DNA ligase IV-deficient cells are more resistant to ionizing radiation in the absence of Ku70: Implications for DNA double-strand break repair

Proc Natl Acad Sci U S A. 2001 Oct 9;98(21):12109-13. doi: 10.1073/pnas.201271098.


Vertebrate cells have evolved two major pathways for repairing DNA double-strand breaks (DSBs), homologous recombination (HR) and nonhomologous DNA end-joining (NHEJ). To investigate the role of DNA ligase IV (Lig4) in DSB repair, we knocked out the Lig4 gene (LIG4) in the DT40 chicken B-lymphocyte cell line. The LIG4(-/-) cells showed a marked sensitivity to X-rays, bleomycin, and VP-16 and were more x-ray-sensitive in G(1) than late S or G(2)/M, suggesting a critical role of Lig4 in DSB repair by NHEJ. In support of this notion, HR was not impaired in LIG4(-/-) cells. LIG4(-/-) cells were more x-ray-sensitive when compared with KU70(-/-) DT40 cells, particularly at high doses. Strikingly, however, the x-ray sensitivity of KU70(-/-)/LIG4(-/-) double-mutant cells was essentially the same as that of KU70(-/-) cells, showing that Lig4 deficiency has no effect in the absence of Ku. These results indicate that Lig4 is exclusively required for the Ku-dependent NHEJ pathway of DSB repair and that other DNA ligases (I and III) do not substitute for this function. Our data may explain the observed severe phenotype of Lig4-deficient mice as compared with Ku-deficient mice.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Antigens, Nuclear*
  • B-Lymphocytes
  • Base Sequence
  • Cell Line
  • Chickens
  • DNA Damage*
  • DNA Helicases*
  • DNA Ligase ATP
  • DNA Ligases / genetics
  • DNA Ligases / physiology*
  • DNA Repair*
  • DNA, Complementary
  • DNA-Binding Proteins / physiology*
  • Gene Targeting / methods
  • Ku Autoantigen
  • Molecular Sequence Data
  • Nuclear Proteins / physiology*
  • Radiation, Ionizing


  • Antigens, Nuclear
  • DNA, Complementary
  • DNA-Binding Proteins
  • LIG4 protein, human
  • Nuclear Proteins
  • DNA Helicases
  • XRCC5 protein, human
  • Xrcc6 protein, human
  • Xrcc6 protein, mouse
  • Ku Autoantigen
  • DNA Ligases
  • DNA Ligase ATP

Associated data

  • GENBANK/AB058600