Described as an inbuilt program for making all the proteins vital for the body to survive and function, DNA or deoxyribonucleic acid is an elaborate molecule first discovered in 1869 containing our unique genetic code. Marred in mystery for nearly a century it wasn’t until 1953 when Watson and Crick determined the structure of DNA was a polymer in a double-helix formation.

This complex double helix structure can be described as a spiral consisting of two DNA strands made of alternating sugar and phosphate groups wound around each other. Within the confines of this spiralized backbone are ‘bases’ attached to each sugar molecule with the double helix held together by the bonds between these base pairs known as adenine, cytosine, guanine, and thymine. It was long thought these bases would only pair with the same set partner until recently the scientific community was surprised by the finding guanine can pair with itself, although the reason for this was not clear.

The discovery of quadruple helix

Sixty years after the discovery of the double-stranded helix a possible explanation was given for this guanine-derived anomaly with researchers proving the existence of the theoretical four-stranded ‘quadruple helix’ DNA structure, known as the G-quadruplex.

Also proposed as having a major role in cancer, these G-quadruplexes are known to appear in regions of DNA rich in guanine to form a four-stranded structure where four guanine bases create a square. There is still much conjecture as to their function, however, with the consensus they are most likely utilized or caused by cancer cell proliferation.

Now, a study from researchers led by Imperial College identifies the formation of quadruple helix DNA in healthy living human cells for the first time, tracking how it works, as well as its possible role in cancer. The team states they engineered a fluorescent marker possessing the ability to attach to quadruplex DNA in living human cells. This allows the team to define for the first time how the structure forms and what role it plays in cells. The study is published in the journal Nature Chemistry

Tracking G-quadruplexes in cells

Previous studies show G-quadruplexes have been detected in cells, however, the technique used to verify these findings either killed the cells or used high concentrations of chemicals to observe the formation of quadruple helix DNA which affected the cell’s stability. Taken together this means the actual presence of quadruplex DNA within normal living cells in vivo has not yet been tracked.

The pioneering study that first discovered the existence of G-quadruplexes posited a higher probability for their occurrence in nuclei of rapidly dividing cancer cells. The scientists based their premise on the fact they traced the quadruple helix DNA structures to telomeres, the protective caps of chromosomes associated with explosive growth, implicated in aging and longevity, and cancer-causing genes. The current study identifies and observes quadruple DNA complexes within anatomically-correct human tissue by attaching a novel fluorescent marker to DNA.

Quadruple helix functionality

The current study develops a single-molecule fluorescent imaging system to track unfolded quadruple helix DNA structures in robust living cells which reveals G-quadruplexes fluctuate between folded and unfolded states. This finding suggests the four-stranded quadruplexes are ‘opening and closing’ their structures to aid in the process of DNA replication, pivotal to cell division and production.

The researchers speculate the G-quadruplexes form to hold the DNA molecule open and allow the transcription of the genetic code to produce proteins, possibly influencing the yield of each amino acid-based molecule.

Usually, this function is performed by chemical tags called epigenetic markers that alter how genes are expressed without changing the underlying DNA sequence. Here results suggest quadruple helix DNA performs a similar role. Data findings indicate there may be communication between the formation of the quadruplex DNA and epigenetic markers, with the G-quadruplexes presenting as an epigenetic mark.

The lab states by using single-molecule microscopy they were able to observe probes at extremely lower concentrations than previously used. This means, unlike previous research, their biosensor only binds to the quadruple helix DNA for milliseconds allowing them to study G-quadruplexes behavior in their natural environment without destabilizing the living cells.

They go on to add their new imaging technique enabled them to prove G-quadruplexes exist in cells as stable structures actualized via normal cellular processes. They conclude their work adds to the growing body of work propounding the quadruple helix DNA is a normal part of DNA regulation and may not have any fixed structure or shape.

G-quadruplexes and cancer

The team surmises they have confirmed the existence of G-quadruplexes in healthy cells and were able to track its conformation and activity. For the future, the researchers state as they can now observe the quadruple helix DNA in real-time they should be able to confirm its exact role in cancer pathology and ultimately how to block its hypothesized carcinogenic functionality.

Source: Imperial College London

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