title: Detection of chromatin-associated single-stranded DNA in regions targeted for somatic hypermutation.
abstract: After encounter with antigen, the antibody repertoire is shaped by somatic hypermutation (SHM), which leads to an increase in the affinity of antibodies for the antigen, and class-switch recombination (CSR), which results in a change in the effector function of antibodies. Both SHM and CSR are initiated by activation-induced cytidine deaminase (AID), which deaminates deoxycytidine to deoxyuridine in single-stranded DNA (ssDNA). The precise mechanism responsible for the formation of ssDNA in V regions undergoing SHM has yet to be experimentally established. In this study, we searched for ssDNA in mutating V regions in which DNA-protein complexes were preserved in the context of chromatin in human B cell lines and in primary mouse B cells. We found that V regions that undergo SHM were enriched in short patches of ssDNA, rather than R loops, on both the coding and noncoding strands. Detection of these patches depended on the presence of DNA-associated proteins and required active transcription. Consistent with this, we found that both DNA strands in the V region were transcribed. We conclude that regions of DNA that are targets of SHM assemble protein-DNA complexes in which ssDNA is exposed, making it accessible to AID.
authors.affiliation: Department of Cell Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
authors.forename: Diana
authors.initials: D
authors.lastname: Ronai
authors.forename: Maria D
authors.initials: MD
authors.lastname: Iglesias-Ussel
authors.forename: Manxia
authors.initials: M
authors.lastname: Fan
authors.forename: Ziqiang
authors.initials: Z
authors.lastname: Li
authors.forename: Alberto
authors.initials: A
authors.lastname: Martin
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Scharff
mesh_terms: D000818:Animals; D001402:B-Lymphocytes / Q000276:immunology / Q000378:metabolism; D001483:Base Sequence; D002460:Cell Line; D002478:Cells, Cultured; D002843:Chromatin / Q000235:genetics*; D003564:Cytidine Deaminase / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D004277:DNA, Single-Stranded / Q000235:genetics* / Q000378:metabolism; D006801:Humans; D019169:Jurkat Cells; D051379:Mice; D018345:Mice, Knockout; D027041:Somatic Hypermutation, Immunoglobulin; D014158:Transcription, Genetic; D000098830:AICDA (Activation-Induced Cytidine Deaminase)
chemical_list: D002843:Chromatin; D004277:DNA, Single-Stranded; D000098830:AICDA (Activation-Induced Cytidine Deaminase); D003564:Cytidine Deaminase