Insight

Fall 2008

Understanding radiation damage and repair

The damaging effects of radiation on the DNA of cells are well documented. What is less understood is why the cells of some individuals are more sensitive to radiation exposure while cells of others are more resistant to those effects, or how low doses of radiation over longer periods of time affect DNA damage and repair. A very small Japanese fish is helping researchers at Colorado State University to answer these very big questions, along with other mysteries of radiation exposure and DNA damage and repair.

The medaka is a small ricefish found in the rice paddies of Southeast Asia and is popular in aquariums (a school of medaka also traveled into space in 1994 aboard the space shuttle Columbia). The medaka, Oryzias latipes, is a common model organism used in biological research because it is simple, tolerant of temperature changes, short-lived, reproductively prolific, hardy, and easy to rear in the laboratory. It can withstand cold and can be shipped easily, important to the research work in Dr. John Zimbrick’s laboratory because his fish routinely have to make the trek from Georgia to Colorado. Working with the medaka fish has the added benefit of using a low-cost model system to conduct experiments that would be cost-prohibitive with a mammalian model.

“In our laboratory, we are using our fish to study the genetic effects of low doses of radiation, doses that would be similar to the doses radiation workers could be allowed to get over several years,” said Dr. Zimbrick, a Professor in the Department of Environmental and Radiological Health Sciences. “In addition, we are searching for so-called transgenerational effects in the form of mutations occurring in the offspring as a result of the exposures to the parents. For many years, the Department of Energy funded research into high-dose effects of radiation, but not too much attention was paid to low-dose effects. Legal limits for the doses workers could be exposed to were established at levels that will not produce detectable harmful effects, but few the studies have been done that show what the genetic effects of those doses might be over longer periods of time covering one or more generations of offspring.”

Dr. Zimbrick is working in collaboration with the University of Georgia’s Savannah River Ecology Laboratory (SREL) where the medaka are exposed to set levels of radiation. The collaboration with SREL gives researchers access to radiation facilities that they otherwise would not have, helping to expand and enhance their research program at a relatively low cost to the laboratory. SREL has a specially designed outdoor facility that consists of a series of large, circular fish tanks each with a radiation source at the end of a rod mounted above the center of the tank. Groups of fish are contained in screen buckets in the tanks and their location in the tanks determines the exposure for each group. Radiation exposure is carefully measured to determine accurate doses for each group.

Following exposure, the fish are shipped to CSU where researchers are conducting transgenerational studies, looking for DNA damage, changes in gene activity and mutations in first through sixth generations of the medaka. To date, they have data for the first three generations, and samples for the fourth and fifth.

“A major early finding is that if we irradiate parents and then examine the first, second and third generations, we see an increase in special mutations called micro-satellite DNA mutations in the offspring, which we can detect if there is a change in even one of the DNA bases,” said Dr. Zimbrick.

Dr. Zimbrick said it is possible that chronic radiation exposure of parents could result in still higher mutation rates in even sixth and seventh generations, but the biological effects from these would have to be of such nature that the future generations survive in spite of the mutations. Over time these offspring may adapt to the radiation-induced mutations, becoming more genetically diverse and more resistant to radiation effects.

“The medaka fish is an excellent model for this type of work in that it is outbred, meaning that its genetic make-up is highly varied and diverse because it has intermingled and bred over many generations, similar to our global human population,” said Dr. Zimbrick.

A new facet of the study is to examine the inter-individual distribution of radiation-induced mutation frequencies which varies based on individual sensitivity to radiation. A study looking at 50 medaka families will measure this distribution of mutation frequencies among families and look at potential genetic markers that could indicate the relative degree of radiosensitivity. The researchers also will select specimens from the lower and upper ends of the sensitivity scale, breed them, profile the genes and look for patterns of gene activities associated with the genes involved in DNA repair. If they are successful and able to find a set of repair genes whose activities relate to individual radiosensitivity, those candidate genes may be elevated for additional study in a more complex model.