Blood Test For Cancer & How DNA Is Repaired

19 October 2003


Scientists in Israel recently announced that they had come up with a blood test that could predict a person's risk of developing lung cancer, particularly if that person is a smoker. Cancers develop when our DNA is mutated, or damaged, by harmful substances in the environment like smoke or ultraviolet rays from the sun. Because there are so many things that can damage DNA during our lives, our cells have evolved ways to sniff out areas of damaged DNA and repair them before they can cause problems. The Israeli team have found that in people who develop lung cancer, the levels of one key DNA repair-enzyme, called OGG1, are much lower than in people who remain cancer free. So a person who smokes and has low levels of the repair enzyme stands a much higher chance of developing lung cancer than a smoker with normal levels (5-10x the chance), or a non smoker. But how do your cells know what to repair, and spot the damaged DNA so quickly ? Indeed there are 3 billion DNA letters in the human genetic blueprint and it's taken us over 10 years to work them all out, yet our cells can do this almost faultlessly day after day. Well, scientist Jacqueline Barton and her team from the California Institute of technology in the US think that our DNA repair systems use electricity to test long sections of our DNA so that they don't have to laboriously check all of it. The researchers have found that normal DNA conducts electricity, but damaged DNA doesn't. They suggest that working together in pairs at each end of a section of DNA, the repair enzymes send an electrical current along the DNA. If the current arrives normally that piece of DNA must be normal and the repair systems look elsewhere. But if the electrical signal doesn't arrive correctly there must be a region of damage somewhere within the section of DNA being tested and the repair enzymes then work their way towards each other checking each part of the DNA sequence as they go until they discover the damaged area, which is snipped out and replaced. The importance of this work is that the more we understand about how DNA repair systems work, the better equipped we are to help people with cancers, and inherited conditions that pre-dispose them to cancers.


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