Delayed Chromosomal Instability Induced by DNA Damage

DNA damage induced by ionizing radiation can result in gene mutation, gene amplification, chromosome rearrangements, cellular transformation, and cell death. Although many of these changes may be induced directly by the radiation, there is accumulating evidence for delayed genomic instability following X-ray exposure. We have investigated this phenomenon by studying delayed chromosomal instability in a hamster-human hybrid cell line by means of fluorescence in situ hybridization. We examined populations of metaphase cells several generations after expanding single-cell colonies that had survived 5 or 10 Gy of X rays. Delayed chromosomal instability, manifested as multiple rearrangements of human chromosome 4 in a background of hamster chromosomes, was observed in 29% of colonies surviving 5 Gy and in 62% of colonies surviving 10 Gy. A correlation of delayed chromosomal instability with delayed reproductive cell death, manifested as reduced plating efficiency in surviving clones, suggests a role for chromosome rearrangements in cytotoxicity. There were small differences in chromosome destabilization and plating efficiencies between cells irradiated with 5 or 10 Gy of X rays after a previous exposure to 10 Gy and cells irradiated only once. Cell clones showing delayed chromosomal instability had normal frequencies of sister chromatid exchange formation, indicating that at this cytogenetic endpoint the chromosomal instability was not apparent. The types of chromosomal rearrangements observed suggest that chromosome fusion, followed by bridge breakage and refusion, contributes to the observed delayed chromosomal instability.