Rada C, Yélamos J, Dean W, Milstein C
MRC Laboratory of Molecular Biology, Cambridge, GB.
Eur J Immunol. 1997 Dec;27(12):3115-20. doi: 10.1002/eji.1830271206.
The hypermutation of antibody genes targets 1-2 kb of DNA which includes the rearranged V(D)J gene segments. The precise nature, location and limits of the targeted region are of considerable interest in terms of the mechanism of hypermutation. We have analyzed the frequency and distribution of mutations in the 5' region of immunoglobulins using several modified kappa transgenes. We found that the position of the boundary, relative to the transcription initiation site, is not affected by the sequence of the V segment or by substituting the kappa chain promoter for a beta-globin promoter. Furthermore, the deletion of the leader intron (containing the hypermutation boundary) does not affect hypermutation per se, but shifts the boundary from the leader intron to the V region such that the distance between the boundary and the site of initiation of transcription remains constant. These results show that the position of the hypermutation boundary (about 185 bases downstream of the site of initiation of transcription) is not defined by the nucleotide sequence but rather by the distance to a fixed upstream position. Although mutations are also observed in the region upstream of the boundary, the frequency at which they occur is one order of magnitude lower relative to the frequency observed in the V segment. Nonetheless this upstream mutation rate remains more than two orders of magnitude higher than that of somatic genes. We discuss possible mechanisms explaining the nature and position of the boundary in the context of an error-prone DNA repair model.
抗体基因的高突变作用于1-2 kb的DNA，其中包括重排的V(D)J基因片段。就高突变机制而言，靶向区域的确切性质、位置和范围备受关注。我们使用了几种修饰的κ转基因分析了免疫球蛋白5'区域突变的频率和分布。我们发现，相对于转录起始位点，边界的位置不受V区段序列的影响，也不受用β-珠蛋白启动子替代κ链启动子的影响。此外，前导内含子（包含高突变边界）的缺失本身并不影响高突变，但会将边界从前导内含子转移到V区域，使得边界与转录起始位点之间的距离保持恒定。这些结果表明，高突变边界的位置（转录起始位点下游约185个碱基处）不是由核苷酸序列定义的，而是由到固定上游位置的距离定义的。尽管在边界上游区域也观察到突变，但其发生频率相对于V区段中观察到的频率低一个数量级。尽管如此，这种上游突变率仍比体细胞基因的突变率高两个多数量级。我们在易错DNA修复模型的背景下讨论了解释边界性质和位置的可能机制。
