The formation of single-strand breaks by near-ultraviolet light at 313 nm and by aerobic γ-rays was compared for skin fibroblast monolayer cultures from 4 normal donors (NF) and 8 patients with Bloom's syndrome (BS) by the alkaline elution method. In 6 of 8 BS strains, the number of breaks induced by near-ultraviolet light, 2.25 kJ/sq m, at 0° was comparable to NF, while elevated breakage was observed in BS strains HG 369 and HG 916. Breakage frequencies were increased substantially in 6 of 8 BS strains relative to NF when the near-ultraviolet light exposure was at 37°. BS strain GM 2520 represents an exception since normal breakage frequencies were induced both at 0° and 37°.

Aerobic γ-rays (75 R) induced comparable numbers of single-strand breaks in BS and NF strains at 0°. The breakage frequencies were reduced an average of 17% in NF when the same dose was given at 30° followed by 6 min incubation. Under the same conditions, the breakage frequencies were on the average reduced by 42% relative to 0° in the BS strains, indicating that they possess normal or possibly slightly increased capacities for the rejoining of γ-ray-induced breaks.

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This work was supported by Grant 3′305.78 from the Swiss National Science Foundation and Grant BC-281 from the American Cancer Society.

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