Targeting Of Somatic Hypermutation By immunoglobulin Enhancer And Enhancer-Like Sequences

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, NFκB, MEF2, or Ets family binding sites—known to be important for the transcriptional role of Ig enhancers—impair 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.体细胞超突变（Somatic hypermutation, SH）可在活化B细胞的重排免疫球蛋白（immunoglobulin, Ig）基因内部诱导点突变，为抗体的亲和力成熟提供遗传多样性。该过程依赖活化诱导胞苷脱氨酶（activation-induced cytidine deaminase, AID）以及突变靶序列的转录，但突变的免疫球蛋白基因特异性是如何实现的，这一问题至今仍未阐明。本研究通过灵敏且严格受控的实验检测发现，尽管免疫球蛋白增强子对转录的激活作用微乎其微，但其可显著活化邻近基因的体细胞超突变过程。已知免疫球蛋白增强子的转录功能依赖特定的E-box、NFκB、MEF2或Ets家族结合位点，若这些位点发生突变，则会削弱甚至完全消除其活化体细胞超突变的能力。体细胞超突变的完全活化通常需要多个免疫球蛋白增强子及类增强子元件协同作用。该机制在进化上具有保守性，哺乳动物免疫球蛋白λ链及重链内含子增强子可在鸡细胞中有效促进体细胞超突变的发生。本研究结果揭示了免疫球蛋白增强子的全新调控功能，表明其要么通过招募活化诱导胞苷脱氨酶，要么通过改变邻近转录单元的染色质可及性来发挥作用。