Ratios of radiation-produced chromosome aberrations as indicators of large-scale DNA geometry during interphase

Chromosome aberrations produced by ionizing radiation are assumed to develop from DNA double-strand breaks (DSBs) which interact pairwise. Stable chromosome aberrations that exemplify inter- and intra-chromosomal exchanges are, respectively, translocations and pericentric inversions. By comparing the number of these for each chromosome one can infer results on the randomness of DSB induction or exchange formation and on large-scale chromosome geometry. We analyze frequencies of translocations and pericentric inversions in lymphocytes from 38 A-bomb survivors, using G-banding. A total of 636 translocations and 102 pericentric inversions were found. The 636/102 ratio of translocations to pericentric inversions is approximately 14 times smaller than predicted by a random model, in general agreement with earlier results and results on the ratio of dicentrics to centric rings for in vitro irradiation. Presumably the excess of intra-chromosomal exchanges is due to a spatial proximity effect, implying a localization of chromosomes within the cell nucleus during and shortly after irradiation. The distribution of the pericentric inversions among different chromosomes indicates this proximity effect is roughly the same for all chromosomes, regardless of DNA content; i.e., the ratio of pericentric inversions for two different chromosomes approximately equals the ratio given by a model which takes into account chromosome lengths and centromere locations but otherwise assumes randomness. Possible exceptions are chromosomes 7 and 12, which show some excess of pericentric inversions. The percentage of translocations involving each chromosome corresponds roughly to the percentage expected assuming randomness, except that for chromosome 1 there is a significant excess.