1. Experimental Immunology Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA
2. Genetics Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA
3. Laboratory of Molecular Pharmacology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA
4. Laboratory of Molecular Immunology, The Rockefeller University, and Howard Hughes Medical Institute, New York, New York 10021, USA
5. Department of Medical Chemistry, Graduate School of Medicine, Kyoto University, Kyoto 606-8501, Japan
6. These authors contributed equally to this work

Correspondence to: André Nussenzweig^1,2 Correspondence and requests for materials should be addressed to A.N. (e-mail: Email: andre_nussenzweig@nih.gov) or M.C.N. (e-mail: Email: nussen@mail.rockefeller.edu).

Abstract

Class switch recombination (CSR) is a region-specific DNA recombination reaction that replaces one immunoglobulin heavy-chain constant region (Ch) gene with another. This enables a single variable (V) region gene to be used in conjunction with different downstream Ch genes, each having a unique biological activity. The molecular mechanisms that mediate CSR have not been defined, but activation-induced cytidine deaminase (AID), a putative RNA-editing enzyme, is required for this reaction¹. Here we report that the Nijmegen breakage syndrome protein (Nbs1) and phosphorylated H2A histone family member X (-H2AX, also known as -H2afx), which facilitate DNA double-strand break (DSB) repair^{2, 3, 4}, form nuclear foci at the Ch region in the G1 phase of the cell cycle in cells undergoing CSR, and that switching is impaired in H2AX^-/- mice. Localization of Nbs1 and -H2AX to the Igh locus during CSR is dependent on AID. In addition, AID is required for induction of switch region (S)-specific DNA lesions that precede CSR. These results place AID function upstream of the DNA modifications that initiate CSR.