Normal Rejoining of DNA Strand Breaks in Ataxia Telangiectasia Fibroblast Lines after Low X-Ray Exposure

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Abstract
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), a rare hereditary disorder whose hallmarks include hypersensitivity to ionizing radiation, both in vivo and in vitro. 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, incubated for suitable times, and lysed on filters; their denatured DNA was sized by its elution rate at alkaline pH. Upon receiving 250 rad, the control fibroblasts from a clinically normal donor rejoined all detectable single-strand breaks (plus alkali-labile bonds) within 30-60 min of incubation. When challenged with 1250 rad the kinetics of strand rejoining by the normal control cells were biphasic; .apprx. 80% of the strand scissions were ligated within 30 min while the remaining 20% were rejoined over the next 2.5-3.5 h. 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 3 mutant cell lines (AT2BE, AT3BI, AT4BI) belonging to the 3 known genetic complementation groups in AT. These results confirm the recent findings of A.J. Fornace, Jr. and J. B. Little. It would thus appear that the enhanced radiosensitivity of cultured AT cells does not stem from faulty rejoining of radiogenic DNA strand breaks.