UPTON, NY -- In research that could help assess the radiation risks faced by astronauts, improve the cancer-killing potential of radiation therapy, and distinguish between DNA damage caused by normal living and that caused by low-level radiation, scientists at the U.S. Department of Energy's Brookhaven National Laboratory have developed a new way to detect and quantify varieties of radiation damage that previously could not be measured.
Scientists have long known that ionizing radiation, such as gamma rays and x-rays, can damage deoxyribonucleic acid (DNA), the genetic-code-carrying molecule that tells cells which proteins to make. The oxygen we breathe can cause damage, too. Most of the time, our bodies repair the damage we receive from everyday radiation sources such as sunlight and from oxygen. But unrepaired or incorrectly repaired damage can be lethal to cells or cause cancer.
"For years, people have been worried about the consequences of double strand breaks," says Betsy Sutherland, who leads the Brookhaven research team. These closely spaced breaks through both strands of the DNA double helix are known to be difficult for cells to repair. Scientists have also hypothesized that radiation might produce other forms of clustered damage on both DNA strands, like oxidation of the bases A, G, C, and T. Could these clustered damage sites be equally, or more, harmful? The problem with finding out, Sutherland says, is that no one has had a way to determine if radiation actually induces these kinds of damage, or to measure their frequencies and assess their repairability -- until now.
"We figured out how to do it," Sutherland says. The idea is fairly simple: Sutherland and her team use special enzymes supplied by collaborators in France that are designed to cut DNA strands at sites of specific kinds of damage. The team first irradiates the DNA (or cells in culture), then chops the DNA with the enzymes, and finally separates and counts the fragments on electrophoretic gels to measure the levels of clusters containing each kind of damage.
The data they've collected so far, both from DNA in solution and from human cell cultures, indicate that radiation does induce clusters of damage such as oxidized DNA bases and so-called abasic sites, where the base is simply knocked off the DNA sugar backbone. Surprisingly, says Sutherland, these other forms of clusters comprise some 80 percent of the complex damage sites observed , with double strand breaks comprising only 20 percent.
"This means that the effects of most types of complex DNA damage in the cells and how they are repaired are completely unknown. The only way to find out about them is to make careful measurements -- and we have a way to do that now," Sutherland says.
The team is currently irradiating cells, measuring various kinds of clustered damage, and assessing how rapidly the cells remove and repair the different forms of damage. Scientists could also use the technique to see if all cells in the body respond the same way, and if there are differences in susceptibility to certain kinds of damage among species, or among different people.
The findings could indicate whether certain people might run a greater risk of radiation damage during long-term missions to Mars or on the Space Station. The National Aeronautics and Space Administration (NASA) estimates that DNA in one-third of astronauts' cells will be hit directly by a heavy charged particle during each year in space. The studies could also help assess the potential of antioxidants to counteract DNA damage, or reveal ways to use radiation more effectively in fighting cancer. "We hope that if we learn how to alter damage to tumor cells, we can improve therapeutic regimens," Sutherland says.
The current research is published in the July 11, 2000 issue of the journal Biochemistry. Related work appeared in the January 4, 2000 issue of the Proceedings of the National Academy of Sciences. The work is funded by the U.S. Department of Energy, NASA, and the Centre National de la Recherche Scientifique and Electricité de France.
The U.S. Department of Energy's Brookhaven National Laboratory creates and operates major facilities available to university, industrial and government personnel for basic and applied research in the physical, biomedical and environmental sciences and in selected energy technologies. The Laboratory is operated by Brookhaven Science Associates, a not-for-profit research management company, under contract with the U.S. Department of Energy.
Note to local editors: Betsy Sutherland lives in Wading River, New York.
Journal
Biochemistry