May 15, 1998 CHAPEL HILL - Canadian scientists collaborating with a University of North Carolina at Chapel Hill researcher have developed the most precise test yet for genetic damage caused by ionizing radiation and cancer-causing chemicals.
The new test promises to be extremely useful because it is some 10,000 to 100,000 times more sensitive than other assays, the scientists say. Experiments with the technique also suggest doctors soon may be able to boost the body's ability to repair genetic injuries.
"Potentially, this assay could measure clinically relevant damage from ionizing radiation even in a clinical situation," said Dr. Steven A. Leadon, professor of radiation oncology at the UNC-CH School of Medicine. "Before long, we also may be able to monitor the effects of irradiating tumors much better than we can now."
A report on the research appears in the May 15 issue of the journal Science. Besides Leadon, a member of the UNC Lineberger Comprehensive Cancer Center, authors are Drs. X. Chris Le, James Z. Xing, Jane Lee and Michael Weinfeld of the University of Alberta.
Ionizing radiation kills cells by breaking or otherwise disrupting segments of genetic material known as DNA, Leadon said. But currently used methods of measuring such damage are not sensitive enough to assess environmental radiation effects. Sometimes testing techniques themselves harm DNA.
"Our new technique employs antibodies that recognize specific forms of DNA damage," Le said. "Those antibodies are then linked to other antibodies that give off fluorescent light and also attach to the damaged DNA."
Scientists feed that mixture of DNA and antibodies through a tiny glass tube to undergo a process called capillary electrophoresis, which Dr. James Jorgenson, professor of chemistry at UNC-CH, pioneered more than a decade ago. A laser beam passes across the tube illuminating the sample, and electronic equipment then monitors the resulting fluorescence. Higher light intensity corresponds to more damage.
In the experiments reported in Science, the researchers demonstrated both the sensitivity of their new method and that DNA repair can be accelerated. They subjected cultured cells to 0.25 Gray - a unit of radiation exposure - four hours before exposing the same cells to a clinical dose of two Gray, eight times as much. Their assay showed pre-treated cells removed genetic damage caused by the larger dose significantly faster than cells not pre-treated.
"We could use this ultra-sensitive assay for other sources of DNA damage, such as those caused by tobacco smoke," Le said. "In this work, we looked at thymine glycol, a kind of damage often caused by radiation."
Besides being less sensitive, existing tests are labor-intensive or require large amounts of sample for analysis, said Leadon, who developed the antibodies the Canadian scientists used. Because those antibodies are so specific, the new assay can reveal a single defect in DNA 300 million base pairs in size. A base pair is a "rung" on a DNA molecule, which is sometimes likened to a long and twisted ladder.
"There is little doubt that, over the foreseeable future, we will see an explosion in the number of reports making use of this seminal technique in the various fields of life sciences, from toxicology to molecular biology," according to an accompanying editorial in Science.
"Armed with such a specific and sensitive assay, one can only begin to imagine some of the possibilities," the editorial writer said. "For instance, scientists should more easily detect the type and frequency of DNA lesions in living tissues after exposure to environmental radiation or chemical carcinogens."
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