Wei Lirong, Chahwan Richard, Wang Shanzhi, Wang Xiaohua, Pham Phuong T, Goodman Myron F, Bergman Aviv, Scharff Matthew D, MacCarthy Thomas
Department of Cell Biology, Albert Einstein College of Medicine, Bronx, NY 10461;
Department of Biosciences, University of Exeter, Exeter EX2 4QD, United Kingdom;
Proc Natl Acad Sci U S A. 2015 Feb 17;112(7):E728-37. doi: 10.1073/pnas.1500788112. Epub 2015 Feb 2.
Activation-induced deaminase (AID) mediates the somatic hypermutation (SHM) of Ig variable (V) regions that is required for the affinity maturation of the antibody response. An intensive analysis of a published database of somatic hypermutations that arose in the IGHV3-2301 human V region expressed in vivo by human memory B cells revealed that the focus of mutations in complementary determining region (CDR)1 and CDR2 coincided with a combination of overlapping AGCT hotspots, the absence of AID cold spots, and an abundance of polymerase eta hotspots. If the overlapping hotspots in the CDR1 or CDR2 did not undergo mutation, the frequency of mutations throughout the V region was reduced. To model this result, we examined the mutation of the human IGHV3-2301 biochemically and in the endogenous heavy chain locus of Ramos B cells. Deep sequencing revealed that IGHV3-23*01 in Ramos cells accumulates AID-induced mutations primarily in the AGCT in CDR2, which was also the most frequent site of mutation in vivo. Replacing the overlapping hotspots in CDR1 and CDR2 with neutral or cold motifs resulted in a reduction in mutations within the modified motifs and, to some degree, throughout the V region. In addition, some of the overlapping hotspots in the CDRs were at sites in which replacement mutations could change the structure of the CDR loops. Our analysis suggests that the local sequence environment of the V region, and especially of the CDR1 and CDR2, is highly evolved to recruit mutations to key residues in the CDRs of the IgV region.
激活诱导的胞嘧啶脱氨酶（AID）介导Ig可变区（V）的体细胞高频突变（SHM），这是抗体应答亲和力成熟所必需的。对已发表的人类记忆B细胞在体内表达的IGHV3-2301人类V区体细胞高频突变数据库进行的深入分析表明，互补决定区（CDR）1和CDR2中的突变热点与重叠的AGCT热点组合、AID冷点的缺失以及大量的聚合酶η热点相吻合。如果CDR1或CDR2中的重叠热点未发生突变，则整个V区的突变频率会降低。为模拟这一结果，我们对人类IGHV3-2301进行了生化研究，并在Ramos B细胞的内源性重链基因座中进行了研究。深度测序显示，Ramos细胞中的IGHV3-23*01主要在CDR2的AGCT中积累AID诱导的突变，这也是体内最常见的突变位点。用中性或冷基序取代CDR1和CDR2中的重叠热点，导致修饰基序内以及在一定程度上整个V区的突变减少。此外，CDR中的一些重叠热点位于替换突变可能改变CDR环结构的位点。我们的分析表明，V区，尤其是CDR1和CDR2的局部序列环境经过高度进化，以将突变募集到IgV区CDR中的关键残基上。
