NHEJ DNA repair pathway is overactivated in resistant B-CLL cells after DNA damage. (A) NHEJ activity in untreated cells (Irradiation -) and cells exposed to 10 Gy irradiation (Irradiation +). Assays were carried out using a resistant B-CLL lymphocyte extract (R), a sensitive B-CLL cell extract (S), and an EBV-transformed B-lymphocyte extract (B cells). The results shown are representative of those obtained with all the cell extracts tested (n = 3 for resistant B-CLL samples, n = 3 for sensitive B-CLL samples, and n = 3 for EBV-transformed B-cell samples). (B) Changes in NHEJ activity after exposure to 10 Gy irradiation were calculated from the mean (± SEM) activities recorded in duplicate experiments on 3 different resistant B-CLL cell extracts (R), 3 different sensitive B-CLL cell extracts (S), and 3 different EBV-transformed B-lymphocyte extracts (B cells). (C) NHEJ activity in cells that were either left untreated (-) or treated (+) with 10 Gy irradiation, 1 μM wortmannin, or 10 μM NU7026. Assays were performed on resistant B-CLL cell extracts (R) and sensitive B-CLL cell extracts (S). A representative experiment is shown. (D) NHEJ activity assay with DNA-PKcs-deficient cell line extract (MO59J) and wild-type cell line extract (MO59K). A representative experiment is shown. See Supplemental Figure S1 for more information.