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

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          Abstract

          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.

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          Extension of life-span by introduction of telomerase into normal human cells.

          Normal human cells undergo a finite number of cell divisions and ultimately enter a nondividing state called replicative senescence. It has been proposed that telomere shortening is the molecular clock that triggers senescence. To test this hypothesis, two telomerase-negative normal human cell types, retinal pigment epithelial cells and foreskin fibroblasts, were transfected with vectors encoding the human telomerase catalytic subunit. In contrast to telomerase-negative control clones, which exhibited telomere shortening and senescence, telomerase-expressing clones had elongated telomeres, divided vigorously, and showed reduced straining for beta-galactosidase, a biomarker for senescence. Notably, the telomerase-expressing clones have a normal karyotype and have already exceeded their normal life-span by at least 20 doublings, thus establishing a causal relationship between telomere shortening and in vitro cellular senescence. The ability to maintain normal human cells in a phenotypically youthful state could have important applications in research and medicine.
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            Identification of a specific telomere terminal transferase activity in Tetrahymena extracts.

            We have found a novel activity in Tetrahymena cell free extracts that adds tandem TTGGGG repeats onto synthetic telomere primers. The single-stranded DNA oligonucleotides (TTGGGG)4 and TGTGTGGGTGTGTGGGTGTGTGGG, consisting of the Tetrahymena and yeast telomeric sequences respectively, each functioned as primers for elongation, while (CCCCAA)4 and two nontelomeric sequence DNA oligomers did not. Efficient synthesis of the TTGGGG repeats depended only on addition of micromolar concentrations of oligomer primer, dGTP, and dTTP to the extract. The activity was sensitive to heat and proteinase K treatment. The repeat addition was independent of both endogenous Tetrahymena DNA and the endogenous alpha-type DNA polymerase; and a greater elongation activity was present during macronuclear development, when a large number of telomeres are formed and replicated, than during vegetative cell growth. We propose that the novel telomere terminal transferase is involved in the addition of telomeric repeats necessary for the replication of chromosome ends in eukaryotes.
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              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.
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                Author and article information

                Journal
                Nucleic Acids Res
                Nucleic Acids Res
                nar
                Nucleic Acids Research
                Nucleic Acids Research
                Oxford University Press
                0305-1048
                1362-4962
                April 2007
                29 March 2007
                29 March 2007
                : 35
                : 7
                : 2440-2450
                Affiliations
                1College of Pharmacy, The University of Arizona, 1703 E. Mabel St, Tucson, AZ 85721, USA, 2Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854, USA, 3Arizona Cancer Center, 1515 N. Campbell Avenue, Tucson, AZ 85724, USA and 4BIO5 Institute, The University of Arizona, 1140 E. South Campus Dr, Tucson, AZ 85721, USA
                Author notes
                *To whom correspondence should be addressed. +1 520 626 5969+1 520 626 6988 yangd@ 123456pharmacy.arizona.edu
                Article
                10.1093/nar/gkm009
                1874667
                17395643
                8f625386-abbd-41de-a8c5-f72586d7cbe1
                © 2007 The Author(s)

                This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 8 September 2006
                : 21 December 2006
                : 27 December 2006
                Categories
                Structural Biology

                Genetics
                Genetics

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