Regulation of somatic hypermutation by higher-order chromatin structure [Tri-C]

The generation of protective antibodies by somatic hypermutation (SHM) is essential for antibody maturation and adaptive immunity. SHM involves co-transcriptional mutagenesis of immunoglobulin variable (V) regions regulated by enhancers located hundreds of kilobases away. How 3D chromatin structure affects SHM is poorly understood. Here, we measure higher-order interactions on single alleles of the human immunoglobulin heavy chain locus (IGH) using Tri-C. We find that SHM is underpinned by a multiway hub wherein the V region is proximal to all enhancers. Cohesin-mediated loop extrusion is dispensable for IGH transcription and hub architecture. Transcription and mutagenesis of IGH switch regions, which are necessary for antibody class-switch recombination, creates new chromatin loops that can form without cohesin. However, these additional loops do not compromise hub integrity, V region transcription or SHM. Thus, antibody maturation occurs within a multiway hub accommodating several gene-enhancer loops wherein transcription and mutagenesis of different segments can occur non-competitively. Examination of the multiway-interactome at the IGH locus through the chromatin conformation caputure technique Tri-C.

体细胞超突变（somatic hypermutation, SHM）介导保护性抗体的产生，该过程是抗体成熟与适应性免疫的核心环节。SHM涉及免疫球蛋白可变区（V区）的共转录诱变，该过程受到位于数百千碱基对之外的增强子调控。目前学界对三维染色质结构如何影响SHM的机制仍知之甚少。本研究采用Tri-C技术，对人类免疫球蛋白重链基因座（IGH）的单等位基因高阶相互作用进行检测。研究发现，SHM的分子基础为一个多向枢纽：可变区与所有增强子均处于近端邻近状态。黏连蛋白介导的环挤出过程对于IGH的转录及枢纽架构并非必需。抗体类别转换重组所必需的IGH开关区的转录与诱变，可形成不依赖黏连蛋白的新型染色质环，但这类额外环并不会破坏枢纽完整性、V区转录或SHM过程。由此可见，抗体成熟发生于一个可容纳多个基因-增强子环的多向枢纽中，不同区段的转录与诱变可在此处非竞争性地开展。本研究通过染色质构象捕获技术Tri-C，对IGH基因座的多向相互作用组进行了分析。