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The effects of 45/sup 0/C hyperthermia and ..gamma.. radiation have been studied in three normal human fibroblast lines (GM38, GM730, WI38) and compared to the effects in two lines derived from patients with the hereditary disease ataxia telangiectasia (AR3BI, AT5BI). All lines, both normal and ..gamma..-sensitive AT, showed a similar resistance to killing by heat alone, suggesting that the defect responsible for the increased radiation sensitivity in AT lines does not confer increased heat sensitivity. Shouldered survival curves were obtained in each case indicating the ability to accumulate sublethal heat damage. All normal and AT cell lines exhibited increased resistance to the lethal effects of heat in response to a thermal stress, indicating that the defect that causes radiosensitivity in AT cell lines does not prevent the induction of thermotolerance. It was hypothesized that in normal cells, this heat treatment inactivates the process which is already defective in AT lines, and that this process may be required for the proper rejoining of double-strand breaks produced during the repair of other radiation-induced lesions.

The alkaline elution method was used to measure the enzymatic repair of x-ray-induced DNA strand breaks in skin fibroblasts derived from human subjects afflicted with ataxia telangiectasia (AT). Monolayer cultures of normal control and AT cell lines were exposed acutely to moderately lethal (250-rad) and highly lethal (1250-rad) doses of 250-kV x rays under aerobic conditions. Upon receiving 250 rad, the control fibroblasts from a clinically normal donor rejoined all detectable single-strand breaks (plus alkali-labile bonds) within 30 to 60 min of incubation. When challenged with 1250 rad the kinetics of strand rejoining by the normal control cells were biphasic. For both exposures, no significant difference in either the rate or the extent of strand rejoining was detected between the normal cell line (GM38) and three mutant cell lines (AT2BE, AT3BI, AT4BI) belonging to the three known genetic complementation groups in AT. It would thus appear that the enhanced radiosensitivity of cultured AT cells does not stem from faulty rejoining of radiogenic DNA strand breaks.