ON April 26, 1986, an accident at the Chernobyl nuclear reactor in the former Soviet Union released an enormous number of fission products into the atmosphere and over a large portion of the planet. With about 100 million curies released in the 10 days following the initial explosion, the accident was the largest single nonmilitary release of radioactivity in history-and one of the largest environmental disasters ever.|
During the first year after the accident, about 25,000 people, mainly Soviet Army troops, were dispatched to the site to clean up the accident. These so-called liquidators were estimated to have received doses of up to 70 centigrays (a gray is the international unit for measuring absorbed ionizing radiation and is equivalent to 100 rads, or 1 joule per kilogram). In the following three years, another half-million people assisted the effort and are estimated to have received lower doses (about 10 to 25 centigrays).
The tasks performed by liquidators included shoveling core material off the roof of the undamaged part of the building, operating heavy equipment to contain contaminated soil, and building a concrete sarcophagus around the destroyed reactor. Depending upon the intensity of radiation exposure associated with their assigned task, most liquidators received radiation exposures over a period of at least several days, and in some cases over many weeks.
Lawrence Livermore biomedical scientists began studying the Chernobyl accident almost as soon as it occurred as part of a Department of Energy effort to help assess the accident's biological effects. The Livermore assistance, which continues today, takes advantage of the Laboratory's longstanding expertise in evaluating human exposures to ionizing radiation and determining their health risks. Livermore scientists have forged numerous and often close scientific relationships with their Russian and Ukrainian counterparts that endure today in collaborations, mutual assistance, informal communications, and visits.
Techniques to Monitor Damage
Applying Biomarkers to Russian Liquidators
Recognizing Statistical Power
The results so far show FISH to be sensitive to the exposures of Chernobyl liquidators, with the HPRT assay being less sensitive and the GPA assay, which proved highly valuable for studies of A-bomb survivors and more highly exposed Chernobyl liquidators, showing no difference between the exposed and control populations (Figure 3). The Livermore team says its population of liquidators received on average a dose of about 15 centigrays, as determined by FISH. Such a radiation dose is roughly equivalent to aging about 10 years or to smoking cigarettes regularly. The expected health consequences to the population under study from such an exposure are small.|
Livermore researcher Jones notes that the sensitivity to detect the effect of radiation exposure is increased by knowing the age and the smoking habits of the individual, because both characteristics contribute to the damage in their cells. However, she says it is impossible to determine the health risk of any one individual who received a specific amount of ionizing radiation, especially at the lower doses that do not cause acute health effects. Each individual has a different complement of genes that determine how well they can repair damage from ionizing radiation and other sources. Personal habits such as smoking, drinking, and diet add to the genetic damage that accumulates in cells. "It is the sum of all damage and the body's response to that damage that determines the risk of cancer and other health effects," she says.
In a separate study led by biomedical scientist Joe Lucas, Livermore researchers applied FISH to a subset of Chernobyl liquidators suspected of receiving a large dose of ionizing radiation. They reconstructed doses for 27 Chernobyl liquidators from the frequency of translocations measured in their lymphocytes. Of the 27 individuals, 15 are being treated for radiation sickness. The remaining 12 show no medical symptoms.
"FISH has worked extremely well on Chernobyl victims," says Lucas, one of the original developers of FISH. He notes that the technique is useful because not every liquidator had a dosimeter, and memories of the nature and duration of work assignments for most workers are not reliable.
Questions Still Unanswered|
Current studies at Livermore and at other research centers are addressing some of the unanswered questions about the assays, such as their sensitivity to low doses, how intensity of radiation exposure affects the response, the persistence of chromosome translocations, and the degree to which factors other than radiation affect them. Jones and her colleagues, for example, are studying the extent to which the type of chromosome aberration affects its persistence in human blood cells, which could change the relationship between translocation frequency determined by FISH and radiation dose as time passes after exposure.
Another major goal of the research will be to understand why people differ in the effect that the same dose of radiation has on their cells. One part of this effort has been started-identifying the differences in the DNA repair gene sequence in people. The next big challenge will be to determine how these differences affect the capacity to repair damaged DNA and if these differences are related to long-term health.
The Lucas group is collaborating with colleagues at Columbia University on a promising method to detect cellular damage among the liquidators. The method is based on measuring the formation of micronuclei, which are secondary and much smaller cell nuclei that form in eye cataract tissue as a result of radiation. The group is also working on an enhancement to FISH that is faster, more accurate, and more sensitive by counting individual chromosomes in liquid suspension instead of on a microscope slide.
In the meantime, Livermore radiation-effects researchers are working with collaborators in Ukraine, Russia, Estonia, and Israel (where some liquidators have immigrated) to apply biodosimeters such as FISH and GPA in their own laboratories.
It seems clear that despite its disastrous environmental consequences, the Chernobyl accident has spawned deeper understanding about the health effects of ionizing radiation and, in the process, spurred stronger international cooperation.
Key Words: biodosimeter, Chernobyl, FISH (fluorescence in situ hybridization), glycophorin A (GPA), hypoxanthine phosphoribosyltransferase (HPRT).
For further information contact irene Jones (925) 423-3626 (firstname.lastname@example.org) or Joe Lucas (925) 422-6283 (email@example.com">).