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Repair of Ionizing Radiation DNA Base Damage in Ataxia-Telangiectasia Cells

Radiobiology Section, Radiation Oncology Branch, Clinical Oncology Program, Division of Cancer Treatment, National Cancer Institute, NIH, Bethesda, Maryland 20892

Micrococcus luteus endonuclease sensitive sites were measured by alkaline elution in normal human and ataxia-telangiectasia (AT) fibroblasts after ionizing radiation. Due to the sensitivity of this assay, repair of base damage after 3 to 6 kilorads has been measured after oxic or hypoxic radiation. With 5.5 kilorads of oxic radiation, more than 50% of the base damage was removed after 1.5 h of repair incubation in all cells, including exr⁺ and exr^- AT cells, and approximately 75% was removed by 4 h. After 3 or 4.5 kilorads of hypoxic X-irradiation, repair was equivalent in normal and exr^- AT cells. This study included three exr^- AT strains which have been reported to be deficient in the removal of -ray base damage at higher doses. Since these strains repaired ionizing radiation base damage normally at lower doses, which are more relevant to survival, it is concluded that the X-ray hypersensitivity of AT cells is probably not related to the repair of base damage.

¹ To whom requests for reprints should be addressed, at National Cancer Institute/NIH, Radiation Oncology Branch, Building 10, Room B3B69, 9000 Rockville Pike, Bethesda, MD 20892.

Received 6/28/85. Revised 12/16/85. Accepted 12/30/85.