Comparison of Gamma-Ray-Induced Chromosome Ring and Inversion Frequencies

Abstract

A method was used to detect chromosome inversions as apparent or false sister chromatid exchanges (SCEs) in the first mitosis after gamma irradiation of human G0 cells. Dose-response relationships for small inversions have not been measured and reported previously, but it has been assumed that these are induced with a frequency equal to that of their easily measured asymmetrical counterpart, the interstitial deletion. Our experiments confirm this expectation. The results also demonstrate, as others have suggested, that in protocols where SCEs have been reported in the first postirradiation mitosis after incorporation of BrdU in the previous cell cycle, the X- or gamma-ray treatment of G0- or G1-phase cells produces virtually no true SCEs.

Insights into the mechanisms of sister chromatid exchange formation

“…1). The hypothesis of false SCEs was substantiated by the results of Mühlmann-Diaz and Bedford (1995). These authors found similar frequencies of interstitial deletions and interstitial SCEs in fibroblasts pre-labeled with BrdU for one round of replication and irradiated with gamma rays in G 1 .…”

Sister chromatid exchanges occur in G2-irradiated cells

“…8,9,14,16 For example, Mühlmann-Diaz and Bedford suggested that SCEs induced after γ-or X-irradiation might represent "false" SCEs that arise from chromosomal aberrations, in particular paracentric inversions, produced in G 0 /G 1 . 20 Color-jumps on chromatids were also observed in cells that were treated with DSB-inducing agents in G 2 , but these jumps were attributed to a two-lesion exchange process. 21 The widely held belief that "true" SCEs are not formed in G 2 was further supported by work of Wojcik et al who observed that ©2 0 1 1 L a n d e s B i o s c i e n c e .…”

Review: Proximity effects in the production of chromosome aberrations by ionizing radiation

“…For example, for the doses speci® ed in Table 1, simulations based on random ness predict, and the data show, near equality of the 2F and 2G aberration types (Table 1). Similarly, centric rings and pericentric inversions appear to occur in approximately equal numbers (Savage and Papworth 1982), as do intra-arm inversions and acentric rings (Muhlmann-Diaz and Bedford 1995), as expected from random ness of illegitim ate reunions. These additional checks involve smaller numbers of aberrations so they are less robust than observations comparing apparently simple dicentrics and translocations.…”