Adaptation of human fibroblasts to radiation alters biases in DNA repair at the chromosomal level

Purpose: To determine whether adaptation to ionizing radiation biases repair of radiation-induced chromosomal breaks. Materials and methods: Normal human fibroblasts were radiationadapted by exposure to 10cGy of gamma-radiation. FISH probes for chromosomes 2, 4, 7, 18 and 19 were used to determine the chromosomal origin of the DNA in micronuclei resulting from a subsequent 4Gy exposure of these cells, and corresponding non-adapted cells. Results: Compared with 4Gy exposed but non-adapted cells, the radiation-adapted cells subsequently exposed to 4Gy showed an overall decrease in the frequency of micronuclei. However, the micronuclei that did form in the adapted cells had a decreased frequency of DNA originating from chromosomes 2 and 18, an increased frequency of DNA from chromosome 19 and no change in frequency of DNA from chromosomes 4 and 7. Conclusions: Adaptation to radiation increased the overall cellular repair of radiation-induced chromosomal breaks, but also created a repair bias such that some chromosomes were preferentially repaired or discriminated against, while the repair of others was unbiased.