Phase separation as an emerging regulatory framework in antibody class switching and genome stability.

The spatial and temporal organization of nuclear processes is increasingly interpreted through principles associated with liquid-liquid phase separation (LLPS), whereby multivalent interactions among proteins and nucleic acids generate dynamic, membraneless assemblies. In DNA repair, such assemblies have been proposed to coordinate damage sensing, signaling, and repair pathway choice; however, their causal contribution in physiological immune contexts remains under active investigation. Antibody class switch recombination (CSR) provides a stringent immunological model in which to examine these concepts, as activated B lymphocytes must efficiently rejoin programmed DNA double-strand breaks (DSBs) across long genomic distances while suppressing aberrant chromosomal rearrangements. Emerging evidence indicates that CSR involves dynamic RNA-protein assemblies enriched for 53BP1, heterogeneous nuclear ribonucleoproteins such as HNRNPU, and transcription-associated RNA scaffolds, with properties consistent with biomolecular condensation. These assemblies are proposed to function as a CSR-specific regulatory hub-or "switchosome"-that concentrates non-homologous end joining factors, enforces repair pathway choice, and integrates transcription, RNA structure, and chromatin architecture at immunoglobulin heavy-chain (