Overview of IgG1 CSR at the murine Igh locus. The locus is drawn to scale. The Igh genes are indicated as well as the Eu, Eg1 and 3'RR enhancers and the 3'CBE which contains ten CTCF-binding sites. Each Igh gene consists of a promoter (only the Iu and Ig1 promoters are shown); repetitive switch (S) sequences (orange for Su, green for Sg1, and red for all others); and C region exons (orange for Cu, green for Cg1, and black for all others). The four enhancers (hypersensitive sites) comprising the 3'RR are indicated as black bars within the 3'RR (hs3a, hs1,2, hs3b, and hs4). In resting, IgM-expressing (IgM+) B cells, the V exon is spliced to the first constant (C) region (Cu) resulting in IgM mRNA expression. A noncoding germline transcript (GLT) is also made from the Iu promoter (not indicated) that lies at the 3' end of the Eu enhancer. In this example, activation of B cells leads to germline transcription from the Ig1 promoter resulting in the Ighg1 GLT. B cell activation also triggers expression of AID which generates lesions in donor S (Su) and acceptor S (Sg1) regions. Processing of these lesions by DNA repair proteins leads to DNA double-strand breaks in the S regions that are used as substrates for deletional recombination between Su and Sg1 via nonhomologous end joining. This results in the excision of the intervening DNA as a circular product and the generation of IgG1 mRNA and IgG1-expressing (IgG1+) B cells. The chromatin loop diagram on the right depicts the current model of CSR wherein cohesin-mediated loop extrusion creates a 3-way conformation that aligns active S regions in the proximity of the 3'RR. This results in the proposed coupling of transcriptional activation and S-S synapsis.15,28
