Since its discovery in the mid-twentieth century, the DNA double-helix has become an iconic motif, encapsulating not only the genius of modern science but also the very essence of what makes us human. However, recent studies have indicated that DNA sometimes arranges itself into a four-stranded structure called a G-quadruplex (G4), and scientists have now observed these strange genetic anomalies interacting with other compounds in human cells.
Though little is known about G4, it seems that they form around sections of DNA that contain high concentrations of the nucleotide base guanine. Of the four types of nucleotide base present in genetic material, guanine is the only one that can bind with itself, and can therefore facilitate the addition of one double helix to another, resulting in a four-stranded molecule.
Exactly what function these structures fulfil in living cells is uncertain, although scientists think they may arise in order to temporarily hold DNA strands apart while they are being read. What we do know, though, is that G4 is more common in cancer cells and has been associated with cancer-related genes, raising suspicions that it may play a role in the formation of tumors.
For this reason, researchers are keen to develop a method to interact with G4 and prevent it from carrying out some of its functions. First, however, it is necessary to find a way to observe the action of G4 within living cells.
Writing in the journal Nature Communications, a team of scientists from Imperial College London describe how they used a molecular probe in order to discern some of the ways in which G4 interacts with other molecules in both human and mouse cells. Known as DAOTA-M2, the chemical probe is able to bind to G4, emitting a fluorescent glow when it does so.
Fluorescence lifetime imaging microscopy map of nuclear DNA in live cells stained with the new probe. Colours represent fluorescence lifetimes between 9 (red) and 13 (blue) nanoseconds. Credit: Imperial College London
In particular, they identified two enzymes – called FancJ and RTEL1 – that significantly affected the amount of time that DAOTA-M2 remained lit up for. Both of these are helicases, meaning they break down DNA helices, and therefore appear to play a role in dismantling G4.
When these two helicases were removed from cells, DAOTA-M2 was able to fluoresce for longer, indicating that the G4 was not being destroyed as quickly. Such a finding would seem to confirm the role of FancJ and RTEL1 in breaking down these four-stranded DNA bundles.
In a statement, study author Ben Lewis explained that "evidence has been mounting that G-quadruplexes play an important role in a wide variety of processes vital for life, and in a range of diseases, but the missing link has been imaging this structure directly in living cells.”
Fortunately, DAOTA-M2 appears capable of highlighting the activity of G4 in cells, and has already revealed two enzymes that can unwind its four-stranded structure. While it’s far too early to say what relevance this will have for the treatment of cancer and other illnesses, it does at least open the door to a whole new approach to tackling certain conditions.
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