Telomeric DNA Sequences Research Articles

The Roles of Telomerase in Regeneration during Aging and the Telomeric DNA Sequence Identification in Aeolosoma viride

Telomeres are the functional chromosome elements that protect the ends of eukaryotic linear chromosomes from degradation and fusion. Incomplete telomere DNA replication results in length shortening during every round of cell division. On the other hand, the enzyme telomerase can extend the ends of telomeres and prevent this situation to happen in certain tissues. With essential function for genome integrity and cell survival, it has been reported that short telomeres and defective telomerase activity are involved in several age‐related diseases. Moreover, recent studies suggest that telomerase may be important for tissue renewal and regeneration after injury. Aeolosoma viride is a kind of fresh water annelid. It has strong ability in regeneration and is easy to manipulate for experiments. After anterior four segments decapitation, A. viride can regenerate its lost parts within five days. However, the regeneration successful rate decreases when the animal gets older and older. The regulation of telomerase gene expression in regeneration and how aging affecting this process are not well understood, so we would like to explore this mysterious aspect. In this study, we cloned the gene of telomerase protein component, avi‐tert, from our A. viride transcriptome database. The results of in situ hybridization and quantitative PCR showed that avi‐tert would express at the regenerating site, the blastema, where cell proliferation is undergoing. Besides, to identify the telomere repeat sequence, we searched the telomerase database (http://telomerase.asu.edu/) and chose two possible sequences, which are TTAGGG and TTAGG, according to the phylogenetic relationship of our species. Then, we extracted the genomic DNA for dot blotting assay and terminal restriction fragment (TRF) analysis. The results suggested that the telomere sequence of A. viride is most possible to be “TTAGGG” (same as human's) rather than “TTAGG” (in some invertebrates). In the next step, we will measure the gene expression profile in the animals of different ages and try to determine whether there are any changes during regeneration process.

Molecular Recognition of Parallel DNA Quadruplex d(TTAGGGT)4 by Mitoxantrone: Binding with 1:2 Stoichiometry Leading to Thermal Stabilization and Telomerase Inhibition

The interaction of the anthraquinone derivative mitoxantrone, a semisynthetic anti-cancer drug with two non-planar side chains, with heptamer G-quadruplex d(TTAGGGT)4 , which contains the human telomere DNA sequence, was evaluated by differential scanning calorimetry, fluorescence Job plotting, absorption, and NMR and CD spectroscopy. Binding led to thermal stabilization of DNA (ΔTm =13-20 °C). The spectra revealed that two mitoxantrone molecules bind externally at two sites of the DNA quadruplex as monomers, by partial insertion of the chromophore and side-chain interaction at the grooves. The inhibition of telomerase (IC50 =2 μM), as determined by a TRAP assay, can be attributed to thermal stabilization of the DNA quadruplex because of the interactions with mitoxantrone. The studies revealed highly specific molecular recognition between a ligand and a parallel-stranded G-quadruplex; this might serve as a platform for the rational design of new drugs.

Reversible pH Switch of Two‐Quartet G‐Quadruplexes Formed by Human Telomere

AbstractA four‐repeat human telomere DNA sequence without the 3′‐end guanine, d[TAGGG(TTAGGG)2TTAGG] (htel1‐ΔG23) has been found to adopt two distinct two G‐quartet antiparallel basket‐type G‐quadruplexes, TD and KDH+ in presence of KCl. NMR, CD, and UV spectroscopy have demonstrated that topology of KDH+ form is distinctive with unique protonated T18⋅A20+⋅G5 base triple and other capping structural elements that provide novel insight into structural polymorphism and heterogeneity of G‐quadruplexes in general. Specific stacking interactions amongst two G‐quartets flanking base triples and base pairs in TD and KDH+ forms are reflected in 10 K higher thermal stability of KDH+. Populations of TD and KDH+ forms are controlled by pH. The (de)protonation of A20 is the key for pH driven structural transformation of htel1‐ΔG23. Reversibility offers possibilities for its utilization as a conformational switch within different compartments of living cell enabling specific ligand and protein interactions.

Reversible pH Switch of Two‐Quartet G‐Quadruplexes Formed by Human Telomere

A four-repeat human telomere DNA sequence without the 3'-end guanine, d[TAGGG(TTAGGG)2 TTAGG] (htel1-ΔG23) has been found to adopt two distinct two G-quartet antiparallel basket-type G-quadruplexes, TD and KDH(+) in presence of KCl. NMR, CD, and UV spectroscopy have demonstrated that topology of KDH(+) form is distinctive with unique protonated T18⋅A20(+) ⋅G5 base triple and other capping structural elements that provide novel insight into structural polymorphism and heterogeneity of G-quadruplexes in general. Specific stacking interactions amongst two G-quartets flanking base triples and base pairs in TD and KDH(+) forms are reflected in 10 K higher thermal stability of KDH(+) . Populations of TD and KDH(+) forms are controlled by pH. The (de)protonation of A20 is the key for pH driven structural transformation of htel1-ΔG23. Reversibility offers possibilities for its utilization as a conformational switch within different compartments of living cell enabling specific ligand and protein interactions.

Small molecule regulated dynamic structural changes of human G-quadruplexes.

The changes in structure and dynamics of oncogenic (c-MYC) and telomeric (h-TELO) G-rich DNA sequences due to the binding of a novel carbazole derivative (BTC) are elucidated using single-molecule Förster resonance energy transfer (sm-FRET), fluorescence correlation spectroscopy (FCS) and NMR spectroscopy. In contrast to the previous reports on the binding of ligands to pre-folded G-quadruplexes, this work illustrates how ligand binding changes the conformational equilibria of both unstructured G-rich DNA sequences and K+-folded G-quadruplexes. The results demonstrate that K+ free c-MYC and h-TELO exist as unfolded and partially folded conformations. The binding of BTC shifts the equilibria of both investigated DNA sequences towards the folded G-quadruplex structure, increases the diffusion coefficients and induces faster end-to-end contact formation. BTC recognizes a minor conformation of the c-MYC quadruplex and the two-tetrad basket conformations of the h-TELO quadruplex.

Expression of human telomerase reverse transcriptase protein in oral epithelial dysplasia and oral squamous cell carcinoma: An immunohistochemical study.

Background:Telomerase is an RNA-dependent DNA polymerase that synthesizes TTAGGG telomeric DNA sequences and almost universally provides the molecular basis for unlimited proliferative potential. The telomeres become shorter with each cycle of replication and reach a critical limit; most cells die or enter stage of replicative senescence. Telomere length maintenance by telomerase is required for all the cells that exhibit limitless replicative potential. It has been postulated that reactivation of telomerase expression is necessary for the continuous proliferation of neoplastic cells to attain immortality. Use of immunohistochemistry (IHC) is a useful, reliable method of localizing the human telomerase reverse transcriptase (hTERT) protein in tissue sections which permits cellular localization. Although there exists a lot of information on telomerase in oral cancer, little is known about their expression in oral epithelial dysplasia and their progression to oral squamous cell carcinoma (OSCC) compared to normal oral mucosa. This study addresses this lacuna.Aims:To compare the expression of hTERT protein in oral epithelial dysplasia and OSCC with normal oral mucosa by Immunohistochemical method.Subjects and Methods:In this preliminary study, IHC was used to detect the expression of hTERT protein in OSCC (n = 20), oral epithelial dysplasia (n = 21) and normal oral mucosa (n = 10). The tissue localization of immunostain, cellular localization of immunostain, nature of stain, intensity of stain, percentage of cells stained with hTERT protein were studied. A total number of 100 cells were counted in each slide.Statistical Analysis:All the data were analyzed using SPSS software version 16.0. The tissue localization, cellular localization of cytoplasmic/nuclear/both of hTERT stain, staining intensity was compared across the groups using Pearson's Chi-square test. The mean percentage of cells stained for oral epithelial dysplasia, OSCC and normal oral mucosa were compared using analysis of variance (ANOVA). A P < 0.05 was considered to be statistically significant.Results:The mean hTERT positive cells in the study groups were as follows, 62.91% in normal oral mucosa samples, 77.06% in oral epithelial dysplasia cases, and 81.48% in OSCC. In 61.9% of oral epithelial dysplasia and 65% of OSCC in our study, staining was visualized within the nucleus predominantly in the dot like pattern. There was a statistically significant difference in the nature of nuclear stain between oral epithelial dysplasia and OSCC (P = 0.023).Conclusions:Our results suggests that the mean percentage of cells showing hTERT expression steadily increased from normal oral mucosa to oral epithelial dysplasia to OSCC. The steady trend of increase in the percentage of cells was evident in different grades of oral epithelial dysplasia group and OSCC. The nature of hTERT staining did show variations among the three groups and promise to be a potential surrogate marker for malignant transformation. Further studies using IHC on larger sample size and clinical follow-up of these patients will be ascertaining the full potential of hTERT as a surrogate marker of epithelial transformation.

Centromere and telomere sequence alterations reflect the rapid genome evolution within the carnivorous plant genus Genlisea.

Linear chromosomes of eukaryotic organisms invariably possess centromeres and telomeres to ensure proper chromosome segregation during nuclear divisions and to protect the chromosome ends from deterioration and fusion, respectively. While centromeric sequences may differ between species, with arrays of tandemly repeated sequences and retrotransposons being the most abundant sequence types in plant centromeres, telomeric sequences are usually highly conserved among plants and other organisms. The genome size of the carnivorous genus Genlisea (Lentibulariaceae) is highly variable. Here we study evolutionary sequence plasticity of these chromosomal domains at an intrageneric level. We show that Genlisea nigrocaulis (1C = 86 Mbp; 2n = 40) and G. hispidula (1C = 1550 Mbp; 2n = 40) differ as to their DNA composition at centromeres and telomeres. G. nigrocaulis and its close relative G. pygmaea revealed mainly 161 bp tandem repeats, while G. hispidula and its close relative G. subglabra displayed a combination of four retroelements at centromeric positions. G. nigrocaulis and G. pygmaea chromosome ends are characterized by the Arabidopsis-type telomeric repeats (TTTAGGG); G. hispidula and G. subglabra instead revealed two intermingled sequence variants (TTCAGG and TTTCAGG). These differences in centromeric and, surprisingly, also in telomeric DNA sequences, uncovered between groups with on average a > 9-fold genome size difference, emphasize the fast genome evolution within this genus. Such intrageneric evolutionary alteration of telomeric repeats with cytosine in the guanine-rich strand, not yet known for plants, might impact the epigenetic telomere chromatin modification.

Molecular dynamics simulation of telomeric single-stranded DNA and POT1

Telomeres, which consist of single-, double- and four-stranded DNA, shorten after each round of cell division. The repeated telomeric DNA sequence 5′-TTAGGG-3′ does not encode genetic information and is not replicated completely. We performed molecular dynamics (MD) simulations of telomeric single-stranded DNA (ssDNA) and protection of telomere 1 (POT1) for 100 ns. We calculated the distance between Cα (POT1) and O5’ (telomeric ssDNA) to verify the binding system for 100 ns MD. We then calculated the distance between the bases of the telomeric DNA ends and the root-mean-square deviation and gyration radius in the single and binding states. Moreover, we compared the root-mean-square fluctuations (RMSFs) between the single and binding states and calculated the number of hydrogen bonds between POT1 and telomeric DNA. There are many hydrogen bonds between Gln94 and the first guanine of the closest 5′-TTAGGG-3′ sequence in telomeric single-stranded DNA, and the RMSF between the single and binding states has a large difference between Gln94 and guanine. Overall, we found that Gln94 and guanine are important components of the binding system and are related to the stability of this system. The molecular dynamics simulations of the telomeric single-stranded DNA (ssDNA) and protection of telomere 1 (POT1) in the single and biding states have been performed for 100 ns to compare the structures of those and to calculate root-mean-deviation, gyration radii, root-mean-square fluctuation and the number of hydrogen bonds. We found that the structures of the telomeric ssDNA in the single and binding states are different, and that one guanine and Gln94 are important for the binding system.

Structural basis of template-boundary definition in Tetrahymena telomerase.

Telomerase is required to maintain repetitive G-rich telomeric DNA sequences at chromosome ends. To do so, the telomerase reverse transcriptase (TERT) subunit reiteratively uses a small region of the integral telomerase RNA (TER) as a template. An essential feature of telomerase catalysis is the strict definition of the template boundary to determine the precise TER nucleotides to be reverse transcribed by TERT. We report the 3 Å crystal structure of the Tetrahymena TERT RNA binding domain (tTRBD) bound to the template boundary element (TBE) of TER. tTRBD is wedged into the base of the TBE RNA stem-loop and each of the flanking RNA strands wraps around opposite sides of the protein domain. The structure illustrates how the tTRBD establishes the template boundary by positioning the TBE the correct distance from the TERT active site to prohibit copying of non-template nucleotides.

Formation and stabilization of the telomeric antiparallel G-quadruplex and inhibition of telomerase by novel benzothioxanthene derivatives with anti-tumor activity.

G-quadruplexes formed in telomeric DNA sequences at human chromosome ends can be a novel target for the development of therapeutics for the treatment of cancer patients. Herein, we examined the ability of six novel benzothioxanthene derivatives S1–S6 to induce the formation of and stabilize an antiparallel G-quadruplex by EMSA, UV-melting and CD techniques and the influence of S1–S6 on A549 and SGC7901 cells through real-time cell analysis, wound healing, trap assay methods. Results show that six compounds could differentially induce 26 nt G-rich oligonucleotides to form the G-quadruplex with high selectivity vs C-rich DNA, mutated DNA and double-stranded DNA, stabilize it with high affinity, promote apoptosis and inhibit mobility and telomerase activity of A549 cells and SGC7901 cells. Especially, S1, S3, S4 displayed stronger abilities, of which S3 was the most optimal with the maximum ΔTm value being up to 29.8 °C for G-quadruplex, the minimum IC50 value being 0.53 μM and the maximum cell inhibitory rate being up to 97.2%. This study suggests that this type of compounds that induce the formation of and stabilize the telomeric antiparallel G-quadruplex, and consequently inhibit telomerase activity, leading to cell apoptosis, can be screened for the discovery of novel antitumor therapeutics.

OP6 - Sirtuins pathways and redox homeostasis: a pilot study on young and old monozygotic twins

Sirtuins are NAD+ dependent deacetylases that play a key role in the regulation of many processes related to homeostasis, such as the regulation of metabolism, apoptosis, DNA repair and inflammatory response. Studies on humans reported a relation between the alteration of their expression and the incidence of various diseases such as metabolic, cardiovascular, cancer and neurodegenerative diseases. According with these findings the maintenance of their optimal level of activity is considered important to ensure a low risk of pathologies related with the aging process. Indeed, SIRT1 has been correlated with the redox homeostasis maintenance and with the levels of oxidative damage, such as those affecting telomeric sequences of DNA. The aim of this study is to verify the correlation between sirtuins’ expression, histone deacetylation and redox status in young and old monozygotic twins. For this study we took advantage from blood samples of young (20-40 years old) and old (70-80 years old) monozygotic twins couple, where we analysed the expression of SIRT1 and SIRT2, Lysine acetylation proteins and Histone H4 acetylation, the protein oxidation through the detection of carbonyl groups have been analysed in PBMCs, plasma level of oxidized and reduced glutathione. The putative increase of SIRT1 and SIRT2 levels that should also lead to an improvement in redox and inflammatory homeostasis, hormonal response to stress, and in the maintenance of telomeric DNA sequences could open an interesting view in this field. Particularly the twin model can help to better understand the interaction between genetic and epigenetic effects in the age-related mechanisms.

Conformation-sensitive nucleoside analogues as topology-specific fluorescence turn-on probes for DNA and RNA G-quadruplexes.

Development of probes that can discriminate G-quadruplex (GQ) structures and indentify efficient GQ binders on the basis of topology and nucleic acid type is highly desired to advance GQ-directed therapeutic strategies. In this context, we describe the development of minimally perturbing and environment-sensitive pyrimidine nucleoside analogues, based on a 5-(benzofuran-2-yl)uracil core, as topology-specific fluorescence turn-on probes for human telomeric DNA and RNA GQs. The pyrimidine residues of one of the loop regions (TTA) of telomeric DNA and RNA GQ oligonucleotide (ON) sequences were replaced with 5-benzofuran-modified 2′-deoxyuridine and uridine analogues. Depending on the position of modification the fluorescent nucleoside analogues distinguish antiparallel, mixed parallel-antiparallel and parallel stranded DNA and RNA GQ topologies from corresponding duplexes with significant enhancement in fluorescence intensity and quantum yield. Further, these GQ sensors enabled the development of a simple fluorescence binding assay to quantify topology- and nucleic acid-specific binding of small molecule ligands to GQ structures. Together, our results demonstrate that these nucleoside analogues are useful GQ probes, which are anticipated to provide new opportunities to study and discover efficient G-quadruplex binders of therapeutic potential.

Telomerase lost?

Telomerase and telomerase-generated telomeric DNA sequences are widespread throughout eukaryotes, yet they are not universal. Neither telomerase nor the simple DNA repeats associated with telomerase have been found in some plant and animal species. Telomerase was likely lost from Diptera before the divergence of Diptera and Siphonaptera, some 260 million years ago. Even so, Diptera is one of the most successful animal orders, making up 11% of known animal species. In addition, many species of Coleoptera and Hemiptera seem to lack canonical telomeric repeats at their chromosome ends. These and other insects that appear to lack canonical terminal repeat sequences account for another 10-15% of animal species. Conversely, the silk moth Bombyx mori maintains canonical telomeric sequences at its chromosome ends but seems to lack a functional telomerase. We speculate that a telomere-specific capping complex that recognizes the telomeric repeats and protects chromosome ends is the determining factor in maintaining canonical telomeric sequences and that telomerase is an early and efficacious mechanism for satisfying the needs of capping complex. There are alternate mechanisms for maintaining chromosome ends that do not depend on telomerase, such as recombination found in some human cancer cells and yeast mutants. These mechanisms may maintain the canonical telomeric repeats or allow the terminal sequence to evolve when specificity of the capping complex for terminal repeat sequences is weak.

Recognition of human telomeric G-quadruplex DNA by berberine analogs: effect of substitution at the 9 and 13 positions of the isoquinoline moiety.

G-quadruplex forming sequences are widely distributed in human genome and serve as novel targets for regulating gene expression and chromosomal maintenance. They offer unique targets for anticancer drug development. Here, the interaction of berberine (BC) and two of its analogs bearing substitution at 9 and 13-position with human telomeric G-quadruplex DNA sequence has been investigated by biophysical techniques. Both the analogs exhibited several-fold higher binding affinity than berberine. The Scatchard binding isotherms revealed non-cooperative binding. 9-ω-amino hexyl ether analog (BC1) showed highest affinity (1.8 × 10(6) M(-1)) while the affinity of the 13-phenylpropyl analog (BC2) was 1.09 × 10(6) M(-1). Comparative fluorescence quenching and polarization anisotropy of the emission spectra gave evidence for a stronger stacking interaction of the analogs compared to berberine. The thiazole orange displacement assay has clearly established that the analogs were more effective in displacing the end stacked dye in comparison to berberine. However, the binding of the analogs did not induce any major structural perturbation in the G-quadruplex structure, but led to higher thermal stability. Energetics of the binding indicated that the association of the analogs was exothermic and predominantly entropy driven phenomenon. Increasing the temperature resulted in weaker binding; the enthalpic contribution increased and the entropic contribution decreased. A small negative heat capacity change with significant enthalpy-entropy compensation established the involvement of multiple weak noncovalent interactions in the binding process. The 9-ω-amino hexyl ether analog stabilized the G-quadruplex structure better than the 13-phenyl alkyl analog.

Involvement of Long‐Lived Intermediate States in the Complex Folding Pathway of the Human Telomeric G‐Quadruplex

AbstractThe energy landscapes of human telomeric G‐quadruplexes are complex, and their folding pathways have remained largely unexplored. By using real‐time NMR spectroscopy, we investigated the K+‐induced folding of the human telomeric DNA sequence 5′‐TTGGG(TTAGGG)3A‐3′. Three long‐lived states were detected during folding: a major conformation (hybrid‐1), a previously structurally uncharacterized minor conformation (hybrid‐2), and a partially unfolded state. The minor hybrid‐2 conformation is formed faster than the more stable hybrid‐1 conformation. Equilibration of the two states is slow and proceeds via a partially unfolded intermediate state, which can be described as an ensemble of hairpin‐like structures.

Involvement of Long-Lived Intermediate States in the Complex Folding Pathway of the Human Telomeric G-Quadruplex.

The energy landscapes of human telomeric G-quadruplexes are complex, and their folding pathways have remained largely unexplored. By using real-time NMR spectroscopy, we investigated the K(+)-induced folding of the human telomeric DNA sequence 5'-TTGGG(TTAGGG)3 A-3'. Three long-lived states were detected during folding: a major conformation (hybrid-1), a previously structurally uncharacterized minor conformation (hybrid-2), and a partially unfolded state. The minor hybrid-2 conformation is formed faster than the more stable hybrid-1 conformation. Equilibration of the two states is slow and proceeds via a partially unfolded intermediate state, which can be described as an ensemble of hairpin-like structures.

Chromosomal characteristics and distribution of rDNA sequences in the brook trout Salvelinus fontinalis (Mitchill, 1814).

Brook trout Salvelinus fontinalis (Mitchill, 1814) chromosomes have been analyzed using conventional and molecular cytogenetic techniques enabling characteristics and chromosomal location of heterochromatin, nucleolus organizer regions (NORs), ribosomal RNA-encoding genes and telomeric DNA sequences. The C-banding and chromosome digestion with the restriction endonucleases demonstrated distribution and heterogeneity of the heterochromatin in the brook trout genome. DNA sequences of the ribosomal RNA genes, namely the nucleolus-forming 28S (major) and non-nucleolus-forming 5S (minor) rDNAs, were physically mapped using fluorescence in situ hybridization (FISH) and primed in situ labelling. The minor rDNA locus was located on the subtelo-acrocentric chromosome pair No. 9, whereas the major rDNA loci were dispersed on 14 chromosome pairs, showing a considerable inter-individual variation in the number and location. The major and minor rDNA loci were located at different chromosomes. Multichromosomal location (3–6 sites) of the NORs was demonstrated by silver nitrate (AgNO3) impregnation. All Ag-positive i.e. active NORs corresponded to the GC-rich blocks of heterochromatin. FISH with telomeric probe showed the presence of the interstitial telomeric site (ITS) adjacent to the NOR/28S rDNA site on the chromosome 11. This ITS was presumably remnant of the chromosome rearrangement(s) leading to the genomic redistribution of the rDNA sequences. Comparative analysis of the cytogenetic data among several related salmonid species confirmed huge variation in the number and the chromosomal location of rRNA gene clusters in the Salvelinus genome.

Cell Death Mechanisms Induced by Altered Telomerase RNA Template Sequence

Due to an inability of DNA polymerases to replicate the extreme end of chromosomes, the telomeres of most somatic cells shorten with each cell division, leading to a state of senescence induced by critically short telomeres. To avoid this, an enzyme called telomerase is required for the synthesis of telomere DNA. In cancer cells, telomerase is upregulated allowing the cancer cells to escape senescence. Due to this unique property of cancer cells, targeting telomerase is a viable therapeutic approach. Telomerase uses an internal RNA template to reverse transcribe telomere DNA with a specific repeating hexameric sequence. Telomere DNA is bound and protected by the shelterin complex. Binding of this complex to telomere DNA is very specific, such that a single nucleotide alteration in telomere DNA sequence results in reduced recognition and binding efficiency. Abrogation of shelterin protein binding induces an immediate DNA damage response. We aim to exploit this mechanism as a putative chemotherapeutic approach by investigating the effects of mutating the template region of telomerase RNA on its overall activity and efficiency. To this end, we created 12 different single nucleotide mutants within the template region of the telomerase RNA that will synthesize non‐native telomere sequences. Using a direct in vitro telomerase assay we identified which sequences affect telomerase function directly. Surprisingly, many of the point mutations resulted in abolished telomerase activity, presumably due to misfolding of the RNA or inhibition of telomerase holoenzyme assembly. Future efforts will include elucidation of the most critical nucleotide within telomere sequence for shelterin binding events by creating stable cell lines with desired mutations at telomerase RNA template.

Nanopore detection of 8-oxoguanine in the human telomere repeat sequence.

The human telomere repeat sequence 5′-TTAGGG-3′ is a hot spot for oxidation at guanine, yielding 8-oxo-7,8-dihydroguanine (OG), a biomarker of oxidative stress. Telomere shortening resulting from oxidation will ultimately induce cellular senescence. In this study, α-hemolysin (α-HL) nanopore technology was applied to detect and quantify OG in the human telomeric DNA sequence. This repeat sequence adopts a basket G-quadruplex in the NaCl electrolyte used for analysis that enters the α-HL channel, slowly unfolds, and translocates. The basket fold containing OG disrupts the structure, leading to >10× increase in the unfolding kinetics without yielding a detectable current pattern. Therefore, detection of OG with α-HL required labeling of OG with aminomethyl-[18-crown-6] using a mild oxidant. The labeled OG yielded a pulse-like signal in the current vs time trace when the DNA strand was electrophoretically passed through α-HL in NaCl electrolyte. However, the rate of translocation was too slow using NaCl salts, leading us to further refine the method. A mixture of NH4Cl and LiCl electrolytes induced the propeller fold that unravels quickly outside the α-HL channel. This electrolyte allowed observation of the labeled OG, while providing a faster recording of the currents. Lastly, OG distributions were probed with this method in a 120-mer stretch of the human telomere sequence exposed to the cellular oxidant 1O2. Single-molecule profiles determined the OG distributions to be random in this context. Application of the method in nanomedicine can potentially address many questions surrounding oxidative stress and telomere attrition observed in various disease phenotypes including prostate cancer and diabetes.

Discrimination between 8-oxo-2'-deoxyguanosine and 2'-deoxyguanosine in DNA by the single nucleotide primer extension reaction with adap triphosphate.

The adenosine derivative of 2-oxo-1,3-diazaphenoxazine (Adap) exhibits a superb ability to recognize and form base pairs with 8-oxo-2'-deoxyguanosine (8-oxo-dG) in duplex DNA. In this study, the triphosphate of Adap (dAdapTP) was synthesized and tested for single nucleotide incorporation into primer strands using the Klenow Fragment. The efficiency of dAdapTP incorporation into 8-oxo-dG-containing templates was more than 36-fold higher than with dG-containing templates, and provides better discrimination than does the incorporation of natural 2'-deoxyadenosine triphosphate (dATP). The selective incorporation of dAdapTP into 8-oxo-dG templates was therefore applied to the detection of 8-oxo-dG in human telomeric DNA sequences extracted from H2 O2 -treated HeLa cells. The enzymatic incorporation of dAdapTP into 8-oxo-dG-containing templates may provide a novel basis for sequencing oxidative DNA damage in the genome.