Analysis of variable (diversity) joining recombination in DNA-dependent protein kinase (DNA-PK)-deficient mice reveals DNA-PK-independent pathways for both signal and coding joint formation

Previous studies have suggested that ionizing radiation causes irreparable DNA double-strand breaks in mice and cell lines harboring mutations in any of the three subunits of DNA-dependent protein kinase (DNA-PK) (the catalyt ic subunit, DNA-PKcs, or one of the DNA-binding subunits, Ku70 or Ku86), In actuality, these mutants vary in their ability to resolve double-strand br eaks generated during variable (diversity) joining [V(D)J] recombination, M utant cell lines and mice with targeted deletions in Ku70 or Ku86 are sever ely compromised in their ability to form coding and signal joints, the prod ucts of V(D)J recombination, It is noteworthy, however, that severe combine d immunodeficient (SCID) mice, which bear a nonnull mutation in DNA-PKcs, a re substantially less impaired in forming signal joints than coding joints, The current view holds that the defective protein encoded by the murine SC ID allele retains enough residual function to support signal joint formatio n. An alternative hypothesis proposes that DNA-PKcs and Ku perform differen t roles in V(D)J recombination, with DNA-PKcs required only for coding join t formation. To resolve this issue, we examined V(D)J recombination in DNA- PKcs-deficient (SLIP) mice. We found that the effects of this mutation on c oding and signal joint formation are identical to the effects of the SCID m utation. Signal-joints are formed at levels 10-fold lower than in wild type , and one-half of these joints are aberrant. These data are incompatible wi th the notion that signal joint formation in SCID mice results from residua l DNA-PKcs function, and suggest a third possibility: that DNA-PKcs normall y plays an important but nonessential role in signal joint formation.