Targeting of Somatic Hypermutation by immunoglobulin Enhancer and Enhancer-Like Sequences
Targeting of Somatic Hypermutation by immunoglobulin Enhancer and Enhancer-Like Sequences
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DOI：
10.1371/journal.pbio.1001831
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发表时间：
2014-04-01
期刊：
影响因子：
9.8
通讯作者：
Schatz, David G.
中科院分区：
文献类型：
--
作者：
Buerstedde, Jean-Marie;Alinikula, Jukka;Schatz, David G.
体细胞超突变（SH）在活化的B细胞的重排免疫球蛋白（IG）基因内产生点突变，为抗体的亲和力成熟提供遗传多样性。SH需要激活诱导的胞苷脱氨酶（AID）蛋白和突变靶序列的转录，但如何实现突变的IG基因特异性仍然是难以捉摸的。我们在这里使用一个敏感的和仔细控制的测定表明，IG增强子强烈激活SH在相邻的基因，即使他们的刺激转录是微不足道的。已知某些E-box、NFB、MEF 2或Ets家族结合位点的突变对IG增强子的转录作用很重要，但这些突变可使增强子的活性减弱或消失。SH的完全激活通常需要多个IG增强子和增强子样元件的组合。该机制在进化上是保守的，因为哺乳动物IG λ和IG重链内含子增强子有效地刺激鸡细胞中的超突变。我们的研究结果表明，一种新的调节功能的IG增强子，表明他们要么招募艾滋病或改变附近的transcription units.Author摘要在B细胞免疫应答，免疫球蛋白（IG）基因受到一个独特的突变过程称为体细胞超突变，使免疫系统产生高亲和力的抗体。相对于其他基因，体细胞超突变优先影响IG基因，这对于防止可能导致B细胞癌症的一般基因组突变的灾难性水平是重要的。我们假设，这种体细胞超突变的优先靶向是由IG基因中或附近的特定DNA序列辅助的，这些DNA序列将突变机制的作用集中在这些基因上。在这项研究中，我们发现人类、小鼠和鸡的IG基因确实含有这种突变靶向序列，并且它们与称为增强子的转录调控区一致。我们发现，IG增强子的组合合作，以实现强大的突变靶向，这种行动取决于众所周知的转录因子结合位点在这些增强子元件。我们的研究结果建立了一个进化上保守的功能，增强子体细胞超突变靶向，其运作机制不同于传统的增强子功能的转录水平的增加。我们提出，IG增强子的组合通过募集突变机制和/或通过使IG基因成为更好的突变底物来靶向IG基因的体细胞突变。
Somatic hypermutation (SH) generates point mutations within rearranged immunoglobulin (Ig) genes of activated B cells, providing genetic diversity for the affinity maturation of antibodies. SH requires the activation-induced cytidine deaminase (AID) protein and transcription of the mutation target sequence, but how the Ig gene specificity of mutations is achieved has remained elusive. We show here using a sensitive and carefully controlled assay that the Ig enhancers strongly activate SH in neighboring genes even though their stimulation of transcription is negligible. Mutations in certain E-box, NFB, MEF2, or Ets family binding sitesknown to be important for the transcriptional role of Ig enhancersimpair or abolish the activity. Full activation of SH typically requires a combination of multiple Ig enhancer and enhancer-like elements. The mechanism is evolutionarily conserved, as mammalian Ig lambda and Ig heavy chain intron enhancers efficiently stimulate hypermutation in chicken cells. Our results demonstrate a novel regulatory function for Ig enhancers, indicating that they either recruit AID or alter the accessibility of the nearby transcription units.Author Summary During the B cell immune response, immunoglobulin (Ig) genes are subject to a unique mutation process known as somatic hypermutation that allows the immune system to generate high-affinity antibodies. Somatic hypermutation preferentially affects Ig genes, relative to other genes, and this is important in preventing catastrophic levels of general genomic mutations that could lead to B cell cancers. We hypothesized that this preferential targeting of somatic hypermutation is assisted by specific DNA sequences in or near Ig genes that focus the action of the mutation machinery on those genes. In this study, we show that Ig genes across speciesfrom human, mouse, and chickendo indeed contain such mutation targeting sequences and that they coincide with transcriptional regulatory regions known as enhancers. We show that combinations of Ig enhancers cooperate to achieve strong mutation targeting and that this action depends on well-known transcription factor binding sites in these enhancer elements. Our findings establish an evolutionarily conserved function for enhancers in somatic hypermutation targeting, which operates by a mechanism distinct from the conventional enhancer function of increasing levels of transcription. We propose that combinations of Ig enhancers target somatic mutation to Ig genes by recruiting the mutation machinery and/or by making the Ig genes better substrates for mutation.