Four-Stranded DNA Complexes Observed In Human Cells

When describing the structure of DNA, a single shape is often used as its descriptor, the double helix. But DNA, as well as, RNA, tends not to be satisfied with this now iconic shape. Under certain conditions, a four stranded structure can arise.

This novel form, known as a quadruplex, has been synthesized for years and has been seen increasingly in the genetic material of many organisms. However, aside from assumptions based on observations in other species, there was no direct evidence that these quadruplexes could form naturally in humans. But a recent paper published in Nature Chemistry has shown not only this, but a possible link between these complex structures and certain forms of cancer.

The idea of four amino acids binding together instead of the usual two (and occasional three) dates back at least to the 1960’s with the publication of a paper by Gellert and colleges.  It was proposed that four Guanines could bond around a central cation through Hoogsteen hydrogen bonding to form a squareish structure called a G-tetrad (where G stands for Guanine).  Then, in 1988, Sen & Gillbert published a paper showing that these G-tetrads could stack forming a four stranded DNA named a G-quadruplex.

At first these structures were seen merely as synthetic curiosities.  It was hypothesized that they could be found in vivo (in living cells) but there was, at least at first, little evidence for this.  Eventually the first G-qudruplexes were observed within the telomeres of certain cells.  Telomeres act as a kind of end cap for chromosomes, preventing damage to the rest of the coiled DNA.  As interest in them grew and techniques improved, these quadruplexes began to be created in vitro (literally, in glass, meaning in a lab setting) from the telomeres from various sources.  From bacteria to vertebrates, these telomeric quadruplexes, as they are known could be produced in a myriad of species.

Fast forward to 2005 when the first G-quadruplexes were observed in vivo.  Now that it was known quadruplexes could form in living cells, the search intensified.  While their purpose was still hotly debated, quadruplexes could no longer be seen as just an artifice.

Knowing that all vertebrates share a common sequence in there telomeres (known as a telomeric repeat), it became an obvious question as to whether human cells could also form quadruplexes.  Using a variety of cancerous cells due to a strong correlation between telomeric damage and the formation of cancer, Giulia Biffi, David Tannahill, John McCafferty, & Shankar Balasubramanian attempted to isolate any potential quadruplexes.

Starting with an antibody known as BG4 which bonds to quadruplexes but not more familiar forms of DNA or RNA, Biffi, et al. then attached a secondary antibody to the BG4 and then a tertiary fluorescent antibody.  When observed, not only did the tell-tale glow the the fluorescent antibodies appear on the telomeres, but also deep within the chromosones and even at similar locations in sister chromatids suggesting a genetic commonality and not just a random dispersal.

As the body of observation grew, it became apparent that these quadruplexes were anything but a rare thing.  In 100 different cells stained with the BG4 complex, 58% showed at least one BG4 focus (point of bonding) with about a third showing multiple such foci.  Even more unexpected than the frequency of appearance of these foci was the observation that roughly only 25% were seen in the telomeres where they were expected to primarily reside.  These foci were observed primarily during the cells S-phase, the point in the cell cycle where the DNA is replicated in anticipation of cell division, with far fewer occuring during the G0/G1 phase (normal functioning state of a cell).  This suggests a link between DNA replication and their formation, which, in healthy cells at least, should be temporary.

But perhaps most interesting were the sites that many of these quadruplexes were observed.  The sites that such quadruplexes form, known as G-quadruplex motifs are also susceptible to the binding action of certain protiens associated with diseases such as Bloom Syndrome, Werner Syndrome, and Alpha-thalassemia.  It is possible that by binding to quadruplexes that form at these locations, the proteins associated with these and possibly other diseases could prolong the duration a quadruplex lasts which would inadvertently cause damage or even cell death.

But this potential cause for disease, especially certain cancers, could be their own undoing.  Biffi, et al. used a small-molecule ligand, a molecule that binds other molecules together, known as PDS.  Previously, it has been shown that PDS can bind to and stabilize quadruplexes as well as clear off proteins that have already bound themselves to it.  Once the PDS binds to the quadruplex, cell death often follows.  By using such a ligand that is designed to bind to quadruplexes at certain locations, it may be possible to destroy certain cancer cells.

While the connection between G-quadruplexes, cancer, and other genetic diseases is still far from concrete, we can at least say with confidence that, at least in certain instances, Human DNA is no stranger to this unusual genetic construct.

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Gellert M.; Lipsett M.N.; Davies D.R. (1962). Helix formation by guanylic acid. Proceedings of National Academy of Science of the United States of America, 48:2013–2018.

Sen, D. & Gilbert, W. (1988). Formation of parallel 4-stranded complexes by guanine-rich motifs in DNA and its implications for meiosis. Nature, 334(6180).

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.

Biffi, G.; Tannahill, D.; McCafferty, J.; & Balasubramanian S. (2013). Quantitative visualization of DNA G-quadruplex structures in human cells. Natural Chemistry, doi:10.1038/nchem.1548.

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