Schrader Carol E, Bradley Sean P, Vardo Joycelyn, Mochegova Sofia N, Flanagan Erin, Stavnezer Janet
Department of Molecular Genetics and Microbiology, University of Massachusetts Medical School, Worcester, MA 01655-0122, USA.
EMBO J. 2003 Nov 3;22(21):5893-903. doi: 10.1093/emboj/cdg550.
Nucleotide substitutions are found in recombined Ig switch (S) regions and also in unrecombined (germline, GL) Smicro 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 Smicro segment and secondarily involves the downstream acceptor S region. Furthermore, the pattern and specificity of mutations in GL and recombined Smicro segments differ, suggesting different repair mechanisms. Mutations in recombined Smicro regions show a strong bias toward G/C base pairs and WRCY/RGYW hotspots, whereas mutations introduced into the GL Smicro do not. Additionally, induction conditions affect mutation specificity within the GL Smicro 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.
在活化的脾B细胞中，核苷酸替换存在于重组的Ig转换（S）区域以及未重组的（种系，GL）Smicro片段中。在此，我们研究在转换重组之前突变是否也被引入到下游的受体S区域，但在活化B细胞的GL Sgamma3和Sgamma1区域中发现极少的突变。这些数据表明转换重组起始于Smicro片段，其次涉及下游的受体S区域。此外，GL和重组的Smicro片段中突变的模式和特异性不同，提示不同的修复机制。重组的Smicro区域中的突变对G/C碱基对和WRCY/RGYW热点有强烈偏向性，而引入到GL Smicro中的突变则没有。另外，诱导条件影响GL Smicro片段内的突变特异性。突变在S-S连接处附近最为频繁，并随着与连接处距离的增加而迅速减少。最后，我们发现表达末端脱氧核苷酸转移酶（TdT）转基因的小鼠在S-S连接处有核苷酸插入，表明重组的DNA末端可被末端加工酶活性作用。
