53BP1 Is Targeted to Sites of DNA Breaks within the Immunoglobulin Heavy Chain Locus and Promotes Class Switch Recombination.

The maintenance of genomic integrity relies on the cellular response to chromosomal damage from both exogenous (e.g. ionizing radiation) and endogenous (e.g. oxidative stress) sources. Various members of the DNA damage-sensing pathway including ATM, H2AX, 53BP1, and MDC1 are necessary to orchestrate the repair of DNA breaks. B cells undergo several programmed DNA alterations during their development: V(D)J recombination, Somatic Hypermutation (SHM), and Class Switch Recombination (CSR). We have previously shown that 53BP1 is relatively dispensable for V(D)J recombination and SHM. In contrast, class switch recombination is largely blocked to all isotypes indicating that regulated DNA breaks in B cells are regarded differentially by the DNA damage response machinery. 53BP1 is thought to promote the joining of DNA ends during CSR thus preventing translocations that could potentially lead to lymphoma. To better understand the damage response to CSR induced DNA breaks, a chromatin immunoprecipitation strategy and a combined immunofluorescence/FISH method was used to examine the components that assemble at IgH switch (S) regions during CSR. H2AX was found at S regions specifically targeted to undergo CSR after in vitro stimulation of B cells, and to a lesser degree, at adjacent S regions that were not activated for a switch event. H2AX was also found at S regions in switch activated 53BP1-deficient B cells. In contrast, 53BP1 was found primarily at S regions specifically targeted for CSR, and not at the adjacent S regions. Moreover, the localization of 53BP1 to S regions appeared to be in part, independent of DNA breaks, and potentially reliant on specialized DNA structures that are generated during CSR. These findings support a differential role for the various components of the DNA damage response program during CSR and have implications for understanding mechanisms of lymphomagenesis.