Identification of X-ray-induced complex chromosome exchanges using fluorescence in situ hybridization: a comparison at two doses.

Complex chromosome exchanges are defined as interactions between three or more breaks, in two or more chromosomes. In this study, a sequential hybridisation technique was developed to visualize a given chromosome pair: green (chromosomes 1, 5 and 7), all centromeres red and the remaining chromosomes blue. Primary human fibroblasts were irradiated in G1 with 4 and 6 Gy 250-kV X-rays. After 4 Gy, a total of 387 simple aberrations (defined as translocations or dicentrics with fragments) and 116 complex aberrations were identified. After 6 Gy these aberrations numbered 225 and 110 respectively. Using a break-rejoin scheme, which describes interactions between breaks within a complex as independent events we modelled the complex 'mix' present after 4 Gy. At 6 Gy the same model could be used for chromosomes 5 and 7, but chromosome 1 frequencies could only be explained if we biased the interactions, such that two-breaks-in-one-chromosome events occur predominantly in chromosome 1. From these predicted interactions we also calculated the number of apparently simple exchanges that are actually complex derived. These accounted for 20% of those observed after 4 Gy and 33% after 6 Gy. Therefore, with this FISH assay, an estimated 35 and 54% of all exchanges are derived from complexes after 4 and 6 Gy respectively.