Mutations occur in the Ig S micro region but rarely in Sgamma regions prior to class switch recombination.

摘要

Nucleotide substitutions are found in recombined Ig switch (S) regions and also in unrecombined (germline, GL) S micro segments in activated splenic B cells. Herein we examine whether mutations are also introduced into the downstream acceptor S regions prior to switch recombination, but find very few mutations in GL Sgamma3 and Sgamma1 regions in activated B cells. These data suggest that switch recombination initiates in the S micro segment and secondarily involves the downstream acceptor S region. Furthermore, the pattern and specificity of mutations in GL and recombined S micro segments differ, suggesting different repair mechanisms. Mutations in recombined S micro regions show a strong bias toward G/C base pairs and WRCY/RGYW hotspots, whereas mutations introduced into the GL S micro do not. Additionally, induction conditions affect mutation specificity within the GL S micro segment. Mutations are most frequent near the S-S junctions and decrease rapidly with distance from the junction. Finally, wefind that mice expressing a transgene for terminal deoxynucleotidyl transferase (TdT) have nucleotide insertions at S-S junctions, indicating that the recombining DNA ends are accessible to end-processing enzyme activities.