On the nature of a defect in cells from individuals with ataxia-telangiectasia.

The cells and tissues of patients with ataxia-telangiectasia (A-T), an inherited disease characterized by a high degree of proneness to cancer, are abnormally sensitive to ionizing radiation. Noncycling cultures of normal human and A-T fibroblasts were exposed to x-rays so that the breakage and rejoining of prematurely condensed chromosomes in the G1 phase could be compared. After a dose of 6.0 grays, both cell types had the same initial frequency of breaks and the same rate for rejoining of the breaks, but the fraction of breaks that did not rejoin was five to six times greater for the A-T cells. The results also show that progression of cells into the S phase is not a prerequisite for the increased frequency of chromosome fragments that appear in mitosis after A-T cells are irradiated in the G1 or G0 phase.