G-quadruplex

In molecular biology, G-quadruplexes (also known as G4-DNA) are structures formed in nucleic acids by sequences that are rich in guanine. Four guanine bases can associate through Hoogsteen hydrogen bonding to form a square planar structure called a guanine tetrad, and two or more guanine tetrads can stack on top of each other to form a G-quadruplex. The quadruplex structure is further stabilized by the presence of a cation, especially potassium, which sits in a central channel between each pair of tetrads.[1] They can be formed of DNA, RNA, LNA, and PNA, and may be intramolecular, bimolecular, or tetramolecular.[2] Depending on the direction of the strands or parts of a strand that form the tetrads, structures may be described as parallel or antiparallel.

Structure of a G-quadruplex. Left: a G-tetrad. Right: an intramolecular G-quadruplex

Quadruplex topology

The length of the nucleic acid sequences involved in tetrad formation determines how the quadruplex folds. Short sequences, consisting of only a single contiguous run of three or more guanine bases, require four individual strands to form a quadruplex. Such a quadruplex is described as tetramolecular, reflecting the requirement of four separate strands. Longer sequences, which contain two contiguous runs of three or more guanine bases, where the guanine regions are separated by one or more bases, only require two such sequences to provide enough guanine bases to form a quadruplex. These structures, formed from two separate G-rich strands, are termed bimolecular quadruplexes. Finally, sequences which contain four distinct runs of guanine bases can form stable quadruplex structures by themselves, and a quadruplex formed entirely from a single strand is called an intramolecular quadruplex.[3]

Depending on how the individual runs of guanine bases are arranged in a bimolecular or intramolecular quadruplex, a quadruplex can adopt one of a number of topologies with varying loop configurations.[4] If the 5' – 3’ direction of all the strands is the same, the quadruplex is termed parallel; that is, all the strands of DNA are proceeding in the same direction. For intramolecular quadruplexes, this means that any loop regions present must be of the edge, or propellor, type, which are positioned to the sides of the quadruplex. If one or more of the runs of guanine bases has a 5’-3’ direction opposite to the other runs of guanine bases, the quadruplex is said to have adopted an antiparallel topology. The loops joining runs of guanine bases in intramolecular antiparallel quadruplexes are either diagonal (joining two diagonally opposite runs of guanine bases) or edge type loops, which join two adjacent runs of guanines.

In quadruplexes formed from double-stranded DNA, possible interstrand topologies have also been discussed [5] .[6] Interstrand quadruplexes contain guanines that originate from both strands of dsDNA.

Telomeric quadruplexes

Telomeric repeats in a variety of organisms have been shown to form these structures in vitro, and subsequently they have also been shown to form in vivo.[7][8] The human telomeric repeat (which is the same for all vertebrates) consists of many repeats of the sequence d(GGTTAG), and the quadruplexes formed by this structure have been well studied by NMR and X-ray crystal structure determination. The formation of these quadruplexes in telomeres has been shown to decrease the activity of the enzyme telomerase, which is responsible for maintaining length of telomeres and is involved in around 85% of all cancers. This is an active target of drug discovery.

Non-telomeric quadruplexes

Recently, there has been increasing interest in quadruplexes in locations other than at the telomere. For example the proto-oncogene c-myc was shown to form a quadruplex in a nuclease hypersensitive region critical for gene activity.[9][10] Since then, many other genes have been shown to have G-quadruplexes in their promoter regions, including the chicken β-globin gene, human ubiquitin-ligase RFP2 and the proto-oncogenes c-kit, bcl-2, VEGF, H-ras and N-ras. This list is ever-increasing.

Genome-wide surveys based on a quadruplex folding rule have been performed, which have identified 376,000 Putative Quadruplex Sequences (PQS) in the human genome, although not all of these probably form in vivo.[11] A similar study has identified putative G-quadruplexes in prokaryotes.[12] There are several possible models for how quadruplexes could influence gene activity, either by upregulation or downregulation. One model is shown below, with G-quadruplex formation in or near a promoter blocking transcription of the gene, and hence de-activating it. In another model, quadruplex formed at the non-coding DNA strand helps to maintain an open conformation of the coding DNA strand and enhance an expression of the respective gene.

Model for quadruplex-mediated down-regulation of gene expression[13]

Quadruplex function

Nucleic acid quadruplexes have been described as "structures in search of a function",[3] as for many years there was minimal evidence pointing towards a biological role for these structures. It has been suggested that quadruplex formation plays a role in immunoglobulin heavy chain switching.[14] As cells have evolved mechanisms for resolving (i.e., unwinding) quadruplexes that form, quadruplex formation may be potentially damaging for a cell; for example, the helicases WRN and Bloom syndrome protein have a high affinity for resolving G4 DNA.[15] More recently, there are many studies that implicate quadruplexes in both positive and negative transcriptional regulation, and in allowing programmed recombination of immunologlobin heavy genes and the pilin antigenic variation system of the pathogenic Neisseria.[16] The roles of quadraduplex structure in translation control are not as well explored. The direct visualization of quadraduplex structures in human cells [17] has provided an important confirmation of their existence. The potential positive and negative roles of quadraduplexes in telomere replication and function remains controversial.

Ligands which bind quadruplexes

One way of inducing or stabilizing G-quadruplex formation, is to introduce a molecule which can bind to the G-quadruplex structure, and a number of ligands, both small molecules and proteins, have been developed which can do so. This has become an increasingly large field of research.

A number of naturally occurring proteins have been identified which selectively bind to G-quadruplexes. These include the helicases implicated in Bloom's and Werner's syndromes and the Saccharomyces cerevisiae protein RAP1. An artificially derived three zinc finger protein called Gq1, which is specific for G-quadruplexes has also been developed, as have specific antibodies.

Cationic porphyrins have been shown to bind intercalatively with G-quadruplexes, as well as the molecule telomestatin.

Quadruplex prediction techniques

Identifying and predicting sequences which have the capacity to form quadruplexes is an important tool in further understanding of their role. Generally, a simple pattern match is used for searching for possible intrastrand quadruplex forming sequences: d(G3+N1-7G3+N1-7G3+N1-7G3+), where N is any base (including guanine).[18] This rule has been widely used in on-line algorithms.

Notes

  1. ↑ Campbell, Nancy H.; Neidle, Stephen (2012). "Chapter 4. G-Quadruplexes and Metal Ions". In Astrid Sigel, Helmut Sigel and Roland K. O. Sigel. Interplay between Metal Ions and Nucleic Acids. Metal Ions in Life Sciences 10. Springer. pp. 119–1134. doi:10.1007/978-94-007-2172-2_4.
  2. ↑ Bochman, Matthew L.; Paeschke, Katrin; Zakian, Virginia A. (2012). DNA secondary structures: stability and function of G-quadruplex structures. Nature Reviews Genetics 13. Nature Publishing Group. pp. 770–780. doi:10.1038/nrg3296.
  3. 1 2 Simonsson, T. (2001). "G-Quadruplex DNA Structures Variations on a Theme". Biological Chemistry 382 (4): 621–628. doi:10.1515/BC.2001.073. PMID 11405224.
  4. ↑ Burge, S.; Parkinson, G. N.; Hazel, P.; Todd, A. K.; Neidle, S. (2006). "Quadruplex DNA: Sequence, topology and structure". Nucleic Acids Research 34 (19): 5402–5415. doi:10.1093/nar/gkl655. PMC 1636468. PMID 17012276.
  5. ↑ Cao, K.; Ryvkin, P.; Johnson, FB. (2012). "Computational detection and analysis of sequences with duplex-derived interstrand G-quadruplex forming potential.". Methods 57 (1): 3–10. doi:10.1016/j.ymeth.2012.05.002. PMC 3701776. PMID 22652626.
  6. ↑ Kudlicki, A. (2016). "G-Quadruplexes Involving Both Strands of Genomic DNA Are Highly Abundant and Colocalize with Functional Sites in the Human Genome.". PLoS ONE 11 (1): e0146174. doi:10.1371/journal.pone.0146174.
  7. ↑ Schaffitzel, C; Berger, I; Postberg, J; Hanes, J; Lipps, H. J.; Plückthun, A (2001). "In vitro generated antibodies specific for telomeric guanine-quadruplex DNA react with Stylonychia lemnae macronuclei". Proceedings of the National Academy of Sciences 98 (15): 8572–7. doi:10.1073/pnas.141229498. PMC 37477. PMID 11438689.
  8. ↑ Paeschke, K.; Simonsson, T.; Postberg, J.; Rhodes, D.; Lipps, H. J. (2005). "Telomere end-binding proteins control the formation of G-quadruplex DNA structures in vivo". Nature Structural & Molecular Biology 12 (10): 847–854. doi:10.1038/nsmb982. PMID 16142245.
  9. ↑ Simonsson, T.; Pecinka, P.; Kubista, M. (1998). "DNA tetraplex formation in the control region of c-myc". Nucleic Acids Research 26 (5): 1167–1172. doi:10.1093/nar/26.5.1167. PMC 147388. PMID 9469822.
  10. ↑ Siddiqui-Jain, A.; Grand, C. L.; Bearss, D. J.; Hurley, L. H. (2002). "Direct evidence for a G-quadruplex in a promoter region and its targeting with a small molecule to repress c-MYC transcription". Proceedings of the National Academy of Sciences 99 (18): 11593–11598. doi:10.1073/pnas.182256799. PMC 129314. PMID 12195017.
  11. ↑ Huppert, J. L.; Balasubramanian, S. (2005). "Prevalence of quadruplexes in the human genome". Nucleic Acids Research 33 (9): 2908–2916. doi:10.1093/nar/gki609. PMC 1140081. PMID 15914667.
  12. ↑ Rawal, P.; Kummarasetti, V. B.; Ravindran, J.; Kumar, N.; Halder, K.; Sharma, R.; Mukerji, M.; Das, S. K.; Chowdhury, S. (2006). "Genome-wide prediction of G4 DNA as regulatory motifs: Role in Escherichia coli global regulation". Genome Research 16 (5): 644–655. doi:10.1101/gr.4508806. PMC 1457047. PMID 16651665.
  13. ↑ Bugaut A, Balasubramanian S (2012). "5'-UTR RNA G-quadruplexes: translation regulation and targeting.". Nucleic Acids Res 40 (11): 4727–41. doi:10.1093/nar/gks068. PMC 3367173. PMID 22351747.
  14. ↑ Sen, D.; Gilbert, W. (1988). "Formation of parallel four-stranded complexes by guanine-rich motifs in DNA and its implications for meiosis". Nature 334 (6180): 364–366. doi:10.1038/334364a0. PMID 3393228.
  15. ↑ Kamath-Loeb, A.; Loeb, L. A.; Fry, M. (2012). Cotterill, Sue, ed. "The Werner Syndrome Protein is Distinguished from the Bloom Syndrome Protein by Its Capacity to Tightly Bind Diverse DNA Structures". PLoS ONE 7 (1): e30189. doi:10.1371/journal.pone.0030189. PMC 3260238. PMID 22272300.
  16. ↑ Maizels, N.; Gray, L. T. (2013). Rosenberg, Susan M, ed. "The G4 Genome". PLoS Genetics 9 (4): e1003468. doi:10.1371/journal.pgen.1003468. PMC 3630100. PMID 23637633.
  17. ↑ Biffi, G.; Tannahill, D.; McCafferty, J.; Balasubramanian, S. (2013). "Quantitative visualization of DNA G-quadruplex structures in human cells". Nature Chemistry 5 (3): 182–186. doi:10.1038/nchem.1548. PMC 3622242. PMID 23422559.
  18. ↑ Todd, A. K.; Johnston, M.; Neidle, S. (2005). "Highly prevalent putative quadruplex sequence motifs in human DNA". Nucleic Acids Research 33 (9): 2901–2907. doi:10.1093/nar/gki553. PMC 1140077. PMID 15914666.

References

External links

Quadruplex websites

Tools to predict G-quadruplex motifs

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