MBD4 Facilitates Immunoglobulin Class Switch Recombination

ABSTRACT

Immunoglobulin heavy chain class switch recombination (CSR) requires targeted formation of DNA double-strand breaks (DSBs) in repetitive switch region elements followed by ligation between distal breaks. The introduction of DSBs is initiated by activation-induced cytidine deaminase (AID) and requires base excision repair (BER) and mismatch repair (MMR). The BER enzyme methyl-CpG binding domain protein 4 (MBD4) has been linked to the MMR pathway through its interaction with MutL homologue 1 (MLH1). We find that when Mbd4 exons 6 to 8 are deleted in a switching B cell line, DSB formation is severely reduced and CSR frequency is impaired. Impaired CSR can be rescued by ectopic expression of Mbd4. Mbd4 deficiency yields a deficit in DNA end processing similar to that found in MutS homologue 2 (Msh2)- and Mlh1-deficient B cells. We demonstrate that microhomology-rich S-S junctions are enriched in cells in which Mbd4 is deleted. Our studies suggest that Mbd4 is a component of MMR-directed DNA end processing.

KEYWORDS:

• Ig class switch recombination
• mismatch repair
• B cells
• Ig class switch
• uracil glycosylase

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Articles of Significant Interest Selected from This Issue by the Editors

Supplemental material for this article may be found at https://doi.org/10.1128/MCB.00316-16.

ACKNOWLEDGMENTS

We thank the UIC Research Resources Center Genomics lab for expert microarray analyses, J. Stavnezer and C. Schrader (University of Massachusetts) for the compendium of DSBs from B cells, F. Alt (Harvard) for pLNTK, and A. Bellacosa (Fox Chase) for Mbd4^Δ2-5/Δ2-5 mice. We thank J. Stavnezer for critical reading of the manuscript and H. Shen for expert technical help.

We declare no competing financial interests.