Detecting ‘hidden’ exchange events within X-ray-induced aberrations using multicolour chromosome paints

In a recent series of papers we pointed out that any complex exchange (an interaction involving three or more breaks) can yield a wide range of aberration configurations. If these aberrations are detected with asingle chromosome paint and total centromere probe, the majority will be scored as either ‘simple’ exchanges,apparently derived from two breaks, which we termpseudosimples, or as complexes which appear to have fewer breaks than were actually involved, which we termreduced complexes. In this study, metaphases from X-irradiated human fibroblasts were hybridized to two distinctly labelled composite probes (paints) for chromosomes 1 and 2. All centromeres were marked by differential 4′,6-diamidino-2-phenylindole (DAPI) staining. Aberrations derived from exchanges involving both paints were characterized. With this method, we are able to identify some of the additional exchange events, so confirming the existence of these two classes of aberration. Ultimately we believe it can be used to estimate their frequency.