The nonhomologous end-joining pathway of DNA repair is required for genomic stability and the suppression of translocations

We have used spectral karyotyping to assess potential roles of three differ ent components of the nonhomologous DNA end-joining pathway in the maintena nce of genomic stability in mouse embryonic fibroblasts (MEFs), MEFs homozy gous for mutations that inactivate either DNA ligase IV (Lig4) or Ku70 disp lay dramatic genomic instability, even in the absence of exogenous DNA dama ging agents. These aberrant events range from chromosomal fragmentation to nonreciprocal translocations that can involve several chromosomes. DNA-depe ndent protein kinase catalytic subunit deficiency also promotes genome inst ability. Deficiency for the p53 cell cycle checkpoint protein has little ef fect on spontaneous levels of chromosomal instability in Lig4-deficient fib roblasts. However, in the context of ionizing radiation treatment, p53 defi ciency allowed visualization of massive acute chromosomal destruction in Li g4-deficient MEFs, which in surviving cells manifested as frequent nonrecip rocal translocations, We conclude that nonhomologous DNA end-joining plays a crucial role as a caretaker of the mammalian genome, and that an alternat ive repair pathway exists that often leads to nonreciprocal translocations.