Regulation of somatic hypermutation by higher-order chromatin structure

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Somatic hypermutation (SHM) is crucial for adaptive immunity and antibody maturation.
• SHM involves mutagenesis of immunoglobulin variable (V) regions, regulated by distant enhancers.
• The role of 3D chromatin topology in SHM is not well understood.

Purpose of the Study:

• To investigate how 3D chromatin topology influences SHM at the human immunoglobulin heavy-chain locus (IGH).
• To elucidate the structural organization of the IGH locus during antibody gene diversification.

Main Methods:

• Utilized Tri-C technology to measure higher-order chromatin interactions on single IGH alleles.
• Analyzed the impact of cohesin and transcription/mutagenesis of switch regions on chromatin structure and SHM.

Main Results:

• Identified a multiway hub structure where the IGH V region is proximal to all enhancers.
• Found cohesin-mediated loop extrusion is not essential for IGH transcription or hub architecture.
• Demonstrated that transcription and mutagenesis of switch regions create new loops without compromising hub integrity or SHM.

Conclusions:

• Antibody maturation occurs within a stable multiway hub that accommodates multiple gene-enhancer loops.
• Transcription and mutagenesis of different IGH segments occur non-competitively within this structural framework.
• The 3D chromatin topology, specifically the multiway hub, is essential for efficient SHM and antibody maturation.