Ionizing radiation-induced chromosomal rearrangements occur in transcriptionally active regions of the genome

Purpose: The mechanism by which ionizing radiation induces chromosomal rearrangements in mammalian cells has for long been a subject of debate. In order to dissect these events at a molecular level, we have studied the sequences involved in gamma irradiation-induced rearrangements. Materials and methods: An inverse polymerase chain reaction (PCR)-based methodology was used to amplify rearrangements that had occurred between one of four target regions (in or neighbouring the avian myelocytomatosis viral oncogene homologue (c-MYC), cyclin-dependent kinase inhibitor 1A (CDKN1A), fibroblast growth factor receptor 2 (FGFR2), or retinoblastoma 1 (RB1) genes) and sequences elsewhere in the genome, following gamma irradiation and subsequent incubation at 37°C of normal human IMR-90 fibroblasts. Results: The sequences of 90 such rearrangements, including both inter- and intra-chromosomal events, have been analysed. Sequence motifs (including DNA topoisomerase recognition sites) were not found to be consistently present either at or near rearrangement breakpoint sites. Statistical analysis suggested that there was significantly more homology between the sites of DNA rearrangement breakpoints than would be expected to occur by chance, however, most DNA rearrangements showed little or no homology between the interacting sequences. The rearrangements were shown to predominantly involve transcriptionally active sequences, a finding that may have significant implications for our understanding of radiation-induced carcinogenesis. Conclusion: The results obtained are difficult to reconcile with most models for ionizing radiation-induced chromosomal aberration formation, but are consistent with the Transcription-Based model.