[Changes of mitochondrial DNA/nuclear DNA ratio in the blood serum following X-ray irradiation of mice at various doses].

Using quantitative real-time PCR, the copy number of mitochondrial and nuclear DNA fragments in mouse blood serum was estimated at different time points following X-ray irradiation at various doses (from 0.5 to 10 Gy). The changes in the correlation between mtDNA and nuclear DNA (mtDNA/nucDNA) in blood serum reflect the degree of radiation injury depending on the dose of irradiation. Exposure to radiation at 10 Gy and massive cell death caused by this lethal dose result in a sharp decrease by an order of magnitude of the mtDNA/nucDNA ratio in the mouse serum; the value of this parameter did not recover within the next 3 days. The opposite effect was revealed when mice were exposed to irradiation at the dose of I Gy, which is not followed by massive cell death, but leads to a higher level of the mtDNA damage as compared with the nuclear DNA protected by histones. Defective mtDNA molecules enter the bloodstream, which results in an increase of the mtDNA/nucDNA ratio in serum. Under irradiation of mice at the intermediate dose of 3 Gy the two processes described above are exhibited at once. During the first hours after irradiation an apoptotic death of radiosensitive cells and release of a large number of nuclear DNA fragments in the serum are initiated, which reduces the mtDNA/nucDNA ratio. However, at later times after irradiation, starting from 5 days, an increase of the mtDNA/nucDNA ratio is observed in the serum, presumably as a result of reparation and elimination of defective mtDNA. Thus, the mtDNA/nucDNA ratio in the serum of irradiated mice reflects the degree of the radiation damage to cells and may be considered as a biological marker of radiation injury in the future.