration in that cell. The main result of the paper is that the proliferative capacity of a cell grows exponentially with the telomerase concentration.

Conclusions/Significance

The theory presented here suggests that long term telomerase inhibition in every cancer progenitor or cancer stem cell is needed for successful telomere targeted cancer treatment. This theory also can be used to plan and asses the results of clinical trials targeting telomerase.

Telomeres do the (un)twist: helicase actions at chromosome termini

Telomeres play critical roles in protecting genome stability, and their dysfunction contributes to cancer and age-related degenerative diseases. The precise architecture of telomeres, including their single-stranded 3' overhangs, bound proteins, and ability to form unusual secondary structures such as t-loops, is central to their function and thus requires careful processing by diverse factors. Furthermore, telomeres provide unique challenges to the DNA replication and recombination machinery, and are particularly suited for extension by the telomerase reverse transcriptase. Helicases use the energy from NTP hydrolysis to track along DNA and disrupt base pairing. Here we review current findings concerning how helicases modulate several aspects of telomere form and function.