Requirement for repair of DNA double-strand breaks by homologous recombination in split-dose recovery

Split-dose recovery has been observed under a variety of experimental condi tions in many cell systems and is believed to be the result of the repair o f sublethal damage. It is considered to be one of tile most widespread and important cellular responses in clinical radiotherapy. To study the molecul ar mechanism(s) of this repair; we analyzed the knockout mutants KU70(-/-), RAD54(-/-), and KU70(-/-)/RAD54(-/-) of the chicken B-cell line, DT40. RAD 54 participates in the recombinational repair of DNA double-strand breaks ( DSBs), while members of the KU family of proteins are involved in nonhomolo gous end joining. Split-dose recovery was observed in the parent DT40 and t he KU70(-/-) cells. Moreover, the split-dose survival enhancement had all o f the characteristics demonstrated earlier for the repair of sublethal dama ge, e,g,, the reappearance of the shoulder on the survival curve with dose fractionation; cyclic fluctuation in cell survival at 37 degreesC; repair a nd no cyclic fluctuation at 25 degreesC. These results strongly suggest tha t repair of sublethal damage is due to DSB repair mediated by homologous re combination, and that these DNA DSBs constitute sublethal damage. (C) 2001 by Radiation Research Society.