Single-strand DNA breaks in Ig class switch recombination that depend on UNG but not AID

• Published in: International immunology Vol. 20; no. 11; pp. 1381 - 1393
• Main Authors: Arudchandran, Arulvathani, Bernstein, Ralph M., Max, Edward E.
• Format: Journal Article
• Language: English
• Published: England Oxford University Press 01.11.2008; Oxford Publishing Limited (England)
• Subjects: Animals; antibodies; B cells; B-Lymphocytes - metabolism; Cells, Cultured; Cytidine Deaminase - genetics; Cytidine Deaminase - metabolism; DNA Breaks, Single-Stranded; gene rearrangement; Genes, Immunoglobulin - immunology; Immunoglobulin Class Switching - genetics; Immunoglobulin Isotypes - biosynthesis; Immunoglobulin Isotypes - genetics; Lymphocyte Activation; Mice; Mice, Inbred C57BL; Mice, Knockout; molecular biology; Polymerase Chain Reaction; Recombination, Genetic - immunology; Uracil-DNA Glycosidase - genetics; Uracil-DNA Glycosidase - metabolism; molecular biology; antibodies; B cells; gene rearrangement
• ISSN: 0953-8178; 1460-2377
• DOI: 10.1093/intimm/dxn097

B lymphocytes switch from secreting IgM to secreting IgG, IgA or IgE through a DNA recombination, class switch recombination (CSR), whose mechanism is incompletely understood. CSR is thought to be triggered by activation-induced deaminase (AID), which is believed to deaminate cytosines to uracil in single-strand regions of switch region DNA. Subsequent excision of uracils by uracil DNA glycosylase (UNG) (product of the UNG gene) generates abasic sites, which are targeted for DNA cleavage, producing DNA breaks that are critical intermediates in CSR. Consistent with this model, CSR-related double-strand breaks (DSBs)––detected by ligation-mediated PCR (LMPCR)––have been reported to be dramatically reduced in B cells from either AID−/− or UNG−/− mice. Here we examine single-strand breaks (SSBs) using LMPCR and report, surprisingly, that CSR-related anti-sense strand breaks in Sγ regions are dependent only on UNG, and not AID, suggesting participation of a cytosine deaminase other than AID. This conclusion is supported by the sequences at these DNA breaks, which show a bias for a consensus sequence different from that reported for AID. The SSBs appear to be part of the normal CSR pathway since in B cells in which CSR is blocked by deletion of Sμ, the content of Sγ SSBs is elevated as though the breaks resolve inefficiently owing to the lack of a recombination partner for completing μ-to-γ CSR. These results suggest a narrower role for AID in CSR than previously recognized and prompt a search for a putative alternative cytosine deaminase participating in CSR.