Nuclear positioning rather than contraction supports ordered rearrangements of immunoglobulin loci. Mus musculus breed:C57BL/6J

Progenitor-B cells recombine V, D and J genes in their immunoglobulin (Ig) loci to create a receptor with unique specificity. Despite a common recombination machinery, Ig heavy and Ig light chain gene rearrangements take place in sequential differentiation stages. To investigate which epigenetic changes are decisive for the stepwise rearrangements, we studied Ig locus contraction, nuclear positioning and long-range chromatin interactions with 3D DNA FISH and 3C-seq in E2A-/- and Rag-/- progenitor-B cells with germline Ig genes. This integrated approach showed that both Ig loci were contracted in pro-B as well as pre-B cells with more frequent interactions between enhancers and V genes than in pre-pro-B cells. A functional Ig intron enhancer was unimportantredundant for this large-scale locus contraction. In contrast, positioning away from the nuclear lamins was tightly regulated for Igh in pro-B cells and Ig in pre-B cells. Thus, on top of Ig locus contraction that juxtaposes genomically distant elements to mediate efficient recombination, sequential positioning of the Igh and Ig loci away from the nuclear periphery enablessupports stage-specific accessibility of Ig loci.