Reduction of the number of mutant copies of mitochondrial DNA in tissues of irradiated mice in the postradiation period

Changes in the number of mutant copies of mitochondrial DNA (mtDNA) were studied in the brain and spleen tissues of mice after their X-irradiation at a dose of 5 Gy. For this purpose, heteroduplexes obtained via hybridization of the products of PCR amplification of mtDNA (ND3 gene and two D-loop regions) from irradiated and control mice were digested with the CelI nuclease capable of specific mismatch cleavage. Heteroduplexes obtained via hybridization of the products of PCR amplification of mtDNA from irrradiated and control mice were digested by the CelI nuclease to a greater degree than heteroduplexes of the PCR products of mtDNA of mice from the control group. This suggests the presence of mutations in mtDNA regions in irradiated mice. Digestion by the CelI nuclease of heteroduplexes obtained via hybridization of the PCR products of mtDNA (ND3 gene and D-loop regions) on day 8 after irradiation is essentially more efficient than digestion of heteroduplexes obtained via hybridization of the PCR products of mtDNA isolated from mouse tissues on days 14 and 28 of the postradiation period. These results indicate a reduction in the number of mtDNA copies with mutations in tissues of irradiated mice by day 28 of the postradiation period. The reduction in the level of mutant mtDNA copies by this term is especially significant in the spleen. The total number of mtDNA copies in the mouse brain and spleen tissues estimated by real-time PCR, relative to the nuclear β-actin gene, is also decreased by 30-50% as compared to the control on days 8 to 28 after irradiation. The results of the study suggest that mutant mtDNA copies are eliminated from tissues of irradiated animals in the postradiation period. This elimination can be regarded either as a result of selective degradation of mitochondria carrying mutant DNA copies or as a result of cell death being continued in tissues of irradiated animals.