Effect of Cranial Irradiation of the Brain on Molecular Genetic Indices of Mitochondrial DNA in Various Tissues In Vivo

This study investigates mitochondrial DNA (mtDNA) as a potential agent causing the “bystander effect” in vivo after radiation exposure. We assessed mtDNA gene expression, total mtDNA copy number, and changes in levels of micronuclei in cells of various tissues in mice exposed to local brain irradiation at doses of 1 and 5 Gy. Two-month-old male Balb/c mice were used in the study. Only the head of the mouse was exposed to X-ray irradiation at doses of 1 and 5 Gy. The following tissues were used for parameter analysis: blood cells, heart, brain, liver, spleen, and bone marrow. Gene expression and total mtDNA copy number were assessed using real-time PCR, and analysis of micronuclei was performed using the micronucleus assay. It was shown that the relative amount of mitochondrial ATP6 gene transcripts increases in nucleated blood cells after irradiation at doses of 1 and 5 Gy. This effect is much more pronounced in granulocytes than in monocytes. At the same time, the amount of mtDNA in nucleated blood cells decreases by 2–3 times relative to the control level. In the brain irradiated at doses of 1 and 5 Gy, an increase in the relative amount of mtDNA transcripts by approximately 2–3 times compared to the control is also observed. 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 24 h after irradiation, sharply decreases and remains at this level 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, peaking at 14–21 days after irradiation. In bone marrow cells of mice after local irradiation of the head with a dose of 1 Gy, increased levels of formation of micronuclei in polychromatophilic erythrocytes (PCE) were recorded. Thus, the results suggest that the observed changes are most likely due to the bystander effect that occurs after local irradiation of the brain with X-rays at doses of 1 and 5 Gy.