Chromatin-intrinsic mechanisms determine orientation-specific class switch recombination

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Antibody class switch recombination (CSR) generates diverse antibody isotypes by replacing Cμ constant region exons with downstream exons.
• The mechanisms controlling orientation-specific productive CSR remain poorly understood.

Purpose of the Study:

• To systematically dissect determinants of productive CSR by analyzing evolved constant regions and recapitulating CSR in jawed vertebrates.
• To elucidate how switch topological configuration influences orientation-specific joining of AID-initiated breaks.

Main Methods:

• Analysis of evolved immunoglobulin heavy chain (Igh) constant regions.
• Construction of diversified constant regions to recapitulate productive CSR.
• Investigation of switch topological configuration (STC) including transcriptional orientation, chromatin distance, and chromatin domain.

Main Results:

• Switch topological configuration (STC) determines orientation-specific joining of activation-induced cytidine deaminase (AID)-initiated breaks for productive CSR.
• Long-distance C_Hs under co-oriented transcription within the Igh domain favor deletional joining-mediated productive CSR.
• Oppositely transcribed and short-distance C_Hs promote diffusion-mediated inversional joining, leading to less efficient CSR.
• AID-initiated breaks in different domains facilitate diffusion-mediated, orientation-unbiased end-joining for non-productive CSR.

Conclusions:

• Chromatin-intrinsic mechanisms safeguard orientation-specific productive CSR throughout evolution.
• STC, encompassing transcriptional orientation, chromatin distance, and domain organization, is a key determinant of productive CSR outcomes.