Mechanisms underlying the conversion of DNA double-strand breaks into chromatid breaks

Abstract

Chromatid breaks are associated with cancer-predisposition but may also provide a mechanism of radiation carcinogenesis via the formation of genomic rearrangements. A model of chromatid breakage has been proposed and supporting evidence presented which derives from work with a genetically-engineered hamster cell line containing a unique dsb site. Chromatid breaks are induced in irradiated mammalian cells as a linear function of dose indicating that they arise from single events, thought to be DNA double-strand breaks (dsb). However, the molecular mechanism of the transition between a dsb (with the loss of only a few base-pairs at most) and a chromatid break (with the apparent loss of 5–40 megabase-pairs) is not yet fully clear. A significant number of chromatid breaks occurring spontaneously or following irradiation clearly involve a genomic rearrangement, manifest as colour-switches between sister chromatids at break-points in harlequin (FPG) stained chromosomes. Furthermore, the kinetics of chromatid break disappearance with time following irradiation does not correspond with the rejoining of dsb; e.g. some mutant cell lines deficient in dsb rejoining show elevated break frequencies but normal chromatid break disappearance with time. The signal model of chromatid breaks is able to accommodate these findings, and in particular allows the dissociation of dsb and chromatid break kinetics with time. Briefly, the model proposes signalling of a single dsb that triggers the cell to make a genomic rearrangement at the crossover points of a looped chromatin domain, possibly a transcription factory. If incomplete, the rearrangement leads to a chromatid break. Thus, rearrangement may occur between sister chromatids, when a colour-switch is seen at the break point, or more frequently (most probably 80% or more) within a single chromatid, leading to an inversion adjacent to the break site. Completion of the latter type of rearrangement would lead to a transmissible inversion, a possible carcinogenic event. We have evidence that the XRCC2 gene is involved in the process since mutation of this gene significantly alters the colour-switch ratio (ratio of colour-switch breaks to the total chromatid breaks). The requirement for only a single dsb has been confirmed by results of experiments with a genetically engineered cell line containing a unique dsb (I-Sce1) cut-site.