Cambridge University scientists have proved the existence of four-stranded ‘quadruple helix’ DNA structures – known as G-quadruplexes – within the human genome.
Visualization of DNA G-quadruplex structure (Jean-Paul Rodriguez / Giulia Biffi)
The discovery marks the culmination of over 10 years investigation by scientists to show these complex structures in vivo – in living human cells – working from the hypothetical, through computational modelling to synthetic lab experiments and finally the identification in human cancer cells using fluorescent biomarkers.
The study, published in the journal Nature Chemistry, shows clear links between concentrations of four-stranded quadruplexes and the process of DNA replication, which is pivotal to cell division and production.
By targeting quadruplexes with synthetic molecules that trap and contain these DNA structures – preventing cells from replicating their DNA and consequently blocking cell division – scientists believe it may be possible to halt the runaway cell proliferation at the root of cancer.
“We are seeing links between trapping the quadruplexes with molecules and the ability to stop cells dividing, which is hugely exciting,” said study senior author Profr Shankar Balasubramanian from the University of Cambridge’s Department of Chemistry and Cambridge Research Institute.
“The research indicates that quadruplexes are more likely to occur in genes of cells that are rapidly dividing, such as cancer cells. For us, it strongly supports a new paradigm to be investigated – using these four-stranded structures as targets for personalised treatments in the future.”
Physical studies over the last couple of decades had shown that quadruplex DNA can form in vitro – in the ‘test tube’, but the structure was considered to be a curiosity rather than a feature found in nature. The researchers now know for the first time that they actually form in the DNA of human cells.
They were able to generate antibody proteins that detect and bind to areas in a human genome rich in quadruplex-structured DNA, proving their existence in living human cells. Using fluorescence to mark the antibodies, they could then identify ‘hot spots’ for the occurrence of four-stranded DNA – both where in the genome and, critically, at what stage of cell division.
Localization of G-quadruplex structures in chromosomes. Scale bars 2.5 µm (Giulia Biffi et al)
While quadruplex DNA is found fairly consistently throughout the genome of human cells and their division cycles, a marked increase was shown when the fluorescent staining grew more intense during the ‘s-phase’ – the point in a cell cycle where DNA replicates before the cell divides.
Cancers are usually driven by genes called oncogenes that have mutated to increase DNA replication – causing cell proliferation to spiral out of control, and leading to tumour growth.
The increased DNA replication rate in oncogenes leads to an intensity in the quadruplex structures. This means that potentially damaging cellular activity can be targeted with synthetic molecules or other forms of treatments.
_______
Bibliographic information: Giulia Biffi et al. Quantitative visualization of DNA G-quadruplex structures in human cells. Nature Chemistry, published online 20 January 2013; doi: 10.1038/nchem.1548
