Storb U, Peters A, Klotz E, Kim N, Shen H M, Hackett J, Rogerson B, O'Brien R, Martin T E
Department of Molecular Genetics and Cell Biology, University of Chicago, IL 60637, USA.
Int J Dev Biol. 1998;42(7):977-82.
This review describes studies on somatic hypermutation of immunoglobulin genes that were started in the mid-80s in collaboration with Ralph Brinster. Almost all of the experiments were carried out using Ig transgenes as targets for the somatic mutation mechanism. Ig transgenes can be very good targets of somatic mutation, despite many different transgene integration sites. Thus, the required cis-acting elements must be present within the approximately 10 kb of the transgene. Only the Ig variable region and its proximate flanks are mutated, not the constant region in unmanipulated sequences. Several Ig gene enhancers are permissive for somatic mutation and they do not have to be associated with the Ig promoter they normally interact with. However, the mutation process does seem to be specific for Ig genes. No mutations were found in several housekeeping genes isolated from cells that had very high levels of somatic hypermutation of their Ig genes. This suggests that the Ig enhancers provide the lg gene specificity. An exception is the Bcl-6 gene, encoding a transcription factor, which was found to be mutated in normal human memory B cells. When the transcriptional promoter that is located upstream of the variable region is duplicated upstream of the constant region, this region is mutated as well. This suggests a transcription coupled model in which a mutator factor associates with the RNA polymerase at the initiation of transcription, travels with the polymerase during elongation, and causes mutations during polymerase pausing. Our recent data with an artificial substrate for somatic mutation suggest that the mutations are increased by increased stability of the secondary structures in the nascent RNA, and the specific nucleotides that are mutated are due to preferences of a mutator factor.
本综述描述了20世纪80年代中期与拉尔夫·布林斯特合作开展的关于免疫球蛋白基因体细胞超突变的研究。几乎所有实验都是以Ig转基因作为体细胞突变机制的靶标进行的。尽管转基因整合位点多种多样，但Ig转基因仍可能是体细胞突变的良好靶标。因此，所需的顺式作用元件必须存在于转基因约10 kb范围内。在未处理的序列中，只有Ig可变区及其紧邻侧翼发生突变，恒定区不发生突变。几种Ig基因增强子允许体细胞突变，它们不必与通常与之相互作用的Ig启动子相关联。然而，突变过程似乎对Ig基因具有特异性。在从具有非常高水平Ig基因体细胞超突变的细胞中分离出的几个管家基因中未发现突变。这表明Ig增强子提供了Ig基因特异性。一个例外是编码转录因子的Bcl-6基因，它在正常人记忆B细胞中发生突变。当位于可变区上游的转录启动子在恒定区上游重复时，该区域也会发生突变。这提示了一种转录偶联模型，即一个诱变因子在转录起始时与RNA聚合酶结合，在延伸过程中与聚合酶一起移动，并在聚合酶暂停时导致突变。我们最近关于体细胞突变人工底物的数据表明，新生RNA二级结构稳定性的增加会增加突变，并且发生突变的特定核苷酸是由于诱变因子的偏好。
