Pre-B cells suppress RAG expression in response to DNA double-strand breaks (HEM1P.225)

The ability of the Rag1/Rag2 (RAG) endonuclease to assemble antigen receptor (AgR) genes is essential for adaptive immunity. However, aberrant induction or repair of RAG-induced DNA double strand breaks (DSBs) can lead to oncogenic AgR translocations. We have previously shown that RAG-induced DSBs in pre-B cells activate the ATM kinase to prevent RAG cleavage of the homologous allele, and to inhibit expression of Rag1 and Rag2. These breaks also suppress expression of Gadd45α, which promotes Rag1 and Rag2 transcription. Since DSBs induced by ionizing radiation (IR) signal through ATM to increase Gadd45α expression in all other cell types including mature B cells, these results indicate that RAG DSBs and/or pre-B cells use unique mechanisms to regulate Gadd45α expression. We now demonstrate that IR-induced DSBs in pre-B cells signal through the ATM kinase to down-regulate Gadd45α expression. We also demonstrate that these IR DSBs signal ATM-dependent suppression of Rag1 and Rag2 transcription, without affecting stability of Rag1 or Rag2 mRNA. Because the presence of multiple DSBs in a cell greatly increases the risk of translocation, we hypothesize that pre-B cells have developed a unique developmental stage-specific DSB response to suppress the induction of RAG DSBs in the presence of other DSBs. Our ongoing studies will determine the role of this pre-B cell specific DSB response in suppressing AgR translocations and enforcing mono-allelic assembly of immunoglobulin genes.