Quantitation of the repair of gamma-radiation-induced double-strand DNA breaks in human fibroblasts

The quantitation and repair of double-strand DNA breaks in human fibroblasts has been determined using a method involving the nondenaturing elution of DNA from a filter. DNA from cells from two human fibroblast lines exposed to γ-radiation from 0 to 10 000 rad showed increasing retention on a filter with decreasing radiation dose, and the data suggest a linear relationship between double-strand breaks induced and radiation dose. The ability of normal human fibroblasts to repair double-strand breaks with various doses of radiation was demonstrated, with a $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of 10 min for repair of 5000 rad exposure and 39 min for repair of 10 000 rad damage. The kinetics of the DNA rejoining were not linear and suggest that, as in the repair of single-strand breaks, both an initial ‘fast’ and a later ‘slow’ mechanism may be involved.