The effects of cysteamine and hypoxia on X-ray-induced mutation in human lymphoblast cells have been investigated. Irradiation of these cells under these two sets of radioprotective conditions resulted in a fourfold lower mutant fraction at the hprt locus, compared to irradiation in the absence of any protection. Using Southern blot analysis, the molecular nature of mutants induced in these cells by X rays delivered under hypoxic conditions, or in the presence of 25 mM cysteamine, has been compared with that of mutants induced by an equally mutagenic treatment of X rays alone. Of 60 mutants from cultures treated with X rays alone, 16 exhibited no change in the restriction fragment pattern and were defined as point mutations, 27 were total gene deletions, and 17 were partial deletions or rearrangements. Of 46 mutants from cultures treated with X rays in the presence of 25 mM cysteamine, 22 were point mutations, 18 were total gene deletions, and only 6 were partial deletions or rearrangements. Of 45 mutants from cultures treated with X rays delivered under hypoxic conditions, 17 were point mutations, 14 were total gene deletions, and 14 were partial deletions or rearrangements. Both radioprotective conditions reduced the level of mutation in each mutational class, when compared to the maximum expected in the absence of protection. The spectrum of mutations induced by X rays in cysteamine was significantly different from X rays alone. However, the mutational spectrum of X rays under hypoxic conditions was not different from X rays alone or from X rays in cysteamine. The extents of the deletions induced at the hprt locus were also examined by hybridizing the Southern blots to X-chromosome markers that have been mapped to within 1200 kb of hprt. There were no significant differences among the three groups; however, many mutants were missing one or more of these markers, indicating that deletions of several hundred kilobases are not uncommon.

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