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.

The comparative induction of chromatid aberrations by 238Pu alpha-particles, or by 250 kVp X-rays was investigated in V79 Chinese hamster cells. Metaphases were sampled at hourly intervals postirradiation up to 8 h and BrdU/FPG staining methods were used to distinguish G2, S and G1 phase cells. Two experiments were performed. In the first, an alpha-particle dose of 0.41 Gy was compared with an X-ray dose of 1.5 Gy used in a previously published study. In the second, an X-ray dose of 1.2 Gy was used in parallel with 0.41 Gy of alpha-particles to produce a similar overall frequency of interchanges, and allow comparative ratios to be derived for other aberration types. At these isoexchange doses, alpha-particles produce relatively less gaps and breaks, particularly in late G2, and significantly more isochromatid deletions. A very high proportion of the isochromatid deletions were incomplete after alpha-particles compared with X-rays, but no difference in incompleteness was found for interchanges. With X-rays, about 6% of interchanges are complex intra-interchange forms. At similar exchange frequencies this increases to 26.7% for alpha-irradiation, suggesting increased multiple lesion interaction. Differences in dose distribution between alpha-particles and X-rays are discussed and mitotic delay is examined after separation of the analysed cells into damaged and undamaged classes.

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The controversy of the Classic versus the Exchange theories for the origin of simple chromatid breaks is outlined. Using BrdU harlequin sister-chromatid differentiation four Revell ratios can be defined and these have been obtained and tested as a block in V79 hamster cells. The values are quite different from the simple predictions. However, values similar to those observed (taken as a block) can be readily simulated from Revell theory by making the assumption that intra-chromatid events are dominant (0.7:0.3). They can also be obtained from contingency modelling of Classic theory using the same assumption plus the additional constraint that there is no contribution from isolated single lesions (Poisson class 1). If this latter assumption is correct, then the frequency of breaks involving a colour-jump (ratio III) should not decline to zero as the dose falls. A dose-response experiment shows that it does not, but remains approximately constant at about 12%, even in the unirradiated control. An added complication arises when we discover for the TB/BB situation, that whilst neither breaks nor gaps show any excess BB involvement (sensitisation), lesions involved in interchanges show at least a 2-fold BB excess. Clearly, the chromatid discontinuities we are scoring are not behaving as would be expected of a residue of unrejoined primary breaks (Classic theory) and we infer also that they are not 'simple'.

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