BRCT-domain protein BRIT1 influences class switch recombination

Significance Class switch recombination (CSR), a B-cell–specific reaction essential for optimal antibody responses, proceeds through the deliberate generation of DNA breaks. Although these breaks are obligatory intermediates of class switching, their improper resolution not only impairs immune responses but also promotes oncogenic translocations. The pathways and mechanisms that participate in the repair of such lesions are yet to be fully elucidated. In this work, we have identified a BRCT domain containing protein BRIT1 as an effector of the DNA repair phase of CSR. We show that B cells lacking BRIT1 are impaired in undergoing class switching due to an inability to repair DNA breaks efficiently. This work thus has significant implications in both immunity and cancer. DNA double-strand breaks (DSBs) serve as obligatory intermediates for Ig heavy chain (Igh) class switch recombination (CSR). The mechanisms by which DSBs are resolved to promote long-range DNA end-joining while suppressing genomic instability inherently associated with DSBs are yet to be fully elucidated. Here, we use a targeted short-hairpin RNA screen in a B-cell lymphoma line to identify the BRCT-domain protein BRIT1 as an effector of CSR. We show that conditional genetic deletion of BRIT1 in mice leads to a marked increase in unrepaired Igh breaks and a significant reduction in CSR in ex vivo activated splenic B cells. We find that the C-terminal tandem BRCT domains of BRIT1 facilitate its interaction with phosphorylated H2AX and that BRIT1 is recruited to the Igh locus in an activation-induced cytidine deaminase (AID) and H2AX-dependent fashion. Finally, we demonstrate that depletion of another BRCT-domain protein, MDC1, in BRIT1-deleted B cells increases the severity of CSR defect over what is observed upon loss of either protein alone. Our results identify BRIT1 as a factor in CSR and demonstrate that multiple BRCT-domain proteins contribute to optimal resolution of AID-induced DSBs.

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