A new technique has the potential to lay bare the history of a cell鈥檚
exposure to cancer-causing chemicals, or carcinogens. Once such chemicals
are identified, it is possible to stop people coming into contact with them,
the first step in any programme to prevent a particular sort of cancer.
David Phillips and his colleagues at London鈥檚 Institute of Cancer Research
have been developing the technique, called 鈥榩ostlabelling鈥�. It uses radioactive
phosphorus to identify chemicals which attach themselves to a cell鈥檚 DNA,
causing cancer.
The DNA in each human cell is the long-chain molecule which contains
the body鈥檚 genes: the human genome. It consists of about 3 billion pairs
of chemical building blocks, or bases.
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Postlabelling exploits the fact that naturally occurring phosphorus
in the nucleotides, that make up DNA, can be replaced, or 鈥榩ostlabelled鈥�,
by radioactive phosphorus. Phillips and his colleagues break down the DNA
into its individual nucleotides, and then run them on an electrophoresis
gel.
Nucleotides that have carcinogens attached have physical properties
which are different from those of nucleotides by themselves, and which characteristic
of that carcinogen. They, therefore, appear as separate spots on the gel.
Each spot鈥檚 position indicates the nature of the carcinogen, and this is
revealed when the radioactive phosphorus acts on a photographic film.
Postlabelling is so sensitive that it can identify just one molecule
of a carcinogen in the genome. And it can achieve this feat with a microscopic
quantity of DNA, obtained from only a few cells.
Until now, it has not been possible to attack the question of environmental
causes for cancer. It has tended to be researched using animal experiments
or epidemiology 鈥� the science of looking for correlations between a population鈥檚
habits and lifestyles and the cancers its members suffer from. But researchers
realised as long as the 1960s that chemical carcinogens cause cancer by
sticking onto the cell鈥檚 DNA, largely as a result of work by Peter Brookes
and Phil Lawley at the Institute of Cancer Research. Now, using postlabelling,
it is possible to discover which chemicals have become attached to the DNA,
and so which ones could cause cancer.
