Normal V(D)J recombination in cells from patients with Nijmegen breakage syndrome.

The majority of antigen receptor diversity in mammals is generated by V(D)J recombination. During this process DNA double strand breaks are introduced at recombination signals by lymphoid specific RAG1/2 proteins generating blunt ended signal ends and hairpinned coding ends. Rejoining of all DNA ends requires ubiquitously expressed DNA repair proteins, such as Ku70/86 and DNA ligase IV/XRCC4. In addition, the formation of coding joints depends on the function of the scid gene encoding the catalytic subunit of DNA-dependent protein kinase, DNA-PK(CS), that is somehow required for processing of coding end hairpins. Recently, it was shown that purified RAG1/2 proteins can cleave DNA hairpins in vitro, but the same activity was also described for a protein complex of the DNA repair proteins Nbs1/Mre11/Rad50. This leaves the possibility that either protein complex might be involved in coding end processing in V(D)J recombination. We have therefore analyzed V(D)J recombination in cells from patients with Nijmegen breakage syndrome, carrying a mutation in the nbs1 gene. We find that V(D)J recombination frequencies and the quality of signal and coding joining are comparable to wild-type controls, as analyzed by a cellular V(D)J recombination assay. In addition, we did not detect significant differences in CDR3 sequences of endogenous Ig lambdaL and kappaL chain gene loci cloned from peripheral blood lymphocytes of an NBS patient and of healthy individuals. These findings suggest that the Nbs1/Mre11/Rad50 complex is not involved in coding end processing of V(D)J recombination.