Differences in Sensitivity to DNA-damaging Agents between XRCC4- and Artemis-deficient Human Cells

Non-homologous end-joining (NHEJ) is the predominant pathway for the repair of DNA double-strand breaks (DSBs) in human cells. XRCC4 is indispensable to NHEJ and functions together with DNA ligase IV in the rejoining of broken DNA ends. Artemis is a nuclease required for trimming of some, but not all, types of broken DNA ends prior to rejoining by the DNA ligase IV/XRCC4 complex. To better understand the roles of these factors, we generated XRCC4- and Artemis-deficient cells from the human colon adenocarcinoma cell line HCT116 by gene targeting and examined their cellular responses to several DNA-damaging agents including X-rays. As anticipated, kinetic analyses of gamma-H2AX foci and chromosomal aberrations after ionizing radiation (IR) demonstrated a serious incompetence of DSB repair in the XRCC4-deficient cells, and relatively moderate impairment in the Artemis-deficient cells. The XRCC4-deficient cells were highly sensitive to etoposide and 5-fluoro-2'-deoxyuridine as well as IR, and moderately sensitive to camptothecin, methyl methanesulfonate, cisplatin, mitomycin C, aphidicolin and hydroxyurea, compared to the parental HCT116 cells. The Artemis-deficient cells were not as sensitive as the XRCC4-deficient cells, except to cisplatin and mitomycin C. By contrast, the Artemis-deficient cells were significantly more resistant to hydroxyurea than the parental cells. These observations suggest that Artemis also functions in some DNA damage response pathways other than NHEJ in human cells.