Estimating the true frequency of X-ray-induced complex chromosome exchanges using fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH) with a series of composite probes for human chromosomes 1, 4, 5, 7 and 13, applied separately with a total centromere probe, was used to identify X-ray-induced, chromosome-type aberrations in primary untransformed human fibroblasts. Visible complex exchanges, i.e. those involving three or more breaks in two or more chromosomes, were classified with reference to possible complex FISH patterns derived from a set of theoretical interactions. At 4 Gy, approximately 20% of all exchanges we observed were visibly complex. Using an interaction scheme, which allows either loose end of a break to restitute with its partner, or join with any other loose end within the potential complex, we modelled a set of interactions that matched the frequencies of the complex FISH patterns identified. The direct score of visible complex patterns is an under-estimate of the true frequency because some apparently 'simple' two-break exchanges (dicentrics and translocations) are actually derived from three or more breaks. Using the model we estimate that these account for 20% of the simples scored. Taking this into account, we estimate the true frequency of complexes at 4 Gy to be 35% of all exchanges scored and we believe that the majority of these involve five breaks in four chromosomes.