Interphase chromosome domain re-organization following irradiation

Abstract

Purpose: To investigate chromosome domain movements during interphase in stationary cell nuclei. Materials and methods: Contact-inhibited primary human fibroblasts were irradiated with 4.0Gy X-rays, BrdU was added, and airdried cell preparations made at intervals up to 48h. 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 1h after radiation. The frequencies observed were in very good agreement with those expected for simple and complex interchanges involving chromosome 4, scored at metaphase in these materials. Conclusions: The observations constitute evidence for significant domain movement and re-organization within a short time of radiation exposure in G0/G1 interphase nuclei, presumably induced by the formation of inter-domain exchanges. Such re-organization must be a very complicated and delicate topological problem for relaxed chromatin, and must have an important bearing on the interpretation of mechanistic premature chromosome condensation experiments performed whilst it is in operation.