Interphase chromosome domain re-organization following irradiation

Purpose: To investigate chromosome domain movements during interphase in st ationary cell nuclei. Materials and methods: Contact-inhibited primary human fibroblasts were irr adiated with 4.0 Gy X-rays, BrdU was added, and air-dried cell preparations made at intervals up to 48 h. Chromosome 4 domain signals (1, 2 and >2) in BrdU negative nuclei (almost exclusively G0/G1) were counted using FISH. A similar experiment was performed using unstimulated human lymphocytes. Results: A very significant rise in nuclei with >2 signals was found within Ih after radiation. The frequencies observed were in very good agreement w ith those expected for simple and complex interchanges involving chromosome 4, scored at metaphase in these materials. Conclusions: The observations constitute evidence for significant domain mo vement and re-organization within a short time of radiation exposure in G0/ G1 interphase nuclei, presumably induced by the formation of inter-domain e xchanges. Such re-organization must be a very complicated and delicate topo logical problem for relaxed chromatin, and must have an important bearing o n the interpretation of mechanistic premature chromosome condensation exper iments performed whilst it is in operation.