Mutations occur in the Ig Sμ region but rarely in Sγ regions prior to class switch recombination

Nucleotide substitutions are found in recombined Ig switch (S) regions and also in unrecombined (germline, GL) Sμ 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 Sγ3 and Sγ1 regions in activated B cells. These data suggest that switch recombination initiates in the Sμ segment and secondarily involves the downstream acceptor S region. Furthermore, the pattern and specificity of mutations in GL and recombined Sμ segments differ, suggesting different repair mechanisms. Mutations in recombined Sμ regions show a strong bias toward G/C base pairs and WRCY/RGYW hotspots, whereas mutations introduced into the GL Sμ do not. Additionally, induction conditions affect mutation specificity within the GL Sμ segment. Mutations are most frequent near the S–S junctions and decrease rapidly with distance from the junction. Finally, we find 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.