Ionizing radiation enhances double-strand-break repair in rapamycin-treated ataxia telangiectasia lymphoblasts

Unlike normal cells, ataxia telangiectasia (A-T) cells do not exhibit enhanced double-strand-break (dsb)-repair fidelity after ionizing radiation (IR) exposure. DNA-repair induction and other responses to IR are mediated by signalling pathways that are defective in A-T cells. Ataxia telangiectasia mutated (ATM) protein, the mutated protein in A-T cells, is homologous to members of the P13-kinase, including the rapamycin target FRAP. Rapamycin kills normal cells more readily in normal than in A-T cells, and inhibits the FRAP target p70 S6 kinase (p70S6K) more readily in normal than in A-T cells. Also, PHAS-1, another FRAP target, may also be a substrate for ATM. Because ATM plays a role in the response of mammalian cells to rapamycin, the effect of rapamycin on IR enhancement of dsb repair was investigated.

Double-strand-break repair by normal and A-T lymphoblasts, either untreated or treated with rapamycin and y-irradiated, was analysed using shuttle pZ189 containing a dsb.

Double-strand-break-rejoining fidelity in linear plasmids (linDNA) processed in either untreated or rapamycin-treated normal cells increased about twofold if transfection occurred immediately after host irradiation. In contrast, radiation did not increase or decrease the repair fidelity by untreated A-T host cells. But, like normal hosts, dsb-repair fidelity improved twofold in A-T hosts treated with rapamycin. Treatment with the P13-kinase inhibitor LY294002 did not alter dsb-rejoining fidelity in normal or in A-T hosts.

These findings demonstrate that rapamycin does not affect IR enhancement of dsb-repair fidelity in normal cells but restores this phenomenon in A-T lymphoblasts, and suggests the involvement of a rapamycin-sensitive pathway in radiation-enhanced dsb repair.