Adam Dicker, M.D., Ph.D., associate professor of radiation oncology at Jefferson Medical College of Thomas Jefferson University and Jefferson's Kimmel Cancer Center in Philadelphia, Mary Frances McAleer, M.D., Ph.D., a resident in the Department of Radiation Oncology at Jefferson Medical College and their co-workers compared the effects on zebrafish embryos of two types of radiation - ionizing radiation, which is the kind given to patients for cancer treatment, and ultraviolet (UV) radiation, which comes naturally from the sun.
The researchers exposed the embryos at different time points in development to different doses of ionizing and UV radiation, comparing the sensitivity of the embryos.
"We found that the zebrafish were very sensitive to the mid-blastula transition, the point in development in which the embryo goes from relying on the maternal mRNA in the yolk sac to the embryo itself controlling development," Dr. McAleer says.
Prior to this transition, the fish are extremely sensitive to ionizing radiation, she says. But when exposed to UV light, the younger embryos were unaffected. But later, after the transition period, the embryos show morphologic damage in their development when exposed to UV radiation.
Dr. McAleer presents the team's findings March 30 at the annual meeting of the American Association for Cancer Research in Orlando.
"We saw something unique," she says. "We hypothesize that this may be due to the gene expression of the embryos." The researchers say that prior to the mid-blastula transition, cells are going through the cell growth cycle without regulation, rapidly dividing. At that point, the cell cycle becomes asynchronous, with certain cells dividing at the same time - which is when differentiation and "the crux of development" occurs.
They found that much of the damage from ionizing radiation is due to breaks in both strands of the cell's double-stranded DNA. When the fish were exposed to UV light, the DNA formed "crosslinks" in which two thymine bases form on the same strand next to each other. The cell uses entirely different repair mechanisms to fix both types of damage.
The Jefferson team performed a microarray analysis to confirm their findings. They looked at normal embryos unexposed to radiation at different time points in their development, examining different groups of genes in normal embryos involved in various types of DNA repair, including base-excision repair, mismatch repair and double-strand break repair.
They found that prior to the mid-blastula transition, the enzymes required for mismatch and base repair are elevated. "Conversely, the double-strand break repair genes aren't expressed until following that time point," Dr. McAleer says. "This supported our observation that this is gene expression-based. The damage we saw early in the fish exposed to ionizing radiation is related to the absence of the double-strand break repair enzymes. There is a low level of repair genes in the later fish, which is when we see UV exposure sensitivity." In earlier work, Dr. Dicker used zebrafish to show that while radiation and some chemotherapeutic agents damage DNA, there were different time periods in development in which the zebrafish were sensitive to either radiation or the drugs.
"In general, drugs targeted for specific enzymes are used in combination with chemotherapy agents," he says. "We can use the zebrafish system to help us understand the mechanisms of how chemotherapy drugs work before we start adding them on."
According to Dr. McAleer, the zebrafish as a vertebrate model with which to study cancer has several advantages. The embryos are optically transparent, meaning researchers can watch organs develop. The fish are easy to manipulate and manage, and develop into adults in a short time. Most importantly, their DNA or genome is very similar to humans.
Next, the researchers plan to use zebrafish to help them test the effectiveness of various drugs in blunting the effects of radiation.
Editors: This release is embargoed for March 30, 2004 at 1 p.m. EST at a Late-Breaking Abstracts session at the annual meeting of the American Association for Cancer Research in Orlando.