Capping of radiation-induced DNA breaks in mouse early embryos

The aim of the study was to investigate the spectrum and frequencies of chromosome aberrations induced by the exposure of different mouse spermatogenic germ cell stages to ionizing radiation. Male mice were exposed in vivo to X-rays. Chromosome aberrations were analyzed in first- and second-embryonic cleavages obtained from mating irradiated males with nonirradiated females at different periods after radiation exposure. A combination of telomeric and centromeric labeling as well as whole Y chromosome painting was used to characterize the rejoining pattern and the telomere status of the radiation-induced DNA breaks. The frequency of chromosome aberrations observed in eggs fertilized with sperm irradiated at the early spermatid stage was markedly higher than the frequency in eggs fertilized with sperm irradiated at the other spermatogenic stages when reference was made to the chromosome aberrations recovered in early embryos. At the first division postirradiation, distal rejoining of broken chromosome ends (in regard to the position of the centromere) was more frequent than proximal rejoining; thus compound acentric fragments were more frequently observed than dicentric chromosomes. The presence of additional telomere signals at the broken chromosome ends in mouse germ cells and early embryos, compatible with de novo formation of telomeres, was not frequent.