Constraints contributed by chromatin looping limit recombination targeting during Ig class switch recombination

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Long-range DNA looping interactions between promoters and distal elements are critical for regulating immunoglobulin class switch recombination (CSR).
• The precise mechanisms governing spatial and regulatory relationships among Igh transcriptional elements are not fully understood.

Purpose of the Study:

• To investigate the chromosome conformation of C region (CH) loci targeted for CSR in mature B lymphocytes.
• To elucidate the roles of transcription factors STAT6 and NF-κB in regulating germline transcription (GLT) and locus:enhancer looping.
• To determine the contribution of promoter elements and transcription elongation to these interactions.

Main Methods:

• Analysis of chromosome conformation in mature B lymphocytes.
• Investigation of germline transcription (GLT) of γ1 and ε CH loci.
• Studies involving STAT6-deficient and NF-κB p50-deleted mice.
• Comparative analysis of endogenous γ1 locus and knock-in heterologous promoter in mice.

Main Results:

• STAT6 deficiency abolished GLT, but NF-κB p50 deletion eliminated both GLT and γ1 locus:enhancer looping.
• Chromatin looping between CH loci and Igh enhancers is independent of GLT and STAT6, but requires NF-κB p50 for establishment and maintenance.
• The promoter itself, not transcription elongation, acts as the looping element.
• Interposition of a heterologous promoter altered GLT and abolished direct IgG2b switching, maintaining a sequential μ→γ3→γ2b format.

Conclusions:

• NF-κB p50 is essential for establishing and maintaining long-range chromatin looping interactions at Igh loci.
• Promoter/enhancer looping can impose negative constraints on distal promoters, influencing germline transcription and CSR targeting.
• These findings clarify the regulatory principles of Igh transcriptional elements and their role in CSR.