Changes in the Copy Number and Gene Expression of mtDNA in Various Tissues of Mice Exposed to Local Brain Irradiation

Purpose: To determine changes in the copy number and gene expression of mtDNA in various tissues of mice subjected to local irradiation of the brain.

Material and methods: Male Balb/c mice aged 2 months were used. Only the head of the mouse was exposed to X-ray irradiation at a dose of 5 Gy (power 2.5 Gy/min). After removal from the animals, the tissues were homogenized on ice, after which the homogenate mass was divided into two parts to isolate nucleic acids. Liquid blood was collected separately, after which nucleated blood cells were separated into granulocyte and monocyte fractions by differential centrifugation by Diacoll density gradient. The total number of mtDNA copies and gene expression were assessed using real-time PCR.

Results: It was shown that in nucleated blood cells, after irradiation, the relative number of transcripts of the mitochondrial gene ATP6 increases. In granulocytes, this effect is much more pronounced than in monocytes. At the same time, the amount of mitochondrial DNA in nucleated blood cells decreases relative to the control level by 2–3 times. In the brain exposed to irradiation, an increase in the relative amount of mtDNA transcripts by about 3 times is also observed, compared to the control. In organs not exposed to irradiation (heart, liver, spleen), the same effect is observed as in the brain, namely, an increase in the relative amount of mtDNA transcripts. The number of copies of mtDNA itself in brain cells, after a sharp increase a day after irradiation, sharply decreases and remains so until the end of the experiment, 30 days later. In liver and heart cells, the opposite process occurs, namely, a significant increase in the number of mtDNA copies, with a maximum at 14–21 days from the moment of irradiation.

Conclusion: Thus, the obtained results allow us to say that the observed changes are due to the occurrence of the “bystander effect” that arose after local irradiation of the brain with X-ray radiation at a dose of 5 Gy.