Chromosomal aberrations induced by defined DNA double-strand breaks: the origin of achromatic lesions

The mechanisms of formation of chromosomal aberrations are poorly understood, despite the common use of aberrations as a measure of the genetic effects of physical and chemical agents. We have used restriction endonucleases to introduce defined DNA double-strand breaks into mammalian cells, and measured chromosomal aberration formation relative to the activity of the endonuclease. The endonucleases AluI and Sau3AI remain active for a relatively short time under simulated cellular conditions and induce achromatic lesions ('gaps') in chromatids only within the first hour or two following treatment. In contrast, the endonuclease MboI (an isoschizomer of Sau3AI) is active for an extremely long time and continues to produce chromatid gaps during the whole 12 hr sampling period. This observation strongly suggests that the aberrations classified as gaps are a manifestation of unrejoined DNA double-strand breaks. The formation of gaps may relate to the opportunities for repair of DNA breaks in relation to cell-cycle position. It is more difficult to relate the formation of structural chromatid aberrations to the endonuclease activity, although at relatively low concentrations all 3 endonucleases gave similar levels of structural aberrations.