X-ray-induced simple, pseudosimple and complex exchanges involving two distinctly painted chromosomes

Purpose: To detect simple, pseudosimple and complex chromosome exchanges in X-ray-induced aberrations involving two distinctly painted chromosomes. Ea ch visibly complex two-paint exchange was analysed to determine the number of breaks and chromosomes necessary to derive the pattern. In addition, the number of associated paint junctions was scored to assess the frequency of non-reciprocal exchanges. Materials and methods: Metaphase spreads were prepared from a human primary fibroblast cell line irradiated with 2, 1 and 6 Gy 250 kV X-rays. FISH-pai nting was performed with distinctly labelled probes for chromosomes 1 and 2 , and a pancentromeric probe. Results: From a total of 78 two-paint exchanges observed, 35 were apparentl y simple, with no additional counterstain chromatin, and 43 were visibly co mplex with two-colour painting, of which 23 contained at least one pseudosi mple exchange. A detailed analysis of the number of two-paint colour juncti ons showed that at least 50% of the visibly complex exchange patterns invol ved non-reciprocal exchanges. The simple and complex exchange dose-response curves were considered to be linear and curvilinear respectively. Conclusion: The frequency of non-reciprocal rejoining events within complex exchanges is consistent with an interaction model based on the free exchan ge of multiple break-ends. In addition, the simple and complex exchanges ha ve distinct dose-response curves, in agreement with previous data for singl e-painted exchanges corrected for pseudosimples.