Mechanisms of induction of SCE and mutatins by BrdU and CldU and the use of inhibitors of DNA repair to study mechanisms of radiation-induced chromosome aberrations

The induction of sister chromatid exchanges (SCE) and specific locus mutations was studied by utilizing incorporation into DNA of the nucleoside analogues 5-bromo- and 5-chlorodeoxyuridine (BrdU and CldU), CldU was found to induce SCE seven-times more efficiently than BrdU at equal extracellular concentrations. This induction was linearly associated with substitution for thymidine from 0.5-20 μM. In these experiments, specific locus mutations were not detected at concentrations less than 50 μM and were not correlated with SCE induction. At concentrations greater than 50 μM, the mutagenicity of CldU and BrdU was similar, although BrdU was slightly more mutagenic than CldU. In experiments designed to examine SCE and mutagenicity as a function of replication of BrdU or CldU-substituted DNA, it was shown that at low analogue concentrations, CldU caused a seven-fold greater rate of mutation and cytotoxicity than BrdU, and that replication of twice as much analogue-substituted DNA was associated with a two-fold increase in SCE for both CldU and BrdU. SCE induction, therefore, appears to be a consequence of replication on templates containing these analogues. Mutations, however, result from two different mechanisms; errors of incorporation where BrdU or CldU substitutes in DNm for deoxycytidine and replication errors which, similar to the mechanisms of SCE induction, require replication on templates substituted with BrdU or CldU.

In the examination of radiation-induced chromosome aberrations in mammalian lymphocytes, 3-aminobenzamide and cytosine arabinoside, which are excision repair inhibitors, were used to show that the induction of chromosome aberrations depends upon the ratio of base damage to directly-induced DNA strand breaks for a particular radiation quality. In addition, it was shown that sensitivity of various mammalian species to X ray-induced aberrations depends upon the rate of repair of base damage. This is in contrast to aberration induction by fission neutrons. Since neutrons produce mainly directly-induced double strand breaks and apparently rather little base damage, it was shown that aberration formation with neutrons is dependent upon mis-rejoining of directly-induced strand breaks, and was therefore, independent of excision repair rate. The relative biological effectiveness of a particular radiation quality therefore, can be accounted for by the ratio of misrepair of directly-induced double strand breaks to base damage and varies for different cell types and species. These results, furthermore, appear to provide an explanation for both the mechanism of and sensitivity to radiation-induced chromosome aberrations.