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Mol Immunol. 2015 Oct;67(2 Pt B):524-31. doi: 10.1016/j.molimm.2015.07.039. Epub 2015 Aug 12.

The repetitive portion of the Xenopus IgH Mu switch region mediates orientation-dependent class switch recombination.

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USC Norris Comprehensive Cancer Ctr. Molecular and Computational Biology Program, Department of Biological Sciences, Departments of Pathology, Biochemistry & Molecular Biology, Molecular Microbiology & Immunology, Urology, University of Southern California Keck School of Medicine, 1441 Eastlake Ave., Rm, 5428, Los Angeles, CA 90089-9176, United States.
Department of Physiology and Pharmacology, State University of New York Downstate Medical Center, New York, NY 11203, United States.
Department of Microbiology and Molecular Genetics, Michigan State University, 5175 Biomedical Physical Sciences, East Lansing, MI 48824, United States.


Vertebrates developed immunoglobulin heavy chain (IgH) class switch recombination (CSR) to express different IgH constant regions. Most double-strand breaks for Ig CSR occur within the repetitive portion of the switch regions located upstream of each set of constant domain exons for the Igγ, Igα or Igϵ heavy chain. Unlike mammalian switch regions, Xenopus switch regions do not have a high G-density on the non-template DNA strand. In previous studies, when Xenopus Sμ DNA was moved to the genome of mice, it is able to support substantial CSR when it is used to replace the murine Sγ1 region. Here, we tested both the 2kb repetitive portion and the 4.6 kb full-length portions of the Xenopus Sμ in both their natural (forward) orientation relative to the constant domain exons, as well as the opposite (reverse) orientation. Consistent with previous work, we find that the 4.6 kb full-length Sμ mediates similar levels of CSR in both the forward and reverse orientations. Whereas, the forward orientation of the 2kb portion can restore the majority of the CSR level of the 4.6 kb full-length Sμ, the reverse orientation poorly supports R-looping and no CSR. The forward orientation of the 2kb repetitive portion has more GG dinucleotides on the non-template strand than the reverse orientation. The correlation of R-loop formation with CSR efficiency, as demonstrated in the 2kb repetitive fragment of the Xenopus switch region, confirms a role played by R-looping in CSR that appears to be conserved through evolution.


AID; Activation-induced deaminase; Amphibian; Antibody; B Cell; Chromosomal rearrangement; Gene rearrangement; Genetic instability; Immune system; Immunoglobulin; Isotype switch; RNA:DNA hybrid; Recombination; Secondary response; Xenopus

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