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Shattered DNA. Fluorescent blobs (green) show DNA double-strand breaks in irradiated cells (blue).

X-rays Meddle With DNA Repair

Ionizing radiation can bust both strands of the DNA double helix--a possible cause of cancer if not fixed properly. The number of these breaks increases with bigger doses of radiation, but new research finds that cells exposed to low doses may have a harder time repairing the damage than cells blasted by high doses. However, the researchers say there's no reason to cancel your next x-ray appointment.

Until recently, researchers had no way to study DNA damage and repair after exposure to low doses of radiation. Instead, the effects had to be extrapolated from examining workers exposed to high radiation in nuclear facilities and Japanese atomic bomb survivors. In the new study, biophysicists Markus Löbrich and Kai Rothkamm at Saarland University in Homburg, Germany, took advantage of a new method that uses fluorescent markers to highlight double-strand breaks.

The pair zapped human fibroblast cells with radiation ranging from 1.2 to 20 milligray (standard x-rays deliver between 1 and 10 milligray). Higher doses caused more damage than lower doses. Löbrich and Rothkamm could watch the breaks being repaired, indicated by markers winking out. To their surprise, the quality of damage control varied. No breaks were repaired for 4 days following exposure to 1.2 milligray. Cells exposed to higher doses steadily repaired breaks, but stopped once roughly one break per 10 cells remained--the same level as in the cells exposed to 1.2 milligray, the team reports online this week in the Proceedings of the National Academy of Sciences. The cells with damaged DNA died when the scientists stimulated them to grow and divide.

The finding suggests that the DNA repair system only works until the number of breaks approaches one per 10 cells. There is no ready explanation for this cutoff point, Löbrich says, but as long as the damage doesn't affect too many cells "it makes perfect sense for an organism not to fix all the double-strand breaks." Because the repairs are often imperfect, it may be safer to simply kill the cells, he says.

"It is a very provocative idea," says William Bonner, a molecular biologist with the National Cancer Institute in Bethesda, Maryland. Tracking double-strand breaks should allow researchers to "really start getting some hard data on effects of low-dose radiation." But the findings don't mean that low doses of radiation are especially unhealthy, says biophysicist Richard Setlow of the Brookhaven National Laboratory in Upton, New York. "It is very little damage and it is not going to have a big effect."

Related sites
Markus Löbrich's site
William Bonner's site
Richard Setlow's site